Critical parameters in the processing of engineered stress profile glasses

A new approach to chemical strengthening is to move the maximum residual compression below the glass surface. With appropriate processing, this approach leads to strengthening and a decrease in the strength variability. This improvement in mechanical reliability is accomplished by introducing stable crack growth into the glass, which leads to multiple cracking of the glass surface prior to failure. In the current study, a fracture mechanics analysis is presented that uses a simple form for the stress profile. By determining the effect of this stress profile on the apparent fracture toughness of the material, it is possible to identify the conditions when spontaneous cracking occurs, the stress for the onset of multiple cracking and the ultimate strength of the glass. In particular, it is shown there is an important interplay between the depth of the maximum compression, the stress gradient and the magnitude of the maximum compressive stress. The current analysis is then used to define a strategy for the processing of glasses with engineered stress profiles.