Laser Ultrasonic Testing for Estimation of Adhesion of Al₂O₃ Plasma Sprayed Coatings

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 K_IC and the strain energy release rate G_IC 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.