Preparation Of SiC Filament By Chemical Vapor Deposition

Because of CVD-SiC fibers' high strength, high modulus, low density and refractory properties, composites reinforced by it have excellent performance. There have been commercial available CVD-SiC fiber productions, due to its military and aerospace application background; CVD-SiC fiber products are an embargo on our country. Development and applicant of it is in the infancy in domestic, so it is necessary to launch preparation research.On the basis of comparing two different kinds of chemical vapor deposition equipments, a new SiC fiber preparation apparatus was set up. In the new apparatus mercury was substituted by cadmium/indium/zinc liquid alloy to seal reactant gas in the reactor, cadmium/indium/zinc liquid alloy not only plays the role of sealant, but also reduces the poisonous volatile and arcing appearance caused by low conductance between mercury and SiC.When tungsten wire with diameters 57μm and 13.5μm were used as substrate to prepare CVD — SiC fiber, both deposition rate and tightness of SiC raise with temperature. However, tubercular and loose deposition would be formed at over temperature. Deposition rate decreases with mole fraction of MTS rising in mixed gas at lower temperature, whole reaction is controlled by surface reaction; on the contrary, deposition rate raise with mole fraction of MTS rising in mixed gas at higher temperature, whole reaction is controlled by mass transmission.Optimum process conditions that temperature is 1570K, deposition time is five minutes and flow rate of hydrogen and nitrogen are 450ml/min and 100ml/min respectively in this study were get by adopting orthogonal experiment. Primary and secondary sequence of parameters is temperature, time, flow rate of hydrogen and nitrogen.Strength of CVD—SiC fiber at room temperature follows Weibull statistics, Weibull shape parameter decreases with rising of gauge length of test sample. Fibers strength has a strong relationship with their morphologies, fibers with high strength has smooth surface, buttubercular fibers has lower strength. Bonding between tungsten wire and SiC deposition of high strength fibers is better than that of lower strength fibers.Except including silicon, carbon elements, prepared fiber also contain heterogeneity elements such as, oxygen, aluminum and calcium. All these heterogeneity elements will result in initiating defects; improving purity of raw materials and increasing leak tightness of reactor can reduce them.