Study on Vapor Phase Synthesis and Sintering of Ultrafine Silicon Nitride Composite Particles

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1. Preparation and sintering of Si_3N_4-SiC composite powder (1) Amorphous Si_3N_4-SiC composite powder with an average particle size of 0.02-0.1mum was produced by the vapor phase reaction of Si(CH_3)_4-NH_3 system (1200ﾟC), or by the plasma gas phase reaction of Si(CH_3)Cl_3-NH_3 system. The structure of composite particles was classified into three types: SiC-coated Si_3N_4, Si_3N_4-coated SiC and mixture of Si_3N_4 and SiC phases. (2) alpha-Si_3N_4 and beta-SiC were crystallized by heat treatment of amorphous composite powder. At 1600ﾟC, the heated powder consisted of coarse Si_3N_4 particles with a size above 3mum and fine SiC particles with a size of about 0.2mum. Hot-pressing of composite powder containing Y_2O_3-Al_2O_3 sintering aid at 1700ﾟC gave a dense sintered body with a relative density above 90%. The crystal phases of sintered body were beta-Si_3N_4 and beta-SiC. The grain size was small as about 0.2mum. The inclusion of SiC suppressed the grain growth of Si_3N_4. SiC content was decreased owing to the reaction with oxides.2. Preparation and sintering of Si_3N_4-BN composite powder Amorphous Si_3N_4-BN composite powder with an average particle size of 0.06-0.1mum was produced by the vapor phase reaction of SiCl_4-BCl_3-NH_3 system (1400ﾟC). The type of composite structure seemed to be a mixture of Si_3N_4 and BN phases. No crystallization was observed in the heat treatment at 1550ﾟC. When composite powder containing sintering aid was hot-pressed at 1700ﾟC, alpha,beta-Si_3N_4 was crystallized but h-BN was little crystallized. The density of sintered body was very low. As described above, the preparation method of composite powder was established and the sintering behavior was elucidated. The sintering technique should be established next to realize high-strength and high-toughness composite materials.