Simulations of friction anisotropy on ordered organic monolayer

A method of molecular dynamics is used to investigate friction anisotropy observed on a hexagonally packed organic monolayer of straight-chain molecules, which tilt in a specific direction. A rigid gold slider with a single atomic protuberance is used as a model of a typical atomic force microscope tip apex. The friction anisotropy is observed at 50 K, which is below the melting point of rotation around a long axis of the molecule. The anisotropic frictional behavior is that sliding in directions normal to the direction of the collective tilt of the molecules results in the maximum friction force. The origin of the anisotropy is attributed to anisotropy in lateral compliance in the monolayer.     

载荷和相对湿度对微摩擦力的影响

利用自制的微摩擦和粘附力测试装置，研究了在微牛量级载荷下，载荷和相对湿度对Si（100）材料与Si3N4材料组成的摩擦副的最大静摩擦和滑动摩擦的影响。实验发现，最大静摩擦力和滑动摩擦力均随着载荷的增大而增大，而最大静摩擦因数和滑动摩擦因数则随载荷增大而降低。相对湿度对摩擦的影响则随不同载荷的范围而有所不同。载荷较小时，相对湿度对摩擦因数的影响较大；载荷较大时，则相反。利用单峰接触的模型，建立了相对湿度与摩擦力关系的公式。     

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.     

基于光纤光栅的机械手指触滑觉传感研究

针对机械手指柔性触滑觉传感问题,提出了一种基于光纤光栅的二维分布式传感阵列的触滑觉传感方法,该方法采用两根平行放置和一根倾斜放置的光纤光栅传感器组成传感单元,利用弹性材料封装增强光纤光栅对压力的灵敏度,并以单元阵列的方式实现对触觉三维力、滑动方向以及滑移位置的感知。仿真和实验结果表明,该方法可以很好地实现对触觉正向压力和剪切力的传感,其中正向压力在0~3.691 9×103Pa的范围内灵敏度为Kp=0.194 pm/Pa,x正向剪切力在0~1.115×103Pa的范围内灵敏度为Kpt=0.03 pm/Pa。通过对传感阵列中不同光栅中心波长漂移的上升沿和峰值时间差实现对物体x轴的滑移方向的判断。该方法可以很好地实现对机械手指触滑觉信息的测量,具有一定的应用价值。     

Influence of Ultrasonic Oscillation on Static and Sliding Friction

Vibrations of varying frequency and amplitude are used in many technological areas to control and reduce friction. In this study we report the results of systematic high-precision measurements of the static and sliding friction under the influence of ultrasonic oscillations. We investigate the effect of ultrasonic oscillations for in-plane and out-of-plane oscillations in the completely relevant interval of oscillation amplitudes and sliding velocities and for various material pairings. The experimental results are interpreted on the basis of both macroscopic and microscopic models. There are two main effects which are of interest for tribological applications. Firstly, the frictional force typically decreases with increasing oscillation amplitude, with an oscillation amplitude of about 0.1???m typically being sufficient for a significant decrease of frictional force. Secondly, the decrease of force is larger at smaller sliding velocities; therefore, at sufficiently large oscillation amplitudes, the frictional force always increases with sliding velocity. This effect can be used to suppress frictionally induced vibrations.     

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.     

Numerical analysis on meshing friction characteristics of nano-gear train

The objective of this research work is to provide a systematic method to perform molecular dynamics simulation or evaluation for meshing friction properties approach of micro/nano-gear train in MEMS. A model for meshing friction of micro/nano-gear train is proposed by using molecular dynamics based on investigation about a pair of meshing nano-teeth. The results show that the frictional characteristics in single meshing region are similar to the situation in the normal macro-gears meshing; however, the zero fictional force is not shown on the node of meshing. The maximum of the frictional force becomes larger and larger and the minimum of the frictional force becomes smaller and smaller when the number of the atomic layer in z direction increases. So the curve of the frictional force becomes steeper than the situation in normal macro-gears meshing. The maximum and the minimum of the frictional force are all invariable but the curve of the frictional force becomes steeper when the angular velocity of the powered gear increases. In the other case, there are no any changes in curve of the frictional force when the resisting torque increases.     

