Metastable chromium-rich oxide formed during plasma spraying of high-alloysteel

During plasma spraying of metals in air, rapid-oxidation reactions occur, i n most cases. In oxidation products of Cr-rich steels, Fe-Cr spinel oxide i s often found as a dominant oxide phase. A thermodynamic analysis of a syst em composed of Fe-13%Cr alloy and water vapor or air showed that the oxidat ion product in a wide range of high temperatures is Fe3-wCrwO4 (w>2). This tetragonally distorted spinel oxide is not stable at room temperature. Wate r vapor and air were considered as limits of the gaseous-phase composition in atmospheric spraying by a water-stabilized plasma gun, where the composi tion of the plasma plume is modified by air entrainment. The equilibrium ca lculations enabled determination of the effects of temperature and gas-to-s olid ratio on w. To show the existence of chromium-rich, tetragonally disto rted spinel oxide experimentally, a typical product resulting from oxidatio n of 13%Cr-steel particles during their flight in the plasma plume was stud ied after rapid solidification. This was made possible by trapping and quen ching the flying particles in liquid nitrogen at a distance from the plasma -gun nozzle corresponding to the nozzle-substrate distance in conventional plasma spraying. The results obtained by X-ray diffraction, Mossbauer spect roscopy, and X-ray fluorescence analysis showed that this oxide, in which w approximate to2.4, constituted the dominant phase in the oxidation product .