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.     

Behavior of real roughness features within mixed lubricated non-conformal contacts

A combination of thin film colorimetric interferometry and phase shifting interferometry was used to study the effect of slide-to-roll ratio on the micro-elastohydrodynamic action and asperity-contact mechanism on the real asperity scale. The behavior of the roughness features of different scales in very thin film, real rough surface elastohydrodynamic contacts was observed from chromatic interferograms evaluated by thin film colorimetric interferometry. Obtained film thickness distribution was compared with undeformed ball surface topography measured by phase shifting interferometry. It was confirmed that the presence of deep grooves within lubricated contact can result in lubrication film breakdown under positive slide-to-roll ratio conditions when the rough surface is moving slower than the smooth surface. Negative slide-to-roll ratio conditions are much less critical from this point of view. Moreover, shallow pits formed naturally on rubbing surface as a result of surface finishing process were observed to significantly influence the film thickness formation. They act as lubricant micro-reservoirs and emit the lubricant into the contact under rolling/sliding conditions that enlarges film thickness. Such a behavior also suggests the possible beneficial tribological effect of surface texturing based on shallow micro-cavities under mixed lubrication of non-conformal surfaces.     

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 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.     

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.     

Thin-Film Lubrication of Dented Surfaces

Surface microfeatures introduced to conformal surfaces have been proved in the last decade to provide beneficial tribological performances. They were found to significantly improve load capacity, wear resistance, or friction coefficient in applications that involve mechanical seals, piston rings, thrust bearings, or ultra-high-density magnetic disc drives. Recent studies have suggested that such an approach could be used to improve the lubrication capabilities under thin-film lubrication of highly loaded non-conformal contacts. However, surface micro-features influence the film thickness and pressure distribution within concentrated contacts that could result in surface failures. In this paper, thin-film colorimetric interferometry has been used to study the effects of an artificially produced micro-dent on film thickness distribution within thin-film lubricated contacts. Obtained results have shown that the behavior of dented surfaces significantly depends on the slide-to-roll ratio. An increase in the lubricant film thickness has been observed just upstream of the trailing edge of the micro-dent when the disc is moving slower than the ball with the micro-dent. In the reverse conditions, for a positive slide-to-roll ratio, the presence of the micro-dent within the concentrated contact results in the film thickness reduction located downstream of the leading edge of the micro-dent. This reduction can cause the local film breakdown of very thin films. Nevertheless, it has been observed that highly viscous boundary films can avoid it and rubbing surfaces have been completely separated using the formulated oil even under very thin lubrication conditions.     

Effect of shot peening on rolling contact fatigue and lubricant film thickness within mixed lubricated non-conformal rolling/sliding contacts

The effect of shot peening on rolling contact fatigue (RCF) and lubricant film thickness within non-conformal rolling/sliding contacts operated under mixed lubrication conditions was observed in this study. Rolling contact fatigue tests and film thickness measurements were carried out using specimens with modified surface topography by shot peening process using glass beads having diameter between 0.07 and 0.11 mm. It has been shown that the effect of shot peening on RCF has no positive effect even if shot peened surface of the roller exhibited somewhat higher hardness in contrast to the grounded surface. The reduction of RCF may be caused due to asperities interactions because after shot peening the surface roughness of the roller was increased. Film thickness measurements confirmed that the contact is realized actually only between asperity peaks of shot peened ball and smooth disc.Conversely, no negative effect on RCF was observed when the shot peened surface of the roller was polished. The polish of asperity peaks causes the creation of lands and micro-cavities, which may be employed as lubricant micro-reservoirs. From film thickness measurements it has been observed that lubricant emitted by shallow micro-cavities can provide the local increase in lubrication film thickness, which thereby reduces asperities interactions. Similar results were obtained for start-up conditions where the squeeze lubricant enlarges film thickness and reduces surface interactions.From the obtained results, it can be suggested that properly designed surface topography modification could help to increase the efficiency of lubrication films leading to the enhancement of contact fatigue life of non-conformal mixed lubricated rolling/sliding contacts.     

The effects of contact configuration and coating morphology on the tribological behaviour of tetrahedral amorphous diamond-like carbon (ta-C DLC) coatings under boundary lubrication

ABSTRACT

Tribological studies were carried out with tetrahedral amorphous diamond-like carbon (ta-C DLC) coatings, varying in thickness and roughness, using two different contact configurations lubricated with seven types of hydraulic oils. Tribopair of cast iron and ta-C coated steel were tested in both non-conformal and conformal, unidirectional sliding contacts. The friction and wear results were mainly affected by the thickness of the coating in the non-conformal contact and the surface roughness of the coating in the conformal contact. Tests done with mineral base oil containing rust inhibitor in the non-conformal contact and with Polyalphaolefins and synthetic ester base oils in the conformal contact resulted in the lowest friction while that with mineral base oil containing zinc resulted in high friction and counterface wear. The results highlight the interdependence of contact configuration, lubricant chemistry, coating’s surface morphology and coating’s thickness in determining the tribological behaviour of ta-C coatings under boundary lubrication.     

