Tribochemistry of fluorinated ZnO nanoparticles and ZDDP lubricated interface and implications for enhanced anti-wear performance at boundary lubricated contacts

Abstract

Surface-capped zinc oxide (ZnO) nanoparticles were engineered to deliver beneficial anti-wear and anti-friction chemistries at sliding interfaces with a focus on potentially reducing environmentally hazardous zinc dialkyl dithiophosphate (ZDDP) additive in automotive lubricants, without compromising tribological performance. A plasma polymerization technique was employed to coat ZnO nanoparticles, initially with fluorine rich polymer films to enhance the formation of surface protective tribofilms, followed by methacrylate-based coatings to reduce agglomeration and, simultaneously, induce stable dispersion of nanoparticles in the base oils. Formulation with an additive mixture of fluorinated ZnO and ZDDP (350 ppm phosphorus level) exhibited a reduction in the coefficient of friction and significantly improved wear performance. The improved tribological performance is attributed to the synergistic interaction between fluorinated ZnO and ZDDP and to the physical and chemical properties of the formed tribofilms. In-situ electrical contact resistance data highlighted the role of fluorinated ZnO nano-additives in reducing the incubation time for stable tribofilm formation. AFM, SEM, XANES, and XPS results revealed that oils with fluorinated ZnO form relatively thick, patchy tribofilms with hierarchical chemical structure enriched with zinc polyphosphates, coupled with minor amounts of metal fluorides, zinc, and iron sulfates and sulfides, further improving wear resistance under low concentration of ZDDP.