True Hardness Measurement by Nanoindentation Using a Single Sharp Indenter

초록

The Oliver-Pharr method has been used in nanoindentation testing to determine contact depth by subtracting the elastic deflection depth from the maximum indentation depth. When severe pile-up/sink-in is generated around the indenter, however, the Oliver-Pharr contact depth leads to significant errors. The relation of pile-up/sink-in to material properties and indentation parameters was investigated by extensive FEM simulation. Two parameters were used to describe the pile-up/sink-in for a sharp indenter. The first is the extent of elastic deformation at some representative strain, such as (@greek o@y or H)/E (ratio of yield strength or hardness to Young`s modulus), hf/hmax (ratio of final indentation depth to maximum indentation depth), We/Wt (ratio of work recovered during elastic unloading to total work input during loading), and the like; the second is the parameter describing the extent of plastic flow, i.e. the work-hardening exponent n. While the first parameter is represented satisfactorily by indentation parameters hf/hmax or We/Wt, the second is not easy to measure in a nanoindentation test using a single sharp indenter. We have now found a relation between the work-hardening exponent n and the characteristic length for the indentation size effect h* that can be measured in a nanoindentation test using a single sharp indenter. We thus propose a novel method to measure true hardness, taking into consideration pile-up/sink-in, with a nanoindentation test using a single sharp indenter.