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We present results of atomic-force-microscopy-based friction measurements on Re-doped molybdenum disulfide (MoS2). In stark contrast to the widespread observation of decreasing friction with increasing number of layers on two-dimensional (2D) materials, friction on Re-doped MoS2exhibits an anomalous, i.e. inverse, dependence on the number of layers. Raman spectroscopy measurements combined withab initiocalculations reveal signatures of Re intercalation. Calculations suggest an increase in out-of-plane stiffness that inversely correlates with the number of layers as the physical mechanism behind this remarkable observation, revealing a distinctive regime of puckering for 2D materials.

Abstract: We present the results of friction experiments performed by manipulation of oxidized platinum nanoislands on highly oriented pyrolytic graphite (HOPG) substrates through atomic force microscopy (AFM). The oxidation of the platinum nanoislands, performed via mild plasma exposure, is confirmed through X-ray photoelectron spectroscopy (XPS) and high-resolution energy-dispersive X-ray spectroscopy (EDX), the latter of which reveals partial oxidation on the sliding surfaces of the nanoislands. Oxidized platinum nanoislands are found to exhibit higher friction than non-oxidized islands, with a ~ 70% increase in mean shear stress over the investigated contact size regime. An increase in chemical interaction forces between the oxidized platinum and the graphite substrate is proposed to explain the increase in friction forces. Our results reveal that alteration of interfacial chemistry through oxidation leads to a noticeable modulation of friction forces, but not a total breakdown of the superlubric state (as evidenced by the signature observation of decreasing shear stress with increasing contact size), providing further feasibility for the design of superlubric mechanical systems to be operated under ambient conditions. Graphic Abstract

We present a comparative study of the tribological properties of Pd-, Pt-, and Zr-based bulk metallic glasses (BMG-Pd, BMG-Pt, and BMG-Zr, respectively) under unlubricated conditions. In particular, micro-tribometry is utilized with a 52,100 steel ball, showing that BMG-Pt exhibits a significantly higher coefficient of friction (COF) (0.58 ± 0.08) when compared with BMG-Pd (0.30 ± 0.02) and BMG-Zr (0.20 ± 0.03). Topographical roughness on and off wear scars is characterized via atomic force microscopy (AFM), with results that do not correlate with the observed frictional behavior. On the other hand, scanning electron microscopy (SEM) is utilized to reveal contrasting wear mechanisms for the three samples: while BMG-Pd and BMG-Zr exhibit predominantly abrasive wear, there is evidence of adhesive wear on BMG-Pt. Consequently, the occurrence of adhesive wear emerges as a potential mechanism behind the observation of relatively high coefficients of friction on BMG-Pt, suggesting stronger interactions with steel when compared with the other BMG samples.

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. 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.

We present results of atomic-force-microscopy-based friction measurements on Re-doped molybdenum disulfide (MoS2). In stark contrast to the widespread observation of decreasing friction with increasing number of layers on two-dimensional (2D) materials, friction on Re-doped MoS2 exhibits an anomalous, i.e. inverse, dependence on the number of layers. Raman spectroscopy measurements combined with ab initio calculations reveal signatures of Re intercalation. Calculations suggest an increase in out-of-plane stiffness that inversely correlates with the number of layers as the physical mechanism behind this remarkable observation, revealing a distinctive regime of puckering for 2D materials. [ABSTRACT FROM AUTHOR]

We perform atomic force microscopy (AFM) experiments on mechanically exfoliated, single-layer and bulk molybdenum disulfide (MoS2) in order to probe friction forces as a function of sliding speed. The results of the experiments demonstrate that (i) friction forces increase logarithmically with respect to sliding speed, (ii) there is no correlation between the speed dependence of friction and the number of layers of MoS2, and (iii) changes in the speed dependence of friction can be attributed to changes in the physical characteristics of the AFM probe, manifesting in the form of varying contact stiffness and tip-sample interaction potential parameters in the thermally activated Prandtl–Tomlinson model. Our study contributes to the formation of a mechanistic understanding of the speed dependence of nanoscale friction on two-dimensional materials. [ABSTRACT FROM AUTHOR]

