Fabrication of Two-Dimensional Functional Covalent Organic Frameworks via the Thiol-Ene “Click” Reaction as Lubricant Additives for Antiwear and Friction Reduction

To address the energy wastage problem caused by friction, novel lubricant additives other than the traditional and basic used additives with outstanding performance are urgently needed. A facile and efficient postsynthetic strategy for modification of two-dimensional (2D) covalent organic frameworks (COFs) was proposed to obtain dialkyl dithiophosphate (DDP)-functionalized COFs (DDP@TD-COF) as lubricant additives. The DDP@TD-COF was prepared by amine-aldehyde condensation reaction of the triazine compound and vinyl-functionalized monomers through a solvothermal process to form a vinyl-functionalized 2D COF (TD-COF), followed by covalent bonding of commercial lubricating molecules (DDP) via the UV-induced thiol-ene "click" reaction. The as-obtained DDP@TD-COF with homogeneous distribution of N, P, and S elements exhibits exceptional dispersion stability in the 500SN base oil, which remains stable for over 6 days. With a trace amount addition of 0.05 wt %, superior friction and wear reduction of DDP@TD-COF are observed with the friction coefficient lessened to 0.096 from 0.19, wear volume loss declined by 94.9%, and load carrying ability increased from 150 to 650 N simultaneously. The mechanism studies show that the shear force can induce interlayer slipping during the friction process, and the stripped DDP@TD-COF can get involved in the contacting interface inducing tribo-chemical reactions via N, P, and S elements forming a protective layer on the surfaces. Consequently, the DDP@TD-COF demonstrated remarkable friction diminution and abrasion resistance abilities even with a trace amount addition, and this work provides a dependable and valid route for the design and preparation of functional COF-based nanoadditives.