Synergistic effects between sulfur- and phosphorus-free organic molybdenums and ZDDP as lubricating additives in PAO 6

In this work, we prepared two environmentally friendly sulfur- and phosphorus-free organic molybdenums, MODE and MGMO, which show excellent wear resistance and improve the tribological properties of PAO 6 by combining with traditional zinc dialkyldithiophosphate (ZDDP). The synthesized additives show good wear resistance with the decreasing WSD by 35.3% and 38.7% for MODE and MGMO, respectively. However, they exhibit no friction-reducing property but increase the COF. Furthermore, MODE and ZDDP have excellent synergistic effects on both friction-reducing and antiwear, decreasing the COF and WSD by 31.6% and 39.3%, respectively, which is attributed to the composite tribofilm composed of MoS2, MoO3, molybdate, sulfide and sulfate generated during the friction process. Our results indicate that the S- and P-free organic molybdenums with prominent tribological performance could be used as environmentally friendly lubricating materials to totally/partially replace ZDDP.