Abstract
Dense, uniform, crystalline mullite (3Al 2O 3·2SiO 2) environmental barrier coatings have been deposited by chemical vapor deposition on SiC substrates, using the AlCl 3–SiCl 4–CO 2–H 2 system. These coatings have exhibited excellent high temperature oxidation and hot-corrosion resistance. For very long-term exposures, it is desirable to reduce the silica activity at the coating surface. A graded coating composition has been achieved in the coatings, with the Al/Si ratio being stoichiometric (∼3) at the coating/substrate interface for coefficient of thermal expansion (CTE) match, and increasing monotonically towards the outer coating surface to some of highest alumina-rich mullite reported to date. The structure of the high alumina mullite has been examined by high-resolution transmission electron microscopy. The effects of the CVD deposition parameters on the coating microstructure and growth kinetics have been investigated.
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