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

Abstract

A cover glass made of a glass ceramic that is silica based and has a main crystal phase of high quartz solid solution or keatite solid solution is provided. The cover glass has a stress profile with at least one inflection point at a depth of the cover glass of more than 10 μm, a thickness from 0.1 mm to 2 mm, and a chemical tempering structure with a surface compressive stress of at least 250 MPa and at most 1500 MPa. A process for producing the cover glass is provided that includes producing a silica based green glass, hot shaping the silica based green glass, thermally treating the silica based green glass with a nucleation step and a ceramization step, and performing an ion exchange at an exchange bath temperature for a duration of time in an exchange bath.

Claims

1. A cover glass comprising: a glass ceramic that is silica based and has a main crystal phase of high quartz solid solution or keatite solid solution; a stress profile that has at least one inflection point at a depth of the glass ceramic of more than 10 μm; a thickness from 0.1 mm to 2 mm; and a chemical tempering structure with a surface compressive stress of at least 250 MPa and at most 1500 MPa.

2. The cover glass as claimed in claim 1, further comprising a transmittance, τ.sub.vis, of more than 80% in a range from 380 nm to 780 nm as determined for thicknesses of 0.4 mm to 0.85 mm.

3. The cover glass as claimed in claim 1, further comprising a transmittance, τ.sub.vis, of more than 85%, in a range from 380 nm to 780 nm as determined for thicknesses of 0.4 mm to 0.85 mm.

4. The cover glass as claimed in claim 1, wherein the glass ceramic is a lithium aluminum silicate glass ceramic and the main crystal phase is keatite solid solution.

5. The cover glass as claimed in claim 1, wherein the glass ceramic comprises the following components in % by weight based on oxide: TABLE-US-00009 SiO.sub.2 55-75 Al.sub.2O.sub.3 18-27 Li.sub.2O 2.8-5. 

6. The cover glass as claimed in claim 1, wherein the glass ceramic comprises the following components in % by weight based on oxide: TABLE-US-00010 SiO.sub.2 62-72 Al.sub.2O.sub.3 18-23 Li.sub.2O  3-5.

7. The cover glass as claimed in claim 1, wherein the glass ceramic comprises the following components in % by weight based on oxide: TABLE-US-00011 SiO.sub.2 55-75 Al.sub.2O.sub.3 18-27 Li.sub.2O 2.8-5.sup.  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   SnO.sub.2 0-2 wherein the TiO.sub.2 and ZrO.sub.2 are in a sum total of greater than 0% and less than 9.5%.

8. The cover glass as claimed in claim 7, wherein the sum total of the TiO.sub.2 and ZrO.sub.2 is greater than 1.2%.

9. The cover glass as claimed in claim 7, wherein the SnO.sub.2, ZrO.sub.2 and TiO.sub.2 are present according to 0≤SnO.sub.2/(ZrO.sub.2+TiO.sub.2)<0.8.

10. The cover glass as claimed in claim 7, wherein the SnO.sub.2, ZrO.sub.2 and TiO.sub.2 are present according to 0.01≤SnO.sub.2/(ZrO.sub.2+TiO.sub.2)<0.7.

11. The cover glass as claimed in claim 7, wherein the glass ceramic comprises the following components in % by weight based on oxide: TABLE-US-00012 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.sup.  B.sub.2O.sub.3 .sup. 0-0.1 Fe.sub.2O.sub.3 0.0001-0.02  SnO.sub.2 0.05-1.6. 

12. The cover glass as claimed in claim 1, further comprising a sharp impact resistance determined in a set drop test between a drop height of at least 120 cm and at most 200 cm.

13. The cover glass as claimed in claim 1, wherein the chemical tempering structure comprises a structure selected from a group consisting of: a sodium ion tempering structure, a potassium ion tempering structure, and a sodium and potassium ion tempering structure.

14. The cover glass as claimed in claim 1, wherein the glass ceramic does not have any lithium metasilicate as crystal phase.

