Effect of Nitrogen Flow Rate on Mechanical Properties of Metallic Coatings by Warm Spray Deposition

Abstract

Abstract In Warm Spraying (WS), the temperature of the combustion flame is reduced and controlled by injecting nitrogen gas into the combustion flame before the injection of spray powders. Thus, 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, Ti, Cu, and Al coatings were fabricated by WS under various nitrogen flow rates. The mechanical properties of the coatings were evaluated by tubular coating tensile (TCT) and micro flat tensile (MFT) tests. For the lower impact temperature regime, the coatings became denser and the ultimate strengths of Ti or Cu coatings increased reaching a maximum by decreasing the nitrogen flow rates. A further decrease of nitrogen flow rates and reaching the upper temperature regime reduced the coating strength. These results clearly demonstrate how particle temperatures affect the microstructures and mechanical properties of WS coatings and that optimum spray conditions have to be balanced between softening and oxidation by adjusting particle temperatures.