COVER GLASS WITH AN ANOMALOUS STRESS PROFILE, PROCESS FOR PRODUCTION THEREOF AND USE THEREOF

Abstract

A cover glass is provided that includes a silica based glass ceramic with a thickness between 0.4 mm and 0.85 mm. The glass ceramic has a transmittance of more than 80% from 380 nm to 780 nm and a stress attribute selected from: an overall compressive stress (CS) of at least 250 MPa and at most 1500 MPa, a compressive stress at a depth of 30 μm (CS30) from one of the two faces of at least 160 MPa and at most 525 MPa, a depth of the compression layer (DoCL) of at least 0.2 times the thickness and less than 0.5 times the thickness, and any combinations thereof. The glass ceramic has at least one silica based crystal phase having in a near-surface layer a unit cell volume of at least 1% by volume larger than that of a core where the crystal phase has minimum stresses.

Claims

1. A cover glass comprising: a chemically tempered silica based glass ceramic having two faces; a thickness between the two faces of at least 0.4 mm and at most 0.85 mm; a transmittance of more than 80% in a range from 380 nm to 780 nm; and a stress attribute selected from a group consisting of: an overall compressive stress (CS) of at least 250 MPa and at most 1500 MPa, a compressive stress at a depth of 30 μm (CS30) from one of the two faces of at least 160 MPa and at most 525 MPa, a depth of the compression layer (DoCL) of at least 0.2 times the thickness and less than 0.5 times the thickness, and any combinations thereof, wherein the chemically tempered silica based glass ceramic comprises at least one silica based crystal phase having in a near-surface layer of 20 μm to 70 μm, determined at a right angle from one of the two faces, a unit cell volume of at least 1% by volume larger than that of a core where the crystal phase has minimum stresses.

2. The cover glass of claim 1, wherein the unit cell volume is at least 2% by volume larger.

3. The cover glass of claim 1, wherein the at least one silica based crystal phase in the near-surface layer has a higher proportion of sodium oxide Na.sub.2O and/or potassium oxide K.sub.2O than in the core.

4. The cover glass of claim 1, wherein the chemically tempered silica based glass ceramic comprises more than 50% by volume of crystal phase and not more than 95% by volume.

5. The cover glass of claim 1, further comprising a color value C* of less than 4.

6. The cover glass of claim 1, further comprising a color value C* of less than 3.

7. The cover glass of claim 1, further comprising a haze of 0.01% to 1%, based on a thickness of the cover glass of 0.7 mm.

8. The cover glass of claim 1, wherein the chemically tempered silica based glass ceramic is an lithium aluminum silicate glass ceramic and the crystal phase is a keatite solid solution.

9. The cover glass of claim 1, wherein the chemically tempered silica based glass ceramic comprises components in % by weight based on oxide: SiO.sub.2 55-75, Al.sub.2O.sub.3 18-27, and Li.sub.2O 2.8-5.

10. The cover glass of claim 1, wherein the chemically tempered silica based glass ceramic comprises components in % by weight based on oxide: SiO.sub.2 62-72, Al.sub.2O.sub.3 18-23, and Li.sub.2O 3-5.

11. The cover glass of claim 1, wherein the chemically tempered silica based glass ceramic comprises components in % by weight based on oxide: SiO.sub.2 55-75, Al.sub.2O.sub.3 18-27, Li.sub.2O 2.8-5, Na.sub.2O 0-4, K.sub.2O 0-4, MgO 0-8, CaO 0-4, SrO 0-4, BaO 0-4, ZnO 0-6, TiO.sub.2 0-4, ZrO.sub.2 0-5, B.sub.2O.sub.3 0-2, Fe.sub.2O.sub.3 0.0001-0.1, and SnO.sub.2 0-2, where the following condition is applicable to a sum total of the TiO.sub.2 and ZrO.sub.2 components: 0<Σ (TiO.sub.2+ZrO.sub.2)<9.5%.

