Quantitative Evaluation of TBC Systems of Gas Turbine Blades Using TFEC

In mechanical engineering and aerospace, gas turbine blades are taken as the crucial components, and need special treatment for protection and long-turn service. Therefore, ceramic Thermal Barrier Coating (TBC), acting as the insulation between gas and the alloying bodies of blades, is utilized to guarantee that the gas turbine blades are able to work in the high-temperature and high-stress environment. It is imperative to non-destructively evaluate TBC systems of blades, since the TBC with thin thickness or delimitation will lead to catastrophic accidents of gas turbines. In this paper, Tri-frequency Eddy Current inspection (TFEC) is proposed for quantitative evaluation of TBC systems, which involves assessment of: (1) TBC thickness to identify possible thinning, and (2) thickness and conductivity of bond coating to detect delimitation and degradation. The Levenberg-Marquardt Algorithm is adopted in the inversion for the parameters, which need to be quantified. In the inverse process, the closed-form expressions of the Jacobian Matrix, which is conventionally obtained by using interpolation functions, are derived based on an analytical modeling. The inversion method is verified by a hybrid numerical modeling, which indicates that the proposed TFEC for quantitative evaluation of TBC is valid and applicable.