Research Of Structure And High Temperature Performance Of Silicon Nitride Prepared By CVD/CVI

Silicon nitride has high strength and hardness, high temperature,corrosion and wear resistance, anti-oxidation, thermal shock resistance, low dielectric constant and a series of excellent performances, it is regarded as the most promising next-generation high-temperature wave-transparent materials. The Si₃N₄ coating on porous Si₃N₄ and quartz was prepared by chemical vapor deposition, BN/Si₃N₄ and C/Si₃N₄ composites were prepared by chemical vapor infiltration. The microstructure of BN/Si₃N₄ and C/Si₃N₄ composites was analysed, the oxyacetylene torch ablation behavior and mechanism of the C/Si₃N₄ composites were studied. BN and C fibers were surrounded and the gap among the fibers was filled by Si₃N₄ matrix, because of the mismatching of the coefficient of the thermal expansion and the poor wettability between the Si₃N₄ coating and the BN fiber, there was gap between the BN fiber and the Si₃N₄ matrix, the Si₃N₄ coating could be easily flaked away. The Si₃N₄ matrix was composed of the spheroid particles, the growth mechanism of Si₃N₄ matrix were investigated using chemical reaction kinetics and supersaturation-condensation-fusion mechanism, the growth mode of the Si₃N₄ matrix accorded with the S–K growth mode. Some islands presented on the surface of the C/Si₃N₄ composite, the matrix was composed of amorphous and nanometerα-Si₃N₄, Fiber pull-out of the carbon fibers in the fracture process was the main toughening mechanism of the C/Si₃N₄ composite. The dielectric constant of the Si₃N₄ matrix was computed with the series mixing model; The rigidity and elastic modulus of the Si₃N₄ matrix were examined with nano-hardness tester; The binding force between Si₃N₄ coating and quartz matrix was examined with scratch tester; The bonds and infrared transmittance of the Si₃N₄ coating before and after RTP were analysed using infrared analyzer; The thermal shock resistance of the Si₃N₄ matrix was tested through cycle thermal shock experiment; The thermal stability and crystal structure evolution of the Si₃N₄ matrix were studied by differential thermal analyzer and high temperature treatment. When the C/Si₃N₄ composite was ablated in the temperature of 2200℃, the carbon fibers was oxidized and the Si₃N₄ matrix sublimated and decomposed in the centre region; In the ring region, a part of the matrix was oxidized to form the liquid SiO₂ layer, some matrix was broken because of the thermal shock, some fibers naked for the mechanical erosion and there were some spherical particles on the surface.