Friction Study of a Ni Nanodot-patterned Surface

Nanoscale frictional behavior of a Ni nanodot-patterned surface (NDPS) was studied using a TriboIndenter by employing a diamond tip with a 1 μm nominal radius of curvature. The Ni NDPS was fabricated by thermal evaporation of Ni through a porous anodized aluminum oxide (AAO) template onto a Si substrate. Surface morphology and the deformation of the NDPS were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM), before and after friction/scratch testing. SEM images after scratching clearly showed that, similar to what was assumed at the macroscale, the frictional force is proportional to the real area of contact at the nanoscale. It was found that adhesion played a major role in the frictional performance, when the normal load was less than 20 μN and plastic deformation was the dominant contributor to the frictional force, when the normal load was between 60 μN and 125 μN. Surprisingly, a continuum contact mechanics model was found to be applicable to the nanoscale contact between the tip and the inhomogeneous Ni NDPS at low loads. The coefficient of friction (COF) was also found to depend on the size of the tip and was four times the COF between a 100 μm tip and the Ni NDPS. Finally, the critical shear strength of the Ni nanodots/Si substrate interface was estimated to be about 1.24 GPa.     

Experimental Technique to Study Tangential-to-Normal Contact Load Ratio

This paper considers the mechanical interaction due to surface roughness and examines the surface theories using the classical definition of coefficient of friction: the tangential-to-normal load ratio. The postulation for maximum static friction is used to experimentally evaluate the contact models. For this purpose, a pin-on-disk test apparatus is employed with the capability of measuring tangential and normal forces for a frictional contact. The tests involve pairs of disks and specimens, that is, steel-on-steel and aluminum-on-aluminum contacts. In each case, profilometer measurements are performed on the disk and the Greenwood and Williamson parameters, are determined. Using the parameters, the theoretical estimates of normal and tangential loads are obtained. The theoretical values of tangential-to-normal contact load ratios are compared with those obtained from measurements for various applied normal loads. The tests utilizing a pin-on-disk apparatus showed a partial agreement between the experimentally obtained load ratios and the predicted upper limit confidence interval using the theoretical elastic and elastic-plastic contact. The result suggested that the elastic-plastic formulations provide better predictions of load ratios than the elastic contact formulations.     

Some Observations on Frictional Force During Fretting Fatigue

Frictional force behavior during fretting fatigue and its interdependence on other fretting variables are investigated. Both coefficient of static friction and the normalized frictional force (i.e., the ratio of frictional force and normal contact load) increase during the earlier part of a fretting fatigue test and then both reach to a stabilized value. The variation of temperature in the contact region and normalized frictional force with increasing cycle numbers and bulk stress show similar trend implying that normalized frictional force represents the average friction in the contact region during a fretting fatigue. An increase in bulk stress, relative slip, and hardness of pad material results in an increase of the normalized frictional force, while an increase in contact load, frequency and temperature decreases the normalized frictional force. The normalized frictional force is also affected by the contact geometry. On the other hand, coefficient of static friction increases with an increase in the hardness of mating material, temperature and roughness from shot-peening treatment, but is not affected by contact geometry and displacement rate. Further, the normalized frictional force is not affected by the contact geometry, roughness and applied bulk stress level when fretting fatigue test is conducted under slip controlled mode, however it increases with increasing applied relative slip and decreasing contact load in this case.     

The Leiden MEMS Tribometer: Real Time Dynamic Friction Loop Measurements With an On-Chip Tribometer

On-chip MEMS tribometer devices until now have been much less sophisticated for dynamically sensing frictional forces than their FFM (friction force microscope) counterparts. In this article, we present a MEMS-based tribometer that can be used to measure dynamically, on-chip and in-situ, the frictional properties of MEMS-scale contact geometries. The device provides the first FFM-like friction loops with contacting MEMS sidewall surfaces. Depending on the normal load two regimes of operation are identified. At low and intermediate loads, the frictional behaviour reflects wear-less relative motion of the silicon oxide surfaces of the MEMS device and we observe repeatable, irregular stick-slip behaviour, related to the surface roughness. At very high loads, wear causes changes in the topography of the contacting surfaces.

