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 degreesC results in dramatic improvements in the tribological properties due to sufficiently large N layer thicknesses and high-N-content solid solution phase, gamma(N). In this paper, post-ion beam processing via isothermal annealing of a low-energy (0.7 keV), high-flux (2.5 mA/cm(2)) N implanted fee 304 stainless steel held at 400 degreesC has been investigated by Mossbauer spectroscopy and X-ray diffraction (XRD). Post-implantation annealing at 400 degreesC demonstrated the metastability and showed that the magnetic gamma(N), produced at lower ion energies and higher fluxes transformed systematically to a paramagnetic gamma(N) phase with less N content and less lattice expansion, thereby destabilizing the magnetic state of gamma(N). The isothermal annealing results in much thicker gamma(N), 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 x 10(-13) cm(2)/s at 400 degreesC, 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 (similar to 19 at.% in 304 stainless steel). (C) 2002 Elsevier Science B.V. All rights reserved.