Nucleation of iron nitrides during gaseous nitriding of iron; the effect of a preoxidation treatment

The nucleation of iron nitrides during gaseous nitriding has been investigated using light microscopy and X-ray diffraction. Initially, the nucleation of gamma'-Fe4N1-x on a pure iron surface starts at grain boundaries meeting the surface, from where the nitride grains grow laterally into the iron grains. On prolonged nitriding, immediate nucleation at the surface of iron grains becomes possible. Calculated incubation times for the nucleation of gamma'-Fe4N1-x during nitriding are generally longer than those observed experimentally in the present work. The incubation time is reduced dramatically after oxidation of the iron surface prior to nitriding. Furthermore, a more uniform distribution of iron-nitride nuclei over the surface is obtained after preoxidation. The enhanced nucleation of iron nitrides on oxidised iron is discussed in terms of the kinetics of the surface reactions, and the development of a metastable precursor for nitride formation.