Publications
30 results found
Song W, Campen S, Shiel H, et al., 2024, Position of carbonyl group affects tribological performance of ester friction modifiers, ACS Applied Materials and Interfaces, Vol: 16, Pages: 14252-14262, ISSN: 1944-8244
The tribological properties of lubricants can be effectively improved by the introduction of amphiphilic molecules, whose performance is largely affected by their polar head groups. In this work, the tribological performance in steel–steel contacts of two isomers, glycerol monostearate (GMS) and stearyl glycerate (SG), a glyceride and a glycerate, were investigated as organic friction modifiers (OFM) in hexadecane. SG exhibits a much lower friction coefficient and wear than GMS despite their similar structures. The same applies when comparing the performance of oleyl glycerate (OG) and its isomer, glycerol monooleate (GMO). Surface chemical analysis shows that SG forms a polar, carbon-based, tribofilm of around tens of nanometers thick, while GMS does not. This tribofilm shows low friction and robustness under nanotribology test, which may contribute to its superior performance at the macro-scale. The reason for this tribofilm formation can be due to the stronger adsorption of SG on the steel surface than that of GMS. The tribofilm formation can be stress-activated since lower friction and higher tribofilm coverage can be obtained under high load. This work offers insights into the lubrication mechanism of a novel OFM and provides strategies for OFM design.
Zhang J, Bolle B, Wong J, et al., 2024, Influence of atmosphere on carbonaceous film formation in rubbing, metallic contacts, Tribology Letters, Vol: 72, ISSN: 1023-8883
Many previous researchers have reported the formation of carbonaceous tribofilms from organic lubricants on rubbing metallic surfaces. This paper shows that a very important factor in the formation of such tribofilms is the presence or absence of molecular oxygen. When steel surfaces are rubbed in saturated hydrocarbon lubricants in the absence of oxygen, for example in nitrogen or hydrogen gas, carbonaceous films form very readily,resulting in low friction and wear. However, when a significant amount of oxygen is present, as is the case in air, carbonaceous tribofilms are not generally formed, so friction and wear are very high, with values comparable to those seen when no lubricant is present.In-situ Raman analysis combined with gas switching experiments shows that the carbonaceous films formed during rubbing when no oxygen is present are rapidly removed during rubbing in air, while tests in which lubricant is removed during a test in N2 indicate that the films are quite weak. This suggests that these carbonaceous films are being continually removed and replenished during rubbing in oxygen-free conditions.It is proposed that these carbonaceous films are formed from hydrocarbyl free radicals that are generated mechanochemically from hydrocarbon molecules during rubbing. In the absence of oxygen, these free radicals then react together to form a carbonaceous film.However, when oxygen is present, the hydrocarbyl free radicals react extremely rapidly with oxygen molecules to produce hydroperoxyl free radicals and so are no longer available to generate a carbonaceous tribofilm.
Zhang J, Wheatley A, Pasaribu R, et al., 2023, Wind turbine lubrication: low temperature fretting wear behaviour of four commercial greases, Tribology International, Vol: 187, ISSN: 0301-679X
Fretting tests on four commercial greases were run on a ball-on-disc machine at 25, 7, − 20, − 40 °C. Post-test wear was measured on the ball and the chemical composition of lubricant films in the wear scar analysed by FTIR (RA-IRS), Raman and SEM-EDS. At 25 and 7 °C the greases had similar friction and wear properties. At − 20 and − 40 °C the highest base-oil viscosity grease gave an inferior friction and wear performance. RA-IRS analysis showed thickener remaining in the wear scar for most test conditions. The highest wear was linked to the absence of thickener in the scar. Better wear and friction performance at low temperatures was associated with thickener present in the lubricated contact and lowest base-oil viscosity.
