Studies On Dense Coating Preparing By Chemical Vapor Deposition

Chemical vapor deposition (CVD) silicon carbide, which exhibits superior mechanical, thermal and optical properties, has become one of the most important technologies for preparing dense surface coating of lightweight space mirror. This paper studied the growth characteristics of CVD coating fabricated by pyrolysis of methyltrichlorosilane (MTS) in hydrogen. The features of defect in SiC coating such as the types, the forming mechanism, and the controlling methods were analyzed in details. In addition, the effect of the processing parameters on the microstructure and performance of SiC coating were studied. The results of these studies are as follows:The effect of the deposition temperature on the growth rate and crystallite size of SiC coating in the temperature region of 1100℃-1230℃has been studied. The controlling mechanism of growth rate of SiC coating has been demonstrated. It was shown that the growth rate of SiC coating increased with the temperature increasing.The activation energy was 116.313KJ/mol, and the growth rate of coating was controlled by chemical reaction rate. The phase composition of the SiC coating had nearly no changes at the preparing temperature of 1100℃,1150℃,1200℃,1230℃. But the crystallite size increased from 22nm to 32nm with the temperature increasing. The types of defects in SiC coating and the forming mechanism of defects have been studied in details. The results indicated that in the deposition process of SiC coating, the main defects were reticulate defect, lamination, crack, orientated growth, pores etc. The form of those defects was related with the processing parameters, the substrate and surface treatment.The deposition temperature influenced the surface microstructure and the density of SiC coating intensively. The grains of SiC coating were equiaxed structure at the temperature of 1100℃, columnar at the temperature 1150℃and faceted columnar at higher temperature of 1230℃. The density of SiC coating was 3.193g/cm3.The ratio of H2 to MTS can influence the phase composition of SiC coating. At the temperature of 1230℃, when the ratio of H2 to MTS was 6, the SiC coating was composed of SiC and C, while the ratio of H2 to MTS was 12, the coating was composed of SiC and Si. Only when the ratio of H2 to MTS was 10, the pure SiC can be obtained. When the ratio of H2 to MTS was 10, the phase composition of SiC coating prepared at the temperature of 1100℃, 1150℃, 1200℃and 1230℃had nearly no changes.The uniformity of the large area coating has been studied. The thickness no uniformity of large area coating can be improved through the improvement of flow distribution. Under optimized condition, the large area dense SiC coating with good performance has been prepared. Through optical polishing, the roughness of SiC coating (Ra) has achieved 0.72nm and the surface figure (RMS) has achieved 0.015λ(λ=0.6328μm).