Study of tribological properties of bis(dimethyl sulfoxide) - tri(N,N-dimethyldithiocarbamate) rare earth in lubricating grease©

Four complexes of bis(dimethyl sulfoxide) - tri(N,N-dimethyldithiocarbamate) rare earth (MDTC-Ln-DMSO) were synthesized and their thermal stabilities were investigated with a thermal gravity analyzer (TGA). Friction and wear behaviors of Ute complexes were evaluated as additives in lithium grease using a four-ball machine and a Timken Tester. As a comparison, the tribiological properties of molybdenum dialkyldithiocarbamate (MoDTC) as an additive in lithium grease were also investigated. Electron Probe Microanalyzer (EPMA), Auger Electron Spectroscopy (AES) end X-ray Photoelectron Spectroecopy (XPS) were employed to elucidate the lubricating mechanism of these rare earth complexes at additives in lithium grease. Friction and wear tests indicated that these rare earth complexes possess excellent load-carrying capacity and good antwear property at concentrations up to 3 wt%. Their extreme-pressure properties were much superior to that of MoDTC at concentrations from 1 wt% to 5 wt%., but the antiwear properties were inferior to that of MoDTC. Results of EPMA analyses revealed the existence of rare earth metals and sulfur on the rubbed surfaces, whereas the results of AES and XPS analyses indicated that these rare earth complexes had formed a protective film containing a rare earth oxide, surface and organic compounds containing sulfur and nitrogen on rubbed surfaces. The lubricating film rich in rare earth and sulfur especially seem to contribute to the good tribological properties of bis(dimethyl sulfoxide) - tri(N,N-dimethyldithiocarbamate) rare earth (MDTC-Ln-DMSO).