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  • Journal article
    Wu P-J, Masouleh MI, Paterson C, Dini D, Török P, Overby DR, Kabakova IVet al., 2019,

    Detection of proteoglycan loss from articular cartilage using Brillouin microscopy, with applications to osteoarthritis

    , Biomedical Optics Express, Vol: 10, Pages: 2457-2466, ISSN: 2156-7085

    The degeneration of articular cartilage (AC) occurs in osteoarthritis (OA), which is a leading cause of pain and disability in middle-aged and older people. The early disease-related changes in cartilage extra-cellular matrix (ECM) start with depletion of proteoglycan (PG), leading to an increase in tissue hydration and permeability. These early compositional changes are small (<10%) and hence difficult to register with conventional non-invasive imaging technologies (magnetic resonance and ultrasound imaging). Here we apply Brillouin microscopy for detecting changes in the mechanical properties and composition of porcine AC. OA-like degradation is mimicked by enzymatic tissue digestion, and we compare Brillouin microscopy measurements against histological staining of PG depletion over varying digestion times and enzyme concentrations. The non-destructive nature of Brillouin imaging technology opens new avenues for creating minimally invasive arthroscopic devices for OA diagnostics and therapeutic monitoring.

  • Journal article
    Geng Z, Shi G, Shao T, Liu Y, Duan D, Reddyhoff Tet al., 2019,

    Tribological behavior of patterned TiAlN coatings at elevated temperatures

    , SURFACE & COATINGS TECHNOLOGY, Vol: 364, Pages: 99-114, ISSN: 0257-8972
  • Journal article
    Hu S, Vladescu S-C, Puhan D, Huang W, Shi X, Peng Zet al., 2019,

    Bi-Gaussian stratified theory to understand wettability on rough topographies

    , Surface and Coatings Technology, ISSN: 0257-8972
  • Journal article
    Hu S, Reddyhoff T, Puhan D, Vladescu S-C, Huang W, Shi X, Dini D, Peng Zet al., 2019,

    Bi-Gaussian stratified wetting model on rough surfaces

    , Langmuir, Vol: 35, Pages: 5967-5974, ISSN: 0743-7463

    Wetting mechanisms on rough surfaces were understood from either a monolayer or a multiscale perspective. However, it has recently been shown that the bi-Gaussian stratified nature of real surfaces should be accounted for when modeling mechanisms of lubrication, sealing, contact, friction, acoustic emission, and manufacture. In this work, a model combining Wenzel and Cassie theories was put forward to predict the static contact angle of a droplet on a bi-Gaussian stratified surface. The model was initially applied to numerically simulated surfaces and subsequently demonstrated on hydrophilic steel and hydrophobic self-assembled monolayer specimens with preset bi-Gaussian stratified topographies. In the Wenzel state, both the upper and the lower surface components are fully wetted. In the Cassie state, the upper component is still completely wetted, while the lower component serves as gas traps and reservoirs. By this model, wetting evolution was assessed, and the existence of different wetting states and potential state transitions was predicted.

  • Journal article
    Fatti G, Righi MC, Dini D, Ciniero Aet al., 2019,

    First-principles insights into the structural and electronic properties of polytetrafluoroethylene in its high-pressure phase (form III)

    , Journal of Physical Chemistry C, Vol: 123, Pages: 6250-6255, ISSN: 1932-7447

    Polytetrafluoroethylene (PTFE), commercially known as Teflon, is one the most effective insulating polymers for a wide range of applications because of its peculiar electronic, mechanical, and thermal properties. Several studies have attempted to elucidate the structural and electronic properties of PTFE; however, some important aspects of its structural and electronic characteristics are still under debate. To shed light on these fundamental features, we have employed a first-principles approach to optimize the two coexisting PTFE structures (monoclinic and orthorhombic) at high pressure by using the characteristic zigzag planar chain configuration. Our electronic analysis of the optimized structures shows charge transfer from carbons to fluorines, supporting the PTFE electronegative character. In addition, band structure calculations show that the band gap is estimated to be around 5 eV, which correlates with previous studies. Moreover, the analysis of the valence and conduction states reveals an intrachain and an interchain character of the charge distribution, suggesting additional insights into the PTFE electronic properties.

  • Journal article
    Ewen J, Gao H, Mueser M, Dini Det al., 2019,

    Shear heating, flow, and friction of confined molecular fluids at high pressure

    , Physical Chemistry Chemical Physics, Vol: 21, Pages: 5813-5823, ISSN: 1463-9076

    Understanding the molecular-scale behavior of fluids confined and sheared between solid surfaces is important for many applications, particularly tribology where this often governs the macroscopic frictional response. In this study, nonequilibrium molecular dynamics simulations are performed to investigate the effects of fluid and surface properties on the spatially resolved temperature and flow profiles, as well as friction. The severe pressure and shear rate conditions studied are representative of the elastohydrodynamic lubrication regime. In agreement with tribology experiments, flexible lubricant molecules give low friction, which increases linearly with logarithmic shear rate, while bulky traction fluids show higher friction, but a weaker shear rate dependence. Compared to lubricants, traction fluids show more significant shear heating and stronger shear localization. Models developed for macroscopic systems can be used to describe both the spatially resolved temperature profile shape and the mean film temperature rise. The thermal conductivity of the fluids increases with pressure and is significantly higher for lubricants compared to traction fluids, in agreement with experimental results. In a subset of simulations, the efficiency of the thermostat in one of the surfaces is reduced to represent surfaces with lower thermal conductivity. For these unsymmetrical systems, the flow and the temperature profiles become strongly asymmetric and some thermal slip can occur at the solid-fluid interface, despite the absence of velocity slip. The larger temperature rises and steeper velocity gradients in these cases lead to large reductions in friction, particularly at high pressure and shear rate.

