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-SiC 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 beta-SiC. At low SiC 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.