Laser-Ultrasonic Characterization of Electrodeposited Chromium Coatings
Metal coatings are of interest in applications where erosion, corrosion, or mechanical degradation result from chemical, temperature and mechanical insults to the surface. The evaluation of a coating nondestructively, responds to the need to estimate its effectiveness prior to actual use, without damaging it. An area of interest is the coatings’ elastic properties. These properties are determined here by measuring bulk and Rayleigh ultrasonic velocities. The material bulk velocities are found by measuring the thickness and shear and longitudinal ultrasonic time-of-flight of the substrate with and without the electroplated chromium coating. Elastic properties of the material are calculated from the bulk velocities measured assuming little or no dispersion within the material. The Rayleigh (surface wave) velocity is measured using a pulsed laser for ultrasonic wave generation in conjunction with a Michelson interferometer for signal detection. Wavelet analysis is used as a signal processing technique to determine the dispersion curves for the laser generated surface acoustic waves. The specimens are produced by electrodepositing chromium of various thicknesses on 2 in. × 1 in. × 0.5 in. steel coupons using controlled plating parameters.