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.

► Adopting a two step method as nitriding plus sulfurizing duplex treatment to get a composite surface layer of nitride and sulfide on the steel parts may be an effective and useful method for further modifying the tribological properties of machine parts. The tribological behaviors of nitrided sample and sulfurized-nitrided sample were tested under base oil lubrication, and following results were drawn:
► (1) A thin FeS layer with a thickness of about 100 nm was formed on the AISI 4135 steel surface using a two-step method as nitriding plus sulfurizing duplex treatment.
► (2) Comparing with the nitrided sample, the sulfurized-nitrided sample displayed much better tribological behaviors under oil lubrication, including load carrying capacity, wear resistance, friction reduction and duration time of tribotest.
► (3) The effect of thin FeS film formed on the sulfurized-nitrided surface can persist a longer time in wear tests is due to its removal and reformation under the local high contact stress and friction heat. In other words, the FeS can be decomposed into Fe and S atoms, the activated S atoms react with the Fe atoms from the substrate, the new FeS chemical reaction film can be reformed again.