Gilles Rolland

Which powders for cold spray

Abstract Since the major technological issues in cold spray are now controlled, the key parameter for successful coating relates to the powder. The influence of the main characteristics of the starting powder on the final coating properties remains rather unknown. This includes primarily metallurgical, morphological and physico–chemical characteristics of the particles. This article focuses on the so-called local approach to these characteristics to show that the current global approach is insufficient. The discussion is based on many examples of cold spray results, including recent work on Ag-based and Ta cold sprayed-coatings. This results in proposed specifications for powders which are claimed to be tailored for cold spray.

Damage study of cold-sprayed composite materials for application to electrical contacts

This work demonstrated the feasibility of cold spray for the achievement of industrial electrical contacts to be used in contactors and circuit breakers. This resulted from the study of cold spray conditions when applied to two types of silver-based particulate composites. Starting materials, primarily powder characteristics and addition content, and spraying conditions were optimized from thorough studies of microstructures and mechanical properties of the cold-sprayed composites. These involve x-ray microtomography and transmission electron microscopy in particular, in addition to tensile tests. Moreover, thermomechanical laser testing of series of cold-sprayed microstructures was successfully developed to investigate into damage mechanisms in coatings. The whole led to a novel phenomenological approach to damage in cold-sprayed materials. Durability tests of the best coatings were carried out using industrial facilities. Cold-sprayed contacts compared more than well with those from conventional powder metallurgy (P/M), which opens a new and cost-attractive production route for electrical contacts.

Lasers and Thermal Spray

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.