SYNTHESIS AND STABILITY OF NITRIDE LAYERS IN IRON BY MEGAELECTRONVOLT NITROGEN IMPLANTATION

Stoichiometric iron nitride layers were fabricated by high-dose, high-energy nitrogen implantation into iron using a two-step implantation process. First, a nitrogen pre-implantation to form nitride precipitates is required. Pre-implantations were performed at 500 keV and at T almost-equal-to 100-degrees-C to restrict the nitrogen mobility. Subsequently, 1 MeV implantation at around 300-degrees-C leads to the formation of a stoichiometric-gamma'-Fe4N layer at the position of the pre-implanted nitrogen atoms. Growth of this nitride layer proceeds by diffusion of the implanted nitrogen through the alpha-Fe matrix. During annealing above 350-degrees-C the gamma'-layers dissolve in a planar fashion, exhibiting an Arrhenius behaviour with an activation energy of 2.3 +/- 0.1 eV. The occurrence of phases after pre-implantation, subsequent annealing, or hot implantation can be understood from a thermodynamical model for the Fe-N system. The experiments and model suggest that gamma' is not a thermodynamically stable phase below 310 +/- 10-degrees-C and should decompose into alpha (ferrite) and epsilon-Fe2N1-x.