  • Journal article
    Stevenson H, Jaggard M, Akhbari P, Vaghela U, Gupte C, Cann Pet al., 2019,

    The role of denatured synovial fluid proteins in the lubrication of artificial joints

    , Biotribology, Vol: 17, Pages: 49-63, ISSN: 2352-5738

    CoCrMo ball-on-flat wear tests were carried out with 25 wt% bovine calf serum (25BCS) and human synovial fluid (HSF) to investigate artificial joint lubricating mechanisms. Post-test the wear scar on the disc was measured and surface deposits in and around the rubbed region were analysed by Micro InfraRed Reflection Absorption Spectroscopy (Micro-IRRAS). In most tests the HSF samples gave higher wear than the 25BCS solution; in some cases, up to 77%. After rinsing a similar pattern of surface deposits was observed in and around the wear scar for both the model and HSF. Micro-IRRAS showed the deposits were primarily denatured proteins with an increased β-sheet content. In some cases, trans-alkyl chain/carbonyl components were also present and these were assigned to lipids. Thioflavin T fluorescent imaging also indicated aggregated non-native β-sheet fibrils were present in the deposits and their presence was associated with lower wear. The formation of insoluble, denatured protein films is thought to be the primary lubrication mechanism contributing to surface protection during rubbing. From this and earlier work we suggest inlet shear induces denaturing of proteins resulting in the formation of non-native β-sheet aggregates. This material is entrained into the contact region where it forms the lubricating film. Patient synovial fluid chemistry appears to influence wear, at least in the bench test, and thus could contributes to increased risk of failure, or success, with metal-metal hips. Finally using 25BCS as a reference screening fluid gives an overly optimistic view of wear in these systems.

  • Journal article
    Limbert G, Masen MA, Pond D, Graham HK, Sherratt MJ, Jobanputra R, McBride Aet al., 2019,

    Biotribology of the ageing skin—Why we should care

    , Biotribology, Vol: 17, Pages: 75-90, ISSN: 2352-5738

    Ageing of populations has emerged as one of the most pressing societal, economic and healthcare challenges currently facing most nations across the globe. The ageing process itself results in degradation of physiological functions and biophysical properties of organs and tissues, and more particularly those of the skin. Moreover, in both developed and emerging economies, population ageing parallels concerning increases in lifestyle-associated conditions such as Type 2 diabetes, obesity and skin cancers. When considered together, these demographic trends call for even greater urgency to find clinical and engineering solutions for the numerous age-related deficits in skin function.From a tribological perspective, detrimental alterations of skin biophysical properties with age have fundamental consequences on how one interacts with the body's inner and outer environments. This stems from the fact that, besides being the largest organ of the human body, and also nearly covering its entirety, the skin is a multifunctional interface which mediates these interactions.The aim of this paper is to present a focused review to discuss some of the consequences of skin ageing from the viewpoint of biotribology, and their implications on health, well-being and human activities. Current and future research questions/challenges associated with biotribology of the ageing skin are outlined. They provide the background and motivation for identifying future lines of research that could be taken up by the biotribology and biophysics communities.

  • Journal article
    Vladescu S-C, Putignano C, Marx N, Keppens T, Reddyhoff T, Dini Det al., 2019,

    The percolation of liquid through a compliant seal - an experimental and theoretical study

    , Journal of Fluids Engineering, Vol: 141, Pages: 031101-031101, ISSN: 0098-2202
  • Journal article
    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.

  • Journal article
    Manieri F, Stadler K, Morales-Espejel GE, Kadiric Aet al., 2019,

    The origins of white etching cracks and their significance to rolling bearing failures

    , International Journal of Fatigue, Vol: 120, Pages: 107-133, ISSN: 0142-1123

    Presence of white etching cracks has been widely associated with early failures of rolling bearings in a number of applications, with wind turbine gearbox bearings being the most frequently cited and practically significant example. Despite the recent research efforts, there is yet no universal agreement on the mechanisms of formation of these cracks and little direct evidence of their significance to bearing reliability. In an attempt to address this, this paper proposes a new theory on the origins and significance of white etching cracks. The paper provides systematic experimental evidence in support of this theory through rolling contact fatigue tests performed with AISI 52100 bearing steel specimens on a triple-disc machine over a wide range of contact conditions. The test results show that white etching cracks can be formed with base oils as well as commercially formulated transmission and engine oils. WECs were generated under slide-roll-ratios ranging from 0.05 to 0.3, under positive and negative sliding, different contact pressures and specific film thicknesses ranging from 0.1 to 0.7. No white etching areas were ever observed without the associate crack being present, and it was also shown that white etching areas themselves can be produced in a pure rubbing contact of bearing steels under both lubricated and unlubricated conditions. These results provide direct evidence that the steel transformations that exhibit themselves as white etching areas are formed through rubbing of the existing crack faces, and that the chemical composition of the lubricant and the magnitude and direction of sliding are not the primary driver of WEC formation, in contrast to literature. Instead, the results presented here show that WECs are formed through the action of a specific stress history in time via the following mechanism: (i) Short-lived high contact stresses, which can be caused by a number of factors, act in the initial stages of the component life to initiate early f

