Non-linear surface deformation of the (10(1)over-bar-0) plane of sapphire:Identification of the linear features around spherical impressions

The work aims to clarify the origin of the characteristic linear features w hich appear in the vicinity of the spherical indentation impressions on the (10 (1) over bar 0) plane of sapphire crystal. Since the location of the f eatures was already predicted by the Resolved Shear Stress Model which poin ted towards twinning as a source of the surface defects, in the present stu dy the electron and atomic force microscopy were employed to verify the res ults of earlier numerical calculations. In order to identify the deformatio n systems which contributed to the formation of the linear features, the pr ecise measurement of the surface slope in the defected region has been acco mplished, which led to the conclusion that the observed eruptions cannot be caused by the rhombohedral twinning exclusively. The presented results ind icate that the basal twinning contributed to the process of linear features formation. It is concluded that the observed surface defects may result fr om the cooperative basal twinning and rhombohedral slip or twinning-somethi ng predicted in an earlier model. The present study supports the conclusion that the high-load pop-ins registered during the loading cycle of the inde ntation into the (10 (1) over bar 0) plane of sapphire are due to twinning- the process being largely overlooked in the recent studies of non-linear in dentation deformation mechanisms. (C) 1999 Acta Metallurgica me. Published by Elsevier Science Ltd. All rights reserved.