Compositionally graded mullite/ZrO 2 coatings, have been tested as environmental barrier coatings (EBCs) for protection against water vapor corrosion of Si-based ceramic components intended for application in turbine engines. Four and five layered systems were engineered by plasma spraying over SiC substrates consisting of a Si bond coat layer, 2 or 3 mullite/ZrO 2 composite graded layers as middle layers and a nanostructured YSZ topcoat. These coatings were heat treated at 1300 °C in both stationary and thermal cycling conditions in a controlled water vapor environment. The effect of these ageing conditions on the coatings was comparatively investigated. Crystallization of the composite coatings and sintering of the YSZ topcoat was perceived. A reduction of SiO 2 content was detected in the composite layers before aging. The porosity of the coating did not change appreciably with the ageing tests and only the evolution of the pre-existing cracks and the growing of a thermally grown oxide layer can be highlighted as the major effect of the ageing tests.

Mullite and mullite/ZrO 2 bi-layer systems are being considered as environment barrier coatings (EBCs) for protection of Si-based (Si 3 N 4 , SiC) substrates against water vapor corrosion for application in forthcoming turbine engines. An approach to reduce the thermal expansion mismatch between mullite and ZrO 2 layers in those coatings would be to tailor intermediate mullite/Y-ZrO 2 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-ZrO 2 (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).

Mullite (Al 6 Si 2 O 13 ) is the basis of efficient environmental barrier coatings (EBCs) for protecting Si-based ceramic matrix composites (CMCs) selected to replace specific hot-section metallic components in advanced gas turbines. Furthermore, YSZ-mullite multilayer architectures with compositional grading between the bond coat and YSZ top coat were envisioned as solutions to ease their coefficient of thermal expansion (CTE) mismatch induced stress. Consequently, a proper understanding of the mechanical properties such as the elastic modulus, hardness or plastic/elastic recovery work serve for an efficient design of such refractory oxide multilayers. In this work, three different mullite powder morphologies (fused and crushed, spray-dried and freeze-granulated) were employed. Using depth-sensing indentation with loads in the range 100 – 500 mN, the role of the microstructure and morphology of the powder feedstock on the mechanical behaviour of air plasma sprayed mullite bond coats deposited on SiC Hexoloy substrates was investigated. Fully crystalline as-sprayed mullite coatings were engineered under controlled deposition conditions. Mechanical properties were measured for the as-sprayed coatings as well as for coatings heat-treated at 1300°C, in water vapour environment, for periods up to 500 h. Both E and H values of the coatings are found to be highly dependent on the morphology of the starting powders.