界面滑移条件下点接触Stribeck曲线的实验研究

采用高黏度聚异丁烯润滑油,在光学弹流实验机上考察球-盘接触纯滑条件下的摩擦因数随卷吸速度和载荷的变化。结果表明,随着卷吸速度的增加,球-盘接触副进入弹流润滑并向流体动压润滑转变的过程中,摩擦因数并不像传统的Stribeck曲线一样,随着卷吸速度的增加而单调增加,而是呈现先上升、后下降、再上升的趋势,其中摩擦因数下降时的起始速度大致为凹陷出现的速度,摩擦因数再上升时的速度大致是润滑进入动压润滑的速度。初步论证界面滑移为产生上述波动的主要原因。     

Lubrication properties of non-adsorbing polymer solutions in soft elastohydrodynamic (EHD) contacts

The in-use performance and processing of many consumer products in the food, home and personal care industries are dependent on their tribological properties. A major component of these products is often a high molecular weight polymer, which is typically used to thicken aqueous systems. Polymer solutions tend to be non-Newtonian, and in particular their viscosity varies with shear rate, such that it is difficult to predict their friction or hydrodynamic film-forming behaviour. The present work relates the tribology of aqueous polymer solutions to their rheological properties in thin films in ‘soft’ contacts at high shear rates. The friction properties of three types of polymers in aqueous solution, polyethylene oxide, PEO; xanthan gum, XG; and guar gum, GG, have been studied as a function of polymer concentration over a wide range of entrainment speeds in a point contact formed between silicone rubber and steel. This has enabled the boundary lubrication and isoviscous-elastic lubrication properties of the solutions to be investigated using both hydrophilic and hydrophobic silicone surfaces.It is found that the friction vs. entrainment speed dependence follows the shape of a classical Stribeck curve. In general, a lower friction is observed with increasing polymer concentration in the mixed-regime. Using scaling factors for the entrainment speed, we have shown that this decrease in friction is likely to be due to viscous effects and that the scaling factors represent effective high shear rate viscosities. In the case of PEO and XG, and GG at low concentrations, a good correlation is found between this effective viscosity and the apparent viscosity measured at the highest shear rates attainable with the available rheometer. However, for GG at concentrations above 0.2%, the effective viscosity decreases with increasing polymer content.The three polymers do not significantly reduce friction in the boundary regime and in general give essentially the same response as water when an effective viscosity is taken into account. However, a slight increase in friction in comparison to pure water has been observed for XG and GG on hydrophobic surfaces. It is suspected that this may be due to a blocking of fluid entrainment, or possibly exclusion of polymer from the contact, due to the large hydrodynamic volume and rigid nature of the two biopolymers. Finally, for PEO solutions with full-film elastohydrodynamic conditions were reached, the measured friction coefficient of the film correlated quite well with the value calculated from the effective viscosity.     

润滑点接触粗糙表面滑动摩擦因数的实验研究

利用多功能微摩擦试验机,在轻载荷条件下测量了具有横向纹理圆盘表面的摩擦因数,得到了包括流体润滑、混合润滑和边界润滑完整的Stribeck曲线。考虑了载荷和时间对实验结果的影响,对不同粗糙度表面的Stribeck曲线进行了比较。实验的初步结果表明:增加载荷与实验时间,会获得更加稳定的实验结果;表面越粗糙,从混合润滑向流体润滑转换的临界速度越大;表面越光滑,Stribeck曲线的“谷底”越明显。     

Nanocomposite UHMWPE–CNT Polymer Coatings for Boundary Lubrication on Aluminium Substrates

