| Turbine blades,as the core components of aeroengines,mainly convert heat energy into kinetic energy.Turbine blades work in extremely harsh environments such as high temperature,high pressure,high stress and high speed.Reliability of turbine blades is crucial to safe production and flight.In order to improve its safety and service life,the blade surface is covered with a thermal barrier coating,which is composed of a ceramic layer,a bonding layer and a nickel-based superalloy substrate.With the increase of service time,both the thickness of the bonding layer and the thickness of the ceramic layer will be reduced and oxidized to produce oxides at high temperature,resulting in poor insulation.More seriously,the ceramic layer will fall off,leading to the failure of the thermal barrier coating.Due to the low conductivity of the adhesive layer in thermal barrier coatings,the resolution of conventional eddy current is not enough to measure their thickness.Therefore,the detection of thermal barrier coating thickness plays a very important role in monitoring the health condition and life prediction of aero engines.The main works of this paper are as follows:Firstly,high-frequency eddy current analytical model of planar spiral coil is established,according to Maxwell's equations,the vector magnetic potential is introduced,the analytical model of the magnetic field of a single coil in air is obtained.Secondly,combination superposition theorem,the analytical model of the magnetic field of planar spiral coil in air is obtained.Then by using the reflection theory of electromagnetic field,the integral analytical model of planar spiral coil vortex field on any layer of conductive specimen is obtained.Subsequently,based on Faraday's law of electromagnetic induction and ohm's law,an integral analytical model of the impedance variation of planar spiral coil with high frequency eddy current is established the experiment and finite element simulations were carried out to verify it.Finally,the influencing factors of thickness measurement are analyzed.The study found that liftoff and conductivity will have an effect on the thickness measurement results of the thermal barrier coating,and the influence of the permittivity in the thermal barrier coatings on the thickness measurement results can be approximately neglected.Then,a method of measuring BC thickness based on peak frequency is proposed,which can suppress the influence of lift-off and the change of the thickness of the TC on the detection result.It is found that when the excitation frequency locate between 1MHz and 20 MHz,the peak value of the imaginary part of the inductance appears at a specific frequency,and the peak value and the peak frequency change with the thickness of the BC.Instead,the thickness of TC does not affect the peak frequency of the imaginary part of inductance.Finally,a TBC thickness measurement method based on analytical model is proposed.The errors of different specimens at different excitation frequencies were analyzed by using sweep frequency 1MHz to 20 MHz.In order to reduce the error of the inversion results,a standard specimen is selected to calibrate the algorithm.The results show that the TBC thickness measurement method based on the analytical model can simultaneously measure the thickness of the TC and the BC,and the measurement error is less than 5%.In this paper,two thickness detection methods are proposed.The first method is to measure the thickness of the bonding layer in the thermal barrier coating by using features of specimens with different thicknesses.This method can only measure the thickness of the BC,but the accuracy of BC thickness measurement is high and the speed is fast.The second method is inversion method based on analytical model,which can measure the TC thickness and BC thickness in thermal barrier coating simutaneously without data processing,but the speed is slow and the accuracy is relatively low.This paper has 63 graphs,26 tables,and 107 references. |
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