Chemically strengthened glass and method of making the same

Abstract

This invention relates to chemically strengthened glass and a method for manufacturing the same. Specifically, this invention relates to a chemically strengthened glass having a thickness t [mm], wherein the stress value CS at a depth x [μm] from the glass surface, measured using a photoelastic stress meter, is... x In the distribution of [MPa], the above stress value CS x The value of the second derivative CS x In CS x Within the range ≥0, the following equation applies: 0 < CS x ≤0.050.

Description

TECHNICAL FIELD

[0001] The present application relates to a chemically strengthened glass and a manufacturing method thereof. BACKGROUND

[0002] Chemically strengthened glass is used as a protective glass of a portable terminal or the like. The chemically strengthened glass is obtained by bringing a glass into contact with a molten salt composition such as sodium nitrate, so that ion exchange occurs between alkali metal ions contained in the glass and alkali metal ions having a larger ionic radius contained in the molten salt composition, thereby forming a compressive stress layer in a surface portion of the glass. The strength of the chemically strengthened glass depends on a stress distribution represented by a compressive stress value (hereinafter, also referred to simply as CS) with a depth from the surface of the glass as a variable.

[0003] The protective glass of a portable terminal or the like sometimes breaks due to deformation at the time of falling or the like. In order to prevent such breakage, that is, breakage caused by bending, it is effective to increase the compressive stress of the surface of the glass. Therefore, recently, cases where a high surface compressive stress of 700 MPa or more is formed are increasing.

[0004] On the other hand, the protective glass of a portable terminal or the like sometimes breaks due to collision with a protrusion when the tip falls on asphalt or sand. In order to prevent such breakage, that is, breakage caused by impact, it is effective to increase the strength by forming a compressive stress layer to a deeper portion of the glass.

[0005] However, when a compressive stress layer is formed in the surface portion of a glass article, tensile stress (hereinafter, also referred to simply as CT) corresponding to the total amount of the compressive stress of the surface is necessarily generated in the central portion of the glass article. When the tensile stress value becomes too large, the glass article breaks violently at the time of breakage, and thus the fragments scatter. When CT is greater than a threshold value (hereinafter, also referred to simply as CT limit), the number of fragments at the time of damage explosively increases.

[0006] Therefore, for a chemically strengthened glass, on the one hand, the compressive stress of the surface is increased, and the compressive stress layer is formed to a deeper portion, and on the other hand, the total amount of the compressive stress of the surface layer is designed so as not to exceed the CT limit. For example, a chemically strengthened glass in which CT is controlled within a certain range is disclosed in Patent Literature 1.

[0007] PRIOR ART DOCUMENTS

[0008] PATENT LITERATURE

[0009] [Patent Literature 1] Japanese Patent Application Laid-Open No. 2017-523110 SUMMARY

[0010] PROBLEMS TO BE SOLVED BY THE INVENTION

[0011] As described above, there is a demand for a chemically strengthened glass having excellent drop strength capable of suppressing breakage caused by impact while forming a compressive stress layer without reaching the CT limit.

[0012] Therefore, an object of the present application is to provide a chemically strengthened glass having excellent drop strength capable of suppressing breakage caused by impact and a method for manufacturing the same.

[0013] Means for solving the problem

[0014] The present inventors have studied the above problem, and as a result, it has been found that a chemically strengthened glass in which a compressive stress in a deep layer effective for impact resistance at the time of dropping is controlled to a certain level or more while reducing a compressive stress in a surface of the glass can be obtained by adding lithium ions in a specific range of concentration to a molten salt composition used in a second chemical strengthening in two-step chemical strengthening of a lithium-containing glass. Furthermore, it has been found that the above problem can be solved by using this chemically strengthened glass, and thus the present application has been completed.

[0015] The present application relates to a chemically strengthened glass having a thickness t [mm] in which a value CS x [MPa] of a second-order differential of a stress value CS x [MPa] in a distribution of the stress value CS x satisfies the following equation in a range where CS x ≥ 0.

[0016] 0 < CS x ” ≤ 0.050.

[0017] The present application relates to a method for manufacturing a chemically strengthened glass, the method comprising the steps of:

[0018] performing a first ion exchange by immersing a lithium-containing glass in a first molten salt composition containing sodium ions and potassium ions, and

[0019] performing a second ion exchange by immersing the lithium-containing glass in a second molten salt composition containing potassium ions, wherein

[0020] a concentration of potassium nitrate in the first molten salt composition is greater than a concentration of sodium nitrate,

[0021] a concentration of potassium nitrate in the second molten salt composition is 85% by mass or more, and a mass ratio of sodium ions to lithium ions is 0 or more and 15 or less.

[0022] Effects of the Invention

[0023] The chemically strengthened glass of the present invention has a specific stress distribution, thereby avoiding reaching the CT limit and exhibiting excellent drop strength. Attached Figure Description

[0024] Figure 1A The stress value CS in this invention is shown. x An example of the distribution. Figure 1B The stress value CS in this invention is shown. x The first differential of the distribution CS x ', Second-order differential CS x An example of the absolute value of "".

[0025] Figure 2 The stress values ​​CS are shown after the first ion exchange and the second ion exchange in this invention. x An example of the distribution.

[0026] Figure 3 The stress values ​​CS after two-step chemical strengthening are shown in the present invention and the prior art. x An example of the distribution.

[0027] Figure 4 The stress value CS shown in the embodiment is illustrated. x The distribution of .

[0028] Figure 5 The stress value CS shown in the embodiment is illustrated. x The first differential of the distribution CS x ', Second-order differential CS x The absolute value of ".

[0029] Figure 6 The results of the drop strength test in the embodiment are shown.

[0030] Figure 7 The stress CS shown in the embodiment x Distribution and stress value CS x The first differential of the distribution CS x ', Second-order differential CS x The absolute value of ".

[0031] Figure 8A and Figure 8B The stress distribution after synthesis in the embodiment is shown. Figure 8A The stress distribution is shown up to a depth of 100 μm. Figure 8B The stress distribution is shown magnified from the surface to 3 μm. Detailed Implementation

[0032] <Stress Measurement Methods>

[0033] In recent years, a protective glass for a smartphone or the like has been mainstreamed with a glass subjected to two-step chemical strengthening in which lithium ions inside the glass are exchanged for sodium ions (Li-Na exchange), and then sodium ions inside the glass are further exchanged for potassium ions (Na-K exchange) in a surface layer portion of the glass.

[0034] In order to obtain a stress distribution of such a two-step chemically strengthened glass in a non-destructive manner, for example, a scattered light photoelastic stress meter (hereinafter also referred to as SLP), a film stress measurement (hereinafter also referred to as FSM), or the like can be used in combination.

[0035] A method using a scattered light photoelastic stress meter (SLP) can measure compressive stress in the glass inside from several tens of μm or more from the surface layer of the glass, which is generated by Li-Na exchange. On the other hand, a method using a film stress measurement (FSM) can measure compressive stress in the surface layer portion of the glass from several tens of μm or less from the surface of the glass, which is generated by Na-K exchange (for example, International Publication No. 2018 / 056121, International Publication No. 2017 / 115811). Therefore, sometimes a stress distribution obtained by combining information of SLP and FSM is used as a stress distribution in the surface layer and the inside of the glass in a two-step chemically strengthened glass.

