The measurement of hardness of very hard materials

It is shown that the classical two-step indentation method of determining hardness can be used to measure the hardness of diamond, long considered the hardest known substance. Experimental results are compared with those of finite element analysis and AFM imaging. It is shown that in very hard materials, unlike metals, the observed projected area of the indent changes significantly when the indenter is withdrawn, giving rise to an apparent increase in the measured hardness of almost a factor of two. This work highlights the significance of elastic recovery on the measurement of hardness that occurs during the indentation of very hard materials.