Successful nitriding of austenitic stainless steel: On the diffusion mechanism of nitrogen and on the role of the surface oxide layer.

In the first part of this paper we present a quantitative trapping model that includes detrapping to describe the diffusion of nitrogen in austenitic stainless steel during successful nitriding. Calculated nitrogen depth profiles, assuming a diffusion pre-exponential factor D0 = 10-3 cm2/s, a diffusion activation energy Ed = 1.1 eV, and a detrapping activation energy Et2d = 1.45 eV, show good agreement with experimental nitrogen depth profiles
obtained from a sample that has been subsequently nitrided with 14N and 15N. The plausibility of a trapping mechanism is also supported by a number of phenomenological and thermodynamical arguments. In the second part of this paper we present a modelling approach for the evolution of the thickness of the surface oxide layer during successful nitriding. The approach bases on the assumption that the surface oxide layer thickness is completely controlled by the interplay of sputtering and oxidation and differentiates between oxide layer growth limited by diffusion and oxide layer growth limited by the rate of oxygen supply. The applicability of this approach is supported by experimental data obtained by real-time elastic recoil detection analysis during ion nitriding at different combinations of ion energy and oxygen partial pressure.