Non-destructive evaluation of degradation in multi-layered thermal barrier coatings by electrochemical impedance spectroscopy

Room temperature electrochemical impedance spectroscopy (EIS) was employed for damage monitoring of multi-layered commercial thermal barrier coatings (TBCs) that consist of a porous ceramic topcoat, thermally grown oxide (TGO), bond coat, and superalloy substrate. To gain a better understanding of the EIS response from multi-layered TBCs, individual constituents of TBCs, namely yttria-stabilized zirconia (YSZ) monoliths with open pores and hot extruded NiAl alloys, were examined as a function of thermal degradation by EIS and microstructural observation. Several types of commercial TBCs were then inspected by EIS, and the changes in the electrochemical resistance and capacitance were correlated to the development of microstructure and damage initiation. EIS response (Bode and Nyquist plots) were acquired from all specimens at room temperature and analyzed with an ac equivalent circuit based on the microstructural constituents to determine the electrochemical resistance and capacitance. The electrochemical resistance of the YSZ was observed to increase initially and then decrease with thermal exposure. This is attributed to the high temperature sintering and crack initiation of the YSZ, respectively. After an initial increase in electrochemical resistance of the TGO, there was a gradual decrease with localized spallation. A large-scale spallation of TGO drastically increased the electrochemical capacitance of the TGO. However, prior to spallation of TGO, the electrochemical capacitance of the TGO decreased inversely as the TGO thickness increased. Evolution in electrochemical resistance and capacitance along with microstructural correlations are discussed with respect to the potential of EIS as a non-destructive evaluation (NDE) technique for TBCs.