We present a review of superlubricity: the state of ultra-low friction between surfaces in relative motion. Various approaches to achieving this state are considered in a broad sense, including structural superlubricity, superlubricity via normal force control, and contact actuation, as well as thermolubricity, liquid superlubricity, and quantum lubricity. An overview of the physical fundamentals associated with each approach is presented, with particular emphasis on recent theoretical and experimental developments that constitute milestones in our scientific understanding. The review also includes a discussion of perspectives on future research in the context of existing challenges. It is projected that interest in superlubricity from the basic science and engineering communities will continue to accelerate in the near future, accompanied by a transition from fundamental studies to technologically relevant applications. [ABSTRACT FROM AUTHOR]

An investigation of the frictional behavior of platinum nanoparticles laterally manipulated on graphite has been conducted to answer the question of whether the recent observation of structural superlubricity under ambient conditions [E. Cihan, S. İPek, E. Durgun, and M. Z. Baykara, Nat. Commun. 7, 12055 (2016)] is exclusively limited to the gold-graphite interface. Platinum nanoparticles have been prepared by e-beam evaporation of a thin film of platinum on graphite, followed by post-deposition annealing. Morphological and structural characterization of the nanoparticles has been performed via scanning electron microscopy and transmission electron microscopy, revealing a crystalline structure with no evidence of oxidation under ambient conditions. Lateral manipulation experiments have been performed via atomic force microscopy under ambient conditions, whereby results indicate the occurrence of structural superlubricity at mesoscopic interfaces of 4000–75 000 nm², with a noticeably higher magnitude of friction forces when compared with gold nanoparticles of similar contact areas situated on graphite. Ab initio simulations of sliding involving platinum and gold slabs on graphite confirm the experimental observations, whereby the higher magnitude of friction forces is attributed to stronger energy barriers encountered by platinum atoms sliding on graphite, when compared with gold. On the other hand, as predicted by theory, the scaling power between friction force and contact size is found to be independent of the chemical identity of the sliding atoms, but to be determined by the geometric qualities of the interface, as characterized by an average “sharpness score” assigned to the nanoparticles. [ABSTRACT FROM AUTHOR]

A non-contact atomic force microscopy-based method has been used to map the static lateral forces exerted on an atomically sharp Pt/Ir probe tip by a graphite surface. With measurements carried out at low temperatures and in the attractive regime, where the atomic sharpness of the tip can be maintained over extended time periods, the method allows the quantification and directional analysis of lateral forces with piconewton and picometer resolution as a function of both the in-plane tip position and the vertical tip–sample distance, without limitations due to a finite contact area or to stick-slip-related sudden jumps of tip apex atoms. After reviewing the measurement principle, the data obtained in this case study are utilized to illustrate the unique insight that the method offers. In particular, the local lateral forces that are expected to determine frictional resistance in the attractive regime are found to depend linearly on the normal force for small tip–sample distances. [ABSTRACT FROM AUTHOR]

We present results of atomic-force-microscopy-based friction measurements on Re-doped molybdenum disulfide (MoS2). In stark contrast to the widespread observation of decreasing friction with increasing number of layers on two-dimensional (2D) materials, friction on Re-doped MoS2 exhibits an anomalous, i.e. inverse, dependence on the number of layers. Raman spectroscopy measurements combined with ab initio calculations reveal signatures of Re intercalation. Calculations suggest an increase in out-of-plane stiffness that inversely correlates with the number of layers as the physical mechanism behind this remarkable observation, revealing a distinctive regime of puckering for 2D materials. [ABSTRACT FROM AUTHOR]

