Tribological mechanisms of the synergistic effect between sulfur- and phosphorus-free organic molybdenum and ZDDP

In this work, three environmentally friendly sulfur- and phosphorus-free organic molybdenums, MODE (amide type), MGMO (ester type) and MGME (ether type) with different structures were prepared, and the relationship between molecular structure and tribological properties was investigated. Furthermore, the tribological properties and mechanisms after compounding with conventional zinc dialkyldithiophosphate (ZDDP) were also investigated, indicating that the synergistic effect on antiwear and antifriction properties has the following order: MODE+ZDDP > MGMO+ZDDP > MGME+ZDDP. We further explored the tribological mechanisms through the in-depth surface and cross-sectional analyses, which manifests that the formation of homogeneous and compact tribofilm composed of molybdenum disulfide, molybdenum oxide, sulfate, phosphate, and polyphosphate with the thickness of 50-200 nm is contributed to the excellent synergistic effect.