Atomic Friction Research Articles

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.

Synthesis and frictional properties of array film of amorphous carbon nanofibers on anodic aluminum oxide

Amorphous carbon nanofiber arrays were synthesized in porous anodic aluminum oxide templates by pyrolysis of acetylene with cobalt nanoparticles as catalyst at 640 °C. The carbon nanofibers have amorphous structures under high-resolution transmission electron microscopy and Raman spectroscopy examination. The aligned amorphous carbon nanofibers grown within the pores of the aluminum oxide membranes are uniform with lengths of about 2 μm and outer diameters of about 85 nm. The frictional properties of the array film of amorphous carbon nanofibers were investigated using an atomic force and friction force microscopy (AFM–FFM) and a ball-on-disk machine in air. The adhesion between the amorphous carbon nanofiber arrays and the anodic aluminum oxide membrane remained intact at relatively low loads. The AFM–FFM measurements indicated that the friction forces on the array film of amorphous carbon nanofibers were uniform. The array film had low friction coefficient and high wear resistance under the micro friction tests. The friction coefficient of the array film dry sliding against a corundum counterface was observed to be constant after an initial transient period and decreased with increasing the sliding velocity.

Introduction to Tribology

Introduction to Tribology

Molecular dynamics simulation of the contact process in AFM surface observations

AbstractIn the present work, several molecular dynamics simulations have been performed to clarify dynamically the contact mechanism between the specimen surface and probe tip in surface observations by an atomic force microscope (SFM) or friction force microscope (FFM). In the simulation, a three‐dimensional model is proposed where the specimen and the probe are assumed to consist of monocrystalline copper and rigid diamond or a carbon atom, respectively. The effect of the cantilever stiffness of the AFM/FFM is also taken into consideration. The surface observation process is simulated on a well‐defined Cu{100} surface. From the simulation results it has been verified that the surface images and the two‐dimensional atomic‐scale stick‐slip phenomenon, just as is the case for real AFM/FFM surface observations, can be detected from the spring force acting on the cantilever. From the evaluation of the behaviour of specimen surface atoms, the importance of the specimen stiffness in deciding the cantilever properties can also be understood. The influence of the probe tip shape on the force images is also evaluated. From the results it can be verified that the behaviour of the specimen surface atoms as well as the solid surface images in AFM/FFM surface observations can be understood using the molecular dynamics simulation of the model presented.

Molecular simulation studies of self-assembled monolayers of alkanethiols on Au(111)

A review is presented of this group's recent molecular simulation studies of self-assembled monolayers (SAMs) of alkanethiols on Au(111) surfaces. SAMs are very useful for the systematic alteration of the chemical and structural properties of a surface by varying chain length, tail group and composition. The scientific and technological importance of SAMs cannot be overestimated. The present work has been centred on studies of atomic scale surface properties of SAMs. First, configurational-bias Monte Carlo simulations were performed in both semigrand canonical and canonical ensembles to investigate the preferential adsorption and phase behaviour of mixed SAMs on Au(111) surfaces. Second, a novel hybrid molecular simulation technique was developed to simulate atomic force microscopy (AFM) over experimental timescales. The method combines a dynamic element model for the tip-cantilever system in AFM and a molecular dynamics relaxation approach for the sample. The hybrid simulation technique was applied to investigate atomic scale friction and adhesion properties of SAMs as a function of chain length. Third, dual-control-volume grand canonical molecular dynamics (DCV-GCMD) simulations were performed of transport diffusion of liquid water and methanol through a slit pore with both inner walls consisting of Au(111) surfaces covered by SAMs under a chemical potential gradient. Surface hydrophobicity was adjusted by varying the terminal group of CH3 (hydrophobic) or OH (hydrophilic) of the SAMs. Finally, ab initio quantum chemical calculations were performed on both clusters and periodic systems of methylthiols on Au(111) surfaces. Based on the ab initio results, an accurate force field capable of predicting c(4×2) superlattice structures over a wide range of temepratures for alkanethiols on Au(111) was developed. The extension of current work is discussed briefly.