  • Journal article
    Yu M, Arana C, Evangelou S, Dini Det al., 2019,

    Quarter-Car Experimental Study for Series Active Variable Geometry Suspension

    , IEEE Transactions on Control Systems Technology, Vol: 27, Pages: 743-759, ISSN: 1063-6536

    In this paper, the recently introduced series active variable geometry suspension (SAVGS) for road vehicles is experimentally studied. A realistic quarter-car test rig equipped with double-wishbone suspension is designed and built to mimic an actual grand tourer real axle, with a single-link variant of the SAVGS and a road excitation mechanism implemented. A linear equivalent modeling method is adopted to synthesize an H-infinity control scheme for the SAVGS, with the geometric nonlinearity compensated. Simulations with a theoretical nonlinear quarter-car indicate the SAVGS potential to enhance suspension performance, in terms of ride comfort and road holding. Practical features in the test rig are further considered and included in the nonlinear model to compensate the difference between the theoretical and testing behaviors. Experiments with a sinusoidal road, a smoothed bump and hole, and a random road are performed to evaluate the SAVGS practical feasibility and performance improvement, the accuracy of the model, and the robustness of the control schemes. Compared with the conventional passive suspension, ride comfort improvements of up to 41% without any deterioration of the suspension deflection are demonstrated, while the SAVGS actuator power is kept very low, at levels below 500 W.

  • Journal article
    Hu S, Reddyhoff T, Wen J, Huang W, Shi X, Dini D, Peng Zet al., 2019,

    Characterization and simulation of bi-Gaussian surfaces induced by material transfer and additive processes

    , Tribology International, ISSN: 0301-679X
  • Journal article
    Wen J, Dini D, Reddyhoff T, 2019,

    Design and optimization of a liquid ring thrust bearing

    , Tribology International, ISSN: 0301-679X

    Liquid menisci at millimeter length scales and smaller exhibit large Laplace pressures. To utilise these effects, liquid ring bearings have recently been developed, which consist of liquid rings confined between alternate superhydrophobic and hydrophilic patterns. We present a detailed experimental and theoretical performance analysis of such bearings. For a single, 100 μm thickness, liquid ring, the maximum supporting force is 0.13 N, which decreases with increasing the ring misalignment. The frictional torque increases linearly with rotational speed until a critical Reynolds number is reached. Above this, an instability occurs due the concave liquid ring meniscus, which further increases friction. These results show how liquid ring bearings can be optimised.

  • Journal article
    Porte E, Cann P, Masen M, 2019,

    Fluid load support does not explain tribological performance of PVA hydrogels

    , Journal of the Mechanical Behavior of Biomedical Materials, Vol: 90, Pages: 284-294, ISSN: 1751-6161

    © 2018 Elsevier Ltd The application of hydrogels as articular cartilage (AC) repair or replacement materials is limited by poor tribological behaviour, as it does not match that of native AC. In cartilage, the pressurisation of the interstitial fluid is thought to be crucial for the low friction as the load is shared between the solid and liquid phase of the material. This fluid load support theory is also often applied to hydrogels. However, this theory has not been validated as no experimental evidence directly relates the pressurisation of the interstitial fluid to the frictional response of hydrogels. This lack of understanding about the governing tribological mechanisms in hydrogels limits their optimised design. Therefore, this paper aims to provide a direct measure for fluid load support in hydrogels under physiologically relevant sliding conditions. A photoelastic method was developed to simultaneously measure the load on the solid phase of the hydrogel and its friction coefficient and thus directly relate friction and fluid load support. The results showed a clear distinction in frictional behaviour between the different test conditions, but results from photoelastic images and stress-relaxation experiments indicated that fluid load support is an unlikely explanation for the frictional response of the hydrogels. A more appropriate explanation, we hypothesized, is a non-replenished lubricant mechanism. This work has important implications for the tribology of cartilage and hydrogels as it shows that the existing theories do not adequately describe the tribological behaviour of hydrogels. The developed insights can be used to optimise the tribological performance of hydrogels as articular cartilage implants.

  • Journal article
    Puhan D, Nevshupa R, Wong J, Reddyhoff Tet al., 2019,

    Transient aspects of plasma luminescence induced by triboelectrification of polymers

    , Tribology International, Vol: 130, Pages: 366-377, ISSN: 0301-679X

    Transient electric gas discharges that occur around sliding interfaces during contact electrification of polymers were studied at millisecond timescales and with micrometre resolution. Deduced vibrational temperatures indicate cold plasma resulting from positive corona discharge. At millisecond timescales, previously unseen rapid discharge events are observed, and modelling suggests that these result from streamer development, triggered by electron emission from the polymer surface. Those which occur over a period of several seconds are shown to be caused by competition between charge generation and the formation of polymer films. The findings explain the interplay between charging and plasma generation and their dependence on wear processes.

