The influence of treatment time on plasma nitrocarburised compound layers for pure iron

Plasma nitrocarburising was carried out in a CO gas atmosphere with 3 mbar at 570degreesC for various times to investigate the effect of treatment time on the structure of the compound layer. The compound layers produced were composed of mainly the gamma' phase for a slow cooled specimen and mixed gamma'/epsilon phases for a fast cooled specimen during the initial stage. With the increasing of the treatment time up to 10 hours, the proportion of die E phase relative to the gamma' phase in the compound layer and the compound layer thickness were increased. When die treatment time was further increased to 30 hours, an anomalous phenomenon was observed: the epsilon phase was not stabilised and the compound layer thickness was decreased. It was found that the anomalous compound layer growth is associated with the removal of surface material by sputtering or the formation of oxynitride.