Cover plate with anomalous stress profile, process for production thereof and use thereof

Abstract

To provide a cover plate having a sufficiently deep compressive stress zone such that a bending load leads to mechanical failure of the cover plate only under a very high load.SOLUTION: A cover plate includes a silica-based glass ceramic and has a thickness from 0.1 mm to 2 mm. The silica-based glass ceramic has a main crystal phase of high quartz mixed crystals or keatite mixed crystals. The cover plate is chemically tempered, has a surface compressive stress of at least 250 MPa and advantageously at most 1500 MPa, and has a stress profile with at least one inflection point, advantageously at a depth of the cover plate greater than 10 μm.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention generally relates to cover plates, and more particularly to cover plates for electronic display devices. Furthermore, the present invention relates to a method for manufacturing such cover plates, and to the use thereof. [Background technology]

[0002] Cover plates for electronic display devices, also known as covers, cover glass, or coating glass, have long been known. They cover the electronic components beneath them and also serve as display plates, and currently often consist of a chemically strengthened glass plate. Such cover glass is often, and can be remarkably, only less than 1 mm thick, because this helps to keep the weight of the display device (e.g., a smartphone or other portable electronic device) equipped with such a cover plate as low as possible. Due to its extremely thin thickness, the mechanical strength of the glass is reduced, so, as already mentioned, the glass needs to be mechanically strengthened by appropriate treatment. In the case of thin glass addressed in the prior art, this is done in a chemical strengthening process in which the glass plate is immersed in an immersion bath containing molten salt. Within the scope of this disclosure, the immersion bath is also synonymously referred to as an exchange bath because ion exchange occurs when immersed in a bath containing molten salt. In this way, smaller cations in the glass or glass plate are exchanged by larger cations in the exchange bath. For example, it is known that sodium ions are exchanged by potassium ions. Based on their size, potassium ions generate compressive stress in the region near the replaced surface of the glass plate, which is balanced by tensile stress within the glass plate. Overall, this increases the glass plate's resistance to mechanical loads.

[0003] It is also known that chemical strengthening can be applied to glass ceramics. Here, the exchange mechanism described above applies in principle to glass ceramics as well. Here, glass ceramics are generally understood within the scope of this disclosure to be materials subjected to controlled crystallization or at least controllable crystallization from green glass, resulting in a structure containing small crystals (or microcrystals) with a very homogeneous size distribution, in which the crystals (or microcrystals) do not, in part, have an average size of 2 μm.

[0004] Such glass-ceramics can be advantageous over chemically strengthenable or chemically strengthened glass because glass-ceramics can usually have greater mechanical resistance due to their special microstructure, which includes microcrystals. However, such glass-ceramics cannot usually be manufactured in the thin thicknesses possible for glass. Therefore, the use of glass-ceramics as a cover plate material may not be particularly advantageous in terms of the resulting weight of portable display devices.

[0005] In chemically strengthened glass or glass plates, the stress profile obtained by ion exchange is a simple ion exchange, i.e., a smaller cation, such as Na. + A larger version of that, for example, K + With respect to the exchange, the stress curve approximately follows a so-called complementary error function, or the resulting stress curve can be approximately described by such a complementary error function.

[0006] However, such a progression of the stress profile has been shown to be unfavorable because, in such a typical stress profile, the stress decreases rapidly from the surface to the interior of the glass article. This is particularly fatal in the case of non-pointed or flat surfaces (so-called blunt impact loading) because bending loads occur here, which can lead to the propagation of lateral cracks. Here, very high compressive stress at the surface beyond the critical depth may be advantageous in counteracting this crack propagation.

[0007] For this reason, it is advantageous to establish the highest possible compressive stress on the surface of the cover plate so that, despite a sharp drop in compressive stress, it leads to a sufficient "DOL" (depth of the compressive stress region, or "depth of layer"). Within the scope of this disclosure, the compressive stress region is referred to as "DOL," or more precisely, "depth of compression layer." However, this can also be disadvantageous, because, as mentioned above, the compressive stress generated on the surface of the cover plate is compensated for by the tensile stress inside the cover plate. The higher the compressive stress in the region near the surface of the cover plate, the higher the tensile stress accumulated inside it. Therefore, if the compressive stress in the region near the surface is very high, it may break under a corresponding load from the accumulated high tensile stress, resulting in the adverse effect of generating many small glass fragments. This is particularly disadvantageous for cover plates of display devices, because the numerous small fragments make it significantly more difficult to see through the plate to the display underneath than if only a few larger fragments were generated. Therefore, this type of failure should be prevented as much as possible, or should only occur under extremely high loads.

[0008] Therefore, a cover plate is required that has a sufficiently deep compressive stress region, and that only under very high loads will a bending load lead to mechanical failure of the cover plate. [Overview of the project] [Problems that the invention aims to solve]

[0009] The object of the present invention is to provide a cover plate that at least reduces the above-mentioned problems of the prior art. Further aspects of the present invention relate to a method for manufacturing such a cover plate, and to the use thereof. [Means for solving the problem]

[0010] The problems of the present invention are solved by the subject matter of the independent claims. Preferred and specific embodiments will become apparent in the dependent claims, the specification and drawings of this disclosure.

[0011] Accordingly, this disclosure relates to a cover plate having a thickness of 0.1 mm to 2 mm, comprising a silica-based glass ceramic, wherein the silica-based glass ceramic comprises a high-temperature quartz mixed crystal or a keytite mixed crystal as the main crystalline phase. The cover plate is chemically strengthened and has a surface compressive stress of at least 250 MPa and preferably up to 1500 MPa, and its stress profile has at least one inflection point at a depth of the cover plate, preferably greater than 10 μm.

[0012] The preferred lower limit of the thickness can generally be 0.4 mm. The preferred upper limit of the thickness can generally be 0.85 mm.

[0013] Such a configuration is extremely advantageous.

[0014] Since the cover plate contains silica-based glass ceramic, it already exists as a chemically very resistant material. Within the scope of this disclosure, silica-based glass ceramic refers to a material containing SiO2, and preferably with SiO4 as a component. 4-It is understood to be a glass ceramic containing a tetrahedron-containing crystalline phase, i.e., so-called crystalline silica. While silica-based glass ceramics inherently possess good mechanical resistance, in the case of different strength requirements, this good mechanical resistance of glass ceramics may not be as pronounced compared to corresponding green glass, for example, in the case of known strengths reported in the glass industry, such as bending strength or impact strength. Furthermore, the cover plate is chemically strengthened, meaning it has a compressive stress (CS) of at least 250 MPa and preferably up to 1500 MPa. Thus, the advantageous properties of the cover plate are achievable in mechanical use tests, such as the so-called "set drop test" concerning the behavior of the cover plate when assembled in a device, and / or in ball drop tests describing resistance to the action of a blunt object or a flat surface, so that a sufficiently thin and, consequently, light cover plate can be used. Therefore, the cover plate has a thickness of 0.1 mm to 2 mm. The preferred lower limit of the thickness can generally be 0.4 mm. The preferred upper limit of the thickness can generally be 0.85 mm.

[0015] Surprisingly, it was found that, generally, cover plates according to the embodiments advantageously yield stress profiles having at least one inflection point at cover plate depths exceeding 10 μm. In this case, the inflection point in the stress profile may be at an arbitrary location, for example, in the tensile stress region (i.e., below DoCL).

[0016] This is unexpected because generally, the stress profile in a cover plate can usually be approximated by a complementary error function and / or by a parabola, that is, in other words, as can be seen from the exemplary drawing of such a conventional stress profile curve in FIG. 11, it has an inverted "half S" shape. Such a stress profile does not have an inflection point at least in the cover plate itself, that is, at a depth of at least 10 μm in the cover plate. The conventional stress profile for a chemically strengthened cover plate (that is, a cover plate strengthened by an ion exchange process) shows a rapid decrease in the compressive stress present on the cover plate surface, which gradually flattens towards the interior of the cover plate and at least partially corresponds to the graph of the complementary error function in the first quadrant of the coordinate system.