  • Journal article
    Dzepina B, Balint D, Dini D, 2019,

    A phase field model of pressure-assisted sintering

    , Journal of the European Ceramic Society, Vol: 39, Pages: 173-182, ISSN: 0955-2219

    The incorporation of an efficient contact mechanics algorithm into a phase field sintering model is presented. Contact stresses on the surface of arbitrarily shaped interacting bodies are evaluated and built into the model as an elastic strain energy field. Energy relaxation through deformation is achieved by diffusive fluxes along stress gradients and rigid body motion of the deforming particles maintain contact between the particles. The proposed model is suitable for diffusion deformation mechanisms occurring at stresses below the yield strength of a defect-free material; this includes Nabarro-Herring creep, Coble creep and pressure-solution. The effect of applied pressure on the high pressure-high temperature (HPHT) liquid phase sintering of diamond particles was investigated. Changes in neck size, particle coordination and contact flattening were observed. Densification rates due to the externally applied loads were found to be in good agreement with a new theory which implicitly incorporates the effect of applied external pressure.

  • Journal article
    Jeffreys S, di Mare L, Liu X, Morgan N, Wong Jet al., 2019,

    Elastohydrodynamic lubricant flow with nanoparticle tracking

    , RSC Advances, Vol: 9, Pages: 1441-1450, ISSN: 2046-2069

    Lubricants operating in elastohydrodynamic (EHD) contacts exhibit local variations in rheological properties when the contact pressure rises. Direct evidence of this behaviour has only been obtained by examining through-thickness velocity profiles U(z) of lubricants in a contact using luminescence-based imaging velocimetry. In the present study, nanoparticles (NPs) are added to polybutene (PB) as tracers to investigate the effect of pressure on the flow of PB in an EHD contact. By tracking NPs in the contact, particle velocity distributions f(U) under various pressures are obtained and found to be pressure dependent. Results show quantitatively that f(U) and U(z) are correlated and thus confirm that U(z) of PB changes from Couette flow to partial plug flow above a critical pressure. This confirmation highlights the complexity of lubricant rheology in a high pressure contact.

  • Journal article
    Ma S, Scaraggi M, Yan C, Wang X, Gorb S, Dini D, Zhou Fet al., 2019,

    Bio-inspired 3D printed locomotion devices based on anisotropic friction

    , Small, Vol: 15, Pages: 1802931-1802931, ISSN: 1613-6810

    Anisotropic friction plays a key role in natural systems, particularly for realizing the purpose of locomotion and strong attachment for the survival of organisms. Of particular interest, here, is the observation that friction anisotropy is promoted numerous times by nature, for example, by wild wheat awn for its targeted and successful seed anchorage and dispersal. Such feature is, however, not fully exploited in man‐made systems, such as microbots, due to technical limitations and lack of full understanding of the mechanisms. To unravel the complex dynamics occurring in the sliding interaction between anisotropic microstructured surfaces, the friction induced by asymmetric plant microstructures is first systematically investigated. Inspired by this, anisotropic polymer microactuators with three‐dimensional (3D) printed microrelieves are then prepared. By varying geometric parameters, the capability of microactuators to generate strong friction anisotropy and controllable motion in remotely stretched cylindrical tubes is investigated. Advanced theoretical models are proposed to understand and predict the dynamic behavior of these synthetic systems and to shed light on the parameters and mechanisms governing their behavior. Finally, a microbot prototype is developed and cargo transportation functions are successfully realized. This research provides both in‐depth understanding of anisotropic friction in nature and new avenues for developing intelligent actuators and microbots.

  • Conference paper
    Spagnoli A, Terzano M, Dini D, 2019,

    Mixed-mode crack propagation during needle penetration for surgical interventions

    , 25th International Conference on Fracture and Structural Integrity, Publisher: ELSEVIER SCIENCE BV, Pages: 775-780, ISSN: 2452-3216
  • Journal article
    Jean-Fulcrand A, Masen MA, Bremner T, Wong JSSet al., 2019,

    Effect of temperature on tribological performance of polyetheretherketone-polybenzimidazole blend

    , Tribology International, Vol: 129, Pages: 5-15, ISSN: 0301-679X

    © 2018 The Authors Polyetheretherketone (PEEK) is one of the most commonly used High Performance Polymers (HPP) although its high temperature performance is poor. In this study, polybenzimidazole (PBI), a HPP with one of the highest glass transition temperatures currently available, is blended to PEEK to form a 50:50 blend (TU60). Tribological performance of the blend (TU60) was investigated by rubbing it against steel at temperatures up to 280 °C. Results obtained are compared to those from neat PEEK and neat PBI. All three polymers were thermally stable during the duration of tests. However chemical analyses on polymeric transfer layers on steel surfaces and polymer debris suggest polymer degradation. The degradation observed is shear-assisted, possibly promoted by shear heating. Indeed the estimated interfacial temperature based on Jaeger model was above the melting point of PEEK in some cases. TU60 outperforms PEEK in all test conditions and PBI at 280 °C. TU60 formed transfer layers on steel similar to that of PEEK. When contact temperature is closed to the melting point of PEEK, PEEK in the TU60 creates a low strength transfer layer which acts as an interfacial lubricant. This reduces friction which in turn reduces PBI degradation in TU60 at high temperature. This work provides a strategy for creating interfacial layers to improve polymer tribological performance while maintaining the integrity of the polymer.

