Foldable substrate, foldable device and manufacturing method

Abstract

In aspects, the lithium oxide concentration at the surface is from about 0.2 mol% to about 2 mol% higher than the lithium oxide concentration at the midpoint. In aspects, the lithium oxide concentration at the surface is from about 0.2 mol% to about 2 mol%. In aspects, the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the surface is from about 5 mol% to about 15 mol%. In aspects, a ratio of a total concentration of the potassium oxide, rubidium oxide, cesium oxide, and francium oxide at a surface to a concentration of lithium oxide at the surface is from about 1 to about 20. The method includes immersing the substrate in a molten salt bath at a temperature of 380 DEG C or higher for about 1 minute to about 10 minutes or about 3 minutes to about 2 hours.

Description

[0001] Cross - reference to related applications

[0002] This application claims priority to U.S. Provisional Application No. 63 / 443,846, filed Feb. 7, 2023, and U.S. Provisional Application No. 63 / 421,241, filed Nov. 1, 2022, under 35 U.S.C. § 119, the content of each of which is hereby incorporated by reference in its entirety and made a part of this application. Technical field

[0003] The present disclosure generally relates to foldable substrates, foldable devices, and manufacturing methods, and more particularly, to foldable substrates including a lithium oxide concentration profile and / or a first central surface region recessed from a first major surface, foldable devices including the foldable substrates, and a method of manufacturing a foldable substrate including a plurality of ion - exchange baths. Background art

[0004] Glass substrates are commonly used, for example, in display devices such as liquid crystal displays (LCDs), electrophoretic displays (EPDs), organic light - emitting diode displays (OLEDs), plasma display panels (PDPs), and the like.

[0005] There is a desire to develop foldable versions of displays and foldable protective covers mounted on foldable displays. The foldable displays and covers should have good impact and puncture resistance. At the same time, the foldable displays and covers should have a small parallel - plate spacing (e.g., about 10 millimeters (mm) or less). However, plastic displays and covers with a small parallel - plate spacing often have poor impact and / or puncture resistance. In addition, it has been conventionally thought that ultra - thin glass sheets with a small parallel - plate spacing (e.g., a thickness of about 75 micrometers (μm or microns) or less) often have poor impact and / or puncture resistance. Moreover, thicker glass sheets with good impact and / or puncture resistance (e.g., greater than 125 micrometers) often have a relatively large parallel - plate spacing (e.g., about 50 millimeters or more). Accordingly, there is a need to develop foldable devices with a small minimum parallel - plate spacing, good impact resistance, good puncture resistance, and no buckling. Summary of the invention

[0006] This document presents a foldable device including a foldable substrate, the foldable substrate, and methods of manufacturing the foldable device and the foldable substrate including the foldable substrate. The foldable substrate includes a first portion, a second portion, and a central portion therebetween. The substrate and / or the portions may include glass and / or ceramic portions, which may provide good dimensional stability, a reduced incidence of mechanical instability, good impact resistance, and / or good puncture resistance. These portions may include glass and / or ceramic portions, which include one or more regions of compressive stress, which may further enhance impact resistance and / or puncture resistance. By providing a substrate including a glass and / or ceramic substrate, such a substrate can also enhance impact resistance and / or puncture resistance while facilitating good folding performance. In various aspects, the substrate thickness may be large enough (e.g., from about 50 micrometers (micrometers or μm) to about 2 millimeters) to further enhance impact resistance and puncture resistance. Providing a foldable substrate including a central portion having a central thickness less than the substrate thickness (e.g., a first thickness of the first portion and / or a second thickness of the second portion) (e.g., less by about 10 μm or more) can achieve a smaller parallel plate spacing (e.g., about 10 millimeters or less) based on the reduced central portion thickness, thereby enabling foldability and / or rollability of the foldable substrate and / or the foldable device.

[0007] In various aspects, the foldable device and / or the foldable substrate may include one or more grooves, e.g., a first central surface region recessed a first distance from a first major surface and / or a second central surface region recessed a second distance from a second major surface. Providing opposing first and second grooves can provide a central thickness less than the substrate thickness. Additionally, providing opposing first and second grooves can reduce the strain caused by the maximum bending of the foldable device, e.g., the strain caused by bending between the central portion and the first portion and / or the second portion, because the central portion including the central thickness can be closer to the intermediate axis of the foldable device and / or the foldable substrate compared to the case of providing only a single groove. Further, making the first distance substantially equal to the second distance can reduce the incidence of mechanical instability in the central portion, e.g., because the foldable substrate is symmetric about a plane including the midpoints of the substrate thickness and the central thickness. Alternatively, providing at least one groove on only one side of the foldable substrate can provide a smooth major surface, e.g., the major surface can face the user and / or provide a uniform tactile feel. Also, providing at least one groove on only one side of the foldable substrate can be completed with a single chemical strengthening process, reducing processing time, space, materials, and cost, and potentially increasing production.

[0008] In various aspects, a foldable device and / or a foldable substrate may include a first transition region attaching a central portion to a first portion and / or a second transition region attaching the central portion to a second portion. Providing a transition region with smooth and / or monotonically decreasing (e.g., continuously decreasing) thickness can reduce stress concentration in the transition region and / or avoid optical distortion. Providing a transition region with sufficient length (e.g., about 0.15 mm or longer, or about 0.3 mm or longer) can avoid optical distortion that would otherwise occur due to a sharp change in the thickness of the foldable substrate. Making the average transition angle of the first transition surface region of the first transition region relative to the first central surface region large enough (e.g., about 167° or greater or about 170° or greater) can avoid optical distortion and / or reduce the visibility of the transition region. Making the average transition angle small enough (e.g., about 179° or smaller or about 176° or smaller) can reduce the number of foldable devices and / or foldable substrates having an intermediate thickness that may reduce impact resistance and / or puncture resistance.

[0009] The present disclosure unexpectedly demonstrates that by providing a Li 2 O surface concentration of about 0.2 mol% to about 2 mol% (e.g., absolute mol% and / or the amount by which the surface concentration is elevated relative to the midpoint concentration), for example, treating a foldable substrate (e.g., for a foldable substrate having a first groove and a second groove opposite the first groove) with a molten salt solution comprising about 0.02 wt% to about 0.08 wt% of a lithium salt, or treating a foldable substrate (e.g., for a foldable substrate having grooves on only one side) with a molten salt solution comprising about 0.5 wt% to about 1.5 wt% (e.g., about 0.75 wt% to about 1.25 wt%) of a lithium salt, the incidence of buckling and / or saddle warping can be reduced. Lithium (e.g., lithium salt, lithium oxide) can reduce the mismatch between the chemically strengthened expansion strains in various parts of the foldable substrate. Exchanging sodium or potassium (or a larger alkali metal) in the foldable substrate for smaller lithium in a molten salt bath ("reverse ion exchange") can counteract (e.g., reduce) the amount of expansion due to chemical strengthening caused by the simultaneous "forward ion exchange" of smaller ions (e.g., sodium) in the foldable substrate with larger ions (e.g., potassium, cesium, francium, rubidium) in the final molten salt bath. As shown in the examples discussed below, including a small amount (e.g., from about 0.02 wt% to about 0.08 wt% or from about 0.5 wt% to about 1.5 wt% depending on the geometry of the foldable substrate described herein) of a lithium salt in the final molten salt bath can unexpectedly reduce the incidence of buckling and / or warping of the foldable substrate (e.g., the central portion). However, a larger amount of lithium salt may result in larger saddle warping, e.g., the different mismatches in the chemically strengthened expansion strains in various parts of the foldable substrate caused by the contraction due to chemical strengthening resulting from the reverse ion exchange of lithium into the foldable substrate. Providing a high concentration (e.g., about 5 mol% or higher) of K2 O (e.g., the increase in absolute mol% and / or surface concentration relative to the midpoint concentration) can provide a large (e.g., about 500 MPa) surface compressive stress, thereby improving fracture resistance.

[0010] The foldable substrate can be used as a rollable substrate, and its central width is greater than the second width. Making the second width of the second portion about 15% or less of the length of the foldable substrate can provide sufficient width to handle the ends of the foldable substrate during processing, fix the foldable substrate and / or foldable device as part of an electronic device, and / or maximize the number of foldable substrates and / or foldable devices to make them part of the display portion visible to the user. Making the central portion account for about 15% to about 50% of the length of the foldable substrate can allow the display portion of the foldable device to adjust when a portion of the rollable substrate moves into and / or out of the user's line of sight, without unnecessarily enlarging the size of the corresponding device when in the fully rolled-up configuration. Making the first width of the first portion about 35% or more of the length of the foldable substrate can provide a larger display portion visible to the user while ensuring that the remaining portion of the foldable substrate (e.g., the central portion and the second portion) is substantially entirely within the occupied space of the first portion.

[0011] Some example aspects of the present disclosure are described below, but it should be understood that any feature of the aspects can be used alone or in combination with each other.

[0012] Aspect 1. A foldable device including a substrate, the substrate comprising:

[0013] A substrate thickness defined between a first major surface and a second major surface opposite the first major surface;

[0014] A first portion including the substrate thickness, a first compressive stress region extending from the first major surface to a first compressive depth, and a second compressive stress region extending from the second major surface to a second compressive depth;

[0015] A second portion including the substrate thickness, a third compressive stress region extending from the first major surface to a third compressive depth, and a fourth compressive stress region extending from the second major surface to a fourth compressive depth;

[0016] A central portion, which is located between the first portion and the second portion, the central portion includes a central thickness defined between a first central surface region and a second central surface region opposite to the first central surface region, a first central compressive stress zone extending from the first central surface region to a first central compressive depth, a second central compressive stress zone extending from the second central surface region to a second central compressive depth, the first central surface region being recessed from the first major surface by a first distance, and the central thickness being less than the substrate thickness; and

[0017] The concentration of lithium oxide at the first central surface region is about 0.2 mol% to about 2 mol% higher than the concentration of lithium oxide at the central midpoint;

[0018] Wherein the first portion includes:

[0019] A midpoint between the first major surface and the second major surface;

[0020] The concentration of lithium oxide at the first major surface;

[0021] The total concentration of potassium oxide, rubidium oxide, cesium oxide and francium oxide at the first major surface;

[0022] The concentration of lithium oxide at the midpoint; and

[0023] The total concentration of potassium oxide, rubidium oxide, cesium oxide and francium oxide at the midpoint, and

[0024] Wherein the substrate is a glass substrate or a ceramic substrate, and the central midpoint is between the first central surface region and the second central surface region, the central portion includes:

[0025] The total concentration of potassium oxide, rubidium oxide, cesium oxide and francium oxide at the first central surface region; and

[0026] The total concentration of potassium oxide, rubidium oxide, cesium oxide and francium oxide at the central midpoint.

[0027] Aspect 2. The foldable device according to aspect 1, wherein the total concentration of potassium oxide, rubidium oxide, cesium oxide and francium oxide at the first major surface is about 5 mol% to about 15 mol% higher than the total concentration of potassium oxide, rubidium oxide, cesium oxide and francium oxide at the midpoint.

[0028] Aspect 3. The foldable device according to any one of aspects 1 to 2, wherein the total concentration of potassium oxide, rubidium oxide, cesium oxide and francium oxide at the first major surface is higher than the concentration of sodium oxide at the first major surface.

[0029] Aspect 4. The foldable device according to any one of Aspects 1 to 3, wherein the total concentration of potassium oxide, rubidium oxide, cesium oxide and francium oxide in the first central surface region is higher than the concentration of sodium oxide in the first central surface region.

[0030] Aspect 5. The foldable device according to any one of Aspects 1 to 4, wherein the ratio of the total concentration of potassium oxide, rubidium oxide, cesium oxide and francium oxide at the first major surface to the concentration of lithium oxide at the first major surface is from about 1 to about 20.

[0031] Aspect 6. The foldable device according to any one of Aspects 1 to 5, wherein the ratio of the total concentration of potassium oxide, rubidium oxide, cesium oxide and francium oxide in the first central surface region to the concentration of lithium oxide in the first central surface region is from about 1 to about 20.

[0032] Aspect 7. The foldable device according to any one of Aspects 1 to 6, wherein the concentration of potassium oxide at the first major surface is from about 5 mol% to about 15 mol%.

[0033] Aspect 8. The foldable device according to any one of Aspects 1 to 6, wherein the concentration of potassium oxide in the first central surface region is about 5 mol% to about 15 mol% higher than the concentration of potassium oxide at the central midpoint.

[0034] Aspect 9. The foldable device according to any one of Aspects 1 to 6, wherein the concentration of potassium oxide in the first central surface region is from about 5 mol% to about 15 mol%.

[0035] Aspect 10. The foldable device according to any one of Aspects 1 to 6, wherein the concentration of potassium oxide in the first central surface region is substantially equal to the concentration of potassium oxide at the first major surface.

[0036] Aspect 11. The foldable device according to any one of Aspects 1 to 6, wherein the ratio of the concentration of potassium oxide at the first major surface to the concentration of lithium oxide at the first major surface is from about 1 to about 20.

[0037] Aspect 12. The foldable device according to any one of Aspects 1 to 6, wherein the ratio of the concentration of potassium oxide in the first central surface region to the concentration of lithium oxide in the first central surface region is from about 1 to about 20.

[0038] Aspect 13. The foldable device according to any one of Aspects 1 to 6, wherein the concentration distribution of potassium oxide in the first part rises relative to the concentration of potassium oxide at the midpoint to a depth of about 10% or more of the substrate thickness from the first major surface.

[0039] Aspect 14. The foldable device according to any one of Aspects 1 to 6, wherein the potassium oxide concentration distribution in the first part increases relative to the potassium oxide concentration at the midpoint to a depth of about 10 micrometers or more from the first major surface.

[0040] Aspect 15. The foldable device according to any one of Aspects 1 to 14, wherein the lithium oxide concentration at the first major surface is about 0.2 mol% to about 2 mol% higher than the lithium oxide concentration at the midpoint of the first part.

[0041] Aspect 16. The foldable device according to any one of Aspects 1 to 14, wherein the lithium oxide concentration at the first major surface is about 1 mol% to about 3 mol% higher than the lithium oxide concentration at the midpoint of the first part.

[0042] Aspect 17. The foldable device according to any one of Aspects 1 to 15, wherein the lithium oxide concentration at the first major surface is about 0.2 mol% to about 2 mol%.

[0043] Aspect 18. The foldable device according to any one of Aspects 1 to 14 and 16 (inclusive), wherein the lithium oxide concentration at the first major surface is about 1.5 mol% to about 2.5 mol%.

[0044] Aspect 19. The foldable device according to any one of Aspects 1 to 18, wherein the lithium oxide concentration in the first central surface region is about 0.2 mol% to about 2 mol%.

[0045] Aspect 20. The foldable device according to any one of Aspects 1 to 18, wherein the lithium oxide concentration in the first central surface region is about 0.75 mol% to about 1.5 mol%.

[0046] Aspect 21. The foldable device according to any one of Aspects 1 to 20, wherein the lithium oxide concentration in the first central surface region is substantially equal to the lithium oxide concentration at the first major surface.

[0047] Aspect 22. The foldable device according to any one of Aspects 1 to 21, wherein the concentration distribution of lithium oxide in the first part increases relative to the lithium oxide concentration at the midpoint to a depth of about 5% or more of the substrate thickness from the first major surface.

[0048] Aspect 23. The foldable device according to any one of Aspects 1 to 21, wherein the concentration distribution of lithium oxide in the first part increases relative to the lithium oxide concentration at the midpoint to a depth of about 3 micrometers to about 15 micrometers from the first major surface.

[0049] Aspect 24. The foldable device according to any one of aspects 1 to 21, wherein the concentration distribution of the lithium oxide in the central portion rises relative to the lithium oxide concentration at the central midpoint to a depth that is about 5% or more of the central thickness away from the first central surface region.

[0050] Aspect 25. The foldable device according to any one of aspects 1 to 21, wherein the concentration distribution of the lithium oxide in the central portion rises relative to the lithium oxide concentration at the central midpoint to a depth of about 3 microns to about 15 microns.

[0051] Aspect 26. The foldable device according to any one of aspects 1 to 25, wherein the first maximum compressive stress at the first major surface is about 500 MPa or greater.

[0052] Aspect 27. The foldable device according to aspect 26, wherein the second maximum compressive stress at the second major surface is substantially equal to the first maximum compressive stress.

[0053] Aspect 28. The foldable device according to any one of aspects 1 to 27, wherein the first central maximum compressive stress at the first central surface region is about 500 MPa or greater.

[0054] Aspect 29. The foldable device according to aspect 28, wherein the second central maximum compressive stress at the second central surface region is substantially equal to the first central maximum compressive stress.

[0055] Aspect 30. The foldable device according to any one of aspects 1 to 29, wherein the substrate thickness is about 50 microns to about 2 mm.

[0056] Aspect 31. The foldable device according to aspect 30, wherein the substrate thickness is about 90 microns to about 200 microns.

[0057] Aspect 32. The foldable device according to any one of aspects 1 to 31, wherein the central thickness is about 25 microns to about 120 microns.

[0058] Aspect 33. The foldable device according to aspect 32, wherein the central thickness is about 25 microns to about 60 microns.

[0059] Aspect 34. The foldable device according to any one of aspects 1 to 33, wherein the first distance is about 20% to about 45% of the substrate thickness.

[0060] Aspect 35. The foldable device according to any one of aspects 1 to 34, wherein the foldable device achieves a parallel plate spacing of 1 millimeter to 10 millimeters.

[0061] Aspect 36. The foldable device according to any one of aspects 1 to 34, wherein the foldable device achieves a parallel plate spacing of 5 millimeters.

[0062] Aspect 37. The foldable device according to any one of aspects 1 to 36, wherein the second central surface region is recessed from the second main surface by a second distance, and the second distance is about 20% to about 45% of the substrate thickness.

[0063] Aspect 38. The foldable device according to aspect 37, wherein the first distance is substantially equal to the second distance.

[0064] Aspect 39. The foldable device according to any one of aspects 1 to 36, wherein the second main surface is coplanar with the second central surface region.

[0065] Aspect 40. The foldable device according to any one of aspects 1 to 36, wherein the second main surface further includes the second central surface region.

[0066] Aspect 41. The foldable device according to any one of aspects 37 to 40, wherein the concentration of lithium oxide at the second central surface region is substantially equal to the concentration of lithium oxide at the first central surface region.

[0067] Aspect 42. The foldable device according to any one of aspects 37 to 40, wherein the concentration of potassium oxide at the second central surface region is substantially equal to the concentration of potassium oxide at the first central surface region.

[0068] Aspect 43. The foldable device according to any one of aspects 1 to 42, wherein the surface profile of the first central surface region along the midline between the first part and the second part exhibits a warpage of 1 millimeter or less.

[0069] Aspect 44. The foldable device according to any one of aspects 1 to 42, wherein the surface profile of the first central surface region along the midline between the first part and the second part exhibits a warpage of 600 micrometers or less.

[0070] Aspect 45. The foldable device according to any one of aspects 1 to 44, wherein the surface profile of the first central surface region has an average gradient of about 0.018 mm / mm or less.

[0071] Aspect 46. The foldable device according to any one of aspects 1 to 44, wherein the surface profile of the first central surface region has an average gradient of about 0.015 mm / mm or less.

[0072] Aspect 47. The foldable device according to any one of aspects 1 to 46, wherein the widths of the first part, the central part, and the second part are measured in a direction corresponding to the size of the substrate, and the percentage of the width of the central part in the size of the substrate is about 15% or more.

[0073] Aspect 48. The foldable device according to aspect 47, wherein the width of the second part is less than the width of the central part.

[0074] Aspect 49. The foldable device according to any one of aspects 47 to 48, wherein the percentage of the width of the central part in the size of the substrate is about 15% to about 50%.

[0075] Aspect 50. A consumer electronic product, comprising:

[0076] A housing including a front surface, a rear surface, and side surfaces;

[0077] An electrical component at least partially located within the housing, the electrical component including a controller, a memory, and a display, the display being located on or adjacent to the front surface of the housing; and

[0078] A cover substrate disposed over the display,

[0079] wherein at least one of a part of the housing or the cover substrate includes the foldable device according to any one of aspects 1 to 49.

[0080] Aspect 51. A substrate, comprising:

[0081] A substrate thickness defined between a first major surface and a second major surface opposite the first major surface;

[0082] A first compressive stress zone extending from the first major surface to a first compressive depth, and a second compressive stress zone extending from the second major surface to a second compressive depth;

[0083] The lithium oxide concentration at the first major surface is about 0.2 mol% to about 2 mol% higher than the lithium oxide concentration at the midpoint between the first major surface and the second major surface; and

[0084] The total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the first major surface is about 5 mol% to about 15 mol% higher than the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the midpoint.

[0085] Aspect 52. A substrate comprising:

[0086] A substrate thickness defined between a first major surface and a second major surface opposite the first major surface;

[0087] A first compressive stress zone extending from the first major surface to a first compressive depth, and a second compressive stress zone extending from the second major surface to a second compressive depth;

[0088] The lithium oxide concentration at the first major surface is about 1 mol% to about 3 mol% higher than the lithium oxide concentration at the midpoint between the first major surface and the second major surface; and

[0089] The total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the first major surface is about 5 mol% to about 15 mol% higher than the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the midpoint.

[0090] Aspect 53. A substrate comprising:

[0091] A substrate thickness defined between a first major surface and a second major surface opposite the first major surface;

[0092] A first compressive stress zone extending from the first major surface to a first compressive depth, and a second compressive stress zone extending from the second major surface to a second compressive depth;

[0093] The lithium oxide concentration at the first major surface is about 0.2 mol% to about 2 mol% higher than the lithium oxide concentration at the midpoint between the first major surface and the second major surface; and

[0094] The total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the first major surface is about 5 mol% to about 15 mol%.

[0095] Aspect 54. The substrate according to any one of aspects 51 to 53, wherein the potassium oxide concentration at the first major surface is about 5 mol% to about 15 mol%.

[0096] Aspect 55. The substrate according to any one of aspects 51 to 53, wherein the potassium oxide concentration at the first major surface is about 5 mol% to about 15 mol% higher than the potassium oxide concentration at the midpoint.

[0097] Aspect 56. The substrate according to any one of aspects 54 to 55, wherein the ratio of the potassium oxide concentration to the lithium oxide concentration at the first major surface is in the range of about 1 to about 20.

[0098] Aspect 57. The substrate according to any one of aspects 51 to 56, wherein the ratio of the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the first major surface to the lithium oxide concentration at the first major surface is in the range of about 1 to about 20.

[0099] Aspect 58. A substrate comprising:

[0100] A substrate thickness defined between a first major surface and a second major surface opposite the first major surface;

[0101] A first compressive stress zone extending from the first major surface to a first compressive depth and a second compressive stress zone extending from the second major surface to a second compressive depth,

[0102] wherein the ratio of the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the first major surface to the lithium oxide concentration at the first major surface is in the range of about 1 to about 20.

[0103] Aspect 59. The substrate according to aspect 58, wherein the lithium oxide concentration at the first major surface is about 0.2 mol% to about 2 mol% higher than the lithium oxide concentration at the midpoint between the first major surface and the second major surface.

[0104] Aspect 60. The foldable device according to aspect 58, wherein the lithium oxide concentration at the first major surface is about 1 mol% to about 3 mol% higher than the lithium oxide concentration at the midpoint of the first part.

[0105] Aspect 61. The substrate according to any one of aspects 51 to 57 and 59 to 60 (inclusive), wherein the lithium oxide concentration profile in the first part rises relative to the lithium oxide concentration at the midpoint to a depth that is about 5% or more of the substrate thickness away from the first major surface.

[0106] Aspect 62. The substrate according to any one of aspects 51 to 57 and 59 to 60 (inclusive), wherein the lithium oxide concentration profile in the first part rises relative to the lithium oxide concentration at the midpoint to a depth that is about 3 microns to about 15 microns away from the first major surface.

[0107] Aspect 63. The substrate according to any one of Aspects 51 to 57 and 59 to 62 (inclusive), wherein the concentration distribution of the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide in the first portion rises relative to the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the midpoint to a depth that is about 10% or more of the substrate thickness away from the first major surface.

[0108] Aspect 64. The substrate according to any one of Aspects 51 to 57 and 59 to 63 (inclusive), wherein the concentration distribution of the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide in the first portion rises relative to the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the midpoint to a depth that is about 10 micrometers or more away from the first major surface.

[0109] Aspect 65. The substrate according to any one of Aspects 51 to 57 and 59 to 64 (inclusive), wherein the lithium oxide concentration at the second major surface is about 0.2 mol% to about 2 mol% higher than the lithium oxide concentration at the midpoint.

[0110] Aspect 66. The substrate according to Aspect 65, wherein the lithium oxide concentration at the second major surface is substantially equal to the lithium oxide concentration at the first major surface.

[0111] Aspect 67. The substrate according to any one of Aspects 51 to 57 and 59 to 66 (inclusive), wherein the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the second major surface is about 5 mol% to about 15 mol%.

[0112] Aspect 68. The substrate according to any one of Aspects 51 to 57 and 59 to 66 (inclusive), wherein the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the second major surface is substantially equal to the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the first major surface.

[0113] Aspect 69. The substrate according to any one of Aspects 51 to 57 and 59 to 68 (inclusive), wherein 95% or more of the lithium oxide in the substrate is within 10 micrometers of the first major surface or within 10 micrometers of the second major surface.

[0114] Aspect 70. The substrate according to any one of Aspects 51 to 57 and 59 to 69 (inclusive), wherein the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the first major surface is higher than the sodium oxide concentration at the first major surface.

[0115] Aspect 71. The substrate according to any one of Aspects 51 to 53 and 58 to 70 (inclusive), wherein the potassium oxide concentration at the first major surface is in the range of about 1 to about 20 with respect to the lithium oxide concentration at the first major surface.

[0116] Aspect 72. The substrate according to any one of Aspects 51 to 53 and 58 to 70 (inclusive), wherein the potassium oxide concentration profile in the first portion rises with respect to the potassium oxide concentration at the midpoint to a depth that is about 10% or more of the substrate thickness away from the first major surface.

[0117] Aspect 73. The substrate according to any one of Aspects 51 to 53 and 58 to 70 (inclusive), wherein the potassium oxide concentration profile in the first portion rises with respect to the potassium oxide concentration at the midpoint to a depth that is about 10 micrometers or more away from the first major surface.

[0118] Aspect 74. The substrate according to any one of Aspects 51 to 53 and 58 to 70 (inclusive), wherein the potassium oxide concentration at the second major surface is from about 5 mol% to about 15 mol%.

[0119] Aspect 75. The substrate according to any one of Aspects 51 to 53 and 58 to 70 (inclusive), wherein the potassium oxide concentration at the second major surface is substantially equal to the potassium oxide concentration at the first major surface.

[0120] Aspect 76. The substrate according to any one of Aspects 51 to 53 and 58 to 70 (inclusive), wherein the potassium oxide concentration at the first major surface is higher than the sodium oxide concentration at the first major surface.

[0121] Aspect 77. The substrate according to any one of Aspects 51 to 76, wherein the first maximum compressive stress at the first major surface is about 500 MPa or greater.

[0122] Aspect 78. The substrate according to Aspect 77, wherein the second maximum compressive stress at the second major surface is about 500 MPa or greater.

[0123] Aspect 79. The substrate according to Aspect 78, wherein the second maximum compressive stress is substantially equal to the first maximum compressive stress.

[0124] Aspect 80. The substrate according to any one of Aspects 51 to 79, wherein the substrate thickness is from about 50 micrometers to about 2 millimeters.

[0125] Aspect 81. The substrate according to Aspect 80, wherein the substrate thickness is from about 90 micrometers to about 200 micrometers.

[0126] Aspect 82. The substrate according to any one of aspects 51 to 81, wherein the substrate is a glass substrate or a ceramic substrate.

[0127] Aspect 83. The substrate according to any one of aspects 42 to 72, further comprising:

[0128] A first portion including the thickness of the substrate;

[0129] A second portion including the thickness of the substrate; and

[0130] A central portion located between the first portion and the second portion, the central portion including a central thickness defined between a first central surface region and a second central surface region opposite the first central surface region, a first central compressive stress zone extending from the first central surface region to a first central compressive depth, a second central compressive stress zone extending from the second central surface region to a second central compressive depth, the first central surface region being recessed from the first major surface by a first distance, and the central thickness being less than the thickness of the substrate,

[0131] wherein the central portion includes the lithium concentration at the first central surface region, the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the first central surface region, and the central midpoint between the first central surface region and the second central surface region.

[0132] Aspect 84. The substrate according to aspect 73, wherein the lithium oxide concentration at the first central surface region is about 0.2 mol% to about 2 mol% higher than the lithium oxide concentration at the central midpoint.

[0133] Aspect 85. The foldable device according to aspect 83, wherein the lithium oxide concentration at the first central surface region is about 0.75 mol% to about 1.5 mol%.

[0134] Aspect 86. The substrate according to any one of aspects 83 to 85, wherein the lithium oxide concentration distribution in the central portion rises relative to the lithium oxide concentration at the central midpoint to a depth that is about 5% or more of the central thickness away from the first central surface region.

[0135] Aspect 87. The substrate according to any one of aspects 83 to 85, wherein the lithium oxide concentration distribution in the central portion rises relative to the lithium oxide concentration at the central midpoint to a depth that is about 3 microns to about 15 microns away from the first central surface region.

[0136] Aspect 88. The substrate according to any one of aspects 83 to 87, wherein the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide in the first central surface region is about 5 mol% to about 15 mol% higher than the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the central midpoint.

[0137] Aspect 89. The substrate according to aspect 88, wherein the concentration distribution of the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide in the central portion rises relative to the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the central midpoint to a depth that is about 10% or more of the central thickness away from the first central surface region.

[0138] Aspect 90. The substrate according to aspect 88, wherein the concentration distribution of the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide in the central portion rises relative to the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the central midpoint to a depth that is about 10 microns or more away from the first central surface region.

[0139] Aspect 91. The substrate according to any one of aspects 83 to 90, wherein the ratio of the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the first central surface region to the concentration of lithium oxide at the first central surface region is in the range of about 1 to about 20.

[0140] Aspect 92. The substrate according to any one of aspects 83 to 91, wherein the concentration of lithium oxide at the first major surface is substantially equal to the concentration of lithium oxide at the first central surface region.

[0141] Aspect 93. The substrate according to any one of aspects 83 to 91, wherein the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the first major surface is substantially equal to the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the first central surface region.

[0142] Aspect 94. The substrate according to any one of aspects 83 to 93, wherein the concentration of potassium oxide at the first central surface region is about 5 mol% to about 15 mol% higher than the concentration of potassium oxide at the central midpoint.

[0143] Aspect 95. The substrate according to aspect 94, wherein the concentration distribution of potassium oxide in the central portion rises relative to the concentration of potassium oxide at the central midpoint to a depth that is about 10% or more of the central thickness away from the first central surface region.

[0144] Aspect 96. The substrate according to aspect 94, wherein the potassium oxide concentration distribution in the central portion rises relative to the potassium oxide concentration at the central midpoint to a depth that is about 10 microns or more from the first central surface region.

[0145] Aspect 97. The substrate according to any one of aspects 94 to 96, wherein the ratio of the potassium oxide concentration at the first central surface region to the lithium oxide concentration at the first central surface region is in the range of about 1 to about 20.

[0146] Aspect 98. The substrate according to any one of aspects 94 to 97, wherein the potassium oxide concentration at the first major surface is substantially equal to the potassium oxide concentration at the first central surface region.

