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.     

Effects of non-flat contact and interference on self-assembled monolayers under sliding friction

The nanotribology mechanism of alkanethiol self-assembled monolayers (SAM) chemisorbed on a gold surface under a non-flat contact by a tilt plane was studied using molecular dynamics (MD) simulations. The molecular trajectories, tilt angles, normal forces, shear forces, and frictional coefficient of the SAM were evaluated during the friction and relaxation processes for various parameters, including the tilt angle of the slider, interference magnitude, and SAM length. At the nanoscale, the magnitude of interface interactional forces is strongly dependent on the magnitude of the contact area, not on the surface geometry. The contact area and the exerted normal force of the SAM increase with decreasing the tilt angle of the slider at the same contact interference. In contrast, the periods in both normal force and shear force are gradually delayed as the tilt angle of the slider increases. Once the contact interference increases, the normal force and shear force increase together. During the sliding friction process with a smaller tilt slider angle, SAM molecules can maintain a better collective ordered structure. Short SAM molecules are more sensitive to a compressive loading and react to a larger normal force under the same contact interference due to the deformation of a larger tilt angle and decrease in chain length. The friction coefficient of SAM is significantly more dependent on the tilt angle of the slider than the contact interference.     

Friction Properties of Bio-mimetic Nano-fibrillar Arrays

Nano-fibrillar arrays are fabricated using polystyrene materials. The average diameter of each fiber is about 300 nm. Experiments show that such a fibrillar surface possesses a relatively hydrophobie feature with a water contact angle of 142°. Nanoscale friction properties are mainly focused on. It is found that the friction force of polystyrene nano-fibrillar surfaces is obviously enhanced in contrast to polystyrene smooth surfaces. The apparent coefficient of friction increases with the applied load, but is independent of the scanning speed. An interesting observation is that the friction force increases almost linearly with the real contact area, which abides by the fundamental Bowden-Tabor law of nano-seale friction.     

Plastic shear response of a single asperity: a discrete dislocation plasticity analysis

We investigate the plastic shear response during static friction of an asperity protruding from a large FCC single crystal. The asperity is in perfectly adhesive contact with a rigid platen and is sheared by tangentially moving the platen. Using discrete dislocation plasticity simulations, we elucidate the plastic shear behaviour of single asperities of various size and shape, in search for the length scale that controls the plastic behaviour. Since plasticity can occur also in the crystal, identification of the length scale that controls a possible size-dependent plastic behaviour is far from being trivial. It is found that scaling down the dimensions of an asperity results in a higher contact shear strength. The contact area is dominant in controlling the plastic shear response, because it determines the size of the zone, in and below the asperity, where dislocation nucleation can occur. For a specific contact area, there is still a dependence on asperity volume and shape, but this is weaker than the dependence on contact area alone.     

Multimode Active Control of Friction,Dynamic Ratchets and Actuators

Active control of friction by ultrasonic vibration is a well-known effect with numerous technical applications ranging from press forming to micromechanical actuators. Reduction of friction is observed with vibration applied in any of the three possible directions (normal to the contact plane, in the direction of motion and in-plane transverse). In this work, we consider the multi-mode active control of sliding friction, where phase-shifted oscillations in two or more directions act at the same time. Our analysis is based on a macroscopic contact-mechanical model that was recently shown to be well-suited for describing dynamic frictional processes. For simplicity, we limit our analysis to a constant, load-independent normal and tangential stiffness and two superimposed phase-shifted harmonic oscillations, one of them being normal to the plane and the other in the direction of motion. As in previous works utilizing the present model, we assume a constant local coefficient of friction, with reduction of the observed force of friction arising entirely from the macroscopic dynamics of the system. Our numerical simulations show that the resulting law of friction is determined by just three dimensionless parameters. Depending on the values of these parameters, three qualitatively different types of behavior are observed: (a) symmetric velocity-dependence of the coefficient of friction (same for positive and negative velocities), (b) asymmetric dependence with respect to the sign of the velocity, but with zero force at zero velocity, and (c) asymmetric dependence with nonzero force at zero velocity. The latter two cases can be interpreted as a "dynamic ratchet" (b) and an actuator (c).     

Influence of the normal force and contact geometry on the static force of friction of an oscillating sample

The paper is devoted to an experimental and theoretical investigation of the static friction force between a rapidly oscillating sample and a steel plate. The static frictional force is studied experimentally as function of the oscillating amplitude, the normal force and the contact geometry. A simplest model of tangent contact with a constant friction coefficient is proposed and shows a good agreement with experiment. The static friction force is proved to be a universal function of the ratio of the oscillation amplitude, the indentation depth and to the friction coefficient.     

