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

Transport phenomena in an original nitriding process are studied. The proce ss 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 nitroge n atom consumption in the gas phase on the one hand by a three-body reactio n and on the other hand on the surfaces. The assumption is made that the at omic nitrogen flux into the rods can be neglected as compared to that to th e surface. Calculations are performed by changing the loss probability of n itrogen atoms on the iron surfaces. It is be established that the previous assumption is valid. Furthermore, it appears that the consumption of nitrog en 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 laye r. This mass transfer boundary layer has a thickness which can be calculate d by the model, taking into account the gas-phase consumption of N atoms. N itrogen transport sufficiently far from the substrates is not drastically m odified by the change in the loss probability at the wall. This result sugg ests that the nitriding of metallic substrates can be carried out using thi s new process even on large surfaces. Treatment of hollow substrates was pe rformed with this process, and led to very satisfying results. (C) 1999 Els evier Science S.A. All rights reserved.