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.     

A Load-Dependent Model to Investigate the Velocity Dependence of Friction Coefficient: ZrO2/Polymer Nanocomposite

Using dynamic finite simulations, we investigate how the friction coefficient of ZrO₂/polymer nanocomposite depends on the sliding speed. The load-dependent model we developed corresponds to common friction systems, where the friction couples are sliding under fixed load for various speeds. Here, we study the effect of the sliding speed on the contact distance between two contacting bodies. In accordance with experimental observations and theoretical arguments, we find the contact distance increased with the sliding speed. We show that the dependence of the reaction force on sliding speed can be rationalized by assuming that the frequency dependence of the polymer chains relaxation times is affected by the damping effects of contact stress. By investigating the energy dissipation, we show how the friction coefficient is affected by the sliding speed. The deformation volume and relaxation times decreased with the increasing sliding speed, which result in the decreasing of energy dissipation. Then, the work in pushing the top cylinder across the bottom plateau decreases, results in a reduction of the mean horizontal reaction force and friction coefficient.     

On the instantaneous and average piston friction of swash plate type hydraulic axial piston machines

Piston friction is one of the important but complicated sources of energy loss of a hydraulic axial piston machine. In this paper, two formulas are derived for estimating instantaneous piston friction force and average piston friction moment loss. The derived formula can be applicable for piston guides with or without bushing as well as for axial piston machines of motoring and pumping operations. Through the formula derivation, a typical curve shape of friction force found from several experimental measurements during one revolution of a machine is clearly explained in this paper that it is mainly due to the equivalent friction coefficient dependent on its angular position. Stribeck curve effect can easily be incorporated into the formula by replacing outer and inner friction coefficients at both edges of a piston with the coefficient given by Manring (1999) considering mixed/boundary lubrication effects. Novel feature of the derived formula is that it is represented only by physical dimensions of a machine, hence it allows to estimate the piston friction force and loss moment of a machine without hardworking experimental test.     

Study of plowing and friction at the surfaces of plastic deformed metals

Experimental and analytical investigations of plowing and friction were conducted at the surfaces of well-polished lead, aluminum, copper, nickel, molybdenum, and tungsten to study the mechanism of the load/penetration dependency. The experimental tests were performed with a Nano-Indenter XP of MTS and a Scanning Probe Microscope (SPM), Nanoscope IIIa of Digital Instruments. In addition to make indentation and measure the hardness and Young’s modulus, the indenter was used to make scratches at the surface of metals under different normal load while the penetration depth and frictional force encountered during the scratching were recorded. The SPM, operated mostly in the contact mode, was used to examine the scratch profile. Under the test conditions, plastic deformation dominated at the surfaces of the metals. An analytical model was established to express plastically deformed contacts, based on plowing of a conical-shaped indenter with a hemispherical tip at a plastic deformed surface. Penetration depth and scratched volume were calculated, which is in good agreement with experimental observation. The frictional coefficient μ was also calculated with the model, which accounted for plowing as well as the adhesion force between the indenter and surface. Beside fair agreement of experimental data and calculated values on μ under the loads applied, the model indicated a dramatic rise in friction coefficient under very low loads, which was not observed in the tests. The discrepancy was discussed, and it was believed that the dramatic increase in μ is for the calculated μ and may be due to the assumed dominant contribution of adhesion force in actual contact load with decreasing external load, and it appears only the adhesion energy Δγ is significant. The actual adhesion energy Δγ between our diamond indenter and metal surface in our test condition might be smaller than the value used in calculation.     

Calculation of the deformation component of the force of friction for a standard elastoviscous base

The deformation component of the force of friction is determined for the standard elastoviscous base using the energy approach. The obtained formula served to calculate the force of friction of a punch with extreme points. It is established that the friction coefficient grows as the punch becomes rougher. When the sliding velocity of the flat punch with the left angle tends to zero, the force of friction has a nonzero limit.     

