Publications
472 results found
Guegan J, Southby M, Spikes H, 2019, Friction modifier additives, synergies and antagonisms, Tribology Letters, Vol: 67, ISSN: 1023-8883
There is growing interest in reducing friction in lubricated machine components to thereby increase the energy efficiency of machines. One important way to minimise friction is to employ friction modifier additives to reduce friction in thin film boundary lubrication conditions. There are currently three main types of friction modifier additive, organic friction modifiers, oil soluble organomolybdenum friction modifiers and functionalised polymers. In common practice, a single such additive is generally employed in a formulated lubricant, but it is of interest to explore whether combinations of two friction modifier additives may prove beneficial. In this study, the performance of eight commercial friction modifier additives spanning all three main types was first measured in three quite different friction tests. The aim was to identify the contact conditions under which each additive was most effective. Additive solutions in both a base oil and a formulated engine oil were investigated. In general, functionalised polymers were most beneficial in sliding–rolling contacts, while oil soluble organomolybdenum friction modifiers worked best in severe, reciprocating sliding conditions. However, all friction modifier additive response was strongly affected by the other additives present in formulated engine oils. The friction performance of combinations of friction modifier additives was then explored. When two different friction modifiers additives were combined in solution, several possible outcomes were observed. The most common was for one of the additives to predominate, to give friction that was characteristic of that additive alone, while in some cases friction lay between the values produced by either additive on its own. In a few cases the additives behaved antagonistically so that the combination gave higher friction than either additive by itself. In a few cases true synergy was observed, where a combination of two additives produced lower friction in a g
Dawczyk J, Morgan N, Russo J, et al., 2019, Film thickness and friction of ZDDP tribofilms, Tribology Letters, Vol: 67, ISSN: 1023-8883
Tribofilm formation by several zinc dialkyl- and diaryldithiophosphate (ZDDP) solutions in thin film rolling-sliding conditions has been investigated. A primary, a secondary alkyl and a mixed alkyl ZDDP show similar rates of film formation and generate films typically 150 nm thick. Another secondary ZDDP forms a tribofilm much faster and the film is partially lost after extended rubbing. An aryl ZDDP forms a tribofilm much more slowly. The films all have a pad-like structure, characterised by flat pad regions separated by deep valleys. Three different techniques have been used to analyse the thickness and morphology of the tribofilms: spacer layer imaging (SLIM), scanning white light interferometry (SWLI) of the gold-coated film and contact mode atomic force microscopy (AFM). The SLIM method measures considerably thicker films than the other two techniques, probably because of lack of full conformity of a glass disc loaded against the rough tribofilm. No evidence of a highly viscous layer on top of the solid tribofilm is seen. SWLI and contact mode AFM measure similar film thicknesses. The importance of coating the tribofilm with a reflective layer prior to using SWLI is confirmed. As noted in previous work, the formation of a ZDDP tribofilm is accompanied by a marked shift in the Stribeck friction curve towards higher entrainment speed. For a given ZDDP this shift is found to correlate with the measured tribofilm roughness, proving that it results from the influence of this roughness on fluid entrainment in the inlet.