[0017] Although such a stress profile is conventional, it has the drawback that at a shallow depth of the cover plate, a compressive stress that is essentially smaller than that of the cover plate surface already occurs. This leads to the cover plate being usually strengthened to a very high compressive stress on the surface, for example, through a mixed exchange using potassium ions and sodium ions, and thus generating a high compressive stress also at the relevant depth. However, this is disadvantageous when a tensile stress that is too high is accumulated in the cover plate in this way, which then results in a fine brittle fracture aspect during the fracture failure of the cover plate, which is in principle inconvenient for the cover plate of a display device.

[0018] However, if the surface compressive stress (CS0) and / or the depth of compressive stress (DoCL) cannot be arbitrarily increased, it is now possible, for a cover plate made of or including at least glass ceramic and under certain conditions, to change the transition of the stress profile so as to delay the decline of the stress curve as much as possible. This is in contrast to the conventional transition that can be approximated or described, at the beginning of the stress profile, by a substantially complementary error function and / or a parabola as described above, leading to a flat portion of stress such as a region of high strengthening, followed by a relatively sharp decline of the stress curve (S-shaped transition) and reaching an inflection point (see, for example, FIG. 9). In short, this transition of the profile, for the same CS0 and DoCL, leads to an increase in compressive stress and thus also an increase in tensile stress by different curve transitions. This increased compressive stress in the front "flat region" has advantages in terms of blunt impact strength (e.g., the so-called ball drop test), because cracks generated by the impact can be effectively stopped in the surface region or the force against crack propagation becomes greater.

[0019] Here, the cover plate according to the present disclosure provides an advantage because its stress profile is configured to advantageously have at least one inflection point at a depth of the cover plate greater than 10 μm. For example, the stress profile of the cover plate according to the present disclosure can be formed "convex" in contrast to the more "concave" stress profiles typical of cover plates of the prior art. Here, "convex" is generally understood as a shape having a shape that "bends upward". Here, "upward" refers to the compressive stress value in the conventional stress profile, where in the coordinate system the compressive stress is "upward", i.e., plotted in the positive y direction over the thickness x of the cover plate.

[0020] Generally, such an anomalous stress profile having at least one inflection point at a cover plate depth greater than 10 μm offers the advantage of achieving a relatively large DoCL without the need for very high surface compressive stress at the same time. A relatively large DoCL when the compressive stress on the surface is not too high at the same time offers the advantage that surface scratches or especially sharp impact damage (e.g., caused by a sharp object piercing the glass surface) do not penetrate to an area of ​​the cover plate already under tensile stress at a very small penetration depth, and therefore do not lead to breakage of the cover plate by fracture. At the same time, there is no need to perform complicated strengthening protocols and mixing exchanges. In particular, it has been found that such a strengthening profile in the cover plate can be produced with just a single ion exchange. This does not preclude that further exchange steps may be performed in some cases to achieve further advantages in the transition of the stress profile. However, this is not always necessary, and according to the embodiment, it is possible to achieve improvement in the strength of the cover plate with just one ion exchange.

[0021] Such anomalous stress profiles having at least one inflection point, and possibly more, at a cover plate depth greater than 10 μm are known in principle. For example, European Patent No. 2819966 (EP2819966 B1), U.S. Patent Application Publication No. 2020 / 0002225 (US2020 / 0002225 A1), and U.S. Patent Application Publication No. 2010 / 0009154 (US 2010 / 0009154 A1) also describe stress profiles in glass cover plates having an anomalous stress profile with one inflection point. However, achieving such an anomalous stress profile requires a complex process involving multiple ion exchanges and heat treatment steps between these ion exchanges. The document in U.S. Patent Application Publication No. 2021 / 0292225 (US2021 / 0292225 A1) also shows an anomalous compressive stress profile having a partially convex transition in stress profile, for example in Figure 1 relating to Example 5 of U.S. Patent Application Publication No. 2021 / 0292225. This anomalous compressive stress profile is obtained for a glass-ceramic cover plate, where glass ceramics having such at least partially convex transition in stress profile are also obtained in a rather complicated process (e.g., Example H of U.S. Patent Application Publication No. 2021 / 0292225), which includes further steps for strengthening other than the ion exchange step in addition to the exchange of mixture (of sodium and potassium), or relates to a special glass ceramic having a high content of Al2O3 and a high content of Li2O of about 10 mol%, along with a high content of Al2O3 and Li2O of about 10 mol%, along with a low content of SiO2 of only 50-53 mol%, and additionally Y2O3. In this case, Y2O3 is contained in the glass ceramic or the original green glass because it improves the meltability of this glass, which has a high content of Al2O3 that increases the melting point.

[0022] Y2O3 may also be present in rather conventional glass ceramics having a higher SiO2 content and a lower Al2O3 content than the aforementioned glass ceramics according to embodiments of U.S. Patent Application Publication No. 2021 / 0292225, in which case it may be used together with other components, such as La2O3 and Nb2O5, to optimize fracture properties.

[0023] However, these components, especially the rare earth components La2O3 and Y2O3, are undesirable because they lead to increased manufacturing costs. Therefore, according to one embodiment, the cover plate, or the glass ceramic contained in the cover plate, contains Y2O3, La2O3, and / or Nb2O5 in the form of unavoidable trace amounts of less than 0.1 mass% each.

[0024] Surprisingly, it was found that a favorable stress profile with at least one inflection point at a cover plate depth of more than 10 μm can already be obtained by ion exchange alone. Furthermore, it is not necessary for the stress profile to have multiple inflection points; in fact, having only one inflection point can even be advantageous.

[0025] The inventors have further discovered that, according to the embodiments, it is possible to obtain a stress profile containing multiple inflection points with only a few exchange steps. In general, the inventors have confirmed that the achieved stress profile contains 2 × n-1 inflection points (where n is the number of ion exchange steps). Thus, according to the embodiments, a stress profile with three inflection points is obtained after a second ion exchange step, and a stress profile with five inflection points is obtained after a third ion exchange step. Here, the number of inflection points is relative to one side of the cover plate. Here, the inflection points are advantageously located at a depth of at least 10 μm each in the transition of the stress profile.

[0026] The anomalous stress profile progression in the glass-ceramic coverplate compared to a coverplate containing glass with the same chemical composition as the glass-ceramic (where the compositions before ion exchange are the same) is due to the strengthening in the glass-ceramic proceeding through a different mechanism than in glass. This mechanism enables a particularly efficient type of strengthening. In other words, it is possible to achieve the same strengthening with fewer available and exchanged ions, or conversely, to achieve higher strengthening with the same strengthening process.

[0027] This can be approximately described, for example, by the dominant strengthening at a particular depth of the coverplate per mole of ions used (or incorporated) for strengthening. In particular, this is the strengthening per mole of sodium ions incorporated into the coverplate according to the embodiment. Within the scope of this disclosure, this value is also referred to as the β value. Generally, the depth x is calculated according to the following formula:

number

[0028] Here, CT represents the central tensile stress value in the cover plate (the maximum tensile stress value in the stress profile), and CS(x) represents the compressive stress value at depth x. c(x) is the concentration of the ion considered at the corresponding depth x, described as an oxide, and c(bulk) is the corresponding ion concentration in the bulk. ν is the Poisson's ratio of the glass or glass-ceramic, and E is the modulus of elasticity. Thus, the strengthening per exchanged ion is identified. Therefore, the β value reflects the strengthening efficiency of the ionic species in the glass or glass-ceramic under investigation in the temperature range where relaxation can be ignored.

[0029] In the case of cover plates according to this disclosure, which include silica-based glass ceramics containing high-temperature quartz mixed crystals or keytite mixed crystals, it has been found that it may be sufficient to consider only sodium ions to determine the β value, because they are exchanged particularly efficiently. However, the given formula is generally applicable and can also be used for other ions used for strengthening, especially potassium.

[0030] This β value can be compared between glass-ceramics and glass of the same composition (i.e., the composition of the bulk material before strengthening).

