Experimental study of the chemical vapor deposition from CH₃SiHCl₂/H₂: Application to the synthesis of monolithic SiC tubes

The aim of the present work is to synthesize high strength monolithic SiC tubes to improve the imperviousness of a SiC/SiC composite structure. A few hundred micrometer-thick tubular coatings were produced by chemical vapor deposition (CVD) at atmospheric pressure from CH₃SiHCl₂/Ar/H₂ mixtures. The CVD-SiC tubes were obtained by deposition on the inner walls of a SiO₂-tube substrate, previously coated with a pyrocarbon interfacial layer to promote delamination. A continuous deposition process was developed to allow the realization of relatively long CVD-SiC tubes by sliding the heating system along the substrate. The chemical composition and the microstructure of the tubes were studied by electron probe microanalysis, Raman spectroscopy and scanning electron microscopy. The deposition rate, composition and microstructure of the CVD-SiC coatings were investigated as a function of the substrate temperature and the gas flow rates. A Fourier transformed infrared (FTIR) spectroscopy analysis was carried out at the reactor outlet to characterize the gas phase reactions. The FTIR analysis of pure species from the Si-C-Cl-H system as well as ab initio calculations at the density functional theory (DFT) level allowed the assignment of the main IR features in the experimental spectra and the quantitative analysis of the complex gas mixture. This study has led to the proposal of a simplified dichloromethylsilane decomposition scheme which is consistent with the influence of the CVD parameters on the nature of the gas phase and the coating. The deposition rate, the Si/C atomic ratio, the SiC crystalline state and the surface morphology are indeed strongly related to the CH₃SiHCl₂ decomposition rate and the further progress of homogeneous reactions. (authors)