Spinel oxides growing on Fe-Cr alloy particles during plasma spraying

Oxidation reactions during plasma spraying of metallic powders give rise to oxide crusts on powder particle surfaces. The first oxidation stage occurs in flight of molten particles. It is usually followed by the second stage after hitting a substrate. To investigate the oxidation products immediately after the first stage, abrupt stopping of in-flight oxidation is possible by trapping and quenching the flying particles in liquid nitrogen. In oxide crusts on plasma sprayed and liquid nitrogen quenched particles of a Fe-12%Cr alloy, two spinel oxides were indicated by Mössbauer spectroscopy and X-ray diffraction. Both are solid solutions of the type Fe₃O₄ - Cr₃O₄ (i.e., Fe_3?x Cr _x O₄, 0 ≤x ≤ 3). One of the oxides, tetragonally distorted spinel, is characterized by the mean value ofx ≈ 2.3. It is only stable at very high temperatures. The other spinel oxide is cubic withx slightly lower than 2, i.e. almost stoichiometric chromite FeCr₂O₄. From thermodynamic considerations it follows that in the Fe₃O₄ - Cr₃O₄ system there is no miscibility gap at high temperatures. The simultaneous existence of both oxides is probably due to non-equilibrium conditions during liquid nitrogen quenching of trapped particles.