非晶質複合系からのナノ構造組織制御

Abstract

Ceramic nanocomposites can be fabricated from composite powders. In particular, the nanostructure is easily formed from the finely-mixed amorphous state. The phase separation and crystallization of the composite powders during heating were investigated in Si3N4-BN and Si3N4-TiN systems. The powders were prepared by a vapor phase reaction method and heat-treated at 1600°C in N2 with and without Y2O3-Al2O3 sintering aid. The crystallization and particle growth of Si3N4 were retarded by BN inclusion. Si3N4-BN composite powder was amorphous even at 1600°C without sintering aid, whereas β-Si3N4 crystallized at 1600°C with sintering aid. IR analysis indicated that Si3N4 and BN separated with a rise of temperature and the crystallization was accelerated by sintering aid. In Si3N4-TiN system, TiN accelerated the crystallization of β-Si3N4. It was found that BN was expelled from Si3N4 particles with and without sintering aid, whereas TiN was surrounded by Si3N4 particles with sintering aid. Si3N4-BN and Si3N4-TiN nanocomposites were fabricated by hot pressing at 1800°C. The excellent thermal-shock resistance was observed in Si3N4-BN system and the high fracture toughness in Si3N4-TiN system in which the growth of Si3N4 rod-like grains was enhanced by fine TiN inclusions.