Publications
472 results found
Ewen J, Gattinoni C, Thakkar F, et al., 2016, Nonequilibrium Molecular Dynamics Investigation of the Reduction in Friction and Wear by Carbon Nanoparticles Between Iron Surfaces, Tribology Letters, Vol: 63, ISSN: 1573-2711
For the successful development and application of novel lubricant additives, a full understanding of their tribological behaviour at the nanoscale is required, but this can be difficult to obtain experimentally. In this study, nonequilibrium molecular dynamics simulations are used to examine the friction and wear reduction mechanisms of promising carbon nanoparticle friction modifier additives. Specifically, the friction and wear behaviour of carbon nanodiamonds (CNDs) and carbon nano-onions (CNOs) confined between α-iron slabs is probed at a range of coverages, pressures, and sliding velocities. At high coverage and low pressure, the nanoparticles do not indent into the α-iron slabs during sliding, leading to zero wear and a low friction coefficient. At low coverage and high pressure, the nanoparticles indent into, and plough through the slabs during sliding, leading to atomic-scale wear and a much higher friction coefficient. This contribution to the friction coefficient is well predicted by an expression developed for macroscopic indentation by Bowden and Tabor. Even at the highest pressures and lowest coverages simulated, both nanoparticles were able to maintain separation of the opposing slabs and reduce friction by approximately 75 % compared to when no nanoparticle was present, which agrees well with experimental observations. CNO nanoparticles yielded a lower indentation (wear) depth and lower friction coefficients at equal coverage and pressure with respect to CND, making them more attractive friction modifier additives. Potential changes in behaviour on harder and softer surfaces are also discussed, together with the implications that these results have in terms of the application of the studied nanoparticles as lubricants additives.
Ewen J, Gattinoni C, Thakkar F, et al., 2016, A Comparison of Classical Force-Fields for Molecular Dynamics Simulations of Lubricants, Materials, Vol: 9, ISSN: 1996-1944
For the successful development and application of lubricants, a full understanding of their complex nanoscale behavior under a wide range of external conditions is required, but this is difficult to obtain experimentally. Nonequilibrium molecular dynamics (NEMD) simulations can be used to yield unique insights into the atomic-scale structure and friction of lubricants and additives; however, the accuracy of the results depend on the chosen force-field. In this study, we demonstrate that the use of an accurate, all-atom force-field is critical in order to; (i) accurately predict important properties of long-chain, linear molecules; and (ii) reproduce experimental friction behavior of multi-component tribological systems. In particular, we focus on n-hexadecane, an important model lubricant with a wide range of industrial applications. Moreover, simulating conditions common in tribological systems, i.e., high temperatures and pressures (HTHP), allows the limits of the selected force-fields to be tested. In the first section, a large number of united-atom and all-atom force-fields are benchmarked in terms of their density and viscosity prediction accuracy of n-hexadecane using equilibrium molecular dynamics (EMD) simulations at ambient and HTHP conditions. Whilst united-atom force-fields accurately reproduce experimental density, the viscosity is significantly under-predicted compared to all-atom force-fields and experiments. Moreover, some all-atom force-fields yield elevated melting points, leading to significant overestimation of both the density and viscosity. In the second section, the most accurate united-atom and all-atom force-field are compared in confined NEMD simulations which probe the structure and friction of stearic acid adsorbed on iron oxide and separated by a thin layer of n-hexadecane. The united-atom force-field provides an accurate representation of the structure of the confined stearic acid film; however, friction coefficients are consistently und
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.