磨粒微观滑动接触过程热效应的有限元分析

利用ANSYS有限元软件分析了磨粒与被磨损材料表面滑动接触过程中,在摩擦热和力场的耦合作用下,接触区表现出的局部温度变化、应力变化等特性。结果表明,在磨粒滑移过程中,磨粒相当于接受固定热源作用,接触区温度逐渐上升,温度存在起伏波动现象,瞬现温升最高点在磨粒接触区两侧,反映出接触状态的不连续性,接触区状态的非稳定性;被磨材料表面的各点在进入接触前、经历接触时、脱离接触时,接触区温度存在先升高再下降的变化过程,同时,接触区的应力、剪应力、接触压力也发生变化。磨粒滑动过程的热效应问题研究将有助于揭示接触过程中材料表面损伤机制。     

Influence of operating conditions on friction and temperature characteristics of a wet clutch engagement

To optimise the gear change in automatic transmissions, more knowledge is needed of the engagement behaviour of wet clutches. A factorial design investigation of the engagement of a wet clutch has been carried out. The friction and temperature characteristics have been studied. The experiments were carried out in an apparatus that can vary sliding velocity, drive torque, inertia, force rate, and lubricant flow. The results show how these parameters affect the response data: engagement time, developed energy, temperature rise, maximum torque, maximum power, static friction, dynamic friction, and initial friction. There are interaction effects between some of the input parameters, but they are relatively small. The friction coefficient varies over time but is independent of the input parameters, except for dynamic friction, which decreases with increasing sliding velocity, drive torque, and inertia. The temperature rise was found to be proportional to developed energy and both these were most affected by sliding velocity. The drive torque and force rate have the greatest effect on the engagement time.     

Chemical Control of Friction: Mixed Lubricant Monolayers

Controlling frictional behavior in nanoscale sheared systems can be made possible when the relationship between the macroscopic frictional response and the microscopic properties of the sheared systems is established. Here, a new approach is proposed for tuning the frictional response and obtaining desirable frictional properties. This tuning is achieved through shear-induced phase transitions in a mixed lubricant monolayer consisting of a base solvent and an additive. The interaction between the solvent and additive molecules and their relative concentrations are shown to be the major parameters in determining the magnitude of the friction force and the nature of the response (stick–slip or sliding).     

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.     

纹理表面滑动摩擦稳态摩擦学性能

基于稳态滑动摩擦系统模型,采用球-盘摩擦副定量分析研究法向载荷、滑动速度、初始表面纹理和摩擦副材料对稳态摩擦因数的影响,得到稳态摩擦因数在不同工况下的变化规律。结果表明：滑动摩擦的稳态摩擦因数与磨损率正相关,周向纹理表面的稳态摩擦因数最大,无纹理表面的稳态摩擦因数次之,径向纹理表面的稳态摩擦因数最小;无论何种初始表面形貌,随着转速的增加,稳态摩擦因数先减小后增大,随着法向载荷的增大,稳态摩擦因数呈增长趋势;较深较宽的表面纹理具有更大的稳态摩擦因数和更大的瞬时波动;稳态摩擦因数也与摩擦副材料的选取有关。     

Molecular Orientation, Crystallinity, and Topographical Changes in Sliding and their Frictional Effects for UHMWPE Film

This paper presents a study on the frictional anisotropy of semi-crystalline UHMWPE polymer film deposited on DLC-overcoated Si substrate. For UHMWPE film slid against a silicon nitride ball, there is a remarkable difference in the coefficient of friction between the forward and reverse directions after the slider has been initially slid against the film for certain number of cycles. The changes in the friction are greatly influenced by the initial number of sliding cycles. This frictional behavior is explained in terms of crystallinity change and molecular orientational effects on UHMWPE and micro-topographical effects due to the initial sliding. Nanoscratch test is conducted to understand the friction of the polymer film in the sliding track and the data are compared with the macroscale friction data. The results show that the friction in the reverse of the initial sliding direction is high in comparison to that in the forward direction and this behavior mainly depends upon the number of initial sliding cycles. The initial sliding cycles affect the crystallinity and molecular orientation of the film, as well as the film topography. This combined effect on the polymer film results in an anisotropic frictional behavior of the film.     

Influence of sliding velocity on friction force in rubber seal rings under reciprocal motion

The influence of sliding velocity on the value of maximum and steady friction force in rubber seal rings under reciprocal motion is studied. It is established that the nature of the change in a static friction force and a steady-state friction force as a function of the sliding velocity is the same all other conditions being equal. The greater the sliding velocity, the faster the decrease and stabilization of a friction force after endurance of rubber seal rings in fixed contact with a cylinder. The maximum of the friction force dependence on the velocity in an investigated pair decreases with increasing temperature of a sealed medium.     

