Mechanism of reaction layer formation in boundary lubrication

Highly loaded moving machine parts under hertzian contact conditions such as gears and roller bearings require lubricants which enable the formation of tribochemical reaction layers to reduce the adhesion component in the interface. The most critical conditions have to be referred to the sliding (slip) portion of such systems which cause microseizure and subsequent high wear. Sliding tests with a pin-on-disc machine were carried out to explore the efficiency of different lubricants with organic and organometallic additives. Two modes of action govern the formation of reaction layers and lead to distinct differences in friction and wear behaviour. Organometallic (e.g. with zinc and lead) dialkyldithiophosphates were found to be the most effective additives compared with organic phosphorus and sulphur compounds even with chemically inert steels, where other lubricants fail at relatively low load levels.     

Tribological behavior of tungsten-doped DLC coating under oil lubrication

Diamond-like carbon (DLC) protective films have received considerable attention in recent years. Beside tools and dies, diamond-like and related coatings are starting to find application in some highly stressed mechanical components, including bearings and gears. Several different gear tests show an increase in carrying load capacity or prolonged lifetime with tungsten carbide-doped DLC (W-DLC)-coated gears. The lubricants have been mostly commercially available gear oils that were primarily developed or tailored for ferrous materials and are probably far from optimal for amorphous W-DLC layers.In the present work, the influence of lubricant chemistry on the friction and wear in W-DLC-coated contacts was investigated using a reciprocating test device. The first six test lubricants were conventional non-inhibited API/ATIEL base stocks followed by selected test blends in combination with different surface-active single additives. Experimental results in coated contact have shown that for friction reduction performance, additives are not necessary, while wear resistance is related with the chemistry of the lubricant. It may be concluded at this stage that different mechanisms of friction-induced interaction between lubricants and W-DLC-coated layers are possible.     

Solid-lubricated ball bearings for use in a vacuum — state-of-the-art

The tribological responses of solid-lubricated ball bearings for vacuum use are described with reference to their friction torque, wear life, performance at high temperatures and high loads, effects of bearing materials other than steel, outgassing, dust generation and cost reduction, based (in part) on the materials offered by Japanese bearing manufacturers. In view of their outgassing and dust-generation characteristics, solid lubricants applied so far are not satisfactory. A new generation of fluorinated-polymer-lubricated bearings provides low contamination properties and can be used in air as well as in vacuum.     

Influence of lubricants on the performance of journal bearings – a review

ABSTRACT

The lubricant properties have a significant influence on the static and dynamic performance characteristics of journal bearing such as load-carrying capability, minimum fluid film thickness, maximum pressure, lubricant flow rate, damping coefficients, stiffness coefficients, etc. The present document reviews the behaviour of various lubricants such as power-law lubricants, couple stress lubricants, micropolar lubricants, ionic liquid lubricants and space lubricants. The influence of these lubricants on the performance of hydrostatic, hydrodynamic and hybrid journal bearings is discussed. An effort is made to develop the understanding to choose the suitable lubricant for journal bearings for different journal bearing configurations. Journal bearings operated with non-Newtonian lubricants have shown better performance compared to Newtonian lubricants. Ionic liquid lubricants have shown high potential in vacuum applications and extreme temperature environment such as in bearings of spacecraft moving mechanical assemblies.     

Lubricant for Heavily Loaded Slow-Speed Journal Bearing

Slow-speed journal bearings subjected to heavy loads operate in a mixed/boundary lubrication regime. Clearance and lubricant play very important roles in reducing the wear and friction in these bearings. In the present article, an experimental study on heavily loaded slow-speed journal bearings with various radial clearances lubricated with three different lubricants is presented. Lubricants with varying viscosities and containing different percentages of antiwear additives have been used. Bearing surface roughness and out-of-roundness are treated as noise parameters. The results of friction coefficient and total wear have been reported. The experimental results suggest that a lubricant with high viscosity and antiwear additives significantly reduces the coefficient of friction and amount of wear under varying bearing clearances, circularity, and cylindricity. The use of such a robust lubricant may obviate the effect of manufacturing uncertainties. This results in reduction of manufacturing and measurement costs.     