Peng B, Spikes H, Kadiric A, 2019, The development and application of a scuffing test based on contra-rotation, Tribology Letters, Vol: 67, ISSN: 1023-8883
Scuffing is a surface failure mode that occurs in sliding–rolling contacts subjected to high loads and high sliding speeds, such as those in gears and cam-followers. Owing to its sudden onset, rapid progression and dependence on both fluid and boundary lubricant films, scuffing is difficult to study in a repeatable manner. This paper describes further development of a recently proposed scuffing test method based on contra-rotation, its extension to higher loads using a new experimental set-up and its application to study the onset of scuffing with a selection of model and fully-formulated oils. The method employs two surfaces moving in opposite directions under rolling–sliding conditions, with a fixed load and step-wise increasing sliding speed. By decoupling the entrainment and sliding speeds, the method allows the effects of lubricant formulation on scuffing performance to be isolated from the influence of viscosity. The approach achieves high sliding speeds in parallel with low entrainment speeds, while minimising the undesirable effects of surface wear and frictional heating. The proposed test is relatively fast and economical, with total test time of about 30 min including specimen cleaning and set-up. Results show that the newly implemented modifications have improved the repeatability of the test method, so that the number of repeat tests required for reliable oil ranking results is minimal. Tests with model and fully-formulated oils show that the onset of scuffing is characterised by a sharp and unrecoverable increase in friction and accompanied by the destruction of any boundary films. All tests show that the relationship load × speedn = constant holds at scuffing, with the exact value of the exponent n being dependent on the oil formulation. Additivised oils were shown to have enhanced scuffing resistance, which arises from their ability to postpone the uncontrollable rise in friction to higher sliding speeds. Finally, the critical maximu
Reddyhoff T, Schmidt A, Spikes H, 2019, Thermal conductivity and flash temperature, Tribology Letters, Vol: 67, Pages: 22-22, ISSN: 1023-8883
The thermal conductivity is a key property in determining the friction-induced temperature rise on the surface of sliding components. In this study, a Frequency Domain Thermoreflectance (FDTR) method is used to measure the thermal conductivity of a range of tribological materials (AISI 52100 bearing steel, silicon nitride, sapphire, tungsten carbide and zirconia). The FDTR technique is validated by comparing measurements of pure germanium and silicon with well-known values, showing discrepancies of less than 3%. For most of the tribological materials studied, the thermal conductivity values measured are reasonably consistent with values found in the literature. However the measured thermal conductivity of AISI 52100 steel (21 W/mK) is less than half the value cited in the literature (46 W/mK). Further bulk thermal conductivity measurements show that this discrepancy arises from a reduction in thermal conductivity of AISI 52100 due to through-hardening. The thermal conductivity value generally cited and used in the literature represents that of soft, annealed alloy, but through-hardened AISI 52100, which is generally employed in rolling bearings and for lubricant testing, appears to have a much lower thermal conductivity. This difference has a large effect on estimates of flash temperature and example calculations show that it increases the resulting surface temperatures by 30 to 50%. The revised value of thermal conductivity of bearing steel also has implications concerning heat transfer in transmissions.
Kontou A, Southby M, Morgan N, et al., 2018, Influence of dispersant and ZDDP on soot wear, Tribology Letters, Vol: 66, Pages: 1-15, ISSN: 1023-8883
Diesel engines and gasoline direct injection (GDI) engines both produce soot due to incomplete combustion of the fuel and some enters the lubricant where it accumulates between drain intervals, promoting wear of rubbing engine components. Currently the most favoured mechanism for this wear is that the anti-wear additives present in engine oils, primarily zinc dialkyldithiophosphates (ZDDPs), react very rapidly with rubbing surfaces to form relatively soft reaction products. These are easily abraded by soot, resulting in a corrosive-abrasive wear mechanism. This study has explored the impact of engine oil dispersant additives on this type of wear using combinations of dispersant, ZDDP and carbon black, a soot surrogate. It has been found that both the concentration and type of dispersant are critical in influencing wear. With most dispersants studied, wear becomes very high over an intermediate dispersant concentration range of ca 0.1–0.4 wt% N, with both lower and higher dispersant levels showing much less wear. However a few dispersants appear able to suppress high wear by ZDDP and carbon black over the whole concentration range. A series of experiments have been carried out to determine the origin of this behaviour and it is believed that high levels of dispersant, and, for a few dispersants, all concentration levels, protect the iron sulphide tribofilm initially formed by ZDDP from abrasion by carbon black.
Dawczyk J, Ware E, Ardakani M, et al., 2018, Use of FIB to study ZDDP tribofilms, Tribology Letters, Vol: 66, Pages: 155-155, ISSN: 1023-8883
Focussed ion beam milling (FIB) followed by TEM has been used to study ZDDP tribofilms on rubbed steel surfaces. It has been found that the impact of high energy platinum and gallium ions during FIB causes significant morphological and structural changes to the uppermost 30–50 nm of a ZDDP tribofilm. This can be prevented by the low energy deposition of a quite thick gold layer prior to installation of the sample in the FIB facility. This problem, and its solution, have been quite widely reported in the non-tribology literature but have not previously been highlighted in the application of FIB to study tribological surfaces. It has also been found, using this gold pre-deposition method, that the bulk of the ZDDP tribofilm studied has a polycrystalline structure.
