Thermodynamic stability of iron nitrides at temperatures below 350 °C

Although iron nitride coatings are applied for their good tribological properties, their thermodynamic stabilities are unknown in the temperature range in which they are used. We have investigated the ɛ and γ’ nitride phases between 0 and 350 °C with ion implantation and find evidence that while the ɛ-${\mathit{Fe}}_2$ ${\mathrm{N}}_{1\mathrm{-}\mathit{x}}$ with x≥2/3 is stable, γ’-${\mathrm{Fe}}_4$N is not. At T≤150 °C, ion irradiation converts γ-${\mathrm{Fe}}_4$N into ɛ-${\mathrm{Fe}}_2$ ${\mathrm{N}}_{1\mathrm{-}\mathit{x}}$, with x=0.5. This ɛ nitride can then be transformed back into γ’-${\mathrm{Fe}}_4$N by annealing at temperatures of 150 °C and higher. Ion irradiation assists this process as well.