Research on Friction Properties of Mineral Lubricants in Thin-Film-Lubricating Regime

On the basis of thin film lubrication theory, the influence of fluid film(disordered film), ordered film and adsorbed film on tribological behavior of lubricating oil in thin-film lubrication(TFL) regime was studied. The μ-L(friction coefficient versus load) curves of different oil viscosity and additive dosage were obtained by a high frequency reciprocating test rig and the adsorption capacity of additive on steel surface were measured by QCM-D. Based on the Stribeck curve and thin film lubrication theory model, some conclusions can be drawn up, namely:(1) The μ-L curves and the parameters of L0 and μ0, obtained from the high frequency reciprocating test rig with ball-disc contact, can be used to study tribological behaviors of lubricating oil under TFL conditions.(2) In comparison with the high viscosity base fluid, the lower one can enter into TFL regime under lower load and keeps a lower friction coefficient in TFL regime.(3) The polar molecules in additive formulation produce ordered adsorbed layer on steel surface to reduce friction coefficient. And in TFL regime, the molecule's polarity, layer thickness and saturation degree on steel surface probably can influence lubricant's tribological behaviors between the moving interfaces. Moreover, the further study would be focused on the competitive adsorption of different additives, the formation of dual- and/or tri-molecular adsorption layers, and other aspects.

Dynamic Characteristics of Ultra Thin Liquid Lubricated Head-Disk Assembly in Magnetic Hard Disk Files

The dynamic characteristics of a liquid thin film lubricated head disk system are analyzed. The shear thinning effect is taken into account by introducing modification coefficients into the lubricant rheological model. The perturbation theory is employed to set up the dynamic pressure equation. The Reynolds equation and dynamic pressure equations are solved by finite difference method. The results obtained by the difference methods agree well with that calculated by the close solutions. IBM3370 slider is employed as a physical model. The slider of the system can keep flying at 20 nm height, which promises a potential application on high density recording device.

Molecular Dynamics Simulation on Pressure and Thickness Dependent Density of Squalane Film

Molecular dynamics（MD） simulations using the polymer consistent force field（PCFF） were adopted to investigate the pressure and thickness dependent density of squalane film in a nanogap at 373 K, with three different initial film thicknesses, and for a wide range of pressures. The equivalent densities predicted by MD simulations were compared with the empirical data. Results show that the squalane atoms tend to form layers parallel to the confining substrates but the orientations of squalane molecules are irregular throughout the film. In addition, distinct excluded volumes are not found at the interfaces of the film and substrates. Furthermore, with the same initial film thickness h_0, the film thickness h and compressibility decrease with increasing pressure, but the compressibility is similar for films with different initial film thicknesses. The equivalent densities predicted by MD simulations with the maximum initial film thickness（9.44 nm） are accurate to the values of Tait equation. The MD simulation with adequate initial film thickness can accurately and conveniently predict the bulk densities of lubricants.

Molecular dynamic simulation of lubricant spreading： effect from the substrate and endbead

Molecular dynamic simulations based on a coarse-gralned, bead-spring model are adopted to investigate the spreading of both nonfunctional and functional perfluoropolyether （PFPE） on solid substrates. For nonfunctional PFPE, the spreading generally exhibits a smooth profile with a precursor film. The spreading profiles on different substrates are compared, which indicate that the bead-substrate interaction has a significant effect on the spreading behaviour, especially on the formation of the precursor film. For functional PFPE, the spreading generally exhibits a complicated terraced profile. The spreading profiles with different endbeads are compared, which indicate that the endbead-substrate interaction and the endbead-endbead interaction, especially the latter, have a significant effect on the spreading behaviour.

Molecular behaviors in thin film lubrication——Part three: Superlubricity attained by polar and nonpolar molecules

In thin-film lubrication(TFL), generally, the viscosity of the lubricant and its coefficient of friction(Co F) increase. Finding a method to reduce the Co F in TFL is a significant challenge for tribologists. In the present work, we report a robust superlubricity attained by using polyalkylene glycols(PAGs, polar molecules) and poly-α-olefins(PAOs, nonpolar molecules) as lubricants on steel/steel friction pairs that have been pre-treated by wearing-in with polyethylene glycol aqueous solution(PEG(aq)). A steady superlubricity state with a Co F of 0.0045 for PAG100 and 0.006 for PAO6 could be maintained for at least 1 h. Various affecting factors, including the sliding velocity, normal load, and viscosity of the lubricants, were investigated. Element analysis proved that composite tribochemical layers were deposited on the worn region after the treatment with PEG(aq). These layers were formed by the tribochemical reactions between PEG and steel and composed of various substances including oxides, iron oxides, Fe OOH, and Fe(OH)3, which contributed to the superlubricity. In addition to the tribochemical layers, ordered layers and a fluid layer were formed by the PAGs and PAOs during the superlubricity periods. All the three types of layers contributed to the superlubricity, indicating that it was attained in the TFL regime. Accordingly, a mechanism was proposed for the superlubricity of the PAGs and PAOs in the TFL regime in this work. This study will increase the scientific understanding of the superlubricity in the TFL regime and reveal, in the future, the potential for designing superlubricity systems on steel surfaces for industrial applications.

Thin film lubrication in the past 20 years

Thin film lubrication (TFL),a lubrication regime that fills the gap between boundary lubrication (BL)and elastohydrodynamic lubrication (EHL) regimes,was proposed 20 years ago.Since it was first recorded in the literature,TFL has gained substantial interest and has been advanced in the fields of theoretical and experimental research.Following the revelation of the TFL phenomenon and its central ideas,many studies have been conducted.This paper attempts to systematically review the major developments in terms of both the history and the advances in TFL.It begins with the description and definition of TFL,followed by the state-of-art studies on experimental technologies and their applications.Future prospects of relevant studies and applications are also discussed.

