PS–PVD Alumina Overlayer on Thermal Barrier Coatings Against CMAS Attack

Glassy deposits mainly comprising of calcium–magnesium–alumina–silicate (CMAS) accelerate the spallation of thermal barrier coatings (TBCs). In this work, an Al₂O₃ layer was produced on yttria-stabilized zirconia (YSZ) coating by plasma spray–physical vapor deposition (PS–PVD). The effects of processing parameters during PS–PVD process on the microstructures of the deposited Al₂O₃ coatings were investigated. A homogeneous Al₂O₃ coating with porosity less than 1% was deposited at the spray distance of 1400 mm, which is much denser than the coating produced by atmospheric plasma spray (APS) and other PS–PVD coatings sprayed at 1000 and 1900 mm. The CMAS corrosion test at 1250 °C showed that the APS Al₂O₃ layer was dissolved in CMAS after 24 h thermal exposure, whereas a dense reaction layer mainly composed of anorthite formed on the top region of the PS–PVD Al₂O₃ layer effectively protected the underlying Al₂O₃ layer from CMAS infiltration. Thermal cycling behavior of the PS–PVD Al₂O₃/YSZ TBC and the APS Al₂O₃/YSZ TBC at 1050 °C was compared. Bulk spallation of the APS TBC occurred within 300 h thermal cycling, while little spallation was observed on the PS–PVD TBC even after 500 h thermal cycling, indicating superior durability of the PS–PVD coating.