Behavior of thin viscous boundary films in lubricated contacts between micro-textured surfaces

Thin film colorimetric interferometry was used to map changes in film thickness in the vicinity of micro-dents of various depths produced on rubbing surfaces. It has been shown in recent studies that shallow micro-features within concentrated contact can increase mean film thickness by supplying more lubricant to the contact; however, this beneficial effect can also be accompanied by a local film thickness reduction. Nevertheless, these observations were done with mineral base oils that exhibited no boundary films formation. In this study the behavior of micro-textured surfaces are observed using formulated lubricant containing polyalcylmethacrylate (PAMA), viscosity index improver with boundary film forming properties. Obtained results show that an enlarged film thickness due to the presence of viscous boundary films is formed within the whole contact and these boundary films minimize the local film thickness reduction caused by micro-dents and further increase the efficiency of surface texturing within non-conformal contacts. It can be suggested from the obtained results that joint action of both boundary film formation and surface texturing combines both contributions that can help to increase tribological performances in different stages of machine parts operation by increasing lubrication film thickness.     

The effect of surface texturing in soft elasto-hydrodynamic lubrication

A theoretical model is developed to study the potential use of laser surface texturing (LST) in the form of spherical micro-dimples for soft elasto-hydrodynamic lubrication (SEHL). The model consists of mutual smooth elastomeric and LST rigid surfaces moving relatively to each other in the presence of viscous lubricant. The pressure distribution in the fluid film and the elastic deformations of the elastomer are obtained from a simultaneous solution of the Reynolds equation and the equation of elasticity for the elastomer. An extensive parametric investigation is performed to identify the main important parameters of the problem, which are the aspect ratio and area density of the dimples. The parametric analysis provides optimum parameters of the surface texturing and shows that LST effectively increases load capacity and reduces friction in SEHL.     

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.     

Thin lubricating films behaviour at very high contact pressure

Thin film colorimetric interferometry has been used to examine the behaviour of thin elastohydrodynamic (EHD) lubricant films under very high contact pressures of the order of 0.5–3 GPa. It has been shown that at moderate pressures, the variation of film thickness with speed follows the Hamrock and Dowson prediction down to one nanometer. As the load is increased, however, thin films behave differently from the prediction of the conventional EHD theory. For a certain lubricant and operational conditions, there is a critical rolling speed below which a reduction of film thickness is observed. This behaviour is very similar to that previously predicted computationally by Zhu.     

Effect of surface texturing on mixed lubricated non-conformal contacts

The behaviour of surface texturing based on shallow micro-dents was observed within mixed lubricated non-conformal contacts and compared with results obtained under thin film elastohydrodynamic conditions. Thin film colorimetric interferometry was used to observe the changes in lubrication film thickness. It was found that lubricant emitted by micro-dents could effectively lift off the real roughness features that provided an increase in average but also the local minimum film thicknesses. On the contrary to smooth contact conditions no film thickness reduction is obvious either downstream or upstream the micro-dent. The possible beneficial effect of surface texturing on mixed lubricated contact was checked through the qualitative wear test. It confirmed that an array of shallow micro-dents reduced asperity interactions of rubbing surfaces. Moreover, the effect of micro-dents on rolling contact fatigue was also considered in this study. It has been shown that individual dents would have to be much deeper compared to those used in surface texturing experiments to cause reduction in contact fatigue life. It can be suggested from the obtained results that properly designed surface texturing could help to increase the separation of rubbing surfaces under mixed lubrication conditions.     

Effect of real longitudinal surface roughness on lubrication film formation within line elastohydrodynamic contact

In many applications (e.g. roller, barrel or needle bearings) surface features exhibit longitudinal alignment to the direction of motion. These features are produced by surface finishing techniques in the circumferential direction and are associated with line or very wide elliptical contact geometries. In such a case, the contact length in the direction of motion is considerably shorter comparing its width and the effect of a longitudinal roughness could significantly influence the lubrication film formation. Recent experimental studies have indicated less severe effect of a longitudinal roughness on lubrication film formation in the comparisons with that observed with transversely orientated roughness caused by the inlet perturbation. Nevertheless, these experimental studies have been focused on the behaviour of artificially produced asperities within a circular contact. The quantitative experimental study of longitudinal real surface roughness within a line contact has not been realized yet. That is why, in this study, the line contact formed between a steel tapered roller and glass disc is observed within an optical test rig and the effects of real surface roughness on lubrication film formation are studied. Experiments carried out under pure rolling conditions have shown that the depth is the key parameter that influences the effect on the film thickness. If the roughness features are shallow, the lubrication film shape within the contact follows the shape of the surface closely. However, the groove having only about 800 nm in depth divided the line contact into two parts that behave as two separate line contacts. Such an effect can increase the risk of the wear of rubbing surfaces as the lubrication film thickness between the real machine components can be significantly lower than expected.     

