Flame Spraying of mullite/ZrO2 Powders

Mullite based compositions have interests for environmental barrier coatings as it has thermal expansion coefficient close to those of Si-based ceramic substrates (SiC and Si₃N₄). Mullite/ZrO₂ coatings have been obtained by flame spraying, which is a low-cost and easily industrial scaling technique. The 25 vol.% of ZrO₂ composition was selected looking for rapid liquid formation as it is close to the eutectic composition. The obtained coatings were studied by X-ray diffraction (XRD) and scanning electron microscopy X-ray dispersive energy procedures (SEM-EDS). Mullite/ZrO₂ coatings were mainly amorphous and formed by different metastable phases. The composition of the amorphous phases was investigated by locating the EDS point analysis on the Al₂O₃-SiO₂-ZrO₂ equilibrium phase diagram. The crystallinity of these coatings was improved by an in-situ surface thermal treatment with the same flame. Hardness and elastic modulus were measured by indentation on as sprayed and on treated coatings as well, both parameters increase with the degree o crystallization up to values of 12 and 180 GPa, respectively. The thermal diffusivity was measured by the laser flash technique up to 800ºC, and the effect of thermal treatment on this property was evaluated.