[0147] Aspect 99. The substrate according to any one of aspects 84 to 98, wherein the central thickness is about 25 microns to about 120 microns.

[0148] Aspect 100. The substrate according to aspect 99, wherein the central thickness is about 25 microns to about 60 microns.

[0149] Aspect 101. The substrate according to any one of aspects 84 to 100, wherein the first distance is about 20% to about 45% of the substrate thickness.

[0150] Aspect 102. The substrate according to any one of aspects 84 to 101, wherein the substrate achieves a parallel plate spacing of 1 mm to 10 mm.

[0151] Aspect 103. The substrate according to any one of aspects 84 to 101, wherein the substrate achieves a parallel plate spacing of 5 mm.

[0152] Aspect 104. The substrate according to any one of aspects 73 to 92, wherein the second central surface region is recessed from the second major surface by a second distance, the second distance being about 20% to about 45% of the substrate thickness.

[0153] Aspect 105. The substrate according to aspect 93, wherein the first distance is substantially equal to the second distance.

[0154] Aspect 106. The foldable device according to any one of aspects 73 to 103, wherein the second major surface is coplanar with the second central surface region.

[0155] Aspect 107. The foldable device according to any one of aspects 73 to 103, wherein the second major surface further includes the second central surface region.

[0156] Aspect 108. The substrate according to any one of aspects 104 to 107, wherein the concentration of lithium oxide in the second central surface region is substantially equal to the concentration of lithium oxide in the first central surface region.

[0157] Aspect 109. The substrate according to any one of aspects 104 to 108, wherein the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide in the second central surface region is substantially equal to the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide in the first central surface region.

[0158] Aspect 110. The foldable device according to any one of aspects 73 to 109, wherein the surface profile of the first central surface region along the midline between the first part and the second part exhibits a warpage of 1 millimeter or less.

[0159] Aspect 111. The substrate according to any one of aspects 73 to 109, wherein the surface profile of the first central surface region along the midline between the first part and the second part exhibits a warpage of 600 micrometers or less.

[0160] Aspect 112. The foldable device according to any one of aspects 73 to 111, wherein the surface profile of the first central surface region has an average gradient of about 0.018 mm / mm or less.

[0161] Aspect 113. The substrate according to any one of aspects 73 to 111, wherein the surface profile of the first central surface region has an average gradient of about 0.015 mm / mm or less.

[0162] Aspect 114. The foldable device according to any one of aspects 73 to 113, wherein the widths of the first part, the central part, and the second part are measured in a direction corresponding to the size of the substrate, and the percentage of the width of the central part in the size of the substrate is about 15% or more.

[0163] Aspect 115. The foldable device according to aspect 114, wherein the width of the second part is less than the width of the central part.

[0164] Aspect 116. The foldable device according to any one of aspects 114 to 115, wherein the percentage of the width of the central part in the size of the substrate is about 15% to about 50%.

[0165] Aspect 117. A consumer electronic product, comprising:

[0166] A housing including a front surface, a rear surface, and side surfaces;

[0167] An electrical component, at least partially located within the housing, the electrical component including a controller, a memory, and a display, the display being located at or adjacent to the front surface of the housing; and

[0168] A cover substrate, disposed over the display,

[0169] wherein at least one of a portion of the housing or the cover substrate includes a substrate according to any one of aspects 51 to 116.

[0170] Aspect 118. A method of forming a foldable device, comprising:

[0171] Chemically strengthening a substrate in a first molten salt bath maintained at a temperature of about 380 °C to about 530 °C for a first period of about 20 minutes to about 8 hours; and then

[0172] Immersing the substrate in a second molten salt bath maintained at a temperature of about 380 °C to about 480 °C for a second period of about 1 minute to about 10 minutes,

[0173] wherein the concentration of the lithium salt included in the second molten salt bath is higher than the concentration of the lithium salt included in the first molten salt bath, the second molten salt bath includes less than 0.4 wt% of the lithium salt, the substrate includes a substrate thickness defined between a first major surface and a second major surface opposite the first major surface, and the substrate includes a glass substrate or a ceramic substrate.

[0174] Aspect 119. The method according to aspect 118, further comprising forming at least one groove in a central portion of the substrate after chemically strengthening the substrate and before immersing the substrate to form a substrate, the first groove being defined between a first central surface region and a first plane defined by the first major surface, the first central surface region being recessed from the first major surface by a first distance, the central portion including a central thickness defined between the first central surface region and a second central surface region opposite the first central surface region, the central portion including a central midpoint intermediate the first central surface region and the second central surface region, and the substrate including a midpoint intermediate the first major surface and the second major surface.

[0175] Aspect 120. The method according to aspect 119, wherein the first distance is about 20% to about 45% of the substrate thickness.

[0176] Aspect 121. The method according to any one of aspects 119 to 120, wherein forming the at least one groove in the central portion further comprises forming a second groove defined between the second central surface region and a second plane defined by the second main surface, and the second central surface region is recessed from the second main surface by a second distance.

[0177] Aspect 122. The method according to aspect 121, wherein the first distance is substantially equal to the second distance.

[0178] Aspect 123. The method according to any one of aspects 119 to 122, wherein the substrate thickness is from about 50 micrometers to about 2 millimeters.

[0179] Aspect 124. The method according to any one of aspects 119 to 123, wherein the central thickness is from about 25 micrometers to about 120 micrometers.

[0180] Aspect 125. The method according to any one of aspects 119 to 124, wherein after immersing the substrate, the lithium oxide concentration at the first central surface region is about 0.2 mol% to about 2 mol% higher than the lithium oxide concentration at the central midpoint.

[0181] Aspect 126. The method according to any one of aspects 119 to 124, wherein the lithium oxide concentration at the first central surface region is from about 0.2 mol% to about 2 mol%.

[0182] Aspect 127. The method according to any one of aspects 125 to 126, wherein after immersing the substrate, the lithium oxide concentration at the second central surface region is substantially equal to the lithium oxide concentration at the first main surface.

[0183] Aspect 128. The method according to any one of aspects 125 to 127, wherein the lithium oxide concentration at the first central surface region is substantially equal to the lithium oxide concentration at the first main surface.

[0184] Aspect 129. The method according to any one of aspects 125 to 128, wherein after immersing the substrate, the lithium oxide concentration distribution in the central portion rises relative to the lithium oxide concentration at the central midpoint to a depth that is about 5% or more of the central thickness away from the first central surface region.

[0185] Aspect 130. The method according to any one of aspects 125 to 128, wherein after immersing the substrate, the lithium oxide concentration distribution in the central portion rises relative to the lithium oxide concentration at the midpoint to a depth that is about 3 micrometers to about 15 micrometers away from the first central surface region.

[0186] Aspect 131. The method according to any one of Aspects 119 to 130, wherein after immersing the substrate, the potassium oxide concentration at the first central surface region is about 5 mol% to about 15 mol% higher than the potassium oxide concentration at the central midpoint.

[0187] Aspect 132. The method according to any one of Aspects 119 to 130, wherein the potassium oxide concentration at the first central surface region is about 5 mol% to about 15 mol%.

[0188] Aspect 133. The method according to any one of Aspects 131 to 132, wherein after immersing the substrate, the potassium oxide concentration at the second central surface region is substantially equal to the potassium oxide concentration at the first central surface region.

[0189] Aspect 134. The method according to any one of Aspects 119 to 133, wherein the potassium oxide concentration at the first central surface region is higher than the sodium oxide concentration at the first central surface region.

[0190] Aspect 135. The method according to any one of Aspects 119 to 134, wherein after immersing the substrate, the first central maximum compressive stress at the first central surface region is about 500 MPa or greater.

[0191] Aspect 136. The method according to Aspect 135, wherein the second central maximum compressive stress at the second central surface region is substantially equal to the first central maximum compressive stress.

[0192] Aspect 137. The method according to any one of Aspects 119 to 136, wherein after immersing the substrate, the surface profile of the first central surface region along the midline of the central portion exhibits a warpage of 600 microns or less.

[0193] Aspect 138. The method according to any one of Aspects 119 to 136, wherein after immersing the substrate, the surface profile of the first central surface region has an average gradient of about 0.015 mm / mm or less.

[0194] Aspect 139. The method according to any one of Aspects 118 to 138, wherein the second molten salt bath comprises about 0.02 wt% to about 0.08 wt% of the lithium salt.

[0195] Aspect 140. The method according to any one of Aspects 118 to 139, wherein the second molten salt bath comprises:

[0196] 0.02 wt% to about 0.08 wt% of the lithium salt;

[0197] 70 wt% to about 99.98 wt% of potassium salt;

[0198] 0 wt% to about 29.98 wt% of sodium salt; and

[0199] 0 wt% to about 1 wt% of silicic acid.

[0200] Aspect 141. The method according to any one of aspects 118 to 140, wherein the first molten salt bath does not contain lithium.

[0201] Aspect 142. The method according to any one of aspects 118 to 141, wherein after immersing the substrate, the concentration of lithium oxide at the first major surface is about 0.2 mol% to about 2 mol% higher than the concentration of lithium oxide at the midpoint, and the midpoint is midway between the first major surface and the second major surface.

[0202] Aspect 143. The method according to any one of aspects 118 to 141, wherein after immersing the substrate, the concentration of lithium oxide at the first major surface is about 0.2 mol% to about 2 mol%.

[0203] Aspect 144. The method according to any one of aspects 118 to 143, wherein after immersing the substrate, the concentration distribution of lithium oxide in the first portion rises relative to the concentration of lithium oxide at the midpoint to a depth that is about 5% or more of the substrate thickness away from the first major surface.

[0204] Aspect 145. The method according to any one of aspects 118 to 144, wherein after immersing the substrate, the concentration distribution of lithium oxide in the first portion rises relative to the concentration of lithium oxide at the midpoint to a depth that is about 3 microns to about 15 microns away from the first major surface.

[0205] Aspect 146. The method according to any one of aspects 118 to 145, wherein after immersing the substrate, the concentration of potassium oxide at the first major surface is about 5 mol% to about 15 mol% higher than the concentration of potassium oxide at the midpoint.

[0206] Aspect 147. The method according to any one of aspects 118 to 145, wherein after immersing the substrate, the concentration of potassium oxide at the first major surface is about 5 mol% to about 15 mol%.

[0207] Aspect 148. The method according to any one of aspects 146 to 147, wherein after immersing the substrate, the concentration of potassium oxide in the first central surface region is substantially equal to the concentration of potassium oxide at the first major surface.

[0208] Aspect 149. The method according to any one of aspects 146 to 148, wherein after immersing the substrate, the potassium oxide concentration at the first major surface is higher than the sodium oxide concentration at the first major surface.

[0209] Aspect 150. The method according to any one of aspects 118 to 149, wherein after immersing the substrate, the potassium oxide concentration profile in the first portion rises relative to the potassium oxide concentration at the midpoint to a depth that is about 10% or more of the substrate thickness away from the first major surface.

[0210] Aspect 151. The method according to any one of aspects 118 to 149, wherein after immersing the substrate, the potassium oxide concentration profile in the first portion rises relative to the potassium oxide concentration at the midpoint to a depth that is about 10 micrometers or more away from the first major surface.

[0211] Aspect 152. The method according to any one of aspects 118 to 151, further comprising, after immersing the substrate, removing from about 100 nanometers to about 2 micrometers from the first major surface.

[0212] Aspect 153. The method according to any one of aspects 118 to 152, wherein the substrate achieves a parallel plate spacing of 1 millimeter to 10 millimeters.

[0213] Aspect 154. The method according to any one of aspects 118 to 152, wherein the substrate achieves a parallel plate spacing of 5 millimeters.

[0214] Aspect 155. A method of forming a foldable device, comprising:

[0215] Immersing a substrate in a molten salt bath maintained at a temperature of about 380 °C to about 480 °C for a period of about 1 minute to about 10 minutes, the molten salt bath comprising:

[0216] 0.02 wt% to about 0.08 wt% of a lithium salt; and

[0217] 70 wt% to about 99.98 wt% of a potassium salt,

[0218] wherein the substrate includes a substrate thickness defined between a first major surface and a second major surface opposite the first major surface.

[0219] Aspect 156. The method according to aspect 155, wherein the molten salt bath further comprises:

[0220] 0 wt% to about 29.98 wt% of a sodium salt; and

[0221] 0 wt% to about 1 wt% of silicic acid.

[0222] Aspect 157. The method according to aspect 156, wherein the molten salt bath consists of the lithium salt, the potassium salt, and optionally silicic acid.

[0223] Aspect 158. The method according to any one of aspects 155 to 157, wherein after immersing the substrate, the lithium oxide concentration at the first major surface is about 0.2 mol% to about 2 mol% higher than the lithium oxide concentration at the midpoint, and the midpoint is between the first major surface and the second major surface.

[0224] Aspect 159. The method according to any one of aspects 155 to 157, wherein after immersing the substrate, the lithium oxide concentration at the first major surface is about 0.2 mol% to about 2 mol%.

[0225] Aspect 160. The method according to any one of aspects 155 to 159, wherein after immersing the substrate, the lithium oxide concentration profile in the first portion rises relative to the lithium oxide concentration at the midpoint to a depth that is about 5% or more of the substrate thickness away from the first major surface.

[0226] Aspect 161. The method according to any one of aspects 155 to 159, wherein after immersing the substrate, the lithium oxide concentration profile in the first portion rises relative to the lithium oxide concentration at the midpoint to a depth that is about 3 micrometers to about 15 micrometers away from the first major surface.

[0227] Aspect 162. The method according to any one of aspects 155 to 161, wherein after immersing the substrate, the potassium oxide concentration at the first major surface is about 5 mol% to about 15 mol% higher than the potassium oxide concentration at the midpoint.

[0228] Aspect 163. The method according to any one of aspects 155 to 161, wherein after immersing the substrate, the potassium oxide concentration at the first major surface is about 5 mol% to about 15 mol%.

[0229] Aspect 164. The method according to any one of aspects 162 to 163, wherein after immersing the substrate, the potassium oxide concentration at the first central surface region is substantially equal to the potassium oxide concentration at the first major surface.

[0230] Aspect 165. The method according to any one of aspects 162 to 164, wherein after immersing the substrate, the potassium oxide concentration at the first major surface is higher than the sodium oxide concentration at the first major surface.

[0231] Aspect 166. The method according to any one of aspects 155 to 165, wherein after immersing the substrate, the potassium oxide concentration distribution in the first portion rises relative to the potassium oxide concentration at the midpoint to a depth that is about 10% or more of the substrate thickness away from the first major surface.

[0232] Aspect 167. The method according to any one of aspects 155 to 165, wherein after immersing the substrate, the potassium oxide concentration distribution in the first portion rises relative to the potassium oxide concentration at the midpoint to a depth that is about 10 micrometers or more away from the first major surface.

[0233] Aspect 168. The method according to any one of aspects 155 to 167, further comprising, after immersing the substrate, removing from about 100 nanometers to about 2 micrometers from the first major surface and removing from about 100 nanometers to about 2 micrometers from the second major surface.

[0234] Aspect 169. The method according to any one of aspects 155 to 168, wherein the substrate thickness is about 50 micrometers to about 2 millimeters.

[0235] Aspect 170. The method according to any one of aspects 155 to 169, wherein the substrate achieves a parallel plate spacing of 1 millimeter to 10 millimeters.

[0236] Aspect 171. The method according to any one of aspects 155 to 169, wherein the substrate achieves a parallel plate spacing of 5 millimeters.

[0237] Aspect 172. The method according to any one of aspects 155 to 171, wherein the substrate comprises a glass substrate or a ceramic substrate.

[0238] Aspect 173. The method according to any one of aspects 155 to 172, wherein the substrate includes at least one groove in a central portion, a first groove being defined between a first central surface region and a first plane defined by the first major surface, the first central surface region being recessed from the first major surface by a first distance, and the central portion including a central thickness defined between the first central surface region and a second central surface region opposite the first central surface region.

[0239] Aspect 174. The method according to aspect 173, wherein the first distance is about 20% to about 45% of the substrate thickness.

[0240] Aspect 175. The method according to any one of aspects 173 to 174, wherein the second central surface region is recessed from the second major surface by a second distance.

[0241] Aspect 176. The method according to aspect 175, wherein the first distance is substantially equal to the second distance.

[0242] Aspect 177. The method according to any one of aspects 173 to 176, wherein the central thickness is from about 25 micrometers to about 120 micrometers.

[0243] Aspect 178. A method of forming a foldable device, comprising:

[0244] chemically strengthening a substrate in a molten salt bath maintained at a temperature of from about 380 °C to about 430 °C for a period of from about 3 minutes to about 2 hours,

[0245] wherein the molten salt bath comprises from about 0.5 wt% to about 1.5 wt% of a lithium salt, the substrate comprises a substrate thickness defined between a first major surface and a second major surface opposite the first major surface, and the substrate comprises a glass substrate or a ceramic substrate.

[0246] Aspect 179. The method according to aspect 178, wherein the substrate is substantially unstrengthened prior to chemically strengthening the substrate.

[0247] Aspect 180. The method according to any one of aspects 178 to 179, further comprising forming at least one groove in a central portion of the substrate prior to chemically strengthening the substrate to form a substrate, the first groove being defined between a first central surface region and a first plane defined by the first major surface, the first central surface region being recessed from the first major surface by a first distance, the central portion comprising a central thickness defined between the first central surface region and a second central surface region opposite the first central surface region, the central portion comprising a central midpoint intermediate the first central surface region and the second central surface region, and the substrate comprising a midpoint intermediate the first major surface and the second major surface.

[0248] Aspect 181. The method according to aspect 180, wherein the second major surface further comprises the second central surface region.

[0249] Aspect 182. The method according to any one of aspects 180 to 181, wherein the first distance is from about 20% to about 45% of the substrate thickness.

[0250] Aspect 183. The method according to any one of aspects 180 to 182, wherein the substrate thickness is from about 50 micrometers to about 2 millimeters.

[0251] Aspect 184. The method according to any one of aspects 180 to 183, wherein the central thickness is from about 25 micrometers to about 120 micrometers.

[0252] Aspect 185. The method according to any one of aspects 180 to 184, wherein after chemically strengthening the substrate, the lithium oxide concentration in the first central surface region is about 0.5 mol% to about 2 mol% higher than the lithium oxide concentration at the central midpoint.

[0253] Aspect 186. The method according to aspect 185, wherein the lithium oxide concentration in the first central surface region is about 0.75 mol% to about 1.5 mol% higher than the lithium oxide concentration at the central midpoint.

[0254] Aspect 187. The method according to any one of aspects 180 to 184, wherein the lithium oxide concentration in the first central surface region is about 0.5 mol% to about 2 mol%.

[0255] Aspect 188. The method according to aspect 187, wherein the lithium oxide concentration in the first central surface region is about 0.75 mol% to about 1.5 mol%.

[0256] Aspect 189. The method according to any one of aspects 185 to 188, wherein the lithium oxide concentration in the second central surface region is substantially equal to the lithium oxide concentration in the first central surface region.

[0257] Aspect 190. The method according to any one of aspects 180 to 189, wherein after chemically strengthening the substrate, the lithium oxide concentration on the first major surface is about 1 mol% to about 3 mol% higher than the lithium oxide concentration at the midpoint.

[0258] Aspect 191. The method according to aspect 190, wherein the lithium oxide concentration on the first major surface is about 1.5 mol% to about 2.5 mol% higher than the lithium oxide concentration at the midpoint.

[0259] Aspect 192. The method according to any one of aspects 180 to 189, wherein after chemically strengthening the substrate, the lithium oxide concentration on the first major surface is about 1 mol% to about 3 mol%.

[0260] Aspect 193. The method according to aspect 192, wherein the lithium oxide concentration on the first major surface is about 1.5 mol% to about 2.5 mol%.

[0261] Aspect 194. The method according to any one of aspects 190 to 193, wherein after chemically strengthening the substrate, the lithium oxide concentration in the second central surface region is substantially equal to the lithium oxide concentration on the first major surface.

[0262] Aspect 195. The method according to any one of Aspects 180 to 194, wherein after chemically strengthening the substrate, the lithium oxide concentration distribution in the central portion rises relative to the lithium oxide concentration at the central midpoint to a depth that is separated from the first central surface region by about 5% or more of the central thickness.

[0263] Aspect 196. The method according to any one of Aspects 180 to 194, wherein after chemically strengthening the substrate, the lithium oxide concentration distribution in the central portion rises relative to the lithium oxide concentration at the midpoint to a depth that is separated from the first central surface region by about 3 micrometers to about 15 micrometers.

[0264] Aspect 197. The method according to any one of Aspects 180 to 196, wherein after chemically strengthening the substrate, the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the first central surface region is about 5 mol% to about 15 mol% higher than the potassium oxide concentration at the central midpoint.

[0265] Aspect 198. The method according to any one of Aspects 180 to 196, wherein the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the first central surface region is about 5 mol% to about 15 mol%.

[0266] Aspect 199. The method according to any one of Aspects 180 to 196, wherein after chemically strengthening the substrate, the potassium oxide concentration at the first central surface region is about 5 mol% to about 15 mol% higher than the potassium oxide concentration at the central midpoint.

[0267] Aspect 200. The method according to any one of Aspects 180 to 196, wherein the potassium oxide concentration of potassium oxide at the first central surface region is about 5 mol% to about 15 mol%.

[0268] Aspect 201. The method according to any one of Aspects 180 to 200, wherein after chemically strengthening the substrate, the potassium oxide concentration at the first major surface is about 5 mol% to about 15 mol% higher than the potassium oxide concentration at the midpoint.

[0269] Aspect 202. The method according to any one of Aspects 180 to 200, wherein after chemically strengthening the substrate, the potassium oxide concentration at the first major surface is about 5 mol% to about 15 mol%.

[0270] Aspect 203. The method according to any one of aspects 180 to 202, wherein after chemically strengthening the substrate, the potassium oxide concentration distribution in the first part rises relative to the potassium oxide concentration at the midpoint to a depth that is about 10% or more of the substrate thickness away from the first major surface.

[0271] Aspect 204. The method according to any one of aspects 180 to 202, wherein after chemically strengthening the substrate, the potassium oxide concentration distribution in the first part rises relative to the potassium oxide concentration at the midpoint to a depth that is about 10 micrometers or more away from the first major surface.

[0272] Aspect 205. The method according to any one of aspects 180 to 204, wherein after chemically strengthening the substrate, the first central maximum compressive stress at the first central surface region is about 500 MPa or greater.

[0273] Aspect 206. The method according to aspect 205, wherein the second central maximum compressive stress at the second central surface region is substantially equal to the first central maximum compressive stress.

[0274] Aspect 207. The method according to any one of aspects 180 to 206, wherein the surface profile of the first central surface region along the midline between the first part and the second part exhibits a warpage of 1 millimeter or less.

[0275] Aspect 208. The method according to any one of aspects 207, wherein the warpage is 600 micrometers or less.

[0276] Aspect 209. The method according to any one of aspects 180 to 208, wherein after chemically strengthening the substrate, the surface profile of the first central surface region has an average gradient of about 0.018 mm / mm or less.

[0277] Aspect 210. The method according to aspect 209, wherein the average gradient is about 0.015 mm / mm or less.

[0278] Aspect 211. The method according to any one of aspects 180 to 210, wherein the time period is about 0.004 minutes / μm2 to about 0.007 minutes / μm2 multiplied by the square of the central thickness in micrometers.

[0279] Aspect 212. The method according to any one of aspects 180 to 211, wherein the molten salt bath comprises about 0.5 wt% to about 1.3 wt% of the lithium salt.

[0280] Aspect 213. The method according to any one of aspects 180 to 211, wherein the molten salt bath comprises:

[0281] from 0.5 wt% to about 1.3 wt% of the lithium salt;

[0282] from 70 wt% to about 99.5 wt% of the potassium salt;

[0283] from 0 wt% to about 29.5 wt% of the sodium salt; and

[0284] from 0 wt% to about 1 wt% of the silicic acid.

[0285] Aspect 214. The method according to any one of aspects 180 to 213, further comprising, after immersing the substrate, removing from about 100 nanometers to about 2 micrometers from the first major surface and removing from about 100 nanometers to about 2 micrometers from the second major surface.

[0286] Aspect 215. The method according to any one of aspects 180 to 214, wherein the widths of the first portion, the central portion, and the second portion are measured in a direction corresponding to the dimension of the substrate, and the percentage of the width of the central portion in the dimension of the substrate is about 15% or greater.

[0287] Aspect 216. The method according to aspect 215, wherein the width of the second portion is less than the width of the central portion.

[0288] Aspect 217. The method according to any one of aspects 215 to 216, wherein the percentage of the width of the central portion in the dimension of the substrate is from about 15% to about 50%.

[0289] Aspect 218. The method according to any one of aspects 180 to 217, wherein the substrate achieves a parallel plate spacing of 1 millimeter to 10 millimeters.

[0290] Aspect 219. The method according to any one of aspects 180 to 217, wherein the substrate achieves a parallel plate spacing of 5 millimeters. BRIEF DESCRIPTION OF THE DRAWINGS

[0291] The above and other features and advantages of aspects of the present disclosure will be better understood when the following detailed description is read with reference to the accompanying drawings, in which:

[0292] Figure 1 is a schematic illustration of an example foldable device in a planar configuration according to aspects, wherein a schematic illustration of the folded configuration can be as Figure 5 shown;

[0293] Figures 2 - 4is a cross-sectional view along line 2-2 of a foldable device according to various aspects including a foldable substrate Figure 1 ;

[0294] Figure 5 is a schematic diagram of an example foldable device according to various aspects of the present disclosure in a folded configuration, where a schematic diagram of a planar configuration can be as shown in Figure 1 ;

[0295] Figure 6 is a cross-sectional view along line 7-7 of a test device for determining the minimum parallel plate spacing of an example foldable substrate along Figure 5 ;

[0296] Figure 7 is a cross-sectional view along line 7-7 of another test device for determining the minimum parallel plate spacing of an example modified foldable device along Figure 5 ;

[0297] Figure 8 is a cross-sectional view along line 7-7 of a test device for determining the minimum parallel plate spacing of an example foldable substrate along Figure 5 ;

[0298] Figure 9 is a cross-sectional view along line 7-7 of another test device for determining the minimum parallel plate spacing of an example modified foldable device along Figure 5 ;

[0299] Figure 10 is a schematic plan view of an example consumer electronic device according to various aspects;

[0300] Figure 11 is Figure 10 a schematic perspective view of an example consumer electronic device;

[0301] Figure 12 is a flowchart showing an example method of manufacturing a foldable device according to various aspects of the present disclosure;

[0302] Figure 13 schematically shows steps in a method of manufacturing a foldable device;

[0303] Figure 14 is a cross-sectional view of a foldable device after the step shown in Figure 13 or 16 and / or before the step shown in Figure 17 ;

[0304] Figures 15 - 17 schematically shows steps in a method of manufacturing a foldable device;

[0305] Figure 18 is after the step shown in Figure 17 and / or before the step shown inFigure 19 Cross-sectional view of a foldable substrate before the steps shown;

[0306] Figure 19 Schematically shows steps in a method of manufacturing a foldable substrate and / or a foldable device;

[0307] Figures 20 - 22 Schematically shows steps in a method of manufacturing a foldable device;

[0308] Figures 23 - 27 Schematically shows the surface profile of a foldable device described in an example;

[0309] Figure 28 Shows the average gradient of a foldable device described in an example;

[0310] Figures 29 - 33 Schematically shows the concentration profile of a glow discharge optical emission spectrometer (GDOES) of a foldable device;

[0311] Figure 34 Is a flowchart showing an example method of manufacturing a foldable device according to aspects of the present disclosure;

[0312] Figures 35 - 36 Schematically shows steps in a method of manufacturing a foldable device;

[0313] Figure 37 Is at Figure 35 Or a cross-sectional view of a foldable device after the steps shown in 36 and / or before the steps shown in Figure 38 ;

[0314] Figures 38 - 41 Schematically shows steps in a method of manufacturing a foldable device;

[0315] Figures 42 - 45 Schematically shows the surface profile of a foldable device described in an example; and

[0316] Figures 46 - 51 Schematically shows the concentration profile of a glow discharge optical emission spectrometer (GDOES) of a foldable device.

[0317] Throughout the present disclosure, the drawings are used to emphasize certain aspects. Therefore, unless otherwise explicitly stated, the relative sizes of the different regions, parts, and substrates shown in the drawings should not be considered to be in proportion to their actual relative sizes. Detailed Description

[0318] Aspects will now be described more fully hereinafter with reference to the drawings, in which example aspects are shown. Whenever possible, the same reference numerals are used throughout the drawings to refer to the same or similar parts. Figures 1 - 4Figures 6 - 9 illustrate views of foldable devices 101, 301, 401, 501, 701, 801, and 901 that include a foldable substrate 201 in accordance with aspects of the present disclosure. Unless otherwise indicated, the discussion of aspects of one foldable device may equally apply to corresponding features of any aspect of the present disclosure. For example, the same component numbers throughout the present disclosure may indicate that the features identified in some aspects are the same, and unless otherwise indicated, the discussion of the identified features of one aspect may equally apply to the identified features of any other aspect of the present disclosure.

[0319] Figures 2 - 4 Exemplary aspects of foldable devices 101, 301, and 401 that include a foldable substrate 201 in accordance with aspects of the present disclosure are schematically illustrated in an unfolded (e.g., planar) configuration, while Figures 6 - 9 exemplary aspects of foldable devices 501, 701, 801, and 901 that include a foldable substrate 201 in accordance with aspects of the present disclosure are illustrated in a folded configuration. Figure 3 Exemplary aspects of foldable device 301, which consists of foldable substrate 201 in accordance with aspects of the present disclosure, are schematically illustrated in an unfolded (e.g., planar) configuration. Foldable devices 101, 301, and 401 and foldable substrate 201 include a first portion 221, a second portion 231, and a central portion 281 located between the first portion 221 and the second portion 231. In aspects, as Figure 2 and 4 shown, foldable devices 101 and 401 may include a release liner 271, but in other aspects, other substrates (e.g., glass substrates and / or ceramic substrates discussed throughout the application) may be used instead of the shown release liner 271. In aspects, as Figure 2 and 7 shown, foldable devices 101 and 701 may include a coating 251. In aspects, as Figure 2 and 4 shown, foldable device 101 may include an adhesive layer 261. In aspects, as Figure 2 and 7 shown, foldable devices 101 and 701 may include a polymer - based portion 289 and / or 299. As Figures 2 - 4 shown, foldable substrate 201 may include a first groove 211. In aspects, as Figures 2 - 3 shown, foldable substrate 201 may further include a second groove 241. It should be understood that any foldable device of the present disclosure may include a second substrate (e.g., a glass substrate and / or a ceramic substrate), a release liner 271, a display device, a coating 251, an adhesive layer 261, and / or a polymer - based portion 289 and / or 299.

