Tribological properties and structure of anodic nitrided layer on AISI 304 austenitic stainless steel

In order to avoid the inherent shortcomings of conventional direct current plasma nitriding such as damage to parts caused by arcing and the edge effect, AISI 304 austenitic stainless steels were anodic nitrided by means of the active screen plasma nitriding. The nitriding experiments were carried out at 440-520℃ for 8 h under anodic potential. The surface morphology, microstructure and phases of the nitrided layer were studied, and tribological properties of the nitrided layer were investigated. The results show that the nitrided layer (S-phase) is formed on the AISI304 austenitic stainless steel surface after nitriding at 440℃. XRD and TEM results show that the surface hardening layer is almost composed of the face-centered cubic structure of S-phase without the CrN precipitated phase. With increasing nitriding temperature to 520℃, the microhardness values reach 1100 HV0.1, which is about 5 times of that of the untreated substrate, while the CrN phase is clearly detected. Under the dry friction, the friction coefficient of the steel after different nitriding treatment is about 0.8, compared to the original sample, the coefficient decreases by 0.2, and the wear rate is only the original one-tenth order of magnitude. The wear mechanisms change from the severe adhesive and abrasive wear for the untreated steel to the slight oxidative and abrasive wear for the treated steel.