Processing and properties of pressureless-sintered Si₃N₄SiC composites

Silicon nitride (Si₃N₄) ceramics are potential candidates for structural application where their high temperature strength and resistance to oxidation and corrosion properties can be utilized. Despite these good properties, the brittle nature of silicon nitride ceramics limits their applications. Reinforcement of ceramics by second phases (whiskers, platelets, fibres, powders) can improve their fracture toughness by the mechanisms related to fibre sliding, crack deflection, crack boving and micro-crack formation. In the present work, composites were prepared using a Si₃N₄ matrix (with Y₂O₃ and Al₂O₃ as the sintering additives) and platelets or powders of SiC up to 30 vol.%. Sintering of these composites was carried out in a N₂ environment at atmospheric pressure at 1700°C (pressureless sintering). The effects of the particle shape and sizes of the SiC reinforcements on the sintering behaviour and fracture toughnesses of the composites were studied and compared to the behaviour of the matrix (Si₃N₄ + Al₂O₃ + Y₂O₃). The results indicate that the presence of second phases lowered the densification, this being more pronounced with the addition of SiC-platelets compared to the addition of SiC-powders, but in both cases the fracture toughness values were greater than that of the matrix. Also, with larger-size particles of SiC the improvement in the fracture-toughness values was greater than that for smaller size-particles.