缸套—活塞环材料在往复滑动中摩擦系数的试验研究

为了研究机车柴油机缸套活塞环材料的摩擦学性能,我们设计并制造了一台往复式摩擦磨损试验机。该试验机可在一定范围内实行载荷、速度、润滑量的单因素控制,并可同时定性和定量的显示运动中的摩擦力大小。我们利用该试验机对美国GE公司采用的软氮化铸铁缸套—表面镀铬铸铁活塞环材料进行了摩擦学性能的试验研究,得出了该配对副在往复滑动中摩擦系数随载荷和速度变化的关系曲线。     

Friction and elasticity on a molecular scale

A molecular scale atomic force microscopy study of friction and elasticity is presented on a one-component lipid bilayer system - a model boundary lubricant. With a real area of contact on the order of the lattice spacings of the sample, the elastic compliances of single lipid molecules are recorded - the first report of a molecularly resolved elasticity map. The anisotropic and highly ordered structure of the lipid bilayer has been observed to cause contrast information in friction on molecularly flat areas, and to be dependent on the sliding direction. This anisotropic behavior of friction has been measured to be independent of normal elastic compliances. Only asymmetric indentation which causes in-plane compliances leads to heterogeneities in the elasticity map. In this fundamental study of friction, the effects of adhesion and elasticity are discussed.     

以汽油和甲醇为燃料时小型二冲程发动机摩擦特性比较

采用往复振动机模拟小型二冲程发动机运转工况，实验研究汽油和甲醇为燃料时发动机气缸和活塞环间的摩擦特性，并比较分别使用润滑油新油、润滑油老化油、润滑油新油和老化油的混合油作为润滑油时气缸和活塞环间的摩擦特性。结果表明，以甲醇为燃料时的摩擦因数和磨损量均小于以汽油为燃料时的摩擦因数和磨损量，特别是使用添加了润滑油新油的燃料时的摩擦因数和磨损量最小。通过黏度和热重（TG）分析，探讨甲醇燃料改善气缸和活塞环间的摩擦特性的原因，结果表明，甲醇燃料具有较高的黏度和较低的摩擦因数，因而以甲醇为燃料时可以降低磨损     

A study on the friction properties of poly(vinyl alcohol) hydrogel as articular cartilage against titanium alloy

Many biomaterials are being developed to repair or replace articular cartilage. One of these materials, poly(vinyl alcohol) (PVA) hydrogel prepared from aqueous solution of the polymer by freezing and thawing method may exhibit the mechanical properties required to withstand the harsh environment of diarthrodial joints. To better understand how PVA hydrogel friction is affected by different variable factors, a three-factor, three-level designed orthogonal experiment was developed. Factors include lubricant, sliding speed, and normal load. Friction coefficient of the PVA hydrogel was found to depend significantly on load and sliding speed. Lubricant had little effects on the friction coefficient. Friction coefficient of the PVA hydrogel decreased with the increase of sliding speed and the friction coefficient approximately increased linearly with the increasing load. Average friction coefficient decreased from 0.0447 to 0.0379 while the sliding speed increased from 0.06 to 0.22 m/s. Average friction coefficient increased from 0.0276 to 0.0546, almost increasing one time, while the load increased from 5 to 15 N.     

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.     

Temperature—friction characteristics of cylinder lubrication in large,slow, cross‐head marine diesel engines under boundary lubrication conditions

