Effect of the wall roughness on slip and rheological properties of hexadecane in molecular dynamics simulation of couette shear flow between two sinusoidal walls

Molecularly thin liquid films of alkanes in extreme conditions in a boundary lubrication regime have been investigated. The wall is modeled as a rough atomic sinusoidal wall. The effect on the boundary condition of the roughness characteristics, given by the period and amplitude of the sinusoidal wall, is studied here. The effect of the molecular length of the lubricating fluid is also examined here. The results show that the relative size of the fluid molecules and wall roughness determines the slip or nonslip boundary conditions. The effect of wall roughness characteristics on the rheological properties of the lubrication film is also studied.     

Contact ratio of rough surfaces with multiple asperities in mixed lubrication at high pressures

Relative optical intensity interference was used to measure the lubrication film thickness when four kinds of polyalphaolefin (PAO) were used as lubricants confined between a smooth sapphire disc surface and a rough steel ball surface. Maximum Hertz contact pressure up to 3 GPa was applied in the central part of the contact region in mixed lubrication. It was found that the contact ratio (the ratio of real contact region to the whole nominal contact region) is related to the film thickness, the applied pressure, the surface roughness and the rolling speed, and so on. Contact ratio evidently reduces as lubrication film thickness or rolling speed increases. Quantitative relationship between the contact ratio and the influence factors was summarized based on the nonlinear fitting of experimental measurements. A formula was put forward to calculate the contact ratio at high pressure conditions according to the current experimental results.     

Transient thermo-mechanical interactions during high-speed slip at metal-on-metal interfaces

The paper describes results obtained from plate-impact pressure-shear friction experiments to investigate the phenomena of high-speed slip at metal-on-metal interfaces. Using a CH tool-steel/Ti-6Al-4V tribo-pair the authors have studied the effects of normal pressure, slip-speed, temperature and surface roughness on the evolution of the dynamic slip-resistance. Moreover, a finite-element procedure is developed to simulate the evolution of thermo-mechanical fields and understand their relationship to the observed slip response.Next, the pressure-shear friction experiments are extended to investigate the behavior of CH tool-steel/7075-T6 Al alloy tribo-pair. This material combination allows dynamic friction characteristics to be studied in the near-melt and the fully melt temperature regime of the lower-melting-point metal (7075-T6 Al alloy) comprising the tribo-pair. Besides providing information on the slip-velocity versus friction-stress relationship under extreme interfacial conditions, the experiments provides critical information on the shearing resistance of confined molten metal films under high pressures and extremely high shearing rates.     

Effects of wettability and interfacial nanobubbles on flow through structured nanochannels: an investigation of molecular dynamics

Solid–fluid boundary conditions are strongly influenced by a number of factors, including the intrinsic properties of the solid/fluid materials, surface roughness, wettability, and the presence of interfacial nanobubbles (INBs). The interconnected nature of these factors means that they should be considered jointly. This paper employs molecular dynamics (MD) simulation in a series of studies aimed at elucidating the influence of wettability in boundary behaviour and the accumulation of interfacial gas. Specifically, we examined the relationship between effective slip length, the morphology of nanobubbles, and wettability. Two methods were employed for the promotion of hydrophobicity between two structured substrates with similar intrinsic contact angles. We also compared anisotropic and isotropic atomic arrangements in the form of graphite and Si(100), respectively. A physical method was employed to deal with variations in surface roughness, whereas a chemical method was used to adjust the wall–fluid interaction energy (?_wf). We first compared the characteristic properties of wettability, including contact angle and fluid density within the cavity. We then investigated the means by which variations in solid–fluid interfacial wettability affect interfacial gas molecules. Our results reveal that the morphology of INB on a patterned substrate is determined by wettability as well as the methods employed for the promotion of hydrophobicity. The present study also illustrates the means by which the multiple effects of the atomic arrangement of solids, surface roughness, wettability and INB influence effective slip length.     

Contact mechanics analysis of oscillatory sliding of a rigid fractal surface against an elastic–plastic half-space