Ewen JP, Gattinoni C, Morgan N, et al., 2016, Nonequilibrium molecular dynamics simulations of organic friction modifiers adsorbed on iron oxide surfaces, Langmuir: the ACS journal of surfaces and colloids, Vol: 32, Pages: 4450-4463, ISSN: 0743-7463
For the successful development and application of lubricants, a full understanding of the nanoscale behavior of complex tribological systems is required, but this is difficult to obtain experimentally. In this study, we use nonequilibrium molecular dynamics (NEMD) simulations to examine the atomistic structure and friction properties of commercially relevant organic friction modifier (OFM) monolayers adsorbed on iron oxide surfaces and lubricated by a thin, separating layer of hexadecane. Specifically, acid, amide, and glyceride OFMs, with saturated and Z-unsaturated hydrocarbon tail groups, are simulated at various surface coverages and sliding velocities. At low and medium coverage, the OFMs form liquidlike and amorphous monolayers, respectively, which are significantly interdigitated with the hexadecane lubricant, resulting in relatively high friction coefficients. At high coverage, solidlike monolayers are formed for all of the OFMs, which, during sliding, results in slip planes between well-defined OFM and hexadecane layers, yielding a marked reduction in the friction coefficient. When present at equal surface coverage, OFMs with saturated and Z-unsaturated tail groups are found to yield similar structure and friction behavior. OFMs with glyceride head groups yield significantly lower friction coefficients than amide and particularly carboxylic acid head groups. For all of the OFMs and coverages simulated, the friction coefficient is found to increase linearly with the logarithm of sliding velocity; however, the gradient of this increase depends on the coverage. The structure and friction details obtained from these simulations agree well with experimental results and also shed light on the relative tribological performance of these OFMs through nanoscale structural variations. This has important implications in terms of the applicability of NEMD to aid the development of new formulations to control friction.
Marx N, Guegan J, Spikes HA, 2016, Elastohydrodynamic film thickness of soft EHL contacts using optical interferometry, Tribology International, Vol: 99, Pages: 267-277, ISSN: 1879-2464
Robust, chromium, semi-reflective coatings have been applied to transparent polymethylmethacrylate and polyurethane discs and this has enabled conventional, normal incidence optical interferometry to be used to measure lubricant film thickness in soft EHL conditions for the first time. High quality interferograms comparable to those obtained from coated glass discs are obtained. Measured film thickness has been compared with existing soft EHL film thickness equations obtained using computer modelling and revised central and minimum film thickness equations have been proposed. These film thickness measurements and measurement technique have applicability to our understanding of the performance and design of lubricated gears and bearings manufactured from polymeric materials.
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
Spikes HA, Costa H, 2015, Impact of ethanol on the formation of antiwear tribofilms from engine lubricants, Tribology International, Vol: 93, Pages: 364-376, ISSN: 1879-2464
This paper investigates the impact of contamination of engine lubricants with ethanol fuel on the growth and stability of anti-wear tribofilms from ZDDP-containing lubricants. The MTM-SLIM technique was used to monitor the effects of blending 5 wt% of both anhydrous and hydrated ethanol on tribofilm thickness in a fully-formulated Group I oil and in a solution of ZDDP anti-wear additive dissolved in Group I base oil. Tribofilm thickness was significantly reduced by the addition of ethanol for both oils, and the reduction was more severe for hydrated than for anhydrous ethanol. When a tribofilm was allowed to form during rubbing using an ethanol-free oil, the subsequent addition of hydrated ethanol showed both the destruction of the pre-formed antiwear tribofilm and damage to the rubbed surfaces.
Spikes HA, 2015, Friction modifier additives, Tribology Letters, Vol: 60, ISSN: 1573-2711
The need for energy efficiency is leading to the growing use of additives that reduce friction in thin film boundary and mixed lubrication conditions. Several classes of such friction modifier additive exist, the main ones being organic friction modifiers, functionalised polymers, soluble organo-molybdenum additives and dispersed nanoparticles. All work in different ways. This paper reviews these four main types of lubricant friction modifier additive and outlines their history, research and the mechanisms by which they are currently believed to function. Aspects of their behaviour that are still not yet fully understood are highlighted.
