The origin of the characteristic discontinuities which were observed during the deformation of the (10•0) plane of sapphire was clarified by performing depth-sensing indentation experiments using a spherical indenter. A sudden increase in the plasticity of the crystal was attributed to a twinning process. Calculations of the distribution of resolved shear stresses under a spherical indenter predicted the observed features of surface deformation. High-resolution microscopic examination of residual impressions confirmed the theoretical predictions.

R.Nowak, T.Sekino, S.Maruno, K.Niihara: Applied Physics Letters, 1996, 68[8], 1063-5