Emerging superlubricity: A review of the state of the art and perspectives on future research

Baykara, Mehmet Z.; Vazirisereshk, Mohammad R.; Martini, Ashlie

doi:10.1063/1.5051445

Cited by 154 publications

(113 citation statements)

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“…This leads to a rotational structural mismatch between basal plane pairs that come into contact with each other at the interfaces that form between the original coating, the transfer film and the third bodies, leading to a systematic cancellation of atomic-scale lateral forces. This results in the establishment of an ultra-low friction regime, in agreement with the theory of structural superlubricity that was spearheaded by Hirano [68] and Sokoloff [69], and is still being investigated in many different experimental systems [67].…”

Section: Mechanisms Of Low Friction and Wearsupporting

confidence: 85%

“…A key feature that leads to ultra-low friction characteristics (mostly referred to as superlubricity [67], with friction coefficients on the order of 0.001) of MoS2 coatings has been determined, via a careful study of the Moiré patterns that emerge during TEM imaging, as the fact that there is significant variability in the rotational registry of individual MoS2 crystallites around the surfacenormal direction. This leads to a rotational structural mismatch between basal plane pairs that come into contact with each other at the interfaces that form between the original coating, the transfer film and the third bodies, leading to a systematic cancellation of atomic-scale lateral forces.…”

Section: Mechanisms Of Low Friction and Wearmentioning

confidence: 99%

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Solid Lubrication with MoS2: A Review

et al. 2019

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Molybdenum disulfide (MoS2) is one of the most broadly utilized solid lubricants with a wide range of applications, including but not limited to those in the aerospace/space industry. Here we present a focused review of solid lubrication with MoS2 by highlighting its structure, synthesis, applications and the fundamental mechanisms underlying its lubricative properties, together with a discussion of their environmental and temperature dependence. An effort is made to cover the main theoretical and experimental studies that constitute milestones in our scientific understanding. The review also includes an extensive overview of the structure and tribological properties of doped MoS2, followed by a discussion of potential future research directions.MoS2 belongs to the family of layered two-dimensional transitional metal dichalcogenides (TMDs). Like graphite and hexagonal boron nitride, its crystal structure consists of covalently bonded sheets, which form stacks that are held together only by weak Van der Waals interactions [16]. It occurs naturally as the mineral molybdenite, an important Mo ore. Due to the strong bonding inplane and weak bonding out-of-plane, mechanical and other properties are highly anisotropic [17], and the stacks can be easily sheared. Single-layer or few-layer (i.e. 2D) MoS2 (analogous to graphene) can be produced and studied individually [18].MoS2 has several different possible structures (polytypes), depending on the bonding within the sheets and the stacks of the sheets. While graphite has a single plane of atoms per sheet, in MoS2 each sheet consists a plane of Mo atoms sandwiched between two planes of S atoms. The single-sheet structure can feature trigonal prismatic coordination around Mo, which is the semiconducting 1H ("hexagonal") structure, or octahedral bonding, which is the metallic 1T ("trigonal") phase. 1H and the ideal 1T each have 3 atoms (MoS2) per unit cell ( Figure 1); however, theoretical calculations with density-functional theory (DFT)[19] and X-ray diffraction (XRD) experiments [20] have shown that in fact 1T is unstable with respect to distortions. The most commonly reported structure is a 3× 3 distortion that triples the unit cell, known as the 1T′ phase [21].Each single-sheet structure can be stacked into a crystal where the next sheet is exactly above the preceding one (AA stacking), producing the 1H, 1T, and 1T′ polytypes. The naturally occurring polytypes in molybdenite, however, are only based on the 1H sheet, and show more complicated stacking [9]. The more common is the 2H structure with 2 sheets per cell in AB stacking, where an S atom in one layer is above an Mo atom in the layer below [22,23]. The less common 3R ("rhombohedral") structure has 3 sheets per cell with ABC stacking [22,24]. These structures are shown in Figure 1. Mo-S bond lengths are around 2.4 Å for all polytypes [25]. DFT calculations show only very small energy differences between the 2H and 3R phases [25]. This insensitivity to stacking, and the low surface energy for 2H-MoS2 of 47 mJ/m 2 =...