[0036] On the other hand, in the present application, in order to investigate the influence of sodium ions introduced by ion exchange, a stress distribution measured by a scattered light photoelastic stress meter (SLP) is mainly used. Note that in the present specification, a stress distribution measured by a scattered light photoelastic stress meter (SLP) is referred to as "SLP stress distribution" or simply "stress distribution", and a stress distribution obtained by combining information of SLP and FSM is referred to as "post-combination distribution" or the like, so as to be distinguished. In addition, in the case of simply referring to a stress value CS x , a tensile stress value CT, a compressive stress layer depth DOL, or the like, a value in the SLP stress distribution is also indicated.

[0037] The scattered light photoelastic stress meter is a stress measuring device having: a polarized light phase difference variable member capable of changing a polarized light phase difference of a laser beam by one wavelength or more with respect to a wavelength of the laser beam; a shooting element that shoots scattered light emitted by causing the laser beam after the change in the polarized light phase difference to be incident to a strengthened glass a plurality of times at a predetermined time interval, and obtains a plurality of images; and an arithmetic unit that measures a periodic brightness change of the scattered light using the plurality of images, calculates a phase change of the brightness change, and calculates a stress distribution in a depth direction from a surface of the strengthened glass based on the phase change.

[0038] As a method for measuring stress distribution using a scattered light optoelastic stress gauge, the method described in International Publication No. 2018 / 056121 can be cited. Examples of scattered light optoelastic stress gauges include the SLP-1000 and SLP-2000 manufactured by Orihara Corporation. When these scattered light optoelastic stress gauges are combined with the accompanying software SlpIV (version 2019.01.10.001), high-precision stress measurement is possible.

[0039] <Stress Distribution>

[0040] The stress distribution in the chemically strengthened glass of the present invention is characterized in that, in the distribution of stress values ​​CSx [MPa] at a depth x [μm] measured from the glass surface using a scattered light photoelastic stress meter as described above, the stress value CS... x The value of the second derivative CS x In CS x Within the range ≥0, 0 < CS x ≤0.050.

[0041] exist Figure 1A An example of stress distribution in the chemically strengthened glass of the present invention is shown in the figure. Figure 1B It shows Figure 1A The graph shows the first and second derivatives of the stress distribution. (See figure.) Figure 1A As shown, the stress value CS of the present invention x It varies depending on the depth x measured from the glass surface. Figure 1B The graph of the first derivative is obtained by applying CS at x. x CS obtained by differentiation x The image is shown in 'CS'. It should be noted that, for ease of observation, CS is used. x The absolute value of ' was plotted.

[0042] The graph of the second derivative is at x with respect to CS. x CS obtained by differentiation x The image is a diagram. (Example) Figure 1B As shown, in this invention, in CS x In the range of ≥0, CS x "Satisfies 0 < CS" x "≤0.050. That is, it has the following characteristics: the stress distribution in the compressive stress layer does not have an inflection point, the amount of stress change is small, and it is linear."

[0043] This shape, as explained below, increases the stress value CS at a depth of 50 μm compared to conventional chemically strengthened glass that has undergone two-step strengthening. 50This improves resistance to fracture from impacts during drop and reduces the total compressive stress in the compressive stress layer. Therefore, it can suppress the stress value of the tensile stress layer corresponding to the total compressive stress, thus avoiding reaching the CT limit.

[0044] Here, the stress value CS at a depth of 50 μm is... 50 This value helps improve the resistance to breakage caused by impact during a drop. When glass objects fall onto asphalt roads or sand, cracks are generated due to the impact with protrusions such as sand. The length of the resulting crack varies depending on the size of the sand that the glass object collides with, but it is more effective when the compressive stress value CS, measured using a photoelastic stress meter at a depth of 50 μm from the glass surface, is increased. 50 When the value of [MPa] is set, for example, a stress distribution with a large compressive stress value is formed near a depth of 50 μm, which can prevent fracture from breaking due to collision with relatively large protrusions.

[0045] Furthermore, the CT limit in this application refers to the maximum tensile stress value CT that marks the boundary between a number of fragments decreasing from less than 10 to more than 10 in the fragment count test described later in the embodiments. It should be noted that the CT limit is an experimentally determined value based on the type of glass and is negatively correlated with the glass thickness.

[0046] The stress distribution in the chemically strengthened glass of the present invention is formed, for example, through a two-step chemical strengthening process. Figure 2 The diagram illustrates an example of the stress distribution after the first step of chemical strengthening (Example 12) and the stress distribution after the second step of chemical strengthening (Example 3) of the chemically strengthened glass of the present invention. In the first step of chemical strengthening, by immersing the glass in a molten salt containing potassium and sodium ions, a "Li-Na exchange" (exchanging lithium ions in the glass with sodium ions in the molten salt) and a "Na-K exchange" (exchanging sodium ions in the glass with potassium ions in the molten salt) mainly occur, and a compressive stress greater than the CT limit is imparted.

[0047] In the subsequent second-step chemical strengthening, a small amount of lithium ions are added to a molten salt containing potassium ions. This allows for the exchange of sodium in the glass with potassium in the molten salt (Na-K exchange) and lithium in the glass with lithium in the molten salt (Li-Na exchange), thereby reducing the sodium content in the glass surface. This moderately relaxes the stress generated in the surface layer, maintaining the stress value CS at a depth of 50 μm. 50 At the same time, avoid reaching the limits of CT scans.

[0048] exist Figure 3 An example of stress distribution in the chemically strengthened glass of the present invention (Example 3) is shown, compared with a conventional two-step strengthening product (Example 9).Figure 3 As shown, in this invention, compared with conventional two-step enhancement products, the CS can be improved. 50 .

[0049] As described above, for the chemically strengthened glass of the present invention, the stress value CS is used to determine the stress value. x The value of the second derivative CS x In CS x Within the range ≥0, 0 < CS x "≤0.050, the stress distribution has no inflection point, the change in stress is small and linear, and it can increase the stress value CS at a depth of 50μm." 50 Furthermore, it can improve resistance to fracture from impacts during drop while avoiding reaching the CT limit. The details of this invention will be described below.

[0050] <Implementation Methods of Chemically Strengthened Glass>

[0051] (Stress distribution in the implementation method)

[0052] As described above, the chemically strengthened glass of the present invention is characterized by a stress value CS at a depth x [μm] measured from the glass surface using a photoelastic stress meter. x In the distribution of [MPa], the stress value CS x The value of the second derivative CS x In CS x Within the range ≥0, 0 < CS x ≤0.050. Here, CS x "When the value is preferably 0.045 or less, more preferably 0.040 or less, and even more preferably 0.035 or less, it becomes more linear and can effectively increase the stress value CS at a depth of 50 μm." 50 On the other hand, CS x "Greater than 0, typically above 0.005."

[0053] Stress value CS x The value of the first derivative CS x The first derivative is -5.3 or higher, more preferably -5.0 or higher, and even more preferably -4.5 or higher. When the value of the first derivative is CS... x 'When within the above range, CS x The change in ' decreases, and the stress value CS x The distribution is linear. Furthermore, for the same CS0, through CS... x 'Large, able to maintain high CS' 50 The effect. CS x Typically below -0.5.