12. The cover glass of claim 11, wherein the following condition is applicable to the sum total of the TiO.sub.2 and ZrO.sub.2 components: 1.2<Σ (TiO.sub.2+ZrO.sub.2)<9.5%.

13. The cover glass of claim 1, wherein the chemically tempered silica based glass ceramic comprises components in % by weight based on oxide: SiO.sub.2 62-72, Li.sub.2O 3-5, Na.sub.2O 0-2, K.sub.2O 0-2, MgO 0-4, CaO 0-2, SrO 0-2, BaO 0-2, ZnO 0-2, TiO.sub.2 0-3, ZrO.sub.2 1.2-4, B.sub.2O.sub.3 0-0.1, Fe.sub.2O.sub.3 0.0001-0.02, and SnO.sub.2 0.05-1.6, where the following condition is applicable to a sum total of the TiO.sub.2 and ZrO.sub.2 components: 0<Σ (TiO.sub.2+ZrO.sub.2)<9.5%.

14. The cover glass of claim 13, wherein the following condition is applicable to a sum total of the TiO.sub.2 and ZrO.sub.2 components: 1.2<Σ (TiO.sub.2+ZrO.sub.2)<9.5%.

15. The cover glass of claim 1, wherein the chemically tempered silica based glass ceramic comprises SnO.sub.2, ZrO.sub.2, and TiO.sub.2, and wherein SnO.sub.2, ZrO.sub.2, and TiO.sub.2 satisfy the following condition 0≤SnO.sub.2/(ZrO.sub.2+TiO.sub.2)<0.8.

16. The cover glass of claim 1, wherein the chemically tempered silica based glass ceramic comprises SnO.sub.2, ZrO.sub.2, and TiO.sub.2, and wherein SnO.sub.2, ZrO.sub.2, and TiO.sub.2 satisfy the following condition 0.01≤SnO.sub.2/(ZrO.sub.2+TiO.sub.2)<0.7.

17. The cover glass of claim 1, further comprising a sharp impact strength determined in a set drop test that is between a drop height of at least 120 cm and up to 200 cm.

18. The cover glass of claim 1, wherein the cover glass is configured for a use selected from a group consisting of a cover glass of an electronic device, a cover glass of an electronic display, a cover glass of a mobile electronic display device, a cover glass of a mobile touch panel, a cover glass of a mobile digital display device, a cover glass of a smartphone, and a cover glass of a smartwatch.

19. A process for producing a cover glass, comprising the steps of: producing a silica based green glass by a melting process; hot shaping the silica based green glass; thermally treating the silica based green glass, wherein at least one nucleation step is conducted within a temperature range of 690° C.-850° C. for a duration of 5 minutes to 8 hours, and at least one ceramization step within the temperature range of 780° C.-1100° C. for a duration of 3 minutes to 60 hours; and performing at least one ion exchange in an exchange bath having a composition of 100% by weight to 0% by weight of KNO.sub.3 and 0% by weight to 100% by weight of NaNO.sub.3 and 0% by weight-5% by weight of LiNO.sub.3 at exchange bath temperature between 370° C. and 500° C. and for a duration between 2 hours and 50 hours.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0084] FIG. 1 shows a schematic diagram (not to scale) of a cover glass according to embodiments of the present disclosure.

[0085] FIG. 2 shows a schematic section diagram (not to scale) of a cover glass according to embodiments of this disclosure.

[0086] FIG. 3 shows an overall structure of a test apparatus for a drop test.

[0087] FIG. 4 shows a sample receptacle of the test apparatus of FIG. 3.

[0088] FIG. 5 shows a cover glass of the test apparatus of FIG. 3.

[0089] FIG. 6 shows the sample in the test apparatus of FIG. 3.

[0090] FIGS. 7A and 7B show the evaluation of EDX measurements on two different samples of a glass-ceramic cover glass.

[0091] FIG. 8 shows a comparison of set drop resistance of different cover glasses.

[0092] FIG. 9 compares illustrative tempering profiles of cover glasses with one another.

