Significant influences of metal reactivity and oxide films at particle surfaces on coating microstructure in cold spraying

Based on large amount of experimental observations, the effects of metal reactivity and oxide films at particle surfaces on coating deposition behavior in cold spraying were presented and discussed. The oxygen contents in as-sprayed Ti, Ti-6Al-4V and Al coatings were higher than those in the corresponding starting powders. The obvious flashing jets outside nozzle exit during deposition of Ti and Ti-6Al-4V were caused by the reaction of the particles with oxygen in the entrained or the adopted air. For Ti and Ti-6Al-4V coatings, their porous structures are predominantly attributed to the surface reactivity (defined as reactivity with oxygen). This surface reaction could be helpful for formation of a metallurgical bonding between the deposited particles. For Al, even though it is more reactive than Ti, the oxide films at Al particle surfaces suppress the surface activity.