Friction and normal forces of model friction modifier additives in simulations of boundary lubrication

ABSTRACT

Interaction forces between solid surfaces are often mitigated by adsorbed molecules that control normal and friction forces at nanoscale separations. Molecular dynamics simulations were conducted of opposing semi-ordered monolayers of united-atom chains on sliding surfaces to relate friction and normal forces to imposed sliding velocity and inter-surface separation. Practical examples include adsorbed friction-modifier molecules in automatic transmission fluids. Friction scenarios in the simulations had zero, one, or two fluid layers trapped between adsorbed monolayers. Sliding friction forces increased with sliding velocity at each stable separation. Lower normal forces were obtained than in most previous nanotribology molecular simulations and were relatively independent of sliding speed. Distinguishing average frictional force from its fluctuations showed the importance of system size. Uniform velocities were obtained in the sliding direction across each adsorbed film, with a gradient across the gap containing trapped fluid. The calculated friction stress was consistent with measurements reported using a surface forces apparatus, indicating that drag between an adsorbed layer and trapped fluid can account sufficiently for sliding friction in friction modifier systems. An example is shown in which changes in molecular organisation parallel to the surface led to a large change in normal force but no change in friction force.     

Superlubricity of graphite

Using a home-built frictional force microscope that is able to detect forces in three dimensions with a lateral force resolution down to 15 pN, we have studied the energy dissipation between a tungsten tip sliding over a graphite surface in dry contact. By measuring atomic-scale friction as a function of the rotational angle between two contacting bodies, we show that the origin of the ultralow friction of graphite lies in the incommensurability between rotated graphite layers, an effect proposed under the name of "superlubricity" [Phys. Rev. B 41, 11 837 (1990)]].     

Departures from Amonton's law of friction

The method of transition from light friction (rolling) to heavy friction (sliding) is used for analyzing the principal departure from Amonton's law (force of sliding friction T proportional to the normal pressure force N) for several metal — nonmetal pairs when the sliding time t approaches zero. It appears that the departure mode is, under these conditions, the same for all pairs which have been tested here. Experimental and theoretical evidence is given to prove that all other modes of departure from Amonton's law represent consequences of changes in the rubbing surfaces which occur during sliding and that, therefore, such departures will appear only after a long enough period of sliding.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 94–98, November, 1974.     

载荷对壁虎刚毛束的摩擦各向异性特性影响的实验研究

用离体壁虎刚毛阵列在自制微黏附摩擦测试台上对预加载荷对刚毛摩擦与黏着的各向异性特性的影响进行了实验研究.实验结果表明,在逆壁虎刚毛自然弯曲方向卷出实现脱附时, 刚毛所受摩擦力与法向力成正比,摩擦系数为0.6;沿顺刚毛自然弯曲方向卷入实现黏附时, 随预载荷增加摩擦力增加,法向力由黏附力变为斥力.在同等预载荷下,卷入方向的摩擦力是卷出方向的2倍以上. 本文提出了摩擦各向异性特征参数,对壁虎刚毛的黏着与摩擦各向异性进行了定量表征, 这种特性是由刚毛的弯曲及多等级结构决定的. 关键词： 壁虎刚毛 黏着 摩擦 各向异性     

An investigation of nanoscale tribological characteristics under different interaction forces

The present study employs a Finite Element Method (FEM) atomic approach to investigate the nanoscale mechanisms of sliding friction. The current investigation chooses diamond-like carbon as the hard material, and copper as the soft material. The atomic configurations following sliding under non-interactive, attractive, and repulsive interaction forces are observed for soft-to-soft, hard-to-soft, and hard-to-hard sliding systems. The relationships between the normal force, the friction force, and the sliding distance are discussed. The current simulation results exhibit a similar trend with the findings of previous studies using molecular dynamics approach.     

