A new look at the influences of load, grain size, and grain boundaries on the room temperature hardness of ceramics

The experimentally measured load (size) effect on the hardness is modelled and explains the decreasing inluence of the testing load at smaller grain sizes. Therefore, microhardness measurements must not be used to investigate grain size effects in the hardness of ceramics. The comparison of the real grain size effect in sintered alumina with the indentation size (load) effect in spphire singel crystals reveals an important contribution of the grain boundaries to the permanent deformation at the indentation site at room temperature even for coarser microstructures. First measurements indicate a possibly different behaviour in binder-free carbides where the covalent character of interface bonding may reduce the degree of grain boundary deformatrion at room temperature as it is known to reduce the microplatic deformability of the crystal lattice.