Tailoring Oil Solubility of 2D Cu(2,6-NDC) Nanosheets via the Solvent Strategy for Synergy-Driven Lubrication in Castor Oil

Two-dimensional metal-organic frameworks (2D MOFs), particularly those with layered architectures, have emerged as promising candidates for advanced lubrication additives. In this work, we developed a solvent-tuned size strategy to synthesize the Cu(2,6-naphthalenedicarboxylate) (2D Cu(2,6-NDC)) nanosheets with the dimensions of 30-45 nm in thickness, 50-200 nm in width, and 600-1000 nm in length. The characterizations confirmed the structure and composition of the 2D Cu(2,6-NDC) nanosheets. Remarkably, the 2D Cu(2,6-NDC) nanosheets exhibited exceptional dispersion stability in castor oil without sedimentation for over 7 days. The tribological evaluation employing a four-ball tester revealed that the 0.08 wt % 2D Cu(2,6-NDC)-castor oil achieved the superior lubrication performance, with a 42.7% reduction in average friction coefficient (ACOF) and a 58.9% decrease in wear scar diameter (AWSD) compared to pure castor oil. The analysis of worn surfaces identified a synergistic lubrication mechanism, which was primarily attributed to the low interlaminar shear and adsorption of the abundant hydroxyl group on the surface of 2D Cu(2,6-NDC) nanosheets.