Durable glass ceramic cover glass for electronic devices

1. An article suitable as a cover glass for a portable electronic device, the article comprising a glass ceramic, the glass ceramic having a primary crystalline phase of transparent spinel or transparent mullite and exhibiting:

a. optical transparency of greater than 60%, as defined by the transmission of light over the range of from 400-750 nm through 1 mm of the glass ceramic;

b. colorlessness, as defined by having the values of L*≧90; 0.1≧a*≧−0.1; and 0.4≧b*≧−0.4 on the CIE 1976 Lab color space as measured in transmission through 1 mm of glass ceramic;

c. at least one of the following attributes:

(i) a fracture toughness of greater than 0.60 MPa·m 1/2;

(ii) a 4-point bend strength of greater than 350 MPa;

(iii) a Vickers hardness of at least 450 kgf/mm 2 ;

(iv) a Vickers median/radial crack initiation threshold of at least 5 kgf;

(v) a Young's Modulus ranging between 50 to 100 GPa; and

(vi) a thermal conductivity of less than 2.0 W/m° C.; and

d. at least one of the following attributes:

(i) a compressive surface layer having a depth of layer (DOL) greater than or equal to 20 μm and a compressive stress greater than 400 MPa, or,

(ii) a central tension of more than 20 MPa.

2. The article of claim 1 wherein the glass ceramic exhibits optical transparency of greater than 80%, as defined by the transmission of light over the range of from 400-750 nm through 1 mm of the glass ceramic.

3. The article of claim 1 , wherein the glass ceramic exhibits colorlessness, as defined by having color space coordinates of L*≧90; 0.08≧a*≧−0.08; and 0.3≧b*≧−0.3 on the CIE 1976 Lab color space as measured in transmission through 1 mm of glass ceramic.

4. The article of claim 1 , wherein the glass ceramic is ion exchanged.

5. The article of claim 4 , wherein the glass ceramic exhibits an overall thickness of 1.2 mm and compressive layer exhibiting a DOL of ranging between 40 to 80 μm and the compressive layer exhibits a compressive stress of 525 MPa.

6. The article claimed in claim 1 wherein the glass ceramic exhibits a Young's Modulus ranging between 50 and 75 GPa.

7. The article claimed in claim 1 wherein the glass ceramic exhibits an 4-point bend strength of greater than 475 MPa.

8. The article claimed in claim 1 wherein the glass ceramic exhibits a Vickers hardness of at least 500 kgf/mm 2 and Vickers median/radial crack initiation threshold of greater than 10 kgf.

9. The article of claim 1 , wherein the glass ceramic exhibits a thermal conductivity of less than 1.5 W/m° C.

10. The article of claim 1 , wherein the glass ceramic is transparent and exhibits at least one surface having a Ra roughness of less than less than 50 nm.

11. The article of claim 1 , wherein the glass ceramic exhibits a near-infra-red transparency of greater than 80% at a wavelength ranging from 750 to 2000 nm.

12. The article of claim 1 , wherein the glass ceramic is fusion formable.

13. An article suitable as a cover glass for a portable electronic device, the article comprising a glass ceramic, the glass ceramic having a primary crystalline phase of transparent spinel or transparent mullite and exhibiting:

a. optical transparency of greater than 60%, as defined by the transmission of light over the range of from 400-750 nm through 1 mm of the glass ceramic;

b. colorlessness, as defined by having the values of L*≧90; 0.1≧a*≧−0.1; and 0.4≧b*≧−0.4 on the CIE 1976 Lab color space as measured in transmission through 1 mm of glass ceramic; and

c. at least one of the following attributes:

(i) a fracture toughness of greater than 1.0 MPa·m 1/2 ;

(ii) an MOR of greater than 135 MPa

(iii) a Knoop hardness of at least 400 kg/mm 2 ;

(iv) a thermal conductivity of less than 4 W/m° C.; and

(v) a porosity of less than 0.1%.

14. An electronic device comprising the article of any of claims 11 and 12 .