Phase transformations in mullite-based nanocomposites

Chemical vapor deposition (CVD) has been used to deposit mullite (3Al2O3 · 2SiO2) based nanocomposite coatings and powders. The nanocomposite coatings typically comprised of Si-rich nanolayer close to the coating-substrate interface which serves as a template for the growth of columnar mullite grains. Finally a third zone composed of Al-rich nanocrystallites is obtained towards the top surface of the coating. The phase stability/transformation of these three zones in the coatings, which is critical for high temperature applications, was investigated in the range 1000–1600 °C. Si-rich nanolayer converts to mullite after a two-step anneal, columnar mullite grains undergo a tetragonal–orthorhombic transformation and the Al-rich nanocrystallite powders completely transform into mullite and α Al2O3 above 1500 °C. Various aspects of these phase transformations which eventually lead to the formation of mullite-α Al2O3 nanocomposite coating are discussed.