A continuous viscosity model for thin film lubrication

The thickness of adsorbed molecular layers is the most critical factor in studying thin film lubrication, and is a key feature that distinguishes thin from thick film lubrication analysis. A method is presented that enables the adsorbed layer thickness to be calculated. This is based on adsorption theory and expressed in terms of molecular interaction energies. A continuous cross-gap viscosity model incorporating the layer thickness is introduced and used to calculate the load capacity and frictional characteristics of a simple bearing operating in the thin film regime.     

薄膜润滑时分层粘度模型的速度场分析

流体的流动将直接影响其润滑特性,特别是润滑剂粘度的分布,速度变化除产生温升外,还将可能使润滑剂产生剪切稀化,在薄膜润滑中,润滑剂量的相对运动速度大,其特性变化将更为突出。本以分层粘模型来研究在此时润滑剂的速度特性,为研究流体温度场和剪切稀化问题提供数据,对揭示薄膜润滑状态下轴承的性能和变化规律具有重要的意义。     

Modeling and analysis of interfacial electro-kinetic effects on thin film lubrication

At the interface between a solid and a following liquid, charge migration within the electric double layer causes an electro-viscous force to develop which manifests itself as an enhanced fluid viscosity. Based on the Poisson–Boltzmann equation, a new mathematical model of the electro-viscosity is developed. Numerical analysis is carried out taking into account the influence of the electric double layer on thin film lubrication. Analysis results show that the electro-viscosity leads to a significant increase in the film thickness in the thin film regime (below 100 nm).     

Research on thin film lubrication: state of the art

Thin film lubrication (TFL) has been well studied as a new lubrication regime since 1990s. TFL is a transition lubrication regime between elastohydrodynamic lubrication (EHL) and boundary lubrication (BL), which has specific lubrication features. Present paper summarizes the advancements of researches on TFL, which involves the origin of the TFL concept, advancement of measuring techniques, TFL film-forming features, investigation on its mechanism, and computation theories.     

On thin film lubrication

Various phenomena are revealed under EHL and micro-EHL conditions, such as the properties of the lubricant under high pressure, traction, and the load-bearing capacity of the lubricant film, and are discussed in the present paper. A new lubrication regime, thin film lubrication, has been discussed. The theoretical and practical significance of research on thin film lubrication is elaborated. Finally, the characteristics describing thin film lubrication and its main research directions are suggested.     

Analytical model for the thermo-hydrodynamic behaviour of a thin lubricant film

The thermo-hydrodynamic behaviour of lubricant films strongly depends on the operating conditions (pressure, speed), on the physical properties of the lubricant, on the temperature level and, furthermore, it is often nonlinear. The thermo-hydrodynamic study of a thin fluid film confined between two parallel planes on relative motion is presented in this paper. The case considered is that of not fully developed thermal conditions and physical properties of the lubricant independent of temperature. A 2D analytical solution is proposed to determine the temperature distribution. The physical parameters affecting the characteristic length, beyond which the flow becomes thermally developed, are determined.     

薄膜润滑状态下的能量方程推导

根据薄膜润滑的基本理论,推导出薄膜状态下流体润滑的能量方程及假设简化,为解决工程实际问题提供了理论基础。     

季戊四醇油酸酯膜厚测量及薄膜润滑研究

运用光干涉法相对光强原理,通过纳米级润滑膜厚度测量仪测量了不同温度、不同载荷下纳米间隙中环境友好润滑剂季戊四醇油酸酯的油膜厚度。探究了膜厚与速度、载荷、温度之间的关系,观察了薄膜润滑现象。结果表明：在对数坐标系下,膜厚与速度具有一定的线性关系,在速度较高时,线性关系更强,润滑剂具有流体效应。载荷对膜厚的影响远小于速度对膜厚的影响,温度对膜厚的影响主要表现在温度越高时,润滑剂的黏度越低,润滑剂的膜厚越薄。     

Full film lubrication of deep drawing

An analytical model for full film lubrication of deep drawing is developed, combining the elastic–plastic membrane finite element code of deep drawing together with full film lubrication theory. In full film lubrication, the surfaces are not in contact, and the gap in between includes two types of lubrication: the thick film lubrication regime and the thin film lubrication regime. The film thickness and the strain distribution of full film lubrication are predicted here. The theoretical results show excellent agreement with the experiment data.     

