Thickness and conductivity of metallic layers from pulsed eddy-current measurements

Abstract

We describe a time-domain (pulsed) eddy-current technique for determining the thickness and conductivity of conductive coatings on metal plates. The pulsed eddy-current instrument records the transient current induced in an absolute, air-cored coil placed next to a layered sample and excited with a step-function change in voltage. Signals are digitized with 16-bit resolution at a sampling rate of 1 megasamples per second, and the excitation is repeated at a rate of 1 kHz. The instrument displays the difference in the transient current measured on the substrate and on the substrate plus coating. We measured pulsed eddy-current signals for a series of metal foils of varying thickness placed over 1 cm thick metal plates. Seven combinations of foil and substrate metals were studied including pure aluminum, copper, and titanium foils over substrates of aluminum, titanium alloy, and stainless steel. We report results for three types of samples: aluminum foils on Ti–6Al–4V substrate, titanium foils on 7075 aluminum alloys, and aluminum foils on AISI 304 stainless steel. Foil thickness ranged from 0.04–1.00 mm. We found that three features of the signal—the peak height, the time of occurrence of the first peak, and a characteristic zero-crossing time—depend sensitively upon the thickness of the layers and the relative electrical conductivity of coating and substrate. Theoretical calculations were compared to the measurements. Absolute agreement between calculated and measured signals was, in most cases, within 3%. No calibration with respect to artifact standards was used. Finally, a feature-based rapid inversion method was developed and used to infer the thickness and conductivity of the layers. The accuracy of the inversion depends upon the thickness of the layer and the contrast in conductivity between layer and substrate. For the materials studied the thickness could be determined within 13%, while the error in determining conductivity was 20%–30%. The time-domain method is much simpler and hundreds of times faster than the frequency-domain method previously reported by Moulder et al. [Rev. Sci. Instrum. 63, 3455 (1992)].

Noncontact Thickness Measurement of Metal Films Using Eddy-Current Sensors Immune to Distance Variation

“…The SEC technique typically provides good estimates to determine the film thickness. SEC requires absolute measurements of the impedance of coil using a computer-controlled impedance analyzer and relies on complicated analytical solutions [3]. The PEC technique contains frequency-rich information and has the potential to detect properties in great depth.…”

Simulation of Transient Eddy-Current Measurement for the Characterization of Depth and Conductivity of a Conductive Plate

“…A close observation of the plots in Figs. 3 and 4 indicates that the logarithm quantities of and time can be approximated by a second-order polynomial fitting (2) where and stands for and , respectively, are fitting coefficients which can be calculated from each against the curve. The contour plot of coefficient is shown in the lower-left part of Fig.…”

“…After appropriate advances in technology, transient eddy-current measurement has recently received interest in many of the common applications in nondestructive evaluation (NDE), such as the detection of defects, the measurement of the thickness of coating or specimen itself, measurement of electrical conductivity, etc. [1], [2]. However, studies on transient eddy-current measurement are still in the early developing stages.…”

Pulsed Eddy Current Method: an Overview

“…For the case of a metal layer on a substrate of a different material, Tai, et aI., [21] prepared tables of the PEC peak amplitude, peak arrival time and zero crossing time as a function of layer thickness and conductivity, with the conductivity of the substrate assumed known. They then used these tables to estimate conductivities and thicknesses of three different layer materials on different substrates.…”