Surface and Coatings Technology 158–159 (2002) 288–294 0257-8972/02/$ - see front matter  2002 Elsevier Science B.V. All rights reserved. PII:S0257-8972 Ž 02 . 00185-8 Thermal stability of the high-N solid-solution layer on stainless steel Orhan Ozturk *, D.L. Williamson a, b ¨ ¨ Department of Physics, Izmir Institute of Technology, Izmir 35487, Turkey a Physics Department, Colorado School of Mines, Golden, CO 80401, USA b Abstract Low-energy, high-flux N ion implantation into austenitic stainless steel held at approximately 400 8C results in dramatic improvements in the tribological properties due to sufficiently large N layer thicknesses and high-N-content solid solution phase, g . In this paper, post-ion beam processing via isothermal annealing of a low-energy (0.7 keV), high-flux (2.5 mAycm ) N 2 N implanted fcc 304 stainless steel held at 400 8C has been investigated by Mossbauer spectroscopy and X-ray diffraction (XRD). ¨ Post-implantation annealing at 400 8C demonstrated the metastability and showed that the magnetic g produced at lower ion N energies and higher fluxes transformed systematically to a paramagnetic g phase with less N content and less lattice expansion, N thereby destabilizing the magnetic state of g . The isothermal annealing results in much thicker g layers but with less N in N N 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 Ds2=10 cm ys at 400 8C, consistent with a model which explains that the trapping by Cr atoms y13 2 in the stainless steel becomes more effective when N contents are low relative to the Cr concentration (;19at.%in304stainless steel).  2002 Elsevier Science B.V. All rights reserved. Keywords: Diffusion; Nitrogen implantation; Ferromagnetism; Stainless steel; Mossbauer spectroscopy; X-Ray diffraction ¨ 1. Introduction Low-energy (F2 keV), high-flux (a few mAycm ) 2 nitrogen ion implantation into 304 stainless steel (SS) at 400 8C is known to generate a metastable, fcc, interstitial, high-N content phase (g ) in the implanted N layer with both magnetic and paramagnetic characteris- tics w1–5x. Under such conditions, the g layers are N found to have N layer depths several microns thick and N contents approaching 30 at.%. Such layers have technological importance due to their high strength (microhardness as high as 20 GPa) and good corrosion resistance w3x. One important characteristic of the g phase is its N metastability, which has been investigated under high- and low-energy N beam conditions, low-temperature plasma nitriding conditions, as well as under thermal annealing studies (post-ion and post-plasma nitriding processing). The metastability has been established based on annealing experiments, which show the g N *Corresponding author. Tel. q90-932-498-7513; fax: q90-232- 498-7509. E-mail address: oozturk@likya.iyte.edu.tr (O. Ozturk). ¨ ¨ phase gradually decomposes upon annealing at 400 8C w6x.Ithasalsobeeninvestigatedinthepresenceoflow- energy N ion beam processing conditions at higher substrate temperatures (above approx. 450 8C) which leads to a phase separated mixture of bcc-FeNi and fcc- CrN w2x. Lower implantation temperatures (;220–350 8C) result in hexagonal nitride, ´-(Fe,Cr,Ni) N with 2qx both paramagnetic and magnetic characteristics w6,7x. The metastability of the g phase has also been N investigated both in the presence and absence of low- temperature plasma nitriding conditions. One such study w8x, a systematic investigation of the thermal stability ofthe g phaseasafunctionofbothtemperature (200– N 600 8C) and time (20–3000h),foundthatthe g phase N is thermodynamically metastable due to its extremely high solubility of nitrogen (N content reported in the plasma nitrided state was ;22 at.%, which is actually quite low in comparison to that found with our low- energy, high-flux N ion beam processing conditions, ;30 at.%). The g phase stability needs further investigation N since under the low-energy N implantation conditions the top surface of fcc-SS becomes magnetic due to very