Oscillatory sliding contact between a rigid rough surface and an elastic–plastic half-space is examined in the context of numerical simulations. Stick-slip at asperity contacts is included in the analysis in the form of a modified Mindlin theory. Two friction force components are considered – adhesion (depending on the real area of contact, shear strength and interfacial adhesive strength) and plowing (accounting for the deformation resistance of the plastically deformed half-space). Multi-scale surface roughness is described by fractal geometry, whereas the interfacial adhesive strength is represented by a floating parameter that varies between zero (adhesionless surfaces) and one (perfectly adhered surfaces). The effects of surface roughness, apparent contact pressure, oscillation amplitude, elastic–plastic properties of the half-space and interfacial adhesion on contact deformation are interpreted in the light of numerical results of the energy dissipation, maximum tangential (friction) force and slip index. A non-monotonic trend of the energy dissipation and maximum tangential force is observed with increasing surface roughness, which is explained in terms of the evolution of the elastic and plastic fractions of truncated asperity contact areas. The decrease of energy dissipation with increasing apparent contact pressure is attributed to the increase of the elastic contact area fraction and the decrease of the slip index. For a half-space with fixed yield strength, a lower elastic modulus produces a higher tangential force, whereas a higher elastic modulus yields a higher slip index. These two competing effects lead to a non-monotonic dependence of the energy dissipation on the elastic modulus-to-yield strength ratio of the half-space. The effect of interfacial adhesion on the oscillatory contact behaviour is more pronounced for smoother surfaces because the majority of asperity contacts deform elastically and adhesion is the dominant friction mechanism. For rough surfaces, higher interfacial adhesion yields less energy dissipation because more asperity contacts exhibit partial slip.     

A dynamic rheological model for thin-film lubrication

In this study, the effects of the non-Newtonian rheological properties of the lubricant in a thin-film lubrication regime between smooth surfaces were investigated. The thin-film lubrication regime typically appears in Stribeck curves with a clearly observable minimum coefficient of friction (COF) and a low-COF region, which is desired for its lower energy dissipation. A dynamic rheology of the lubricant from the hydrodynamic lubrication regime to the thin-film lubrication regime was proposed based on the convected Maxwell constitutive equation. This rheology model includes the increased relaxation time and the yield stress of the confined lubricant thin film, as well as their dependences on the lubricant film thickness. The Deborah number (De number) was adopted to describe the liquid-solid transition of the confined lubricant thin film under shearing. Then a series of Stribeck curves were calculated based on Tichy's extended lubrication equations with a perturbation of the De number. The results show that the minimum COF points in the Stribeck curve correspond to a critical De number of 1.0, indicating a liquid-to-solid transition of the confined lubricant film. Furthermore, the two proposed parameters in the dynamic rheological model, namely negative slipping length b (indicating the lubricant interfacial effect) and the characteristic relaxation time λ 0 , were found to determine the minimum COF and the width of the low-COF region, both of which were required to optimize the shape of the Stribeck curve. The developed dynamic rheological model interprets the correlation between the rheological and interfacial properties of lubricant and its lubrication behavior in the thin-film regime.     

Study of Friction between Liquid Crystals and Crystalline Surfaces by Molecular Dynamic Simulations

The lubrication characteristics of liquid crystal (LC) molecules sheared between two crystalline surfaces obtained from molecular dynamics (MD) simulations are reported in this article. We consider a coarse-grained rigid bead-necklace model of the LC molecules confined between two atomic surfaces subject to different shearing velocities. A systematic study shows that the slip length of LC lubrication changes significantly as a function of the LC-surface interaction energy, which can be well described though a theoretical curve. The slip length increases as shear rate increases at high LC-surface interaction energy. However, this trend can not be observed for low interaction energy. The orientation of the LC molecules near the surface is found to be guided by the atomics surfaces. The influence of temperature on the lubrication characteristics is also discussed in this article.     

Association of polyethylene friction and thermal unfolding of interfacial albumin molecules

Under the articulation of artificial joints, ultra-high molecular weight polyethylene (UHMWPE) acts as a bearing surface under the lubrication of synovial fluid containing various proteins. Albumin is the most abundant composition and acts as the interfacial molecule in the boundary lubrication regime. The dissipated energy including thermal energy from the tribological process may lead to the conformational change of albumin molecules.In this study, a series of experiments were designed and carried out to investigate the association of thermal unfolding albumin and the frictional characteristics of highly-crosslinked UHMWPE (x-UHMWPE). An accelerated oxidation experiment was used to prepare x-UHMWPE with an oxidized surface. Analysis of the albumin protein by circular dichroism (CD) spectroscopy was performed to detect the conformational changes during a thermal process. In addition, a molecular simulation was performed to understand the structural change of albumin at various temperatures and the exposed hydrophobic contact areas. Linear reciprocating frictional tests were carried out to obtain the start-up friction coefficients. The results indicate that a decrease of α-helix content and an unfolding of the secondary structure of albumin were observed with increasing temperatures which may come from the frictional heat of joint articulation process. The conformational change of albumin differentiates the frictional characteristics for x-UHMWPE with different oxidation levels. A model, describing that the properties of the lubricating molecules and articulating surfaces may affect the adsorption of the boundary lubrication thin film which is critical to the tribological behavior, is proposed.     

