Functionally graded chemical vapor deposited mullite environmental barrier coatings for Si-based ceramics

Dense, uniform, crystalline mullite (3Al2O3·2SiO2) environmental barrier coatings have been deposited by chemical vapor deposition on SiC substrates, using the AlCl3–SiCl4–CO2–H2 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.