Construction of phase diagram up to 2 GPa and 200°C for thermo‐reversible gel lubricants by diamond anvil cell

The objective of this research was to make the phase diagram up to 2.0 GPa and 200°C of thermo‐reversible gel lubricants (TR Gel‐Lube) using diamond anvil cell (DAC). TR Gel‐Lube consisted of base fluid and gelling agent. The most important characteristic of TR Gel‐Lube is the reversible change of its physical state. The DAC is able to concerning visual observation of the sol/gel and liquid/viscoelastic–solid transition at high pressure and high temperature. DAC enables the visual observation of the photoelastic effect in order to generate local principal stress difference under the compression field for liquid/viscoelastic‐solid transition of base oil poly‐α‐olefin. The direct observation for sol/gel transition of TR Gel‐Lube at high pressure was also made possible. The phase diagram corresponding to transition from liquid to viscoelastic solid for base oil poly‐α‐olefin and sol to gel for TR Gel‐Lube was made by using DAC. Copyright © 2009 John Wiley & Sons, Ltd.     

Bearing Fatigue Life Tests of Two Advanced Base Oils for Space Applications Under Vacuum and Atmospheric Environments

Four series of rolling-element bearing fatigue tests were conducted with 51104 size thrust ball bearings with three balls made from SUJ2 (AISI 52100) steel lubricated with two advanced synthetic base oils used for space applications. The test lubricants were perfluoropolyether (PFPE) and multiply alkyated cyclopentane (MAC). Each oil was tested with bearings under vacuum and atmospheric environments. The bearings were tested at a maximum Hertzian stress of 4 GPa on the inner and outer races. The outer race was rotated at a speed of 250 rpm. A pool lubrication system was used. Fresh lubricant was used for each test bearing. Testing in vacuum conditions was at 5 × 10^?2 Pa. The test oils were analyzed to determine whether changes occurred as a result of operating in air and in a vacuum. In a vacuum environment, the PFPE 815Z oil exhibited a longer fatigue life than the MAC 2001A oil. However, in an air environment, the MAC 2001A oil had a longer L₁₀ fatigue life than the PFPE 815Z oil. The fatigue life tests of PFPE 815Z oil in vacuum resulted in a longer L₁₀ life than when tested in an air environment. In an air environment, hydrogen fluoride was generated in the bearing tests with the PFPE 815Z oils. Under vacuum conditions, hydrogen fluoride was not generated with the PFPE 815Z oil, resulting in longer bearing fatigue lives. The fatigue life tests of MAC 2001A oil in a vacuum resulted in shorter L₁₀ fatigue life than in an air environment. The shorter life was attributed to the lower elastohydrodynamic oil film formation with the MAC 2001A oil because of a higher operating temperature and decomposition of the oil in vacuum.     

Rolling Bearing Performance and Film Formation Behavior of Four Multiply Alkylated Cyclopentane (MAC) Base Greases for Space Applications

Four multiply alkylated cyclopentane (MAC)-based greases, used for space applications, were used for rolling contact fatigue tests employing thrust ball bearings. These greases were R2000, ML, MU, and 5200 and were made from the same base oil 2001A. Each grease had a different thickener, which were sodium soap, Li soap, urea, and polytetrafluoroethylene (PTFE), respectively. The effect of the grease thickener on the ball bearings' fatigue life in vacuum and atmospheric environments was investigated via testing. For the greases R2000, MU, and ML it was found that the oil film formation in the experiments performed under vacuum conditions declined remarkably in comparison to under atmospheric conditions. For the 5200 grease, the oil film formation was almost the same as under atmospheric conditions. These results were supported by the occurrence of cage wear in the ball bearing fatigue tests. In the absence of sufficient films, high wear rates were measured.     

Evaluation of Ion-Sputtered Molybdenum Disulfide Bearings for Spacecraft Gimbals

High-density, sputtered molybdenum disulfide films (MoS₂) were investigated as lubricants for the next generation of spacecraft gimbal bearings where low torque signatures and long life are required. Low friction in a vacuum environment, virtually no out-gassing, insensitivity to low temperature, and radiation resistance of these lubricant films are valued in such applications. One hundred and twenty five thousand hours of accumulated bearing lest time were obtained on 24 pairs of flight-quality bearings ion-sputtered with three types of advanced MoS₂ films. Life tests were conducted in a vacuum over a simulated duty cycle for a space pay bad gimbal. Optimum retainer and ball material composition were investigated. Comparisons were made with test bearings lubricated with liquid space lubricants.