粗糙纳通道内流体流动与传热的分子动力学模拟研究

为研究粗糙表面对纳尺度流体流动和传热及其流固界面速度滑移与温度阶跃的影响,本文建立了粗糙纳通道内流体流动和传热耦合过程的分子动力学模型,模拟研究了粗糙通道内流体的微观结构、速度和温度分布、速度滑移和温度阶跃并与光滑通道进行了比较,并分析了固液相互作用强度和壁面刚度对界面处速度滑移和温度阶跃的影响规律. 研究结果表明,在外力作用下,纳通道主流区域的速度分布呈抛物线分布,由于流体流动导致的黏性耗散使得纳通道内的温度分布呈四次方分布. 并且,在固体壁面处存在速度滑移与温度阶跃. 表面粗糙度的存在使得流体剪切流动产生了额外的黏性耗散,使得粗糙纳通道内的流体速度水平小于光滑通道,温度水平高于光滑通道,并且粗糙表面的速度滑移与温度阶跃均小于光滑通道. 另外,固液相互作用强度的增大和壁面刚度的减小均可导致界面处速度滑移和温度阶跃程度降低. 关键词： 速度滑移 温度阶跃 流固界面 粗糙度     

The definition and calculation of interfacial energies for thin films

We provide a new definition of the interfacial energy which eliminates three physically extraneous contributions from the conventional definition: (1) the strain or stress energy due to lattice mismatch between film and substrate; (2) the surface energy of the film-vacuum interface; and, (3) the substrate surface energy contribution from substrate layers below the film layers. This new interface energy then quantifies the variation in interactions among film/substrate, film/film and substrate/substrate bonding. Using this new definition, we derive the equations for evaluation of the interfacial energy in terms of the interaction energy for any atom in each layer of the film/substrate, film/film and substrate/substrate systems. With this formulation, it is simple to determine the dependence of the interfacial energy on the film thickness using virtually any interaction potential. Using a corrected effective medium theory, we present results for a few pseudomorphic film systems containing Ni/Cu, Ni/Ag, Cu/Ag and Rh/Ag on (111) and (100) surfaces. These systems cover a wide range of lattice mismatch and alloy formation energies. The results demonstrate that the new definition of interfacial energy converges after only 3–4 film layers, regardless of the degree of lattice mismatch. We also show that the interfacial energies at (100) and (111) interfaces differ and that the interfacial energy for a given pair of materials depends on which of the materials is the film.     

Direct experimental evidence of slip in hexadecane: solid interfaces

The boundary condition for the flow velocity of a Newtonian fluid near a solid wall has been probed experimentally with a novel setup using total internal reflection-fluorescence recovery after photobleaching leading to a resolution from the wall of the order of 80 nm. For hexadecane flowing on a hydrocarbon/lyophobic smooth surface, we give what we think to be the first direct experimental evidence of noticeable slip at the wall. We show that the surface roughness and the strength of the fluid-surface interactions both act on wall slip, in antagonist ways.     

A numerical study on stick-slip motion of a brake pad in steady sliding

A numerical model for an elastic brake pad sliding under constant load and with constant velocity over a rigid surface is investigated by finite element analysis. The geometry is taken to be two-dimensional, the contact is assumed to follow the laws of continuum mechanics and temporal and spatial resolution are such that dynamical effects localized at the interface are resolved. It turns out that at the contact interface localized slip events occur either in the form of long-lasting slip pulses, or in the form of brief local relaxations. Macroscopically steady sliding, macroscopic stick-slip motion or slip-separation dynamics occurs, depending on the macroscopic relative velocity. While structural oscillations of the brake pad do not seem to play a significant role during steady sliding at least one structural oscillation mode becomes synchronized with the interfacial dynamics during stick-slip or slip-separation motion. Assuming a given friction law for the interface, the macroscopically observed friction coefficient depends considerably on the underlying dynamics on the interface.     

Nanopolishing by colloidal nanodiamond in elastohydrodynamic lubrication

In this paper, the feasibility of using explosion synthesized diamond nanoparticles with an average particle size (APS) of 3–5 nm with a concentration of 1 % by weight for improving lubrication and friction in elastohydrodynamic lubrication (EHL) was investigated. Owing to the orders of magnitude increase in the viscosity of the lubricant in the EHL contact zone, diamond nanoparticles in the lubricant polish the surfaces at the nanoscale which decreases the composite roughness of contacting surfaces. The reduced composite roughness results in an increased film thickness ratio which yields lower friction. In the numerical analysis, governing equations of lubricant flow in the full elastohydrodynamic lubrication were solved, and the shear stress distribution over the fluid film was calculated. Using an abrasion model and the shear stress distribution profile, the material removal by the nanofluid containing nanoparticles and the resultant surface roughness were determined. The numerical analysis showed that in full EHL regime, the nanolubricant can reduce the composite roughness of moving surfaces. Experimental results from prior studies which exhibited surface polishing by such nanolubricants in boundary, mixed, and full elastohydrodynamic lubrication were used for comparison to the numerical model.     

