93 results found
Vladescu S-C, Olver AV, Pegg IG, et al., 2016, Combined friction and wear reduction in a reciprocating contact through laser surface texturing, Wear, Vol: 358-359, Pages: 51-61, ISSN: 0043-1648
Vladescu S-C, Medina S, Olver AV, et al., 2016, The Transient Friction Response of a Laser-Textured, Reciprocating Contact to the Entrainment of Individual Pockets, Tribology Letters, Vol: 62, ISSN: 1573-2711
To shed light on the mechanisms with which surface texture improves the tribological performance of piston–liner contacts, we have measured the transient friction response as individual pockets pass through a reciprocating sliding contact. Tests were performed at different sliding speeds and results compared to those from a non-textured, reference specimen under different lubrication regimes. At low speed when the contact is in the boundary regime, friction force falls abruptly as each pocket leaves the contact zone, before gradually returning to an approximately steady-state value. This suggests that each pocket acts to temporarily increase the film thickness, which then decays to its non-textured value as oil is squeezed out. At higher speeds, friction is seen to reduce in a stepwise fashion, since the period between pockets being entrained is less than the time taken for the film to decay. In addition, friction results obtained when the contact is operating in the middle of the mixed regime point to a temporary film thickness collapse as the pocket enters the contact, and this agrees with recent modelling predictions. At higher speeds, the compound effect of successive pockets is to shift the contact to the right on Stribeck curve. These results imply that each pocket gives rise to an increase in film thickness that is both short-lived and small in magnitude (we estimate a few tens of nm). However, the resulting effect on friction can be significant (up to 82 % in this study) for two reasons: (1) provided the pocket frequency is sufficiently high, each successive pocket entrainment builds the film up without there being time for it to reduce back to its steady-state value; (2) when the contact is in the mixed regime, the Stribeck curve is at its steepest and friction is therefore most sensitive to film thickness changes. This has important practical implications in that pocket spacing on piston liners should be varied as a function of reciprocating sliding
Putignano C, Reddyhoff T, Dini D, 2016, The influence of temperature on viscoelastic friction properties, Tribology International, Vol: 100, Pages: 338-343, ISSN: 0301-679X
Viscoelastic friction strongly depends on temperature, which determines the material stiffness and, therefore, given a constant load, the volume that is deformed and dissipates energy. We compare the results obtained by a numerical approach introduced by Carbone and Putignano (2013)  with measurements that separate viscoelastic losses from Coulomb contribution. This is done for a range of temperatures. We show that viscoelastic friction curves for different temperatures can be arranged into a single master curve using a frequency shift coefficient, which can be found from the characterization of the viscoelastic material response. This shows that it is possible to accurately (a) use dynamic material analysis data to extrapolate viscoelastic friction measurements to values outside the tested range, and (b) use a tribometer to obtain fundamental viscoelastic material properties.
Vladescu S, medina S, olver A, et al., 2016, Lubricant film thickness and friction force measurements in a laser surfacetextured reciprocating line contact simulating the piston ring – linerpairing, Tribology International, Vol: 98, Pages: 317-329, ISSN: 1879-2464
Applying surface texture to piston liners may provide an effective means of controlling friction and hence improving engine efficiency. However, little is understood about the mechanisms by which pockets affect friction, primarily because of a lack of reliable experimental measurements. To address this, the influence of surface texture on film thickness and friction force was measured simultaneously in a convergent-divergent bearing, under conditions that closely replicate an automotive piston ring-liner conjunction. Film thicknesses were measured using a modified version of the ultra-thin film optical interferometry approach, enabling film thicknesses <50 nanometres to be measured under transient, mixed lubrication conditions. This involved using the out-of-contact curvature of the specimens in place of a spacer layer and analysing multiple interference fringes to avoid fringe ambiguity. Tests were performed on both a textured sample (with features oriented normal to the direction of sliding) and a non-textured reference sample, while angular velocity, applied normal load and lubricant temperature were controlled in order to study the effect of varying lubrication regime (as typically occurs in service). Results showed that the presence of surface pockets consistently enhances fluid film thickness in the mixed lubrication regime by approximately 20 nm. Although this is only a modest increase, the effect on friction is pronounced (up to 41% under these conditions), due to the strong dependence of friction on film thickness in the mixed regime. Conversely, in the full film regime, texture caused a reduction in film thickness and hence increased friction force, compared with the non-textured reference. Both textured and non-textured friction values show nearly identical dependence on film thickness, (showing that, under these conditions, texture-induced friction reduction results entirely from the change in film thickness). These results are important in providing
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.
