Numerical and Experimental Studies on Tribological Behaviors of Cu-Based Friction Pairs from Hydrodynamic to Boundary Lubrication

When studying the tribological behaviors of a Cu-based friction pair in different lubrication regimes, calculation of the real contact area of asperity contacts is crucial but difficult. In this work, a mixed lubrication model in plane contacts is developed, and pin-on-disc tests are carried out. The real contact area ratio, load sharing ratio, and friction coefficient are investigated. Effects of sliding velocity, temperature, and pressure are considered. The results show that when the maximum contact area ratio is about 14.6%, the load sharing ratio of asperity contacts is about 95%. The friction coefficient obviously increases from less than 0.04 to about 0.15 as the regime changes from hydrodynamic to boundary lubrication. Asperities have a significant influence on the local lubrication of a Cu-based friction pair, and the action of hydrodynamic pressure cannot be ignored.     

超声振动对不同黏度润滑油摩擦学性能的影响

研究GCr15/45#钢摩擦副在4种不同黏度的润滑油润滑时,有和无超声振动下的摩擦磨损性能,采用扫描电子显微镜分析磨痕表面形貌,探讨在不同黏度润滑油作用下,超声振动对润滑油摩擦学性能的影响机制。结果表明:超声振动对不同黏度润滑油摩擦学性能的影响是不同的;超声振动可以提高低黏度润滑油润滑的减摩抗磨性能,如在6#白油润滑时施加超声振动后,摩擦副间的摩擦因数和磨损体积分别减小了13.6%和17.5%;高黏度润滑油润滑时,超声振动会加剧摩擦副的摩擦磨损,如在150BS润滑时施加超声振动后,摩擦副间的摩擦因数和磨损体积分别增加了10.4%和50%。     

An investigation of fretting behaviour of several metallic materials under grease lubrication

Fretting wear tests under grease lubrication have been carried out on an aluminium alloy, 52100 steel and low-alloy steel. The sphere–flat contact configuration is used. The influence of the displacement amplitude and normal load is investigated. Comparison between dry and lubricated contact of aluminium alloy, between 52100/52100 steel and 52100/low-alloy steel contact with grease lubrication has been carried out. Results show that grease lubrication strongly affects fretting behaviour. Base oil that separated from the grease during friction may result in accelerated contact wear by fretting.     

Thin film lubrication of sliding point contacts of AISI 52100 steel

The mechanism of thin film lubrication of sliding point contacts of AISI 52100 steel has been studied as a function of load, sliding speed, composition and temperature of the lubricant.Below certain critical combinations of Hertzian pressure, speed and temperature the surfaces are kept apart by an elastohydrodynamic lubricant film. The load carrying capacity of this film depends primarily on the effective viscosity of the lubricant in the contact region which decreases with bulk oil temperature and with increasing sliding speed, because of friction induced thermal effects. After breakdown of the EHD film, boundary lubrication may still prevent severe adhesive wear. The transition from the boundary lubricated regime towards the regime of severe adhesive wear is a function of load (normal force), speed and bulk oil temperature and possibly depends on the conjunction temperature. Irrespective of the initial lubrication condition, oxidation of the steel surfaces leads to the (re)establishment of low friction, mild wear conditions.     

高载荷条件下石墨-石墨摩擦副的摩擦学特性研究

利用研制的高载荷条件下摩擦因数测试装置,研究了石墨/石墨摩擦副在空气、水和油介质中的摩擦学特性。结果表明在4~15MPa范围内,随着载荷的增加,摩擦副在空气、水和油介质中的摩擦因数都逐渐降低;在油介质中摩擦副的摩擦因数最小,在水介质中摩擦因数变化最平稳,在空气中摩擦因数最大,且随载荷的增加变化幅度最大。磨损表面原始形貌对比分析表明,在空气中,摩擦副表面处于边界润滑状态,主要磨损机制是粘着磨损和犁削;水润滑条件下为轻微犁削;油润滑条件下,摩擦副表面处于为边界润滑和流体润滑状态,油中的减摩剂对试样表面有抛光作用。     

Experimental study of wear of model friction pairs lubricated with contaminated oil

The results of tribotests of block-on-ring friction pairs, which are analogs of piston–liner friction pairs of axial-piston pumps, are presented. It has been shown that, when the pairs are lubricated with oil that contains inorganic contaminants, the wear rate of the steel specimen is more than 200 times higher than that of the bronze specimen. It has been found that the concentration of contaminants in oil of 0.02–0.03% is permissible for block-on-ring friction pairs. The dependence of the wear rate of the specimens on the concentration of the solid contaminations is described by a logarithmic function, while the dependences of the coefficient of friction and the contact temperature on this concentration are described by hyperbolic functions. This has made it possible to assume that the wear of the specimens is determined by the hydroerosion component, which enhances the abrasive effect of the solid contaminations.     

