Study On Electromagnetic/Capacitive Compound Testing Technology Of Thermal Barrier Coatings

With the development of aeronautics and space technology,demand for aero-engines with higher performance is very urgent,especially for the engines with large ratio of thrust and weight,even the most advanced single crystal superalloy cannot meet the requirements of the high operating temperatures.Within this context,thermal barrier coating technology emerges as the times require and it improves the thermal resistance of the engine blade and greatly improves the thermal efficiency of the aero-engines.However,the structure of thermal barrier coating system is rather complex,and their working environment is very harsh.Most importantly,the failure of thermal barrier coating system will lead to very serious consequences.Therefore,technologies used for nondestructive testing and assessment of the thermal barrier coating are highly demanded for their practical applications.In view of the present situation that the existing methods are simple and difficult to realize the comprehensive evaluation of thermal barrier coating system,a detection method combining electromagnetic eddy current testing with planar capacitive detection is proposed in the thesis,and the involved detection mechanisms are studied as well.The optimization design method of compound sensor,the calibration method and compound method of measuring system,and the information fusion of array sensor are studied,the main research work and innovation are as follows:Firstly,the method of combining electromagnetic eddy current testing with planar capacitive testing is proposed,providing a feasible solution for the comprehensive evaluation of thermal barrier coating system.The mechanisms of electromagnetic eddy current testing and planar capacitive testing is studied.The electromagnetic eddy current method is suitable for measuring the characteristics of conductive materials and the thickness of non-conductive materials,including the properties of bonding layer and superalloy substrate as well as the thickness of ceramic layer in thermal barrier coating system.The planar capacitive testing method is suitable for detecting the characteristics of non-conductive ceramic coatings.The combination of these two testing methods can achieve complementary advantages,enhance the detection ability of the sensor,and overcome the shortcoming that the conventional nondestructive testing method can only realize the local detection of thermal barrier coating system.Secondly,the design of electromagnetic/capacitive compound sensor is studied,whose finite element simulation model is also established.In addition,the interaction between different testing units in the sensor is analyzed,and a set of optimization design method of compound sensor is formed.In order to meet the testing requirements,a variety of compound sensors with different structures are designed,and the finite element model is established.The performance of the sensor is simulated and analyzed in several aspects such as penetration depth of electric and magnetic fields,detection sensitivity,and resolution.A reasonable compound sensor structure is determined.In different operating modes,the interactions between the electromagnetic testing unit and the capacitive testing unit are analyzed.The structural parameters of the sensor are optimized according to the analysis results,and a set of optimization design methods of compound sensor areformed.Thirdly,a micro capacitance(pF)measurement system based on impedance analyzer and multi-channel switching unit is developed,and the calibration method based on equivalent circuit is proposed.The experimental results show that the measurement error of micro capacitance is greatly reduced.Based on the high-precise impedance analyzer,a multi-channel switching circuit is designed to measure the impedance of electromagnetic/capacitive compound sensors in different operating modes.In order to overcome the influence of channel impedance,parasitic capacitance and bypass capacitance,and to obtain more accurate measurement values,a calibration method based on equivalent circuit is proposed.The measurements and calibrated experiments are carried out to verify the validity of the calibration method and the accuracy of the measuredresults.Fourthly,a multi-mode,multi-frequency and multi-channel information fusion method is proposed to realize the integrated detection of thermal barrier coating system by electromagnetic/capacitive compound sensor.The characteristic detection of dielectric constant and thickness of ceramic layer is realized by the information fusion of electromagnetic and capacitance.The frequency characteristic quantity which can be used to detect the thickness change of adhesive layer is established by the information fusion of multi-frequency electromagnetic testing.The defect detection of superalloy substrate is realized by multi-channel information fusion and scanning imaging of sensor array.