Synergistic Interactions between Silver and Palladium Nanoparticles in Lubrication

The reactivity and functionality of nanoparticles (NPs) are determined by the material- and size-dependent physicochemical properties at nanoscale. While single-type metallic NPs have been extensively studied for many potential applications, combination of multitype NPs and the interactions among them have received less attention. Here, an interesting synergistic effect between silver and palladium NPs is presented for using them together as lubricant additives. Dodecanethiol-modified Ag and Pd NPs were discovered to interact actively in a solution, changing their sizes, shapes, and electronic structures. A mixture of these two NPs was able to form a stable suspension in a lubricating oil and effectively improve tribological performance in the concentration range of 0.1-1 wt %. The Ag-Pd NP blend further reduced the friction and wear by >30% and >80%, respectively, compared with either NP alone at the same total NP concentration. The NPs and worn surfaces were characterized using a set of comprehensive analytical tools and the superior lubricating behavior was attributed to the formation of two types of protective tribofilms on the contact surfaces.