Microstructure evolution and water vapor resistance of multi-layer EB-PVD yttrium-based EBCs

Abstract

Electron beam physical vapor deposition (EB-PVD) is a promising deposition technique to produce environmental barrier coatings (EBCs) to protect ceramic matrix composites (CMCs) for complex components in aircraft engines with sharp edges such as vanes or blades. The main focus of this study was establishing an initial parameter set for depositing a multi-layer yttrium mono−/di-silicate (YMS/YDS) EBCs using EB-PVD. YMS and YDS layers show a dense microstructure without feather-arms and opened inter-columnar gaps in the as deposited state. EB-PVD EBCs were found to be amorphous in the as-coated condition, and a crystallization heat treatment was needed. Oxidation tests of EBCs were also carried out under “air condition” (1400 °C, 100 h, 100 wt% air) and “wet condition” (1300 and 1400 °C, 100 h, 30 wt% H₂O/70 wt% air) to get the preliminary assessment of the degradation. The YMS top layer has undergone slight morphological changes such as porosity and crack network formation. The YMS layer has exhibited a phase separation into Y₂O₃ and X2-YMS after the oxidation. No noticeable degradation of YDS due to water vapor oxidation was found. However, few polymorphs were observed in the YDS layer along with few cracks during various stages of the heat treatments.