Surface processes and diffusion mechanisms of ion nitriding of stainless steel and aluminium

Model experiments with low-energy ions under controlled vacuum conditions have been performed in order to identify the surface processes and diffusion mechanism associated with nitrogen transport during ion nitriding of stainless steel 316 and pure aluminium. A necessary condition for efficient nitriding is the transmission of the implanted ions through a surface oxide layer, which results from a balance of ion sputtering and re-oxidation from the residual gas. For ion energies of approximately 1 keV and ion current densities of approximately 0.2 mA/cm2, oxygen partial pressures of less than approximately 3×10−6 and 3×10−7 mbar are required for stainless steel and aluminium, respectively. Diffusion under the influence of traps controls the transport of nitrogen in stainless steel, with dynamic trapping at the Cr atoms of the alloy. In contrast, stoichiometric AlN grows on aluminium, due to Al diffusion from the underlying bulk. From the surface and diffusion mechanisms, limitations of plasma nitriding and plasma immersion ion nitriding are derived, and the implications for industrial applications are discussed.