[0320] Throughout this disclosure, reference is made to Figure 1 , the width 103 of the foldable devices 101, 301, 401, 501, 701, 801, and / or 901 is considered to be the dimension of the foldable device obtained between opposite edges of the foldable device along a direction 104 along the folding axis 102 of the foldable device, where the direction 104 also includes the direction of the width 103. Additionally, throughout this disclosure, the length 105 of the foldable device is considered to be the dimension of the foldable device obtained between opposite edges of the foldable device along a direction 106 perpendicular to the folding axis 102 of the foldable device. It should be understood that the direction 104 of the width 103 and / or the direction 106 of the length 105 may correspond to the corresponding directions in the foldable substrate 201. In various aspects, as Figures 1 - 2 shown, a foldable device of any aspect of this disclosure may include a folding plane 109 that contains the folding axis 102 when the foldable device is in a planar configuration (see Figure 2 ). In other aspects, as Figure 2 shown, when the foldable device is in a planar configuration (see Figure 2 ), the folding plane 109 may extend along the folding axis 102 and in the direction of the substrate thickness 207. The folding plane 109 may include a central axis 107 of the foldable device. In various aspects, the foldable device may be folded about the folding axis 102 extending in the direction 104 of the width 103 along a direction 111 (see Figure 1 ) to form a folded configuration (see Figure 5 , 7 and 9). Similarly, folding the foldable substrate 201 about the folding axis (see Figure 3 ) may form a folded configuration (see Figure 6 and 8 ). As shown, the foldable device and / or the foldable substrate may include a single folding axis such that the foldable device and / or the foldable substrate can include a double fold, where for example the foldable device and / or the foldable substrate can be folded in half. In other aspects, the foldable device and / or the foldable substrate may include two or more folding axes, where each folding axis includes a central portion similar or identical to the central portion 281 discussed herein. For example, providing two folding axes may enable the foldable device and / or the foldable substrate to include a triple fold, where for example the foldable device and / or the foldable substrate can be folded into a first portion 221, a second portion 231, and a third portion similar or identical to the first portion or the second portion, with the central portion 281 and another central portion similar or identical to the central portion located between the first portion and the second portion and between the second portion and the third portion, respectively.

[0321] The foldable substrate 201 may include a glass substrate and / or a ceramic substrate having a pencil hardness of 8H or higher, e.g., 9H or higher. As used herein, pencil hardness is measured using ASTM D 3363-20 standard lead grade pencils. Providing a glass foldable substrate and / or a ceramic foldable substrate can enhance puncture resistance and / or impact resistance. Throughout the present disclosure, the elastic modulus (e.g., Young's modulus) is measured using ISO 527-1:2019. In various aspects, the foldable substrate 201 may include an elastic modulus in the range of about 10 GPa to about 150 GPa, about 40 GPa to about 100 GPa, about 60 GPa to about 80 GPa, or any range or sub-range therebetween.

[0322] In various aspects, the foldable substrate 201 may include a glass substrate. As used herein, "glass" encompasses both glass and glass-ceramics, where the glass-ceramics have one or more crystalline phases and an amorphous residual glass phase. The glass material (e.g., the glass substrate) may include an amorphous material (e.g., glass) and optionally one or more crystalline materials (e.g., ceramics). The amorphous material and the glass material can be strengthened. As used herein, the term "strengthened" may refer to a material that has been chemically strengthened, e.g., by ion-exchanging smaller ions with larger ions in the substrate surface, as discussed below. However, other strengthening methods may also be utilized, such as thermal tempering, or by taking advantage of the mismatch in the coefficient of thermal expansion between various parts of the substrate to generate compressive stress and a central tension zone, thereby forming a strengthened substrate. Exemplary glass materials include soda-lime glass, alkali aluminosilicate glass, alkali borosilicate glass, alkali aluminoborosilicate glass, alkali phosphosilicate glass, and alkali aluminophosphosilicate glass, whether or not containing lithium oxide. In various aspects, the glass material may include an alkali-containing glass or an alkali-free glass, either of which may be free of lithium oxide or contain lithium oxide. In various aspects, the glass material may be alkali-free and / or include a low content of alkali metals (e.g., about 10 mol% or less of R 2 O, where R 2 O includes Li 2 O, Na 2 O, K 2 O, or the broader list provided below). In one or more aspects, in mole percent (mol%), the glass material may include: SiO 2 in the range of about 40 mol% to about 80 mol%, Al 2 O 3 in the range of about 5 mol% to about 30 mol%, B 2 O 3 in the range of 0 mol% to about 10 mol%, ZrO 2, P in the range of 0 mol% to about 15 mol% 2 O 5 , TiO in the range of 0 mol% to about 2 mol% 2 , R in the range of 0 mol% to about 20 mol% 2 O, and RO in the range of 0 mol% to about 15 mol%. As used herein, R 2 O may refer to alkali metal oxides, for example, Li 2 O, Na 2 O, K 2 O, Rb 2 O and Cs 2 O. As used herein, RO may refer to MgO, CaO, SrO, BaO, and ZnO. "Glass-ceramic" includes materials made by controlled crystallization of glass. In various aspects, the glass-ceramic has a crystallinity of about 1% to about 99%. Examples of suitable glass-ceramics may include Li 2 O-Al 2 O 3 -SiO 2 -based (i.e., LAS-based) glass-ceramics, MgO-Al 2 O 3 -SiO 2 -based (i.e., MAS-based) glass-ceramics, ZnO×Al 2 O 3 ×nSiO 2 (i.e., ZAS-based), and / or glass-ceramics comprising a major crystalline phase comprising β-quartz solid solution, β-spodumene, cordierite, petalite, and / or lithium disilicate. The glass-ceramic substrate can be strengthened using a chemical strengthening process. In one or more aspects, the MAS-based glass-ceramic substrate can be strengthened in a Li 2 SO 4 molten salt, where an exchange of 2Li + with Mg 2+ can occur.

[0323] In various aspects, the foldable substrate 201 may include a ceramic substrate. As used herein, "ceramic" includes both ceramics and glass-ceramics, where the glass-ceramic has one or more crystalline phases and an amorphous residual glass phase. The ceramic material can be strengthened (e.g., chemically strengthened). In various aspects, the ceramic material can be formed by heating a glass material to form a ceramic (e.g., crystalline) portion. In other aspects, the ceramic material may include one or more nucleating agents that can promote the formation of the crystalline phase. In various aspects, the ceramic material may include one or more oxides, nitrides, oxynitrides, carbides, borides, and / or silicides. Example aspects of ceramic oxides include zirconia (ZrO 2) Zircon (ZrSiO 4 ) Titanium dioxide (TiO 2 ) Hafnium oxide (Hf 2 O), Yttrium oxide (Y 2 O 3 ), Iron oxide, Beryllium oxide, Vanadium oxide (VO 2 ), Fused quartz, Cristobalite, Mullite (a mineral of a composition including alumina and silica), and Spinel (MgAl 2 O 4 ). Example aspects of ceramic nitrides include Silicon nitride (Si 3 N 4 ), Aluminum nitride (AlN), Gallium nitride (GaN), Beryllium nitride (Be 3 N 2 ), Boron nitride (BN), Tungsten nitride (WN), Vanadium nitride, Alkaline earth metal nitrides (e.g., Magnesium nitride (Mg 3 N 2 )), Nickel nitride, and Tantalum nitride. Example aspects of oxynitride ceramics include Silicon oxynitride, Aluminum oxynitride, and SiAlON (a composition of alumina and silicon nitride and may have Si 12-m-n Al m+n O n N 16-n , Si 6-n Al n O n N 8-n or Si 2-n Al n O 1+n N 2-n and other chemical formulas, where m, n, and the resulting subscripts are all non-negative integers).

[0324] In various aspects, the foldable substrate 201 can be optically transparent. As used herein, "optically transparent" or "optically clear" means that the average transmittance is 70% or higher in the wavelength range from 400 nm to 700 nm through a 1.0 mm thick material. In various aspects, through a 1.0 mm thick material, in the wavelength range from 400 nm to 700 nm, the average transmittance of the foldable substrate 201 can be 75% or higher, 80% or higher, 85% or higher, or about 90% or higher, 92% or higher, 94% or higher, 96% or higher. The average transmittance in the wavelength range from 400 nm to 700 nm is calculated by measuring the transmittance at integer wavelengths from about 400 nm to about 700 nm and averaging the measured values.

[0325] As Figures 2 - 4As shown, the foldable devices 101, 301, and 401 include a foldable substrate 201 that includes a first major surface 203 and a second major surface 205 opposite the first major surface 203. As Figures 2 - 4 shown, the first major surface 203 may extend along a first plane 204a. The second major surface 205 may extend along a second plane 206a. In various aspects, as shown, the second plane 206a may be parallel to the first plane 204a. As used herein, the substrate thickness 207 may be defined between the first major surface 203 and the second major surface 205 as the distance between the first plane 204a and the second plane 206a. In various aspects, the substrate thickness 207 may be about 10 micrometers (μm) or greater, about 25 μm or greater, about 50 μm or greater, about 70 μm or greater, about 80 μm or greater, about 90 μm or greater, about 100 μm or greater, about 125 μm or greater, about 150 μm or greater, about 200 μm or greater, about 300 μm or greater, about 2 millimeters (mm) or less, about 1 mm or less, about 800 μm or less, about 500 μm or less, about 300 μm or less, about 200 μm or less, about 180 μm or less, or about 160 μm or less. In various aspects, the substrate thickness 207 may be in the range of about 10 μm to about 2 mm, about 25 μm to about 2 mm, about 50 μm to about 2 mm, about 70 μm to about 2 mm, about 70 μm to about 1 mm, about 70 μm to about 800 μm, about 80 μm to about 500 μm, about 90 μm to about 500 μm, about 100 μm to about 200 μm, about 125 μm to about 200 μm, about 150 μm to about 200 μm or any range or sub-range therebetween.

[0326] As Figures 2 - 4 shown, a first portion 221 of the foldable substrate 201 may include a first surface region 223 and a second surface region 225 opposite the first surface region 223. The first portion 221 will now be described with reference to Figure 2 the foldable device 101, it being understood that unless otherwise stated, this description of the first portion 221 may also apply to any aspect of the present disclosure, such as Figures 3 - 4The foldable devices 301, 401, 501, 701, 801, and / or 901 shown in FIGS. 6 - 9. In various aspects, as shown, the first surface region 223 may include a flat surface, and / or the second surface region 225 of the first portion 221 may include a flat surface. In other aspects, as shown, the second surface region 225 may be parallel to the first surface region 223. In various aspects, as shown, the first major surface 203 may include the first surface region 223, and the second major surface 205 may include the second surface region 225. In other aspects, the first surface region 223 may extend along the first plane 204a. In other aspects, the second surface region 225 may extend along the second plane 206a. In various aspects, the substrate thickness 207 may correspond to the distance between the first surface region 223 and the second surface region 225 of the first portion 221. In various aspects, the substrate thickness 207 may be substantially uniform across the first surface region 223. In various aspects, the first thickness defined between the first surface region 223 and the second surface region 225 may be within one or more of the ranges discussed above with respect to the substrate thickness 207. In other aspects, the first thickness may include the substrate thickness 207. In other aspects, the first thickness of the first portion 221 may be substantially uniform between the first surface region 223 and the second surface region 225 across its corresponding length (i.e., in the direction 106 of the length 105 of the foldable device) and / or its corresponding width (i.e., in the direction 104 of the width 103 of the foldable device).

[0327] As Figures 2 - 4 shown, the second portion 231 of the foldable substrate 201 may include a third surface region 233 and a fourth surface region 235 opposite the third surface region 233. The second portion 231 will now be described with reference to Figure 2 the foldable device 101, wherein it should be understood that this description of the second portion 231 may also apply to any aspect of the present disclosure, such as Figures 3 - 4The foldable devices 301, 401, 501, 701, 801, and / or 901 shown in FIGS. 6 - 9. In various aspects, as shown, the third surface region 233 of the second portion 231 may include a flat surface, and / or the fourth surface region 235 of the second portion 231 may include a flat surface. In other aspects, the third surface region 233 of the second portion 231 may be in a common plane with the first surface region 223 of the first portion 221. In other aspects, as shown, the fourth surface region 235 may be parallel to the third surface region 233. In other aspects, the fourth surface region 235 of the second portion 231 may be in a common plane with the second surface region 225 of the first portion 221. The second thickness may be defined between the third surface region 233 and the fourth surface region 235 of the second portion 231. In various aspects, the second thickness may be within the range discussed above with respect to the substrate thickness 207. In other aspects, the second thickness may include the substrate thickness 207 and / or be substantially equal to the substrate thickness 207 (e.g., the first thickness). In various aspects, the second thickness of the second portion 231 may be substantially uniform between the third surface region 233 and the fourth surface region 235.

[0328] As Figures 2 - 4 shown, the foldable substrate 201 may include a central portion 281 located between the first portion 221 and the second portion 231. The central portion 281 includes a first central surface region 213 and a second central surface region 243 opposite the first central surface region 213. As shown, the first central surface region 213 may be located between the first surface region 223 and the third surface region 233. In other aspects, the first central surface region 213 may correspond to the central region 248 of the central portion 281. In other aspects, as shown, when the foldable devices 101, 301, and / or 401 are in a planar configuration, the first central surface region 213 may extend along the third plane 204b. The first groove 211 may be defined between the first central surface region 213 (e.g., the third plane 204b) and the first plane 204a.

[0329] In various aspects, the third plane 204b may be substantially parallel to the first plane 204a and / or the second plane 206a. In other aspects, as Figures 2 - 3As shown, the first central surface region 213 may be recessed from the first major surface 203 by a first distance 219. In other aspects, the first distance 219 by which the first central surface region 213 is recessed from the first plane 204a may be about 5 μm or greater, about 10 μm or greater, about 25 μm or greater, about 40 μm or greater, about 80 μm or greater, about 100 μm or greater, about 125 μm or greater, about 150 μm or greater, about 1 mm or less, about 800 μm or less, about 500 μm or less, about 300 μm or less, about 200 μm or less, about 180 μm or less, or about 150 μm or less. In other aspects, the first distance 219 may be in the range of about 5 μm to about 1 mm, about 5 μm to about 500 μm, about 10 μm to about 300 μm, about 25 μm to about 200 μm, about 40 μm to about 200 μm, about 80 μm to about 200 μm, about 100 μm to about 180 μm, about 125 μm to about 150 μm or any range or sub-range therebetween. In other aspects, the first distance 219 expressed as a percentage of the substrate thickness 207 may be about 1% or greater, about 5% or greater, about 10% or greater, about 15% or greater, about 20% or greater, about 25% or greater, about 75% or less, about 60% or less, about 50% or less, about 40% or less, about 35% or less, or about 30% or less. In other aspects, the first distance 219 expressed as a percentage of the substrate thickness 207 may be in the range of about 1% to about 75%, about 1% to about 60%, about 5% to about 60%, about 10% to about 50%, about 15% to about 45%, about 20% to about 35%, about 25% to about 30% or any range or sub-range therebetween.

[0330] As Figures 2 - 4 shown, the second central surface region 243 of the central portion 281 is located between the second surface region 225 and the fourth surface region 235. In various aspects, as Figure 4 shown, the second central surface region 243 may extend along the second plane 206a. In other aspects, as shown, the second central surface region 243 may be coplanar with the second major surface 205 (i.e., extend along the second plane 206a). In other aspects, as shown, in addition to the second surface region 225 and the fourth surface region 235, the second major surface 205 may further include the second central surface region 243. Alternatively, in various aspects, as Figures 2 - 3 shown, when the foldable device 101 and / or 301 is in a planar configuration, the second central surface region 243 may extend along the fourth plane 206b (e.g., different from the second plane 206a). In other aspects, the second groove 241 is defined between the second central surface region 243 (e.g., the fourth plane 206b) and the second plane 206a.

[0331] In various aspects, such as Figures 2 - 3 shown, the second central surface region 243 can be recessed from the second major surface 205 by a second distance 249. In other aspects, the second distance 249 can be within one or more of the ranges discussed above for the first distance 219. In other aspects, the first distance can be greater than the second distance. In yet other aspects, the second distance 249 by which the second central surface region 243 is recessed from the second plane 206a, expressed as a percentage of the substrate thickness 207, can be about 1% or greater, about 2% or greater, about 5% or greater, about 10% or greater, about 12% or greater, about 30% or less, about 25% or less, about 20% or less, about 18% or less, or about 15% or less. In yet other aspects, the second distance 249, expressed as a percentage of the substrate thickness 207, can be in the range of about 1% to about 30%, about 2% to about 25%, about 5% to about 20%, about 10% to about 18%, about 12% to about 15%, or any range or sub-range therebetween. In other aspects, such as Figure 2 shown, the first distance 219 can be substantially equal to the second distance 249. Making the first distance substantially equal to the second distance can further reduce the incidence of mechanical instability in the central portion, for example because the foldable substrate is symmetric about a plane that includes the midpoint of the substrate thickness and the central thickness. In other aspects, such as Figure 4 shown, the second central surface region 243 can be coplanar with the second surface region 225 and / or the fourth surface region 235, for example, thereby forming a flat second major surface 205 that extends along the second plane 206a.

[0332] The center thickness 209 can be defined as the distance between a third plane 204b and a fourth plane 206b between a first central surface region 213 and a second central surface region 243. In various aspects, the center thickness 209 can be about 10 μm or greater, about 25 μm or greater, about 40 μm or greater, about 200 μm or less, about 120 μm or less, about 100 μm or less, about 80 μm or less, about 60 μm or less, or about 50 μm or less. In various aspects, the center thickness 209 can be in the range of about 10 μm to about 200 μm, about 25 μm to about 120 μm, about 25 μm to about 100 μm, about 25 μm to about 80 μm, about 40 μm to about 60 μm or any range or sub-range therebetween. In various aspects, the center thickness 209 can be about 10 μm or more, about 20 μm or more, about 30 μm or more, about 40 μm or more, about 50 μm or more, or about 60 μm or more less than the substrate thickness 207. In various aspects, the center thickness 209 expressed as a percentage of the substrate thickness 207 can be about 0.5% or greater, about 1% or greater, about 2% or greater, about 5% or greater, about 40% or less, about 30% or less, about 20% or less, about 13% or less, about 10% or less, or about 8% or less. In various aspects, the center thickness 209 expressed as a percentage of the substrate thickness 207 can be in the range of about 0.5% to about 40%, about 0.5% to about 20%, about 1% to about 13%, about 2% to about 10%, about 5% to about 8% or any range or sub-range therebetween. In various aspects, the central region 248 of the central portion 281 can correspond to the region including the center thickness 209. By making the first central surface region 213 of the central portion 281 extending along the third plane 204b parallel to the second central surface region 243 of the central portion 281 extending along the fourth plane 206b, a uniform center thickness 209 can extend over the central portion 281, thereby providing enhanced folding performance at a predetermined thickness of the center thickness 209. The uniformity of the center thickness 209 on the central portion 281 can prevent stress concentration from occurring because a part of the central portion 281 is thinner than the rest of the central portion 281, thereby improving the folding performance.

[0333] In various aspects, as Figures 2 - 4 shown, the central portion 281 of the foldable substrate 201 can include a first transition region 212, which includes a first transition surface region 215 extending between a first surface region 223 and a first central surface region 213. In other aspects, as shown in the figure, the width of the first transition region 212 (e.g., the first transition width 214) corresponds to a part extending along the third plane 204b in the first central surface region 213 and a part of the first surface region 223 in the direction 106 of the length 105 (see Figure 1) The minimum distance on []. In still other aspects, the first transition width 214 of the first transition region 212 can be about 0.15 mm or greater, about 0.3 mm or greater, about 0.5 mm or greater, about 0.6 mm or greater, about 0.7 mm or greater, about 0.8 mm or greater, about 2 mm or less, about 1.8 mm or less, about 1.5 mm or less, about 1.2 mm or less, about 1 mm or less, about 0.8 mm or less, about 0.7 mm or less, or about 0.5 mm or less. In still other aspects, the first transition width 214 of the first transition region 212 can be in the range of about 0.15 mm to about 2 mm, about 0.3 mm to about 2 mm, about 0.5 mm to about 1.8 mm, about 0.6 mm to about 1.5 mm, about 0.7 mm to about 1.2 mm, about 0.8 mm to about 1 mm or any range or sub-range therebetween. In various aspects, as Figures 2 - 3 shown, the first transition region 212 can include a second transition surface region 245 extending between the second surface region 225 and the second central surface region 243. The width of the second transition surface region 245 corresponds to the minimum distance between a portion extending along the fourth plane 206b in the second central surface region 243 and a portion of the second surface region 225 in the direction 106 of the length 105 (see Figure 1 ) and can be within one or more of the ranges discussed above for the first transition width 214 and / or be substantially equal to the first transition width 214.

[0334] In various aspects, as Figures 2 - 4 shown, the thickness of the first transition region 212 can decrease between the substrate thickness 207 of the first portion 221 and the central thickness 209 of the central portion 281. In other aspects, as shown in the figure, the thickness of the first transition region 212 can decrease smoothly, monotonically, and / or smoothly and monotonically between the substrate thickness 207 of the first portion 221 and the central thickness 209 of the central portion 281. As used herein, a smooth decrease in thickness means that the change in cross-sectional area is smooth (e.g., gradual), rather than a sudden change in thickness (e.g., stepped). As used herein, a monotonic decrease in thickness in one direction means that the thickness decreases for some period of time and remains constant, decreases, or both (i.e., the thickness decreases in this direction but never increases) for the remainder of the time. The smooth shape of the first transition region and / or the second transition region can reduce optical distortion.

[0335] In various aspects, as Figures 2 - 4As shown, the first transition surface region 215 may include a linear inclined surface extending between the first central surface region 213 and the first surface region 223. In various aspects, although not shown, the first transition surface region may include an upwardly concave shape, for example, where the local slope of the first transition surface region smoothly transitions to the slope of the first central surface region 213, but the local slope of the first transition surface region is significantly different from the slope of the first surface region 223. In various aspects, although not shown, the first transition surface region may include an S-shaped configuration. In various aspects, although not shown, the local slope of the first transition surface region at the midpoint of the first transition surface region may be greater than the locations where the first transition surface region meets the first central surface region 213 and where the first transition surface region meets the first surface region 223. In various aspects, although not shown, the first transition surface region may include an upwardly convex shape, for example, where the local slope of the first transition surface region smoothly transitions to the slope of the first surface region 223, but the local slope of the first transition surface region is significantly different from the slope of the first central surface region 213. In various aspects, the second transition surface region may include one of the shapes or characteristics discussed hereinabove for the first transition surface region. For example, as Figure 2 shown, the second transition surface region 245 may include a linear inclined surface extending between the second central surface region 243 and the second surface region 225.

[0336] In various aspects, as Figures 2 - 4 shown, the thickness of the first transition region 212 may decrease from the substrate thickness 207 to the central thickness 209 at a constant rate (e.g., varies linearly). In various aspects, although not shown, the rate of decrease of the thickness of the first transition region at the location where the first transition surface region meets the first central surface region 213 may be slower than the midpoint of the first transition region and / or the location where the first transition surface region meets the first surface region 223 (e.g., the first portion 221). In various aspects, although not shown, the rate of decrease of the thickness of the first transition region at the location where the first transition surface region meets the first central surface region 213 may be faster than the midpoint of the first transition region and / or the location where the first transition surface region meets the first surface region 223. Making the slopes of the surface regions of the first transition region and / or the second transition region non-uniform may reduce the number of corresponding transition regions that include an intermediate thickness, such as an expansion strain due to chemical strengthening, that is less than a portion of the corresponding transition region that is closer to the first central surface region and / or the second central surface region and / or the first central surface region and / or the second central surface region.

[0337] Throughout the present disclosure, the average angle of a transition surface region relative to a central surface region is measured as the angle between the transition surface region and the central surface region. The angle is calculated with respect to positions on the corresponding transition surface region relative to the corresponding central surface region, where the position of the corresponding central surface region is approximated by a plane fitted from measurements of 20 positions uniformly spaced along a direction 106 along a length 105 on the corresponding central surface region. The measured angle is the exterior angle of the foldable substrate, i.e., extending from the plane fitted to the corresponding central surface region to the position of the corresponding transition surface region without passing through the material of the foldable substrate. The average angle is calculated based on 10 positions in a region of the corresponding transition surface region that lies within 80% of the distance by which the corresponding central surface region is recessed from the corresponding major surface, where the region is centered on the midpoint between the corresponding central surface region and the corresponding major surface in a direction 202 of thickness (e.g., substrate thickness 207, central thickness 209). In various aspects, as Figures 2 - 4 shown, a first transition surface region 215 of a first transition region 212 extends between a first surface region 223 and a first central surface region 213 at a first average angle 282 relative to the first central surface region 213. As described above, the first average angle 282 is the exterior angle since it does not pass through the material of the foldable substrate 201 except for a negligible amount possibly associated with the endpoints. In other aspects, the first average angle 282 can be about 160° or greater, about 162° or greater, about 165° or greater, about 167° or greater, about 170° or greater, about 171° or greater, about 172° or greater, about 179° or less, about 176° or less, about 175° or less, about 174° or less, or about 173° or less. In other aspects, the first average angle 282 can be in the range of about 160° to about 179°, about 162° to about 176°, about 165° to about 176°, about 167° to about 175°, about 170° to about 175°, about 171° to about 174°, about 172° to about 173°, or any range or sub-range therebetween. For example, a first transition surface including a linear (e.g., flat) surface region has a first transition width of 500 μm and a height of 30 μm (i.e., the difference between the first central surface region 213 and the first major surface 203, corresponding to a first distance 219), corresponding to a first average angle of about 176.6°.

[0338] In various aspects, as Figures 2 - 4As shown, the third transition surface region 217 of the second transition region 218 extends between the third surface region 233 and the first central surface region 213 at a third average angle 286 relative to the first central surface region 213. In other aspects, the third average angle 286 may be within one or more of the ranges discussed above for the first average angle 282. In other aspects, the first average angle 282 may be substantially equal to the third average angle 286.

[0339] In various aspects, as Figures 2 - 4 shown, the central portion 281 of the foldable substrate 201 may include a second transition region 218, which includes a third transition surface region 217 extending between the third surface region 233 and the first central surface region 213. In other aspects, as shown, the width of the second transition region 218 (e.g., the second transition width 216) may be measured as the minimum distance between a portion of the first central surface region 213 extending along the third plane 204b and a portion of the third surface region 233 in the direction 106 of the length 105 (see Figure 1 ). In still other aspects, the second transition width 216 of the second transition region 218 may be within one or more of the ranges discussed above for the first transition width 214. In yet other aspects, the second transition width 216 of the second transition region 218 may be substantially equal to (e.g., equal to) the first transition width 214.

[0340] In various aspects, as Figures 2 - 3 shown, the second transition region 218 may include a fourth transition surface region 247 extending between the fourth surface region 235 and the second central surface region 243. In other aspects, the width of the fourth transition surface region 247 may be measured as the minimum distance between a portion of the second central surface region 243 extending along the fourth plane 206b and a portion of the fourth surface region 235 in the direction 106 of the length 105 (see Figure 1 ). In still other aspects, the width of the fourth transition surface region 247 may be substantially equal to (e.g., equal to) the second transition width 216. In various aspects, as Figures 2 - 3 shown, the thickness of the second transition region 218 may decrease between the substrate thickness 207 of the second portion 231 and the central thickness 209 of the central portion 281. In other aspects, as shown, the thickness of the first transition region 212 may decrease smoothly, monotonically, or smoothly and monotonically between the substrate thickness 207 of the second portion 231 and the central thickness 209 of the central portion 281. In various aspects, as Figure 4 shown, the portion of the second transition region 218 extending between the fourth surface region 235 and the second central surface region 243 may be coplanar with one or both of the surface regions.

[0341] In various aspects, such as Figures 2 - 4 as shown in, the third transitional surface region 217 may include a linear inclined surface extending between the first central surface region 213 and the third surface region 233. In various aspects, the third transitional surface region 217 and / or the fourth transitional surface region 247 may include one of the shapes or characteristics discussed above with reference to the first transitional surface region. In various aspects, the fourth transitional surface region 247 may include one of the shapes or characteristics discussed above in this paragraph with respect to the first transitional surface region. For example, as Figures 2 - 4 as shown in, the fourth transitional surface region 247 may include a linear inclined surface extending between the second central surface region 243 and the fourth surface region 235. In various aspects, such as Figures 2 - 4 as shown in, the thickness of the second transitional region 218 may decrease from the substrate thickness 207 to the central thickness 209 at a constant rate (e.g., vary linearly). In various aspects, although not shown, the rate of decrease of the thickness of the second transitional region at the position where the third transitional surface region meets the first central surface region 213 may be slower than the midpoint of the second transitional region and / or the position where the third transitional surface region meets the third surface region 233 (e.g., the first portion 221). In various aspects, although not shown, the rate of decrease of the thickness of the second transitional region at the position where the third transitional surface region meets the first central surface region 213 may be faster than the midpoint of the second transitional region and / or the position where the third transitional surface region meets the third surface region 233.

[0342] In various aspects, such as Figures 2 - 3 as shown in, the second transitional surface region 245 of the first transitional region 212 extends between the second surface region 225 and the second central surface region 243 at a second average angle 284 with respect to the second central surface region 243. In other aspects, the second average angle 284 may be within one or more of the ranges discussed above for the first average angle 282 and / or may be substantially equal to the first average angle 282. Providing an average angle within one of the above ranges may reduce the visibility of the transitional region. In various aspects, such as Figures 2 - 3 as shown in, the fourth transitional surface region 247 of the second transitional region 218 extends between the fourth surface region 235 and the second central surface region 243 at a fourth average angle 288 with respect to the second central surface region 243. In other aspects, the fourth average angle 288 may be within one or more of the ranges discussed above for the second average angle 284. In other aspects, the second average angle 284 may be substantially equal to the fourth average angle 288. In other aspects, the first average angle 282 and / or the third average angle 286 may be substantially equal to the fourth average angle 288.

[0343] As used herein, if a first layer and / or component is described as being "disposed over a second layer and / or component", there may or may not be other layers between the first layer and / or component and the second layer and / or component. Additionally, as used herein, "disposed over" does not refer to a relative position with respect to gravity. For example, when a first layer and / or component is located below, above, or to one side of a second layer and / or component, the first layer and / or component may be considered to be "disposed over the second layer and / or component". As used herein, a first layer and / or component is described as being "joined to" a second layer and / or component means that the layers and / or components are joined to each other by direct contact and / or a bond between the two layers and / or components or via an adhesive layer. As used herein, a first layer and / or component is described as "contacting" a second layer and / or component or "in contact with a second layer and / or component" means direct contact and includes the case where the layers and / or components are joined to each other.

[0344] As Figure 2 and 4 shown in, the foldable device 101 may include an adhesive layer 261. As shown, the adhesive layer 261 may include a first contact surface 263 and a second contact surface 265 that may be opposite the first contact surface 263. In various aspects, as Figure 2 and 4 shown in, the second contact surface 265 of the adhesive layer 261 may include a flat surface. In various aspects, as Figure 2 and 4 shown in, the first contact surface 263 of the adhesive layer 261 may include a flat surface. The adhesive thickness 267 of the adhesive layer 261 may be defined as the minimum distance between the first contact surface 263 and the second contact surface 265. In various aspects, the adhesive thickness 267 of the adhesive layer 261 may be about 1 μm or greater, about 5 μm or greater, about 10 μm or greater, about 100 μm or less, about 60 μm or less, about 30 μm or less, or about 20 μm or less. In various aspects, the adhesive thickness 267 of the adhesive layer 261 may be in the range of about 1 μm to about 100 μm, about 5 μm to about 60 μm, about 10 μm to about 30 μm, about 10 μm to about 20 μm or any range or sub-range therebetween.

