Cold spray is a promising process to fabricate high-quality metallic coatings. However, it is necessary to improve some properties, especially the adhesive strength of the coating to the substrate to clarify deposition mechanism of the solid particles onto substrate surface. In this study, deposition behavior of the cold sprayed copper fine particles was observed precisely and the adhesive strength of the coating was evaluated. The deposition behavior of the sprayed individual copper particles on mirror polished stainless steel substrate was fundamentally investigated. The interface microstructure between sprayed particle and substrate revealed that an amorphous-like band region was recognized at interface during coating fabrication at high power conditions. For the deposition mechanism of the cold sprayed particles onto substrate surface, it was indicated that the deformation of the particles initially induce the destruction of its surface oxide and an appearance of the active fresh surface of the material may enhance the bonding between particles and substrate. On the other hand, in coating fabrication at high power condition, bonding between particle and substrate may be possibly formed via oxygen-rich amorphouslike layer at interface.

To improve the deposition efficiency of copper fine particles mean diameter around 5 mm onto metallic substrate surface in cold spray process, optimization in nozzle design was performed by numerical simulation. Particles velocity reached up to 585 m/s under the optimum conditions with originally designed nozzle based on the simulation results. In the spraying of copper particles onto normal steel substrate, lamellar-like microstructure was formed near the interface in the steel substrate. Correspondingly, remarkable hardness increase in this lamellarlike region of the steel substrate was recognized due to the higher velocity of the particles attained. Moreover, to reduce the bow shock effect especially for fine particles on the substrate surface in cold spray process, special nozzle was newly designed. The deposition efficiency, Vickers hardness and coating adhesion strength increased significantly especially in case of fine particles, as well as at higher pressure level of the working gas, while nominal particle velocity decreased with the special nozzle. Numerical simulation indicated that the pressure levels on the substrate surface decreased effectively in the newly designed special nozzle. In the observation of sprayed individual particles onto the substrate, extended metal jet was recognized at the splat's periphery when the particle was sprayed with the special nozzle. The results indicate that the decrease of particles velocity due to bow shock was suppressed effectively in the special nozzle as compared to the conventional one.

This study investigates the deposition behavior of cold sprayed copper particles on flat surfaces. In the experiments, a Laval-barrel nozzle was used to spray water atomized spherical copper particles with a mean diameter of 5 µm onto mirror-polished stainless steel. The particles were similar in morphology regardless of spraying conditions with an average bonding strength of 60 MPa as determined by nano scratch (shear) testing. An amorphous-like layer at the particle-substrate interface indicates that the deformation of the particles initially destroys their surface oxide, revealing an active fresh surface that facilitates metallic bonding. At higher spray velocities, metal jetting is observed at the periphery of flattened particles and its relationship with deposition efficiency is statistically analyzed and put forth as a potential method for controlling the cold spray process.

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