DETAILED DESCRIPTION

[0093] The invention is elucidated in detail hereinafter by examples.

[0094] The compositions of inventive glass-ceramic materials can be found in TABLE 1.

[0095] The materials listed in TABLE 1 were melted and refined using raw materials customary in the glass industry at temperatures of about 1600 to 1680° C. The batch was first melted here in sintered silica glass crucibles and was then decanted into Pt/Rh crucibles with inner silica glass crucibles and homogenized by stirring at temperatures of about 1550° C. for 30 minutes. After being left to stand at 1640° C. for 2 h, castings of about 140 mm×100 mm×30 mm in size were made and annealed in a cooling oven at about 620 to 680° C. and cooled down to room temperature. The castings were used to prepare the test specimens for the measurement of the properties in the vitreous state and for the ceramizations.

[0096] For the ceramizations, in general, two-stage programs were used, which are specified in TABLE 1. In these, the starting glasses are heated from room temperature firstly to a nucleation temperature above T.sub.g, and kept at that temperature for a period sufficient for nucleation. Subsequently, the samples are heated to the ceramization temperature and likewise kept at that temperature. It is also possible to use three- or multistage programs (Example 2 in TABLE 1). Hold times may also be replaced by slow heating rates.

[0097] The ceramized samples were used to determine, with the aid of XRD, crystal phases and the contents thereof and transmittance in the visible region τ.sub.vis (on samples having thickness 0.7 mm) and color values in the Lab system (standard illuminant C).

[0098] The crystal phase contents reported in TABLE 1 were determined with the aid of x-ray diffraction measurements using a Panalytical X'Pert Pro diffractometer (Almelo, the Netherlands). The x-radiation used was CuKα radiation generated by means of an Ni filter (λ=1.5060 Å). The standard x-ray diffraction measurements on powder samples and solid-state samples were conducted using Bragg-Brentano geometry (θ-2θ). The x-ray diffraction diagrams were measured between 10° and 100° (2θ angle). The relative crystalline phase fractions were quantified, and the crystallite sizes determined, via a Rietveld analysis. Measurement was effected on ground sample material, as a result of which the volume fraction of the core region is distinctly dominant. The measured phase fractions therefore correspond to the phase distribution in the core of the glass ceramic. The “V” samples correspond to comparative examples. The examples that have merely been numbered are examples of embodiments.