[0031] Within the scope of this disclosure, the β value is considered a constant that occurs specifically in glass or glass-ceramic, and may sometimes exhibit small variations over thickness. To calculate the β value, within the scope of this disclosure, its value was identified at various depths of glass or glass-ceramic articles, and the arithmetic mean was finally taken. Within the scope of this disclosure, the inventors considered glass depths up to 150 μm to identify the β value. This β value in glass-ceramic is 1.1 to 1.5 times, and preferably 1.1 to 1.4 times, greater than in glass of the same chemical composition. For example, a ratio of 1.26 was obtained for a cover plate according to one embodiment. Therefore, the construction of stress by ion exchange in a glass-ceramic cover plate according to the embodiment is clearly more efficient than in a cover plate made of or containing glass of a corresponding composition to the glass-ceramic. For the cover plate considered herein, an exemplary β value is 3 × 10⁻⁶ -4 / mol ~9 x 10 -4 / mol, e.g., 4 × 10 -4 / mol ~8 x 10 -4 This is within the range of / moles.

[0032] Here, regarding the above description, the β value relates to sodium as the ion that causes strengthening. Therefore, this is particularly appropriate especially when considering the ratio of the β values in the strengthened glass-ceramic cover plate according to an embodiment and the cover plate containing glass having the same chemical composition as the glass-ceramic, respectively, because the inventors have discovered that in many glass-ceramics according to the embodiment, sodium particularly constitutes an essential proportion of the strengthening and is also particularly efficiently exchanged. However, depending on the basic composition of the glass or the corresponding glass-ceramic used, it is also possible to perform this consideration for other ions used for strengthening, such as potassium. However, for the glass-ceramic considered according to the embodiment, since the influence of sodium on strengthening is significant, for the sake of simplicity, it is permissible to consider only the strengthening achieved by sodium ions.

[0033] Therefore, the present disclosure generally relates to a cover plate having a thickness of 0.1 mm to 2 mm, including a silica-based glass-ceramic, without being limited to specific embodiments according to the present disclosure, wherein the silica-based glass-ceramic includes high-temperature quartz mixed crystal or keatite mixed crystal as the main crystal phase. The cover plate is chemically strengthened and has a surface compressive stress of at least 250 MPa and preferably up to 1500 MPa. The cover plate is characterized by a β value specified by the formula

Number

[0034] According to one embodiment, the stress profile has exactly one inflection point. This can be advantageous because such a stress profile can be obtained in a single ion exchange step alone, thus achieving already favorable strengthening properties. Particularly in the strength of the cover plate against the effects of blunt objects (e.g., blunt impacts tested in a ball drop test), very favorable properties can be achieved, here, from the favorable transition of the stress profile having at least one inflection point, preferably at least 10 μm in depth of the cover plate, because there is no abrupt decrease in surface compressive stress. Thus, an embodiment in which only one such inflection point is achieved can already result in a significant improvement in mechanical properties.

[0035] Therefore, a cover plate with a thin thickness of 0.1 mm to 2 mm is even more advantageous because it can achieve high light transmittance in this way. Here, generally, light transmittance is τ vis As shown, according to one embodiment of the cover plate, the light transmittance exceeds 80% in the wavelength range of 380 nm to 780 nm, and advantageously exceeds 85%. Advantageously, the light transmittance value is specified for comparison with a cover plate thickness of 0.7 mm. To compare whether they meet this condition, thinner plates can be stacked to achieve a corresponding thickness, and thicker plates can be made thinner. Generally, this light transmittance value is achieved for cover plate thicknesses of 0.1 mm to 2 mm according to one embodiment.

[0036] The cover plate is generally constructed in a plate-like manner, meaning its thickness is at least an order of magnitude smaller than its length and width. Thus, it has two sides (or "sides"), the dimensions of which are defined by length and width, and extending perpendicularly inward from there toward the center of the cover plate, a near-surface layer is defined. This near-surface layer is formed on both sides of the cover plate. It is advantageously a layer with a depth of 20 μm to 70 μm.

[0037] In a preferred embodiment, the glass ceramic contains keytite or keytite mixed crystals as the main crystalline phase, and it is understood that more than 50 volume percent of the crystalline phase contained in the glass ceramic exists with a keytite crystalline structure. Advantageously, up to 98.5 volume percent, or even 100 volume percent, of the crystalline phase contained in the glass ceramic may exist with a keytite crystalline structure, i.e., as keytite or keytite mixed crystals. However, the glass ceramic may also contain a secondary phase, such as a nucleating agent present in crystal form.

[0038] According to a further embodiment, the cover plate has a chrominance of less than 4, preferably less than 3 C * This is characterized by the following: In other words, since the cover plate has only a very slight color cast, it is possible to view the display behind it through the cover plate without any interfering color distortion. Saturation C * or C ab * It is also called chroma, and saturation a * , b * The following is calculated from this:

number

[0039] In a further embodiment, the cover plate is characterized by a haze of 0.01% to 1% relative to the cover plate thickness of 0.7 mm. Haze is understood as cloudiness. In other words, the cover plate is formed with only slight cloudiness.

[0040] To achieve low chromaticity and / or low cloudiness, it may be advantageous to limit the TiO2 content of the glass ceramic. TiO2 is a known component of silica-based glass ceramics, such as so-called lithium aluminum silicate glass ceramics, and can be particularly useful for efficient nucleation. However, it has been found that this component, even if it does not color itself, can contribute to the coloration of the resulting glass ceramic through cloudiness. Therefore, according to one embodiment, the glass ceramic contains TiO2, advantageously up to 4 mass% of TiO2, and particularly preferably up to 3 mass% of TiO2.

[0041] Advantageously, according to one embodiment, the glass ceramic contained in the cover plate is formed as lithium aluminum silicate glass ceramic, and its crystalline phase is formed as a keytite mixed crystal. Lithium aluminum silicate glass ceramic is well known as a material that offers clear advantages in the manufacture of glass ceramics. Therefore, forming the glass ceramic to include a keytite mixed crystal as the crystalline phase (or crystalline phase) is even more advantageous because it has been found that not all crystalline phases in a lithium aluminum silicate glass ceramic system are configured to be strengthenable. However, keytite or keytite mixed crystals in particular clearly have a crystalline structure in which ion exchange, particularly lithium to sodium and / or sodium and / or lithium to potassium, is available. However, a drawback of known keytite mixed crystal glass ceramics, especially those that already possess inherently high strength, is that these glass ceramics often have a strong cloudiness. However, surprisingly, it has been found that cover plates containing keytite mixed crystal glass ceramics that simultaneously have slight cloudiness, only slight coloration, and even higher light transmittance are possible. The inventors do not yet fully understand the reason for this. However, the inventors believe that this may be due to a specific configuration of the crystalline phase, particularly in the enhanced state, such that the optical properties of the crystalline phase and the residual glass phase are optimally matched to each other in such a way that there is almost no difference in refractive index between the two phases. This reduces the effect of clouding and explains why such a configuration has advantageous optical properties.

[0042] In general, without being limited to a specific embodiment, according to one embodiment, the glass ceramic consists of the following components: SiO255-75, preferably 62-72 Al2O3 18-27, preferably 18-23 Li2O 2.8-5, preferably 3-5 It may contain in mass percent based on oxides.

[0043] This is a silica-based glass ceramic that is sufficiently meltable as glass and does not tend to crystallize immediately and disorderly. Within this general composition range, known lithium aluminum silicate glass ceramics, particularly those well known with respect to melting and ceramicization conditions, can be manufactured. A lithium content in the glass ceramic is even more advantageous because it allows for the exchange of sodium and / or potassium for lithium.

[0044] According to one embodiment, the glass ceramic contains only the components of La2O3, Y2O3, and / or Nb2O5 in the form of an unavoidable trace amount of less than 0.1% by mass, each of which is used.

