DLC coating of boundary lubricated components—advantages of coating one of the contact surfaces rather than both or none

The influence of conventional extreme-pressure (EP) and anti-wear (AW) additives on the wear and friction behaviour of DLC coatings has been investigated. Special emphasis was put on exploring if it is most beneficial to coat only one or both the contacting surfaces and on when and how the coatings may improve the friction situation in sliding contact boundary lubricated systems. Tests were performed in a load-scanning test rig, which allows the normal load to gradually increase during the forward stroke and to correspondingly decrease during the reverse stroke. The sliding speed was set to 0.1 m/s, while the normal load was in the range between 140 and 1700 N (2.4–5.6 GPa).This investigation indicates that, under boundary lubrication conditions, addition of commercial AW and EP additives to PAO oil may significantly improve the tribological properties of DLC coatings. Furthermore, the DLC/steel combination was found to give a smoother running-in process and a better tribological performance than the DLC/DLC and steel/steel combination.     

Extreme Pressure Lubricant Additives Interacting on the Surface of Steel- and Tungsten Carbide–Doped Diamond-Like Carbon

Certain diamond-like carbon (DLC) coatings offer excellent tribological properties under both dry and oil-lubricated sliding conditions. However, the underlying mechanisms under lubricated conditions are generally not fully understood, especially when performance depends on strong tribochemical interactions with lubricant additives. The aim of the present work is to explore the friction and wear performance of steel and tungsten carbide (WC)-doped DLC (WC-DLC) surfaces in the presence of different types of extreme pressure (EP) and nitrogen–sulfur-based (NS) additives. Tribological tests were performed on a ball-on-disc test rig, and X-ray photoelectron spectroscopy (XPS) was used for physical and chemical characterization of the tribofilms. It was observed that EP and NS additives significantly reduced the wear of WC-DLC surfaces in comparison with tests conducted on steel surfaces. XPS indicated that the additive interactions on the WC-DLC surface formed a distinctive tribofilm that promoted better friction and wear performance. The higher concentration of carbon compounds and lower concentration of oxygen compounds in the tribofilm significantly improved friction and wear characteristics.     

Assessment of lubricity of compositions of fuel oil with biocomponents derived from rape-seed

This paper presents an assessment of tribological properties of compositions of fuel oil containing fatty acid methyl esters derived from rape-seed oil. An HFRR (high-frequency reciprocating rig – ball-on-flat) apparatus was used in this investigation. The studies were carried out for a steel–steel pair in the concentrated contact.The tribological properties were determined by motion resistances, wear and lubricant film concentration. The surfaces were analysed after tribological tests. The aim of the investigation was an attempt to obtain a correlation between wear quantities being measured and percentage content of bioadditives in fuel oil. It was found that at as little as 5% of the additive the coefficient of friction decreases by about 20% while wear decreases twofold relative to fuel oil, which may result from an increase in the stability of the lubricant film.     

Influence of surface texture on boundary lubricated sliding contacts

The friction and wear behaviour of boundary lubricated sliding surfaces is influenced by the surface texture. By introducing controlled depressions and undulations in an otherwise flat surface, the tribological properties can be improved. Lubricant can then be supplied even inside the contact by the small reservoirs, resulting in a reduced friction and a prolonged lifetime of the tribological contact.In the present paper, well-defined surface textures were produced by lithography and anisotropic etching of silicon wafers. The wafers were subsequently PVD coated with thin wear resistant TiN or DLC coatings, retaining the substrate texture. The size and shape of the depressions were varied and evaluated in reciprocating sliding under dry and boundary lubricated conditions.     

