Palladium Nanoparticle-Enabled Ultrathick Tribofilm with Unique Composition

There is a consensus that savings of 1.0-1.4% of a country's gross domestic product may be achieved through lubrication R&D. Recent studies have shown great potential for using surface-functionalized nanoparticles (NPs) as lubricant additives to enhance lubricating performance. NPs were reported with ability of producing a low-friction antiwear tribofilm, usually 20-200 nm in thickness, on the contact surface. In contrast, this study reports an unexpected 10 times thicker (2-3 μm) tribofilm formed by dodecanethiol-modified palladium NPs (core size: 2-4 nm) in boundary lubrication of a steel-cast iron contact. Adding 0.5-1.0 wt % such NPs to a lubricating oil resulted in significant reductions in friction and wear by up to 40 and 97%, respectively. Further investigation suggested that the PdNP core primarily was responsible for the improvement in both friction and wear, whereas the thiolate ligand only contributed to the wear protection but had little impact on the friction behavior. In addition, unlike most previously reported tribofilms that contain a substantial amount of metal oxides, this PdNP-induced tribofilm is clearly dominated by Pd/S compounds, as revealed by nanostructural examination and chemical analysis. Such a ultrathick tribofilm with unique composition is believed to be responsible for the superior lubricating behavior.