Publications
472 results found
Leong JY, Reddyhoff T, Sinha SK, et al., 2013, Hydrodynamic friction reduction in a MAC-hexadecane lubricated MEMS contact, Tribology Letters, Vol: 49, Pages: 217-225, ISSN: 1023-8883
Recent research has shown that hydrodynamic lubrication is an effective means of reducing friction in high sliding micro-electromechanical systems (MEMS). At high speeds, however, such lubrication can lead to increased friction due to viscous drag. This article describes a series of hydrodynamic tests on a silicon MEMS contact lubricated with a blend of hexadecane and a multiplyalkylated cyclopentane (MAC). Results show that the presence of the MAC reduces hydrodynamic friction compared with neat hexadecane. Such behaviour is contrary to conventional hydrodynamic theory, since the viscosity of the MAC - a mixture of di and tri(2 octyldodecyl)cyclopentane - is significantly higher than that of neat hexadecane. This effect increases with MAC concentration up to an optimum value of 3 wt%, where the hydrodynamic friction coefficient at 15,000 rpm is reduced from 0.5 to 0.3. Above this concentration, friction begins to rise due to the overriding effect of increasing viscosity. The viscosity of the blended lubricant increased monotonically with MAC concentration, when measured using both a Stabinger and an ultrahigh shear viscometer. In addition to this, no reduction in friction was observed when a squalane-hexadecane blend of equal viscosity was tested. This suggests that some property of the MAC-hexadecane lubricant, other than its viscosity, is influencing hydrodynamic lubrication. A tentative explanation for this behaviour is that the MAC induces the liquid to slip, rather than shear, close to the silicon surfaces. This hypothesis is supported by the fact that the friction reducing ability of the MAC blend was inhibited by the inclusion of octade-cylamine - a substance known to form films on silicon surfaces. Furthermore, the MAC reduces friction in the mixed regime, in a manner suggesting that the formation of a viscous boundary layer. This unusual behaviour may have useful implications for reducing hydrodynamic friction in liquidlubricated MEMS devices. © Sprin
Leong JY, Reddyhoff T, Sinha SK, et al., 2013, Hydrodynamic Friction Reduction in a MAC-Hexadecane Lubricated MEMS Contact, TRIBOLOGY LETTERS, Vol: 49, Pages: 217-225, ISSN: 1023-8883
- Author Web Link
- Cite
- Citations: 24
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.
Holtzinger J, Green J, Lamb G, et al., 2012, Peer-reviewed: New method of measuring permanent viscosity loss of polymer-containing lubricants, Tribology and Lubrication Technology, Vol: 68, ISSN: 1545-858X
An ultrahigh shear rate viscometer (USV) was used to measure the viscosity of polymer solutions. It was found that some polymer solutions in base oil, including those used as engine oil viscosity modifiers, show permanent viscosity loss when subjected to very high shear rates above 106 s-1. The USV was modified to automatically carry out a series of viscosity measurements on the same test lubricant sample. This enabled the accumulation of permanent viscosity loss to be measured over successive strain cycles. As expected, permanent viscosity loss increased with both strain rate and molecular weight. When carried out at 5 × 106 s-1 and 100°C, the test was more severe than the Kurt Orbahn test because samples of lubricants subjected to the latter underwent further shear thinning in the USV. The USV test appears to be a rapid and convenient way to quantify the permanent viscosity loss of polymer-containing lubricants for engine use, and a protocol to assess permanent viscosity loss (PVL) and permanent shear stability index (PSSI) based on viscosity measurements at 106 s-1 before and after shear thinning is outlined. The study also shows that it is important to take into account possible permanent viscosity loss when measuring the viscosity of polymer solutions in veiy high shear rate viscometers such as the USV. This can be done by minimizing the amount of shear to which the lubricant is subjected or by taking successive measurements and subtracting the permanent viscosity loss taking place in each of the first few strain rate cycles.
