Ultrasonic characterization of plasma-sprayed coatings

Ultrasonic methods are developed for characterizing plasma-sprayed coatings. First, ultrasonic spectroscopy is used to determine the acoustic properties of various plasma-sprayed materials. The spectrum of overlapping back-echoes reflected on a thin plate is measured. The velocity and the attenuation coefficient of longitudinal waves are deduced from the characteristics of the spectrum at the resonant frequencies of the plate. The results show these ultrasonic properties are a good indicator to reveal differences between microstructures of coatings prepared with various plasma-spraying conditions as well as to follow their evolution after thermal treatments. Second, a pulse-echo method is developed for examining the interfaces between the components of a duplex coating through the substrate. The knowledge of the acoustic properties of the plasma sprayed materials is shown to be essential for explaining responses of coated systems. Finally, a scanning technique is used to build images of model specimens with various interface preparations.