Film forming behavior in thin film lubrication at high speeds

The film forming condition may transit into thin film lubrication(TFL) at high speeds when it is under severe starvation. Central film thicknesses and film thickness profiles are obtained via a technique of relative optical interference intensity. These profiles show a critical film thickness lower than which the absolute values of the film thickness gradient against speed or time decrease. It is possible to be in the thin film lubrication mode under such conditions. The high speed flow drives the lubricant molecules to rearrange in TFL and critical film thickness higher than 100 nm is achieved. The viscosity is one of the main factors controlling the decreasing rate and the critical film thickness. This paper is designed to investigate the thin film lubrication behavior at high speeds.

Advances in thin film lubrication(TFL): From discovery to the aroused further researches

Thin film lubrication(TFL) is known as a mode of fluid-film lubrication regime bridging the gap between elastohydrodynamic lubrication(EHL) and boundary lubrication(BL). Since the first recorded literature published in 1995, TFL has become one of the landmarks in the development of lubrication theory over the last twenty years. This article presents an overview of the advances in the research of TFL regime. We first begin with a brief introduction of the discovery of the phenomenon, followed by the discussion of the relevant essential aspects, such as the performing approach, theory and physical model. The state-of-the-art studies on both the theoretical and experimental fields are also presented, also latter with emphasis on the review of researches in several promising applications.

Dynamic Response of IBM3370 Slider in High Density Recording Systems

A liquid lubricated head disk system is introduced. Subjected to high shear rate the rheology of the ultra thin film is different from that of the bulk continuum theory. The shear thinning effect is considered in setting up the mathematical model of the ultra thin film rheology. The Reynolds equation and the perturbation theory are employed to set up the static pressure distribution model and to deduce the dynamic pressure equation. The static and dynamic equations are solved by finite difference method. Based on the dynamic analysis the dynamic response of the slider is simulated and some valuable results are obtained about the static and dynamic characteristics of the liquid lubricated head disk systems.

Static and Dynamic Numerical Simulation of High Density Recording Disk Drivers Based on Short Bearing Theory

Static and dynamic numerical simulations of high recording density disk drivers are presented in this paper. The shear thinning effect on the viscosity is taken into account in the lubricant rheological model. The perturbation theory and short bearing theory are employed to set up the static and dynamic lubrication model on the head disk interface. Close solutions are given for the pressure distribution, load capacity and the dynamic characteristics such as stiffness and damping coefficients, which provide a simple method to analyze the dynamic response of the slider supported by the ultra thin fluid film lubricated bearing. Based on the close solutions the static and dynamic responses of the IBM 3380 slider are simulated. Some interesting results are obtained for the analysis of the slider’s flying stability.

Modelling of spreading process: effect from hydrogen bonds

Lubricant spreading on solid substrates has drawn considerable attention not only for the microscopic wetting theory but also for the dramatic application in head-disk interface of magnetic storage drive systems. Molecular dynamic simulation based on a coarse-grained bead-spring model has been used to study such a spreading process. The spreading profiles indicate that the hydrogen bonds among lubricant molecules and the hydrogen bonds between lubricant molecules and polar atoms of solid substrates will complicate the spreading process in a tremendous degree. The hydrogen bonds among lubricant molecules will strengthen the lubricant combination intensity, which may hinder most molecules from flowing down to the substrates and diffusing along the substrates. And the hydrogen bonds between lubricant molecules and polar atoms of solid substrates will confine the lubricant molecules around polar atoms, which may hinder the molecules from diffusing along the substrates and cause precursor film to vanish.

On the relevance of analytical film thickness EHD equations for isothermal point contacts: Qualitative or quantitative predictions?

Thin film and elastohydrodynamic lubrication regimes are rather young domains of tribology and they are still facing unresolved issues.As they rely upon a full separation of the moving surfaces by a thin (or very thin) fluid film,the knowledge of its thickness is of paramount importance,as for instance to developing lubricated mechanisms with long lasting and efficient designs.As a consequence,a large collection of formulae for point contacts have been proposed in the last 40 years.However,their accuracy and validity have rarely been investigated.The purpose of this paper is to offer an evaluation of the most widespread analytical formulae and to define whether they can be used as qualitative or quantitative predictions.The methodology is based on comparisons with a numerical model for two configurations,circular and elliptical,considering both central and minimum film thicknesses.

Investigation of the tribological performance of ionic liquids in non-conformal EHL contacts under electric field activation

Real-time external alteration of the internal properties of lubricants is highly desirable in all mechanical systems. However, fabricating a suitable and effective smart lubricant is a long-lasting experimental process. In this study, the film thickness and frictional response of ionic liquid-lubricated non-conformal contacts to an electric field excitation under elastohydrodynamic conditions were examined. Film thickness was evaluated using a "ball-on-disc" optical tribometer with an electric circuit. Friction tests were carried on a mini traction machine (MTM) tribometer with a "ball-on-disc" rotation module and an electric circuit for contact area excitation. The results demonstrate that there is a difference in the behaviour of the ionic liquid during electric field excitation at the evaluated film thicknesses. The results of evaluated film thicknesses demonstrate that there is a difference in the behaviour of the ionic liquid during electric field excitation. Therefore, the ionic liquids could be a new basis for the smart lubrication of mechanical components. Moreover, the proposed experimental approach can be used to identify electrosensitive fluids.