Abstract
A two‐step ion‐exchange process that introduces strengthening and multiple cracking into glasses has been developed. The experimental parameters in the second step must be carefully controlled to avoid spontaneous cracking, because the compressive stress is reduced in the surface region. Control of the near‐surface stress gradient is an important role of the second ion‐exchange step. Double ion exchange leads to strength values as high as those that 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 spacing 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 measured residual stresses to calculate the apparent‐toughness curves for surface cracks. The duration of the first ion‐exchange step has been shown to be important in determining the degree of strengthening; however, the duration of the second step controls the extent of the crack stability.
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