Friction Loop Research Articles

Nanotribology on Polymer Blend Surface by Atomic Force Microscopy

Nanotribological properties of polystyrene/poly(vinyl methyl ether) blend surface mainly at its melt state were investigated using frictional loop (FL) measurement by atomic force microscopy (AFM). The energy dissipation estimated from the area inside the FL showed each peak against both temperature and scan rate. Indentation depth profile against scan rate was simultaneously detected. The indentation was largely restricted above the scan rate where the dissipation energy curve had a peak. We could succeed in drawing both master curves for the energy dissipation and the indentation depth using the same shift factor aT. It was concluded that AFM is a powerful tool for evaluating the nanotribological properties of polymer melt.

Numerical simulation for the friction mechanism of a powder bed using two-dimensional distinct element method

Abstract The motion of particles under shear stress has been numerically simulated using two-dimensional distinct element method. The calculated results well describe the stick-slip phenomena caused by the friction in the particle bed. The results also show that the motions that govern these phenomena are particle rotations which give rise to the slip motion from the static stick state of the particle bed. These motions of particle rotations are transmitted fairly deep inside the particle bed due to particle-particle interaction. Based on these calculated results, we have also obtained the friction loop which is qualitatively in good agreement with the data measured by Nasuno et al. However, the calculated friction coefficient of the particle bed is about 30% of that measured by Nasuno et al. The main cause seems to be the difference in the clearance between the cover plate particles and the bed particles from the experimental state in the powder bed.

The inhibition by dodecylamine of enhanced dissolution of Cu‐Ni alloy initiated by an AFM tip/surface interaction

PurposeTo investigate the influence of an organic corrosion inhibitor on the enhanced dissolution of metal, initiated by AFM tip scratching in corrosive media.Design/methodology/approachThe test solutions were 1.5 M NaCl and 0.01 M HCl. AFM tip scratching experiments were performed for Cu‐Ni alloys in solutions with or without 0.005 M dodecylamine. AFM frictional loop tests were also performed to investigate the effect of dodecylamine on the tip‐surface frictional interaction.FindingsEnhanced dissolution of Cu‐Ni alloy was observed as a result of AFM tip scratching both in NaCl and HCl solutions, and in HCl the effect was more severe than was the case in NaCl. Enhanced dissolution was inhibited markedly by adding 0.005 M dodecylamine to the corrosive media. The results of frictional loop tests indicated that frictional interaction between the tip and the alloy surface was diminished by the adsorption of dodecylamine on the sample surface. The weakening of tip‐surface frictional interaction and the elevation of the ionization energy of metal atoms were responsible for the notable inhibition effect of dodecylamine on the accelerated dissolution.Originality/valueIn this paper, the influence of an organic corrosion inhibitor on the corrosion of metal induced by outside forces was investigated. This was carried out initially by AFM scratching skill and the inhibition mechanism of dodecylamine on the enhanced dissolution of Cu‐Ni alloy initiated by AFM tip scratching.

Theoretical Substantiation of the Slip Index Approach to Fretting

A theoretical basis is presented for a previous fully empirical development of the unified approach to fretting that was based on a new similarity criterion termed slip index. It is shown that the slip index can be analytically derived from any friction loop geometry and hence, can be used for the characterization of any fretting system.

粉体層の摩擦機構の３次元Ｄｉｓｔｉｎｃｔ Ｅｌｅｍｅｎｔ Ｍｅｔｈｏｄによる数値シミュレーションと実験による検証

The motion of particle bed under shear stress has been numerically simulated using 3-Dimensional Distinct Element Method. The calculated results quantitatively well describe the stick-slip phenomena caused by the friction between a shear plate and particle bed. Homogeneous and closed configuration of contact particles in the friction plane is important for the prediction of stick-slip phenomena. The present results confirm that the motions governing these phenomena are particle rotations which suddenly arise the slip motion from the static stick state of the particle bed. The results also show that the close packed particles at stick state change to loose packed particles at the slip state due to the shear force. This phenomenon corresponds to the solid to liquid phase transition in the solid and the thin liquid film frictions.The friction loop calculated based on these results is fairly in good agreement with the measured data. The friction forces for example during the stick-slip can be correctly obtained using the friction coefficients along the friction loop.