A comparison of the tribological behaviour of steel/steel, steel/DLC and DLC/DLC contacts when lubricated with mineral and biodegradable oils

Diamond-like carbon (DLC) coatings, which can nowadays be applied to many highly loaded mechanical components, sometimes need to operate under lubricated conditions. It is reasonable to expect that in steel/DLC contacts, at least the steel counter body will behave according to conventional lubrication mechanisms and will interact with lubricants and additives in the contact. However, in DLC/DLC contacts, such mechanisms are still unclear. For example, the “inertness” of DLC coatings raises several questions about whether they are able to provide real boundary “lubrication” or whether they are just a “passive” member in these contacts. On the other hand, biodegradable oils, in particular vegetable base oils, possess a good lubricating ability, often much better than mineral or conventional synthetic oils as a result of the large amount of un-saturated and polar components that can promote the lubricity of DLC coatings. Accordingly, in this study, we present the results of experiments under severe boundary-lubrication conditions during reciprocating sliding. We look at the effect of the type of mating surfaces - steel/DLC, DLC/DLC and steel/steel - and the type of oil on the tribological performance of DLC coatings. We compare the wear and friction behaviours of two types of DLC coatings, i.e., a “pure” non-doped a-C:H DLC coating (denoted as a-DLC) and a WC-containing multilayer coating (denoted as W-DLC) tested with a mineral oil and a biodegradable vegetable oil. These oils, which have very different chemical compositions, were used as base oils and also with mild AW and strong EP additives. Among other things, the results confirm the following: (1) coating/coating lubricated contacts can resemble metal-lubrication mechanisms; (2) additives reduce wear in coating/coating contacts by up to 80%; (3) better wear and friction performance are obtained with oils that contain large amounts of polar and un-saturated molecules; (4) a coating/coating combination generally results in less wear than a steel/coating combination.     

Tribological Characterisation of Air-Sprayed Epoxy-CNT Nanocomposite Coatings

Epoxy-carbon nanotube (CNT) composites are promising coating materials for wear and corrosion critical applications such as marine bearings, shafts, bolts and gears. However, there are insufficient tribological data available for design. This article described the fabrication and tribological testing of an epoxy-CNT composite coating composed of a commercial epoxy primer and commercial CNT filler. The CNT filler was pre-treated so that it was compatible with epoxy resin and was dispersed using a ball milling process. A reciprocating sliding test rig was built for the measurement of friction and wear of the coatings which were subjected to multi-pass testing using the ball-on-plate sliding geometry. The rig allowed testing with either constant or varying normal force, along with measurement of normal and tangential forces. Thus, the coefficient of friction (COF) under ramping or constant normal force could be determined. Following testing, the samples were examined using an optical microscopy to determine the severity of any galling which had taken place. The coatings were found to display encouraging properties in all aspects of testing. COF values of around 0.2 were recorded under a nominal contact pressure up to 1 GPa. This coating can be used for components which require anti-corrosion and low friction properties.     

An investigation on tribological properties of graphite nanosheets as oil additive

Graphite nanosheets with average diameter of 500 nm and thickness about 15 nm were prepared by stirring ball milling. The tribological behavior of the graphite nanosheets as additive in paraffin oil were investigated with a four-ball and a pin-on-disk friction and wear tester. The wear surfaces of the steel ball lubricated with the additive-containing paraffin oil were analyzed by means of scanning electron microscopy (SEM). It has been found that the graphite nanosheets as additive in oil at proper concentration show better tribological properties than pure paraffin oil. The load-carrying capacity and antiwear ability of the lubricating oil were improved. Moreover, the friction coefficient of the lubricating oil was decreased by the addition of the graphite nanosheets. The optimal concentration of the additive in paraffin oil is about 1.0 × 10⁻² wt.%.     

The Combined Role of Soot Aggregation and Surface Effect on the Friction of a Lubricated Contact

One of the considered research paths to reduce friction loss consists in optimizing the interactions between surfaces and lubricants. The latter may significantly change with the lubricant ageing. In this framework, the tribological behaviour of aged formulated lubricant is analysed for various low-speed reciprocating motions and with different nature of surfaces. This paper focuses on soot aggregate formation processes in a lubricated contact and on their correlation to friction. Although no aggregates have been observed in pure rolling conditions, pure sliding conditions may lead to the appearance of aggregates moving through the contact as a function of the nature of the surfaces. The analysis of their displacement within the contact is used to discuss their interactions with the surfaces. Moreover, we show that the velocity and the dwell time of the aggregate depend on the sliding speed. The morphology of these aggregates evolves over time, affecting friction behaviour. An additive law combining a contribution from the shear of the aggregates with another one due to the shear of a thin lubricant film surrounding the aggregate is then proposed to interpret friction origin and friction evolution with time of shear. The aggregate motion also varies with the nature of the surfaces: in particular, DLC–DLC couple reduces aggregation phenomena and maintains a low friction without apparent wear.     

