When Is Structural Lubricity Load Independent? The Role of Contact Geometry and Elastic Compliance

Abstract

Using molecular dynamics simulations of an incommensurate Au(111)/graphite interface, we investigate the conditions under which structural lubricity produces load-independent friction. We show that strict load independence occurs only in laterally infinite, area-filling contacts, where dissipation is governed by phonon-mediated viscous coupling and the shear stress scales linearly with sliding velocity. Finite contacts with explicit boundary terminations exhibit substantially higher friction yet remain load independent up to a critical load. Load dependence arises only when elastic out-of-plane deformation near the contact line exceeds a critical amplitude, activating additional dissipation channels. These results demonstrate that contact geometry and local elastic compliance, rather than normal load itself, determine the onset and breakdown of load-independent structural lubricity.

Figures (4)

• Figure 1: Simulation models of (a) the area-filling and (b) contact-line geometries. The graphite substrate has in-plane dimensions of 11.5$\times$11.6 nm$^2$ with periodic boundary conditions applied laterally. The topmost layer of the Au(111) slab is treated as rigid, with the normal load $F_n$ applied uniformly, and the slab is driven at a constant velocity $v_x$. The remaining gold atoms and the top graphene layer evolve freely without positional constraints.
• Figure 2: Spatial power spectral density $S(k)$ of the lateral force for the area-filling (AF) and contact-line (CL) geometries. In both cases, the Au(111) slab slides along the graphene armchair direction at a velocity of 10 m/s under a normal load of 0.1 MPa.
• Figure 3: Interfacial shear stress $\tau$ as a function of sliding velocity $v$ for the area-filling (AF) and contact-line (CL) geometries at a normal load of 0.1 MPa. The AF interface exhibits a linear viscous response, $\tau=\eta_{\rm vis}v$ with $\eta_{\rm vis}=0.365$ kPa$\cdotp$s/m. In contrast, the CL geometry follows the composite form $\tau(v)=\eta_{\rm vis}v+\eta_{\rm def}v^c$, with $\eta_{\rm vis}=0.090$ kPa$\cdotp$s/m, $\eta_{\rm def}=5.03$ kPa$\cdotp$(s/m)$^c$, and $c=0.61$.
• Figure 4: Interfacial shear stress $\tau$ as a function of (a) normal load $F_n$ and (b) out-of-plane deformation amplitude $\Delta z$ of the top graphene layer for the area-filling (AF) and contact-line (CL) geometries. Solid and open triangle symbols denote the low-load ($F_n<$100 MPa) and high-load ($F_n\geq$100 MPa) regimes, respectively. The dashed curve in (b) represents a fit to the CL data using Eq. \ref{['eqn:stress_dz']}, yielding $\beta=3.4$. Error bars smaller than the symbol size are omitted.