粉体の摩擦機構の２次元Ｄｉｓｔｉｎｃｔ Ｅｌｅｍｅｎｔ Ｍｅｔｈｏｄによる数値シミュレーション

The motion of particles under the shear stress has been numerically simulated using 2-dimensional Distinct Element Method. The calculated results well describe the stick-slip phenomena caused by the friction in the particle bed. The results also show that the motions governed these phenomena are particle rotations which arise the slip motion from the static stick state of the particle bed. These motions of particle rotations are transmitted to the fairly deep inside area of the particle bed due to the particle-particle interaction.Based on these calculated results we also have obtained the friction loop which is qualitatively in good agreement with the data measured by Nasuno et al (Physical Review E, 58 Part B, 2161-2171 (1998)). However the calculated friction coefficient of particle bed is about 30% of measured value by Nasuno et al. The main cause seems to be the difference of the clearances between the cover plate particles and the bed particles from the real state in the powder bed.

Friction measurements using force versus distance friction loops in force microscopy

The atomic force microscope (AFM) allows investigation of the properties of surfaces and interfaces at atomic scale resolution. However, several different operational modes (imaging, force versus distance and lateral force), need to be deployed in order to gain insight into the structure, tribiological and mechanical properties. A new method, based on a variation of the force versus distance mode, has been developed. In essence, a coupling of the deformational modes of the probe is exploited whereby the tip is induced to undergo lateral travel in response to application of an out-of-plane force (and thus normal bending of the force-sensing lever). The lateral travel induces in-plane forces that are then measurable as a consequence of stimulation of the ‘buckling’ deformational mode of the lever. Outcomes will be demonstrated for atomically flat surfaces of WTe 2 and highly oriented pyrolytic graphite.

Surface topography and surface chemistry of radiation‐patterned P(tBuMA)—analysis by atomic force microscopy

AbstractPoly‐(tert‐butyl methacrylate) (P(tBuMA)) thin‐film surfaces were patterned by UV radiation at doses in the range 10–100 mJ cm−2, in order to induce laterally differentiated surface chemistry with µm resolution. The most likely pathway for the radiation chemistry predicts a transition from hydrophobicity to hydrophilicity. Outcomes of analysis by atomic force microscopy under air ambient conditions were consistent with that prediction. Topographic and lateral force imaging, in combination with friction loop analysis, revealed shrinkage and increased friction arising from exposure. Force versus distance analysis revealed greater adhesion in hydrophilic regions, due to greater meniscus force acting on the tip. The thickness of adsorbed moisture, increased by a factor of 2.5 from ca 0.8 nm for the unirradiated surface, as a result of greater hydrophilicity induced by radiation. The latter observation shows that the increased friction was due principally to the greater normal force on the tip from an additional meniscus force. Copyright © 2003 Society of Chemical Industry

Atomic Stick-Slip Friction Between Commensurate Self-Assembled Monolayers

Molecular dynamics (MD) simulations have been used to examine the friction between commensurate self-assembled monolayers (SAMs) on Au (111). The discrepancy between sawtoothed friction force and the discontinuous molecular movements reveals the complex dynamics of the closed-packing chains under shearing. Molecules in the lower monolayer can be divided into two groups with a phase difference of π. The periodic motion of the molecules is not synchronous with the frictional stick-slip loops, which result in a second-order valley in friction curves.

Dynamic simulations of adhesion and friction in chemical force microscopy.

A hybrid molecular simulation approach has been applied to investigate dynamic adhesion and friction between a chemical force microscope (CFM) tip and a substrate, both modified by self-assembled monolayers (SAMs) with hydrophobic methyl (CH(3)) or hydrophilic hydroxyl (OH) terminal groups. The method combines a dynamic model for the CFM tip-cantilever system and a molecular dynamics (MD) relaxation technique for SAMs on Au(111) at room temperature. The hybrid simulation method allows one to simulate force-distance curves (or adhesion) and friction loops (or friction coefficient) in the CFM on the experimental time scale for the first time. The simulation results also provide valuable molecular information at the interface that is not accessible in CFM experiments, such as the actual tip position with respect to the cantilever support position, molecular and hydrogen-bonding structures at the interface, and load distributions among different molecular chains (or single-molecule forces). Results show that the adhesion force and friction coefficient for the OH/OH contact pair are much larger than those for the CH(3)/CH(3) pair due to the formation of hydrogen bonds. During the retraction of a CFM tip from a surface, the CFM tip is away from the sample surface slightly while the spring undergoes dramatic elongation in the normal direction before rupture occurs. Single-molecule forces are distributed unevenly at the contact area. Surface energies calculated for functionalized surfaces compare well with those determined by experiments.

