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Catheter surgery is an effective treatment for vascular disease and has been investigated from a tribological perspective to prevent vascular damage. Endothelial glycocalyx layer (EGL), which is present on the most superficial surface of vascular endothelial cells plays an important role in maintaining vascular homeostasis, but, during catheter surgery, they are affected by frictional stimulation caused by direct contact with the catheter. In this study, to investigate and discuss the effects of frictional stimulation on the surface properties of vascular endothelial cells, we conducted nano friction tests using atomic force microscopy (AFM) on the surface of vascular endothelial cells. and evaluated mechanical properties, interaction structure, and surface properties of vascular endothelial cells and EGL. The results show that the elastic modulus of endothelial cells increased after the nano friction test. It was also found that EGL was worn away after several slides and the friction coefficient of vascular endothelial cells increased with the wear of the EGL. Furthermore, the adhesion force of endothelial cells increased by the wear of EGL. Besides that, the amount of adsorption to fatal bovine serum (FBS) was also examined using the QCM-D method, and a trend toward increased adsorption was observed for cells without EGL compared to cells with EGL.

In order to reveal the friction mechanism of low-dimensional nanomaterials, and effectively regulate the friction process, the interlayer friction of coaxial double-walled carbon nanotubes(DWNTs)with different interlayer distances under axial tension was studied by molecular dynamics simulations. The simulation results show that the friction behavior of the abnormal DWNTs is quite different from that of a normal DWNT(interlayer distances d=0. 34 nm). The interlayer friction force of the abnormal DWNTs(d<0. 34 nm)could be reduced by increasing the tensional strain, but the friction force changes between the layers of the normal DWNT are relatively small. It is found that the interlayer van der Waals interaction of the abnormal DWNTs decreases gradually with increasing the strain, which leads to an obvious decrease in the interlayer resistance and the interlayer friction force. The research indicates that the interlayer frictional behavior of abnormal DWNTs can be effectively regulated by strain engineering. The results are of great significance for further understanding the frictional behavior of low-dimensional nanomaterials. [ABSTRACT FROM AUTHOR]

Despite extensive research on the tribological properties of MoS2, the frictional characteristics of other members of the transition-metal dichalcogenide (TMD) family have remained relatively unexplored. To understand the effect of the chalcogen on the tribological behavior of these materials and gain broader general insights into the factors controlling friction at the nanoscale, we compared the friction force behavior for a nanoscale single asperity sliding on MoS2, MoSe2, and MoTe2 in both bulk and monolayer forms through a combination of atomic force microscopy experiments and molecular dynamics simulations. Experiments and simulations showed that, under otherwise identical conditions, MoS2 has the highest friction among these materials and MoTe2 has the lowest. Simulations complemented by theoretical analysis based on the Prandtl-Tomlinson model revealed that the observed friction contrast between the TMDs was attributable to their lattice constants, which differed depending on the chalcogen. While the corrugation amplitudes of the energy landscapes are similar for all three materials, larger lattice constants permit the tip to slide more easily across correspondingly wider saddle points in the potential energy landscape. These results emphasize the critical role of the lattice constant, which can be the determining factor for frictional behavior at the nanoscale.

Ultralow friction can be achieved with 2D materials, particularly graphene and MoS2. The nanotribological properties of these different 2D materials have been measured in previous atomic force microscope (AFM) experiments sequentially, precluding immediate and direct comparison of their frictional behavior. Here, friction is characterized at the nanoscale using AFM experiments with the same tip sliding over graphene, MoS2, and a graphene/MoS2 heterostructure in a single measurement, repeated hundreds of times, and also measured with a slowly varying normal force. The same material systems are simulated using molecular dynamics (MD) and analyzed using density functional theory (DFT) calculations. In both experiments and MD simulations, graphene consistently exhibits lower friction than the MoS2 monolayer and the heterostructure. In some cases, friction on the heterostructure is lower than that on the MoS2 monolayer. Quasi-static MD simulations and DFT calculations show that the origin of the friction contrast is the difference in energy barriers for a tip sliding across each of the three surfaces.

Friction force microscopy (FFM) explores the interaction in a sliding contact on the nanoscale, providing information on the frictional dynamics and lateral contact stiffness with lattice resolution. Recent FFM measurements on a NaCl crystal immersed in liquid (ethanol) surroundings displayed an increase of the effective contact stiffness, Keff, with the applied load, differently from similar measurements performed under ultra-high vacuum (UHV) conditions, where Keff showed negligible load dependency. Additionally, under UHV conditions multiple slip length friction with increasing load was reported, while in ethanol surroundings only single (lattice unit length) slips were observed. Our current understanding of this behavior relates the transition from single jumps to multiple jumps dynamics to the normal load (manifested through the amplitude of the interaction potential at the contact, U0) and to the damping of the system. Here we have incorporated the effect of the load dependency on both U0 and Keff within Prandtl—Tomlinson based simulations, accompanied by variations in the damping coefficient of the system. Introducing the experimentally observed load dependency to Keff resulted indeed in single slip jumps at critical damping, while multiple slip jumps were obtained at constant Keff. The average slip length increased with the normal load, particularly when the system became underdamped. Our work provides a glimpse on the relation between the characteristic observables in atomic-scale sliding friction (maximal slip forces, stiffness, and slip dynamics) with respect to their governing parameters (corrugation energy, effective stiffness, and damping). While common understanding in nanotribology relates the effect of surrounding media mainly to the interaction potential at the contact, here we show that the media can also greatly affect the elastic interaction, and consequently play an important role on the transition from single to multiple stick-slip. [ABSTRACT FROM AUTHOR]

