A comparison of three microindentation hardness scales at low and ultralow loads

Indentation hardness tests were performed on thick, fine-grained, electro-formed deposits of copper and nickel using Knoop, Vickers, and Berkovich indenters. The latter type of indenter was used for shallow penetrations (85–1750nm), and results are reported in terms of nanoscale hardness (NH) numbers. Knoop and Vickers indenters were used with applied loads of between 0.15 and 0.98 N, and at the lowest load, produced indentation depths comparable to the larger ones obtained with the Berkovich indenter. The NH numbers became very sensitive to penetration depth when the penetration depth was less than certain critical values. NH numbers for Cu and Ni were higher than those for Knoop and Vickers testing at comparable penetration depths. Applying indenter area function corrections to calculate hardness numbers (i.e., considering projected area versus facet contact area) resulted in a closer correlation between microhardness and nanohardness scales; however, changes in the tip shape because of wear or other imperfections can lead to inaccurate calculation of NH numbers at the lowest loads. Results also suggest that the interconversion of lowload hardness numbers from one scale to another can be material-dependent.