[0045] In further embodiments, the glass ceramic contains MgO, with a preferred upper limit of 8% by mass. Particularly preferably, the glass ceramic contains 4% by mass or less of MgO. MgO is a preferred component because it promotes the formation of keytite mixed crystals. This means that a certain percentage of MgO leads to a decrease in the ceramicization temperature. However, if the MgO content is too high, undesirable secondary phases, such as spinel and / or magnesium titanate, may be formed. This adversely affects the permeability of the resulting glass ceramic material, particularly with respect to its scattering. Therefore, the MgO content of the glass ceramic is advantageously limited to the limits described above.

[0046] According to one embodiment, the glass ceramic further contains ZnO in an advantageous maximum of 6% by mass, and particularly advantageously 2% by mass or less. Such a ZnO content can be advantageous because ZnO lowers the viscosity of the glass, making the green glass of the glass ceramic more easily meltable. However, too much ZnO leads to the formation of other phases, such as zinc spinel, and consequently, increased scattering.

[0047] Other alkaline earth metal oxides, such as CaO and BaO, can also have a positive effect on melting properties. However, the amount of such components RO (alkaline earth metal oxides, including ZnO) should generally be limited to avoid the formation of heterogeneous phases that can lead to greater scattering and, consequently, a decrease in transmittance. Furthermore, the use of heavy alkaline earth oxides, such as BaO and SrO, in particular, can be used to match the refractive index of the residual glass phase to that of the crystalline phase, thereby optimizing transmittance.

[0048] A special component of the glass ceramic according to one embodiment is SnO2. SnO2 can act, for example, as a clarifying agent in the molten material and then as a nucleating agent in the glass ceramic itself. Therefore, according to one embodiment, the glass ceramic of the cover plate preferably contains SnO2, more preferably up to 2% by mass. A SnO2 content of at least 0.05% by mass, and preferably up to 1.6% by mass, is particularly preferred. A higher SnO2 content leads to a strong devitrification tendency, which worsens the productivity of the glass ceramic.

[0049] The components of ZrO2 and TiO2 can also act as nucleating agents in the glass ceramic according to the embodiment. Nucleation, particularly the content and relative ratio of the nucleating agents in the glass ceramic, has been found to be important for the formation of lightly colored silica-based glass ceramics having good light transmittance and slight cloudiness. Accordingly, according to one embodiment, the glass ceramic contains TiO2, advantageously up to 4 mass% of TiO2, particularly preferably up to 3 mass% of TiO2.

[0050] Furthermore, ZrO2 is also a very efficient nucleating agent in glass ceramics according to one embodiment. Accordingly, according to one embodiment, the glass ceramic contains ZrO2 in an advantageous amount of up to 5% by mass, particularly advantageously up to 4% by mass, and especially preferably at least 1.2% by mass.

[0051] The glass ceramic may further contain Fe2O3 up to a particularly large amount of 0.1 mass%. Fe2O3 is often present in the form of an unavoidable impurity in the glass ceramic according to the embodiment, but at the same time, it is beneficial for nucleation, so a certain content of Fe2O3 can also be beneficial. However, in order to obtain a glass ceramic that is as color neutral as possible, the content of Fe2O3 should be limited, and is advantageously 0.02 mass% or less. In particular, a content of 0.0001 mass% to 0.1 mass%, preferably 0.0001 to 0.02 mass%, is possible. In other words, generally, the Fe2O3 content of the glass ceramic of the cover plate according to one embodiment is less than 0.02 mass%.

[0052] Regarding the ratio of the important components TiO2 and ZrO2, according to one embodiment, the following relationship applies: 0 < (TiO2 + ZrO2) < 9.5%, preferably 1.2 < (TiO2 + ZrO2) < 9.5%.

[0053] In the case of such a combined content of both TiO2 and ZrO2 as nucleating components, it has been found that particularly good values ​​regarding cloudiness and slight coloration can be achieved.

[0054] This can generally be achieved even better when the ratio of nucleating agents also incorporates an additional nucleating component, SnO2. According to preferred embodiments, the following generally applies: 0 ≤ SnO2 / (ZrO2 + TiO2) < 0.8, preferably 0.01 ≤ SnO2 / (ZrO2 + TiO2) < 0.7.

[0055] In particular, the glass ceramic according to one embodiment consists of the following components: SiO255-75, preferably 62-72 Al2O3 18~27 Li2O 2.8-5, preferably 3-5 Na2O 0-4, preferably 0-2 K2O 0-4, preferably 0-2 MgO 0-8, preferably 0-4 CaO 0-4, preferably 0-2 SrO 0-4, preferably 0-2 BaO 0-4, preferably 0-2 ZnO 0-6, preferably 0-2 TiO20-4, preferably 0-3 ZrO20~5, preferably 1.2~4 B2O30~2, preferably 0~0.1 Fe2O3 0.0001~0.1, preferably 0.0001~0.02 SnO20~2, preferably 0.05~1.6 This can be included in mass% on an oxide basis, where, advantageously, the total of the TiO2 and ZrO2 components, 0 < Σ(TiO2 + ZrO2) < 9.5%, preferably 1.2 < Σ(TiO2 + ZrO2) < 9.5% This applies.

[0056] According to one embodiment, the glass ceramic of the cover plate contains a crystalline phase having a microcrystal size of 120 nm or less. Advantageously, the microcrystals contained in the glass ceramic are up to 90 nm in size.

[0057] Embodiments in which the glass ceramic of the cover plate is As2O3 and / or Sb2O3-free are particularly preferred. Within the scope of this disclosure, “free” of these components is understood to mean that these components are present only in the form of unavoidable impurities or in trace amounts, at a maximum content of 500 ppm, preferably 100 ppm, of the mass.

[0058] In a further embodiment, the cover plate is characterized by a sharp impact strength at drop heights of at least 120 cm to 200 cm, as specified in a set drop test, preferably with a thickness of 0.7 mm. To investigate the sharp impact strength of untempered glass or glass ceramic, a representable value is preferably obtained using 180 grit sandpaper. In contrast, for tempered glass or glass ceramic, the sharp impact strength is preferably investigated using 60 grit sandpaper.

[0059] So-called "sharp impact" strength is understood, within the scope of this disclosure, to mean that a smartphone dummy containing the plate to be tested is dropped onto a rough surface using a drop device so that a number of small, sharp objects (e.g., grains of sand on asphalt, concrete, or sandpaper) can pierce the plate to be tested. In other words, this is the effect of one or more sharp objects, i.e., particles with a very small radius of curvature or with an angle of less than 100° on part of the protrusion.

[0060] In this case, a key-tight glass-ceramic coverplate according to one embodiment having chemical strengthening of the crystalline phase achieves an average drop height of approximately 172 cm, which is about four times the height of a standard non-ceramicized but chemically strengthened glass, where chemical strengthening is conventionally constructed in the glass phase and has an average drop height of 42 cm. A similarly strengthened key-tight glass-ceramic coverplate according to another embodiment has been observed to have a high drop height of approximately 156 cm (see Figure 8). According to one embodiment, the coverplate exists as a coverplate strengthened with sodium ions. The coverplate can be strengthened with sodium ions alone, which is based on the high selectivity of strengthening through sodium in the glass-ceramic according to the embodiment, especially in the case of salts of so-called industrial purity or salt mixtures containing a high proportion of K (e.g., up to about 90% KNO3). However, according to a further embodiment, it may also be advantageous to include a specific proportion of lithium ions in the exchange bath, for example, 0.1% by mass of lithium salt, for example, 0.1% by mass of LiNO3, in the exchange bath otherwise of NaNO3. In a further embodiment, the cover plate exists as a potassium ion-reinforced cover plate, and more particularly as a cover plate reinforced solely with potassium ions. Here, the purity of the potassium exchange bath is 99.9% (relative to mass).

[0061] In general, according to one embodiment, the cover plate may also exist as a cover plate reinforced with sodium and potassium ions.

[0062] According to one embodiment, only one ion exchange is performed, in which the exchange bath preferably contains NaNO3 and optionally up to 0.1% by mass of LiNO3, or contains KNO3 with a purity of 99.9% by mass.

