Molecular asperity theory of boundary friction

A computer investigation of the hypothesis that boundary friction is caused by molecular forces between the tails of long-chain molecules attached vertically to the sliding surfaces is reported. It has previously been shown that the interaction statements which apply to this system are the Slater and Bartell scattering centre potentials. The Slater scattering centre potential was less accurate but was considered to see how much it affects the calculated friction.The frictional force was calculated and shown to be a two-term relation. The first term is due to the interaction energy barrier encountered when moving one surface from one equilibrium position to the next, together with the work done against internal rotation barriers. The second term is due to the lifting of one end group over the opposite one as the chains move, a molecular “asperity” friction. The calculations show that the friction rises with normal load in a manner similar to that found experimentally. As with almost all theoretical predictions of strength calculated from intermolecular potentials, these forces are some ten times too large, a discrepancy which is usually attributed to dislocations.     

Thermal activation in boundary lubricated friction

The friction coefficients for copper pairs lubricated with fatty acids and fluorinated fatty acids have been measured over a wide range of sliding speeds and temperatures. Sliding speeds in the range 10⁻⁷−10⁻² m s⁻¹ and temperatures in the range 4.2–300 K were used. The friction coefficients near 300 K are generally low and increase with sliding speed, while the friction coefficients at low temperatures are markedly higher and relatively independent of velocity. Each lubricant's friction vs. velocity behavior over the temperature range 150–300 K can be described by a friction-velocity master curve derived from a thermal activation model for the lubricant's shear strength. The activation energies deduced from this friction model are identical to those obtained in the same temperature range for a vibrational mode associated with low temperature mechanical relaxations in similarly structured polymers. These results suggest that thermally activated interfacial shear is responsible for the fatty acids' positive-sloped friction vs. velocity characteristics at low sliding speeds near room temperature.     

The influence of boundary layers on friction

The influence of boundary layers on friction was investigated using a Bartel lubrimeter. Boundary layers have only a limited life and are re-formed in the contact area. A minimum specific pressure is required to deposit a solid lubricant film. The addition of MoS₂ lowers friction and frictional temperature. Some antiwear extreme pressure additives increase friction and others reduce friction and friction temperature. Some boundary layers tenaciously adhere to the surface and are only slowly worn away.     

Synergetic representation of stick-slip mode of boundary friction

The melting of an ultrathin lubricating film clamped between two atomically smooth solid surfaces that are in relative motion is studied based on the Lorentz model for the approximation of a viscoelastic medium. An equation of motion for the stresses has been derived in the form of a three-order differential equation and analyzed at various friction surface temperatures. In all cases, the phase portraits and the time dependences of the stresses have been plotted. It has been found that, depending on the temperature and the lubricant parameters, either the damped oscillation mode or the stochastic oscillation mode may occur. The stochastic oscillation mode is presented in the phase plane as a strange attractor. It has been shown that initial conditions have a critical effect on the system behavior. Based on the model, the behavior of two types of tribosystems, i.e., with the unidirectional shear of the surfaces and under an alternating external effect, has been described.     

The friction of sliding surfaces carrying boundary layers

Conventional boundary films are thought of as ``solid' layers in which shear strength is directly proportional to local contact pressure. However, recent studies suggest both that the properties of adsorbed or deposited surface films may be much more com- plex than this and that the details of surface topography cannot be ignored in determining the overall coefficient of friction of real engineering contacts. In this contribution we examine both these effects.     

Nonlinear friction compensation in mechatronic servo systems

Friction, especially its nonlinear component, may degrade the tracking performance of robots. Based on Kang’s method, a novel compensation method for nonlinear friction is presented in this paper, which modified Southward’s traditional compensation method for nonlinear friction. The stability of the systems which adopt the novel compensation method is proved with Layapunov’s stability theorem, and is enhanced further. Having estimated the nonlinear friction model using an identification method, the effect caused by its nonlinear component can be compensated, and enhanced tracking performance is verified under the SCARA robot experimental platform using Windows NT and VenturCom’s real-time extension module (RTX) environment.     

精密实验平台的非线性摩擦建模与补偿

非线性摩擦是影响高精度机械伺服系统动静态性能的主要因素之一。针对精密实验平台随行程位置不同表现出不同的摩擦特性,提出了一种基于LuGre模型的改进型摩擦建模方法,以速度和行程位置信号作为模型的输入变量,并用遗传算法对该模型的动静态参数进行辨识。基于改进型摩擦模型,分别通过精密实验运动平台及其相应的伺服仿真平台进行了摩擦现象和摩擦前馈补偿的实验和仿真。实验结果表明,摩擦补偿后的跟踪误差值约为补偿前的1/3,系统的静差也由原来1.4μm减小到0.4μm,与仿真平台摩擦补偿前后的现象基本一致。该改进型LuGre摩擦模型能直观、精确地描述实验平台的摩擦特性,基于该摩擦模型的前馈补偿减小了系统的跟踪误差,提高了系统的定位精度。     

Processes in heat dynamics and modelling of friction and wear (dry and boundary friction)

Theoretical postulates of heat dynamics and modelling of friction and wear are formulated based on a system of equations including basic dependences of friction behaviour on load, velocity, time and temperature.     

Nonequilibrium kinetics of phase transitions in the boundary friction mode

A nonequilibrium evolution thermodynamic model is presented, which describes the processes developing in the boundary friction mode. The excessive volume parameter is introduced to describe the lubricant state; it has a minimum value in the case of the solid-like structure of the lubricant and increases during melting. The source of entropy growth which results from the external energy inleak during the deformation of the lubricant owing to the shear of the rubbing surfaces, is taken into account. It is shown that the stick-slip mode of motion occurs within wide ranges of the parameters; this is caused by periodic phase transitions of the first-order between the structural states of the lubricant. The effect of the shear velocity, load, and temperature on the pattern of stick-slip friction is studied.     

