Investigation on the lubrication advantages of MoS₂ nanosheets compared with ZDDP using block-on-ring tests

The purpose of this work was to expand the applications of molybdenum disulfide (MoS₂) nanosheet additives by investigating their effectiveness and advantages for lubricating line contacts between bearing steel and medium carbon steel surfaces. The friction and wear effects of MoS₂ nanosheet additives were investigated using block-on-ring tests, and compared with those from zinc dialkyldithiphosphates (ZDDP). Both additives were combined with ISO VG 32 white oil as base fluid. Results suggest 0.25 wt% MoS₂ nanosheets exhibited similar friction coefficient, oil temperature and wear scar width with those of ZDDP at low loads; while the reductions of friction coefficient and wear scar width were 28.6% and 34.3%, respectively, compared to the behavior of ZDDP at high load (459 N). Furthermore, the wear scar widths of the ZDDP containing oil significantly increased in 2 h tests, while they remained constant with a smaller size for 0.25 wt% additives of MoS₂ nanosheets at a load of 326 N. Scanning electron microscope observations showed some pits on the wear scar lubricated with ZDDP, compared with spalls for 0.06 wt% MoS₂ nanosheets. The wear scar was smooth and had a patched film for tests using 0.25 wt% MoS₂ nanosheets. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analysis revealed that the patched surface film was composed of MoS₂ and phosphate, and that film reduced friction coefficient and wear. The results highlight how high concentration MoS₂ nanosheets exhibit significant advantages over ZDDP in reducing friction and wear in high load in line contact.