Quantitative evaluation of thermal barrier coating based on eddy current technique

Thermal Barrier Coating (TBC), as the insulation between high-temperature gas and the alloying bodies of gas turbine blades, is adopted for protection and long-term service of gas turbines. It is imperative to non-destructively evaluate TBC of blades, since the TBC with thin thickness or delamination will lead to catastrophic accidents of gas turbines. In this paper, Eddy Current inspection (EC) is proposed for quantitative evaluation of TBC, which involves estimation of: (1) thickness of the top coating to identify possible thinning and (2) thickness and conductivity of bond coating to detect delamination and degradation. In an effort to implement the fast inversion, the closed-form expressions of the Jacobian matrix are derived based on Truncated Region Eigenfunction Expansion (TREE) modeling. The Levenberg–Marquardt (LM) algorithm is also adopted in the inversion for the parameters. The proposed inverse algorithm is verified by hybrid numerical modeling and experiments, which indicates that the quantitative evaluation of TBC by using EC in conjunction with the proposed inverse algorithm is valid and applicable to the engineering application.

► LM-based inversion for retrieving the parameters of TBC has been realized.
► Closed-form expressions of the Jacobian matrix in inversion have been formulated.
► Proposed inversion was proved efficient and applicable.