Nondestructive measurement of elastic modulus for thermally sprayed WC-Ni coatings based on acoustic wave mode conversion by small angle incidence

a nondestructive method for characterization of thermally sprayed coatings is developed using an ultrasonic small angle incidence scheme in coatings of four layers and three interfaces. Through wave mode conversion analysis, the longitudinal and the transverse waves at the interfaces are simultaneously derived at an incident angle of 4.1°. The ultrasonic reflection coefficient amplitude spectrum (URCAS) is used to determine the thickness and longitudinal wave velocity of specimens through a two-parameter inversion utilizing the cross-correlation algorithm. The elastic modulus and Poisson's ratio of coatings were calculated utilizing the inversion results and modified density. Ultrasonic experiments were carried out on four WC-Ni specimens sprayed using the high velocity oxygen fuel (HVOF) method at spray angles of 30°, 45°, 60°, and 90°. The thicknesses measured by the ultrasonic method were in good agreement with those observed by optical microscopy with less than 10% error. The porosities determined from cross-section SEM photographs were 4.67%, 1.76%, 0.92%, and 0.19%, respectively with increasing spray angle. The elastic moduli of the specimens measured by the ultrasonic method were in the range of 315 GPa-351 GPa, and the Poisson's ratios were during 0.221-0.245. Metallurgical analysis indicated that increasing the spray angle increases both the density and the bond strength between particles which leads to an enhancement of the elastic modulus of the coatings. The proposed ultrasonic method is valid for nondestructive characterization of the elastic modulus and Poisson's ratio of the coatings.