Experimental study of air squeeze effect on high-frequency friction contact

A special tribometer was developed which was used to test sliding friction force between PTFE-based composites and bronze with normal (out-of-plane) or transverse (in-plane) high-frequency vibrations under three different environmental pressures. The influences of environmental pressure, vibration amplitude and sliding velocity on sliding friction coefficient were studied. The results show that the effect of environmental pressure on reduction of sliding friction is outstanding. With the increase of vacuum, the reduction of sliding friction by high-frequency vibrations decreased, especially the reduction of sliding friction by normal vibration. The sliding friction coefficient with high-frequency vibrations slowed down as the vibration amplitude increased. With increase of sliding velocity, the time-averaged friction coefficient with transverse vibration increased.     

Analytical determination of friction resistance as a function of normal load and geometry of surface irregularities

A mathematical modeling and simulation of friction during steady state sliding of metals, based on the upper-bound approach, is demonstrated. The existence of wedge-shaped protrusions on the tool surface is assumed. Pressing these protrusions onto the workpiece and sliding the tool along the workpiece produces asperities on the surface of the workpiece. These asperities move in a wave-like motion along the surface layer and cause plastic deformation through a specified depth under the surface. This plastic deformation combines with local friction between the tool and the workpiece along the asperity interface to produce resistance to sliding. The relation between the normal pressure and the sliding resistance is established for the entire range of pressure levels from zero to infinity. The apparent Coulomb coefficient of friction for lower levels of normal pressure and the constant friction factor for excessive load levels are determined. The transition region from Coulomb coefficient of friction to constant friction factor also becomes clear. A mathematical determination is obtained by means of a force equilibrium considering the concept of a contact surface friction ratio. The force of resistance to sliding is related both to the geometry of the asperity of the surface of the tool and to the constant friction factor, which is used for measuring a local frictional force along the interface of each asperity.     

Molecular dynamics studies of atomic-scale tribological characteristics for different sliding systems

A slider-slab sliding model for hard-to-soft and soft-to-soft sliding systems with abrasive and non-abrasive wear conditions is used to investigate atomic-scale friction. The molecular dynamics simulation uses the Morse potential to calculate interatomic forces between atoms. Separation distance between the slider and the slab is changed to simulate repulsive and attractive interactive force fields exerted on interface between two sliding components. Effects of the interaction potential parameters on the sliding friction are investigated. The relationship of frictional force, normal force and temperature rise of the slider and the slab during sliding are established. Comparison of the hard-to-soft and the soft-to-soft sliding system are carried out and shows different tribological phenomena.     

Lowering friction coefficient under low loads by minimizing effects of adhesion force and viscous resistance

Conditions (normal load, sliding speed, ambient conditions, and material) to obtain the lower friction coefficient were studied by measuring the friction and pull-off forces between a metal pin (copper or gold) and a plate (steel or single crystal silicon). First, a pin was rubbed against a plate under a normal load between −12 and 870 μN at a sliding speed between 0.012 and 9.6 μm/s. The friction force was measured during reciprocating sliding motion. The pull-off force was measured before and after each friction force measurement. All the force measurements were taken in high vacuum at 10⁻⁵ Pa, dry argon at 1 atm, and ambient humid air of 38 and 60% relative humidity. Then, the friction coefficient was calculated by dividing friction force by the sum of normal load and pull-off force. In high vacuum, when a copper pin was rubbed against either a silicon or steel plate, the friction coefficient decreased to less than 0.05 with decreasing sliding speed. The effect of sliding speed on the friction coefficient suggests that under a low normal load the viscous resistance of liquid contributed to the friction force. When a gold pin was rubbed against a silicon plate, the friction coefficient was not affected by sliding speed.     

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.     

A numeric investigation of friction behaviors along tool/chip interface in nanometric machining of a single crystal copper structure

The interaction between the tool rake face and the chip is critical to chip morphology, cutting forces, surface quality, and other phenomena in machining. A large body of existing literature on nanometric machining or nano-scratching only considers the overall friction behavior by simply regarding the total force along tool movement direction as the friction force, which is not suitable for describing the intriguing friction phenomena along the tool/chip interface. In this study, the molecular dynamic (MD) simulation is used to model the nanometric machining process of single crystal copper with diamond tools. The effects of three factors, namely, tool rake angle, depth of cut, and tool travel distance, are considered. The simulation results reveal that the normal force and friction force distributions along tool/chip interface for all cases investigated are similarly shaped. It is found that the normal force consistently increases along the entire tool/chip interface when a more negative rake angle tool is used. However, the friction force increases as the rake angle becomes more negative only in the contact area close to the tool tip, and it reverses the trend in the middle of tool/chip interface. Meanwhile, the increase of depth of cut overall increases the normal force along the tool/chip interface, but the friction force does not necessarily increase. Also, the progress of tool into the work material does not change the patterns of normal force, friction force, or friction coefficient distributions to a great extent. More importantly, it is discovered that the traditional sliding model with a constant friction coefficient can be used to approximate the later section of friction distributions. However, no friction model for traditional machining is appropriate to describe the first section of friction distributions obtained from the MD simulation.     

