Friction Force Microscopy Research Articles

Anisotropy of nanoscale friction: Influence of lattice structure, temperature, and wear

The anisotropy of nanoscale friction is analyzed for different samples using friction force microscopy under ultrahigh-vacuum conditions with varying scan directions. For both $\mathrm{Mo}{\mathrm{S}}_{2}$ and highly oriented pyrolytic graphite surfaces, our experiments reveal a sixfold symmetry of the friction vector plots in accordance with solutions of the Langevin equation, where the atomic force microscope tip is treated as a point mass elastically driven on the hexagonal surface lattices. In addition, the effect of temperature is analyzed both by experiment and by simulations. In both cases we find that increased temperature not only lowers the absolute friction values but also decreases the friction anisotropy. In contrast, experiments on NaCl cannot be explained on a similar basis. While the square symmetry is clearly revealed in the first measurements, it progressively deteriorates with repeated scanning, which is possibly due to abrasive wear occurring at the interface.

Liquid-Phase Friction of Two-Dimensional Molybdenum Disulfide at the Atomic Scale.

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 (MoS2) as a model system, we explored the nanoscale friction of MoS2 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.

Moiré-Tile Manipulation-Induced Friction Switch of Graphene on a Platinum Surface.

Friction control and technological advancement are intimately intertwined. Concomitantly, two-dimensional materials occupy a unique position for realizing quasi-frictionless contacts. However, the question arises of how to tune superlubric sliding. Drawing inspiration from twistronics, we propose to control superlubricity via moiré patterning. Friction force microscopy and molecular dynamics simulations unequivocally demonstrate a transition from a superlubric to dissipative sliding regime for different twist angles of graphene moirés on a Pt(111) surface triggered by the normal force. This follows from a novel mechanism at superlattice level where, beyond a critical load, moiré tiles are manipulated in a highly dissipative shear process connected to the twist angle. Importantly, the atomic detail of the dissipation associated with the moiré tile manipulation─i.e., enduring forced registry beyond a critical normal load─allows the bridging of disparate sliding regimes in a reversible manner, thus paving the road for a subtly intrinsic control of superlubricity.

Ultrahigh Lubricity between Two-Dimensional Ice and Two-Dimensional Atomic Layers.

Low temperature and high humidity conditions significantly degrade the performance of solid-state lubricants consisting of van der Waals (vdW) atomic layers, owing to the liquid water layer attached/intercalated to the vdW layers, which greatly enhances the interlayer friction. However, using low temperature in situ atomic force microscopy (AFM) and friction force microscopy (FFM), we unveil the unexpected ultralow friction between two-dimensional (2D) ice, a solid phase of water confined to the 2D space, and the 2D molybdenum disulfides (MoS2). The friction of MoS2 and 2D ice is reduced by more than 30% as compared to bare MoS2 and the rigid surface. The phase transition of liquid water into 2D ice under mechanical compression has also been observed. These new findings can be applied as novel frictionless water/ice transport technology in nanofluidic systems and promising high performance lubricants for operating in low temperature and high humidity environments.

Crystal lattice imaging of two-dimensional materials by friction force microscopy

摩擦力显微镜(friction force microscopy, FFM)是一种基于摩擦力信号的原子力显微镜，它能够对二维材料晶格进行快速、无损的高分辨成像。然而，由于热漂移、黏附力、表面静电等因素的影响，环境条件下二维材料的高分辨FFM成像仍面临着巨大挑战。基于以上问题，本文以高定向热解石墨为标准样品，通过对探针在样品表面黏滑行为的分析，系统研究了探针弹性常数、正应力和扫描速度对高分辨FFM成像的影响，并建立了一套可靠的二维材料晶格结构表征方法。该方法能够获得精确的结构信息，所测得的二维材料晶格常数平均误差小于2.3%。此外，该方法还适用于化学气相沉积法和剥离法制备的多种二维材料，展现出较高的普适性。本文的研究结果为环境条件下二维材料晶格结构的精确表征提供了新思路。

Frictional behaviour of plant proteins in soft contacts: unveiling nanoscale mechanisms.

