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.

Superlubricity behaviors of Nitinol 60 alloy under oil lubrication

The tribological tests were performed using Nitinol 60 alloy pin sliding over GCr15 steel disc in the tribometer system. Four kinds of oils were experimentally investigated as lubrication oils for lubricating Nitinol 60 alloy in the boundary lubrication regime. The experimental results were compared with a reference dry friction. It was found that Nitinol 60 alloy can be lubricated significantly and has shown remarkable lubrication performance. A superlubricity behavior of Nitinol 60 alloy was observed under castor oil lubrication. An ultra-low coefficient of friction of Nitinol 60 alloy about 0.008 between Nitinol 60 alloy and GCr15 steel was obtained under castor oil lubrication condition after a running-in period. Accordingly, the present study is focused on the lubrication behaviors of castor oil as potential lubrication oil for Nitinol 60 alloy. In the presence of castor oil, coefficient of friction is kept at 0.008 at steady state, corresponding to so-called superlubricity regime （when sliding is then approaching pure rolling）. The mechanism of superlubricity is attributed to the triboformed OH-terminated surfaces from friction-induced dissociation of castor oil and the boundary lubrication films formed on the contact surface due to high polarity and long chain of castor oil allowing strong interactions with the lubricated surfaces.

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.

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.

Experimental Study on Molecular Arrangement of Nanoscale Lubricant Films——A Review

In order to understand lubrication mechanism at the nanoscale, researchers have used many physical experimental approaches, such as surface force apparatus, atomic force microscopy and ball-on-disk tribometer. The results show that the variation rules of the friction force, film thicknessand viscosity of the lubricant at the nanoscale are different from elastohydrodynamic lubrication （EHL）. It is speculated that these differences are attributed to the special arrangement of the molecules at the nanoscale. However, it is difficult to obtain the molecular orientation and distribution directly from the lubricant molecules in these experiments. In recent years, more and more attention has been paid to use new techniques to overcome the shortcomings of traditional experiments, including various spectral methods. The most representative achievements in the experimental research of molecular arrangement are reviewed in this paper： The change of film structure of a liquid crystal under confinement has been obtained using X-ray method. The molecular orientation change of lubricant films has been observed using absorption spectroscopy. Infrared spectroscopy has been used to measure the anisotropy of molecular orientation in the contact region when the lubricant film thickness is reduced to a few tens of nanometers. In situ Raman spectroscopy has been performed to measure the molecular orientation of the lubricant film semi-quantitatively. These results prove that confinement and shear in the contact region can change the arrangement of lubricant molecules. As a result, the lubrication characteristics are affected. The shortages of these works are also discussed based on practicable results. Further work is needed to separate the information of the solid-liquid interface from the bulk liquid film.

ANALYSIS OF DEPOSITED PRODUCTS AND REANODIZED POROUS LUBRICATING FILM ON A1 AFTER(NH_4)_2MoS_4 SOLUTION TREATMENT

Observation was carried out of the structure of sulphuric,oxalic or phosphoric film on Al after treatment of reanodizing and electrolytic depositing lubricant in (NH_4)_2MoS_4 solution,as well as of the deposited products by means of EMPA,TEM and energy spectro-scopic analysis.The deposited products are mixture of compounds of S and Mo rather than single MoS_2 and most of them dcposited near the surface layer of the film.Some regular long pores without barrier layer occurred in the film,but the regular fine channels without relation to the structural element parameters of original anodized film were found in the thickened barrier layer of phosphoric film.Sulphur may be remained as Mo sulphide in the film during heating under Ar protective environment.

Non-linear Dynamics of Inlet Film Thickness during Unsteady Rolling Process

The inlet film thickness directly affects film and stress distribution of rolling interfaces. Unsteady factors, such as unsteady back tension, may disturb the inlet film thickness. However, the current models of unsteady inlet film thickness lack unsteady disturbance factors and do not take surface topography into consideration. In this paper, based on the hydrodynamic analysis of inlet zone an unsteady rolling film model which concerns the direction of surface topography is built up. Considering the small fluctuation of inlet angle, absolute reduction, reduction ratio, inlet strip thickness and roll radius as the input variables and the fluctuation of inlet film thickness as the output variable, the non-linear relationship between the input and output is discussed. The discussion results show that there is 180° phase difference between the inlet film thickness and the input variables, such as the fluctuant absolute reduction, the fluctuant reduction ratio and non-uniform inlet strip thickness, but there is no phase difference between unsteady roll radius and the output. The inlet angle, the steady roll radius and the direction of surface topography have significant influence on the fluctuant amplitude of unsteady inlet film thickness. This study proposes an analysis method for unsteady inlet film thickness which takes surface topography and new disturbance factors into consideration.

