Ultrasonic nondestructive evaluation of thin (sub-wavelength) coatings

Abstract

This paper describes a technique for ultrasonic nondestructive evaluation (NDE) of a thin coating on a thick substrate; here thin means the thickness of the coating is less than the wavelength of the ultrasonic wave used to interrogate it. A plane longitudinal wave that is normally incident upon the coating is considered. Transfer functions have been derived for both the coating-side and the substrate-side insonification. A systematic analysis of the sensitivity of the transfer functions to the thickness and wave speed has been carried out. An inverse algorithm, which utilizes the well-known Newton–Raphson method, has been developed to reconstruct the thickness and the phase velocity through a comparison of the theoretical and the measured transfer functions; fortunately, this algorithm is independent of the substrate thickness. Using this technique both the thickness and the wave speed of the coating can be extracted from the same measurement, without knowing either. The technique is fully automated and computer-controlled and can be easily used for in-situ NDE applications. The technique was used to measure the thickness and wave speed of epoxy and Plexiglas coatings (50–100 μm) on an aluminum substrate using low-frequency (10- and 20-MHz) transducers; the ratio of thickness/wavelength was about 1/3. The precision in the measurement of the thickness and the wave speed was found to be ±2 μm and ±3%, respectively.