Self-lubricating PTFE retainers were required for long life, i.e., > 40 million gimbal cycles. Bearings with polyimide retainers, silicon nitride ceramic balls, or steel balls sputtered with MoS₂ film suffered early torque failure, irrespective of the type of race-sputtered MoS₂ film. Failure generally resulted from excess film or retainer debris deposited in the ball track which tended to jam the bearing. Both grease lubricated and the better MoS₂ film lubricated bearings produced long lives, although the torque with liquid lubricants was lower and less irregular.     

Lubricating properties of TR Gel-lube—Influence of chemical structure and content of gel agent

Tribological properties of a new and unique thermo-reversible Gel-Lubricant (TR Gel-lube) have been investigated. TR Gel-lube that includes an amide-type gel agent in base fluid is repeatedly able to change from gel state to liquid state at the melting point of its gel agent. Very low friction coefficient is indicated because of adsorption on a metal surface, and the gel agent works as an oiliness agent. In this study, the correlation between the lubricating property and the gel agent's characteristics (chemical structure, content, etc.) was clarified. DSC tester was applied to evaluate the stability of the gel state by change of entropy and the thermodynamic consideration was examined. In addition, lubricating properties under practical conditions such as at high temperature and in the presence of water were proved.     

Atomic Force Microscopy and Raman Spectroscopy Investigation of Additive Interactions Responsible for Anti-Wear Film Formation in a Lubricated Contact

Rational formulation of lubricants requires an understanding of additive interactions that impact antiwear film qualities such as thickness, topography, and friction. In an effort to understand the complex additive interactions responsible for formation of anti-wear and friction-reducing films, atomic force microscopy (AFM) in conjunction with Raman microscopy has been used to conduct a nanoscale investigation of the wear tracks formed by a high-frequency reciprocating rig (HFRR) in the presence of various commercial lubricant additives combinations. Of the additives examined, zinc dithiophosphate (ZnDTP)-based additives are found to be solely responsible for the formation of a thick (hundreds of nm) film that exhibits a pitted topography. Addition of a molybdenum-based friction modifier to the lubricant blend reduces the film thickness considerably and reacts to produce MoS ₂ on the surface, suggesting an interaction with the zinc dithiophosphate–based additive that prevents antiwear film formation. Formation of MoS ₂ , found only in the wear track, is consistent with a dramatic reduction of friction coefficient measured in the HFRR. Subsequent addition of borated dispersants to the lubricant reveals a further reduction in friction coefficient and a modest return of anti-wear film. However, addition of detergents to the formulation increases the friction coefficient and also promotes the formation of an anti-wear film. Nanoindentation measurements on the bulk properties of the anti-wear films determined that all of the anti-wear films had similar modulus and hardness measurements which were lower than that of the parent steel material, but did not correlate with the friction measurements obtained from the HFRR. This indicates that nanoscale measurements on material properties of the film are necessary to elucidate friction properties of the interface, and that these properties cannot be determined from macroscale measurements on the bulk film.

     

A study of the tribological behaviour of heterocyclic derivatives as additives in rape seed oil and mineral oil

The tribological properties of two novel heterocyclic derivatives as additives in rape seed oil and in mineral oil were investigated using a four‐ball machine. The morphologies and chemical features of the worn steel surfaces were observed and examined by means of X‐ray photoelectron spectroscopy and scanning electron microscopy. Both heterocyclic compounds improved the load‐carrying and extreme‐pressure capacities and the friction‐reducing and antiwear properties of rape seed oil more than they improved those of the mineral oil. Surface analyses of the rubbed surfaces revealed the formation of a protective film containing FeSO₄, organosulphur compound, FeS₂, polymerised ester, and organonitrogen compound when the surfaces were lubricated by rape seed oil containing the additives. The protective film formed during sliding processes contributed to the increase in the wear resistance.     

The tribological behavior of thiophosphates as additives in rapeseed oil

Two kinds of thiophosphate, tri-n-octyl thiophosphate and tri-n-octyl tetrathiophosphate, were synthesized. The tribological behavior of the synthetic compounds and tricresyl phosphate as additives in rapeseed oil for steel-steel frictional pair was investigated with a four-ball machine. The relationship between the additive structure and tribological properties was explored, while the lubrication mechanisms of the additives were investigated as well. Thus the worn surfaces of the steel ball lubricated with the additive-containing rapeseed oil were analyzed by means of X-ray photoelectron spectrometry and scanning electron microscopy, and the elemental compositions and distributions in the worn steel surfaces were observed and determined with an EPMA-810Q electron probe micro-analyzer. It has been found that the synthetic thiophosphates as additives in rapeseed oil at proper concentrations show better tribological properties than tricresyl phosphate. Tribochemical reactions were involved for steel–steel frictional pair lubricated with the rapeseed oil containing thiophosphates, with the formation of a boundary lubricating and protecting film composed of glyceride of rapeseed oil and tribochemical reaction products of the lubricants. This contributes to improve the tribological properties of the rapeseed oil base stock.     