Behaviour of MoDTC in DLC/DLC and DLC/steel contacts

Diamond-like carbon (DLC) coatings are beginning to be used on machine components parts because of their excellent friction and wear resistance properties. It is hence important to be able to formulate lubricants able to work effectively with these coatings. This requires knowledge of how the various surface-reactive additives generally employed in lubricants behave with DLCs. This paper compares the behaviour of seven types of DLC, a-C, a-C:H, a-C:H:W, a-C:H:WC, Si-DLC, ta-C, ta-C:H, lubricated with molybdenum dialkyldithiocarbamate (MoDTC) solution. It is found that a-C and a-C:H:WC give lower boundary friction than the other types of DLC. MoDTC improves the wear resistance of DLC/DLC contacts but appears to greatly degrade the wear resistance properties of some DLCs in DLC/steel contacts, even though Mo-derived tribofilms form on all DLCs.     

硼酸酯与聚醚复配水溶液的摩擦学性能研究

水的润滑性能较差,因此需要选取性能良好的水溶性添加剂对其摩擦性能进行改善。研究选取椰油酰胺聚氧乙烯醚(CPOE)和三乙醇胺硼酸酯(BN)作为添加剂,分别对CPOE和CPOE-BN水溶液的摩擦学性能进行研究。使用NGY-6纳米级膜厚测量仪测试两种溶液的成膜性能,结果表明两种溶液在低浓度下均具有良好的成膜能力。利用四球试验机对两种溶液的减摩抗磨性能、稳定性能和极压性能进行了研究,结果表明两种溶液在低浓度下均具有良好且稳定的减摩抗磨性能,BN的加入可以大大提高CPOE水溶液的减摩性能和高载荷下的抗磨性能。利用XPS技术对CPOE-BN磨痕表面的化学元素进行了分析,结果表明在磨痕的表面生成了氮化硼、硼化铁等化合物,起到了良好的减摩抗磨效果。研究表明,CPOE和BN具有良好的协同润滑效果,是一种具有良好工业应用前景的绿色水基润滑添加剂。     

高速角接触球轴承速度性能试验研究

以内圆磨床砂轮轴中定压预紧成对串列背对背安装的角接触球轴承为例，通过试验研究了结构型式，润滑参数和润滑油对高速性能的影响，研究结果为高速轴承的设计和应用，以及润滑参数的选择提供了依据。     

Ionic Liquid Lubricants: Basics and Applications

Interest in the tribological performance of ionic liquids (ILs) has increased significantly since they were first introduced as lubricants in 2001. The primary advantages of ILs over conventional lubricants lie in their better ability to form tribofilms, higher thermal stability, environmental friendliness, and adaptability to various applications. A remarkable reduction in friction and wear has been observed after the addition of ILs in oil- or water-based media and in grease, suggesting that ILs are promising candidate materials as neat lubricants as well as lubricant additives. Despite the relatively common utilization of ILs as lubricating media, their wider use is limited by their high cost and corrosive properties. This article provides a brief introduction to relevant IL structures and properties, focusing on recent applications of the materials in engineering tribology.     

Tribology,lubricants and lubrication engineering — a review

In this review paper more than 200 papers published between 1974 and 1976 are evaluated. These papers deal with the fundamentals of friction, wear and lubrication, including the state of friction, lubrication and materials. Another important subject is the analytical data and properties of mineral oil and synthetic lubricants, lubricating greases, additives and solid lubricants. Lubrication systems and lubricating devices as well as the lubrication of bearings, gears and internal combustion engines are also covered. Lubricants for machining processes and for metalworking are reviewed, as well as questions concerning the testing and evaluation of lubricants.     

DLC solid lubricant coatings on ball bearings for space applications

The environment of space offers special challenges for the lubrication of components in sliding and rolling mechanisms. Hydrogenated diamond-like carbon (DLC) films are being studied as solid lubricant coatings to simultaneously fulfil specifications regarding wear resistance and low friction behaviour under ambient atmosphere and in vacuum.In this paper, the tribological behaviour of highly hydrogenated DLC coatings (50 at% hydrogen) is assessed. Coating composition was optimised on flat AISI 52100 steel substrates based on ball-on-disc tribotest results in air, vacuum and dry nitrogen environments. The developed DLC coatings can be tailored to yield ultra-low friction values in vacuum (μ=0.008). The average friction coefficient range obtained in humid air, dry nitrogen and vacuum for the range of applied loads were, respectively, 0.22 to 0.27, 0.02 to 0.03, and 0.007 to 0.013.New in this work is that optimised DLC coatings were applied to ball bearings for space applications. The torque and life tests of coated pairs of angular contact bearings in air revealed that relatively high bearing torques are generated which increase with time, but the amount of coating wear generated during in-air operation appears relatively light. In vacuum, low torques are generated after a prolonged running-in period. Low-torque life exceeds that observed for MoS₂ by a factor of about two. It is concluded that, in contrast to MoS₂ coated bearings, DLC-coated bearings for space applications might therefore be capable of undergoing in-air ground testing without too much disruption of the subsequent in-space performance.     

