436 results found
Peng B, Spikes H, Kadiric A, 2019, The Development and Application of a Scuffing Test Based on Contra-rotation, TRIBOLOGY LETTERS, Vol: 67, ISSN: 1023-8883
Reddyhoff T, Schmidt A, Spikes H, 2019, Thermal Conductivity and Flash Temperature, TRIBOLOGY LETTERS, Vol: 67, ISSN: 1023-8883
Vladescu S-C, Marx N, Fernandez L, et al., 2018, Hydrodynamic Friction of Viscosity-Modified Oils in a Journal Bearing Machine, TRIBOLOGY LETTERS, Vol: 66, ISSN: 1023-8883
Marx N, Fernández L, Barceló F, et al., 2018, Shear Thinning and Hydrodynamic Friction of Viscosity Modifier-Containing Oils. Part II: Impact of Shear Thinning on Journal Bearing Friction, Tribology Letters, Vol: 66, ISSN: 1023-8883
© 2018, The Author(s). In a companion paper, the temporary shear thinning behaviour of a series of viscosity-modifier (VM)-containing blends was studied over a wide shear rate and temperature range [Marx et al. in Tribol Lett, https://doi.org/10.1007/s11249-018-1039-5]. It was found that for almost all VMs the resulting data could be collapsed on a single viscosity versus reduced strain rate curve using time–temperature superposition. This made it possible to derive a single equation to describe the viscosity–shear rate behaviour for each VM blend. In the current paper, these shear thinning equations are used in a Reynolds-based hydrodynamic lubrication model to explore and compare the impact of different VMs on the film thickness and friction of a lubricated, isothermal journal bearing. It is found that VMs reduce friction and especially power loss markedly at high shaft speeds, while still contributing to increased hydrodynamic film thickness at low speeds. The model indicates that VMs can contribute to reducing friction in two separate ways. One is via shear thinning. This occurs especially at high bearing speeds when shear rates are large and can result in a 50% friction reduction compared to the equivalent isoviscous oil at low temperatures for the blends studied. The second is via their impact on viscosity index, which means that for a set viscosity at high temperature the low-shear-rate (and thus the high shear rate) viscosity of a high-VI oil, and consequently its hydrodynamic friction, will be lower at low temperatures than that of a low-VI oil. The identification and quantification of these two alternative ways to reduce friction should assist in the design of new, fuel-efficient VMs.
Marx N, Fernandez L, Barcelo F, et al., 2018, Shear Thinning and Hydrodynamic Friction of Viscosity Modifier-Containing Oils. Part I: Shear Thinning Behaviour, TRIBOLOGY LETTERS, Vol: 66, ISSN: 1023-8883
Reddyhoff T, Underwood RJ, Sayles RS, et al., 2018, Temperature measurement of debris particles in EHL contacts, SURFACE TOPOGRAPHY-METROLOGY AND PROPERTIES, Vol: 6, ISSN: 2051-672X
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
Spikes H, 2018, Stress-augmented thermal activation: Tribology feels the force, FRICTION, Vol: 6, Pages: 1-31, ISSN: 2223-7690
Kontou A, Southby M, Spikes HA, 2017, Effect of steel hardness on soot wear, Wear, Vol: 390-391, Pages: 236-245, ISSN: 0043-1648
Due to incomplete combustion, high levels of soot can accumulate in engine lubricants between drain intervals. This soot can promote wear of engine parts such as timing chains and cam followers. One standard approach to reducing wear is to increase the hardness of the rubbing components used. According to the Archard wear equation, wear rate should be broadly inversely proportional to hardness. To explore this approach for controlling soot wear, wear tests have been conducted in a High Frequency Reciprocating Rig (HFRR) with HFRR steel discs of various hardness against a hard steel ball. Carbon black (soot surrogate) dispersions in model lubricants based on solutions of ZDDP and dispersant in GTL base oils have been studied. Wear volumes have been measured and wear scars and tribofilms analysed using scanning white light interferometry and SEM-EDS. It is found that, while most oils show wear that reduces with increasing hardness, for blends that contain both ZDDP and carbon black, wear rate markedly increases with disc hardness as the latter approaches the hardness of the ball. The results support the prevalence of a corrosive-abrasive wear mechanism when carbon black and ZDDP are both present in a lubricant and suggests that selection of very hard surfaces may not be a useful way to control soot.
