Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Superlubricity of carbon nanostructures

Superlubricity has been developing very rapidly in recent years as a new and important area in tribology. Many new phenomena and materials, as well as some new mechanisms in both liquid and solid superlubricity have been obtained. In liquid superlubricity, tens of new kinds of liquids with superlubricity have been found (e.g., water-based liquids, oil-based lubricants, and liquids combined with additives of two-dimensional (2D) materials that exhibit very good superlubricity properties under high pressure). In the field of solid superlubricity, more materials with superlubricity have been observed, including graphene-to-graphene surfaces, highly oriented pyrolytic graphite to graphene surfaces, and heterostructure surfaces where a friction coefficient as low as 0.00004 has been obtained. However, superlubricity is still under laboratory research. What is the future of superlubricity? What is the barrier restricting superlubricity from industrial applications? How do we transfer superlubricity from scientific research to industrial application? These questions and application fields of superlubricity in near future have been analyzed, and the concept of “superlubricitive engineering” has been proposed in the present work.

/pdf/superlubricitive-engineering-future-industry-nearly-getting-2dkefrc0zm.pdf

Superlubricitive engineering—Future industry nearly getting rid of wear and frictional energy consumption

Pristine graphene has been considered one of the most promising materials because of its excellent physical and chemical properties. However, various defects in graphene produced during synthesis or fabrication hinder its performance for applications such as electronic devices, transparent electrodes, and spintronic devices. Due to its intrinsic bandgap and nonmagnetic nature, it cannot be used in nanoelectronics or spintronics. Intrinsic and extrinsic defects are ultimately introduced to tailor electronic and magnetic properties and take advantage of their hidden potential. This article emphasizes the current advancement of intrinsic and extrinsic defects in graphene for potential applications. We also discuss the limitations and outlook for such defects in graphene.

https://pubs.rsc.org/en/content/articlepdf/2022/ra/d2ra01436j

Various defects in graphene: a review

Achieving structural superlubricity in graphitic samples of macroscale size is particularly challenging due to difficulties in sliding large contact areas of commensurate stacking domains. Here, we show the presence of macroscale structural superlubricity between two randomly stacked graphene layers produced by both mechanical exfoliation and chemical vapour deposition. By measuring the shifts of Raman peaks under strain we estimate the values of frictional interlayer shear stress (ILSS) in the superlubricity regime (mm scale) under ambient conditions. The random incommensurate stacking, the presence of wrinkles and the mismatch in the lattice constant between two graphene layers induced by the tensile strain differential are considered responsible for the facile shearing at the macroscale. Furthermore, molecular dynamic simulations show that the stick-slip behaviour does not hold for incommensurate chiral shearing directions for which the ILSS decreases substantially, supporting the experimental observations. Our results pave the way for overcoming several limitations in achieving macroscale superlubricity using graphene. Superlubricity in macro-scale graphitic samples is hampered by commensurate stacking domains that prevent facile sliding between adjacent graphene layers. Here, the authors show the presence of macroscale structural superlubricity between two randomly stacked graphene layers produced by both mechanical exfoliation and CVD upon the imposition of a tensile stress.

/pdf/tunable-macroscale-structural-superlubricity-in-two-layer-4k330fq8jm.pdf

Tunable macroscale structural superlubricity in two-layer graphene via strain engineering.

Structural superlubricity, a nearly frictionless state between two contact solid surfaces, has attracted rapidly increasing attention during the past few years. Yet a key problem that limits its promising applications is the high anisotropy of friction which always leads to its failure. Here we study the friction of a graphene flake sliding on top of a graphene substrate using molecular dynamics simulation. The results show that by applying strain on the substrate, biaxial stretching is better than uniaxial stretching in terms of reducing interlayer friction. Importantly, we find that robust superlubricity can be achieved via both biaxial and uniaxial stretching, namely for stretching above a critical strain which has been achieved experimentally, the friction is no longer dependent on the relative orientation mainly due to the complete lattice mismatch. The underlying mechanism is revealed to be the Moire pattern formed. These findings provide a viable approach for the realization of robust superlubricity through strain engineering.

Robust superlubricity by strain engineering.

The sliding friction of a graphene flake atop strained graphene substrates is studied using molecular dynamics simulation. We demonstrate that in this superlubric system, friction can be reduced nonmonotonically by applying strain, which differs from previously reported results on various 2D materials. The critical strain needed for significant reduction in friction decreases drastically when the flake size increases. For a 250 nm flake, a 0.1% biaxial strain could lead to a more than 2-order-of-magnitude reduction. The underlying mechanism is revealed to be the evolution of Moire patterns. The area of the Moire pattern relative to the flake size plays a central role in determining friction in strain engineering and other scenarios of superlubricity as well. This result suggests that strain engineering could be particularly efficient for friction modification with large contacts.

Strain Engineering Modulates Graphene Interlayer Friction by Moiré Pattern Evolution

Structural superlubricity, which results from the cancellation of lateral force between contacted smooth solid surfaces with weak interaction, achieves ultralow friction and wear, inspiring disruptive innovations in applications on microscale. It is generally believed that this phenomenon requires the contact interface being ultra-clean, ensured by either a clean environment or mechanical cleaning of the contact. However, in this paper, the experimental observation of structural superlubricity with a contaminant-rich interface is unambiguously observed. After intentionally introducing airborne contaminants into microscale structural superlubric incommensurate graphite contacts, we find that the important features of structural superlubricity, the nearly-zero friction coefficient, ultralow friction, and symmetry in friction, are all well preserved. Moreover, contaminants are found to reduce the friction of a commensurate contact but increase the friction of an incommensurate contact, i.e., playing opposite roles. A theoretical model is proposed to fully grasp the contaminant effect. Our results bring challenges to the current understanding about the mechanism of structural superlubricity and provide solid support for its application under practical conditions. 

Structural superlubricity with a contaminant-rich interface

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Robust superlubricity by strain engineering.

Strain Engineering Modulates Graphene Interlayer Friction by Moiré Pattern Evolution

Structural superlubricity with a contaminant-rich interface