Demonstration of atomic scale stick-slip events stimulated by the force versus distance mode using atomic force microscopy

It has been shown that longitudinal deformation of the force-sensing/imposing lever can be stimulated by the conventional force versus distance (F-d), analytical mode of a scanning force microscope. Accordingly it is possible to measure simultaneously both in-plane and out-of-plane force components acting between a tip and a surface. Discrete atomic scale stick-slip events have been observed by F-d generated friction loop analysis of cleaved WTe2, Mica and HOPG single crystals, and of a Langmuir-Blodgett film. Due to the lever geometry, the lateral resolution arising from z-stage movement is better by an order of magnitude than that obtained from translation of the x-y-stage.

Atomic Stick-Slip Friction Between Commensurate Self- Assembled Monolayers

Molecular dynamics (MD) simulations have been used to examine the friction between commensurate self-assembled monolayers (SAMs) on Au (111). The discrepancy between sawtoothed friction force and the discontinuous molecular movements reveals the complex dynamics of the closed-packing chains under shearing. Molecules in the lower monolayer can be divided into two groups with a phase difference of π. The periodic motion of the molecules is not synchronous with the frictional stick-slip loops, which result in a second-order valley in friction curves.

Topography-induced contributions to friction forces measured using an atomic force/friction force microscope

Most friction studies using an atomic force/friction force microscope, while concentrating on material-induced effects, often present users with conflicting and confusing interpretations of the topography-induced friction forces. It has been generally reported that topography-induced contributions are independent of scanning direction and can be removed by subtracting friction data from forward and backward scans. In this article, we present friction studies on samples with well-defined topography variations and find that the above-given statement is not generally true. At surface locations involving significant changes in topography, the topography-induced contributions to friction forces are found to be different between forward and backward scanning directions. This is explained by the ratchet mechanism of friction and due to the additional torsion generated by “collision” of the tip when traversing up an increase in topography, which is absent in the downward travel. Topography-induced contributions to the friction force always correspond to transitions in the surface slope. Moreover, these contributions will be of the same sign in both Trace and Retrace friction profiles of the friction loop whereas changes due to material effects are in opposite directions. These characteristics of topography-induced friction forces will aid in differentiating them from other effects.

Load dependence of lateral force and energy dissipation at NaF(001) surface

We report the load dependence of the lateral force and the energy dissipation occurring at a tip–NaF(001) interface. Clear sublattice periodic maps of the lateral force consisting of large and small peaks appear at loading forces below tens of pN and even at negative loading forces. As loading force is increased, only large peaks remain, representing lattice periodicity. The behavior of the stick–slip lateral forces is completely consistent with that of the energy dissipation calculated from the frictional loops. At a load of around 6nN, an abrupt increase in the lateral force occurs, causing a large deformation of the surface. The load dependence of the lateral force is attributed to that of the contact area between the tip–NaF(001) interface.

Probing the interface potential in stick/slip friction by a lateral force modulation technique

The influence of the shape of the interaction potential is investigated on details in stick/slip friction as encountered between an AFM tip and a substrate. Based on qualitative arguments of stick/slip systems, a novel technique is introduced in which the AFM tip is brought into a lateral resonance mode. In comparison to a direct measurement in the stick/slip signal, we suggest that the method is preferable to highlight these non-linear characteristics. In combination with the shape of the surface potential involved in stick/slip friction, this modulation diminishes the friction loop amplitude in a controlled way. Furthermore, a partial stick/slip behavior is observed above a certain threshold level of driving amplitude, where the tip alternates periodically between a zero-friction and a non-zero-friction state.