Vladescu SC, Marx N, Fernández L, et al., 2018, Hydrodynamic friction of viscosity-modified oils in a journal bearing machine, Tribology Letters, Vol: 66, ISSN: 1023-8883
The friction properties of a range of viscosity modifier-containing oils in an engine bearing have been studied in the hydrodynamic regime using a combined experimental and modelling approach. The viscometric properties of these oils were previously measured and single equations derived to describe how their viscosities vary with temperature and shear rate (Marx et al. Tribol Lett 66:92, 2018). A journal bearing machine has been used to measure the friction properties of the test oils at various oil supply temperatures, while simultaneously measuring bearing temperature using an embedded thermocouple. This shows the importance of taking account of thermal response in journal bearings since the operating oil film temperature is often considerably higher than the oil supply temperature. For Newtonian oils, friction coefficient measurements made over a wide range of speeds, loads and oil supply temperatures collapse onto a single Stribeck curve when the viscosity used in determining the Stribeck number is based on an effective oil film temperature. Journal bearing machine measurements on VM-containing oils show that these give lower friction than a Newtonian reference oil. A thermo-hydrodynamic model incorporating shear thinning has been used to explore further the frictional properties of the VM-containing oils. These confirm the findings of the journal bearing experiments and show that two key factors determine the friction of the engine bearing; (i) the low shear rate viscosity of the oil at the effective bearing temperature and (ii) the extent to which the blend shear thins at the high shear rate present in the bearing.
Marx N, Fernández L, Barceló F, et al., 2018, Shear thinning and hydrodynamic friction of viscosity modifier-containingoils. Part I: shear thinning behaviour, Tribology Letters, Vol: 66, Pages: 92-92, ISSN: 1023-8883
Viscosity versus shear rate curves have been measured up to 107 s−1 for a range of VM solutions and fully formulated oils of known composition at several temperatures. This shows large differences in the shear thinning tendencies of different engine oil VMs. It has been found that viscosity versus shear rate data at different temperatures can be collapsed onto a single master curve using time–temperature superposition based on a shear rate shift factor. This enables shear thinning equations to be derived that are able to predict the viscosity of a given oil at any shear rate and temperature within the range originally tested. One of the tested lubricants does not show this time temperature superposition collapse. This fluid also exhibits extremely high viscosity index and shear thins more easily at high than at low temperature, unlike all the other solutions tested. This unusual response may originate from the presence on the VM molecules of two structurally and chemically different components. In a companion paper, the master shear thinning curves obtained in this paper are used to explore how VMs impact film thickness and friction in a steadily loaded, isothermal journal bearing [1].
Reddyhoff T, Underwood R, Sayles R, et al., 2018, Temperature measurement of debris particles in EHL contacts, Surface Topography: Metrology and Properties, Vol: 6, ISSN: 2051-672X
Dents caused by entrained debris are now the main cause of fatigue failure in rolling element bearings. It is therefore important to be able to understand and predict the deformation behaviour of particles in elastohydrodynamic contacts. This paper describes a new method to study debris entrainment. This uses a sensitive infrared microscope to map the temperature of a contact between a steel ball and coated sapphire disc as lubricant dispersed with bearing dust is entrained. Full-field thermal maps were acquired at a sufficient rate to monitor the deformation of a single particle on its journey through the contact.Under the low-speed, high-sliding conditions studied, the temperature rise increases from when the particle is trapped by the inlet to reach a peak near the contact centre, where shearing is a maximum. Under these conditions, temperature rises are typically of the order of 10 °C, which is significantly lower than has been predicted theoretically. Even lower temperature rises were observed under pure rolling conditions, since minimal shearing occurs.Experimental results are also compared with existing models used to predict particle behaviour. Measured radiation distributions confirm qualitatively the ductile particle deformation mechanisms originally proposed by Hamer et al.
