Oxygen‐Defect‐Mediated Nanoconfined Water Dynamics Govern Nonmonotonic Friction in MoS ₂

Understanding how environmental molecules dissipate energy at solid‐lubricant interfaces is pivotal for reliable friction control under ambient conditions. As a benchmark two‐dimensional solid lubricant, exhibits excellent lubricity in dry or vacuum conditions, yet its friction rises rapidly in humid environments, and the underlying atomistic dissipation mechanism remains under debate. Here, large‐scale ab initio molecular dynamics simulations are used to capture the sliding dynamics of humid / interfaces and resolve the atomistic evolution of interfacial hydrogen bonds (H‐bonds), yielding friction coefficients consistent with experimental magnitudes. Oxygen defects are identified as preferential trapping sites for , and the sliding‐induced rupture of strong ‐ H‐bonds emerges as a dominant dissipation pathway underlying friction increase. A nonmonotonic dependence of friction on interfacial water coverage is uncovered, originating from the competition between the increasing H‐bond density and the concurrent softening of the nanoconfined water network at higher coverage, in contrast to the cooperative strengthening in bulk water. Systematic ball‐on‐plate friction experiments corroborate the predicted oxygen‐dependent friction rise and lubricity recovery at high humidity, validating the proposed framework. A predictive link is thereby established between quantum‐scale interfacial bonding dynamics and macroscopic tribological response, providing design guidance for environmentally robust 2D solid‐lubricant interfaces.