Coating-Bond Evaluation Using Dispersion Curves and Laser-Ultrasonics

摘要

The dispersion characteristics of Rayleigh surface waves are used to distinguish the quality of the bond between a coating and the metallic substrate. The velocity versus frequency curves for good or 'welded' bonds differ significantly from the correspondent dispersion curves for poorer or 'smooth' bonds. A theoretical dynamic model of the ultrasonic experiments was developed and compared with experimental results. As examples of welded contacts, we used coatings of tantalum sputtered on steel, or of chromium electrodeposited on steel. As examples of smooth contacts, we used thin films of nickel and tantalum weakly bonded to copper and steel substrates, respectively. The experiments were performed using laser generation and detection of surface waves traveling along the coated surface of a flat sample. The ultrasonic waves were generated in an annular ring pattern, and the converging waves at the center of the ring were detected using an optical interferometer. Wavelet analysis was used on the acquired ultrasonic signals to obtain the experimental dispersion (velocity-frequency) curves. An analytical proof is presented herein as to their applicability for this purpose. 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.