Conventional micro-Vickers hardness as determined by instrumented indentation technique

Abstract

In instrumented indentation testing (IIT), assessing the correct contact depth is indispensable since contact area is determined from the contact depth evaluated from indentation load-depth curve without the optical observation of residual indent. When pile-up/sink-in of materials around an indenter is produced severely, it is difficult to determine the contact depth only in IIT, that is, additional observation of the residual indent is needed to revise the contact depth for pile-up/sink-in. In this study, we developed the algorithm to evaluate the conventional micro-Vickers hardness values only with IIT, for which the height for pile-up/sink-in at the corner of residual indent should be determined from the IIT using Vickers indenter. We used two parameters to describe the pile-up/sink-in of materials; hf/hmax (ratio of final indentation depth to maximum indentation depth) or We/Wt (ratio of work recovered during elastic unloading to total work input during loading) corresponding to the portion of elastic deformation at some total (representative) strain induced by the indentation, and the characteristic length for indentation size effect corresponding to the degree of plastic flow. With these two parameters, the equation to evaluate the height for pile-up/sink-in was derived and the evaluations of the micro-Vickers hardness from IIT was verified based on the experimental results of conventional micro-Vickers tests and instrumented indentation tests for about 20 metallic materials.