Influence of reduced current density on diffusion and phase formation during PIII nitriding of austenitic stainless steel and CoCr alloys

The formation of expanded austenite in austenitic stainless steel and CoCr alloys is a well established fact while the underlying mechanisms are still contended. When inserting nitrogen by plasma immersion ion implantation (PIII), additional auxiliary heating allows continuous increase of the average current density by either incrementing the voltage or the pulse repetition frequency. For the nitrogen content and layer thickness, monotonously increasing values are observed, indicating an additive process for the nitrogen uptake from low energy plasma ions adsorbing on the surface and high energy ions implanted below the surface. A supply limited diffusion is apparently occurring throughout the whole ion flux range as no saturation is observed at the processing temperature of 400 °C. A much stronger influence of the ion fluence on the surface concentration in stainless steel than in CoCr alloys is observed. On the other hand, while the lattice expansion scales directly with the surface nitrogen content, a non-monotonous influence of the ion flux on the detailed peak shape, respective stress and stress gradients is observed.