Tribological behaviors and mechanism of sulfur- and phosphorus-free organic molybdate ester with zinc dialkyldithiophosphate

An oil-soluble sulfur- and phosphorus-free organic molybdate ester (ME) was synthesized. The antiwear and friction-reducing properties of ME with zinc dialkyldithiophosphate (ZDDP) in base oils were evaluated by four-ball tester. The results show that ME addition effectively reduced wear scar diameter (WSD) and friction coefficient (μ) as well as good antiwear synergism with ZDDP. The topography, composition and chemical states of typical elements on the worn scar were analyzed by scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) and X-ray photoelectron spectrometer (XPS). Smooth and light topography of worn scar further confirms the good antiwear synergism of ME with ZDDP. EDX and XPS analyses indicate that tribo-chemically boundary films formed on the rubbing surface consist of metal oxides, sulfides and phosphates, leading to enhancement of the antiwear and friction-reducing properties of the lubricants, and that the considerable MoS2 layer especially plays an important role in improving antiwear and friction-reducing properties of oils. The proposed antiwear mechanism involves a synergy between ME and ZDDP.