Study On The Processing And Structure Of The SiBCN Interphase Coatings Prepared By Chemical Vapor Deposition

As is well-known, Si Cf/Si C composites are made up of Si C fiber, Si C matrix and the interphase between them. Among these compositions, interphase is the key component. Pyrocarbon(Py C) and BN with layer structure are the most widely used interphase materials for Si Cf/Si C composites. However, the two interphases both have poor high-temperature oxidation resistance, and Py C interphase with high electrical conductivity can lead to strong microwave reflction of Si Cf/Si C composites. Researches have shown that Si BCN interphase fabricated by doping of BN with Si and C can increase the oxidation and moisture resistance of BN in Si Cf/Si C composites. Based on the above, Si BCN ceramic is used as interphase of the high-temperature radar absorbing materials in this thesis.Si BCN ceramic was prepared by using Borazine(BZ)-Liquid polycarbosilane(LPCS)-Ar system and chemical vapor deposition(CVD). The effect of temperature upon depositon process, morphology, phase composition, element content and bonding states was invesgated. The results showed that from 600? to 900?, the deposition rate increased with temperature rising and peaked height velocity in 900?. The activation energy was 89.8k J/mol. However, the deposition rate decreased with temperature rising from 900? to 1100?. The composition and structure of the deposition showed the existence of residual C-H, B-H, N-H and Si-H bondings due to unfinished precursor decomposition below 800?. The as-deposited Si BCN ceramic showed completely inorganic and amorphous containing Si-C, B-N, Si-N and C-C bondings above 900?. The morphology of the deposition presented island disjunction between 600? and 700?; Between 600? and 700?, the deposition presented uniform and smooth surfaces; The particle sizes of the deposition were different and the particles didn't accumulated tightly.The effect of pressure upon as-deposited coatings was also invesgated. The morphology of the coatings presented compact structure while heterogeneous nucleation reaction dominated the coating growth in low pressure(1k Pa~2k Pa); The surface of the coatings was loose and coarse while homogeneous nucleation reaction dominated the coating growth in high pressure(3k Pa~4k Pa).The effect of total flow rate of reaction gas upon as-deposited coatings showed that in low flow rate(10sccm~15sccm), the deposition rate increased with flow rate rising and peaked height velocity in the deposition rate of 15 sccm. The deposition rate significantly decreased with flow rate further rising due to gas phase nucleation reaction and the coatings were loose and porous containing powder particles.The result of the effect of BZ/LPCS upon as-deposited coatings indicated that the contents of element Si and element C decreased gradually with BZ/LPCS rising. No significant change exsited upon the morphology and deposition rate.The optimized process conditions through experiment were as below: the temperatures were 900?~1100?; The total pressure was 2k Pa; The total total flow rate of reaction gas was 10sccm~15sccm; BZ/LPCS was 10:1.The effects of Si BCN coatings upon the tensile strength and the electrical resistivity of Si C fiber were investigated. The results showed that the the tensile strength and the electrical resistivity of Si C fiber remained about the same before and after depositoin. The experiment indicated the fiber with various coating thickness could improve the oxidation resistance of the fiber effectively and the strength retention rate of the fiber reached maximum(89.6%) while the coating thickness was 1000 nm.The influences of Si BCN interphase on mechanical property, oxidation resistance and dielectric property of Si Cf/Si BCN/Si C composites were studied. The results were as followed:(1) Comparing with the fracture strength of Si Cf/Si C composites without interphase(60.4MPa), the fracture strength of Si Cf/Si C composites with Si BCN interphase reached 86.4MPa~126.1MPa.(2) Comparing with Si Cf/Si C composite and Si Cf/BN/Si C composite, Si Cf/Si BCN/Si C composite had more excellent oxidation resistance. After 1h oxidation in the oxidation temperature of 1000?, the strength retention rates were 53.77%, 63.36% and 77.06%, respectively.(3) The Si BCN interphase had little effect on the permittivity of Si Cf/Si C composite indicating it was ideal interphase material of the high-temperature radar absorbing materials.