Long Time Dynamics of Met-Enkephalin: Comparison of Explicit and Implicit Solvent Models

Met-enkephalin is one of the smallest opiate peptides. Yet, its dynamical structure and receptor docking mechanism are still not well understood. The conformational dynamics of this neuron peptide in liquid water are studied here by using all-atom molecular dynamics (MD) and implicit water Langevin dynamics (LD) simulations with AMBER potential functions and the three-site transferable intermolecular potential (TIP3P) model for water. To achieve the same simulation length in physical time, the full MD simulations require 200 times as much CPU time as the implicit water LD simulations. The solvent hydrophobicity and dielectric behavior are treated in the implicit solvent LD simulations by using a macroscopic solvation potential, a single dielectric constant, and atomic friction coefficients computed using the accessible surface area method with the TIP3P model water viscosity as determined here from MD simulations for pure TIP3P water. Both the local and the global dynamics obtained from the implicit solvent LD simulations agree very well with those from the explicit solvent MD simulations. The simulations provide insights into the conformational restrictions that are associated with the bioactivity of the opiate peptide dermorphin for the δ-receptor.

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.

411 摩擦力顕微鏡による原子摩擦過程の分子動力学シミュレーション

In order to dynamically clarify the friction mechanism between specimen surface and probe tip in the surface observation by atomic force microscope (AFM) or friction force microscope (FFM), several molecular dynamics simulations have been performed. In the simulation, a 3-dimensional simulation model is proposed where the specimen and the probe are assumed to consist of mono-crystalline copper and rigid diamond or carbon atom, respectively. The effect of cantilever stiffness of AFM/FFM is also taken into considered. The surface observation process on a well-defined Cu {100} is simulated. From the simulation results, it is verified that the surface images and the 2-dimensional atomic-scale stick-slip phenomenon just as observed through the real AFM/FFM surface observation can be detected from spring force acting on the cantilever. From the evaluation, it is clarified that the double slip and the dispersion in the stick-slip period occurs when the stiffness in the scan direction of the cantilever is low.

Thermal effects on atomic friction.

We model friction acting on the tip of an atomic force microscope as it is dragged across a surface at nonzero temperatures. We find that stick-slip motion occurs and that the average frictional force follows (absolute value lnv)(2/3), where v is the tip velocity. This compares well to recent experimental work, permitting the quantitative extraction of all microscopic parameters. We calculate the scaled form of the average frictional force's dependence on both temperature and tip speed as well as the form of the friction-force distribution function.

Frictional properties of molecular thin films

AbstractIn this paper, we discuss the frictional behaviour of molecular thin films on a mica surface at a nanometre scale. Carbon 60 (C60)‐based macromolecules were synthesized experimentally. Analytical techniques such as Fourier transform infrared spectroscopy, UV–VIS spectroscopy and gel permeation chromatography were used to analyze the synthesized product. The Langmuir–Blodgett technique was used in making the macromolecules in the thin film on the mica surface. Atomic force microscopy and friction force microscopy were used for surface characterization. The properties, novel structures and their effects on frictional properties are discussed based on experiment results. It is found that the molecular thin films can be transferred onto the mica surface. These thin films possess significant tribological properties. Copyright © 2001 John Wiley & Sons, Ltd.

VISUALIZATION OF ATOMIC FORCE MICROSCOPY FROM MOLECULAR DYNAMICS SIMULATIONS

A computer visualization method is used to present the details of the imaging action by the surface scanning equipment—Atomic Force Microscope ~AFM! from the Molecular Dynamics ~MD! simulations. The purpose of the scanning process is to resolve ~image! the surface details, to observe the tip-substrate interactions, and to record the dynamics of transport processes in the film down to the atomic or molecular level. The device uses a nondestructive tip to probe the interatomic and intermolecular forces and is schematically shown in Fig. 1. Figures 2 through 5 show results of MD simulations of an AFM cantilever tip that probes a sample covered with the lubricant film. The cantilever tip is made of nickel ~Ni, shown in red color!, the sample is gold ~Au, shown in gold color!, and the lubricant film consists of several long chain molecules of the polymer ~shown in green color!. Figure 2 presents an instantaneous snapshot of the polymer molecular motion around the AFM tip induced by intermolecular interactions between the molecules of the substrate, film, and the cantilever tip. In Fig. 3, we show the topology of the surface atomic structure of the substrate via mapping the local height field by the color image to generate the rubbersheet-like surface. This is virtual representation of the instantaneous topographic image of the surface generated by AFM. Figure 4 presents computer imaging of the short range axial intermolecular forces ~i.e., atomic normal stresses! induced by atoms located nearby to the cutting plane, while the atomic level friction forces induced by the tip and polymer molecules on the sample surface are demonstrated in Fig. 5. For the purpose of visualization of the unstructured data irregularly located in space, the mapping surfaces are created to interpolate the scattering data points into correct field values on the mapping surface ~Fig. 4!.