Effect of surface texturing on the elastohydrodynamic lubrication analysis of metal-on-metal hip implants

With the development of surface processing techniques, applications of bearing components have been introduced with specific surface textures to take advantage of lubrication.An advanced numerical model was established to simulate the mixed EHL problem of a metal-on-metal hip prosthesis with dimpled surface texturing. The surface texture with simple cylindrical dimples was numerically simulated, under both steady state and walking conditions.The present results showed that surface texturing may have a potentially beneficial effect on the reduction of asperity contact ratio and the improvement of lubrication performance of metal-on-metal hip replacements, particularly under predominant boundary lubrication conditions.     

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.     

Study of surface roughness effects in elastohydrodynamic lubrication of rolling line contacts using a deterministic model

A sinusoidal surface roughness model is adopted for the analysis of the effects of roughness amplitude and wavelength on pressure profile, film shape, minimum film thickness and coefficient of friction in a steady state EHL line contact. The influence coefficients used for the evaluation of surface displacements are calculated by utilizing a numerical method based on Fast Fourier Transform. Significant reduction is observed in the minimum film thickness due to surface roughness. Such reduction is quantified by roughness correction factor, C_R, and a relationship between C_R and non-dimensional surface roughness amplitude A is derived as: C_R=1−0.7823A^0.8213. This equation may prove to be of interest from designer's viewpoint. The friction coefficient is found to increase appreciably with increasing amplitude and decreasing wavelength of surface roughness.     

A measurement system for thin elastohydrodynamic lubrication films

An elastohydrodynamic lubrication (EHL) film measurement system using multi-beam interferometry is introduced in this paper. The measurement principle and the instrumentation are discussed. A simple and efficient method is suggested to obtain the fringe order of measured points. It is demonstrated that the presented measurement system can provide continuous measurement of lubricating films from nano to micro scales at a nano-level resolution, and can be used to investigate ultra-thin EHL films and tiny variations in EHL films. Translated from Tribology, 2006, 26(2): 150–153 [译自: 摩擦学学报]     

The effect of tapered edges on lubrication regimes in surface-textured elastomer seals

The paper focuses on modeling a hydraulic elastomer seal cross-section for improving the effect of surface texturing in Soft Elasto Hydrodynamic Lubrication. The model consists of textured elastomer seal having a cross section combined of composite tapered and flat land, sliding relative to a rigid smooth counterpart in the presence of viscous lubricant. The hydrodynamic lubrication and the elasticity problems are solved simultaneously. A parametric analysis is performed to obtain the optimum surface texturing parameters and the improved cross section. Ranges of different possible lubrication regimes are also discussed.     

Effect of surface texturing on rolling contact fatigue within mixed lubricated non-conformal rolling/sliding contacts

The rolling contact fatigue (RCF) life of highly loaded machine components is significantly influenced by the surface roughness features so that there is a continuous effort to design the topography of rubbing surfaces to enhance lubrication efficiency and prolong the operation of machine components. It can be suggested from the recent experimental results that lubricant emitted from shallow micro-dents could effectively lift off the real roughness features and reduce the asperities interactions within rolling/sliding mixed lubricated contacts. Thereby the additional supply of lubricant from surface features could help to reduce the risk of surface damage through the reduction of the interaction of rubbing surfaces during start-up or starvation. However, the introduction of such roughness features into the rubbing surfaces of highly loaded non-conformal contacts should consider not only the effects on lubrication film thickness but also on RCF.That is why this study is focused on the effects of surface texturing on RCF within non-conformal rolling/sliding contacts operated under mixed lubrication conditions. The principal task has been whether possible beneficial effect on film thickness is not accompanied by the reduction in RCF life. Textures with various sizes of micro-dents and their arrangement within the contacts have been considered. It has been found that results obtained with textured surfaces have exhibited no obvious reduction in RCF. Conversely, some increase in RCF using textured surfaces was observed that could be attributed to the positive contribution of micro-dents working as lubricant micro-reservoirs that reduce asperities interactions. Nevertheless, further experiments are necessary to confirm this possible beneficial contribution of surface texturing on RCF.     