Marx N, Fernández L, Barceló F, et al., 2018, Shear Thinning and Hydrodynamic Friction of Viscosity Modifier-ContainingOils. Part II: Impact of Shear Thinning on Journal BearingFriction, Tribology Letters, Vol: 66, Pages: 91-91, ISSN: 1023-8883
Zhang J, Spikes H, 2018, Mechanochemistry in high shear stress elastohydrodynamic contacts, 255th National Meeting and Exposition of the American-Chemical-Society (ACS) - Nexus of Food, Energy, and Water, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727
Spikes HA, 2018, Stress-augmented thermal activation: Tribology feels the force, Friction, Vol: 6, Pages: 1-31, ISSN: 2223-7690
In stress-augmented thermal activation, the activation energy barrier that controls the rate of atomic and molecular processes is reduced by the application of stress, with the result that the rate of these processes increases exponentially with applied stress. This concept has particular relevance to Tribology, and since its development in the early twentieth century, it has been applied to develop important models of plastic flow, sliding friction, rheology, wear, and tribochemistry. This paper reviews the development of stress-augmented thermal activation and its application to all of these areas of Tribology. The strengths and limitations of the approach are then discussed and future directions considered. From the scientific point of view, the concept of stress-augmented thermal activation is important since it enables the development of models that describe macroscale tribological performance, such as friction coefficient or tribofilm formation, in terms of the structure and behaviour of individual atoms and molecules. This both helps us understand these processes at a fundamental level and also provides tools for the informed design of lubricants and surfaces.
Zhang J, Wong J, Dini D, et al., 2018, Mechanochemical film formation by ZDDP, 6th Asia International Conference on Tribology (ASIATRIB), Publisher: MALAYSIAN TRIBOLOGY SOC-MYTRIBOS, Pages: 138-139
Kontou A, Southby M, Spikes HA, 2017, Effect of steel hardness on soot wear, Wear, Vol: 390-391, Pages: 236-245, ISSN: 0043-1648
Due to incomplete combustion, high levels of soot can accumulate in engine lubricants between drain intervals. This soot can promote wear of engine parts such as timing chains and cam followers. One standard approach to reducing wear is to increase the hardness of the rubbing components used. According to the Archard wear equation, wear rate should be broadly inversely proportional to hardness. To explore this approach for controlling soot wear, wear tests have been conducted in a High Frequency Reciprocating Rig (HFRR) with HFRR steel discs of various hardness against a hard steel ball. Carbon black (soot surrogate) dispersions in model lubricants based on solutions of ZDDP and dispersant in GTL base oils have been studied. Wear volumes have been measured and wear scars and tribofilms analysed using scanning white light interferometry and SEM-EDS. It is found that, while most oils show wear that reduces with increasing hardness, for blends that contain both ZDDP and carbon black, wear rate markedly increases with disc hardness as the latter approaches the hardness of the ball. The results support the prevalence of a corrosive-abrasive wear mechanism when carbon black and ZDDP are both present in a lubricant and suggests that selection of very hard surfaces may not be a useful way to control soot.
Ewen J, 2017, Molecular dynamics simulations of lubricants and additives
Shimizu Y, Spikes HA, 2017, The Influence of Aluminium–Silicon Alloy on ZDDP Tribofilm Formation on the Counter-Surface, Tribology Letters, Vol: 65, ISSN: 1023-8883
In order to reduce vehicle weight and thus improve fuel economy, aluminium (Al) alloys have been increasingly adopted as contact surfaces such as piston skirts and cylinder liners in current engines. In general, hypereutectic Al–Si alloys are used, in which hard silicon grains are embedded in a softer Al–Si single phase matrix. It is reported that after rubbing, the matrix is removed to leave silicon grains protruding from the surface. However, the response of the counter-surface by these silicon grains is rarely investigated. In this study, mini traction machine–space layer imaging (MTM–SLIM) has been used to monitor tribofilm formation in situ and investigate the evolution of both surfaces in the contact of a steel ball on an Al–Si disc lubricated by ZDDP solution. In low-load conditions, the top layer of aluminium on the Al–Si disc is removed physically to leave silicon grains protruding from the surface, while ZDDP tribofilm pads are formed mainly on the grains. On the counter-surface (steel ball), ZDDP tribofilms are formed and build up with no wear scars. In high-load conditions, deep gaps are observed to form initially round the silicon grains on the disc. During rubbing, these become shallower, while the silicon grains start to protrude. On the steel ball, ZDDP tribofilm is generated initially over the whole rubbing track, but then the tribofilm in the middle of the track is almost completely removed by rubbing against the protruding silicon grains. Wear of the underlying steel surface then ensues.
