Structural transition from ordered molecular layers to disordered arrangement triggers nonmonotonic friction at n-hexadecane-graphite interfaces.

Friction is generally understood to increase with the normal load according to classical tribological theory; however, a few nanoscale studies have reported deviations from this well-known behavior. Here, we report nonmonotonic friction behavior at the n-hexadecane-graphite interface, characterized by a transition from negative to positive load dependence in single-asperity atomic force microscopy (AFM) measurements. Combined analyses of experiments and molecular dynamics (MD) simulations attribute this unusual trend to load-induced structural evolution of the confined hexadecane ordered layers. At low loads, load-induced molecular rearrangement of confined hexadecane layers suppresses friction by reducing the effective pathways for shear transmission and interfacial energy dissipation, resulting in a negative friction coefficient (NFC). Further loading progressively leads to the collapse of molecular layering which promotes direct tip-substrate contact, resulting in enhanced interfacial shear strength and increased energy dissipation. These findings demonstrate that the dynamic stability of confined molecular ordering controls friction at nanoscale, thereby opening new opportunities for the development of nanoscale energy-efficient lubrication systems.