Preparation and tribological properties of the N‐containing heterocyclic borate esters and Cu microparticles as lubricant additives in base oil

In this paper, four kinds of N‐containing heterocyclic borate esters and polyvinyl pyrrolidone‐protected Cu microparticles were synthesised and characterised. Their tribological properties as lubricant additives in industrial white oil were evaluated using a four‐ball tribometer, and their lubrication mechanisms were investigated by X‐ray photoelectron spectroscopy. The results showed that the anti‐friction and anti‐wear performance of the base oil can be significantly improved by the addition of N‐containing borate esters and Cu microparticles, and they present synergistic tribological effect. Moreover, X‐ray photoelectron spectroscopy results showed a lubricating tribochemical reaction film containing B₂O₃, FeB, FeO, Fe₂O₃ and so on is formed on the worn surface. In addition, Cu microparticles as rolling bearings, which transform sliding friction to rolling sliding and the formation of the Cu microparticles deposited film, are probably responsible for the improvement of tribological performance. Copyright © 2017 John Wiley & Sons, Ltd.     

Effect of Molecular Weight Distribution of Mineral Oils on Life of Thrust Ball Bearings

In order to investigate the effect of molecular weight distributions of mineral oils on bearing life, the authors carried out systematic tests using thrust ball bearings, type 51104.

Besides the general trend of increasing life with viscosity, blended oils characterized by wide-ranged molecular weight distributions exhibited longer life compared with base oils with narrow-ranged distributions corresponding to the same viscosity grade. These results were obtained by estimating states of oil film and oxide film formations, and furthermore, by measuring coefficients of friction, oil temperatures, and viscosity at the glass transition points.     

铜型添加剂摩擦修复作用的可行性研究

通过实验室实验和实际应用试验考察了铜型添加剂的摩擦修复作用,结果表明：铜型添加剂在摩擦条件下可在表面生成单质铜,随着负荷和温度升高,铜微粒、铁的磨损微粒、铜铁合金相互熔合扩散共晶焊接在摩擦面上形成修复膜。对梯级负荷１６００Ｎ时修复膜的厚度进行了估算,并建立了修复膜的作用模型。     

Friction Characteristics and Topography of Tribofilms from Anti-Wear Additives Applied to Metal V-Belt Type CVT Fluids

This study has investigated potential links between tribological performance and the morphology of tribofilms formed from anti-wear additives with application to metal V-belt pushing type continuously variable transmission fluids (B-CVTFs). The influence of metal–metal tribological properties of anti-wear additives was evaluated using a ball on plate tribometer, enabling friction and lubricant film formation to be monitored during reciprocating tests. Contact mode atomic force microscopy was utilised to investigate the nature of tribofilms at the nanometre scale. As a result, an additive formulation composed of hydrogen phosphite and over-based calcium sulphonate in a hydro-cracked mineral Group II base oil demonstrated a synergism with 8% higher friction coefficient and more stable film formation than the individual additive cases, providing a positive outcome for a B-CVTF. Tribofilm species produced by a chemical reaction between hydrogen phosphite and over-based calcium sulphonate were densely deposited on the wear scar to form a rougher surface, which may explain the higher friction observed.     

滚珠型弧面凸轮机构的摩擦学设计

在滚珠型弧面凸轮机构的设计中引入摩擦学设计方法,推导出钢球和凸轮滚道接触的最小油膜厚度计算公式。为了保证机构的润滑状态,采用膜厚比进行润滑状态的判断,并以此作为设计准则。同时为了防止油膜温升导致润滑失效,计算出润滑油膜的最大温升,利用润滑油的黏温特性,计算出油膜厚度降低系数,对常温下油膜厚度进行修正。在设计中兼顾强度设计和压力角校核,满足机构的使用性能,减少摩擦和磨损。     

Wear and friction behaviour of CaCO3 nanoparticles used as additives in lubricating oils

