Nanostructure control from amorphous composite system

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 rodlike grains was enhanced by fine TiN inclusions.