EFFECTS OF NITRATE SINTERING ADDITIVES ON PRESSURELESS SINTERING OF MONOLITHIC SI3N4 AND SI3N4 10-30 MASS-PERCENT-SIC COMPOSITES AND THEIR MECHANICAL-PROPERTIES/

Effects of the amount of sintering additives and that of SiC as a seco nd phase on the densification and mechanical properties of Si3N4 and S i3N4/10-30 mass%SiC composites were examined, Yttrium and aluminum nit rates as sintering additives were added to raw mixtures as methanol so lutions, Powder compacts were pressurelessly sintered at 1750 degrees C for 2h in N-2 atmosphere, The bulk density of the monolithic Si3N4 w ith 4 mass% sintering additives reached 99% T.D. Both of the Si3N4/30 mass%SiC with 8 mass% sintering additives and the Si3N4/20 mass%SiC wi th 6 mass% sintering additives were sintered to 94% T.D. Fracture toug hness of the dense Si3N4/SiC composites decreased compared with that o f the monolithic Si3N4, due to the inhibitation of beta-Si3N4 grain gr owth caused by the addition of SiC. Subsequently, the bending strength of these composites decreased, Furthermore, hardness of these composi tes increased. On the other hand, in the case of large amount of SiC a ddition or small amount of sintering additives, composites were not de nsified sufficiently, and bending strength, fracture toughness and har dness of these composites degraded markedly as compared with these of the monolithic Si3N4.