Compatibility of DLC coatings with formulated oils

In the present study, the tribological performance and compatibility of hydrogenated amorphous carbon coating (a-C:H) and metal-doped diamond-like carbon (DLC) coating (Me-C:H) with formulated oils under the boundary lubrication regime was investigated. The investigation employed ball-on-flat contact geometry in reciprocating sliding motion and six formulated oils (manual gearbox oil, automatic gearbox oil, hydraulic oil, compressor oil, and normal and high performance motor oil), with pure poly-alpha-olefin (PAO) oil used as a reference. In addition, DLC coatings behavior in diesel and gasoline fuel was evaluated.Compared with the uncoated steel surfaces a-C:H coatings give improved wear resistance in base PAO as well as in fully formulated oils and fuels. On the other hand, W-doped DLC coatings show the lowest steady-state friction under boundary lubrication, especially when using oils with high additive contents.     

Shear characteristics of a boundary film for a paper-based wet friction material: friction and real contact area measurement

A paper-based wet friction material lubricated with automatic transmission fluids (ATFs) produces plateau and positive slope regions in the friction–sliding velocity diagram. This feature observed at less than 1 mm/s of sliding velocity is discussed in this paper. The relationship between the friction and contact pressures of less than 5 MPa was determined by a reciprocating friction test rig lubricated with mineral oil including phosphoric acid ester, one of the additives for ATFs. The real contact area for dry condition was also determined by optical interferometry. The consistency of the real contact pressure dependency of the shear strength of the boundary film was indicated at pressures of less than 1 GPa obtained by steel-on-steel boundary friction (oiliness) test. This suggests that the generation of the positive slope region is due to thin film lubrication. To the plateau region, possibility of application of the phase transition model is considered.     

甘油润滑下CrCN涂层的超低摩擦特性

为开发与CrCN涂层具有良好配伍润滑性能的绿色润滑剂,使用磁控溅射技术在304不锈钢表面沉积CrCN涂层,利用场发射扫描电子显微镜、原子力显微镜、纳米压痕仪、维氏硬度计、X射线衍射仪、X射线光电子能谱仪分别对其表面形貌、涂层厚度、力学性能、物相组成以及元素化学价态进行分析,并借助多功能摩擦磨损试验机评价在甘油润滑下CrCN涂层的摩擦学性能,并与PAO6润滑下结果进行比较。利用磁控溅射技术在不锈钢表面构筑的CrCN涂层表面光滑致密,粗糙度仅为1.01 nm,硬度可达14.39 GPa。对比钢-钢和钢-CrCN体系的摩擦学性能发现,钢-CrCN体系在甘油润滑下展现出优异的润滑性能;当负载为0.5 N时,钢-CrCN体系在甘油润滑下的摩擦因数可低至0.01,大大低于PAO6润滑下的摩擦因数。对磨痕的XPS分析表明,在摩擦过程中,甘油发生摩擦化学反应,在CrCN涂层的接触表面生成一层FeOOH层,甘油分子及其降解产物可能进一步吸附在FeOOH层,形成流体润滑层,有效降低了摩擦和磨损。     

Tribological Performance of Halogen-Free Ionic Liquids in Steel–Steel and DLC–DLC Contacts

The tribological performance of halogen-free ionic liquids at steel–steel and diamond-like carbon (DLC)–DLC contacts was investigated. Hydrogenated amorphous carbon (a-C:H) and tetrahedral amorphous carbon (ta-C) were used as test specimens. Friction tests were carried out on steel–steel, a-C:H–a-C:H, and ta-C–ta-C contacts by using a reciprocating cylinder-on-disk tribotester lubricated with two different types of halogen-free ionic liquids: 1-ethyl-3-methylimidazolium dicyanamide ([BMIM][DCN]) and 1-butyl-3-methylimidazolium tricyanomethanide ([BMIM][TCC]). From the results of friction tests, the ta-C–ta-C tribopair lubricated with [BMIM][DCN] or [BMIM][TCC] exhibited an ultralow friction coefficient of 0.018–0.03. On the other hand, ultralow friction was not observed at the steel–steel and a-C:H–a-C:H contacts. Measurements obtained with a laser scanning microscope and an atomic force microscope (AFM) showed that a chemical reaction film, derived from the ionic liquid lubricant used, was formed on the steel surfaces. However, this chemical reaction film was not observed on either of the DLC surfaces. The AFM results showed that there were high-viscosity products on the ta-C surfaces, that the wear tracks on the ta-C surfaces exhibited low frictional properties, and that the ta-C surfaces were extremely smooth after the friction tests. Based on these results, it was concluded that an ionic liquid–derived adsorbed film formed on the ta-C surface and resulted in the ultralow friction when lubricated with a halogen-free ionic liquid.     

