Synthesis of mullite coatings by chemical vapor deposition

Formation of mullite on ceramic substrates via chemical vapor deposition was investigated. Mullite is a solid solution of Al₂O₃ and SiO₂ with a composition of 3Al₂O₃ · 2SiO₂. Thermodynamic calculations performed on the AlCl₃–SiCl₄–CO₂–H₂ system were used to construct equilibrium chemical vapor deposition (CVD) phase diagrams. With the aid of these diagrams and consideration of kinetic rate limiting factors, initial process parameters were determined. Through process optimization, crystalline CVD mullite coatings have been successfully grown on SiC and Si₃N₄ substrates. Results from the thermodynamic analysis, process optimization, and effect of various process parameters on deposition rate and coating morphology are discussed.