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RESEARCH PRODUCT

Caractérisation et prédiction du rendement de projection lors du procédé de projection à froid – cas de la métallisation des polymères

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This thesis work deals with the cold spray metallization of polymer substrates by aiming at an optimal deposition efficiency using experimental analysis combined with numerical simulation. Experimental observations of the powder jet using an ombroscopic analysis have revealed a generic behavior consisting of a uniform confined zone at the immediate exit of the nozzle followed by a dispersed zone. The supersonic flow at the nozzle exit being essentially axial, the powders are entrained to form a uniform jet. Beyond a certain distance, a dispersion appears. This distance is very important because it characterizes the behavior of the powders. It can alter the conditions for the elaboration of coatings since a deviation of the powder trajectory can lead to oblique collisions on the substrate surface and reduce the normal component of the collision speed. It has been clearly observed that this critical distance is influenced by the combination of powder density and diameter (ρpDp) although there is no generalizable trend when extending the study over a large variance of ρpDp.A CFD simulation study has provided a better understanding of the phenomenological behaviour of the fluid/powder flow. At the nozzle outlet, the shearing of the surrounding air by the supersonic jet creates self-sustaining vortices that generate an oscillation of the local properties of the fluid. This turbulence is repetitive and produces instability in the upper and lower parts of the flow. The drag force exerted by the fluid does not cause the radial deviation of the powder trajectory. From a phenomenological point of view, it is the consideration of a Magnus effect that allows to reproduce qualitatively and also quantitatively the experimental dispersion. The simulated diameter of the powder jet corresponds relatively well to the experimental diameter and thus corroborates the role of the Magnus effect. Powders of fine sizes are less sensitive to this effect. However, this sensitivity deserves further analysis. Nevertheless, the understanding of this powder kinematic phenomenology has helped to explain the limitation of the spray yield at low pressure and to show the necessity to work at higher pressures to allow an optimal growth of the coating.The metallization of PEEK at high pressure could therefore be undertaken with operating conditions that avoid substrate deterioration. The spraying parameters were then adjusted from an efficiency of 70% to a maximum threshold of 91%. The adjustment of the parameters consists in reducing the phenomena of erosion or delamination. A lack of speed favours erosion and too high a speed causes spalling when the coating adhesion is made up of an interfacial layer of poor cohesion (8MPa). According to the best processing conditions, porosity rates of less than 0.27 ± 0.179% and oxides of 2.21 ± 1.150% were obtained. Electrical conductivity measurements showed relatively good values (62.3% IACS). If the achievement of an efficiency higher than 90% is now possible with cold spraying at high pressure, the problem of interfacial adhesion remains nevertheless a limiting factor that leads to new perspectives for improving the metallization of organic substrates.