Holtzinger J, Green J, Lamb G, et al., 2012, New Method of Measuring Permanent Viscosity Loss of Polymer-Containing Lubricants, TRIBOLOGY & LUBRICATION TECHNOLOGY, Vol: 68, Pages: 66-+, ISSN: 1545-858X
Campen S, Green J, Lamb G, et al., 2012, On the Increase in Boundary Friction with Sliding Speed, TRIBOLOGY LETTERS, Vol: 48, Pages: 237-248, ISSN: 1023-8883
- Author Web Link
- Cite
- Citations: 81
Vengudusamy B, Green JH, Lamb GD, et al., 2012, Behaviour of MoDTC in DLC/DLC and DLC/steel contacts, TRIBOLOGY INTERNATIONAL, Vol: 54, Pages: 68-76, ISSN: 0301-679X
- Author Web Link
- Cite
- Citations: 86
Timm K, Myant C, Nuguid H, et al., 2012, Investigation of friction and perceived skin feel after application of suspensions of various cosmetic powders, INTERNATIONAL JOURNAL OF COSMETIC SCIENCE, Vol: 34, Pages: 458-465, ISSN: 0142-5463
- Author Web Link
- Cite
- Citations: 13
Fowell MT, Medina S, Olver AV, et al., 2012, Parametric study of texturing in convergent bearings, TRIBOLOGY INTERNATIONAL, Vol: 52, Pages: 7-16, ISSN: 0301-679X
- Author Web Link
- Cite
- Citations: 86
Quinchia LA, Delgado MA, Franco JM, et al., 2012, Low-temperature flow behaviour of vegetable oil-based lubricants, INDUSTRIAL CROPS AND PRODUCTS, Vol: 37, Pages: 383-388, ISSN: 0926-6690
- Author Web Link
- Cite
- Citations: 80
Spikes H, 2012, Boundary lubrication and boundary lubricating films, Handbook of Lubrication and Tribology, Volume II: Theory and Design, Second Edition, ISBN: 9781420069082
In 1919, it was found that the addition of oleic acid to mineral oils improved the latters’ friction and wear performance to equal that of natural oils [1]. This sparked considerable research and in the early 1920s, Langmuir, Hardy, and others showed that the property of oiliness could be produced by monolayers of long-chain surfactants adsorbed or reacted on solid surfaces. In 1922, Hardy and Doubleday coined the word “boundary lubrication” to describe this effect [2]. Nowadays, the superiority of natural oils in terms of boundary lubrication is considered to be due to their partial decomposition to form long-chain surfactants, either carboxylic acids or partial esters, that act as “natural” friction modifiers.
Heyes DM, Smith ER, Dini D, et al., 2012, Pressure dependence of confined liquid behavior subjected to boundary-driven shear, J. Chem. Phys, Vol: 136, Pages: 134705-134705
Ku ISY, Reddyhoff T, Wayte R, et al., 2012, Lubrication of microelectromechanical devices using liquids of different viscosities, Journal of Tribology, Vol: 134, ISSN: 0742-4787
Lubrication of contacting and sliding surfaces in MEMS (microelectromechanical systems) is particularly challenging because of the predominance of surface forces at the microscale. The current paper explores the possibility of using liquid lubrication in this application. Measurements of friction and lubricant film thickness have been made for liquid lubricants of different viscosities, including low viscosity silicone oil, hexadecane, squalane, and water. Testing was carried out using a newly developed MEMS tribometer in which a rotating silicon disk is loaded against a stationary silicon disk. Two different test setups were used: one where both disks are flat, and the other where the stationary disk is structured as in a thrust pad bearing. In all tests the disks were fully submerged in the lubricant. With the flat-on-patterned disk combination, the variation of friction with rotation speed was found to follow classical Stribeck curves for all the lubricants tested. The friction at high speeds also decreased with increasing normal load, in accordance with hydrodynamic lubrication theory. For the least viscous lubricants, it was found that the hydrodynamic friction coefficients remained relatively low even at higher speeds. In particular, for water the friction coefficient for water was around 0.1 at 10,000 rpm. However, boundary friction was found to be unacceptably high at low speeds where there was insufficient lubricant entrainment. The experimental results have been compared with a finite difference solution of Reynolds equation and reasonable agreement is seen between theory and experiment. The results indicate that liquid lubrication is potentially an effective means of lubricating MEMS components with high levels of sliding. 2012 American Society of Mechanical Engineers.
