Non-destructive thickness determination of metallic coatings using ultrasonic Rayleigh waves

A non destructive method for the thickness determination of metallic coatings is proposed which is based on the measurement of the velocity and the attenuation of ultrasonic Leaky Rayleigh waves (LRW). The considered coatings are Vacuum Plasma Sprayed (VPS) NiCoCrAlY coatings (thickness ranging from 0.06 to 0.33 mm) deposited on AISI 316L and on UDIMET 520 (Ni-based alloy) substrates. The ultrasonic technique used is based on the measurement of an acoustic signature (the so called v(z)- curve) of the coated specimen. Two highly-damped flat transducers working in transmission are translated along a z- axis, perpendicular to the sample surface (see fig.1). The half angle between the axis of the transducers is close to the Rayleigh angle. The sample is immersed in water. For each height z of the transducer, the signal is digitized and Fourier transformed. Thus, for each frequency, complex v(z)- curves are obtained. In the appropriate z- range, the v(z)- variations (both in phase and amplitude) depend only on the velocity and the attenuation of the Leaky Rayleigh wave (also called surface wave) propagating in the layered media and no longer depend on the shape of the beam. According to its frequency, the LRW propagate mainly either in the substrate (low frequency), in the coating (high frequency) or in both materials (intermediate frequency). As a consequence, a change in the characteristics of the wave occurs because of the different elastic constants and microstructure of the coating and the substrate materials. This change is used to determine the thickness of the coating. The ''ultrasonic'' layer thickness is then compared to that optically measured on cross-section photomicrographs. Two cases were observed. In the first one, the relative difference between the RW velocities in the coating and in the substrate materials is about 15%, allowing the phase analysis of the v(z)- curve to be well adapted. In the second one, this difference is lower (less than 5%) so that the amplitude analysis of the v(z)- curve, related to the difference of microstructure, has also been performed, giving however a lower accuracy on the thickness.