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.