Optimizing the Cold Spray Process

Abstract

Abstract Cold spray is a new coating technique that does not involve significant heating of the sprayed material. Thus, it can be used to deposit thermally sensitive materials. Due to the fact that complex processes, such as combustion and ionization of the process gas are avoided, cold spray becomes very amenable to analytical modeling. To deposit with high efficiency, and to yield low porosity and high bond strength coatings, developers of the cold spray process have tried to maximize the impact velocity. Earlier studies have clearly shown that low molecular weight gases, long nozzles, high gas temperatures and pressures yield high impact velocities. Use of smaller particles of low-density materials also can increase the impact velocity. However, there is a limit to the gains that can be obtained with smaller particles due to the deceleration of the particles in the near stagnant region in front of the substrate. This study analytically estimates the optimum particle size in terms of other input values. To investigate the conventional wisdom that a higher impact velocity results in a better coating, a small experimental program was initiated. Experimental results clearly demonstrate the advantages of high impact velocities. These results confirm that the deposition efficiency, bond strength, porosity and surface roughness are all improved as the impact velocity increases. Limited experimental data also demonstrate that some coating properties depend not only on the particle impact velocity but also the particle impact temperature. The bond strength showed a dramatic increase when the particle impact temperature was increased. Small improvements were also observed for the deposition efficiency and the porosity.