纳米接触行为的三维多尺度数值模拟

应用分子动力学与有限元耦合的桥域多尺度算法,模拟三维刚性球形压头与光滑基体表面的纳米尺度接触行为,并与全原子分子模拟结果比较.考察在一定载荷下的系统弛豫行为、两种模型桥接区位移和应力的连续性、法向力和接触面积随压头位移变化等,结果表明:一定外载荷下,桥域多尺度算法能较快达到平衡状态,且压头的振荡幅度更小,系统初始温度为0 K时该算法的相对误差最小.在准静态加载过程中,该算法能够将原子区的位移、应力等连续的过渡到连续介质区,具有较好的耦合效果;法向力-压头位移和接触半径-压头位移曲线几乎与分子模拟结果重合,表明算法具有较高的计算精度.     

纳米接触过程中黏着规律的变化

本文应用大规模分子动力学方法, 模拟了两种具有不同粗糙形貌的、刚性球形探头与弹性平面基体之间的纳米尺度接触, 计算了探头与基体之间的拉离力和黏着功, 研究了接触过程中界面黏着力随载荷的变化规律, 分析了接触界面原子的法向应力分布. 研究发现, 原子级光滑接触的黏着力随着载荷的增大而线性增大, 而原子级粗糙接触的黏着力-载荷曲线分为以不同斜率增长的两个阶段. 相比于原子级光滑探头, 原子级粗糙探头与基体之间具有较小的拉离力和黏着功, 却在接触过程中形成了较大的黏着力. 因此, 拉离力和黏着功不能表征出纳米接触过程中原子吸引作用对界面法向力的贡献大小.     

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     

Surface modifications induced by the friction of graphites against steel

Graphite is a well-known, multipurpose material used in many applications including tribology, electrochemistry and metallurgy. In this study, we focused our attention on the modifications of the surfaces of graphite pins, one natural and two synthetic, in dynamic contact against steel. Specifically, we explored the effect of the normal load applied to the contact, on the friction behaviour of graphites.The results of the series of tests under “low” loads indicated that the friction coefficient and the transfer film forming tendency increased with increasing load, due to the modification of the morphology of the contact area. Whereas experiments under “high” loads revealed the existence of a critical load at which a sudden decrease of both the coefficient of friction and the transfer film forming tendency is observed. The reorientation of the graphitic planes is supposed to be at the origin of this phenomenon.     

Solute friction and forest interaction

Solute friction is known to prevail in crystals where a solution of point defects results in a diffuse resistance to dislocation motion. This property is often used to strengthen materials. In this paper we show that it also affects dislocation–dislocation interactions. Dislocation dynamics simulations are used to investigate and quantify this property. The solute friction results in a shielding of elastic interactions leading to a significant decrease of the intrinsic strengths of junction and annihilation reactions. Simulations in static and dynamic conditions show that the interaction stability decreases with the friction stress. A model is proposed to account for the modification of the interaction coefficient predicted by massive simulations in latent hardening conditions. Results suggest that the observed softening is mainly due to the decrease of the line tension of dislocations involved in the dislocation–dislocation interactions.     

A model for the characterization of friction contacts in turbine blades

Stresses produced by the forced vibrations can lead to a significant reduction of the life of turbo engine blades. To predict the vibration amplitudes of this components an accurate dynamic analysis is necessary. The forced response calculation of these dynamic systems is strongly affected by the presence of the contact interfaces (i.e., underplatform dampers, shrouds, root joints). Different contact models are available in literature. These models make use of contact parameters, contact stiffness and friction coefficient to evaluate the damping and stiffness related to the contact interfaces. In this paper a model is proposed to characterize friction contact of non-spherical contact geometries obeying the Coulomb friction law with constant friction coefficient and constant normal load. The hysteresis curves of the oscillating tangential contact forces vs. relative tangential displacements and the dissipated energy at the contact are obtained for different contact geometries. The developed model is suitable to be implemented in numerical solvers for the calculation of the forced response of turbine blades with embedded friction contacts.     

On the dependence of the static friction force between a rigid,randomly rough fractal surface and a viscoelastic body on the normal force

The present investigation is based on the assumption that the static coefficient of friction can be characterized roughly by the rms value of the surface gradient in the contact region. We numerically investigate the contact between a rigid, randomly rough surface and an elastic half-space and determine the dependence of the rms slope in the contact region on the normal force. For fractal surfaces with a Hurst exponent of H ≈ 1, the rms slope can be approximated very well by a logarithmic function of the normal force. Parameters of the approximation have been determined as functions of the Hurst exponent. The rms value of the slope always increases with the normal force, which indicates that the coefficient of friction should be an increasing function of the normal force.     

