Tentative modelling of nitriding with ion beams, the role of surface effects

Nitriding of austenitic stainless steel under high ion current density (approximate to 1 mA cm(-2)) and at elevated temperatures (350-430 degreesC) leads to the formation of a relatively thick solid solution phase (several microns) with a high nitrogen content (20-30 at.%), which is known to have beneficial effects on the proper-ties of the stainless steel. The model presented in this work is an attempt of a new approach to investigate the fundamental driving mechanisms of anomalous penetration depth of nitrogen into the austenitic stainless steel. It is based on balance equations which include processes of preferential sputtering, surface atoms displacements, creation of defects by irradiation and thermal diffusion. The development of the surface roughness and the formation of an altered layer highly enriched by nitrogen is analyzed and it is concluded that the transport of nitrogen into the bulk could result from a flux of matrix atoms driven by mobile vacancies at elevated temperatures. This behavior is consistent with an altered layer 'growth' that is controlled by the ion beam induced displacements of surface atoms rather by thermal diffusion. (C) 2002 Elsevier Science B.V. All rights reserved.