A note on a common mistake in the analysis of nanoindentation data

It is shown that to analyze the load versus displacement data obtained from a nanoindentation experiment on a flat surface, it is incorrect to use the so called "reduced modulus," which includes the elastic properties of both the indenter and the test solid. It is suggested that, until correct analytical solutions become available, the indenter should always be of a much stiffer material than the test solid and it should be approximated to a rigid indenter in the analysis. Furthermore, when the indenter and the test surface are of comparable elastic moduli, the measured indenter displacement is the distance of mutual approach of the two contacting bodies rather than the penetration of the indenter below the original surface of the test solid.