Measurements of tribological properties of poly(pyrrole) thin film bearings

We report the results of a recent study on the tribological properties of electropolymerised thin films at light loads and low speeds. Poly(pyrrole) films incorporating different counter-ions have been electrochemically deposited onto gold electrodes on the plano-convex glass substrates and studied extensively. The measuring apparatus has been greatly improved from that reported earlier and now provides simultaneous monitoring of frictional force and wear. High precision capacitive gauging is employed to provide high resolutions of frictional force of better than 100 μN and height variation (wear) of 2 nm. A large number of specimens of poly(pyrrole) grown from five different counter-ions were prepared and their performances evaluated. The film morphology of each type of film was examined by atomic force microscopy (AFM) for control of the variability of film formation. Results are presented for the friction coefficients and wear rates observed for the films typically at a load of 2 N and a sliding speed of 5 mm s⁻¹. The effects of normal loading force and sliding speed on the friction coefficient are also discussed with a load range of 0.2–5 N and a sliding speed up to 30 mm s⁻¹.     

Nanofriction Mechanisms Derived from the Dependence of Friction on Load and Sliding Velocity from Air to UHV on Hydrophilic Silicon

This paper examines friction as a function of the sliding velocity and applied normal load from air to UHV in a scanning force microscope (SFM) experiment in which a sharp silicon tip slides against a flat Si(100) sample. Under ambient conditions, both surfaces are covered by a native oxide, which is hydrophilic. During pump-down in the vacuum chamber housing the SFM, the behavior of friction as a function of the applied normal load and the sliding velocity undergoes a change. By analyzing these changes it is possible to identify three distinct friction regimes with corresponding contact properties: (a) friction dominated by the additional normal forces induced by capillarity due to the presence of thick water films, (b) higher drag force from ordering effects present in thin water layers and (c) low friction due to direct solid–solid contact for the sample with the counterbody. Depending on environmental conditions and the applied normal load, all three mechanisms may be present at one time. Their individual contributions can be identified by investigating the dependence of friction on the applied normal load as well as on the sliding velocity in different pressure regimes, thus providing information about nanoscale friction mechanisms.     

Optimum film thickness of thin metallic coatings on silicon substrates for low load sliding applications

The frictional behaviour of thin metallic films on silicon substrates sliding against 52100 steel balls is presented. The motivation of this work is to identify an optimum film thickness that will result in low friction under relatively low loads for various metallic films. Dry sliding friction experiments on silicon substrates with soft metallic coatings (silver, copper, tin and zinc) of various thickness (1–2000 nm) were conducted using a reciprocating pin-on-flat type apparatus under a controlled environment. A thermal vapour deposition technique was used to produce pure and smooth coatings. The morphology of the films was examined using an atomic force microscope, a non-contact optical profilometer and a scanning electron microscope. Following the sliding tests, the sliding tracks were examined by various surface characterization techniques and tools. The results indicate that the frictional characteristics of silicon are improved by coating the surface with a thin metallic film, and furthermore, an optimum film thickness can be identified for silver, copper and zinc coatings. In most cases ploughing marks could be found on the film which suggests that plastic deformation of the film is the dominant mode by which frictional energy dissipation occurred. Based on this observation, the frictional behaviour of thin metallic coatings under low loads is discussed and friction coefficients are correlated with an energy based friction model.     

Effects of film characteristics on frictional properties of carboxylic acid monolayers

Monolayers of long-chain carboxylic acids, with various fluorination ratios, were deposited on solid substrates to investigate the effects of surface properties on frictional behavior. It is found that a lower surface free energy correlates to a lower friction coefficient for the monolayers of partially fluorinated carboxylic acids. However, a stearic acid (C₁₇H₃₅COOH) monolayer shows the lowest friction coefficient, although its surface free energy is relative high. The two-dimensional elastic modulus, which might be used as a parameter to quantitatively characterize the film strength, was developed. Friction coefficients of the monolayers show a strong correlation with their elastic modulus, that is, the higher the elastic modulus, the lower the friction coefficient. The research results indicate that frictional properties of ultrathin films are mainly dependent on film strength. Enhancement of intermolecular attractive force might be a more effective way to improve the lubricating properties of ultrathin films.     

Investigation of adhesive and frictional behavior of GeSbTe films with AFM/FFM

Frictional force microscope (FFM) was used to investigate the nanoscale frictional behavior of GeSbTe films deposited by magnetron sputtering. The effects of relative humidity, scanning velocity and surface roughness on friction were taken into account. Besides, the frictional behavior of GeSbTe films with different compositions was analyzed. Experimental results show that the coefficient of friction of GeSbTe films is almost independent of scanning velocity, while the frictional force decreases with increasing velocity. Both the relationship of friction vs. normal load and that of friction vs. RMS keep relatively linear, and the coefficient of friction increases with the increase in RMS. The influence of humidity on adhesion between the tip and the GeSb₂Te₄ film is more significant than that between the tip and the Ge₂Sb₂Te₅ film.     

