Functionally Graded Alumina/Mullite Coatings for Protection of Silicon Carbide211 Ceramic Components from Corrosion

摘要

During the six months of this reporting period, we accomplished the following:211u001ePreparatory work was done on the development of the feed supply system (for 211u001emixtures of AlCl(sub 3), SiCl(sub 4), H(sub 2) and CO(sub 2)) and effluent 211u001etreatment section for the CVD system we plan to employ for coating preparation. A 211u001ecomprehensive literature survey of past work done on the chemical vapor 211u001edeposition of silica, alumina and aluminosilicates (mullite) was carried out, and 211u001ework was initiated on the study of thermochemical equilibrium in the 211u001eAl/Si/Cl/C/O/H system so as to identify the boundaries of the region of the space 211u001eof operating parameters and conditions where preparation of functionally graded 211u001emullite/ alumina coatings through CVD from metal chloride, CO(sub 2), and H(sub 211u001e2) is feasible. Since the alumina/mullite films that are proposed to be developed 211u001ecan also be applied to carbon matrix composites provided that a layer that 211u001ebridges the gap that exists between the thermal expansion coefficient of carbon 211u001eand that of mullite is employed, experiments were conducted on the preparation of 211u001ecompositionally graded carbon/silicon carbide coatings. Deposition from mixtures 211u001eof ethylene and methyltrichlorosilane or tetrachlorosilane (silicon 211u001etetrachloride) in hydrogen was used for the preparation of SiC/C coatings, and 211u001eour experiments focused on the study of the occurrence of multiple steady states 211u001ein the deposition process and the effects of the type of chlorosilane on the 211u001edeposition rate and the deposit composition and their variation along the length 211u001eof the reactor. The results showed that when operation is carried out outside the 211u001emultiplicity region, codeposition of SiC and C from ethylene and chlorosilanes is 211u001ea feasible route for preparation of SiC/C graded coatings.