FEM Calculation Of Thermal Barrier Coatings Impedance Spectroscopy: Influence Of Measuring Conditions And TBCs Geometric Structure

As a kind of adiabatic ceramic coating, Thermal Barrier coating(Thermal Barrier Coatings, TBCs) successfully application in aero engine turbine blade, improving the service temperature and corrosion resistance of the turbine blades. However, due to the complexity of TBCs structure, service conditions, its structure changes in the process of serving in aero engine turbine, as a result, falling off and fail. Thus, Impedance Spectroscopy(hereinafter referred to as IS) was applied to test the structure and predict the life of the thermal barrier coating, as a non-destructive testing method IS. But, there were two questions about the application of IS in thermal barrier coating of current aviation engine: 1. the lack of stable tools and reliable basis in analysis of IS result; 2. the asymmetric electrode was widely used in IS experiment, but its influence on IS was not clear. Therefore, in this article, the finite element method was introduced to simulate IS of thermal barrier coatings, in order to further study the influence complex of measurement environment and TBCs structure changing on APS thermal barrier coating IS. The main contents were as follows:1. According to the FEM results, experimental results and the IS theory, the influence of measuring voltage, temperature and electrode size on impedance spectroscopy was analyzed in detail. Then, the simulation result was fitted with equivalent circuit, obtaining the YSZ and TGO measuring thickness. Finally, the comprehensive influence of measuring voltage, temperature and electrode size on impedance spectroscopy and fitting results were considered, As a result: the TBCs IS optimal measuring voltage is 1 v, temperature is 400°C, and Pt electrode diameter is 3 ~ 5 mm.2. According to the FEM results, experimental results and the IS theory, the influence of YSZ, TGO thickness and conductivity on TBCs impedance spectrum was analyzed. Moreover, by comparing the fitting YSZ and TGO thickness and which in FEM model, the error of TBCs IS testing was identified. As a result: when the impedance spectrum was used to fitting YSZ and TGO thickness, a value bigger than Pt electrode area instead of Pt electrode area should not be used as the area of currents flows through. For example, As the Pt electrode diameter is 3 mm, the area of currents flows through value 10% larger than Pt electrode area for fitting YSZ thickness, and value 70% larger for fitting TGO thickness.3. In this article, TBCs IS simulation was carried out with COMSOL, except obtaining TBCs impedance spectrum, the electric field distribution in TBCs was also obtained. analyst the distribution of electric field under different conditions in the TBCs, get the following conclusion: the using of asymmetric electrode leads to divergence of electric field line in TBCs, and the electric field divergence is one of the most important reason which make IS detecting techniques inaccuracy; The influence of Measuring temperature, electrode size and thickness and conductivity of YSZ, TGO on impedance spectra can be explained from two ways,(1) Its value changing affect the impedance spectroscopy of TBCs directly.(2) It effects TBCs impedance spectra by impact the electric field distribution in TBCs.