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Section: Mechanisms Of Low Friction and Wearsupporting

confidence: 85%

Section: Mechanisms Of Low Friction and Wearmentioning

confidence: 99%

Solid Lubrication with MoS2: A Review

et al. 2019

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“…Although these ratios can sensitively depend on the exact tip configuration, our experiments demonstrate that electronic friction must explicitly be taken into account in models describing singleasperity sliding friction and probably also plays a more important role than so far anticipated for larger multiasperity tribocontacts. Electronic friction may also be an important factor for recent emerging applications of superlubricity (28), which relies on vanishing potential energy barriers for sliding.…”

Section: Discussionmentioning

confidence: 99%

“…However, there is no experimental evidence to support this hypothe sis. Knowledge about the relative contribution of electronic friction contributions is also highly relevant for the emerging field of super lubricity (28), where a structural mismatch between atomically flat interfaces leads to a vanishing corrugation of the periodic energy sur face during sliding and electronic friction should dominate. From a more fundamental point of view, especially, the transition regime around the superconducting phase transition must be considered a key element to understand the role of charge carriers in the friction process.…”

Section: Introductionmentioning

confidence: 99%

Single-asperity sliding friction across the superconducting phase transition

2020

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In sliding friction, different energy dissipation channels have been proposed, including phonon and electron systems, plastic deformation, and crack formation. However, how energy is coupled into these channels is debated, and especially, the relevance of electronic dissipation remains elusive. Here, we present friction experiments of a single-asperity sliding on a high-Tc superconductor from 40 to 300 kelvin. Overall, friction decreases with temperature as generally expected for nanoscale energy dissipation. However, we also find a large peak around Tc. We model these results by a superposition of phononic and electronic friction, where the electronic energy dissipation vanishes below Tc. In particular, we find that the electronic friction constitutes a constant offset above Tc, which vanishes below Tc with a power law in agreement with Bardeen-Cooper-Schrieffer theory. While current point contact friction models usually neglect such friction contributions, our study shows that electronic and phononic friction contributions can be of equal size.

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“…In principle, strucural lubricity is a fundamental concept, where strongly reduced or even vanishing friction is anticipated, if two surfaces in relative motion do not share the same lattice structure and consequently, energy dissipation (i.e., friction) related to instabilities from interlocking surface potentials is strongly reduced. But despite this intriguingly simple mechanism, the analysis of structural lubricity still remains a demanding problem of vibrant research [1][2][3], which usually consists of two main challenges that have received particular attention during recent years:…”

Section: Introductionmentioning

confidence: 99%

Friction vs. Area Scaling of Superlubric NaCl-Particles on Graphite

et al. 2019

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Structural lubricity is an intriguing tribological concept, where extremely low friction is anticipated, if two surfaces in relative motion do not share the same lattice structure and consequently instabilities originating from interlocking surface potentials are strongly reduced. Currently, the challenges related to the phenomenon of structural lubricity are considered to be twofold. On one hand, experimental systems suitable for showing structural lubricity must be identified, while at the same time, it is also crucial to understand the intricate details of interface interaction. Here, we introduce a new material combination, namely NaCl-particles on highly oriented pyrolithic graphite (HOPG), where the nanoparticles coalesce under the influence of ambient humidity. Our experiments reveal that the interfacial friction can be described by the concept of structural lubricity despite the seemingly unavoidable contamination of the interface. By systematically analyzing the friction versus area scaling, this unlikely candidate for structural lubricity then shows two separate friction branches, with distinct differences of the friction versus area scaling. The exact tribological behavior of the nanoparticles can ultimately be understood by a model that considers the influence of nanoparticle preparation on the interface conditions. By taking into account an inevitable water layer at the interface between particle and substrate that can exist in different crystalline configurations all friction phenomena observed in the experiments can be understood.

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Emerging superlubricity: A review of the state of the art and perspectives on future research

Cited by 154 publications

References 154 publications

Solid Lubrication with MoS2: A Review

Solid Lubrication with MoS2: A Review

Single-asperity sliding friction across the superconducting phase transition

Friction vs. Area Scaling of Superlubric NaCl-Particles on Graphite

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