Measuring Ultrathin Metallic Coating Properties Using Swept-Frequency Eddy-Current Technique

Thin metallic coatings are used in a wide range of industrial settings to protect or enhance the properties of the underlying material. We propose a new swept-frequency eddy-current-based technique to simultaneously measure the thickness and conductivity of metallic coatings, which thickness in the order of tens of micrometers. Variations in coil impedance with lift-off were discussed when the coil was placed above a conductive plate with and without a thin metallic coating. Then, an impedance coordinate transformation method was presented to eliminate the lift-off effect. In the new impedance coordinate system, the impedance of the coil placed above the plate under testing was normalized by the impedance of the coil placed above two reference plates to form a new model containing the variables that are the coating thickness and conductivity, and the conductivity of the underlying plate. A combination of the least-squares method and the Levenberg-Marquardt (LM) algorithm was applied to solve it. The proposed method can be extended to measure the parameters of a conductive plate with multilayer metallic coatings. The results of the experiments in this study showed the proposed method has high accuracy and sensitivity.