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 Si
3N
4-BN and Si
3N
4-TiN systems. The powders were prepared by a vapor phase reaction method and heat-treated at 1600°C in N
2 with and without Y
2O
3-Al
2O
3 sintering aid. The crystallization and particle growth of Si
3N
4 were retarded by BN inclusion. Si
3N
4-BN composite powder was amorphous even at 1600°C without sintering aid, whereas β-Si
3N
4 crystallized at 1600°C with sintering aid. IR analysis indicated that Si
3N
4 and BN separated with a rise of temperature and the crystallization was accelerated by sintering aid. In Si
3N
4-TiN system, TiN accelerated the crystallization of β-Si
3N
4. It was found that BN was expelled from Si
3N
4 particles with and without sintering aid, whereas TiN was surrounded by Si
3N
4 particles with sintering aid. Si
3N
4-BN and Si
3N
4-TiN nanocomposites were fabricated by hot pressing at 1800°C. The excellent thermal-shock resistance was observed in Si
3N
4-BN system and the high fracture toughness in Si
3N
4-TiN system in which the growth of Si
3N
4 rod-like grains was enhanced by fine TiN inclusions.
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