Spikes H, Tysoe W, 2015, On the Commonality Between Theoretical Models for Fluid and Solid Friction, Wear and Tribochemistry, Tribology Letters, Vol: 59, ISSN: 1573-2711
Tribology is concerned with the influence ofmechanically applied forces on interfacial phenomena thataccompany and control sliding. A wide range of modelshas been developed to describe these phenomena, whichinclude frictional dissipation, wear and tribochemical reactions.This paper shows that these apparently disparatemodels are based on the same fundamental concept that anexternally applied force accelerates the rate of thermaltransition of atoms or molecules across energy barrierspresent in solid and liquid materials, thereby promotingflow, slip or bond cleavage. Such ‘‘stress-assisted’’ effectsand the associated thermal activation concepts were developedindependently and in different forms by Prandtl (ZAngew Math Mech 8:85, 1928) and Eyring (J Chem Phys4(4):283–291, 1936). These two works have underpinnedsubsequent theories of dry friction, boundary lubrication,EHD rheology, tribochemistry and nanoscale wear modelling.This paper first reviews the historical developmentof the concepts, focussing in particular on the models ofPrandtl and Eyring and how they have subsequently beenused and adapted by others. The two approaches are thencompared and contrasted, noting that although superficiallysimilar, they contain quite different assumptions and constraints.First, the Prandtl model assumes that the force isexerted through a compliant spring, while constant forcesliding is assumed by Eyring. Second, different approximationsare made in the two models to describe the change in energy barrier with external force. Prandtl exploresthe asymptotic behaviour of the energy barrier as theapplied force become sufficiently high to reduce it to zero,while Eyring assumes that the energy barrier is reduced byan amount equal to the external work carried out on thesystem. The theoretical underpinnings of these differencesare discussed along with the implications of compliantcoupling and constant force sliding on the velocity andtemperature dependence of t
Guegan J, Kadiric A, Spikes HA, 2015, A Study of the Lubrication of EHL Point Contact in the Presence of Longitudinal Roughness, Tribology Letters, Vol: 59, ISSN: 1023-8883
This work investigates the effect on elastohydrodynamic lubrication of roughness ridges oriented along the rolling–sliding direction, such as may be present on rolling bearing raceways. The roughness of the three specimens tested is characterised by the RMS of surface heights and a dominant wavelength. Optical interferometry and a ball-on-disc set-up were employed to map the oil film thickness. The paper first describes a novel procedure to carry out optical interferometry measurements on rough surfaces. Film thickness maps from the central part of the contact were obtained for a range of speeds in pure rolling and rolling–sliding conditions. The evolution of the film distribution with increasing speed along with the in-contact RMS and the real area of contact was calculated. The film maps show that the lift-off speed increases when roughness is introduced compared with smooth surfaces, while the average film thickness remains very close to the smooth case. The general horseshoe film shape that becomes visible at higher speeds is discussed. Using an inverse solution approach based on measured in-contact roughness, the pressure distribution is estimated in a rough, lubricated contact and its evolution with speed is explained. The findings provide important insights into the transition from boundary, through mixed, to full EHL lubrication for longitudinal roughness.
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.
Campen S, Green JH, Lamb GD, et al., 2015, In situ study of model organic friction modifiers using liquid cell AFM: self-assembly of octadecylamine, Tribology Letters, Vol: 58, ISSN: 1573-2711
Liquid cell AFM has been applied to study in situ the formation and properties of self-assembled films formed on mica surfaces by octadecylamine from alkane solution. Mica surfaces immersed in hexadecane or dodecane at room temperature show no identifiable surface films. However, when octadecylamine solution is injected into the cell, a boundary film forms almost immediately. This film takes the form of irregular islands of mean diameter approximately 1–3 µm and thickness typically 1.5 nm when measured in contact mode. These islands are believed to correspond to patches of vertically oriented but tilted octadecylamine or ammonium salt held together primarily by van der Waals forces between adjacent alkyl chains. These films are quite labile in that during scanning of the tip in both tapping mode and contact mode changes to the shape of the islands take place, including consolidation of the island density in the scanned region and depletion from around this area. In situ experiments in which the temperature of the cell is varied over time show that the initially formed islands disappear at a temperature of ca. 35 °C but are reformed when the cell is re-cooled. Similar tests on samples that remain immersed in solution for extended periods show more stable films, with islands being lost only above about 50–60 °C. The work shows that liquid cell AFM has great promise for studying the formation and properties of the boundary films formed by organic friction modifiers.
