Understanding Topographic Dependence of Friction with Micro- and Nano-Grooved Surfaces

The work reported in this paper aims at understanding sliding friction anisotropy at the nano-, micro-, and macroscales with respect to surface asperity orientation and exploring the mechanisms behind this phenomenon. Experiments were conducted by probing surfaces with grooves parallel or perpendicular to the direction of relative motion. Continuum mechanics analyses with the FEM and a semi-analytical static friction model and the atomic molecular dynamics simulation were performed for the mechanism exploration. Friction anisotropy was understood from the differences in contact area, surface stiffness, stiction length, and energy barrier from the continuum mechanics prospective and from that in the stick–slip phenomena at the atomic level.     

Simulations of the kinetic friction due to adsorbed surface layers

Simulations of the kinetic friction due to a layer of adsorbed molecules between two crystalline surfaces are presented. The adsorbed layer naturally produces friction that is consistent with Amontons' laws and insensitive to parameters that are not controlled in experiments. The kinetic friction rises logarithmically with velocity as in many experimental systems. Variations with potential parameters and temperature follow variations in the static friction. This correlation is understood through analogy with the Tomlinson model and the trends are explained with a hard-sphere picture.     

Modeling the effect of skewness and kurtosis on the static friction coefficient of rough surfaces

Engineering surfaces possess roughnesses that exhibit asymmetrical height distributions. However, the Gaussian distribution is most often used to characterize the topography of surfaces, and is also used in models to predict contact and friction parameters. In this paper, the effects of kurtosis and skewness on different levels of surface roughness are investigated independently. This is accomplished by adopting the Pearson system of frequency curves and used in conjunction with a static friction model for rough surfaces to calculate the friction force and friction coefficient. This study is the first attempt to independently model the effect of kurtosis and skewness on the static friction and friction coefficient. It is predicted that surfaces with high kurtosis and positive skewness exhibit lower static friction coefficient compared to the Gaussian case. More importantly, it is predicted that, for high kurtosis values, the static friction coefficient decreases with decreasing external force rather than increasing as seen with increasing skewness. This is a very promising result for applications involving smooth lightly loaded contacts such as magnetic storage devices and microelectromechanical systems. The practical significance of the present model is specifically demonstrated on static friction predictions in magnetic storage head–disk interfaces. Such predictions can be used to determine the optimal characteristics of such devices prior to fabrication to achieve lower friction in terms of surface roughness, mechanical properties, apparent contact area, and operational environment.     

Dynamic friction measurements with an atomic force microscope on polymer surfaces

The combination of scanning friction force microscopy (SFFM) and lock‐in techniques leads to dynamic SFFM (DSFFM) and provides great advantages in friction force studies with sub‐micrometre resolution. In this paper are presented measurements on thin adsorbed organic films on polymers (polymer blend of 75% poly(allylaminehydrochloride) (PAA) and 25% poly(diallyl‐dimethylammonium chloride) (PDDAC)) and on mica (as a reference). The amplitude and phase response as a function of the excitation amplitude can be explained on hard surfaces by a simple static and dynamic friction model. This model allows us further to distinguish static friction forces and kinetic friction forces in a quantitative way. Furthermore, we demonstrate the use of these spectra to determine the correct modulation amplitude of the excitation to achieve the optimal friction contrasts directly. Polymer data suggest that the viscoelastic shear flow under the atomic force microscope (AFM) tip is responsible for the shape of the phase and amplitude spectrum. Lastly, we demonstrate that DSFFM is a useful technique for surface characterisation in situations where SFFM may not be adequate.     

Effects of Anisotropy and Substrate Shape on Atomic Friction Force in Two-Dimensional Model

Understanding the effects of anisotropy and substrate shape on atomic friction force is critically needed for the designed development of nanoscale friction devices. The simulation of atomic force microscope on various substrate shapes using the 2D Prandtl–Tomlinson model is investigated in the framework of three representative surface lattices: \({\hbox {MoS}}_2\), NaCl and highly oriented pyrolytic graphite surfaces. The results show that the lateral force map reveals a significant contrast between different surface lattice shapes yielding lattice rows which differ from their neighboring ones. Careful analysis of the friction force during the individual friction scanning revealed that the friction forces over the narrow maxima domains were lower than those over the narrow wells domains. Depending on crystal orientation and the potential shape, variations in the frictional force can also be seen in the simulations. It has been numerically observed that frictional force depends on the crystal orientation as well as on the shape of the substrate potential. Velocity dependence of the kinetic friction force has the form of a power law \(F_{k}-F_{k0}={\hbox {cst}} \, v_{\mathrm{s}}^{2/3}\), for small scanning velocities. The effects of the shape parameter r on this law have been shown.     