  • Journal article
    Dench J, di Mare L, Morgan N, Wong Jet al., 2018,

    Comparing the molecular and global rheology of a fluid under high pressures

    , Physical Chemistry Chemical Physics, Vol: 20, Pages: 30267-30280, ISSN: 1463-9076

    The viscosity of liquids is a strong function of pressure. While viscosity is relatively easy to measure at low pressure, high-pressure rheology presents significant experimental challenges. As a result, rheological models are often used to extrapolate viscosity from low pressure measurements to higher pressures. Techniques to obtain data over a wide range of pressures and shear rates, as well as understanding the validity and limitations of methods to fill the gaps in the available data, are therefore of crucial practical and theoretical importance. This work examines the viscosity of polyalphaolefin (PAO) by combining average global area averaged measurements at high pressure and local molecular viscosity measurements at moderate pressures. Viscosities spanning five orders of magnitude are examined at pressures up to 720 MPa. High pressure results were obtained with friction measurements where the fluid is sheared between two surfaces in a loaded point contact. The local molecular microviscosity at medium and low pressures was measured by applying a technique based on fluorescence anisotropy, which probes the rotational motion of dye molecules in a nanoscale film under shear. Both sets of measurements are taken in the same configuration, an elastohydrodynamic (EHD) contact. This is the first set of quantitative local viscosity measurements that have been verified against both friction and high pressure rheometry measurements. Commonly used rheological models were compared to experimental results. Our work shows that fluorescence anisotropy and friction measurements can be used to determine the viscosity of liquids over a wide range of conditions from a single experimental setup. The results obtained match results from low- and high-pressure rheometry for PAO. The importance of correcting friction data for pressure non-uniformity, temperature and shear thinning is also highlighted.

  • Journal article
    Tan Z, Dini D, Rodriguez y Baena F, Forte Aet al., 2018,

    Composite hydrogel: A high fidelity soft tissue mimic for surgery

    , Materials and Design, Vol: 160, Pages: 886-894, ISSN: 0264-1275

    Accurate tissue phantoms are difficult to design due to the complex non-linear viscoelastic properties of real soft tissues. A composite hydrogel, resulting from a mix of poly(vinyl) alcohol and phytagel, is able to reproduce the viscoelastic responses of different soft tissues due to its compositional tunability. The aim of this work is to demonstrate the flexibility of the composite hydrogel in mimicking the interactions between surgical tools and various soft tissues, such as brain, lung and liver. Therefore compressive stiffness, insertion forces and frictional forces were used as matching criteria to determine the hydrogel compositions for each soft tissue. A full map of the behaviour of the synthetic material is provided for these three characteristics and the compositions found to best match the mechanical response of brain, lung and liver are reported. The optimised hydrogel samples are then tested and shown to mimic the behaviour of the three tissues with unprecedented fidelity. The effect of each hydrogel constituent on the compressive stiffness, needle insertion and frictional forces is also detailed in this work to explain their individual contributions and synergistic effects. This study opens important opportunities for the realisation of surgical planning and training devices and tools for in-vitro tissue testing.

  • Journal article
    Ewen J, Heyes D, Dini D, 2018,

    Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

    , Friction, Vol: 6, Pages: 349-386, ISSN: 2223-7704

    Nonequilibrium molecular dynamics (NEMD) simulations have provided unique insights into the nanoscale behaviour of lubricants under shear. This review discusses the early history of NEMD and its progression from a tool to corroborate theories of the liquid state, to an instrument that can directly evaluate important fluid properties, towards a potential design tool in tribology. The key methodological advances which have allowed this evolution are also highlighted. This is followed by a summary of bulk and confined NEMD simulations of liquid lubricants and lubricant additives, as they have progressed from simple atomic fluids to ever more complex, realistic molecules. The future outlook of NEMD in tribology, including the inclusion of chemical reactivity for additives, and coupling to continuum methods for large systems, is also briefly discussed.

  • Journal article
    Kontou A, Southby M, Morgan N, Spikes HAet al., 2018,

    Influence of Dispersant and ZDDP on Soot Wear

    , Tribology Letters, Vol: 66, 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.

  • Journal article
    Dawczyk J, Ware E, Ardakani M, Russo J, Spikes Het 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.

  • Journal article
    Vladescu SC, Marx N, Fernández L, Barceló F, Spikes HAet 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.

  • Journal article
    Tajabadi-Ebrahimi M, Dini D, Balint DS, Sutton AP, Ozbayraktar Set al., 2018,

    Discrete crack dynamics: a planar model of crack propagation and crack-inclusion interactions in brittle materials

    , International Journal of Solids and Structures, Vol: 152-153, Pages: 12-27, ISSN: 0020-7683

    The Multipole Method (MPM) is used to simulate the many-body self-consistentproblem of interacting elliptical micro-cracks and inclusions in single crystals. Acriterion is employed to determine the crack propagation path based on the stressdistribution; the evolution of individual micro-cracks and their interactions withexisting cracks and inclusions is then predicted using what we coin the DiscreteCrack Dynamics (DCD) method. DCD is fast (semi-analytical) and particularlysuitable for the simulation of evolving low-speed crack networks in brittle orquasi-brittle materials. The method is validated against finite element analysispredictions and previously published experimental data.