Song W, Zhang J, Campen S, et al., 2023, Lubrication mechanism of a strong tribofilm by imidazolium ionic liquid, Friction, Vol: 11, Pages: 425-440, ISSN: 2223-7690
Friction modifiers are surface-active additives added to base fluids to reduce frictionbetween rubbing surfaces. Their effectiveness depends on their interactions with rubbingsurfaces and may be mitigated by the choice of the base fluid. In this work, theperformance of an imidazolium ionic liquid (ImIL) additive in polyethylene-glycol (PEG)and 1,4-butanediol for lubricating steel/steel and diamond-like-carbon/diamond-likecarbon (DLC-DLC) contacts were investigated. ImIL containing PEG reduces frictionmore effectively in steel-steel than DLC-DLC contacts. In contrast, adding ImIL in1,4-butanediol results in an increase in friction in steel-steel contacts. Results fromRaman spectroscopy, XPS and FIB-TEM reveal that a surface film is formed on steelduring rubbing in ImIL containing PEG. This film consists of two layers. The top layer iscomposed of amorphous carbon and are easily removed during rubbing. The bottom layer,which contains iron oxide and nitride compound, adheres strongly on the steel surface.This film maintains its effectiveness in a steel-steel contact even after ImIL additives are2depleted. Such film is not observed in 1,4-butanediol where the adsorption of ImIL ishindered, as suggested by QCM measurements. No benefit is observed when the basefluid on its own is sufficiently lubricious, as in the case of DLC surfaces.This work provides fundamental insights on how compatibilities among base fluid,friction modifier and rubbing surface affect performance of IL as surface active additives.It reveals the structure of an ionic liquid surface film, which is effective and durable. Theknowledge is useful for guiding future IL additive development.
Zhang J, Yu M, Joedicke A, et al., 2022, Characterising the effects of simultaneous water and gasolinedilution on lubricant performance, Tribology International, ISSN: 0301-679X
Yang X, Liu H, Dhawan S, et al., 2022, Digitally-enhanced lubricant evaluation scheme for hot stamping applications, Nature Communications, Vol: 13, ISSN: 2041-1723
Digitally-enhanced technologies are set to transform every aspect of manufacturing. Networks of sensors that compute at the edge (streamlining information flow from devices and providing real-time local data analysis), and emerging Cloud Finite Element Analysis technologies yield data at unprecedented scales, both in terms of volume and precision, providing information on complex processes and systems that had previously been impractical. Cloud Finite Element Analysis technologies enable proactive data collection in a supply chain of, for example the metal forming industry, throughout the life cycle of a product or process, which presents revolutionary opportunities for the development and evaluation of digitally-enhanced lubricants, which requires a coherent research agenda involving the merging of tribological knowledge, manufacturing and data science. In the present study, data obtained from a vast number of experimentally verified finite element simulation results is used for a metal forming process to develop a digitally-enhanced lubricant evaluation approach, by precisely representing the tribological boundary conditions at the workpiece/tooling interface, i.e., complex loading conditions of contact pressures, sliding speeds and temperatures. The presented approach combines the implementation of digital characteristics of the target forming process, data-guided lubricant testing and mechanism-based accurate theoretical modelling, enabling the development of data-centric lubricant limit diagrams and intuitive and quantitative evaluation of the lubricant performance.
Zhang J, Ewen J, Spikes H, 2022, Substituent effects on the mechanochemical response of zinc dialkyldithiophosphate, Molecular Systems Design & Engineering, Vol: 7, Pages: 1045-1055, ISSN: 2058-9689
Mechanochemistry is known to play a key role in the function of some lubricant additives, such as the tribofilm growth of zinc dialkyldithiophosphate (ZDDP). This raises the intriguing possibility of tailoring the mechanochemical response of additives by modifying their alkyl substituents. Here, we study the tribofilm formation rate of ZDDPs containing several different alkyl groups on steel surfaces from a high-friction base oil. We use macroscale tribometer experiments under full-film elastohydrodynamic lubrication conditions to enable careful control of the temperature and stress during tribofilm growth. We show how the chain length and the presence of branches or bulky cycloaliphatic groups can lead to large differences in the temperature- and stress-dependencies of the tribofilm formation rate, which can be explained through variations in packing density, steric hindrance, and stress transmission efficiency. Our rate data are successfully fitted using the Bell model; a simple modification of the Arrhenius equation that is commonly employed to model the kinetics of mechanochemical processes. Using this model, we observe large differences in the activation energy, pre-exponential factor, and activation volume for the various ZDDPs. Our findings show how structure–performance relationships can be identified for lubricant additives, which may be useful to optimise their molecular structure.