A Theory of Boundary Lubrication

The mechanism of boundary friction is assumed here to be due to the molecular forces between hydrocarbon molecules adsorbed on the surfaces rather than to welding and tearing of the opposing surface roughness. On this assumption the frictional force, which arises when two orientated layers are moved over each other, can be calculated. The formulae for the Van der Waals and the repulsion forces are those successfully used by Müller for paraffins. The different values of the kinetic and static frictions can be explained by the forces being due to the tails of the chains in kinetic and the sides of the chains in static lubrication.

The value of the coefficient of friction, as calculated by this method, is of the correct order of magnitude.     

Contact Analysis of Non-Gaussian Surfaces for Minimum Static and Kinetic Friction and Wear

Most statistical contact analyses assume that surface heights and peak (summit) height distributions follow a Gaussian distribution. However, engineering surfaces are frequently non-Gaussian with a degree of non-Gaussian character dependent upon materials and surface finishing processes used. For example, magnetic rigid disk surfaces used in magnetic storage industry are highly non Gaussian. The use of a Gaussian analysis in such cases can lead to erroneous results. This study for the first time presents a method to carry out a statistical analysis of non-Gaussian surfaces. Real area of contact, number of contacts, contact pressure and meniscus force (in wet interfaces) are calculated for probability density functions having different skewness and kurtosis. From these curves, the optimum value of skewness and kurtosis can be predicted for minimum static/kinetic friction. It is found that a range of positive skewness (between 0.3–0.7) and a high kurtosis (greater than five) significantly lower the real area of contact and meniscus contribution implying low friction and wear. Also, sensitivity of film thickness to static friction goes down for a surface with a positive skewness and a high kurtosis.     

Wetting study of patterned surfaces for superhydrophobicity

Superhydrophobic surfaces have considerable technological potential for various applications due to their extreme water-repellent properties. A number of studies have been carried out to produce artificial biomimetic roughness-induced hydrophobic surfaces. In general, both homogeneous and composite interfaces are possible on the produced surface. Silicon surfaces patterned with pillars of two different diameters and heights with varying pitch values were fabricated. We show how static contact angles vary with different pitch values on the patterned silicon surfaces. Based on the experimental data and a numerical model, the trends are explained. We show that superhydrophobic surfaces have low hysteresis and tilt angle. Tribological properties play an important role in many applications requiring water-repellent properties. Therefore, it is important to study the adhesion and friction properties of these surfaces that mimic nature. An atomic/friction force microscope (AFM/FFM) is used for surface characterization and adhesion and friction measurements.     

Stick-Slip Motion in Boundary Lubrication

Using dynamical analysis for a pin-on-disk sliding system and the consideration of meniscus formation at the sliding interface, a wide range of experimental observations on stick-slip motion can be explained. It is shown that when the initial growth rate of the static friction force is larger than about half the product of the substrate speed and the spring constant, slick-slip motion occurs in that sliding system. The critical substrate speed or the critical spring constant, above which stick-slip motion ceases, can thus be determined. It is also shown that the saturation substrate speed, below which stick-slip motion retains its maximum stick-slip amplitude, is inversely proportional to the total growth time of the static friction force. The maximum stick-slip amplitude is proportional to the final difference between the static and kinetic friction force. For a thicker surface liquid-film, the initial growth rate and the final static friction force are larger but the total growth time is shorter, resulting in a larger critical speed, a larger stick-slip amplitude, and a larger saturation speed. For rougher contact surfaces, the initial growth rate is larger but the final static friction force and the total growth lime are smaller, resulting in a larger critical speed, a smaller stick-slip amplitude, and a larger saturation speed.     

粗糙度各向异性对润滑接触面摩擦的影响

齿轮、轴承、凸轮等重载接触副的性能受表面粗糙度的显著影响。高负载情况下的摩擦因数与润滑接触面粗糙度的各向异性相关。测量的表面粗糙度可以分解为一系列具有不同波长、幅值的正弦表面粗糙度,因此,考虑各向异性正弦表面粗糙度,构建粗糙表面点接触瞬态弹性流体动力润滑(TEHL)模型,提出基于多重网格算法的粗网格构造新方法,提高粗糙表面润滑问题求解的稳健性。研究表面粗糙度各向异性对高负载情况下摩擦因数的影响规律。结果表明,粗糙度的各向异性影响接触面压力、油膜厚度分布、粗糙度形变量,从而影响摩擦因数。提出一个组合函数来量化粗糙度各向异性对摩擦因数的影响,表明全膜润滑到混合润滑的过渡不仅与载荷、速度等工况参数相关,还与粗糙度各向异性相关。     