A Model for Wear and Friction in Cylinder Liners and Piston Rings

Abstract

A one-dimensional elstohydrodynamic mixed lubrication wear and friction model is developed. The model can predict the effects of surface roughness, asperity contact, temperature-pressure-viscosity on wear, lubrication, and friction of the piston rings and cylinder liner. Wear is predicted based on the surface asperity contact pressure. The cylinder bore wear and the ring pack friction during an engine break-in are simulated and compared with the experimental results. The influence of cylinder wall temperature and surface roughness on friction and wear is investigated. The ring pack friction due to oil viscous shearing and asperity contact is found to reach its minimum at a certain oil temperature.     

润滑和载荷状态对聚甲醛齿轮服役性能的影响

基于齿轮耐久性能试验台开展了一系列干接触/油润滑下POM(聚甲醛)-POM齿轮副承载能力试验,并测量了服役过程中的轮齿温度、磨损量、齿廓精度和齿面形貌。试验发现,POM齿轮失效形式与载荷和润滑方式有关。通过对齿面微观形貌和磨屑表征,确认干接触下POM齿轮主要磨损模式为黏着磨损与磨粒磨损,而油润滑下POM齿轮失效形式为接触疲劳失效。由于润滑油减少了齿面摩擦,降低了运行温度,延缓了齿面劣化程度,因此POM齿轮在油润滑下的承载能力明显提高。     

An Adhesion-Dominated Rolling Friction Regime Unique to Micro-scale Ball Bearings

We demonstrate that micro-scale rolling bearings exhibit friction and wear properties markedly different from their macro-scale counterparts. A microfabricated testing platform uses variable rolling element diameters or vapor-phase lubricated interfaces to independently test friction force with varying contact area and surface energy. A linear, consistent, relationship between friction force and contact area is observed among different rolling element diameters. When surface free energy is altered through the introduction of vapor-phase lubrication, an 83 % decrease in friction is observed. When coupled with observed ball material adhered to the raceway, there is strong evidence for adhesion-dominated rolling friction regime at the micro-scale.     

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.     

菜籽油润滑条件下TiN涂层摩擦学特性*

为探究TiN涂层在油润滑条件下的摩擦磨损性能,通过磁控溅射技术在AISI304不锈钢上制备TiN涂层,借助扫描电镜、纳米划痕仪、XRD和摩擦磨损试验机探究不同氮通比下TiN涂层的微观结构、力学性能及其在植物菜籽油润滑下的摩擦学性能。实验结果表明：在氮通比为0.45下制备的TiN涂层具有最致密的晶状结构,且力学性能最优;在菜籽油润滑下,随着氮通比的增大TiN涂层摩擦因数和磨损率呈现先降低后增加的趋势,在氮通比为0.45时两者均最小。XPS分析表明,TiN涂层表面存在的氧化层,增加了菜籽油在涂层表面的润湿性,从而更好地形成润滑膜而起到润滑特性。与橄榄油、聚α烯烃(PAO6)基础油、5W40润滑油3种润滑介质相比,TiN菜籽油润滑下涂层表现出更优的润滑性能。     

Tribological Effects of Vegetable Oil as Alternative Lubricant: A Pin-on-Disk Tribometer and Wear Study

The investigation of lubricated friction and wear is an extended study. The aim of this study is to investigate the friction and wear characteristics of double fractionated palm oil (DFPO) as a biolubricant using a pin-on-disk tribotester under loads of 50 and 100 N with rotating speeds of 1, 2, 3, 4, and 5 ms^?1 in a 1-h operation time. In this study, hydraulic oil and engine oil (SAE 40) were used as reference base lubricants. The experiment was conducted using aluminum pins and an SKD 11(alloy tool steel) disc lubricated with test lubricants. To investigate the wear and friction behavior, images of the worn surface were taken by optical microscopy. From the experimental results, the coefficient of friction (COF) rose when the sliding speed and load were high. In addition, the wear rate for a load of 100 N for all lubricants was almost always higher compared to lubricant with a load of 50 N. The results of this experiment reveal that the palm oil lubricant can be used as a lubricating oil, which would help to reduce the global demand for petroleum-based lubricants substantially.     

