Compositionally graded mullite-based chemical vapor deposited coatings

Dense, crystalline mullite (3Al₂O₃ċ2SiO₂) coatings have been deposited by chemical vapor deposition on Si-based substrates using the AlCl₃–SiCl₄–CO₂–H₂ 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* Al₂O₃ is nucleated. Experimental evidence is presented in this study that this phase has some Si-incorporation in it and has been called δ*^(Si)Al₂O₃. Like the other known aluminosilicates, δ*^(Si)Al₂O₃ converts to mullite on heating at elevated temperatures.