TABLE-US-00002 TABLE 1 Examples V1 V2 1 2 3 Al.sub.2O.sub.3 19.95 19.95 22.50 19.86 21.70 AS.sub.2O.sub.3 0.85 0.85 B.sub.2O.sub.3 0.20 BaO 0.84 0.84 2.26 0.55 CaO 0.02 0.43 0.25 Cl Fe.sub.2O.sub.3 0.012 0.015 HfO.sub.2 K.sub.2O 0.19 0.19 0.20 0.27 0.32 L.sub.i2O 3.67 3.67 4.40 3.95 3.64 MgO 1.07 1.07 1.02 0.27 0.32 Na.sub.2O 0.15 0.15 0.65 0.61 0.15 Nd.sub.2O.sub.3 0.06 0.06 0.24 0.05 P.sub.2O.sub.5 0.20 0.03 Sb.sub.2O.sub.3 SiO.sub.2 67.31 67.31 65.50 66.64 66.40 SnO.sub.2 0.54 0.12 0.07 SrO 0.50 TiO.sub.2 2.29 2.29 1.60 2.20 2.20 V.sub.2O.sub.5 ZnO 1.70 1.70 0.44 1.50 1.94 ZrO.sub.2 1.76 1.76 1.95 1.90 1.85 NaCl 0.12 TOTAL 99.84 99.84 99.58 100.02 99.99 SnO.sub.2/(TiO.sub.2 + ZrO.sub.2) 0.00 0.00 0.15 0.03 0.02 Density 2.512 [g/cm.sup.3] Ceramization 760° C./30 min. + 760° C./30 min. + 760° C./60 min. + 795° C./60 min. + 760° C./60 min. + 900° C./10 min. 990° C./5 min. 990° C./8 min. 930° C./60 min. + 1000° C./12 min 975° C./5 min. Main crystal HQMK KMK KMK KMK KMK + phases HQMK Relative n.d. n.d. 96 96 86 proportion of KMK τ.sub.vis (C/2) 0.7 mm 90.3 76.6 80.2 87.6 84.9 YI (std. ill. C 1.7 yellow) L* 96.13 90.1 91.8 95 94.1 a* −0.14 −0.1 −1.2 −0.1 1.2 b* 0.77 8.6 9.1 1.5 4.4 C* 0.78 8.6 9.2 1.5 4.6 Examples V3 V4 4 5 6 Al.sub.2O.sub.3 20.50 21.98 22.02 19.35 19.35 AS.sub.2O.sub.3 BaO 1.92 1.23 CaO 0.02 0.02 0.03 0.20 Fe.sub.2O.sub.3 0.014 0.008 0.008 0.018 0.010 HfO.sub.2 K.sub.2O 0.50 Li.sub.2O 3.20 3.59 3.68 4.25 4.50 MgO 0.10 0.75 1.20 1.26 0.50 Na.sub.2O 0.52 0.38 0.39 0.06 0.50 P.sub.2O.sub.5 1.37 1.36 SiO.sub.2 67.40 67.70 65.90 70.90 68.50 SnO.sub.2 0.07 1.22 1.24 1.59 1.50 SrO 0.01 0.50 TiO.sub.2 2.31 0.01 0.01 0.02 ZnO 1.42 1.50 ZrO.sub.2 1.80 2.92 2.93 2.44 2.50 TOTAL 99.25 99.94 100.00 99.92 100.06 SnO.sub.2/(TiO.sub.2 + ZrO.sub.2) 0.02 0.42 0.42 0.65 0.60 T.sub.g 701 725 700° C. Ceramization 760° C./60 min. + 760° C./60 min. + 760° C./60 min. + 760° C./60 min. + 740° C./3 h + 1000° C./12 min 1000° C./12 min 980° C./8 min 990° C./12 min. 830° C./10 min Main crystal HQMK HQMK KMK KMK KMK phases Relative — — 97.4 94.5 98.2 proportion of KMK KMK crystallite n.d. n.d. 68 69 69 size (nm) Transmittance D = n.d. n.d. n.d. n.d. n.d. 0.8 mm τ.sub.vis (C/2) 0.7 mm 88.9 83.4 80.6 90 82.5/4 mm YI (std. ill. C yellow) Standard illuminant C L* 95.5 93.2 91.9 96 92.8 a* −0.3 −0.1 0.4 −0.1 −0.7 b* 2.4 5.6 3.2 1.2 6.1 C* 2.4 5.6 3.2 1.2 6.2 Examples 7 8 9 Al.sub.2O.sub.3 19.35 26.20 30.71 As.sub.2O.sub.3 0.50 BaO CaO 0.20 2.44 Fe.sub.2O.sub.3 0.010 HfO.sub.2 0.10 K.sub.2O 0.50 6.13 Li.sub.2O 4.50 4.32 MgO 0.50 0.25 Na.sub.2O 0.50 0.51 12.36 P.sub.2O.sub.5 SiO.sub.2 68.50 59.20 41.25 SnO.sub.2 1.50 0.47 SrO 0.50 TiO.sub.2 7.37 ZnO 1.50 1.81 ZrO.sub.2 2.50 4.16 1.67 TOTAL 100.06 99.46 100.00 SnO.sub.2/(TiO.sub.2 + ZrO.sub.2) 0.60 0.11 0.00 T.sub.g 700° C. Ceramization 740° C./3 h + 760° C./60 min. + 850° C./4 h + 850° C./30 min 980° C./8 min. 900° C./30 min. Main crystal KMK KMK Nepheline, phases rutile (tr) Relative 98.3 96.5 — proportion of KMK KMK crystallite 72 81 Nepheline = size (nm) 62 nm Transmittance D = n.d. 0.8 mm τ.sub.vis (C/2) 0.7 mm 90.2 82.8 82.2 YI (std. ill. C yellow) Standard illuminant C L* 96.1 92.9 92.7 a* −0.2 0.1 −1.3 b* 1.2 2.9 8.5 C* 1.2 2.9 8.6

