An investigation of the fretting behaviour of low friction coatings on steel

Three different low friction coatings on steel (electrolytically deposited cadmium, PTFE solid lubricant in epoxy resin and PTFE solid lubricant in polyimide resin) were studied in order to relate their fretting behaviour with mechanical properties. Particular importance was given to adhesion which was measured using a scratch test. Fretting tests were carried out on the steel substrate and on the coatings under the same conditions. The major parameters of the tribological system were identified and then quantified. The values of the parameters obtained for each coating were compared with the corresponding values for uncoated steel. The mechanical characteristics of the coatings and their fretting parameters were plotted using a polar diagram in order to give an overview of the fretting behaviour of each coating. Differences were noted and the corresponding parameters were identified. The influence of the adherence of the coating and of the fretting test load on the lifetime of the coating was determined.     

Investigation on MoS2 and graphite coatings and their effects on the tribological properties of the radial spherical plain bearings

With constant enlargement of the application areas of the spherical plain bearings, higher quality lubrication of the bearings is required. To solve the lubricating problems of spherical plain bearings under high temperature, high vacuum, high speed, heavy loads and strong oxidation conditions, it is urgent for us to develop more excellent self-lubricating technologies. In this paper, the bonded solid lubricant coatings, which use inorganic phosphate as the binder, the mixture of MoS2 and graphite with two different weight proportions as the solid lubricant, are prepared by spraying under three different spray gun pressures. The bonding strength tests on the coatings show that the best spraying pressure is 0.2 MPa and the better mixing proportion of MoS2 to Graphite is 3:1. Then for the radial spherical plain bearings with steel/steel friction pair, after the coatings are made on the inner ring outer surfaces, the friction coefficient, the wear loss and the friction temperature of the bearings under four oscillating frequencies are investigated by a self-made tribo-tester. The test results, SEM of the worn morphologies and EDS of worn areas show that tribological properties of the bearing are obviously improved by the bonded solid lubricant coatings. When sprayed under the spray gun pressure of 0.2 MPa, the bearings have better anti-friction and anti-wear properties than those sprayed under 0.1 MPa and 0.3 MPa. Further as proved from the XPS analysis, between the coating with 3:1 mixing ratio of MoS2 to Graphite and the coating with 1:1 ratio, the former has less oxidation occurred on the surface and therefore has better tribological characteristics than the latter. This paper provides a reference to developing a new product of the radial spherical plain bearings with high bonding strength, oxidation resistance and abrasion resistance.     

Solid Lubricant Coatings: Recent Developments and Future Trends

In recent years, great strides have been made in the formulation of solid lubricant coatings for a wide range of industrial applications. These coatings are now available in nano-structured and/or -composite forms to provide better performance and durability even under very severe sliding conditions. By coupling these coatings with smart surface engineering strategies (such as micro-texturing and/or -patterning), researchers have achieved higher levels of performance and durability in demanding tribological applications. Some of these advanced coatings are now commercially available and can meet the ever-increasing performance and durability requirements of severe tribological applications. This paper will provide a historical overview of recent developments in solid lubricant coatings and will expand on the lubrication mechanisms of both traditional and new solid lubricants. Special emphasis will be placed on modern practices that are aimed at enhancing the properties of these coatings and expanding their uses in practical applications.     

边界润滑摩擦面固体润滑涂层技术发展的若干趋势

随着表面工程与涂层技术的发展,固体润滑涂层在机械零件上得到了广泛的应用,涂层材料与涂层工艺取得了很大的进展。本文论述了边界润滑摩擦面固体润滑涂层技术发展的若干趋势,指出开发新的涂层材料、涂层结构、涂层工艺,实现固/液混合润滑,以及利用涂层激光纹理技术提高润滑效果是固体润滑涂层技术发展的重要方向。     

Antiwear Properties of Bonded MoS2 Solid Lubricant Coating under Dual-Rotary Fretting Conditions

Bonded MoS₂ solid lubricant coatings are widely used in tribology for their friction-reducing and antiwear properties. However, such coatings have been rarely investigated in complex fretting conditions, such as dual-rotary fretting (DRF). DRF is a complex fretting wear mode that combines torsional fretting with rotational fretting. In this work, the antiwear properties of bonded MoS₂ solid lubricant coating under dual-rotary fretting conditions were studied. Results indicated that the MoS₂ coating had better friction-reducing and antiwear properties than the substrate for alleviating DRF wear. The coating can greatly influence the fretting regimes and reduce the coefficient of friction. Furthermore, the service life of the coating was strongly dependent on the competition of the two fretting components and was reduced as the rotational fretting component increased.     