Marx N, Ponjavic A, Taylor RI, et al., 2017, Study of permanent shear thinning of VM polymer solutions, Tribology Letters, Vol: 65, ISSN: 1023-8883
The ultrashear viscometer (USV) has been adapted and employed to investigate the permanent shear thinning of polystyrene solutions in a series of phthalate ester base fluids. The permanent shear stability index based on viscosities measured at 106 s−1, PSSI(106), has been found to be a convenient way to express the magnitude of permanent shear thinning. When comparing permanent shear thinning at various shear rates in the USV, it is very important to take account of the different times of shear that are present at different shear rates. The PSSI(106) value divided by the total time of shear is then a useful way of quantifying and comparing permanent shear thinning rates. Tests using polystyrene in different viscosity base fluids have shown that this rate of permanent shear thinning depends on shear stress and not shear rate and varies linearly with polymer concentration. The rate of permanent shear thinning also varies exponentially with shear stress, suggestive of a stress-promoted polymer breakdown process. By using a small volume of test fluid in the USV and solvent extraction after a test, it has proved possible to obtain molecular weight distributions of polymer after shear using gel permeation chromatography (GPC). This indicates that the polymer breakdown process is different at low and high polymer concentrations, with molecule fragmentation at low polymer concentration but mid-chain scission at high concentration. A key feature of the USV is that, unlike other methods currently used to measure permanent shear thinning behaviour of engine oils, it subjects the test fluid to well-defined, controllable high shear conditions. Coupled with the use of GPC, this makes it possible for the first time to relate quantitatively the permanent shear thinning of engine oils to shear conditions and to polymer degradation response.
Ewen J, Gattinoni C, Zhang J, et al., 2017, On the effect of confined fluid molecular structure on nonequilibrium phase behaviour and friction, Physical Chemistry Chemical Physics, Vol: 19, Pages: 17883-17894, ISSN: 1463-9084
A detailed understanding of the behaviour of confined fluids is critical to a range of industrial applications, for example to control friction in engineering components. In this study, a combination of tribological experiments and confined nonequilibrium molecular dynamics simulations has been used to investigate the effect of base fluid molecular structure on nonequilibrium phase behaviour and friction. An extensive parameter study, including several lubricant and traction fluid molecules subjected to pressures (0.5–2.0 GPa) and strain rates (104–1010 s−1) typical of the elastohydrodynamic lubrication regime, reveals clear relationships between the friction and flow behaviour. Lubricants, which are flexible, broadly linear molecules, give low friction coefficients that increase with strain rate and pressure in both the experiments and the simulations. Conversely, traction fluids, which are based on inflexible cycloaliphatic groups, give high friction coefficients that only weakly depend on strain rate and pressure. The observed differences in friction behaviour can be rationalised through the stronger shear localisation which is observed for the traction fluids in the simulations. Higher pressures lead to more pronounced shear localisation, whilst increased strain rates lead to a widening of the sheared region. The methods utilised in this study have clarified the physical mechanisms of important confined fluid behaviour and show significant potential in both improving the prediction of elastohydrodynamic friction and developing new molecules to control it.
