Electrical and Dielectric Properties of Thermally Grown Oxide (TGO) on Fecralloy Substrate Studied by Impedance Spectroscopy

Abstract

Thermally grown oxide (TGO) formed on Fecralloy substrate exposed to an oxidizing environment of 1100°C for 4 hours was characterized by impedance spectroscopy method. Electrical and dielectric properties of TGO were investigated. AC conductivities and dielectric constants were studied as a function of frequency and temperature. AC conductivity analysis indicates a change in conduction mechanism around 400°C. Dielectric constant shows remarkable dispersions at low frequencies and high temperatures, which is considered to be originated from the Maxwell-Wagner type polarization from the interface of grain and grain boundary. INTRODUCTIONThermal barrier coatings (TBCs) have been widely used in the hot section of aeroturbine engines to increase turbine efficiency and to extend the life of metallic components '" 3 TBCs have a multi-layered structure, typically consisting of a ceramic topcoat yttria stabilized zirconia (YSZ), a metallic bond coat, and a thermally grown oxide (TGO) layer formed at the topcoat/bond coat interface due to the oxidation of the bond coat at high temperature. The TGO, predominantly alumina, provides oxidation protection and has a major influence on the TBC durability 4 . Since TGO is a dominant factor leading to failure of TBCs 5 , a deep understanding of the properties of the TGO is a critical issue in TBCs.Impedance spectroscopy is a powerful method to characterize the electrical and dielectric properties of solid materials and their interfaces. Since 1999, impedance spectroscopy has been developed to examine degradation of TBCs as a non-destructive evaluation tool, which is critical for prediction of TBCs lifetime during service 6 " 16 . Much effort has been devoted to characterize the TGO layer in TBCs by impedance spectroscopy. For example, Song et al. studied the formation and growth of TGO layer in air plasma sprayed (APS) TBCs by impedance spectroscopy and found the formation, thickening, densification and composition change of the TGO layer during TGO growth can be qualitatively evaluated by the peak shifts on the Bode plot ", and the results were confirmed by finite element simulation by Deng et al. "; Wang et al. also studied the TGO growth in TBCs by impedance spectroscopy and found a linear relationship between the TGO thickness and the diameter of the semicircle on the electrical modulus spectra, which proved the thickness of TGO layer in TBCs can be estimated by measuring its impedance response 18 . The above studies may provide useful information in estimating the remaining lifetime of the TBC system. However, it is still necessary to characterize the TGO itself to obtain a better and clear understanding of the TGO layer inside the TBCs.Fecralloy is a high temperature oxidation-resistant alloy. Under high temperature, a continuous, adherent, slowly-growing alumina surface scale is formed on Fecralloy, which provides excellent oxidation resistance against a high temperature and a corrosive atmosphere 9 . A preliminary impedance spectroscopy study on thermally oxide alumina scal...