Holtzinger J, Green J, Lamb G, et al., 2012, New Method of Measuring Permanent Viscosity Loss of Polymer-Containing Lubricants, TRIBOLOGY TRANSACTIONS, Vol: 55, Pages: 631-639, ISSN: 1040-2004
- Author Web Link
- Cite
- Citations: 11
Timm K, Myant C, Spikes HA, et al., 2011, Particulate lubricants in cosmetic applications, Tribology International, Vol: 44, Pages: 1695-1703, ISSN: 1879-2464
Polymer powders are commonly added to cosmetic formulations to improve product performance and skin feel. This study investigates the effect of particle concentration and size on the lubricating properties of powder suspensions. Results are reported for various particle sizes and concentrations.When the tribological contact was fully immersed the addition of particles had no effect. However different behaviour was observed when the contact was only partially lubricated. In this case, a three-stage friction coefficient curve was observed. By varying the particle size and concentration it was shown that the duration and magnitude of each stage can be controlled.
Müller M, Topolovec-Miklozic K, Dardin A, et al., 2011, Peer reviewed: The design of boundary film-forming PMA viscosity modifiers, Tribology and Lubrication Technology, Vol: 67, ISSN: 1545-858X
Previous research has shown that some viscosity modifier additives are able to adsorb from oil solution on to metal surfaces to produce thick, viscous boundary films. These films enhance lubricant film formation in slow-speed and high temperature conditions and thus produce a significant reduction in friction. This article describes a systematic study of this phenomenon, which makes use of the versatile nature of polymethacrylate (PMA) chemistry. Dispersant polymethacrylates with a range of different functionalities, molecular weights, and architectures have been synthesized using controlled radical polymerization techniques. The influence of each of these features on boundary film formation and friction has been explored using optical interferometry and friction versus speed measurement. From the results, guidelines have been developed for designing PMAs having optimal boundary lubricating and, thus, friction-reducing properties.
Timm K, Myant C, Spikes HA, et al., 2011, Cosmetic powder suspensions in compliant, fingerprintlike contacts, BIOINTERPHASES, Vol: 6, Pages: 126-134, ISSN: 1934-8630
- Author Web Link
- Cite
- Citations: 7
Mueller M, Topolovec-Miklozic K, Dardin A, et al., 2011, The Design of Boundary Film-Forming PMA Viscosity Modifiers, TRIBOLOGY & LUBRICATION TECHNOLOGY, Vol: 67, Pages: 50-+, ISSN: 1545-858X
- Author Web Link
- Cite
- Citations: 4
Ku ISY, Reddyhoff T, Holmes AS, et al., 2011, Wear of silicon surfaces in MEMS, Wear, Vol: 271, Pages: 1050-8, ISSN: 0043-1648
High levels of friction and wear are problems which currently limit the development of sliding micro-electro-mechanical systems (MEMS) - devices which would otherwise offer significant technological advancement. The current paper focuses on the wear of MEMS silicon surfaces, and specifically looks at the effect of environment and surface preparation on wear behaviour. Included in the study is the assessment of two self-replenishing lubrication mechanisms; namely liquid and vapour phase lubrication. All tests were carried out using a tribometer which operated and measured friction and wear under conditions representative of MEMS.It is shown that friction and wear behaviour depend strongly on subtle changes of the silicon surfaces prior to testing. Greatest wear was measured when the surfaces were tested immediately after plasma-cleaning, while subsequent exposure to ambient air for 15h reduced wear to negligible levels. Exposure of plasma-cleaned surfaces to water-saturated argon prior to testing prevented wear to a limited extent. Based on this, and TOF-SIMS analysis, it is suggested that the observed wear reduction after exposure to air is caused by tiny amounts of lubricious long chain hydrocarbon contaminants present in ambient air.Tests carried out with the specimens submerged in a liquid bath show that the presence of liquid water reduces friction and wear, but only if specimens have been plasma-cleaned beforehand. This behaviour is tentatively attributed to the hydrophilic nature of plasma treated silicon, reducing the corrosive action of water. When hexadecane or 1-pentanol was used as a liquid lubricant, friction was minimal, and wear was undetectable under all sliding conditions. This was the case even though the contact operated in the mixed lubrication regime, suggesting a boundary film is formed on the silicon surfaces by both of these organic liquids.Results of tests carried out with the lubricant being supplied in the form of pentanol vapour also showe
Vengudusamy B, Mufti RA, Lamb GD, et al., 2011, Friction properties of DLC/DLC contacts in base oil, TRIBOLOGY INTERNATIONAL, Vol: 44, Pages: 922-932, ISSN: 0301-679X
- Author Web Link
- Cite
- Citations: 62
Hoshino K, Yagishita K, Tagawa K, et al., 2011, Tribological properties of sulphur-free antiwear additives zinc dialkylphosphates (ZDPs), SAE Technical Papers, Vol: 5, Pages: 504-510, ISSN: 1946-3952
The film-forming, friction, and antiwear properties of zinc dialkylphosphates (ZDPs) were investigated and compared with the corresponding zinc dialkyldithiophosphates (ZDDPs). The primary ZDPs generally show similar friction and antiwear performance to the primary ZDDPs, although some differences are seen between them in film-forming properties. For the secondary ZDP and ZDDP, there are some clear differences in their tribological properties. This indicates that the properties of the primary ZDPs and ZDDPs may be controlled predominantly by adsorbed films consisting the intact additives and/or their decomposition compounds, and that the properties of the secondary ones may be controlled by glassy reaction films consisting zinc/iron polyphosphates. © Copyright 2011 Society of Automotive Engineers of Japan, Inc. and SAE International.
