HREM investigation of the constitution and the crystallography of thin thermal oxide layers on iron

Oxide layers formed at 573 K in O2 at atmospheric pressure, both on a clean iron surface and on an iron surface covered with an etching induced (hydro)oxide film, were investigated with high-resolution transmission electron microscopy (HREM). Cross-sections of oxidised samples were prepared by a specially developed technique, consisting of, consecutively, jet electropolishing, oxidation and ion milling.

Oxidation of clean iron surfaces yielded an oxide layer of uniform thickness with Fe3O4 adjacent to a-Fe and a-Fe2O3 on top of Fe3O4. A Bain-type orientation relationship for magnetite/ferrite and a Shoji-Nishiyama-type orientation relationship for hematite/magnetite were observed. At several locations cracks close to and parallel to the interface with the substrate were observed within Fe3O4.

Oxidation of iron samples that were covered with an (hydro)oxide film prior to oxidation yielded thin, nanocrystalline oxide layers, mainly composed of a-Fe2O3, of variable thickness. The oxide grains of these oxide layers did not exhibit a specific orientation relationship with the underlying iron grains.