Titanium particles were deposited on a steel substrate by the impact of high velocity in warm spraying. In the process, nitrogen gas at various flow rates was mixed to control the temperature of a supersonic gas flow generated by combustion. TEM and other techniques were used to analyze the microstructure of the interface between the titanium coatings and the substrate. At the lower nitrogen flow rate, thick oxide double layers in the interface region were observed. The adhesive strength of the coating was high even at lower particles’ velocity possibly because the mechanical interlocking between the titanium particle and the substrate could be enhanced by the high deformability of heated particles. As the nitrogen flow rate increased, however, just a little oxide and a very thin oxide layer covering on the titanium splats were locally detected. The highly localized pressure and the resultant intensive shear stress generated within a titanium particle by the impact could reveal the fresh metal surface through break-up of the thin oxide films on the particle and the substrate. As a result, the metallic bonding between the deposited particle and the substrate was formed and increased the adhesive strength remarkably beyond a certain impact velocity.

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