Despite the significance of nanotribology in the design of functional biomaterials, little is known about nanoscale friction in the presence of protein-coated soft contact surfaces. Here, we report a detailed investigation of frictional behaviour of sustainable plant proteins at the nanoscale for the first time, using deformable bio-relevant surfaces that achieve biologically relevant contact pressures. A combination of atomic force microscopy, quartz crystal microbalance with dissipation monitoring, and friction force microscopy with soft polydimethylsiloxane (PDMS, 150 kPa) surfaces was employed to elucidate the frictional properties of model plant proteins, i.e. lupine, pea, and potato proteins at the nanoscale while systematically varying the pH and ionic strength. Interactions of these plant proteins with purified mucins were also probed. We provide the much-needed direct experimental evidence that the main factor dictating the frictional properties of plant proteins is their affinity towards the surface, followed by the degree of protein film hydration. Proteins with high surface affinity, such as pea and potato protein, have better lubricating performance than lupine at the nanoscale. Other minor factors that drive lubrication are surface interactions between sliding bodies, especially at low load, whilst jamming of the contact area caused by larger protein aggregates increases friction. Novel findings reveal that interactions between plant proteins and mucins lead to superior lubricating properties, by combining high surface affinity from the plant proteins and high hydration by mucins. We anticipate that fundamental understanding gained from this work will set the stage for the design of a plethora of sustainable biomaterials and food with optimum nanolubrication performance.

Temperature-dependent friction coefficient on flat graphite plane

Investigating the frictional properties of two-dimensional layered materials with dynamic evolution of contact quality advances understanding the mechanism underlying puckering effects, offering a new pathway for the rational control of friction. In this letter, friction force microscopy (FFM) in ultra-high vacuum (UHV) conditions has been used to probe the nanoscale friction on a freshly cleaved flat graphite surface (0001) as a function of normal force and surface temperature. It is found that friction force shows a nonmonotonic change with the decrease of temperature with the maximum friction appeared around Tmax=140±5 K. Besides, coefficients of friction (COFs) defined as the slope of friction-normal force curves remains nearly constant before the friction peak while surprisingly change into negative values with the further decrease of temperature. The loading–unloading results reveal that, the abnormal COFs behavior originates from the temperature induced contact quality evolution which leads to the ripple formation at cryogenic temperatures in the front of scanning tip. The results reported here provide a deep insight into the effects of temperature on the frictional behaviors of graphite, which give a new thrust in the field of friction on lamellar materials.

Velocity Dependence of Moiré Friction.

Friction force microscopy experiments on moiré superstructures of graphene-coated platinum surfaces demonstrate that in addition to atomic stick-slip dynamics, a new dominant energy dissipation route emerges. The underlying mechanism, revealed by atomistic molecular dynamics simulations, is related to moiré ridge elastic deformations and subsequent relaxation due to the action of the pushing tip. The measured frictional velocity dependence displays two distinct regimes: (i) at low velocities, the friction force is small and nearly constant; and (ii) above some threshold, friction increases logarithmically with velocity. The threshold velocity, separating the two frictional regimes, decreases with increasing normal load and moiré superstructure period. Based on the measurements and simulation results, a phenomenological model is derived, allowing us to calculate friction under a wide range of room temperature experimental conditions (sliding velocities of 1-104 nm/s and a broad range of normal loads) and providing excellent agreement with experimental observations.

Scanning Drop Friction Force Microscopy.

Wetting imperfections are omnipresent on surfaces. They cause contact angle hysteresis and determine the wetting dynamics. Still, existing techniques (e.g., contact angle goniometry) are not sufficient to localize inhomogeneities and image wetting variations. We overcome these limitations through scanning drop friction force microscopy (sDoFFI). In sDoFFI, a 15 μL drop of Milli-Q water is raster-scanned over a surface. The friction force (lateral adhesion force) acting on the moving contact line is plotted against the drop position. Using sDoFFI, we obtained 2D wetting maps of the samples having sizes in the order of several square centimeters. We mapped areas with distinct wetting properties such as those present on a natural surface (e.g., a rose petal), a technically relevant superhydrophobic surface (e.g., Glaco paint), and an in-house prepared model of inhomogeneous surfaces featuring defined areas with low and high contact angle hysteresis. sDoFFI detects features that are smaller than 0.5 mm in size. Furthermore, we quantified the sliding behavior of drops across the boundary separating areas with different contact angles on the model sample. The sliding of a drop across this transition line follows a characteristic stick-slip motion. We use the variation in force signals, advancing and receding contact line velocities, and advancing and receding contact angles to identify zones of stick and slip. When scanning the drop from low to high contact angle hysteresis, the drop undergoes a stick-slip-stick-slip motion at the interline. Sliding from high to low contact angle hysteresis is characterized by the slip-stick-slip motion. The sDoFFI is a new tool for 2D characterization of wetting properties, which is applicable to laboratory-based samples but also characterizes biological and commercial surfaces.