Observed transition from linear to non-linear friction-load behavior using a lateral force microscope

The most commonly observed friction behavior for sliding systems is that described by Amontons laws of friction. In this case, sliding friction is independent of the gross or apparent area of contact between the materials and a linear function of the applied normal load, where the constant of proportionality is called the friction coefficient. However, for dry sliding solids in contact via a single-asperity junction, Amontons (linear) friction-load behavior is not strictly relevant. In experiments measuring sliding friction between a silicon tip and a quartz surface using an atomic force microscope (AFM), a transition from linear to non-linear friction-load behavior has been observed. This is proposed to result from a nanoscale ‘conditioning’ of a multiple-contact tip-surface interface to form a single-asperity contact.     

基于滑动势能面的二维材料原子尺度摩擦行为的量化计算

近年来,二维材料优异的摩擦特性成为人们关注的焦点,然而目前缺乏理论上对其摩擦力进行快速、有效、精确的计算预测方法.本文提出采用密度泛函理论计算真实体系的滑动势能面,利用得到的"数值型势能面"替代传统的解析势函数,并结合Prandtl-Tomlinson模型,量化求解具有复杂形状势能面的真实二维材料体系的摩擦行为.基于该方法,揭示了原子力显微镜实验中观察到的石墨烯Moire纹超晶格结构的双周期"黏-滑"摩擦现象;理论预测了二维材料异质结构的层间超低摩擦现象,相对于同质材料,其静摩擦力和滑动摩擦力均成数量级降低,发现势能面起伏和驱动弹簧刚度均会影响层间相对滑动路径,进而对层间的摩擦行为产生影响.该方法同样可拓展到其他van der Waals作用主导的界面摩擦体系.     

摩擦力演示装置

为了辅助摩擦力的教学,自制了摩擦力演示装置,通过模拟电路与数字电路的转换可以直接显示出滑块与传送带间的摩擦力.该装置不仅可以探究静摩擦力和滑动摩擦力,而且还可直观地显示出物体间摩擦力的渐变过程.     

掺硅类金刚石薄膜摩擦过程的分子动力学模拟

掺硅类金刚石(Si-DLC) 薄膜表现出优异的摩擦学性能, 在潮湿空气和高温中显示出极低的摩擦系数和很好的耐磨性, 但是许多实验表明Si-DLC膜的摩擦性能受其硅含量的影响很大. 因此, 本文采用分子动力学模拟的方法分别研究干摩擦和油润滑两种情况下不同硅含量的Si-DLC膜的摩擦过程. 滑移结果表明干摩擦时DLC膜和掺硅DLC膜之间生成了一层转移膜, 而油润滑时则为边界膜. 因此干摩擦时的摩擦力明显大于油润滑时的摩擦力. 少量添加硅确实能降低DLC膜的摩擦力, 但是硅含量大于20%后对DLC膜的摩擦行为几乎无影响. 干摩擦时硅含量对转移膜内键的数量影响很大, 转移膜内CC键和CSi键都先增加后减少, 滑移结束时几乎不含CSi键.     

Frictional coupling between sliding and spinning motion

The tangential motion at the contact of two solid objects is studied. It consists of a sliding and a spinning degree of freedom (no rolling). We show that the friction force and torque are inherently coupled. As a simple test system, a sliding and spinning disk on a horizontal flat surface is considered. We calculate, and also measure, how the disk slows down and find that it always stops its sliding and spinning motion at the same moment. We discuss the impact of this coupling between friction force and torque on the physics of granular materials.     

Molecular dynamics simulation of nano-scale interfacial friction characteristic for different tribopair systems

3D non-equilibrium molecular dynamics (NEMD) simulations using embedded atom potentials method (EAM) are performed to identify the dynamics processes of atomic-scale interfacial friction taking places in metal tribopairs. A block-block sliding simulation model for soft-to-hard (Cu/Fe) and soft-to-soft (Cu/Ag) tribopairs with is built. The microstructural evolution and temperature variation of the two tribopairs are analyzed at different sliding speeds. The results show that the average temperature of the two different tribopairs both increases rapidly during the transient sliding period. The different microstructural changes for the two tribopairs, including extensive plastic deformation, mechanical mixing and material transfer are observed when the temperature rapidly increases. The characteristics of the friction effects for the two tribopairs are also revealed by analyzing the friction force evolution as a function of time and velocity.     