Assessment and application of TiB2 coating in sliding pair under lubrication conditions

The aim of the present work is to determine the influence of TiB₂ coating on the friction parameters in sliding pairs under lubricated friction conditions. The TiB₂ coating deposited on modified surface layers of ring specimens made of AISI 5045 steel was matched under test conditions with counterparts made from SAE-783 and SAE-48 bearing alloys. Tested sliding pairs were lubricated with 5 W/40 Lotos synthetic engine oil. Tribological properties of the TiB₂ coating were measured using a block-on-ring tribometer. The applied modification technology of the surface layer of steel allowed for obtaining construction material with pre-determined tribological characteristics required for the elements of sliding pairs in lubricated contact. The results showed differences in the wear of bearing alloys, as a result of the interaction between co-operating surface layers and of the physiochemical changes of their surfaces, induced by external forces. Friction resistance and temperature in the friction area in the pair with TiB₂ coating and the SAE-783 bearing alloy are considerably higher than in the pair with the SAE-48 bearing alloy. The SAE-48 bearing alloy is subjected to more intensive wear processes in contact with the TiB₂ coating than the SAE-783 bearing alloy.     

Effect of Flash Temperature on Tribological Properties of Bulk Metallic Glasses

The tribological properties of Cu-based and Zr-based bulk metallic glasses (BMGs) sliding against Si₃N₄ under dry and water lubrication were studied on a pin-on-disc tribometer. The wear mechanisms of bulk metallic glasses were investigated based on the calculated flash temperature. The friction coefficients if fully amorphous alloy are about 0.7, while those of BMGs with nanocrytalline are a little higher. The wear rates of Cu-based BMG (V101) are about one order of magnitude lower than those of Zr-based BMG (Vit1) under dry friction, even two orders of magnitude lower under water lubrication. The wear resistance of bulk metallic glasses was influenced by the flash temperature. The calculated flash temperature (3,337 K) on the friction surface of Zr-based amorphous alloy exceeds its glass transition temperature, even its melting temperature. The high flash temperature leads to glass transition accompanied with viscous flow and material transfer, which is responsible for the poor wear resistance of Zr-based BMGs.     

浸渍磷酸盐石墨与9Cr18Mo不锈钢配副的摩擦磨损性能研究

针对航空发动机石墨密封常用的摩擦副浸渍磷酸盐石墨(M234Ao)和9Cr18Mo不锈钢材料,在UMT-TriboLab试验机上进行球-盘、销-盘接触摩擦试验,研究其低速轻载、高速重载工况以及干摩擦、油润滑下的摩擦磨损性能,利用接触式形貌仪、金相显微镜等对摩擦副表面进行观察分析,并分析其磨损机制。结果表明：在油润滑及面-面接触下的摩擦因数和磨损率明显低于干摩擦和点-面接触下;添加油介质可以降低界面摩擦剧烈程度,抑制金属氧化以及降低摩擦因数,特别是在高速重载工况下;M234Ao和9Cr18Mo配副间的磨损机制以磨粒磨损和黏着磨损为主,伴随有犁沟、微裂纹及擦伤现象。     

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.     

Correlation Studies of WS 2 Fullerene-Like Nanoparticles Enhanced Tribofilms: A Scanning Electron Microscopy Analysis

