Effects of Nitrate Sintering Additives on Pressureless Sintering of Monolithic Si₃N₄ and Si₃N₄/10-30mass% SiC Composites and Their Mechanical Properties

Abstract

Effects of the amount of sintering additives and that of SiC as a second phase on the densification and mechanical properties of Si₃N₄ and Si₃N₄/10-30mass% SiC composites were examined. Yttrium and aluminum nitrates as sintering additives were added to raw mixtures as methanol solutions. Powder compacts were pressurelessly sintered at 1750°C for 2h in N₂ atmosphere. The bulk density of the monolithic Si₃N₄ with 4mass% sintering additives reached 99% T. D. Both of the Si₃N₄/30mass% SiC with 8mass% sintering additives and the Si₃N₄/20mass% SiC with 6mass% sintering additives were sintered to 94% T. D. Fracture toughness of the dense Si₃N₄/SiC composites decreased compared with that of the monolithic Si₃N₄, due to the inhibitation of β-Si₃N₄ grain growth caused by the addition of SiC. Subsequently, the bending strength of these composites decreased. Furthermore, hardness of these composites increased. On the other hand, in the case of large amount of SiC addition or small amount of sintering additives, composites were not densified sufficiently, and bending strength, fracture toughness and hardness of these composites degraded markedly as compared with these of the monolithic Si₃N₄.