Real area of contact and boundary friction in metal forming

Upper-bound models for asperity flattening on a workpiece surface undergoing bulk plastic deformation are developed. It is found that the effective hardness of the surface can be greatly reduced by the presence of underlying plastic flow. Theoretical predictions of the variation of real area of contact with strain show excellent agreement with experiments using model asperities in rolling. Friction models which allow for the reduction in effective hardness are developed for cases in which roughness is concentrated on either the workpiece or tooling.     

三维无摩擦接触问题的边界元法

主要讲述三维无摩擦接触问题的边界元理论的建立,以及离散积分方程的求解,目的是描述接触问题边界元方法建立的全过程。     

A friction model for microforming

For the simulation of metal forming processes, input data relating to the tool–workpiece interface is necessary. For microforming applications, this input data becomes very much more critical and traditional methods are not realistic. This paper describes an approach that seeks to describe friction by modelling the geometric surface roughness of the tool. This finite-element-based model has been validated experimentally in terms of loads and metal forming using the ring test and actual surface measurements. It enables more accurate and also more flexible modelling of friction.     

温度变化下的伺服系统非线性摩擦建模

针对机械伺服系统因温度变化产生的非线性摩擦变化,提出了一种反映温度因素的摩擦建模方法来实现对伺服系统的摩擦补偿。首先,分析了温度和摩擦的关系,并结合修正黏性摩擦的LuGre模型,讨论了该模型各参数与温度之间的关系。利用单隐层BP神经网络描述了随温度变化的各个参数,并确定了神经网络的输入、输出以及传递函数。然后,通过神经网络训练获得神经网络参数,从而得到与温度相关的摩擦模型。最后,改变运行条件,验证了提出的模型对摩擦的估计能力。建立的摩擦模型在不同运行条件、不同温度状态下的最大相对估计偏差小于2.5%,表明其能很好地估计系统摩擦力矩,满足高精度摩擦补偿。     

Molecular orbital indexes criteria for friction modifiers in boundary lubrication

Friction modifiers, due to their adsorption onto metal surfaces, play an important role in boundary lubrication. The molecular orbital indexes were used as the criteria to study the interaction between lubricant polar end groups and metal surfaces. By comparing the net electric charge of bonding atoms, the highest occupied molecular orbital energy (E_HOMO) and the lowest unoccupied molecular orbital energy (E_LUMO) of interactive molecules, one could find that the hydrogen bond strength between alcohol and oxide metal surface which is hydroxylated was stronger than that between a hydroxylated oxide metal surface and ester. On the other hand, the interaction between naked aluminium atoms and ester is stronger than that between naked aluminium atoms and alcohol. Thus, lubricants consisting of alcohol and ester show a combination friction-reducing effect because each of the two components has its own advantages in interaction with the hydroxylated aluminium oxide and naked aluminium atoms on the lubricated interface.     

A stochastic model of stick-slip boundary friction with account for the deformation effect of the shear modulus of the lubricant

The melting of an ultrathin lubricating film during the friction of two solid atomically smooth surfaces is studied within the limits of the Lorentz model that approximates a viscoelastic medium, the deformation effect of the shear modulus being taken into account. It is shown that the action of a random force representing additive non-correlated noise results in the sustained oscillation mode that corresponds to stickslip friction. The numerical modeling of the process yields the ratios between the relaxation times at which the stick-slip mode is characterized by a high amplitude. The amplitude of stick-slip transitions is found to decrease as the shear modulus of the lubricant increases.     

The behaviour of friction modifiers under boundary and mixed EHD conditions

The behaviour of a range of model and commercial friction modifiers (FMs) has been evaluated under elastohydrodynamic (EHD) and boundary lubrication conditions. Using a series of long‐chain carboxylic acids, it has been shown that measured boundary friction coefficients (BFCs) decrease with increasing chain length, unsaturation level, temperature, and concentration. Base oil polarity was found to have no effect under these conditions. Commercial oleate esters in synthetic base fluids gave lower BFCs than nitrogen‐containing compounds under the same conditions. This difference was observed over a range of concentrations and temperatures. The friction performance of formulated oils under mixed and full‐film EHD conditions was found to be dependent on FM, base oil, and detergent type. Under boundary conditions, friction was found to vary with FM type, but the effect of changing the base oil and the detergent system was negligible.     

Application of the generalized boundary condition to solving thermal friction problems

A new generalized boundary condition is advanced to describe the nonideal thermal contact in the tribosystem body-counterbody with high heat conductivity. The solutions of one-dimensional problems of nonstationary heat conductivity in half-bounded and bounded bodies with a generalized boundary condition are presented. The applicability of this condition to simulation of the contact temperature is exemplified by the polymer-metal tribosystem.     

Triboengineering properties of ceramic oxide coatings under boundary friction against steel

The triboengineering properties of ceramic-oxide coatings are studied during boundary friction against steel 45. Ceramic-oxide coatings formed by microplasma oxidation can be used within a wide range of pressures (8–36 MPa) and sliding velocities (0.05–2 m/s) to ensure lower friction coefficient and wear rate of the materials in contact as compared to a steel-steel pair. The improved triboengineering characteristics of tribojoints with ceramic oxide are attributed to the formation of nanostructures (with 40–100 nm elements) and surface smoothing up to R _a = 5.1 nm. The nanostructure and frictional surface morphology of steel are found to be a result of oil I-40A modification by high-dispersed wear products of ceramics in the friction zone playing the role of a nanosize additive to the lubricating material.