Ultrahardness: Measurement and Enhancement

In the quest for novel superhard materials with hardness comparable to or even higher than that of natural diamond, two issues are of central concern: how hardness can be reliably measured and how hardness can be enhanced. Here, we analyze the specific stress states of the indenter and the tested sample during an indentation hardness measurement and reveal that the traditional prerequisite that the indenter should be harder than the measured sample is not necessary to ensure a reliable hardness measurement. In fact, the indentation hardness can be reliably measured as long as the shear strength of the sample is less than the compressive strength of the diamond indenter. Also, we suggest nanostructuring (nanocrystallinity and nanotwinning) as an effect means of hardness enhancement.