Oxidation of Iron Particles During APS: Effect of the Process on Formed Oxide; Wetting of Droplets on Ceramic Substrates

Abstract

Abstract Investigations have been carried out to study the influence of Atmospheric Plasma Spray process (APS) on the in-flight oxidation of pure iron particles. After collecting the molten droplets in-flight, XRD, SEM and Mössbauer spectroscopy are used to determine the amount and distribution of formed oxides. The results indicate that the Wüstite, Fe0.95O, is the only oxide formed during the APS. In the jet core and at the beginning of its plume, oxidation is controlled by convection within molten droplets and then for the downstream of the plasma by diffusion where the solubility of oxygen through the external oxide layer governs the growth of Wüstite. Calculations have shown that the convective movement is due to the drastic velocity difference between the plasma jet and particles. Wüstite granules can be distinguished within the particles due to the immiscibility between Fe and FeO in liquid phase. This oxide phase represents about 13 wt% of the collected particles at 100 mm stand-off distance in an Ar-H2 plasma jet (50:10 SLM) with 18 kW effective power. The amount of oxide decreases when the H2 volume percentage of the plasma, the internal diameter of the anode nozzle and the effective power increase. Sessile drop studies of molten iron on ceramic substrates are carried out to simulate the wetting of oxidized iron particles during coating formation. It is found that the oxidation state of iron particles during APS has a significant effect on the observed contact angles. A strong decrease of the contact angle is observed in the case of oxidized iron particles. In-flight oxidation of iron particles allows a better splat covering.