In-flight oxidation of high-alloy steels during plasma spraying

In-flight oxidation of high-alloy steel powder particles sprayed by a water stabilized plasma gun was studied. The particles were collected in liquid nitrogen, which enabled rapid cooling and prevention from further oxidation. Shrouding by two different gases (nitrogen-hydrogen mixture and acetylene) was carried out. The influence of both gases on the oxidation reaction was assessed by determining the oxygen content in particles quenched in liquid nitrogen. Acetylene was found to be significantly more effective in decreasing the oxygen content. The main method to study the composition of the oxidation products was Mossbauer spectroscopy. The dominant oxide phase found in the oxidation products was non-stoichiometric chromium-rich FeCr2O4. A comparison with previous results shows that if the particles are deposited onto a solid substrate instead of being quenched in liquid nitrogen, this oxide is converted in non-stoichiometric iron-rich FeCr2O4 during the relatively slow cooling of the plasma sprayed deposit.