Electrically Tunable Tribological Behavior of Hierarchical Flower-Like HF-COF Nanocontainers.

An ever-growing demand for high-performance lubricant additives has been raised, and covalent organic frameworks (COFs) are regarded as smart nanocontainers due to their abundant active sites and ordered porous structures. However, poor dispersibility in base oils and inadequate film-forming performance under harsh working conditions are still observed for these materials. In the present work, a novel synthetic route for the controllable fabrication of hierarchical flower-like covalent organic framework (HF-COF) nanocontainers was proposed. The prepared HF-COF containing 1-(4-fluorophenyl)-1,3-butanedione (HF-COF-D) used as a lubrication modifier was accordingly constructed. The optimal hierarchical flower-like morphology of HF-COF was obtained under the catalytic condition of 2.4 M acetic acid, as verified by morphological characterizations. Tribological measurements showed that the best friction-reducing and antiwear performances were realized when 1 wt % HF-COF nanocontainers were dispersed into polyalphaolefin (PAO) base oil. The friction coefficient was stably reduced from approximately 0.10 of neat PAO to about 0.085, and an 86.7% decrease in wear volume was recorded simultaneously. Current-carrying tribological tests were further conducted, and a prominent electrically triggered antiwear behavior was identified for the composite system at a current of 0.5 A, where the wear volume was minimized to 318.1 μm3. The superior lubrication performance was primarily explained by the synergistic action of electric field excitation and mechanical shear. Friction-regulating diketone molecules contained in the composite were in situ released at the friction interface, and a robust organic-inorganic composite tribochemical film enriched with Fe-F, Fe-O, and organic species was synchronously formed on the worn surface. A novel insight into the structural design and mechanistic investigation of high-performance current-carrying lubricant additives is provided by this work.