In-Situ Measurement of Thermal Barrier Coating Properties via Induction Phase Radiometry - Methodology Development

This research introduces a new method for accurately measuring the thermal resistance of thermal barrier coatings (TBCs), as typically used in gas turbine engines. The proposed method involves periodic internal heat generation inside the airfoil in the vicinity of the TBC-alloy boundary using low frequency modulated induction heating through a coil. The phase-lag between the radiation flux emitted to the surroundings from the exposed side of the TBC and the recorded voltage input to the modulated induction coil is proportional to the thermal resistance (L^2/a) of the thin film. A simplified analytical model is developed to quantify the relationship between the lag as a function of the thermal resistance and induction modulation frequency. Numerical experiments are conducted to solve the full physics. Comparing the output with the simplified model, precision of 0.8% under ideal conditions can be observed. Moreover, the robustness of the methodology to recover properties is characterized for differing levels and types of noise levels, including Gaussian and constant-lag biases. For the relevant application, it is found that the suggested approach maintains a recovery error bound to range of 1-10%, dependent on input noise level.