Evaluation of degradation of thermal barrier coatings using impedance spectroscopy

Impedance spectroscopy (IS) was used as a non-destructive tool to evaluate the degradation of thermal barrier coatings (TBCs) due to oxidation at 1150 C. The spallation of TBCs was found to be induced by the development of thermally grown oxides (TGO) resulting from the oxidation of the TBCs. The change in the electrical properties of the TGOs was found to be related to the change in their microstructure and microchemistry. The resistivity due to oxidation from 10 to 1000 h decreased rapidly, corresponding to the increase in porosity and the compositional change of the TGO from alpha-Al2O3 to a mixture of alpha-Cr2O3 and (Ni,Co)(Cr,Al)2O4 spinel. The slow decrease in the resistivity upon oxidation for 1000-2000 h showed that there was little change in the composition of the mixed oxides, although the growth rate of the TGOs was relatively fast during this oxidation period. The disappearance of alpha-Al2O3 in the TGO caused rapid oxide growth during oxidation and led to the spallation of the TBCs. 31 refs.