Initial stages of contact-induced plasticity in sapphire. I. Surface traces of slip and twinning

Abstract

The present study focuses on the effects of surface orientation on the peculiarities of the earliest stages of nanoindentation-induced plasticity in sapphire (Al₂O₃) single crystal surfaces. The previous theoretical analyses do not account for all the experimentally observed trends. Additional considerations are required to bridge the gap between experimental results and theoretical predictions. Of key importance are 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, (0001)], rhombohedral [R,(1102)] and prism [A,(1210) and M,(1010)] surfaces. Surface patterns of slip and twining are 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.

Acknowledgements

The authors are grateful to Prof. A.H. Heuer and Prof. W.D. Scott for valuable insights on the crystallography of rhombohedral twinning in sapphire. Special thanks to Prof. W. Scott for providing the data of his calculations of Schmid factor for rhombohedral twinning as a function of the direction of applied uniaxial load. The data was used by the present authors to verify their calculations. NT thanks R.R. Carlson for inspiration, support, helpful discussions, and for editing the manuscript.