3.8 Hardness

The simplest way of determining the resistance of a metal to plastic deformation is through a hardness test. Indentation tests constitute the vast majority of hardness tests. They are essentially divided into three classes, commonly called nanoindentation, microindentation, and macroindentation tests, but improperly referred to as micro-hardness and macrohardness tests. The division between micro and macro occurs for a load of approximately 200 gf (~2 N). In nano-indentation testing, the load is of the order of mN. The indentation tests in metals measure the resistance to plastic deformation; both the yield stress and the work-hardening characteristics of the metal are important in determining the hardness. In spite of the theoretical studies done on hardness, hardness cannot be considered a fundamental property of a metal. Rather, it represents a quantity measured on an arbitrary scale. ^[15] Hardness measurements should not be taken to mean more than what they are: an empirical, comparative test of the resistance of the metal to plastic deformation. Any correlation with a more fundamental parameter, such as the yield stress, is valid only in the range experimentally determined. Similarly, comparisons between different hardness scales are meaningful only through experimental verification. For steels, Table 3.2 gives a fair conversion of hardness and the tensile strength equivalents.