In-plane and out-of-plane interaction analysis of adsorbates on multilayer graphene and graphite by multifrequency atomic force microscopy

Graphene and graphite are generally considered as materials exhibiting friction anisotropy. However, it was demonstrated by friction force microscopy experiments that friction anisotropy most likely originates from adsorbates aligning in well-ordered stripes on graphitic surfaces. In this study, we analyzed the evolution of adsorbates on a few-layer graphene-/graphite-flake on top of a Si/SiO 2 -substrate after different times of storage using a multifrequency force microscopy approach. The method allows for the simultaneous in-plane and out-of-plane sample analysis as well as the discrimination of conservative and dissipative tip-sample interactions directed parallel and perpendicular to the surface and thus offers a myriad of complementary physical information. We found that friction anisotropy was observable at distinct areas of the graphene-/graphite-flake after 14 days of storage when exposed to air, albeit not throughout the whole flake, indicating different types of adsorbates. High-resolution imaging revealed that friction anisotropy occurred not only at positions where stripe-like structures were visible, but also on areas which were completely covered with adsorbates. Additionally, we demonstrated that after removal of the adsorbate material by oxygen-plasma treatment, no friction anisotropy was observable on the graphene-/graphite-flake, corroborating adsorbate driven nature of friction anisotropy on graphitic surfaces. • Combined in-plane and out-of-plane force microscopy for interpretation of ordered adsorbate formation on graphite/graphene. • Discrimination of different types of adsorbates by tip-sample interactions parallel and perpendicular to the surface. • 90° rotation of the graphene/graphite flake revealed that friction anisotropy is only caused by certain types of adsorbates.

Effect of temperature-induced contact quality evolution on nanoscale friction

Multiple subcontacts formation/rupture and puckering the evolution of contact quality play crucial roles in modulating friction on two-dimensional lamellar materials but have up to now been studied separately in the field of nanotribology. In this work, by using variable temperature friction force microscopy in ultrahigh-vacuum conditions, we investigate friction on graphite surfaces with different numbers of free edges as a function of temperature from 295.0 to 100.0 K. We introduce subcontacts formation/rupture simply by changing temperature and tune the out-of-plane deformation compliance by changing the number of free edges. It is found that friction on a plane with large deformation compliance is constantly larger than that with small deformation compliance at cryogenic temperatures. While appearing to be influenced secondly by deformation compliance, the coefficients of friction increase, however, linearly with temperature. By performing systematic stick-slip measurements, the aforementioned separate friction mechanisms have been linked, where the temperature-induced increase of subcontacts rupture force can give rise to the frictional contact quality which additionally becomes more prominent on a plane with large deformation compliance at cryogenic temperatures. These results emphasize a way to vary the contact quality via temperature and demonstrate that the subcontacts formation/rupture and the contact quality must be correlatively accounted for.

Natural two-dimensional pyrophyllite: Nanoscale lubricant, electrical insulator and easily-machinable material

Pyrophyllite, with the chemical formula Al2Si4O10(OH)2, is a naturally occurring and abundant van der Waals mineral belonging to the group of phyllosilicates. It is very soft, layered crystal used for sculpting and an excellent electrical and thermal insulator aimed for the operation at high pressure and temperature. Here, for the first time, two-dimensional (2D) pyrophyllite obtained by both mechanical and liquid phase exfoliation is presented and investigated at the nanoscale. The layered structure provides low friction coefficient of around 0.1 as measured by friction force microscopy. The wear properties, studied by atomic force microscope (AFM) based scratching, are distinctly different from graphene. Since the wear is initiated at low normal forces, 2D pyrophyllite can be routinely carved by the AFM tip and it is suitable for scratching based nanolithography. According to our optical measurements, 2D pyrophillite is an insulator with a band gap of ∼5.2eV. Local current measurements by conductive AFM reveal that 2D pyrophyllite flakes behave as efficient electrical insulators with a breakdown voltage of around 6MV/cm. Therefore, the obtained results indicate possible applications of 2D pyrophyllite as a low-cost electric insulator and lubricant, as well as an easily-machinable material at the nanoscale.

