Nitrogen transport mechanisms in low temperature ion nitriding

The aim of the study was to find whether the basic phenomena that occur during the glow discharge assisted nitriding of metal differ from those occurring during gaseous nitriding. To define the specificity of a glow discharge assistance a simple model system such as an iron cathode nitrided in d.c. glow discharge in nitrogen at a low temperature was examined. The Fe4N layer of thickness 0.8 mum and 1.5 mum was produced in 5 h at 275 degreesC and 3 h at 350 degreesC, respectively. The results were compared with experimental data of gaseous nitriding and computer aided simulations based on the Lehrer (NH3-H-2)(1.5)(Fe-N) equilibrium diagram. The thickness of the Fe4N layer produced in our experiment was greater than the thickness of the Fe4N layer produced in the gaseous process observed experimentally and as the estimated values from the model based on the equilibrium diagram, in spite of the fact that the total number of N atoms in the gaseous process was 40 divided by 80 times greater than in the glow discharge assisted process. Based on the relation DeltaN = k root Dt we proposed that the difference between the gaseous nitriding process and the glow discharge assisted nitriding process results from the difference in the values of k, which in turn depend on the N level at the diffusion front. The higher N level can be explained in terms of a shallow ion implantation. (C) 2001 Elsevier Science B.V. All rights reserved.