Coupling Relationship and Decoupling Method for Thickness and Conductivity Measurement of Ultra-Thin Metallic Coating Using Swept-Frequency Eddy-Current Technique

The detection of parameters such as the thickness and conductivity of the near-surface materials of metallic plates is significant to ensure the safe operation of the related equipment. Eddy current testing is widely used for near-surface defect or parameter measurement of metallic materials. However, it is still difficult to detect the thickness and conductivity of ultra-thin metallic coating with several tens of micrometers when both parameters are unknown, and the reasons are not elucidated. In this article, we first established parameter measurement models of metallic plate with ultra-thin coating using eddy current testing and theoretically deduced a coupling relationship between ultra-thin coating thickness and conductivity, which explained why ultra-thin coating parameters were difficult to detect simultaneously. Then, a decoupling feature, which is the coil resistance change at high frequencies of tens of megahertz, was extracted to decouple the coating thickness and conductivity of the thick metallic plate with an ultra-thin coating. After that, the ultra-thin coating thickness and conductivity, as well as the substrate conductivity, were measured on the basis of the decoupling feature by solving the least squares problem between theoretical and experimental values of combining both the coil resistance and reactance change. Finally, the proposed method was used to conduct parameter measurement experiments on thick metallic plates with coating thicknesses of 23, 32, and 52 mu m, respectively. The results show that the detection errors of coating thickness, coating conductivity, and substrate conductivity are all less than 15%.