Guegan J, Kadiric A, Gabelli A, et al., 2017, Reply to the 'Comment on "The Relationship Between Friction and Film Thickness in EHD Point Contacts in the Presence of Longitudinal Roughness'' by Guegan, Kadiric, Gabelli, & Spikes' by Scott Bair, Tribology Letters, Vol: 65, ISSN: 1023-8883
Taylor LJ, Spikes HA, 2017, Friction-Enhancing Properties of ZDDP Antiwear Additive: Part II-Influence of ZDDP Reaction Films on EHD Lubrication, TRIBOLOGY & LUBRICATION TECHNOLOGY, Vol: 73, Pages: 50-56, ISSN: 1545-858X
Spikes HA, 2017, Comment on: Rheology of an Ionic Liquid with Variable Carreau Exponent: A Full Picture by Molecular Simulation with Experimental Contribution, by Nicolas Voeltzel, Philippe Vergne, Nicolas Fillot, Nathalie Bouscharain, Laurent Joly, Tribology Letters (2016) 64: 25, Tribology Letters, Vol: 65, ISSN: 1023-8883
Ewen J, Gattinoni C, Spikes H, et al., 2017, Nonequilibrium molecular dynamics simulations of organic friction modifiers, 253rd National Meeting of the American-Chemical-Society (ACS) on Advanced Materials, Technologies, Systems, and Processes, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727
Taylor LJ, Spikes HA, 2017, Friction-enhancing properties of ZDDP antiwear additive: Part I-friction and morphology of ZDDP reaction films, Tribology and Lubrication Technology, Vol: 73, Pages: 54-62, ISSN: 1545-858X
Many phosphorus-based antiwear films, including those formed by zinc dialkyl dithiophosphates (ZDDP), cause a significant increase in friction in thin film, high-pressure, lubricated contacts. This can have a deleterious effect on engine oil fuel efficiency. Previous work has shown that friction is increased not under boundary, but under mixed lubrication conditions and it has been suggested that this phenomenon results from an effective roughening of the rubbing surfaces by the formation of unevenly-distributed reaction films. In the current paper it is shown that, when other additives commonly used in engine oils are added to ZDDP solutions, quite smooth ZDDP reaction films can result. Despite this, the ZDDP still produces a marked increase in friction in mixed lubrication conditions, which suggests that surface roughening is not the main origin of friction enhancement by ZDDP reaction films. In a companion paper, Part II, it is shown that ZDDP reaction films, whether rough or smooth, enhance friction by inhibiting the entrainment of liquid lubricant into rubbing contacts, thereby reducing the elastohydrodynamic oil film thickness (13).
Ponjavic A, Lemaigre T, Southby M, et al., 2017, Influence of NOx and Air on the Ageing Behaviour of MoDTC, TRIBOLOGY LETTERS, Vol: 65, ISSN: 1023-8883
Molybdenum dialkyldithiocarbamates (MoDTCs) are very effective friction modifier additives for use in engine oils and other lubricants. However, as engine oils age during extended drain intervals, MoDTCs can lose some or all of their ability to reduce friction and this is generally believed to result from their oxidative degradation. In this study, MoDTC solutions in base oil have been subjected to oil ageing in a controlled NOx/air flow rate, controlled temperature test apparatus and the effect of ageing on the ability of the MoDTC to reduce friction has been explored. As shown in previous studies, the additive’s friction-reducing properties are completely lost after a quite short period of ageing at 160 °C. However, it was found that at this temperature the presence of NOx has little if any influence on the rate of friction loss, indicating that the latter is controlled primarily by the rate of reaction of oxygen with the base oil and thus the rate of consumption of MoDTC as a peroxide decomposer. By contrast, when ageing tests are carried out at lower temperatures it is found that NOx has a very strong effect on the rate at which MoDTC loses its ability to reduce friction, so that at 100 °C NOx accelerates this rate by two orders of magnitude compared to air alone. This suggests that at low temperatures the rate at which MoDTC is consumed is controlled by its reaction as a radical inhibitor with NOx species.
