A new look at grain size and load effects in the hardness of ceramics

A simple model describes the load effect (size effect) in the hardness, assuming an increasing microplastic deformability, when for the further extension of the plastic zone growth and multiplication of pre-existing elements of plasticity are more effective than the generation of new dislocations or twins in the virgin material around the indentation site. The model explains experiments with sintered alumina which indicate a reduced load effect in increasingly fine-grained microstructures due to a grain size effect that is more pronounced at higher testing loads (larger indents) than in the microhardness range. A large difference between the hardness of plastically deformed volumes in single crystals and in polycrystalline microstructures consisting of grains with the same size, respectively, reveals a substantial contribution of the grain boundaries to inelastic deformation at the indentation site even at room temperature and even for coarser microstructures.