Chemical bonding of nitrogen in low energy high flux implanted austenitic stainless steel

AISI 304L austenitic stainless steel was implanted at 400 degreesC with 1.2 keV nitrogen ions using a high beam current density of 1 mA/cm(2). The nitrogen depth profile, structure, and chemical composition in the modified surface layer were determined by nuclear reaction analysis (NRA) and x-ray diffraction (XRD). The chemical bonding of Fe, Cr atoms with nitrogen was investigated by x-ray photoelectron spectroscopy (XPS). For a treatment time of 1 h, the formation of a thick nitrided layer of about 3.5 mum with a high nitrogen content (similar to20 at. %) is observed by NRA. The nitrogen depth profile is characterized by a nearly flat shape over a thickness of 2.5 mum followed by an abrupt decrease. XRD spectra show the formation in the nitrided layer of a phase usually called expanded austenite gamma(N), which corresponds fairly well with a nitrogen solid solution of the fcc structure containing a high density of stacking faults. The XPS study of the Cr 2p(3/2), Fe 2p(3/2), and N 1s binding states indicate clearly the preferential bonding of chromium with nitrogen with a binding energy of about 1 eV. This value, which is lower than the expected one for chromium nitride CrN, would be characteristic of the binding energy of nitrogen with Cr in the gamma(N) expanded austenite phase. Moreover, it has been found that the atomic ratio N/Cr in the nitride layer deduced from both NRA and XPS is very close to 1. These experimental results support the specific role of chromium in the mechanisms of atomic transport of nitrogen over long distances at moderate temperature in austenitic stainless steels. (C) 2002 American Institute of Physics.