In large, slow, cross‐head marine diesel engines research has increasingly shown that the lubrication regime between piston rings and cylinder liner at top dead centre is of the boundary lubrication type due to the high gas pressure, low sliding speed, and high temperature. This means that the tribological properties of piston ring, cylinder liner, and cylinder lubricant in these types of engine under boundary lubrication conditions should be considered simultaneously when friction and wear between the piston ring and cylinder liner are studied. Until now there has been no standard method to evaluate boundary lubrication performance. There are a few traditional methods used in lubricant research, but their results are not correlated with service conditions. It is important to find a suitable method to evaluate the boundary lubrication performance of lubricants at the laboratory testing stage or before the engine testing stage. The important parameters, such as sliding speed, normal load, materials of the contacting pairs, and lubricant, need all to be controlled. In this paper a systematic experimental procedure, the ‘five times heating and cooling test’, is introduced to assess lubricant properties under boundary lubrication conditions. Most of the parameters mentioned above are controlled. The model contact, of pin‐on‐plate form, is made from the actual piston and liner materials used in a large‐bore, slow, cross‐head marine diesel engine. The temperature characteristics of different blends of lubricants are investigated under boundary lubrication conditions using a pin‐on‐plate reciprocating test rig. These blends of lubricants have the same additives but different base fluids; they nevertheless fulfil the physical and chemical requirements of a real marine diesel engine. The test temperature range is from room temperature to the working temperature of the top piston ring. The experiments show that there are different temperature—friction characteristics for lubricants with different bases and the same additive package and there are also different temperature—friction characteristics during heating up and cooling down for each blend. Single‐base lubricants have more promising temperature—friction characteristics than those of a blend of a high‐viscosity base and a low‐viscosity base at high temperature.     

Contributions of adhesion and hysteresis to coefficient of friction between shoe and floor surfaces: effects of floor roughness and sliding speed

Abstract

Improving shoe–floor friction in order to reduce slip and fall accidents requires thorough understanding of the factors that contribute to friction. The friction between a sliding viscoelastic material (shoe) and a hard surface (floor) has two major components: adhesion and hysteresis. This study aimed to quantify the effects of floor roughness and sliding speed on adhesion and hysteresis to determine how each component contributes to the coefficient of friction. Experiments were conducted on a pin on disc tribometer using ceramic tiles with three levels of roughness, six sliding speeds, two common shoe materials and four liquid lubricants. Hysteresis was measured using a lubricant that minimised adhesion. Dry and lubricated adhesion was measured by subtracting hysteresis from the coefficient of friction. Analysis of variance regression models were used to determine the contributions of hysteresis, dry adhesion, sliding speed and fluid to lubricated coefficient of friction. Increased floor roughness led to increased hysteresis, while increased sliding speed reduced both adhesion and hysteresis. These findings are consistent with theory that states that larger asperities increase hysteretic deformation and that sliding speed affects deformation and real area of contact between a viscoelastic material and a hard surface. The model correctly predicted 83% of variation in coefficient of friction based on dry adhesion, hysteresis and fluid dependent constants. The sensitivity of hysteresis friction to shoe material and floor roughness indicates that optimising these parameters may be effective at reducing slip accidents on oily floor surfaces.     

Velocity-Dependent Adhesion with Lubricants on Thin-Film Disks

The well-known problem of stiction in a magnetic disk drive largely depends on the forces induced by the presence of a thin liquid film. It is commonly recognized that both adhesive and viscous effects contribute to the magnitude of the stiction force, but is is not known what relative roles the two effects have in a lubricated contact. In the present work, the nature of adhesive and viscous effects is investigated for the slider/disk interface under conditions of constant-speed sliding.

Friction measurements are conducted over a range of sliding speeds, 0.25-250 mm/s, with eight perfluoropolyether (PFPE) lubricants applied in various thicknesses, 0-6.6 nm, to carbon-coated magnetic thin-film disks. The lubricants were selected to cover a broad range of viscosities. For several sliding speeds and lubricant film thicknesses, the friction force is found to decrease significantly with increasing sliding speed for all lubricants. In several instances, large friction forces are observed at the lowest sliding speeds, indicating stiction-like behavior, whereas, at higher speeds, the friction is reduced to even below unlubricated friction levels. At the highest film thickness and sliding speed, the friction was found to increase with speed for some lubricants. The implications of these results on current models of lubricant-mediated adhesion are discussed.     