The boundary lubrication regime plays a very important role in determining the life span of any of the two mating parts under liquid-lubricated conditions. It is during the start\stop cycles when insufficient fluid is available to fully separate the surfaces in relative motion and thus unusual wear takes place; a case of boundary lubrication. The aim of this work is to study the feasibility of using polymer coatings as boundary lubricants. This study investigates the friction and wear properties of ultra-high molecular weight polyethylene (UHMWPE) films coated on aluminium substrates under dry and base oil (without any additives)-lubricated conditions. In order to increase the load bearing capacity of the UHMWPE coatings, 0.1 wt% of single-walled carbon nanotubes are added. Experiments are carried out on a custom-built tribometer simulating a line contact between a polymer-coated cylindrical Al surface (shaft) and a flat uncoated Al plate as the counterface. The experimental parameters such as the normal load and the sliding speed are selected to simulate the boundary and mixed lubrication regimes for comparison purposes. Specific wear rates of the polymer films and bare Al surface under lubricated conditions are also calculated. Stribeck curves have been generated to evaluate the effectiveness of the pristine UHMWPE and the nanocomposite coatings in the various regimes of lubrication, especially the boundary lubrication regime. It is observed that the selected polymer coatings are effective in protecting the metallic surfaces without causing any observable oil contamination with wear debris.     

The Frictional Properties of Newtonian Fluids in Rolling–Sliding soft-EHL Contact

A combined experimental and numerical study has been carried out to explore friction in rolling–sliding, soft-EHL contact. Experimental work has employed corn syrup solutions of different concentrations in water to provide a range of lubricant viscosities and has measured Couette friction in mixed rolling–sliding conditions over a wide range of entrainment speeds. A Stribeck curve has been generated, ranging from the boundary to full film, isoviscous-elastic lubrication regime. In the latter regime, friction coefficient is approximately proportional to the product of (entrainment speed × viscosity) raised to the power 0.55. Numerical solution of the isoviscous-elastic lubrication regime has been used to derive predictive equations for both Couette and Poiseuille friction in circular, soft-EHL contacts. This shows that in soft-EHL the Poiseuille or “rolling” friction can have magnitude comparable to the Couette friction. The calculated Poiseuille friction coefficient can be predicted from non-dimensional load and speed using a simple power law expression similar to that used for film thickness. However accurate prediction of calculated Couette friction coefficient requires a two-term power law expression. Comparison of experimental and numerical Couette friction coefficients shows quite good agreement between the two, with a similar non-dimensional speed dependence, but slightly lower predicted than measured values.     

Effects of Ethanol Contamination on Friction and Elastohydrodynamic Film Thickness of Engine Oils

The use of ethanol as engine fuel has increased for environmental reasons, both in flex-fuel engines and as increasing amounts of ethanol blended with gasoline in conventional engines. This article describes an investigation into the effects of ethanol contamination of lubricants during engine use with ethanol fuel. To facilitate this, a new technique was developed to measure small amounts of ethanol in lubricants. Elastohydrodynamic film thickness measurements and Stribeck curves were obtained for Group I base and formulated oils containing small added amounts of ethanol. The effect of the water present in hydrated ethanol was evaluated by carrying out tests using both hydrated and anhydrous ethanol. Measurements were also carried out using a Group II base oil with added ethanol. These measurements showed that in the low entrainment speed region, where the elastohydrodynamic film is very thin so that boundary lubrication prevails, the addition of ethanol produced a boundary film, which was not present for the base oils. By contrast, the addition of ethanol to formulated oil reduced film thickness in all lubrication regimes. The friction tests showed friction reduction due to addition of ethanol to the base oil, in particular at low speeds. For the formulated oil, ethanol reduced friction at high speeds, which was associated with a reduction in the viscosity of the lubricant, but at low speeds, ethanol reduced the formation of a boundary layer, increasing friction. The presence of water in hydrated ethanol did not significantly change the film thickness and friction when compared with anhydrous ethanol.     

Thin-Film Rheology and Tribology of Magnetorheological Fluids in Isoviscous-EHL Contacts

