Non-linear surface deformation of the (1010) plane of sapphire: Identification of the linear features around spherical impressions

The work aims to clarify the origin of the characteristic linear features which appear in the vicinity of the spherical indentation impressions on the (1010) plane of sapphire crystal. Since the location of the features was already predicted by the Resolved Shear Stress Model which pointed towards twinning as a source of the surface defects, in the present study the electron and atomic force microscopy were employed to verify the results of earlier numerical calculations. In order to identify the deformation systems which contributed to the formation of the linear features, the precise measurement of the surface slope in the defected region has been accomplished, which led to the conclusion that the observed eruptions cannot be caused by the rhombohedral twinning exclusively. The presented results indicate that the basal twinning contributed to the process of linear features formation. It is concluded that the observed surface defects may result from the cooperative basal twinning and rhombohedral slip or twinning - something predicted in an earlier model. The present study supports the conclusion that the high-load pop-ins registered during the loading cycle of the indentation into the (1010) plane of sapphire are due to twinning -the process being largely overlooked in the recent studies of non-linear indentation deformation mechanisms.