[0054] Note that, as a differential method of stress distribution, in the present application, as shown in the following formula, the rate of change of CS x at a change amount of x of 0.5 μm is used as the value of CS x , the rate of change of CS x at a change amount of x of 0.5 μm is used as the value of CS x , and the rate of change of CS x at a change amount of x of 0.5 μm is used as the value of CS

[0055] CS x ' = (CS x+0.5 - CS x ) / 0.5

[0056] CS x " = (CS x+0.5 ' - CS x ' ) / 0.5

[0057] For the value of CS 50 [MPa] of the chemically strengthened glass of the present application, when the maximum tensile stress value is set to CT and the depth of compressive stress layer is set to DOL at a thickness of t [mm], the value of CS 50 / (CT x (t - 2 x DOL)) / t is preferably 4.90 or greater, more preferably 5.0 or greater, further preferably 5.1 or greater, more further preferably 5.5 or greater, and most preferably 6.0 or greater.

[0058] CS 50 is larger in accordance with the maximum tensile stress value CT, on the other hand, the physical phenomenon that the thinner the thickness of the sheet, the higher the maximum tensile stress value CT occurs, and thus in order to generalize by removing the influence of the thickness, as described above, the value of CS 50 / (CT x (t - 2 x DOL)) / t is used for comparison. For the chemically strengthened glass of the present application, by the value of CS 50 / (CT x (t - 2 x DOL)) / t being 4.90 or greater, the drop strength can be improved.

[0059] The chemically strengthened glass of the present application preferably has a thickness of t [mm] and a value of the maximum tensile stress value CT2 [MPa] measured by a scattered light photoelastic stress meter of (-120t + 164) or less. By having a thickness of t [mm] and a value of CT2 [MPa] of (-120t + 164) or less, it becomes below the CT limit, and it is possible to suppress the occurrence of violent breakage.

[0060] On the other hand, the value of the maximum tensile stress CT2 [MPa] measured by the scattered light photoelastic stress meter is preferably (-120t + 150) or greater. When the value of CT2 [MPa] is (-120t + 150) or greater, a compressive stress sufficient to improve the strength can be introduced from the compressive stress layer.

[0061] The value of the compressive stress CS0 at the glass surface of the chemically strengthened glass of the present application measured by the scattered light photoelastic stress meter is preferably 400 MPa or less, more preferably 350 MPa or less, and further preferably 300 MPa or less. By making CS0 400 MPa or less, the stress sum of the compressive stress layer can be reduced, the maximum tensile stress CT can be suppressed, and the CT limit can be avoided.

[0062] In the case where the thickness is t [mm], the value of the compressive stress layer depth DOL [μm] of the chemically strengthened glass of the present application is preferably greater than (100t + 25), more preferably (100t + 35) or greater, and further preferably (100t + 40) or greater. By making the thickness t [mm] and the compressive stress layer depth DOL greater than (100t + 25), the compressive stress is introduced to a deep portion in the thickness direction of the glass, which is advantageous in preventing breakage caused by impact. In addition, in order to make the balance of the total amount of the compressive stress and the tensile stress uniform in the entire thickness direction of the glass, in the case where the thickness is t [mm], the value of DOL [μm] is preferably (100t + 70) or less, more preferably (100t + 60) or less, and further preferably (100t + 55) or less.

[0063] (Basic composition of chemically strengthened glass)

[0064] The basic composition of the chemically strengthened glass of the present application is the same as the composition of the glass before chemical strengthening described later. The basic composition refers to the composition of a region not affected by ion exchange, and is the composition of a region deeper than the compressive stress layer depth DOL of the chemically strengthened glass except in the case where an extreme ion exchange treatment is performed.

[0065] <Manufacturing method of chemically strengthened glass>

[0066] The chemical strengthening treatment is a treatment in which a glass is brought into contact with a metal salt (e.g., potassium nitrate) containing a metal ion having a large ionic radius (typically, sodium ion or potassium ion) by immersing in a molten liquid of the metal salt, etc., so that a metal ion having a small ionic radius (typically, lithium ion or sodium ion) in the glass is replaced with a metal ion having a large ionic radius (typically, sodium ion or potassium ion for lithium ion, and potassium ion for sodium ion) in the metal salt.

[0067] The manufacturing method of the chemically strengthened glass of the present application has the following features.

[0068] (Step 1) The first ion exchange is performed by immersing the lithium-containing glass in a first molten salt composition in which the concentration of potassium nitrate is higher than the concentration of sodium nitrate.

[0069] (Step 2) The second ion exchange is performed by immersing the above-mentioned lithium-containing glass in a second molten salt composition in which the concentration of potassium nitrate is 85 mass% or more and the mass ratio of sodium ion to lithium ion is 0 to 15. Hereinafter, the details of each step will be described.

[0070] (Step 1) First ion exchange

[0071] In the first step in the method for producing a chemically strengthened glass according to the present application, the first ion exchange is performed by immersing the lithium-containing glass in a first molten salt composition containing sodium ions and potassium ions. The details of the lithium-containing glass used for chemical strengthening will be described later.

[0072] In Step 1, by "Li-Na exchange" in which lithium ions in the glass are exchanged with sodium ions in the molten salt, sodium is introduced into the deep layer of the glass, and a deep compressive stress layer can be formed. Further, in Step 1, since the concentration of potassium nitrate in the first molten salt composition is higher than the concentration of sodium nitrate, "Na-K exchange" in which sodium ions in the glass are exchanged with potassium ions in the molten salt also occurs at the same time, and potassium is introduced into the surface layer of the glass. Thus, in the following second ion exchange, it is possible to significantly suppress the phenomenon of excessive reduction of sodium in the surface layer of the glass due to "Na-Li exchange" in which sodium in the glass is exchanged with lithium in the molten salt, and CS 50 decrease.

[0073] Here, the "molten salt composition" refers to a composition containing a molten salt. As the molten salt contained in the molten salt composition, for example, nitrates, sulfates, carbonates, chlorides, and the like can be listed. As the nitrates, for example, lithium nitrate, sodium nitrate, potassium nitrate, cesium nitrate, rubidium nitrate, silver nitrate, and the like can be listed. As the sulfates, for example, lithium sulfate, sodium sulfate, potassium sulfate, cesium sulfate, rubidium sulfate, silver sulfate, and the like can be listed. As the chlorides, for example, lithium chloride, sodium chloride, potassium chloride, cesium chloride, rubidium chloride, silver chloride, and the like can be listed. These molten salts can be used alone or in combination of a plurality of kinds.

[0074] As the molten salt composition, a molten salt composition in which nitrates are the main component is preferred, and a molten salt composition in which sodium nitrate and potassium nitrate are the main component is more preferred. Here, the "main component" refers to a content of 80 mass% or more in the molten salt composition, and preferably the total amount of sodium nitrate and potassium nitrate is 90 mass% or more, and more preferably 100 mass%.

