Gas Phase Impregnation/Densification of Carbon-carbon and other High-temperature Composite Materials

Abstract

Chemical vapour deposition (CVD) is a process in which a solid product nucleates and grows on a substrate, by decomposition or reaction of gaseous species, and involves the heating of a fibre preform in a gaseous environment so that the matrix is deposited from the gas phase. The technology developed to date allows fine control over the composition and morphology of the solid deposit. Various processes have been used for the production of thin film semiconductor devices for the communications industry and the production of hard, abrasion-resistant coatings on cutting tools and coatings on radioactive pellets. The CVD techniques have also been widely used for the preparation of oxidation- and wear-resistant coatings to carbon-carbon composites. Well-processed CVD-derived composites generally possess excellent mechanical properties as a consequence of the slow, steady build-up of matrix material around the fibre network. The CVD method has proven especially useful for the production of ceramic matrix composites, where melt-processing techniques are inapplicable and conventional powder-processing methods — such as those used for the production of monolithic ceramics — result in serious fibre degradation. The major drawback of CVD is the very slow rate of deposition, leading to large material/energy inputs and a high final cost.