Measurement of the friction of paper by the strip-on-drum method

The friction between paper surfaces has been studied in a strip-on-drum geometry, under quasistatic and slow sliding conditions. The method provides a better simulation of some practical applications than other more widely used tests. The mechanics of the test have been analysed for two cases: with constant coefficient of friction; and with the friction force related to normal pressure by a power law. The behaviour of four different types of paper was examined. In all cases, the coefficient of friction of the sample fell with repeated sliding; this was associated with progressive damage to the paper surface, and has implications for the design of standard friction test methods for paper. The coefficient of friction (COF) also depended significantly on the contact pressure, in a way that can be correlated with the relative compressibilities of the different paper structures. The pressure dependence of friction, at low pressures, was accurately modelled by a power-law relationship.     

Pre-sliding Behaviour of Single Asperity Contact

The pre-sliding and static friction force behaviour at asperity level between a smooth ball and a smooth flat surface at different normal loads, as well as friction behaviour during full slip has been studied. The normal load dependence of the friction force and the preliminary displacement is discussed when the mean contact pressure is kept under 100 MPa. The theoretical model to calculate the shear stress and the preliminary displacement in the contact is discussed and the experimental data were used to verify the model. The results show that for low applied normal loads the adhesion force has an influence on the friction force measurements. Furthermore, the results for the friction force and preliminary displacement show good agreement with the theoretical trends. The experiments along with the model can be used to analyse the tangential traction in the contact and the behaviour of the stick–slip area. The measurement results along with the model were used to calculate the maximum shear stress at the point of sliding for different applied normal loads. It is also shown that at low applied normal loads the shear stress is not constant as compared to relatively high applied normal loads due to the presence of adhesion force.     

Effects of Friction on the Contact and Deformation Behavior in Sliding Asperity Contacts

Finite-element analyses are carried out to study the effects of friction on the contact and deformation behavior of sliding asperity contacts. In the analysis, on elastic-perfectly-plastic asperity is brought in contact with a rigid flat at a given normal approach. Two critical values of the normal approach are used to describe the asperity deformation. One is the approach corresponding to the point of initial plastic yielding, and the other at the point of full plastic flow. Additional variables used to characterize the deformation behavior include the shape and size of the plastic zone and the asperity contact size, pressure, and load capacity. Results from the finite-element analysis show that the two values of critical normal approach decrease significantly as the friction in the contact increases, particularly the approach that causes plastic flow of the asperity. The size of the plastically deformed zone is reduced by the friction when the contact becomes fully plastic. The reduction is very considerable with a high friction coefficient, and the plastic deformation is largely confined to a small thin surface layer. For a low friction coefficient, the contact size, pressure and load capacity of the asperity are not very sensitive to the friction coefficient. For a moderate friction coefficient, the contact pressure is reduced and the junction size increased; the load capacity of the asperity is not significantly affected due to the compensating effects of the pressure reduction and the junction growth. For a high friction coefficient, the pressure-junction compensation is not longer sufficient and the asperity load capacity is reduced. The degree of the friction effects on these contact variables depends on the applied force or the normal approach. Although the analyses are conducted using a line-contact model, the authors believe that the effects of friction in sliding asperity contacts of three-dimensional geometry are essentially the same and the same conclusions would have been reached. These results may provide some guidance to the modeling of rough surfaces in boundary lubrication, in which the asperity friction coefficient can be high and vary significantly both in time and from one micro-contact to another.     

