Abstract
Air engulfment by the plasma jet in Air Plasma Spraying (APS) causes in-flight oxidation of metallic particles. This oxidation, often complex and difficult to explain by classical diffusion-controlled oxidation, is governed by several mechanisms. This paper highlights the possible in-flight oxidation mechanisms in metallic particles with a focus on convective oxidation. Two different types of austenitic stainless steel particles, Metco 41C (-106+45 µm) and Techphy (-63+50 µm) were air plasma sprayed using a dc plasma gun (PTF4 type) and were collected in an argon atmosphere. Preliminary experiments indicated that different mechanisms are likely to occur during the in-flight oxidation of particles. Mass transfer from surface to interior of particle occurred forming oxide islands in particles. The mass transfer is governed by convective movements inside liquid particles within plasma jet core due to higher plasma-particle kinematic viscosities ratio and particles Reynolds number higher than 20. The islands were composed of metastable phases consisting of mixed oxide of Fe and Cr, likely in a nonstoichiometric form of FeCr2O4. Convective movements within particles cease roughly outside of the plasma jet core and classical surface oxidation was found to be the dominating phenomenon forming the surface oxide layer. Moreover, the molten surface oxide outside the jet core is entrained to the tail of the particle if plasma conditions promote higher particle temperature, velocity and Re number.