[0075] The content of potassium nitrate in the first molten salt composition in the first ion exchange can be exemplified as being greater than 50 mass%. In the first embodiment, the content of potassium nitrate in the first molten salt composition is more preferably 55 mass% or greater, 60 mass% or greater, in this order. When the content of potassium nitrate is within the above range, in the subsequent second ion exchange, it is possible to significantly suppress the phenomenon of excessive reduction of sodium in the glass surface layer due to "Na-Li exchange", CS 50 On the other hand, the content of potassium nitrate is preferably 80% or less, more preferably 70% or less. When the content of potassium nitrate is within the above range, it is possible to sufficiently introduce sodium ions into the glass interior.

[0076] On the other hand, the content of sodium nitrate in the first molten salt composition is less than 50 mass%. In the first embodiment, the content of sodium nitrate in the first molten salt composition is more preferably 45 mass% or less, 40 mass% or less, in this order. On the other hand, the content of sodium nitrate is preferably 20% or more, more preferably 30% or more. When the content of sodium nitrate is within the above range, it is possible to sufficiently introduce sodium ions into the glass interior.

[0077] In the manufacturing method of the present application, it is preferable that the maximum tensile stress value CT1 of the stress distribution formed in the glass interior by the first ion exchange be greater than the CT limit. By making the maximum tensile stress value CT1 after the first ion exchange greater than the CT limit, it is possible to sufficiently introduce compressive stress by the first ion exchange, and even after the stress value of the glass surface layer is reduced in the subsequent second ion exchange process, it is possible to maintain a high CS 50 .

[0078] It is more preferable to perform ion exchange in such a manner that the maximum tensile stress value CT1 of the chemically strengthened glass after the first ion exchange is greater than (-120t + 164). By making the maximum tensile stress value CT1 greater than (-120t + 164), it is possible to introduce compressive stress in a manner greater than the CT limit.

[0079] In the process 1, it is preferable to immerse the lithium-containing glass in the first molten salt composition, which is preferably 380°C or higher. When the temperature of the first molten salt composition is 380°C or higher, ion exchange is easily performed, and it is possible to introduce compressive stress up to a range greater than the CT limit. The temperature of the first molten salt composition is more preferably 421°C or higher, and further preferably 430°C or higher. In addition, from the viewpoint of the danger due to evaporation, and the change in the composition of the molten salt, the temperature of the first molten salt composition is generally 450°C or lower.

[0080] In the process 1, when the time for which the lithium-containing glass is immersed in the first molten salt composition is 0.5 hours or more, the surface compressive stress becomes large, and thus is preferable. The immersion time is more preferably 1 hour or more. When the immersion time is too long, not only the productivity decreases, but also the compressive stress sometimes decreases due to a relaxation phenomenon. Therefore, the immersion time is usually 8 hours or less.

[0081] (Step 2) Second ion exchange

[0082] The process 2 is a process of performing second ion exchange by immersing the glass subjected to the first ion exchange treatment in the process 1 in a second molten salt composition containing a small amount of lithium ions in addition to potassium nitrate as the main component. In the second ion exchange, "Na-K exchange" in which sodium ions in the glass are exchanged with potassium ions occurs, and thus potassium ions are introduced in a region of several tens of μm in the surface layer of the glass. At the same time, the sodium ions in the surface layer of the glass decrease due to "Na-Li exchange", and thus the compressive stress caused by sodium relaxes. In the second ion exchange, the compressive stress of the chemically strengthened glass is relaxed while maintaining the CS 50 in a state where the CT limit is exceeded, and is adjusted to be below the CT limit. Note that the effect of the stress of the surface layer of the glass in which potassium ions are introduced is not reflected in the stress profile measured using SLP. Therefore, by using the stress profile measured using SLP, the decrease in the tensile stress caused by the decrease in sodium ions can be confirmed.

[0083] The concentration of potassium nitrate in the second molten salt composition is 85% by mass or more, more preferably 90% by mass or more, and further preferably 95% by mass or more. There is no particular limitation on the upper limit, but it is usually 99.9% by mass or less. Furthermore, the sodium ion / lithium ion ratio in the second molten salt composition is 0 or more and 15 or less in terms of mass ratio. The sodium ion / lithium ion ratio in the above range means that lithium ions are intentionally added to the molten salt composition. For example, in the case where sodium nitrate is added to the second molten salt composition, lithium ions in the glass are exchanged with sodium ions in the second molten salt (Li-Na exchange), and thus lithium ions are mixed into the second molten salt. The amount of lithium ions mixed at this time increases in accordance with the amount of sodium ions in the molten salt. On the other hand, the exchange (Na-Li exchange) of sodium ions in the glass with lithium ions mixed into the molten salt is suppressed by sodium ions in the molten salt. Therefore, in the manufacturing method of the present application, by intentionally adding lithium ions in an amount of at least the amount mixed due to the addition of sodium nitrate, the exchange (Na-Li exchange) of sodium ions in the glass with lithium ions in the molten salt can be effectively caused to occur, and the compressive stress generated in the surface layer of the glass in the process (1) can be weakened.

[0084] The second molten salt composition preferably contains 0.1 mass% or more and 10 mass% or less of lithium nitrate. By the second molten salt composition containing lithium nitrate in the above range, in the process (1), exchange of sodium ions introduced into the vicinity of the surface of the glass with lithium ions in the second molten salt composition occurs in parallel with exchange of the sodium ions with potassium ions in the second molten salt composition, and thus it is possible to reduce stress on the surface of the glass. The content of lithium nitrate in the second molten salt composition is more preferably 0.3 mass% or more and 5 mass% or less, and further preferably 0.5 mass% or more and 2.5 mass% or less.

[0085] The second molten salt composition can contain sodium nitrate. In the case of containing sodium nitrate, the concentration of sodium nitrate is preferably greater than 0.1 mass%, and more preferably 0.5 mass% or more. When the sodium nitrate is in the above range, the effect of improving CS 50 is improved. By the presence of sodium ions in the second molten salt, Li-Na exchange also occurs in the second ion exchange, and thus CS 50 is improved. Furthermore, when the sodium nitrate is in the above range, it is possible to extend the time during which the effect of the present application is exerted without replacing the second molten salt, and it is possible to increase the glass processing amount. When the concentration of sodium nitrate in the second molten salt composition is preferably 5 mass% or less, more preferably 3 mass% or less, further preferably 2 mass% or less, and most preferably 1 mass% or less, it is easy to suppress the CT2 value within the CT limit.

[0086] The second molten salt composition can further contain an additive other than a nitrate salt. As the additive, for example, silicic acid, a specific inorganic salt, or the like can be listed. By the second molten salt composition having an additive, it is possible to increase CS0 in the post-combination distribution of FSM and SLP. Details will be described below.

[0087] The second molten salt composition can contain silicic acid as an additive. Silicic acid refers to a compound containing silicon, hydrogen, and oxygen represented by the chemical formula nSiO2-xH2O. Here, n and x are natural numbers. As one of such silicic acids, for example, metasilicic acid (SiO2-H2O), disilicic acid (2SiO2-H2O), orthosilicic acid (SiO2-2H2O), pyrosilicic acid (2SiO2-3H2O), silica gel [SiO2-mH2O (m is a real number of 0.1 to 1)], or the like can be listed.