[0063] It has been found that particularly advantageous embodiments can be obtained in the case of pure strengthening using sodium. In particular, based on the high selectivity of ion exchange, favorable stress profiles and corresponding mechanical properties can be obtained with relatively short strengthening times, especially in the case of so-called set-drop strength. However, when other strengths, such as four-point bending strength, are considered, potassium strengthening or generally mixed strengthening may also be advantageous.

[0064] In a further embodiment, the glass ceramic of the cover plate does not contain lithium metasilicate as a crystalline phase. This is advantageous because, in this way, the glass ceramic is configured to contain a crystalline phase, particularly keytite mixed crystals, which are capable of utilizing the advantageous and selective replacement process described above.

[0065] This disclosure also relates to a method. A method for manufacturing a cover plate, and in particular a cover plate according to one embodiment, is as follows: • The process of manufacturing silica green glass through a melting process and subsequent hot forming, The heat treatment step of the silica green glass, wherein at least one nucleation step is performed at a temperature range of 650°C to 850°C, preferably 690°C to 850°C, for a period of 5 minutes to 60 hours, preferably up to 8 hours, and particularly preferably 30 minutes to 2 hours, and at least one ceramicization step is performed at a temperature range of 700°C to 1100°C, preferably 780°C to 1100°C, for a period of 3 minutes to 120 hours, preferably up to 60 hours, and preferably 3 minutes to 8 hours. The step of performing at least one ion exchange in an exchange bath having a composition of 100% to 0% by mass of KNO3, and / or 0% to 99.9% by mass of NaNO3, and 0% to 5% by mass of LiNO3, at a temperature of 360°C to 500°C and for a period of 2 to 50 hours. Includes.

[0066] The cooling rate is advantageous if it is between 2°C and 50°C / minute.

[0067] Generally, one or more further exchange steps can be carried out at a temperature of 360°C to 500°C for a period of 30 minutes to 20 hours, using an exchange bath having a composition of 90% by mass of KNO3 and 10% by mass of NaNO3 to 100% by mass of KNO3, or 70, preferably 50% by mass of NaNO3 and 30, preferably 50% by mass of LiNO3 to 100% by mass of NaNO3.

[0068] Chemical strengthening characteristic values ​​CS0 (compressive stress on the surface of the cover plate), CS 30 The compressive stress (or general stress) at a depth of 30 μm, as well as the DoCL (depth of the compressive stress layer, sometimes referred to as the exchange depth), can be determined using suitable measuring devices, such as the SLP-1000 and FSM6000. However, DoCL is not the same as the ion exchange depth.

[0069] Accordingly, this disclosure also generally relates to cover plates that have been manufactured or can be manufactured by a method according to one embodiment.

[0070] Furthermore, this disclosure relates to the use of a cover plate according to one embodiment and / or a cover plate manufactured by a method according to one embodiment in electronic devices, particularly in electronic display devices, particularly in mobile electronic display devices, such as mobile touch panels and / or mobile digital display devices, such as smartphones or smartwatches, and generally in touch panels. Furthermore, this disclosure also relates to display devices, particularly digital display devices, such as touch panels or smartwatches or smartphones, that include a cover plate according to one embodiment and / or a cover plate manufactured by a method according to one embodiment.

[0071] An exchange bath is understood to be a molten salt used in an ion exchange procedure for glass or glass articles. Within the scope of this disclosure, the terms exchange bath and ion exchange bath are used synonymously.

[0072] Typically, industrial-grade salts are used for exchange baths. This means that even if only sodium nitrate is used as the starting material for the exchange bath, certain impurities may also be present in the bath. In this case, the exchange bath is a molten salt, such as sodium nitrate, or a mixture of salts, such as a mixture of sodium and potassium salts. Here, the composition of the exchange bath is described in terms of its nominal composition, without taking into account any impurities that may be present. Therefore, within the scope of this disclosure, when 100% molten sodium nitrate is mentioned, this means that only sodium nitrate was used as the raw material. However, the actual sodium nitrate content of the exchange bath may deviate from this, and this is normal, because industrial raw materials, in particular, have a certain proportion of impurities. However, this is usually less than 5% by mass, and especially less than 1% by mass, relative to the total mass of the exchange bath.

[0073] However, in certain situations, it may be possible and even advantageous to use particularly pure salts, that is, salts of analytical quality rather than industrial purity. This can be especially advantageous when a cover plate that allows for highly selective ion exchange should be enhanced. In other words, certain glass ceramics can be used in precisely this way, and it has been found that, advantageously, only specific ions, such as sodium, are exchanged, while potassium is not. This high selectivity of certain embodiments of glass ceramics or cover plates leads to the exchange of only impurities in the presence of small amounts of impurities, that is, only sodium is exchanged, even in an industrial-purity KNO3 exchange bath with a NaNO3 content of 0.5 mass%.

[0074] Therefore, according to exemplary embodiments, it may be conceivable to use a salt having 3n purity (99.9% purity relative to mass), especially when only exchange with specific ions is envisioned. When such a salt is used, this is specifically noted. However, it is advantageous to use an industrial-purity salt because it is clearly cheaper than a high-purity salt. Here, the purity is typically 2n, i.e., 99.5% relative to mass.

[0075] In one embodiment, advantageously, only one ion exchange is performed, where the exchange bath is advantageously made of pure NaNO3 or pure KNO3, and may contain unavoidable impurities in a total content of up to 0.01% by mass. This is a particularly simple method and therefore may be preferred. However, especially in the case of NaNO3 strengthening, it may be preferable to have lithium ions in addition to sodium ions, for example, lithium salts up to a concentration of 0.1% by mass, in order to avoid the appearance of fine, brittle fracture in the event of breakage.

[0076] Accordingly, in the case of exchange baths having mixtures of different salts, the nominal content of these salts is stated without taking into account industrially occurring impurities in the starting materials. Therefore, an exchange bath having 90% by mass of KNO3 and 10% by mass of NaNO3 may also have small amounts of impurities, but these are due to the raw materials and should normally be less than 5% by mass, and especially less than 1% by mass, of the total mass of the exchange bath.

[0077] The aforementioned embodiment regarding the composition of the exchange bath is applicable here accordingly.

[0078] Furthermore, the composition of the exchange bath also changes during the ion exchange process, because continuous ion exchange causes lithium ions, in particular, to move from the glass or glass article into the exchange bath. However, such changes in the composition of the exchange bath due to aging are not taken into consideration here unless explicitly stated. Rather, within the scope of this disclosure, when the composition of the exchange bath is described, it is to match the nominal original composition. [Brief explanation of the drawing]

[0079] [Figure 1] This figure shows a cover plate 1 according to an embodiment of the present disclosure. [Figure 2] This figure shows a cover plate 1 according to an embodiment of the present disclosure. [Figure 3] This is a diagram illustrating the implementation of a set-drop test. [Figure 4] This is a diagram illustrating the implementation of a set-drop test. [Figure 5] This is a diagram illustrating the implementation of a set-drop test. [Figure 6] This is a diagram illustrating the implementation of a set-drop test. [Figure 7] This figure shows the stress profile (top) and EDX curve (bottom) of a cover plate reinforced with potassium ions. [Figure 8] This figure shows the stress profile (top) and EDX curve (bottom) of a cover plate reinforced with sodium ions. [Figure 9] This figure shows the stress profiles obtained after two ion exchange cycles. [Figure 10] This figure shows a comparison of set drop intensities. [Figure 11] This figure shows an exemplary stress profile of a cover plate corresponding to conventional technology. [Figure 12] This figure illustrates the changes in the stress profile for a cover plate containing silica-based glass ceramic, and for a cover plate containing silica-based glass with a chemical composition corresponding to the chemical composition of the glass ceramic. [Figure 13] This figure shows a comparison of sodium oxide concentrations in glass and glass ceramics under different strengthening processes using sodium as a strengthening ion, where glass and glass ceramics have the same chemical composition. [Figure 14] This figure shows a stress curve using SLP. [Figure 15]This figure shows the values ​​obtained in the set-drop test. [Examples]

[0080] The present invention will be described in more detail below using examples.