Ewen J, 2017, Molecular dynamics simulations of lubricants and additives
Shimizu Y, Spikes HA, 2017, The Influence of Aluminium–Silicon Alloy on ZDDP Tribofilm Formation on the Counter-Surface, Tribology Letters, Vol: 65, ISSN: 1023-8883
In order to reduce vehicle weight and thus improve fuel economy, aluminium (Al) alloys have been increasingly adopted as contact surfaces such as piston skirts and cylinder liners in current engines. In general, hypereutectic Al–Si alloys are used, in which hard silicon grains are embedded in a softer Al–Si single phase matrix. It is reported that after rubbing, the matrix is removed to leave silicon grains protruding from the surface. However, the response of the counter-surface by these silicon grains is rarely investigated. In this study, mini traction machine–space layer imaging (MTM–SLIM) has been used to monitor tribofilm formation in situ and investigate the evolution of both surfaces in the contact of a steel ball on an Al–Si disc lubricated by ZDDP solution. In low-load conditions, the top layer of aluminium on the Al–Si disc is removed physically to leave silicon grains protruding from the surface, while ZDDP tribofilm pads are formed mainly on the grains. On the counter-surface (steel ball), ZDDP tribofilms are formed and build up with no wear scars. In high-load conditions, deep gaps are observed to form initially round the silicon grains on the disc. During rubbing, these become shallower, while the silicon grains start to protrude. On the steel ball, ZDDP tribofilm is generated initially over the whole rubbing track, but then the tribofilm in the middle of the track is almost completely removed by rubbing against the protruding silicon grains. Wear of the underlying steel surface then ensues.
Ewen JP, 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: 1463-9076
Marx N, Ponjavic A, Taylor RI, et al., 2017, Study of permanent shear thinning of VM polymer solutions, Tribology Letters, Vol: 65, ISSN: 1023-8883
The ultrashear viscometer (USV) has been adapted and employed to investigate the permanent shear thinning of polystyrene solutions in a series of phthalate ester base fluids. The permanent shear stability index based on viscosities measured at 106 s−1, PSSI(106), has been found to be a convenient way to express the magnitude of permanent shear thinning. When comparing permanent shear thinning at various shear rates in the USV, it is very important to take account of the different times of shear that are present at different shear rates. The PSSI(106) value divided by the total time of shear is then a useful way of quantifying and comparing permanent shear thinning rates. Tests using polystyrene in different viscosity base fluids have shown that this rate of permanent shear thinning depends on shear stress and not shear rate and varies linearly with polymer concentration. The rate of permanent shear thinning also varies exponentially with shear stress, suggestive of a stress-promoted polymer breakdown process. By using a small volume of test fluid in the USV and solvent extraction after a test, it has proved possible to obtain molecular weight distributions of polymer after shear using gel permeation chromatography (GPC). This indicates that the polymer breakdown process is different at low and high polymer concentrations, with molecule fragmentation at low polymer concentration but mid-chain scission at high concentration. A key feature of the USV is that, unlike other methods currently used to measure permanent shear thinning behaviour of engine oils, it subjects the test fluid to well-defined, controllable high shear conditions. Coupled with the use of GPC, this makes it possible for the first time to relate quantitatively the permanent shear thinning of engine oils to shear conditions and to polymer degradation response.
