The constitution and crystallography of thin thermal oxide layers on epsilon-Fesub2Nsub1-x: a HREM investigation

Oxide layers formed on epsilon-Fe2N1-x were investigated with X-ray photoelectron spectroscopy, X-ray diffraction, and in particular with high-resolution transmission electron microscopy. Prior to oxidation, the epsilon-Fe2N1-x substrates were either exposed to air at room temperature, or subjected to a sputter-cleaning pretreatment, or to a sputter cleaning with an additional annealing pretreatment. The samples were oxidized at 400 or 573 K in pure O-2 at 8.0 x 10(-2) Pa or at 10(5) Pa. All oxide films contained magnetite (Fe3O4) as a major constituent. On samples that were sputter-cleaned and annealed prior to oxidation as well as on the air-exposed sample, wustite, (Fe1-deltaO) was observed between the Fe3O4 and epsilon-Fe2N1-x. On the basis of HREM, this wustite phase, which on alpha-Fe does nor develop at temperatures below 843 K, was concluded to have a preferred crystallographic orientation with respect to the supporting epsilon-Fe2N1-x grains. No specific orientation relation was found between Fe3O4 and epsilon-Fe2N1-x. The implications of the development of Fe1-deltaO for the evolution of the oxide film are discussed.