PROCESSING AND PROPERTIES OF PRESSURELESS-SINTERED SI3N4-SIC COMPOSITES

Silicon nitride (Si3N4) ceramics are potential candidates for structur al application where their high temperature strength and resistance to oxidation and corrosion properties can be utilized. Despite these goo d properties, the brittle nature of silicon nitride ceramics limits th eir 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 bovin g and micro-crack formation. In the present work, composites were prep ared using a Si3N4 matrix (with Y2O3 and Al2O3 as the sintering additi ves) and platelets or powders of SiC up to 30 vol.%. Sintering of thes e composites was carried out in a N-2 environment at atmospheric press ure at 1700 degrees C (pressureless sintering). The effects of the par ticle shape and sizes of the SiC reinforcements on the sintering behav iour and fracture toughnesses of the composites were studied and compa red 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 compar ed to the addition of SiC-powders, but in both cases the fracture toug hness values were greater than that of the matrix. Also, with larger-s ize particles of SiC the improvement in the fracture-toughness values was greater than that for smaller size-particles.