Nanotribology of Mo–Se–C films

Transition metal dichalcogenides with a layered structure are well known for their self-lubricating properties, particularly in a vacuum or dry atmosphere. The macrotribological properties of these films have been studied extensively. However, the tribological behaviour of these films in the nanonewton load range has hardly been reported. Study of tribological properties with load in the nanonewton range is required for applications related to microelectromechanical systems or nanoelectromechanical systems. In view of the above, the hardness, surface force, friction force, etc. of Mo–Se–C films were investigated at an applied load in the nanonewton range using a nanoindenter and atomic force microscopy. The effect of carbon content, applied load and scanning speed on the friction coefficient was determined. Data pertaining to topography, lateral force and pull-off force of various surfaces are illustrated. The observed nanotribological behaviour of these films is analysed in the light of their nanohardness. The results indicate that the friction force of all the films is very low and in general dependent on surface force. However, a film having the highest carbon content exhibits the maximum friction force. With increasing carbon content of the films tested, the hardness increases and wear decreases. The above results pertain to investigations under ambient conditions.     

表面黏附导致的接触行为转变

应用大规模分子动力学方法, 模拟了具有不同原子级粗糙形貌的两种刚性球形探头与弹性平面基体的黏附接触行为. 研究了载荷与真实接触面积、接触界面排斥力与真实接触面积, 以及黏附力与真实接触面积之间的关系. 分子模拟得到的载荷与真实接触面积的关系, 与连续力学接触理论预测很好地定性一致. 无论是原子级光滑探头还是粗糙探头, 黏附接触下的排斥力与真实接触面积的关系, 都与无黏附接触时的规律相一致, 即黏附力对接触行为的影响作用, 可以等效为附加在真实外载荷基础上的虚拟载荷, 将对黏附接触行为的分析转变为无黏附接触分析. 两种探头的黏附力随真实接触面积都呈幂函数形式的增长, 但是, 原子级光滑探头的幂指数大于1, 而原子级粗糙探头的幂指数小于1. 关键词： 接触行为 表面黏附 分子动力学模拟     

A dislocation mechanism of friction between a nanoprobe and solid surface

A dislocation mechanism of friction between an atomic-force microscope (AFM) probe and an atomically smooth solid surface is put forward. In this mechanism, the contact region is represented by an edge dislocation. The triboacoustic emission measured with an AFM shows the dislocation nature of friction. The friction force is calculated for a parabolic tip.     

Interaction potential and hopping dynamics governing sliding friction

The friction force on a nanometer-sized tip sliding on a surface is related to the thermally activated hopping of the contact atoms on an effective atomic interaction potential. A general analytical expression relates the height of this potential and the hopping attempt frequency to measurements of the velocity dependence of the friction force performed with an atomic force microscope. While the height of the potential is roughly proportional to the normal load, the attempt frequency falls in the range of mechanical eigenfrequencies of the probing tip in contact with the surface.     

Deformation of the contact area and adhesive friction between a scanning frictional microscope probe and the atomic-flat surface

The determination of the equilibrium atomic structure of a nanotribocontact, formed by a hard probe to be viewed as a paraboloid of revolution and subjected to an external load, with the soft surface modeled by a set of parallel atomic planes is considered. Structural, energy, and load characteristics are calculated. In addition, dissipative static adhesive friction as a function of the normal load and the radius of probe curvature for the diamond-graphite system is derived. A number of approximations of the interatomic potentials is used. It is shown that an allowance for the deformation of the contact area causes the adhesive frictional force in the tensile (negative) load range to decrease. For positive loads in a range of 0–80 nN, the variation of the frictional force (when deformation is taken into account) depends on the radius of the probe curvature and the used approximation of the interaction potential. The dependence of friction on the radius of the probe curvature is close to a direct proportionality. The calculated results are compared with the available experimental data.     

Study of AFM-based nanometric cutting process using molecular dynamics

Three-dimensional molecular dynamics (MD) simulations are conducted to investigate the atomic force microscope (AFM)-based nanometric cutting process of copper using diamond tool. The effects of tool geometry, cutting depth, cutting velocity and bulk temperature are studied. It is found that the tool geometry has a significant effect on the cutting resistance. The friction coefficient (cutting resistance) on the nanoscale decreases with the increase of tool angle as predicted by the macroscale theory. However, the friction coefficients on the nanoscale are bigger than those on the macroscale. The simulation results show that a bigger cutting depth results in more material deformation and larger chip volume, thus leading to bigger cutting force and bigger normal force. It is also observed that a higher cutting velocity results in a larger chip volume in front of the tool and bigger cutting force and normal force. The chip volume in front of the tool increases while the cutting force and normal force decrease with the increase of bulk temperature.