Study On Thickness Measurement Of Multi-layer Conductive Coatings Via Pulsed Eddy Current Testing Technology

The multi-layer conductive coatings can improve the heat resistance,wear resistance and corrosion resistance of the equipment and key components,therefore it is applied widely in aerospace,petrochemical,metallurgical machinery and other industries,such as aircraft engine blades,oil pipe,metal castings.The thickness and uniformity of the coating will directly affect the service life and reliability of the equipment and key components.Therefore,the thickness measurement of the coating is an urgent problem to be solved.At present,the sinusoidal excitation is employed in the traditional eddy current testing methods to measure the thickness of multi-layer conductive coatings.Compared to the sinusoidal excitation signal,the excitation signal of the pulsed eddy current testing(PECT)is a square wave signal,which is composed by many of sinusoidal signal.The advantages of PECT include frequency-rich,simple operation and low cost,so it has become a research hotspot in the field of non-destructive testing.In this paper,the PECT problem of thickness measurement of multi-layer conductive coatings is studied by simulation and experiment method.The main research contents are as follows:(1)According to the theory of PECT,a finite element model is built.On the basis of this model,the influence of the signal’s generation approach,frequency and ampitude on the measurement results is analyzed.Consequently,the optimal excitation generation method,excitation frequency,excitation ampitude and the location of detection point are determined.(2)According to the analytical solution,the fitting function between the thickness,the conductivity of the conducting layers and the detected signal is proposed.Based on this function,the testing method of the single coating thickness is proposed when the substrate is non-ferromagnetic metal,the proposed measurement approach can be applied when the coating conductivity is unknown and known;while when the substrate material is the ferromagnetic metal,the proposed measurement approach is avaible when the coating conductivity is known.The PECT experimental platform is designed to verify this method,and the experimental results show the proposed approach can measure the single coating thickness with high accuracy.(3)For the thickness measurement of two-layer coatings,the thickness inversion method is studied when the coating conductivity is known.The solution equation algorithm and the decoupling algorithm are employed to obtain the coating thickness.And the results are verified by the expriment.Furthermore,the two algorithms are applied to measure the thickness of two-layer coatings with unknown coating conductivity,the thickness of three-layer coatings and more than three-layer coatings with known coating conductivity.Finally,the study shows that both the solution equation algorithm and the decoupling algorithm can be used for the thickness measurement of multi-layer conductive coatings,while the decoupling algorithm is better than the solution equation algorithm for its higher accuracy.