Scandium and Yttrium/Ytterbium Disilicates as Environmental Barrier Coatings: Microstructural Characterization and High-Temperature Water Vapor Exposure Tests

Ceramic matrix composites (CMCs), particularly SiC/SiC, exhibit outstanding properties making them ideal for applications in the hot sections of aircraft engines. However, they require protection against high-temperature water vapor corrosion: To address this challenge, environmental barrier coatings (EBCs) are currently one of the most widely used solutions for protecting CMC components. This work proposes the deposition and characterization of rare earth (RE) silicates produced using the atmospheric plasma spray (APS) technique. Four different multilayer RE silicate APS coatings were deposited on SiC substrates and a bond layer of Si, obtained by APS: (i) yttrium/ytterbium disilicate (YbYSi2O7), (ii) scandium disilicate (Sc2Si2O7), (iii) ytterbium disilicate (Yb2Si2O7) and (iv) ytterbium disilicate with ytterbium monosilicate as a topcoat (Yb2Si2O7/Yb2SiO5). Since few research works focus on thermal spray-deposited YbYSi2O7 and Sc2Si2O7, it was necessary to test several sets of APS parameters in order to find an optimized deposition strategy. EBC samples were exposed to a mixture of 90 mol.% H2O and 10 mol.% air at 1300 °C for up to 500 h. All samples were characterized using SEM/EDS, XRD and image analysis (crack density and porosity) to assess the quality of the as-sprayed coatings and to analyze the microstructural modifications induced by the high-temperature exposure tests. The results highlight the strong dependence of the protective capability on the microstructural features, particularly in terms of coating cohesion and crack density.