PTFE固体润滑膜研究进展

聚暇氟乙烯PTFE是广泛应用于机械工程中的润滑材料。文中综述了粘结型、复合镀型和Almitei型3种主要类型的PTFE固体润滑膜,分析了各自的成膜特征与作用机理,指出了PTFE固体润滑膜的主要发展方向。     

热喷涂宽温域耐磨自润滑涂层研究现状与展望

在航空、航天、冶金、电力、石化、矿采等领域,零件高温摩擦磨损特性是影响装备寿命的重要因素,在关重零部件表面设计制备宽温域耐磨自润滑涂层是装备零件强化改性和再制造修复的重要手段。首先阐述了宽温域耐磨自润滑涂层设计中过渡层、基础相、增强相的材料选择依据;其次针对单一固体润滑剂适用温度范围窄的问题,梳理了从低温润滑剂发生氧化反应原位生成高温润滑剂,低温润滑剂与高温润滑剂长时协同作用,添加抑制剂减缓润滑相的损耗退化等三种宽温域耐磨自润滑涂层材料设计方法;而后总结了宽温域耐磨自润滑涂层的制备工艺,分析了不同喷涂工艺的技术特点和涂层制备实例,介绍了宽温域耐磨自润滑涂层在军事装备和工业设备上的典型应用;最后在此基础上对宽温域耐磨自润滑涂层的未来发展方向进行了展望。     

高速重载下三种固体润滑膜耐磨性的实验研究

用销盘摩擦实验机研究三种固体润滑涂层；喷涂NF-1、粘结PEF及粘结Teflon-S涂层在高速（5.6m/s）、重载（初始接触应力400MPa)干摩擦条件下的摩擦磨损性能。结果表明：这三种涂都具有接近30min的使用寿命，具有较低的摩擦系数和磨损率，提高了抗咬合能力，基点NF-1和Teflon的摩擦系数从0.08分别增加到0.14和0.15。PEF涂层摩擦系数从0.1降低到0.08。     

Some Improvements in Solid Lubricants Coatings For High Temperature Operations

In order to satisfy the growing request in solid lubrication for high temperatures, new solid lubricants are being developed. In this field the “quasi” solid lubricants and the soft nonabrasive film-forming lubricants constitute two important families.

In the family of “quasi” solid lubricants an evaluation of lead monoxide-lead silicate coatings has been performed, particularly through the examination of the effect of minor addition of metallic materials, such as aluminum, iron and stainless steel, on the lubricant properties of coatings. These coatings are obtained through the melting and partial devitrification of lubricants. The lubricating properties of the coatings have been evaluated at different temperatures, with different load, with the rider wear and friction test.

The system PbO-SiO₂-Fe has shown the best properties (f = 0.2 at 650 C), and it may be utilized as a lubricant coating at temperatures to 650 C.

In the family of soft nonabrasive film-forming lubricants, an evaluation of CaF₂-base systems has been performed. In this case micrographic examinations, rider wear, and friction determinations have been carried out for evaluating the lubricating properties of the coatings.

For applications that foresee temperatures over a wide range, the CaF₂-BaF₂-A g system is the most promising for lubricant coatings (f = 0.32 at 25 C₁ f = 0.18 at 700 C); it may be used up to 800 C. For the temperature range 400–800 C, CaF₂-BaF₂ 60–40 w/o system may be utilized for its low friction coefficient (at 600–700 C, f = 0.20). Some improvements in solid lubricants coatings for high temperature operations are discussed.     