Effect of surface structure on frictional behaviour of a tongue/palate tribological system

The effect of surface structure on the friction and lubrication properties of a model tribological system representing the tongue/palate contact was investigated under dry conditions and in the presence of oil and aqueous solution having the same viscosity at mouth temperature. To this end, several soft silicone surfaces with well-defined surface structures, based on hemisphere pillars of different dimensions in the sub-millimetre range, were fabricated by a moulding technique in order to cover the different scales roughness of the human tongue. The friction experiments were performed on a reciprocating motion sliding tribometer involving contact between a relatively hard ball (steel or PCTFE) representing the palate and one of the soft silicone surfaces simulating the tongue. Test conditions were designed to represent those encountered in the mouth when thin films of food residues coat the oral mucosa surfaces. The results show that the frictional behaviour of the investigated model tribosystem is strongly affected by the topographical structure of the contacting surfaces. Under dry conditions, the coefficient of friction decreases significantly with increase of hemispherical pillar density. For lubricated surfaces, higher pillars with an optimal high density increase the friction coefficient. Further, it was observed that a minimal wetting of at least one contacting surface is essential for establishing effective lubrication.     

Tribological behaviour of carbon and low alloy steels: effect of mechanical properties and test conditions

Abstract

In this paper, the effects of mechanical properties and test conditions on the tribological behaviour of ISO C45 carbon steel and ISO 42CrMo4 low alloy steel were studied. The tribological tests were carried out, without lubrication, on a reciprocating friction tester. Cylinder on flat contact configuration was adopted. The results showed that there is no obvious relationship between the mechanical properties and the friction ones. However, the variation in the coefficient of friction depends on the test conditions. In contrary to normal load, the effect of sliding speed on the coefficient of friction is not the same for the two steel nuances. The tribological properties are dependent, however, on the nature of wear debris.     

Tribological performance of silicone composite coatings filled with wax-containing microcapsules

The effect of wax-containing microcapsules incorporated in silicone composite coatings deposited on aluminum (Al) alloy substrates on the tribological performance of the coatings was systematically investigated. The wax-containing microcapsules were prepared via in situ polymerization. The tribological behavior of the composite coatings was evaluated using ball-on-disk tribological test. It was found that the increase in microcapsule concentration in the composite coatings apparently reduced the friction coefficient of the coatings because the lubricant released from the broken microcapsules during the tribological test of the coatings lubricated the rubbing surfaces. The results showed that the silicone composite coatings rubbed by a smaller Cr6 steel ball (3 mm diameter) under a lower normal load (100 mN) produced higher friction coefficients via reduced complication of their underlying strong substrates compared to the same coatings tested against a larger Cr6 steel ball (6 mm diameter) under a higher normal load (1 N).     

Tribological behaviour of W‐alloyed carbon‐based coatings in dry and lubricated sliding contact

Carbon‐based coatings with different W contents were deposited by direct current magnetron sputtering in reactive and non‐reactive atmospheres. All deposited coatings have compact morphologies with amorphous (tungsten‐free) or nanocrystalline structures (tungsten‐doped). The latter one was indicated by very broad peaks in X‐ray Diffraction spectra in the position of tungsten carbide suggesting W‐carbide nanoparticles embedded in an amorphous carbon matrix. The hardness increased from 10 to 15 GPa with increasing W content. The coatings were tribological tested at dry and lubricated conditions with increasing temperature in a coating/steel configuration. In dry sliding, the friction coefficient increases with the increase of the temperature reaching values higher than 1.0. The friction is significantly lower in lubricated contact using three different oils: poly‐alpha‐olefin, paraffin and olive oil. The olive oil shows promising lubricating properties at the temperature lower than 70°C; however, at higher temperature, the coatings were quickly worn through. Copyright © 2014 John Wiley & Sons, Ltd.     