Effects of film thickness and contact load on nanotribological properties of sputtered amorphous carbon thin films

The nanotribological properties of amorphous carbon (a-C) films of thickness in the range of 5-85 nm sputtered on Si(1 0 0) substrates were investigated with a surface force microscope (SFM), using a Berkovich diamond tip of nominal radius of curvature approximately equal to 200 nm and contact (normal) loads between 10 and 1200 μN. The dependence of the friction and wear behaviors of the a-C films on normal load and film thickness was studied in terms of nanomechanical properties, images of scratched surfaces, and numerical results obtained from a previous analytical friction model. The increase of the contact load caused the coefficient of friction to decrease initially to a minimum value and, subsequently, to increase to a maximum value, after which, it either remained constant or decreased slightly. The dominant friction mechanism in the low-load range was adhesion, while both adhesion and plowing mechanisms contributed to the friction behavior in the intermediate- and high-load ranges. Thinner (thicker) a-C films yielded higher (lower) friction coefficients for normal loads less than 50 μN (low-load range) and lower (higher) friction coefficients for normal loads greater than 150 μN (high-load range). Elastic and plastic deformation, microcracking, and delamination of the a-C films occurred, depending on the contact load and film thickness ranges. The reduced load-carrying capacity, relatively low effective hardness (strength) obtained with thinner films, and dominant friction and wear mechanisms at each load range illustrate the film thickness and contact load dependence of the nanotribological properties of the sputtered a-C films.     

Clutch Shudder Correlated to ATF Degradation through Local Friction vs. Velocity Measurements by a Scanning Force Microscope

Shudder in continuously slipping wet clutch (CSWC) systems is an instability that manifests itself as a vibration of the automobile's drive train. Dynamometer experiments imply that the shudder is connected to degradation of the frictional properties of the clutch friction material-automatic transmission fluid (ATF) system. The poor friction characteristics are revealed in negatively sloped torque vs. slip-speed curves which can result in dynamic friction oscillation instabilities. The authors report local measurements of friction vs. velocity with the use of a scanning force microscope (SFM) on new and glazed wet clutch friction material with either new or degraded ATF acting as a lubricant. The friction material was a commonly used cellulose/phenolic resin material. Results in the very low-speed dynamic regime indicate that the fluid condition plays a critical role in determining the slope characteristics of the friction vs. velocity curve. ATF degraded in service imparts a flat or negative slope to the friction vs. velocity curve, whether on new or glazed (degraded) friction material. A positive slope is measured for fresh ATF. The SFM measurements are therefore consistent with dynamometer and vehicle experiments which measure gross properties of the friction clutch system. The ability of the SFM to reproduce the dynamometer results on a local scale implies that shudder is not due to the gross fluid flow or mechanical effects that are present in a complete torque converter. The frictional characteristics of the clutch plate materials and the ATF are paramount. This technique allows continuous monitoring of ATF fluid degradation in test fleet vehicles without ATF replenishment since only a few ccs of fluid are necessary for these tests.     

Observations of Stick-Slip Friction in Velcro®

Friction, and in particular stick-slip friction, occurs on every length scale, from the movement of atomic force microscope tips at the nanoscale to the movement of tectonic plates of the Earth’s crust. Even with this ubiquity, there still appears to be outstanding fundamental questions, especially on the way that frictional motion varies generally with the mechanical parameters of a system. In this study, the frictional dynamics of the hook-and-loop system of Velcro^® in shear is explored by varying the typical parameters of driving velocity, applied load, and apparent contact area. It is demonstrated that in Velcro^® both the maximum static frictional force and the average kinetic frictional force vary linearly with apparent contact area (hook number), and moreover, in the kinetic regime, stick-slip dynamics are evident. Surprisingly, the average kinetic friction force is independent of velocity over nearly two-and-a-half orders of magnitude (~2 × 10^?4 to ~6 × 10^?2 m/s). The frictional force varies as a power law on the applied load with an exponent of 0.28 and 0.24 for the maximum static and kinetic frictional forces, respectively. Furthermore, the evolution of stick-slip friction to more smooth sliding, as controlled by contact area, is demonstrated by both a decrease in the spread of the kinetic friction and the spread of the fluctuations of the average kinetic friction when normalized to the average kinetic friction; these decreases follow power-law behaviors with respect to the increasing contact area with exponents of approximately ?0.3 and ?0.8, respectively. Lastly, we note that the coefficients of friction μ _s and μ _k are not constant with applied load but rather decrease monotonically with power-law behavior with an exponent of nearly ?0.8. Phenomenologically, this system exhibits interesting physics whereby in some instances it follows classical Amontons–Coulomb (AC) behavior and in others lies in stark contrast and hopefully will assist in the understanding of the friction behavior in dry surfaces.     

