Non-Destructive Characterisation of Thermal Barrier Coatings Using Impedance Spectroscopy
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Non-destructive Characterisation of Thermal Barrier Coatings Using Impedance Spectroscopy Junfa Mei, Xin Wang, Ping Xiao Department of Mechanical Engineering, Brunel University, Uxbridge, UB8 3PH, UK ABSTRACT Impedance spectroscopy (IS) has been used to characterise the degradation of thermal barrier coatings (TBCs) due to thermal treatments at 1100oC for a period up to 200 hrs. The growth of the oxide layer at the yttria stabilised zirconia (YSZ)/bond coat interface in TBCs can be examined by measuring the impedance diagrams. From the analysis of the impedance diagrams of TBCs, the electrical properties of YSZ were found to be nearly constant during the thermal treatments, indicating there was little change in the microstructure and composition of YSZ. However, there was a clear change in the electrical properties of the oxide layer in the TBCs after thermal treatments, suggesting both microstructural and composition changes occurred in the oxide layer. These studies indicate that the IS is a very useful method in non-destructive characterisation of the degradation of TBCs. INTRODUCTION Thermal barrier coatings (TBCs) have been extensively applied to engine components to allow engines to operate at higher temperatures; hence achieving higher thrusts and greater thermodynamic efficiency of the engine [1]. A TBC normally consists of a top coat of YSZ and a bond coat typically of the formula MCrAlY where M represents Co, Ni or Fe. As an oxygen ion conductor at high temperature, the YSZ top coat does not block the oxidation of the underlying bond coat. Alumina layers form at the interface between the bond-coat and the YSZ due to oxidation of the bond coat at high temperature [2]. As the alumina layer grows to a thickness of about 10-15µm, the TBC fails, either at the bond coat /YSZ interface or in the YSZ close to the interface due to thermal mismatch between the alumina layer and the bond coat. Failure of TBCs will expose the underlying metallic components to hostile environments, which might result in the catastrophic failure of the component. For the successful application of TBCs on engine components, it is vital to predict the coating lifetime which is controlled by the oxide growth at the bond coat/YSZ interface. Finding a suitable method of measuring and monitoring the thickness of the alumina layer non-destructively will provide a way of predicting the lifetime of TBCs. Impedance spectroscopy has been used to characterise the oxide scales formed on nickel and nickel superalloys [3,4]. In characterisation of NiO scales, a previous study indicated that the IS was an effective method to study the electrical conductivity and defect structure in NiO scales [3]. In our previous study [4], impedance measurements were used to examine the composition of oxide scales on the surface of superalloys, where the relaxation frequency was found to correspond to the complex composition of the oxide layer. In addition, resistance was found to increase with the increase of the oxide layer thickness, but it decreased significantly
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