[0345] In various aspects, as Figure 2 and 4 shown in, the second contact surface 265 of the adhesive layer 261 may face and / or contact a first major surface 273 (described below) of the release liner 271. In various aspects, as Figure 2 shown, the first contact surface 263 of the adhesive layer 261 may face and / or contact a second surface region 225 of the first portion 221. In various aspects, as Figure 2As shown, the first contact surface 263 of the adhesive layer 261 may face and / or contact the fourth surface region 235 of the second portion 231. In various aspects, as Figure 2 shown, the first contact surface 263 of the adhesive layer 261 may face the second central surface region 243 of the central portion 281. In various aspects, as Figure 4 shown, the first contact surface 263 of the adhesive layer 261 may face and / or contact the first surface region 223 of the first portion 221. In various aspects, as Figure 4 shown, the first contact surface 263 of the adhesive layer 261 may face and / or contact the third surface region 233 of the second portion 231. In various aspects, as Figure 4 shown, the first contact surface 263 of the adhesive layer 261 may face the first central surface region 213 of the central portion 281. In various aspects, as Figure 2 shown, the first contact surface 263 of the adhesive layer 261 may face the second central surface region 243 of the central portion 281. In other aspects, although not shown, the first contact surface 263 of the adhesive layer 261 may contact the second central surface region 243 of the central portion 281, for example, by filling Figure 2 the area indicated as being occupied by the second polymer-based portion 299 (e.g., the second groove 241). In various aspects, although not shown, the second groove may not be completely filled, for example, to leave space for an electronic device and / or a mechanical device. In various aspects, although not shown, Figure 4 the foldable substrate 201 may be configured such that the adhesive layer 261 contacts the second major surface 205 instead of the first major surface 203, while the second polymer-based portion 299 or the coating 251 replacing the second polymer-based portion 299 may be at least partially positioned in the first groove 211.

[0346] In various aspects, the adhesive layer 261 may include one or more of polyolefins, polyamides, halogen-containing polymers (e.g., polyvinyl chloride or fluoropolymers), elastomers, polyurethanes, phenolic resins, parylene, polyethylene terephthalate (PET), and polyetheretherketone (PEEK). Exemplary aspects of polyolefins include low molecular weight polyethylene (LDPE), high molecular weight polyethylene (HDPE), ultra-high molecular weight polyethylene (UHMWPE), and polypropylene (PP). Exemplary aspects of fluoropolymers include polytetrafluoroethylene (PTFE), polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF), perfluoropolyether (PFPE), perfluorosulfonic acid (PFSA), perfluoroalkoxy (PFA), fluorinated ethylene propylene (FEP) polymers, and ethylene tetrafluoroethylene (ETFE) polymers. Exemplary aspects of elastomers include rubbers (e.g., polybutadiene, polyisoprene, chloroprene rubber, butyl rubber, nitrile rubber) and block copolymers (e.g., styrene-butadiene, high impact polystyrene, polydichlorophosphazene). In other aspects, the adhesive layer 261 may include optically clear adhesives. In still other aspects, the optically clear adhesives may include one or more optically transparent polymers: acrylics (e.g., polymethyl methacrylate (PMMA)), epoxies, silicones, and / or polyurethanes. Examples of epoxies include bisphenol-based epoxies, phenol-based epoxies, cycloaliphatic-based epoxies, and glycidylamine-based epoxies. In still other aspects, the optically clear adhesives may include, but are not limited to, acrylic adhesives such as 3M 8212 adhesive, or optically clear liquid adhesives such as LOCTITE optically clear liquid adhesives. Exemplary aspects of optically clear adhesives include transparent acrylic resins, epoxies, silicones, and polyurethanes. For example, the optically clear liquid adhesives may include one or more of LOCTITE AD 8650, LOCTITE AA3922, LOCTITE EAE-05MR, LOCTITE UK U-09LV, all of which may be obtained from Henkel Corporation.

[0347] Throughout the present disclosure, the tensile strength, ultimate elongation (e.g., break strain), and yield point of the polymeric material (e.g., adhesive, polymer-based portion) are determined using ASTM D638, at 23 °C and 50% relative humidity, with a Type I dogbone specimen, using a tensile testing machine such as an Instron 3400 or Instron 6800. In various aspects, the adhesive layer 261 can include a modulus of elasticity of about 0.001 megapascals (MPa) or greater, about 0.01 MPa or greater, about 0.1 MPa or greater, about 1 MPa or less, about 0.5 MPa or less, about 0.1 MPa or less, or about 0.05 MPa or less. In various aspects, the adhesive layer 261 can include a modulus of elasticity in the range of about 0.001 MPa to about 1 MPa, about 0.01 MPa to about 0.5 MPa, about 0.1 MPa to about 0.5 MPa, or any range or sub-range therebetween. In various aspects, the adhesive layer can include a modulus of elasticity within one or more of the ranges discussed below for the modulus of elasticity of the polymer-based portion 289 and / or 299.

[0348] As Figure 2 and 4 shown, the polymer-based portion 289 and / or 299 of the foldable device 101 can be located between the first portion 221 and the second portion 231. In various aspects, as shown, the polymer-based portion can include a first polymer-based portion 289 that is at least partially located in and / or fills the first groove 211. In various aspects, as Figure 2 shown, the polymer-based portion can include a second polymer-based portion 299 that is at least partially located in and / or fills the second groove 241. In various aspects, as Figure 4 shown, the polymer-based portion can include a second polymer-based portion 299 that is at least partially located in and / or fills the first groove 211. In various aspects, although not shown, the second groove can be not completely filled, for example, to leave room for an electronic device and / or a mechanical device.

[0349] As Figure 2 shown, the first polymer-based portion 289 can include a fourth contact surface 285 that is opposite the third contact surface 283. In various aspects, as shown, the third contact surface 283 can include a flat surface, for example, that is substantially coplanar with the first surface area 223 and the third surface area 233 (e.g., extends along a common plane, i.e., the first plane 204a). In various aspects, as Figure 2As shown, the fourth major surface 255 of the coating 251 may face and / or contact the third contact surface 283 of the first polymer-based portion 289. In various aspects, the fourth contact surface 285 may include a flat surface, for example, being substantially coplanar with the first central surface region 213 (e.g., extending along a common plane, i.e., the third plane 204b). In other aspects, the fourth contact surface 285 may contact the first central surface region 213, the first transition surface region 215, and / or the third transition surface region 217.

[0350] As Figure 2 and 4 shown, the second polymer-based portion 299 may include a fourth contact surface 295 opposite to the third contact surface 293. In other aspects, as Figure 2 shown, the third contact surface 293 may contact the second central surface region 243, the second transition surface region 245, and / or the fourth transition surface region 247. In various aspects, as Figure 2 shown, the third contact surface 293 may include a flat surface, for example, being substantially coplanar with the second central surface region 243 (e.g., extending along a common plane with the fourth plane 206b). In various aspects, as Figure 2 shown, the fourth contact surface 295 may include a flat surface, for example, being substantially coplanar with the second surface region 225 and the fourth surface region 235 (e.g., extending along a common plane with the second plane 206a).

[0351] In various aspects, as Figure 4 shown, the third contact surface 293 may contact the first central surface region 213, the first transition surface region 215, and / or the third transition surface region 217. In various aspects, as Figure 4 shown, the third contact surface 293 may include a flat surface, for example, being substantially coplanar with the first central surface region 213 (e.g., extending along a common plane with the third plane 204b). In various aspects, as shown, the third contact surface 293 may include a flat surface, for example, being substantially coplanar with the first central surface region 213 (e.g., extending along a common plane with the third plane 204b). In various aspects, as Figure 4 shown, the fourth contact surface 295 may be coplanar with the first surface region 223 and the third surface region 233 (e.g., extending along a common plane with the first plane 204a). In various aspects, as Figure 2 and 4 shown, the first contact surface 263 of the adhesive layer 261 may face and / or contact the fourth contact surface 295 of the second polymer-based portion 299.

[0352] In various aspects, the polymer-based portion 289 and / or 299 includes a polymer (e.g., an optically transparent polymer). In other aspects, the polymer-based portion 289 and / or 299 may include one or more of the following optically transparent materials: acrylic (e.g., polymethyl methacrylate (PMMA)), epoxy resin, silicone, and / or polyurethane. Examples of epoxy resins include bisphenol-based epoxy resins, phenolic-based epoxy resins, alicyclic-based epoxy resins, and glycidylamine-based epoxy resins. In other aspects, the polymer-based portion 289 and / or 299 may include one or more of polyolefin, polyamide, halogen-containing polymer (e.g., polyvinyl chloride or fluoropolymer), elastomer, polyurethane, phenolic resin, parylene, polyethylene terephthalate (PET), and polyetheretherketone (PEEK). Exemplary aspects of elastomers include rubber and block copolymers, such as including one or more of polystyrene, polydichlorophosphazene, and poly(5-ethylidene-2-norbornene). In various aspects, the polymer-based portion may include a sol-gel material. Exemplary aspects of polyurethane include thermosetting polyurethane, such as Dispurez 102 available from Incorez, and thermoplastic polyurethane, such as KrystalFlex PE505 available from Huntsman. In still other aspects, the second portion may include an ethylene acid copolymer. Exemplary aspects of ethylene acid copolymers include SURLYN available from Dow (e.g., Surlyn PC-2000, Surlyn 8940, Surlyn 8150). Additional exemplary aspects of the second portion include Eleglass w802-GL044 available from Axalta, where the crosslinker is 1 wt% to 2 wt%. In various aspects, the polymer-based portion 289 and / or 299 may further include nanoparticles, such as carbon black, carbon nanotubes, silica nanoparticles, or nanoparticles including a polymer. In various aspects, the polymer-based portion may further include fibers to form a polymer-fiber composite.

[0353] In various aspects, the polymer-based portion 289 and / or 299 can include an elastic modulus that is about 0.001 megapascals (MPa) or greater, about 0.01 MPa or greater, about 1 MPa or greater, about 10 MPa or greater, about 20 MPa or greater, about 100 MPa or greater, about 200 MPa or greater, about 1,000 MPa or greater, about 5,000 MPa or less, about 3,000 MPa or less, about 1,000 MPa or less, about 500 MPa or less, or about 200 MPa or less. In various aspects, the elastic modulus included in the polymer-based portion 289 and / or 299 can be in the range of about 0.001 MPa to about 5,000 MPa, about 0.01 MPa to about 3,000 MPa, about 0.01 MPa to about 1,000 MPa, about 1 MPa to about 200 MPa, about 10 MPa to about 200 MPa, about 100 MPa to about 200 MPa, or any range or sub-range therebetween. In various aspects, the adhesive layer 261 includes an elastic modulus that is greater than the elastic modulus of the polymer-based portion 289 and / or 299, and this arrangement improves the puncture resistance. In various aspects, the elastic modulus of the polymer-based portion 289 and / or 299 can be less than the elastic modulus of the foldable substrate 201. In various aspects, the adhesive layer 261 can include an elastic modulus within the ranges listed hereinabove in this paragraph. In other aspects, the elastic modulus included in the adhesive layer 261 can be substantially the same as the elastic modulus of the polymer-based portion 289 and / or 299. In various aspects, the elastic modulus of the polymer-based portion 289 and / or 299 can be less than the elastic modulus of the foldable substrate 201.

[0354] In various aspects, as Figure 2As shown, the coating 251 can be disposed over the first major surface 203 of the foldable substrate 201. In other aspects, the coating 251 can be disposed over the first portion 221, the second portion 231, and the central portion 281. In various aspects, the coating 251 can include a third major surface 253 and a fourth major surface 255 opposite the third major surface 253. In other aspects, the coating 251 (e.g., the fourth major surface 255) can contact the foldable substrate 201 (e.g., the first major surface 203). In other aspects, at least a portion of the coating 251 can be located within the first groove 211. In yet other aspects, the coating 251 can fill the first groove 211. In other aspects, the coating 251 can include a coating thickness 257 defined between the third major surface 253 and the fourth major surface 255. In other aspects, the coating thickness 257 can be about 0.1 μm or greater, about 1 μm or greater, about 5 μm or greater, about 10 μm or greater, about 20 μm or greater, about 25 μm or greater, about 40 μm or greater, about 80 μm or greater, about 200 μm or less, about 100 μm or less, or about 50 μm or less, about 25 μm or less, about 20 μm or less, about 20 μm or less, about 15 μm or less, or about 10 μm or less. In various aspects, the coating thickness 257 can range from about 0.1 μm to about 200 μm, from about 1 μm to about 100 μm, from about 10 μm to about 50 μm, from about 20 μm to about 50 μm, or any range or sub-range therebetween.

[0355] In various aspects, the coating 251 can include a polymer hard coating. In other aspects, the polymer hard coating can include one or more of an ethylene acid copolymer, a polyurethane-based polymer, an acrylate resin, and a thiol ester resin. Exemplary aspects of ethylene acid copolymers include ethylene-acrylic acid copolymers, ethylene-methacrylic acid copolymers, and ethylene-acrylic acid-methacrylic acid terpolymers (e.g., Nucrel produced by DuPont), ionomers of ethylene acid copolymers (e.g., Surlyn produced by DuPont), and ethylene-acrylic acid copolymer amine dispersions (e.g., Aquacer produced by BYK). Exemplary aspects of polyurethane-based polymers include waterborne modified polyurethane dispersions (e.g., ) produced by Axalta Coating Systems. Exemplary aspects of UV-curable acrylate resins include acrylate resins (e.g., resin produced by Allinex), cyanoacrylate adhesives (e.g., UV620) and UV free radical acrylic resins (e.g., Ultrabond windshield repair resin such as Ultrabond (45CPS)). Example aspects of thiol ester resins include thiol ester triallyl isocyanurate (e.g., Norland Optical Adhesive NOA61). In other aspects, the polymer hard coat can include ethylene-acrylic copolymers and ethylene-methacrylic acid copolymers, which can be ionomerized by neutralizing carboxylic acid residues with typical alkali metal ions (such as sodium, potassium, and zinc) to form ionomer resins. Such ethylene-acrylic and ethylene-methacrylic acid ionomers can be dispersed in water and coated onto a substrate to form an ionomer coating. Alternatively, such acid copolymers can be neutralized with ammonia, which is released after coating and drying, reforming the acidic copolymer into a coating. By providing a coating including a polymer coating, the foldable device can achieve low-energy fracture.

[0356] In various aspects, the coating can include a polymer hard coat, including an optically clear polymer hard coat. Materials suitable for an optically clear polymer hard coat include, but are not limited to, cured acrylate resin materials, inorganic-organic hybrid polymer materials, aliphatic or aromatic hexa-functional polyurethane acrylates, siloxane-based hybrid materials, and nanocomposites, such as epoxy resins and polyurethane materials with nanosilicates. In various aspects, the optically clear polymer hard coat can consist essentially of one or more of these materials. In various aspects, the optically clear polymer hard coat can consist of one or more of these materials. As used herein, "inorganic-organic hybrid polymer material" refers to a polymer material including monomers containing both inorganic and organic components. Inorganic-organic hybrid polymers are obtained through polymerization reactions between monomers having inorganic groups and organic groups. Inorganic-organic hybrid polymers are not nanocomposites including separate inorganic and organic components or phases, e.g., inorganic particles dispersed in an organic matrix. More specifically, materials suitable for an optically transparent polymer (OTP) hard coat include, but are not limited to, polyimides, polyethylene terephthalate (PET), polycarbonate (PC), polymethyl methacrylate (PMMA), organic polymer materials, inorganic-organic hybrid polymer materials, and aliphatic or aromatic hexa-functional polyurethane acrylates. In various aspects, the OTP hard coat can consist essentially of an organic polymer material, an inorganic-organic hybrid polymer material, or an aliphatic or aromatic hexa-functional polyurethane acrylate. In various aspects, the OTP hard coat can consist of polyimide, an organic polymer material, an inorganic-organic hybrid polymer material, or an aliphatic or aromatic hexa-functional polyurethane acrylate. In various aspects, the OTP hard coat can include a nanocomposite. In various aspects, the OTP hard coat can include at least one of an epoxy resin and a polyurethane material with nanosilicates. Compositions suitable for such OTP hard coats are described in U.S. Patent Publication No. 2015 / 0110990, which is hereby incorporated herein by reference in its entirety. As used herein, "organic polymer material" refers to a polymer material including monomers containing only organic components. In various aspects, the OTP hard coat can include an organic polymer material produced by Gunze Limited and having a hardness of 9H, such as Gunze's "Highly Durable Transparent Film". As used herein, "inorganic-organic hybrid polymer material" refers to a polymer material including monomers containing both inorganic and organic components. Inorganic-organic hybrid polymers are obtained through polymerization reactions between monomers having inorganic groups and organic groups. Inorganic-organic hybrid polymers are not nanocomposites including separate inorganic and organic components or phases, e.g., inorganic particles dispersed in an organic matrix. In various aspects, the inorganic-organic hybrid polymer material can include polymerized monomers including inorganic silicon-based groups (e.g., silsesquioxane polymers).For example, the silsesquioxane polymer can be an alkylsilsesquioxane, arylsilsesquioxane, or arylalkylsilsesquioxane having the following chemical structure: (RSiO. 1.5 ) n , where R is an organic group such as, but not limited to, methyl or phenyl. In various aspects, the OTP hard coat can include a silsesquioxane polymer bound to an organic matrix, such as SILPLUS manufactured by Nippon Steel Chemical Co., Ltd. In various aspects, the OTP hard coat can include 90 wt% to 95 wt% of an aromatic hexa-functional polyurethane acrylate (e.g., PU662NT (aromatic hexa-functional polyurethane acrylate) manufactured by Miwon Specialty Chemical Co.) and 10 wt% to 5 wt% of a photoinitiator (e.g., Darocur 1173 manufactured by Ciba Specialty Chemicals Corporation), and has a hardness of 8H or higher. In various aspects, an OTP hard coat composed of an aliphatic or aromatic hexa-functional polyurethane acrylate can be formed as a separate layer by spin-coating this layer on a polyethylene terephthalate (PET) substrate, curing the polyurethane acrylate, and removing the polyurethane acrylate layer from the PET substrate. In various aspects, the OTP hard coat can be a layer of an aliphatic or aromatic hexa-functional polyurethane acrylate material having a thickness within one or more of the thickness ranges discussed above for coating thickness 257.

[0357] In various aspects, the coating 251 (if provided) can also include one or more of an easy-to-clean coating, a low-friction coating, an oleophobic coating, a diamond-like coating, a scratch-resistant coating, or an abrasion-resistant coating. The scratch-resistant coating can include an oxynitride having a thickness of about 500 microns or greater, such as aluminum oxynitride or silicon oxynitride. In these aspects, the abrasion-resistant layer can include the same material as the scratch-resistant layer. In various aspects, the low-friction coating can include a highly fluorinated silane coupling agent, such as an alkylfluorosilane having an oxy-methyl group pendant on a silicon atom. In these aspects, the easy-to-clean coating can include the same material as the low-friction coating. In other aspects, the easy-to-clean coating can include a protonatable group, such as an amine, such as an alkylaminosilane having an oxy-methyl group pendant on a silicon atom. In these aspects, the oleophobic coating can include the same material as the easy-to-clean coating. In various aspects, the diamond-like coating includes carbon and can be produced by applying a high voltage potential in the presence of a hydrocarbon plasma.

[0358] In various aspects, as Figure 2 and 4As shown, the foldable device 101 may include a release liner 271, but in other aspects other substrates may be used (e.g., the glass substrates and / or ceramic substrates discussed throughout this application) instead of the release liner 271 shown. In other aspects, as shown, the release liner 271 or another substrate may be disposed over the adhesive layer 261. In still other aspects, as shown, the release liner 271 or another substrate may directly contact the second contact surface 265 of the adhesive layer 261. The release liner 271 or another substrate may include a first major surface 273 and a second major surface 275 opposite the first major surface 273. As shown, the release liner 271 or another substrate may be disposed on the adhesive layer 261 by attaching the second contact surface 265 of the adhesive layer 261 to the first major surface 273 of the release liner 271 or another substrate. In aspects, as shown, the first major surface 273 of the release liner 271 or another substrate may include a flat surface. In aspects, as shown, the second major surface 275 of the release liner 271 or another substrate may include a flat surface. The substrate including the release liner 271 may include paper and / or polymers such as polyester (e.g., polyethylene terephthalate (PET)) and polyolefin.

[0359] Aspects of the present disclosure may include consumer electronic products. The consumer electronic product may include a front surface, a rear surface, and side surfaces. The consumer electronic product may further include electrical components at least partially located within a housing. The electrical components may include a controller, a memory, and a display. The display may be located at or adjacent to the front surface of the housing. The display may include a liquid crystal display (LCD), an electrophoretic display (EPD), an organic light emitting diode (OLED) display, or a plasma display panel (PDP). The consumer electronic product may include a cover substrate disposed over the display. In aspects, at least one of a portion of the housing or the cover substrate includes a foldable device discussed throughout the present disclosure. The consumer electronic product may include a portable electronic device such as a smart phone, a tablet computer, a wearable device, or a laptop computer.

[0360] The foldable devices disclosed herein may be incorporated into another article, e.g., an article having a display (or display article) (such as a consumer electronic device, including a mobile phone, a tablet computer, a computer, a navigation system, a wearable device (such as a watch), etc.), a building article, a transportation article (such as an automobile, a train, an airplane, a ship, etc.), a household appliance article, or any article that may benefit from a certain degree of transparency, scratch resistance, abrasion resistance, or a combination thereof. Exemplary articles having any of the foldable devices disclosed herein are shown in Figures 10 - 11 Specifically, Figures 10 - 11A consumer electronic device 1000 is shown that includes a housing 1002 having a front surface 1004, a rear surface 1006, and side surfaces 1008. Although not shown, the consumer electronic device may include electrical components that are at least partially within the housing or entirely within the housing. For example, the electrical components at least include a controller, a memory, and a display. As Figures 10 - 11 shown, the display 1010 may be located at or adjacent to the front surface of the housing 1002. The consumer electronic device may include a cover substrate 1012 located at or above the front surface of the housing 1002 such that it is above the display 1010. In various aspects, at least one of the cover substrate 1012 or a portion of the housing 1002 may include any of the foldable devices disclosed herein, such as a foldable substrate.

[0361] In various aspects, the foldable substrate 201, including a glass substrate and / or a ceramic substrate, as well as a first portion 221, a second portion 231, and / or a central portion 281, may include one or more compressive stress regions. In various aspects, the compressive stress regions may be generated by chemical strengthening. Chemical strengthening may include an ion exchange process in which ions in the surface layer are replaced or exchanged with larger ions having the same valence or oxidation state. The method of chemical strengthening will be discussed later. Without being bound by theory, chemically strengthening the first portion 221, the second portion 231, and / or the central portion 281 can achieve good impact resistance and / or puncture resistance (e.g., not being damaged when the pen drop height reaches about 15 centimeters (cm) or higher, about 20 cm or higher, about 50 cm or higher). Without being bound by theory, chemically strengthening the first portion 221, the second portion 231, and / or the central portion 281 can achieve a smaller parallel plate spacing (e.g., less than about 10 mm or smaller), because the compressive stress generated by chemical strengthening can counteract the tensile stress caused by bending on the outermost surface of the substrate. The compressive stress region may extend into a portion of the first portion and / or the second portion, and the depth reached is called the depth of compression (DOC). As used herein, the depth of compression refers to the depth at which the stress in the chemically strengthened substrate and / or portion described herein changes from compressive stress to tensile stress. The depth of compression can be measured by a surface stress meter or a scatter light polarimeter (SCALP, where the values reported here use the SCALP-5 produced by Glasstress of Estonia), depending on the ion exchange process and the thickness of the item being measured. If the stress in the substrate and / or portion is generated by exchanging potassium ions into the substrate, a surface stress meter, such as the FSM-6000 (Orihara Industrial Co., Ltd. (Japan)), is used to measure the depth of compression. Unless otherwise specified, the compressive stress (including surface CS) is measured by a surface stress meter (FSM) using a commercially available instrument such as the FSM-6000 manufactured by Orihara. Surface stress measurement depends on the accurate measurement of the stress optical coefficient (SOC) related to the birefringence of the glass. Unless otherwise specified, the SOC is measured according to Procedure C (glass disk method) of the standard test method named "Standard Test Method for Measurement of Glass Stress-Optical Coefficient" described in ASTM standard C770-16, the content of which is incorporated herein by reference in its entirety. If the stress is generated by exchanging sodium ions into the substrate and the thickness of the item being measured exceeds about 400 μm, then the depth of compression and the central tension (CT) can be measured using SCALP.If the stress in the substrate and / or part is generated by exchanging potassium and sodium ions into the substrate and / or part and the thickness of the article being measured exceeds about 400 μm, then the compression depth and CT are measured by SCALP. Without being bound by theory, the exchange depth of sodium ions can indicate the compression depth, and the exchange depth of potassium ions can indicate the change in the magnitude of the compressive stress (rather than the change from compressive stress to tensile stress). A graphical representation of the stress distribution can also be obtained using the refraction near field (RNF; the RNF method is described in U.S. Patent No. 8,854,623, entitled "Systems and methods for measuring a profile characteristic of a glass sample", which is incorporated herein by reference in its entirety). When obtaining a graphical representation of the stress distribution using the RNF method, the maximum center tension value provided by SCALP is utilized in the RNF method. The graphical representation of the stress distribution obtained by RNF is force balanced and calibrated based on the maximum center tension value provided by the SCALP measurement. As used herein, "depth of layer" (DOL) refers to the depth to which ions (e.g., sodium, potassium) have been exchanged into the substrate and / or part. Throughout this disclosure, the DOL is measured in accordance with ASTM C-1422. Without being bound by theory, the DOL is generally greater than or equal to the corresponding DOC. Through this disclosure, when the maximum center tension cannot be directly measured by SCALP (e.g., when the thickness of the article being measured is less than about 400 μm), the maximum center tension can be approximated by dividing the product of the maximum compressive stress and the compression depth by the difference between the substrate thickness and twice the compression depth, where the compressive stress and the compression depth are measured by FSM.

[0362] In various aspects, the first portion 221 may include a first compressive stress zone extending from the first surface region 223 to a first compressive depth at the first surface region 223, and / or a second compressive stress zone extending from the second surface region 225 to a second compressive depth at the second surface region 225. In various aspects, the first compressive depth and / or the second compressive depth, expressed as a percentage of the substrate thickness 207, may be about 5% or greater, about 10% or greater, about 12% or greater, about 15% or greater, about 30% or less, about 25% or less, about 22% or less, about 20% or less, about 17% or less, or about 15% or less. In various aspects, the first compressive depth and / or the second compressive depth, expressed as a percentage of the substrate thickness 207, may be in the range of about 5% to about 30%, about 10% to about 25%, about 10% to about 22%, about 12% to about 20%, about 15% to about 17%, or any range or sub-range therebetween. In various aspects, the first compressive depth and / or the second compressive depth may be about 1 μm or greater, about 10 μm or greater, about 15 μm or greater, about 20 μm or greater, about 25 μm or greater, about 30 μm or greater, about 200 μm or less, about 150 μm or less, about 100 μm or less, about 60 μm or less, about 45 μm or less, about 30 μm or less, or about 20 μm or less. In various aspects, the first compressive depth and / or the second compressive depth may be in the range of about 1 μm to about 200 μm, about 1 μm to about 150 μm, about 10 μm to about 100 μm, about 15 μm to about 60 μm, about 20 μm to about 45 μm, about 20 μm to about 30 μm, or any range or sub-range therebetween. Good impact resistance and / or puncture resistance can be achieved by making the first portion including the first glass and / or ceramic portion include a first compressive depth and / or a second compressive depth in the range of about 1% to about 30% of the first thickness.

[0363] In various aspects, the first compressive stress zone may include a first maximum compressive stress, and / or the second compressive stress zone may include a second maximum compressive stress. In other aspects, the first maximum compressive stress and / or the second maximum compressive stress may be about 100 megapascals (MPa) or greater, about 300 MPa or greater, 400 MPa or greater, about 500 MPa or greater, about 600 MPa or greater, about 700 MPa or greater, about 1,500 MPa or less, about 1,200 MPa or less, about 1,000 MPa or less, or about 800 MPa or less. In other aspects, the first maximum compressive stress and / or the second maximum compressive stress may be in the range of about 100 MPa to about 1,500 MPa, about 300 MPa to about 1,200 MPa, about 400 MPa to about 1,000 MPa, about 500 MPa to about 900 MPa, about 600 MPa to about 900 MPa, about 700 MPa to about 800 MPa, or any range or sub-range therebetween. By having the first maximum compressive stress and / or the second maximum compressive stress in the range of about 100 MPa to about 1,500 MPa, good impact resistance and / or puncture resistance can be achieved.

[0364] The first portion 221 may include a first tensile stress zone located between the first compressive stress zone and the second compressive stress zone. In various aspects, the first maximum tensile stress included in the first tensile stress zone may be about 10 MPa or greater, about 20 MPa or greater, about 30 MPa or greater, about 100 MPa or less, about 80 MPa or less, or about 60 MPa or less. In other aspects, the first maximum tensile stress may be in the range of about 10 MPa to about 100 MPa, about 20 MPa to about 80 MPa, about 30 MPa to about 60 MPa, or any range or sub-range therebetween. Having the first maximum tensile stress in the range of about 10 MPa to about 100 MPa can achieve good impact resistance and / or puncture resistance while providing low energy consumption fracture.

[0365] In various aspects, the second portion 231 may include a third compressive stress zone extending from the third surface region 233 to a third compressive depth at the third surface region 233, and / or the second portion 231 may include a fourth compressive stress zone extending from the fourth surface region 235 to a fourth compressive depth at the fourth surface region 235. In various aspects, the third compressive depth and / or the fourth compressive depth are expressed as a percentage of the substrate thickness 207 and may be within one or more of the ranges discussed above for the first compressive depth and / or the second compressive depth. In other aspects, the third compressive depth may be substantially equal to the fourth compressive depth. The third compressive stress zone may include a third maximum compressive stress, and / or the fourth compressive stress zone may include a fourth maximum compressive stress. In various aspects, the third maximum compressive stress and / or the fourth maximum compressive stress may be within one or more of the ranges discussed above for the first maximum compressive stress and / or the second maximum compressive stress. The second portion 231 may include a second tensile stress zone located between the third compressive stress zone and the fourth compressive stress zone. In various aspects, the second tensile stress zone may include a second maximum tensile stress, and the second maximum tensile stress may be within one or more of the ranges discussed above for the first maximum tensile stress. In other aspects, the first maximum tensile stress may be substantially equal to the second maximum tensile stress.

[0366] In various aspects, the first compressive depth may be substantially equal to the third compressive depth. In various aspects, the second compressive depth may be substantially equal to the fourth compressive depth. In various aspects, the first maximum compressive stress may be substantially equal to the third maximum compressive stress. In various aspects, the second maximum compressive stress may be substantially equal to the fourth maximum compressive stress. In various aspects, the depth of the first layer of one or more alkali metal ions may be substantially equal to the depth of the third layer of one or more alkali metal ions. In various aspects, the depth of the second layer of one or more alkali metal ions may be substantially equal to the depth of the fourth layer of one or more alkali metal ions.