Spikes HA, 2017, Comment on: Rheology of an Ionic Liquid with Variable Carreau Exponent: A Full Picture by Molecular Simulation with Experimental Contribution, by Nicolas Voeltzel, Philippe Vergne, Nicolas Fillot, Nathalie Bouscharain, Laurent Joly, Tribology Letters (2016) 64: 25, TRIBOLOGY LETTERS, Vol: 65, ISSN: 1023-8883
Ponjavic A, Lemaigre T, Southby M, et al., 2017, Influence of NOx and Air on the Ageing Behaviour of MoDTC, TRIBOLOGY LETTERS, Vol: 65, ISSN: 1023-8883
Guegan J, Kadiric A, Gabelli A, et al., 2017, Reply to the 'Comment on "The Relationship Between Friction and Film Thickness in EHD Point Contacts in the Presence of Longitudinal Roughness'' by Guegan, Kadiric, Gabelli, & Spikes' by Scott Bair, Tribology Letters, Vol: 65, ISSN: 1023-8883
Delgado MA, Quinchia LA, Spikes HA, et al., 2017, Suitability of ethyl cellulose as multifunctional additive for blends of vegetable oil-based lubricants, JOURNAL OF CLEANER PRODUCTION, Vol: 151, Pages: 1-9, ISSN: 0959-6526
Taylor LJ, Spikes HA, 2017, Friction-Enhancing Properties of ZDDP Antiwear Additive: Part II-Influence of ZDDP Reaction Films on EHD Lubrication, TRIBOLOGY & LUBRICATION TECHNOLOGY, Vol: 73, Pages: 50-56, ISSN: 1545-858X
Ewen J, Gattinoni C, Spikes H, et al., 2017, Nonequilibrium molecular dynamics simulations of organic friction modifiers, 253rd National Meeting of the American-Chemical-Society (ACS) on Advanced Materials, Technologies, Systems, and Processes, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727
Taylor LJ, Spikes HA, 2017, Friction-enhancing properties of ZDDP antiwear additive: Part I-friction and morphology of ZDDP reaction films, Tribology and Lubrication Technology, Vol: 73, Pages: 54-62, ISSN: 1545-858X
© 2017, Society of Tribologists and Lubrication Engineers. All rights reserved. Many phosphorus-based antiwear films, including those formed by zinc dialkyl dithiophosphates (ZDDP), cause a significant increase in friction in thin film, high-pressure, lubricated contacts. This can have a deleterious effect on engine oil fuel efficiency. Previous work has shown that friction is increased not under boundary, but under mixed lubrication conditions and it has been suggested that this phenomenon results from an effective roughening of the rubbing surfaces by the formation of unevenly-distributed reaction films. In the current paper it is shown that, when other additives commonly used in engine oils are added to ZDDP solutions, quite smooth ZDDP reaction films can result. Despite this, the ZDDP still produces a marked increase in friction in mixed lubrication conditions, which suggests that surface roughening is not the main origin of friction enhancement by ZDDP reaction films. In a companion paper, Part II, it is shown that ZDDP reaction films, whether rough or smooth, enhance friction by inhibiting the entrainment of liquid lubricant into rubbing contacts, thereby reducing the elastohydrodynamic oil film thickness (13).
Zhang J, Tan A, Spikes H, 2017, Effect of Base Oil Structure on Elastohydrodynamic Friction, TRIBOLOGY LETTERS, Vol: 65, ISSN: 1023-8883
Spikes HA, 2017, Additive-additive and additive-surface interactions in lubrication., IN: 6TH INT. COLLOQUIUM ON INDUSTRIAL LUBRICANTS - PROPERTIES, APPLICATION, DISPOSAL., (OSTFILDERN, FED. REP GERMANY: JAN 12-, Vol: 1 , Ostfildern, Fed. Rep. Germany, Esslingen Tech. Akad., 1988, Paper 4, p.4.2-1-4.2-12. (ISBN 3-92481
Combinations of additives are often found to exhibit antagonistic or synergistic effects compared to the performance of the individual additives. This paper reviews the state of our knowledge concerning such effects. Direct interactions, where two additives combine at a molecular level are distinguished from complementary of exclusary effects where the individual contributions of separate additives enhance or reduce overall performance, but with no direct interaction. Additive interactions takes place in solution and at surface and both are discussed.
Guegan J, Kadiric A, Gabelli A, et al., 2016, The Relationship Between Friction and Film Thickness in EHD Point Contacts in the Presence of Longitudinal Roughness, TRIBOLOGY LETTERS, Vol: 64, ISSN: 1023-8883
Shimizu Y, Spikes HA, 2016, The Tribofilm Formation of ZDDP Under Reciprocating Pure Sliding Conditions, TRIBOLOGY LETTERS, Vol: 64, ISSN: 1023-8883
Shimizu Y, Spikes HA, 2016, The Influence of Slide-Roll Ratio on ZDDP Tribofilm Formation, TRIBOLOGY LETTERS, Vol: 64, ISSN: 1023-8883
Ewen JP, Gattinoni C, Thakkar FM, 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: 1023-8883
Zhang J, Spikes H, 2016, On the Mechanism of ZDDP Antiwear Film Formation, TRIBOLOGY LETTERS, Vol: 63, ISSN: 1023-8883
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