Surface Temperatures at Unlubricated Sliding Contacts

This is a theoretical paper concerned with the rise in the surface temperatures caused by frictional heating at sliding contacts. By taking into account in the theoretical model, the growth of junctions between contacting asperities due to the action of the friction force, a large improvement in the correlation with experimental surface temperature measurements at dry friction contacts has been obtained. These results are felt to support the use of the usual flash temperature technique for predicting the surface temperatures in lubricated or low friction conditions.     

Long-term lubrication of momentum wheels used in spacecrafts—An overview

Momentum wheels are spacecraft actuators of large angular momentum used for the attitude control and stabilization of spacecrafts. The on-orbit performance of spacecrafts depends largely on the performance of the momentum wheels which in turn depends on its bearings and its lubrication. Currently, the life cycle of spacecrafts are aimed to be around 20–30 years. However, the increases in size, complexity and life expectancy of spacecrafts demand advanced technologies especially in tribology and in turn the development of more innovative lubrication systems for long-term operation. This article reviews the tribological requirements of momentum wheels and the various lubrication systems used in the past and raises a necessity to develop a new system to cope with the new requirements.     

Seizure mechanisms of wheel–rail contacts under lubricated conditions using a transient ball-on-disc test method

This study focuses on the transition from mild to severe wear in the wheel and rail contact. Such a transition has been observed at increased loading (normal load, sliding velocity, or bulk temperature) which can be compared to a change from a wheel thread–rail head contact to a wheel flange–rail gauge contact. This transition was experimentally studied using a transient test method of ball-on-disc type at different sliding velocities, contact pressures, and lubricants. It can be seen in the results that different seizure mechanisms are active for different sliding velocities. Also the amount of applied lubricant clearly affects the transition to seizure.     

Temperature Analysis in Lubricated Simple Sliding Rough Contacts

A temperature analysis of dry sliding fully plastic contact is extended to calculate the asperity temperatures between a sliding lubricated rigid smooth plane and a stationary elastic rough surface. First, surface roughness is generated numerically to have a Gaussian height distribution and a bilinear autocorrelation function. Lai and Cheng's elastic rough contact computer program is then used to determine the asperity contact loads and geometries of real contact areas. Assuming different frictional coefficients for shearing the lubricant film at the noncontact areas, shearing the surface film at the asperity contacts and shearing the oxide film as the asperity temperature exceeds a critical temperature, asperity temperature distributions can be calculated. Eight cases in Durkee and Cheng's scuffing tests of lubricated simple sliding rough contacts are simulated by using 20 computer-generated rough surfaces. The results show that scuffing is correlated to high-temperature asperities which are above the material-softening temperature.     

改善表面润湿性的研究

论述了表面润湿性在流体润滑中的作用,分析了固体表面能与表面润湿性的关系,通过实验对润湿性能不同的试片进行了研究。实验结果表明,金属表面能显著地影响着润滑效果。     

普通导轨润滑机理探讨

本文在对导轨滑动表面润滑机理进行研究中，首次提出了微观表面动压润滑的概念，在此基础上深入探讨了导轨滑动表面的润滑机理，从而指出导轨滑动表面润滑状态不应是介于边界润滑与干摩擦之间，而应是介于边界润滑与动压润滑之间。     

An Evaluation of Several Metals and Ceramics in Lubricated Sliding

A numerical scheme that has been successfully used to solve a wide variety of compressible flow problems, including flows with large and small discontinuities, entitled the space-time conservation element and solution element (CE/SE) method, is extended to compute compressible viscous flows in pressurized thin fluid films. This method is applied to calculate the pressure distribution in a hybrid gas journal bearing. The formulation of the problem is presented, including the modeling of the feeding system. The numerical results obtained are compared with experimental data. Good agreement between the computed results and the test data were obtained, and thus, validate the CE/SE method to solve such problems.     

Au/Cr Sputter Coating for the Protection of Alumina During Sliding at High Temperatures

A sputter-deposited bilayer coating of gold and chromium was investigated as a potential solid lubricant to protect alumina substrates in applications involving sliding at high temperatures. The lubricant was tested in a pin-on-disk tribometer with coated alumina disks sliding against uncoated alumina pins. Three test parameters—temperature, load and sliding velocity—were varied over a wide range in order to determine the performance envelope of the Au/Cr solid lubricant film. The tribo-tests were run in air at temperatures of 25° to 1000°C, under loads of 4.9 to 49.0 N and at sliding velocities from 1 to 15 ms^?1. Posttest analyses included surface profilometry, wear factor determination and SEM/EDS examination of worn surfaces.