Surface characterization using Friction Force Microscopy and the Jarzynski equality

The Jarzynski equality is a fundamental result from non-equilibrium statistical mechanics that provides the difference in free energy between two states from the work done on the system by processes that can be carried out far from equilibrium. We evaluate the applicability of Jarzynski equality to map the potential energy of a model graphene surface using data from simulated Friction Force Microscopy (FFM). We model the scanning process of the FFM using the Prandtl-Tomlinson model and Langevin dynamics. By varying the simulation parameters, we verify the “stick–slip” and thermolubricity friction regimes, as well as the crossover between them. We then calculate the surface potential energy using the Jarzynski equality for these regimes. A new method for properly evaluating the free energy of the cantilever is introduced. We observe that the applicability of Jarzynski's equality is linked to the friction regimes: For thermolubricity, a very accurate potential energy curve is obtained for relatively few repetitions, but for the “stick–slip” movement, it is only possible to use Jarzynski’s equation in a small fraction of the scanning distance. For the crossover regime, it is possible to obtain a relatively accurate potential energy curve for a sufficiently large number of sampling repetitions.

Molecular dynamics analysis of friction-triggering process with spherical probe

Molecular dynamics simulation with embedded atom method/Finnis–Sinclair potential function was utilised to study the friction properties of the friction-triggering process at the mesoscopic scale. The sliding friction process of a spherical probe with different diameters on the surface of a single-crystal aluminum matrix was simulated and the influences of sliding friction on the atomic behaviour of the surface under different contact conditions were analysed. The relationship between contact force and friction coefficient with spherical diameter was studied. The research shows that the plastic groove, normal force and friction force increase with the increase in probe diameter in the sliding friction process. The contribution of increasing spherical diameter to normal force is greater than that of friction force. The coefficient of friction varies with the ratio of the actual contact area to the normal force. After the running-in process is stable, the friction coefficient fluctuates around a certain value as theoretically verified by the two-term friction law. The increase in the diameter of the spherical probe leads to the increase in the number of dislocation atoms in the workpiece and the formation of dislocation rings, which result in the difference in the sliding friction process under indenters with different diameters. The results provide valuable reference for friction-triggering measurement based on 3D nanometre probes such as friction force microscopy.

Effects of interfacial interactions between metal and process control agents during ball milling on the microstructure of the milled Fe-based nanocrystalline alloy powder

Fe–Si–B–P–Cu nanocrystalline alloy were treated with ball-mill using a lubricant as a process control agent (PCA). The resulting alloy powder is a strong candidate material for soft magnetic composites. Two ball milling methods (continuous and interval) were employed to control the interactions between the PCA and the alloy surface, and their effect on the microstructure of the prepared alloy particles was investigated. The alloy sheet was broken into small pieces and deformed plastically into flake-shaped particles regardless of the ball milling method implemented. Friction-force microscopy of the alloy immersed in the PCA revealed that the friction coefficient of the alloy surface exposed to air for a certain period was higher than that of the unexposed alloy surface (immediately after polishing). During ball milling, the ratio of the newly generated surface to the oxidized surfaces of the alloy subjected to interval milling was smaller than that of the alloy subjected to continuous milling. Therefore, the friction coefficient of the surface of the alloy subjected to interval milling was higher than that of the alloy subjected to continuous milling. Synchrotron radiation analysis revealed that the alloy subjected to interval milling exhibited enhanced surface friction, showing an obvious steepness and inflection in the diffraction intensity as a function of the tilt angle based on the Schulz reflection method. This indicates formation of crystallographic texture in α-Fe grains in an amorphous matrix. Hence, we demonstrated successfully that the ball milling process induced a crystallographic texture in the Fe-based nanocrystalline alloy due to plastic deformation due to the enhanced surface friction. The surface of the alloy was prepared based on the effect of the interfacial interactions between the alloy surface and the PCA.

Layer-dependent frictional properties of Ti3C2Tx MXene nanosheets

MXenes are an emerging class of two-dimensional lamellar materials with exceptional architectural variety, tunable chemical compositions, and chemical inertness. Owing to their unique structure, with a highly tunable interlayer distance and binding properties, MXenes can potentially exhibit excellent nanoscale solid lubrication properties. Herein, we report the layer dependence of the frictional characteristics of atomically thin Ti3C2Tx MXene nanosheets. The frictional properties of the isolated Ti3C2Tx nanosheets deposited on a clean Si surface were investigated using friction force microscopy. The friction decreased with an increase in the number of layers, starting from monolayer Ti3C2Tx. The layer dependence of friction was attributed to the reduced elastic compliance with an increase in the number of Ti3C2Tx layers. A partially suspended Ti3C2Tx monolayer exhibited a higher degree of friction than the substrate-supported Ti3C2Tx monolayer, indicating the role of elastic compliance in the friction mechanism of Ti3C2Tx nanosheets. The reduced elastic compliance of Ti3C2Tx nanosheets decreases the localized out-of-plane puckering-type deformation surrounding the sliding indenter, thereby reducing the resistance to frictional sliding. The results indicate that tuning their out-of-plane mechanical properties by varying the chemical architecture, chemical composition, surface terminations, and interlayer intercalations can render MXenes as potential candidates for versatile nanoscale solid lubrication applications.

