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 furt her extension of the plastic zone growth and multiplication of pre-exi sting elements of plasticity are more effective than the generation of new dislocations or twins in the virgin material around the indentati on site. The model explains experiments with sintered alumina which in dicate a reduced load effect in increasingly fine-grained microstructu res due to a grain size effect that is more pronounced at higher testi ng loads (larger indents) than in the microhardness range. A large dif ference 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 o f the grain boundaries to inelastic deformation at the indentation sit e even at room temperature and even for coarser microstructures. (C) 1 998 Elsevier Science S.A. All rights reserved.