An Advanced Rough Surface Continuum-Based Contact and Sliding Model in the Presence of Molecularly Thin Lubricant

A model of molecularly thin lubricant layer behavior for rough, sliding contact is presented in this work as a function of lubricant layer morphology. Building on previous work by the authors where the lubricant layer was assumed to be uniform in thickness and morphology, lubricant contributions to contact are presently treated at the asperity level and the effects of lubricant bonding ratio and coverage are accounted for. Effective stiffnesses for lubricated asperities are used to calculate the bearing and shear forces, while variable surface energy is modeled at the asperity level and used within an improved continuum adhesive formulation. Contributions from asperities in lubricant and solid contact for partial coverage are determined within the context of a statistical mechanics model. The proposed model can be used to study the mixed nanolubrication regime expected during light contact or “surfing” recording in magnetic storage, where sustained nanolubricant contact would partially deplete mobile molecules from the contact interface.     

The Development of a Spacer Layer Imaging Method (SLIM) for Mapping Elastohydrodynamic Contacts

Optical interferometry has proved to be a valuable experimental tool in the study of elastohydrodynamic lubrication (EHD). It is a technique that gives detailed information on the lubricant film distribution within the contact; however, the sensitivity is limited and it is only recently, with the development of the spacer layer optical technique, that the study of the thin film lubrication regime has been possible. The limitation of the spacer layer technique is that generally only one measurement is taken from the center of the contact. The next logical step in the development of this technique is, therefore, a system that combines the mapping capabilities of the original optical method with the thin film capabilities of the spacer layer approach.

This paper describes the development of a contact mapping technique that uses the spacer layer approach to visualize, and measure, thin lubricant films in concentrated contacts. The development of the technique is described and its application to both static and moving contacts reported. Thin EHD films (down to 10 nm) have been measured and mapped.     

Modeling Bearing and Shear Forces in Molecularly Thin Lubricants

Under the effects of high shear rate and confinement between solid surfaces, the behavior of a thin lubricant film deviates from that of the bulk, resulting in significant increases of lubricant viscosity and interfacial slip. A semi-empirical model accounting for the breakdown of continuum theory at the nanoscale is proposed—based on film morphology and chemistry from available experimental and molecular dynamics simulation data—to describe lubricant behavior under shear. Viscosity stiffening and interfacial slip models are introduced into the formulations of the normal (bearing) and shear forces acting on a sphere that moves within a thin lubricant film parallel to a rigid plane. The experimentally measured ‘apparent’ viscosity confounding the effects of both stiffening and slip is used to predict the hydrodynamic forces acting on a fully or partially submerged sphere for the purposes of describing lubricant contact in magnetic storage. The proposed sphere-on-flat model forms the basis of a future, dynamic contact with friction model that will account for lubricant contact in the context of molecularly thin lubricated rough surface contact.     

Influence of viscoelastic coatings on the contact of lubricated bodies

The contact of elastic bodies with viscoelastic coatings is considered, in the presence of lubrication. The one-dimensional Kelvin model is employed for the viscoelastic coatings. The pressure distribution in the lubricant layer and the layer thickness are investigated on transition from low to large loads.     

Molecular Dynamics of Thin Lubricant Films on Sol-Gel SiO2 Surfaces for Thin Film Magnetic Disks

Mobility of molecularly thin lubricant film is an important issue in understanding boundary lubrication mechanisms and to develop reliable magnetic disk media. Intra-molecular mobility for a perfluorinated poly ether (PFPE), which is used as a disk lubricant, with two hydroxyl groups on a sol-gel SiO₂ surface, which is used for a protective overcoat for plated magnetic disks, was studied using nuclear magnetic resonance (NMR). Thin film viscosities for molecular segments were derived from a relaxation time. The viscosity for the hydroxyl segment is 1.8 to 11 times as much as that for a bulk lubricant at room temperature, and the viscosity rate increased with increasing temperature. For example, it increased 15 times at 100°C. The viscosities for the segments in a main chain were not different from that of bulk PFPE.

A spin-off calculation for the molecularly thin lubricant film with thin film viscosity, derived from the NMR method, shows that there is no thickness decrease after seven years.     

Application of Spectroscopic Reflectometry to Elastohydrodynamic Lubrication Films Study

Optical measurement techniques have been successfully used for elastohydrodynamic (EHD) lubricant films studies for several decades and have significantly helped to understand the lubrication mechanisms within highly loaded machine contacts. Nevertheless, there are still many phenomena waiting for the explanation and new experimental approaches and measurements techniques are developed. Recent studies have provided promising results as to the application of spectroscopic reflectometry to the study of EHD films. Nevertheless, some simplifications were introduced. The main aim of this study was to develop a physically correct approach that could provide the additional information about the properties of lubricant film within highly loaded contacts. The principal part of this article was devoted to the effort to develop and verify the optical design suitable for such applications. This verification was carried out within lubricated contact formed between a steel barrel and sapphire disc without any semi-reflective layer. This simplified optical arrangement has enabled to obtain the correct lubricant film data and verify the applicability of the spectroscopic reflectometry for EHD lubrication films study. It represents the first step in this application of spectroscopic reflectometry and further research in the field of the behavior of thin solid films under high contact pressures is necessary to enable thin film measurements.     

