Oxygen-induced nickel segregation in nitrogen plasma implanted AISI 304 stainless steel

Austenite stainless steel is widely used commercially due to its superior corrosion resistance. Plasma surface treatment has been shown to improve the wear resistance of the materials without degrading the corrosion resistance. Plasma immersion ion implantation (PIII) is a special form of plasma treatment in which the ion energy can be adjusted easily and its non-line-of-sight characteristic makes it suitable for large industrial components possessing an irregular geometry. We observe nickel segregation beneath the top surface in nitrogen plasma immersion ion implanted AISI 304 stainless steel. The amount of segregated nickel and the location depend on the implantation conditions. The phenomenon can be attributed to oxygen-induced surface segregation despite the use of high-purity (99.999%) nitrogen in our experiments. The Auger results indicate that the sample surface has been unexpectedly oxidized in spite of a very small amount of oxygen in the residual vacuum. This is due to the non-UHV (ultra-high vacuum) nature of PIII instruments and the reactive plasma environment. It is believed that the movement of the nickel atoms away from the surface is due to the higher affinity of oxygen to Cr or Fe than Ni. Our investigation also shows that the phenomenon is not related to nitrogen incorporation. As the properties of the treated sample depend on many factors, nickel segregation must be considered in designing PIII experiments.