Nucleation of iron nitrides during gaseous nitriding of iron; Effect of a preoxidation treatment

The nucleation of iron nitrides during gaseous nitriding has been investiga ted using light microscopy and X-ray diffraction. Initially, the nucleation of gamma ' -Fe4N1-x on a pure iron surface starts at grain boundaries meet ing the surface, from where the nitride grains grow laterally into the iron grains. On prolonged nitriding, immediate nucleation at the surface of iro n grains becomes possible. Calculated incubation times for the nucleation o f gamma ' -Fe4N1-x during nitriding are generally longer than those observe d experimentally in the present work. The incubation time is reduced dramat ically after oxidation of the iron surface prior to nitriding. Furthermore, a more uniform distribution of iron-nitride nuclei over the surface is obt ained after preoxidation. The enhanced nucleation of iron nitrides on oxidi sed iron is discussed in terms of the kinetics of the surface reactions, an d the development of a metastable precursor for nitride formation.