Tribological evaluation of piston skirt/cylinder liner contact interfaces under boundary lubrication conditions

The friction and wear between the piston and cylinder liner significantly affects the performance of internal combustion engines. In this paper, segments from a commercial piston/cylinder system were tribologically tested using reciprocating motion. The tribological contact consisted of aluminium alloy piston segments, either uncoated, coated with a graphite/resin coating, or an amorphous hydrogenated carbon (a‐C : H) coating, in contact with gray cast iron liner segments. Tests were conducted in commercial synthetic motor oils and base stocks at temperatures up to 120°C with a 2 cm stroke length at reciprocating speeds up to 0.15 m s⁻¹. The friction dependence of these piston skirt and cylinder liner materials was studied as a function of load, sliding speed and temperature. Specifically, an increase in the sliding speed led to a decrease in the friction coefficient below approximately 70°C, while above this temperature, an increase in sliding speed led to an increase in the friction coefficient. The presence of a coating played an important role. It was found that the graphite/resin coating wore quickly, preventing the formation of a beneficial tribochemical film, while the a‐C : H coating exhibited a low friction coefficient and provided significant improvement over the uncoated samples. The effect of additives in the oils was also studied. The tribological behaviour of the interface was explained based on viscosity effects and subsequent changes in the lubrication regime, formation of chemical and tribochemical films. Copyright © 2010 John Wiley & Sons, Ltd.     

The Effect of Humidity on Low-Load Frictional Properties of a Bonded Solid Film Lubricant

Low-load slow-speed sliding friction tests were conducted on a MoS₂/graphite (90/10) bonded solid film lubricant in controlled-humidity environments. The average steady-state coefficient of friction increased from 0.23 to 0.28 as relative humidity (RH) was raised from 10 percent to 50 percent, and it increased to 0.6 when RH reached 90 percent.

At RH ≥80 percent, friction response was strongly influenced by chemically reactive metals used for slider and lubricant substrate. Thus, friction with carbon steel sliders was significantly lower than with stainless steel sliders, although corrosion of the former caused rapid destruction of the lubricant film. Also, continued sliding by stainless steel on lubricant applied to phosphor-bronze eventually resulted in a significant friction decrease and simultaneous formation of a dark red-brown film in the wear track. The proposed explanation for these effects is based on oxidation of MoS₂ at the sliding interface and reaction of the oxidation products with slider and substrate metals.     

Tribopolymerization of n-butyl acrylate on the steel–steel rubbing surface

Antiwear property of n-butyl acrylate (BA) in hexadecane for steel–steel friction elements was studied. Scanning electronic microscopy (SEM) images of worn scars of balls, and width measurements of worn tracks of disks indicate that BA has good antiwear property.Tribopolymerization (polymerization initiated by the rubbing surface) tests of BA used as lubricant instead of an additive were conducted, to classify antiwear mechanisms. Infrared spectroscopy (IR) of washing solution and thermogravimetry (TG) traces of wear debris confirmed that tribopolymers were generated on steel–steel interface in situ. Also considerable wear products were precipitated from vacuum-condensed worn fluids with methanol as the non-solvent, characterized by IR, gel permeation chromatography (GPC), nuclear magnetic resonance (NMR) spectroscopy, showing that the precipitates were poly(n-butyl acrylate) (PBA) with very high molecular weight.To study tribopolymerization mechanisms of BA, effects of sliding velocities and adding 1 wt% n-butyl alcohol to BA fluids on the mass of tribopolymers were investigated, respectively. Expectedly, the mass of tribopolymers dramatically enhanced with the sliding velocity increasing, showing that the tribopolymers were generated just due to friction processes. Additionally, tribopolymerization of styrene (easy to thermopolymerize) was studied. Unexpectedly—but not unreasonably—no substantial worn products were precipitated from vacuum-condensed worn fluids. Based on these experimental results, an exoelectron-radical-tribopolymerization mode, consistent with some Kajdas’ tribochemistry theories, for BA was proposed.     

