Metallic coating thickness assessment over nonmagnetic metals using eddy current technology

Abstract

Recent improvements in apparent eddy current conductivity (AECC) spectroscopy lends itself as a practical approach to assess metallic coating thicknesses over nonmagnetic metals. It can potentially offer one-order of magnitude improvement in coating thickness estimation when compared to existing phase-sensitive single-frequency and impedance-based multiple-frequency measurement systems. Existing methods require the lift-off distance between the sample and calibration blocks to be consistent to ensure that the obtained measurement is accurate. Building on AECC measurement capabilities, which offer a significantly reduced sensitivity to those deviations, a robust forward and inverse approach is developed in this thesis using single-frequency AECC measurements. The proposed inversion algorithm is developed using the Bisection method, a numerical interpolation technique used for approximating solutions where determining the exact answer is not possible. The logarithmic nature of the problem as well as the requirement of two initial guesses to start the iteration process while covering the entire coating thickness range of interest further justifies the use of the selected method. This not only reduces AECC spectroscopy measurements from a broad frequency range to single-frequency measurements, but also reduces the coil diameter by one-order of magnitude while complying with the plane-wave approximation in the lift-off range of interest. The accuracy in estimating coating thicknesses using both single- and multiple-frequency AECC measurement techniques are compared numerically and experimentally in agreement with the plane-wave approximation for different coating thicknesses relevant to the industry. It is demonstrated that AECC measurement technology delivers 3% uncertainty in metallic coating thickness estimation over nonmagnetic metals in a ±25.4 μm liftoff range. This makes it a practical approach to replace commercially available measurement systems while offering one-order of magnitude improvement in coating thickness estimation.