[0367] In various aspects, the central portion 281 may include a first central compressive stress zone extending from the first central surface region 213 to a first central compressive depth at the first central surface region 213, and / or the central portion 281 may include a second central compressive stress zone extending from the second central surface region 243 to a second central compressive depth at the second central surface region 243. In other aspects, the first central compressive depth and / or the second central compressive depth, expressed as a percentage of the central thickness 209, may be within one or more of the ranges discussed above for the first compressive depth and / or the second compressive depth expressed as a percentage of the substrate thickness 207. In other aspects, the first central compressive depth and / or the second central compressive depth, expressed as a percentage of the central thickness 209, may be about 1% or greater, about 2% or greater, about 5% or greater, about 8% or greater, about 10% or greater, about 12% or greater, about 25% or less, about 20% or less, about 17% or less, about 15% or less, about 12% or less, about 10% or less, about 7% or less, or about 5% or less. For example, expressed as a percentage of the central thickness 209, the first central compressive depth and / or the second central compressive depth may be in the range of about 1% to about 25%, about 2% to about 20%, about 5% to about 17%, about 7% to about 12%, or any range or sub-range therebetween. In other aspects, the first central compressive depth may be substantially equal to the second central compressive depth. In other aspects, the first central compressive depth and / or the second central compressive depth may be within one or more of the ranges discussed above for the first compressive depth and / or the second compressive depth. In other aspects, the first central compressive depth and / or the second central compressive depth may be about 1 μm or greater, about 2 μm or greater, about 4 μm or greater, about 6 μm or greater, about 20 μm or less, about 15 μm or less, about 10 μm or less, or about 8 μm or less. For example, the first central compressive depth and / or the second central compressive depth may be in the range of about 1 μm to about 20 μm, about 2 μm to about 15 μm, about 4 μm to about 10 μm, about 6 μm to about 8 μm, or any range or sub-range therebetween. Good impact resistance and / or puncture resistance can be achieved by having the central portion, which includes the glass and / or ceramic portion, include a first central compressive depth and / or a second central compressive depth in the range of about 1% to about 25% of the central thickness.

[0368] The first central compressive stress zone may include a first central maximum compressive stress, and / or the second central compressive stress zone may include a second central maximum compressive stress. In various aspects, the first central maximum compressive stress and / or the second central maximum compressive stress may be within one or more of the ranges discussed above for the first maximum compressive stress and / or the second maximum compressive stress. By having the first central maximum compressive stress and / or the second central maximum compressive stress within the range of from about 100 MPa to about 1,500 MPa, good impact resistance and / or puncture resistance can be achieved.

[0369] The central portion 281 may include a central tensile stress zone located between the first central compressive stress zone and the second central compressive stress zone. In various aspects, the central maximum tensile stress included in the central tensile stress zone may be about 125 MPa or greater, about 150 MPa or greater, about 200 MPa or greater, about 375 MPa or less, about 300 MPa or less, or about 250 MPa or less. In other aspects, the central maximum tensile stress may be within the range of from about 125 MPa to about 375 MPa, from about 125 MPa to about 300 MPa, from about 125 MPa to about 250 MPa, from about 150 MPa to about 375 MPa, from about 150 MPa to about 300 MPa, from about 150 MPa to about 250 MPa, from about 200 MPa to about 375 MPa, from about 200 MPa to about 300 MPa, from about 200 MPa to about 250 MPa, or any range or sub-range therebetween. Having the central maximum tensile stress within the range of from about 125 MPa to about 375 MPa may result in a smaller minimum parallel plate spacing.

[0370] In various aspects, the central portion 281 and / or the central region 248 of the foldable devices 101, 301, 401, 501, 701, 801, and / or 901 and / or the foldable substrate 201 do not buckle. A foldable device and / or a foldable substrate buckles when the surface profile of a first central surface region intercepted along the midline of the central portion that is equally spaced from the first portion and the second portion includes a non-parabolic shape. As used herein, the profilometer profile is measured using a SpecGAGE3D available from Irsa Vision and using default settings. The raw profilometer measurements correspond to an array of gradients within the measurement area. The measured gradients are integrated by software provided by SpecGAGE3D to generate a 3D surface. The zero point of the 3D surface is set such that the average height of the entire 3D surface is 0. A line profile corresponding to the midline of the central portion (i.e., midway between the first portion and the second portion) is extracted from this 3D surface and used as the surface profile (i.e., the profilometer profile). For example, Figure 23 a surface profile 2305 that shows buckling, has multiple peaks, and is non-parabolic in profile; in contrast, Figures 25 - 26The shown surface profile 2505 or 2605 is non-buckled and has a parabolic surface profile. However, the general shape of the surface profile may not be precise enough to determine whether the sample is buckled or non-buckled. As discussed below with reference to Figure 28 it has been found that the average value of the absolute value of the surface profile gradient (i.e., the "average gradient") can distinguish between buckled and non-buckled samples, with the average gradient of buckled samples being greater than that of non-buckled samples. As used herein, the average gradient is calculated by averaging all gradient measurements, where each gradient measurement is calculated between adjacent extreme values (e.g., a local maximum and an adjacent local minimum). In various aspects, the average gradient included in the surface profile obtained by intercepting the first central surface region 213 along the midline of the central region 248 can be about 0.018 mm / mm or less, about 0.017 mm / mm or less, about 0.016 mm / mm or less, about 0.015 mm / mm or less, about 0.013 mm / mm or less, about 0.12 mm / mm or less, about 0.011 mm / mm or less, or about 0.010 mm / mm or less. In various aspects, the average gradient included in the surface profile obtained by intercepting the first central surface region 213 along the midline of the central region 248 can be from about 0.001 mm / mm to about 0.018 mm / mm, from about 0.002 mm / mm to about 0.017 mm / mm, from about 0.003 mm / mm to about 0.016 mm / mm, from about 0.005 to about 0.015 mm / mm, from about 0.008 mm / mm to about 0.013 mm / mm, from about 0.010 mm / mm to about 0.012 mm / mm, or any range or sub-range therebetween. For non-buckled samples, the central portion and / or the central region may exhibit warping, which is tolerable in most applications. The warping degree is taken as the maximum height difference (longitudinal axis) of the surface profile along the midline width, excluding measurements within 1 mm of the edge of the surface profile. In various aspects, the surface profile obtained by intercepting the first central surface region 213 along the midline of the central region 248 can include a tolerable warping degree of about 1,000 μm or less, about 700 μm or less, about 600 μm or less, about 500 μm or less, about 400 μm or less, about 350 μm or less, about 320 μm or less, about 300 μm or less, about 280 μm or less, about 250 μm or less, or about 200 μm or less. In various aspects, the warping degree (μm / mm) of the surface profile of each segment of the midline can be about 10 μm / mm or less, about 9 μm / mm or less, about 8 μm / mm or less, about 7 μm / mm or less, about 6 μm / mm or less, or about 5 μm / mm or less.

[0371] Buckling is a mechanical instability. Without being bound by theory, buckling occurs when a portion of a foldable substrate experiences a strain greater than the critical buckling strain of that portion. The critical buckling strain increases with increasing thickness; thus, the central portion may be most prone to buckling. When the strain experienced by the central portion increases but remains less than the critical buckling strain, the central portion exhibits an increasingly large saddle-shaped warp. One source of strain in the central portion is the expansion strain due to chemical strengthening, which is caused by the expansion when larger ions in the foldable substrate replace existing smaller ions. Specifically, there may be a mismatch between the expansion strains due to chemical strengthening in the central portion, the first portion, and the second portion because the thickness (e.g., volume) of these portions (central portion, first portion, and second portion) is different, and the amount of chemical strengthening experienced by these portions may also be different.

[0372] The present disclosure shows that the mismatch between the expansion strains due to chemical strengthening in portions of a foldable substrate can be reduced by adding a small amount (e.g., about 0.02 wt% to about 0.08 wt% when manufacturing a foldable substrate 201 (with two grooves opposite each other in the foldable substrate) - see Figures 2 - 3 , or about 0.5 wt% to about 1.5 wt% when manufacturing a foldable substrate 201 (with a groove only on one side of the central portion) - see Figure 4 of a lithium salt to the final molten salt bath, which increases the surface concentration of lithium oxide in the foldable substrate. Sodium or potassium in the foldable substrate is exchanged with the smaller lithium in the molten salt bath ("reverse ion exchange") to counteract (e.g., reduce) the amount of expansion due to chemical strengthening caused by the "forward ion exchange" of smaller ions (e.g., sodium) in the foldable substrate with larger ions (e.g., potassium, cesium) in the final molten salt bath at the same time. As shown in the examples discussed below, adding a small amount (e.g., about 0.02 wt% to about 0.08 wt%) of a lithium salt to the final molten salt bath can unexpectedly reduce the incidence of buckling and / or warping of the foldable substrate (e.g., the central portion). However, a larger amount of lithium salt may result in a larger saddle-shaped warp. For example, the chemical strengthening-induced shrinkage resulting from the reverse ion exchange of lithium into the foldable substrate can cause different mismatches in the expansion strains due to chemical strengthening in portions of the foldable substrate.

[0373] Throughout the present disclosure, lithium oxide (Li 2 O), sodium oxide (Na 2 O), and potassium oxide (K 2The concentration distribution of O) was measured using glow discharge optical emission spectroscopy (GDOES). Although surface concentration can be measured using secondary ion mass spectrometry (SIMS), the surface concentration discussed herein will use the measured values of GDOES; however, the "surface concentration" or "concentration at the surface" in the GDOES measurements is taken as the concentration at a depth of 1 μm from the surface to avoid any false readings or surface contamination at the start of the GDOES measurement. As used herein, the concentration in mol% reported in the concentration distribution of GDOES refers to the amount of the specified compound at a specific depth from the surface relative to other compounds detected at the same specific depth from the surface. As discussed below, Figures 29 - 33 and FIGS. 46 - 51 illustrate the Li measured using GDOES 2 O, Na 2 O and K 2 O concentration distributions.

[0374] In various aspects, the Li at the first major surface 203 (e.g., the first surface region 223 in the first portion 221, the third surface region 233 in the second portion 231) 2 O concentration can be about 0.1 mol% or higher, about 0.2 mol% or higher, about 0.3 mol% or higher, about 0.4 mol% or higher, about 0.5 mol% or higher, about 0.75 mol% or higher, about 1 mol% or higher, about 2 mol% or lower, about 1.8 mol% or lower, about 1.5 mol% or lower, about 1.2 mol% or lower, or about 1 mol% or lower. In various aspects, the Li at the first major surface 203 (e.g., the first surface region 223 in the first portion 221, the third surface region 233 in the second portion 231) 2 O concentration can be in the range of about 0.1 mol% to about 2 mol%, about 0.2 mol% to about 2 mol%, about 0.2 mol% to about 2 mol%, about 0.3 mol% to about 1.8 mol%, about 0.4 mol% to about 1.5 mol%, about 0.5 mol% to about 1.2 mol%, about 0.75 mol% to about 1 mol% or any range or sub - range therebetween. In various aspects, the Li at the first major surface 203 2 O concentration can be about 0.2 mol% to about 2 mol%, about 0.5 mol% to about 1.8 mol%, about 0.75 mol% to about 1.5 mol% or any range or sub - range therebetween. In various aspects, the Li at the second major surface 205 (e.g., the second surface region 225 in the first portion 221, the fourth surface region 235 in the second portion 231) 2The O concentration can be within one or more of the ranges discussed hereinabove in this paragraph and / or be substantially equal to the Li 2 O concentration at the first major surface 203. In various aspects, the Li 2 O concentration at the first central surface region 213 and / or the second central surface region 243 can be within one or more of the ranges discussed hereinabove in this paragraph. In other aspects, the Li 2 O concentration at the first central surface region 213 and / or the second central surface region 243 can be substantially equal to the Li 2 O concentration at the first major surface 203 and / or the second major surface 205. Having a Li 2 O surface concentration (e.g., expressed as absolute mol% and / or the amount by which the surface concentration is elevated relative to the midpoint concentration) of from about 0.2 mol% to about 2 mol% can reduce (e.g., mitigate, counteract) swelling due to chemical strengthening in the foldable substrate and the resulting strain.

[0375] In various aspects, the K 2 O concentration at the first major surface 203 (e.g., the first surface region 223 in the first portion 221, the third surface region 233 in the second portion 231) can be about 5 mol% or higher, about 6 mol% or higher, about 7 mol% or higher, about 8 mol% or higher, about 9 mol% or higher, about 10 mol% or higher, about 15 mol% or lower, about 14 mol% or lower, about 13 mol% or lower, about 12 mol% or lower, about 11 mol% or lower, or about 10 mol% or lower. In various aspects, the K 2 O concentration at the first major surface 203 (e.g., the first surface region 223 in the first portion 221, the third surface region 233 in the second portion 231) can be in the range of from about 5 mol% to about 15 mol%, from about 6 mol% to about 14 mol%, from about 7 mol% to about 13 mol%, from about 8 mol% to about 12 mol%, from about 9 mol% to about 11 mol%, from about 9 mol% to about 10 mol%, or any range or sub-range therebetween. In various aspects, the K 2 O concentration at the second major surface 205 (e.g., the second surface region 225 in the first portion 221, the fourth surface region 235 in the second portion 231) can be within one or more of the ranges discussed hereinabove in this paragraph and / or be substantially equal to the K 2 O concentration at the first major surface 203. In various aspects, the K 2 O concentration at the first central surface region 213 and / or the second central surface region 243 can be within one or more of the ranges discussed hereinabove in this paragraph. In other aspects, the K2 The concentration of K₂O can be substantially equal to that at the first major surface 203 and / or the second major surface 205. 2 O concentration. Alternatively or additionally, the total concentration of potassium oxide (K₂O), rubidium oxide (Rb₂O), cesium oxide (Cs₂O), and francium oxide (Fr₂O) at the first major surface (e.g., the first surface region 223 in the first part 221, the third surface region 233 in the second part 231), the second major surface 205 (e.g., the second surface region 225 in the first part 221, the fourth surface region 235 in the second part 231), the first central surface region 213, and / or the second central surface region 243 can be within one or more of the ranges discussed above in this paragraph for the K₂O concentration. Providing a high concentration (e.g., about 5 mol% or higher) of K₂O (e.g., the absolute mol% and / or the increase in surface concentration relative to the midpoint concentration) can provide a relatively large (e.g., about 500 MPa) surface compressive stress, thereby improving fracture resistance. 2 O), rubidium oxide (Rb₂ 2 O), cesium oxide (Cs₂ 2 O), and francium oxide (Fr₂ 2 O) can be within one or more of the ranges discussed above in this paragraph for the K₂ 2 O concentration. Providing a high concentration (e.g., about 5 mol% or higher) of K₂ 2 O (e.g., the absolute mol% and / or the increase in surface concentration relative to the midpoint concentration) can provide a relatively large (e.g., about 500 MPa) surface compressive stress, thereby improving fracture resistance.

[0376] In various aspects, the concentration of K₂O at the first major surface 203 (e.g., the first surface region 223 in the first part 221, the third surface region 233 in the second part 231) can be higher than the concentration of Na₂O at the first major surface 203. In other aspects, the ratio of the concentration of K₂O to the concentration of Na₂O at the first major surface 203 can be about 1 or greater, about 2 or greater, about 3 or greater, about 4 or greater, about 5 or greater, about 20 or less, about 15 or less, about 12 or less, or about 10 or less. In other aspects, the ratio of the concentration of K₂O to the concentration of Na₂O at the first major surface 203 can be in the range of about 1 to about 20, about 2 to about 15, about 3 to about 12, about 4 to about 10, about 5 to about 10, or any range or sub-range therebetween. In various aspects, the concentration of Na₂O at the second major surface 205 (e.g., the second surface region 225 in the first part 221, the fourth surface region 235 in the second part 231) can be less than the concentration of K₂O at the second major surface 205 (e.g., the second surface region 225 in the first part 221, the fourth surface region 235 in the second part 231). In other aspects, the concentration of K₂O at the second major surface 205 2 O concentration can be higher than the Na₂ 2 O concentration at the first major surface 203. In other aspects, the K₂ 2 O concentration at the first major surface 203 and the Na₂ 2 O concentration ratio at the first major surface 203 can be about 1 or greater, about 2 or greater, about 3 or greater, about 4 or greater, about 5 or greater, about 20 or less, about 15 or less, about 12 or less, or about 10 or less. In other aspects, the K₂ 2 O concentration at the first major surface 203 and the Na₂ 2 O concentration ratio at the first major surface 203 can be in the range of about 1 to about 20, about 2 to about 15, about 3 to about 12, about 4 to about 10, about 5 to about 10, or any range or sub-range therebetween. In various aspects, the Na₂ 2 O concentration at the second major surface 205 (e.g., the second surface region 225 in the first part 221, the fourth surface region 235 in the second part 231) can be less than the K₂ 2 O concentration at the second major surface 205 (e.g., the second surface region 225 in the first part 221, the fourth surface region 235 in the second part 231). In other aspects, the K₂ 2The ratio of the O concentration to the Na concentration at the second major surface 205 can be within one or more of the ranges discussed above in this paragraph. In various aspects, the K concentration at the first central surface region 213 and / or the second central surface region 243 2 can be higher than the Na concentration at the corresponding surface. In other aspects, if the surface is the first central surface region 213 and / or the second central surface region 243, then the ratio of the Na concentration to the K concentration at the surface 2 can be within one or more of the ranges discussed above in this paragraph. Alternatively or additionally, the total concentration of potassium oxide (K 2 O), rubidium oxide (Rb 2 O), cesium oxide (Cs 2 O), and francium oxide (Fr 2 O) at the first major surface 203 can be higher than the Na concentration at the first major surface. Similarly, the ratio of the total concentration of K 2 O, Rb 2 O, Cs 2 O, and Fr 2 O at the first major surface to the Na concentration at the first major surface can be within one or more of the corresponding ranges discussed above in this paragraph. Having more potassium oxide than sodium oxide at the surface (or the ratio within one or more of the ranges discussed in this paragraph) can provide a relatively large (e.g., about 500 MPa) surface compressive stress, thereby improving fracture resistance because most of the smaller alkali metals in the substrate have been exchanged with potassium. 2 O, Rb 2 O, Cs 2 O, and Fr 2 O at the first major surface 203 can be higher than the Na concentration at the first major surface. Similarly, the ratio of the total concentration of K 2 O, Rb

[0377] In various aspects, the K concentration at the first major surface 203 2 can be higher than the Li concentration at the first major surface 203. In other aspects, the ratio of the K concentration to the Li concentration at the first major surface 203 2 can be about 5 or greater, about 7 or greater, about 8 or greater, about 9 or greater, about 10 or greater, about 12 or greater, about 20 or less, about 15 or less, about 12 or less, or about 10 or less. In other aspects, the ratio of the K concentration to the Li concentration at the first major surface 203 2 can be in the range of about 5 to about 20, about 7 to about 20, about 8 to about 20, about 9 to about 15, about 10 to about 12, or any range or subrange therebetween. Alternatively or additionally, the total concentration of potassium oxide (K 2 O), rubidium oxide (Rb 2 O), cesium oxide (Cs 2 O), and francium oxide (Fr 2 O) at the first major surface 2032 O), cesium oxide (Cs 2 O), and francium oxide (Fr 2 O) can be higher than the Li 2 O concentration at the first major surface. Similarly, the total concentration of K 2 O, Rb 2 O, Cs 2 O, and Fr 2 O at the first major surface and the Li 2 O concentration can be within one or more of the corresponding ranges discussed hereinabove in this paragraph. Having more potassium oxide than lithium oxide at the surface (or the ratio within one or more of the ranges discussed in this paragraph) can provide a relatively large (e.g., about 500 MPa) surface compressive stress, thereby enhancing fracture resistance, because most of the smaller alkali metals in the substrate have been exchanged with potassium.

[0378] As used herein, the total amount of alkali metals is determined by integrating the concentration profile obtained from GDOES (as described herein) over half the thickness. In various aspects, the ratio of the total amount of K 2 O to the total amount of Li 2 O can be about 100 or greater, about 120 or greater, about 150 or greater, about 170 or greater, about 180 or greater, about 300 or less, about 250 or less, about 200 or less, about 190 or less, or 180 or less. In various aspects, the ratio of the total amount of K 2 O to the total amount of Li 2 O can be in the range of about 100 to about 300, about 120 to about 250, about 150 to about 200, about 170 to about 200, about 180 to about 190, or any range or sub-range therebetween. Alternatively or additionally, the ratio of the total amount of potassium oxide (K 2 O), rubidium oxide (Rb 2 O), cesium oxide (Cs 2 O), and francium oxide (Fr 2 O) to the total amount of Li 2 O can be within one or more of the corresponding ranges discussed hereinabove in this paragraph.

[0379] As used herein, the midpoint of the foldable substrate 201 (e.g., the first portion 221, the second portion 231) is defined as the position midway between the first major surface 203 and the second major surface 205. For example, if the substrate thickness 207 is 100 μm, then the midpoint (e.g., in the first portion) is 50 μm from the first major surface 203 and 50 μm from the second major surface 205. Similarly, as used herein, the midpoint of the central portion 281 (e.g., the central region 248) is defined as the position midway between the first central surface region 213 and the second central surface region 243. For example, if the central thickness 209 is 30 μm, then the central midpoint (e.g., in the central region) is 15 μm from the first central surface region 213 and 15 μm from the second central surface region 243. In various aspects, when the first distance 219 is equal to the second distance 249, the midpoint of the foldable substrate 201 in the first portion 221 and in the second portion 231 can be coplanar with the central midpoint.

[0380] In various aspects, the Li 2 O concentration at the first major surface 203 (e.g., the first surface region 223 in the first portion 221, the third surface region 233 in the second portion 231) can be about 0.1 mol% or more, about 0.2 mol% or more, about 0.3 mol% or more, about 0.4 mol% or more, about 0.5 mol% or more, about 0.75 mol% or more, about 1 mol% or more, about 2 mol% or less, about 1.8 mol% or less, about 1.5 mol% or less, about 1.2 mol% or less, or about 1 mol% or less higher than the Li 2 O concentration at the midpoint (e.g., in the first portion, in the second portion). In various aspects, the Li 2 O concentration at the first major surface 203 (e.g., the first surface region 223 in the first portion 221, the third surface region 233 in the second portion 231) can be about 0.1 mol% to about 2 mol%, about 0.2 mol% to about 2 mol%, about 0.2 mol% to about 2 mol%, about 0.3 mol% to about 1.8 mol%, about 0.4 mol% to about 1.5 mol%, about 0.5 mol% to about 1.2 mol%, about 0.75 mol% to about 1 mol% or any range or sub-range therebetween higher than the Li 2 O concentration at the midpoint (e.g., in the first portion, in the second portion). In various aspects, the Li 2 O concentration at the first major surface 203 can be higher than the Li 2The O concentration is about 0.2 mol% to about 2 mol%, about 0.5 mol% to about 1.8 mol%, about 0.75 mol% to about 1.5 mol%, or any range or sub-range therebetween. In various aspects, the Li 2 O concentration at the second major surface 205 (e.g., the second surface region 225 in the first portion 221, the fourth surface region 235 in the second portion 231) is higher than the Li 2 O concentration at the midpoint (e.g., in the first portion, in the second portion) by an amount that can be within one or more of the ranges discussed hereinabove in this paragraph, and / or is substantially equal to the amount by which the Li 2 O concentration at the first major surface 203 is higher than the concentration at the midpoint. In various aspects, the Li 2 O concentration at the first central surface region 213 and / or the second central surface region 243 can be higher than the Li 2 O concentration at the central midpoint by an amount within one or more of the ranges discussed hereinabove in this paragraph. In various aspects, the Li 2 O concentration at the first central surface region 213 and / or the second central surface region 243 is higher than the Li 2 O concentration at the central midpoint by an amount that can be substantially equal to the amount by which the Li 2 O concentration at the first major surface 203 is higher than the Li 2 O concentration at the midpoint. Having a Li 2 O surface concentration (e.g., expressed as absolute mol% and / or the amount by which the surface concentration is elevated relative to the midpoint concentration) of about 0.2 mol% to about 2 mol% can reduce (e.g., mitigate, counteract) swelling due to chemical strengthening in the foldable substrate and the resulting strain.

[0381] As used herein, a concentration profile being "elevated" relative to a reference concentration means that the concentration is higher than the reference concentration by at least one of the following values: (1) 10% of the difference between the surface concentration and the reference value, or (2) 0.1 mol%. In various aspects, the Li 2 O concentration profile in the first portion 221 relative to the Li 2 O concentration at the midpoint, expressed as a percentage of the substrate thickness 207, can be about 5% or greater, about 7% or greater, about 10% or greater, about 12% or greater, about 15% or greater, about 30% or less, about 25% or less, about 23% or less, about 20% or less, about 18% or less, or about 15% or less. In various aspects, the Li 2 O concentration profile in the first portion 221 relative to the Li 2The distance of the increase in Li₂O concentration, expressed as a percentage of the substrate thickness 207, can be in the range of about 5% to about 30%, about 7% to about 25%, about 10% to about 22%, about 12% to about 20%, about 15% to about 17%, or any range or sub-range therebetween. In various aspects, the Li₂O concentration distribution in the first portion 221 relative to the Li₂O concentration at the midpoint 2 The distance of the increase in Li₂O concentration 2 can be about 3 μm or greater, about 4 μm or greater, about 5 μm or greater, about 6 μm or greater, about 15 μm or less, about 12 μm or less, about 10 μm or less, or about 8 μm or less. In various aspects, the Li₂O concentration distribution in the first portion 221 relative to the Li₂O concentration at the midpoint 2 The distance of the increase in Li₂O concentration 2 can be in the range of about 3 μm to about 15 μm, about 4 μm to about 12 μm, about 5 μm to about 10 μm, about 6 μm to about 9 μm, or any range or sub-range therebetween. In various aspects, the Li₂O concentration distribution in the central region 248 relative to the Li₂O concentration at the central midpoint 2 The distance of the increase in Li₂O concentration 2 expressed as a percentage of the central thickness 209 or an absolute distance can be in one or more of the ranges discussed above for the distance of the increase in Li₂O concentration expressed as a percentage or absolute distance of the substrate thickness 2 The Li₂O concentration distribution can be in one or more of the ranges discussed for the distance of the increase in concentration relative to the midpoint concentration.

[0382] In various aspects, the K₂O concentration at the first major surface 203 (e.g., the first surface region 223 in the first portion 221, the third surface region 233 in the second portion 231) can be about 5 mol% or more, about 6 mol% or more, about 7 mol% or more, about 8 mol% or more, about 9 mol% or more, about 10 mol% or more, about 15 mol% or less, about 14 mol% or less, about 13 mol% or less, about 12 mol% or less, about 11 mol% or less, or about 10 mol% or less higher than the K₂O concentration at the midpoint (e.g., in the first portion, in the second portion). In various aspects, the K₂O concentration at the first major surface 203 (e.g., the first surface region 223 in the first portion 221, the third surface region 233 in the second portion 231) 2 is higher than the K₂O concentration at the midpoint (e.g., in the first portion, in the second portion) 2 by about 5 mol% or more, about 6 mol% or more, about 7 mol% or more, about 8 mol% or more, about 9 mol% or more, about 10 mol% or more, about 15 mol% or less, about 14 mol% or less, about 13 mol% or less, about 12 mol% or less, about 11 mol% or less, or about 10 mol% or less. In various aspects, the K₂O concentration at the first major surface 203 (e.g., the first surface region 223 in the first portion 221, the third surface region 233 in the second portion 231) 2 is higher than the K₂O concentration at the midpoint (e.g., in the first portion, in the second portion) 2The O concentration is about 5 mol% to about 15 mol%, about 6 mol% to about 14 mol%, about 7 mol% to about 13 mol%, about 8 mol% to about 12 mol%, about 9 mol% to about 11 mol%, about 9 mol% to about 10 mol%, or any range or sub-range therebetween. In various aspects, the K at the second major surface 205 (e.g., the second surface region 225 in the first portion 221, the fourth surface region 235 in the second portion 231) 2 The O concentration is higher than the K at the midpoint (e.g., in the first portion, in the second portion) 2 The amount by which the O concentration is higher can be within one or more of the ranges discussed hereinabove in this paragraph and / or is substantially equal to the K at the first major surface 203 2 The amount by which the O concentration is higher than the concentration at the midpoint. In various aspects, the K at the first central surface region 213 and / or the second central surface region 243 2 The O concentration can be higher than the K at the central midpoint by an amount within one or more of the ranges discussed hereinabove in this paragraph. In various aspects, the K at the first central surface region 213 and / or the second central surface region 243 2 The O concentration is higher than the K at the central midpoint by an amount within one or more of the ranges discussed hereinabove in this paragraph. In various aspects, the K at the first central surface region 213 and / or the second central surface region 243 2 The O concentration is higher than the K at the central midpoint 2 The amount by which the O concentration is higher can be substantially equal to the K at the first major surface 203 2 The amount by which the O concentration is higher than the K at the midpoint 2 Providing a high concentration (e.g., about 5 mol% or higher) of K 2 O (e.g., absolute mol% and / or the increase in surface concentration relative to the midpoint concentration) can provide a relatively large (e.g., about 500 MPa) surface compressive stress, thereby improving fracture resistance. Alternatively or additionally, the total concentration of potassium oxide (K 2 O), rubidium oxide (Rb 2 O), cesium oxide (Cs 2 O), and francium oxide (Fr 2 O) at the first major surface (e.g., the first surface region 223 in the first portion 221, the third surface region 233 in the second portion 231) can be higher than the total concentration of K 2 O, Rb 2 O, Cs 2 O, and Fr 2 O at the midpoint by an amount within one or more of the ranges discussed hereinabove in this paragraph. Alternatively or additionally, the total concentration of potassium oxide (K 2 O), rubidium oxide (Rb 2 O), cesium oxide (Cs 2 O), and francium oxide (Fr 2 O) at the first central surface region 213 can be higher than the K at the central midpoint2 O, Rb 2 O, Cs 2 O and Fr 2 The total concentration of O is high by a certain amount, and the amount is within one or more of the ranges discussed in the preceding paragraphs herein.

[0383] In each aspect, the K in the first part 221 2 O concentration distribution relative to the K at the midpoint 2 The distance over which the O concentration increases, expressed as a percentage of the substrate thickness 207, can be about 10% or greater, about 12% or greater, about 14% or greater, about 15% or greater, about 25% or less, about 23% or less, about 20% or less, about 18% or less, or about 15% or less. In each aspect, the K in the first part 221 2 O concentration distribution relative to the K at the midpoint 2 The distance over which the O concentration increases, expressed as a percentage of the substrate thickness 207, can be in the range of about 10% to about 25%, about 12% to about 23%, about 14% to about 20%, about 15% to about 18%, or any range or sub - range therebetween. In each aspect, the K in the first part 221 2 O concentration distribution relative to the K at the midpoint 2 The distance over which the O concentration increases can be about 10 μm or greater, about 12 μm or greater, about 15 μm or greater, about 30 μm or less, about 25 μm or less, about 20 μm or less, or about 18 μm or less. In each aspect, the K in the first part 221 2 O concentration distribution relative to the K at the midpoint 2 The distance over which the O concentration increases can be in the range of about 10 μm to about 30 μm, about 12 μm to about 25 μm, about 15 μm to about 20 μm, or any range or sub - range therebetween. In each aspect, the K in the central region 248 2 O concentration distribution relative to the K at the central midpoint 2 The distance over which the O concentration increases, expressed as a percentage of the central thickness 209, can be within one or more of the ranges discussed above for the K in the first part expressed as a percentage of the substrate thickness 2 The O concentration distribution can be within one or more of the ranges discussed for the distance over which the concentration increases relative to the midpoint concentration. In each aspect, the K in the central region 248 2 O concentration distribution relative to the K at the central midpoint 2 The distance over which the O concentration increases can be about 5 μm or greater, about 7 μm or greater, about 10 μm or greater, about 12 μm or greater, about 15 μm or greater, about 20 μm or less, about 18 μm or less, or about 15 μm or less. In each aspect, the K in the central region 248 2 O concentration distribution relative to the K at the central midpoint2 The distance of the increase in O concentration can be in the range of about 5 μm to about 20 μm, about 7 μm to about 18 μm, about 10 μm to about 15 μm, or any range or sub - range therebetween. Alternatively or additionally, at the first major surface (e.g., the first surface region 223 in the first part 221, the third surface region 233 in the second part 231), the total concentration of potassium oxide (K 2 O), rubidium oxide (Rb 2 O), cesium oxide (Cs 2 O), and francium oxide (Fr 2 O) can increase by a distance relative to the total concentration of K 2 O, Rb 2 O, Cs 2 O, and Fr 2 O at the mid - point, and the distance is within one or more of the ranges discussed above in this paragraph with respect to the absolute distance or percentage of the substrate thickness 207. Similarly, at the first central surface region 213, the total concentration of K 2 O, Rb 2 O, Cs 2 O, and Fr 2 O can increase by a distance relative to the total concentration of K 2 O, Rb 2 O, Cs 2 O, and Fr 2 O at the central mid - point, and the distance is within one or more of the ranges discussed above in this paragraph with respect to the absolute distance or percentage of the central thickness 209.