[0088] By containing silicic acid, since the silicic acid adsorbs lithium ions and potassium ions easily enter the glass, it is possible to increase the stress in the surface layer of the post-synthesis distribution of FSM and SLP in a state in which CT is suppressed. Lithium ions react with sodium ions through "Na-Li exchange", so it is possible to suppress the progress of "Na-K exchange". Therefore, in order to promote "Na-K exchange", silicic acid can be added. The addition amount of silicic acid is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and most preferably 0.5% by mass or more. In addition, the addition amount of silicic acid is preferably 3% by mass or less, more preferably 2% by mass or less, and most preferably 1% by mass or less. When the addition amount of silicic acid is within the above range, it is possible to significantly reduce the surface stress in the SLP stress distribution through "Na-Li exchange". That is, it is possible to achieve the effect of the increase in compressive stress in the post-synthesis distribution at the surface layer of μm due to the introduction of potassium and the reduction in stress in the SLP distribution up to DOL due to the reduction in sodium.

[0089] Silicic acid is preferably silica gel [SiO2-mH2O (m is a real number of 0.1 to 1)]. Silica gel has the advantages that it easily settles in molten salt, is easily charged and recovered, because the secondary particles are relatively large. In addition, there is no concern about dust flying, and it is possible to ensure the safety of the operators. Furthermore, since it is a porous body, it is easy to supply molten salt to the surface of the primary particles, so the reactivity is excellent, and the effect of adsorbing lithium ions is great.

[0090] The second molten salt can contain a specific inorganic salt as an additive (hereinafter referred to as a flux). As the flux, carbonates, bicarbonates, phosphates, sulfates, hydroxides, chlorides are preferred, preferably contains at least one salt selected from the group consisting of K2CO3, Na2CO3, KHCO3, NaHCO3, K3PO4, Na3PO4, K2SO4, Na2SO4, KOH, NaOH, KCl, NaCl, particularly more preferably at least one salt selected from the group consisting of K2CO3, Na2CO3, and further preferably K2CO3.

[0091] The lithium ions in the second molten salt can weaken the compressive stress in the glass caused by sodium through "Na-Li" exchange, and on the other hand, can hinder "Na-K exchange". When "Na-K exchange" is hindered, it is difficult to obtain the effect of introducing potassium into the glass surface layer μm to increase CS0 in the post synthesis distribution. Here, in the second molten salt, the interaction with the anion produced from the flux increases in the order of lithium ions, sodium ions, potassium ions. By containing the flux in the second molten salt, the anion attracts lithium ions, and thus the hindrance of "Na-K exchange" caused by lithium ions can be suppressed, and it is easy to introduce potassium into the glass. On the other hand, since the anion does not suppress "Na-Li exchange", the stress in the glass caused by sodium can be weakened. Thus, it is possible to obtain the effect of increasing CS0 in the post synthesis distribution while the effect of weakening the compressive stress in the range from the glass surface layer to DOL in the SLP distribution is maintained.

[0092] When the flux is preferably 0.1% by weight or more, it is easy to obtain the effect of increasing CS0. On the other hand, in order to suppress the change in the properties of the glass surface, the flux is preferably 2% by weight or less, and more preferably 1% by weight or less.

[0093] The second molten salt composition preferably contains any one of silicic acid or a carbonate. More preferably, when both silicic acid and a carbonate are contained, it is particularly easy to obtain the effect of increasing CS0.

[0094] In step 2, it is preferable to immerse the lithium-containing glass in the second molten salt composition, which is preferably 380°C or higher. When the temperature of the second molten salt composition is 380°C or higher, ion exchange is easily performed. In addition, from the viewpoint of the danger caused by evaporation, the change in the composition of the molten salt, the temperature of the second molten salt composition is usually 450°C or lower, and from the viewpoint of preventing excessive reduction of stress caused by "Na-Li exchange", it is more preferably 400°C or lower.

[0095] In step 2, when the time for which the lithium-containing glass is immersed in the second molten salt composition is 0.1 hour or more, exchange of sodium ions introduced into the vicinity of the glass surface in step (1) with lithium ions in the second molten salt composition sufficiently occurs, and it is easy to weaken the stress of the glass surface. The immersion time is more preferably 0.3 hour or more. From the viewpoint of preventing excessive reduction of stress caused by "Na-Li exchange", the immersion time is preferably 3 hours or less.

[0096] The time t2 (minutes) for which the above-described lithium-containing glass is immersed in the above-described second molten salt composition with respect to the temperature T (°C) of the second molten salt composition preferably satisfies the following formula. Thus, it is possible to moderately weaken the stress of the glass surface.

[0097] -0.35T + 173 < t2 < -1.4T + 650

[0098] t2 (min) is preferably greater than (-0.38T + 173), more preferably (-0.36T + 167) or more, and further preferably (-0.35T + 167) or more. In addition, t2 (min) is preferably less than (-1.4T + 650), more preferably (-1.3T + 600) or less, and further preferably (-1.2T + 550) or less.

[0099] In the process 2, the temperature of the second molten salt composition for impregnating the lithium-containing glass and the impregnation time are preferably adjusted. Specifically, for example, in the case where the temperature of the second molten salt composition for impregnating the lithium-containing glass is 380°C, the impregnation time is preferably 10 minutes or more and 120 minutes or less. In the case where the temperature of the second molten salt composition for impregnating the lithium-containing glass is 390°C, the impregnation time is preferably 7 minutes or more and 100 minutes or less. In the case where the temperature of the second molten salt composition for impregnating the lithium-containing glass is 400°C, the impregnation time is preferably 5 minutes or more and 60 minutes or less. In the case where the temperature of the second molten salt composition for impregnating the lithium-containing glass is greater than 400°C, the impregnation time is preferably 60 minutes or less.

[0100] As described above, the second ion exchange is preferably performed in such a manner that the maximum tensile stress value CT2 of the chemically strengthened glass after the second ion exchange is CT limit or less.

[0101] Further, the chemical strengthening is preferably performed in such a manner that the maximum tensile stress value CT2 [MPa] of the chemically strengthened glass after the second ion exchange is a value of 50% to 93% of the maximum tensile stress value CT1 [MPa] of the chemically strengthened glass after the first ion exchange. More preferably, it is 60% or more, and further preferably 75% or more. On the other hand, it is preferably 90% or less.

[0102] More preferably, the chemical strengthening is performed in such a manner that the maximum tensile stress value CT2 [MPa] after the second ion exchange is (-120t + 164) or less.

[0103] (Third and subsequent ion exchange processes)

[0104] In the manufacturing method of the present application, the ion exchange treatment can be further performed multiple times before and after the process 2. For example, in the case where the third ion exchange is performed after the process 2, the chemical strengthening is preferably performed at 380°C to 420°C for 5 minutes to 30 minutes using a molten salt composition containing 95% by mass or more of potassium nitrate. The concentration of potassium nitrate in this case is preferably 95% by mass or more, more preferably 97% by mass or more, and further preferably 99% by mass or more. By such a third ion exchange, an effect of increasing the compressive stress of the outermost layer in the distribution after synthesis can be obtained.

[0105] (lithium-containing glass)

[0106] In the method for producing a chemically strengthened glass of the present application, a lithium-containing glass is subjected to chemical strengthening. The glass for chemical strengthening in the present application is preferably a lithium-aluminum-silicate glass. The glass for chemical strengthening in the present application can be a glass-ceramic or an amorphous glass.

