Reactive in-situ formation and self-assembly of MoS2 nanoflakes in carbon tribofilms for low friction

Abstract

Modern lubricants require additives for improving their frictional and wear performance. The most effective and widely used additives rely on organo-metallic compounds, which lead to ash formation and pose serious environmental concerns. Despite intensive research, a cost-effective alternative cannot be foreseen in the immediate future.On the quest for an alternative concept, the reactive formation and self-assembly of few-layer MoS2 nanoflakes in a carbon-based tribofilm is studied during reciprocating sliding contact of molybdenum substrates lubricated with oils containing sulfurized olefin extreme-pressure (EP) additive. Based on a combination of Raman spectroscopy and transmission electron microscopy it can be concluded that nanoflakes of well-adherent 002-oriented MoS2 layers form in the presence of S-containing EP additive. This leads to a reduction in friction from 0.3 to 0.08. The reaction rate to form MoS2 nanoflakes increases with temperature and EP concentration. At temperatures over 100 °C, the MoS2 nanoflakes are accompanied by carbon-based tribofilms. These carbon-based tribofilms are catalytically formed by dissociating hydrocarbon molecules of the lubricant.These results suggest that applying Mo alloyed materials with sulfur containing lubricants provides an alternative to conventional organo-metallic compounds. The presented lubrication concept can be utilized for further developments of materials (like protective hard coatings) and machine designs.