Yu M, Zhang J, Kirkby T, et al., 2022, Electrical impedance spectroscopy enabled in-depth lubrication condition monitoring, The 2022 STLE Annual Meeting & Exhibition
Electrical contact resistance or capacitance as measured between twointerfaces of a lubricated contact has been used in tribometers, partiallyreflecting the lubrication condition. In contrast, the electrical impedancespectroscopy (EIS) provides rich information of magnitude/phase spectrum,which is thoroughly investigated using a combination of electrical circuitmodels (equivalent to the lubricated contact) and in-situ measurements with aball-on-disc contact. Results indicate a promising potential of EIS inlubrication condition monitoring, including the variation of lubricant filmthickness as estimated using high-frequency magnitude response; thetransition between full-film, mixed, and boundary lubrication regimes, asdifferentiated using extracted electrical resistance together with phasespectrum; the forming of anti-wear boundary film, where extraresistor/capacitor are added; and the degradation of lubricant, such as fueldilution, oil oxidization, and water emulsifying.
Zhang J, Campen S, Wong J, et al., 2022, Oxidational wear in lubricated contacts – or is it?, Tribology International, Vol: 165, Pages: 1-9, ISSN: 0301-679X
This study examines the influence of inert gas atmosphere on the wear behaviour of rubbing steel-on-steel contacts lubricated by two hydrocarbon base fluids, isooctane and hexadecane. It is found that for both fluids, wear and mean friction in nitrogen and argon atmospheres are considerably lower than in dry air. As the oxygen content in nitrogen is increased, mean friction and wear both increase, to level out above about 10% oxygen (an O2 partial pressure of 10 kPa). Raman analysis of rubbed surfaces shows the presence of a carbon film on surfaces rubbed in inert gas and at low O2 levels. This film is not observed at high O2 levels.These findings indicate that the prevailing model of oxidational wear in lubricated contacts, that states that wear is greater in air than in inert gas because of corrosion by oxygen, is largely incorrect. Instead, the deleterious effect of oxygen on lubricated wear is primarily due to it preventing the formation of a lubricious, carbon-based boundary film that is generated from hydrocarbon base fluids on rubbing steel surfaces in inert gas conditions.The ability of organic fuels and lubricants to form carbon-based films on rubbing steel surfaces in inert atmospheres may provide a mechanism for reducing friction and wear of fuel- and oil-lubricated machine components. The study also provides a platform from which to design lubricant formulations for use in inert atmospheres.
Wang L, 2021, Experimental and modelling studies of the transient tribological behaviour of a two-phase lubricant under complex loading conditions, Friction, Vol: 10, ISSN: 2223-7690
The transient tribological phenomenon and premature lubricant breakdown have beenwidely observed in metal forming, leading to excessive friction at the contact interfaces. Inthis research, the transient tribological behaviour of a two-phase lubricant were studiedunder complex loading conditions, featuring abrupt interfacial temperature, contact load andsliding speed changes, thus representing the severe interfacial conditions observed inwarm/hot metal forming applications. The strong experimental evidence indicates that theevolution in friction was attributed to the physical diminution and chemical decompositioneffects. As such, a visco-mechanochemical interactive friction model was developed toaccurately predict the transient tribological behaviour of the two-phase lubricant undercomplex loading conditions. The new friction model exhibited close agreements between themodelling and experimental results.
Yu M, Zhang J, Joedicke A, et al., 2021, Experimental investigation into the effects of diesel dilution on engine lubrication, Tribology International, Vol: 156, Pages: 1-9, ISSN: 0301-679X
The dilution of lubricant due to contamination with diesel fuel is an increasingly prevalent, potentially importantand poorly understood issue. Thisstudy addressestwo fundamental questions: 1) How doesthe change in lubricantrheology due to diesel dilution affect engine lubrication? 2) How is the chemical performance of lubricantcomponents (base oil and performance additives) impacted by diesel dilution under different lubrication regimes(boundary/full film, hydrodynamic/elastohydrodynamic). This is achieved by testing three lubricant samples: 1)neat fully formulated 0W-30 engine oil, 2) fully formulated 0W-30 oil diluted with diesel at a concentration of15%, denoted “0W-30D”, and 3) neat, fully-formulated 0W-16, with the same base oil components andperformance additives as the 0W-30, but blended to give a viscosity equal to that of the diluted an equivalent“0W-30D”. Tribometer tests, including 1) low pressure, low shear viscosity, 2) Ultra-high Shear Viscosity (USV),3) elastohydrodynamic film thickness, 4) Stribeck friction and 5) boundary friction and wear, are then conducted.To further emulate engine lubrication conditions, Stribeck curve measurements are performed on the threelubricants using a journal bearing test rig, fitted with a connecting-rod and commercial diesel engine shells.Results suggest that diesel dilution only slightly affects chemical additive performance (with friction modifiersbeing more inhibited than anti-wear additives) but does reduce both viscosity and film thickness. However, caremust be taken in using viscometrics to predict dilution behaviour because 1) the pressure viscosity coefficient isalso affected by diesel dilution which has implications for elastohydrodynamically lubrication contacts, 2) shearthinning means that viscosity modifier additives effects lose their functions at high shear rates; whereas dieselcontamination affects viscosity behaviour throughout the whole shear rate range.