[0107] As the composition of the lithium-containing glass, more specifically, a lithium-aluminum-silicate glass is preferred,

[0108] The lithium-aluminum-silicate glass contains, in terms of mol% on an oxide basis:

[0109] 52% to 75% of SiO2,

[0110] 8% to 20% of Al2O3,

[0111] 5% to 16% of Li2O.

[0112] Note that, in the present specification, "~" indicating a numerical range is used in the meaning that the numerical values recited before and after it are included as lower limit values and upper limit values, unless otherwise specified, and the same is used in the present specification below.

[0113] In the present specification, unless otherwise specified, the glass composition is indicated in terms of mol% on an oxide basis, and mol% is simply noted as "%". In addition, in the glass composition of the present specification, "substantially not containing" means below the level of impurities contained in raw materials and the like, i.e., not intentionally contained. Specifically, for example, less than 0.1%.

[0114] Hereinafter, the preferred glass composition is described.

[0115] SiO2is a component constituting the glass framework. In addition, SiO2is a component that improves chemical durability, and is a component that reduces crack generation when damage occurs on the surface of the glass.

[0116] The content of SiO2is preferably 55% or more, further preferably 60% or more, and particularly preferably 65% or more. On the other hand, from the viewpoint of improving meltability, the content of SiO2is preferably 75% or less, more preferably 72% or less, further preferably 70% or less, and particularly preferably 68% or less.

[0117] Al2O3is an effective component from the viewpoint of improving ion exchange performance at the time of chemical strengthening and increasing surface compressive stress after strengthening.

[0118] The content of Al2O3 is preferably 8% or more, more preferably 9% or more, further preferably 10% or more, particularly preferably 11% or more, and typically 12% or more. On the other hand, when the content of Al2O3 is too much, crystals easily grow in melting, and the yield is easily reduced due to devitrification defects. In addition, the viscosity of the glass increases, and the meltability decreases. The content of Al2O3 is preferably 20% or less, more preferably 19% or less, and further preferably 18% or less.

[0119] Both SiO2 and Al2O3 are components that stabilize the structure of the glass, and in order to reduce the brittleness, the total content of SiO2 and Al2O3 is preferably 65% or more, more preferably 70% or more, and further preferably 75% or more.

[0120] Li2O is a component that forms surface compressive stress by ion exchange, and is a component that improves the meltability of the glass. The glass containing Li2O is chemically strengthened, and a stress distribution having a large surface compressive stress and a large compressive stress layer can be obtained by a method in which lithium ions on the surface of the glass are ion-exchanged with sodium ions, and further the sodium ions are ion-exchanged with potassium ions. From the viewpoint of easily obtaining a preferred stress distribution, the content of Li2O is preferably 5% or more, more preferably 7% or more, further preferably 9% or more, particularly preferably 10% or more, and most preferably 11% or more.

[0121] On the other hand, when the content of Li2O is too much, the crystal growth rate during glass molding becomes fast, and the problem of the yield reduction due to devitrification defects becomes large. The content of Li2O is preferably 20% or less, more preferably 16% or less, further preferably 14% or less, and particularly preferably 12% or less.

[0122] Both Na2O and K2O are not essential, but are components that improve the meltability of the glass and reduce the crystal growth rate of the glass, and in order to improve the ion exchange performance, it is preferable to contain Na2O and K2O in a total of 2% or more. In addition, the total is preferably 10% or less, preferably 9% or less, more preferably 8% or less, further preferably 7% or less, and particularly preferably 5% or less.

[0123] Na2O is a component that forms a surface compressive stress layer in chemical strengthening treatment using a potassium salt, and is a component that can improve the meltability of the glass. In order to obtain this effect, the content of Na2O is preferably 1% or more, more preferably 2% or more, further preferably 3% or more, and particularly preferably 4% or more. On the other hand, from the viewpoint of avoiding the reduction of surface compressive stress (CS) in strengthening treatment using a sodium salt and achieving a linear distribution without a knee, it is preferably 8% or less, more preferably 7% or less, further preferably 6% or less, and particularly preferably 5% or less. 50 high, achieving a linear distribution without a knee, it is preferably 8% or less, more preferably 7% or less, further preferably 6% or less, and particularly preferably 5% or less.

[0124] K2O can be contained for the purpose of improving ion exchange performance, etc. In the case of containing K2O, the content of K2O is preferably 0.1% or more, more preferably 0.15% or more, and particularly preferably 0.2% or more. In order to further prevent devitrification, the content of K2O is preferably 0.5% or more, and more preferably 1.2% or more. On the other hand, since brittleness and surface stress reduction due to reverse exchange at the time of strengthening are caused by the inclusion of a large amount of K, the content of K2O is preferably 5% or less, and more preferably 3% or less.

[0125] MgO can be contained for the purpose of reducing viscosity at the time of melting, etc. The content of MgO is preferably 1% or more, more preferably 2% or more, and further preferably 3% or more. On the other hand, when the content of MgO is too much, it is difficult to increase the compressive stress value at the time of chemical strengthening treatment. The content of MgO is preferably 15% or less, more preferably 10% or less, further preferably 8% or less, and particularly preferably 6% or less.

[0126] ZrO2 can not be contained, but from the viewpoint of increasing the surface compressive stress of the chemically strengthened glass, it is preferable to contain ZrO2. The content of ZrO2 is preferably 0.1% or more, more preferably 0.15% or more, further preferably 0.2% or more, particularly preferably 0.25% or more, and typically 0.3% or more. On the other hand, when the content of ZrO2 is too much, devitrification defects are easily generated, and it is difficult to increase the compressive stress value at the time of chemical strengthening treatment. The content of ZrO2 is preferably 2% or less, more preferably 1.5% or less, further preferably 1% or less, and particularly preferably 0.8% or less.

[0127] The content of Y2O3 is preferably 0.1% or more, more preferably 0.2% or more, further preferably 0.5% or more, and particularly preferably 1% or more. On the other hand, when the content of Y2O3 is too much, it is difficult to increase the compressive stress value at the time of chemical strengthening treatment. The content of Y2O3 is preferably 5% or less, more preferably 3% or less, further preferably 2% or less, and particularly preferably 1.5% or less.

[0128] The composition of the lithium-containing glass for chemical strengthening according to the present application has the composition described above. The glass raw materials are appropriately adjusted to obtain a glass having the above-mentioned composition, and are heated and melted in a glass melting furnace. Then, the glass is homogenized by bubbling, stirring, addition of a fining agent, etc., and is formed into a glass sheet of a predetermined thickness, and is slowly cooled. Alternatively, it can also be formed into a sheet shape by a method of forming into a block shape and slowly cooling, and then cutting.

[0129] As a method of forming into a plate shape, for example, there can be listed a float method, a press method, a fusion method, and a down-draw method. In particular, in the case of manufacturing a large glass plate, the float method is preferred. In addition, a continuous forming method other than the float method, such as the fusion method and the down-draw method, is also preferred.

[0130] In addition, the lithium-containing glass can be a glass-ceramic. In the case of a glass-ceramic, a glass-ceramic containing one or more kinds of crystals selected from the group consisting of lithium silicate crystals, lithium aluminosilicate crystals, and lithium phosphate crystals is preferred. As the lithium silicate crystals, lithium metasilicate crystals, lithium disilicate crystals, and the like are preferred. As the lithium phosphate crystals, lithium orthophosphate crystals and the like are preferred. As the lithium aluminosilicate crystals, β-spodumene crystals, petalite crystals, and the like are preferred.

