Elastic and hardness anisotropy and the indentation size effect of pyrite (FeS2) single crystal

The Knoop hardness of single crystal pyrite, FeS2, was investigated on the (1 1 1) and the (1 0 0) planes by indentation testing from 10 to 500 gf (0.10–4.90 N). An energy balance was applied to analyse the hardness and the indentation size effect (ISE). The hardness anisotropy is directly attributed to the anisotropy of the Young’s modulus (E). The isotropic force-independent hardness for these two planes are 962 kgf mm−2 (94.34 N mm−2) for the (1 1 1) plane and 900 kgf mm−2 (88.26 N mm−2) for the (1 0 0) plane. When the ISE is prevalent at low indentation test forces, the surface area to volume ratio of the indentation is high because a part of these phenomena is attributable to the friction between the indenter and the specimen.