液压往复密封的摩擦特性分析

基于往复式密封的弹性流体动力润滑的数学模型,对影响密封性能的因素进行了综合分析。综合考虑了形变理论、接触力学理论以及流体-固体耦合理论,采用MATLAB数值分析法,通过数学迭代计算完成最终求解。深入研究了润滑油黏度、界面摩擦系数以及密封圈的表面粗糙度对密封性能的综合影响。结果表明:随着耦合界面摩擦系数的增加,接触摩擦力都呈现出增大的趋势;总摩擦力随着粗糙度的增加呈现出抛物线式变化趋势;润滑油的黏度存在一个临界值,当润滑油黏度小于此临界值时,随着润滑油黏度的增加,总摩擦力先增加后降低;当润滑油黏度超过此临界值时,接触摩擦力呈现出单调增加的趋势;润滑油黏度和界面粗糙度的增加会导致流体泄漏的增大。     

Tribological Behavior of NiAl–1.5 wt% Graphene Composite Under Different Velocities

The progress in aerospace field requires a new NiAl matrix composite that can stand against wear and decrease the energy dissipation through decreasing friction. In this study, the tribological behavior of NiAl–1.5 wt% graphene composite is investigated at room temperature under a constant load of 12 N and different sliding velocities. The results show that the friction coefficient and wear rate increase with increasing sliding velocity from 0.2 to 0.4 m/s due to the adhesion between the sliding bodies and tearing of the graphene layer. The friction coefficient and wear rate tend to decrease at a sliding velocity of 0.6 m/s as a result of severe plastic deformation and grain refinement of the worn surface. However, at 0.8 m/s the friction coefficient reaches a minimum value and the wear rate increases and changes the wear mechanism to fatigue wear. It can be concluded that various wear mechanisms lead to different tribological performance of NiAl–1.5 wt% graphene composite.     

An analytical model of dissipated viscous and hysteretic energy due to interaction forces in a pneumatic tire: Theory and experiments

When in use, a tire dissipates energy according to various mechanisms: rolling resistance, viscosity, hysteresis, friction energy, etc. This dissipation of energy contributes to influencing tire temperature, contact conditions and the resulting friction coefficient.This research project deals with viscoelastic and hysteretic mechanisms, and presents an explicit expression of the energy dissipated by tire-road interactions caused by these mechanisms. It is based on the Dahl model with regard to the hysteretic force together with a spring and a frequency variable damping coefficient with regard to the viscoelastic one. The energy expression found in this way can be used in tire thermal models to determine one of the heat flows needed to estimate the contact temperature and to find out the actual friction coefficient to be used in real time tire-road interaction models.Experimental tests were carried out, for longitudinal interaction only, in order to evaluate the effectiveness of the proposed expression by identifying the parameters and validating the results.     

Atomic friction studies on well-defined surfaces

Atomic friction studies have been performed by means of a friction force microscope (FFM) in ultrahigh vacuum, where well-defined surfaces can be prepared. A home-built FFM allows us to study lateral forces as low as 0.05 nN using rectangular silicon cantilevers. Furthermore, comparison with dissipation measurements performed in non-contact mode are possible. Recent experimental results are presented and discussed in the framework of a one-dimensional Tomlinson model which includes thermal activation. Atomic-scale stick–slip processes on a metallic surface could be repeatedly measured on Cu(111), while the Cu(100) surface was distorted by the tip during the scanning process. A logarithmic velocity dependence of atomic friction has been measured on Cu(111) and NaCl(100) for low scanning velocities. The dissipation found in stick–slip measurements is compared to the power loss detected in dynamic non-contact measurement.     

A6061薄壁圆锥管轴向压缩性能的数值分析和理论研究

采用LS-DYNA软件对锥倾角0°~40°、壁厚1 ~2.5 mm的A6061圆锥管进行轴向压缩仿真,研究锥倾角和壁厚对圆锥管吸能特性和变形规律的影响;基于Mamalis圆锥管瞬时载荷压溃模型,提出大范围锥倾角（15°~40°）瞬时载荷修正公式。研究结果表明：变形模式的临界角为15°,当锥倾角小于等于15°时,变形模式为堆叠模式,当锥倾角大于15°时,变形模式转变为嵌套模式;平均载荷与比吸能均随着锥倾角的增大缓慢减小,而初始峰值载荷则大幅度减小;随着壁厚增大,压溃模式由钻石模式转变为环形对称模式,平均载荷、初始峰值载荷和比吸能均增大。基于仿真结果,采用皮尔逊相关系数分析法修正了Mamalis瞬时载荷公式,修正值与仿真值的最大误差小于10%,通过试验验证了修正公式的准确性。修正公式为揭示大范围锥倾角薄壁管压溃机理提供了理论依据。     

