The influence of chemical vapour deposition conditions on the structural stability and cracking of silicon carbide coatings on carbon-carbon composites

Abstract

Low pressure chemical vapour deposition (LPCVD) was used to apply silicon carbide coatings to a carbon-carbon (C/C) composite using precursors of methyl-trichloro-silane (MTS) and dichloro-methyl-silane (DMS). The influence of the deposition conditions on the structure and cracking behaviour of these coatings was studied using the techniques of scanning and transmission electron microscopy, X-ray diffraction and laser ionization mass spectroscopy. The coatings develop cracks when cooled from the deposition temperature owing to thermal expansion mismatches. The deposition conditions influence the degree of cracking due to structural differences in the coatings and hence govern the surface area of exposed substrate. Deposition from DMS at 1000°C produces the lowest exposed surface area and protects the composite from oxidation at temperatures below 1000°C. At higher temperatures this coating has been found to be structurally unstable, resulting in an increase in the surface area of exposed substrate.