We present experiments involving the structural and frictional characterization of gold nanoparticles (AuNP) thermally deposited on highly oriented pyrolytic graphite (HOPG). The effect of thermal deposition amount, as well as post-deposition annealing on the morphology and distribution of gold on HOPG is studied via scanning electron microscopy (SEM) measurements, while transmission electron microscopy (TEM) is utilized to confirm the crystalline character of the nanoparticles. Lateral force measurements conducted via atomic force microscopy (AFM) under ambient conditions are employed to investigate the nanotribological properties of the gold nanoparticles as a function of normal load. Finally, the increase in lateral force experienced at the edges of the nanoparticles is studied as a function of normal load, as well as nanoparticle height. As a whole, our results constitute a comprehensive structural and frictional characterization of the AuNP/HOPG material system, forming the basis for nanotribology experiments involving the lateral manipulation of thermally deposited AuNPs on HOPG via AFM under ambient conditions. [ABSTRACT FROM AUTHOR]

Surface texturing and solid lubricants have demonstrated the ability to substantially reduce friction and wear under dry conditions. In recent decades, these two technologies have been combined to leverage the advantages of both for superior tribological performance. This review article first summarizes the state-of-the-art regarding surface texturing and solid lubricants, including soft metals, polytetrafluorethylene, diamond-like carbon and 2D layered materials. Then, the synergy between surface textures and solid lubricants is discussed, with particular emphasis on the underlying mechanisms. Finally, gaps in the existing understanding of these synergies are identified and opportunities for future research are suggested. Image 1 • This review focuses on the combination of surface textures and solid lubricants. • The limitations of surface textures and solid lubricants for dry sliding are identified. • Soft metals, PTFE, DLC and 2d layered materials as solid lubricants are considered. • Synergistic mechanisms between texturing and solid lubricants are discussed. • Current short-comings and opportunities for future research are identified. [ABSTRACT FROM AUTHOR]

[Display omitted] • A combined QCM/AFM approach is presented for high-speed nanotribology. • Proof-of-principle experiments are performed on graphene and gold surfaces. • Simultaneously acquired maps of lateral force and frequency shift show spatial alignment. • Frequency shift maps exhibit improved contrast when compared with lateral force maps. • Stick and partial slip regimes are observed via speed-dependent experiments. Forming a fundamental understanding of tribological processes on the atomic scale has the potential to revolutionize the control of friction and wear in macroscopic mechanical systems. On the other hand, methods such as atomic force microscopy (AFM) that provide nanoscale spatial resolution are severely limited in terms of scanning speed when compared with macroscopic mechanical processes, leading to a "speed gap" between fundamental and applied tribology that spans several orders of magnitude. Here, we propose a new method combining AFM experiments with simultaneous quartz crystal microbalance (QCM) measurements for high-speed nanoscale tribology. In particular, scanning speed and vibration amplitude are controlled by QCM whereas normal loads are controlled by AFM. Complementary data are simultaneously recorded in the form of nanoscale, two-dimensional maps of frequency shifts and lateral forces provided by the QCM and AFM, respectively. Proof-of-principle results are presented on a gold-coated QCM sensor surface patterned with a graphene array, whereby stick and partial slip regimes are observed as a function of sliding speed. [ABSTRACT FROM AUTHOR]

Friction on few-layer graphene is known to exhibit unique layer dependence where friction measured via atomic force microscopy (AFM) on the nanometer scale is generally observed to decrease with increasing number of layers. However, this trend is not always observed for AFM probe tips with different sizes and for graphene on different substrates. Within this context, the precise role played by the interface, in particular, the size of the contact and substrate roughness, in the layer dependence of friction on graphene is not yet completely understood. Here, we probe the origins of the roughness dependence of layer-dependent friction on graphene by a combination of AFM measurements and molecular dynamics (MD) simulations. In the experiments, friction is observed to monotonically decrease with increasing number of graphene layers for tips with various apex radii, while the roughness of the sample surface is observed to decrease. In the simulations, two opposite layer-dependence trends for friction are observed on few-layer graphene on substrates with different roughness values. The underlying mechanisms are investigated using atomistic details obtained from the simulations, where the different friction trends are found to originate from an interplay between surface roughness, the trajectory of the tip, and the number of atoms in contact. Finally, the effect of topographical correlation length on the layer dependence of friction on graphene is discussed. [ABSTRACT FROM AUTHOR]