Friction-reducing mechanisms of molybdenum dithiocarbamate/zinc dithiophosphate combination: New insights in MoS2 genesis

Abstract

The friction-reducing properties of the Modtc/Zndtp combination were studied using a ultrahigh vacuum (UHV) analytical tribometer. The friction tests were performed on a previously formed tribofilm. Transmission electron microscopy observation shows that this tribofilm is composed of a mixture of zinc phosphate zones containing molybdenum and carbon-rich zones containing zinc and ${\mathrm{MoS}}_{\mathrm{2}}$ single sheets. After UHV friction, the wear scars of both the sliding surfaces were characterized by in situ analytical tools like Auger electron spectroscopy, scanning Auger spectroscopy, and microspot x-ray photoelectron spectroscopy. Low friction is associated to the transfer of a thin ${\mathrm{MoS}}_{\mathrm{2}}$ film to the pin. Zinc, phosphorous, and oxygen are localized in the wear debris surrounding the pin wear scar. The friction-reducing phenomena is explained on the basis of the hard and soft acids and bases principle.