Friction analysis according to pretension of laparoscopy surgical robot instrument

Surgical robot instruments are driven by using cable-pulley structure. The elasticity at cable-pulley structure causes slack effect of cable. Elastic region of cable must be compensated for preventing the slack. The surgical instrument inputs a pretension for compensation at cable’s elastic region. Therefore pretension is an important factor in the design of a surgical instrument. However, the pretension is related with friction force which is affected by normal force at the driving shaft pulley. The shaft and contact surface of the pulley consist of sliding bearing structure. Typically, the friction coefficient in the sliding bearing structure is determined by the Petroff equation. The pressure of Petroff equation is related to the denominator of the equation. Therefore increasing normal force brings about an effect which decreases friction coefficient. Eventually, in theoretically, that means the pretension can be increased to infinity to decrease friction coefficient. However actual result comes out different. Therefore in this paper, the friction analysis method according to a pretension is proposed and an experiment method for the analysis of the friction is introduced. The correlation between pretension and friction is estimated by the analysis. The estimated result is compared with the experiment one and is analyzed. Also finally this paper suggests an appropriate pretension for surgical robot instrument by the friction analysis result.     

Method of Constructing a 3D Friction Map for a Rubber Tire Tread Sliding Over a Rough Surface

An experimental methodology for constructing a 3D friction map, which depends on normal pressure, sliding velocity, and temperature, for tire tread rubber has been presented. The methodology of constructing a 3D friction map is based on the principle of temperature–velocity superposition, which makes it possible to combine the obtained experimental data into one sculpted surface. Two types of the tread rubbers were investigated that are used in summer passenger tires and heavy-loaded all-metal tires. The results of analyzing the obtained 3D friction maps have been presented. The influence of normal pressure and sliding velocity to the amount and position of the values of coefficient of friction of the rubber sliding over the sandpaper surface with the specified abrasive grit has been considered.     

Microtribological investigations of stick/slip phenomena using a novel oscillatory friction and adhesion tester

A novel friction and adhesion tester bridging the gap between macro- and nanotribology is introduced. A friction and/or adhesion induced deflection of a spring is detected using a high-resolution laser interferometer. Unlike force microscopes, where a sharp tip interacts with the surface, this approach allows two plane substrates to be brought into contact. In this way, the exact tribological analysis of microtechnological devices is possible. Since the tester can be operated in air as well as under high vacuum conditions, the environment can be controlled over a wide range. Using this tester, micro-stick/slip phenomena have been investigated as a function of sliding velocity, surface morphology, normal force and contact area. All experiments presented in this paper were carried out on air. This revised version was published online in June 2006 with corrections to the Cover Date.     

Friction and Wear at High Sliding Speeds

Sliding tests have been carried out using a variety of soft metal and nonmetal pins on a rotating steel disk at speeds up to 150 m/s. A new high-speed friction apparatus in which the normal force, the friction force and the friction coefficient are recorded, was used. In general, the wear rate increased drastically, and the friction coefficient decreased moderately as the sliding speed was raised, these changes being especially pronounced when pin materials of low melting temperature were used. The friction data are in good agreement with those obtained by others using the pin-on-disk geometry. However, although in many cases the interface reached the melting temperature of the lower melting sliding material, the very low friction coefficient values of under 0.05 reported by some investigators were not reached.     

Multi-layer models of friction between solids

Recent simulations of processes in surface layers of rubbing solids have shown the formation of a boundary layer, called quasi-fluid layer. To better understand the physical nature of the quasi-fluid layer, we investigate the processes occurring in the surface layer of rubbing solids in the frame of a simple multi-layer model, which also leads to the formation of a quasi-fluid layer. The multi-layer model can further be used to investigate how the friction force is determined by the material and loading parameters. It has been shown that the friction force and the thickness of the quasi-fluid layer depend on sliding velocity, the viscosity of the material and on a microscopic parameter (layer thickness). Subsequently, two different ways of extending the discussed model are proposed aiming at a model that can also predict the influence of normal pressure and surface topography on the friction force and on the formation of the quasi-fluid layer.     

Kinetic friction characterizations of the tubular rubber seals

In this paper, both the kinetic friction characterizations and the stick–slip motion phenomena for the tubular rubber seals are studied. First, the kinetic friction model of the rubber seal is established to explain the kinetic friction mechanism of the tubular rubber seals. Second, both the measurement principle and the test instrument for the kinetic friction properties of the tubular rubber seals are developed, and then both the normal force curve and the friction force curve are obtained. Finally, the influences of the sliding velocity and the compressive displacement on the kinetic friction properties and the stick–slip motion of the tubular rubber seals are analyzed. The results will play an important role for designing and evaluating advanced rubber seal components.     

