Superlubricity refers to a phenomenon where interfacial sliding occurs with ultra-low friction, typically with kinetic friction coefficients (the change in the measured friction force with the applied normal force during sliding) lower than 10^-3. At the van der Waals interface formed between rigid incommensurate crystalline surfaces, superlubric sliding emerges, even in the absence of lubricants, due to effective cancellation of lateral forces. The term structural superlubricity was coined to emphasize the geometric nature of this behavior. Interfacial lattice incommensurability can be achieved by twisting or straining homogeneous crystalline interfaces or by constructing heterogeneous contacts incorporating surfaces of different lattice parameters.

Illustration of an AFM tip sliding over a corrugated graphene grain boundary.

Since its theoretical inception and experimental demonstrations on nano- and micro-scale layered material interfaces (e.g. graphene and h-BN), structural superlubricity has emerged as a promising approach for effectively reducing energy dissipation and wear across various length scales. However, on larger scales, the polycrystalline nature of layered materials, exhibiting a mosaic pattern of randomly oriented grains, separated by grain boundary (GB) dislocation chains, poses challenges to the scaling up of structural superlubricity towards macroscopic applications. While the random grain orientation increases incommensurability (and hence supports superlubric behavior), GBs typically present significant out-of-plane surface deformations (often termed protrusions) that may impede superlubricity and affect wear resistance. Therefore, understanding the impact of GBs on the frictional behavior of layered material contacts is crucial for achieving superlubricity over large layered material interfaces.

The complex friction behavior of the entire mosaic structure is determined by the dynamics of individual GB protrusion and by their cross talk with adjacent counterparts. A recent experimental-computational-theoretical collaborative study between the University of Basel and Tel Aviv University aimed at uncovering and understanding energy dissipation mechanisms of individual GB protrusions.

Shear induced buckling/unbuckling transition of a corrugated graphene GB

The researchers measured the friction force of polycrystalline graphene GBs grown on a Pt(111) surface, using high-resolution atomic force microscopy (AFM) under ultrahigh vacuum conditions. They found that the corrugated GBs exhibit negative differential friction coefficient (NFC), where the friction force decreases with increasing applied load. This counterintuitive behavior contradicts our daily experience, where the friction force is proportional to the normal load, as stated by the famous Amontons’ law. They also observed that the friction force exhibits an unexpected non-monotonic velocity dependence. These behaviors were markedly different from those previously observed on moiré superstructures in surface grain areas, where low (<10 pN) and nearly constant friction force was observed at low normal loads and sliding velocities, followed by a linear or a logarithmic increase. The researchers therefore concluded that different mechanisms underly the frictional behavior at moiré grain regions and at corrugated GBs.

To explain these fascinating experimental results, the researchers conducted molecular dynamics (MD) simulations, revealing that the main energy dissipation mechanism of GB protrusions involves a dynamic out-of-plane buckling process (see Movie). When the AFM tip slides over a GB it buckles any underlying protrusion downwards, similar to an ironing process. As the tip moves past the buckled protrusion, the later undergoes unbuckling to its original configuration. This buckling and unbuckling process dissipates energy, leading to enhanced friction. Notably, with increasing normal load GB corrugation is suppressed and the out-of-plane motion becomes smooth with low energy dissipation and reduced friction. Accordingly, intrinsically flat GBs, where dynamic buckling is nearly absent, exhibit friction behavior very similar to that of pristine layered interfaces.

While such MD simulations provide valuable insights on the underlying mechanisms that are manifested in the experimental observations, with present computational resources they remain limited to velocity regimes that are orders of magnitude larger than those accessible  experimentally. Hence, the researchers harnessed the MD simulation results to develop a physically motivated phenomenological two-state model that capture the main ingredients required to describe the underlying mechanism while enabling extrapolation of the results to experimentally relevant conditions.

The revealed energy dissipation mechanism and its dependence on experimental parameters is not limited to the case of graphitic interfaces and is expected to be present in numerous other polycrystalline 2D material surfaces. Hence, by exploiting the unconventional frictional properties of GBs, such as the negative friction coefficient and non-monotonic velocity dependence, it may be possible to achieve large-scale structural superlubricity under dry lubrication conditions.