涂层材料热特性参数对点接触弹流润滑的影响

为探究涂层材料热特性参数对点接触弹流润滑的影响,选择3种不同方法制备的类金刚石（DLC）涂层和氧化锆陶瓷涂层,构建考虑涂层热特性的点接触弹流润滑模型,分析涂层材料、涂层厚度和润滑剂的流变性对接触区润滑性能的影响。结果表明：在弹流润滑状态下具有不同热特性的4种表面涂层导致了膜厚的差异,固体表面温度及润滑区温度场会随涂层热惯性变化;热惯性最小的DLC涂层加在快速运动表面能获得更高的膜厚;随着涂层厚度的增加,会引起固体表面的温度升高,使摩擦因数降低;非牛顿流体对压力、膜厚的影响很小,但与牛顿流体相比,能获得相对较低的温度。在弹流润滑状态下,涂层覆在快表面对于减小摩擦、提高膜厚是有益的。     

Influence of Polyfluo-Wax on the Friction and Wear Behavior of Polyimide/Epoxy Resin–Molybdenum Disulfide Bonded Solid Lubricant Coating

Polyimide/Epoxy resin–molybdenum disulfide bonded solid lubricant coatings (denoted as PI/EP-MoS₂) were prepared. The influence of polyfluo-wax (denoted as PFW) on the microhardness and friction and wear behavior of as-prepared PI/EP-MoS₂ lubricant coating was measured using a microhardness tester and a reciprocating ball-on-disc tribometer, respectively. The worn surfaces of the lubricant coatings were observed with a scanning electron microscope, and their wear rate was determined with a Micro XAM surface mapping microscope. Moreover, the transfer films formed on the counterpart steel ball surfaces were analyzed by X-ray photoelectron spectroscopy. Results indicate that the incorporation of a proper content of PFW filler is effective at improving the antifriction performance of the PI/EP-MoS₂ lubricant coating while maintaining better wear resistance. Moreover, the friction coefficient of the lubricant coating decreases with increasing content of PFW from 2 to 10%, and the one with a filler content over 6% PFW has a steady friction coefficient of 0.07. The improvement in the antifriction performance of the lubricant coating with the incorporation of the PFW filler is attributed to the excellent lubricity of homogeneously distributed PFW.     

The influence of lubricant composition on the lubricant breakdown in upsetting between overhanging dies

Upsetting experiments were conducted on aluminum alloy workpieces with a variety of lubricants. Measurements of the width of the unlubricated zone with a mineral oil were in good agreement with Wilson and Carpenter's theory. The addition of surface active materials to the oil did not affect the extent of the unlubricated zone but did reduce the forming load. The breakdown process was also observed with solid lubricant coatings.     

Sliding wear response of zinc based alloy as affected by suspended solid lubricant particles in oil lubricant

Abstract

Wear behaviour of a zinc based alloy has been studied in partially lubricated condition. The test environment comprised a mixture of oil plus graphite/talc particles. The composition of the lubricant mixture was varied by changing the concentration of the solid lubricant particles suspended in the oil lubricant. Wear response of the alloy was noted to improve in terms of decreased wear rate, frictional heating and friction coefficient initially with the increasing concentration of the solid lubricant particles suspended in the oil lubricant. A critical content of the solid lubricant led to the best wear performance of the samples. This was followed by a reversal in the trend at concentrations of the solid lubricant particles in the lubricant mixture that were greater than the critical one. Wear behaviour of the alloy has been substantiated through the characteristics of wear surfaces, subsurface regions and debris particles.     

Effect of low temperatures on the wear mechanism of solid lubricant coatings in vacuum

The conditions of transition to mild wear and the regularities of secondary structure formation in solid lubricant coatings of various compositions were studied using a variety of methods. It was found that the fatigue mechanism of fracture of these coatings is realized in the case of a high degree of texturing of the lamellar lubricant component within the active layer, owing to plasticization of the binder during sliding. The parameters of transition to a mild mode of wear are defined by the testing temperature and by the nature of the polymer binder.     

Ways of improving the tribological properties of steel gas-flame coatings

The paper reports the results of study of the structure and properties of steel gas-flame coatings. The effect of the formation of an abnormally great amount of retained austenite during the deposition of coatings from steel wire is found. Techniques for implementing the “austenite effect” to improve the wear resistance of the coatings are proposed. A method for introducing a solid lubricant into the coating during its deposition is developed. A possibility of enhancing the tribological properties of the coatings for friction members is shown. It is based on thermochemical treatment (nitrocarburizing) and tribomodification of the friction surfaces during running-in with a lubricant containing nanosized modifiers.     

