Mechanical Behaviour of Silicon-Silicon Carbide Composites

The dependence on the composition of Young 's modulus, fracture toughness and flexural strength of a reaction-sintered (RS) silicon-silicon carbide (Si­-SiC) composite was determined at room temperature over a wide range of SiC content (0-90 vol%). The results were compared with those of two commercial reaction-bonded (RB) Si-Si C materials. Young 's modulus follows two-phase models over the whole compositional range when a value of 432 GPa is assumed for the Young's modulus of ?-SiC. At low Si C contents (<60 vol%), the RS composites show fracture behaviour consistent with a crack­ deflection toughening model, while at SiC content higher than about 70 vol%, they exhibit much higher surface energies than the equivalent RB com­mercial Si-SiC. Between 60 and 70 vol% SiC an abrupt change of fracture behaviour is observed. Such differences in surface energies are attributed to quite different microstructures and crack propa­gation mechanisms.