Papers by Author: L. Berthe

Abstract: Basically, thermal spray and laser processing can be considered as half brothers since they show many common features due to the use of a (more or less) high-energy source for both. Their combination can therefore be very fruitful and prominent to achieve coatings, which results in their most recent and advanced applications. In the materials processing development story, the laser will thus have moved from cutting to coating. This keynote presentation focuses on the recently-developed coupling of laser processing to cold spray). In this dual process, a cold spray gun is combined to a laser head in a single device, e.g. on a robot. Series of coating experiments using various laser irradiation conditions, primarily pulse frequency, were carried out for Al-based and Ni-based alloys. Laser pre-treatment of the substrate just prior to cold spray, was shown to be beneficial for adhesion of cold-sprayed coatings. Adhesion improvement was exhibited and studied from LASATesting (LASAT for “LAser Shock Adhesion Test”). Incidentally, through LASAT also, the role of lasers in the development of thermally-sprayed coatings can be considered as major. Results are discussed in the light of a TEM (Transmission Electron Microscope) study of the coating-substrate interface with and without laser pre-treatment.

Abstract: Three interface factors may influence thermally-sprayed coatings adhesion: interface morphology (as usual), thermal and chemical features. It was shown that these three aspects of adhesion mechanisms are shown to be dependent and very local. It is especially true for cold spray which is one of the most promising spray processes. As this spraying technique is based on rapid deformation, cold spray coating/substrate interfaces show local morphological, thermal and chemical features, in a way that none of them can be neglected. LASAT is particularly suitable for testing these coatings because it can be applied to small areas (~1 mm²). From this, it has the outstanding advantage to be sensitive to fine-scaled phenomena responsible for coating adhesion.

Abstract: MCrAlY-typed coatings are conventional for applications to land-based turbines against hightemperature oxidation and corrosion. However, improvements are still currently expected from innovations in the coating process and/or in the selection of the starting materials. Both types of innovations were studied in the present work. The former consisted in cold spray as a substitute for plasma spray which is conventionally used as the coating process. The latter consisted in developing mechanically-alloyed powders to be suitable for the targeted application especially. In this study, coating-substrate adhesion was considered as the justice of the peace to assess improvements from these innovations. This was determined using the LAser Shock Adhesion Test, namely LASAT, which was recently developed as innovative adhesion testing of thermal spray coatings. Among the main results, mechanical alloying was shown to be satisfactory to result in an homogeneous powder from the mixing of CoNiCrAlY with Mo. This powder could be coldsprayed, all the more easily because of a fine grain size. Results were compared with those obtained from conventional commercial pre-alloyed powders. As a general result, it was shown that cold spray could lead to highly-dense and high-adhesion MCrAlY-typed coatings onto Inconel 625 even though the process is usually claimed to be convenient for high-ductility materials such as copper. Incidentally, LASAT was confirmed to be a flexible and powerful testing tool to study adhesion; which resulted in the ranking of the various types of coatings involved in the work. Results are discussed in the light of an experimental simulation of the impinging of cold-sprayed particles using so-called “laser flier impact experiments”. In this development of this simulation approach to cold spray, the flier was made of a 50μm-thick disc machined from HIP’ed CoNiCrAlY.