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.