INFLUENCE OF PROCESS PARAMETERS ON THE NITRIDING OF STEELS BY PLASMA IMMERSION ION-IMPLANTATION

Nitriding of steels by Plasma Immersion Ion Implantation (PI3) allows access to a large process parameter space. Although the parameters ass ociated with the high-energy ion bombardment (implantation energy, hig h-voltage pulse length and frequency, ion current density and time-ave raged dose rate) are important, the treatment temperature and plasma p arameters such as ion density, excited neutral density and plasma pote ntial also play a vital role. Previous investigations have been hamper ed by the use of the high-energy ion bombardment to heat the workpiece . In this paper, we present the results of a study in which the treatm ent temperature and the ion bombardment were decoupled by radiatively heating. The effect of varying high-voltage pulse length, repetition r ate, total implanted dose, plasma density and potential on the nitroge n uptake during PI3 treatment depends strongly on whether nitrides are formed in the surface (e.g. Ck45 mild steel) or nitrogen is incorpora ted in solid solution (e.g. X6CrNiTi 1810 austenitic stainless steel). In the first case, nitride formation can be suppressed by increasing the high-voltage pulse frequency or can be enhanced by treating at a h igh pressure or plasma potential. In the second case, the thickness of the modified layer can be increased by increasing the ion current den sity or time-averaged dose rate. In both cases, nitrogen uptake by dir ect thermochemical absorption from the plasma is significant. (C) 1998 Elsevier Science S.A.