Tribological properties of sulfurized-nitrided layer prepared by a two-step method

The tribological properties of nitrided layer and sulfurized-nitrided layer of AISI 4135 steel were investigated under oil lubrication, and the layers were prepared by the ion nitriding treatment and a two-step method as the ion nitriding plus sulfurizing duplex treatment, respectively. A ball-on-disc friction and wear tester was adopted to evaluate the tribological performance. Scanning electron microscope (SEM), scanning Auger microprobe (SAM) and X-ray photoelectron spectroscope (XPS) were used to identify the morphologies and chemical compositions of the treated layer and the worn surface. It was presented that the sulfurized-nitrided sample with a thin FeS layer possessed much better tribological behaviors than the nitrided sample, including load carrying capacity, wear resistance, friction reduction and duration time. The mechanism was supposed that the decomposed activated S atoms of FeS layer promoted a new FeS chemical reaction film formation, which induced to the thin FeS layer playing as a solid lubricant for a longer time.