Factors affecting the measurement of hardness and hardness anisotropy

Abstract

By considering the observed hardness anisotropies of two different materials (001) single-crystal MgO and an aligned Al-CuAl₂ eutectic), this paper discusses some of the factors controlling the shapes and sizes of microhardness indentations. Both Vickers and Knoop profile indenters have been used. In the Vickers case, the responses of differing materials along symmetrically equivalent indenter diagonals have been observed while, in the Knoop case, indentations were often observed to have width∶length ratios different from that of the indenter. The observed behaviour has been interpreted in terms of differential elastic recovery on withdrawal of the indenter, and of changes in surface topography resulting from the accommodation of material displaced from the indentation (e.g. pile-up). It is demonstrated that both effects can seriously affect the sizes and shapes of hardness impressions. Further, these “extrinsic” effects are superimposed upon the “intrinsic” mechanical response and anisotropy of the test material itself. Thus, measured hardness anisotropies are a superposition of a number of effects, each important in its own right and each with its own anisotropy. Approaches have been devised which attempt to separate these extrinsic and intrinsic components of the observed hardness response. The results allow some important conclusions to be drawn concerning the interpretation of hardness values and hardness anisotropies.