Simultaneous determination of the coating thickness and its longitudinal velocity by ultrasonic nondestructive method

The ultrasonic reflection coefficient amplitude spectrum (URCAS) has been extended to obtain the coating thickness and its longitudinal velocity at the same time on thick substrates. A model was set up first to represent the ultrasonic waves reflected from a coating system at normal incident, and the expression of URCAS was derived in order to obtain the coating thickness and velocity. Then, an inverse algorithm based on the Gauss–Newton method was introduced to determine the thickness and velocity by comparing the theoretical and measured URCAS. Experimental validation was conducted on the homogeneous epoxy coatings on the aluminum substrates and inhomogeneous ZrO2−7wt%Y2O3 (YSZ) coatings on the superalloy substrates. The relative errors of the thickness and velocity measurement were in the ranges of 2.15–2.35% and 2.67–4.40% for epoxy coatings, and between 5.33–5.96% and 8.95–9.66% for YSZ coatings. It is concluded that the URCAS combined with inversion technique can be applied to obtain the thickness and longitudinal velocity of coatings simultaneously.