EVOLUTION OF THE SURFACE CONCENTRATION DURING POSTDISCHARGE NITRIDING

During microwave post-discharge nitriding, the surface concentration q uickly evolves until it reaches the equilibrium value with the surroun ding atmosphere. When the surface concentration reaches the value corr esponding to the equilibrium with gamma' and then epsilon, the formati on of these nitrides is observed on the surface. In this work, a model has been developed to follow the evolution of the surface concentrati on during post-discharge nitriding. The model considers the dissociati on kinetics of the N-2 molecule to solve the mass transport equations into the solid. Experimental results were incorporated into the model to establish a lower limit for the nucleation conditions of the gamma' nitride phase. Preliminary results of the model indicate fast dissoci ation kinetics of the N-2 molecule, which results in a quick saturatio n of atomic nitrogen at the surface during post-discharge nitriding. ( C) 1997 Elsevier Science S.A.