A review on thermo-optical properties of complex rare-earth thermal barrier oxides in extreme environments: Measurement methodology, materials and applications

The temperature at turbine inlets of advanced engines has been steadily rising, posing greater challenges to thermal barrier coatings (TBCs) used to protect the underlying substrates made of superalloys or ceramic matrix composites. The thermal insulation and durability performance of TBCs must be significantly enhanced with the increasing need in advanced propulsion systems such as aviation, aerospace, naval vessel, energy, etc. Measurements of thermo-optical properties such as thermal conductivity, emissivity and reflectivity at elevated temperatures are extremely critical for rare-earth thermal barrier oxides operated in extreme environments. This review outlines the state-of-the-art progress on new rare-earth thermal barrier oxides, measurement methods for thermo-optical properties, and two strategies to inhibit thermal radiation penetration through TBCs, viz. improving reflectivity through designing novel material and coating structures, and enhancing emissivity via doping and compositing strategies. This paper also provides new insights into consideration of potential applications and challenges for rare-earth thermal barrier oxide coatings in various fields of modern industries.