SURFACE DEFORMATION OF SAPPHIRE CRYSTAL

The anisotropic surface deformation of sapphire has been studied by me ans of depth-sensing indentation experiments with a small-radius spher ical indenter loaded onto the basal (0001) and the prismatic (<10(1)ov er bar 0>) planes of the crystal. The results of the low-speed high-ac curacy continuous-indentation test have revealed significant differenc es in the mechanical behaviour of alumina crystals when indented on di fferent crystallographic planes. A special effort has been made to cla rify the origin of the characteristic discontinuities appearing in the loading part of the indentation hysteresis loops observed for the exp eriments performed on the (0001) as well as (<10(1)over bar 0>) planes of sapphire. It was concluded that the large 'step-in' reflects the f ormation of a twin in g-alumina crystals deformed with a blunt indente r. A new resolved shear stress model has been pro-posed in order to me et the requirements of spherical indentation. The results of numerical calculations predicted the deformation systems activated in the vicin ity of spherical indentation. The prediction correlated with the obser vations of residual impressions.