[0384] In various aspects, in the foldable substrate 201 (e.g., the foldable device 101 or 301) having the first groove 211 and the second groove 241 opposite the first groove 211, the surface concentration of (1) Li 2 O, (2) Na 2 O, (3) K 2 O, and / or the total concentration of (4) K 2 O, Rb 2 O, Cs 2 O, and Fr 2 O at the first major surface 203 and / or the first central surface region 213 can be within one or more of the corresponding ranges discussed in the previous set of paragraphs. In various aspects, in the foldable substrate 201 (e.g., the foldable device 101 or 301) having the first groove 211 and the second groove 241 opposite the first groove 211, for (1) Li 2 O, (2) Na 2 O, (3) K 2 O, and / or (4) K 2 O, Rb 2 O, Cs2 O and Fr 2 Regarding the total concentration of O, the difference between the concentration at the first major surface 203 or the first central surface region 213 and the corresponding concentration at the midpoint or central midpoint can be within one or more of the corresponding ranges discussed in the previous set of paragraphs. In various aspects, in the foldable substrate 201 (e.g., foldable device 101 or 301) having the first groove 211 and the second groove 241 opposite the first groove 211, in the first portion 221 or the central portion 281 (1) Li 2 O, (2) Na 2 O, (3) K 2 The concentration distribution of O and / or (4) K 2 O, Rb 2 O, Cs 2 O and Fr 2 The total concentration of O can be elevated by a depth relative to the corresponding concentration at the midpoint or central midpoint, and the depth can be within one or more of the corresponding ranges discussed in the previous set of paragraphs. Alternatively, in the foldable substrate 201 (e.g., foldable device 401) having the first groove 211 but not having a second groove opposite the first groove (e.g., the second major surface 205 includes the second central surface region 243), at the first major surface 203 and / or the first central surface region 213 (1) Li 2 O, (2) Na 2 O, (3) K 2 The surface concentration of O and / or (4) K 2 O, Rb 2 O, Cs 2 O and Fr 2 The total concentration of O can be within one or more of the corresponding ranges discussed in the following set of paragraphs. In various aspects, in the foldable substrate 201 (e.g., foldable device 401) having the first groove 211 but not having a second groove opposite the first groove (e.g., the second major surface 205 includes the second central surface region 243), regarding (1) Li 2 O, (2) Na 2 O, (3) K 2 O and / or (4) K 2 O, Rb 2 O, Cs 2 O and Fr 2For the total concentration of O, the difference between the concentration at the first major surface 203 or the first central surface region 213 and the corresponding concentration at the midpoint or central midpoint can be within one or more of the corresponding ranges discussed in the following set of paragraphs. In various aspects, in a foldable substrate 201 (e.g., a foldable device 401) having a first groove 211 but not having a second groove opposite the first groove (e.g., the second major surface 205 includes a second central surface region 243), in the first portion 221 or the central portion 281, (1) Li 2 O, (2) Na 2 O, (3) K 2 O concentration distribution and / or (4) K 2 O, Rb 2 O, Cs 2 O and Fr 2 O total concentration can increase by a depth relative to the corresponding concentration at the midpoint or central midpoint, and the depth can be within one or more of the corresponding ranges discussed in the previous set of paragraphs. Due to the different geometries, the surface concentration or concentration difference of a foldable substrate with two opposing grooves is different from that of a foldable substrate having at least one groove on only one side of the foldable substrate. Additionally, as discussed below, the method of manufacturing Figure 4 the foldable device 401 shown can employ a single chemical strengthening process, which may result in different concentrations between the first portion and / or the second portion and the central portion between the surface and the corresponding midpoint due to the different thicknesses of the corresponding portions, but employing multiple chemical strengthening processes can also make the first portion, the second portion, and / or the central portion reach similar concentrations.

[0385] In various aspects, the Li 2 O concentration of the first major surface 203 (e.g., the first surface region 223 in the first portion 221, the third surface region 233 in the second portion 231) can be about 0.8 mol% or higher, about 1.0 mol% or higher, about 1.25 mol% or higher, about 1.5 mol% or higher, about 1.75 mol% or higher, about 2 mol% or higher, about 2.1 mol% or higher, about 2.2 mol% or higher, about 3.5 mol% or lower, about 3.0 mol% or lower, about 2.75 mol% or lower, about 2.5 mol% or lower, about 2.4 mol% or lower, or about 2.3 mol% or lower. In various aspects, the Li 2The O concentration can be in the range of from about 0.8 mol% to about 3.5 mol%, from about 1 mol% to about 3 mol%, from about 1.25 mol% to about 2.75 mol%, from about 1.5 mol% to about 2.5 mol%, from about 1.75 mol% to about 2.4 mol%, from about 2 mol% to about 2.3 mol%, from about 2.2 mol% to about 2.3 mol%, or any range or sub-range therebetween. In various aspects, the Li at the second major surface 205 (e.g., the second surface region 225 in the first portion 221, the fourth surface region 235 in the second portion 231) 2 O concentration can be within one or more of the ranges discussed hereinabove in this paragraph and / or be substantially equal to the Li at the first major surface 203 2 O concentration. In various aspects, the Li at the first central surface region 213 and / or the second central surface region 243 2 O concentration can be within one or more of the ranges discussed hereinabove in this paragraph. In other aspects, the Li at the first central surface region 213 and / or the second central surface region 243 2 O concentration can be substantially equal to the Li at the first major surface 203 and / or the second major surface 205 2 O concentration. Having a surface concentration of Li 2 O (e.g., expressed as absolute mol% and / or the amount by which the surface concentration is elevated relative to the midpoint concentration) of from about 1 mol% to about 3 mol% can reduce (e.g., mitigate, counteract) swelling due to chemical strengthening in the foldable substrate and the resulting strain.

[0386] In various aspects, the K at the first major surface 203 (e.g., the first surface region 223 in the first portion 221, the third surface region 233 in the second portion 231) 2 O concentration can be about 5 mol% or higher, about 6 mol% or higher, about 7 mol% or higher, about 8 mol% or higher, about 9 mol% or higher, about 10 mol% or higher, about 15 mol% or lower, about 14 mol% or lower, about 13 mol% or lower, about 12 mol% or lower, about 11 mol% or lower, or about 10 mol% or lower. In various aspects, the K at the first major surface 203 (e.g., the first surface region 223 in the first portion 221, the third surface region 233 in the second portion 231) 2The concentration of K₂O can be in the range of about 5 mol% to about 15 mol%, about 6 mol% to about 14 mol%, about 7 mol% to about 13 mol%, about 8 mol% to about 12 mol%, about 8 mol% to about 11 mol%, about 8 mol% to about 10 mol%, about 9 mol% to about 10 mol%, or any range or sub - range therebetween. In various aspects, the K₂O concentration at the second major surface 205 (e.g., the second surface region 225 in the first portion 221, the fourth surface region 235 in the second portion 231) 2 The concentration of K₂O can be within one or more of the ranges discussed herein in this paragraph and / or be substantially equal to the K₂O concentration at the first major surface 203. 2 In various aspects, the K₂O concentration at the first central surface region 213 and / or the second central surface region 243 2 The concentration of K₂O can be within one or more of the ranges discussed herein in this paragraph. In other aspects, the K₂O concentration at the first central surface region 213 and / or the second central surface region 243 2 The concentration of K₂O can be substantially equal to the K₂O concentration at the first major surface 203 and / or the second major surface 205. 2 Alternatively or additionally, the total concentration of potassium oxide (K₂O), rubidium oxide (Rb₂O), cesium oxide (Cs₂O), and francium oxide (Fr₂O) at the first major surface (e.g., the first surface region 223 in the first portion 221, the third surface region 233 in the second portion 231), the second major surface 205 (e.g., the second surface region 225 in the first portion 221, the fourth surface region 235 in the second portion 231), the first central surface region 213, and / or the second central surface region 243 2 can be within one or more of the ranges discussed herein for the K₂O concentration in this paragraph. Providing a high concentration (e.g., about 5 mol% or higher) of K₂O (e.g., absolute mol% and / or the increase in surface concentration relative to the mid - point concentration) 2 can provide a relatively large (e.g., about 500 MPa) surface compressive stress, thereby improving fracture resistance. 2 2 2 2

[0387] In various aspects, the K₂O concentration at the first major surface 203 (e.g., the first surface region 223 in the first portion 221, the third surface region 233 in the second portion 231) 2 can be higher than the Na₂O concentration at the first major surface 203. In other aspects, the K₂O concentration at the first major surface 203 2 2 and the Na₂O concentration at the first major surface 203 2 ​​​​​The ratio of the K₂O concentration can be about 1 or greater, about 1.2 or greater, about 1.4 or greater, about 1.5 or greater, about 1.7 or greater, about 2 or greater, about 3 or greater, about 4 or greater, about 5 or greater, about 20 or less, about 15 or less, about 12 or less, about 10 or less, about 5 or less, about 3 or less, about 2.5 or less, about 2.2 or less, or about 2 or less. In other aspects, the K₂O concentration at the first major surface 203 2 and the Na₂O concentration at the first major surface 203 2 The ratio can be in the range of about 1 to about 20, about 1 to about 15, about 1.2 to about 12, about 1.2 to about 10, about 1.5 to about 10, about 1.5 to about 5, about 1.5 to about 3, about 1.5 to about 2.5, about 1.7 to about 2.2, about 2 to about 2.2 or any range or sub-range therebetween. Or any range or sub-range therebetween. In various aspects, the Na₂O concentration at the second major surface 205 (e.g., the second surface region 225 in the first portion 221, the fourth surface region 235 in the second portion 231) can be less than the K₂O concentration at the second major surface 205 (e.g., the second surface region 225 in the first portion 221, the fourth surface region 235 in the second portion 231) 2 In other aspects, the ratio of the K₂O concentration to the Na₂O concentration at the second major surface 205 can be within one or more of the ranges discussed in this paragraph above. In various aspects, the K₂O concentration at the first central surface region 213 and / or the second central surface region 243 can be higher than the Na₂O concentration at the corresponding surface 2 In other aspects, if the surface is the first central surface region 213 and / or the second central surface region 243, then the ratio of the Na₂O concentration to the K₂O concentration at the surface can be within one or more of the ranges discussed in this paragraph above. Having more potassium oxide than sodium oxide at the surface (or the ratio within one or more of the ranges discussed in this paragraph) can provide a relatively large (e.g., about 500 MPa) surface compressive stress, thereby improving fracture resistance, because most of the smaller alkali metals in the substrate have been exchanged with potassium. Alternatively or additionally, the total concentration of potassium oxide (K₂O), rubidium oxide (Rb₂O), cesium oxide (Cs₂O), and francium oxide (Fr₂O) at the first major surface 203 can be higher than the Na₂O concentration at the first major surface 2 and the Na₂O concentration at the second major surface 205 2 The ratio can be within one or more of the ranges discussed in this paragraph above. In various aspects, the K₂O concentration at the first central surface region 213 and / or the second central surface region 243 can be higher than the Na₂O concentration at the corresponding surface 2 In other aspects, if the surface is the first central surface region 213 and / or the second central surface region 243, then the ratio of the Na₂O concentration to the K₂O concentration at the surface can be within one or more of the ranges discussed in this paragraph above. Having more potassium oxide than sodium oxide at the surface (or the ratio within one or more of the ranges discussed in this paragraph) can provide a relatively large (e.g., about 500 MPa) surface compressive stress, thereby improving fracture resistance, because most of the smaller alkali metals in the substrate have been exchanged with potassium. Alternatively or additionally, the total concentration of potassium oxide (K₂O), rubidium oxide (Rb₂O), cesium oxide (Cs₂O), and francium oxide (Fr₂O) at the first major surface 203 can be higher than the Na₂O concentration at the first major surface 2 In other aspects, if the surface is the first central surface region 213 and / or the second central surface region 243, then the Na₂O concentration at the surface 2 and the K₂O concentration at the surface 2 The ratio can be within one or more of the ranges discussed in this paragraph above. Having more potassium oxide than sodium oxide at the surface (or the ratio within one or more of the ranges discussed in this paragraph) can provide a relatively large (e.g., about 500 MPa) surface compressive stress, thereby improving fracture resistance, because most of the smaller alkali metals in the substrate have been exchanged with potassium. Alternatively or additionally, the total concentration of potassium oxide (K₂O), rubidium oxide (Rb₂O), cesium oxide (Cs₂O), and francium oxide (Fr₂O) at the first major surface 203 can be higher than the Na₂O concentration at the first major surface. Similarly, the total concentration of K₂O, Rb₂O 2 at the first major surface, rubidium oxide (Rb₂O) 2 cesium oxide (Cs₂O) 2 and francium oxide (Fr₂O) 2 can be higher than the Na₂O concentration at the first major surface 2 Similarly, the total concentration of K₂O at the first major surface 2 Rb₂O, Cs₂O, and Fr₂O2 O, Cs 2 O and Fr 2 The total concentration of O and the Na at the first major surface 2 The ratio of the O concentration can be within one or more of the corresponding ranges discussed hereinabove in this paragraph.

[0388] In various aspects, the K 2 O concentration at the first major surface 203 can be higher than the Li 2 O concentration at the first major surface 203. In other aspects, the K 2 O concentration and the Li 2 O concentration at the first major surface 203 can be about 5 or greater, about 7 or greater, about 8 or greater, about 9 or greater, about 10 or greater, about 12 or greater, about 20 or less, about 15 or less, about 12 or less, or about 10 or less. In other aspects, the K 2 O concentration and the Li 2 O concentration at the first major surface 203 can be in the range of about 5 to about 20, about 7 to about 20, about 8 to about 20, about 9 to about 15, about 10 to about 12, or any range or sub-range therebetween. Alternatively or additionally, the total concentration of potassium oxide (K 2 O), rubidium oxide (Rb 2 O), cesium oxide (Cs 2 O), and francium oxide (Fr 2 O) at the first major surface can be higher than the Li 2 O concentration at the first major surface. Similarly, the total concentration of K 2 O, Rb 2 O, Cs 2 O, and Fr 2 O and the Li 2 O concentration at the first major surface can be within one or more of the corresponding ranges discussed hereinabove in this paragraph. Having more potassium oxide than lithium oxide at the surface (or the ratio being within one or more of the ranges discussed in this paragraph) can provide a relatively large (e.g., about 500 MPa) surface compressive stress, thereby improving fracture resistance, since most of the smaller alkali metals in the substrate have been exchanged with potassium.

[0389] As used herein, the total amount of alkali metals is determined by integrating the concentration profile obtained from GDOES (as described herein) over half of the thickness. In various aspects, the total amount of K 2 O and the Li 2The ratio of the total amount of O can be about 100 or greater, about 120 or greater, about 150 or greater, about 170 or greater, about 180 or greater, about 300 or less, about 250 or less, about 200 or less, about 190 or less, or 180 or less. In various aspects, K 2 The total amount of O and Li 2 The ratio of the total amount of O can range from about 100 to about 300, about 120 to about 250, about 150 to about 200, about 170 to about 200, about 180 to about 190, or any range or sub-range therebetween. Alternatively or additionally, the total amount of potassium oxide (K 2 O), rubidium oxide (Rb 2 O), cesium oxide (Cs 2 O), and francium oxide (Fr 2 O) and the total amount of Li 2 The ratio of the total amount of O can be within one or more of the corresponding ranges discussed herein in this paragraph.

[0390] In various aspects, the concentration of Li 2 O at the first major surface 203 (e.g., the first surface region 223 in the first part 221, the third surface region 233 in the second part 231) can be higher than the concentration of Li 2 O at the midpoint (e.g., in the first part, in the second part) by about 0.8 mol% or more, about 1.0 mol% or more, about 1.25 mol% or more, about 1.5 mol% or more, about 1.75 mol% or more, about 2 mol% or more, about 2.1 mol% or more, about 2.2 mol% or more, about 3.5 mol% or less, about 3.0 mol% or less, about 2.75 mol% or less, about 2.5 mol% or less, about 2.4 mol% or less, or about 2.3 mol% or less. In various aspects, the concentration of Li 2 O at the first major surface 203 (e.g., the first surface region 223 in the first part 221, the third surface region 233 in the second part 231) can be higher than the concentration of Li 2 O at the midpoint (e.g., in the first part, in the second part) by about 0.8 mol% to about 3.5 mol%, about 1 mol% to about 3 mol%, about 1.25 mol% to about 2.75 mol%, about 1.5 mol% to about 2.5 mol%, about 1.75 mol% to about 2.4 mol%, about 2 mol% to about 2.3 mol%, about 2.2 mol% to about 2.3 mol%, or any range or sub-range therebetween. In various aspects, the concentration of Li 2The amount by which the Li₂O concentration is higher at the midpoint (e.g., in the first portion, in the second portion) can be within one or more of the ranges discussed hereinabove in this paragraph, and / or can be substantially equal to the amount by which the Li₂O concentration at the first major surface 203 is higher than the Li₂O concentration at the midpoint. In various aspects, the Li₂O concentration at the first central surface region 213 and / or the second central surface region 243 can be higher than the Li₂O concentration at the central midpoint by an amount within one or more of the ranges discussed hereinabove in this paragraph. In various aspects, the amount by which the Li₂O concentration at the first central surface region 213 and / or the second central surface region 243 is higher than the Li₂O concentration at the central midpoint can be substantially equal to the amount by which the Li₂O concentration at the first major surface 203 is higher than the Li₂O concentration at the midpoint. Alternatively, the amount by which the Li₂O concentration at the first central surface region 213 and / or the second central surface region 243 is higher than the Li₂O concentration at the central midpoint can be less than the amount by which the Li₂O concentration at the first major surface 203 is higher than the Li₂O concentration at the midpoint, such as in the case where chemical strengthening increases the Li₂O concentration at the central midpoint by an amount that exceeds the corresponding concentration at the midpoint. Having a surface concentration of Li₂O (e.g., expressed as absolute mol% and / or the amount by which the surface concentration is elevated relative to the midpoint concentration) of from about 1 mol% to about 3 mol% can reduce (e.g., mitigate, counteract) swelling and the resulting strain in the chemically strengthened foldable substrate. 2 The amount by which the Li₂O concentration is higher at the midpoint (e.g., in the first portion, in the second portion) can be within one or more of the ranges discussed hereinabove in this paragraph, and / or can be substantially equal to the amount by which the Li₂O concentration at the first major surface 203 is higher than the Li₂O concentration at the midpoint. 2 In various aspects, the Li₂O concentration at the first central surface region 213 and / or the second central surface region 243 can be higher than the Li₂O concentration at the central midpoint by an amount within one or more of the ranges discussed hereinabove in this paragraph. 2 The Li₂O concentration 2 In various aspects, the Li₂O concentration at the first central surface region 213 and / or the second central surface region 243 can be higher than the Li₂O concentration at the central midpoint by an amount within one or more of the ranges discussed hereinabove in this paragraph. 2 The Li₂O concentration 2 The amount by which the Li₂O concentration at the first central surface region 213 and / or the second central surface region 243 is higher than the Li₂O concentration at the central midpoint can be substantially equal to the amount by which the Li₂O concentration at the first major surface 203 is higher than the Li₂O concentration at the midpoint. 2 The amount by which the Li₂O concentration is higher at the midpoint (e.g., in the first portion, in the second portion) 2 The amount by which the Li₂O concentration at the first central surface region 213 and / or the second central surface region 243 is higher than the Li₂O concentration at the central midpoint can be substantially equal to the amount by which the Li₂O concentration at the first major surface 203 is higher than the Li₂O concentration at the midpoint. 2 The Li₂O concentration 2 The amount by which the Li₂O concentration at the first central surface region 213 and / or the second central surface region 243 is higher than the Li₂O concentration at the central midpoint can be less than the amount by which the Li₂O concentration at the first major surface 203 is higher than the Li₂O concentration at the midpoint. 2 The amount by which the Li₂O concentration is higher at the midpoint (e.g., in the first portion, in the second portion) 2 The amount by which the Li₂O concentration at the first central surface region 213 and / or the second central surface region 243 is higher than the Li₂O concentration at the central midpoint can be less than the amount by which the Li₂O concentration at the first major surface 203 is higher than the Li₂O concentration at the midpoint, such as in the case where chemical strengthening increases the Li₂O concentration at the central midpoint by an amount that exceeds the corresponding concentration at the midpoint. 2 Having a surface concentration of Li₂O (e.g., expressed as absolute mol% and / or the amount by which the surface concentration is elevated relative to the midpoint concentration) of from about 1 mol% to about 3 mol% can reduce (e.g., mitigate, counteract) swelling and the resulting strain in the chemically strengthened foldable substrate. 2 Having a surface concentration of Li₂O (e.g., expressed as absolute mol% and / or the amount by which the surface concentration is elevated relative to the midpoint concentration) of from about 1 mol% to about 3 mol% can reduce (e.g., mitigate, counteract) swelling and the resulting strain in the chemically strengthened foldable substrate.

[0391] In various aspects, the Li₂O concentration profile in the first portion 221 2 relative to the Li₂O concentration at the midpoint 2 The distance by which the Li₂O concentration profile in the first portion 221 is elevated relative to the Li₂O concentration at the midpoint, expressed as a percentage of the substrate thickness 207, can be about 5% or greater, about 7% or greater, about 10% or greater, about 12% or greater, about 15% or greater, about 20% or greater, about 25% or greater, about 30% or greater, about 35% or less, about 34% or less, about 33% or less, about 32% or less, about 31% or less, about 30% or less, about 25% or less, about 23% or less, about 20% or less, about 18% or less, or about 15% or less. In various aspects, the Li₂O concentration profile in the first portion 221 2 relative to the Li₂O concentration at the midpoint 2The distance of the increase in Li2O concentration, expressed as a percentage of the substrate thickness 207, can be in the range of about 5% to about 35%, about 7% to about 35%, about 10% to about 34%, about 12% to about 34%, about 15% to about 33%, about 20% to about 33%, about 25% to about 32%, about 30% to about 32% or any range or sub-range therebetween. In various aspects, the Li2O concentration distribution in the first portion 221 relative to the Li2O concentration at the midpoint 2 The distance of the increase in Li2O concentration 2 can be about 3 μm or greater, about 5 μm or greater, about 8 μm or greater, about 10 μm or greater, about 15 μm or greater, about 20 μm or greater, about 25 μm or greater, about 30 μm or greater, about 40 μm or less, about 38 μm or less, about 36 μm or less, about 35 μm or less, about 34 μm or less, about 33 μm or less, about 32 μm or less, about 30 μm or less, about 25 μm or less, or about 20 μm or less. In various aspects, the Li2O concentration distribution in the first portion 221 relative to the Li2O concentration at the midpoint 2 The distance of the increase in Li2O concentration 2 can be in the range of about 3 μm to about 40 μm, about 5 μm to about 40 μm, about 8 μm to about 38 μm, about 10 μm to about 36 μm, about 15 μm to about 35 μm, about 20 μm to about 34 μm, about 25 μm to about 33 μm, about 30 μm to about 32 μm or any range or sub-range therebetween. In various aspects, the Li2O concentration distribution in the central region 248 relative to the Li2O concentration at the central midpoint 2 The distance of the increase in Li2O concentration 2 expressed as a percentage of the central thickness 209 or an absolute distance can be respectively within one or more of the ranges discussed above for the distance of the increase in Li2O concentration expressed as a percentage of the substrate thickness or an absolute distance 2 The Li2O concentration distribution can be within one or more of the ranges discussed for the distance of the increase in concentration relative to the midpoint concentration

[0392] In various aspects, the K2O concentration at the first major surface 203 (e.g., the first surface region 223 in the first portion 221, the third surface region 233 in the second portion 231) can be higher than the K2O concentration at the midpoint (e.g., in the first portion, in the second portion) 2 The K2O concentration 2The concentration of K₂O is about 5 mol% or more, about 6 mol% or more, about 7 mol% or more, about 8 mol% or more, about 9 mol% or more, about 10 mol% or more, about 15 mol% or less, about 14 mol% or less, about 13 mol% or less, about 12 mol% or less, about 11 mol% or less, or about 10 mol% or less. In various aspects, the K₂O concentration at the first major surface 203 (e.g., the first surface region 223 in the first part 221, the third surface region 233 in the second part 231) 2 is higher than the K₂O concentration at the midpoint (e.g., in the first part, in the second part) 2 by about 5 mol% to about 15 mol%, about 6 mol% to about 14 mol%, about 7 mol% to about 13 mol%, about 8 mol% to about 12 mol%, about 9 mol% to about 11 mol%, about 9 mol% to about 10 mol%, or any range or sub - range therebetween. In various aspects, the K₂O concentration at the second major surface 205 (e.g., the second surface region 225 in the first part 221, the fourth surface region 235 in the second part 231) 2 is higher than the K₂O concentration at the midpoint (e.g., in the first part, in the second part) 2 by an amount that can be within one or more of the ranges discussed hereinabove in this paragraph and / or is substantially equal to the amount by which the K₂O concentration at the first major surface 203 2 is higher than the concentration at the midpoint. In various aspects, the K₂O concentration at the first central surface region 213 and / or the second central surface region 243 2 can be higher than the K₂O concentration at the central midpoint 2 by an amount within one or more of the ranges discussed hereinabove in this paragraph. In various aspects, the K₂O concentration at the first central surface region 213 and / or the second central surface region 243 2 is higher than the K₂O concentration at the central midpoint 2 by an amount that can be substantially equal to the amount by which the K₂O concentration at the first major surface 203 2 is higher than the K₂O concentration at the midpoint. Alternatively or additionally, the total concentration of potassium oxide (K₂O), rubidium oxide (Rb₂O), cesium oxide (Cs₂O), and francium oxide (Fr₂O) at the first major surface (e.g., the first surface region 223 in the first part 221, the third surface region 233 in the second part 231) 2 can be higher than the total concentration of K₂O, Rb₂O, Cs₂O, and Fr₂O at the midpoint 2 by an amount within one or more of the ranges discussed hereinabove in this paragraph. In various aspects, the K₂O concentration at the first central surface region 213 and / or the second central surface region 243 2 is higher than the K₂O concentration at the central midpoint 2 by an amount that can be substantially equal to the amount by which the K₂O concentration at the first major surface 203 2 is higher than the K₂O concentration at the midpoint. Alternatively or additionally, the total concentration of potassium oxide (K₂O), rubidium oxide (Rb₂O), cesium oxide (Cs₂O), and francium oxide (Fr₂O) at the first major surface (e.g., the first surface region 223 in the first part 221, the third surface region 233 in the second part 231) 2 can be higher than the total concentration of K₂O, Rb₂O, Cs₂O, and Fr₂O at the midpoint 2 by an amount within one or more of the ranges discussed hereinabove in this paragraph. In various aspects, the K₂O concentration at the first central surface region 213 and / or the second central surface region 243 2 is higher than the K₂O concentration at the central midpoint 2The total concentration of O is a certain amount higher, and the amount is within one or more of the ranges discussed in the previous paragraphs herein. Alternatively or additionally, the total concentration of potassium oxide (K 2 O), rubidium oxide (Rb 2 O), cesium oxide (Cs 2 O), and francium oxide (Fr 2 O) at the first central surface region 213 can be a certain amount higher than the total concentration of K 2 O, Rb 2 O, Cs 2 O, and Fr 2 O at the central midpoint, and the amount is within one or more of the ranges discussed in the previous paragraphs herein. Providing a high concentration (e.g., about 5 mol% or higher) of K 2 O (e.g., absolute mol% and / or the increase in surface concentration relative to the midpoint concentration) can provide a relatively large (e.g., about 500 MPa) surface compressive stress, thereby improving fracture resistance.

[0393] In various aspects, the distance of the K 2 O concentration distribution in the first part 221 relative to the increase in the K 2 O concentration at the midpoint, expressed as a percentage of the substrate thickness 207, can be about 5% or greater, about 6% or greater, about 7% or greater, about 8% or greater, about 9% or greater, 10% or greater, about 12% or greater, about 14% or greater, about 15% or greater, about 25% or less, about 23% or less, about 20% or less, about 18% or less, about 15% or less, about 12% or less, about 10% or less, or about 9% or less. In various aspects, the distance of the K 2 O concentration distribution in the first part 221 relative to the increase in the K 2 O concentration at the midpoint, expressed as a percentage of the substrate thickness 207, can be in the range of about 5% to about 25%, about 5% to about 23%, about 6% to about 20%, about 6% to about 18%, about 7% to about 15%, about 7% to about 12%, about 8% to about 10%, about 9% to about 10%, or any range or sub-range therebetween. In various aspects, the distance of the K 2 O concentration distribution in the first part 221 relative to the increase in the K 2 O concentration at the midpoint can be about 5 μm or greater, about 6 μm or greater, about 7 μm or greater, about 8 μm or greater, about 9 μm or greater, about 10 μm or greater, about 12 μm or greater, about 15 μm or greater, about 30 μm or less, about 25 μm or less, about 20 μm or less, or about 18 μm or less, about 15 μm or less, about 12 μm or less, or about 10 μm or less. In various aspects, the K 2 O concentration distribution in the first part 221 relative to the midpoint K2 The distance of the increase in K 2 O concentration can be in the range of about 5 μm to about 30 μm, about 5 μm to about 25 μm, about 6 μm to about 20 μm, about 6 μm to about 18 μm, about 7 μm to about 15 μm, about 7 μm to about 12 μm, about 8 μm to about 10 μm, about 9 μm to about 10 μm or any range or sub-range therebetween. In various aspects, the K in the central region 248 2 O concentration distribution relative to the K at the central midpoint 2 The distance of the increase in O concentration, expressed as a percentage of the central thickness 209, can be within one or more of the ranges discussed above for the first part expressed as a percentage of the substrate thickness for the distance of the increase in the midpoint concentration of K 2 O concentration distribution relative to the K at the central midpoint 2 The distance of the increase in O concentration can be about 5 μm or greater, about 7 μm or greater, about 10 μm or greater, about 12 μm or greater, about 15 μm or greater, about 20 μm or less, about 18 μm or less, or about 15 μm or less. In various aspects, the K in the central region 248 2 O concentration distribution relative to the K at the central midpoint 2 The distance of the increase in O concentration can be in the range of about 5 μm to about 20 μm, about 7 μm to about 18 μm, about 10 μm to about 15 μm or any range or sub-range therebetween. Alternatively or additionally, the total concentration of potassium oxide (K 2 O), rubidium oxide (Rb 2 O), cesium oxide (Cs 2 O), and francium oxide (Fr 2 O) at the first major surface (e.g., the first surface region 223 in the first part 221, the third surface region 233 in the second part 231) can increase by a distance relative to the total concentration of K 2 O, Rb 2 O, Cs 2 O, and Fr 2 O at the midpoint, said distance being within one or more of the ranges discussed above in this paragraph with respect to the absolute distance or the percentage of the substrate thickness 207. Similarly, the total concentration of K 2 O, Rb 2 O, Cs 2 O, and Fr 2 O at the first central surface region 213 can be relative to the total concentration of K 2 O, Rb 2 O, Cs 2 O, and Fr 2The total concentration of O increases by a distance within one or more of the ranges discussed above in this paragraph in terms of absolute distance or percentage of the center thickness 209.

