Growth and Microstructure of Iron Nitride Layers and Pore Formation in ϵ-Fe3N

Abstract Layers of ϵ-Fe3N and γ′-Fe4N on ferrite were produced by nitriding iron single crystals or rolled sheets of iron in flowing ammonia at 520°C. The nitride layers were characterised using X-ray diffraction, light microscopy as well as scanning and transmission electron microscopy. The compound layer consists of ϵ-Fe3N at the surface and of γ′-Fe4N facing the ferrite. After 4 h of nitriding, pores develop in the near surface region of ϵ-Fe3N showing more or less open porosity. Growth of the entire compound layer as well as of the massive and the porous ϵ-Fe3N sublayer is diffusion-controlled and follows a parabolic growth rate. The γ′-Fe4N layer is formed as a transition phase within a narrow interval of nitrogen activity, and it shows little growth in thickness. The transformation of γ′-Fe4N to ϵ-Fe3N is topotactic, where the orientation of the closed-packed iron layers of the crystal structures is preserved. Determination of lattice plane spacings was possible by X-ray diffraction, and this was co...