The friction behavior of clay particles plays an essential role in controlling its mechanical properties, but remains unclear in microscale. As one of the major clay particles, the nanoscale friction properties of hydrated montmorillonite have been studied using steered molecular dynamics (SMD) simulation method, considering the coupling effect of water content of 0 ∼ 30 % and normal load of 1 atm ∼ 10 GPa. The SMD friction along the y -direction has been performed to investigate the frictional mechanism of hydrated montmorillonite and its structural properties of interlayer water. The evolution of average friction load with water content, friction coefficients, and the distribution of interlayer water molecules were obtained. Furthermore, the SMD pulling along the z -direction was conducted to study the effect of interlayer water molecules on the interaction between montmorillonite layers. All simulation results showed that more energy and a higher friction load were required to slide at a higher normal load. The friction coefficient of 0.058 ∼ 0.17 and internal friction angle of 3.3°∼9.6° for hydrated montmorillonite agreed well with previous studies in microscale, showing the weak friction properties. The evolution of friction coefficient with water content was different at different normal loads, where the friction coefficient of 5 % hydrated montmorillonite was the highest at 3 ∼ 10 GPa. Moreover, the increasing normal load could change the distribution of interlayer water molecules. [ABSTRACT FROM AUTHOR]

Graphite or graphene due to its excellent tribological properties is highly expected in solid lubrications, where the friction behaviors are intimately related to interfacial adhesion; however, the fundamental mechanism governing their correlation remains scientifically intriguing and largely unexplored. Through the combination of atomic force microscopy (AFM) measurements and density functional theory (DFT) calculations, we demonstrated that the friction of graphite surface slid against various AFM probes is generally enlarged with stronger adhesive interaction due to the higher interfacial charge density. The correlation between friction and adhesion mainly originates from the sliding induced charge evolution which governs the energy dissipation of solid lubrication between graphite and bulk materials. A general strategy that suppresses charge flowed into contact region enables the superlubricity of graphene or graphite surface owing to the feeble adhesion of the sliding systems. It is thereby proposed that the superlubricity is generally favored by weak interaction of the sliding interfaces. [Display omitted] • Nonlinear dependence of friction on adhesion is shown in tests and simulations. • Superlubricity of graphite against a Al 2 O 3 AFM tip is demonstrated. • Friction of HOPG is enlarged by adhesion due to higher interfacial charge density. • Graphene modification for an active SiO 2 tip enables superlubricity on HOPG surface. [ABSTRACT FROM AUTHOR]

Several key features of nanoscale friction phenomena observed in experiments, including the stick-slip to smooth sliding transition and the velocity and temperature dependence of friction, are often described by reduced-order models. The most notable of these are the thermal Prandtl–Tomlinson model and the multibond model. Here we present a modified multibond (mMB) model whereby a physically-based criterion—a critical bond stretch length—is used to describe interfacial bond breaking. The model explicitly incorporates damping in both the cantilever and the contacting materials. Comparison to the Fokker–Planck formalism supports the results of this new model, confirming its ability to capture the relevant physics. Furthermore, the mMB model replicates the near-logarithmic trend of increasing friction with lateral scanning speed seen in many experiments. The model can also be used to probe both correlated and uncorrelated stick slip. Through greater understanding of the effects of damping and noise in the system and the ability to more accurately simulate a system with multiple interaction sites, this model extends the range of frictional systems and phenomena that can be investigated. This article is part of the theme issue 'Nanocracks in nature and industry'. [ABSTRACT FROM AUTHOR]

Moiré superstructures arising at twisted 2D interfaces have recently attracted the attention of the scientific community due to exotic quantum states and unique mechanical and tribological behaviors that they exhibit. Here, we predict the emergence of chiral distortions in twisted layered interfaces of finite dimensions. This phenomenon originates in intricate interplay between interfacial interactions and contact boundary constraints. A metric termed the fractional chiral area is introduced to quantify the overall chirality of the moiré superstructure and to characterize its spatial distribution. Despite the equilibrium nature of the discovered energetic and structural chirality effects, they are shown to be manifested in the twisting dynamics of layered interfaces, which demonstrates a continuous transition from stick-slip to smooth rotation with no external trigger.

