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.

To improve the deposition efficiency of copper particles, especially fine particle, in cold spray process onto metallic substrate, optimization of nozzle shape and dimension was performed by numerical simulation. Maximum velocity of the particle reached up to 685 m/s under the optimum conditions by using self-designed nozzle based on the simulation results. In the spraying of copper particle onto steel substrate, lamellar-like unique micro-structure was observed near the interface region of the steel substrate. Correspondingly, hardness increase in this region of steel substrate was recognized. Work hardening was induced in the steel substrate due to the higher velocity of copper particles. Furthermore, to reduce the bow shock effect on the substrate surface region in cold spray process, special shaped nozzle was newly developed. While nominal particle velocity decreased in the special nozzle, deposition efficiency, Vickers hardness and adhesion strength increased significantly especially in the case of fine particle and higher pressure levels of the working gas. Numerical simulation showed that pressure level on the substrate surface decreased effectively in the special nozzle. In the observation of a cold sprayed individual particle onto mirror polished substrate, extended metal jet was recognized at particle’s periphery when the particle was sprayed by the special nozzle. The results indicate that the decrease of particles velocity by bow shock was suppressed effectively in the special nozzle as compared with conventional nozzle.