[0131] In order to improve the mechanical strength, the crystallization rate of the glass-ceramic is preferably 10% or more, more preferably 15% or more, further preferably 20% or more, and particularly preferably 25% or more. In order to improve the transparency, the crystallization rate of the glass-ceramic is preferably 70% or less, more preferably 60% or less, and particularly preferably 50% or less. When the crystallization rate is low, it is also excellent in terms of being easy to bend and form by heating and the like. The crystallization rate can be calculated by the Rietveld method from the X-ray diffraction intensity. The Rietveld method is described in the "Crystalline Analysis Handbook" edited by the Crystalline Society of Japan (Korin Publishing, published in 1999, pp. 492 to 499).

[0132] In order to improve the transparency, the average particle diameter of the precipitated crystals of the glass-ceramic is preferably 300 nm or less, more preferably 200 nm or less, further preferably 150 nm or less, and particularly preferably 100 nm or less. The average particle diameter of the precipitated crystals can be calculated from a transmission electron microscope (TEM) image. In addition, it can be estimated from a scanning electron microscope (SEM) image.

[0133] [Example]

[0134] Hereinafter, the present application will be described by way of examples, but the present application is not limited thereto.

[0135] The glass raw material was prepared so as to obtain the following composition expressed in terms of molar percentage on an oxide basis, and the glass raw material was weighed so as to reach 400 g in terms of glass. Subsequently, the mixed raw material was put in a platinum crucible and was put into an electric furnace at 1500°C to 1700°C, was melted for about 3 hours, and was subjected to deaeration and homogenization.

[0136] Glass composition: SiO2 68.9%, Al2O3 12.4%, Y2O3 1.3%, ZrO2 0.3%, Li2O 10.8%, Na2O 4.8%, K2O 1.2%, other components 0.3%.

[0137] The obtained molten glass was cast into a mold, held at a temperature of about 50°C higher than the glass transition temperature for 1 hour, and then cooled to room temperature at a rate of 0.5°C / min, thereby obtaining a glass block. The obtained glass block was subjected to cutting, grinding, and finally mirror polishing on both surfaces, thereby obtaining a glass sheet having a thickness of 600 μm.

[0138] The chemical strengthening treatment described in Tables 1 to 3 was performed using the obtained glass sheet, thereby producing the chemically strengthened glasses of Examples 1 to 18 below. The chemical strengthening treatment was performed using the first molten salt composition shown in the first ion exchange column of Tables 1 to 3 at the temperature shown in the temperature column of the first ion exchange for the time shown in the time column of the first ion exchange. Then, the second molten salt composition shown in the second ion exchange column was used at the temperature shown in the temperature column of the second ion exchange for the time shown in the time column of the second ion exchange, thereby obtaining a chemically strengthened glass.

[0139] The results are shown in Tables 1 to 3. Examples 1 to 6 and 17 to 19 are examples, and Examples 7 to 16 are comparative examples. Note that, in Table 3, the amount of the additive in the second molten salt composition is expressed as a proportion with respect to the total amount of the second molten salt composition excluding the additive, which is taken as 100%.

[0140] The obtained chemically strengthened glass was evaluated by the following methods.

[0141] [Measurement of stress using a scattered light photoelastic stress meter]

[0142] The stress was measured by the method described in International Publication No. 2018 / 056121 using a scattered light photoelastic stress meter (manufactured by Oryuka Mfg., SLP-1000). In addition, the stress was measured using the attached software [SlpIV (2019.01.10.001 version)] of the scattered light photoelastic stress meter (manufactured by Oryuka Mfg., SLP-1000). In addition, from the obtained stress distribution, the compressive stress value CS0, the maximum tensile stress value CT, the compressive stress layer depth DOL, the value CS 50 ’ of the first-order differential of the compressive stress value, the value CS x ” of the second-order differential of the compressive stress value, and the value of the function CS x were calculated by the above-described methods.

[0143] The function for obtaining the stress distribution was σ(x) = (a1×erfc(a2×x) + a3×erfc(a4×x) + a5). a i ( i = 1 to 5) are fitting parameters, and erfc is the complementary error function. The complementary error function is defined by the following formula.

[0144]

[0145] In this evaluation, following the instructions for using the accompanying software, the fitting parameters were optimized by minimizing the sum of squared residuals of the obtained raw data and the aforementioned functions. The measurement processing conditions were set to single-shot. Regarding the processing adjustments for the measurement area, the edge method was specified for the surface, 6.0 μm for the inner surface end, automatic for the inner left and right ends, and automatic (center of sample film thickness) for the inner deep end. Furthermore, the extension of the phase curve up to the center of the sample thickness was selected as the fitted curve.

[0146] In addition, the concentration distribution of alkali metal ions (sodium and potassium ions) in the cross-sectional direction was measured using SEM-EDX (EPMA), confirming that it did not contradict the obtained stress distribution.

[0147] [Stress Measurement Using a Glass Surface Stress Gauge]

[0148] Stress was measured using a glass surface stress gauge (Orihara Manufacturing Co., Ltd., FSM-6000) via a non-destructive testing method utilizing the optical waveguide effect and photoelastic effect. The stress distribution map was then synthesized using software called PMC, also manufactured by Orihara Manufacturing Co., Ltd., along with the results obtained from the stress measurement using the scattered light photoelastic stress gauge.

[0149] The results are shown in Tables 1-3 and Figures 4-7 , Figure 8A and Figure 8B .exist Figure 4 The stress values ​​CS measured using SLP are shown in Examples 1 to 16. x The distribution of, in Figure 5 Examples 1 to 16 show CS. x '、CS x The image is titled "". Figure 7 The stress values ​​CS measured using SLP in Examples 17 to 19 are shown in the figure. x Distribution and CS x '、CS x The image is in Figure 8A and Figure 8B The diagram shows the stress distribution obtained by combining the stresses measured using SLP and FSM in Examples 17-19. It should be noted that, for ease of observation, CS is used... x Plot the absolute value of '.

[0150] [Drop Strength Test]

[0151] In the drop strength test, for the samples of Examples 2 and 9, the resulting glass samples of 120 mm x 60 mm x thickness 0.7 mm were embedded in a structure adjusted in mass and rigidity in accordance with the size of a general smartphone currently in use, prepared to simulate a smartphone, and then allowed to freely fall onto #180 SiC sandpaper. Regarding the drop height, in the case where it was not broken when allowed to fall from a height of 5 cm, the operation of increasing the height by 5 cm and allowing it to fall again was repeated until it was broken, and the height at the time of the first breakage was taken as the drop height. The results at the time of the 19th drop test for each example are shown in a box plot of FIG. 1. Figure 6

[0152] [Fragment number test]

[0153] Glass processed into a square of one side 50 mm was chemically strengthened, and the resulting glass was subjected to a breakage test by a diamond indenter having a 90-degree indenter tip angle. In the case where the glass was not broken, the test was repeated while gradually increasing the load applied to the indenter, and the number of fragments at the minimum load at which breakage occurred was counted as the fragment number. In the case where the fragment number was greater than 10, it was judged to be greater than the CT limit.

