ION-BEAM SYNTHESIS OF NITRIDE LAYERS IN IRON

Stoichiometric iron nitride layers have been synthesized by high dose, high energy nitrogen implantation into Fe using a two-step implantation process. First, a nitrogen preimplantation at -100-degrees-C is used to form nitride precipitates. A low temperature is necessary to restrict the nitrogen mobility. Second, 1 MeV implantation at approximately 300-degrees-C leads to the formation of a stoichiometric gamma'-Fe4N layer at the position of the preimplanted N atoms. Growth of this nitride layer proceeds by diffusion of the implanted N through either the alpha-Fe matrix (for 200 or 500 keV preimplantations) or the nitride layer itself (for 1 MeV preimplantation). During annealing above 350-degrees-C the gamma' layers dissolve in a planar fashion, characterized by an activation energy of 2.3 eV. Phase formation during preimplantation and phase transformations during subsequent annealing or hot implantation can be understood from the thermodynamics for the Fe-N system, while the kinetics of layer growth is influenced by the beam-induced defects. The experiment and model suggest that gamma' is not a thermodynamically stable phase below 310 +/- 10-degrees-C and should decompose into alpha (ferrite) and epsilon nitride.