Theoretical analysis and experimental study on the influence of electric double layer on thin film lubrication

A new mathematical model for thin film lubrication is established by taking into account the effect of an electric double layer. In the present paper, experiments are carried out on a self-made tester. With a composite block and a rotating disk, influence of electric double layer on thin film lubrication is studied. Two different methods are used to reconstruct the field of electric double layer so as to change its effect. One is to change the ionic concentration of lubricants by adding additives, and the other is to apply an external electric field on friction pairs. According theoretical analysis, both the methods will apparently change the electro-viscosity of the lubricant film so as to change the lubrication performances. After theoretical calculation of electro-viscosity is amended according to the experimental results, the equations of electro-viscosity are presented. The results show that the equivalent viscosity of fluid induced by the effect of electric double layer apparently increases with the decrease of thickness of the film while the lubrication film is thin enough. The effect of electro-viscosity is weakened as the thickness of the film increases. Moreover, the effect of electro-viscosity increases with the increase of external electric field at first. When the voltage reaches a certain value, the electro-viscosity begins to decrease. __________ Translated from Tribology, 2005, 25(6) (in Chinese)     

Global and local analysis of gear scuffing tests using a mixed film lubrication model

Gear tests were performed in a FZG test rig in order to evaluate the influence of the operating conditions (torque, speed and oil bath temperature), gear geometry and base oil viscosity on gear scuffing.A mixed film lubrication model was used to evaluate the normal pressures and shear stresses in several points along the gear meshing line, for each load stage and for all the gear scuffing tests performed.The gear scuffing results were analyzed using two different approaches: one considering global gear parameters defined at the meshing line scale and another based on local parameters at the roughness asperity scale, determined using the mixed film lubrication model.The analysis at the roughness asperity level was used to complete the scuffing study performed with global gear contact parameters, explaining the occurrence of scuffing during ‘running-in’, justifying the zones in teeth flanks where the first scuffing marks appear and supplying indicators for low scuffing resistance at high oil bath temperatures.     

Effects of surface forces on pure squeeze thin film EHL motion of circular contacts

The pure squeeze thin film elastohydrodynamic lubrication (thin film EHL) motion of circular contacts with effects of surface forces taken into account is explored under constant load conditions. The difference between thin film EHL model and EHL model is apparent as the film thickness is thinner than 5 nm. The oscillation phenomena in pressure and film thickness come mainly from the action of solvation forces. The effects of surface forces become significant as the film thickness becomes thinner. Moreover, the viscosity is oscillatory due to its dependency on the hydrodynamic pressure which is affected by surface forces.     

Measurements of friction in strip drawing under thin film lubrication

Strip drawing is used to investigate the friction behaviour under thin film lubrication in metal forming with plastic deformation. Friction coefficients are measured under a wide range of tribological conditions. The surface roughness is measured on an interferometric profilometer. The results show that the friction coefficient decreases with increasing oil film thickness h_w, as estimated using a formula appropriate for smooth tool and workpiece. Measurements of the surface topography show that change in friction is associated with a change in contact ratio between the tool and strip. The effect of strip reduction, strip roughness and die roughness on the friction coefficient is also investigated.     

The effect of surface texturing on thin EHD lubrication films

Surface texturing has been successfully used for conformal contacts in many tribological applications in an effort to diminish friction and wear. However, the use of such a surface modifications are still in nascent as far as highly loaded contacts between non-conformal surfaces are concerned. It is mainly caused by the fact that the presence of such micro-features within these contacts can significantly influence the pressure distribution within the contact. Nevertheless, it has been shown in recent studies that the surface texturing can also have beneficial tribological effects if the depth of micro-features is properly designed. This paper is devoted to the experimental study of the effect of the micro-dents of various depths on thin lubrication films to find an experimental evidence of the micro-feature depth threshold for surface texturing applications in highly loaded non-conformal surfaces. The behaviour of an array of micro-dents within thin EHD contacts has been studied by thin film colorimetric interferometry. The influence of surface texturing on lubricant film formation has been observed under sliding/rolling conditions. The significant effect of micro-dents depth on lubricant film thickness is observed for positive slide-to-roll ratio when the disc is moving faster than the micro-textured ball. The presence of deep micro-dents within lubricated contact results in film thickness reduction downstream. As the depth of micro-dents is reduced, this effect diminishes and beneficial effect of micro-dents on film thickness formation has been observed. No significant influence of micro-dents depth on lubricant film shape has been observed in case of negative slide-to-roll conditions when micro-dents do not cause film thickness reduction regardless of their depths.     

