Structural and magnetic characterization of nitrogen ion implanted stainless steel and CoCrMo alloys

Abstract

Ion beam surface modification methods can be used to create hard and wear resistant surface layers with enhanced corrosion resistance on austenitic stainless steels (SS) and CoCr base alloys using nitrogen ions. This is mainly due to the formation of high N content phase, γN, at relatively low substrate temperatures from about 350 to 450 ºC. This surface layer is known as an expanded austenite layer. Different N contents and diffusion rates depending on grain orientations as well as anisotropic lattice expansion and high residual stresses are some peculiar properties associated with the formation of this phase. Another peculiar feature of the expanded austenite phase is related to its magnetic character. In this study, new data related to the magnetic nature of the expanded austenite layers on austenitic stainless steel (304 SS) and CoCrMo alloy by nitrogen plasma immersion ion implantation (PIII) are presented. Magnetic behaviour, nitrogen distribution, implanted layer phases, surface topography, and surface hardness were studied with a combination of experimental techniques involving magnetic force microscopy, SIMS, XRD, SEM, AFM and nanoindentation method. The experimental analyses indicate that the low temperature samples clearly show the formation of the expanded austenite phase, while the decomposition of this metastable phase into CrN precipitates occurs at higher temperatures. As a function of the processing temperature, phase evolution stage for both alloys follows the same trend: (1) initial stage of the expanded phase, γN, formation; (2) its full development, and (3) its decomposition into CrN precipitates and the Cr-depleted matrix, fcc γ-(Co,Mo) for CoCrMo and bcc α-(Fe,Ni) for 304 SS. MFM imaging reveals distinct, stripe-like ferromagnetic domains for the fully developed expanded austenite layers both on 304 SS and CoCrMo alloys. Weak domain structures are observed for the CoCrMo samples treated at low and high processing temperatures. The images also provide strong evidence for grain orientation dependence of magnetic properties. The ferromagnetic state for the γN phase observed here is mainly linked to large lattice expansions due to high N content.