Oxygen-induced frictional evolution and saturation mechanisms in hydrogenated a-C/PEEK interfaces

Reactive molecular dynamics simulations were performed to investigate the tribological behavior of hydrogenated a-C/PEEK interfaces under different oxygen concentrations. The results show that the friction force increases with oxygen concentration and saturates beyond ~300 O₂ molecules. This behavior arises from two coupled mechanisms. First, sliding shear drives oxygen diffusion into the PEEK structure. At high concentrations, oxygen penetration and subsequent saturation diminish the intrinsic lubricity of PEEK. Second, excess oxygen accumulates at the interface as an oxygen-rich layer, which interacts with surface hydrogen on a-C and further enhances interfacial adhesion. In addition, dissociated O atoms preferentially bond with aromatic carbons, ether oxygens, and surface hydrogens of PEEK, disrupting π-conjugation and introducing polar functional groups that increase adhesion. Surface hydrogen, instead of acting as an inert barrier, reduces lubricity through chemical interactions at both low and high oxygen concentrations. These findings provide mechanistic insights into the oxygen-induced increase and eventual stabilization of friction force in a-C/PEEK systems.