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.
Campen S, Green JH, Lamb GD, et al., 2015, In Situ Study of Model Organic Friction Modifiers Using Liquid Cell AFM; Saturated and Mono-unsaturated Carboxylic Acids, TRIBOLOGY LETTERS, Vol: 57, ISSN: 1023-8883
Liang H, Guo D, Reddyhoff T, et al., 2015, Influence of thermal effects on elastohydrodynamic (EHD) lubrication behavior at high speeds, SCIENCE CHINA-TECHNOLOGICAL SCIENCES, Vol: 58, Pages: 551-558, ISSN: 1674-7321
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- Citations: 11
Ingram M, Hamer C, Spikes H, 2015, A new scuffing test using contra-rotation, Wear, Vol: 328-329, Pages: 229-240, ISSN: 1873-2577
The mode of lubricant failure known as scuffing provides a significant design constraint in high sliding gears, cams and metal cutting and forming processes. It is therefore important to have an effective test method to measure the scuffing resistance of lubricant formulations. In most existing scuffing bench tests, a moving surface is rubbed against a stationary one at a fixed sliding speed and the load at which scuffing occurs is determined. This approach has two disadvantages. One is that wear of the stationary surface can lead to a large decrease in effective contact pressure during a test. The second is that viscous lubricants often generate significant elastohydrodynamic films at the sliding speeds employed. This means that the scuffing tests measure a complex combination of the influence of the fluid and boundary film-forming properties of the lubricant on scuffing rather than reflecting solely the influence of lubricant formulation.This paper describes a new scuffing test method in which the two metal surfaces are rubbed together in mixed rolling–sliding with the two surfaces moving in opposite directions with respect to the contact, i.e. in contra-rotation. This enables the sliding speed to be decoupled from the entrainment speed so that the scuffing properties of a lubricant can be determined in boundary lubrication conditions over a wide range of sliding speeds. Also, because both surfaces move relative to the contact, wear is distributed and this minimises changes in contact pressure during a test.
Campen S, Green JH, Lamb GD, et al., 2015, In situ study of model organic friction modifiers using liquid cell AFM; saturated and mono-unsaturated carboxylic acids, Tribology Letters, Vol: 57, Pages: 1-20, ISSN: 1023-8883
Fatty acids and their derivatives have been used as model organic friction modifiers for almost a century, but there is still debate as to the nature of the boundary films that they form on rubbed surfaces. In this study, in situ liquid cell atomic force microscopy (AFM) is used to monitor the self-assembly of boundary films from solutions of fatty acids in alkanes on to mica surfaces. Because the mica surfaces are wholly immersed in solution, it is possible to study directly changes in the morphology and friction of these films over time and during heating and cooling. It has been found that stearic acid and elaidic acid, which are able to adopt linear molecular configurations, form irregular islands on mica that are tens to hundreds of microns in diameter and typically 1.6 nm thick, corresponding to domains of tilted single monolayers. At a relatively high concentration of 0.01 M, stearic acid in hexadecane forms an almost complete monolayer, but at lower concentrations, in dodecane solution and for elaidic acid solutions, these films remain incomplete after prolonged immersion of more than a day. The films formed by fatty acids on mica are displaced by repeated scanning in contact mode AFM but can be imaged without damage using tapping mode AFM. Rubbed quartz surfaces from a sliding ball-on-disc test were also scanned ex situ using AFM, and these showed that stearic acid forms similar monolayer island films on quartz in macro-scale friction experiments as are found on mica. Oleic acid solutions behave quite differently from stearic acid and elaidic acid, forming irregular globular films on both mica and rubbed quartz surfaces. This is believed to be because its cis-double bond geometry means that, unlike its trans-isomer elaidic acid or saturated stearic acid, it is unable to adopt a linear molecular configuration and so is less able to form close-packed monolayers.