Load Induced Microstructure Evolution and Friction in an Organic Monolayer Self-assembled on a Silicon Substrate

In sliding of organic self-assembled monolayer against a probe the friction force is generally found to vary linearly with normal load. Here, lateral force microscopy is used to track the physical changes at the interface brought about when an octadecyltrichlorosilane monolayer, self-assembled on a silicon wafer, is slid against a Si₃N₄ tip in the 0–30 nN load range. Regarding a morphologically heterogeneous monolayer domain to be made up of tiles of characteristic friction forces, each tile is in a unique physical state; the variation of area fraction (in a scan area) of each tile is tracked as a function of normal load. The area averaged friction force at a load is obtained by summing the fractional forces of constituent friction tiles. The friction force obtained thus, is found to vary linearly with normal tip load. It is observed that this force is dominated by the low-friction crystalline tiles at low loads and by the high friction more amorphous tiles at high loads. This suggests that for a self- assembled monolayer the load governance of friction as implied by the Amontons Law may be attributed to the physical changes that are brought about at the interface by changing the normal load.     

MD simulation of the frictional behavior of CNTs with respect to orientation

In this work, the frictional behaviors of carbon nanotubes under various sliding conditions were investigated using Molecular Dynamics Simulation. A single CNT and multiple aligned CNTs were modeled for friction simulation under a normal load ranging from 3 nN to 32 nN. Three sliding directions were chosen to investigate the effects of orientation. Results showed that a lower friction coefficient was obtained under high normal loads for all cases. When a single nanotube was used, the friction coefficient along the 0° direction was the smallest, while simulations with multiple aligned CNTs exhibited the smallest friction coefficient along the 90° direction.     

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.     

Dry Sliding Contact Between Rough Surfaces at the Atomistic Scale

Although, a lot is known about the factors contributing to friction, a complete physical understanding of the origins of friction is still lacking. At the macroscale several laws have long since described the relation between load (Amontons, Coulomb), apparent and real area of contact (Bowden and Tabor), and frictional forces. But it is not yet completely understood if these laws of friction extend all the way down to the atomistic level. Some current research suggests that a linear dependence of friction on the real contact area is observed at the atomistic level, but only for specific cases (indentors and rigid substrates). Because continuum models are not applicable at the atomic scale, other modeling techniques (such as molecular dynamics simulations) are necessary to elucidate the physics of friction at the small scale. We use molecular dynamics simulations to model the friction of two rough deformable surfaces, while changing the surface roughness, the sliding speed, and the applied normal load. We find that friction increases with roughness. Also all sliding cases show considerable surface flattening, reducing the friction close to zero after repetitive sliding. This questions the current view of (static) roughness at the atomistic scale, and possibly indicates that the macroscopic laws of friction break down several orders of magnitude before reaching the atomic scale.     

A procedure for evaluating the positioning accuracy of reciprocating friction drive systems

The dynamic characteristics of stick-slip motion in reciprocating friction drive systems are investigated under dry contact using a 0.45% carbon steel pair. Based on this dynamic analysis, the stick-slip motion can be eliminated under certain experimental conditions depending upon driver speed, normal load and spring constant. The effects of normal load, driver speed, and spring constant on the positioning accuracy of the reciprocating friction drive system are examined under harmonic oscillation without stick-slip motion. Results show that at very low spring constants, the slip increases with increasing driver speed such that higher normal load has better positioning accuracy or smaller slip than does lower normal load. However, at high spring constants and high normal loads, there exhibits severe wear or peak at two limit positions on the sliding region. As a result, smaller normal loads have a better positioning accuracy than larger normal loads due to severe wear at high normal loads.     

Enhanced friction model for high-speed right-angle gear dynamics

The modeling of elastohydrodynamic lubrication friction and the analysis of its dynamic effect on right-angle gears, such as hypoid and spiral bevel types are performed in the present study. Unlike the classically applied empirical constant coefficient of friction at the contacting tooth surfaces, the enhanced physics-based gear mesh friction model is both spatial and time-varying. The underlying formulation assumes mixed elastohydrodynamic lubrication (EHL) condition in which the division and load distribution between the full film and asperity contact zones are determined by the film thickness ratio and load sharing coefficient. In the proposed time-varying friction model, the calculation of friction coefficient is performed at each contact grid inside the instantaneous contact area that is being subjected to mineral oil lubrication. The effective friction coefficient and directional parameters synthesized from the net frictional and normal contact forces are then incorporated into a nonlinear time-varying right-angle gear dynamic model. Using this model, the effect of friction on the gear dynamic response due to the transmission error and mesh excitations is analyzed. Also, parametric studies are performed by varying torque, surface roughness and lubrication properties to understand the salient role of tooth sliding friction in gear dynamics. The simulation results are included. But experimental verification is needed.     

