Nitrogen diffusion in medical CoCrNiW alloys after plasma immersion ion implantation

CoCr alloys are widely used for medical applications, e.g. total hip replacements or coronary stents. Nevertheless, an increase in the surface hardness and a reduction of the wear rate is still desirable to improve the biocompatibility. Plasma immersion ion implantation (PIII) at different temperatures, acceleration voltages and working pressures is used to determine the nitrogen diffusivity in the CoCr alloys SY21med, L605 and HS188. Depending on the temperature, two different treatment regimes can be distinguished, independent of the ion energy. At low temperatures, a diffusion process with an activation energy of 1.0–1.1 eV is present, indicative of interstitial nitrogen diffusion. Beyond 350 °C, a reduced activation energy of 0.5–0.7 eV is observed. Additionally, a strong dependency of the layer thickness on the working pressure in the range 0.3–0.8 Pa was observed for the temperature dependent diffusion regime, which suggests a synergistic interplay of adsorption and implantation during PIII. Below 500 °C, an increase of the diffusion coefficient by three orders of magnitude was observed for PIII, compared with pure plasma nitriding.