Study of transport phenomena by 3D modeling of a microwave post-discharge nitriding reactor

Transport phenomena in an original nitriding process are studied. The process is based on the use of Ar-N-2-H-2 post-discharges. Iron rods are located downstream from the plasma in a large-volume (0.1 m(3)) reactor. In order to control the nitriding process, a 3D model of this reactor is developed. The mass, momentum and heat-balance equations are solved, including nitrogen atom consumption in the gas phase on the one hand by a three-body reaction and on the other hand on the surfaces. The assumption is made that the atomic nitrogen flux into the rods can be neglected as compared to that to the surface. Calculations are performed by changing the loss probability of nitrogen atoms on the iron surfaces. It is be established that the previous assumption is valid. Furthermore, it appears that the consumption of nitrogen atoms on the surfaces of the rods affects the transport of this species only in a zone around the substrates, i.e. in a mass transfer boundary layer. This mass transfer boundary layer has a thickness which can be calculated by the model, taking into account the gas-phase consumption of N atoms. Nitrogen transport sufficiently far from the substrates is not drastically modified by the change in the loss probability at the wall. This result suggests that the nitriding of metallic substrates can be carried out using this new process even on large surfaces. Treatment of hollow substrates was performed with this process, and led to very satisfying results. (C) 1999 Elsevier Science S.A. All rights reserved.