Atomic friction studies on well-defined surfaces

Atomic friction studies have been performed by means of a friction force microscope (FFM) in ultrahigh vacuum, where well-defined surfaces can be prepared. A home-built FFM allows us to study lateral forces as low as 0.05 nN using rectangular silicon cantilevers. Furthermore, comparison with dissipation measurements performed in non-contact mode are possible. Recent experimental results are presented and discussed in the framework of a one-dimensional Tomlinson model which includes thermal activation. Atomic-scale stick–slip processes on a metallic surface could be repeatedly measured on Cu(111), while the Cu(100) surface was distorted by the tip during the scanning process. A logarithmic velocity dependence of atomic friction has been measured on Cu(111) and NaCl(100) for low scanning velocities. The dissipation found in stick–slip measurements is compared to the power loss detected in dynamic non-contact measurement.

Analyzing Elastic-Plastic Real Rough Surface Contact in Running-in

A numerical method is presented for evaluating the elastic-elastic contact of real rough surface contacts during running-in. For the surface contact, an elastic-plastic model based on the variational method is applied to analyze the pressure distribution and contact area of worn surfaces during running-in. In conjunction with the classical statistic model of Greenwood and Williamson, the numerical result showed that the plasticity index Ψ was decreased to one in the elastic range as running-in proceeded. In comparison with the Hertzian solution, the influence of the asperities is very significant on the pressure distribution, thereafter causing a higher peak value of contact pressure. For the subsurface, the interior stress from the von Mises criterion was calculated to evaluate the subsurface stress field subject to both normal and tangential forces. In the calculated of the interior stress, the total stress is decomposed into a fluctuating component and a smooth component. The fluctuating part is solved by using FFT from the concept of the convolution theorem while the smooth part is obtained directly by analytical solution. Calculations of contact area and subsurface stress on experimentally produced surfaces whose topography has been determined using an atomic force microscope and friction coefficient front sliding have been carried out. The results showed that asperities and friction coefficient gave rise to stress increase in the near-surface stress field and produced a high stress zone towards the surface. As a result, transverse asperity cracking was produced. The calculations and supporting experimental evidence clearly confirmed that the reduction of peak pressure during running-in decreased the plastic deformation of contact.

Atomic indentation and friction of self-assembled monolayers by hybrid molecular simulations

This paper focuses on the atomic indentation and friction properties of self-assembled monolayers (SAMs) by a novel hybrid molecular simulation approach. By introducing a sliding dynamics for the tip-cantilever assembly in atomic force microscopy (AFM) and a fast molecular dynamics relaxation algorithm for SAMs, we simulate the scanning process of the assembly over SAMs in the time scale of AFM experiments. For the atomic indentation of SAM surfaces, we find that elastic modulus is chain-length independent, and has a value of 20±10 GPa. However, under shear, effective shear modulus is found to be chain-length dependent, which explains the SAM chain-length dependence of friction observed in AFM experiments. The calculated surface energy of methyl terminated SAMs is consistent with many experimental results.

