Research On Deterioration Mechanism Of Gas Turbine Thermal Barrier Coating Based On AC Impedance

Gas turbine engine is a key equipment related to national energy security and aviation safety.Its blades are important structural components that bear high temperature and high load.Thermal barrier coating technology is usually used on gas turbine blades to ensure safe and stable operation at high temperatures.Thermal barrier coating is a ceramic material with high temperature resistance,low thermal conductivity and corrosion resistance,which is combined with the blade substrate through a metal bonding layer.In a high temperature working environment,an oxide growth layer（mainly Al₂O₃）is formed between the thermal barrier coating and the adhesion layer,and will continue to thicken with increasing the number of thermal cycles.Due to the difference in thermal expansion coefficients and internal and external stress,the thermal barrier coating can be cracked eventually,which will seriously affect the safety of operation of gas turbine engine and aircraft.Shutting down for maintenance is usually necessary to check the growth of oxides in the thermal barrier coating,while this process is complicated and will affects production directly.In this subject a method of using impedance technology was proposed to monitor the growth of oxides in thermal barrier coatings under actual machine operating temperature.Based on the principle of impedance spectroscopy,the structure of thermal barrier was obtained by analyzing the response of AC impedance and phase angle information in different frequencies.By combining the growth information of the oxide and the SEM microscopic morphology,a prediction model was built to calculate the degradation process of the thermal barrier coating.After 50 hours of high temperature oxidation at 1100℃,it was found that the thermal barrier coating cracked at the interface between the ceramic layer（Yttria stabilized zirconia,YSZ）and the oxide growth layer（Thermal Grown Oxide,TGO）,and there were many micro-cracks in the ceramic layer.After high temperature oxidation at 900℃for 300h,peeling of the coating was observed under the electron microscope.After high temperature oxidation at 1000℃ for different time,a dense dark oxide layer（TGO）was found between the ceramic layer and the bonding layer,and the thickness increased with the oxidation time.The EDS test results showed that the components of TGO layer were in the form of mixed oxides,of which Al₂O₃ is the main component;through the measurement of the thickness of TGO,the relationship between thickness and time was simulated,indicating that the growth of TGO thickness followed a good parabolic law.By using a new type of platinum glue,the impedance spectra of the Thermal Barrier Coating（TBC）after oxidation at 1000°C for different times were tested with the AC impedance method.The impedance spectra showed two semicircles,and the radius increased with the increase of time,which indicated that the thickness of TGO increasee with time gone.As the oxidation time increased,the composition of TGO changes from Al₂O₃ at the initial stage of oxidation to the mixed oxide of Al₂O₃（mainly）,Cr₂O₃,Co O,and Ni O,which lead to the decrease of the inflection point in Bode phase angle diagrams.Therefore,by fitting the impedance spectrum of the thermal barrier coating,information on resistance,capacitance,etc.can be obtained,and at the same time the thickness and composition of the TGO can be determined.By establishing an appropriate circuit model to fit the impedance spectrum data,the parabolic relationship between the R_T value of the circuit element and the oxidation time was established,thus confirming the feasibility of the impedance measurement technology in the non-destructive detection of the deterioration process of the thermal barrier coating at the actual working temperature of the thermal barrier coating.