Coatings tribology—contact mechanisms and surface design

The fundamentals of coating tribology are presented by using a generalised holistic approach to the friction and wear mechanisms of coated surfaces in dry sliding contacts. It is based on a classification of the tribological contact process into macromechanical, micromechanical, nanomechanical and tribochemical contact mechanisms, and material transfer. The important influence of thin tribo- and transfer layers formed during the sliding action is shown. Optimal surface design regarding both friction and wear can be achieved by new multi-layer techniques which can provide properties such as reduced stresses, improved adhesion to the substrate, more flexible coatings and harder and smoother surfaces. The differences between contact mechanisms in dry, water- and oil-lubricated contacts with coated surfaces is illustrated by experimental results from diamond-like coatings sliding against a steel and an alumina ball. The mechanisms of the formation of dry transfer layers, tribolayers and lubricated boundary and reaction films are discussed.     

Tribological performance of α-Fe(Cr)-Fe2B-FeB and α-Fe(Cr)-h-BN coatings obtained by laser melting

α-Fe(Cr)-h-BN and α-Fe(Cr)-Fe₂B-FeB coatings on X30Cr13 stainless steel are synthesized by laser melting with incorporation of hexagonal boron nitride, or by alloying of boron. The additive powders are deposited on steel before pulsed irradiation by Nd-YAG laser beam. The solidification structures of the obtained coatings are investigated by optical microscopy and X-ray diffractometry. The mechanical properties are investigated by nanoindentation and the tribological behaviour is characterized on pin-on-disc tribometer, under dry-sliding conditions with different loads and a temperature range 25–500 °C. h-BN-α-Fe(Cr) and Fe₂B-α-Fe(Cr) coatings have average hardnesses 10.0 and 14.5 GPa, respectively, while hardness of untreated stainless steel is 4.2 GPa. In comparison with this untreated steel, the sliding contact on ceramic (ruby) of such coating shows a lower coefficient friction and a definitively better wear resistance.     

Tribological behaviour of steel‐alumina sliding pairs under boundary lubrication conditions

The tribological behaviour of oil‐lubricated steel‐alumina sliding pairs was investigated using a ball‐on‐disc tribometer at room temperature. Commercial bearing balls of 10 mm diameter were mated to 99.7% Al₂O₃ discs, and additive‐free mineral oil was fed into the contact area. The sliding speed and the applied normal load were varied, and the initial surface roughness of the Al₂O₃ disc was altered using different polishing and grinding procedures. The results showed that the surface roughness of the ceramic discs dominated the tribological behaviour under the given experimental conditions. The sliding speed as well as the normal load showed less effect on the friction behaviour, but the amount of wear depended strongly on the normal load. From the results it was concluded that improvement of the surface roughness and optimised surface machining of the ceramic material can be essential for improving the tribological performance for boundary‐lubricated steel‐ceramic sliding pairs.     

Friction and wear of blended polyoxymethylene sliding against coated steel plates

Measurements were made of the dynamic friction coefficients and specific wear rates of several thermoplastics rubbing against relatively soft coatings on steel plates. Polyoxymethylene (POM)-based composites were investigated using reciprocating, line contact tests against two types of corrosion-protected steel plates (electro-deposited cathodic epoxy layers, called “E-coatings”, and galvanised plates). In addition to virgin POM, composites containing glass fibres, polytetrafluoroethylene (PTFE) fibres, PTFE micro-powder, and high-viscosity silicon oil were investigated. Sliding speeds ranged from 0.05 to 0.3 m/s, and normal loads ranged from 5 to 30 N. The E-coating failed at high loads and velocities. The beneficial effects of lubricating additives in tests with uncoated steel counterfaces were also observed with the coated steel surfaces. POM with glass fibre additives was found to be more abrasive than the base material. The considered non-conformal contact produced similar friction and wear trends than those obtained for the conformal contact.