摩擦导致的聚合物表层微观结构改变

应用大规模分子动力学方法, 模拟了锥形探头在非晶态聚合物薄膜表面的滑动摩擦过程, 研究了摩擦导致的聚合物薄膜表层微观结构改变, 以及探头与基体间黏着作用、滑动速度和分子链长度对基体表层微观结构改变的影响. 当探头与基体之间为黏着作用时, 摩擦导致基体表面滑痕区域的键取向沿滑动方向重新取向, 导致表层分子链回转半径沿滑动方向伸长, 并且这些表层微观结构的改变程度随滑动速度的减小而增大. 在摩擦导致结构改变的过程中, 链端单体和链中单体的贡献作用不同, 形成了不同的分子链拉伸变形机制. 当样本缠结度较大或探头滑动速度较小时, 相比于链中单体, 探头对链端单体的拖曳作用使更多分子链发生拉伸变形. 研究还发现, 在探头与聚合物薄膜系统中, 使薄膜表层微观结构发生改变是摩擦能量耗散的重要途径.     

Ultrasonic excitation affects friction interactions between food materials and cutting tools

In the food industry, ultrasonic cutting is used to improve separation by a reduction of the cutting force. This reduction can be attributed to the modification of tool–workpiece interactions at the cutting edge and along the tool flanks because of the superposition of the cutting movement with ultrasonic vibration of the cutting tool. In this study, model experiments were used to analyze friction between the flanks of a cutting tool and the material to be cut. Friction force at a commercial cutting sonotrode was quantified using combined cutting–friction experiments, and sliding friction tests were carried out by adapting a standard draw-off assembly and using an ultrasonic welding sonotrode as sliding surface. The impact of material parameters, ultrasonic amplitude, and the texture of the contacting food surface on friction force was investigated. The results show that ultrasonic vibration significantly reduces the sliding friction force. While the amplitude showed no influence within the tested range, the texture of the contact surface of the food affects the intensity of ultrasonic transportation effects. These effects are a result of mechanical interactions and of changes in material properties of the contact layer, which are induced by the deformation of contact points, friction heating and absorption heating because of the dissipation of mechanical vibration energy.     

Nanoscale interaction mechanism between a solid tip and a layered material while in relative motion: the boric acid case

The interaction occurring between a layered material (boric acid) and an atomic force microscope tip is discussed. It is shown that images containing the periodicity of a boric acid crystal, and the low friction occurring between the tip and the crystal surface, are caused by an effective tip composed of boric acid molecules. The friction at the sliding system decreases with an increase of the scanning velocity, suggesting that the dependence of friction on the velocity can be caused by a change of the energy dissipation regime from the nonlinear dynamics of a sliding system to phonon excitation.     

Reconstruction of bowing point friction force in a bowed string

A method is presented for reconstructing the friction force and the velocity at the bowing point of a string excited by a rosined bow sliding transverse to the string. Two versions of the method of reconstruction are presented, each approximate in different ways, but both capable of sufficient accuracy to allow useful application to problems of understanding frictional interactions in this dynamical system. The method is illustrated with simulated data to verify its accuracy, and results are shown for two contrasting cases of observed stick-slip string motion. As has been found in other investigations, the friction force during sliding is not determined by the instantaneous sliding speed. The results seem to be compatible with a thermally based model of rosin friction.     