Self-lubricating polymer composites and polymer transfer film lubrication for space applications

The use of self-lubricating polymers and polymer composites in space is somewhat limited at present. In general, they are only used when other methods are inadequate. There is potential, however, for these materials to make a significant impact on future space missions, if properly utilized. This paper surveys some of the different polymers and fillers used to make self-lubricating composites. It delves into the mechanisms of composite lubrication and wear, the theory behind transfer film lubricating mechanisms, and some factors which affect polymer composite wear and transfer. In addition, some of the current space tribology application areas for self-lubricating polymer composites and polymer transfer are mentioned.     

Microscopic study of thin film lubrication and its contributions to macroscopic tribology

This review article summarizes recent progress in investigation of nano‐rheology and thin film lubrication, as well as their contributions to conventional tribology. As the thickness of a lubricating film becomes comparable to molecular dimensions, a lubricant confined between solid walls undergoes a dramatic transition in its rheological properties and physical state, including the formation of ordered structure, enhanced viscosity and slow relaxation, glass transition or solidification, and consequent stick‐slip motion. As a result, it is recognized that there is special regime between EHL and boundary lubrication, identified as thin film lubrication, where lubricant flow and hydrodynamics are still in action but behave differently from expectations of the classical theory. Generalized theories of thin film lubrication are under development. Microscopic studies of thin film lubrication provide a solid theoretical basis to the development of high‐tech and micro devices, the understanding of lubrication failure, the generalization of classical lubrication theory, and friction control and interface design. This revised version was published online in June 2006 with corrections to the Cover Date.     

The investigation of contact ratio in mixed lubrication

In order to describe the mixed lubrication in nano-scale which is constituted from dry friction, boundary lubrication, and thin film lubrication, a contact ratio between surfaces of a glass disk and a steel ball in a pure rolling process has been measured by the technique of Relative Optical Interference Intensity (ROII) with a resolution of 0.5 nm in the vertical direction and 1 μm in the horizontal direction. The relationships between the contact ratio and its influence factors have been investigated. Experimental results indicate that the contact ratio in the static state is related to the combined surface roughness, maximum Hertz pressure, and the combined elastic module of tribo-pair in an exponential function. The decrease of rolling speed or lubricant viscosity, and the increase of the pressure will enhance the dynamic contact ratio which is the contact ratio measured in the rolling process. The addition of polar additives into basic oil will reduce the contact ratio. The contact ratio between rough surfaces is larger than that between smooth surfaces in the higher speed region. However, the former becomes smaller than the later after speed decreases below a critical value. A formula for calculating the dynamic contact ratio is given in the end of the paper.     

指数型粘度修正模型的提出及实用分析

根据吸附理论和分子间能量的变化规律来确定表面相互作用程度 ,以此可定义吸附层 ,得出吸附层厚度的分析计算公式。根据分子相互作用的基本理论及流体力学的基本定义得出指数型粘度修正的表达式 ,用于确定微小间隙内流体粘度的变化规律 ,用此修正模型对薄膜润滑条件下轴承的性能计算 ,同时与实验比较 ,得到比较吻合的效果     

The influence of transverse roughness in thin film, mixed elastohydrodynamic lubrication

The Spacer Layer Imaging (SLIM) method has been used to investigate the influence of transverse roughness on the thickness and shape of elastohydrodynamic (EHD) films. The effects of entrainment speed and lubricant viscosity on film distribution are shown for three distinct asperity heights over a wide range of lambda ratio (ratio of lubricant film thickness, separating two contacting surfaces, to their combined RMS roughnesses). Subsequently, the behaviour of film distribution for a range of mixed rolling–sliding conditions is also studied for both thin and thick film conditions. This study provides an estimate of how and when transverse asperities decompress and an indication of conditions under which these asperities cease to affect lubricant film formation.     

薄膜润滑条件下指数粘度修正模型的速度场分析

在薄膜润滑轴承分析中，流体的流动状态将直接影响其润滑特性，其主要反映在润滑剂粘度的分布，以指数粘度修正模型来研究在微小间隙内润滑剂的速度特性，得出速度分布规律，为薄膜流体的温度场和剪切稀化等问题提供计算依据。