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 m scale the cleavage structure with monoatomic steps of 3.5 ± 0.3 Å is revealed. On the atomically flat terraces, atomic-scale resolution is achieved. The resolved square lattice shows a periodicity of 4.7 Å and corrugations of 0.3–0.7 Å 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.     

Frictional properties of sputtered tin film

The structural changes and frictional properties of sputtered tin films on a glass plate were investigated as a function of the effects of film thickness and load and the influence of oxidizing, reducing and inert atmospheres. X-ray analysis of surface texturing showed that crystal rotation was a fundamental change in the structure of the surface, the rotation axis being dependent on the direction of rubbing. It was found that the coefficient of friction of tin films rubbing against SUS 304 steel decreased with decreasing ambient pressure and reached a minimum in the range 1–10 Torr.The coefficients of friction of thin films of SnO and SnO₂ were also investigated and discussed relative to the oxidation of the tin film during friction. It was concluded that the coefficient of friction of tin is controlled by gases adsorbed on the interface rather than by the formation of oxides during friction.     

Contact mechanics and friction of fractal surfaces probed by atomic force microscopy

We investigated the contact mechanics and friction forces between atomic force microscope (AFM) probes and self-affine fractal carbon films. We studied single-asperity contacts by means of conventional nanometric conical tips whilst custom-designed micrometric flat tips were adopted to form multiple junctions between the probe and the sample. By varying the externally applied load we found that the average frictional force follows a power-law behavior in the single-asperity regime and a linear behavior in the multi-asperity regime. The friction coefficient was the same for carbon specimens having different fractality. We also acquired quasi-static load-displacement curves on micrometric scale, revealing a strong dependence of the average indentation depth on the values of fractal parameters. A comparison of experimental data with contact theories for randomly rough surfaces is provided.     

An investigation of the tribological properties of thin KCl films on iron in ultrahigh vacuum: modeling the extreme-pressure lubricating interface

The frictional properties of thin KCl films deposited onto clean iron are measured in ultrahigh vacuum using a tungsten carbide tribotip, where the observed initial rapid decrease in friction coefficient with film thickness is proposed to be due to the formation of a complete KCl monolayer where the friction coefficient of this film is ∼0.27. A 1800 Å thick KCl film shows a hardness and friction coefficient similar to those for bulk KCl when the width of the surface height distribution of the tribotip measured by atomic force microscopy (AFM) is 2000–3000 Å. This implies that the KCl film behaves like the bulk material when the film thickness exceeds the roughness of the interfaces.     

Extremely low coefficient of friction of diamond sliding against Ag thin films on Si(1 1 1) surface under ultrahigh vacuum condition

The tribological properties of a diamond (1 1 1) pin slid on subnanometer thick Ag films, which were deposited on a cleaned Si(1 1 1) substrate, were studied using a pin-on-plate tribometer. The preparation of Ag ultrathin films and frictional experiments were performed in an ultrahigh vacuum (UHV) chamber at a pressure of 10⁻⁸ Pa. The frictional experiments were carried out at a sliding speed of 0.1 mm/s and at a normal load of 250 mN. An extremely low coefficient of friction, less than 0.01, was obtained when the pin was slid on Ag films, whose thicknesses were 1 and 2.6 monolayer (ML) under reciprocal motion. The minimum coefficient of friction was less than 0.004 for Ag 1 ML film. After the extremely low coefficient of friction was obtained, Ag remained on the worn track without any transfer to the diamond pin, as confirmed by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS).The mechanisms of extremely low coefficients of friction are discussed in terms of the chemical bonding force between atoms of the topmost layers, and Ag coverage on the Si surface.     

Atomic Friction Investigations on Ordered Superstructures

We review recent friction measurements on ordered superstructures performed by atomic force microscopy. In particular, we consider ultrathin KBr films on NaCl(001) and Cu(001) surfaces, single and bilayer graphene on SiC(0001), and the herringbone reconstruction of Au(111). Atomically resolved friction images of these systems show periodic features spanning across several unit cells. Although the physical mechanisms responsible for the formation of these superstructures are quite different, the experimental results can be interpreted within the same phenomenological framework. A comparison between experiments and modeling shows that, in the cases of KBr films on NaCl(001) and of graphene films, the tip-surface interaction is well described by a potential with the periodicity of the substrate which is modulated or, respectively, superimposed with a potential with the symmetry of the superstructure.     

