Impact-damage-resistant glass sheet

Abstract

Impact-damage-resistant glass sheet comprising at least one chemically etched surface in combination with a tempering surface compression layer, the glass sheet exhibiting a high standardized ball drop failure height and a high flexural modulus of rupture strength, useful to provide damage-resistant glass cover sheets for consumer electronic video display devices, is provided by subjecting thin glass sheet to a combination of a surface tempering treatment and a surface etching treatment that improves strength while maintaining the optical glass sheet properties required for video display applications.

What is claimed is: 
     
         1 . A method of making an impact-damage-resistant glass sheet comprising the steps of (i) subjecting at least one surface of a glass sheet to a tempering treatment to develop a compressive surface layer thereon, and (ii) contacting the at least one surface of the glass sheet with a chemical glass etching medium to remove a layer of surface glass therefrom. 
     
     
         2 . A method in accordance with  claim 1  wherein the step of subjecting at least one surface of the glass sheet to a tempering treatment comprises subjecting the glass sheet to a tempering treatment effective to provide a surface compression level of at least 400 MPa in the surface of the sheet, and wherein the step of contacting the at least one surface of the glass sheet with a chemical glass etching medium comprises contacting the surface with an acidic etching medium for a time and at a temperature effective to remove not more than 4 μm of surface glass from the sheet. 
     
     
         3 . A method in accordance with  claim 1  wherein the step of subjecting the glass surface to a tempering treatment precedes the step of subjecting the glass surface to the etching medium. 
     
     
         4 . A method in accordance with  claim 1  wherein the glass sheet is of alkali aluminosilicate composition, and wherein the surface of the glass sheet subjected to the tempering treatment is substantially free of surface flaws of a depth greater than about 2 μm. 
     
     
         5 . A method in accordance with  claim 1  wherein the tempering treatment is selected from the group consisting of thermal tempering and chemical tempering. 
     
     
         6 . A method in accordance with  claim 5  wherein the tempering treatment comprises chemical tempering and includes a step of contacting at least one surface of the glass with an ion-exchange strengthening medium. 
     
     
         7 . A method in accordance with  claim 1  wherein the etching medium is an aqueous solution comprising a fluoride compound. 
     
     
         8 . A method in accordance with  claim 7  wherein the etching efficiency of an etching bath containing the etching medium is maintained by:
 removing a volume of the medium from the bath that (i) contains HF at a concentration below a predetermined minimum level and/or (ii) contains dissolved glass at a concentration above a predetermined maximum level, and 
 replacing the volume with an equivalent volume of an HF solution containing a concentration of HF exceeding the predetermined minimum level. 
 
     
     
         9 . A method of making a display cover sheet for a video display device comprising the steps of selecting an alkali aluminosilicate glass sheet of a thickness not exceeding  1  mm, contacting at least one surface of the glass sheet with an ion-exchange strengthening medium to develop a compressive surface layer thereon, and contacting the at least one surface of the glass sheet with a chemical glass etching medium to remove a layer of surface glass therefrom. 
     
     
         10 . A method in accordance with  claim 9  wherein the step of contacting the at least one surface of the glass sheet with the glass etching medium comprises contacting the surface with an acidic fluoride-containing glass etching medium and is carried out at a temperature and for a time not exceeding a time effective to remove a layer of surface glass not exceeding 4 μm in thickness from the glass sheet. 
     
     
         11 . A method in accordance with  claim 9  wherein the step of contacting the at least one surface of the glass sheet with the acidic glass etching medium is carried out at a temperature and for a time effective to remove a layer of glass not exceeding about 2 μm in thickness from the surface of the glass sheet. 
     
     
         12 . A method in accordance with  claim 9  wherein the step of contacting the at least one surface of the glass sheet with the acidic glass etching medium is carried out for a time not exceeding a time effective to reduce the compressive stress level in the glass sheet surface by 3%. 
     
     
         13 . A method in accordance with  claim 9  wherein the step of contacting at least one surface of the glass sheet with an ion-exchange strengthening medium comprises contacting the surface with a source of alkali metal ions of larger ionic diameter than at least one alkali metal ion component present in the glass. 
     
     
         14 . A method in accordance with  claim 13  wherein the step of contacting the at least one surface of the glass sheet with an ion-exchange strengthening medium is carried out at a temperature below the strain point of the glass, and for a time at least sufficient to develop a compressive stress layer with a depth in excess of 40 μm and a peak compressive stress level in excess of 650 MPa in the at least one surface. 
     
     
         15 . A method in accordance with  claim 9  wherein the step of contacting the at least one surface of the glass sheet with the glass etching medium is carried out for a time and at a temperature such that (i) not more than a 2 μm thickness of surface glass is removed from the sheet surface; (ii) a compressive stress of at least 650 MPa glass is retained in the sheet surface; (iii) the glass sheet retains a final haze level not exceeding 0.1%; and (iv) the glass sheet retains a final light transmission level and a final surface gloss level that differ by less than 1% from, respectively, an initial light transmission level and an initial surface gloss level for the glass sheet. 
     
     
         16 . A method in accordance with  claim 9  which comprises a further step, prior to the step of contacting at least one surface of the glass sheet with an ion-exchange strengthening medium, of treating at least one surface of the glass sheet to provide an anti-glare surface thereon. 
     
     
         17 . A method in accordance with  claim 9  which comprises a further step, prior to the step of contacting at least one surface of the glass sheet with an ion-exchange strengthening medium, of pre-treating the at least one surface of the glass sheet to reduce the population and/or size of surface flaws thereon. 
     
     
         18 . A method in accordance with  claim 17  wherein the step of pre-treating comprises contacting the at least one surface of the glass with an etching solution to remove surface glass therefrom.