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。     

Effect of Molecularly Thin Lubricant on Roughness and Adhesion of Magnetic Disks Intended for Extremely High-Density Recording

For extremely high-density recording using conventional technologies, the fly-height needs to decrease to less than ten nanometers. To allow such operation, disk and slider surfaces must become extremely smooth, down to root-mean-square (RMS) roughness values of a few angstroms. For super-smooth disks, molecularly thin lubricants are applied to improve tribological performance of head/disk interfaces. The focus of this study is to quantify the effect of lubricant thickness in terms of detailed roughness parameters and to evaluate the effect of roughness and molecularly thin lubricant on adhesion of magnetic disks intended for extremely high-density recording. Three identical ultra-low-flying disks have been fabricated from the same batch for this particular experiment. To investigate the effect of molecularly thin lubricants on disk roughness, super-smooth magnetic disks with increasing lubricant thickness have been measured and studied, using a primary roughness parameter set. It describes amplitude, spatial, hybrid, and functional aspects of surface roughness and is used to quantify the extremely smooth disk roughness as a function of lubricant thickness. It is found that in addition to simple amplitude parameters, hybrid and functional parameters also capture small features on the disk roughness and show distinct trends with increasing lubricant thickness. Subsequently, a continuum-based adhesion model that uses three parameters from the primary roughness parameter set, is used to predict how the varying thickness of molecularly thin lubricant and the resulting disk roughness affect intermolecular forces at ultra-low-flying head-disk interfaces. It is found that a thicker lubricant layer of 2nm causes higher adhesion forces for ultra-low-flying-heights in the range of 1–3 nm     

Prediction of surface roughness in metal forming with liquid lubricant

In the paper an experimental study of compression of rough surface in the presence of lubricant and in dry state is presented. The evolution of roughness parameters was analyzed. In the lubrication conditions an existence of asymptotic state was observed where roughness parameters and real contact area stabilize due to hydrostatic response of lubricant. The simplified model was proposed allowing to predict the roughness parameters and real contact area of the deformed surface in the asymptotic state. The model is based on analysis of topography of the undeformed surface and compression of a singular lubricant pit. The analysis was performed both numerically and experimentally.     

Estimation of lubricant interlayer shear strength under conditions of imperfect boundary lubrication

This paper considers the use of contact electrical conductivity for investigations into molecular tribology. The shear strength of thin lubricant films has been studied at nominal point contact between two crossed cylindrical steel probes. The interface was simultaneously monitored using electrical contact resistance. It is shown that the binomial law of friction holds for the sliding path portions where electrical measurements point to a continuous lubricant film. The experimental data can be used to evaluate the molecular friction parameters. An algorithm and software with which to evaluate shear strength and its components under conditions of imperfect boundary lubrication have been formulated.     

Lubrication of sliding point contacts of AISI 52100 steel — the influence of curvature

The lubrication mechanism of fully submerged sliding point contacts of non-oxidized AISI 52100 steel has been studied as a function of radius of curvature in the contact area. The results of experiments, performed with hemispherically tipped pins with radii of curvature R₁ ranging from 1 mm to 23 mm in contact with the curved surfaces of rings of 76 mm diameter, corroborate the existence of three well-defined lubrication regimes, i.e. a regime of thin oil film lubrication, a regime of boundary lubrication and a regime of virtually unlubricated contact. At small radii of curvature (i.e.R₁ = 1 mm and 5 mm), the contacts derive their load-carrying capacity almost entirely from the presence of a boundary lubricant film. At R₁ = 23 mm, lubrication is mainly due to the presence of a thin oil film.     

Contact Problem for a Viscoelastic Half-Space and a Rough Rigid Cylinder under the Conditions of Hydrodynamic Lubrication

A model has been proposed for use in studying the combined effect of the roughness of a rigid punch and the viscous properties of a base separated by a thin lubricant layer exerted on the characteristics of contact interactions and the sliding friction force. The problem of the motion of a thin a lubricant layer between a fixed rigid cylinder with a regular relief, as well as the surface of a moving viscoelastic half-space, the rheological properties of which are described by an integral operator with an exponential creep kernel, has been considered. The pressure and thickness of the lubricating layer, as well as the deformation component of the frictional force depending on the sliding velocity, have been analyzed. A comparison of the results of solutions of contact problems for viscoelastic and elastic rough bodies in the presence of a lubricant has been presented.     