[0081] Table 1 shows the composition of the glass ceramic material according to the present invention.

[0082] The materials listed in Table 1 were melted at a temperature of approximately 1600-1680°C using standard raw materials in the glass industry, and then clarified. The melting of the mixture was first carried out in a sintered quartz glass crucible, then poured into a Pt / Rh crucible equipped with an inner quartz glass crucible, and homogenized by stirring at approximately 1550°C for 30 minutes. After standing at 1640°C for 2 hours, a mold measuring approximately 140mm × 100mm × 30mm was poured, relaxed in a cooling furnace at approximately 620-680°C, and then cooled to room temperature. From this mold, test samples were prepared for measuring the properties in the glass state and for ceramicization.

[0083] For ceramicization, a two-step program, as described in Table 1, was typically used. In this case, the starting glass was first heated from room temperature. g The sample is heated to a nucleation temperature above a certain level and held there for a sufficient time for nucleation. Subsequently, the sample is heated to the ceramicization temperature and held there as well. Programs with three or more steps may also be used (Example 2 in Table 1). Furthermore, the holding time can be replaced by a slower heating rate. In the ceramicized sample, the crystalline phase and its content, as well as the transmittance τ in the visible range, are determined using XRD. vis The chrominance (in a sample with a thickness of 0.7 mm) and the chrominance in the Lab system (standard light source C) were determined.

[0084] The crystalline phase content listed in Table 1 was determined using X-ray diffraction measurements with a Panalytical X'Pert Pro Diffraktometer (Almelo, Netherlands). The generated CuKα rays (λ=1.5060 Å) were used as X-rays, filtered through a Ni filter. Standard X-ray diffraction measurements were performed on powder and solid samples under a Bragg-Brentano configuration (θ-2θ). X-ray diffraction patterns were measured from 10° to 100° (2θ angle). The relative proportion of the crystalline phase and the determination of microcrystal sizes were performed via Rietveld analysis. These measurements were performed on pulverized sample materials where the volume proportion of the central region was clearly dominant. Therefore, the measured phase proportions correspond to the phase distribution in the center of the glass ceramic. Samples marked with "V" correspond to comparative examples. Examples with only a number are examples of embodiments.

[0085] [Table 1-1]

[0086] [Table 1-2]

[0087] [Table 1-3]

[0088] For strengthening tests, ceramicized glass ceramic plates with a thickness of 0.7 mm were strengthened in various salt baths. Table 2 shows the changes in crystallographic data during the strengthening of glass ceramics according to the present invention.

[0089] [Table 2]

[0090] The aforementioned samples contained keytite mixed crystals as the main crystalline phase after ceramicization (96% keytite mixed crystals, 3% ZrTiO4). After strengthening (7.5–18 hours at 420–440°C), all samples, regardless of the selected salt bath, showed unit cell expansion of more than 1% in the near-surface layer compared to unstrengthened samples. Samples strengthened in 100% KNO3 even showed the formation of two different keytite mixed crystal structures in the near-surface layer, both of which had expanded unit cell volumes compared to unstrengthened keytite. All 0.7 mm samples with increased set drop intensity further showed DoCL of 140 μm or 135 μm. CS 30 The values ​​for this ranged from 150 MPa to 360 MPa.

[0091] A sample having the composition according to Example 9 was similarly prepared, ceramicized, and strengthened as described in Example 9 in Table 1. It contains nepheline ((Na,K)[AlSiO4]) as the main crystalline phase and trace amounts of rutile. From XRD measurements, the following crystallographic data were obtained for nepheline (hexagonal structure): a = 10.026 (5) Å, c = 8.372 (5) Å, unit cell volume V = 728.8 (10) Å 3 The following was obtained. After strengthening (100% KNO3, 500°C for 8 hours), calsilite (potassium-substituted end component of the nepheline mixed crystal system, KAlSiO4): a = 5.170 (5) Å, c = 8.730 (5) Å was obtained. For direct comparison, the lattice constant of the a axis must be doubled here, based on the difference in the size of the unit cells of the two structures (therefore, there are the same number of formula units in the unit cell). Thus, the unit cell volume V for calsilite is 808.3 (10) Å. 3 This yields a result that corresponds to an expansion of approximately 10%.

[0092] The reinforcement conditions and, consequently, the reinforcement parameters achieved for different cover plates are shown in the table below (Table 3).

[0093] [Table 3]

[0094] Here, CT represents the internal tension and is expressed in MPa. "K DoL" is the depth of the compressive stress due to potassium (if applicable) and is expressed in μm, CS0 is the height of the compressive stress at the surface of the cover plate and is expressed in MPa, CS 30 This represents the compressive stress measured from the surface of the cover plate at a depth of 30 μm (expressed in MPa).

[0095] Strengthening with a sodium-lithium mixture can be advantageous in improving the fracture pattern, that is, in obtaining a pattern of less fine, brittle fracture.

[0096] Description of the drawing Figure 1 shows a schematic, not to scale, diagram of the cover plate 1 according to an embodiment of the present disclosure. The cover plate 1 is formed in the form of a plate or sheet, in the sense that its thickness d (not shown in Figure 1) is at least an order of magnitude smaller than the length l and width b of the cover plate 1. The cover plate 1 may be flat or planar, as illustrated exemplary in Figure 1, or it may be a curved or bent plate. Generally, embodiments in which the cover plate has a small curvature only in the edge region are also conceivable. Both the length and width dimensions determine the two main faces or sides (sometimes referred to as "surfaces") of the cover plate 1.

[0097] Figure 2 shows a schematic, not-to-scale, cross-sectional view of the cover plate 1 according to an embodiment of this disclosure. The cover plate 1 has two sides 10 and 12 (these sides may also be referred to as the “surface” or “main surface” of the cover plate 1), with side 10 here being the top surface and side 12 being the bottom surface. Furthermore, the thickness d of the cover plate 1 is shown. The cover plate 1 has a layer 101 located between both sides 10 and 12, which is also referred to in the scope of this disclosure as the “near-surface layer”. The near-surface layer 101 is formed on both sides of the cover plate 1 and can be formed similarly, i.e., having the same thickness, for example, within the range of measurement accuracy. However, it is possible, and even preferable, for the thickness of the near-surface layer 101 belonging to one of the sides, for example side 10, to be different from the near-surface layer belonging to side 12. This may be the case, for example, when the chemical strengthening of the cover plate 1 is carried out in such a way that uneven replacement occurs.

[0098] A central region 102 is located between the two near-surface layers 101. Between the near-surface layers 101 and the central region 102, there may be further regions where ion exchange has occurred but which do not contribute to, for example, compressive stress (though not shown in Figure 2). The central region is generally the region of the cover plate 1 with the minimum stress. In contrast, the near-surface layers 101 have higher stress, and in particular, they can be under compressive stress. The cover plate 1 generally comprises a silica-based glass ceramic, where the cover plate 1 generally has a thickness d of 0.1 mm to 2 mm. The preferred lower limit of the thickness can generally be 0.4 mm. The preferred upper limit of the thickness can generally be 0.85 mm. According to one embodiment, the light transmittance τ of the cover plate 1 visThe coating is preferably greater than 80% in the 380nm to 780nm range, and more preferably greater than 85%, with a thickness of 0.1mm to 2mm, and particularly preferably greater than 0.7mm. The cover plate 1 exists as a chemically strengthened cover plate by chemical strengthening that leads to the presence of compressive stress in at least one or both near-surface layers 101, particularly in layers of 20μm to 70μm perpendicular to the sides 10 and 11 of the cover plate 1, and its stress profile has at least one inflection point at a depth of the cover plate that is preferably greater than 10μm. Advantageously, the crystalline phase contained in the glass ceramic of the cover plate 1 can be a silica-based crystalline phase. By chemical strengthening, the cover plate 1 has a CS of at least 250MPa and preferably up to 1500MPa.

[0099] Figures 3-6 illustrate the implementation of a so-called set drop test to determine the intensity of set drop.

