The studying of splats in thermal spray is prominent to improve coating properties due to better understanding of coating build-up mechanisms. Most of studies, however, devoted to splats rest on physical aspects of the build-up, which feature the behaviour of a single particle impinging on a substrate. These studies involve the nature, velocity and intrinsic characteristics of the particle. The present work deals with the study of the splat-substrate interface using an ultra-sonic (U.S.) technique. This technique is based on the determining of splat-substrate adhesion (for an elementary splat or a group of splats) to help in the optimising of spraying conditions. APS copper splats were studied for Al-based substrates. This U.S. technique was already tested successfully for adhesion control of thermally-sprayed coatings in Basel, Switzerland. Further development then related to the specific experimental set-up for splat analysis using several transducers. This involved the use of various frequencies, various sizes for the focal spot and various scan velocities. In this study, results from this U.S. technique were discussed in the light of the observation of cross-sections of U.S.- controlled splats. U.S. C-Scan images were shown to fit well with cross-sectional images. In a general conclusion, the work showed the feasibility and efficiency of advanced U.S. control of splats to promote thermally-sprayed coatings whatever the process, i.e. plasma spray, flame spray, HVOF or cold spray.

The present work deals with the application to actual in industrial conditions of the promising results of the study on adhesion, as presented at the previous edition of ITSC’2004 in Osaka, Japan. These industrial parts were typical of those produced in workshops for aircraft-aerospace, petro-chemical, printing works industries etc…. Thermal spray processing of industrial parts tested in this study are numerous, primarily air plasma spraying (APS), HVOF and conventional flame spray with or without remelting. Ultrasonic NDT facilities are now installed in an actual industrial workshop and show the reliability of the testing method and related results. For every tested part, digital ultrasonic testing was carried out to be compared to results from conventional adhesion tests according to EN 582 standard. U.T. results were quite in keeping with those from EN 582 testing. However, in addition, ultrasonic testing exhibited the any heterogeneity when existing, (local weak zones) at the coating-substrate interface. This resulted in easy discriminating of adhesive areas from cohesive areas. Results of this work obtained in a real workshop environment leaves promising expectations for ultrasonic testing to meet results from conventional EN 582 adhesion testing when applied to industrially-produced parts. Ultrasonic testing results in a low-cost NDT method which should compete with current high-cost control. Moreover, ultrasonic NDT looks more reliable in so far as the whole part can be controlled due to scanning of the whole surface.

To address drawbacks with conventional destructive adhesion tests, two advanced nondestructive methods were developed to characterize the acoustic response of coating-substrate bonding. These new tests, immersion ultrasonic testing and laser shock adhesion testing, are discussed in this contribution. The paper describes the test process and results as compared to traditional destructive testing. The advantages and drawbacks of the techniques are addressed.