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)     

Thickness Change in Molecularly Thin Lubricant Under Flying Head in Hard Disk Drives

In hard disk drives (HDDs), lubricants on disks are very important material to reduce head and disk wear. Thus, it is necessary to know changes in lubricant thickness to keep lubricant thickness constant on rotating disks. For this purpose, we have to know changes in lubricant thickness during HDD operations. We developed a simulation program to simulate changes in lubricant thickness during HDD operations numerically. First, we had simulated the changes in lubricant thickness of 10-nm-thick non-polar lubricant film under a flying head. The result corresponded to a reported experimental result. In recent HDDs, a lubricant thickness has become molecularly thin and lubricants with polar end groups have been used. In molecularly thin polar lubricants, diffusion depends on their thickness and their viscosity becomes very high. Next, we simulated the change in the lubricant thickness of 2-nm-thick polar lubricant film considering the effects of lubricant initial thickness. The simulated results showed that the changes were very small in 2-nm-thick lubricant film, but they were not confirmed with the experiment. In this paper, experimental results of the change in the thickness of molecularly thin non-polar and polar lubricants under a flying head were first measured. The simulations that took account of thickness-dependent diffusion and thin-film viscosity were then performed with the simulation parameters based on the experiments. The simulated results of lubricant distribution were in good agreement with the experimental results.     

Molecular dynamics characterization of thin film viscosity for EHL simulation

Molecular simulations were used to characterize changes in lubricant viscosity that may occur during thin film elastohydrodynamic lubrication (EHL). Molecular dynamics simulations were performed at variable wall speed and film thickness such that the effects of both parameters could be evaluated. Using this approach it was found that the viscosity of thin films under large shear is subject to both shear thinning and oscillation with film thickness. A composite model was developed that incorporated both effects. The expected impact that this model might have on an EHL interface was evaluated using a continuum simulation. An overall decrease in viscosity with some oscillation near the interface edges was predicted due to the molecularly modeled thin film effects.     

Experimental Study on Lubrication Film Thickness Under Different Interface Wettabilities

This paper describes some experimental studies about the effect of interface wettability on hydrodynamic lubrication film thickness by a custom-made slider bearing tester. The lubricated contact pair consists of a fixed-incline slider and a transparent disc, and a thin lubrication film can be generated when the disc rotates. The film thickness was measured by interferometry. The wettability of different slider surfaces was evaluated by the contact angle of the lubricant on them. The relationship of film thickness versus disc speed was measured under different liquid–solid interfaces, and the results showed that slider surfaces with strong wettability to the lubricant could generate higher film thickness. Furthermore, case experiments were carried out to validate the hydrodynamic effect by tailored-slippage. By numerical simulations, the experimental findings were tentatively explained with the phenomenon of wall slippage.     

Effect of surface texturing on elastohydrodynamically lubricated contact under transient speed conditions

Effect of surface topography modifications on lubrication film thickness within non-conformal lubricated contact operated under transient speed conditions is observed. Optical test rig is used to observe the lubricant film behaviour between the flat surface of a chromium coated glass disc and a steel ball under simplified operational conditions modelling the cam and tappet contact. Numerical simulation was used to be able to choose the operating conditions suitable for experiments. An array of micro-dents was produced on the ball surface to be able to demonstrate the effect of surface topography on lubrication film formation. Experiments were carried out under elastohydrodynamic lubrication conditions. Obtained results have shown that surface texturing could represent the way how to increase lubrication efficiency of rolling/sliding non-conformal contacts under transient operational conditions through the lubricant emitted from micro-dents. It was found that the lubricant emitted from the micro-dents helps to separate rubbing surfaces especially under thin film lubrication conditions where the rubbing surfaces moves in the opposite direction.     

用指数型粘度模型计算径向滑动轴承

本文应用指数型粘度修正模型进行薄膜润滑径向滑动轴承的计算分析，得出了考虑润滑剂与金属壁面相互作用时径向轴承的特性，同时进行与常规计算轴承的比较，为进行表面沉积层润滑性能分析和机理分析提供理论指导。     

Drag Reduction by Dimples on Surfaces in Plane–Plane Contact Lubrication

An experimental system was designed to evaluate the drag reduction by dimples on surfaces in plane–plane contact lubrication. Because of the dimples on the surfaces, in some of the experiments, it was observed that stable gas bubbles existed in the lubricant film. The results of the experiments showed that the existence of gas was decided by the depth of dimples and the volume of lubricant supply; the deeper the dimples were and the less the lubricant supply, the higher the probability of gas existence. The reason for the existence of gas, based on a model analysis, was the meniscus force of the gas–liquid interface, which caused the gas to be enclosed in the dimples. When a small amount of lubricant was supplied, the friction force was reduced because of a mixed lubrication film of oil and gas. For different amounts of lubricant supply, an optimal depth of dimples existed in plane–plane contact lubrication, which could obviously reduce the friction force. An optimization model for full film lubrication was presented to determine the optimal depth of dimples. Based on the model, the optimal dimple depth is approximately proportional to the distance between the two surfaces. The theoretical analysis agreed with the experiments.     

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.