Deformation of sapphire induced by a spherical indentation on the (1010) plane

This work clarifies the origin of the characteristic discontinuities registered during the deformation of the (101̄0) plane of sapphire, by means of depth sensing indentation experiments with a spherical indenter. The sudden increase of the plasticity of the crystal was found to be caused by a twinning process. The calculation and analysis of the distribution of resolved shear stresses under a spherical indenter predicted the features of the surface deformation. The high resolution microscopic examination of the residual impressions confirmed the theoretical prediction.