[0154]

[0155]

[0156] [Table 3]

[0157]

[0158] As shown in Tables 1 and 2, it was found that in Examples 1 to 6 as the examples, CS x was greater than 0 and less than or equal to 0.050, and CS 50 was increased relative to the comparative example.

[0159] In addition, in Examples 1 to 6, in the first ion exchange, the maximum tensile stress value CT was introduced to be greater than or equal to -120t + 164, and CT at this time was greater than the CT limit. On the other hand, in the second ion exchange, the maximum tensile stress value CT was less than -120t + 164, and CT at this time was less than the CT limit. The ratio of CT after the second ion exchange to CT after the first ion exchange (two-step strengthening CT / one-step strengthening CT of Table 1) was 0.95 or less. It was found that in Examples 1 to 6, by the first ion exchange, compressive stress was sufficiently introduced, by the second ion exchange, the total amount of compressive stress was reduced while maintaining a high CS 50 , and the CT limit was avoided.

[0160] ​On the other hand, in Example 9 and Example 13, which are comparative examples, the tensile stress value CT after the second ion exchange is larger than the tensile stress value CT after the first ion exchange, but the value of CS after the second ion exchange is smaller than the value of CS after the first ion exchange. 50 50

[0161] Further, as shown in Table 3, it is known that in Example 17 to Example 19, which are examples, by adding silica gel or silica gel and potassium carbonate in the second molten salt, the “Na-K exchange” is promoted in the distribution after the synthesis of SLP and FSM, and CS0 is improved.

[0162] This application is based on Japanese Patent Application No. 2020-131057 filed on July 31, 2020, and Japanese Patent Application No. 2021-030729 filed on February 26, 2021, the contents of which are incorporated herein by reference.​​

Claims

1. A chemically strengthened glass having a thickness t in mm, wherein the chemically strengthened glass satisfies the following formulae: the chemically strengthened glass has a basic composition containing, in terms of mol% on an oxide basis: 52 to 75% of Si02, 5 to 16% of Li20, 8 to 18% of Al203, 0.1 to 1.2% of K20, Na20 and K20 in total are 7% or less, a value CS x of a second-order differential of the distribution of stress values CS x of the chemically strengthened glass determined by a scattered light photoelastic stress meter at a depth x from a glass surface x satisfies the following equation in a range where CS x ≥ 0. 0 < CS x " < 0.050, The depth x is in units of pm, and the stress value CS x is in units of MPa.

2. The chemically strengthened glass of claim 1, wherein, a tensile stress value CT2 of the chemically strengthened glass satisfies the following formula using the thickness t: CT2 < -120t + 164, the tensile stress value CT2 is in MPa and the thickness t is in mm.

3. The chemically strengthened glass of claim 1 or 2, wherein, a tensile stress value CT2 of the chemically strengthened glass satisfies the following formula using the thickness t: -120t + 150 < CT2, the tensile stress value CT2 is in MPa and the thickness t is in mm.

4. The chemically strengthened glass of claim 1 or 2, wherein, a stress value CS at a depth of 50 μm from the surface of the chemically strengthened glass 50 using the thickness t, the tensile stress value CT2 and the compressive stress layer depth DOL, the following equation is satisfied: CS 50 (CT2 x (t - 2 x DOL)) / t > 4.90, the thickness t is in mm, the tensile stress value CT2 is in MPa, and the depth of the compressive stress layer DOL is in mm.

5. The chemically strengthened glass of claim 1 or 2, wherein, In the distribution of the stress value CS x , the value CS x of the first derivative of the stress value CS x is -5.3 or more, and the unit of the stress value CS x is MPa.

6. The chemically strengthened glass of claim 1 or 2, wherein, the chemically strengthened glass is a chemically strengthened glass obtained by performing chemical strengthening in two or more steps, and the tensile stress value CT2 of the chemically strengthened glass is a value of 50 to 93% of a tensile stress value CT1 of the chemically strengthened glass after the first step of the chemical strengthening in the two or more steps, the tensile stress value CT2 is in MPa, and the tensile stress value CT1 is in MPa.

7. A method for producing a chemically strengthened glass, the method comprising the steps of: performing a first ion exchange by immersing a lithium-containing glass in a first molten salt composition containing sodium ions and potassium ions, and performing a second ion exchange by immersing the lithium-containing glass in a second molten salt composition containing potassium ions, wherein the potassium nitrate concentration in the first molten salt composition is greater than the sodium nitrate concentration, the potassium nitrate concentration in the second molten salt composition is 85 mass% or more, and the mass ratio of sodium ions / lithium ions is 0 or more and 15 or less, the second molten salt composition contains more than 0 mass% and 5 mass% or less of sodium nitrate, the lithium-containing glass contains, in terms of mol% on an oxide basis: 52 to 75% of Si02, 5 to 16% of Li20, 8 to 18% of Al203, 0.1 to 1.2% of K20, Na20 and K20 in total are 7% or less.

8. The method of producing chemically strengthened glass according to claim 7, wherein The potassium nitrate concentration in the first molten salt composition is greater than 50 mass%.

9. The method of manufacturing chemically strengthened glass according to claim 7 or 8, wherein, The second molten salt composition contains 0.1 mass% or more and 10 mass% or less of lithium ions.

10. The method of producing a chemically strengthened glass according to claim 7 or 8, wherein, In the first ion exchange, the temperature of the first molten salt composition is 380°C or more and 450°C or less.

11. The method of producing chemically strengthened glass according to claim 10, wherein In the first ion exchange, the lithium-containing glass is immersed in the first molten salt composition for 0.5 hours or more and 8 hours or less.

12. The method of producing chemically strengthened glass according to claim 7 or 8, wherein, In the second ion exchange, the temperature of the second molten salt composition is 380°C or more and 450°C or less.

13. The method of producing chemically strengthened glass according to claim 12, wherein, In the second ion exchange, the immersion time t2 of the lithium-containing glass in the second molten salt composition satisfies the following formula using the temperature T of the second molten salt: -0.38T + 173 < t2 < -1.4T + 650, the unit of the immersion time t2 is minute, and the unit of the temperature T is °C.

14. The method of producing a chemically strengthened glass according to claim 7 or 8, wherein, The chemical strengthening is performed in such a manner that the tensile stress value CT2 of the chemically strengthened glass after the second ion exchange is a value of 50% to 93% of the tensile stress value CT1 of the chemically strengthened glass after the first ion exchange, the unit of the tensile stress value CT2 is MPa, and the unit of the tensile stress value CT1 is MPa.

15. The method of producing a chemically strengthened glass according to claim 7 or 8, wherein, The tensile stress value CT1 of the chemically strengthened glass after the first ion exchange satisfies the following formula using the thickness t of the chemically strengthened glass: CT1 > -120t + 164, the unit of the tensile stress value CT1 is MPa, and the unit of the thickness t is mm.

16. The method of producing chemically strengthened glass according to claim 7 or 8, wherein, The second molten salt composition contains a silicate.

17. The method of producing chemically strengthened glass according to claim 7 or 8, wherein, The second molten salt composition contains a carbonate.

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