8. Chemical Strengthening of Glass

A basic ternary sodium aluminosilicate glass system is described since this simple system forms the basis for glasses readily ion-exchanged to the high surface compressive stress and deep compressive stress layer. The ionic interdiffusion of monovalent alkali ions within an aluminosilicate glass is described and the complementary error function form of the invading ion concentration profile is established. The generation of the stress profile from the concentration gradient is then described mathematically. The basics of fracture mechanics are reviewed and then used to describe the advantages of ion-exchanged glasses, namely imparting high surface strength to allow highly flexible and bendable thin glass sheets and for thicker glass, the retention of strength following deep contact damage. A simple model is described that can accurately predict the retained strength as a function of flaw depth for a known stress profile. The frangibility behavior of ion-exchanged glasses is also described in terms of stored strain energy and cracking responses are shown. The sharp contact failure mode for cover glasses is also described and the use of a Vickers diamond indenter to replicate this type of failure mode is demonstrated. Experimental data show that the resistance to sharp contact strength-limiting flaw generation is improved both with high compressive stress enveloping the deformation region and by utilizing glass compositions that are more resistant to subsurface damage during sharp contact events. Sliding Knoop and Vickers indenter scratch testing shows that ion-exchanged glasses with resistance to subsurface damage do not produce highly visible lateral cracks at loads that readily produce this type of damage in typical ion-exchanged aluminosilicate glasses.