A recent systematic experimental characterisation of technological thin films, based on elaborated design of experiments as well as probe calibration and correction procedures, allowed for the first time the determination of nanoscale friction under the concurrent influence of several process parameters, comprising normal forces, sliding velocities, and temperature, thus providing an indication of the intricate correlations induced by their interactions and mutual effects. This created the preconditions to undertake in this work an effort to model friction in the nanometric domain with the goal of overcoming the limitations of currently available models in ascertaining the effects of the physicochemical processes and phenomena involved in nanoscale contacts. Due to the stochastic nature of nanoscale friction and the relatively sparse available experimental data, meta-modelling tools fail, however, at predicting the factual behaviour. Based on the acquired experimental data, data mining, incorporating various state-of-the-art machine learning (ML) numerical regression algorithms, is therefore used. The results of the numerical analyses are assessed on an unseen test dataset via a comparative statistical validation. It is therefore shown that the black box ML methods provide effective predictions of the studied correlations with rather good accuracy levels, but the intrinsic nature of such algorithms prevents their usage in most practical applications. Genetic programming-based artificial intelligence (AI) methods are consequently finally used. Despite the marked complexity of the analysed phenomena and the inherent dispersion of the measurements, the developed AI-based symbolic regression models allow attaining an excellent predictive performance with the respective prediction accuracy, depending on the sample type, between 72% and 91%, allowing also to attain an extremely simple functional description of the multidimensional dependence of nanoscale friction on the studied variable process parameters. An effective tool for nanoscale friction prediction, adaptive control purposes, and further scientific and technological nanotribological analyses is thus obtained. [ABSTRACT FROM AUTHOR]

Surface defects and their mutual interactions are anticipated to affect the superlubric sliding of incommensurate layered material interfaces. Atomistic understanding of this phenomenon is limited due to the high computational cost of ab initio simulations and the absence of reliable classical force-fields for molecular dynamics simulations of defected systems. To address this, we present a machine-learning potential (MLP) for bilayer defected graphene, utilizing state-of-the-art graph neural networks trained against many-body dispersion corrected density functional theory calculations under iterative configuration space exploration. The developed MLP is utilized to study the impact of interlayer bonding on the friction of bilayer defected graphene interfaces. While a mild effect on the sliding dynamics of aligned graphene interfaces is observed, the friction coefficients of incommensurate graphene interfaces are found to significantly increase due to interlayer bonding, nearly pushing the system out of the superlubric regime. The methodology utilized herein is of general nature and can be adapted to describe other homogeneous and heterogeneous defected layered material interfaces.

Clay minerals are platelike particles that play a critical role in problems involving swelling, deformation, and failure. Fundamental understanding of these phenomena and the parameters that influence them requires studies at the nanoscale. The nanoscale mechanism of the sliding of clay sheets at different states of hydration and hydrostatic stress was studied here using molecular dynamics in an isobaric–isothermal ensemble. The hydration state lay in the range of crystalline swelling (0–400 mgwater/gclay), and the hydrostatic pressure was varied in the range 5–12 GPa. Under hydrostatic pressures as high as several gigpascals, the mobility and the molecular structure of water are comparable to those of supercooled water. Despite high hydrostatic stresses, the failure was located in the shear-dominated zone. Examination of the molecular structure of interlayer water, number of hydrogen bonds, and their configuration during shear loading showed evidence of stick-slip phenomena similar to those found in thin films. The number of hydrogen bonds between interlayer water molecules increased as the clay–water system approached the slip point. Hydration state and hydrostatic stress influenced the stress–strain behavior of system and the average shear stress. The hydration states, in which the maximum average shear stresses occurred, coincided with the formation of the second, third, and fifth water layers. The shear strength decreased with the increase of water content above 340 mgwater/gclay. The nanoscale cohesion and friction angle of the layers were calculated using the Mohr–Coulomb failure criterion and were found to be in good agreement with previous studies. [ABSTRACT FROM AUTHOR]

A structured transdisciplinary method for the experimental determination of friction in the nanometric domain is proposed in this paper. The dependence of nanoscale friction on multiple process parameters on these scales, which comprise normal forces, sliding velocities, and temperature, was studied via the lateral force microscopy approach. The procedure used to characterize the stiffness of the probes used, and especially the influence of adhesion on the obtained results, is thoroughly described. The analyzed thin films were obtained by using either atomic layer or pulsed laser deposition. The developed methodology, based on elaborated design of experiments algorithms, was successfully implemented to concurrently characterize the dependence of nanoscale friction in the multidimensional space defined by the considered process parameters. This enables the establishment of a novel methodology that extends the current state-of-the-art of nanotribological studies, as it allows not only the gathering of experimental data, but also the ability to do so systematically and concurrently for several influencing variables at once. This, in turn, creates the basis for determining generalizing correlations of the value of nanoscale friction in any multidimensional experimental space. These developments create the preconditions to eventually extend the available macro- and mesoscale friction models to a true multiscale model that will considerably improve the design, modelling and production of MEMS devices, as well as all precision positioning systems aimed at micro- and nanometric accuracy and precision. [ABSTRACT FROM AUTHOR]

A generic model has been developed to simulate the effect of phonon interactions during nanoscale sliding with an incommensurate interface. A rigid slider or array of sliders is translated across a 3D elastic slab whose mass elements are harmonically coupled, either in a simple cubic structure (for vast majority of cases) or in a face-centered cubic structure. Each slider interacts with the slab via the Lennard-Jones 6–12 intermolecular potential. Elastic waves are allowed to propagate without any damping and no energy is removed from the system. Boundary conditions are set sufficiently remotely that no significant wave energy returns to the interface from boundary reflection. Simulation results demonstrate that for such nanoscale contacts, (1) the presence of one slider can affect the friction felt by another slider through phonon generation; (2) friction force scales with contact width rather than with contact area; and (3) the friction force may be sensitive to the number of contact regions that comprise a given total area.

