Flaw-insensitive ion-exchanged glass: II, production and mechanical performance

A two-step ion-exchange process that introduces strengthening and multiple cracking into glasses has been developed. The experimental parameters in th e second step must be carefully controlled to avoid spontaneous cracking, b ecause the compressive stress is reduced in the surface region. Control of the near-surface stress gradient is an important role of the second ion-exc hange step. Double ion exchange leads to strength values as high as those t hat are obtained after the first ion-exchange step but remarkably produces multiple cracking on the specimen surface on loading. The multiple cracking , which acts as a failure warning, is initiated at 300 MPa, and the crack s pacing decreases as the applied stress increases. The multiple cracking is shown to be a result of the crack stability that is induced by the designed residual-stress profiles. This behavior has been confirmed, using the meas ured residual stresses to calculate the apparent-toughness curves for surfa ce cracks. The duration of the first ion-exchange step has been shown to be important in determining the degree of strengthening; however, the duratio n of the second step controls the extent of the crack stability.