Influence of Mosaicity on the Fracture Behavior of Sapphire

The fracture resistance of sapphire samples of different crystallographic orientations and produced with the Kyropoulos and Verneuil processes was measured by using four-point bending tests and the single-edge V-notch beam (SEVNB) method. The fracture strength and the fracture toughness are up to two times higher in Verneuil than in Kyropoulos sapphire. Moreover, the scatter of these two properties is generally higher in Verneuil than in Kyropoulos sapphire, as emphasized by the differences in the Weibull modulus values. Among the several analyzed potential factors, the presence of mosaicity in Verneuil and its absence in Kyropoulos sapphire seems to be the main responsible for this behavior. By using the Read-Shockley model to estimate the energy of low-angle grain boundaries, together with the Griffith energy criterion for intergranular crack propagation, it can be demonstrated that intergranular fracture along the mosaic block boundaries is energetically more favorable than transgr