Effect of nitrogen flow rate on microstructures and mechanical properties of metallic coatings by warm spray deposition

In Warm Spraying (WS), the temperature of the combustion flame is controlled by injecting nitrogen gas into the combustion flame before the injection of spray powder. Temperatures of spray particles are kept under their melting points with moderately heated and thermally softened states. As compared to HVOF-spraying, the oxidation of particles can be significantly suppressed due to lower deposition temperatures, whereas, as compared to cold spraying, the degree of particle deformation upon impact can be enhanced by attaining higher particle temperatures. In the present study, the effects of particle temperatures on the coating microstructures and the mechanical properties were investigated for three metal coatings (Ti, Cu, and Al) by varying nitrogen flow rates during WS deposition. The mechanical properties of the coatings were evaluated by tubular coating tensile (TCT) and micro flat tensile (MFT) tests. For Ti and Cu coatings, a maximum in ultimate strength was reached for medium nitrogen flow rates, i.e. medium impact temperatures. For Cu, the maximum reflects a balance between the amount of bonded interfaces and softening by annealing. At lowest N2 flow rates and thus highest impact temperatures, the elongation to failure of Cu coatings reached ~ 3%, and decreased with increasing nitrogen flow rates. Finally brittle fracture behavior was observed with the highest nitrogen flow rate. These results revealed how particle temperatures affect the microstructures and mechanical properties of WS coatings, and demonstrated that optimum spray conditions have to be balanced by adjusting particle temperatures, facilitating sufficient deformability while avoiding oxide formation.