The beneficiary effects of tungsten disulphide (WS₂) inorganic fullerene-like nanoparticles (IFLNPs) in the lubrication industry were shown in recent years. However, their successful incorporation into lubricants (oils, greases) is not straightforward. In practice, the lubricant contains several components for different purposes, e.g. reducing the oxidization of the oil (antioxidant), minimizing the wear rate (anti-wear additive), dispersants, etc. These additives can contain chemically active compounds, which under the lubrication process (where locally extreme conditions can develop: high pressure and flash temperatures) can change the chemistry in the contact zone and block the beneficial effects of the inorganic nanoparticles. In this investigation, poly-alpha-olefin (PAO) is being used as base oil in which the WS₂ nanoparticles and different additives are mixed. A ball-on-disc sliding test revealed that certain additives inhibit the nanoparticles to reduce friction (less than 5 % decrease in friction coefficient), while in other cases, the friction reduction was above 50 %. The comparison is being made between PAO + additive and PAO + additive + IFLNPs. Scanning electron microscope and energy dispersive X-ray spectroscopy were used to investigate the elemental composition of the tribofilms formed on the wear marks. Further analysis was made in order to reveal correlations between elemental compositions of the tribofilms and external parameters such as the friction coefficient and wear rate. For instance, a strong correlation between tungsten content of the tribofilm and the friction coefficient was found.     

Recent research on tribology in Japan

Recent research on tribology in Japan is outlined. There have been many papers on the following problems.

In fluid film lubrication, the effects of surface irregularities on the lubricating performance and increase in friction in high speed bearings due to turbulent flow are highlighted to date. Elastohydrodynamic lubrication and its related problems such as traction play an important role in the lubrication of cams and followers, rolling contact bearings and traction drives. The rheology of lubricants at high pressures is one of the most important properties governing lubricated concentrated contacts. Regarding basic studies on wear, the mechanism of the adhesive wear process is being studied. It is interesting to investigate the deformation behaviours of materials and structures by finite element methods. The friction and wear characteristics of plastics and their composites have been studied in basic terms.

The chemical reactions of lubricating oil additives with frictional surfaces play important roles in lubricated concentrated contacts. Recently there have been many studies on the correlation between the lubricating properties and the chemical reactivity of lubricant additives. It is very interesting that there are many papers on the mutual effects between lubricant additives.     

Morphological Influence of Molybdenum Disulfide on the Tribological Properties of Rapeseed Oil

The morphological influence of molybdenum disulfide (MoS₂) on the tribological properties of rapeseed oil (RO) was investigated. MoS₂ nano-vesicles improved the tribological properties of RO, especially the wear resistance. However, nano-platelets and micro-platelets weakened the wear resistance because of their bad dispersibility in RO. Positive synergistic lubrication was observed between nano-vesicles and nano-platelets, but not between nano-vesicles and micro-platelets or nano-platelets and micro-platelets. Abrasive plowing is the main wear manner for the steel balls lubricated by RO or ROs with platelet-like MoS₂. However, chemical corrosive wear occurred on the friction surface as lubricated by RO with nano-vesicles. The chemical corrosive wear confirmed the entering of MoS₂ nano-particles to the contact area for lubrication. This lubrication of nano-vesicles was ascribed to their spherical structure and good dispersibility in RO. The nano-vesicles did not influence the stability of RO and assembled beside furrows to alleviate the wear. Thus, potential applications of MoS₂ nano-vesicles in vegetable oils were revealed.     

On the behaviour of an oil lubricated fretting contact

Although many engineering situations involving fretting damage are lubricated, comparatively little has been reported on this aspect of fretting wear. The viscosity of the lubricating oil and its boundary layer performance are expected to influence fretting behaviour, in addition to the normal fretting parameters, such as stroke and contact force.

This paper examines the effect of lubrication regime, oil viscosity and stroke on the behaviour of a ball-against-flat specimen arrangement. Ball and flat specimens were both manufactured from a bearing steel (SUJ2). Polybutane oils, without additives, covering a range of viscosities from 1 to 10 000 cSt, and fretting strokes up to 35 μm were investigated. The lubricating oil was added to the fretting interface after 0, 3 and 2000 fretting cycles had been completed. Lubrication regime, oil viscosity and stroke were all found to affect fretting behaviour in terms of both coefficient of friction (or traction coefficient) and wear. For strokes less than 9 μm, i.e. for conditions approaching almost complete ‘stick’, coefficient of friction values under oil lubrication were well in excess of double those observed without it. These high values suggest that the oil was unable to penetrate into the fretting contact region, but did maintain a shield around it, so that metal-on-metal contact was maintained under oxygen deprived conditions. The lowest values of steady state coefficient of friction (≈ 0.2) were observed when oil lubrication was applied after 2000 cycles had been completed, indicating that surface roughening and the presence of oxide films and oxidised debris assisted penetration of the lubricant into the fretting contact zone.