Evaluation of Thermal Barrier Coatings Exposed to Hot Corrosion Environment by Impedance Spectroscopy

Abstract

In this investigation, impedance spectroscopy (IS) is used, as a non-destructive tool, to examine the hot corrosion behavior of thermal barrier coatings (TBCs) exposed to a mixture of 25wt% NaCl and 75wt% Na2SO4. The results show that the thermally grown oxide (TGO) formed along the top coat/bond coat interface is a mixed oxide layer, i.e. Cr2O3, (Ni, Co)(Cr, Al)2O4 spinel and NiO. The growth of TGO layer appears to follow the parabolic law. The resistance of TGO increases due to the increase of thickness when the number of cycles is less than 110. While for more than 110 cycles, the resistance of TGO decreases with the increase of porosity of TGO layer, even though the thickness of TGO layer increases. The nucleation and propagation of cracks within top coat increase the electrical resistance of top coat. The parameters in equivalent circuit could be used to characterize the degradation of TBCs.