Durable glass ceramic cover glass for electronic devices

Abstract

The invention relates to glass articles suitable for use as electronic device housing/cover glass which comprise a glass ceramic material. Particularly, a cover glass comprising an ion-exchanged glass ceramic exhibiting the following attributes (1) optical transparency, as defined by greater than 90% transmission at 400-750 nm; (2) a fracture toughness of greater than 0.6 MPa·m 1/2 ; (3) a 4-point bend strength of greater than 350 MPa; (4) a Vickers hardness of at least 450 kgf/mm 2 and a Vickers median/radial crack initiation threshold of at least 5 kgf; (5) a Young's Modulus ranging between about 50 to 100 GPa; (6) a thermal conductivity of less than 2.0 W/m° C., and (7) and at least one of the following attributes: (i) a compressive surface layer having a depth of layer (DOL) greater and a compressive stress greater than 400 MPa, or, (ii) a central tension of more than 20 MPa.

We claim: 
     
       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 or  
 (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 a 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 a 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 or  
 (v) a porosity of less than 0.1%. 
 
 
     
     
       14. An electronic device comprising the article of any of  claims 11  and  12 . 
     
     
       15. An article suitable as a cover glass for a portable electronic device, the article comprising a glass ceramic, the glass ceramic being titania-free and having a primary crystalline phase of b-spodumene, 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; or 
 (vi) a thermal conductivity of less than 2.0 W/m° C. 
   
     
     
       16. The article of claim 15 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. 
     
     
       17. The article of claim 15, 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. 
     
     
       18. The article of claim 15, wherein the glass ceramic is ion exchanged. 
     
     
       19. The article of claim 18, wherein the glass ceramic exhibits an overall thickness of 1.2 mm and a compressive layer exhibiting a DOL ranging between 40 to 80 μm and the compressive layer exhibits a compressive stress of 525 MPa. 
     
     
       20. The article claimed in claim 15 wherein the glass ceramic exhibits a Young's Modulus ranging between 50 and 75 GPa. 
     
     
       21. The article claimed in claim 15 wherein the glass ceramic exhibits an 4-point bend strength of greater than 475 MPa. 
     
     
       22. The article claimed in claim 15 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. 
     
     
       23. The article of claim 15, wherein the glass ceramic exhibits a thermal conductivity of less than 1.5 W/m° C. 
     
     
       24. The article of claim 15, wherein the glass ceramic is transparent and exhibits at least one surface having a Ra roughness of less than less than 50 nm. 
     
     
       25. The article of claim 15, wherein the glass ceramic exhibits a near-infra-red transparency of greater than 80% at a wavelength ranging from 750 to 2000 nm. 
     
     
       26. The article of claim 15, wherein the glass ceramic is fusion formable. 
     
     
       27. An article suitable as a cover glass for a portable electronic device, the article comprising a glass ceramic, the glass ceramic being titania-free and having a primary crystalline phase of b-spodumene, 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.; or 
 (v) a porosity of less than 0.1%. 
   
     
     
       28. An electronic device comprising the article of any of claims 15-27.