Zhang J, Ueda M, Campen S, et al., 2021, Boundary friction of ZDDP tribofilms, Tribology Letters, Vol: 69, Pages: 1-17, ISSN: 1023-8883
The frictional properties of ZDDP tribofilms at low entrainment speeds in boundary lubrication conditions have been studied in both rolling/sliding and pure sliding contacts. It has been found that the boundary friction coefficients of these tribofilms depend on the alkyl structure of the ZDDPs. For primary ZDDPs, those with linear alkyl chains give lower friction those with branched alkyl chain ZDDPs, and a cyclohexylmethyl-based ZDDP gives markedly higher friction than non-cyclic ones. Depending on alkyl structure, boundary friction coefficient in rolling-sliding conditions can range from 0.09 to 0.14. These differences persist over long duration tests lasting up to 120 h. For secondary ZDDPs, boundary friction appears to depend less strongly on alkyl structure and in rolling-sliding conditions stabilises at ca 0.115 for the three ZDDPs studied. Experiments in which the ZDDP-containing lubricant is changed after tribofilm formation by a different ZDDP solution or a base oil indicate that the characteristic friction of the initial ZDDP tribofilm is lost almost as soon as rubbing commences in the new lubricant. The boundary friction rapidly stabilises at the characteristic boundary friction of the replacement ZDDP, or in the case of base oil, a value of ca 0.115 which is believed to represent the shear strength of the bare polyphosphate surface. The single exception is when a solution containing a cyclohexylethyl-based ZDDP is replaced by base oil, where the boundary friction coefficient remains at the high value characteristic of this ZDDP despite the fact that rubbing in base oil removes about 20 nm of the tribofilm. XPS analysis of the residual tribofilm reveals that this originates from presence of a considerable proportion of C-O bonds at the exposed tribofilm surface, indicating that not all of the alkoxy groups are lost from the polyphosphate during tribofilm formation. Very slow speed rubbing tests at low temperature show that the ZDDP solutions give boundar
Zhang J, Ueda M, Campen S, et al., 2021, Boundary friction of ZDDP tribofilms (vol 69, 8, 2021), Tribology Letters, Vol: 69, Pages: 1-1, ISSN: 1023-8883
Yu M, Zhang J, Reddyhoff T, 2021, Characterizing Fuel Dilution Effects on Rheological and Tribological Behavior of Engine Lubricant, the 7th World Tribology Congress (WTC2021)
Zhang J, Spikes H, 2020, Measurement of EHD friction at very high contact pressures, Tribology Letters, Vol: 68, Pages: 1-12, ISSN: 1023-8883
EHD friction curves have been measured up to very high pressure (pmean = 5 GPa, pmax = 7.5 GPa) using a newly developed, rolling-sliding, ball on disc machine, the ETM. Six base fluids have been studied, spanning the API base oil categories Group I to Group V. At high pressures, thermal effects become substantial even at quite modest slide-roll ratios, and these must be considered when analysing friction measurements in terms of the underlying rheological properties of the oils. By comparing measurements from steel/steel and WC/WC ball and disc combinations with very different thermal conductivities, the use of thermal correction to derive isothermal friction curves has been validated. At relatively low pressures (mean pressure = 1 GPa), there are substantial differences between the EHD friction properties of the various API Group base oils, but as pressure is raised these diminish and the EHD friction coefficients of all the Groups approach a similar maximum value at a given temperature. EHD friction continues to be quite strongly temperature dependent even at very high pressure. As pressure is increased, EHD friction curves become progressively steeper, so that friction coefficients at very low slide-roll ratios (1 to 2% SRR) become several times greater at high than at low pressure. This has important practical implications for the efficiency of rolling element bearings at high pressures since these components normally operate in this SRR range. There is no evidence of any of the base oils reaching a limiting shear stress over the whole pressure and temperature range studied. Instead, shear stress continues to increase with log(strain rate) in accord with the Eyring-activated flow model up to very high pressures.
