Disturbance induced by surface preparation on instrumented indentation test

Surface preparation, which may induce considerable sample disturbance, plays an important role in instrumented indentation test (IIT). In this study, the sample disturbance (mainly divided into residual stresses and plastic strain) induced by the surface preparation process of instrumented indentation test specimens were investigated with both experimental tests and numerical simulations. Grazing incidence X-ray diffractions (GIXRD) and uniaxial tensile tests were conducted for characterizing the residual stresses and high plastic strain in the top surface layers of a carefully mechanically polished indentation sample, which, in the present work, is made of commercially pure titanium. Instrumented indentation tests and the corresponding finite element simulations were performed as well. For comparison, a reference sample (carefully mechanically polished & electrolytically polished) which represents the raw material was prepared and tested. Results showed that a careful mechanical polishing procedure can effectively reduce the level of residual stresses induced by this process. However, the high plastic strain in the surface region imposed by the polishing process is significant. The induced plastic strain can affect a depth up to 5 µm, which is deeper than the maximum penetration depth hmax (3 µm) used for the instrumented indentation tests. In the near surface layer (in the range of depth about 350 nm), the plastic strain levels are fairly high. In the very top layer, the plastic strain was even estimated to reach more than 60%. The simultaneous use of indentation tests and numerical simulations showed that the existence of high plastic strain in the surface region will make the load vs depth (P-h) curve shift upwards, the contact hardness (H) increase and the contact stiffness (S) decrease.