Influence of inserting AlN between AlSiON and 4H-SiC interface for the MIS structure

A control of interface between gate insulating film and semiconductor is required to achieve high-power field effect transistors (FET) using SiC. To improve the interface between the high-k layer and SiC, we propose inserting an AlN layer as an interfacial layer. The reason for selecting AlN film is that it has a wide bandgap, as well as almost the same lattice constant as that of 4H-SiC. The insertion of AlN film between 4H-SiC and the insulating film effectively reduces the interfacial roughness. The roughness of the interface between AlN and SiC can be suppressed compared with that of the thermal oxidized SiC. Moreover, the AlSiON film was deposited on the AlN layer as a high dielectric gate insulating film with low leakage current at high temperature and low space charge. The C-V characteristics of the AlSiON/AlN/SiC MIS structure with an AlN buffer layer are improved by increasing the deposition temperature of the AlN film. This demonstrates that AlSiON/AlN/SiC is one of attractive MIS structures for SiC devices.     

Very low friction state of a dodecane film confined between mica surfaces

The existence of a very low friction state of a lubrication film is demonstrated in nonequilibrium molecular dynamics simulations of a six-layer dodecane film between mica walls. We argue that this low friction state is thermodynamically stable with respect to the well documented high friction film, the latter being a metastable state. These results are in striking accord with the recent report of Zhu and Granick [Phys. Rev. Lett. 93, 096101 (2004).]. The extreme low friction is the result, not of wall slip, but of layer sliding throughout the film, a mechanism similar to solid lubrication.     

波后滑移线引起的界面变形和R-M不稳定性

实验研究复杂波形结构引起平面界面变形和反射激波冲击下的R-M不稳定性的问题.在竖直激波管中生成稳定的N₂/SF₆平面界面,激波在圆柱绕射后,冲击平面界面,由此研究复杂激波引起的界面变形.平面激波在圆柱绕射后的流场,演化成具有初始入射波、三波点、弯曲反射波、Mach波和Mach反射产生的滑移线等复杂结构.研究复杂结构激波对界面的作用,对认识界面扰动的生成具有较大帮助.绕柱激波冲击后,平面界面仅在两对滑移线内部发生变形.绕柱激波冲击界面后,两对滑移线将界面分成"内界面"和"外界面",界面变形形态同滑移线和界面相交位置相关.反射激波二次冲击下,界面扰动的增长与Jacobs-Sheeley涡量模型较吻合.      

粗糙接触界面超声非线性效应的概率模型

提出描述粗糙接触界面超声非线性效应的概率模型,利用分段均匀概率函数描述粗糙接触界面劲度系数变化,结合表面粗糙峰的几何分布特征,得到界面粗糙度和两侧表面相对运动对声波的非线性调制作用。实验观测了铝合金材料的粗糙接触界面的高次谐波现象和阈值现象,进一步分析了归一化非线性参数与界面加载压力、粗糙度之间的关系。实验测量结果与概率模型的理论预测一致,证明了该模型的正确性,为利用超声非线性效应评价粗糙接触界面提供了理论依据。      

Complete boundary element formulation for normal and tangential contact problems

The boundary element method as a numerical tool in contact mechanics is widely used and allows for surface roughness to be investigated with very fine grids. However, for every two grid points, influence coefficients have to be employed for every force-displacement combination. In this paper, we derive the matrixes of influence coefficients for the deformation of an elastic half space, starting from the classical solutions of Boussinesq and Cerruti. We show how to overcome complexity problems by using FFT-based fast convolution. A comprehensive algorithm is given for solving the case of dry Coulomb friction with partial slip. The resulting computer program can be used effectively in iterative schemes also in similar problems, such as mixed lubrication and notably improves the applicability of the boundary element method in contact mechanics.     

Investigation of the influence of oil film thickness on helical gear defect detection using Acoustic Emission

This paper reports an investigation into the use of Acoustic Emission (AE) for monitoring gear teeth defects under varying lubrication regimes in helical gears. The investigation used a back-to-back gearbox test-rig with oil-bath lubrication. Variation in oil film thickness was achieved by decreasing the gear metal temperature with nitrogen gas whilst the gears were in operation. Results demonstrate a clear relationship between AE activity, operating temperature and specific film thickness. In addition, results show that there are lubricating conditions that may prevent AE from identifying the presence of gear defects.