Flame Spraying of Mullite/ZrO₂ Powders

Mullite based compositions have interest for thermal barrier coatings because they have thermal expansion coefficients close to those of silicon ceramic substrates. In this work, mullite-zirconia coatings are obtained by flame spraying and characterized based on microstructure, crystal phases, hardness, elastic modulus, and thermal conductivity. Crystallinity is improved by in-situ heating with a flame torch, which is also shown to increase hardness and elastic modulus. Thermal diffusivity measurements show that the thermal properties of mullite-zirconia coatings are relatively stable over a wide temperature range and adequate for many thermal barrier applications.