Failure mechanisms in model thermal and environmental barrier coating systems

• Published in: Journal of the European Ceramic Society 2022-09, Vol.42 (12), p.5129-5144
• Main Authors: Deijkers, Jeroen A., Begley, Matthew R., Wadley, Haydn N.G.
• Format: Article
• Language: English
• Subjects: Atmospheric plasma spray; Ceramic matrix composites; Environmental barrier coating; Multi-layer; Thermal barrier coating; Vapor deposition
• ISSN: 0955-2219; 1873-619X
• DOI: 10.1016/j.jeurceramsoc.2022.04.046

The durability of environmental barrier coating (EBC) systems in gas turbine engine environments depends upon their temperature dependent rates of degradation by processes such as steam volatilization and bond coat oxidation. While addition of a thermal barrier coating (TBC) reduces the temperature within the EBC system, it introduces new failure mechanisms. Deposition of a segmented HfO2 TBC with a reduced in-plane Young’s modulus is essential to avoid bifurcated TBC channel cracking into a Yb2Si2O7 EBC, and delamination, as a result of an approximately 50% difference in coefficients of thermal expansion (CTE) of the coating layers. During prolonged high temperature steam cycling, a thin fluorite phase reaction layer is observed to develop at the HfO2-YbDS interface consistent with recent thermochemical assessments. The CTE of the fluorite phase is shown to be substantially higher than that of either of the layers to which it is bonded, resulting in tunnel cracking of the fluorite, and eventual coating delamination of the TBC at either the fluorite-HfO2 or YbDS-fluorite interfaces upon cooling. The study highlights the importance of matching the CTEs of the TBC and EBC layers during coating system design, and those of the reaction products that may form between them. •TBC with high Young’s modulus on Si-Yb2Si2O7 EBC system applied using APS results in bifurcated channel cracks.•Segmented TBC with low Young’s modulus adheres to EBC layer without cracking.•Prolonged high temperature steam cycling results in the formation of a thin fluorite phase reaction layer.•High CTE of the fluorite phase results in tunnel cracking and delamination of TBC from EBC.•Matching the CTE and chemical stability of the TBC and EBC layers is important for survival of the coating system.