The effects of nitrogen additions to a cobalt-chromium surgical implant alloy

The determination of the feasibility of adding nitrogen to a cobalt-chromium implant alloy was undertaken with the ultimate goal of the work being the improvement of the static and fatigue properties of the alloy. Nitrogen additions were made using high-temperature heat treatments in a nitrogen-containing gas atmosphere. The effects of the nitrogen additions were characterized in this study using several techniques. The maximum solid solubility of nitrogen in the alloy at 1200° C (the heat-treatment temperature) was found to be approximately 0.35 wt % N. X-ray diffraction using nitrogen heat-treated powder samples indicated that the addition of nitrogen in solution resulted in a lattice dilation lying in the range of 0.0021 to 0.0035 nm per wt % N. Above the solubility limit, Cr₂N and Cr₂(CN) were present at the nitrogen heat-treatment temperatures in the form of large second-phase particles. Ageing of the alloy containing approximately 0.35 wt % N at 400° C resulted in the precipitation of CrN. A study of the nitrogen distribution suggested that the diffusion of nitrogen ms affected by the carbon content of the cobalt-base alloy.