  • Journal article
    Verschueren J, Gurrutxaga-Lerma B, Balint D, Sutton A, Dini Det al., 2018,

    Instabilities of high speed dislocations

    , Physical Review Letters, Vol: 121, ISSN: 0031-9007

    Despite numerous theoretical models and simulation results, a clear physical picture of dislocations traveling at velocities comparable to the speed of sound in the medium remains elusive. Using two complementary atomistic methods to model uniformly moving screw dislocations, lattice dynamics and molecular dynamics, the existence of mechanical instabilities in the system is shown. These instabilities are found at material-dependent velocities far below the speed of sound. We show that these are the onset of an atomistic kinematic generation mechanism, which ultimately results in an avalanche of further dislocations. This homogeneous nucleation mechanism, observed but never fully explained before, is relevant in moderate and high strain rate phenomena including adiabatic shear banding, dynamic fracture, and shock loading. In principle, these mechanical instabilities do not prevent supersonic motion of dislocations.

  • Journal article
    Hartinger M, Reddyhoff T, 2018,

    CFD modeling compared to temperature and friction measurements of an EHL line contact

    , Tribology International, Vol: 126, Pages: 144-152, ISSN: 0301-679X

    In this paper, predictions from CFD modeling are compared against measurements of surface temperatures and friction for an EHL line contact lubricated with the fluid Santotrac 50. Two slide-to-roll-ratios (SRR), 50% and 100%, and entrainment velocities ranging from 0.211 to 1.13 m/s are considered. Very good agreement is shown for the 50% SRR cases, with only a 3% deviation in friction coefficient values. At 100% SRR, the deviation in friction increases to 3–7% which is attributed to deficiencies in the modeling approach with regard to shear-thinning. The temperature profiles agree reasonably well at 50% SRR and show larger deviations at 100% SRR. For all cases, the formation of a shear-band in the center of the fluid film is predicted. This is very pronounced for 100% SRR, although likely to be over-estimated by this CFD-approach. The data presented here serve as a basis from which further refinements in the modeling and measurements shown can be made.

  • Journal article
    Yu M, Arana C, Evangelou S, Dini D, Cleaver Get al., 2018,

    Parallel active link suspension: a quarter car experimental study

    , IEEE/ASME Transactions on Mechatronics, Vol: 23, Pages: 2066-2077, ISSN: 1083-4435

    In this paper, a novel electro-mechanical active suspension for cars, the Parallel Active Link Suspension (PALS), is proposed and then experimentally studied. PALS involves the introduction of a rotary-actuator-driven rocker-pushrod mechanism in parallel with the conventional passive suspension assembly, to exert an additional controlled force between the chassis and the wheel. The PALS geometric arrangement is designed and optimized to maximize the rocker torque propagation onto the tire load increment. A quarter car test rig with double wishbone suspension is utilized for the PALS physical implementation. Based on a linear equivalent model of the PALS quarter car, a conservative and an aggressive robust H∞ control schemes are synthesized separately to improve the ride comfort and the road holding, with different levels of control effort allowed in each of the control schemes. Simulations with a theoretical nonlinear model of the PALS quarter car are performed to evaluate the potential in suspension performance enhancement and power demand in the rocker actuator. Experiments with a harmonic road, a smoothed bump and hole, and swept frequency are conducted with the quarter car test rig to validate the practical feasibility of the novel PALS, the ride comfort enhancement, as well as the accuracy of the theoretical model and of a further nonlinear model in which practical features existing in the test rig are identified and included.

  • Journal article
    Gattinoni C, Ewen JP, Dini D, 2018,

    Adsorption of Surfactants on alpha-Fe2O3(0001): A Density Functional Theory Study

    , JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 122, Pages: 20817-20826, ISSN: 1932-7447
  • Journal article
    Marx N, Fernández L, Barceló F, Spikes HAet 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].

  • Journal article
    Vakis A, Yastrebov V, Scheibert J, Nicola L, Dini D, Minfray C, Almqvist A, Paggi M, Lee S, Limbert G, Molinari JF, Anciaux G, Aghababaei R, Echeverri Restrepo S, Papangelo A, Cammarata A, Nicolini P, Putignano C, Carbone G, Stupkiewicz S, Lengiewicz J, Costagliola G, Bosia F, Guarino R, Pugno NM, Müser MH, Ciavarella Met al., 2018,

    Modeling and simulation in tribology across scales: An overview

    , Tribology International, Vol: 125, Pages: 169-199, ISSN: 0301-679X

    This review summarizes recent advances in the area of tribology based on the outcome of a Lorentz Center workshop surveying various physical, chemical and mechanical phenomena across scales. Among the main themes discussed were those of rough surface representations, the breakdown of continuum theories at the nano- and micro-scales, as well as multiscale and multiphysics aspects for analytical and computational models relevant to applications spanning a variety of sectors, from automotive to biotribology and nanotechnology. Significant effort is still required to account for complementary nonlinear effects of plasticity, adhesion, friction, wear, lubrication and surface chemistry in tribological models. For each topic, we propose some research directions.

