Tribological properties of sulfur- and phosphorus-free organic molybdenum compound as additive in oil

A sulfur- and phosphorus-free organo-molybdenum (SPFM) was successfully synthesized. The friction-reducing and anti-wear properties of SPFM combined with zinc dialkyldithiophosphate (ZDDP) in base oil were assessed, revealing SPFM additive efficiently reduces the friction coefficient and exhibits good synergistic anti-wear relationship with ZDDP. The smoother wear scar surface further supports the improvement in the anti-wear behavior and synergy effect between SPFM and ZDDP. Energy dispersive x-ray spectroscopy unravels the existence of Mo and S in the tribofilms formed on the rubbing surface, and the formation of MoS2 crystalline is verified by x-ray photoelectron spectroscopy and high-resolution transmission electron microcopy. The mechanism promoting the in situ formation of MoS2 is proposed via the synergistic chemical reaction between SPFM and ZDDP during rubbing.