On the kinetics of the initial oxidation of iron and iron nitride

The initial oxidation of alfa-Fe and epsilon Fe2N1-x was investigated. Prior to oxidation the sample surfaces were either sputter cleaned with Ar+ ions or sputter cleaned followed by annealing. It was shown that the sputter cleaning pretreatment of epsilon Fe2N1-x led to a reduction of the N concentration in the surface region; subsequent annealing at 573 K led to restoration of the N concentration. Oxidation in pure O2 (generally pO2 1•10 4 Pa) was performed at temperatures ranging from 300 K to 600 K. The oxidised samples were investigated with X-ray photoelectron spectroscopy (XPS), ellipsometry and high-resolution transmission electron microscopy (HREM). The oxidation kinetics, as determined with ellipsometry, were described theoretically with the model due to Fromhold and Cook, adopting time-dependent work functions of the metal-oxide and oxide-oxygen interfaces. The calculated evolution of the work functions of alfa-Fe could be related to the change of the oxide film composition, as determined with XPS: it evolved with increasing film thickness from approximately FeO to a composition close to Fe3O4. Upon oxidation of epsilon Fe2N1-x, the nitrogen atoms accumulated underneath the oxide film. For sputter-cleaned + annealed epsilon Fe2N1-x this lead to a nitrogen concentration larger than the maximum solubility of nitrogen in epsilon Fe2N1-x. The excess nitrogen at the metal-oxide interface, which is negatively charged as was indicated by the corresponding XPS N 1s peak, lead to a lower initial oxidation rate of sputter cleaned + annealed epsilon Fe2N1-x than that of alfa-Fe and sputter cleaned epsilon Fe2N1-x. Upon prolonged oxidation the oxidation rate of sputter cleaned + annealed epsilon Fe2N1-x was found to exceed the oxidation rate of sputter cleaned epsilon Fe2N1-x. This was related to the crystallographic constitution of the oxide, as determined with HREM. It was shown that the oxide film on sputter cleaned epsilon Fe2N1-x consisted of only Fe3O4, whereas the oxide film on sputter cleaned + annealed epsilon Fe2N1-x additionally contained Fe1-dO.