Spikes H, 2015, Basics of EHL for practical application, LUBRICATION SCIENCE, Vol: 27, Pages: 45-67, ISSN: 0954-0075
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- Citations: 36
Myant CW, Fowell M, Spikes H, et al., 2014, A study of lubricant film thickness in compliant contacts of elastomeric seal materials using a laser induced fluorescence technique, Tribology International, Vol: 80, Pages: 76-89, ISSN: 0301-679X
A laser induced fluorescence technique was used to investigate the build-up of lubricant films in compliant contacts operating in the isoviscous elasto-hydrodynamic regime (I-EHL). The described technique utilises an optimised optical set-up with a relatively high signal-to-noise ratio and was shown to be able to produce film thickness maps of the complete contact area and measure a very wide span of thicknesses, from 50 nm to 100 μm. Maps of film thickness were obtained over a range of entrainment speeds and loads for three different contact configurations and two elastomer materials, polydimethylsiloxane (PDMS) and a fluorocarbon rubber (FKM) which is typically used in rotary seal applications. In a model contact of a nominally smooth PDMS ball sliding on a glass flat, a crescent shaped area of reduced film thickness was observed towards the contact exit. In contrast to typical elasto-hydrodynamic contacts, no side-lobes of reduced film thickness were recorded, while the central film region exhibited a converging wedge shape. The elliptical contact of an FKM O-ring rolling on a flat glass showed a central region of flat film while areas of minimum film thickness were located near the contact edges either side of the centre. The highly conformal contact of relatively rough FKM O-ring sliding against a concave glass lens, a geometry more representative of that found in elastomeric seals, showed discrete regions of reduced film, corresponding to surface roughness asperities. With rising entrainment speed, some lift-off was observed, with surface roughness asperities appearing to be increasingly compressed. Measured films thicknesses were compared to existing theoretical predictions for I-EHL contacts and the level of agreement was found to be highly dependent on contact geometry and applied conditions.
Costa HL, Spikes H, 2014, Effects of Ethanol Contamination on Friction and Elastohydrodynamic Film Thickness of Engine Oils, TRIBOLOGY TRANSACTIONS, Vol: 58, Pages: 158-168, ISSN: 1040-2004
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- Citations: 15
Liu X, Spikes H, Wong JSS, 2014, In-situ pH responsive fluorescent probing of localized iron corrosion, Corrosion Science, ISSN: 0010-938X
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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- Citations: 92
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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- Citations: 1
Quinchia LA, Delgado MA, Reddyhoff T, et al., 2014, Tribological studies of potential vegetable oil-based lubricants containing environmentally friendly viscosity modifiers, TRIBOLOGY INTERNATIONAL, Vol: 69, Pages: 110-117, ISSN: 0301-679X
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- Citations: 160
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.
Yang S, Reddyhoff T, Spikes H, 2013, Influence of Lubricant Properties on ARKL Temperature Rise and Transmission Efficiency, TRIBOLOGY TRANSACTIONS, Vol: 56, Pages: 1119-1136, ISSN: 1040-2004
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- Citations: 7
Vengudusamy B, Green JH, Lamb GD, et al., 2013, Durability of ZDDP Tribofilms Formed in DLC/DLC Contacts, TRIBOLOGY LETTERS, Vol: 51, Pages: 469-478, ISSN: 1023-8883
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- Citations: 35
Vengudusamy B, Green JH, Lamb GD, et al., 2013, Influence of hydrogen and tungsten concentration on the tribological properties of DLC/DLC contacts with ZDDP, WEAR, Vol: 298, Pages: 109-119, ISSN: 0043-1648
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- Citations: 52
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