Mohs's Hardness Scale - A Physical Interpretation

Abstract

The Mohs hardness scale consists of ten minerals arranged in increasing order of hardness such that each mineral will scratch the one on the scale below it, but will not scratch the one above it. Although these minerals are generally considered to be brittle, experiments show that under local indentation and during sliding their behaviour is primarily determined by the plastic properties of the minerals. This suggests that there should be some relation between the Mohs hardness and the indentation hardness of the minerals. Experiments with metals show that in general a metal surface of indentation hardness H_s will be scratched by a point of hardness H_p if H_p gt-or-equal, slanted 1.2 H_s. Thus a scratch hardness scale can be constructed in which each standard has an indentation hardness at least 1.2 times greater than the preceding standard. The scratch hardness number is then proportional to the logarithm of the indentation hardness. Excluding diamond, which is anomalous, this type of relation is found to apply to the Mohs standards, and it is found that each standard is approximately 60% harder than the preceding one.