Oxygen-Defect-Mediated Nanoconfined Water Dynamics Govern Nonmonotonic Friction in MoS₂

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, ${\mathrm{MoS}}_2$ 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 ${\mathrm{MoS}}_2$ / ${\mathrm{MoS}}_2$ 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 $H_{2}O$ , and the sliding-induced rupture of strong $O_d$ - $H_{2}O$ 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.