[0100] Here, the set-drop trial is advantageously conducted as follows:

[0101] The cover plate is fixed to the sample container and dropped onto a defined surface from a cumulative drop height. An overview of the overall structure is shown in Figure 3. The cover plate used in the set drop test in Figure 5 has a length of 99 mm and a width of 59 mm and is magnetically fixed to the sample dummy in the sample container, as shown in Figure 4. However, for the experiments described in this disclosure, a cover plate format of 49.5 mm × 49.5 mm is used, which does not affect the principle configuration of the test implementation in Figures 3-6, but the weight of the dummy is reduced accordingly.

[0102] First, a plastic plate is attached to a metal housing having the shape and weight of a holder for a mobile device, such as a smartphone, using double-sided adhesive tape. In this case, a plastic plate having a thickness of, for example, 4.35 mm to 4.6 mm is suitable (see Figure 5). The attachment is preferably done using double-sided adhesive tape having a thickness of approximately 100 μm (for investigations with reinforced cover plates containing glass or glass ceramic) or 295 μm (for investigations with unreinforced cover plates containing glass or glass ceramic). Next, the plate-shaped glass article to be tested is attached to the plastic plate using double-sided adhesive tape, preferably double-sided adhesive tape with a thickness of 295 μm, in particular double-sided adhesive tape of trademark tesa®, product number 05338, such that a gap of 350 μm to 450 μm is maintained between the upper end of the housing or holder and the upper end of the glass article. The cover plate is higher than the frame of the housing, and there should be no direct contact between the cover plate and the aluminum housing. This simulates the attachment of a cover plate to a mobile device, and is a kind of "dummy" for a real mobile device, in this case a smartphone, resulting in a weight of 60.5g (cover glass format 49.5×49.5mm). 2A “set” weighing 37.5g (for investigation with tempered glass or glass ceramic) or 37.5g (for investigation with untempered glass or glass ceramic) is then dropped vertically, i.e., downward from zero, with a certain initial velocity, on a surface the size of DIN A4, the so-called impact surface, glass-side down. The impact surface is then prepared as follows: Sandpaper with an appropriate grit size, e.g., grit 60 (#60), is attached to the base plate using double-sided adhesive tape, e.g., 100 μm thick (for investigation with tempered cover plates containing glass or glass ceramic) or 3 × 100 μm thick (for investigation with untempered cover plates containing glass or glass ceramic). Transparent double-sided Tesa (10m / 15mm), product number 05338, was used as the adhesive tape. Within the scope of this disclosure, particle size is defined in accordance with the standards of the Federation of European Producers of Abrasives (FEPA), for example see DIN ISO 6344, particularly DIN ISO 6344-2:2000-04, and Abrasives on substrates - Particle size analysis - Part 2: Specification of the particle size distribution of macroparticles pp. 12-220 (ISO 6344-2: 1998). In the case of the values ​​disclosed herein, the weight of the aluminum substrate base plate reaches approximately 3 kg.

[0103] The base plate must be rigid and preferably made of aluminum, or alternatively, steel. The sandpaper must be completely attached with adhesive tape and glued without air bubbles. The impact surface may be used only for five drop tests and should be replaced after five drop tests. The sample, i.e., the resulting set, is placed in the test apparatus and adjusted using a 2D level (circular bubble tube) so that the set is horizontal, with the cover plate facing the floor, i.e., the direction of the impact surface (see Figure 6). The initial drop height is 20 cm, and if no fracture occurs, the drop height is increased by 10 cm increments until glass fracture occurs. The fracture height, the point of fracture, and the appearance of the fracture are recorded. The test is performed on 10 to 15 samples and the average value is taken.

[0104] Figure 7 shows the stress profile of a first cover plate according to one embodiment of the present disclosure, reinforced with potassium ions, in the upper region and the EDX curve in the lower region. The approximate locations of the inflection points in the stress profile are shown in the upper part of Figure 7. The lower part of Figure 7 shows the progression of potassium oxide concentration over the depth of the cover plate (plotted on the x-axis). In addition to the measurement points (black squares), a “convex fitting” (dotted line) to this data is also shown, which clearly illustrates the anomalous progression of the concentration curve (which is converted to a stress profile as is known). For comparison, a “typical” expected progression of the concentration profile is also shown, which can conventionally be described by a complementary error function (dashed line in the lower part of Figure 7).

[0105] Figure 8 shows the stress profile of a first cover plate according to one embodiment of the present disclosure, reinforced with sodium ions, in the upper region and the EDX curve in the lower region. The approximate locations of the inflection points in the stress profile are shown in the upper part of Figure 8. The lower part of Figure 8 shows the progression of sodium oxide concentration over the depth of the cover plate (plotted on the x-axis). In addition to the measurement points (black squares), a “convex fitting” to this data (dotted line) is also shown, which clearly illustrates the anomalous progression of the concentration curve (which is converted to a stress profile as is known). For comparison, a “typical” expected progression of the concentration profile is also shown, which can conventionally be described by a complementary error function (dashed line in the lower part of Figure 8).

[0106] Finally, Figure 9 shows the stress profile obtained after two ion exchanges. This stress profile has three inflection points, which are approximately shown in the graph in Figure 9.

[0107] In Figures 7-9, at least one inflection point is included in the stress profile at a depth of at least 10 μm in the cover plate.

[0108] Finally, Figure 10 shows a comparison of the set drop strength of different cover plates. Here, 2 shows the results obtained for the drop height for conventionally chemically strengthened glass. 3 shows the results for a cover plate according to the first embodiment, strengthened for 14 hours at 440°C in a pure sodium bath (100% NaNO3). Finally, 4 shows the results for a cover plate according to a further embodiment, strengthened for 9 hours at 440°C in a pure sodium exchange bath (100% NaNO3). The set drop test results are also summarized in the table below. Drop heights are given in cm.

[0109] [Table 4]

[0110] Samples 2-4 were obtained using the following exchange conditions: [Table 5]

[0111] Figure 11 shows an example stress profile of a cover plate that is consistent with conventional technology and whose curve progression can be approximated by a complementary error function in the near-surface region, up to DoCL, or by a parabola. The central region, where the stress is minimum, is indicated by 102.

[0112] Figure 12 exemplifies the progression of the stress profile for a cover plate containing a silica-based glass ceramic according to one embodiment, and for a cover plate containing silica-based glass with a chemical composition corresponding to that of the glass ceramic. Here, Figure 12 shows the progression of the curve investigated using the SLP measuring device, and the dependence of the smoothed stress curves on the depth of the cover plate. As can be seen, particularly in the region near the surface of the cover plate, the strengthening in the cover plate according to one embodiment containing glass ceramic is clearly greater than the strengthening in a cover plate made of or containing glass, corresponding to efficient strengthening in glass ceramic, i.e., efficient stress construction by ion exchange. In particular, in the example shown, the compressive stress in glass ceramic is greater than the compressive stress in glass, and is greater over almost the entire compressive stress region (due to inaccuracies caused by the measurement technique, it is not entirely clear whether the measured stress profile curve intersects before or after the stress zero crossing). In this case, correspondingly, the tensile stress (CT) finally obtained in glass ceramic is also greater than in glass.

[0113] Figure 13 shows a comparison of sodium oxide concentrations in glass and glass ceramics under different strengthening processes using sodium as the "strengthening ion," where glass and glass ceramics have the same chemical composition. The corresponding stress curve for the cover plate investigated here corresponds to that shown in Figure 12. As can be seen, the sodium oxide concentration is always higher in glass up to a certain depth (e.g., about 140 μm). This roughly corresponds to the DoCL of the sample shown in Figure 12. Furthermore, regarding the concentration progression, a characteristic curve progression (or a so-called "convex" progression) with at least one inflection point is observed in glass ceramics.

[0114] The ion exchange advantage of the cover plate according to the embodiment compared to a cover plate made of or containing glass of the same chemical composition is also shown in Figure 14, where stress curves or measurements using SLP are described for the sample. Here, ion exchange was performed for a cover plate containing silica-based glass ceramic according to one embodiment, and for a cover plate containing a corresponding silica-based glass, so that the same values ​​were obtained for CS, i.e., for the compressive stress on the surface of the cover plate, and for DoCL.

