Nano- versus macro-hardness of liquid phase sintered SiC

Silicon carbide polycrystalline materials were prepared by liquid phase sintering. Different rare-earth oxides (Y2O3, Yb2O3, Sm2O3) and AlN were used as sintering additives. The final microstructure consists of core-rim structure owing to the incorporation of AlN into the rim of SiC grains by solid solution. Nano- versus macro-hardness of polycrystalline SiC materials were investigated in more details. The nano-hardness of SiC grains was in the range of 32-34 GPa and it depends on the chemical compositions of grains. The harness followed the core-rim chemistry of grains, showing lower values for the rim consisting of SiC-AlN solid solution. The comparison of nano- and macro-hardness showed that nano-hardness is significantly higher, generally by 5-7 GPa. The macro-hardness of tested samples had a larger scatter due to the influence of several factors: hardness of grains (nano-hardness), indentation size effect (ISE), microstructure, porosity, and grain boundary phase. The influence of grain boundary phase on macro-hardness is also discussed.