This article is concerned with an investigation of the tribological performance of magnetorheological (MR) fluids in pure sliding soft-elastohydrodynamic lubrication (EHL) steel/polytetrafluoroethylene (PTFE) point contacts. The lubricating properties of MR fluids were measured in thin film, lubricated conditions using a ball-on-three-plates tribometer and compared to base fluids in the form of Stribeck curves. A range of techniques was employed to interpret the possible mechanisms of friction and wear of dispersed iron microparticles. The friction surfaces were investigated using optical microscopy, environmental scanning electron microscopy (ESEM), X-ray photoelectron spectroscopy, and energy-dispersive spectroscopy (EDS). In all cases investigated, the friction coefficient is found to strongly depend on the viscosity of the lubricant oil. In the case of low-viscosity liquids, iron microparticles are entrapped in the contact and plow the PTFE surfaces resulting in a sensibly constant friction coefficient. For intermediate viscosities, friction decreases at low speeds because of the so-called ??ball-bearing?? effect, and later, friction increases as particles become embedded in the PTFE matrix. Finally, for high-viscosity fluids, iron particles either accumulate around the rubbing zone as a barrier that reduces the supply of oil available to the contact for boundary lubrication or the particles indent PTFE surfaces.     

The Effect of Morphology on the Tribological Properties of MoS<Subscript>2</Subscript> in Liquid Paraffin

The tribological properties of liquid paraffin (LP) containing molybdenum disulfide (MoS₂) additives, including nano-balls, nano-slices, and bulk 2H-MoS₂, are evaluated using a four-ball tribometer. Results show that all MoS₂ additives used can improve the tribological properties of LP, and that nanosized MoS₂ particles function as lubrication additives in LP better than micro-MoS₂ particles do. The LP with nano-balls presents the best antifriction and antiwear properties at the MoS₂ content of 1.5 wt%. This is ascribed to the chemical stability of the layer-closed spherical structure of nano-balls. The Stribeck curves confirm that the rotation speed of 1,450 rpm used is located at the mixed lubrication region under 300 N. MoS₂ nano-slices have small sizes and easily enter into the interface of the friction pair with a roughness of 0.032 μm, functioning as a lubricant in LP better than nano-balls do at the MoS₂ content of 1.0 wt%. The Stribeck curves also show that the differences between the two nano samples were magnified at high rotation speeds in hydrodynamic lubrication region. The application of nano-slices in high sliding speeds will be more advantageous. This work furthers the understanding of the relationship between the tribological properties and morphology of MoS₂.     

Water Lubrication of Silicon Nitride in Sliding

Several studies have shown that the coefficient of friction of self-mated silicon nitride in water decreases from an initially high value to about 0.002 after a certain run-in period. Since the worn surfaces become extremely smooth, the low friction is attributed to the initiation of hydrodynamic lubrication by a thin water film at the interface. The possibility of mixed lubrication, i.e., hydrodynamic lubrication by water and boundary lubrication due to the presence of colloidal silica on the wearing surfaces, has also been proposed. The purpose of our study is to investigate the influence of load, speed, and surface roughness on the duration of the run-in period. The results confirmed that a low coefficient of friction is obtained following a run-in period when a wear scar of sufficient size is developed to reduce the contact stress. The run-in period, during which the coefficient of friction is fairly high, was shorter for smoother surfaces and at higher loads and speeds. The low friction behavior was found to be unstable and occasional high friction spikes were observed. The surfaces of the wear tracks and wear scars contained a series of striations parallel to the sliding direction and exhibiting plastic deformation, delamination and fracture. The striations that appeared to be associated with the high friction spikes, could form as a result surface film breakdown. Although these results are consistent with the proposed mechanisms of hydrodynamic lubrication or mixed lubrication, it is proposed that the low friction behavior may be also related to fundamental interactions between two hard and elastically deforming surfaces covered with hydrogen-terminated oxide films.     

The comparisons of sliding speed and normal load effect on friction coefficients of self-mated Si3N4 and SiC under water lubrication

The effects of sliding speed and normal load on friction coefficients of self-mated Si₃N₄ and SiC sliding in water after running-in in water were investigated with pin-on-disk apparatus at sliding speeds of 30 to 120 mm/s, normal loads of 1 to 14 N in ambient condition. The results showed that, after running-in in water, for two kinds of self-mated ceramics, friction coefficient increases with both decreasing sliding speed and increasing normal load when normal load is larger than a critical normal load. Friction coefficient was independent of normal load when normal load is smaller than the critical load. The lubrication film of Si₃N₄ under water lubrication exhibited larger load carrying capacity than that of SiC did. Stribeck curves indicated that, for self-mated Si₃N₄ ceramics, hydrodynamic lubrication will change into boundary lubrication abruptly when the sommerfeld number is less than a critical value; while for self-mated SiC ceramics, hydrodynamic lubrication will change into mixed lubrication and then into boundary lubrication gradually when the sommerfeld number is below critical value.     