Zhang J, Ewen JP, Ueda M, et al., 2020, Mechanochemistry of zinc dialkyldithiophosphate on steel surfaces under elastohydrodynamic lubrication conditions, ACS Applied Materials & Interfaces, Vol: 12, Pages: 6662-6676, ISSN: 1944-8244
Zinc dialkyldithiophosphate (ZDDP) is added to engine lubricants to reduce wear and ensure reliable operation. ZDDP reacts under rubbing conditions to form protective zinc/iron phosphate tribofilms on steel surfaces. Recently, it has been demonstrated that this process can be promoted by applied stresses in lubricated contacts, as well as temperature, and is thus mechanochemical in origin. In this study, a tribology test rig capable of applying very high loads has been developed to generate ZDDP tribofilms under full-film elastohydrodynamic lubrication (EHL) conditions in steel/steel ball-on-disk contacts. This provides a well-defined temperature and stress environment with negligible direct asperity contact in which to study mechanochemical processes. ZDDPs with branched primary and secondary alkyl substituents have been studied in three base oils, two with high EHL friction and one with low EHL friction. In the high EHL friction base oils, the tribofilm growth rate increases exponentially with shear stress and temperature for both ZDDPs, as predicted by a stress augmented thermal activation model. Conversely, under otherwise identical conditions, negligible ZDDP tribofilm formation takes place in the low EHL friction base oil. This confirms that the ZDDP reaction is driven by macroscopic shear stress rather than hydrostatic pressure. The secondary ZDDP forms tribofilms considerably faster than the primary ZDDP under equivalent conditions, suggesting that the initial decomposition reaction is the rate determining step for tribofilm formation. The rate of tribofilm growth is independent of ZDDP concentration over the range studied, indicating that this process follows zero-order kinetics. Under full-film EHL conditions, ZDDP tribofilm formation is promoted by macroscopic shear stress applied through the base oil molecules, which induces asymmetric stress on adsorbed ZDDP molecules to promote their decomposition and initiate rapid phosphate polymerisation.
Zhang J, Spikes H, 2018, Mechanochemistry in high shear stress elastohydrodynamic contacts, 255th National Meeting and Exposition of the American-Chemical-Society (ACS) - Nexus of Food, Energy, and Water, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727
Ewen J, Gattinoni C, Zhang J, et al., 2017, On the effect of confined fluid molecular structure on nonequilibrium phase behaviour and friction, Physical Chemistry Chemical Physics, Vol: 19, Pages: 17883-17894, ISSN: 1463-9084
A detailed understanding of the behaviour of confined fluids is critical to a range of industrial applications, for example to control friction in engineering components. In this study, a combination of tribological experiments and confined nonequilibrium molecular dynamics simulations has been used to investigate the effect of base fluid molecular structure on nonequilibrium phase behaviour and friction. An extensive parameter study, including several lubricant and traction fluid molecules subjected to pressures (0.5–2.0 GPa) and strain rates (104–1010 s−1) typical of the elastohydrodynamic lubrication regime, reveals clear relationships between the friction and flow behaviour. Lubricants, which are flexible, broadly linear molecules, give low friction coefficients that increase with strain rate and pressure in both the experiments and the simulations. Conversely, traction fluids, which are based on inflexible cycloaliphatic groups, give high friction coefficients that only weakly depend on strain rate and pressure. The observed differences in friction behaviour can be rationalised through the stronger shear localisation which is observed for the traction fluids in the simulations. Higher pressures lead to more pronounced shear localisation, whilst increased strain rates lead to a widening of the sheared region. The methods utilised in this study have clarified the physical mechanisms of important confined fluid behaviour and show significant potential in both improving the prediction of elastohydrodynamic friction and developing new molecules to control it.
Wang P, Zhang J, Spikes HA, et al., 2016, Development of hydrodynamic micro-bearings, 16th International Conference on Micro and Nanotechnology for Power Generation and Energy Conversion Applications (PowerMEMS 2016), Publisher: IOP Publishing, ISSN: 1742-6588
This paper describes the modelling and testing of mm-scale hydrodynamic bearings which are being developed to improve the efficiency of a cm-scale turbine energy harvester, whose efficiency was previously limited by poorly lubricated commercial jewel-bearings. The bearings were fabricated using DRIE and their performance was assessed using a custom built MEMS tribometer. Results demonstrate that acceptably low friction is achieved when low viscosity liquid lubricants are used in combination with an appropriate choice of friction modifier additive. Further reduction in friction is demonstrated when the step height of bearing is adjusted in accordance with hydrodynamic theory. In parallel with the experiments, hydrodynamic lubricant modelling has been carried out to predict and further optimize film thickness and friction performance. Modelling results are presented and validated against experimental friction data.