外界压力对粗糙表面摩擦与接触的影响研究

为研究外界压力对粗糙表面摩擦的影响,建立斜面滑块在不同真空度下的模型并进行试验测量在不同真空度下不同摩擦副配伍材料发生初始滑动时的斜面倾角。试验结果表明：随着外界压力增加,滑块发生初始滑动时的倾角增大,静摩擦力也增大。对于硬度小、重量轻以及接触面积大的摩擦副材料和零部件,其所受外界压力的影响更大。当外界压力从真空度-0.095 MPa增加到0 MPa,轴承钢/轴承钢的最大静摩擦力增大11%、轴承钢/聚碳酸酯增大12%,而聚碳酸酯/聚碳酸酯增大39%。由于通常情况下物体间的摩擦性能总是会受到外界压力的影响,因此真实摩擦角应在绝对真空环境下进行测量,而在大气中进行的测量都存在误差。同时可用文中方法测量两物体的实际接触面积。试验表明随着载荷增加实际接触面积增加,跟摩擦学的经验理论一致。     

Formation mechanism of electrical damage on sliding lubricated contacts for steel pair under DC electric field

A dynamic electrical pitting tester has been developed to investigate the effects of supply voltage, supply current, and oil film thickness on the electrical behavior, the normal and frictional forces, and the formation mechanism of electric damage for the sliding lubricated contacts of steel pair under DC electric field. Based on the experimental results for the normal and frictional forces, and the observations of the pitted surface, the process by which electrical damage occurs on the sliding lubricated surfaces is deduced. In this process, the molten metals are attracted to each other, leading to the formation of a molten metal column, which becomes a semisolid metal column due to the cooling action of the oil film. The normal force is significantly affected by this semisolid column, which grows and pushes against the lubricated contacts. Results also show that the frictional force primarily comes from the electrostriction force. The correlation formula for the damage width in terms of supply voltage, supply current, and oil film thickness is derived, but the supply current is the most important parameter affecting the width of the electric damage.     

Calculating frictional force with considering material microstructure and potential on contact surfaces

A method based on the energy dissipation mechanism of an Independent Oscillator model is used to calculate the frictional force and the friction coefficient of interfacial friction. The friction work is calculated with considering the potential change of contact surfaces during sliding. The potential change can be gained by a universal adhesive energy function. The relationships between frictional force and parameters of a tribo-system, such as surface energy and microstructure of interfacial material, are set up. The calculation results of the known experimental data denote that the frictional force is nearly proportional to the surface energy of the material, nearly inversely proportional to the scaling length, and independent of the lattice constant. The results agree with that of adhesion friction equations. They also agree with the experimental results performed with an atomic-force microscope under the ultra high vacuum condition. __________ Translated from Tribology, 2006, 26(2): 159–162 [译自: 摩擦学学报]     

Dry friction between flat surfaces: multistable elasticity vs. material transfer and plastic deformation

A generic model for frictional forces between two monoatomic crystals is investigated by molecular dynamics simulations. Two solids, each composed of several atomic layers, are brought into contact and moved against each other. The mechanisms that lead to finite pinning (static friction) forces are analyzed by varying the geometry, the interfacial interaction, and the externally applied force. Material transfer leading to welded junctions is seen to be responsible for friction between strongly adhering surfaces. Chemically passivated surfaces pin if they deform plastically. In no region of the model's parameter range can finite frictional forces be attributed to multistable elasticity. Such wearless pinning mechanisms play the predominant role in Frenkel–Kontorova and Tomlinson models. In the parameter range where pinning is observed, externally driven sliding induces wear at the interface.     

超声马达各向异性摩擦材料性能的试验研究

根据超声马达对摩擦材料切向和纵向性能的不同要求,采用在酚醛树脂胶中添加玻璃丝布和玻璃纤维增强的方法制备出不同弹性模量匹配的各向异性摩擦材料,并对其性能进行了测试。结果表明:当摩擦材料垂直方向与水平方向的弹性模量的比值为0.71~1.42时,摩擦材料受振动头的正压力和驱动力先增大后减小,比值为1.24时,马达的正压力和驱动力最大,驱动效果最好。     

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.