Influence of Sputtering and Redeposition on Compound Layer Growth during Plasma Nitriding

Abstract

Apparently anomalous surface compound layer growth is observed during plasma nitriding, and this has previously been ascribed to the removal of surface material by sputtering. Previous work has produced results which clearly indicated that during plasma nitriding non-uniform sputtering of material occurs on the workpiece and that this gives rise to a net loss of material from high sputtering sites such as edges and corners and to a net redeposition of material on flat faces. In the present work, the γ’–Fe4N compound layers on the flat faces of pure iron workpieces were found to display anomalous growth behaviour, in which the compound layer initially increases and subsequently decreases in thickness. This observation cannot be explained by the removal of layer material by sputtering. The nature of the redeposition of material was examined and it was found that there is a time delay associated with its onset. In addition, examination of the redeposited material by quantitative scanning Auger microprobe depth profiling revealed that oxygen accumulation occurs in the redeposited material. The anomalous growth behaviour of the compound layer can be ascribed to the formation of the redeposited material layer which inhibits further nitriding of the substrate.