Surface roughness effects on curved pivoted slider bearings with couple stress fluid

In this paper the effect of surface roughness on the performance of curved pivoted slider bearings is studied. A more general type of surface roughness is mathematically modelled by a stochastic random variable with nonzero mean, variance and skewness. The averaged modified Reynolds type equation is derived on the basis of Stokes microcontinuum theory for couple stress fluids. The closed‐form expressions for the mean pressure, load‐carrying capacity, frictional force and the centre of pressure are obtained. Numerical computations show that the performance of the slider bearing is improved by the use of lubricants with additives (couple stress fluid) as compared to Newtonian lubricants. Further, it is observed that the negatively skewed surface roughness increases the load‐carrying capacity and frictional force and reduces the coefficient of friction, whereas the positively skewed surface roughness on the bearing surface adversely affects the performance of the pivoted slider bearings. Copyright © 2006 John Wiley & Sons, Ltd.     

Measurement of static friction coefficient between flat surfaces

An experimental device, called the centrifugal friction apparatus (CFA), was developed which is capable of measuring the static coefficient of friction, μ, between flat surfaces with very low normal forces. Experiments were performed with the CFA using the polished surface of n-type arsenic doped silicon wafers to investigate static frictional behavior under differing light load conditions. For loads less than 100 mgf, the average μ and the standard deviation were found to increase as the normal load decreased. For loads between 100 mgf and 1.1 gf μ was found to be a constant in the range of 0.30−0.40. Results suggest that despite extreme smoothness the real area of contact is still significantly smaller than the apparent area of contact and μ is still a function of load. Tests with solid particles between the silicon mating surfaces showed that under low load conditions ( <0.9 Pa), the change in μ was negligible for both abrasive particles and for solid lubricants. At the higher loads used ( ≈475–575 Pa) the expected changes in μ were starting to occur for both types of solid particles.     

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.     

Effect of Various Lubricants and Base Materials on Friction at Ultrahigh Loads

A series of high load, low-speed sliding friction tests was made on eight greases and eighteen dry lubricants at normal unit loads from 10,000 psi to 150,000 psi. Four different substrate materials were used having a range of hardnesses from Rockwell C 18 to Rockwell C 55. The ultimate load capability of both greases and dry films is a function of substrate hardness with the best ultimate load capability being provided by inorganically bonded molybdenum disulfide films with small amounts of graphite added. The coefficient of friction of the greases appears to be an inverse function of substrate hardness and a direct function of the normal load. The coefficient of friction of the dry lubricants is an inverse function of the normal load, but does not appear to be related to the substrate hardness.     

Surface roughness effects in porous inclined stepped composite bearings lubricated with micropolar fluid

In this paper, the effect of surface roughness on the performance characteristic of porous inclined stepped composite bearings is studied. A generalised form of surface roughness is mathematically modelled by a stochastic random variable with non‐zero mean, variance and skewness. The generalised average Reynolds‐type equation is derived for the rough porous inclined stepped composite bearings with micropolar fluid. The closed‐form expressions are obtained for the fluid film pressure, load‐carrying capacity and frictional force. The results are presented for three different types of bearing system. The numerical computations of the results show that the negatively skewed surface roughness pattern increases fluid film pressure and load‐carrying capacity and decreases the coefficient of friction, whereas adverse effects were found for the positively skewed surface roughness pattern. Further, the rough porous inclined stepped composite bearing provides the largest load‐carrying capacity and the least coefficient of friction as compared with the porous plane slider and porous composite tapered concave bearings. Copyright © 2012 John Wiley & Sons, Ltd.     