15. A cover glass comprising: a glass ceramic that is silica based and has a main crystal phase of high quartz solid solution or keatite solid solution; a thickness from 0.1 mm to 2 mm; a chemical tempering structure with a surface compressive stress of at least 250 MPa and at most 1500 MPa; and a β-value that is given according to the following equation: $\beta \left(x\right)=\frac{CS\left(x\right)+CT}{{c\left(x\right)}_{\mathrm{Ion}}-{c\left(\mathrm{Bulk}\right)}_{\mathrm{Ion}}}⨯\frac{1-v}{E}$ wherein CT is a central tension, CS(x) is a compressive stress value at depth x, c(x).sub.Ion is a concentration of a respective ion, given as oxide, in the depth, and c(bulk).sub.Ion is a concentration of the ion in the bulk, v is Poisson's ratio, and E is Young's modulus of the glass ceramic.

16. The cover glass as claimed in claim 15, wherein the chemical tempering structure is sodium ion chemical tempering structure and the β-value is in a range from 3*10.sup.−4/mol to 9*10.sup.−4/mol.

17. A process for producing a cover glass, the process comprising: producing a silica based green glass by a melting process; hot shaping the silica based green glass; thermal treating of the silica based green glass with a nucleation step within a temperature range of 650° C.-850° C. for a duration of 5 min to 60 h and a ceramization step within a temperature range of 700° C.-1100° C. for a duration of 3 min to 120 h; and performing an ion exchange at an exchange bath temperature between 360° C. and 500° C. and for a duration of 2 hours to 50 hours in an exchange bath, wherein the exchange bath has a composition selected from a group consisting of 100% by weight to 0% by weight of KNO.sub.3, 0% by weight to 99.9% by weight of NaNO.sub.3, 0% by weight to 5% by weight of LiNO.sub.3, and any combinations thereof.

18. The process as claimed in claim 17, wherein the ion exchange is a single ion exchange and the exchange bath comprises NaNO.sub.3 and up to 0.1% by weight of LiNO.sub.3 or comprises KNOB having a purity of 99.9%, based on weight.

19. The method of claim 17, further comprising applying the cover glass to an electronic device.

20. The cover glass as claimed in claim 1, wherein the glass ceramic is sized and configured for application to an electronic device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0086] FIG. 1 shows a cover glass according to the present disclosure.

[0087] FIG. 2 is a section view of a cover glass according to the present disclosure.

[0088] FIG. 3 shows an overall view of the set drop test setup.

[0089] FIG. 4 shows a sample receptacle and trigger mechanism of the set drop test setup.

[0090] FIG. 5 shows an aluminum housing and plastic sheet as a sample receptacle and sample dummy.

[0091] FIG. 6 shows alignment of the sample dummy in the sample receptacle of FIG. 4 by means of 2D water level.

[0092] FIGS. 7a to 11 show data and graphs according to the present disclosure.

DETAILED DESCRIPTION

[0093] 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 can, 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.

[0094] 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 can also be referred to as “surfaces” or “main surfaces” of the cover glass 1), with the side 10 designed here as top side and the 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 can be the same, i.e., for example, have an equal thickness within the scope of measurement accuracy. It can 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 can be the case, for example, when the chemical tempering of the cover glass 1 has been executed such that exchange is unequal.

[0095] The core 102 lies between the two near-surface layers 101. There can 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 0.1 mm and 2 mm. A preferred lower thickness limit can generally be 0.4 mm. A preferred upper thickness limit can generally be 0.85 mm. The transmittance, τ.sub.vis, of the cover glass 1 according to one embodiment is more than 80%, preferably more than 85%, in the range from 380 nm to 780 nm, preferably determined for thicknesses between 0.1 mm and 2 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 a compressive stress is obtained, at least in a near-surface layer 101 or in the two near-surface regions 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, is present as a chemically tempered cover glass, with the stress profile having at least one inflection point, preferably at a cover glass depth of more than 10 μm. The crystal phase encompassed by the glass ceramic of the cover glass 1 can 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 of at most 1500 MPa.

[0096] FIGS. 3 to 6 relate to the performance of what is called the set drop test for determination of set drop resistance.

[0097] The set drop test is preferably conducted as follows:

[0098] 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, while the weight of the dummies were reduced accordingly.

