Evaluation of degradation of thermal barrier coatings using impedance spectroscopy

In this work, impedance spectroscopy (IS) has been used to evaluate the deg radation of thermal barrier coatings (TBCs) due to oxidation at 1150 degree sC. The spallation of TBCs was found to be induced by the development of th ermally grown oxides (TGO) produced from the oxidation of TBCs. The change in the electrical properties of the TGOs was found to be related to the cha nge in the microstructure and microchemistry of the TGO. The resistivity of the TGO due to oxidation from 10 to 1000 h decreased rapidly, which corres ponded to the increase in porosity in the TGO and the compositional change of the TGO from alpha -Al2O3 to a mixture of alpha -Cr2O3 and (Ni,Co)(Cr,Al )(2)O-4 spinel. The slow decrease in the resistivity of the TGO from oxidat ion for 1000-2000 h indicated that there was little change in the compositi on of the mixed oxides, although the growth rate of the TGOs was relatively fast during this oxidation period. The disappearance of alpha -Al2O3 in th e TGO caused a rapid oxide growth during oxidation and led to the spallatio n of TBCs. (C) 2001 Elsevier Science Ltd. All rights reserved.