The change of microstructure in an atmospheric plasma sprayed thermal barrier coating (APS TBC) by isothermal heat exposure has been studied. Used 8 massY 2 O 3  ZrO 2 TBC layer, a Ni Co Cr Al Y bond coat (BC) layer, and an IN738 superalloy substrate. The TBC system is subjected to heat exposure at 1423 K in ambient air for 10, 50, 100, 200, 300, and 400 h. Polished transverse sections of heat exposed TBCs are examined and the changes of constitutes are discussed. Both sintering and cracking of the TBC layer are observed and these behaviors are strongly correlated with the morphology of as sprayed intersplat boundaries and heat exposure time, respectively. A thermally grown oxide (TGO) layer is formed after heat exposure, and the thickness increases with the increase in heat exposure time. The chemical composition and morphology of the TGO layer depend on the heat exposure time. Micro cracking in the TGO layer and TBC layer is observed after formation of a spinel, which is attributed to a lack of Al in the BC layer. The composition and phases in the BC layer are strongly correlated with the growth behavior of the TGO layer.

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The electrical and mechanical properties of atmospheric plasma sprayed (APS) yttria stablised zirconia (YSZ) thermal barrier coatings (TBCs) were determined using impedance spectroscopy and an indentation technique, respectively. Upon thermal exposure, sintering and phase transformation occurs in the YSZ TBCs, leading to changes in both the mechanical and electrical properties of the TBCs. After the thermal treatment, the formation of the monoclinic phase in the YSZ TBCs reduced the density of the TBCs and thus affected both the mechanical properties and conductivity of the TBCs. In this study, a relationship between the electrical and mechanical properties of APS TBCs has been established, allowing us to evaluate the properties of TBCs nondestructively with the use of impedance spectroscopy.

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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...