Tribology of diamond-like carbon films from generic fabrication routes investigated by lateral force microscopy

The tribological characteristics of diamond-like carbon (DLC) films have been studied by lateral force microscopy (LFM). Specimens from two fabrication routes, ion-beam assisted deposition and chemical vapour deposition, have been investigated. Thick (micrometres) and thin (a few nanometres) films from both routes have been considered, as have the service environments of ambient air and vacuum. Lateral force data were calculated from `friction loops', obtained as functions of load, surface topography, scan speed and service environment. An identical methodology and LFM probe were used throughout the series of measurements in order to ensure internal consistency, and the validity of the methodology was checked against measurements on epitaxially grown Si. A linear dependence was observed between lateral force and force loading up to ca , in accord with a multi-asperity model, thus allowing determination of coefficients of friction that ranged from 0.05 to 0.15. The results showed that adhesive interactions contributed up to to the overall dynamic load. Meniscus interaction played a minor role in comparison to that from tribo-generated electrostatic forces. The experiments show that LFM methodologies have value and relevance to the science and technology of tribology, especially when the required spatial resolution cannot be obtained with the traditional macroscopic techniques.

Energy dissipation in scanning force microscopy-friction on an atomic scale

Stick-slip behaviour for a typical scanning force microscope setup operated in the wearless friction regime is modelled. Not only the deflection of the cantilever but also the local elastic deformation of tip and sample are taken into account. The combined effect of macroscopic spring and microscopic elastic deformation is a key feature to the scanning motion of the tip. Within this model, energy dissipation arises naturally due to mechanical instabilities either of the macroscopic cantilever or of the microscopic tip-sample contact. Our model reproduces all features of atomically resolved friction loops, which can be calculated from interatomic potentials. Moreover, a general scheme is introduced which allows the exact response of the tip-sample system to be calculated from the different interacting potentials.

Lock-in technique for measuring friction on a nanometer scale

A method for measuring friction forces on a nanometer scale is described. This method combines a lock-in technique with scanning force and friction microscopy. Essentially, a lock-in amplifier is used to determine the amplitude of the friction loop, which is measured at high frequency. To demonstrate the capability of this method, the dependence of the friction force with normal load is measured and a two dimensional image is presented.

Friction force microscopy in ultrahigh vacuum: an atomic-scale study on KBr(001)

We present an atomic-scale study on friction performed by a bidirectional atomic force microscope operated in ultrahigh vacuum. Experiments on surfaces of in situ cleaved KBr crystals are presented. On a μm scale the cleavage structure with monoatomic steps of 3.5 ± 0.3 A is revealed. On the atomically flat terraces, atomic-scale resolution is achieved. The resolved square lattice shows a periodicity of 4.7 A and corrugations of 0.3–0.7 A and exhibits the cubic symmetry of KBr(001). The lateral (frictional) force map shows all characteristics of the stick-slip movement of the probing tip. From analysis of the friction loops, the kinetic friction force was determined as a function of load. For a load regime of -4 to 10 nN, lateral force corrugations ranging from 1 to 5 nN were found. A comparison with a novel theoretical model is discussed qualitatively.

Simultaneous Measurement of Surface Topography and Friction Force by a Single-Head Lateral Force Microscope

Although friction force measurements using one sensor to detect both the normal deflection and rotation angle of a scanning probe are convenient and popular, the critical issues regarding the calibration of the instruments have not been fully studied. A Lateral Force Microscope (LFM), modified from the Point Contact Microscope (PCM), is used to simultaneously measure the surface topography and friction force. An optical head is used to measure the normal bending deflection and rotation angle of the cantilever that carries the diamond tip. Emphasis is put on the development of reliable calibration procedures for obtaining the normal deflection and rotation sensitivities of the optical head as well as the spring constants in the bending and torsion modes. The friction loop, which is essential for friction measurements, is investigated in detail. The LFM is used to measure a two-phase composite to show its ability to distinguish different materials on a surface. Wear tests on a single-crystal silicon <100> surface show different friction coefficient regimes, depending on the applied load. For small loads, there is no wear and the friction coefficient is constant. For larger loads, the friction coefficient and wear depth increase with normal load.