激光微造型表面摩擦磨损性能研究

采用声光调Q二级管泵浦固体光源（DPSS）Nd：YAG激光器对缸套试件表面进行了,微造型州纹加工。在往复式活塞环-缸套摩擦磨损模拟试验机上进行了激光造型缸套试件与未造型光滑缸套试件的摩擦磨损性能对比试验研究。试验结果表明,在重载高速条件下,激光网纹试件与未造型试件相比,摩擦因数降低23％,磨损量降低66％。说明激光网纹沟槽具有贮油、积屑和动压润滑作用,同时激光加工的网纹淬火效应也有利于提高支承表面的耐磨抗擦伤性能。     

润滑油粘度对缸套/活塞环摩擦学性能的影响

在内燃机实际运行中,润滑油的粘度直接影响到润滑油膜的状态,因而活塞环在缸套中不同位置时的摩擦、润滑状态各不相同。文中以缸套活塞环为研究对象,建立了润滑计算模型,并运用该模型对缸内压力、温度、油膜厚度和摩擦系数进行了分析。结果表明,润滑油膜厚度和摩擦系数随转速改变而发生变化,而剪切稀化导致润滑油粘度减小是引起该变化的主要原因。最后,通过对计算结果的分析,提出了适用于缸套活塞环的润滑油粘度指标。     

A Load-Dependent Model to Investigate the Velocity Dependence of Friction Coefficient: ZrO2/Polymer Nanocomposite

Using dynamic finite simulations, we investigate how the friction coefficient of ZrO₂/polymer nanocomposite depends on the sliding speed. The load-dependent model we developed corresponds to common friction systems, where the friction couples are sliding under fixed load for various speeds. Here, we study the effect of the sliding speed on the contact distance between two contacting bodies. In accordance with experimental observations and theoretical arguments, we find the contact distance increased with the sliding speed. We show that the dependence of the reaction force on sliding speed can be rationalized by assuming that the frequency dependence of the polymer chains relaxation times is affected by the damping effects of contact stress. By investigating the energy dissipation, we show how the friction coefficient is affected by the sliding speed. The deformation volume and relaxation times decreased with the increasing sliding speed, which result in the decreasing of energy dissipation. Then, the work in pushing the top cylinder across the bottom plateau decreases, results in a reduction of the mean horizontal reaction force and friction coefficient.     

Effect of Sliding Speed on Surface Modification and Tribological Behavior of Copper–Graphite Composite

In practice, the sliding speed is an important parameter for materials applied in sliding condition. We have conducted an experimental study to explore the effect of sliding speed on friction and wear performance of a copper–graphite composite. The sliding tests were carried out over a wide range of speeds with a pin-on-disc configuration. The results show that there is a critical speed at which there is a transition of the friction and wear regimes of the composite. In addition, the formation of a lubricant layer on the contact surface (surface modification) determines the actual tribological performance of the composite. The wear mechanisms in different wear regimes are also discussed.     

A laboratory simulation for stick-slip phenomena on the hydraulic cylinder of a construction machine

Stick-slip properties between steel and the glass fiber-reinforced nylon employed in the hydraulic cylinder of a construction machine were studied with a reciprocating tribometer as a laboratory simulator. A stick-slip evaluation method was established by introducing the running-in operation under certain operating conditions. Through studying the effect of sliding speed, the normal load and the viscosity, it was found that the sliding surfaces were operated in the mixed lubrication regime, where some friction modifiers were effective for elimination of stick-slip. Surface observation and surface analysis of the specimens after the tests indicated that stick-slip was mainly caused by an increase in the static friction coefficient due to transfer of iron oxide on to the seal surface from the steel surface. Subsequently, an attempt was made to correlate the results by the simulation test results for the hydraulic oil with those obtained by the hydraulic cylinder test.     

