Ultra-microhardness testing procedure with Vickers indenter

Abstract

Depth-sensing indentation equipment is widely used for evaluation of the hardness and Young's modulus of materials. The depth resolution of this technique allows the use of ultra-low loads. However, aspects related to the determination of the contact area under indentation should be cautiously considered when using this equipment. These are related to the geometrical imperfections of the tip, the diamond pyramidal punch and the formation of pileup or the presence of sink-in, which alter the shape and size of the indent. These and other aspects, such as the thermal drift of the equipment and the scattering at the zero indentation depth position related to surface finishing, are discussed in this work. A study concerning the hardness and the Young's modulus results determined by Vickers indentation on different materials was performed. Samples of fused silica, BK7 glass, aluminium, copper and mild steel (for which the values of Young's modulus were previously known) were tested using indentation loads in the range 10-1000 mN. Moreover, two methods are proposed for performing the indentation geometrical calibration of the contact area; these are compared with a former method proposed by Oliver and Pharr (OP). The present methods are based on: (i) analysis of the punch profile using atomic force microscopy (AFM); and (ii) a linear penetration-depth function correction (LM), based on knowledge of the values of the Young's modulus of several materials. By applying these methods to the indentation load/indentation depth results, it was possible to draw some conclusions about the benefit of the AFM and LM methods now under proposal.