Non-destructive evaluation of thermal barrier coatings using impedance spectroscopy

Abstract

Determining the oxidation of thermal barrier coatings (TBCs) non-destructively is essential for monitoring the performance of TBCs and predicting the lifetime of TBCs in service. In this research, impedance spectroscopy was used, as a non-destructive tool, to examine the oxidation of thermal barrier coatings (TBCs). Impedance diagrams obtained from impedance measurements at 623 K, were analysed according to equivalent circuit models of the oxidised TBCs. TBCs after oxidation at 1373 K consist of yttria stablised zirconia as a top coat, an alumina layer from the oxidation and a MCrAlY alloy as a bond coat. When the oxidation time is less than 200 h, the impedance spectra of TBCs can be fitted by using an equivalent circuit model, which represents a discontinuous layer of alumina between the top coat and bond coat. When the oxidation time is between 400 and 1500 h, the impedance spectra of TBCs can be fitted by using a simple equivalent circuit model, which represents a three-layer structure consisting of a continuous alumina layer, a mixed oxide layer and the top coat. In this case, the thickness of the alumina layer can also be measured by simulating the electrical modulus spectra of TBCs. Therefore, impedance spectroscopy is a powerful tool for non-destructive evaluation of oxidation of TBCs.