Surface characteristics and tribological behavior of 3D-printed 316 L steel after plasma assisted low temperature carburizing

Conventional carburizing treatments of steels are usually carried out above 550 °C thus they are not suitable for austenitic stainless steels because surface hardening is achieved to detriment of corrosion resistance. To overcome this drawback different carburizing treatments at lower temperature have been developed and among them one of the most efficient is a plasma assisted process at 475 °C. With respect to austenitic stainless steels produced through traditional processes additive manufacturing allows to obtain mechanical parts of complex shape with enhanced strength and without loss of ductility. However, for certain applications the components still suffer of limited hardness and wear resistance. In this work the plasma assisted process at 475 °C has been employed to treat the 316 L steel manufactured by Laser Powder Bed Fusion (L-PBF) with the aim of improving its tribological behavior. The treatment time was 7 h and gas mixtures (CH4 + H2) with different amounts of CH4 have been used. The samples were then submitted to wear tests in pin-on-flat (POF) mode at room temperature with applied loads of 10 N and 20 N. The surface characteristics were examined by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), Raman spectroscopy (RS), glow discharge optical emission spectroscopy (Rf-GDOES), and X-ray photoelectron spectroscopy (XPS). The results show that plasma treatments induce the formation of a ~ 25 μm thick layer of expanded austenite (S-phase) that is covered by a diamond like carbon (DLC) over-layer of ~2 μm. Such over-layer exhibits a complex sub-structure: the inner part consists of amorphous C (sp2 bonds) with a degree of topological disorder depending on the gas mixture whereas in the external part both sp2 (graphite-like) and sp3 (diamond-like) bonds are present with relative amounts changing with the distance from the surface. Independently on the gas mixture used in plasma treatment, all the samples exhibit a wear resistance much better than that of the untreated material. Moreover, wear resistance also depends on plasma treatment conditions: the greater the CH4 content, the lower the wear resistance. The results have been discussed by considering the intrinsic DLC brittleness that fractures and detaches from the metal surface during the tests thus hard debris take part to the wear process acting as abrasive particles.