1D Model of Precursors to Frictional Stick-Slip Motion Allowing for Robust Comparison with Experiments

In this article, we study the dynamic behaviour of 1D spring-block models of friction when the external loading is applied from a side, and not on all blocks like in the classical Burridge–Knopoff-like models. Such a change in the loading yields specific difficulties, both from numerical and physical viewpoints. To address some of these difficulties and clarify the precise role of a series of model parameters, we start with the minimalistic model by Maegawa et al. (Tribol. Lett. 38: 313, 2010) which was proposed to reproduce their experiments about precursors to frictional sliding in the stick-slip regime. By successively adding an (i) internal viscosity, (ii) interfacial stiffness and (iii) initial tangential force distribution at the interface, we manage to (i) avoid the model’s unphysical stress fluctuations, (ii) avoid its unphysical dependence on the spatial resolution and (iii) improve its agreement with the experimental results, respectively. Based on the behaviour of this improved 1D model, we develop an analytical prediction for the length of precursors as a function of the applied tangential load. We also discuss the relationship between the microscopic and macroscopic friction coefficients in the model.     

On the scratch behaviour of self-lubricating WSe2 films

Transition metal dichalcogenides films are well known for their self-lubricating properties. These films are having lamellar structure – whereby weak “van der Walls” forces act between the layers – commonly believed to be responsible for their excellent self-lubricating properties. Among these films, diselenoids have shown less sensitivity to humidity and they are more oxidation resistant in humid environment than sulphides. In view of the above, a comprehensive work is undertaken to study friction properties in macro- and micro-scale. The present work deals with preliminary study and critical examination of friction due to scratching of WSe₂ film. WSe₂ film is deposited using sputtering technique. The composition of the film is determined by means of energy dispersive spectrometry (EDS) and X-ray photoelectron spectrometry (XPS). The micro-structural features, topography and mechanical properties of the film are evaluated using transmission electron microscopy (TEM), atomic force microscopy (AFM) and nano-indenter. The film is scratched at different constant loads and also with increasing load using a scratch tester with a spherical indenter in macro-loading regime. A 3D confocal microscope is used to study the scratched portion. In micro-loading regime the film is scratched with an angular indenter. The results show that even though self-lubricating effect comes into play in macro-loading regime, this effect cannot be seen in micro-loading regime. Further coefficient of friction in different loading regime is independent of applied load.     

Frictional behaviour of Pb‐Sn thin‐film lubrication

The frictional properties of lead‐tin thin films (thickness of 0.05–0.19 μm) with two types of copper interlayer were investigated. The thin film and the interlayer were formed on a silicon wafer surface by vacuum deposition. Friction tests were carried out using a ball‐on‐disc apparatus in a vacuum chamber. The thin copper interlayer reduced the friction coefficient and prolonged the film life. The effect of load on the friction coefficient is explained by an equation derived using the Hertzian contact area between a sphere and a plate. The thicker copper interlayer did not reduce the friction coefficient but markedly extended the life of the film. In this case, the dependence of the friction coefficient on the load is explained by an equation derived using the Hertzian contact area between a sphere with surface roughness of second order and a plate.     

A scanning conduction microscopic method for probing abrasion of insulating thin films

The use of scanning force microscopy (SFM) to probe wear processes at interfaces is of considerable interest. We present here a simple modification of the SFM which allows us to make highly spatially resolved measurements of conductivity changes produced by abrasion of thin insulating films on metal substrates. The technique is demonstrated on fluorocarbon polymer thin films deposited on stainless steel substrates.     

Comparative studies of perfluorinated lubricants adsorbed on hydrogenated amorphous carbon and amorphous carbon nitride

Temperature-programmed desorption and scanning force microscopy have been used to probe the interaction of a perfluorinated lubricant (Fomblin ZDOL) with hydrogenated amorphous carbon (a:C-H _x ) and amorphous carbon nitride (a:C-N _x ) substrates, two materials used as hard coatings in disk drive products. Temperature-programmed desorption measurements indicate that the nitride surfaces are more reactive toward this perfluorinated lubricant and, as a result, the thin lubricant film is more tightly bound to this substrate. Frictional force microscopy has been used to measure the coefficient of friction of the lubricated surfaces, 0.18 ± 0.02 for both substrate materials, and finds that frictional properties of these interfaces in the low load regime are influenced more by the presence of the lubricant rather than the adsorbed state of the film. Likewise, similar disjoining pressures were measured for the lubricant adsorbed on the different coating materials and suggest that the ultrathin nature of the adsorbed lubricant film dominates this property rather than adsorption states.     

High load AFM friction and wear experiments on V2O5 thin films

High load friction and wear experiments by means of atomic force microscopy were carried out at the surface of highly (0 0 1) oriented vanadium oxide V₂O₅ thin films deposited on silicon by reactive magnetron sputtering. Microscopic friction coefficient was estimated for wide range of loads. The nature of surface wear due to multiple, high load scanning is presented and discussed.