Towards a new non-destructive-evaluation of coating-substrate adhesion by laser-ultrasounds

The aim of the present study is to investigate the potentiality of using laser ultrasonics for the estimation of the practical adhesion of transparent coatings (Al₂O₃ 80 μm-340 μm), deposited onto stainless steel substrates by atmospheric plasma spraying (APS). Using an already validated model adapted to a two-layers structure suitable to calculate the in- and out-of-plane components of the mechanical displacement versus time, the interfacial stress field is computed versus the absorbed laser-energy and especially for the critical value corresponding to the interfacial fracture. A complete contactless system is used to validate the computations, using a Q-switched Nd:YAG laser coupled with a heterodyne interferometer. Therefore, the adhesion has been determined, showing that this study (i) leads to the entire stress-tensor components; (ii) gives consistent results compared to other methods; (iii) yields to a new approach of the interfacial failure mechanisms