Thermal stability of the high-N solid-solution layer on stainless steel

Low-energy, high-flux N ion implantation into austenitic stainless steel held at approximately 400 °C results in dramatic improvements in the tribological properties due to sufficiently large N layer thicknesses and high-N-content solid solution phase, γN. In this paper, post-ion beam processing via isothermal annealing of a low-energy (0.7 keV), high-flux (2.5 mA/cm2) N implanted fcc 304 stainless steel held at 400 °C has been investigated by Mössbauer spectroscopy and X-ray diffraction (XRD). Post-implantation annealing at 400 °C demonstrated the metastability and showed that the magnetic γN produced at lower ion energies and higher fluxes transformed systematically to a paramagnetic γN phase with less N content and less lattice expansion, thereby destabilizing the magnetic state of γN. The isothermal annealing results in much thicker γN layers but with less N in solid solution due to the N diffusion into the substrate. Based on the XRD data, the N diffusivity under isothermal annealing conditions is found to be D=2×10−13 cm2/s at 400 °C, consistent with a model which explains that the trapping by Cr atoms in the stainless steel becomes more effective when N contents are low relative to the Cr concentration (∼19 at.% in 304 stainless steel).