Surface deformation of sapphire crystal

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Abstract
The anisotropic surface deformation of sapphire has been studied by means of depth-sensing indentation experiments with a small-radius spherical indenter loaded onto the basal (0001) and the prismatic (1010) planes of the crystal. The results of the low-speed high-accuracy continuous-indentation test have revealed significant differences in the mechanical behaviour of alumina crystals when indented on different crystallographic planes. A special effort has been made to clarify the origin of the characteristic discontinuities appearing in the loading part of the indentation hysteresis loops observed for the experiments performed on the (0001) as well as (1010) planes of sapphire. It was concluded that the large 'step-in' reflects the formation of a twin in a-alumina crystals deformed with a blunt indenter. A new resolved shear stress model has been proposed in order to meet the requirements of spherical indentation. The results of numerical calculations predicted the deformation systems activated in the vicinity of spherical indentation. The prediction correlated with the observations of residual impressions.