[0099] For tempering tests, ceramized glass-ceramic panes having a thickness of 0.7 mm were tempered in various salt baths. TABLE 2 shows the change in the crystallographic data on tempering of a glass ceramic of the invention.

TABLE-US-00003 TABLE 2 a (Å) C (Å) V (Å.sup.3) ΔV Literature value for 7.505 9.070 510.91 Li.sub.0.75Al.sub.0.75Si.sub.2.25O.sub.6 Non-tempered 7.499 9.099 511.64 0 100% KNO.sub.3 7.501 9.199 517.56 1.16% 7.443 9.687 536.59 4.88% 80% KNO.sub.3 + 20% NaNO.sub.3 + 7.499 9.261 520.76 1.78% 100% KNO.sub.3 99% KNO.sub.3 + 1% NaNO.sub.3 7.497 9.275 521.35 1.90% 95% KNO.sub.3 + 5% NaNO.sub.3 7.502 9.295 523.09 2.24% 90% KNO.sub.3 + 10% NaNO.sub.3 7.503 9.303 523.77 2.37% (Literature value ICDD-PDF# 00-035-0794)

[0100] The sample after ceramization contains keatite solid solutions as the main crystal phase (96% keatite solid solution, 3% ZrTiO.sub.4). After tempering (at temperatures of 420-440° C. for 7.5-18 h), all samples, irrespective of the salt bath selected, had an increase in the size of the unit cell in the near-surface layer of more than 1% compared to the non-tempered sample. The sample that was tempered in 100% KNO.sub.3, in the near-surface layer, even showed the formation of two different keatite solid solution structures, both of which had a greater unit cell volume compared to the non-tempered keatite. All samples additionally showed an increase in strength with DoCL values of 102 μm (100% KNO.sub.3)-154 μm (80% KNO.sub.3/20% NaNO.sub.3). The CS 30 values were between 195 MPa and 360 MPa.

[0101] Samples with a composition according to example 9 were produced in an analogous manner, ceramized as specified in TABLE 1, ex. 9, and tempered. They contain nepheline ((Na,K)[AlSiO.sub.4]) as the main crystal phase and traces of rutile. An XRD measurement for the nepheline (hexagonal structure) gave the following crystallographic data: a=10.026 (5) Å, c=8.372(5) Å, unit cell volume: V=728.8(10) Å.sup.3. The tempering (100% KNO.sub.3, 8 h at 500° C.) gave rise to kalsilite (potassium-substituted end member of the nepheline solid solution series, KAlSiO.sub.4): a=5.170(5) Å, c=8.730(5) Å. For direct comparison, it is necessary here on account of the different unit cell size of the two structures to double the a lattice constant (for there to be the same number of formula units in the unit cell). For the kalsilite, this results in a unit cell volume of V=808.3(10) Å.sup.3, corresponding to an increase in size of about 10%.

[0102] Tempering conditions and tempering parameters thus achieved are listed for different cover glasses in the table below.