Nonlinear bending vibration of a prestressed thick plate

In this paper, the flexural vibration frequency in the antisymmetric mode of a thick plate as a function of the amplitude of the vibration and the axial force applied is investigated. With this aim, the theory of geometrically nonlinear deformation of second order and an optimized three-dimensional Ritz method are used. The plate is homogeneous, elastically linear, free from any constraints, and subjected to axial forces uniformly distributed on two of its opposite sides. Several approaches are discussed. First, the problem based on finite stress and infinitesimal strains is solved. Second, the deformation energy is assumed as the energy in the initial state plus the vibration energy of small or large amplitude. Third, without assumptions about the size of the deformation and of the vibration amplitude, the theory of nonlinear deformation is employed. Finally, numerical calculations for free vibration are compared with experimental results, including their systematic uncertainties.     

带颗粒减振剂碰撞阻尼的减振特性

传统碰撞阻尼的工作原理大都建立在动量交换、摩擦耗能的范围内,动量交换并没有将振动能量永久地消耗掉,摩擦对于高频振动具有较好的减振效果,而对于低频振动效果较差。为此提出一种以微细颗粒塑性变形将振动能量永久消耗掉的新型的碰撞阻尼,称为带颗粒减振剂碰撞阻尼。分别对在传统单体碰撞阻尼和带颗粒减振剂碰撞阻尼作用下悬臂梁减振效果进行试验研究。试验结果表明：以微细颗粒塑性变形消耗振动能量的带颗粒减振剂碰撞阻尼具有优秀的减振效果,远远超过传统单体碰撞阻尼器。带颗粒减振剂碰撞阻尼在低频振动(低于50 Hz)中仍然具有良好减振性能,这是其他碰撞阻尼所缺乏的特性。机械振动多为低频振动,带颗粒减振剂碰撞阻尼具有广阔的应用前景。     

Study on the energy dissipation mechanism of atomic-scale friction with composite oscillator model

A model called composite oscillator model is proposed for studying energy dissipation mechanism of atomic-scale wearless friction. The model consists of the whole macroscopic oscillator and the micro oscillators of interfacial atoms. The different influences of the two oscillators on the energy dissipation process of friction are discussed. It is found that the frequency of the interfacial exciting force is the key factor to energy conversion in the friction process by analyzing the dynamics characteristic of interfacial atoms. In the equilibrium stage, the interfacial force acts integrally and uniformly on each atom because its frequency is near zero. In the non-equilibrium stage, however, the distribution of energy received by the interfacial atoms is non-uniform because the frequency of the interfacial acting force is very high. Therefore, the extra energy may be easily transferred to the adjacent atoms to have the energy dissipated. Then, the formulas are derived to calculate a frictional force. The calculated force is found to be close to the experimental one. The comparisons show that the composite oscillator model can explain energy dissipation mechanism in a frictional process and it can be used to control friction as well.     

Tribological and nanoscale research on model friction couples intended for hip joint prostheses

Abstract

The paper presents the results of tribological and nanoscale research on model friction couples intended for hip joint prostheses. The tribological tests were performed by means of reciprocating pin on plate testing machine. The investigated friction pairs contained plates rubbing against polymer pins. The test plates were made from seven kinds of ceramics containing different concentrations of ZrO₂ and Al₂O₃, and two kinds of Co–Cr alloy. The test pins were made from UHMWPE. Tribological tests were performed in conditions of Ringer solution circulation. On the basis of friction force measurements, for each investigated friction couple, the average coefficient of friction was calculated. On the basis of total wear measurements, for each investigated couple, the wear intensity was calculated. Before and after every test, the plates and pins were analysed by means of atomic force microscopy. The difference in plate surface roughness was determined by the results of the atomic force microscopy analyses.