Investigation on microtribological behavior of thin films using friction force microscopy

A purpose-built atomic force and friction force microscope (AFM/FFM) was employed to study micro friction and wear behavior of thin films of Langmiur–Blodgett (L–B), gold (Au), polytetrafluoroethylene (PTFE), silicon nitride (Si 3N 4) and PTFE/Si 3N 4 multi-layers prepared by different deposition techniques. The results show that the L–B film has a low friction coefficient but can be worn easily under a light load less than 20 nN; Au film and silicon wafer has a higher friction coefficient than that of L–B film and can be worn under a higher load of approximately 50 nN; the PTFE/Si 3N 4 multilayer has a lower friction coefficient than that of Si 3N 4 and has a higher micro wear resistance than that of PTFE. The friction force of the PTFE/Si 3N 4 multilayer is linear with the load at the nanometer scale. The worn track is formed in PTFE film and PTFE/Si 3N 4 multilayers when the load is greater than 70 nN.

Formation Mechanism of Multilayer Polyamic Acid Alkylamine Salt Langmuir–Blodgett Films

In this paper we present atomic force microscopy (AFM) and friction force microscopy (FFM) analyses on the surface structure of mono- and multilayer films prepared from two types of polyamic acid with long alkylamine chains, which were utilized as the precursors of polyimide Langmuir–Blodgett (LB) films. The characteristic domain structures on the monolayer films, namely, the plateau type and the hollow type, were examined while changing the film preparation conditions and the chemical structures of the precursors. The surface structures of the multilayer LB films were also analyzed by AFM, FFM and FT-IR techniques. It was found that the domain structures that appeared on the monolayer film were transformed to a smooth domain-free structure by multilayer deposition of the LB film. The formation and smoothing mechanism of the domain structures was discussed taking into account the fluidity of the polymer groups controlled by the frictional force at the film interface.

Atomic force microscopy and friction force microscopy studies of ferroelastic crystal surfaces

Topography and frictional properties of freshly cleaved surfaces of ferroelastic crystals: K3Na(SeO4)2 (KNSe), and NH4LiH3(SO4)4 (ALHS), and Gd2(MoO4)3 (GMO) were investigated by combined scanning and friction force microscopy (FFM) under ambient and UHV conditions. Atomic force microscopy (AFM) images revealed stepped surfaces with step heights corresponding to the multiplication height of crystal lattice constant fractions e.g., c/2 or c/4. A frictional contrast was observed between terraces which are separated by steps and between different domains of ferroelastic crystals. We suggest that the frictional contrast is due to different molecular orientations at different terraces and/or at different domains of a ferroelastic crystal, in the case of GMO, surface bends at the domain boundary with the angle estimated to be about 2.35°. It was also shown that the ALHS crystal surface can be modified (e.g., by changing the step profile and creation of the artificial bumps) by an AFM tip imaging with normal force above 100 nN.

Microtribological properties of ultrathin C60 films grown by molecular beam epitaxy

Molecular beam epitaxy (MBE) has been applied to a condensation of thin C60 films with high crystallinity. In situ measurements with reflection high energy electron diffraction (RHEED) showed an epitaxial growth of C60 film with a lattice constant of 1.0 nm on a MoS2 substrate. Atomic force microscopy (AFM) and friction force microscopy (FFM) revealed a low friction coefficient of 0.012, the lowest value reported to date for C60.

Analysis of Microstructure of Polyamic Acid Alkylamine Salt Monolayers by Atomic Force Microscopy and Friction Force Microscopy

The surface microstructures of monolayers of polyamic acid alkylamine salts having long alkyl chains were analyzed by atomic force microscopy (AFM) and friction force microscopy (FFM). Two different aromatic structures of polyamic acids and four different single chain alkylamine structures were used for the preparation of polyamic acid alkylamine salt monolayers. Although single chain alkylamines tend to form an island-like domain structure, the homogeneous island-free surface structure of monolayers were observed for the long alkyl chain dimethylamine derivative. The island-like structure was formed by the coagulation of alkylamine during the film preparation process. It was found that the coagulation of alkylamine chains could be eliminated by the selection of the starting single chain alkylamine structure and by the conditions of monolayer deposition, such as surface pressure.

Load dependence of sticking-domain distribution in two-dimensional atomic scale friction of NaF(100) surface

Remarkable transitions were found in atomic-scale friction-images of the NaF(100) surface and the corresponding sticking-domain distribution by detailed investigation of the load dependence using the two-dimensional frictional force microscope. The tip-position map reveals a new type of sticking-domain distribution pattern, which is different from the simple lattice periodicity.