Kinetic frictional behaviour of alumino-silicate ceramics

The frictional behaviour was experimentally investigated of alumino-silicate ceramics (3Al₂O₃, 2SiO₂) rubbing against a hard steel surface under static and kinetic friction conditions. Tests were carried out on a pin-on-disc machine under both dry and wet contact conditions. Results showed that the frictional behaviour under either static or kinetic conditions was highly dependent on the ceramic body phase transformation which in turn was controlled by the firing temperature during ceramic processing and treatment. Lower friction values were evident when using specimens of ceramic bodies containing a high mullite crystalline phase, which are attained at high firing temperatures. Both the running speed and applied loads had insignificant effects at high loads.During kinetic friction tests lower frictional values were displayed than for static friction tests under wet contact conditions, and under dry conditions when using high mullite ceramic bodies. For specimens of ceramics fired at relatively low temperatures, kinetic friction tests produced higher frictional values than static friction tests.     

Studies on the friction and transfer of semicrystalline polymers

The friction and transfer of various semi-crystalline polymers were studied in several experiments. The cylindrical surfaces of polymers were slid over glass plates at low speed and under constant load. The kinetic friction of PTFE in repeated traverses did not vary with the number of traverses and the transfer of PTFE occurred successively on previously transferred PTFE films. The film transferred at each traverse was extremely thin (< 50 Å). The friction of PTFE decreased with increased humidity in the environment and appeared to be independent of crystalline transitions. Other polymers exhibited higher friction than PTFE and their transfer was generally as small lumps or short streaks. HDPE displayed a very low friction, although the friction of ultrahigh molecular weight polyethylene (UHDPE) was somewhat higher than that of HDPE. With HDPE and UHDPE, as well as with PTFE, long films stretched from one side of the abrasion grooves produced on the polymer frictional surfaces to the other side, like a bridge. The static friction of the three different polymers was very sensitive to the direction of prerubbing on the frictional surfaces and the static friction in sliding parallel to the pre-rubbing direction was much smaller than that perpendicular to it. The roles of the molecular profile and of the banded or spherulitic structure of the polymers in the polymer transfer mechanism are discussed on the basis of the experimental results obtained.     

Anisotropic Friction of the Ventral Scales in the Snake Lampropeltis getula californiae

Since the ventral body side of snakes is in almost continuous contact with the substrate during undulating locomotion, their skin is presumably adapted to generate high friction for propulsion and low friction to slide along the substrate. In this study, the microstructure of ventral scales was analyzed using scanning electron microscopy, atomic force microscope and confocal laser scanning microscopy. Dynamic friction was investigated by a microtribometer. The ventral scales demonstrated anisotropic frictional properties. To analyze the role of the stiffness of underlying layers on the frictional anisotropy, two different types of scale cushioning (hard and soft) were tested. To estimate frictional forces of the skin surface on rough substrates, additional measurements with a rough surface were performed. Frictional anisotropy for both types of scale cushioning and rough surfaces was revealed. However, for both types of surface roughness, the anisotropy was stronger expressed in the soft-cushioned sample. This effect could be caused by (1) the stronger interaction of the microstructure with the substrate in soft-cushioned samples due to larger real contact area with the substrate and (2) the composite character of the skin of this snake species with embedded, highly ordered fiber-like structures, which may cause anisotropy in material properties.     

A Study of the Static/Kinetic Friction Behavior of PTFE-Based Fabric Composites

Three different polytetrafluoroethylene (PTFE)-based fabric composites were prepared. The static/kinetic friction behaviors of these composites under different loads and speeds were studied. A 3D laser microscope and profile measurement apparatus were used for analysis of the morphology and weave structure of the composites, and the contact temperature of these composites under different loads and speeds was monitored continuously using a high-precision thermal resistor. In addition, a dynamic mechanical analysis (DMA) apparatus was used to explore the thermal and mechanical properties of PTFE-based fabric composites. The results demonstrated that speed/load, weave structure, and fiber form have an important influence on static and kinetic friction behavior of the fabric composites. Generally, the static friction coefficient is greater than the kinetic friction coefficient, except when considering light load conditions. Under light load conditions, the static friction coefficient is equal to the kinetic friction coefficient. In addition, the kinetic friction coefficient first increased and then decreased with increased speed, but the static coefficient increased first and then remained at an almost constant value. At all sliding speeds, the static and kinetic friction coefficients of tape yarn composites are better than those of the multifilament yarn composites. Weave structure has no effect on the static friction coefficient, but it has a significant influence on the kinetic friction coefficient.     

