Plasma immersion ion implantation: duplex layers from a single process

Plasma immersion ion implantation (PI³) is an alternative non-line-of-sight technique for implanting ions directly from a plasma which surrounds the component to be treated. In contrast to plasma source ion implantation, the PI³ system uses an inductively coupled r.f. plasma. It is shown that nitrogen can be retained during implantation at elevated temperatures, even for unalloyed steels. This allows controlled diffusion of nitrogen to greater depths, thereby improving the load bearing capacity of the implanted layer. Components can be heated directly, using the energy deposited by the incident ions during the pulsed implantation. The necessary temperature control can be accomplished simply by regulating the frequency and length of the high voltage pulses applied to the component. Chemical depth profiles and microstructural data obtained from H13 tool steel are used to show that PI³ can, in a single process, effectively produce a duplex subsurface structure. This structure consists of an outer non-equilibrium layer typical of nitrogen implantation (containing in excess of 20 at. percent nitrogen) backed by a substantial diffusion zone of much lower nitrogen content. The relationship between implantation temperature and the resultant subsurface microstructure is explored