Lubricant Effects in the Transition from Boundary to Microelastohydrodynamic Lubrication

Lubricant effects in the friction transition from boundary to microelastohydrodynamic lubrication were investigated by using a ballon-flat tribotester at sliding speeds from 0.02 to 0.88 mm/sec. Three lubricants—cyclophosphazine (X-IP), poly-alpha-olefin (PAO) and Z-DOL—were used, in this investigation. When X-IP was used at room temperature, a drop in friction coefficient from 0.22 to 0.12 at sliding speeds ≥0.10 mm/sec. (an unusually low speed) was observed, accompanied by a rise in the contact electrical resistance across the ball-fiat interface. The friction drop did not occur at temperatures ≥100°C. The friction transition was achieved at lower speeds when sliding perpendicular to the surface roughness texture. No transition occurred when PAO and Z-DOL were substituted as the lubricant. The latter him lubricants were working in the boundary lubrication regime as indicated by the contact resistance measurement.     

Wear durability studies of ultra-thin perfluoropolyether lubricant on magnetic hard disks

This paper presents wear and friction studies on ultra-thin (~2 nm) film of perfluoropolyether (PFPE) coated on glass substrate magnetic hard disks. The lubricant was coated on the disk by the dip-coating method and the tribological tests were carried out by sliding a 3 mm diameter glass ball slider (normal load=20 mN) on the rotating disk surface. Lube thickness and lube wear profile were measured using an ellipsometer whereas the worn disk surface was studied using a surface reflectivity analyzer. The sliding speed and the lube bonding conditions were varied during the test. From the results, it is concluded that about 80% bonding of the lube to the disk surface leads to an increase in the wear durability of the lubricant by a factor of 2 when compared to the as-lubed condition. Lube bonding has an effect on increasing the coefficient of friction. Initially, increasing sliding speed increases both friction and wear but for very high sliding speed these values tend to decrease. The glass ball surface showed wear due to asperity interactions as well as lube transfer from the disk to the glass surface.     

Tribochemical Effects on Tribological Properties of Self-Mated Zirconia Ceramic in HFC-134a Gas

Tribochemical effects on the tribological properties of self-mated zirconia ceramic in CF₃CH₂F (HFC-134a) were investigated using a ball-on-disk type environmental tribometer. The friction chamber of the tribometer was attached to a micro-spot X-ray Photoelectron Spectrometer (XPS) for ensuring that surface analysis be conducted without exposuring the frictional surfaces to air. It was found that HFC-134a gas was an effective lubricant for zirconia ceramic, especially at a pressure higher than 10³ Pa. The products of tribochemical reactions between zirconia and HFC-134a molecules were detected. The amount and chemical state of the tribochemical products seemed to control the tribological behaviors. Thus, the role of tribochemical products on the tribological properties of zirconia in HFC-134a gas at 10⁴ Pa was studied in detail under applied loads of 0.6–5.0 N and sliding speed of 0.04–0.35 m/s. It was found that severe tribochemical reactions occurred at low speeds and high loads. The formation of ZrF₄ accelerated the chemical wear of zirconia, and raised the friction. Zirconia ceramic is suitable for use at moderate load and sliding speed under a reactive environment.     

Tribological behavior of reciprocating friction drive system under lubricated contact

The dynamic friction and wear behaviors are investigated in reciprocating friction drive system using a 0.45% carbon steel pair. The effects of various operating parameters on the traction force, stick and slip time, and friction modes are examined under the lubricated contacts. Moreover, the critical operating conditions in classifying three friction modes are also established. Results show that the fluid friction induced by the shearing of lubricant dominates the variation of traction force and produces the positive slope γ at the first period of slip in the traction force–relative sliding velocity curve. The γ value decreases at higher driver speed during stick-slip motion due to the thicker fluid film and shear thinning effect. The γ value increases due to the asperity interactions as the friction region is transferred from stick-slip to sticking with normal load from 196 to 980 N. Furthermore, it is also found that the static friction force is independent of stick time for the tangential loading rate ranged from 1.12 to 16.8 s⁻¹. The transition region produces the severest wear under the different driver speeds, but the wear is insensitive to the friction regions and the severe wear only occurs at higher normal load due to the action of Hertzian contact.