Volume 109 (2001) Issue 1271 Pages 602-606
Fracture toughness, fracture surface energy and crack initiation load of single 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 with B and C additives>single crystals. The properties of single crystals depended on crystal orientation, while little difference of properties was observed between 4H and 6H structures. High fracture toughness and fracture surface energy of Al2O3 containing SiC resulted from crack deflection and branching at grain boundaries. Slightly higher fracture toughness and fracture surface energy of SiC sintered with B and C additives, in comparison with single crystals, was caused by crack deflection which was observed even when transgranular propagation occurred due to the different crystal orientation of cleavage planes in each grain. A microstructure favorable for high toughness involves a more severe local damage beneath the indentation.