TABLE-US-00004 Ion exchange measured CS 0 CS 30 DoCL CT Sample Step 1 Step 2 with [MPa] [MPa] [μm] [MPa] V5 7.5 h 420° C. 4 h 405° C. SLP 1000 842 179 150 110 80/20% 100% K FSM 6000 K/Na V6 10 h 400° C. SLP 1000 294 225 143 148 80/20% K/Na 10 7.5 h 440° C. SLP 1000 552 363 112 147 100% Na 11 7.5 h 440° C. SLP 1000 341 219 108 81 99.5/0.5% K/Na 12 7.5 h 440° C. SLP 1000 506 300 107 149 100% Na 13 7.5 h 420° C. 4 h 405° C. SLP 1000 360 252 121 119 80/20% 100% K K/Na 14 12 h 440° C. SLP 1000 438 335 145 217 80/20 K/Na 15 7.5 h 490° C. SLP 1000 987  27 28.1/34 35 100% K FSM 6000 16 7.5 h 440° C. FSM 6000 1088 nd  11 29 100% K 17 7.5 h 440° C. SLP 1000 341 240 121 118 99/1% Na/Li

[0103] CT stands here for center tension and is reported in MPa.

[0104] FIG. 1 shows the schematic diagram (not to scale) of a cover glass 1 according to embodiments of the present disclosure. The cover glass 1 in the present case is in the form of a pane or sheet in that its thickness d (not identified in FIG. 1) is at least one order of magnitude less than the length l and width b of the cover glass 1. The cover glass 1 may, as shown by way of example in FIG. 1, be flat or planar or in the form of a curved or bent pane. Other conceivable embodiments are those in which the cover glass has merely slight curvature in the edge region. The two dimensions of length and width determine the two main areas or sides (in some cases also called “surfaces”) of the cover glass 1.

[0105] FIG. 2 shows a schematic section diagram (not to scale) of a cover glass 1 according to embodiments of this disclosure. The cover glass 1 has two sides 10, 12 (these sides may also be referred to as “surfaces” or “main surfaces” of the cover glass 1), with side 10 designed here as top side and side 12 as bottom side. In addition, the thickness d of the cover glass 1 is identified. The cover glass 1 has a layer 101 disposed between the two sides 10, 12, which is also referred to as “near-surface layer” in the context of the present disclosure. The near-surface layer 101 is formed on either side of the cover glass 1 and may be the same, i.e., for example, have an equal thickness within the scope of measurement accuracy. It may alternatively be possible and even preferable for the thickness of the near-surface layer 101 facing one of the two sides, for example side 10, to have a different thickness than the near-surface layer facing side 12. This may be the case, for example, when the chemical tempering of the cover glass 1 has been executed such that exchange is unequal.

[0106] The core 102 lies between the two near-surface layers 101. There may be a further adjoining region between the near-surface layer 101 and the core 102, although not identified in FIG. 2, in which there has been ion exchange, but without contributing anything to compressive stress, for example. The core is generally the region of minimum stress in the cover glass 1. The near-surface layers 101, by comparison, have higher stress; they may especially be under compressive stress. The cover glass 1 generally comprises a silica based glass ceramic, with the cover glass 1 generally having a thickness d between 0.4 mm and 0.85 mm. The transmittance, τ.sub.vis, of the cover glass 1 is more than 80%, preferably more than 85%, in the range from 380 nm to 780 nm, preferably determined for thicknesses between 0.4 mm and 0.85 mm, especially preferably at a thickness of 0.7 mm. The cover glass 1, as a result of a chemical tempering, the effect of which is that, at least in a near-surface layer 101 or in the two near-surface layers 101, especially in a layer of 20 μm to 70 μm, determined at a right angle from one of the lateral faces 10, 12 of the cover glass 1, a crystal phase encompassed by the cover glass 1 or by the glass ceramic that encompasses it, preferably averaged by the tempering process, has a unit cell volume that is at least 1% by volume, preferably at least 2% by volume, greater than that of the crystal phase in the core 102. The crystal phase encompassed by the glass ceramic of the cover glass 1 may preferably be a silica based crystal phase. As a result of the chemical tempering, the cover glass 1 has a CS of at least 250 MPa and preferably at most 1500 MPa and/or a CS30 of at least 160 MPa and preferably at most 525 MPa and/or a DoCL, based on the cover glass 1, of at least 0.2 times the thickness d of the cover glass 1 and preferably less than 0.5 times the thickness d of the cover glass 1.

