Effect of applied pulse voltage on nitrogen plasma immersion ion implantation of AISI 316 austenitic stainless steel

Plasma immersion ion implantation (PIII) of nitrogen was performed for 4 h on AISI 316 austenitic stainless steel substrates at a temperature of 400 °C. The substrate bias was varied between −1 and −20 kV with different duty cycles so as to keep the substrate temperature fixed. The implanted ions were evident to the depths of a few microns, which is much greater than their implantation depths, resulting in an enhancement of surface microhardness. A load dependent surface microhardness is observed which is nearly independent of the applied bias. Glancing angle X-ray diffraction patterns indicate the formation of expanded austenite at all biases. X-ray photoelectron spectroscopy indicates the formation of CrN in the first 175 Å of the surface for high bias. The results indicate that if implantation accompanies diffusion, then PIII at low bias is suitable enough for nitrogen incorporation in materials. Lower voltages also mean greater conformity between the ion sheath and the sample surface, and hence better treatment uniformity.