Hoshino K, Yagishita K, Tagawa K, et al., 2011, Tribological properties of sulphur-free antiwear additives zinc dialkylphosphates (ZDPs), ISSN: 0148-7191
The film-forming, friction, and antiwear properties of zinc dialkylphosphates (ZDPs) were investigated and compared with the corresponding zinc dialkyldithiophosphates (ZDDPs). The primary ZDPs generally show similar friction and antiwear performance to the primary ZDDPs, although some differences are seen between them in film-forming properties. For the secondary ZDP and ZDDP, there are some clear differences in their tribological properties. This indicates that the properties of the primary ZDPs and ZDDPs may be controlled predominantly by adsorbed films consisting the intact additives and/or their decomposition compounds, and that the properties of the secondary ones may be controlled by glassy reaction films consisting zinc/iron polyphosphates. © Copyright 2011 Society of Automotive Engineers of Japan, Inc. and SAE International.
Stoehr T, Eisenberg B, Suchert E, et al., 2011, Advances in tribological design of poly(alkyl methacrylate) viscosity index improvers, SAE Technical Papers, ISSN: 0148-7191
Fuel economy has become the dominant criterion in the design of new automobiles. The globally enacted targets for fleet average emissions pose true challenges to automobile manufacturers. Increasing fuel economy requires enhancements both in hardware as well as in lubricant performance. As a key component of the lubricant, poly(alkyl methacrylate) PAMA viscosity index improvers have been identified as crucial design element due to their multiple modes of action. In their original application, they serve the well-known mechanism of polymer coil expansion at high temperatures and collapse at low temperatures. They help to flatten the viscosity/temperature relationship of the lubricant and allow for reduced low temperature viscosities and reduced internal friction, which directly translates into fuel economy. In addition to this bulk application, interfacial tribological phenomena contribute significantly to efficiency and fuel economy. Poly(alkyl methacrylates), with their versatile chemistry, can be designed with a sequential architecture. Such structures adsorb onto the metal surface as thick polymer layers, and as a consequence reduce friction in the fuel economy relevant mixed lubrication regime of the Stribeck curve. The present paper now describes how to further fine-tune the tribological design of PAMA film-formers towards particular requirements of the tribo system, might it be lubrication regime (e.g. boundary lubrication), hardware surface (e.g. P-tribolayer, DLC), and component interaction (e.g. vs. poly(isobutene) succinimide PIBSI). With these improvements, the avenue towards broad usage of PAMA film-formers tribo-tailored to the individual fluid constraints in driveline, industrial gear, engine or hydraulic application - has been opened up. © Copyright 2011 Society of Automotive Engineers of Japan, Inc. and SAE International.
Reddyhoff T, Ku ISY, Holmes AS, et al., 2011, Friction Modifier Behaviour in Lubricated MEMS Devices, Tribology Letters, Vol: 41, Pages: 239-46, ISSN: 1023-8883
Low viscosity fluids could provide reliable lubrication for certain microelectromechanical system's (MEMS) applications where high-sliding speeds and/or high sliding distances occur. However, while the use of low viscosity fluids leads to reduced hydrodynamic friction, high boundary friction can be a significant issue at low entrainment speeds. This article describes a series of tests of low viscosity fluids, blended with a friction modifier additive so as to provide a combination of both low hydrodynamic and low boundary friction at MEMS scales. The low viscosity fluids tested were hexadecane, low viscosity silicone oil, toluene and water. With the exception of water, the addition of the organic friction modifier octadecylamine to all these lubricating fluids produced a significant reduction in boundary friction. For a MEMS contact lubricated with silicone oil for instance, boundary friction was reduced from 0.5 to close to 0.05. The presence of the amine dissolved in the toluene had the effect of reducing boundary friction from 0.75 to 0.55; this was further reduced to 0.25 after the specimens had been immersed in the toluene-additive blend for 48 h. A water-soluble additive, diethylamine, was added to de-ionized water, at 0.1% by weight concentration. Although an initial reduction in boundary friction was observed (0.45-0.25), under these conditions the rapid onset of severe wear negated these effects. It is suggested that corrosion of silicon by water, followed by abrasion, is the cause of this accelerated wear.
