In situ tribochemical sulfurization of molybdenum oxide nanotubes

MoS₂ nanoparticles are typically obtained by high temperature sulfurization of organic and inorganic precursors under a S rich atmosphere and have excellent friction reduction properties. We present a novel approach for making the sulfurization unnecessary for MoO₃ nanotubes during the synthesis process for friction and wear reduction applications while simultaneously achieving a superb tribological performance. To this end, we report the first in situ sulfurization of MoO₃ nanotubes during sliding contact in the presence of sulfur-containing lubricant additives. The sulfurization leads to the tribo-chemical formation of a MoS₂-rich low-friction tribofilm as verified using Raman spectroscopy and can be achieved both during sliding contact and under extreme pressure conditions. Under sliding contact conditions, MoO₃ nanotubes in synergy with sulfurized olefin polysulfide and pre-formed zinc dialkyl dithiophosphate tribofilms achieve an excellent friction performance. Under these conditions, the tribochemical sulfurization of MoO₃ nanotubes leads to a similar coefficient of friction to the one obtained using a model nanolubricant containing MoS₂ nanotubes. Under extreme pressure conditions, the in situ sulfurization of MoO₃ nanotubes using sulfurized olefin polysulfide results in a superb load carrying capacity capable of outperforming MoS₂ nanotubes. The reason is that while MoO₃ nanotubes are able to continuously sulfurize during sliding contact conditions, MoS₂ nanotubes progressively degrade by oxidation thus losing lubricity.