Confinement-limited hydration and tribofilm continuity in water-contamination-tolerant lubrication by 2D COFs

Water intrusion into organic base fluids frequently induces boundary lubrication failure. To address this challenge, two-dimensional covalent organic frameworks (2D COFs) were investigated as water-tolerant nano-additives. Two COFs with distinct linkages were compared: the rigid imine-linked TFB-DB and the flexible β -ketoenamine-linked TFP-ODA. A combined experimental and multiscale simulation strategy was employed to correlate nanoscale interfacial dynamics with mesoscale tribofilm morphology. Although both COFs exhibited similar macroscopic wettability, their lubrication behaviors differed significantly. Multiscale simulations revealed a confinement-limited geometric saturation effect under extreme local contact pressures. Because the confined-water hydrogen-bond network reached geometric saturation, functional-group abundance alone could not proportionally enhance lubrication. Instead, structural adaptability was identified as the critical factor. Computations demonstrated that the flexible TFP-ODA framework provided favorable local water affinity and dynamically stabilized the confined water layer under severe shear, preventing hydration network collapse. Morphological analyses confirmed that this microscale flexibility directly facilitated mesoscale tribofilm continuity. While the rigid TFB-DB produced discontinuous deposits, TFP-ODA formed a continuous, strongly adhered tribofilm that effectively shielded the high-shear metallic substrate. Consequently, TFP-ODA demonstrated exceptional water-contamination tolerance, achieving an optimal friction coefficient of ∼0.15 in a 50% ethylene glycol/water mixture. These results indicate that the boundary lubrication performance of COF additives is governed synergistically by dynamic hydration-layer stabilization and continuous tribofilm formation, rather than by macroscopic wettability or functional-group density alone.