[0107] FIGS. 3 to 6 relate to the performance of what is called the set drop test for determination of set drop resistance.

[0108] The set drop test is preferably conducted as follows:

[0109] A cover glass is fixed on a sample receptacle and allowed to fall from accumulating drop heights onto a defined floor. An overview of the overall structure is shown in FIG. 3. The cover glass used in the set drop test in FIG. 5 has a length of 99 mm and a width of 59 mm, and, as shown in FIG. 4, is fixed magnetically with a sample dummy in the sample receptacle. For the studies outlined in the present disclosure, in a departure from the sample representation in FIG. 4, however, cover glass formats of 49.5 mm×49.5 mm were used, without affecting the basic construction of the test procedure in FIGS. 3 to 6.

[0110] First of all, a plastic sheet is stuck with the aid of a double-sided adhesive tape into a metal housing having the shape and weight of a holder for an ultimate mobile device, for example a smartphone. Suitable plastic sheets here are for example those having thicknesses between 4.35 mm and 4.6 mm (see FIG. 5). They are preferably stuck in by means of a double-sided adhesive tape having a thickness of about 100 μm. Then, by means of a double-sided adhesive tape, preferably a double-sided adhesive tape of thickness 295 μm, especially a double-sided adhesive tape of the tesa® brand, product number 05338, the glass article to be tested and in the form of a pane is stuck onto the plastic sheet in such a way that a distance between 350 μm and 450 μm is obtained between the top edge of the housing/holder and the top edge of the glass article. The cover glass lies higher than the housing frame, and there must be no occurrence of direct contact between cover glass and aluminum housing. The set thus obtained with a weight of 177.5 g, which simulates the incorporation of a cover glass into an ultimate mobile device and is a kind of dummy for a real ultimate mobile device, a smartphone here in particular, is subsequently allowed to drop downward onto an area of DIN A4 size, called the impact area, by the glass side with an initial speed in vertical direction, and hence a fall direction of zero. The impact area is produced here as follows: Sandpaper with an appropriate grain size, for example grain size 60 (#60), is stuck onto a base plate by means of a double-sided adhesive tape, for example an adhesive tape of thickness 100 μm. The adhesive tape used was Tesa (10 m/15 mm), transparent, double-sided, product number 05338. Grain size in the context of the present disclosure is defined according to the standards of the Federation of European Producers of Abrasives (FEPA); for examples thereof see also DIN ISO 6344, especially DIN ISO 6344-2:2000-04, Coated abrasives—Grain size analysis—Part 2: Determination of grain size distribution of macrogrits P 12 to P 220 (ISO 6344-2:1998). The weight of the base plate, which, with the values disclosed in the present context, is an aluminum base, is about 3 kg.

[0111] The base plate must be firm and is preferably formed from aluminum or else alternatively from steel. The sandpaper must be completely covered with adhesive tape and stuck down without bubbles. The impact area must be used only for five drop tests and should be exchanged after the fifth drop test. The sample, i.e., the set obtained, is inserted into the test apparatus and aligned by means of a 2D water level (circular level) such that the set is horizontal, with the cover glass facing the floor, i.e., in the direction of the impact area (see FIG. 6). The first drop height is 25 cm, then the drop is from a height of 30 cm. If breakage still does not occur, the drop height is increased in 10 cm steps until glass breakage occurs. The breakage height, the breakage origin and the breakage appearance are noted. The test is conducted on 10 to 15 samples, and an average is formed.

[0112] FIGS. 7A and 7B show the evaluation of EDX measurements on two different samples of a glass-ceramic cover glass. The glass-ceramic material of the cover glass was of identical composition, but was subjected to different tempering protocols.

