Study On Thickness Measurement Of Conductive Coatings Via Eddy Current Nondestructive Testing Technology

To ensure key structural performance in high-temperature, high-stress and hostile environments, coatings are widely adopted in the aerospace, shipbuilding, nuclear industries etc. For coatings, its thickness is very important because it is related to the lifetime of the coating, stress, bonding strength, material consumption, costs etc. In light of this, the nondestructive evaluation(NDE) of the coating thickness is urgently demanded for the safety assurance of various applications. Among the NDE technologies, eddy current(EC) techniques are widely used in the measurement of coating thickness since it has fast, high tolerance to harsh environments, non-contact, low cost and high bandwidth. In this paper, the basic principles of eddy current sensor, analytic model of multi-layer conductive structures and multi-frequency multi-parameter theory are analyzed first. And then, around the measurement of multi-layer conductive coating thickness, some studies are carried out, which can be summarised as follow:(1) A qualitative and quantitative analysis in the structural design of the probe for the measurement of thin coating thickness is performed. The optimization procedure of the coil is simplified based on the analyzed of coil sensitivity, and the criteria of coil geometry design is developed. Moreover, I-type, E-type, U-type, Ring-type four core structures are compared, and the effect of core size on the sensitivity is studied.(2) The thickness measurement of the case that single coating on the substrate is studied, and the effect of the variation of coating thickness and conductivity on the detection is investigated. Moreover, when the coating thickness is fixed, the influence of the conductivity changes on the magnetic field strength is analyzed. Then three measurement approaches, calibration appraoch, least square approach and slope approach, are proposed to measure single coating thickness. For the slope approach, experimental studies are also performed and the results are with good agreement each other. It verified the proposed approach can effectively and accurately measure the thickness of coating without knowing conductivity.(3) The thickness measurement of the case that two-layer coatings on the substrate is studied, and the effect of the variation of coating thickness and conductivity on the detection is also investigated. Then the polygon approach is presented to measure two-layer conductive coatings thickness according to the influence rules. Further on, this approach is extended to measure the thickness of two-layer conductive coatings without knowing conductivity and the thickness of multi-layer conductive coatings. Finally, the experimental studies are performed to verify the correctness of the polygon approach. Experimental results show that the new approach can be effectively applied to thickness measurement of multi-layer conductive coatings.