Electrochemical impedance spectroscopy of porous ZrO2–8 wt.% Y2O3 and thermally grown oxide on nickel aluminide

Electrochemical impedance spectroscopy (EIS) is being developed as a non-destructive evaluation (NDE) technique for life-remain assessment and quality control of thermal barrier coatings (TBCs). In this investigation, EIS was employed to non-destructively examine impedance and capacitance behavior of monolithic ZrO2–8 wt.%Y2O3 (8YSZ) with open pores as a function of thickness and density. Also, the growth and spallation of thermally grown oxide (TGO) on NiAl as a function of cyclic oxidation at 1121 °C were examined by EIS. The porous 8YSZ is the material representing ceramic topcoat and NiAl is an alumina former like many commercial bond coats for TBCs. Electrochemical impedance response was acquired at room temperature and analyzed with an ac equivalent circuit. Physical and microstructural features of the specimens were also examined by optical and scanning electron microscopy. With an increase in the thickness of the YSZ, the resistance and the capacitance of the 8YSZ increased and decreased, respectively. Higher capacitance of the 8YSZ was observed with those specimens with lower density, given similar thickness. As the TGO thickness increased with oxidation, the resistance of the TGO increased and capacitance of the TGO decreased correspondingly. Opposite trend in the resistance and capacitance of TGO was observed due to the spallation of the TGO scale that results in the exposure of conductive NiAl surface. The parabolic growth (e.g. thickness) of TGO during high temperature oxidation was inversely proportional to the capacitance of TGO, excluding the abrupt changes associated with the spallation.