Estimation of Thermal Conductivity of Thermal Barrier Coatings through Measured Electronic and Structural Characteristics

For thin films, by reason of their specific nature, the determination of many properties intrinsic to compact materials is a well-known problem. This is particularly the case with one of the most important functional properties of thermal barrier coatings, namely, thermal conductivity. Therefore, the problem addressed in the present work is to develop the method of estimation of thermal conductivity of relatively thin coatings (up to 200 μm) through characteristics that can be easily measured in the experiment. This method includes the experimental measurement of the Volta potential difference of coatings and calculations of their thermal conductivity through the actual elemental and phase composition, which is determined by instrumental methods of metallography, X-ray diffraction, and spectroscopy. The investigation is performed on Nb-Ti-Al- and Zr-Ti-Al-based thermal barrier coatings about 80 μm in thickness deposited by the vacuum ion-plasma technology on a substrate made of nickel superalloy Inconel 713LC, which is used in the manufacture of first-stage blades of aircraft, locomotive and ship gas turbine engines. Despite the close chemical composition, the coatings have fundamentally different structures and phase compositions after long-term deposition and subsequent heat treatment. Structural features of the coatings affect the Volta potential difference measured by the original method. The developed calculation and analytical model for thermal conductivity also accounts for structural characteristics. Thermal conductivity values calculated by the presented model are highly correlated with the experimentally measured values of the Volta potential difference for single-phase alloys (Inconel 713LC) and single-phase layered coatings (Nb-Ti-Al). For multiphase coatings with a 3D composite structure (Zr-Ti-Al), the model shows insufficient correlation between the calculated and experimental values and needs to be corrected.