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Palladium Nanoparticle-enabled Ultrathick Tribofilm with Unique Composition

Publication Type
Journal Name
ACS Applied Materials & Interfaces
Publication Date
Page Numbers
31804 to 31812

Oil-suspendable palladium and silver nanoparticles (NPs) were synthesized by dodecanethiol surface modification. Characterization revealed the particle sizes in the ranges of 2-3.5 and 3-6 nm and the organic contents of 35% and 30% for the Pd and Ag NPs, respectively. Addition of 0.5-1.0 wt% of either Pd or Ag NPs into a lubricating base oil formed a stable emulsion and demonstrated significant friction (by 25-40%) and wear (by 90-97%) reductions in boundary lubrication of a steel-cast iron contact. Particularly, the Pd NPs clearly outperformed the Ag NPs with additional 10-15% friction reduction and 20-50% wear reduction. The roles of the metallic NP core and the organic ligand in lubrication were revealed: the thiolate ligand provides wear protection to some extent but had little impact on friction, and the metallic NP core is primarily responsible for the friction and wear reductions but it relied on the ligand to get dispersed in the oil to reach the contact interface. The superior lubricating performance of the Pd NPs is attributed to the ultra-thick tribofilm (2-3 m vs. 20-200 nm commonly seen) with unique composition (dominated by Pd-S with little oxides vs. metal oxides rich commonly seen) as discovered by the surface morphology examination and composition analysis from both the top surface and in cross-section