MECHANISM OF BOUNDARY LUBRICATION UNDER POINT CONTACT

The acid number of the mixed solution of 150SN oil and oleic acid characterizes the volume content of oleic acid in the solution, based on which the adsorptive capability of oleic acid is studied on the 45 steel balls and disks. Boundary lubrication tests are carried out on a self designed ball-on-disk machine, The base oil is pure 150SN oil, and oleic acid as additive are added into the lubricant. Disks have surface roughness values （Ra） of 0.8 μn and 0.4 μn. The electrical contact resistance method is used to determine the lubrication status. Hypothesize that the molecular film is monomolecular layer in condensed state and the opposing surfaces are completely separated by molecular film. A boundary lubrication model is established according to experimental results and hypothesizes. The experimental and calculatienal results show that the adsorption of polar molecules on steel surface is the main factor to form the boundary lubrication film. Load and sliding speed contribute little to the friction coefficient of boundary lubrication. The properties of steel surface and additive for the lubricant significantly influence on the characters of boundary lubrication. The smaller the surface roughness value is, the smaller the friction coefficient of the boundary lubrication is.

Lubricant film flow and depletion characteristics at head/disk storage interface

The characteristics of lubricant film at head/disk interface （HDI） are essential to the stability of hard disk drives. In this study, the theoretical models of the lubricant flow and depletion are deduced based on Navier-Stokes （NS） and continuity equations. The air bearing pressure on the surface of the lubrication film is solved by the modified Reynolds equation based on Fukui and Kaneko （FK） model. Then the lubricant film deformations for a plane slider and double-track slider are obtained. The equation of lubricant film thickness is deduced with the consideration of van der Waals force, the air bearing pressure, the surface tension, and the external stresses. The lubricant depletion under heat source is simulated and the effects of different working conditions including initial thickness, flying height and the speed of the disk on lubricant depletion are discussed. The main factors that cause the lubricant flow and depletion are analyzed and the ways to reduce the film thickness deformation are proposed. The simulation results indicate that the shearing stress is the most important factor that causes the thickness deformation and other terms listed in the equation have little influence. The thickness deformation is dependent on the working parameter, and the thermal condition evaporation is the most important factor.

Molecular Dynamics of Ultra-thin Lubricating Films under Confined Shear

The molecular dynamics simulation of ultra-thin films under confined shear was performed to investigate the relation between dynamic properties of ultra-thin films and their microstructure. The solid walls were modelled using an Au crystal and the fluid molecules were modeled using decane. The simulation results indicate that the microstructure of ultra-thin films is a kind of solid-like layering structure. The density and velocity profiles of the fluid molecules are symmetric. The slip and shear thinning behavior was founded and interpreted.A mathematic model was set up according to the results of the simulation and experiments.

Effects of surface nanostructure on boundary lubrication using molecular dynamics

Molecular dynamics simulations are used to study the boundary lubrication behaviors of squalane lubricant between two iron wall structures during shearing at different pressures and temperatures.Boundary lubrication models with a smooth iron wall and a nanostructured iron wall,respectively,are constructed,and the density distribution of the lubricating film and the velocity distribution in the shearing process are analyzed.The mechanical response of the solid wall is output,and the friction coefficient is calculated.A tribological test is performed with a UMT-2 tribometer under sliding conditions to evaluate the reliability of the simulation method.The results show that the surface nanostructure has a significant effect on the film thickness and delamination of the lubricating film but little effect on the velocity distribution of the lubricating film.The nano strip groove helps to reduce the friction coefficient of the boundary lubrication system.

THE LUBRICATING PROPERTIES OF ION-PLATING FILMS OF Pb-Sn ALLOY

Experimental results of the lubricating behaviour of Pb-Sn alloy films formed by ion-plating on brass substrates are given. It is shown that the film microhardness, friction coefficient and wear life per thickness are under the influence of the substrate. The wear failure of film appears to be film adhesion and transferring by the mating surface.

SCUFFING MODEL CONSIDERING INFLUEN OF HYDRODYNAMIC LUBRICATION

This paper attempted to solve the problem of scuffing mechanism on the basis of the-ories in three fields: Boundary lubrication , Hydrodynamic lubrication and Surface contact. forlineasr contacting, a simple solution has been obtained.

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.

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.

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.

Three-dimensional modeling of effect of surface intermetallic phase on surface defects of Al-Fe-Si aluminum foils during twin-roll casting

Scanning electron microscopy and X-ray energy dispersive spectrum analysis show that the clusters of intermetallic AlFeSi particle are distributed on or near the aluminum foil stock surfaces heterogeneously. 3D finite element modeling shows that these clusters of hard particles induce the fracture of the nano-scale lubricant oil film at first and further lead to severe deformation in the nearby aluminum foil substrate along the rolling direction. Consequently, the optical property in this region differs from that in the surroundings, resulting in surface defects.