[0099] First of all, a polymer sheet is stuck with the aid of 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 (for studies with chemically tempered cover glasses comprising glass or glass ceramic) or 295 μm, respectively (for studies with cover glasses comprising glass or glass ceramic that had not been tempered). 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 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 baseplate by means of a double-sided adhesive tape, for example an adhesive tape of thickness 100 μm (for studies with chemically tempered cover glasses comprising glass or glass ceramic) or 3*100 μm, respectively (for studies with cover glasses comprising glass or glass ceramic that had not been tempered). 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 baseplate, which, with the values disclosed in the present context, is an aluminum base, is about 3 kg.

[0100] The baseplate 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 surface 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 20 cm; if no breakage occurs, 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.

[0101] FIG. 7a shows a stress profile and FIG. 7b shows an EDX curve of a first cover glass according to one embodiment of the disclosure, in which tempering was effected by means of potassium ions. The approximate position of the inflection point in the stress profile is identified in FIG. 7a. FIG. 7b shows the progression of the potassium oxide concentration versus the depth of the cover glass (plotted on the x axis). As well as the measurement points (filled square), the convex fit to these data (dotted line) is shown, which clearly shows the anomalous progression of the concentration curve (which, as is well known, translates into the stress profile). By way of comparison, the “conventional” expected progression of a concentration profile is also shown, which can typically be described by a complementary error function, i.e., the dotted line in FIG. 7b.

[0102] FIG. 8a shows a stress profile and FIG. 8b shows an EDX curve of a first cover glass according to one embodiment of the disclosure, in which tempering was effected by means of sodium ions. The approximate position of the inflection point in the stress profile is identified in FIG. 8a. FIG. 8b shows the progression of the sodium oxide concentration versus the depth of the cover glass (plotted on the x axis). As well as the measurement points (filled square), the convex fit to these data (dotted line) is shown, which clearly shows the anomalous progression of the concentration curve (which, as is well known, translates into the stress profile). By way of comparison, the “conventional” expected progression of a concentration profile is also shown, which can typically be described by a complementary error function, i.e., the dotted line in FIG. 8b.

[0103] FIG. 9, finally, shows a stress profile that has been obtained after a double ion exchange. The stress profile has three inflection points that are given approximately in the diagrams of FIG. 9.

[0104] In FIGS. 7a to 9, at least one inflection point at a depth of the cover glass of at least 10 μm or more is encompassed by the stress profile.

[0105] FIG. 10, finally, shows a comparison of the set drop resistance of different cover glasses. 2 here denotes the results that are obtained with drop heights for prior art chemically tempered glass. 3 denotes results for a cover glass according to a first embodiment that has been tempered in a pure sodium bath (100% NaNO.sub.3) at 440° C. for 14 h. Finally, 4 denotes the result for a cover glass according to a further embodiment that has been tempered in a pure sodium exchange bath (100% NaNO.sub.3) at 440° C. for nine hours. The results of the set drop test are also compiled in the table below. The drop heights are each reported in cm.

EXAMPLES

[0106] The compositions of the glass ceramic materials according to the disclosure can be found in Table 1.

[0107] The materials listed in Table 1 were melted and refined using raw materials customary in the glass industry at temperatures of 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.

[0108] For the ceramizations, in general, two-stage programs were used, which are specified in Table 1. In these, the starting glasses were 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 can also be replaced by slow heating rates.

[0109] 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).

[0110] 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 was conducted using Bragg-Brentano geometry (θ−2θ). The x-ray diffraction diagrams were measured between 10° and 100° (2θangle). The relative crystalline phase components 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-00003 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 Li.sub.2O 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 [g/cm.sup.3] 2.512 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 phases HQMK KMK KMK KMK KMK + HQMK Relative proportion of KMK n.d. n.d. 96 96 86 τ.sub.vis (C/2) 0.7 mm 90.3 76.6 80.2 87.6 84.9 YI (std. ill. C yellow) 1.7 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 phases HQMK HQMK KMK KMK KMK Relative proportion of KMK — — 97.4 94.5 98.2 KMK crystallite size (nm) n.d. n.d. 68 69 69 Transmittance D = 0.8 mm n.d. n.d. n.d. n.d. n.d. 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 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 phases KMK KMK Nepheline, rutile (tr) Relative proportion of KMK 98.3 96.5 — KMK crystallite size (nm) 72 81 Nepheline = 62 nm Transmittance D = 0.8 mm n.d. 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

[0111] 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 disclosure.

