Coating-bond evaluation using dispersion curves and laser-ultrasonics

Good or “welded” and poorer or “smooth” contact thin coatings are studied by measuring the dispersion characteristics of Rayleigh surface waves. As examples of welded contacts, tantalum was sputtered onto steel and HC chromium was electrodeposited on steel; as examples of the smooth contacts thin sheets of nickel and tantalum were epoxied to copper and steel substrates respectively. The experimental dispersion curves are compared to curves predicted by theory to give insight into the bond quality. We have used the experimental method of Cielo et al. (1985). This consists of laser generation of the surface waves in an annular ring and detection at the center. We have also performed the dynamic analysis for interaction of layer and half space for the annular source. Wavelet techniques are used on the surface wave detected signals in order to obtain the experimental dispersion (velocity-frequency) curves. Dispersion curves obtained from the experiment and signal processing correlate with theory for the welded contact and show features of the theory for the smooth contact