Characterization of Friction and Heat Partition Coefficients during Machining of a TiAl6V4 Titanium Alloy and a Cemented Carbide

This article aims at characterizing the frictional behavior of a TiAl6V4 alloy and a carbide tool under extreme conditions corresponding to those occurring at the cutting tool–work material interface. A specially designed open tribometer was used to characterize the macroscopic friction coefficient, heat partition coefficient, and adhesion in the contact versus sliding velocity and contact pressure. It has been shown that titanium leads to intense adhesion, which seems to be even more intensive with high contact pressure and high sliding velocity, which limits the local sliding movement at the interface (stuck layer). However, the tribometer provides the evolution of an apparent friction coefficient and a macroscopic heat partition coefficient related to the shearing of titanium between the adhesive layer and the bulk material. An increase in sliding velocity or contact pressure induces a small decrease in the apparent friction coefficient as well as the heat partition coefficient. It has been shown that adhesion is thermally activated by a combination of contact pressure and sliding velocity, which leads to a threshold effect. Furthermore, the application of an emulsion showed a small decrease in the apparent friction coefficient associated to a decrease in adhesion. Finally, this work provides quantitative data on the apparent friction and heat partition coefficients versus sliding velocity and contact pressure that can support the development of macroscopic cutting models for titanium alloys.     

The frictional pattern of tactile sensations in anthropomorphic fingertip

In this report, to consider the technology of frictional tactile sensations (FTS) for a prosthetic fingertip, the anthropomorphic fingertip (silicone) was compared with the human fingertip in terms of sliding friction (reciprocating sliding on an acrylic plate under a load force of ∼2.5 N max.) and the prototype of a FTS was demonstrated. In the results, the friction coefficient of the human fingertip changed according to the contact location more than the anthropomorphic fingertip. In the demonstration, the prototype FTS that faked the human fingertip could generate a high friction coefficient and the peculiar signal pattern at each contact location. Moreover, the prototype FTS recognized a slight difference in the surface roughness of the copier paper. In conclusion, the friction coefficient in the human fingertip increases when increasing the contact area under the same load force. Increasing the contact area induces stiction and stick-slip phenomena, generating a high friction coefficient and vibrations. Moreover, the high friction coefficient amplifies slight contact signals and supports FTS in a prosthetic fingertip.     

Orientation and relocation of biphenyl thiol self-assembled monolayers under sliding

Recent studies have emphasized the use of biphenyl thiol (BPT) monolayers as a resist in electron lithography. In this paper, friction and wear properties of BPT monolayers have been studied by atomic force microscopy. The BPT molecular chains are compliant and experience orientation under normal load, which in turn reduces the friction force. During sliding it is observed that after the first several scans, the friction force of BPT is significantly reduced, but the surface height does not show any apparent change. These observations suggest that the orientation is reversible and can be facilitated by initial sliding. Relocation and accumulation of BPT occurs during the first several scans, which lead to the formation of larger terrace. Based on the wear studies of a single BPT terrace, it is found that the wear lives of self-assembled monolayers increase exponentially with terrace size.     

Thin film lubrication of sliding point contacts of AISI 52100 steel

The mechanism of thin film lubrication of sliding point contacts of AISI 52100 steel has been studied as a function of load, sliding speed, composition and temperature of the lubricant.Below certain critical combinations of Hertzian pressure, speed and temperature the surfaces are kept apart by an elastohydrodynamic lubricant film. The load carrying capacity of this film depends primarily on the effective viscosity of the lubricant in the contact region which decreases with bulk oil temperature and with increasing sliding speed, because of friction induced thermal effects. After breakdown of the EHD film, boundary lubrication may still prevent severe adhesive wear. The transition from the boundary lubricated regime towards the regime of severe adhesive wear is a function of load (normal force), speed and bulk oil temperature and possibly depends on the conjunction temperature. Irrespective of the initial lubrication condition, oxidation of the steel surfaces leads to the (re)establishment of low friction, mild wear conditions.     

表面粗糙度对滑动电接触磨损率的影响

在电气化铁路弓网系统中,磨损率是衡量列车运行状态与接触导线使用状态的重要指标。为了充分模拟弓网系统中磨损率情况,利用自行搭建的滑动电接触摩擦磨损试验机对滑板和接触导线进行摩擦磨损试验,分析滑板表面粗糙度、法向压力、接触电流与运行速度对磨损率的影响。得出结论:滑板磨损率随滑板初始表面粗糙度、接触电流、法向压力、运行速度的增加而增加,而高载荷下粗糙度对于磨损率的影响降低;滑板摩擦从磨合期进入稳定摩擦期存在一个临界表面粗糙度,当滑板初始表面粗糙度值等于临界粗糙度值时,其磨损率最低;不同初始表面粗糙度的滑板在跑合期内磨损过程不同,在稳定摩擦期内磨损过程趋于一致,且摩擦试验后滑板表面粗糙度也接近。