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  • Conference paper
    Rycerz P, Kadiric A, Pasaribu R, Espejel GM, Olver AVet al., 2012,

    EFFECT OF ADDITIVES ON SURFACE PERFORMANCE

    , 15th International Conference on Experimental Mechanics (ICEM), Publisher: INEGI-INST ENGENHARIA MECANICA E GESTAO INDUSTRIAL
  • Conference paper
    Hajishafiee A, Dini D, Zaki T, Kadiric A, Ioannides Set al., 2012,

    MODELLING ELASTO-HYDRODYNAMIC LUBRICATION USING CFD

    , 15th International Conference on Experimental Mechanics (ICEM), Publisher: INEGI-INST ENGENHARIA MECANICA E GESTAO INDUSTRIAL
  • Journal article
    Bertocchi L, Giacopini M, Strozzi A, Fowell MT, Dini Det al., 2012,

    A MASS-CONSERVING COMPLEMENTARITY FORMULATION TO STUDY FLUID FILM LUBRICATION IN THE PRESENCE OF CAVITATION FOR NON-NEWTONIAN AND COMPRESSIBLE FLUIDS

    , PROCEEDINGS OF THE ASME 11TH BIENNIAL CONFERENCE ON ENGINEERING SYSTEMS DESIGN AND ANALYSIS, VOL 4, Pages: 629-635
  • Journal article
    Medina S, Dini D, 2012,

    A FAST DETERMINISTIC MODEL TO STUDY ADHESION IN ROUGH CONTACTS

    , PROCEEDINGS OF THE ASME 11TH BIENNIAL CONFERENCE ON ENGINEERING SYSTEMS DESIGN AND ANALYSIS, VOL 4, Pages: 637-643
  • Journal article
    Ku ISY, Reddyhoff T, Wayte R, Choo JH, Holmes AS, Spikes HAet 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.

  • Journal article
    Oldfield M, Dini D, Giordano G, Rodriguez y Baena Fet al., 2012,

    Detailed finite element modelling of deep needle insertions into a soft tissue phantom using a cohesive approach

    , ISSN: 1476-8259

    Detailed finite element modelling of needle insertions into soft tissue phantoms encounters difficulties of large deformations, high friction, contact loading and material failure. This paper demonstrates the use of cohesive elements in high-resolution finite element models to overcome some of the issues associated with these factors. Experiments are presented enabling extraction of the strain energy release rate during crack formation. Using data from these experiments, cohesive elements are calibrated and then implemented in models for validation of the needle insertion process. Successful modelling enables direct comparison of finite element and experimental force-displacement plots and energy distributions. Regions of crack creation, relaxation, cutting and full penetration are identified. By closing the loop between experiments and detailed finite element modelling, a methodology is established which will enable design modifications of a soft tissue probe that steers through complex mechanical interactions with the surrounding material.

  • Journal article
    Heyes DM, Smith ER, Dini D, Spikes HA, Zaki TAet al., 2012,

    Pressure dependence of confined liquid behavior subjected to boundary-driven shear

    , J. Chem. Phys, Vol: 136, Pages: 134705-134705
  • Journal article
    Ku ISY, Reddyhoff T, Wayte R, Choo JH, Holmes AS, Spikes HAet 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.

  • Journal article
    Timm K, Myant C, Spikes HA, Grunze Met 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.

  • Journal article
    Masen MA, 2011,

    A systems based experimental approach to tactile friction

    , JOURNAL OF THE MECHANICAL BEHAVIOR OF BIOMEDICAL MATERIALS, Vol: 4, Pages: 1620-1626, ISSN: 1751-6161
  • Journal article
    Myant CW, underwood R, fan J, cann Pet al., 2011,

    Lubrication of metal-on-metal hip joints: The effect of proteincontent and load on film formation and wear

    , Journal of the Mechanical Behavior of Biomedical Materials, Vol: 6, Pages: 30-40, ISSN: 1751-6161