Direct Observation of Atomic-Scale Gliding on Hydrophilic Surfaces.

Nanoscale friction behavior on hydrophilic surfaces (HS), influenced by a probe gliding on a confined water layer, has been investigated with friction force microscopy under various relative humidity (RH) conditions. The topographical and frictional responses of the mechanically exfoliated single-layer graphene (SLG) on native-oxide-covered silicon (SiO2/Si) and mica were both influenced by RH conditions. The ordinary phenomena at ambient conditions (i.e., higher friction on a HS than on a SLG due to different hydrophilicity), nondistinguishable height, friction of SLG with SiO2/Si at high RH (>98%), and the superlubricating behavior of friction on a HS were observed. Furthermore, the subdomain within SLG, consisting of an ice-like water layer intercalated between SLG and SiO2/Si, showed friction enhancement. These results suggest that the abundant water molecules at the interface of the probe and a HS can make a slippery surface that overcomes capillary and viscosity effects through the gliding motion of the probe.

Observation of robust superlubricity of MoS2 on Au(111) in ultrahigh vacuum

The structural and superlubric properties of single layer MoS2 on Au(111) forming moiré superlattice structures have been investigated by means of ultrahigh vacuum atomic force microscope with bimodal and contact modes. We synthesize epitaxial monolayer MoS2 flakes on the Au(111) surface in ultrahigh vacuum. Using friction force microscopy, atomic friction measurements indicate a superlubric regime between the tip apex and the moiré corrugated MoS2 surface in which the friction force remains at an ultralow value and is independent from normal load. Superlubricity conditions are observed for different loads and velocities which indicates the absence of out-of-plane deformations. We find that the MoS2 layer including the moiré superlattice modulation originating from the natural misfit between MoS2 and the Au(111) substrate is relatively rigid. We also demonstrate a low friction coefficient of the MoS2 surface crossing a single Au(111) step. Our results open up a new avenue for minimizing friction in nanoscale electronic devices and other dry rigid contacts used in aerospace lubrication.

Temperature dependence of nanoscale friction on topological insulator Bi2Se3 surfaces

The topological insulator Bi2Se3 is an insulator in the bulk and has unusual metallic surface states that consist of spin-polarized Dirac fermions. Although the electronic properties of topological insulators have been extensively studied, the friction characteristic that is a key factor for further applications is barely known. In this study, conductive friction force microscopy (c-FFM) in ultrahigh vacuum (UHV) conditions was used to probe the nanoscale friction on freshly cleaved Bi2Se3 planes (0001) as a function of surface temperature from 105.0 K to 300.0 K. The experimental results demonstrate a non-monotonic enhancement of dry friction, with distinct friction peaks emerging at K and K. While the widths of the friction peaks are nearly independent of the normal force, the relative peak height increases and the peak temperature decreases with normal load. We reveal that this anomalous friction behavior originates from the formation and rupture of multiple thermally activated sub-contacts, which is further confirmed by velocity dependence measurements. These results for multiple thermally activated sub-contact formation and rupture provide a deep insight into the frictional behaviors of Bi2Se3.

Nanoscale contact mechanics of the interactions at monolayer MoS2 interfaces with Au and Si

Molybdenum disulfide (MoS2) is a two-dimensional material that exhibits unique interfacial interactions with gold (Au) and silicon (Si). These interfaces have gained attention due to their potential to be integrated into a broad range of applications, where they involve direct contact that affects their performance. Here we investigate the nanoscale mechanical contact interaction of MoS2 monolayers with Au and Si, using adhesion measurements and friction force microscopy. MoS2-Au contact exhibits pronounced adhesive interaction, manifested by higher adhesion pull-off forces and energies, compared to MoS2-Si. Influenced by the adhesive contacts, the friction forces recorded demonstrate stronger lateral interactions and stronger interfacial shear strength between MoS2 and Au. Analysis with the Prandtl–Tomlinson model reveals that the MoS2-Au direct interaction is also stiffer, suggesting higher elastic energy at the contact. This information on the mechanistic nature of MoS2-Au and MoS2-Si contacts under shear and normal loads is of potential importance in applications where such interfaces are utilized, such as in the design of coatings, sensors and mechanoelectrical devices.