Damage resistant, chemically toughened protective cover glass

Abstract

The invention is directed to a high strength, chemically toughened protective glass article, the glass article having a high damage tolerance threshold of at least 1500 g as measured by the lack of radial cracks when the load is applied to the glass using a Vickers indenter; preferably greater than 2000 g is measured by the lack of initiation of radial cracks when the load is applied to the glass using a Vickers indenter

We claim: 
     
         1 . A method of designing the ion-exchange parameters in thin glass articles for use as protective cover glasses, said method comprising the steps of:
 choosing the depth of the compression layer required to achieve the desired level of damage resistance as measured by a Vickers indenter test and/or a scratch resistant test using a Knoop diamond indenter;   selecting the compressive stress that will allow a designed maximum tensile stress to develop in the center of the glass article;   diluting an ion-exchange bath containing alkali metal ions having a diameter larger than that of sodium ions with sodium ions to achieve the desired compressive stress;   providing a glass sheet, said glass sheet being made of a glass selected from the group consisting of alkali containing aluminosilicate glasses, alkali containing aluminoborosilicate glasses, alkali containing borosilicate glasses and alkali containing glass-ceramics;   chemically strengthening the glass sheet by ion-exchanging Na and/or Li ions in the surface of the glass for larger ions, said chemical exchange being to a depth of at least 40 μm from the surface of the sheet; and   finishing the sheet by cutting and polishing as required to make the glass article;   wherein, when finished, said glass article has damage tolerance threshold of at least 2000 g as measured by the lack of the presence of radial cracks when the load is applied to the glass using a Vickers indenter.   
     
     
         2 . The method according to  claim 1 , wherein providing a glass sheet means provided a glass sheet whose composition comprising 64 mol %≦SiO 2 ≦68 mol %; 12 mol %≦Na 2 O≦16 mol %; 8 mol %≦Al 2 O 3 ≦12 mol %; 0 mol %≦B 2 O 3 ≦3 mol %; 2 mol %≦K 2 O≦5 mol %; 4 mol %≦MgO≦6 mol %; and 0 mol %≦CaO≦5 mol %, 0-0.5% (As 2 O 3 , SnO 2 ); wherein: 66 mol %≦SiO 2 +B 2 O 3 +CaO≦69 mol %; Na 2 O+K 2 O+B 2 O 3 +MgO+CaO+SrO>10 mol %; 5 mol %≦MgO+CaO+SrO≦8 mol %; (Na 2 O+B 2 O 3 )−Al 2 O 3 ≦2 mol %; 2 ol %≦Na 2 O−Al 2 O 3 ≦6 mol %; and 4 mol %≦(Na 2 O+K 2 O)−Al 2 O 3 ≦10 mol %. 
     
     
         3 . The method according to  claim 1 , wherein providing a glass sheet means provided a glass sheet whose composition comprises 64-68% SiO 2 , 10-12% Al 2 O 3 , 0-2% B 2 O 3 , 12-15% Na 2 O, 2-4% K 2 O, 5-7% MgO, >0-1% CaO, 0-0.5% (As 2 O 3 , SnO 2 ), 0-1% (Sb 2 O 3 , SnO 2 ), and >0-1% TiO 2 .17. (Rejected) A high strength, chemically toughened protective cover glass article, said cover glass article having a thickness of less than 1.7 millimeters and having been chemically toughened to provide a surface compression layer of a depth (DOL) of at least 40 μm, the level of surface compression (CS) in the article being limited such that the article exhibits a level of central tension (CT) not exceeding 54 MPa at a thickness of 1.0 mm, and the article exhibiting a high damage tolerance threshold load of at least 2000 g as measured by the absence of radial cracking when the load is applied to the glass using a Vickers indenter.