Microhardness anisotropy and the indentation size effect on the basal plane of single crystal hematite

The Knoop microhardness anisotropy of single crystal hematite, Fe2O3, was investigated on the basal plane (0001) at indentation test loads from 50 to 1000 g. Microhardness maxima were observed for low indentation test loads when the long axis of the Knoop indenter was parallel to the <2110>. Microhardness minima occurred for the <1010> orientations. The microhardness anisotropy at low indentation test loads is associated with slip dominance on the primary slip system of the corundum structure, the {0001}<1120>. An energy balance was applied to the indentation size effect (ISE) and also to address the microhardness anisotropy. The load independent, orientation independent, Knoop microhardness for the basal plane is 5.35 GPa. The ISE was further investigated by lubrication of the specimen test surface just prior to the Knoop microindentation process. Results from the lubricated microhardness tests are compared with the standard dry Knoop microhardness test results and reveal a significant reduction in the ISE. This indicates that friction between the test specimen and the indenter facets is a major contribution to the ISE.