[0394] As used herein, "total thickness variation" (TTV) of the central portion refers to the absolute value of the difference between the minimum thickness and the maximum thickness of the central portion 281 (e.g., the central region 248). The maximum thickness and the minimum thickness are measured by combining the 3D surfaces measured for each surface of the region (e.g., the central portion 281, the central region 248) using SpecGAGE3D as described above. In various aspects, the TTV included in the central portion 281 and / or the central region 248 can be about 6 μm or less, about 5 μm or less, about 4 μm or less, about 3.8 μm or less, about 3.6 μm or less, or about 3.4 μm or less. In various aspects, the TTV included in the central portion 281 and / or the central region 248 can be in the range of about 0.5 μm to about 6 μm, about 1 μm to about 5 μm, about 1.5 μm to about 4 μm, about 2 μm to about 3.8 μm, about 2.5 μm to about 3.6 μm, about 3 μm to about 3.4 μm or any range or sub-range therebetween. Providing a lower TTV (e.g., about 6 μm or less) can reduce the variation in chemical strengthening-induced strain associated with expansion or contraction caused by ion exchange, thereby further reducing the incidence of buckling.

[0395] As used herein, the refractive index is measured at a wavelength of 589 nm in accordance with ASTM E1967-19. In various aspects, the polymer-based portion 289 and / or 299 can be optically clear and / or include a first refractive index. In various aspects, the first refractive index of the polymer-based portion 289 and / or 299 can be about 1.3 or greater, about 1.4 or greater, about 1.45 or greater, about 1.49 or greater, about 2 or less, or about 1.7 or less, about 1.6 or less, or about 1.55 or less. In various aspects, the first refractive index of the polymer-based portion 289 and / or 299 can be in the range of about 1 to about 2, about 1.3 to about 1.7, about 1.4 to about 1.6, about 1.45 to about 1.55, about 1.49 to about 1.55 or any range or sub-range therebetween.

[0396] In various aspects, the foldable substrate 201 may include a second refractive index. In various aspects, the second refractive index of the foldable substrate 201 may be about 1.4 or greater, about 1.45 or greater, about 1.49 or greater, about 1.7 or less, about 1.6 or less, or about 1.55 or less. In various aspects, the second refractive index of the foldable substrate 201 may be in the range of about 1.4 to about 1.7, about 1.4 to about 1.6, about 1.45 to about 1.55, about 1.49 to about 1.55, or any range or sub-range therebetween. In various aspects, the absolute value of the difference equal to the difference between the second refractive index of the foldable substrate 201 and the first refractive index of the polymer-based portion 289 and / or 299 may be about 0.1 or less, about 0.07 or less, about 0.05 or less, about 0.001 or greater, about 0.01 or greater, or about 0.02 or greater. In various aspects, the difference is in the range of about 0.001 to about 0.1, about 0.01 to about 0.07, about 0.02 to about 0.05, or any range or sub-range therebetween. In various aspects, the second refractive index of the foldable substrate 201 may be greater than the first refractive index of the polymer-based portion 289 and / or 299. In various aspects, the second refractive index of the foldable substrate 201 may be less than the first refractive index of the polymer-based portion 289 and / or 299.

[0397] In various aspects, the adhesive layer 261 may include a third refractive index. In various aspects, the third refractive index of the adhesive layer 261 may be within one or more of the ranges discussed above with respect to the first refractive index of the polymer-based portion 289 and / or 299. In various aspects, the absolute value of the difference equal to the difference between the third refractive index of the adhesive layer 261 and the first refractive index of the polymer-based portion 289 and / or 299 may be within one or more of the ranges discussed above for the difference between the second refractive index and the first refractive index. In various aspects, the third refractive index of the adhesive layer 261 may be greater than the first refractive index of the polymer-based portion 289 and / or 299. In various aspects, the third refractive index of the adhesive layer 261 may be less than the first refractive index of the polymer-based portion 289 and / or 299. In various aspects, the absolute value of the difference equal to the difference between the third refractive index of the adhesive layer 261 and the second refractive index of the foldable substrate 201 may be within one or more of the ranges discussed above for the difference between the second refractive index and the first refractive index. In various aspects, the third refractive index of the adhesive layer 261 may be greater than the second refractive index of the foldable substrate 201. In various aspects, the third refractive index of the adhesive layer 261 may be less than the second refractive index of the foldable substrate 201.

[0398] In various aspects, the coating 251 can include a fourth refractive index. In various aspects, the fourth refractive index of the coating 251 can be within one or more of the ranges discussed above with respect to the first refractive index of the polymer-based portion 289 and / or 299. In various aspects, the absolute value of the difference equal to the difference between the fourth refractive index of the coating 251 and the first refractive index of the polymer-based portion 289 and / or 299 can be within one or more of the ranges discussed above for the difference between the second refractive index and the first refractive index. In various aspects, the fourth refractive index of the coating 251 can be greater than the first refractive index of the polymer-based portion 289 and / or 299. In various aspects, the fourth refractive index of the coating 251 can be less than the first refractive index of the polymer-based portion 289 and / or 299. In various aspects, the absolute value of the difference equal to the difference between the fourth refractive index of the coating 251 and the second refractive index of the foldable substrate 201 can be within one or more of the ranges discussed above for the difference between the second refractive index and the first refractive index. In various aspects, the fourth refractive index of the coating 251 can be greater than the second refractive index of the foldable substrate 201. In various aspects, the fourth refractive index of the coating 251 can be less than the second refractive index of the foldable substrate 201. In various aspects, the absolute value of the difference equal to the difference between the fourth refractive index of the coating 251 and the third refractive index of the adhesive layer 261 can be within one or more of the ranges discussed above for the difference between the second refractive index and the first refractive index. In various aspects, the fourth refractive index of the coating 251 can be greater than the third refractive index of the adhesive layer 261. In various aspects, the fourth refractive index of the coating 251 can be less than the third refractive index of the adhesive layer 261.

[0399] Figures 6 - 9 As shown schematically in aspects of the foldable devices 501, 701, 801, and / or 901 in accordance with various aspects of the present disclosure in a folded configuration. As Figure 6 shown, after the foldable device 501 is folded, the second major surface 205 of the foldable substrate 201 is located inside the folded foldable device 501. For example, the foldable device 301 can be folded to form the foldable device 501. For example, a display can be located on the side of the second major surface 205, and a viewer can view the display from the side of the first major surface 203. Alternatively, the display can be located on the side of the first major surface 203, and a viewer can view the display from the side of the second major surface 205. As Figure 8 shown, after the foldable device 801 is folded, the second major surface 205 of the foldable substrate 201 is located inside the folded foldable device 501. For example, a display can be located on the side of the second major surface 205, and a viewer can view the display from the side of the first major surface 203. Alternatively, the display can be located on the side of the first major surface 203, and a viewer can view the display from the side of the second major surface 205.

[0400] AsFigure 7 As shown, Figure 1 the foldable device 101 (modified as described in the parallel plate test below) is folded to form a folded foldable device 701 such that the first major surface 203 of the foldable substrate 201 is located inside the folded foldable device 701. In Figure 7 , the user can view the display device through the foldable substrate 201 rather than the PET sheet 707, and thus the display device will be located on the side of the first major surface 203. In various aspects, as Figure 7 shown, the foldable device 701 may include a coating 251 disposed on the foldable device 701 (e.g., the second major surface 205). In other aspects, the user will view the display device through the coating 251 rather than the PET sheet 707. In various aspects, as Figure 7 shown, the polymer-based portion 289 and / or 299 may be disposed on the foldable substrate 201. In other aspects, although not shown, there may be additional substrates (e.g., glass substrates and / or ceramic substrates, for replacing the release liner 271 or the PET sheet 707), and the additional substrates may be disposed on the display device. As Figure 9 shown, Figure 4 the foldable device 401 (modified as described in the parallel plate test below) is folded to form a folded foldable device 901 such that the first major surface 203 of the foldable substrate 201 is located inside the folded foldable device 901. In Figure 9 , the user can view the display device through the foldable substrate 201 rather than the PET sheet 707, and thus the display device will be located on the side of the first major surface 203. In various aspects, as Figure 9 shown, the second polymer-based portion 299 may be disposed on the foldable substrate 201 (e.g., in the first groove 211). In other aspects, although not shown, there may be additional substrates (e.g., glass substrates and / or ceramic substrates, for replacing the release liner 271 or the PET sheet 707), and the additional substrates may be disposed on the display device. It should be understood that the foldable device can be designed such that after folding, the display device is located inside the bend, outside the bend, or the foldable device can be folded in either direction.

[0401] As used herein, "foldable" encompasses fully folding, partially folding, bending, flexing, or a combination of functions. As used herein, the terms "rupture", "breakage", etc. refer to fracture, damage, delamination, or crack propagation. Similarly, if a foldable device maintains a parallel plate spacing "X" for 24 hours without being damaged at about 85 °C and about 85% relative humidity, then it can be said that the foldable device achieves a parallel plate spacing "X" or has a parallel plate spacing "X" or includes a parallel plate spacing "X".

[0402] As used herein, the "parallel plate spacing" of a foldable device and / or a foldable substrate is measured using a parallel plate apparatus 601 (see Figures 6 - 9 ) with the following test configuration and procedure. The parallel plate apparatus includes a pair of parallel rigid stainless steel plates 603, 605, including a first rigid stainless steel plate 603 and a second rigid stainless steel plate 605. When measuring the "parallel plate spacing" of a foldable substrate 201 (e.g., Figure 3 the foldable device 301 composed of the foldable substrate 201 as shown), as Figure 6 and 8 shown, the foldable substrate 201 is placed between the pair of plates 603 and 605 such that the first major surface 203 contacts the pair of plates 603 and 605. When measuring the "parallel plate spacing" of a foldable device similar to Figure 2 and 4 the corresponding foldable devices 101 and 401 shown, the adhesive layer 261 is removed and replaced with a test adhesive layer 709 having a thickness of 50 μm. In addition, the test is performed using a 100-μm thick polyethylene terephthalate (PET) sheet 707 instead of the Figure 2 and 4 release liner 271. Thus, when testing to determine the "parallel plate spacing" of the configuration of a foldable device, the foldable device 701 is fabricated using a 100-μm thick PET sheet 707 instead of the Figure 2 and 4 release liner 271.

[0403] When preparing the foldable device 701, the 100-μm thick PET sheet 707 is attached to the test adhesive layer 709 in the same manner as the release liner 271 is attached to the second contact surface 265 of the adhesive layer 261, as Figure 2 shown. To test the Figure 7 foldable device 701, the test adhesive layer 709 and the PET sheet 707 can likewise be installed in accordance with the Figure 7 configuration for testing on the foldable device 701. The foldable device 701 is placed between the pair of parallel rigid stainless steel plates 603 and 605 such that the foldable substrate 201 is located inside the bend, similar to the Figure 7 configuration shown. Similarly, when preparing the foldable device 901, the Figure 4The foldable device 401 shown is ready for testing. When determining the "parallel plate spacing", the spacing between the parallel plates is decreased at a rate of 50 μm / second until the parallel plate spacing 611 or 711 is equal to the "parallel plate spacing" to be tested. Then, the parallel plates are maintained at the "parallel plate spacing" to be tested for 24 hours at about 85 °C and about 85% relative humidity. As used herein, the "minimum parallel plate spacing" is the minimum parallel plate spacing that the foldable device can tolerate without being damaged under the conditions and configurations described above.

[0404] In various aspects, the foldable devices 101, 301, 401, 501, 701, 801, and / or 901 and / or the foldable substrate 201 can achieve a parallel plate spacing of 100 mm or less, 50 mm or less, 20 mm or less, 10 mm or less, 5 mm or less, or 3 mm or less. In other aspects, the foldable device and / or the foldable substrate can achieve a parallel plate spacing of 50 millimeters (mm), 20 mm, 10 mm, 5 mm, or 3 mm. In various aspects, the minimum parallel plate spacing included in the foldable device and / or the foldable substrate can be about 40 mm or less, about 20 mm or less, about 10 mm or less, about 5 mm or less, about 3 mm or less, about 1 mm or less, about 1 mm or greater, about 3 mm or greater, about 5 mm or greater, or about 10 mm or greater. In various aspects, the minimum parallel plate spacing included in the foldable device and / or the foldable substrate can be in the range of about 1 mm to about 40 mm, about 1 mm to about 20 mm, about 1 mm to about 10 mm, about 1 mm to about 5 mm, about 1 mm to about 3 mm. In various aspects, the minimum parallel plate spacing achieved by the foldable device and / or the foldable substrate can be in the range of about 2 mm to about 40 mm, about 2 mm to about 20 mm, about 3 mm to about 10 mm, about 3 mm to about 5 mm, or any range or sub-range therebetween.

[0405] The central width 287 of the central portion 281 of the foldable substrate 201 is defined along the direction 106 of the length 105 between the first portion 221 and the second portion 231. In various aspects, the central width 287 of the central portion 281 of the foldable substrate 201 may extend from the first portion 221 to the second portion 231. The widths 210 of the first central surface region 213 and the second central surface region 243 of the foldable substrate 201 are defined along the direction 106 of the length 105 between the first transition region 212 and the second transition region 218, for example, defined as a portion including the central thickness 209. In various aspects, the central width 287 of the central portion 281 of the foldable substrate 201 and / or the width 210 of the first central surface region 213 of the foldable substrate 201 may be about 1.4 times or greater, about 1.6 times or greater, about 2 times or greater, about 2.2 times or greater, about 3 times or less, or about 2.5 times or less of the minimum parallel plate spacing. In various aspects, the central width 287 of the central portion 281 of the foldable substrate 201 and / or the width 210 of the first central surface region 213 of the foldable substrate 201, expressed as a multiple of the minimum parallel plate spacing, may be in the range of about 1.4 times to about 3 times, about 1.6 times to about 2.5 times, about 2 times to about 2.5 times, about 2.2 times to about 2.5 times or any range or sub-range therebetween. Without being bound by theory, the length of the curved portion between the parallel plates arranged in a circular configuration may be about 1.6 times the parallel plate spacing. Without being bound by theory, the length of the curved portion between the parallel plates arranged in an elliptical configuration may be about 2.2 times the parallel plate spacing. In various aspects, the central width 287 of the central portion 281 of the foldable substrate 201 and / or the width 210 of the first central surface region 213 of the foldable substrate 201 may be about 1 mm or greater, about 3 mm or greater, about 5 mm or greater, about 8 mm or greater, about 10 mm or greater, about 15 mm or greater, about 20 mm or greater, about 60 mm or less, about 50 mm or less, about 40 mm or less, about 35 mm or less, about 30 mm or less, or about 25 mm or less. In various aspects, the central width 287 of the central portion 281 of the foldable substrate 201 and / or the width 210 of the first central surface region 213 of the foldable substrate 201 may be in the range of about 1 mm to about 100 mm, about 3 mm to about 60 mm, about 5 mm to about 50 mm, about 8 mm to about 40 mm, about 10 mm to about 40 mm, about 15 mm to about 35 mm, about 20 mm to about 30 mm or any range or sub-range therebetween. In various aspects, the central width 287 of the central portion 281 of the foldable substrate 201 and / or the width 210 of the first central surface region 213 of the foldable substrate 201 may be about 2.8 mm or greater, about 6 mm or greater, about 9 mm or greater, about 60 mm or less, about 40 mm or less, or about 24 mm or less.In various aspects, the center width 287 of the center portion 281 of the foldable substrate 201 and / or the width 210 of the first center surface area 213 of the foldable substrate 201 can be in the range of from about 2.8 mm to about 40 mm, from about 6 mm to about 24 mm, or any range or sub-range therebetween. In various aspects, the first center surface area 213, the center portion 281 (e.g., the center line of the center portion 281), and / or the folding plane 109 can correspond to the midpoint between opposite ends of the foldable substrate and / or the foldable device in the direction 106 of the length 105. Having the width within the range presented above for the center portion can facilitate folding of the foldable device without breakage.

[0406] In various aspects, the foldable substrate and / or the foldable device can be rollable. As used herein, a corresponding foldable substrate or foldable device is "rollable" if the threshold parallel plate spacing achievable over the length of the foldable substrate and / or the foldable device is 10 mm or 10% of the length of the corresponding foldable substrate and / or foldable device, whichever is greater. For example, as Figures 3 - 4 shown, when the center width 287 of the center portion 281 is greater than 10% of the length 105 extending in the direction 106 of the length 105 (see Figure 1 ), the foldable substrate 201 is considered "rollable". In various aspects, as Figures 3 - 4 shown, the foldable substrate 201 can include a first width 227, a center width 287, and a second width 237 in the direction 106 of the length 105. The sum of the first width 227, the center width 287, and the second width 237 can be substantially equal to and / or equal to the length of the foldable substrate 201 (e.g., Figure 1 the length 105 of the foldable device 101 shown).

[0407] In other aspects, the second width 237 is expressed as a percentage of the length of the foldable substrate 201 and / or the foldable device 101, and can be about 15% or less, about 12% or less, about 10% or less, about 8% or less, about 6% or less, about 5% or less, about 4.5% or less, about 4% or less, about 1% or greater, about 1.5% or greater, about 2% or greater, about 2.5% or greater, about 3% or greater, or about 3.5% or greater. In other aspects, the second width 237 is expressed as a percentage of the length of the foldable substrate 201 and / or the foldable device 101, and can be in the range of about 1% to about 15%, about 1% to about 12%, about 1.5% to about 10%, about 1.5% to about 8%, about 2% to about 6%, about 2.5% to about 5%, about 3% to about 4.5%, about 3.5% to about 4%, or any range or sub-range therebetween. Having the second width within one or more of the ranges mentioned in this paragraph above can provide sufficient width to handle the ends of the foldable substrate during processing, secure the foldable substrate and / or the foldable device as part of an electronic device, and / or maximize the number of the foldable substrate and / or the foldable device that becomes part of the display portion visible to the user. As used herein, a "display portion" refers to a portion of the foldable device corresponding to a location where an image can be displayed by a display device and viewed by a viewer through the foldable substrate (e.g., a rollable substrate).

[0408] In other aspects, the first width 227 is expressed as a percentage of the length of the foldable substrate 201 and / or the foldable device 101, and can be 35% or greater, about 40% or greater, about 45% or greater, about 50% or greater, about 75% or less, about 70% or less, about 65% or less, about 60% or less, or about 55% or less. In other aspects, the first width 227 is expressed as a percentage of the length of the foldable substrate 201 and / or the foldable device 101, and can be in the range of about 35% to about 75%, about 40% to about 70%, about 45% to about 65%, about 50% to about 60%, about 50% to about 55%, or any range or sub-range therebetween. In other aspects, the first width 227 can be about 35 mm or greater, about 40 mm or greater, about 45 mm or greater, about 50 mm or greater, about 75 mm or less, about 70 mm or less, about 65 mm or less, about 60 mm or less, or about 55 or less. In various aspects, the first width 227 can be in the range of about 35 mm to about 75 mm, about 40 mm to about 70 mm, about 45 mm to about 65 mm, about 50 mm to about 60 mm, about 50 mm to about 55 mm, or any range or sub-range therebetween. Having the first width within one or more of the ranges mentioned hereinabove in this paragraph can make the display portion visible to the user larger, while ensuring that the remaining portions of the foldable substrate (e.g., the central portion and the second portion) can be substantially all within the occupied space of the first portion.

[0409] Additionally or alternatively, the center width 287 can be greater than the second width 237. In various aspects, the center width 287 is expressed as a percentage of the length of the foldable substrate 201 and / or the foldable device 101, and can be about 15% or greater, about 20% or greater, about 25% or greater, about 30% or greater, about 35% or greater, about 40% or greater, about 42% or greater, about 44% or greater, about 45% or greater, about 50% or less, about 49% or less, about 48% or less, about 47% or less, about 46% or less, about 45% or less, about 38% or less, or about 32% or less. In various aspects, the center width 287 is expressed as a percentage of the length of the foldable substrate 201 and / or the foldable device 101, and can be in the range of about 15% to about 50%, about 20% to about 50%, about 25% to about 49%, about 30% to about 49%, about 35% to about 48%, about 40% to about 48%, about 42% to about 47%, about 43% to about 46%, about 44% to about 45% or any range or sub-range therebetween. In other aspects, the second width 237 can be less than the center width 287. Having the center width within one or more of the ranges mentioned in this paragraph above can enable the display portion of the foldable device to be adjusted when a portion of the rollable substrate is moved into and / or out of the user's line of sight, without unnecessarily increasing the size of the corresponding device when in the fully rolled-up configuration.

[0410] The foldable devices 101, 301, 401, 501, 701, 801, and / or 901 can have impact resistance, defined as the ability of a region of the foldable device (e.g., a region including the first portion 221, a region including the second portion 231, a region including the polymer-based portion 289 and / or 299 and / or the central portion 281) to not be damaged at a certain pen-drop height (e.g., 5 centimeters (cm) or higher, 10 centimeters or higher, 20 cm or higher), where the pen-drop height is measured according to the "pen-drop test". As used herein, the "pen-drop test" refers to testing a sample of the foldable device under a load (i.e., a pen dropped from a certain height) applied to an external major surface (e.g., Figures 2 - 3 the first major surface 203 of the foldable substrate 201 of the foldable device 101 or 301 as shown, Figures 3 - 4 the second major surface 205 of the foldable substrate 201 of the foldable device 301 or 401 as shown), where the foldable device is configured as in a parallel-plate test, i.e., attaching a 100-μm-thick PET sheet 707 to a 50-μm-thick test adhesive layer 709, rather than Figure 2The release liner 271 shown. Thus, the PET layer in the pen drop test is intended to simulate a foldable electronic display device (e.g., an OLED device). During the test, the foldable device bonded to the PET layer is placed on an aluminum plate (6063 aluminum alloy, polished to a surface roughness with 400 grit sandpaper), where the PET layer contacts the aluminum plate. The side of the sample placed on the aluminum plate is not taped.

[0411] The pen drop test uses a tube to guide the pen to the outer surface of the foldable device. For Figures 2 - 4 the foldable devices 101, 301, 401, 501, 701, 801, and / or 901 in FIGS. 6-9, the pen is guided to the outer major surface (e.g., Figures 2 - 3 the first major surface 203 of the foldable substrate 201 of the foldable device 101 or 301 shown, Figures 3 - 4 the second major surface 205 of the foldable substrate 201 of the foldable device 301 or 401 shown), and the tube is placed in contact with the outer major surface 205 of the foldable substrate 201 such that the longitudinal axis of the tube is substantially perpendicular to the outer major surface, where the longitudinal axis of the tube extends in the direction of gravity. The outer diameter of the tube is 1 inch (2.54 cm), the inner diameter is nine-sixteenths of an inch (1.4 cm), and the length is 90 cm. Each time the test is performed, an acrylonitrile-butadiene (ABS) spacer is used to fix the pen at a predetermined height. After each drop, the tube is repositioned relative to the sample to guide the pen to a different impact location on the sample. The pen used in the pen drop test is a BIC EasyGlide pen, fine type, tungsten carbide ballpoint tip, with a diameter of 0.7 mm (0.68 mm) and a weight of 5.73 grams (g), including a pen cap.

[0412] In the pen drop test, the pen cap is attached to the top end of the pen (i.e., the end opposite the tip), and then the pen is dropped so that the ballpoint pen can interact with the test sample. According to the drop sequence of the pen drop test, a pen drop is performed at an initial height of 1 cm, and then pen drops are continuously performed in 0.5 cm increments until 20 cm, and after 20 cm, pen drops are performed in 2 cm increments until the test sample is damaged. After each drop, it is recorded whether there are any observable signs of fracture, damage, or other damage to the sample, as well as the specific pen drop height. Using the pen drop test, multiple samples can be tested according to the same drop sequence to generate a population with higher statistical accuracy. When performing the pen drop test, a new pen is used every 5 drops and every time a new sample is tested. Additionally, all pens are dropped at random positions at or near the center of the sample, and no pen is dropped at or near the edge of the sample.

[0413] In the pen drop test, "failure" means the formation of visible mechanical defects in the laminate. Mechanical defects may be cracks or plastic deformations (such as surface indentations). Cracks may be surface cracks or through cracks. Cracks can form on the inner or outer surface of the laminate. Cracks can extend through all or part of the foldable substrate 201 and / or the coating. The minimum size of the visible mechanical defect is 0.2 mm or greater.

[0414] In various aspects, when the pen drop height is 10 centimeters (cm), 12 cm, 14 cm, 16 cm, or 20 cm, if the pen drops on the area including the first part 221 or the second part 231, the foldable device will not be damaged. In various aspects, the maximum pen drop height that the foldable device can withstand without damage to the area including the first part 221 or the second part 231 can be about 10 cm or higher, about 12 cm or higher, about 14 cm or higher, about 16 cm or higher, about 40 cm or lower, or about 30 cm or lower, about 20 cm or lower, about 18 cm or lower. In various aspects, the maximum pen drop height that the foldable device can withstand without damage to the area including the first part 221 or the second part 231 can be in the range of about 10 cm to about 40 cm, about 12 cm to about 30 cm, about 14 cm to about 20 cm, about 16 cm to about 20 cm, about 18 cm to about 20 cm or any range or sub-range therebetween.

[0415] In various aspects, when the pen drop height is 1 cm, 2 cm, 3 cm, 4 cm, 5 cm or higher, if the pen drops in a region (e.g., the central portion 281) between the first portion 221 and the second portion 231 that includes the polymer-based portion 289 and / or 299, the foldable device is not damaged. In various aspects, the maximum pen drop height that the foldable device can withstand without damaging the region between the first portion 221 and the second portion 231 that includes the polymer-based portion 289 and / or 299 can be about 1 cm or higher, about 2 cm or higher, about 3 cm or higher, about 4 cm or higher, about 20 cm or lower, about 10 cm or lower, about 8 cm or lower, or about 6 cm or lower. In various aspects, the maximum pen drop height that the foldable device can withstand without damaging the region between the first portion 221 and the second portion 231 that includes the polymer-based portion 289 and / or 299 can be in the range of about 1 cm to about 20 cm, about 2 cm to about 10 cm, about 3 cm to about 8 cm, about 4 cm to about 8 cm, about 4 cm to about 6 cm or any range or sub-range therebetween. In various aspects, the maximum pen drop height that the foldable device can withstand without damaging the region between the first portion 221 and the second portion 231 that includes the polymer-based portion 289 and / or 299 can be in the range of about 1 cm to about 10 cm, about 1 cm to about 8 cm, about 2 cm to about 5 cm, about 3 cm to about 5 cm, about 4 cm to about 5 cm or any range or sub-range therebetween.

[0416] Aspects of a method of manufacturing a foldable device and / or a foldable substrate according to various aspects of the present disclosure will be discussed with reference to Figure 12 and 34 the flowcharts, Figure 13 , 15 the example method steps shown in -17, 19-22, 35-36, and 38-41, and Figure 14 , 18 the cross-sectional views shown in and 37.

[0417] Reference will now be made to Figure 12 the flowcharts in, Figure 13 , 15 the example method steps shown in -17 and 19-22, and Figure 14 and 18 the cross-sectional views shown in to discuss example aspects of manufacturing Figures 2 - 3 the foldable devices 101, 301, 501, and / or 701 and / or the foldable substrate 201 shown in 6-7. In a first step 1201 of the method of the present disclosure, the method can begin with obtaining a foldable substrate 1301 (see Figures 13 - 14)。In various aspects, the foldable substrate 1301 can be provided by purchasing or otherwise obtaining a substrate or by forming a foldable substrate. In various aspects, the foldable substrate 1301 can include a glass substrate and / or a ceramic substrate. In other aspects, the glass substrate and / or the ceramic substrate can be provided by various tape forming processes, such as slot drawing, down-drawing, fused down-drawing, up-drawing, press roll, re-drawing, or floating. In other aspects, the ceramic substrate can be provided by heating the glass substrate to crystallize one or more ceramic crystals. The foldable substrate 1301 can include an existing first major surface 1303, and the first major surface can extend along a first plane 1304. The foldable substrate 1301 can include an existing second major surface 1305, and the second major surface can extend along a second plane 1306. In various aspects, as Figure 13 shown, in step 1201, the foldable substrate 1301 can include an existing first central surface region 1313 that is coplanar with the existing first major surface 1303. For example, the existing first major surface 1303 includes the existing first central surface region 1313. In various aspects, as Figure 13 shown, in step 1201, the foldable substrate 1301 can include an existing second central surface region 1343 that is coplanar with the existing second major surface 1305. For example, the existing second major surface 1305 includes the existing second central surface region 1343. The central portion 281 includes the existing first central surface region 1313 and the existing second central surface region 1343. Alternatively, in various aspects, as Figures 17 - 18 shown, the central portion 281 can include a first central surface region 1613 that is recessed from the existing first major surface 1303 and / or a second central surface region 1643 that is recessed from the existing second major surface 1305 at the end of step 1201. In various aspects, as Figure 14 and 18 shown, at the end of step 1201, the foldable substrate 201 can include one or more compressive stress regions.

[0418] After step 1201, as Figure 13 shown, the method can proceed to step 1203, including initially chemically strengthening the foldable substrate 1301. In various aspects, prior to the chemical strengthening of step 1203, the foldable substrate 1301 can be substantially unstrengthened. As used herein, substantially unstrengthened means that the substrate does not include a layer depth, does not include a compressive depth, the layer depth is in the range of 0% to about 5% of the substrate thickness, or the compressive depth is in the range of 0% to about 5% of the substrate thickness. In various aspects, as Figure 13As shown, chemically strengthening the foldable substrate 1301 may include contacting at least a portion of the foldable substrate 1301 that includes lithium cations and / or sodium cations with a first molten salt bath 1361 that includes a first molten salt solution 1365. When a first cation within a depth of the surface of the foldable substrate 1301 exchanges with a second cation in the first molten salt solution 1365 that has a radius larger than the first cation, the foldable substrate 1301 (e.g., a glass substrate, a ceramic substrate) can be chemically strengthened through ion exchange. For example, lithium cations within the depth of the surface of the foldable substrate 1301 can exchange with sodium cations or potassium cations in the first molten salt solution 1365. Accordingly, the surface of the foldable substrate 1301 is compressed and is thus chemically strengthened through the ion exchange process because the radius of the lithium cation is smaller than the radius of the sodium cation or potassium cation exchanged in the first molten salt solution 1365. Chemically strengthening the foldable substrate 1301 may include contacting at least a portion of the foldable substrate 1301 that includes lithium cations and / or sodium cations with a first molten salt bath 1361 that includes a first molten salt solution 1365, the first molten salt solution including potassium nitrate, potassium phosphate, potassium chloride, potassium sulfate, sodium chloride, sodium sulfate, sodium nitrate, and / or sodium phosphate, wherein the lithium cations and / or sodium cations diffuse from the foldable substrate 1301 into the first molten salt solution 1365 contained in the first molten salt bath 1361. In various aspects, the first molten In various aspects, prior to immersion of the foldable substrate 1301, and / or during step 1203, the first molten salt solution may be lithium-free. In various aspects, the first molten salt solution 1365 may further include silicic acid, e.g., within one or more of the ranges discussed below for the amount of silicic acid in the second molten salt solution 1703. In various aspects, the molten salt solution may consist of a potassium salt and optionally silicic acid.