[0115] The values ​​obtained in the so-called set drop test for both of these samples, along with the results obtained for the unreinforced samples, are shown in Figure 15 (Figures 15a and 15b). Here, 7 and 8 represent the values ​​obtained for sample groups including unreinforced silica-based glass (7) or unreinforced silica-based glass ceramic (8) cover plates having a chemical composition corresponding to the glass of the sample 7. As can be seen, the corresponding drop height or break height with #180 sandpaper is equivalent for sample groups 7 or 8, at 36.3 cm and 31.3 cm (arithmetic mean), or 27.5 cm and 25 cm (median). It is particularly noteworthy that the values ​​for unreinforced glass ceramics are not as good as those for unreinforced glass.

[0116] However, this situation changes when examining the values ​​in the #60 set-drop test for appropriately reinforced coverplates, where 9 represents a sample of a coverplate containing reinforced silica-based glass, and 10 represents a sample of a coverplate containing appropriate silica-based glass ceramic. As mentioned above, the stress values ​​(CS, DoCL) are identical within the range of measurement accuracy (see Figure 14). Here, for the embodiment, i.e., the coverplate containing silica-based glass ceramic, a stress profile with at least one inflection point is clearly advantageous, as can be seen from Figure 14. Therefore, stress construction by ion exchange occurs more efficiently in the coverplate according to the embodiment and leads to better results in application-relevant tests.

[0117] The values ​​that form the basis of Figure 15 are also listed in the table below: [Table 6]

[0118] Here, all values ​​for drop height are given in centimeters (cm). [Explanation of Symbols]

[0119] 1 Cover plate 10, 12 Side of the cover plate 101 Surface-neighboring layer of the cover plate 102 Center d. Thickness of the cover plate l Length of the cover plate b Width of the cover plate 7, 8, 9, 10 Results of set-drop tests on different sample populations 6. Enhanced Profile of Comparative Example

Claims

1. 1. A cover plate having a thickness of 0.1 mm to 2 mm, comprising a silica-based glass ceramic, the silica-based glass ceramic comprising high quartz or keatite mixed crystals as a predominant crystalline phase, the cover plate being chemically strengthened and having a surface compressive stress of at least 250 MPa and preferably at most 1500 MPa, the stress profile having at least one inflection point preferably at a depth of the cover plate greater than 10 μm.

2. The cover plate preferably has a light transmittance τ of more than 80%, preferably more than 85%, in the range from 380 nm to 780 nm, for a thickness of 0.4 mm to 0.85 mm, particularly preferably for a thickness of 0.7 mm. vis The cover plate of claim 1 , comprising:

3. 2. The cover plate according to claim 1, wherein the glass ceramic is formed as a lithium aluminum silicate glass ceramic and the crystalline phase is formed as a keatite mixed crystal.

4. The glass-ceramic may comprise: SiO 2 55 to 75, preferably 62 to 72 Al 2 O 3 18 to 27, preferably 18 to 23 Li 2 O 2.8 to 5, preferably 3 to 5 The cover plate of claim 1 , comprising, in mass % on an oxide basis:

5. The glass ceramic comprises the following components: SiO 2 55 to 75, preferably 62 to 72 <h2 style=";text-align:left;direction:ltr">Al<h2 style=";text-align:left;direction:ltr"> 2 <h2 style=";text-align:left;direction:ltr"> O<h2 style=";text-align:left;direction:ltr"> 3 <h2 style=";text-align:left;direction:ltr"> 18222 Li 2 O 2.8 to 5, preferably 3 to 5 Na 2 O 0 to 4, preferably 0 to 2 K 2 O 0 to 4, preferably 0 to 2 MgO 0 to 8, preferably 0 to 4 CaO 0-4, preferably 0-2 SrO 0-4, preferably 0-2 BaO 0-4, preferably 0-2 ZnO 0 to 6, preferably 0 to 2 TiO 2 0 to 4, preferably 0 to 3 ZrO 2 0 to 5, preferably 1.2 to 4 B 2 O 3 0 to 2, preferably 0 to 0.1 Fe 2 O 3 0.0001 to 0.1, preferably 0.0001 to 0.02 SnO 2 0 to 2, preferably 0.05 to 1.6 in wt. % on an oxide basis, where advantageously TiO 2 and ZrO 2 Regarding the sum of the components, 0<(TiO 2 + ZrO 2 ) < 9.5%, preferably 1.2 < (TiO 2 + ZrO 2 ) <9.5% 2. The cover plate of claim 1, wherein:

6. Component SnO 2 , ZrO 2 and TiO 2 Regarding 0≦SnO 2 / (ZrO 2 + TiO 2 ) <0.8, preferably 0.01≦SnO 2 / (ZrO 2 + TiO 2 ) <0.7 2. The cover plate according to claim 1, wherein:

7. 2. The cover plate of claim 1, characterized in that it has a sharp impact strength determined in a set drop test up to a drop height of at least 120 cm to 200 cm.

8. 10. The cover plate of claim 1, wherein the cover plate is present as a cover plate strengthened with sodium ions and / or the cover plate is present as a cover plate strengthened with potassium ions.

9. The cover plate of claim 1 , wherein the glass-ceramic of the cover plate does not include lithium metasilicate as a crystalline phase.

10. 1. A cover plate having a thickness of 0.1 mm to 2 mm, comprising a silica-based glass ceramic, the silica-based glass ceramic comprising high quartz mixed crystals or keatite mixed crystals as the predominant crystalline phase, the cover plate being chemically strengthened and having a surface compressive stress of at least 250 MPa and preferably at most 1500 MPa, the cover plate comprising a material having a surface compressive stress of at least 250 MPa and preferably at most 1500 MPa, the material ... having a surface compressive stress of at least [Equation 1] where x is the depth of the cover plate, CS is the compressive stress at this depth, CT is the maximum tensile stress (internal tension), c(x) イオン is the concentration of the ion used for reinforcement, c (bulk) イオン is the concentration in the bulk of the ion used for strengthening, ν is the Poisson's ratio, and E is the elastic modulus of the glass ceramic, preferably the ion considered and used for strengthening is the sodium ion, and the β value is 3×10 －4 / mol ~ 9 × 10 －4 / mol, e.g., 4 x 10 －4 / mol ~ 8 x 10 －4 / mol.

11. 11. A method for manufacturing a cover plate according to any one of claims 1 to 10, comprising the following steps: - producing silica green glass by a melting process and subsequent hot forming; a heat treatment step of the silica green glass, in which at least one nucleation step is carried out at a temperature in the range from 650°C to 850°C, advantageously from 690°C to 850°C, for a time period of from 5 minutes to 60 hours, advantageously up to 8 hours, particularly preferably from 30 minutes to 2 hours, and at least one ceramming step is carried out at a temperature in the range from 700°C to 1100°C, advantageously from 780°C to 1100°C, for a time period of from 3 minutes to 120 hours, advantageously up to 60 hours, preferably from 3 minutes to 8 hours, 100% by mass to 0% by mass of KNO 3 and / or 0% by weight to 99.9% by weight of NaNO 3 and 0% to 5% by weight of LiNO 3 carrying out at least one ion exchange in an exchange bath having a composition of the formula: The method comprising:

12. Only one ion exchange is carried out, the exchange bath preferably being NaNO 3 and optionally up to 0.1% by weight of LiNO 3 or KNO having a purity of 99.9% by mass 3 The method of claim 11 , comprising:

13. A cover plate produced or producible by the method of claim 11.

14. Use of a cover plate according to any one of claims 1 to 10 in an electronic device, in particular in an electronic display device, in particular in a mobile electronic display device, such as a mobile touch panel and / or a mobile digital display device, such as a smartphone or a smartwatch.

15. A display device, in particular a digital display device, comprising at least one cover plate according to any one of claims 1 to 10.

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