Lateral Friction Force Research Articles

Amplitude Dependence of the Lateral-Vibration Wear Test for Perpendicular Recording Magnetic Disks Treated by Heat Curing

The tribological characteristics of perfluoropolyether (PFPE) and heat-treated PFPE lubricant films deposited on magnetic thin diamond-like carbon (DLC) film-coated disks by dip coating were studied using lateral modulation frictional force microscopy (LM-FFM). The topography and microstructural properties of the DLC film disk before PFPE lubricant dip coating were investigated by transmission electron microscopy (TEM) and atomic force microscopy (AFM) in the dynamic force modulation (DFM) and frictional force modulation (FFM) modes. The TEM and AFM images show valleys and fine spaces formed between grain boundaries. It is possible that free lubricants reserved in these valleys and spaces during dip coating replenished the surface as a result of tip sliding. In wear tests, the friction properties of the PFPE-DLC disk without heat curing were improved due to the supply of lubricant.

Nanoindentation and nanowear of extremely thin protective layers of C–N and B–C–N

Carbon nitride (C–N) and boron and carbon nitride (B–C–N) films, 1 and 3 nm thick, were deposited on magnetic disks by means of a complex treatment method involving plasma irradiation and CN or BCN reactive sputtering in nitrogen and helium mixed gas using two targets of h-BN and graphite. The properties of these extremely thin coatings were evaluated by indentation using an atomic force microscope and nanowear tests using a lateral modulation friction force microscope. The extremely thin B–C–N coatings show highest indentation hardness and good wear resistance properties. It is proposed that this is due to their graduated composition and interfacial properties.

A 3D Brush-type Dynamic Tire Friction Model

The use of advanced dynamic friction models can improve the brush-type tire friction models. This paper presents a 3D dynamic brush model based on the LuGre friction model. The model describes the dynamics of longitudinal and lateral tire friction forces, as well as the self aligning torque dynamics. It has been originally derived in a distributed-parameter form, and then transformed to a simpler lumped-parameter form with only three internal states. Both uniform and non-uniform normal pressure distributions are considered. The model has analytical solution for steady-state conditions. The steady-state behavior is validated with respect to “magic” formula static model, which served as an “ideal” benchmark. The lumped model dynamic behavior is validated by comparing its time-responses with original distributed model responses. The model parameterization with respect to normal force and other tire/road parameters is considered as well.

Tribological behavior of W 2C nano-particle reinforced Si 3N 4 matrix composite

W 2C nano-particle reinforced Si 3N 4 matrix composite was fabricated by sintering tungsten solution-infiltrated porous Si 3N 4, and the microstructures, sliding wear and micro-tribological behaviors of W 2C–Si 3N 4 composite and sulphided W 2C–Si 3N 4 composite were studied. By incorporating the W 2C nano-particles into the grain boundary phases of Si 3N 4, not only the mechanical properties, but also the tribological behaviors were improved. The W 2C–Si 3N 4 composite showed the lower friction resistance compared with the Si 3N 4, and the W 2C particles had an effect to prevent the wear debris from adhering to the wear scar. After sulphide treatment, the nano-meter sized WS 2 particles were observed from the surface of W 2C–Si 3N 4 composite. The average friction coefficient and specific wear rate of the sulphided W 2C–Si 3N 4 composite were 0.41 and 5.6×10 −6 mm 3/Nm, under dry condition, 34, 47% lower than those for the Si 3N 4, respectively. The micro-tribological behavior of sulphided W 2C–Si 3N 4 composite was evaluated using the lateral force modulation friction force microscope, and the WS 2 particles with very low friction resistance were confirmed from the surface. The formation of the solid lubricant WS 2 on the surface of the W 2C–Si 3N 4 composite lowered both the friction coefficient and specific wear rate, and exhibited self-lubricated behavior during the sliding wear.