Ingram M, Reddyhoff T, Spikes HA, 2011, Thermal behaviour of a slipping wet clutch contact, Tribology Letters, Vol: 41, Pages: 23-32, ISSN: 1023-8883
Wet clutches are used in automatic transmissions to enable gear changes and also to reduce energy loss in the torque converter. These friction devices are susceptible to stick-slip effects, which result in the vehicle giving an unsteady ride. Stick-slip effects can be avoided by ensuring the wet clutch and lubricant combination produces a friction coefficient that increases with sliding speed. Although wet clutches have been studied throughout the industry for many decades, the mechanism of the generated friction is still not fully understood. It is known that, because of the fibrous nature and thus very large roughness of friction material, the overall contact between clutch plates actually consists of many small, independent, contact units, which are the sites of the generated friction. Some authors have suggested that a temperature rise due to friction either at these contact units or of the overall clutch plate may be important in controlling friction behaviour. In this study, the flash temperatures at the contact units formed in the wet clutch contact have been measured using an infrared camera. Three friction materials have been tested. It was found that measured flash temperature in a pure sliding system similar to that present in a slipping clutch remain well below 5 C, and are therefore unlikely to play a major role in the observed friction-speed dependency of slipping wet clutches at speeds below 2 m/s.
Ingram M, Noles J, Watts R, et al., 2011, Frictional Properties of Automatic Transmission Fluids: Part I-Measurement of Friction-Sliding Speed Behavior, TRIBOLOGY TRANSACTIONS, Vol: 54, Pages: 145-153, ISSN: 1040-2004
- Author Web Link
- Cite
- Citations: 37
Ingram M, Noles J, Watts R, et al., 2011, Frictional Properties of Automatic Transmission Fluids: Part II-Origins of Friction-Sliding Speed Behavior, TRIBOLOGY TRANSACTIONS, Vol: 54, Pages: 154-167, ISSN: 1040-2004
- Author Web Link
- Open Access Link
- Cite
- Citations: 32
Spikes H, 2010, Boundary lubrication; History and recent developments, Pages: 821-880
Benedet J, Green JH, Lamb GD, et al., 2010, Tribological characteristics of low and zero SAPS antiwear additives, Pages: 323-325
The tribological properties of a range of low and zero sulphated ash, phosphorus and sulphur antiwear additives that have been suggested as possible alternatives to, or partial replacements for ZDDP, were studied. Zinc dialkyldithiophosphate (ZDDP)1 rapidly developed a thick film, while the low and zero SAPS antiwear additives formed thinner or even negligible tribofilms compared to ZDDP. This is an abstract of a paper presented at the STLE Annual Meeting and Exhibition 2010 (Las Vegas, NV 5/16-20/2010).
Vengudusamy B, Green JH, Lamb GD, et al., 2010, Behaviour of lubricant additives in DLC/DLC and DLC/steel contacts, Pages: 376-378
The friction properties of diamond-like coating (DLC), including W-DLC, WC-DLC, Si-DLC, taC, a-C:H and a-C, were studied in DLC/DLC and DLC/Steel sliding-rolling contacts lubricated by additivated mineral oils. DLC showed a reduction in friction when additivated, indicating the presence of tribofilm on the DLC surface. Only W-DLC formed a pad-like structure similar to that of steel. Other DLC either formed very thin film or debris-like species on the surface. ta-C coatings give the lowest boundary friction, followed by a-C:H and a-C coatings. DLC that can form ZDDP pads showed good anti-wear properties. This is an abstract of a paper presented at the STLE Annual Meeting and Exhibition 2010 (Las Vegas, NV 5/16-20/2010).
This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.