Zhang J, Tan A, Spikes H, 2016, Effect of base oil structure on elastohydrodynamic friction, Tribology Letters, Vol: 65, ISSN: 1023-8883
The EHD friction properties of a wide range of base fluids have been measured and compared in mixed sliding–rolling conditions at three temperatures and two pressures. The use of tungsten carbide ball and disc specimens enabled high mean contact pressures of 1.5 and 2.0 GPa to be obtained, comparable to those present in many rolling bearings. The measurements confirm the importance of molecular structure of the base fluid in determining EHD friction. Liquids having linear-shaped molecules with flexible bonds give considerably lower friction than liquids based on molecules with bulky side groups or rings. EHD friction also increases with viscosity for liquids having similar molecular structures. Using pure ester fluids, it is shown that quite small differences in molecular structure can have considerable effects on EHD friction. The importance of temperature rise in reducing EHD friction at slide–roll ratios above about 5% has been shown. By measuring EHD friction at several temperatures and pressures as well as EHD film thickness, approximate corrections to measured EHD friction data have been made to obtain isothermal shear stress and thus EHD friction curves. These show that under the conditions tested most low molecular weight base fluids do not reach a limiting friction coefficient and thus shear stress. However, two high traction base fluids appear to reach limiting values, while three linear polymeric base fluids may also do so. Constants of best fit to a linear/logarithmic isothermal shear stress/strain rate relationship have been provided to enable reconstruction of isothermal EHD friction behaviour for most of the fluids tested.
Zhang J, Spikes HA, 2016, On the mechanism of ZDDP antiwear film formation, Tribology Letters, Vol: 63, Pages: 1-15, ISSN: 1573-2711
Zinc dialkyldithiophosphate additives are used to control wear and inhibit oxidation in almost all engine oils as well as many other types of lubricant. They limit wear primarily by forming a thick, protective, phosphate glass-based tribofilm on rubbing surfaces. This film formation can occur at low temperatures and is relatively indifferent to the chemical nature of the substrate. There has been considerable debate as to what drives ZDDP tribofilm formation, why it occurs only on surfaces that experience sliding and whether film formation is controlled primarily by temperature, pressure, triboemission or some other factor. This paper describes a novel approach to the problem by studying the formation of ZDDP films in full film EHD conditions from two lubricants having very different EHD friction properties. This shows that ZDDP film formation does not require solid-solid rubbing contact but is driven simply by applied shear stress, in accord with a stress-promoted thermal activation model. The shear stress present in a high pressure contact can reduce the thermal activation energy for ZDDP by at least half, greatly increasing the reaction rate. This mechanismexplains the origins of many practically important features of ZDDP films; their topography, their thickness and the conditions under which they form.The insights that this study provides should prove valuable both in optimising ZDDP structure and in modelling ZDDP antiwear behaviour. The findings also highlight the importance of mechanochemistry to the behaviour of lubricant additives in general.
Leong J, Zhang J, Reddyhoff T, et al., 2016, Prevention of spreading of lubricant on silicon surfaces, Pages: 533-536
Leong JY, Zhang J, Reddyhoff T, et al., 2016, Confining of liquids under induced motion, Pages: 555-558
Leong JY, Zhang J, Sinha SK, et al., 2015, Confining Liquids on Silicon Surfaces to Lubricate MEMS, Tribology Letters, Vol: 59, ISSN: 1573-2711
Liquid lubrication may provide a solution tothe problem of high friction and wear in micro-electromechanicalsystems. Although the effectiveness of thisapproach has been demonstrated in laboratory-based frictiontests, practical constraints prevent it from being appliedin commercial devices. The main problem is how toposition the lubricant on a silicon surface in order to limitspreading and evaporation. This paper describes twotechniques to address this issue. First, low concentrationsof additives are used to promote autophobic behaviour.Tests’ results show that certain concentrations of bothmultiply alkylated cyclopentane and amine additives areeffective in halting the spread of hexadecane on silicon,and, in the latter case, cause the hexadecane drop to subsequentlyretract. The second approach involves applying amicro-contact printing technique previously used on goldsurfaces. Here, silicon surfaces are coated with octadecyltrichlorosilanemono-layers that are then selectively removed,using oxygen plasma, to leave regions ofcontrasting surface energy. Results from spin tests showthat surfaces treated in this way can anchor 1 ll drops ofhexadecane and water when forces of up to 22 and 230 lN,respectively, are applied.
