Damage Detection Of Thermal Barrier Coatings By Electrochemical Impedance Spectroscopy

Thermal barrier coatings (TBCs), consisting of yttria-stabilized zirconia (YSZ) on NiCoCrAlY or aluminide bond coats, protect hot-section components, such as blades and vanes of advanced gas turbine engines. Electrochemical impedance spectroscopy (EIS) is being developed as a non-destructive evaluation (NDE) technique for TBCs. For better understanding of electrochemical impedance response of TBCs and their damage after high temperature exposure (isothermal/cyclic), monolithic YSZ with open pores and alumina-forming NiAl specimens were independently examined by EIS as a function of isothermal oxidation. Impedance of the YSZ and oxidized NiAl increased with high temperature sintering and oxide-scale growth, respectively. However, the electrochemical impedance was significantly reduced when quench-cracks in sintered YSZ or spallation of oxide-scale from the NiAl were observed. These observations are correlated to the electrochemical impedance response of a TBC system consisting of electron beam physical vapor deposited (EB-PVD) YSZ and (Ni,Pt)Al bond coats as a function of isothermal oxidation