Low temperature tribometers and the behaviour of ADLC coatings in cryogenic environment

In a cryogenic environment components with interacting surfaces in relative motion (tribosystems) like bearings, seals and valves often generate undesired heat and experience high wear. Because conventional lubricants like oils or greases cannot be used in this temperature range, solid lubricants or materials with good frictional properties in unlubricated operation must be used. To test the suitability of conventional and advanced materials for tribosystems in this extreme environment and to obtain reliable materials data, model friction tests at low temperatures are performed. In most of the tests the samples are in ball-on-disc-configuration but various methods for cooling, loading, and measurement are applied. As an example the test results for amorphous carbon coatings in the temperature range 10–77 K are presented. These coatings can be suitable for cryogenic tribosystems, but their behaviour strongly depends on the composition and deposition method.     

舷外机用环境友好润滑油及其生物毒性的研究

利用发光细菌作为受试生物对研制的舷外机用TC-W3环境友好润滑油生物毒性进行了试验研究。试验结果表明以三羟甲基丙烷酯(TMP)为主调制的基础油具有低生物毒性、高润滑性能等优点,而以传统矿物油为主的基础油生物毒性较高,大量添加不利于实现低生物毒性的环境友好指标。润滑油的生物毒性主要取决于其添加剂的生物毒性,而添加剂生物毒性差异很大,合理选用添加剂及其使用比例是研制环境友好润滑油的关键。通过对润滑油基础油和添加剂的筛选及配比试验,成功地研制出低生物毒性、可生物降解的TC-W3环境友好润滑油,经润滑性、生物降解性和生物毒性试验,各项指标均满足设计要求。     

A study on the tribological characteristics of graphite nano lubricants

Many researchers have tried to improve the tribological characteristics of lubricants to decrease friction coefficients and wear rates. One approach is simply the use of additives in the base lubricant to change its properties. Recently, nanoparticles have emerged as a new kind of additive because of their size, shape and other properties. A nano lubricant is a new kind of engineering lubricant made of nanoparticles, dispersant, and base lubricant. In this study, graphite nanoparticles were used to fabricate nano lubricants with enhanced tribological properties and lubrication characteristics. The base lubricant used was industrial gear oil, which has a kinematic viscosity of 220 cSt at 40°C. To investigate the physical and tribological properties of nano lubricants, friction coefficients and temperatures were measured by a disk-on-disk tribotester. The surfaces of the fixed plates were observed by a scanning electron microscope and an atomic force microscope to analyze the characteristics of the friction surfaces. The results show that when comparing fixed plates coated with raw and nano lubricants, the plate coated with a nano lubricant containing graphite nanoparticles had a lower friction coefficient and less wear. These results indicate that graphite nanoparticle additives improve the lubrication properties of regular lubricants.     

The tribological behavior of Ni–Cu–Ag-based PVD coatings for hybrid bearings under different lubrication conditions

In a cryogenic environment, components like bearings with interacting surfaces in relative motion (tribosystems) often generate undesired heat and experience high wear. Because the properties of conventional bearing materials like stainless steel cannot be applied to this temperature range, the PVD coating based on metal–metal pairs with better frictional properties must be employed. To test the suitability of the Ni–Cu–Ag-based PVD coatings of hybrid bearings for liquid rocket engine turbopumps and to obtain reliable coating material data in the extreme environment, the tribological behaviors of coatings under the cryogenic fluid (liquid oxygen and liquid nitrogen) and water lubricated conditions are studied, respectively. In the paper, the specimens are in a vibrocryotribometer with the ball-on-plane contact type, and various running conditions in terms of lubricants, contacting loading, and contacting velocity are examined. The simulated experiment for testing the actual tribological performance of Ni–Cu–Ag-based PVD coatings for hybrid bearings is tested. The results of the tests indicate that the coatings can be suitable for cryogenic tribosystems of turbopumps. In the cryogenic environment, the volume wear rate of coatings increases rapidly with the contacting loading, but 15 min later, the volume wear volume of coatings turns into 2.5–15×10⁻⁴ mm³. Besides, under the liquid oxygen condition in simulating the liquid rocket engine turbopumps environment, the friction coefficients are 0.03–0.1.     

