FUNCTIONALLY GRADED ALUMINA/MULLITE COATINGS FOR PROTECTION OF SILICON CARBIDE CERAMIC COMPONENTS FROM CORROSION

The main objective of this research project is 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. Mullite will be employed as the inner (base) layer and the composition of the film will be continuously changed to a layer of pure alumina, which will function as the actual protective coating of the component. Chemical vapor deposition reactions of silica, alumina, and aluminosilicates (mullite) through hydrolysis of aluminum and silicon chlorides in the presence of CO{sub 2} and H{sub 2} will be employed to deposit compositionally graded films of mullite and alumina. Our studies will include the kinetic investigation of the silica, alumina, and aluminosilicate deposition processes, characterization of the composition, microstructure, surface morphology, and mechanical behavior of the prepared films, and modeling of the various deposition processes. During this reporting period, the construction and development of the chemical vapor deposition system was completed, and experiments were conducted on the deposition of alumina, silica, and aluminosilicates (such as mullite) from mixtures of AlCl{sub 3} and CH{sub 3}SiCl{sub 3} in CO{sub 2} and H{sub 2}. Work was mainly done on the investigation of the effects of the reaction temperature on the deposition kinetics. It was found that the temperature had a positive effect on the single oxides deposition rates and the codeposition rate. The apparent activation energy values extracted from the deposition rate vs. temperature curves in the high temperature region were similar for the three deposition processes, having a value around 20 kcal/mol. The codeposition rates were higher, by a more than a factor of 2 in some cases, than the sum of the deposition rates of the two oxides in the independent experiments at the same operating conditions, and this result led to the conclusion that there should exist additional surface reaction steps in the codeposition process, that lead to solid formation and involve both silicon-containing and aluminum-containing species. The elemental analysis (EDXA) of films deposited from MTS-AlCl{sub 3}-CO{sub 2}- H2 mixtures showed that silicon oxide was the main component, and comparison of the deposition rates of SiO{sub 2} and Al{sub 2}O{sub 3} during codeposition with those seen in single species deposition experiments at the same conditions revealed that the codeposition process was characterized by a dramatic enhancement of the deposition of SiO{sub 2} and an equally dramatic reduction in the rate of Al{sub 2}O{sub 3} deposition. Since the enhanced codeposition rate was caused by increased silicon oxide deposition, it was concluded that the main deposition product of the additional surface reaction steps in codeposition must be silicon oxide. A comprehensive investigation of the effects of the other operating parameters on the kinetics of the codeposition process will be carried out in the next reporting period