表面织构对水润滑轴承混合润滑性能的影响

为分析表面织构对水润滑轴承混合润滑性能的影响,基于平均Reynolds方程及JFO空化边界条件建立带有表面织构的水润滑轴承混合润滑模型并数值求解,获得不同织构参数下水润滑轴承的Stribeck曲线。研究结果表明:表面织构是否能改善润滑性能与其深径比及面密度参数密切相关,织构的引入并不一定能降低水润滑轴承的摩擦因数;表面织构的面密度和深径比存在最优值,能使水润滑轴承获得最大的膜厚比与最小的摩擦因数,并在较低的转速下由混合润滑状态进入流体动压润滑状态。     

Calculation of Stribeck curves for line contacts

A mixed lubrication model is presented by which Stribeck curves can be calculated. By means of the Stribeck curve the transitions from boundary lubrication to mixed lubrication and the transitions from mixed lubrication to elasto-hydrodynamic lubrication can be predicted and subsequently the lubrication regime in which a particular contact operates can be predicted. Calculations are restricted to line contacts, because of the wide range of applications of line contacts or of very wide elliptical contacts.     

Si3N4 as a biomaterial and its tribo‐characterization under water lubrication

Hip implant wear is recognised as the main cause of hip implant failure therefore has been widely investigated both experimentally and clinically, demonstrating the coexistence of abrasive, adhesive, fatigue and corrosive wear. Many clinical in vivo and bulk material wear rate data from published literature have been presented for non‐oxide ceramic implants. Several studies have shown that the coefficient of friction of self‐mated silicon nitride in water decreases from an initially high value to about 0.002 after a certain run‐in period. Since the worn surfaces become extremely smooth, the low friction is attributed to the initiation of hydrodynamic lubrication by a thin water film at the interface. The possibility of mixed lubrication, i.e. hydrodynamic lubrication by water and boundary lubrication due to the presence of colloidal silica on the wearing surfaces, has also been proposed. Influence of load, speed and surface roughness on the duration of the run‐in period of silicon nitride under water lubrication was investigated in this study. The results confirmed that a low coefficient of friction is obtained following a run‐in period when a wear scar of sufficient size is developed to reduce the contact stress. The run‐in period, during which the coefficient of friction is fairly high, is shorter for smoother surfaces and at higher loads and speeds. The striations that appeared to be associated with the high‐friction spikes can be formed as a result of surface film breakdown. Although the results are consistent with the proposed mechanisms of hydrodynamic lubrication or mixed lubrication, it is proposed that the low‐friction behaviour may also be related to fundamental interactions between two hard and elastically deforming surfaces covered with hydrogen‐terminated oxide films. Copyright © 2016 John Wiley & Sons, Ltd.     

Tribological behaviour of MoS2 and inorganic fullerene-like WS2 nanoparticles under boundary and mixed lubrication regimes

Abstract

For several years different types of nanoparticles have been considered and studied as potential friction modifying lubricant additives. Some nanoparticles can reduce the friction coefficient by 30–70%, depending on the base oil and the experimental conditions. In the present study, an experimental analysis on tribological properties of inorganic fullerene-like metal dichalcogenides was performed in comparison with MoS₂ 2H layered structures. Tribological tests were carried out on a pin on disc tribometer in ambient air. Several contact conditions are analysed in order to realise boundary and mixed lubrication regimes. The experimental study was performed on a mineral base oil, and particle concentration effects were analysed. Antifriction properties were evaluated by measuring the friction coefficient and are presented as generalised Stribeck diagrams. Inorganic fullerene-like WS₂ and MoS₂ nanoparticles present interesting friction reduction properties when tested in boundary and mixed lubrication.     

