Phase Composition and Microstructural Responses of Crystalline Mullite/YSZ Coating Under Water Vapor Environments

Mullite and mullite/ZrO₂ bi-layer systems are being considered as environment barrier coatings (EBCs) for protection of Si-based (Si₃N₄, SiC) substrates against water vapor corrosion for application in forthcoming turbine engines. An approach to reduce the thermal expansion mismatch between mullite and ZrO₂ layers in those coatings would be to tailor intermediate mullite/Y-ZrO₂ composite layers. The feasibility of these composite layers is studied in a comparative manner by plasma spraying both single mullite and bi-layer coatings of mullite and of mullite/ Y-ZrO₂ (75/25 vol %.) over Hexoloy SiC substrates. All feedstock materials are equally prepared using spray drying methods as the mix powders are not commercially available. Singular spraying conditions are used to assure enhanced crystallization of the mullite phase. Coatings are aged for 100 h at 1300 °C in a controlled water vapor environment. The effect of water corrosion on the exposed coatings is comparatively investigated, determining changes in crystalline phase by X-ray diffraction (XRD), the crystallization of amorphous phases is highlighted by the use of differential thermal analysis (DTA) tools and the microstructure of the polished coatings is analyzed by scanning electron microscopy (SEM).