NITROGEN MASS TRANSFER DURING PLASMA NITRIDING

摘要

In the present paper, attempts have been made to analyse the mass transfer problem occurring during plasma nitriding using the classic thermodynamic approach, with the aim to specify the surface boundary conditions during the process. Experiments involving weight gain measurements and nitrogen concentration profile measurements, as well as numerical evaluation of the obtained data have been carried out to evaluate the nitrogen mass transfer rate during various plasma nitriding processes. Theoretical analysis reveals that nitrogen mass transfer during plasma nitriding is determined by the forward reactions and backward reactions. The forward reactions involve the interactions of various plasma species including nitrogen carrying ions and neutrals with the specimen surface; whilst the backward reactions involve sputtering and denitriding. Experiments showed that during plasma nitriding in nitrogen and hydrogen mixtures, the forward reactions proceed much faster than the backward reactions such that the contribution of the latter to the overall nitrogen mass transfer can be negligible at least at the early stage of nitriding, i.e. Bright Nitriding. Accordingly, nitrogen mass transfer during the early stage of plasma nitriding before compound layer formation can be approximated by a constant nitrogen flux supplied from the plasma to the component surface.