Friction and wear of powdered composites impregnated with WS2 inorganic fullerene-like nanoparticles

The impregnation of inorganic fullerene-like nanoparticles of WS₂ (IF) allows to improve effectively the tribological properties of powdered materials in comparison to the impregnation of oil or commercially available layered WS₂ (2H) particles. The main goal of this work was to determine the dominant lubrication regimes under friction of the bronze, iron, and iron–nickel porous matrixes impregnated with 2H and IF solid lubricants. The tribological tests were performed at laboratory atmosphere (humidity ∼50%) using a ring-on-block tester at the sliding speed of V=1 m/s, and the loads of 150–1000 N. The wear of the metal bodies was measured by an eddy current probe system and by weighting of the samples before and after the test. In order to evaluate the radial clearance, the profiles of wear blocks were analyzed by profile projector. Than these data were used in the calculation of the Sommerfeld reciprocal values. Friction and wear results were presented as the Stribeck curves. The critical Sommerfeld reciprocals were evaluated from these curves. The Stribeck curves were compared with the Morgan’s curves for different ranges of non-dimensional permeability, Ψ. These results are used then in calculation of the permeability, Φ. Three lubrication regimes as: quasi-hydrodynamic, boundary or mixed, and dry friction were revealed under friction of porous samples in the load-range studied. It was found that the impregnation of IF nanoparticles provides the regime of quasi-hydrodynamic lubrication in the widest range of loads in comparison to the reference sample and the sample impregnated with 2H–WS₂. Fe–Ni samples exhibited the highest wear resistance and provided the widest range of quasi-hydrodynamic lubrication in comparison to bronze and iron powdered composites. The effect of IF on the regimes of lubrication is explained on the base of the third-body model. It is expected that the sliding/rolling of the IF nanoparticles in the boundary of the first bodies and in between the wear particles (third-body) facilitate the shear of the lubrication film and thus provide the quasi-hydrodynamic regime of friction. It is supposed that the roll shape of IF nanoparticles allows to release the IF from the pores to contact surfaces.     

The Failure and Damage Mechanisms Under Friction of Copper in the EHL and Mixed EHL Regions

Recently, friction and wear behaviour of Copper (Cu)–steel pairs rubbed under different lubrication conditions were studied. The Stribeck curve was used to identify the different regimes of friction of copper with different virgin grain sizes: the elasto-hydrodynamic lubrication (EHL), mixed lubrication and boundary lubrication (BL) regimes. The aim of this work is the detailed analysis of the damage evolution under friction in the EHL and mixed EHL regions. The effects of load on the friction and the wear and damage mechanisms have been studied. The surfaces of Cu samples before and after friction have been analysed using SEM and AFM techniques as well as roughness and hardness measurements. It was shown that the mechanisms of damage and failure of Cu samples under friction in the EHL region are similar to the damage and failure mechanisms in Very High Cycle Fatigue. Friction in the EHL region is accompanied by initiation and coalescence of pores and microcracks. The effects of the coalescence of pores and microcracks observed in the EHL region are enlarged with a subsequent loading in the mixed EHL region. The effect of the loading rate on the transition from the EHL to BL region has been studied. It was shown that decreasing the loading rate increases significantly the load of the transition from the EHL to BL region. The pore and microcrack coalescence remain as the dominant damage mechanism under friction in the mixed EHL region with the low loading rate, while a lot of ploughing tracks and large delaminated regions appeared on the surface of Cu sample after friction with the higher loading rate.     