    Lubricant films were measured for a series of bovine serum and protein containing (albumin, globulin) saline solutions for CoCrMo femoral component sliding against a glass disc. Central film thickness was measured by optical interferometry as a function of time (constant mean speed: 0 and 10 mm/s) and variable mean speed (0–50 mm/s). The effect of load (5–20 N) on film thickness was also studied. The development of the wear scar on the CoCrMo surface was monitored by measuring the width of the contact zone during the film thickness tests. The results showed film thickness increased with time for both the static and sliding tests. Films formed in the static, loaded test were typically in the range of 3–40 nm. The globulin containing solutions formed the thickest films. In the sliding tests a wear scar rapidly formed on the implant component for the bovine serum and albumin fluids, negligible wear was observed for the globulin solutions. Film thickness increased with sliding time for all test solutions and was much greater than predicted by isoviscous EHL models. The film increase was found to correlate with increasing wear scar size and thus decreasing contact pressure. A new lubricating mechanism is proposed whereby during sliding the fluid undergoes bulk phase separation rheology, so that an elevated protein phase forms in the inlet zone. This protein phase is a high-viscosity biphasic matrix, which is periodically entrained into the contact forming a thick protective hydro-gel film. One of the main findings of this study is that film thickness was very sensitive to load; to a much greater extent than predicted by EHL models. Thus film formation in MoM hip joints is very susceptible to high contact pressures which might be due to implant misalignment and edge-loading.

  • Journal article
    Timm K, Myant C, Spikes HA, Schneider M, Ladnorg T, Grunze Met al., 2011,

    Cosmetic powder suspensions in compliant, fingerprintlike contacts

    , BIOINTERPHASES, Vol: 6, Pages: 126-134, ISSN: 1934-8630
  • Journal article
    Mueller M, Topolovec-Miklozic K, Dardin A, Spikes Het al., 2011,

    The Design of Boundary Film-Forming PMA Viscosity Modifiers

    , TRIBOLOGY & LUBRICATION TECHNOLOGY, Vol: 67, Pages: 50-+, ISSN: 1545-858X
  • Journal article
    Scaraggi M, Carbone G, Dini D, 2011,

    Experimental Evidence of Micro-EHL Lubrication in Rough Soft Contacts

    , TRIBOLOGY LETTERS, Vol: 43, Pages: 169-174, ISSN: 1023-8883
  • Journal article
    Ku ISY, Reddyhoff T, Holmes AS, Spikes HAet 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

  • Journal article
    Vengudusamy B, Mufti RA, Lamb GD, Green JH, Spikes HAet al., 2011,

    Friction properties of DLC/DLC contacts in base oil

    , TRIBOLOGY INTERNATIONAL, Vol: 44, Pages: 922-932, ISSN: 0301-679X
  • Journal article
    Hills DA, Dini D, 2011,

    Characteristics of the process zone at sharp notch roots

    , INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES, Vol: 48, Pages: 2177-2183, ISSN: 0020-7683
  • Journal article
    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%.

  • Journal article
    Balcombe R, Fowell MT, Olver AV, Ioannides S, Dini Det al., 2011,

    A coupled approach for rolling contact fatigue cracks in the hydrodynamic lubrication regime: The importance of fluid/solid interactions

    , WEAR, Vol: 271, Pages: 720-733, ISSN: 0043-1648
  • Journal article
    Mulvihill DM, Kartal ME, Olver AV, Nowell D, Hills DAet al., 2011,

    Investigation of non-Coulomb friction behaviour in reciprocating sliding

    , WEAR, Vol: 271, Pages: 802-816, ISSN: 0043-1648
  • Journal article
    Accardi MA, Dini D, Cann PM, 2011,

    Experimental and numerical investigation of the behaviour of articular cartilage under shear loading-Interstitial fluid pressurisation and lubrication mechanisms

    , TRIBOLOGY INTERNATIONAL, Vol: 44, Pages: 565-578, ISSN: 0301-679X
  • Journal article
    Reina S, Dini D, Hills DA, Iida Yet al., 2011,

    A quadratic programming formulation for the solution of layered elastic contact problems: Example applications and experimental validation

    , EUROPEAN JOURNAL OF MECHANICS A-SOLIDS, Vol: 30, Pages: 236-247, ISSN: 0997-7538
  • Journal article
    Wong JSS, Hong L, Bae SC, Granick Set al., 2011,

    Polymer Surface Diffusion in the Dilute Limit

    , Macromolecules, Vol: 44
  • Journal article
    Hart AJ, Ilo K, Underwood R, Cann P, Henckel J, Lewis A, Cobb J, Skinner Jet al., 2011,

    The relationship between the angle of version and rate of wear of retrieved metal-on-metal resurfacings A PROSPECTIVE, CT-BASED STUDY

    , JOURNAL OF BONE AND JOINT SURGERY-BRITISH VOLUME, Vol: 93B, Pages: 315-320, ISSN: 0301-620X
  • Journal article
    Scaraggi M, Carbone G, Dini D, 2011,