It was stated, that in the case of investigated friction joints, working under reciprocating motion, the wear and friction coefficient correlates with the surface roughness of plate specimens. For the plates with higher surface roughness, the lower friction coefficient and also lower UHMWPE pin wear intensity were observed. The friction coefficient and wear intensity were increasing with decreasing surface roughness. The correlation is confirmed by the differences in material transfer process. Considering investigated friction couples, the pin polymer material is smeared on the ceramic plates with the highest surface roughness creating a thin polymer film. In the case of ceramic surfaces with the lowest surface roughness, the strong adhesive bounds are created and some large particles of polymer are transferred to ceramic surface.     

新型弧面摩擦阻尼器力学性能研究

为克服传统摩擦阻尼器自适应能力差及耗能能力低的缺点,提出了一种新型弧面摩擦阻尼器,该阻尼器的结构特征在于其摩擦板和滑块的滑移面均为弧形,两滑块之间装有压缩橡胶,阻尼器通过摩擦板与滑块之间的移动产生摩擦力实现耗能。建立了阻尼器的机理模型,并采用数值模型验证了机理模型的合理性,分析了加载频率和橡胶弹簧初始压缩量对阻尼器滞回阻尼特性的影响规律。研究结果表明：该新型阻尼器具有马鞍形滞回曲线,其摩擦力具有位移随变性;该阻尼器的耗能能力比传统摩擦阻尼器强,耗能能力最多提高了23.84%;其力学性能与加载频率相关性较小,而橡胶弹簧预紧力越大,阻尼器的滞回耗能能力越强。     

The Relationship Between Contact Mechanics and Adhesion in Nanoscale Contacts Between Non-Polar Molecular Monolayers

Atomic and friction force microscopy were employed to examine adhesion and friction between dodecanethiol self-assembled monolayers in pure media as well as in two-component heptane/acetone mixtures. In media that did not contain hydrogen bond donors, the pull-off forces were found to be in very good agreement with theoretic predictions based on the Lifshitz theory. As the hydrogen bond donor ability of the medium increased, the adhesion energy was found to be increasingly underestimated by the model, illustrating the importance of the medium–medium interactions outside the contact area in determining the adhesive properties of the contact at the nanoscale. Exceptionally, in n-octanol, the pull-off forces were considerably lower than predicted and a dual slope linear friction–load relation was observed. These observations were rationalized by the formation of physisorbed layers of octanol on the surfaces. The friction–load relationship in the other media was found to be dependent on the magnitude of adhesion. For weakly adhering systems, the friction–load relationship was linear, but as adhesion increased, a sublinear relationship was observed. The data were rationalized by treating the friction as the sum of an adhesion-dependent shear term characterized by a surface shear strength τ and a molecular plowing term characterized by a coefficient of friction μ. Thus, Amontons’ law appears to describe the limiting case of very weak adhesion where viscoelastic plowing is primarily responsible for energy dissipation, while a sublinear friction–load relationship emerges in other situations due to the dissipation of energy in shearing adhesive contacts.     

冷轧摩擦热计算及温升模型研究

冷轧时轧件塑性变形热和摩擦热是导致轧件温度升高的两个主要原因。本文采用包含滑动摩擦和粘着摩擦在内的混合摩擦模型精确地计算了轧制区的摩擦热，综合考虑轧件塑性变形热，推导了轧件温升计算公式。     

摩擦系数影响摩擦噪声发生的机理研究

结合摩擦系数测量和磨痕表面形貌的观察，认为摩擦力的波动成分是引发摩擦噪声的主要机理。对于往复滑动过程，在摩擦系数一往复循环次数的关系图上一般有一个驼峰状的变化。应用Suh的理论，说明了在摩擦系数的驼峰状上升沿靠顶部的地方既是摩擦力接近最大值，又是摩擦力波动最激烈的地方，因而在这地方很容易出现摩擦噪声。