Dry friction of steel under high pressure in quasi-static conditions

Measurements were carried out for static and kinetic friction coefficients for steel as a function of the normal pressure for two surface roughness conditions of the matrix: ground and sand blasted. The samples were interstitial free steels, the tests were done at room temperature, in quasi-static and dry contact conditions. Very high pressures were applied in the range of 230–1100 MPa in order to simulate the conditions of testing in severe plastic deformation processes of metals. A new device was designed for this purpose. The results showed a decrease of the friction coefficients with the applied normal stress with stronger dependence for sand blasted surfaces.     

The effect of Coulomb friction on the static performance of foil journal bearings

In foil bearings, the friction between bumps and their mating surfaces is the major factor which exerts great influence on the bearing performance. From this point of view, many efforts have been made to improve the understanding of the influence of the friction on the foil bearing performance by developing a number of analytical models. However, most of them did not consider the hysteretic behavior of the foil structure resulting from the friction. The present work developed the static structural model in which hysteretic behavior of the friction was considered. The foil structure was modeled using finite element method and the algorithm which determines the conditions of the contact nodes and the directions of the friction forces was used to take into account the friction. The developed model was integrated into the foil bearing prediction code to investigate the effects of the friction on the static performance of the bearing. The results of analysis show that multiple static equilibrium positions are presented for the one static load due to the friction, inferring its great effects on the dynamic performance. However, the effect of friction on the minimum film thickness which determines the load capacity of the bearing is negligible.     

Experimental characterization and analysis of wet belt friction and the vibro-acoustic behavior

The robustness and noise warranty costs of rubber belts used for power transmission are directly affected by the frictional properties under varying environmental conditions. This paper presents an experimental characterization and analysis of the friction and vibro-acoustic behavior of automotive ribbed rubber belts under wet conditions. The experimental results show that the static friction under wet condition is higher than the corresponding kinetic friction by 40%-1040% for different belts; and the wet static friction is also much higher than the dry static friction. The wet kinetic friction is lower than the dry kinetic friction by about 30-40%. The occurrence of wet static friction is associated with the strong noise of the belt system. The spectrogram analysis of recorded sound demonstrates that the sound exhibits an impulsive sound pattern with broadband frequency extending to 20 kHz. In this study, the belt vibration is also measured and the spectrum results correlated with those of the sound measurement. The capillary effect, dry adhesive effect and the boundary lubrication effect are discussed based on adhesion models, which are used to correlate with experimental results and to interpret the effects of relevant parameters. The presented results are based on the start-up running of a newly developed belt-pulley test rig, which are different from some published results based on SAE Standard J2432. The test rig based on SAE Standard J2432 is actually operated as water lubricated coast-down, which is not applicable to characterizing the friction properties of belt in wet start-up running.     

Investigation on the Molecular Shear-Induced Organization in a Molecularly Thin Film of N-hexadecane

The frictional properties of a thin hexadecane film confined between two atomically smooth surfaces of mica were studied using the surface forces apparatus equipped with a 3D actuator–sensor attachment specially designed to investigate static and dynamic forces in three orthogonal directions simultaneously. The use of this attachment allows the relative alignment of the reciprocal sliding motion to be changed by an angle of 90° while maintaining the film under the same confinement conditions. The effects of the commensurability of the confining mica surfaces as well as the relative sliding direction on the frictional behavior of the hexadecane film were determined for different temperatures (18–29 °C) and sliding velocities (4 nm/s to 4 μm/s). The confined hexadecane film exhibited smooth sliding friction whose amplitude increased with the commensuration of the surfaces. A progressive evolution in the kinetic friction force toward a steady-state value was observed over reciprocal sliding motion for given experimental conditions of applied load, sliding velocity and environmental temperature. This friction evolution shows to be dependent on the sliding history of the film and could result from a partial molecular ordering, occurring during shear.     

The effect of surface roughness in elastohydrodynamic lubrication

The effect of surface roughness on the load capacity and friction in bearings operating in the elastohydrodynamic regime is predicted. A perturbation expansion is used to expand the pressure in powers of e (the ratio of the standard deviation of the surface roughness to the central film thickness). Two-dimensional roughness on both bearing surfaces with arbitrary anisotropy is considered. It is shown how the leading-order effects of roughness are insensitive to the precise form of the surface autocorrelation function and depend only on the variance of the height distribution and a parameter which describes the extent and nature of the anisotropy.     

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.