FUNCTIONALY 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 CO2 and H2 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 six-month reporting period, the experimental work on the investigation of the deposition of alumina, silica, and aluminosilicates from mixtures of methyltrichlorosilane (MTS), aluminum trichloride, carbon dioxide, and hydrogen was continued. Experiments were also conducted on the deposition processes of the simple oxides, alumina and silica, from mixtures containing only one chloride (AlCl3 and MTS, respectively). Deposition rate data were obtained in a relatively broad range of operating conditions: temperatures in the range 800-1000 o C, 100 Torr pressure, 0.006-0.015 AlCl3 feed mole fraction, 0.011- 0.027 CH3SiCl3 feed mole fraction, and 0.004-0.07 CO2 feed mole fraction, and various positions along the axis of the deposition reactor. Since the effect of temperature had been examined in detail in the previous reporting period, our efforts were mainly concentrated on the investigation of the effects of the other parameters on the three deposition processes. The results showed that the mole fraction of CO2 had a strong influence on all deposition rates, with the alumina deposition rate and the codeposition rate showing a tendency to increase significantly with increasing concentration of CO2 in the feed at low mole fraction values and reach a limiting value at high CO2/chloride ratios. The increase in the concentration of carbon dioxide had, in general, a negative effect on the rate of silica deposition. These effects were in agreement with the conclusions reached from the thermodynamic equilibrium analysis of the deposition processes in past studies. The increase in the mole fraction of AlCl3 had a positive effect on the rate of codeposition and the deposition of alumina. In the case of alumina deposition, the deposition rate leveled off in some cases as the AlCl3 concentration was increased, and this behavior was consistent with the results of past experimental studies. The deposition rate and the deposit stoichiometry were influenced strongly by the substrate position in the reactor, and the deposition rate could increase or decrease with increasing distance from the entrance of the reactor depending on the reaction temperature