Delgado MA, Quinchia LA, Spikes HA, et al., 2017, Suitability of ethyl cellulose as multifunctional additive for blends of vegetable oil-based lubricants, JOURNAL OF CLEANER PRODUCTION, Vol: 151, Pages: 1-9, ISSN: 0959-6526
In a previous study, ethyl cellulose was successfully blended with castor oil and the results demonstrated its suitability to be used as additive to expand the range of operating conditions under which fluid film lubrication is sustained, mainly at high temperature. However, apparent solubility problems were detected when mixed with other vegetable oils with lower polarity than castor oil. In this work, a suitable combination of ethyl cellulose (EC) with both high oleic sunflower (HOSO) and castor (CO) oils was found able to reach stable and non gel-like blends with viscosities at 40 °C ranging between 62 and 493 cSt, and viscosity indexes fitting into group III (VI ≥ 120) of API classification of base fluids. The ternary HOSO/CO/EC blend showed an important reduction in friction coefficient at low entrainment speed, and generated a stable EHD-film at 100 °C of around 20 nm, which suggests better boundary properties than HOSO/EC or CO/EC blends. On the other hand, ethyl cellulose hindered wax crystallization process of these vegetable oil-based lubricants at 5 °C, yielding comparable results to those obtained with standard polymethacrylate backbone additives. Therefore, the suitable combination of both castor and HOSO with EC as multifunctional additive allow a set of eco-friendly base fluids to be formulated with a wide kinematic viscosity range, better viscosity-temperature dependence than many mineral or synthetic oils lubricants and excellent boundary lubrication properties, making them suitable for many lubricant applications.
Wang P, Zhang J, Spikes HA, et al., 2016, Development of hydrodynamic micro-bearings, 16th International Conference on Micro and Nanotechnology for Power Generation and Energy Conversion Applications (PowerMEMS 2016), Publisher: IOP Publishing, ISSN: 1742-6588
This paper describes the modelling and testing of mm-scale hydrodynamic bearings which are being developed to improve the efficiency of a cm-scale turbine energy harvester, whose efficiency was previously limited by poorly lubricated commercial jewel-bearings. The bearings were fabricated using DRIE and their performance was assessed using a custom built MEMS tribometer. Results demonstrate that acceptably low friction is achieved when low viscosity liquid lubricants are used in combination with an appropriate choice of friction modifier additive. Further reduction in friction is demonstrated when the step height of bearing is adjusted in accordance with hydrodynamic theory. In parallel with the experiments, hydrodynamic lubricant modelling has been carried out to predict and further optimize film thickness and friction performance. Modelling results are presented and validated against experimental friction data.
Zhang J, Tan A, Spikes H, 2016, Effect of base oil structure on elastohydrodynamic friction, Tribology Letters, Vol: 65, ISSN: 1023-8883
The EHD friction properties of a wide range of base fluids have been measured and compared in mixed sliding–rolling conditions at three temperatures and two pressures. The use of tungsten carbide ball and disc specimens enabled high mean contact pressures of 1.5 and 2.0 GPa to be obtained, comparable to those present in many rolling bearings. The measurements confirm the importance of molecular structure of the base fluid in determining EHD friction. Liquids having linear-shaped molecules with flexible bonds give considerably lower friction than liquids based on molecules with bulky side groups or rings. EHD friction also increases with viscosity for liquids having similar molecular structures. Using pure ester fluids, it is shown that quite small differences in molecular structure can have considerable effects on EHD friction. The importance of temperature rise in reducing EHD friction at slide–roll ratios above about 5% has been shown. By measuring EHD friction at several temperatures and pressures as well as EHD film thickness, approximate corrections to measured EHD friction data have been made to obtain isothermal shear stress and thus EHD friction curves. These show that under the conditions tested most low molecular weight base fluids do not reach a limiting friction coefficient and thus shear stress. However, two high traction base fluids appear to reach limiting values, while three linear polymeric base fluids may also do so. Constants of best fit to a linear/logarithmic isothermal shear stress/strain rate relationship have been provided to enable reconstruction of isothermal EHD friction behaviour for most of the fluids tested.