Multilayered YSZ–Ag–Mo/TiN adaptive tribological nanocomposite coatings

Adaptive nanocomposite coatings provide low friction in broad ranges of environmental conditions through the formation of lubricious surfaces resulting from interactions with the ambient atmosphere. Designing adaptive coatings to withstand wear through repeated thermal cycles is particularly difficult since most demonstrate irreversible changes in surface composition and morphology. This permanent change in the condition of the surface limits the utility of adaptive coatings in applications where thermal cycling is expected. Moreover, some lubrication mechanisms occur over the entire coating surface in addition to the area experiencing wear, which results in a significant waste of lubricant. In an effort to increase the wear lifetime and move toward thermal cycling capabilities of solid adaptive lubricants, a multilayer coating architecture incorporating two layers of adaptive YSZ–Ag–Mo nanocomposite lubricant separated by a TiN diffusion barrier was produced. The multilayer coating lasted over five times longer than a monolithic adaptive coating of identical composition and total thickness for dry sliding tests at 500 °C in air. Analysis of the structure and composition of the films after heating suggests directed, lateral diffusion of lubricant beneath the diffusion barrier toward the wear scar as a mechanism for the increased wear life of the multilayer coating.     

Investigation of the tribological properties of the different textured DLC coatings under reciprocating lubricated conditions

In this study, we propose the use of laser surface texturing methods to improve the wear resistance of diamond-like-carbon (DLC) coatings. First, the application of dimples perpendicular to an engineering surface is introduced by using laser in a controlled manner. The solid DLC lubricant is subsequently deposited on the textured surface using the magnetron sputtering technique. In the experiments, the dimple densities were varied from 0 to 30%, and the dimple diameters were varied from 40 to 300 μm. The effect of the geometric parameters on the wear performance was studied using a reciprocating sliding-wear tester under oil lubrication conditions. The results showed that the DLC coatings with the appropriate dimple density (10%) and diameter (∼100 μm) demonstrated an obvious improvement in terms of the friction coefficient and wear rate compared with that of the un-textured DLC coatings. The experimental treatment produced respective reductions of 20% in friction and nearly 52% reduction of wear rate. This improvement could be explained by the action of reservoirs that enhance lubricant retention and trap the wear particles during sliding motions. In addition, the relationship between the dimple diameter, the contact width ratio and the wear performance is discussed.     

Low temperature tribology at the B. Verkin Institute for Low Temperature Physics & Engineering (historical review)

This review presents the history of creation and evolution of the Tribology Laboratory at Special Research & Development Bureau of the B. Verkin Institute for Low Temperature Physics & Engineering. It describes the low temperature tribometers designed at the laboratory, gives our understanding of the processes of protective secondary structure formation during friction providing higher wear resistance of materials under extreme frictional stressing conditions, reports the main results on the analysis of friction and wear in metals and alloys, solid lubricants and coatings and other engineering materials at low temperatures in vacuum and in low-pressure gases and also in various cryogenic liquids (liquid nitrogen, liquid hydrogen, liquid helium).     

Direct visualization of sliding-induced tribofilm on Au/MoS<Subscript>2</Subscript> nanocomposite coatings by c-AFM

The nanoscale lubrication mechanism of nanocomposite Au/MoS₂ solid lubricant coatings has been studied by conductive atomic force microscopy (c-AFM). A direct visualization of the lubricating process suggests tribomechanical formation of a MoS₂ tribofilm to be a key mechanism. The sliding-induced tribofilm formation was visualized by a reduction in local friction and conductivity in nanoscale AFM images. The tribofilm was found to possess considerable crystallinity and orientation, which was not observed in the as-deposited coatings. The observed mechanism is broadly applicable to a range of nanocomposite metal/MoS₂ coatings.     

Particle Generation by Solid-Lubricated Bearings and Ball Screws in Vacuum Environments©

Rolling bearings used for transporting wafers and liquid crystals in the film-forming process of semiconductor and LC panel manufacturing are required to minimize the particles (originating from both lubricant and bearing material) they generate while in operation. The research reported in this paper focuses on the particle generation under different rotational conditions of ball bearings and ball screws coated with various solid lubricants and operating in a vacuum environment. The results of the tests indicated that a specially composed and applied polytetrafluoroethylene (PTFE) coating was superior in minimizing particle generation by ball bearings and ball screws. Observation of the bearing interior surfaces indicated that particle generation is primarily caused by wear of cage-pocket surfaces, while sudden and large increases in wear are the result of the balls making direct surface-to-surface contact with the raceways and cages due to erosion of the solid lubricant coatings.