    Lubrication in soft rough contacts: A novel homogenized approach. Part II - Discussion

    , SOFT MATTER, Vol: 7, Pages: 10407-10416, ISSN: 1744-683X
  • Journal article
    Scaraggi M, Carbone G, Persson BNJ, Dini Det al., 2011,

    Lubrication in soft rough contacts: A novel homogenized approach. Part I - Theory

    , SOFT MATTER, Vol: 7, Pages: 10395-10406, ISSN: 1744-683X
  • Journal article
    Fan J, Myant CW, Underwood R, Cann PM, Hart Aet al., 2011,

    Inlet protein aggregation: a new mechanism for lubricating film formation with model synovial fluids

    , Proceedings of the Institution of Mechanical Engineers Part H - Journal of Engineering in Medicine, Vol: 225, Pages: 696-709, ISSN: 0954-4119

    This paper reports a fundamental study of lubricant film formation with model synovial fluid components (proteins) and bovine serum (BS). The objective was to investigate the role of proteins in the lubrication process. Film thickness was measured by optical interferometry in a ball-on-disc device (mean speed range of 2–60 mm/s). A commercial cobalt–chromium (CoCrMo) metal femoral head was used as the stationary component. The results for BS showed complex time-dependent behaviour, which was not representative of a simple fluid. After a few minutes sliding BS formed a thin adherent film of 10–20 nm, which was attributed to protein absorbance at the surface. This layer was augmented by a hydrodynamic film, which often increased at slow speeds. At the end of the test deposited surface layers of 20–50 nm were measured. Imaging of the contact showed that at slow speeds an apparent ‘phase boundary’ formed in the inlet just in front of the Hertzian zone. This was associated with the formation of a reservoir of high-viscosity material that periodically moved through the contact forming a much thicker film. The study shows that proteins play an important role in the film-forming process and current lubrication models do not capture these mechanisms.

  • Journal article
    Dickinson AS, Browne M, Wilson KC, Jeffers JRT, Taylor ACet al., 2011,

    Pre-clinical evaluation of ceramic femoral head resurfacing prostheses using computational models and mechanical testing

    , PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART H-JOURNAL OF ENGINEERING IN MEDICINE, Vol: 225, Pages: 866-876, ISSN: 0954-4119
  • Journal article
    Ingram M, Noles J, Watts R, Harris S, Spikes HAet 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
  • Journal article
    Ingram M, Noles J, Watts R, Harris S, Spikes HAet 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
  • Journal article
    Hoshino K, Yagishita K, Tagawa K, Spikes Het 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.

  • Journal article
    Stoehr T, Eisenberg B, Suchert E, Spikes H, Fan Jet 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.

  • Journal article
    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.

  • Journal article
    Reddyhoff T, Ku ISY, Holmes AS, Spikes HAet 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.

  • Journal article
    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 stiffness 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 ms, 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%. 2011 American Society of Mechanical Engineers.

  • Journal article
    Ku ISY, Reddyhoff T, Holmes AS, Spikes HAet al., 2011,

    Wear of silicon surfaces in MEMS

    , Wear, Vol: 271, Pages: 1050-1058, 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 15. h 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 sho

  • Journal article
    Reddyhoff T, Ku ISY, Holmes AS, Spikes HAet al., 2011,

    Friction modifier behaviour in lubricated MEMS devices

    , Tribology Letters, Vol: 41, Pages: 239-246, 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. 2010 Springer Science+Business Media, LLC.

  • Journal article
    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. 2010 Springer Science+Business Media, LLC.

  • Journal article
    Heyes DM, Smith ER, Dini D, Zaki TAet al., 2011,

    The equivalence between Volume Averaging and Method of Planes deļ¬nitions of the pressure tensor at a plane

    , J. Chem. Phys, Vol: 135, Pages: 024512-024512
  • Journal article
    Wong JSS, Hong L, Bae SC, Granick Set al., 2010,

    Fluorescence Recovery after Photobleaching Measurements of Polymers in a Surface Forces Apparatus

