Eddy Current Instrumentation of Bond Coating Thickness by Developing Three-Winding Transformer Model

The thickness of the metal bond coating (BC) is an important indicator to characterize the quality of thermal barrier coating (TBC). The previous eddy current (EC) testing could hardly establish a direct method to characterize the BC thickness and reduce the influence of the ceramic coating (TC). In this article, an efficient, simple, and direct method of the BC thickness measurement was proposed, which could reduce the influence of the TC. First, the three-winding transformer model was creatively built to analyze the relationships among the impedance signal, the electromagnetic parameters, and the geometric structure of the TBC, and it was found that the phase signature has a linear relationship with the exponential square of the BC thickness. Second, the influences of the TC thickness were considered using the finite-element model in the BC thickness measurement. The intercept of the phase signature could reflect the change of the TC thickness linearly. The measurement errors of the BC thicknesses were greatly reduced after compensating the TC thicknesses. Finally, an EC detection instrument was designed, and the experiments were carried out to evaluate the BC thickness. The results show that the maximum relative error is less than 10%.