The influence of thin hard coatings on frictional behaviour in the orthogonal cutting process

New knowledge about the tribological response deriving from the interaction of the substrate/coating-chip system, with special attention to the orthogonal cutting process when chatter-free end turning using natural contact tools, is developed. In order to evaluate the frictional behaviour of this process under modified contact conditions, experimental investigations including the contact temperature, the contact loads, friction and the frictional heat flux per unit area were carried out. In contrast to the most obvious approach, the coefficient of sliding friction versus the cutting speed, the contact temperature, the normal pressure and the interface control factor is considered. A number of different coating structures, starting from single up to three and four layer films, in combination with medium carbon and austenitic stainless steels, were tested. It is pointed out that the results obtained provide a modified approach to the frictional behaviour of the cutting process and its controllability. Among various responses, specific for such tribo-contact pairs, of particular interest is the self-adaptation resulting in controlled generation of friction energy and conduction of the frictional heat flux.     

The effect of friction on the usability of touchpad

To investigate the degradation of the touchpad usability by surface wear, the touchpad with new, worn, plastics and paper surfaces are examined in terms of performance and frictional response. It is found that the friction coefficient of the surface affect the usability. It is concluded that the major factors controlling the friction was surface roughness. It is also found that friction coefficient is affected by the difficulty of the task. The reasons for this is the human tendency to be cautious, he/she try to reduce the load to be precise. Then the load dependence of friction coefficient results in high friction. The other reason is mental sweating, which will be increased when the task is difficult.     

A frictional force microscope controlled with an electromagnet

Frictional forces are usually measured by detecting spring displacement. To obtain high-resolution measurements of frictional force distributions, a sharp tip and a light load are required. In measuring frictional force on relatively rough surfaces, using very sharp tips (submicron radii), significant stick-slip motions are observed, and continuously varying dynamic frictional forces can not be measured. To measure continuous friction distributions between sharp tips and surfaces with light loads, a new frictional force microscope (FFM) is developed. This FFM has an electromagnet to maintain the tip suspension spring in a non-deflected position. The frictional force is then measured from the magnet current. Using this FFM, continuous friction distributions between 0·1 μm radius diamond tips and magnetic disk surfaces with light loads (less than 10 μN) are obtained.     

Rolling Friction of Single Balls in a Flat-Ball-Flat-Contact as a Function of Surface Roughness

This paper reports on the dependence of rolling resistance on surface roughness of single balls in a flat-ball-flat configuration at sub-Newton loads using an specially designed rolling resistance tribometer. The tribometer setup features an ultra-stable drive and a two-dimensional force transducer with a high ratio between load and friction force sensitivity. Using this setup, the rolling resistance was examined for different materials as well as a self-mated sapphire contact. Results indicate a very strong dependence of rolling resistance on surface roughness for the tested loads. Rolling resistance was found to decrease monotonically with decreasing roughness. A comparison was also attempted between rolling resistance and sliding friction. Sliding friction measurements show that decreased roughness does not necessarily decrease the coefficient of friction. In fact, a friction minimum is reached at a certain roughness. A further decrease of roughness increases the friction. This indicates that sliding friction measurements do not seem to be transferable to rolling resistance in every case, especially when the average surface roughness is in the range of a few nanometres.     

Shear-induced boundary film formation from dialkyl sulfides on copper

The friction of a copper surface, which is exposed to diethyl disulfide (DEDS), dimethyl disulfide (DMDS) or dimethyl trisulfide (DMTS) while rubbing by a tungsten carbide pin, are explored in an ultrahigh vacuum tribometer to investigate the effect of the sulfur-to-carbon stoichiometry on gas-phase lubrication. Surface analyses by Auger spectroscopy of the wear scars after rubbing reveal that the amount of sulfur increases with sulfur content of the gas-phase lubricant, as anticipated. It is found that the friction reduction depends on the normal load, where the friction coefficient tends to decrease more at lower loads, and that the load at which the most friction-reducing tribofilm is formed depends on the stoichiometry of the gas-phase lubricant. DEDS (with a sulfur to carbon ratio of 0.5) only reduces the friction coefficient to a minimum value of ～0.28 at a normal load of 0.25 N, for DMDS (S/C = 1.0) friction is reduced to ～0.28 for loads below ～0.7 N, and DMTS (S/C = 1.5) reduces friction to this value for all loads tested.