The present study focuses on the effects of surface orientation on the peculiarities of the earliest stages of nano-indentation induced plasticity in sapphire (Al2O3) single crystal surfaces. The previous theoretical analyses did not account for all the experimentally observed trends. Additional considerations were required to bridge

the gap between experimental results and theoretical predictions. Of key importance were accounting for the sense of twinning shear, the multiplicity of slip and twinning systems involved and an appropriate criterion for the transition from elastic to elastic-plastic regime. The present study supplements a continuum-based stress analysis with the above considerations and compares the resulting theoretical predictions with the experimental results for basal [C, (00•1)], rhombohedral [R,(11•2)] and prism [A,(12•0) and M,(10•0)] surfaces. Surface patterns of slip and twining were scrutinized in Part I. Previously unexplained features justified by the results obtained by the present authors include the distribution of the linear surface features ascribed to twinning and the symmetry of indentation pile-up. Part II focuses on the mechanisms of the transition between the elastic and elastic-plastic regimes.

Initial Stages of Contact-Induced Plasticity in Sapphire I - Surface Traces of Slip and Twinning. N.L.Tymiak, W.W.Gerberich: Philosophical Magazine, 2007, 87[33], 5143-68