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).     

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.     

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.     

The influence of thin boundary films on real surface roughness in thin film, mixed EHD contact

The behaviour of thin viscous boundary films in the rough surface rolling–sliding point contact operated under thin film lubrication conditions have been observed by thin film colorimetric interferometry. Changes in film thickness distribution within the lubricated contact between steel ball and glass disc were studies with both mineral base oil and mineral oil formulated with non-functionalized polyalkylmethacrylate (PAMA). Recent studies by other researchers showed that this polymer-containing viscosity index improver exhibits some friction-reducing capabilities even though it forms only very thin boundary films on rubbing surfaces. Results obtained in the current study proved that thin viscous boundary films formed on rubbing surfaces can reduce asperities interactions of rubbing surfaces under very thin film conditions. Even though these boundary films do not separate rubbing surfaces completely, they still can provide some protection of contacting bodies against excessive friction and wear.     

Velocity analysis for layered viscosity model under thin film lubrication

The lubrication of bearings is directly affected by the flow properties of the liquid lubricant. One key feature of lubricant is its viscosity distribution in the bearing. The distribution is dependent on the adsorbent properties of the surface and the velocity distribution of the lubricant. This paper is only connected with the velocity analysis on the cross-gap flow. Because of increasing the speed, apart from increasing the temperature, the other features of lubricant will be changed. When fluid film is thin, the speed effects become important due to the increased shear rate. This paper describes the fluid velocity profiles produced for the case where layers of greater viscosity than the bulk occur on the bearing surfaces.     

A non-Newtonian model based on Ree-Eyring theory and surface effect to predict friction in elastohydrodynamic lubrication

The traction behaviour of various lubricants is investigated in elastohydrodynamic regime using a tribometre that simultaneously allows the contact visualization and the friction measurement under controlled contact kinematics. First, the film formation capability is determined: although the base oil and the polymer, via their viscosity, govern the film thickness in EHL regime, the additives control the existence of a boundary film at low entrainment speed. Second, the relative role of the additives and of the base oil on the frictional behaviour is studied for various experimental conditions: this clearly demonstrates that the piezo-viscous response of the lubricant controls the Newtonian/non-Newtonian transition and the friction level. The role of thermal effects is discussed. A strong influence of the entrainment speed, over the frictional response is unexpectedly observed, even in full film regime. A rheological model based on Ree-Eyring theory supposing a heterogeneous flow due to adsorbed surface layers, is proposed to discuss the organization of the molecules within the contact and its effect on the frictional response of the contact.     

A Generalized Differential Colorimetric Interferometry Method: Extension to the Film Thickness Measurement of Any Point Contact Geometry

Whereas industrial elastohydrodynamic (EHD) contacts are generally noncircular, most experimental observations are made on sphere-on-plane conjunctions. The circular case is indeed a specific elliptical case, and it was widely investigated. The differential colorimetric interferometry (DCI) technique was often used to perform precise film thickness measurements in circular EHD contacts. From a single picture of the dynamic contact, it enables mapping the film thickness of the full conjunction, and postprocessing can be applied afterwards. Moreover, it is possible to record sequences at relatively high-frequency sampling. However, until now, the method could not be directly applied to noncircular conjunctions. In the present article, a generalized DCI method is proposed and assessed by several static and EHD validation cases for elliptical and torus-on-plane contact geometries. This new method no longer necessitates particular requirements on the contact shape while retaining the advantages of the original DCI method. It allows precise film thickness measurements in realistic industrial EHD contacts and opens the way for new experimental observations.     

A computer model of mixed lubrication in point contacts

A numerical model of mixed lubrication is presented in this paper. The idea introduced here is that asperity contact may be viewed as a result of a continuous decrease in film thickness, so that the transition between contact and non-contact is continuous and the same mathematical model should work for both regions. The pressure over the thin films is assumed to obey the Reynolds equation, and the solution of the equation, under the condition of h→0, is expected to be the same as that predicted by the theory of elasticity. To achieve convergent and stable solutions, the left-hand side terms of the Reynolds equation are switched off when the local film thickness approaches zero, leading to a reduced Reynolds equation. Pressure distributions over the entire computation domain are thus obtained through solving a unified equation system without identifying hydrodynamic or asperity contact regions. Computations were conducted for several example cases and results show that convergent solutions are achievable on different types of roughness, over a wide range of λ ratios (0.01 to infinity), and for different slide-to-roll ratios (0.0–2.0).