Shimizu Y, Spikes HA, 2016, The tribofilm formation of ZDDP under reciprocating pure sliding conditions, Tribology Letters, Vol: 64, ISSN: 1573-2711
The anti-wear and anti-seizure performance and action mechanisms of zinc dithiophosphate (ZDDP) have been investigated under reciprocating pure sliding conditions to simulate piston ring and cylinder liner assembly, using new techniques. The Mini Traction Machine–Space Layer Imaging is a useful method for monitoring tribofilm formation by ZDDPs. However, tests are generally carried out in mixed sliding–rolling conditions and ZDDP film formation in reciprocating pure sliding conditions is rarely investigated. In this paper, the authors describe an investigation of ZDDP film formation in stationary ball on reciprocating disc pure sliding conditions and compare the results to those obtained in unidirectional pure sliding conditions. In unidirectional pure sliding conditions, the worn area on the ball expands with test time. By contrast, in reciprocating pure sliding conditions, tribofilm forms on the stationary ball and no significant damage occurs. In the initial tribofilm formation under reciprocating pure sliding conditions, solid particulate tribofilm with a high concentration of S forms initially in the contact area and subsequently breaks up. During further rubbing, a Zn- and P-rich tribofilm forms on the comminuted sulphur-rich tribofilm and also the area where the initial tribofilm was removed.
Guegan J, Kadiric A, Gabelli A, et al., 2016, The relationship between friction and film thickness in EHD point contacts in the presence of longitudinal roughness, Tribology Letters, Vol: 64, ISSN: 1573-2711
This study investigates friction and film thickness in elastohydrodynamic contacts of machined, rough surfaces, where roughness is dominated by longitudinal ridges parallel to the rolling/sliding direction. A ball-on-disc tribometer was used to simultaneously measure friction and film thickness in rough contacts as well as with nominally smooth specimens for comparison. The studied rough surfaces were selected so that the influence of the root-mean-square roughness and roughness wavelength can be assessed. Friction and film measurements were taken over a range of slide–roll ratios and speeds and with two lubricating oils with different viscosities, hence covering a wide range of specific film thicknesses. The measurements with the nominally smooth specimens show that friction is strongly influenced by thermal effects at high SRRs and that the transition from mixed/boundary to full EHD lubrication occurs at lambda ratios greater than three. At low speeds, the rough specimens are found to generate higher friction than the smooth ones for all the roughness structures considered, and this is shown to be related to the thinner minimum film thickness. Comparison of friction in rough and smooth contacts shows that the total friction in rough contacts can be divided into two components: one that is equivalent to friction in smooth contacts under the same conditions and is dependent on the slide–roll ratio, and the other that is due to the presence of roughness and is independent of the slide–roll ratio under the conditions tested. Further analysis of the minimum film thickness on tops of roughness ridges indicates that even after the full lift-off, an effect of the roughness on friction persists and is most likely related to the local shear stress in the micro-EHD contacts on the top of roughness ridges. At even higher speeds, the difference in friction between the rough and smooth specimens vanishes.
Shimizu Y, Spikes HA, 2016, The influence of slide–roll ratio on ZDDP tribofilm formation, Tribology Letters, Vol: 64, ISSN: 1573-2711
The anti-wear performance and action mecha-nisms of zinc dithiophosphate (ZDDP) have been investi-gated under various test conditions. The Mini TractionMachine–Space Layer Imaging (MTM–SLIM) is a widelyused and useful method for monitoring tribofilm formationby ZDDPs. However, tests are generally carried out inmixed sliding–rolling conditions, typically between 50 %SRR (slide–roll ratio) and 100 % SRR. In this paper, theauthors describe an investigation of ZDDP film formationat SRRs much higher than 100 % SRR, including puresliding conditions using a novel MTM–SLIM technique. Athigh SRRs, ZDDP tribofilms form without damaging theball surfaces so long as both surfaces move above athreshold speed with respect to the contact, regardless ofwhether the two surfaces move in the same or opposingdirections. In pure sliding conditions, although the wornarea expands with time under pure sliding conditionsshowing that wear takes place, tribofilms are still built upthroughout a test and the ZDDP has a beneficial effect onwear rate. The very early stages of film formation arestudied to show that a tribofilm with a high concentrationof S is formed initially and then replaced with a film havinga high concentration of Zn and P.
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