Influence of mosaicity on the fracture behavior of sapphire

The resistance to breakage of sapphire samples of different crystallographic orientations and produced by the Kyropoulos and Verneuil processes was investigated. The fracture strength was determined by using four-point bending tests, whereas the single-edge V-notch beam method was used to measure the fracture toughness. For both mechanical properties, it was found that (i) Verneuil sapphire has values up to two times higher than Kyropoulos sapphire and (ii) the data scatter is also generally higher for Verneuil sapphire. The main factor responsible for this behavior is believed to be the presence of mosaicity in Verneuil and its absence in Kyropoulos sapphire. 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 is demonstrated that intergranular fracture along the mosaic block boundaries is energetically more favorable than transgranular fracture for crack-to-boundary orientations smaller than a critical deflection angle. Nevertheless, both the location of the crack in respect to the boundary, as well as the 3-D nature of these features, must be taken into consideration for a complete description of the intergranular deflection toughening mechanism.