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

A cover glass comprising:

• a lithium aluminum silicate glass ceramic that has a main crystal phase of keatite solid solution;
• wherein the glass ceramic has SnO2 in an amount 0.05-1.6% by weight, ZnO in an amount 0.44-6% by weight, and MgO in an amount 0.1-8% by weight;
• a haze value of 0.01-1%;
• 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:
• β
• ⁡
• (
• x
• )
• =
• CS
• ⁢
• (
• x
• )
• +
• CT
• c
• ⁡
• (
• x
• )
• Ion
• -
• c
• ⁡
• (
• Bulk
• )
• Ion
• ⨯
• 1
• -
• v
• E
• wherein CT is a central tension, CS(x) is a compressive stress value at depth x, c(x)ion is a concentration of a respective ion, given as oxide, in the depth, and c(bulk)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.

A process for producing a cover glass, the process comprising:

• producing a lithium aluminum silicate glass by a melting process,
• wherein the lithium aluminum silicate glass has a composition that comprises SnO2 in an amount of 0.05-1.6% by weight, ZnO in an amount of 0.44-6% by weight, and MgO in an amount of 0.1-8% by weight;
• hot shaping the lithium aluminum silicate glass to a thickness from 0.1 mm to 2 mm;
• thermal treating the lithium aluminum silicate 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 to convert the lithium aluminum silicate glass into a lithium aluminum silicate glass ceramic that has a main crystal phase of keatite solid solution;
• 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 KNO3, 0% by weight to 99.9% by weight of NaNO3, 0% by weight to 5% by weight of LiNO3, and any combinations thereof,
• wherein the ion exchange yields a chemical tempering structure with a surface compressive stress of at least 250 MPa and at most 1500 MPa, and
• wherein the cover glass has a haze value of 0.01-1% and a stress profile that has at least one inflection point at a depth of the glass ceramic of more than 10 μm.