For some years, reports have been published on adding solid lubricant powder to oil to improve the tribological properties of the latter, but the results have not been satisfactory. In this paper, we describe the preparation of CaCO₃ nanoparticles in a microemulsion consisting of sodium dodecyl‐sulphate (SDS)/isopentanol/cyclohexane/water, and assessment of the tribological behaviour of CaCO₃ nanoparticles as additives for lubricating oils. The CaCO₃ nanoparticles were characterised by transmission electron microscopy (TEM), and their tribological performance was tested in a four‐ball machine; the rubbing surface was analysed with X‐ray photoelectron microscopy (XPS). The results indicate that the size of CaCO₃ nanoparticles increased with the concentration of aqueous reactant, and that CaCO₃ nanoparticles exhibited good load‐carrying capacity, antiwear and friction‐reducing properties. The tribological properties of lubricating oils could be improved significantly by dispersing CaCO₃ nanoparticles in 500SN base oil containing dispersants such as polyisobutene‐butanediimide (T154), calcium alkylsulphonate (T101) and methyl‐tricaprylamine chloride (aliquat 336). The improvements in friction and wear were concluded to be due to the formation of a film containing CaCO₃ and CaO in the rubbing region, and the presence of nanoparticles, which may act in the same way as ball bearings, to facilitate sliding.     

Tribochemical effects on the friction and wear behaviors of diamond-like carbon film under high relative humidity condition

Friction and wear behaviors of diamond-like carbon (DLC) film in humid N₂ (RH-100%) sliding against different counterpart ball (Si₃N₄ ball, Al₂O₃ ball and steel ball) were investigated. It was found that the friction and wear behaviors of DLC film were dependent on the friction-induced tribochemical interactions in the presence of the DLC film, water molecules and counterpart balls. When sliding against Si₃N₄ ball, a tribochemical film that mainly consisted of silica gel was formed on the worn surface due to the oxidation and hydrolysis of the Si₃N₄ ball, and resulted in the lowest friction coefficient and wear rate of the DLC film. The degradation of the DLC film catalyzed by Al₂O₃ ball caused the highest wear rate of DLC film when sliding against Al₂O₃ ball, while the tribochemical reactions between DLC film and steel ball led to the highest friction coefficient when sliding against steel ball.     

Tribological properties of ultra-thin ionic liquid films on single-crystal silicon wafers with functionalized surfaces

Two kinds of room temperature ionic liquid (RTIL) films carrying vinyl and hydroxyl functional groups were prepared on single-crystal Si wafers by spin coating. The tribological properties of the RTIL films sliding against AISI-52100 steel ball and Si₃N₄ ball in a ball-on-plate configuration were investigated on a dynamic–static friction coefficient measurement apparatus, using perfluoropolyether (PFPE) film as a comparison. The tribological behaviors of the ionic liquid films sliding against the same counterparts at extended test durations were also evaluated using a universal UMT-2MT test rig. The morphologies of the wear tracks of the RTIL films and the counterparts were examined using a scanning electron microscope equipped with an energy-dispersive X-ray analyzer attachment. It was found that the tribological performances of the ionic liquid films were closely related to the chemical structures of the RTILs and the chemical characteristics of the substrate surfaces. The films of vinyl group functionalized ionic liquids on hydroxylated substrate and vinyl group modified substrate exhibited very good friction-reduction and wear-resistant properties. It was assumed that there were enough strong forces between the films and substrate in these cases, and the ionic liquid molecules maintained good flexibility simultaneously. The films on hydrogen-terminated and methyl-terminated substrate showed poor tribological performance, which could be related to the relatively weak forces between the films and substrates. Moreover, the films on hydroxylated substrate showed lower friction at higher sliding velocities, which was assumed to be governed by the more rapid adsorption of the ionic liquid molecules on the steel ball at a higher sliding velocity. In addition, the ionic liquid films also had excellent tribological properties as they slid against silicon nitride ball. Therefore, it was supposed that the ionic liquid films could be used as a kind of universal lubricant for various combinations of the frictional pair.     

Functional mechanism of formation of friction polymer with dihydroxydocosanoic acid

The results of wear tests with a four-ball machine showed that dihydroxydocosanoic acid has good antiwear properties, comparatively approaching those of zinc dialkyldithiophosphate (ZDDP): the four-ball wear-scar diameters, d 30 min/294 N, with dihydroxydocosanoic acid, ZDDP, and 400 neutral base oil are 0.36, 0.35, and 0.62 mm, respectively. In order of effectiveness the antiwear ability of the C₂₂ acids is: 13,14-dihydroxydo-cosanoic acid (I) 13(14)-mono-hydroxydocosanoic acid (II) docosanoic acid (III), showing that their antiwear properties are remarkably improved after introduction of the hydroxy group into fatty acids. Work on dihydroxy acid as an antiwear agent is not much reported in the literature up to now. In the present work, it was verified with infrared spectroscopy that polyester is formed on the rubbing surfaces with I and II by tribochemical reaction. An oxygen-rich protective film on the rubbing surfaces was identified with Auger Electron Spectroscopy analysis. These results are identical to the high oxygen-containing polyester film shown by IR analysis. It is preliminarily confirmed that the functional antiwear mechanism is, principally, the formation of a polymeric friction film on the rubbing surfaces.     

