Characterizing tantalum sputtered coatings on steel by using eddy currents

With the goal of building a system for fast inspection of coatings, we have developed a method that uses induced eddy currents to characterize tantalum alpha and beta phases in a layer of thin sputtered tantalum on steel. The detection of the tantalum phases is based on the large difference in electrical conductivity between them. Measurements based on the method agree well with values based on theoretical calculations. We applied the method in a two-probe differential system having higher sensitivity and less noise than a one-probe system. The probe uses pulsed eddy currents with a pulsewidth of 1 μs, allowing us to scan at rates of up to 10⁵ pulses per second on a computer-controlled XY table for fast data acquisition. When the system was used to scan steel samples coated with 12.5-30 μm of tantalum, a clear difference between alpha and beta phases was observed. The system was also used to measure the conductivity of the alpha and beta phases. We present here a conductivity map of the sample.