Frictional Forces and Amontons' Law: From the Molecular to the Macroscopic Scale

We review the historical and modern understanding of the most basic equation of friction, Amontons' law, which describes phenomena that were already understood and studied by Leonardo da Vinci 500 years ago. This law states that for any two materials the (lateral) friction force is directly proportional to the (normal) applied load, with a constant of proportionality, the friction coefficient, that is constant and independent of the contact area, the surface roughness, and the sliding velocity. No theory has yet satisfactorily explained this surprisingly general law; all attempts have been model or system dependent. We review the experimental evidence and find, for example, that the same friction coefficient is often measured for the same system of materials with junctions whose areas differ by more than 6 orders of magnitude. The trends obtained through molecular dynamics (MD) simulations agree with recent and past experiments and with Amontons' law, and they suggest that the local energy-dissipating mec...

AFM Nanodissection Reveals Internal Structural Details of Single Collagen Fibrils

We utilize the atomic force microscope (AFM) to perform a careful nanodissection procedure on a single collagen fibril. By precisely incising the fibril and then peeling away exterior layers, its internal structure was successfully exposed for direct visualization. This methodology has the advantage of avoiding unnecessary rough contact with the specific area being examined as well as high lateral frictional forces, both of which can hinder attempts to dissect delicate biological specimens with the AFM.

Measurement of mobility of adsorbed colloids by lateral force microscopy

The surface mobility of colloidal latex particles adsorbed on mica was measured by moving the particles with an AFM tip in the lateral force microscopy mode. The same particle was repeatedly scanned while the normal force was gradually increased, until the particle was displaced from its location on the substrate. The lateral (friction) force curve obtained for that scan was then used to determine the force needed to displace the particle. The data accumulated for approximately 100 particles indicate a wide distribution in the lateral force required. However, the data show that the mean lateral force is proportional to the particle diameter, with the effect of electrostatic interactions on the mobility of adsorbed particles seen to be weak. These results are consistent with classical theories of friction in macroscopic systems.

Dynamic Contact Characteristics During Chemical Mechanical Polishing (CMP)

AbstractIn chemical mechanical polishing (CMP), it is critical to understand dynamic contact at the pad-particles-wafer interface for desired CMP performance. The dynamic contact is dependent on process variables (platen velocity and down pressure) and particle characteristics (size and concentration), which in turn affect friction force. In this study, we have characterized the dynamic contact at the pad-particles-wafer interface as a function of platen velocity and down pressure. In situ lateral friction force measurements were carried out for silica slurry / sapphire wafer system in order to investigate the dynamic contact during polishing. As solids loading increases, the slope in the friction force vs. platen velocity curve changes from a negative to a positive value. Friction force increases with down pressure for different solids loading conditions. Consequently, friction force is determined as a function of down pressure and platen velocity, validating a dynamic contact mechanism during CMP.

Atomic Force Microscopy Observation of the Phase Separation in Acidic Phospholipid/Neutral Phospholipid Supported Bilayers

Phase separation induced by Ca2+ in the DPPS (dipalmitoyl phosphatidylserine)/DPPC (dipalmitoyl phosphatidylcholine) or DPPS/DOPC (dioleoyl phosphatidylcholine) system was observed by atomic force microscopy (AFM). Lateral modulation frictional force microscopy (LM-FFM) revealed phase separation in supported bilayer membranes prepared by the transfer of the monolayers of DPPS/DPPC with a small amount of Ca2+ in the subphase, while the phase separation was not shown in their topographic images obtained by contact mode AFM. Phase separation induced by Ca2+ in DPPS/DOPC mixed systems was observed in topographic images and the time course of the appearance and development of DPPS/DOPC (1/9) phase separation after the addition of Ca2+ (0.1 mM) was revealed.

Characterization of nanotubes as microscale beam manipulators: transmission of neutral atoms and low-energy ions

Physical processes related to the passage of low-energy ions and atoms (plasma particles) in cylindrical channels (nanotubes) are discussed. It is shown that dynamics of particle motion inside the channel (at angles of incidence up to 0.2 rad) is governed by initial entrance conditions. The transverse motion is well described by broken-line trajectories. Different contributions to the lateral (frictional) forces are compared and shown to be dependent on the charge state of particles. For long channels, the particles are neutralized and experience very low lateral forces. As a result, the penetration depth grows by one to two orders of value.