The Influence of DLC Coating on EHL Friction Coefficient

High hardness, high elastic modulus, low friction characteristics, high wear and corrosion resistance, chemical inertness, and thermal stability are factors that make diamond-like carbon (DLC) coatings the subject of many studies. For the same reasons they also seem suitable for use in, amongst others, machine components and cutting tools. While most studies in the literature focus on the influence of coatings on wear and friction in boundary lubrication and pure sliding contacts, few studies can be found concerning rolling and sliding elastohydrodynamic lubrication (EHL) friction, especially in the mixed and full film regime. In this article tests are carried out in a Wedeven Associates Machine tribotester where an uncoated ball and disc pair is compared to the case of coated ball against uncoated disc, coated disc against uncoated ball, and coated disc against coated ball. The tests are conducted at two different temperatures and over a broad range of slide-to-roll ratios and entrainment speeds. The results are presented as friction maps as introduced in previous work (Bj?rling et al. in J Eng Tribol 225(7):671, 2011). Furthermore a numerical simulation model is developed to investigate if there is a possibility that the hard, thin DLC coating is affecting the friction coefficient in an EHL contact due to thermal effects caused by the different thermal properties of the coating compared to the substrate. The experimental results show a reduction in friction coefficient in the full film regime when DLC-coated surfaces are used. The biggest reduction is found when both surfaces are coated, followed by the case when either ball or disc is coated. The thermal simulation model shows a substantial increase of the lubricant film temperature compared to uncoated surfaces when both surfaces are coated with DLC. The reduction in friction coefficient when coating either only the ball or the disc are almost the same, lower than when coating both the surfaces but still higher than the uncoated case. The findings above indicate that it is reasonable to conclude that thermal effects are a likely cause for the decrease in coefficient of friction when operating under full film conditions, and in the mixed lubrication regime when DLC-coated surfaces are used.     

Enhancement of Lubrication Properties of Nano-oil by Controlling the Amount of Fullerene Nanoparticle Additives

In this article, the tribological properties of fullerene nanoparticles-added mineral oil were investigated as a function of volume concentration of fullerene nanoparticle additives (e.g., 0.01, 0.05, 0.1, and 0.5 vol.%). The lubrication tests were performed at the disk-on-disk type tester under the various normal forces and fullerene volume concentrations. Tribological properties were evaluated by measuring the friction surface temperature and friction coefficient, and then interpreted in terms of Stribeck curves. At the same time the friction surfaces tested were evaluated by observing their SEM images, surface roughness, and AFM images. The results showed that the nano-oil containing the higher volume concentration of fullerene nanoparticles resulted in the lower friction coefficient and less wear in the fixed plate, indicating that the increase of fullerene nanoparticle additives improved the lubrication properties of regular mineral oil.     

Influence of Mixed-Lubrication and Rough Elastic-Plastic Contact on the Performance of Small Fluid Film Bearings

A previously developed deterministic elastohydrodynamic (EHD) numerical model for small fluid film bearings functioning in the mixed lubrication regime is extended in this work by considering the rough contact. Several simplifying hypotheses are made: the shaft is considered rigid and smooth, turning at low speeds (isothermal regime), and the pad is assumed to have an elastic-perfectly-plastic behavior. The Reynolds equation is solved on a very fine mesh and the elasto-plastic pad deformation caused by the hydrodynamic pressure is taken into account. A deterministic active set-based method is used for determining the contact pressure, the contact extent, and the corresponding deformation. The results are presented for a partial journal bearing configuration, with a linear shaft velocity of 0.47 m/s under specific pressures varying up to 50 MPa. Two pad materials are considered, and the lubricant is either isoviscous or piezoviscous oil. The simulation results, presented as a function of the bearing specific pressure, include eccentricity, the film thickness, the friction torques, the contact extent, etc. Stribeck curves showing the evolution of the friction coefficient in the hydrodynamic and mixed lubrication regimes are also discussed.     

Friction and wear of copper samples in the steady friction state

The effect of sliding velocity and load on the friction and wear of Cu-steel pairs was studied. Elasto-hydrodynamic (EHL), mixed (ML) and boundary lubrication (BL) regions were analyzed using the Stribeck curve. The lubrication number of Schipper, Z, was used in the analysis of the Stribeck curve. Steady friction states were observed in the mixed EHL and BL regions, however two types of the ML region are revealed. The first type is the stable ML range. The second one is the range of unstable friction and wear when a decrease of the lubricant film leads to abrupt change of all controlled parameters. It was found that a transition to the unstable ML region occurs within a narrow range of Z parameter. Wear modes in the lubrication regions were studied. Deformation hardening in the lubricant regimes is discussed.