Performance Characteristics of Perfluoroalkylpolyether Synthetic Lubricants

An extensive test program was conducted to determine the performance characteristics (friction, wear, temperature) of a linear high temperature perfluoroalkylpolyether (PFPE) and two branched PFPE lubricants under normal and severe operating conditions. These include temperatures up to 200° C and Hertzian pressures as high as 2.8 GPa. A four-ball wear tester was used to measure wear scar and a two-disk machine was used to measure friction. The linear perfluoroalkylpolyether lubricant demonstrated low friction and wear at elevated temperatures under the test conditions considered. A bearing testing apparatus was used to measure friction and temperature of tapered roller bearings under various operating conditions for a branched PFPE and the results are reported.     

Antiwear Additives in Neopentyl Ester Oils

A variety of organophosphorus and other selected compounds were studied as antiwear additives for neopentyl polyol ester lubricants on 440C stainless steel rubbing surfaces using a four-ball wear test machine. Their effectiveness as additives was compared with that of tricresyl phosphate. Although 440C steel is relatively unreactive, it was possible to improve strikingly its wear and friction characteristics with organophosphorus additives. Some of the best results were obtained with acid phosphates. Alkyl chain length also appeared to be a significant factor. It was demonstrated that small concentrations of extraneous acidic substances are responsible for the antiwear action of phosphites at high load and that larger amounts of such acidic materials are needed as the load increases. Experiments involving addition of amines to neutralize formulations containing an acid phosphate confirmed that free acids promote antiwear action at the higher loads. Vinyl stearate and selected compounds containing chlorine or sulfur proved ineffective as additives for 440C steel in the antiwear region.     

耐高温复合固体润滑涂层的研究

MoS2、WS2、Sb2O3三种物质都是常见的金属基固体润滑剂,且性能优良,但作为单一涂层的局限性较大,因此,可将三种物质复配组成一种耐高温复合固体润滑涂层,设计正交试验,优化涂层配比,通过高温摩擦磨损试验和电镜探讨该耐高温复合涂层的润滑性能、分析其润滑机理。     

A Review of Lubrication Conditions for Wear Simulation in Artificial Hip Replacements

It is essential to evaluate the wear behavior of joint replacement materials and designs. While synovial fluid is the biological joint lubricant, it is present in too small quantities to use for wear simulations. A variety of lubricants have been used, such as water, saline solutions, synovial fluid components (hyaluronic acid, lubricin, and phospholipids), serum, and custom-made lubricants. We review studies from our Loma Linda University Medical Center lab and other labs using metal-on-polyethylene, exploring the benefits and deficiencies of these pseudo-synovial fluids. Serum is used extensively as the lubricant because its results most closely match clinical results with polyethylene bearings. We summarize laboratory experiments that have explored the effects of serum with differences in proteins (concentration, type, and ratio), volume, viscosity, temperature, and additives and their wear response. Protein precipitates seem to protect the implants from wear. The properties of current lubricants, in particular the roles of proteins, are the current focus in the search for an ideal joint analogue fluid.

     

Boundary Lubrication of Steel Surfaces with Borate,Phosphorus, and Sulfur Containing Lubricants at Relatively Low and Elevated Temperatures

Prolonging the life of engineering components through lubricant formulation to achieve better wear resistance and higher oxidation stability is of paramount importance to many mechanical systems, such as automotive gears and bearings. This can be accomplished with formulated lubricants that limit the generation of wear debris causing severe abrasion and protect the contacting surfaces through the formation of wear-resistant tribofilms. In this study, a ball-on-disk tribometer was used to characterize the friction and wear properties of steel surfaces slid in the boundary lubrication regime. An experimental scheme was developed to allow the statistical screening of various lubricant formulations. Sliding experiments were performed in baths of different lubricants at relatively low and elevated temperatures, approximately 32 and 100°C, respectively, under conditions of constant load and sliding speed. Surface profilometry, optical microscopy, and scanning electron microscopy were used to characterize the dominant friction and wear mechanisms. The tribological properties were found to strongly depend on the temperature and the additives (e.g., borate, phosphorus, and sulfur) present in the blended lubricants. The superior high-temperature wear performance of the lubricant with the higher borate content is indicative of the formation of a durable tribofilm that reduces metal-to-metal adhesion, material transfer, and surface plowing by wear debris.