Influence of surface roughness features on mixed‐film lubrication

An optical technique (three‐dimensional spacer layer imaging) has been developed to map accurately lubricant film thickness in thin‐film elastohydrodynamic (EHD) contacts. This experimental technique has been used to study the influence of surface roughness features, asperity height, and slope on EHD film thickness and pressure. Single ridges transverse to the entrainment direction were used to represent asperities. It was found that the ridges with lower slopes generate films of greater minimum thickness. Below a certain entrainment speed, the minimum film thickness declined at a rate dependent on the ridge slope. At low speeds, the ridges with higher slopes entrapped a larger volume of lubricant ahead of the ridge and along the entrainment direction. For all speeds, the highest ridges entrapped the most lubricant. Both ridge slope and ridge height had a negligible effect on mean film thickness in the contact. Asperity pressure increased with higher ridge slope, but was not influenced by entrainment speed. An increase in pressure was found where lubricant is entrapped upstream of a ridge.     

Twin disc micropitting tests

Gears are important power transmission components and their efficiency and durability strongly depend on their tribological performance. The demand for higher efficiency has further increased the requirements for controlled tribological performance of gear contacts. One of the typical tribological failures that gears experience is micropitting, which is a fatigue failure of the surface strongly associated with low lubricant film thickness. Micropitting is claimed to be the dominant life limitation for ground hardened gears operating under modest conditions. In this work the factors affecting micropitting performance of gears were studied. Twin disc experiments were carried out to investigate the effect of surface roughness, lubricant type and surface treatment on micropitting performance of two case hardening steels. The test results showed that the roughness of the gear has a major impact on the micropitting performance. Also the use of DLC coating or carbonitriding surface treatment and the use of ester type lubricant had a beneficial effect compared to untreated or mineral oil lubricated surfaces.     

Tribological study on the surface modification of metal-on-polymer bioimplants

The tribological performance of artificial joints is regarded as the main factor of the lifespan of implanted prostheses. The relationship between surface roughness and coefficient of friction (COF) under dry and lubricated conditions is studied. Results show that under dry test, friction coefficient is not reduced all the time with a decrease in surface roughness. On the contrary, a threshold of roughness value is observed, and frictional force increases again below this value. This critical value lies between 40 and 100 nm in S_a (roughness). This phenomenon is due to the transfer of friction mechanisms from abrasion to adhesion. Under wet test, COF always decreases with reduction in surface roughness. This result is mainly attributed to the existence of a thin layer of lubricant film that prevents the intimate contact of two articulating surfaces, thus greatly alleviating adhesion friction. Furthermore, surface texturing technology is successful in improving the corresponding tribological performance by decreasing friction force and mitigating surface deterioration. The even-distribution mode of texturing patterns is most suitable for artificial joints. By obtaining the optimal surface roughness and applying texturing technology, the tribological performance of polymer-based bioimplants can be greatly enhanced.     

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.     

Some aspects of the design of high speed roller bearings

High speed lightly loaded cylindrical roller bearings are prone to slip and lubricant starvation at the inner race - roller contacts. Both factors affect the oil film thickness which, in turn, influence the bearing life. Existing oil film thickness - life factors do no adequately consider these parameters It can be shown that the most important single factor is the cage speed; almost any variables that modify the conditions in the bearing do so by adjusting the speed of the cage itself. Curves are presented illustrating the influence of the bearing and chamber design and lubricant properties on the starved oil film thickness at the bearing inner contacts. It is shown that these values can be used to obtain improved life estimates     

Thermal point contact EHL analysis of rolling/sliding contacts with experimental comparison showing anomalous film shapes

The paper presents an experimental and numerical investigation of non-conformal lubricated contacts in which anomalous film shapes occur. The experiments were concerned with the contact between a steel ball and the plane surface of a glass disc at various slide-roll ratios. A paraffin base mineral oil was used as a lubricant and friction coefficients and film thicknesses were measured. It was found that for slide-roll ratios with the disk moving faster anomalous elastohydrodynamic lubrication (EHL) films were obtained characterized by a “dimple” in the central region of the contact. Numerical thermal-elastohydrodynamic analyses were carried out to simulate both film thickness and friction corresponding to the experimental conditions using Newtonian and Ree-Eyring rheological models. Initial results from this study suggest that neither of these lubricant models predict the correct detailed film shape and the experimental friction at the same time. An alternative lubricant model including both thermal and limiting shear stress effects (wall slippage) is currently under development.     