TABLE-US-00004 TABLE 2 a (Å) C (Å) V (Å.sup.3) ΔV Literature value for Li.sub.0.75Al.sub.0.75Si.sub.2.25O.sub.6 7.505 9.070 510.91 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)

[0112] 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 0.7 mm samples that had an increase in set drop resistance additionally showed a DoCL of 140 μm or 135 μm. The CS 30 values were between 150 MPa and 360 MPa.

[0113] Samples with a composition according to example 9 were produced in an analogous manner, ceramized as specified in Table 1, example 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) Λ, c=8.372(5) Λ, 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, KAISO.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%.

[0114] Tempering conditions and tempering parameters thus achieved are listed for different cover glasses in the Table 3 below.

TABLE-US-00005 TABLE 3 stress profile parameters obtained via SLP1000 and FSM6000 for different tempered cover glasses comprising glass ceramic Thickness Ion exchange CS.sub.0 K DoL CS 30 DoCL Sample [mm] Step 1 Step 2 [MPa] [μm] [MPa] [μm] 10 0.71 48 h 490° C. 100% KNO.sub.3 57 245 57 11 0.71 2.5 h 440° C. 100% Na NO.sub.3 404 98 12 0.7 9 h 440° C. 100% NaNO.sub.3 536 404 140 13 0.71 4 h 440° C. 100% KNO.sub.3 1076 12.8 12.8 14 0.71 15 h 440° C. 100% KNO.sub.3 27.5 27.5 15 0.71 9 h 440° C. 99.5% NaNO.sub.3 0.5% LiNO.sub.3 313 127 16 0.71 9 h 440° C. 99% NaNO.sub.3 1% LiNO.sub.3 232 120 17 0.71 9 h 440° C. 98.5% NaNO.sub.3 1.5% LiNO.sub.3 165 120 18 0.51 2.5 h 440° C. 100% NaNO.sub.3 1 h 440° C. 100% KNO.sub.3 1086 7.7 238 98 19 0.51 2.5 h 440° C. 100% NaNO.sub.3 4 h 440° C. 100% KNO.sub.3 1091 14.1 78 86

[0115] CT stands here for center tension and is reported in MPa. “K DoL” is the depth of compressive strength resulting from sodium (if applicable) and is reported in μm; CS.sub.0 is the level of compressive stress at the surface of the cover glass and is given in MPa; CS.sub.30 is the compressive stress at a depth of 30 μm, measured from the surface of the cover glass (given in MPa).

[0116] Mixed tempering by sodium and lithium can be advantageous in order to improve the fracture profile, i.e. to obtain a less fine crumbly fracture profile.

TABLE-US-00006 Samples 2 Samples 3 Samples 4 20 110 180 50 170 180 30 160 180 20 150 160 50 180 180 60 140 180 30 180 180 50 100 150 60 140 150 70 180 180 40 150 20 180 Mean 42 153 172 Median 45 155 180 Min 20 100 150 Max 70 180 180

[0117] Samples 2 to 4 were obtained here by means of the following exchange conditions:

TABLE-US-00007 Sample Thickness CS.sub.0 CS 30 DoCL No. [mm] Exchange 1 Exchange 2 [MPa] [MPa] [μm] 2 0.7 7.5 h 20/80% K/NaNO.sub.3 410° C. 3 h 100% KNO.sub.3 395° C. 840 134 150 3 0.7 9 h; 440 C.; 100% NaNO.sub.3 — 536 404 140 4 0.7 14 h; 440° C.; 100% NaNO.sub.3 — 425 315 135

[0118] FIG. 11 shows an illustrative stress profile of a cover glass which corresponds to the prior art, the curve progression of which can be approximated in the near-surface region or up to the DoCL by means of a complementary error function or by a parabola. 102 denotes the core, the region in which stress assumes a minimum value.