Self-assembled monolayer protective films for hybrid sliding contacts

Abstract

Silicon nitride as an energy efficient material is replacing conventional steels for new generation engineering components such as bearings, cutting tools, electronics and engine parts in automotive, aerospace and wind industries. Compared with steel bearings, silicon nitride bearings can be operated at much higher temperatures and speeds with >60% weight reduction and up to 80% friction reduction. These are all due to its unique material properties, including high wear and corrosion resistance, low density and heat generation. Current lubrication solutions for hybrid contacts, where silicon nitride balls and steel races are used, are mostly relying on the protection film formed on the metal surfaces. Self-assembled monolayers (SAMs) have been found very useful in modifying surfaces, especially for microelectromechanical system and nanoscale applications, e.g. atomic force microscopy tips, etc. This study aims to investigate the feasibility of forming a SAM protection film on industrial grade bearing material silicon nitride to reduce the friction for the oil lubricated hybrid contacts. Four silanes with different functional head groups, including octadecyltrichlorosilane (OTS), octyltrichlorosilane, chlorodimethyloctadecylsilane and octadecyltrimethoxysilane, were initially investigated to form SAMs on industrial grade silicon nitride surfaces. The effects of concentration and immersion time of the silanes on the formation of SAMs on the silicon nitride surface were evaluated using contact angle measurements. The preliminary results show that the wetting properties of the silicon nitride surface can be effectively modified by the formation of SAMs from the silane solutions. OTS can form an order and compact SAM on the silicon nitride surfaces within 2 min at the concentration of 2··5 mM in decane solution, while the other three alkylsilanes can also effectively modify silicon nitride surfaces given sufficient immersion time, e.g. over 1 h. Tribological tests were subsequently carried out on a ball on disc rig where a steel ball and a silicon nitride disc were used. The effect of the formation of alkylsilane SAMs on the friction between the sliding contacts has been evaluated in two different methods. The first method was to test preformed SAM films under dry conditions, and the second was to premix one of the surfactants with Shell Vitrea ISO 32 mineral base oil and then spray the mixture to the contacts during the ball on disc testing. The test results show that an average of over 40 and 30% friction reduction was achieved for the hybrid contact when lubricated with the base oil mixed with OTS (>2··5 mM) and octadecyltrimethoxysilane (5 mM) respectively compared with that of the sliding contact lubricated by the base oil only. Since OTS may produce corrosive byproducts during SAM formation, octadecyltrimethoxysilane may be a more suitable additive for the hybrid contacts.     

石墨层间化合物的成膜性能及耐压抗磨性

采用南墅小鳞片石墨(<4μm)进行溶剂法插入FeCl_3,得到石墨氯化铁三阶层间化合物(GIC)。以其在石蜡油中滚涂和在空气中擦涂于不同钢底材上,用动静摩擦系数精密测定仪等考察了钢表面上的GIC成膜性、并在四球试验机等摩擦磨损试验机上对GIC膜进行了极压性和耐磨性能的评价,给出钢表面上形成一定厚度和GIC极压膜和润滑膜可使承载能力和耐磨寿命提高。     

Effects of Ethanol Contamination on Friction and Elastohydrodynamic Film Thickness of Engine Oils

The use of ethanol as engine fuel has increased for environmental reasons, both in flex-fuel engines and as increasing amounts of ethanol blended with gasoline in conventional engines. This article describes an investigation into the effects of ethanol contamination of lubricants during engine use with ethanol fuel. To facilitate this, a new technique was developed to measure small amounts of ethanol in lubricants. Elastohydrodynamic film thickness measurements and Stribeck curves were obtained for Group I base and formulated oils containing small added amounts of ethanol. The effect of the water present in hydrated ethanol was evaluated by carrying out tests using both hydrated and anhydrous ethanol. Measurements were also carried out using a Group II base oil with added ethanol. These measurements showed that in the low entrainment speed region, where the elastohydrodynamic film is very thin so that boundary lubrication prevails, the addition of ethanol produced a boundary film, which was not present for the base oils. By contrast, the addition of ethanol to formulated oil reduced film thickness in all lubrication regimes. The friction tests showed friction reduction due to addition of ethanol to the base oil, in particular at low speeds. For the formulated oil, ethanol reduced friction at high speeds, which was associated with a reduction in the viscosity of the lubricant, but at low speeds, ethanol reduced the formation of a boundary layer, increasing friction. The presence of water in hydrated ethanol did not significantly change the film thickness and friction when compared with anhydrous ethanol.