Abstract
In this study, a new laser based technique was evaluated for the characterization of plasma-sprayed oxide coatings. It uses the contactless laser generation and detection of ultrasonic waves in the bi-layered systems. For this purpose, a nanosecond pulsed Nd:YAG laser (λ : 1064 nm, τ =14 ns) was used for irradiating the ceramic coating, whilst the longitudinal displacements of the rear surface of the metallic substrate were detected at the epicenter using a laser heterodyne interferometer. The acoustic signal recorded at the rear surface of the substrate was found to be characteristic of the different events taking place within the irradiated system. In this way, the longitudinal wave velocity, the porosity, as well as the Young's modulus of the coatings can be easily determined, whilst the coating/ substrate adhesion strength can be calculated, taking into account both the thermal, as well as the acoustic effects of the laser radiation. The proposed technique was applied to alumina coatings deposited onto stainless steel coupons by Atmospheric Plasma Spraying and the results were found to be in accordance with those obtained by the techniques commonly used for testing thermal spray coatings (interfacial indentation test, porosity measurement, etc.).