[0119] FIG. 12 schematically depicts stress profiles obtained for a cover glass comprising a silica based glass ceramic according to an embodiment as well as for a cover glass comprising a silica based glass of the chemical composition corresponding to that of the glass ceramic. FIG. 12 depicts the raw data curve obtained by means of mess equipment SLP as well as the smoothed stress profile as a function of depth within the cover glass. As can be seen, the stress in the cover glass according to an embodiment, comprising glass ceramic, is, especially in a near-surface layer of the cover glass, significantly greater than in a cover glass comprising or consisting of glass, corresponding to the more efficient tempering, that is, the more efficient building up of stress due to ion exchange, in the glass ceramic. In particular, in the depicted example, compressive stress in the glass ceramic is larger than in the glass, and this throughout nearly the entire compressive stress zone (due to technical measurement reasons, it cannot be ascertained that the intersection of measurement curves is before or after zero crossing). Accordingly, the resulting central tension (CT) within the glass ceramic is larger than in the glass.

[0120] FIG. 13 depicts the comparison between the concentration of sodium oxide in a glass and a glass ceramic of identical chemical composition, but a different chemical tempering process, wherein sodium has been used as tempering ion. Stress profiles that correspond to the depicted cover glasses are depicted in FIG. 12. As can be seen, concentration of sodium oxide is up to a certain depth—in the exemplary depiction this is at about 140 μm—always above the concentration in the glass. This corresponds approximately to the DoCL of the sample of FIG. 12. Further, the concentration profile for the glass ceramic shows the same characteristic shape, having at least one inflection point (or, so to say, a “convex” shape).

[0121] The advantage of an ion exchange in cover glasses according to embodiments compared to that in cover glasses consisting of or comprising a glass of identical chemical composition can also be seen in samples whose stress profiles or measurement values obtained via SLP are depicted in FIG. 14. Here, ion exchange was conducted in such a way that the cover glass comprising a silica based glass ceramic according to an embodiment and for the cover glass comprising the corresponding silica based glass both have nearly identical values for CS, that is, compressive stress at the surface of the cover glass, and for DoCL.

[0122] FIG. 15 depicts for these two samples values obtained in a set drop test, as well as results obtained for samples that had not been tempered (a, b in FIG. 14). 7 and 8 relate to values that have been obtained for sample entities comprising cover glasses comprising a silica based glass that had not been tempered respectively a silica based glass ceramic that had not been tempered that had a chemical composition identical to those of the samples of entity 7 (8). As can be seen, the respective drop heights for #18ß0 sandpaper for entities 7 and 8 are with values of 36.3 cm and 31.3 cm, respectively, nearly the same. In particular, it is remarkable that the results obtained for a non-tempered glass ceramic are not better than those for the non-tempered glass.

[0123] This is different, however, for the values obtained for the corresponding tempered cover glasses in the #60 sandpaper set drop test, wherein 9 refers to the samples of cover glasses comprising tempered silica based glass and 10 shows the values for the samples of cover glasses comprising the corresponding silica based glass ceramic. As has been explained above, the stress values (CS, DoCL) are, taking into account precision of measurements, identical (see FIG. 14). As can be seen, cover glasses according to embodiments, that is, comprising a silica based glass ceramic having a stress profile, as also depicted in FIG. 14, that has at least one inflection point, have significant advantages. Tempering is achieved much more efficiently in the cover glasses according to embodiments and leads to better results in application-based tests.

[0124] Values depicted in FIG. 15 are also listed in the following table:

TABLE-US-00008 Sample Glass (not Glass ceramic Tempered Tempered np tempered) (not tempered) glass glass ceramic 1 90 20 25 90 2 25 25 20 70 3 25 25 40 50 4 25 70 70 25 5 30 25 40 100 6 25 30 30 70 7 30 50 30 90 8 30 20 60 25 9 20 20 80 120 10 25 50 70 40 11 60 20 50 50 12 50 20 25 110 13 40 100 14 30 150 15 90 100 MW 36.3 31.3 46.7 79.3 Median 27.5 25.0 40.0 90.0 All values for drop height are given in centimeters (cm)..