Electrochemical impedance spectroscopy of thermal barrier coatings as a function of isothermal and cyclic thermal exposure

Electrochemical impedance spectroscopy (EIS) is being developed as a non-destructive evaluation technique of thermal barrier coatings (TBCs) for life-remaining assessment and quality control. In this investigation, EIS was employed to non-destructively examine TBCs, as a function of thermal exposure during isothermal and cyclic oxidation in air at 1121 °C. TBCs examined in this study include electron beam physical vapor deposited (EB-PVD) and air-plasma sprayed ZrO2–7wt.%Y2O3 (YSZ) on NiCoCrAlY or (Ni,Pt)Al bondcoat. Electrochemical impedance response of TBC specimens at room temperature were acquired and analyzed with an AC equivalent circuit based on the multi-layered micro-constituents of TBCs. Microstructural characteristics of TBCs were also examined by optical and electron microscopy. Initial increase in the resistance of YSZ with thermal exposure was observed and related to the high temperature sintering of YSZ. The parabolic growth of TGO during high temperature oxidation was inversely proportional to the capacitance of TGO. An explanation based on electrolyte penetration into sub-critical damage is proposed for the gradual decrease in the resistances of YSZ and TGO with prolonged thermal exposure. Observation of exposed metallic bondcoat surface on the fracture surface, which provides conduction readily through electrolyte contact, was related to the abrupt and large increase in the capacitance of YSZ and TGO.