[0113] In FIG. 7A, a silica based glass ceramic was chemically tempered in an exchange bath composed of 80% by weight of KNO.sub.3 and 20% by weight of NaNO.sub.3 at 440° C. for 18 hours. According to the EDX evaluation, it is apparent that the sodium oxide content at the sample surface is about 8 mol %. For the region of the core or of the bulk 102, a content of about 1 mol % of Na.sub.2O can be assumed. This is also identified as such in FIG. 7A at the top. By means of the tempering mentioned of the glass ceramic in the cover glass, at least in a near-surface region 101, it is possible to distinctly increase the Na.sub.2O content, in some cases by up to 7 mol % in absolute terms or—preferably averaged over the thickness of the near-surface region 101 from FIG. 2—by up to 6 mol % in absolute terms.

[0114] As already set out further up, it is more difficult with regard to the larger potassium ion to arrive at such an increase in the potassium oxide content. According to the exact configuration of the glass ceramic and/or the tempering protocol, exchange depths peaking at about 30 μm are achieved here; the inventors assume that greater exchange depths should also be possible with optimized processes and materials. Although potassium exchange is more difficult to accomplish, it is simultaneously also more efficient, such that CS 0 values of, for example, up to 1500 MPa, for example 1200 MPa or 1100 MPa, appear possible. In the case of other tempering protocols, as depicted by way of example in FIG. 7B, a very high potassium concentration was employed in the exchange bath. The composition of the exchange bath here was 99.5% by weight of KNO.sub.3 and only 0.5% by weight of NaNO.sub.3. The exchange bath temperature was 420° C.; the duration of tempering was 7.5 hours. Here, as can be inferred from FIG. 7B, there is an increase in the Na.sub.2O content in the near-surface region 101 of the cover glass, peaking at at least 5 mol %, even though only a very low NaNO.sub.3 content is present in the exchange bath. However, the K.sub.2O content is also demonstrably increased and peaks at up to nearly 3 mol %. In the core 102, by contrast, there is a much lower content of K.sub.2O and Na.sub.2O, since there has been no corresponding exchange here.

[0115] It is pointed out that there is a transition region between the region 101 and the core 102 in which exchange has taken place, and at least to some degree also an increase in the size of the unit cell volume. This “intermediate region” is a region in which ion exchange has still taken place, but an increase in compressive stress need not necessarily have taken place.

[0116] FIG. 8, finally, shows a comparison of set drop resistance of different cover glasses having the same chemical composition, but different crystal phase content or different ceramization. In the stress curve, 102 denotes the core, the region of the stress minimum in the cover glass. 2 here denotes the results that are obtained at drop heights for a green glass that is in chemically tempered form. 3 denotes the results for a glass ceramic comprising a crystal phase—in this case high quartz solid solution—that is not amenable to tempering in the crystal phase. The strength achieved in the set drop test is entirely inadequate. Finally, 4 denotes the result for a cover glass according to an embodiment, here comprising keatite as crystal phase. The results of the set drop test are also compiled in the table below. The drop heights are each reported in cm.

TABLE-US-00005 Samples 2 Samples 3 Samples 4 60 20 180 70 20 150 60 20 180 100 20 140 100 20 180 60 20 180 120 20 140 110 20 170 100 20 180 50 20 140 50 20 180 60 20 160 60 Mean 77 20 165 Median 60 20 175 Min 50 20 140 Max 120 20 180

[0117] FIG. 9 compares illustrative tempering profiles of cover glasses with one another, namely a tempering profile of a cover glass according to one embodiment with a tempering profile for a cover glass comprising a material that is of the same chemical composition but unceramized. Tempering profile 5 corresponds to that of a cover glass according to one embodiment, tempering profile 6 to that of a cover glass comprising a material of the same chemical composition, but in vitreous and unceramized form. 102 denotes the core, the region in which stress assumes a minimum value.

LIST OF REFERENCE NUMERALS

[0118] 1 cover glass [0119] 10, 12 sides of the cover glass [0120] 101 near-surface layer of the cover glass [0121] 102 core [0122] d thickness of the cover glass [0123] l length of the cover glass [0124] b width of the cover glass [0125] 2, 3, 4 results of set drop tests for different sample entities [0126] 5 tempering profile according to embodiment [0127] 6 tempering profile of a comparative example