Adhesion, friction, and mechanical properties of functionalized alkanethiol self-assembled monolayers.

We have used interfacial force microscopy to study the adhesion, friction, and mechanical properties of molecular monolayers self-assembled on Au surfaces. This quantitative and stable scanning-probe technique permits detailed studies of these factors. By systematic variation of the chemical nature of the end groups on the monolayers and utilization of standard and intuitive contact-mechanics models, quantitative results are presented of inter- and intrafilm bonding strength as well as the relationship between mechanical behavior and the lateral friction force.

Effects of Particle Concentration in CMP

ABSTRACTThis paper reports on characterization of the surface coverage of particles by in-situ lateral friction force measurement during chemical mechanical polishing. The lateral friction force apparatus was made to operate close to real CMP conditions. For these experiments a sapphire wafer of constant surface roughness was used. For both 2psi and 4psi down force we observed increase in lateral friction forces with increasing solid loading. The lateral friction forces have been found to be significantly dependent on the contact area at the wafer-pad-slurry interface, thus showing that in-situ dynamic friction force changes in the surface coverage of particles. From these results, we conclude that the enhancement of frictional force is due to increased contact area at the wafer-pad-slurry interfaces. The lateral friction force measurement can provide an understanding of wafer-pad-slurry interactions.

Study of Ultrafine Iron Powders by Atomic Force Microscopy

The particles of ultrafine iron powders obtained by three different methods (electrolytic deposition, reduction in hydrogen flow, and grinding in a planetary ball mill in heptane medium) were studied by the atomic force microscopy and the results were compared with the data of electron microscopy and X-ray diffraction analysis. The shape and size of particles were determined from three-dimensional images obtained by atomic force microscopy, and the grain structure of the particle surface layer was studied by measuring the lateral friction forces.

Transverse Elastic–Friction Vibrations of a Running Aircraft

An analytical model of the plane-parallel running of an aircraft under crosswind load is proposed. The compliance of the internal constraints of the aircraft and the nonlinearity of the side force on the landing gear wheels are taken into account. The model is intended to study the transverse elastic friction vibrations of the aircraft hull and landing gears. The steady motion of the aircraft is considered. The dependence of the lateral friction force on the slip angle is assumed to have the form of a sinusoidal segment. The Routh–Hurwitz instability conditions are derived in an explicit form

Lateral Force Microscopy Study of the Friction between Silica Surfaces

The friction between a single silica glass sphere and a flat silica glass surface has been investigated using lateral force microscopy. Two types of silica surface were investigated, one fully hydroxylated and the other partially dehydroxlated. These samples have markedly different wetting properties and are known as hydrophilic and hydrophobic silica, respectively. The lateral friction force as a function of applied normal load has been collected for five different-sized spheres for both surface types. Analysis of the friction versus load data indicates that the hydrophilic surfaces have a molecularly smooth contact. For the hydrophobic surfaces, it is not clear whether the friction-load response is caused by a smooth or an asperity-dominated contact zone.

Frictional Force Microscopy of Oxidized Polystyrene Surfaces Measured Using Chemically Modified Probe Tips

Chemical force microscopy (CFM) using probe tips that have been chemically functionalized to give hydroxylated (polar) or methylated (apolar) surfaces has been used to investigate frictional properties of polystyrene (PS) films that have been oxidatively modified to varying degrees using an ultraviolet-ozone (UVO) treatment. Oxygen chemisorption levels and functional group chemistry of the PS films have been followed using X-ray photoelectron spectroscopy (XPS), and the resulting data have been correlated with lateral (frictional) force measurements for each surface type. XPS analysis showed that UVO treatments introduced polar species at the film surfaces via the formation of C−O, CO, and O−CO functional groups. CFM was performed on the native and treated films using tips that were either hydroxyl- or methyl-terminated. The treated films exhibit substantially higher friction than untreated films when imaging with the hydroxyl-terminated tips. Friction is reduced when nonpolar methyl-functionalized tips a...

