Nucleation mechanisms in chemically vapor-deposited mullite coatings on SiC

Dense, uniform, and adherent chemically vapor-deposited mullite coatings were deposited on SiC substrates using the AlCl₃–SiCl₄–H₂–CO₂ system. Typical coating morphology consisted of a thin interfacial layer of γ–Al₂O₃ nanocrystallites embedded within a vitreous SiO₂-based matrix. When a critical Al/Si ratio of 3.2 ± 0.29 was reached within this nanocrystalline layer, mullite crystals nucleated and grew as columnar grains. The thickness of the nanocrystalline layer decreased as the input AlCl₃/SiCl₄ ratio was increased. In all cases, the Al/Si composition in the coating increased from the coating/substrate interface to the coating surface. Critical factors leading to the nucleation and growth of mullite crystals are discussed in this article.