Abstract
Brittleness of sapphire has been studied by four point bending and ball on three ball tests. A significantly larger flexural stress was observed for crystals grown by the Verneuil process compared to EFG (Edge-defined Film-fed Growth) crystals. Crystallographic defects were characterized by X-ray topography and it was shown that subgrain boundaries found in Verneuil crystals do not impact the fracture behavior. Larger amounts of basal dislocations in Verneuil boules appeared to account for the differences in brittleness. The dislocation densities have been related to the temperature fields experienced by the crystals during the growth processes. Characterization of point defects by thermo-stimulated luminescence revealed that they are more numerous in Verneuil crystals, especially after annealing, and that they pin dislocations, contributing to the mechanical response of the crystal.
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