Mechanical behavior of single crystalline and polycrystalline silicon carbides evaluated by vickers indentation

Fracture toughness, fracture surface energy and crack initiation load of si ngle crystal and polycrystalline SiC sintered with different additives were evaluated by Vickers indentation technique. Resistance to crack initiation and propagation in sintered SiC with Al2O3 additive was > sintered SiC wit h B and C additives > single crystals. The properties of single crystals de pended on crystal orientation, while little difference of properties was ob served between 4H and 6H structures. High fracture toughness and fracture s urface energy of Al2O3 containing SiC resulted from crack deflection and br anching at grain boundaries. Slightly higher fracture toughness and fractur e surface energy of SiC sintered with B and C additives, in comparison with single crystals, was caused by crack deflection which was observed even wh en transgranular propagation occurred due to the different crystal orientat ion of cleavage planes in each grain. A microstructure favorable for high t oughness involves a more severe local damage beneath the indentation.