Investigation of Subsurface Damage Behaviors in Single-crystal Ge by Multi-cyclic Nanoindentation

Single-crystal Ge shows various machining-induced phase transformation behaviors which are accompanied by significant change of material characteristics. Therefore, revealing the mechanism of phase transformation may open up new ways of developing low-damage machining processes. Although the phase transformation of single-crystal Ge has been investigated in a few previous studies, the mechanisms of phase transformation and crystal defect nucleation are very complicated and still unclear. In this study, multi-cyclic nanoindentation of single-crystal Ge was performed with varying load holding processes to investigate the change of subsurface damaging behaviors. The experimental results showed that phase transformation behaviors of single-crystal Ge were classified into three types according to the holding load. A low holding load promoted phase transformation from dc-Ge to r8-Ge, whereas a middle holding load prevented the phase transformation. At a low holding load, a mixed region of phase transformation and crystal defects was observed around a densely phase-transformed region immediately underneath the indented surface. It was demonstrated that the middle holding load promoted nucleation of wide twins, around which no phase transformation occurred. For a high holding load, however, a densely phase-transformed region was widely formed and phase transformation to r8-Ge was confirmed more frequently than a middle holding load. These results will deepen the understanding of phase transformation and subsurface damaging behaviors of Ge, and contribute to the mechanical machining process optimization.