T. Kulkarni

Compositionally graded mullite-based chemical vapor deposited coatings

Dense, crystalline mullite (3Al 2 O 3 ċ2SiO 2 ) coatings have been deposited by chemical vapor deposition on Si-based substrates using the AlCl 3 –SiCl 4 –CO 2 –H 2 system. A graded coating composition has been achieved in the coatings, with the Al/Si ratio being stoichiometric (∼3) at the coating/substrate interface, and increasing monotonically toward the outer coating surface. The highest reported Al-rich mullite has been deposited in the process. At high Al/Si ratios, the mullite structure breaks down and an aluminosilicate phase similar to the metastable δ* Al 2 O 3 is nucleated. Experimental evidence is presented in this study that this phase has some Si-incorporation in it and has been called δ* (Si) Al 2 O 3 . Like the other known aluminosilicates, δ* (Si) Al 2 O 3 converts to mullite on heating at elevated temperatures.

Advanced Environmental Barrier Coatings (EBC’s)

Dense, uniform and crack-free mullite (3Al2O3·2SiO2) coatings were deposited on Sibased substrates by chemical vapor deposition using the AlCl3–SiCl4–H2–CO2 system. The coatings were compositionally graded, with the Al/Si ratio increasing towards the outer surface of the coatings for improved corrosion resistance. Mullite grains nucleated when the surface composition of the growing coating was in a narrow range close to that of stoichiometric mullite. The growth rate and crystal structure of mullite were dependent upon temperature, pressure, reactant concentration, and reactant ratios. The phase transformations occurring in these coatings during high-temperature anneals in the range 1100–1400 °C were studied.

Effect of Deposition Parameters on the Microstructure and Growth Rate of CVD Mullite Environmental Barrier Coatings

Si-based ceramics such as SiC require environmental barrier coatings to protect against hot-corrosion and recession in gas turbine applications. Dense, crystalline mullite coatings of uniform thickness have been deposited by hot-wall chemical vapor deposition (CVD) on SiC substrates, using the AlCl 3 -SiCl 4 -CO 2 -H 2 system. The effects of the CVD deposition parameters such as temperature, total reactor pressure, and metal-chloride partial pressure on the coating microstructure and growth kinetics have been investigated, and are discussed in this paper.