Low temperature plasma carburizing of additive manufactured 316 L steel: the effect of treatment time on tribological behavior

A low temperature (-475 degrees C) plasma assisted carburizing treatment has been employed to improve the tribological behavior of 316 L steel manufactured by Laser Powder Bed Fusion (L-PBF). The treatments have been carried out in a carburizing atmosphere consisting of 2.5 % CH4 + 97.5 % H2 with increasing treatment time, from 30 to 120 min. For all the treatments, the result is the formation of a layer of expanded austenite (S-phase), responsible for surface hardening. If the treatment time is longer than 90 min, an overlayer of Diamond-Like Carbon (DLC) has been observed on the surface. Wear tests in pin-on-flat (POF) mode at room temperature with applied loads of 10 N and 20 N have been employed to identify the treatment conditions leading to the optimal wear resistance. The wear resistance of all the samples resulted to be greater than the one of the not treated material, but it is negatively affected by the formation of the DLC overlayer. In fact, due to its hardness and brittleness, it detaches from the surface during the wear tests and contributes to increasing the wear rate. The best tradeoff is a 90 min treatment leading to the formation of a -25 mu m thick S-phase layer without the deposition of DLC. In this treatment condition, the wear rate is about 20 times lower than the one of the not treated 316L steel produced by L-PBF and tested in the same conditions.