Compared to unlubricaled Al₂O₃ sliding, the use of the Au/Cr film reduced friction by 30 to 50 percent and wear by one to two orders of magnitude. Increases in test temperature resulted in lower friction and the Au/Cr film continued to provide low friction, about 0.3, even at 1000°C. Pin wear factors and friction were largely unaffected by increasing loads up to 29.4 N. Sliding velocity had essentially no effect on friction, however, increased velocity reduced coaling life (total sliding distance). Based upon these research results, the Au/Cr film is a promising lubricant for moderately loaded, low-speed applications operating at temperatures as high as 1000°C.     

Development of a Centrifugal Oil Lubricator for Long-Term Lubrication of Spacecraft Attitude Control Systems—Design and Theory

Spacecraft, irrespective of their mission's purpose, contain a number of moving mechanical assemblies such as momentum/reaction wheels, control moment gyros, solar array drives, etc. Most problems encountered with these moving systems are related to tribology and, specifically, lubrication. Lubrication problems result from the loss of lubricant from the working zone by evaporation, surface migration, etc. Therefore, to ensure long-term uninterrupted performance of these systems, an efficient lubricant replenishment system is essential. This article describes the development of a novel lubricant supply system that can supply lubricant for more than 30 years at a controlled rate of few micrograms/hour. A mathematical model of a such lubrication system is developed to predict its useful life and performance. In addition, a computational fluid dynamics (CFD) simulation study of the lubricator is carried out and the results are used to verify and refine the theoretical model. Finally, the refined model is used to predict the performance of the lubricator for a period of 30 years.     

材料弹性模量对滑动高副润滑性能的影响

本文通过理论分析和实验研究证明，降低滑动高副中材料的弹性模量，可显著地改善其润滑性能，易于实现全膜润滑。     

Sliding friction at elastomer/glass contact: Influence of the wetting conditions and instability analysis

The aim of this paper is to simulate and give a better understanding of the behavior of rubber blade/windscreen contact: the friction behavior of the rubber/glass contact, the wiping quality and the characterization of friction instabilities are investigated. Therefore, the tribological prospect of a rough natural compound rubber sliding over a smooth glass in various wetting conditions is analyzed. This study is carried out using a modified elastohydrodynamic tribometer that allows us to investigate the rubber dynamical behavior under controlled conditions, thanks to the coupling of dedicated dynamical sensors and the contact visualization. Wetting conditions are varied as the impact of velocity and normal load is studied and a specific interest has been given to the transition between wet and dry regimes. Frictional instabilities are observed and correlated to the contact vibrations.     

Interfacial Temperature Distribution Within a Sliding Hertzian Contact

An analytic expression/or the steady-state interfacial temperature field in a sliding circular Hertzian contact is derived, taking into account the ellipsoidal distribution of the frictional power and the difference between the bulk temperatures of the two bodies, for the case where one surface is stationary and the other rapidly moving with respect to the contact. Other cases may be treated in a similar manner. It is shown that the temperature at any point on the interface can be approximated, to an accuracy improving with velocity, by half the harmonic mean of the two surface temperatures attained at that point if each body were to receive all the frictional power. The resulting maximum flash temperature is 33–38% higher than that given by Blok's widely used formula.     

铝材轧制工艺润滑与表面质量

本文用系统方法进行了铝材润滑轧制过程的摩擦学分析，论述了铝材热轧与冷轧过程的工艺润滑机理及润滑剂评价，最后，提出了铝材轧制表面质量控制系统。     

On the Mechanisms of MoS2-Film Failure in Sliding Friction

The effect of oxygen on the life expectancy of a run-in molybdenum disulfide film under heavy load has been studied. Tested in argon, with only small quantities of oxygen present, the smooth running period is increased by at least two decades as compared to a test in oxygen.

Blister formation is an important phase in the gradual destruction of the lubricant film. Blisters of submicroscopic size can be traced with the electron microscope, and in the presence of oxygen macroscopic blisters are rapidly formed; their further history has been followed by cine-film. Oxygen promotes sintering of the individual particles 10 a continuous, smooth, and therefore highly reflective lubricant layer. This process is an essential condition for blistering.

No wear occurs during the smooth running period of MoS₂ lubrication. Graphite differs from MoS₂ in this respect and in its much lower load-carrying capacity. The cine-film of graphite-lubricated surfaces indicated a rougher surface and the formation of smaller blisters. On addition of graphite to MoS₂, a sudden change in surface features occurs within narrow limits of graphite concentration. It seems that the addition of 15% by weight of graphite increases the total life expectancy of the MoS₂ film, but reduces the length Of the smooth running period.     

