Load-independent structural lubricity of graphite controlled by contact geometry and elastic compliance

Using molecular dynamics simulations of incommensurate Au(111)/graphite interfaces, we investigate how contact geometry and the out-of-plane compliance of graphite control the robustness of structural lubricity under normal loading. In laterally periodic, area-filling contacts, the shear stress remains load independent over four orders of magnitude in pressure and increases linearly with sliding velocity, consistent with phonon-mediated interfacial damping. Introducing a finite contact termination substantially increases friction through an additional boundary-mediated contribution, while load independence persists at low and moderate pressures. At higher pressures, substantial out-of-plane deformation develops near the contact termination, coincident with the emergence of load-dependent friction. Variations in graphite thickness, contact width, substrate length, and sliding direction show that the quantitative friction and deformation depend on finite-contact geometry and elastic boundary conditions, while the qualitative high-pressure friction enhancement persists. Similar deformation amplitudes can nevertheless coexist with different friction levels, demonstrating that the global deformation amplitude alone does not uniquely determine friction. Together, these results associate the breakdown of load independence with the boundary-mediated elastic response, without evidence for a substantial pressure-induced reduction of the lateral lattice mismatch, and reveal how in-plane structural mismatch and out-of-plane elastic compliance jointly determine the robustness of structural lubricity in finite graphitic contacts.