Ku ISY, Chong WWF, Reddyhoff T, et al., 2015, Frictional characteristics of molecular length ultra-thin boundary adsorbed films, Meccanica, Vol: 50, Pages: 1915-1922, ISSN: 1572-9648
The paper presents measurements of friction of any ultra-thin film entrained into the contact of a pair of very smooth specimen subjected to entrainment in a converging micro-wedge of a special-purpose micro-tribometer. An ultra-thin film is expected to form at the boundary solids through adsorption of boundary active molecules. Fluids with linear and branched molecules are used in the investigation. It is found that the frictional characteristics of these films can be adequately described through use of Eyring thermal activation energy and a potential energy barrier to sustain conjunctional sliding motion. The combined experimental measurement and the simple activation energy approach shows that the thin molecular adsorbed films act like hydro Langmuir–Blodgett layers, the formation and frictional characteristics of which are affected by the competing mechanisms of adsorption, forced molecular re-ordering and discrete-fashion drainage through the contact by the solvation effect. This process is a complex function of the contact sliding velocity as well as a defined Eyring activation density (packing density of the molecules within the conjunction). It is shown that the contribution of solvation to friction is in the form of energy expended to eject layers of lubricant out of the contact, which unlike the case of micro-scale hydrodynamic films, is not a function of the sliding velocity.
Reddyhoff T, Dobre O, Le Rouzic J, et al., 2015, Friction Induced Vibration in Windscreen Wiper Contacts, Journal of Vibration and Acoustics-Transactions of the ASME, Vol: 137, ISSN: 1048-9002
This research is aimed at understanding the mechanisms that give rise to friction induced noise in automotive windscreen wipers, with a focus on frequencies between 500 and 3500 Hz. To study this phenomenon, experimental friction, sound, and high-speed video measurements are combined with finite element modeling of a rubber wiper/glass contact. In agreement with previous research, simultaneous sound and friction measurements showed that wiper noise in this frequency range results from the negative damping effect caused by the dependence of friction on speed in the mixed lubrication regime. Furthermore, during sliding, the friction induced noise recorded by the microphone occurred in one of two frequency ranges (close to 1000 Hz and between 2000 and 2500 Hz). These coincided closely with the eigen-frequencies of first two bending modes, predicted by finite element modeling. Experimental observations also showed the wiper to be oscillating backward and forward without any torsional motion and that the thickness of the glass had no effect on the emitted noise. These observations highlight how friction induced noise—although caused by conditions within contact—has characteristics that are determined by the structure of the excited component. A number of additional findings are made. Most importantly, both experiment and finite element modeling showed that the presence of water in contact with the wiper modulates the frequency and amplitude of the emitted noise by effectively adding mass to the vibrating system. While this is occurring, Faraday-like standing waves are observed in the water. In addition to this, friction induced vibration is shown only to occur for glass surfaces with intermediate surface energies, which is possibly due to high contact angles preventing water reaching the contact. Based on the understanding gained, a number of suggestions are made regarding means of reducing windscreen wiper noise.
