Characterization of a magnetron sputtered γ″-FeN coating by electron microscopies

We were interested in characterizing films of cubic γ″-FeN not only by X-ray diffraction, but also by scanning electron microscopy and transmission electron microscopy. Samples were fabricated by dc magnetron sputtering of an iron target in N2 atmosphere at high pressure. The X-ray diffraction technique indicated that films were composed of a single phase, i.e. the γ″-FeN phase. Scanning electron microscopy and transmission electron microscopy showed the presence of a strong 〈111〉 texture in the coatings. Film microstructure consisted of triangular cross-section rods perpendicular to the substrate. These rods contain {111} twin defects. We also frequently observed supplementary diffraction spots on electron diffraction patterns of some rods. The intensity of these spots is much less than those of the γ″-FeN phase. These weak spots also lie at half the distance between two consecutive spots of the γ″-FeN phase. These two facts suggest that these additional spots correspond to a superstructure of the γ″-FeN. This new phase is face-centered cubic and its lattice parameter is twice the one of γ″-FeN, i.e. a=0.864 nm. Darkfield transmission electron microscopy experiments performed with both fundamental and superstructure spots indicate that the new phase is located on the surface of the rods and form nanodomains.