The friction characteristics of single-layer h-BN, MoS 2 , and graphene were systematically investigated via friction force microscopy measurements at various operational (e.g., normal force and sliding speed) and environmental (e.g., relative humidity and thermal annealing) conditions. The low friction characteristics of these single-layer materials were clearly observed from the normal force-dependent friction results, and their interfacial shear strengths were further estimated using a Hertz-plus-offset model. In addition, speed-dependent friction characteristics clearly demonstrated two regimes of friction as a function of sliding speed – the first is the logarithmic increase in friction with sliding speed regime at sliding speeds smaller than the critical speed and the second is the friction plateau regime at sliding speeds greater than the critical speed. Fundamental parameters such as effective shape of the interaction potential and its corrugation amplitude for these single-layer materials were characterized using the thermally-activated Prandtl-Tomlinson model. Moreover, friction of single-layer h-BN, MoS 2 , and graphene was found to increase with relative humidity and decrease with thermal annealing; these trends were attributed to the diffusion of water molecules to the interface between the single-layer materials and their substrates, which leads to an increase in the puckering effect at the tip-material interface and interaction potential corrugation. The enhanced puckering effect was verified via molecular dynamics simulations. Overall, the findings enable a comprehensive understanding of friction characteristics for several classes of two-dimensional materials, which is important to elucidate the feasibility of using these materials as protective and solid-lubricant coating layers for nanoscale devices. Unlabelled Image • Mechanics behind friction properties of single-layer h-BN, MoS 2 , and graphene. • Interfacial shear strength of single-layer h-BN, MoS 2 , and graphene from normal force-dependent friction. • Intrinsic friction properties of single-layer h-BN, MoS 2 , and graphene as a function of speed. • Effects of relative humidity and thermal annealing on thickness and friction of single-layer h-BN, MoS 2 , and graphene. [ABSTRACT FROM AUTHOR]

The possibility of strain engineering of friction in carbon nanotube is explored in this paper. The intertube friction in double-walled carbon nanotube under torsional strain is calculated by employing the molecular dynamics simulations. By studying the intertube friction in double-walled carbon nanotube subjected to torsional strain, it is found that the friction can be tuned by torsional strain. It shows that the temperature has great influence on torsional strain effects on intertube friction. Mechanism and physical insight of torsional strain effects are revealed by numerical and phonon spectra analysis. It is found that the torsional strain accelerates of the exchange from order kinetic energy to disorder thermal energy. The work in this paper implies the potential of strain engineering in nanoscale friction and brings new insight into fundamental understanding of strain-effect mechanism in nanoscale friction. [ABSTRACT FROM AUTHOR]

The nanoscale friction behavior of quartz-quartz and quartz-kaolinite interfaces is investigated through Steered Molecular Dynamics (SMD) simulations. The effects of normal load, sliding velocity, temperature, and hydration on the friction behavior are discussed, and the friction mechanism of quartz and quartz/kaolinite interface is revealed. The friction coefficients of all systems at different cases are obtained and compared with other experimental results for validation. The simulation results show that the stick-slip effect in all interfaces was found during the friction process, where the higher the sliding velocity and hydration, or the lower the normal load, the weaker the stick-slip effect. The friction load increased with the rising normal load, and the relationship between shear stress and normal load was approximately linear. The friction coefficient and cohesion of the quartz-quartz interface could rise with the increasing sliding velocity or the decreasing temperature. Moreover, the friction coefficient of quartz-kaolinite was significantly smaller than that of quartz-quartz, indicating that the presence of clay could weaken the frictional strength of quartz. The effect of the interlayer water film on friction behavior was rather complex, showing the lubricating or bonding role, which has been discussed and analyzed in the present study. [Display omitted] • The stick-slip effect was found in the quartz-quartz interface during the friction process. • The higher the sliding velocity and hydration, or the lower the normal load, the weaker the stick-slip effect. • The presence of clay could weaken the frictional strength of quartz. • Friction coefficient and cohesion of quartz rose with the rising sliding velocity or decreasing temperature. • The relationship between shear stress and normal load was linear in nanoscale friction. [ABSTRACT FROM AUTHOR]

The influence of vibration on friction at the nanoscale was studied via molecular dynamics (MD) simulations. The results show that average friction increases in a high-frequency range. This can be attributed to the vibration of the tip following vibration excitation, which results in peaks of repulsive interaction between tip and substrate and leads to higher friction. However, when the frequency is lower than a certain value, friction decreases. This is because vibration excitation results not in an obvious vibration of the tip but in a slightly larger interface distance, which leads to a decrease in friction. [ABSTRACT FROM AUTHOR]

