Chemical vapor deposition of silica, alumina, and aluminosilicates from mixtures of aluminum trichloride, chlorosilane, carbon dioxide, and hydrogen

The objective of this study is the comprehensive investigation of the kinetics of the codeposition of silica (SiO₂), alumina (Al ₂O₃), mullite (3Al₂O₃·2SiO ₂), and other aluminosilicates from mixtures of chlorosilane (SiCl ₄ or MTS), aluminum trichloride, carbon dioxide, and hydrogen in order to prepare coatings for the protection of SiC-based ceramics from oxidation in high-temperature applications. In an attempt to elucidate some aspects of the codeposition process, the deposition of silica and alumina from chlorosilane and aluminum trichloride, respectively, in carbon dioxide and hydrogen is also studied. Experiments are conducted in a tubular, hot-wall chemical vapor deposition reactor, coupled to an electronic microbalance, using various substrates. The effects of process parameters on deposition rate, film morphology, and film composition are examined over a wide range of experimental conditions. Among the most interesting results of this study is that the presence of AlCl ₃ has a catalytic effect on the incorporation of silica in the deposit, leading to codeposition rates that are higher than the deposition rates that are obtained when only one of the two chlorides (chlorosilane or AlCl ₃ is present in the feed. The results of the deposition experiments also show that manipulation of the temperature of the reaction and the residence time of the mixture in the reactor offers a way to control the composition of the codeposited films in SiO₂ and Al₂O₃, obtaining deposits with significant alumina and aluminosilicate (e.g., mullite) content. In order to account for the complex chemistry of the formation of the oxide films, detailed homogeneous and heterogeneous mechanisms are developed for the deposition of silica, the deposition of alumina, and the codeposition process. The kinetic mechanisms encompass several reaction sequences for the generation of deposition precursors and the formation of solid phases, and are incorporated into the reaction and transport model of a plug flow reactor. The capability of the models to reproduce the behavior patterns observed in the experiments is demonstrated. The mechanism of the codeposition process and the results of the kinetic studies are employed to formulate routes for the preparation of mullite and functionally graded mullite/alumina coatings from SiCl₄-AlCl₃-CO₂-H₂ mixtures.