In-flight oxidation of iron particles sprayed using gas and water stabilized plasma torch

This paper is devoted to the study of the oxidation of iron (low carbon) particles during their flight in d.c. plasma jets flowing in air as well as the oxidation after the impact and solidification of particles onto the substrate. Two types of torches have been studied: a conventional one: gas stabilized plasma (GPS) with an effective power Peff=18 kW and a more exotic one: water stabilized plasma (WSP) with Peff=139 kW. Besides their power levels, their main difference lies in the plasma gas specific mass downstream of the anode attachment almost 10 times lower in GSP than in WSP with correspondingly the plasma temperature 10,000 K higher for WSP. The GSP oxidation is promoted by the convective movement induced within the liquid metal droplets by the plasma flow in the jet core. In-fight oxidation shows strong similarities for both techniques with mainly the formation of non-stoichiometric FexO. In contrast in coatings depending on the temperature during spraying, Fe2O3 or Fe3O4 are observed.