Friction at the nanoscale differs markedly from that between surfaces of macroscopic extent. Characteristically, the velocity dependence of friction between apparent solid/solid contacts can strongly deviate from the classically assumed velocity independence. Here, we show that a nondestructive friction between solid tips with radius on the scale of hundreds of nanometers and solid hydrophobic self-assembled monolayers has a strong velocity dependence. Specifically, using laterally oscillating quartz tuning forks, we observe a linear scaling in the velocity at the lowest accessed velocities, typically hundreds of micrometers per second, crossing over into a logarithmic velocity dependence. This crossover is consistent with a general multicontact friction model that includes thermally activated breaking of the contacts at subnanometric elongation. We find as well a strong dependence of the friction on the dimensions of the frictional probe. [ABSTRACT FROM AUTHOR]

Single molecule experiments on bacteriophages show an exponential scaling for the dependence of mobility on the length of DNA within the capsid. It has been suggested that this could be due to the “capstan mechanism” – the exponential amplification of friction forces that result when a rope is wound around a cylinder as in a ship's capstan. Here we describe a desktop experiment that illustrates the effect. Though our model phage is a million times larger, it exhibits the same scaling observed in single molecule experiments. [ABSTRACT FROM AUTHOR]

Phononic coupling can have a significant role in friction between nanoscale surfaces. We find frictional dissipation per atom in carbon nanotube (CNT) oscillators to depend significantly on interface features such as contact area, commensurability, and by end-capping of the inner core. We perform large-scale phonon wavepacket MD simulations to study phonon coupling between a 250 nm long (10,10) outer tube and inner cores of four different geometries. Five different phonon polarizations known to have dominant roles in thermal transport are selected, and transmission coefficient plots for a range of phonon energies along with phonon scattering dynamics at specific energies are obtained. We find that the length of interface affects friction only through LA phonon scattering and has a significant nonlinear effect on total frictional force. Incommensurate contact does not always give rise to superlubricity: the net effect of two competing interaction mechanisms shown by longitudinal and transverse phonons decides the role of commensurability. Capping of the core has no effect on acoustic phonons but destroys the coherence of transverse optical phonons and creates diffusive scattering. In contrast, the twisting and radial breathing phonon modes have perfect transmission at all energies and can be deemed as the enablers of ultralow friction in CNT oscillators. Our work suggests that tuning of interface geometries can give rise to desirable friction properties in nanoscale devices. [ABSTRACT FROM AUTHOR]

The relationship between the friction behavior and surface properties such as surface chemistry, morphology and phase distribution is not fully understood. In the case of surface treatments involving steels, the alloying elements introduce non-controllable variables in tribological experiments. We further advance this discussion by designing experiments where the friction behavior of a conical diamond tip sliding on different plasma nitrided and post-oxidized pure iron substrates was determined. Also, a detailed chemical and microstructural surface characterization using discharge optical emission spectroscopy, X-ray diffraction, and atom probe tomography was performed. Implementing composition profiling and localized elemental distribution analysis throughout the outermost layer, the formation of iron oxide with non-homogenous morphology was detected. Although the coefficient of friction decreases as a function of the post-oxidation time, the friction force is under a relative great dispersion at intermediate post-oxidation times. The dispersion of the friction force obtained from the sliding analyses can be understood as a combination of surface chemistry and different phases (structure) at the surface. [ABSTRACT FROM AUTHOR]

This review discusses the important concept of cotton fiber friction at both the macro- and nanoscale. First, the technological importance of fiber friction and its role in fiber breakage during fiber processing is discussed. Next, previous studies on frictional properties of cotton fibers are reviewed and different experimental procedures to measure friction between fibers or against another surface are evaluated. Friction models developed to explain friction process during various experimental procedures are considered and their limitations are discussed. Since interpretation of friction processes at the macroscale can be challenging (mainly due to difficulties in analyzing the multiple asperities in contact), a separate section is devoted to surveying studies on the emerging field of single-asperity friction experiments with atomic force microscope (AFM). Special attention is given to studies on nanoscale frictional characteristics of rough viscoelastic surfaces (e.g., plant cuticular biopolymers and cotton fibers). Due to the close relationship between friction and adhesion hysteresis at the nanoscale, adhesion studies with AFM on viscoelastic surfaces are also reviewed. Lastly, recommendations are made for future research in the field of frictional properties of cotton fibers.

The influence of vibration on friction at the nanoscale was studied via molecular dynamics (MD) simulations. The results show that average friction increases in a high-frequency range. This can be attributed to the vibration of the tip following vibration excitation, which results in peaks of repulsive interaction between tip and substrate and leads to higher friction. However, when the frequency is lower than a certain value, friction decreases. This is because vibration excitation results not in an obvious vibration of the tip but in a slightly larger interface distance, which leads to a decrease in friction.