[0419] In various aspects, the temperature of the first molten salt solution 1365 can be about 380 °C or higher, about 400 °C or higher, about 420 °C or higher, about 430 °C or lower, about 530 °C or lower, about 500 °C or lower, about 480 °C or lower, or about 450 °C or lower. In various aspects, the temperature of the first molten salt solution 1365 can be in the range of about 380 °C to about 530 °C, about 400 °C to about 500 °C, about 420 °C to about 480 °C, about 430 °C to about 450 °C or any range or sub-range therebetween. In various aspects, the foldable substrate 1301 can be in contact with the first molten salt solution 1365 for about 30 minutes or longer, about 20 minutes or longer, 30 minutes or longer, about 45 minutes or longer, about 1 hour or longer, about 8 hours or shorter, about 4 hours or shorter, about 2 hours or shorter, or about 1.5 hours or shorter. In various aspects, the foldable substrate 1301 can be in contact with the first molten salt solution 1365 for a time in the range of about 20 minutes to about 8 hours, about 30 minutes to about 4 hours, about 45 minutes to about 2 hours, about 1 hour to about 1.5 hours or any range or sub-range therebetween.

[0420] In various aspects, as Figure 14 shown, the initial chemical strengthening of the foldable substrate 1301 in step 1203 can include chemically strengthening the existing first major surface 1303 in the first portion 1321 and the second portion 1331 to form an initial first compressive stress zone extending from the existing first major surface 1303 to an initial first compressive depth 1413. In various aspects, as Figure 14 shown, chemically strengthening the foldable substrate 1301 in step 1205 can form an initial second compressive stress zone extending from the existing second major surface 1305 to an initial second compressive depth 1415. As Figure 14 indicated, any chemical strengthening at the existing first central surface region 1313 will be removed when forming the first groove (indicated by the dashed line 1417) and / or the second groove (indicated by the dashed line 1419) in step 1205 (discussed below), since the first distance 1427 and / or the second distance 1429 of the groove is greater than the initial first compressive depth 1413. In various aspects, the initial first compressive depth 1413 and / or the initial second compressive depth 1415, expressed as a percentage of the initial substrate thickness, can be about 10% or greater, about 11% or greater, about 12% or greater, about 13% or greater, about 20% or less, about 18% or less, about 16% or less, or about 14% or less. In various aspects, the initial first compressive depth 1413 and / or the initial second compressive depth 1415, expressed as a percentage of the initial substrate thickness, can be in the range of about 10% to about 20%, about 11% to about 18%, about 12% to about 16%, about 13% to about 14% or any range or sub-range therebetween.

[0421] After step 1201 or 1203, as Figure 16 shown, the method can proceed to step 1205, including at least etching an existing first central surface region 1313 to form a first central surface region 1613. In various aspects, step 1205 may include disposing an etch mask on the foldable substrate 1301, but not covering the entire existing first central surface region 1313. In various aspects, as Figure 18 shown, a first portion 1503b may be disposed on a first portion 221 (e.g., a first surface region 1507b of the first portion 1503b may contact an existing first major surface 1303 in the first portion 221). In other aspects, as Figure 15 shown, the first portion 1503b may extend a length 1519 into the central portion 281 (relative to the dimensions of the resulting foldable substrate), which may enable the resulting foldable substrate to achieve a predetermined size, e.g., by considering undercut during the etching process. In various aspects, as Figure 15 shown, a second portion 1503a may be disposed on a second portion 231 (e.g., a second surface region 1507a of the second portion 1503a may contact an existing first major surface 1303 in the second portion 231). In other aspects, as Figure 15 shown, the second portion 1503a may extend into the central portion 281, e.g., by an extension length equal to the length 1519. In various aspects, as Figure 15 shown, a third portion 1505b may be disposed on an existing second major surface 1305 (e.g., a first portion 221, where a third surface region 1509b of the third portion 1505b contacts the existing second major surface 1305 in the first portion 221), and / or a fourth portion 1505a may be disposed on an existing second major surface 1305 (e.g., a second portion 231, where a fourth surface region 1509a of the fourth portion 1505a contacts the existing second major surface 1305 in the second portion 231). In various aspects, the etch mask (e.g., the first portion 1503b, the second portion 1503a, the third portion 1505b, the fourth portion 1505a) may include a polymer (e.g., an acid-resistant polymer) or an inorganic material. Exemplary aspects of the polymer include polyolefins, polyamides, halogen-containing polymers (e.g., polyvinyl chloride or fluoropolymers), elastomers, polyurethanes, phenolic resins, parylene, polyethylene terephthalate (PET), and polyetheretherketone (PEEK). Exemplary aspects of the inorganic materials for the etch mask include titanium dioxide (TiO 2 ), zirconium oxide (ZrO 2 ), tin oxide (SnO 2 ), aluminum oxide (Al 2 O 3) Silicon dioxide (SiO 2 ), silicon nitride (Si 3 N 4 ), and / or combinations thereof, but other mask materials can be used in other respects. In various aspects, the etch mask can be placed by curing a precursor dispensed from a container onto a foldable substrate or by attaching a tape including an acid-resistant polymer and an adhesive. Alternatively, another method (e.g., chemical vapor deposition (CVD) (e.g., low-pressure CVD, plasma-enhanced CVD), physical vapor deposition (PVD) (e.g., evaporation, molecular beam epitaxy, ion plating), atomic layer deposition (ALD), sputtering, spray pyrolysis, chemical bath deposition, sol-gel deposition) can be used to form the etch mask.

[0422] By comparing Figures 14 - 15 with Figure 16 it is shown that step 1205 includes etching an existing first central surface region 1313 (see Figures 14 - 15 ) by bringing an existing first central surface region 1313 into contact with an etchant 1603 to form a first central surface region 1613. For example, the foldable substrate 1301 or 201 can be immersed in an etch bath 1601 containing the etchant 1603. In other aspects, the etchant 1603 can include one or more acids (e.g., HCl, HF, H 2 SO 4 , HNO 3 ). In various aspects, the etchant 1603 can undercut a first portion 1503b and / or a second portion 1503a of the etch mask, e.g., an undercut length 1519 (see Figure 15 ). Step 1205 can form a first central surface region 1613 that can be recessed from a first plane 1304 by a first distance 1427. In various aspects, as Figures 16 - 17As shown, step 1205 may further form a first transition region 212 including a first transition surface region 215 and / or a second transition region 218 including a third transition surface region 217. In other aspects, as shown, the angle between the first transition surface region 215 and the first central surface region 1613 may be substantially equal to the first average angle 282, and / or the angle between the third transition surface region 217 and the first central surface region 213 may be substantially equal to the third average angle 286. Step 1205 may form a second central surface region 1643, which may be recessed from the second plane 1306 by a second distance 1429. In other aspects, as shown, the angle between the second transition surface region 245 and the second central surface region 1643 may be substantially equal to the second average angle 284, and / or the angle between the fourth transition surface region 247 and the second central surface region 243 may be substantially equal to the fourth average angle 288. In various aspects, as shown, the minimum distance 1641 between the portions of the etch mask (e.g., between the first portion 1503b and the second portion 1503a, between the third portion 1505b and the fourth portion 1505a) may be less than the width of the central portion 281 (e.g., the sum of the widths 1620, 1624, and 1626) due to undercutting. In other aspects, the width 1620 of the first central surface region 1613 may be less than the minimum distance 1641. In other aspects, the widths 1624 and 1626 of the transition regions 212 and 218 may be substantially equal to the corresponding first transition width 214 and / or second transition width 216, e.g., as Figures 2 - 3 shown. Similarly, the width 1620 of the first central surface region 1613 may be substantially equal to the width 210.

[0423] After forming the first central surface region 1613 and / or the second central surface region 1643, as Figure 17 shown, step 1205 may further include removing the one or more etch masks. In various aspects, removing the etch mask may be accomplished by using tools (e.g., grinding, sweeping, scraping, pushing, etc.), by cleaning the foldable substrate (e.g., using a cleaning agent solution, using an alkaline solution), or by a combination thereof.

[0424] After step 1201 or 1205, as Figures 17 - 18As shown, the method can proceed to step 1207, which includes further chemically strengthening the foldable substrate 201. In various aspects, as shown, step 1207 can include contacting the foldable substrate 201 with a second molten salt solution 1703 for a second period of time. In other aspects, as shown, step 1207 can include dipping the foldable substrate 201 into a second molten salt bath 1701 containing the second molten salt solution 1703. The lithium salt contained in the second molten salt solution 1703 is greater than 0 wt% to less than about 0.4 wt%. In various aspects, the second molten salt solution 1703 can include a lithium salt in an amount of about 0.01 wt% or more, about 0.02 wt% or more, about 0.03 wt% or more, about 0.04 wt% or more, about 0.05 wt% or more, about 0.2 wt% or less, about 0.1 wt% or less, about 0.08 wt% or less, about 0.07 wt% or less, or about 0.06 wt% or less, or about 0.05 wt% or less. In various aspects, the lithium salt included in the second molten salt solution 1703 can be about 0.01 wt% to about 0.2 wt%, about 0.01 wt% to about 0.1 wt%, about 0.02 wt% to about 0.08 wt%, about 0.03 wt% to about 0.07 wt%, about 0.04 wt% to about 0.06 wt% or any range or sub-range therebetween. In various aspects, the second molten salt solution 1703 includes potassium ions in an amount of about 70 wt% or more, about 80 wt% or more, about 85 wt% or more, about 90 wt% or more, about 95 wt% or more, about 97 wt% or more, about 98 wt% or more, about 99 wt% or more, about 99.92 wt% or more, about 99.98 wt% or less, about 99.92 wt% or less, about 98.98 wt% or less, about 98.92 wt% or less, about 97.98 wt% or less, about 97.92 wt% or less, about 96.98 wt% or less, about 96.92 wt% or less, about 95.98 wt% or less, about 95.92 wt% or less, about 94.98 wt% or less, about 94.92 wt% or less, about 89.98 wt% or less, about 89.92 wt% or less, about 84.98 wt% or less, about 84.92 wt% or less. In various aspects, the potassium ions included in the second molten salt solution 1703 can be about 70 wt% to about 99.98 wt%, about 80 wt% to about 99.92 wt%, about 85 wt% to about 98.98 wt%, about 90 wt% to about 97.98 wt%, about 95 wt% to about 96.98 wt% or any range or sub-range therebetween.In various aspects, the second molten salt solution 1703 may optionally include sodium salts in an amount greater than 0 wt%, about 1 wt% or more, about 2 wt% or more, about 3 wt% or more, about 29.98 wt% or less, about 28.98 wt% or less, about 27.98 wt% or less, about 24.98 wt% or less, about 19.98 wt% or less, about 14.98 wt% or less, about 9.98 wt% or less, or about 5 wt% or less. In various aspects, the second molten salt solution 1703 may optionally include sodium salts in an amount in the range of greater than 0 wt% to about 29.98 wt%, about 1 wt% to about 28.98 wt%, about 2 wt% to about 24.98 wt%, about 3 wt% to about 19.98 wt%, about 4 wt% to about 9.98 wt% or less, about 4 wt% to about 5 wt%, or any range or sub-range therebetween. In various aspects, the second molten salt solution 1703 may optionally include silicic acid in an amount greater than 0 wt% to about 0.1 wt% or more, about 0.2 wt% or more, about 0.3 wt% or more, about 0.4 wt% or more, about 1 wt% or less, about 0.9 wt% or less, about 0.8 wt% or less, about 0.7 wt% or less, or about 0.6 wt% or less. In various aspects, the second molten salt solution 1703 may optionally include silicic acid in an amount in the range of greater than 0 wt% to 1 wt%, about 0.1 wt% to about 0.9 wt%, about 0.2 wt% to about 0.8 wt%, about 0.3 wt% to about 0.7 wt%, about 0.4 wt% to about 0.6 wt%, or any range or sub-range therebetween. Exemplary aspects of the anions of the lithium salts, potassium salts, and / or sodium salts are nitrates, but in other aspects other anions may be used (as discussed above for the first molten salt solution). Alternatively, the total concentration of the potassium salts, cesium salts, francium salts, and rubidium salts may be within one or more of the ranges discussed above for the potassium salts.

[0425] In various aspects, the second molten salt solution 1703 can be maintained at a temperature of about 380 °C or higher, about 400 °C or higher, about 410 °C or higher, about 480 °C or lower, about 460 °C or lower, or about 440 °C or lower. In various aspects, the second molten salt solution 1703 can be maintained at a temperature in the range of about 380 °C to about 480 °C, about 400 °C to about 460 °C, about 410 °C to about 440 °C or any range or sub-range therebetween. In various aspects, the second time period during which the second molten salt solution 1703 contacts the foldable substrate 201 can be less than the first time period. In other aspects, the second time period can be about 1 minute or longer, about 2 minutes or longer, about 4 minutes or longer, about 10 minutes or shorter, about 8 minutes or shorter, or about 6 minutes or shorter. In various aspects, the second time period can be in the range of about 1 minute to about 10 minutes, about 2 minutes to about 8 minutes, about 4 minutes to about 6 minutes or any range or sub-range therebetween.

[0426] At the end of step 1207, as Figure 18 shown, the compressive stress zone extending from the existing first major surface 1303 can increase from an initial first compressive depth (as indicated by the hollow circle 1803) to about the first compressive depth 1809 (as indicated by the solid dot 1805). At the end of step 1207 or at the end of step 1209 (discussed below), the first compressive stress zone in the first portion 221 can extend to the first compressive depth 1809 and / or the third compressive stress zone in the second portion 231 can extend to the first compressive depth 1809. At the end of step 1207, the compressive stress zone extending from the existing second major surface 1305 can increase from an initial second compressive depth (as indicated by the hollow circle 1813) to the second compressive depth 1819 (as indicated by the solid dot 1815). At the end of step 1207 or at the end of step 1209 (discussed below), the second compressive stress zone in the first portion 221 can extend to the second compressive depth 1819 and / or the fourth compressive stress zone in the second portion 231 can extend to the second compressive depth 1819. At the end of step 1207, the first central compressive stress zone (as indicated by the solid dot 1807) extending from the first central surface region 1613 in the central portion 281 can extend to the first central compressive depth 1829, and / or the second central compressive stress zone (as indicated by the solid dot 1817) extending from the second central surface region 1643 in the central portion 281 can extend to the second central compressive depth 1839.

[0427] As Figure 18As shown, the first surface layer 1801 and / or the second surface layer 1811 may be removed in step 1209 (discussed below). In various aspects, the first surface layer 1801 may be substantially uniform across the existing first major surface 1303, the first transition surface region 215, the third transition surface region 217, and / or the first central surface region 1613. In various aspects, the second surface layer 1811 may be substantially uniform across the existing second major surface 1305 (e.g., including the second central surface region 1643). In other aspects, the second surface layer 1811 may be substantially equal to the first surface layer 1801. After step 1209, the foldable substrate may correspond to Figures 2 - 3 the foldable substrate shown.

[0428] After step 1207, as Figure 19 shown, the method may proceed to step 1209, including etching away a uniform thickness substantially uniformly from the foldable substrate 201. In various aspects, as shown, step 1209 may include contacting the foldable substrate 201 with an etchant 1903. In other aspects, as shown, step 1209 may include dipping the foldable substrate 201 into an etch bath 1901 containing the etchant 1903. In various aspects, the etchant 1903 may be the same as the etchant 1603 discussed above with reference to step 1205. In various aspects, the concentration (e.g., molar concentration) included in the etchant 1903 may be lower than that of the etchant 1603. In various aspects, the thickness removed substantially uniformly from the foldable substrate 201 in step 1209 may be about 0.1 μm or thicker, about 0.2 μm or thicker, about 0.5 μm or thicker, about 5 μm or thinner, about 2 μm or thinner, about 1 μm or thinner, or about 0.8 μm or thinner. In various aspects, the thickness removed substantially uniformly from the foldable substrate 201 in step 1209 may be in the range of about 0.1 μm to about 5 μm, about 0.1 μm to about 2 μm, about 0.2 μm to about 1 μm, about 0.5 μm to about 0.8 μm, or any range or sub-range therebetween. (After further chemically strengthening the foldable substrate in step 1207) etching the substrate in step 1209 may remove defects near or on the surface of the foldable substrate 201, thereby improving the strength (e.g., pen drop height) and / or flexibility (e.g., the ability to achieve a specific parallel plate spacing) of the foldable substrate 201. In various aspects, as shown, at the end of step 1209, the etching may produce a foldable substrate having a substrate thickness 207 and a central thickness 209. In various aspects, as shown, at the end of step 1209, the etching may form a first surface region 223, a second surface region 225, a third surface region 233, a fourth surface region 235, a first central surface region 213, and / or a second central surface region 243, the characteristics of which are as described above with reference to Figures 2 - 3 that discussed.

[0429] After step 1209, as Figures 20 - 22 shown, the method can proceed to step 1211, including assembling a foldable device including a foldable substrate. In various aspects, as Figures 20 - 22 shown, step 1209 can include assembling the foldable device by disposing polymer-based portions (e.g., a first polymer-based portion 289, a second polymer-based portion 299), an adhesive layer 261, and / or a coating 251 over the foldable substrate 201. In other aspects, as Figure 20 shown, the first polymer-based portion 289 can be disposed in the first recess 211 and / or over the first central surface region 213. In other aspects, as Figures 20 - 21 shown, the coating 251 can be disposed over the first major surface 203 (e.g., the first surface region 223 and the third surface region 233), e.g., by dispensing a first liquid 2003 from a container 2001 (e.g., a conduit, a flexible tube, a micropipette, or a syringe) over the first major surface 203, which can cure to form the coating 251. In still other aspects, the first liquid 2003 can include a coating precursor, a solvent, particles, nanoparticles, and / or fibers. In additional other aspects, the coating precursor can include, but is not limited to, one or more of monomers, accelerators, curing agents, epoxy resins, and / or acrylates. Curing the first liquid 2003 can include heating the first liquid 2003, irradiating the first liquid 2003 with ultraviolet (UV) radiation, and / or waiting for a predetermined amount of time (e.g., about 30 minutes to 24 hours, about 1 hour to about 8 hours). In various aspects, although not shown, the coating 251 can be disposed in the first recess 211 (e.g., filling the first recess 211) without contacting the first major surface 203 (e.g., the first surface region 223, the third surface region 233), e.g., instead of Figures 20 - 22 the first polymer-based portion 289. In other aspects, as Figures 20 - 22 shown, the second polymer-based portion 299 can be disposed in the second recess 241, e.g., by dispensing a second liquid 2103 from a container 2101 (e.g., a conduit, a flexible tube, a micropipette, or a syringe) over the second central surface region 243, which can cure to form the second polymer-based portion 299. Curing the second liquid 2103 can include heating the second liquid 2103, irradiating the second liquid 2103 with ultraviolet (UV) radiation, and / or waiting for a predetermined amount of time (e.g., about 30 minutes to 24 hours, about 1 hour to about 8 hours). In other aspects, as Figure 22As shown, the adhesive layer 261 can contact the second major surface 205 (e.g., the second surface region 225 and the fourth surface region 235). For example, the adhesive layer 261 can include one or more sheets of adhesive material. In various aspects, there can be an integral interface between the one or more sheets including the adhesive layer 261, which can reduce (e.g., avoid) optical diffraction and / or optical discontinuities when light propagates between the sheets, since the one or more sheets can include substantially the same refractive index. In various aspects, although not shown, at least a portion of the adhesive layer can be disposed in the second groove. In various aspects, a release liner (see the release liner 271 in Figure 2 ) or the display device can be disposed on the adhesive layer 261 (e.g., the second contact surface 265).

[0430] After step 1207, 1209, or 2111, the method can proceed to step 1213, where the method of manufacturing the foldable substrate and / or the foldable device can be completed. In various aspects, the method of manufacturing the foldable substrate and / or the foldable device according to various aspects of the present disclosure can proceed sequentially along the steps 1201, 1203, 1205, 1207, 1209, 1211, and 1213 of the flowchart in Figure 12 , as discussed above. In various aspects, for example, if the foldable substrate 1301 already includes an initial compressive stress zone at the end of step 1201, then the method can proceed from step 1201 to step 1205 along arrow 1202. In various aspects, for example, if the foldable substrate already includes an initial compressive stress zone at the end of step 1201 and has an etch mask disposed thereon, then the method can proceed from step 1201 to step 1207 along arrow 1204. In various aspects, for example, if the method has been completed at the end of step 1207, then the method can proceed from step 1207 to step 1213 along arrow 1206. In various aspects, for example, if the method has been completed at the end of step 1209, then the method can proceed from step 1209 to step 1213 along arrow 1208. According to embodiments of the present disclosure, any of the above options can be combined to manufacture the foldable device.

[0431] Reference will now be made to the flowchart in Figure 34 , the example method steps shown in Figures 35 - 36 and 38 - 41, and the cross-sectional views shown in Figure 37 to discuss example aspects of manufacturing the foldable devices 401, 801, and / or 901 and / or the foldable substrate 201 shown in Figure 4 and 8 - 9. In the first step 3401 of the method of the present disclosure, the method can begin with obtaining the foldable substrate 201 or 1301 (see the foldable substrate 1301 in Figure 13 or Figures 35 - 36Collapsible substrate 201). In various aspects, the collapsible substrate 201 or 1301 can be provided by purchasing or otherwise obtaining a substrate or by forming a collapsible substrate. In various aspects, the collapsible substrate 201 or 1301 can include a glass substrate and / or a ceramic substrate. In other aspects, the glass substrate and / or the ceramic substrate can be provided by various strip forming processes, such as slot drawing, down drawing, fusion down drawing, up drawing, press roll, redrawing, or floating. In other aspects, the ceramic substrate can be provided by heating the glass substrate to crystallize one or more ceramic crystals. The collapsible substrate 201 or 1301 can include an existing first major surface 1303, which can extend along a first plane 1304. The collapsible substrate 201 or 1301 can include an existing second major surface 1305, which can extend along a second plane 1306. In various aspects (e.g., see Figure 13 ), in step 3401, the collapsible substrate 1301 can include an existing first central surface region 1313, which is coplanar with the existing first major surface 1303. For example, the existing first major surface 1303 includes the existing first central surface region 1313. In various aspects, as Figure 35 shows, in step 3401, the collapsible substrate 201 or 1301 can include an existing second central surface region 1343, which is coplanar with the existing second major surface 1305. For example, the existing second major surface 1305 includes the existing second central surface region 1343. The central portion 281 includes the existing first central surface region 1313 (see Figure 13 ) and the existing second central surface region 1343. Alternatively, in various aspects, as Figures 35 - 36 shows, the central portion 281 can include a first central surface region 213 or 3513 that is recessed from the existing first major surface 1303 at the end of step 3401. In various aspects, at the end of step 3401, the collapsible substrate 201 or 1301 can be substantially unstrengthened. Alternatively, in various aspects, the collapsible substrate 201 or 1301 can include one or more compressive stress zones (e.g., see Figure 14 and the related discussion above), and it should be understood that any initial compressive stress zones in the first part and / or the second part will be increased by chemical strengthening in step 3405 discussed below.

[0432] After step 3401, as Figure 35 shows, the method can proceed to step 3403, including at least etching the existing first central surface region 1313 (see Figure 13 ) to form the first central surface region 3513. In various aspects, step 3403 can include disposing an etching mask on the collapsible substrate 201 or 1301, but not covering the entire existing first central surface region 1313. In various aspects, asFigure 35 As shown, the first part 3523b can be disposed on the first part 221 (e.g., contacting the existing first major surface 1303 in the first part 221). In other aspects, as Figure 35 shown, the first part 1503b can extend a length 3529 into the central part 281 (relative to the dimensions of the resulting foldable substrate 201), which can enable the resulting foldable substrate to achieve a predetermined size, e.g., by considering undercut during the etching process. In various aspects, as Figure 35 shown, the second part 3523a can be disposed on the second part 231 (e.g., contacting the existing first major surface 1303 in the second part 231). In other aspects, as Figure 35 shown, the second part 3523a can extend into the central part 281, e.g., with an extension length equal to the length 3529. In various aspects, as Figure 35 shown, the third part 3505 can be disposed on the existing second major surface 1305 (e.g., the first part 221, the second part 231, and / or the central part 281, contacting the existing second major surface 1305). In other aspects, as shown in the figure, the third part 3505 can cover the entire existing second major surface 1305 (e.g., be disposed thereon). In various aspects, the etching mask (e.g., the first part 3523b, the second part 3523a, the third part 3505) can include a polymer (e.g., an acid-resistant polymer) or an inorganic material, such...

Claims

1. A foldable device, which includes a substrate, and the substrate comprises: a substrate thickness defined between a first major surface and a second major surface opposite to the first major surface; a first portion, which includes the substrate thickness, a first compressive stress zone extending from the first major surface to a first compressive depth, and a second compressive stress zone extending from the second major surface to a second compressive depth; a second portion, which includes the substrate thickness, a third compressive stress zone extending from the first major surface to a third compressive depth, and a fourth compressive stress zone extending from the second major surface to a fourth compressive depth; a central portion, which is located between the first portion and the second portion, and the central portion includes a central thickness defined between a first central surface region and a second central surface region opposite to the first central surface region, a first central compressive stress zone extending from the first central surface region to a first central compressive depth, and a second central compressive stress zone extending from the second central surface region to a second central compressive depth, the first central surface region is recessed from the first major surface by a first distance, and the central thickness is less than the substrate thickness; and the lithium oxide concentration at the first central surface region is about 0.2 mol% to about 2 mol% higher than the lithium oxide concentration at the central midpoint, wherein the first portion includes: a midpoint in the middle of the first major surface and the second major surface; the lithium oxide concentration at the first major surface; the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the first major surface; the lithium oxide concentration at the midpoint; and the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the midpoint, and wherein the substrate is a glass substrate or a ceramic substrate, and the central midpoint is in the middle of the first central surface region and the second central surface region, and the central portion includes: the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the first central surface region; and the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the central midpoint.

2. The foldable device according to claim 1, wherein the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the first major surface is about 5 mol% to about 15 mol% higher than the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the midpoint.

3. The foldable device according to any one of claims 1 to 2, wherein the total concentration of potassium oxide, rubidium oxide, cesium oxide, and francium oxide at the first major surface is higher than the sodium oxide concentration at the first major surface.

4. The foldable device according to any one of claims 1 to 3, wherein the potassium oxide concentration distribution in the first portion rises relative to the potassium oxide concentration at the midpoint to a depth that is about 10% or more of the substrate thickness away from the first major surface.

5. The foldable device according to any one of claims 1 to 3, wherein the potassium oxide concentration distribution in the first portion rises relative to the potassium oxide concentration at the midpoint to a depth that is about 10 microns or more away from the first major surface.

6. The foldable device according to any one of claims 1 to 5, wherein the lithium oxide concentration at the first major surface is about 0.2 mol% to about 2 mol% higher than the lithium oxide concentration at the midpoint of the first portion.

7. The foldable device according to any one of claims 1 to 5, wherein the lithium oxide concentration at the first major surface is about 1.5 mol% to about 2.5 mol%.

8. The foldable device according to any one of claims 1 to 5, wherein the concentration profile of the lithium oxide in the first portion rises relative to the lithium oxide concentration at the midpoint to a depth that is about 5% or more of the substrate thickness away from the first major surface.

9. The foldable device according to any one of claims 1 to 5, wherein the concentration profile of the lithium oxide in the first portion rises relative to the lithium oxide concentration at the midpoint to a depth that is about 3 microns to about 15 microns away from the first major surface.

10. The foldable device according to any one of claims 1 to 9, wherein: the first maximum compressive stress at the first major surface is about 500 MPa or greater; the substrate thickness is about 50 microns to about 2 mm; the center thickness is about 25 microns to about 120 microns; and the first distance is about 20% to about 45% of the substrate thickness.

11. The foldable device according to any one of claims 1 to 10, wherein the foldable device achieves a parallel plate spacing of 5 mm.

12. The foldable device according to any one of claims 1 to 11, wherein the second central surface region is recessed from the second major surface by a second distance that is about 20% to about 45% of the substrate thickness.

13. The foldable device according to any one of claims 1 to 11, wherein the second major surface is coplanar with the second central surface region.

14. The foldable device according to any one of claims 1 to 12, wherein the surface profile of the first central surface region has an average gradient of about 0.015 mm / mm or less.

15. A method of forming a foldable device, which comprises: chemically strengthening a substrate in a first molten salt bath maintained at a temperature of about 380 °C to about 530 °C for a first period of about 20 minutes to about 8 hours; and then immersing the substrate in a second molten salt bath maintained at a temperature of about 380 °C to about 480 °C for a second period of about 1 minute to about 10 minutes, wherein the concentration of the lithium salt included in the second molten salt bath is higher than the concentration of the lithium salt included in the first molten salt bath, the second molten salt bath includes less than 0.4 wt% of the lithium salt, the substrate includes a substrate thickness defined between a first major surface and a second major surface opposite the first major surface, and the substrate includes a glass substrate or a ceramic substrate.

16. The method according to claim 15, further comprising forming at least one groove in a central portion of the substrate after chemically strengthening the substrate and before immersing the substrate to form a foldable substrate, a first groove being defined between a first central surface region and a first plane defined by the first major surface, the first central surface region being recessed from the first major surface by a first distance, the central portion including a central thickness defined between the first central surface region and a second central surface region opposite the first central surface region, the central portion including a central midpoint intermediate the first central surface region and the second central surface region, and the substrate including a midpoint intermediate the first major surface and the second major surface.

17. The method according to any one of claims 15 to 16, wherein the second molten salt bath comprises from about 0.02 wt% to about 0.08 wt% of the lithium salt.

18. The method according to any one of claims 15 to 17, wherein the first molten salt bath is lithium-free.

19. A method of forming a foldable device, which comprises: chemically strengthening a substrate in a molten salt bath maintained at a temperature of from about 380 °C to about 430 °C for a period of from about 3 minutes to about 2 hours, wherein the molten salt bath comprises from about 0.5 wt% to about 1.5 wt% of a lithium salt, the substrate includes a substrate thickness defined between a first major surface and a second major surface opposite the first major surface, and the substrate includes a glass substrate or a ceramic substrate.

20. The method according to claim 19, further comprising forming at least one groove in a central portion of the substrate before chemically strengthening the substrate to form a substrate, a first groove being defined between a first central surface region and a first plane defined by the first major surface, the first central surface region being recessed from the first major surface by a first distance, the central portion including a central thickness defined between the first central surface region and a second central surface region opposite the first central surface region, the central portion including a central midpoint intermediate the first central surface region and the second central surface region, and the substrate including a midpoint intermediate the first major surface and the second major surface.

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