Dynamics of friction: superlubric state
Abstract
Dynamics in friction is studied from an atomistic point of view. Friction is formulated as a problem of whether or not a given kinetic energy for the translational motion dissipates into the kinetic energies for the internal motions during sliding. From the study of the Frenkel-Kontorova model with kinetic energy terms, it is found that two different regimes appear in the parameter space specifying the model: the superlubricity and the friction regimes. The friction exactly vanishes in the superlubric regime and appears in the friction regime. The conditions for the superlubricity to occur are described. It is emphasized that a high dimensionality in the friction system is a key to understanding the physics of superlubricity. For high dimensional systems, superlubricity is a generic phenomenon, appearing for a wide class of (strong or weak) adhesion such as the metallic bonding and the van der Waals interaction. The results are discussed in comparison with those obtained by assuming the case where the upper surface slides quasi-statically against the lower surface.
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Reaching near-zero friction is one of the jewels on the crown of tribology, and structural superlubricity is a crucial mechanism to achieve it. Previous works focus mainly on the structural superlubricity at incommensurate crystalline interfaces. However, realizing such interfaces on a large scale without defects and contaminations is a formidable challenge. Here, we report a charge-induced robust macroscale superlubricity between graphite and atomically flat surfaces in the ambient condition. We transferred graphite flakes on Si3N4 balls and used them to measure the friction properties on pristine and charged atomically flat surfaces such as 300 nm SiO2/Si and sapphire. We found that the surface charge can dramatically reduce the coefficient of friction between graphite and substrates by two orders of magnitude to 10−4, and the sliding is wearless even under harsh contact conditions (∼1.1 GPa center pressure and >100 m). We demonstrate that the surface charge is critical in achieving superlubricity possibly because it can reduce adhesion between graphite and substrate surfaces and make the substrate surfaces resistant to contaminations. Our method offers a ready-to-use solution to superlubricity alternative to achieve incommensurate crystalline interfaces. Thus, it can reduce the difficulty of realizing macroscale superlubricity for applications.
Superlubricity modulation by molecular structure of two-dimensional zeolite imidazole frameworks
2023, Materials Today NanoZeolite imidazole frameworks (ZIFs) have the advantages of high surface stiffness of inorganic materials and low surface free energy of organic materials, which is expected to be an ideal material to achieve solid superlubricity. In this paper, based on the evolution law of the adhesion and surface stiffness of ZIF@mIm and its complexes with structural parameters, superlubricity was obtained on ZIF@mIm + eIm surface (friction coefficient as low as 0.0076), and the influence mechanism of adhesion and surface stiffness on the lubrication properties was revealed. Firstly, the adhesion work between ZIFs and probe is positively correlated with the surface free energy of ZIFs, and the essence is that its ligand affects the anchoring effect strength between metal atom and the tip. Secondly, increasing Zn–N coordination bond strength improves surface stiffness, thereby reducing the out-of-plane deformation. By adjusting the molecular structure, the friction energy dissipation will be reduced from the above two aspects, so as to achieve superlubricity modulation. This is helpful to establish the structure-function relationship between structural parameters and lubrication performance, promising for the precise design of two-dimensional superlubricity materials.
Edge-pinning effect of graphene nanoflakes sliding atop graphene
2023, Materials Today PhysicsEdge effect is one of the detrimental factors preventing superlubricity in laminar solid lubricants. Separating the friction contribution from the edge atom and inner atom is of paramount importance for rational design of ultralow friction across scales in van der Waals heterostructures. To decouple these contributions and provide the underlying microscopic origin at the atomistic level, we considered two contrast models, namely, graphene nanoflakes with dimerized and pristine edges sliding on graphene monolayer based on extensive ab initio calculations. On one hand, we found edge pinning effect by dimerization is obvious for misaligned contact. This case providing local commensuration along edges is reminiscent of Aubry’s pinned phase. The contribution of per dimerized edge carbon atom to the sliding potential energy corrugation is even 1.5 times more than that of an atom in bilayer graphene under commensurate contact. Thus, the dynamic and random edge dimerization during sliding will impact the tribological properties significantly and may be the important sources to account for the marked discrepancies in measured friction parameters [Qu et al., Phys. Rev. Lett. 2020, 125, 126102]. On the other hand, we found the edge contribution to friction is lattice orientation dependent and suppressed in aligned contact. This rationalizes the experimental finding where friction is dominated by interior atoms rather than edge pinning [Liao et al., Nat. Mater. 2022, 21, 47–53]. To eliminate the undesirable edge effects, we adopt strain engineering and edge fluorination to the tribological system constructed here. However, dimerized edges as high frictional pinning sites are robust to both approaches. We hope the detailed atomic information identified here would help for improving superlubric systems.
Review of two-dimensional nanomaterials in tribology: Recent developments, challenges and prospects
2023, Advances in Colloid and Interface ScienceFrom our ordinary lives to various mechanical systems, friction and wear are often unavoidable phenomena that are heavily responsible for excessive expenditures of nonrenewable energy, the damages and failures of system movement components, as well as immense economic losses. Thus, achieving low friction and high anti-wear performance is critical for minimization of these adverse factors. Two-dimensional (2D) nanomaterials, including transition metal dichalcogenides, single elements, transition metal carbides, nitrides and carbonitrides, hexagonal boron nitride, and metal-organic frameworks have attracted remarkable interests in friction and wear reduction of various applications, owing to their atomic-thin planar morphologies and tribological potential. In this paper, we systematically review the current tribological progress on 2D nanomaterials when used as lubricant additives, reinforcement phases in the coatings and bulk materials, or a major component of superlubricity system. Additionally, the conclusions and prospects on 2D nanomaterials with the existing drawbacks, challenges and future direction in such tribological fields are briefly provided. Finally, we sincerely hope such a review will offer valuable lights for 2D nanomaterial-related researches dedicated on tribology in the future.
Kinetic friction of structurally superlubric 2D material interfaces
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Robust and universal macroscale superlubricity with natural phytic acid solutions
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