3 GPa熔融盐固体介质三轴高温压力容器的轴压摩擦力标定

 在温度标定和围压标定的基础上,采用轴压循环方法,对3 GPa固体介质三轴高温高压实验系统的轴压摩擦力进行了标定,分析了围压、温度、轴向位移速率、装样方式（盐套类型）等实验条件对轴压摩擦力的影响。结果表明：静摩擦力、挤压摩擦力和滑动摩擦力3种轴压摩擦力对轴向应力的影响不同,其中静摩擦力和挤压摩擦力对轴向应力的影响很小,影响应力精度的主要是滑动摩擦力。静摩擦力及滑动摩擦力与围压正相关;静摩擦力与轴向位移速率正相关,但受其影响较小,滑动摩擦力不受其影响;静摩擦力和滑动摩擦力与温度负相关,并且受其影响较显著;盐套类型对轴压摩擦力的影响较大,当实验条件接近盐套熔点时,轴压摩擦力显著降低,当样品周围的盐套处于熔融状态时,轴压摩擦力最小。基于此,确定了标定轴压摩擦力的具体步骤,并对角闪岩的应力-应变曲线进行了轴压摩擦力标定。对比轴压摩擦力校正前、后的应力-应变曲线发现,经过轴压摩擦力校正的应力-应变曲线能更好地反映样品的实际变形情况。     

Tribological behavior of micro/nano-patterned surfaces in contact with AFM colloidal probe

In effort to investigate the influence of the micro/nano-patterning or surface texturing on the nanotribological properties of patterned surfaces, the patterned polydimethylsiloxane (PDMS) surfaces with pillars were fabricated by replica molding technique. The surface morphologies of patterned PDMS surfaces with varying pillar sizes and spacing between pillars were characterized by atomic force microscope (AFM) and scanning electron microscope (SEM). The AFM/FFM was used to acquire the friction force images of micro/nano-patterned surfaces using a colloidal probe. A difference in friction force produced a contrast on the friction force images when the colloidal probe slid over different regions of the patterned polymer surfaces. The average friction force of patterned surface was related to the spacing between the pillars and their size. It decreased with the decreasing of spacing between the pillars and the increasing of pillar size. A reduction in friction force was attributed to the reduced area of contact between patterned surface and colloidal probe. Additionally, the average friction force increased with increasing applied load and sliding velocity.     

Transverse thermal depinning and nonlinear sliding friction of an adsorbed monolayer

We study the response of an adsorbed monolayer under a driving force as a model of sliding friction phenomena between two crystalline surfaces with a boundary lubrication layer. Using Langevin-dynamics simulation, we determine the nonlinear response in the direction transverse to a high symmetry direction along which the layer is already sliding. We find that below a finite transition temperature there exist a critical depinning force and hysteresis effects in the transverse response in the dynamical state when the adlayer is sliding smoothly along the longitudinal direction.     

不同织构形状及分布下滑动摩擦副的多目标驱动优化及敏感性分析

为改善滑动摩擦副润滑性能,引入了表面微织构技术。首先采用计算流体力学(Computational Fluid Dynamics,CFD)方法建立了四种不同织构形状下滑动摩擦副的数值计算模型,并对其流动状态进行研究,其次以承载力最高和摩擦力最低为目标,基于非支配排序遗传算法(NSGA-Ⅱ)对织构结构及分布进行多目标驱动优化,最后采用局部敏感性分析方法研究了目标参数对各设计变量的敏感度。结果表明:优化后滑动摩擦副润滑性能明显提高,最优分布参数与织构形状无关,而最优织构深度随着织构形状不同有所变化,且优化前后矩形织构型滑动摩擦副润滑性能均最优;环向距离对承载力影响程度最大,织构深度次之,而摩擦力对各设计变量均不敏感。     

纳米级随机粗糙表面微观滑动摩擦力的计算研究

表面形貌很大程度上决定了摩擦副的摩擦性能, 而所有的表面都不可能是绝对光滑的.由于摩擦表面形貌的随机性, 决定了实际的摩擦过程具有随机性的特点, 因此为了获得与随机形貌对应的摩擦特性, 建立合理的随机摩擦模型是必要的. 本文基于Lennard-Jones势能建立了纳米级随机粗糙表面和原子级光滑的刚性平面间的随机摩擦模型; 模型中, 界面势能由法向载荷和界面间平衡距离决定.通过数值计算的方法, 推导了微观滑动摩擦力的计算公式和摩擦力与法向载荷之间的关系. 研究结果表明摩擦力随着法向载荷的增加而增加, 但不是线性增长. 结果也说明界面间的表面势能可能是微观摩擦力的本质起源. 关键词： 随机粗糙表面 Lennard-Jones势能 微滑动摩擦力 微摩擦