Superhydrophobic double roughening structure of DLC film was prepared by 2.45 GHz surface wave-excited plasma CVD with the mixture of methane (CH4) and tetramethylsilane (TMS: Si(CH3)4) gases on the undulated DLC film by a series of plasma Ar etching, coating process and plasma Ar etching. Static wetting angle of water was observed that double roughening structure of DLC was superhydrophobicity such as wetting angle 161°. This approach also increased in air pockets easily trap among the needle-like posts. For the low friction at nanoscale, the surface wettability of the solid lubrication played a significant role, when the DLC film modified from flat to double roughening structure, the friction was constantly inner humidity conditions. Results generally showed that humidity had insignificant effect on the nanoscale friction at superhydrophobic DLC surface. The effect of the superhydrophobic double roughening DLC and friction were discussed with the following factors; the surface morphology affinity to needle-like shape, a reduction of the real area of contact, graphitization and easily occur to slip at small contact interface due to superhydrophobicity.

Tribological properties depend strongly on environmental conditions such as temperature, humidity, and operation liquid. However, the origin of the liquid effect on friction remains largely unexplored. Herein, taking molybdenum disulfide (MoS 2 ) as a model system, we explored the nanoscale friction of MoS 2 in polar (water) and nonpolar (dodecane) liquids through friction force microscopy. The friction force exhibits a similar layer-dependent behavior in liquids as in air; i.e., thinner samples have a larger friction force. Interestingly, friction is significantly influenced by the polarity of the liquid, and it is larger in polar water than in nonpolar dodecane. Atomically resolved friction images together with atomistic simulations reveal that the polarity of the liquid has a substantial effect on friction behavior, where liquid molecule arrangement and hydrogen-bond formation lead to a higher resistance in polar water in comparison to that in nonpolar dodecane. This work provides insights into the friction on two-dimensional layered materials in liquids and holds great promise for future low-friction technologies.

Using nonequilibrium molecular dynamics simulations and a coarse-grained description of a system, we have investigated frictional properties of nanojunctions including atomically thin sheets embedded between metal surfaces. We found that the frictional properties of the junctions are determined by the interplay between the lattice mismatch of the contacting surfaces and out-of-plane displacements of the sheet. The simulations provide insight into how and why the frictional characteristics of the nanojunctions are affected by the commensurate-incommensurate transition. We demonstrated that in order to achieve a superlow friction, the graphene sheet should be grown on or transferred to the surface with morphology, which is close to that of the graphene (for instance, Cu), while the second confining surface should be incommensurate with the graphene (e.g., Au). Our results suggest an avenue for controlling nanoscale friction in layered materials and provide insights in the design of heterojunctions for nanomechanical applications. [ABSTRACT FROM AUTHOR]

This work investigates the friction and adhesive forces at the single-asperity contact between hydrogen-free diamond-like carbon (DLC) coated surfaces in humid air and in argon. High power impulse magnetron sputtering deposition has been used to produce hydrogen-free DLC thin films on commercial silicon AFM tips and silicon wafers. The structure and surface morphology of the deposited DLC films has been investigated by X-ray photoelectron spectroscopy and tapping AFM, investigations that showed very smooth DLC films with about 29% of sp 3 C − C bonds. The as-deposited hydrogen-free DLC thin films were hydrophilic (water contact angle about 65°) due to incorporation of a small amount of oxygen (about 9% of C = O bonds) at the film surfaces. The friction force measurements show that at low normal loading force the contact friction is much lower in the humid air than in the argon, while at large values of the loading force the contact friction does not depend on the measurement medium. A qualitative friction model based on the concept of dry and wet friction regimes, when the layer of water adsorbed on DLC film surfaces plays the role of lubricator, is proposed to explain these findings. [ABSTRACT FROM AUTHOR]

Highlights: [•] Friction force microscopy measurements as a function of load for WSC/Cr coatings. [•] Non-linear contact area dependence on load in solid–solid nanocontacts. [•] Friction coefficient decreases with increasing applied load. [•] Low shear strength – formation of an easy-shear layer at the sliding interface. [ABSTRACT FROM AUTHOR]

A new experimental capability for elevated temperature nanoscale friction measurement is described. Its stability and resolution were demonstrated in two case studies up to 750 °C. A stainless steel probe was used to study friction between steel and glass at 25, 200 and 400 °C. Friction forces were calibrated at temperature. The friction coefficient increased between 25 and 200 °C, but stick-slip was dominant at 400 °C due to chemical interaction between the stainless steel probe and the glass. This was verified by scanning Energy Dispersive X-ray Spectroscopy analysis. A WC-Co probe was used to study friction on a range of TiN-based and CrAlC (so named MAX-phase composition) coatings at 25, 400 and 750 °C. A maximum in friction coefficient was observed at 400 °C. The decrease in friction at 750 °C was associated with the formation of lubricating surface oxides and oxidation-associated surface roughening. [ABSTRACT FROM AUTHOR]