Wear Behavior of Thin Film Heads for VCR in Sliding with Metal Evaporated Tapes

In order to apply thin film heads to digital video cassette recorders, wear behavior, machinability of head materials, and level difference of the heads were evaluated. The tests, performed with metal evaporated tapes, were divided into two stages. First, wear rate and machinability of individual materials were evaluated. It was found that adhesive wear was dominant for every material. Wear damage was especially-severe for metal magnetic films. Machinability was satisfactory for most bulk materials except for those with a hardness of more than 1000 kgf/mm², ZrO₂ ceramics and devitalized glass. Secondly, wear damage and level difference were evaluated using model heads. After the test, a number of flows were formed on sliding surfaces, mainly on the film surfaces, indicating that abrasive wear occurred. With bulk material of larger wear rate, the level difference reached the final value faster and its deviation was smaller. With Co-Zr-Nb magnetic metal and an Al₂O₃ protective layer, the level difference was less than 20 nm using CaTiO₃ or ZrO₂-Ta₂O₅, bulk substrates.     

Influence of Lubricant Additive and Surface Texture on the Sliding Friction Characteristics of Steel under Varying Speeds Ranging from Ultralow to Moderate

A newly developed tribometer that undergoes significant changes in sliding speed, ranging from ultralow (5 μm/s) to moderate (17 cm/s), was used to study the lubricated friction characteristic of steel. In this study, the friction characteristics of stearic acid-formulated oil were studied to clarify the effects of surface roughness or surface roughness texture on friction. Several kinds of specimens having isotropic and anisotropic surface roughness with different textures were used. For an isotropic surface, a rougher surface resulted in low friction under low-speed conditions. The same surface produced high friction under high-speed conditions, where macroscopic hydrodynamic action was predominant. Remarkably less friction was observed in the transverse than in the longitudinal direction when the specimen had anisotropic roughness. This difference was particularly notable under ultra-low-speed conditions. Two other parameters of skewness and kurtosis of roughness distributions show that low friction was obtained when surface roughness distribution approached normal. It appears that the low friction observed with a rough surface or a transverse roughness direction could be explained by the microscopic hydrodynamic action of fluid together with the lubricity of the adsorbed molecular layer.     

On Prediction of Wear Coefficients in Sliding Wear

The wear rates and wear coefficients of metals are analytically predicted based on the delamination theory of wear when the wear rates are controlled by the subsurface crack propagation rate. The wear rate and the wear coefficient are predicted to be directly proportional to the depth of crack location and the crack growth rate. The numerical values of wear coefficients are obtained through finite element analysis of crack propagation in elastoplastic solids. The agreement between typical experimental results and theoretical prediction is excellent.     

钢蜗轮传动齿面润滑的理论与实践

本文分析研究了钢蜗轮副的齿面润滑机理,并进行了润滑剂对钢蜗轮传动性能影响的试验研究.结果表明齿面润滑是实现钢蜗轮传动的关键技术之一,且齿面润滑对钢蜗轮传动性能影响很大.为钢蜗轮传动的推广与应用提供了可靠的依据.     

Measurements of Temperature Distributions around Longitudinally Grooved Rough Surfaces in Sliding Elastohydrodynamic Point Contacts

This paper describes the temperature measurements in the EHL conjunction area comprising a longitudinally grooved steel ball and a sapphire disk under high slip conditions. The authors measured the temperatures of the oil film as well as both the disk and ball surfaces; furthermore, they estimated the temperature profile across the oil film by means of experimental values. The experimental results show that the temperature of the grooved ball surface increased considerably compared with that of a non-grooved ball. The temperatures of the faster surface for the grooved ball became sensitive to the slip ratio, whereas that for the non-grooved surface was almost constant. The temperature distribution had a higher value at the land zones and a lower one at the grooved zones. The temperature rise in the grooved zones varied qualitatively depending on the thermal conditions of both the sliding surfaces.     

钢背衬碳纤维织物/环氧复合材料干摩擦特性研究

研究了钢背衬碳纤维织物/环氧复合材料在环-环端面干摩擦状态下的摩擦学特性,考察了MoS2与石墨粉及其配比、衬层厚度、法向载荷对衬层干摩擦性能的影响,用扫描电子显微镜对衬层的磨损表面及对偶件45^#钢环表面进行了观察与分析。结果表明：厚度为1.5mm的试环衬层在摩擦过程中主要表现出粘结磨损特性,而含20%（质量分数）MoS2粉的0.6mm衬层表现出疲劳磨损与磨粒磨损特性。摩擦因数-时间特性曲线表明MoS2粉在降低衬层摩擦因数的同时能够抑制环氧树脂向对偶钢环表面的粘结;石墨对衬层的减摩效果优于MoS2粉,但摩擦温升引起树脂向偶件表面转移增多使得减摩效果大大降低;质量分数为33%的MoS2与石墨粉衬层表现出最佳的摩擦学性能,衬层摩擦因数具有随载荷先减小后上升的趋势。