Synergy between UiO-66-NH2 and a Hierarchical Aluminum Reservoir for Durable, Self-Replenishing Dry Lubrication

Friction and wear limit the reliability and energy efficiency of mechanical systems, motivating solid lubricants for conditions where liquid lubricants fail. Metal-organic frameworks (MOFs) are attractive owing to their tunable structures and ability to form protective tribofilms, but their low-yield synthesis demands strategies that maximize performance at minimal loading. Herein, we developed a synergistic dry-lubrication strategy in which a hierarchical structure (HS) acts as an integrated reservoir for UiO-66-NH2. Drop-cast MOF particles were anchored within the interconnected concavities of an etched aluminum (Al 5052) substrate bearing a micro/nano hierarchical surface. The anchored UiO-66-NH2 was continuously supplied to the contact interface as a rolling third body, infilling worn regions. Post-test SEM-EDS detected Zr across the wear track, suggesting that the MOF was compacted into a protective MOF-derived layer. The amino-functionalized UiO-66-NH2 showed slightly better wear resistance than carboxyl-functionalized UiO-66-(COOH)2, and performance was further optimized by tuning the MOF concentration (7 mg/mL) and HCl etching time (2 min). Using only 0.56 mg/cm2 of MOF, the surface reduced wear-track width and maximum wear depth by 46.6% and 62.6%, respectively, relative to bare Al sliding against a stainless-steel pin under 1 N. The friction reduction was mainly provided by the HS, whereas UiO-66-NH2 primarily suppressed wear. This durable, self-replenishing approach offers a scalable route to practical MOF-based tribological surfaces.