    , Journal of Polymer Science. Part B, Polymer Physics, Vol: 48
  • Journal article
    Ahmad M, Alexandrou I, Al-Nuaimy W, Amavasai BP, An YY, Ariwa E, Arteche J, Audrino F, Ayesh A, Baber C, Bailey C, Balkan N, Barria J, Bartosova J, Benkrid K, Bleijs H, Bluck M, Bose I, Bouzas PR, Braiden PM, Brdys M, Burriesci G, Cannataro M, Carvalho A, Chang CC, Chen D, Chen GG, Chen YS, Chiclana F, Cooke A, Das DB, Davis DN, Dayoub I, Deb S, Demetriou IC, Devai F, Dilmaghani RS, Dini D, Drikakis D, Durkan C, Durodola J, Etebar K, Fenn P, Figueiredo A, Florou G, Freear S, Gabrys B, Galbraith GH, Gaskell PH, Gaura E, Ge ZQ, Ghafouri-Shiraz H, Ghavami M, Giannopoulos K, Gonzalez REP, Gracia AM, Grecos C, Guan L, Gulpinar N, Guo R, Guo Y, Hardalupas Y, He L, Herrero JR, Hicks BJ, Hines EL, Hodgson S, Horsfall A, Hosein P, Hu F, Hu H, Ijomah W, Ming J, James A, Jancovic P, Jhumka A, Kamareddine F, Kannan R, Karsligil ME, Katircioglu ST, Khalid A, Kokossis A, Kontis K, Kulekci MO, Laukaitis A, Leeson M, Limbachiya MC, Li L, Li L, Lin P, Ling WK, Macias Lopez EM, Lovas T, Luglio M, Mainardi S, Mahanti PK, Marinos I, Maropoulos P, Mativenga P, Mavrommatis Get al., 2010,

    WCE 2010 - World Congress on Engineering 2010: Preface

    , WCE 2010 - World Congress on Engineering 2010, Vol: 2
  • Journal article
    Oldfield M, Dini D, Rodriguez Y Baena F, 2010,

    Detailed finite element simulations of probe insertion into solid elastic material using a cohesive zone approach.

    , Conference proceedings : ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference, Pages: 3198-3201, ISSN: 1557-170X

    In this paper a method is presented for detailed finite element modelling of probe insertion into an elastic material. This is part of an ongoing investigation into the mechanics of a novel, biomimetic, soft-tissue probe currently under development at Imperial College, London. Analysis is performed using a 'cohesive zone' approach by integrating multiple cohesive elements into a finite element mesh using Abaqus software. Cohesive zones with variable crack paths, generated by both remote tensile and contact loading, and substantial probe penetration along an arbitrarily curved crack path are demonstrated. These advances are critical to understanding probe interactions for the development of an existing prototype and control strategy.

  • Conference paper
    Vengudusamy B, Green JH, Lamb GD, Spikes HAet 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).

  • Conference paper
    Benedet J, Green JH, Lamb GD, Spikes HAet 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).

  • Journal article
    Spikes HA, 2010,

    Fourth World Tribology Conference

    , LUBRICATION SCIENCE, Vol: 22, Pages: 415-416, ISSN: 0954-0075
  • Journal article
    Myant C, Fowell M, Spikes HA, Stokes JRet al., 2010,

    An Investigation of Lubricant Film Thickness in Sliding Compliant Contacts

    , TRIBOLOGY & LUBRICATION TECHNOLOGY, Vol: 66, Pages: 46-+, ISSN: 1545-858X
  • Journal article
    Giacopini M, Fowell MT, Dini D, Strozzi Aet al., 2010,

    A Mass-Conserving Complementarity Formulation to Study Lubricant Films in the Presence of Cavitation

    , JOURNAL OF TRIBOLOGY-TRANSACTIONS OF THE ASME, Vol: 132, ISSN: 0742-4787
  • Journal article
    Arora H, Cann PM, 2010,

    Lubricant film formation properties of alkyl imidazolium tetrafluoroborate and hexafluorophosphate ionic liquids

    , TRIBOLOGY INTERNATIONAL, Vol: 43, Pages: 1908-1916, ISSN: 0301-679X
  • Journal article
    Reina S, Hills DA, Dini D, 2010,

    Contact of a rigid cylinder indenting an elastic layer sliding over a rigid substrate

    , EUROPEAN JOURNAL OF MECHANICS A-SOLIDS, Vol: 29, Pages: 772-783, ISSN: 0997-7538
  • Journal article
    Smeeth M, Spikes H, Gunsel S, 2010,

    Boundary Film Formation by Viscosity Index Improvers

    , TRIBOLOGY & LUBRICATION TECHNOLOGY, Vol: 66, Pages: 54-+, ISSN: 1545-858X

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