Ma G, Wang L, Gao H, et al., 2015, The friction coefficient evolution of a TiN coated contact during sliding wear, Applied Surface Science, Vol: 345, Pages: 109-115, ISSN: 0169-4332
Spikes H, Zhang J, 2015, Reply to the Comment by Scott Bair, Philippe Vergne, Punit Kumar, Gerhard Poll, Ivan Krupka, Martin Hartl, Wassim Habchi, Roland Larson on "History, Origins and Prediction of Elastohydrodynamic Friction" by Spikes and Jie in Tribology Letters, Tribology Letters, Vol: 58, ISSN: 1573-2711
Bair, accompanied by some of his past co-authors, has commented on our paper “History, Origins and Prediction of Elastohydrodynamic Friction”. We believe that our paper presents a balanced summary of current understanding of EHD friction, its origins and prediction. However, Bair et al. appear to have chosen to interpret our paper as an assault on the high-stress viscometric approach that Bair has championed for some years and also as an espousal of the Eyring model of rheology which he has consistently denigrated over the same period. To combat this perceived assault, they suggest that our paper somehow misrepresents the literature and indeed contains numerous misstatements of fact and misrepresentations by omission. Our reply refutes this.
Spikes H, Jie Z, 2014, History, Origins and Prediction of Elastohydrodynamic Friction, TRIBOLOGY LETTERS, Vol: 56, Pages: 1-25, ISSN: 1023-8883
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Zhang J, Yamaguchi E, Spikes H, 2014, The Antagonism between Succinimide Dispersants and a Secondary Zinc Dialkyl Dithiophosphate, TRIBOLOGY & LUBRICATION TECHNOLOGY, Vol: 70, Pages: 60-+, ISSN: 1545-858X
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Zhang J, Yamaguchi E, Spikes H, 2014, The antagonism between succinimide dispersants and a secondary zinc dialkyl dithiophosphate, Tribology Transactions, Vol: 57, Pages: 57-65, ISSN: 1040-2004
Recent years have seen an increase in the concentration of dispersant present in formulated engine oils, while the concentration of antiwear additives has been progressively reduced. However, it is known that the presence of dispersant can, in some cases, detract from the performance of antiwear additives in lubricant blends. In this article, the influence of three succinimide dispersants on the film formation and wear-reducing properties of a secondary zinc dialkyldithiophosphate (ZDDP) has been studied. Both posttreated and non-post-treated dispersants reduce steady-state ZDDP tribofilm formation to a certain extent depending on the dispersant concentration. At very high dispersant concentrations, ZDDP film formation is suppressed almost entirely. This can be restored only marginally by increasing ZDDP concentration, which implies that the absolute dispersant concentration rather than the dispersant : ZDDP ratio controls the impact of the dispersant on ZDDP film formation.Addition of dispersant to ZDDP also caused an increase in wear rate for all three dispersants tested. For one succinimide reported in detail in this article, it is has been shown that the wear rate increases approximately linearly with dispersant concentration and is largely independent of ZDDP concentration over the P weight percentage range studied.
Zhang J, Yamaguchi E, Spikes H, 2013, Comparison of three laboratory tests to quantify mild wear rate, Tribology Transactions, Vol: 56, Pages: 919-928, ISSN: 1040-2004
Because the viscosities of engine and transmission lubricants are lowered in order to reduce hydrodynamic friction and thus energy consumption, it is important to ensure that wear rates do not increase and thus machine durability is not impaired. In practical terms this means that we require reliable methods for measuring the mild wear rates present in most lubricated machine components.This article compares three mild wear reciprocating laboratory tests, one based on the high-frequency reciprocating rig (HFRR) and two on the mini-traction machine (MTM), in order to explore the extent to which wear rate is determined by the test configuration. The results show that some additive-containing lubricants including blends of antiwear additive and dispersant give quite consistent wear rates, independent of whether the surface is in continuous or intermittent contact, whereas others such as two friction modifiers do not. Possible reasons for these differences are discussed. The importance of accounting for wear during running-in and the need to remove any thick tribofilms present before quantifying wear volume are also confirmed.
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