  • Journal article
    Campen S, Smith B, Wong J, 2018,

    Deposition of asphaltene from destabilized dispersions in heptane-toluene

    , Energy and Fuels, Vol: 32, Pages: 9159-9171, ISSN: 0887-0624

    Deposition of carbonaceous materials, such as asphaltene, is a major problem in petroleum production. During production, changing environmental conditions destabilize asphaltene, resulting in dispersions that are out of equilibrium, where asphaltene is aggregating or flocculating. Key to developing the most effective strategies for tackling this problem is a fundamental understanding of asphaltene deposition behavior. A quartz crystal microbalance with dissipation monitoring (QCM-D) is used to study asphaltene deposition from destabilized dispersions generated by in-line mixing of asphaltene in toluene (a solvent) with n-heptane (a precipitant). The effects of heptane:toluene ratio and destabilization time are investigated. At high heptane:toluene ratio, the rate of asphaltene aggregation is faster, and large flocs form by the time the flowing liquid reaches the QCM cell. In this case, the rate of deposition decreases with deposition time. At low heptane:toluene ratio, the rate of asphaltene aggregation is slower; hence large flocs do not form before the flowing liquid reaches the QCM cell, and deposition of smaller aggregates occurs. Here, the deposition rate is constant with time. The deposited mass is greatest before the formation of large flocs and at short destabilization times, where the particle distribution is furthest from equilibrium. Destabilized small particles existing immediately after a destabilization event pose a greater deposition problem than the flocs that subsequently form. This may be a contributing factor in the existence of deposition “hotspots” at certain locations in the production pipeline. Pushing destabilized dispersions to their new equilibrium distributions as quickly as possible may be a preventative strategy to combat deposition. The dissipation–frequency relationship monitored by QCM-D is sensitive to the nature of deposited asphaltene films and may be used as a diagnostic tool.

  • Journal article
    Shen L, Denner F, Morgan N, Van Wachem B, Dini Det al., 2018,

    Capillary waves with surface viscosity

    , Journal of Fluid Mechanics, Vol: 847, Pages: 644-663, ISSN: 0022-1120

    Experiments over the last 50 years have suggested a tentative correlation between the surface (shear) viscosity and the stability of a foam or emulsion. We examine this link theoretically using small-amplitude capillary waves in the presence of a surfactant solution of dilute concentrations where the associated Marangoni and surface viscosity effects are modelled via the Boussinesq-Scriven formulation. The resulting integro-differential initial value problem is solved analyticallyand surface viscosity is found to contribute an overall damping effect on the amplitude of the capillary wave with varying degrees depending on the lengthscale of the system.Numerically, we find the critical damping wavelength to increase for increasing surface concentration but the rate of increase remains different for both the surface viscosity and the Marangoni effect.

  • Journal article
    Reddyhoff T, Underwood R, Sayles R, Spikes Het 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.

  • Journal article
    Marx N, Fernández L, Barceló F, Spikes HAet 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
  • Journal article
    Kanca Y, Milner P, Dini D, Amis Aet al., 2018,

    Tribological evaluation of biomedical polycarbonate urethanes against articular cartilage

    , Journal of the Mechanical Behavior of Biomedical Materials, Vol: 82, Pages: 394-402, ISSN: 1751-6161

    This research investigated the in-vitro wear and friction performance of polycarbonate urethane (PCU) 80A as they interact with articular cartilage, using a customised multidirectional pin-on-plate tester. Condyles were articulated against PCU 80A discs (Bionate® I and Bionate® II) (configuration 1) and the results arising from these tests were compared to those recorded during the sliding of PCU pins against cartilage plates (configuration 2). Configuration 1 produced steadily increasing coefficient of friction (COF) (up to 0.64 ± 0.05) and had the same trend as the cartilage–on–stainless steel articulation (positive control). When synovial fluid rather than bovine calf serum was used as lubricant, average COF significantly decreased from 0.50 ± 0.02–0.38 ± 0.06 for condyle–on–Bionate® I (80AI) and from 0.41 ± 0.02–0.24 ± 0.04 for condyle–on–Bionate® II (80AII) test configurations (p < 0.05). After 15 h testing, the cartilage–on–cartilage articulation (negative control) tests showed no cartilage degeneration. However, different levels of cartilage volume loss were found on the condyles from the positive control (12.5 ± 4.2 mm3) and the PCUs (20.1 ± 3.6 mm3 for 80 AI and 19.0 ± 2.3 mm3 for 80AII) (p > 0.05). A good correlation (R2 =0.84) was found between the levels of average COF and the volume of cartilage lost during testing; increasing wear was found at higher levels of COF. Configuration 2 showed low and constant COF values (0.04 ± 0.01), which were closer to the negative control (0.03 ± 0.01) and significantly lower than configuration 1 (p < 0.05). The investigation showed that PCU is a good candidate for use in hemiarthroplasty components, where only one of the two articulating surfaces is replaced, as long as the synthetic material is implanted in a region where migrating cartilage contact is achieved. Bio

  • Journal article
    Stevenson H, Parkes M, Austin L, Jaggard M, Akhbri P, Vaghela U, Williams H, Gupte C, Cann PMet al., 2018,

    The development of a small-scale wear test for CoCrMo specimens with human synovial fluid

    , Biotribology, Vol: 14, Pages: 1-10, ISSN: 2352-5738

    A new test was developed to measure friction and wear of hip implant materials under reciprocating sliding conditions. The method requires a very small amount of lubricant (<3 ml) which allows testing of human synovial fluid. Friction and wear of Cobalt Chromium Molybdenum (CoCrMo) material pairs were measured for a range of model and human synovial fluid samples. The initial development of the test assessed the effect of fluid volume and bovine calf serum (BCS) concentration on friction and wear. In a second series of tests human synovial fluid (HSF) was used. The wear scar size (depth and volume) on the disc was dependent on protein content and reduced significantly for increasing BCS concentration. The results showed that fluid volumes of <1.5 ml were affected by evaporative loss effectively increasing the protein concentration resulting in anomalously lower wear. At the end of the test thick deposits were observed in and around the wear scars on the disc and ball; these were analysed by Infrared Reflection-Absorption Spectroscopy. The deposits were composed primarily of denatured proteins and similar IR spectra were obtained from the BCS and HSF tests. The analysis confirmed the importance of SF proteins in determining wear of CoCrMo couples.