Fractionation of liquid lubricants at solid surfaces

The film-forming properties of lubricant base fluid mixtures in elastohydrodynamic contacts have been studied using ultrathin film interferometry. It has been shown that in binary mixtures where one of the components is more polar than the other, the EHD film thickness formed in the very thin film (< 10 nm region) is controlled by the viscosity of the polar component rather than the viscosity of the blend. This means that a mixture of a highly viscous ester in less viscous hydrocarbon gives thicker than predicted lubricant films in the sub 20 nm region and vice versa. This phenomenon can be ascribed to the fractionation of the lubricant mixtures close to the surface caused by lubricant molecule/surface van der Waals forces.     

The influence of the slide-to-roll ratio on the friction coefficient and film thickness of EHD point contacts under steady state and transient conditions

The effect of different slide-to-roll ratios has been experimentally investigated under steady state and transient conditions using a steel ball in contact with the plane surface of a glass disc. Under transient conditions the entraining velocity has been varied with a sinusoidal law at two different frequencies. Measurements of the friction force and film thickness using optical interferometry have been made.For the same working conditions, different friction coefficient trends found for positive and negative slide-to-roll ratios can be related to different film thickness values and shapes. The combination of different thermal effects could be a possible explanation for the obtained results.     

Porosity Effect of Sintered Steel on the Frictional Performance of Conformal and Nonconformal Lubricated Contacts

Abstract

The effect of surface topography on lubricated systems plays a crucial role in terms of friction performance, because surface micro-irregularities can improve the load-carrying capacity of mechanical parts in lubricated conformal and nonconformal contacts. Sintered materials, which can be applied to manufacturing several mechanical components such as gears, axial thrust bearings, and disc brake pads, are interesting candidates, because they present pores that could be somewhat compared to microcavities produced by surface texturing techniques. This work aims at studying the influence of surface pores originated from the sintering process on the frictional performance of lubricated contacts under different lubrication regimes and slide-to-roll ratios (SRR). The research contributes to understanding how random micro-irregularities could change lubrication conditions and promote effects similar to those of more expensive and precise surface features produced by texturing techniques. The experimental results showed that a decrease in porosity led to a reduction in the coefficient of friction. Furthermore, less porous samples promoted friction reduction compared to nonporous materials due to the probable additional load support caused by small-scale surface pores. Therefore, in addition to the traditional appeal of the use of sintered materials to reduce production costs, the present contribution reveals that this type of material could also be used to reduce friction in contacting mechanical components operating under certain tribological conditions.     

Effect of surface topography on mixed lubrication film formation during start up under rolling/sliding conditions

The start up operation of mixed lubricated contacts represents one of the transient conditions that bring the risk of the surface damage because of asperities interactions. This paper focuses on the effects of both artificially produced and real roughness features on mixed lubrication film formation during start up of non-conformal contacts operated under rolling/sliding conditions. Chromatic interferograms captured during start up enabled the detailed changes in lubrication film caused by surface features to be observed. The observation of the effects of surface dents artificially produced on the ball surface helped to better understand the behaviour of real surface topography. It was found that the presence of shallow surface features can help to separate mixed lubricated rubbing surfaces more efficiently than it could be suggested from the results obtained with smooth surfaces. Lubricant emitted from surface features appears to spread more easily within the mixed lubricated contact because of local decrease of pressure in the vicinity of real surface features. Conversely, high pressure within smooth contacts restricts the migration of lubricant emitted from surface pits much more. It can be suggested from the results obtained that the properly designed topography of the rubbing surfaces can help to reduce the asperities interactions under transient operational conditions.     

Influence of surface texture shape,geometry and orientation on hydrodynamic lubrication performance of plane‐to‐plane slider surfaces

This paper investigates the effect of surface texturing on the tribological performance of hydrodynamic slider surfaces. The geometric parameters of the texture considered were the texture shape, bottom profile, orientation and depth. A theoretical model based on the steady‐state Reynolds equation was solved numerically by finite difference method to analyse the effect of surface texturing. The optimum geometry of each texture shape was determined, for which, the film thickness was maximised and the friction coefficient was minimised. Among all the shapes, the square with a single wedge bottom profile outperformed. The performance of the triangle, the chevron and the ellipse was highly influenced by their orientation with respect to the sliding direction. Texture density was found to be more dominant in impacting the tribological performance than the texture aspect ratio. The results obtained were further discussed with respect to the studies available in literature to emphasise the significance of the findings. Copyright © 2016 John Wiley & Sons, Ltd.