The aim of the present study is to investigate a laser-ultrasonic method to determine the oxide-coating's adhesion strength on a metallic substrate. In order to demonstrate this method a completely contactless system is used, combining laser generation and detection of ultrasonic waves. A nanosecond pulsed Nd:YAG laser is used to irradiate the transparent oxide coatings, while the normal displacement of the rear surface of the metallic substrate is detected at the epicenter by a laser heterodyne interferometer. Increasing the laser beam energy, the generated acoustic wave forms are correlated with the thermoelastic regime, the fracture of the coating-substrate interface, and finally the expulsion of the coating. The generation of ultrasonic signals in the thermoelastic regime is described by a two-dimensional model suitable to calculate the in-and out-of-plane components of the mechanical displacement versus time. The values of the so-called practical adhesion found are in agreement with those obtained by classic contact techniques ͑tensile adhesion test, indentation, bending test͒. This work demonstrates the suitability of this quantitative and contactless test to evaluate the coating-substrate adhesion.

The aim of the present study was to investigate the possibility of using a laser ultrasonic technique for the estimation of the adhesion of ceramic coatings, deposited onto metallic substrates by thermal spraying techniques. For this purpose, a pulsed Nd : YAG laser (λ = 1064 nm, 14 ns) was used to irradiate Al2O3 coatings, of different thickness (30–350 µm), deposited onto stainless steel substrates by atmospheric plasma spraying. The laser generated ultrasonic waves were in situ recorded at the epicentre using a laser heterodyne interferometer. The acoustic waveforms were correlated with the interaction between the pulsed laser radiation and the coated metal, taking into account the physical properties and the geometry of both the coating and the substrate, as well as the porosity of the coating. For the critical value of laser beam energy for which debonding just occurred, the stress field, owing to the combined thermal and acoustic effects of laser radiation, was calculated. The obtained values of the stress intensity factor KIC and the strain energy release rate GIC were correlated with previous relative data concerning the three-point bending test, and were found to be in very good agreement, demonstrating the potential of the proposed technique for the estimation of the adhesion strength of transparent ceramic coatings deposited on metallic substrates by thermal spraying techniques.

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