The hardness of solids

TLDR

Brittle solids experience high hydrostatic pressures during deformation that can suppress brittle fracture. The review examines the fundamental physical interpretation of hardness. Indentation and scratch hardness of ductile and brittle solids primarily reflect plastic properties, providing a physical basis for the Mohs scale.

Abstract

This review is concerned with the basic physical meaning of hardness. It is shown that indentation hardness of ductile materials is essentially a measure of their plastic properties. With brittle solids the high hydrostatic pressures around the deformed region are often sufficient to inhibit brittle fracture. Under these conditions both indentation and scratch hardness are essentially a measure of the plastic rather than the brittle properties of the solid. This provides a simple physical basis for the Mohs scratch hardness scale