The use of the atomic force microscope (AFM) as a tool to manipulate matter at the nano-scale has received a large amount of research interest in the last decade. Experimental and theoretical investigations have showed that the AFM cantilever can be used to push, cut, or pull nanosamples. However, AFM-based nanomanipulation suffers a lack of repeatability and controllability because of the complex mechanics in tip-sample interactions and the limitations in AFM visual sensing capabilities. In this paper, we will investigate the effects of the tip-sample interactions on nanopushing manipulation. We propose the use of an interaction model based on the Maugis-Dugdale contact mechanics. The efficacy of the proposed model to reproduce experimental observations is demonstrated via numerical simulations. In addition, the coupling between adhesion and friction at the nanoscale is analyzed. [ABSTRACT FROM AUTHOR]

This work deals with the effect of surface roughness parameters on the frictional properties of nanowire-based lubrication systems (NBLS) across Cu surfaces with various topographies. The friction coefficient was discussed in the context of surface roughness parameters including the rms height, inter-island separation and a combined roughness parameter related to the pressure experienced by each nanowire. It was concluded that the rms height of asperity should not be lower than the radius of nanoparticles for effective lubrication. In addition, when the inter-island separation is an integer multiple of the nanowire length, nanowires perform as effective lubricants. Furthermore, the friction coefficient increased when the mean pressure experienced by the nanowires increased. The results obtained in this original study offer some interesting insights into the frictional properties of NBLS as a function of surface roughness parameters. This could lead to a great impact on the selection of nanoparticle-based lubricant aimed at reducing wear and energy losses for various applications. [ABSTRACT FROM AUTHOR]

Abstract: The nanoscale friction of partially oxidized silicon nitride thin films deposited by reactive magnetron sputtering was investigated. Post deposition thermal annealing in O2, trying to simulate the oxidation by atmospheric oxygen in working conditions, formed a partially oxidized layer at the surface with maximum thickness around 10nm. Unidirectional sliding tests showed a decrease of the low-load friction coefficients of the sliding pair for the samples annealed in oxygen as compared to the non-annealed ones. The results are discussed on the lights of our extension of the crystal chemistry model, which establishes a relationship between ionic potential and friction coefficient. [Copyright &y& Elsevier]

Abstract: We recently proposed a new nanoscale friction model based on the bristle interpretation of single asperity contacts. The model is mathematically continuous and dynamic which makes it suitable for implementation in nanomanipulation and nanorobotic modeling. In the present paper, friction force microscope (FFM) scans of muscovite mica samples and vertically aligned multi-wall carbon nanotubes (VAMWCNTs) arrays are conducted. The choice of these materials is motivated by the fact that they exibit different stick-slip behaviors. The corresponding experimental and simulation results are compared. Our nanoscale friction model is shown to represent both the regular and reverse frictional sawtooth characteristics of the muscovite mica and the VAMWCNTs, respectively. [Copyright &y& Elsevier]

We synthesized Mo x W 1−x S 2 alloy by chemical vapor deposition. Raman characterization results showed that the sample is a monolayer with a special structure which transforms from MoS 2 to WS 2. Atomic force microscopy measurements suggested that the friction force decreases from MoS 2 region to WS 2 region. Molecular dynamics simulations verified that interfacial interaction is the determining factor resulting in the variation in the friction properties of monolayer Mo x W 1−x S 2. Green's function calculations demonstrated that phonons are excited at both washboard frequency and its harmonics. Besides, the number and total energy of the excited phonons were calculated. The decreasing of the number and total energy of the excited phonons with the weakening interfacial interaction, leading to the decreasing dissipated energy and reduced friction force. • Mo x W 1-x S 2 alloy, a monolayer TMDs with unique structural properties, was synthesized by chemical vapor deposition. • AFM measurements suggested that the friction decreases from MoS 2 to WS 2 region with the reduced molybdenum concentration. • MD simulations showed that interfacial interaction is the determining factor leading to the friction variation of Mo x W 1-x S 2. • The number as well as total energy of the excited phonons were calculated quantitatively by Green's functions calculations. [ABSTRACT FROM AUTHOR]

Abstract: Tribological behavior of diamond-like carbon (DLC) is believed to be controlled by an interfacial transfer layer of low shear strength, formed by a friction-induced transformation of the top layer of the DLC film. Nanoscale friction, adhesion, durability and wear studies indicating such a phase transformation in DLC films are discussed. [Copyright &y& Elsevier]

[Display omitted] • The frictional properties of monolayer and few-layer MoSe 2 flakes are investigated. • The angular dependence of the friction is highly anisotropic for both types of MoSe 2. • The friction force anisotropy decreases with applied normal load. • The anisotropy demonstrates an opposite angular dependence for mono- and few-layer MoSe 2. The frictional properties of two-dimensional (2D) materials are strongly dependent on the crystallographic orientation and number of layers. Although friction anisotropy caused by crystallographic orientations has been reported for various 2D materials, the flexural deformations and different defects complicate the insight into the mechanism of the in-plane friction anisotropy of these materials. Here, the anisotropic friction behavior between an atomic force microscopy tip and monolayer (ML) and few-layer (FL) MoSe 2 flakes grown by CVD was performed by nanofriction measurements at different crystallographic directions, applied loads (F A), and tip scanning velocities. Our results reveal that the angular dependence of the friction forces is highly anisotropic for both types of MoSe 2 flakes, and the anisotropy decreases with F A. Importantly, the anisotropies demonstrate opposite angular dependence for ML and FL MoSe 2 flakes. As confirmed by the friction scanning velocity dependences, this difference seems to originate from different friction mechanisms for ML and FL MoSe 2 flakes. The friction of FL flakes was predominantly influenced by atomic stick–slip motion. In contrast, ML MoSe 2 is characterized by a coexistence of deformation-induced and atomic stick–slip motion. The experimental results presented here extend the understanding of the tribological properties of dry lubricants operating at the nanoscale. [ABSTRACT FROM AUTHOR]

