The initial oxidation of ε-Fe2N1−x: an XPS investigation

The initial oxidation of α-Fe and ε-Fe2N1−x, subjected either to a sputter cleaning pretreatment or a sputter cleaning plus additional annealing pretreatment, was investigated with XPS. The samples were oxidised at pO2=8·10−5 Pa and temperatures ranging from 300 to 600 K. From the Fe 2p and O 1s spectra the thickness and composition of the oxide film was determined. The composition of the oxide films formed on α-Fe and on ε-Fe2N1−x was only a function of oxidation temperature and film thickness and was independent of the composition or pretreatment of the substrate. Analysis of the N 1s spectra provided information on the electric charge on nitrogen atoms and the depth distribution of nitrogen. A linear relation was found between the N 1s electron binding energy and the nitrogen concentration in the substrate. Upon oxidation of the iron nitride, nitrogen atoms accumulated underneath the oxide film. If the nitrogen concentration at that location exceeded the maximum solubility of nitrogen in ε-Fe2N1−x an additional N peak appeared in the N 1s spectrum, which indicated the formation of a nitrogen containing phase other than ε-Fe2N1−x at the nitride–oxide interface. For the oxygen exposures applied, the oxide-film thickness decreased with increasing nitrogen concentration in the substrate. The effect of nitrogen in the substrate on the initial oxidation was evaluated from the results.