Coatings are widely used in industry. They provide good electrical conductivity, wear resistance, thermal and electrical insulation, and corrosion protection. Recently sub-micron range coatings and sophisticated layered composite structures have emerged from the high technology advanced research. A nondestructive evaluation (NDE) of coatings has an immediate goal to ensure satisfactory properties of coated metals, and save often expensive materials. For this purpose two most powerful methods acoustic microscopy and eddy-currents have been used in industry for many years. Two techniques for NDE of micron and sub-micron range coatings by both methods are given in this paper; performance, advantages and limitations are outlined. There are a number of quantitative acoustic microscopy (AM) methods for investigation of thin layers, in this work one of them is discussed. The Doppler continuous wave scanning acoustic microscope has been used for the evaluation of 0.3-5.0 μm thick titanium nitride coatings on steel substrate. Thickness errors are typically within 10 percent. The method has an obvious advantage for nonconductive coatings and substrates and for coatings only slightly different in mechanical properties from the substrate, as nitrogen implantation hardening layers. This potential of the method has been illustrated on tool steel samples having a 0.2 μm thick nitrogen implantation coating. Eddy currents (EC): Operating at a single frequency, using various coils, thickness of conductive coatings on conductive and nonconductive substrates has been determined. Analytical solutions obtained for long coils and surface coils, are mathematically very compact, and allow a real-time evaluation. Aluminum foils of 32-64 μm thick, 0.3-0.8 μm thick metal films sputtered on nonconductive substrates, 15-45 μm thick aluminum coatings on stainless steel, and 2.8-58.5 μm thick zinc coatings on steel substrate have been measured. Agreement between theory and experiment is excellent. Discrepancies between the eddy current thickness and that determined using other methods are typically within few fractions of micron. Encountered problems with measurements on sub-micron range coatings are reported.
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