The interparticle friction behavior of saturated clay controls its mechanical properties, but remains unclear at nanoscale. As one of major clay minerals, the hydrated montmorillonite (MMT) is selected to investigate the nanoscale friction characteristics using Molecular Dynamics simulation method. Two portions of MMT representing two particles with a water film in the middle are created to simulate an undrained system. A virtual spring is applied on the upper MMT portion to provide the sliding with a constant velocity relative to the bottom portion. The effects of normal load and sliding velocity on the frictional behavior are then investigated. The friction coefficients of hydrated MMT at different cases were measured and compared with other experimental and simulation results for the validation. The evolution of normal load with the number of hydrogen bonds for hydrated MMT was finally analyzed. All simulation results indicated that the friction load fluctuated periodically with a cycle of about 9.10 Å at sliding velocity inferior 0.001 Å•fs−1, which was nearly equal to montmorillonite's lattice constant along the sliding direction; the fluctuation amplitude of the friction load increased with the decreasing sliding velocity; the relationship between the average friction load and the logarithm of sliding velocity followed a power function; the friction coefficient and the cohesion were found to increase approximately linearly with sliding velocity. [Display omitted] • The friction load fluctuates periodically at low sliding velocity. • The cycle equals to montmorillonite's lattice constant along the sliding direction. • Friction coefficient and cohesion increase almost linearly with sliding velocity. • Bound water increases with normal load till a certain level. [ABSTRACT FROM AUTHOR]

[Display omitted] • MD simulation facilitates analysis of nanotribology to the atomic scale. • LAMMPS, AMBER, GROMACS, CHARMM, and GROMOS, are used for molecular analysis. • MD simulation is used to measure the interacting force of the molecules. • The script for nanoindentation provides guidance to the LAMMPS software. • MD simulations includes interfacial layer, nanoparticle's plastic energy etc. To improve the efficiency of a machinery component, it is necessary to enhance the quality of lubricants. Lubricants help to reduce the friction and wear between the two surfaces which are in relative motion. The study of friction and wear covers in the field of tribology. Recently tribological behavior has been studied in micro and nanoscale. Nanotribology deals with the study friction, adhesion, wear and lubrication of metal crystal, analyze dynamics and single asperity contacts during sliding, formation and propagation of voids, fracture and nucleation and motion of dislocations properties at atomic level and also study the interfacial properties between the two surfaces. The expensive nanotribological experimental studies and limited availability of experimental facility, motivates researchers to use simulation approach. Molecular Dynamic (MD) simulation facilitate to analysis at molecular level and nanoscale. Present review paper is elucidating the application of MD simulations in the field of Nanotribology. In addition, this study provides utter knowledge about the LAMMPS commands and also confabulate the capabilities of MD simulation in nanotribology area. This review paper is fully dedicated to all those readers who are fascinated to do research in field of nanotribology using molecular dynamics simulation in nanotribology with LAMMPS. [ABSTRACT FROM AUTHOR]

Metallic glasses are excellent materials for micromechanical systems, where miniature components involving mechanical contact require control of friction at the microscopic scale. We report on an in-situ study of the structure of oxide films formed upon electrochemical polarization and their role in nanoscale friction on a metallic glass in aqueous environment using atomic force microscopy. The oxide film has a bilayer structure, as revealed by repeated scanning with the tip of an atomic force microscope. The dependence of friction on electrochemical potential reveals the growth mechanism and highlights the role of the oxide films for the frictional response of metallic glasses. The chemical sensitivity of nanotribology studies under electrochemical control contributes to the understanding of corrosion mechanisms on metallic glasses. [Display omitted] • Surface oxide films strongly affect nanoscale friction of metallic glasses in corrosive environments. • Nanoscale friction experiments reveal the structure of electrochemically formed surface oxide films. • The growth mechanism of oxide films as function of applied potential is discussed based on friction results. [ABSTRACT FROM AUTHOR]

The motion of a nanoscale MoO3 crystal on an MoS2 substrate is an ideal case for the study of the effects of microstructure on macroscopic phenomena like friction, because both components of this system can be prepared with atomically flat surfaces. We apply a recently developed real-space density functional theory method to investigate the energetics of sliding an MoO3 crystal on an MoS2 substrate. We then link the results to simple models based on continuum elastic theory, in order to study the mechanical behavior of the oxide crystals under loads that correspond to realistic situations. Based on these results, we extract estimates of the force which must be applied with an atomic force microscope in order to move the oxide crystal on the substrate, for different orientations of the two components. [ABSTRACT FROM AUTHOR]