Effect of austenitic stainless steel composition on low-energy, high-flux, nitrogen ion beam processing

A collection of 16 metals with the face-centered-cubic (fcc) crystal structure, including stainless steels, Fe-Ni alloys and pure Nil have been subjected to the same nitrogen ion beam processing conditions to examine the role of alloy composition in the surface modification behavior. A low-energy (700 eV), high-flux (2 mA cm(-2)) beam of ions was used with each sample held at 400 degrees C during a 15 min treatment. The near surface regions have been characterized by conventional and grazing-incidence X-ray diffraction, Auger electron spectroscopy, conversion electron Mossbauer spectroscopy, and microhardness measurements. There is a clear distinction in the modifications depending on whether the alloys are Fe-rich or Ni-rich. Fe-rich samples all yield relatively thick (2.5-3.5 mu m) layers with high N content in solid solution. The large lattice expansions lead to ferromagnetism in these surfaces. A novel double-layer structure has been induced in all the Fe-rich alloys, corresponding to two rather well-defined N contents: high (20-26 at%) in the surface layer, and medium (4-10 at%) in the subsurface layer. It is suggested that this substructure is caused by stress-assisted diffusion. The Ni-rich alloys have much thinner N-containing layers (less than or equal to 1 mu m) and a much lower amount of N retained in the (111) planes oriented parallel to the surface compared to those in the Fe-rich alloys. (C) 1998 Elsevier Science S.A.