Medina S, Fowell MT, Vladescu S-C, et al., 2015, Transient effects in lubricated textured bearings, Proceedings of the Institution of Mechanical Engineers Part J - Journal of Engineering Tribology, Vol: 229, Pages: 523-537, ISSN: 1350-6501
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
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
Vladescu S-C, Olver AV, Pegg IG, et al., 2015, The effects of surface texture in reciprocating contacts - An experimental study, TRIBOLOGY INTERNATIONAL, Vol: 82, Pages: 28-42, ISSN: 0301-679X
Putignano C, Le Rouzic J, Reddyhoff T, et al., 2014, A theoretical and experimental study of viscoelastic rolling contacts incorporating thermal effects, PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART J-JOURNAL OF ENGINEERING TRIBOLOGY, Vol: 228, Pages: 1112-1121, ISSN: 1350-6501
Le Rouzic J, Reddyhoff T, 2014, Spatially Resolved Triboemission Measurements, Tribology Letters, Vol: 55, Pages: 245-252, ISSN: 1573-2711
Hernandez Battez A, Viesca JL, Gonzalez R, et al., 2014, Effect of Shear Rate, Temperature, and Particle Concentration on the Rheological Properties of ZnO and ZrO2 Nanofluids, TRIBOLOGY TRANSACTIONS, Vol: 57, Pages: 489-495, ISSN: 1040-2004
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
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
Moorhouse BS, Reddyhoff T, Ward-Close M, et al., 2013, Formation of wear resistant coatings on Ti-6Al-4V by calciothermic reduction, SURFACE & COATINGS TECHNOLOGY, Vol: 221, Pages: 214-221, ISSN: 0257-8972
Le Rouzic J, Reddyhoff, Tom, 2013, Development of Infrared Microscopy for Measuring Asperity Contact Temperatures, Journal of Tribology, Vol: 135, Pages: 021504-021504, ISSN: 0742-4787
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
Putignano C, Reddyhoff T, Dini D, et al., 2013, Viscoelastic contact mechanics: Numerical simulations with experimental validation, Pages: 1511-1514
Guégan J, Kadiric A, Reddyhoff T, et al., 2013, Friction and lubrication of textured surfaces in elasto-hydrodynamic contacts, Pages: 1659-1661
Hartinger M, Reddyhoff T, 2013, CFD modeling and infrared measurement of an EHL line contact, Pages: 999-1002
Putignano D, 2013, Experimental investigation of viscoelastic rolling contacts: A comparison with theory, Tribology Letters, Vol: 51, Pages: 105-113
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.
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.
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
Dwyer-Joyce RS, Reddyhoff T, Zhu J, 2011, Ultrasonic measurement for film thickness and solid contact in elastohydrodynamic lubrication, Journal of Tribology, Vol: 133, ISSN: 0742-4787
The reflection of ultrasound can be used to determine oil film thickness in elastohydrodynamic lubricated (EHL) contacts if the opposing surfaces are fully separated by the liquid layer. The proportion of the wave amplitude reflected depends on the stiffness of the liquid layer, which is a function of its bulk modulus and thickness. However, in many practical applications, boundary or mixed film lubrication is a common occurrence as the nominal thickness of the separating film is of a similar order to the height of the surface asperities. The reflection is then dependent on both the liquid contact and solid contact parts and the total interfacial stiffness is the controlling parameter. In this paper an investigation was carried to study the reflection of ultrasonic waves from the lubricated contact between a sliding steel ball and a flat steel disc when substantial solid contact occurs. To interpret the ultrasonic reflection results, a mixed regime model for a circular point contact was established. The liquid film stiffness was calculated by using a predicted film thickness and a bulk modulus estimated from published rheological models of lubricants under high pressure. Solid contact stiffness was predicted using a statistical rough surface contact model. Under all operating conditions, the prediction of fluid stiff ness was found to be much greater than the solid contact stiffness. The total stiffness predicted by the model showed good agreement with experimental measurements for kinematic cases. The model was used to separate the stiffness contributions from the asperity contact part and lubricant layer part from the experimental data. For contact pressures ranging from 0.42 to 0.84 GPa and sliding speed from zero to 2 m/s, the film thickness was found to vary from 0.01 to 0.8 m, and the proportion of the load supported by asperity contact varied from 50% to 0%.
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.
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.
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