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

The measured load (size) effect on the hardness is modelled assuming that for the extension of the plastically deformed zone, growth and multiplication of pre-existing elements of plasticity are more effective than the generation of new dislocations and twins in the virgin material around. This idea also explains the decreasing load effect at smaller grain sizes. Therefore, microhardness approaches must not be used to investigate grain size effects in ceramic microstructures. The comparison of the real grain size effect in sintered Al₂O₃ with the indentation size (load) effect in sapphire single crystals (shown to simulate the grain size effect in polycrystals but avoiding the influence of grain boundaries) reveals an important contribution of the grain boundaries to the permanent deformation at the indentation site at room temperature even for coarser microstructures and rules out the chances for a strong hardness increase in oxide ceramics at grain sizes < 100 nm. However, first results indicate a possibly different behavior in binder-free carbides where the covalent character of interface bonding may reduce the degree of grain boundary deformation at room temperature as it is known to reduce the microplastic deformability of the crystal lattice.