Nanotribology of behenic acid monolayer and its aggregation state

The tribological properties of behenic acid monolayers in CS and L 2 phases were investigated on the basis of scanning probe microscopic observations. The lateral (friction) force of the monolayers in CS and L 2 phases increased consistently with applied force. The increment in CS phase comes from an increase in contact area between a probe and the monolayer owing to elastic deformation, while that in L 2 phase does from not only increase in contact area owing to plastic deformation but wear of the monolayer. The lateral force of the monolayer in CS phase was independent of the scan velocity. On the other hand, the monolayer in L 2 phase exhibited the scan velocity dependence of the lateral force. The difference in tribological properties of the monolayer is attributable to the static mechanical properties of elasticity and plasticity which are governed by the molecular packing in monolayer.

Atomic force microscopy study of the morphology of polythiophene films grafted onto the surface of a Pt microelectrode array

This work involves the application of atomic force microscopy (AFM) to the study of the morphology of polythiophene (PT) films grafted onto an array of six-band microelectrodes used for the measurements of in situ electronic conductivity of polymeric films. The following two types of the polymeric film can be distinguished: one film is related to the layer just above the microelectrodes, whereas the other type of film bridges the gap between the two microelectrodes. We have shown that the gaps located in the interior part of the array are completely bridged by the PT film; however, the thickness of the film above the microelectrodes is about one order of magnitude larger than that related to the center of the gap. The Z-profile of the film across the gap has a rounded shape. The lateral (friction) force image taken from both types of the PT films shows that they are similar. The AFM images prove that the PT film possesses mainly a granular-type morphology.

CURVE SQUEAL OF TRAIN WHEELS, PART 1: MATHEMATICAL MODEL FOR ITS GENERATION

A mathematical model is presented for the squeal noise generated by trains when traversing tight curves. Curve squeal is presumed to arise from lateral crabbing of the wheels across the rail head. This induces a lateral friction force acting at the contact of each wheel with the rail. An individual wheel then performs out-of-plane oscillations which are radiated and heard as squeal. This phenomenon is modelled by considering a flat round disc, with several out-of-plane modes, excited at one point along the edge by a dry-friction force (typically a stick/slip force) which is dependent on the disc velocity. An iteration scheme is developed which gives the time history of the disc velocity. The iteration is straightforward and only requires the impulse response (or the Green's function) of the disc and the functional dependence between friction force and disc velocity (friction characteristic). The numerical simulations produce time histories that show transient phenomena, such as exponential amplitude growth and the onset of limit cycles. The way these phenomena are influenced by some parameters, in particular the modal loss factors of the disc and its crabbing speed, will be examined. Practical methods to reduce or eliminate curve squeal will be discussed.

Scanning force microscopy experiments probing micromechanical properties on polymer surfaces using harmonically modulated friction techniques II. Investigations of heterogeneous systems

AbstractApplying a high‐frequency lateral vibration between tip and sample in a scanning force microscope (SFM), a harmonically modulated lateral (friction) force image can be obtained using lock‐in techniques. Harmonically modulated lateral force microscopy (HM‐LFM) offers several advantages compared with standard lateral force microscopy (LFM). After a brief investigation of the scan velocity dependence of LFM and HM‐LFM, two samples were investigated. First, the surface of a poly(acrylonitrile‐co‐styrene)/polybutadiene blend (ABS) was used to demonstrate the ability of the new technique to decrease the stick effects of the SFM tip. Second, an interface between two chemically very similar polymers was prepared by melting polypropylene (PP) and poly(propene‐block‐ethene) (PP‐block‐PE) films on each other. After cutting, the surface roughness of this sample was very high. It is shown that only HM‐LFM clearly resolves the local micromechanical properties without artefacts.