Functionally Graded Alumina/Mullite Coatings for Protection of Silicon Carbide Ceramic Components From Corrosion

Abstract

ii EXECUTIVE SUMMARYThe main objective of this research project was the formulation of processes that can be used to prepare compositionally graded alumina/mullite coatings for protection from corrosion of silicon carbide components (monolithic or composite) used or proposed to be used in coal utilization systems (e.g., combustion chamber liners, heat exchanger tubes, particulate removal filters, and turbine components) and other energy-related applications. Since alumina has excellent resistance to corrosion but coefficient than silicon carbide, the key idea of this project has been to develop graded coatings with composition varying smoothly along their thickness between an inner (base) layer of mullite in contact with the silicon carbide component and an outer layer of pure alumina, which would function as the actual protective coating of the component. (Mullite presents very good adhesion towards silicon carbide and has thermal expansion coefficient very close to that of the latter.)A comprehensive investigation of the chemical vapor codeposition (CVD) of alumina and mullite through hydrolysis of aluminum and silicon chlorides in the presence of CO 2 and H 2 as a route for the preparation of composite alumina/mullite coatings was carried out. The kinetics of the codeposition of silica, alumina, and aluminosilicates from mixtures of methyltrichlorosilane or silicon tetrachloride, aluminum trichloride, carbon dioxide, and hydrogen was studied experimentally. In order to elucidate some aspects of the codeposition process, the deposition of pure silica and the deposition of pure alumina from mixtures of methyltrichlorosilane or silicon tetrachloride and aluminum trichloride, respectively, with CO 2 and H 2 were also investigated. Kinetic data were obtained by carrying out chemical vapor deposition experiments on SiC substrates in a hot-wall reactor of tubular geometry, which permits continuous monitoring of the deposition rate through the use of a microbalance. Experiments were conducted over relatively broad temperature and pressure ranges around 1300 K and 100 Torr, respectively, and the effects of feed composition, flow rate, and distance from the entrance of the reactor on the deposition rate and deposit composition were investigated. Thermodynamic equilibrium computations were performed on the Al/Si/Cl/C/O/H, Al/Cl/C/O/H, and Si/Cl/C/O/H systems at the conditions used in the deposition experiments, and the results were used to explain the observations made in the experiments. Among the most interesting findings of the kinetic studies was that in the codeposition problem there was a dramatic increase of the deposition rate of SiO2 in the codeposition process, relative to the rate seen in a silica deposition experiment through the hydrolysis of silicon chloride at the same conditions. This enhancement was accompanied by a reduction of the rate of deposition of Al 2 O 3 , relative to the deposition rate in an independent alumina deposition experiment at the same conditions. The overall deposition rate was by a...

Advances in Functionally Graded Ceramics – Processing, Sintering Properties and Applications

“…By exploiting the myriad possibilities inherent in the ceramic/ceramic FGCs concept, it is anticipated that the properties of materials will be optimized and new uses for them will be discovered. Examples of these FGCs are alumina/zirconia, a material used in biomedical and structural applications, mullite/alumina, which is used as a protective coating for SiC components in corrosive environments [2,5]. Zirconia-mullite/alumina FGCs can be used as refractory materials in high temperature applications, as well as being suitable for engineering and tribological applications [6,7].…”