Thickness Measurement of Thermal Barrier Coating Based on Mutual Inductance of Eddy Current System

Thickness of thermal barrier coating (TBC) plays an important role in the insulation property of coating. Eddy current (EC) testing could be used to measure the thickness of ceramic coating (TC) for TBCs. However, previous EC testing has made it difficult to decouple the relationship between TC, bond coating (BC), and substrate. In this study, an accurate and simple method of TC thickness measurement is proposed based on the mutual inductance and the transformer model, which is immune to the BC and the substrate. First, an EC system is analyzed using the transformer model. Second, mutual inductance is formulated with the Neumann formula and the Taylor series, and the coupling coefficient is established according to the approximate and simplified mutual inductance expression. Third, the coupling relationship is revealed, and the combination of the change in inductance and the phase signature serves as the new feature to characterize the thickness of TC immune to the variation in BC and substrate. Finally, an EC detection system is designed, and experiments are carried out to evaluate the thickness of TC. The results show that the relative errors are less than 5% and the method has a good inhibition effect on the change in temperature.