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

Dense, uniform, crystalline mullite (3Al(2)O(3)-2SiO,) environmental barrier coatings have been deposited by chemical vapor deposition on SiC substrates, using the AlCl3-SiCl4-CO2-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 (similar to 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. (c) 2007 Elsevier Ltd. All rights reserved.