  • Journal article
    Heyes D, Dini D, Smith E, 2018,

    Incremental viscosity by non-equilibrium molecular dynamics and the Eyring model

    , Journal of Chemical Physics, Vol: 148, ISSN: 0021-9606

    The viscoelastic behavior of sheared fluids is calculated by Non-Equilibrium Molecular Dynamics(NEMD) simulation, and complementary analytic solutions of a time-dependent extension of Eyring’smodel (EM) for shear thinning are derived. It is argued that an “incremental viscosity,”ηi, or IV whichis the derivative of the steady state stress with respect to the shear rate is a better measure of the physicalstate of the system than the conventional definition of the shear rate dependent viscosity (i.e., the shearstress divided by the strain rate). The stress relaxation function,Ci(t), associated withηiis consistentwith Boltzmann’s superposition principle and is computed by NEMD and the EM. The IV of the Eyringmodel is shown to be a special case of the Carreau formula for shear thinning. An analytic solutionfor the transient time correlation function for the EM is derived. An extension of the EM to allow forsignificant local shear stress fluctuations on a molecular level, represented by a gaussian distribution,is shown to have the same analytic form as the original EM but with the EM stress replaced by its timeand spatial average. Even at high shear rates and on small scales, the probability distribution functionis almost gaussian (apart from in the wings) with the peak shifted by the shear. The Eyring formulaapproximately satisfies the Fluctuation Theorem, which may in part explain its success in representingthe shear thinning curves of a wide range of different types of chemical systems.

  • Journal article
    Yang S, Wong J, Zhou F, 2018,

    Ionic liquid additives for mixed and elastohydrodynamic lubrication

    , Tribology Transactions, Vol: 61, Pages: 816-826, ISSN: 1040-2004

    Ionic liquids (ILs), both as pure lubricants and lubricant additives, have been demonstrated extensively to exhibit excellent tribological performance in terms of friction and wear reduction in the boundary lubrication (BL) regime. Since engineering contacts experience boundary and mixed, as well as full film lubrication depending on operating conditions, it is crucial to examine if lubrication regimes other BL regime can also benefit from the use of ILs. The objective of this work is to investigate the tribological performance of IL additives in the mixed lubrication (ML) and the elastohydrodynamic lubrication (EHL) regimes. Polyethylene glycol (PEG) was used as the base fluid. ILs were synthesized in situ by dissolving lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in PEG. Friction and film thickness measurements were employed to investigate the effectiveness of IL additives at room temperature, 60°C and 80°C; at various loads and slide-roll ratios (SRRs). The effect of IL additives on the rheological behavior of PEG was also investigated. The EHL film thickness increases with increasing IL concentration. EHL friction is however only mildly affected by IL additives. In the ML regime, IL additives can reduce friction and metal wear as compared to pure PEG in mild conditions. It is conjectured that IL forms sacrificial layers and protects the rubbing surfaces.

  • Journal article
    Ewen JP, Kannam SK, Todd BD, Dini Det al., 2018,

    Slip of Alkanes Confined between Surfactant Monolayers Adsorbed on Solid Surfaces

    , Langmuir, Vol: 34, Pages: 3864-3873, ISSN: 0743-7463

    © 2018 American Chemical Society. The slip and friction behavior of n-hexadecane, confined between organic friction modifier surfactant films adsorbed on hematite surfaces, has been studied using nonequilibrium molecular dynamics simulations. The influence of the surfactant type and coverage, as well as the applied shear rate and pressure, has been investigated. A measurable slip length is only observed for surfactant films with a high surface coverage, which provide smooth interfaces between well-defined surfactant and hexadecane layers. Slip commences above a critical shear rate, beyond which the slip length first increases with increasing shear rate and then asymptotes toward a constant value. The maximum slip length increases significantly with increasing pressure. Systems and conditions which show a larger slip length typically give a lower friction coefficient. Generally, the friction coefficient increases linearly with logarithmic shear rate; however, it shows a much stronger shear rate dependency at low pressure than at high pressure. Relating slip and friction, slip only occurs above a critical shear stress, after which the slip length first increases linearly with increasing shear stress and then asymptotes. This behavior is well-described using previously proposed slip models. This study provides a more detailed understanding of the slip of alkanes on surfactant monolayers. It also suggests that high coverage surfactant films can significantly reduce friction by promoting slip, even when the surfaces are well-separated by a lubricant.

  • Journal article
    Guo Y, di Mare L, Li R, Wong Jet al., 2018,

    Cargo release from polymeric vesicles under shear

    , Polymers, Vol: 10, Pages: 336-336, ISSN: 2073-4360

    In this paper we study the release of cargo from polymeric nano-carriers under shear. Vesicles formed by two star block polymers— A12B6C2 ( ABC ) and A12B6A2 ( ABA )—and one linear block copolymer— A14B6 ( AB ), are investigated using dissipative particle dynamics (DPD) simulations. A - and C -blocks are solvophobic and B -block is solvophilic. The three polymers form vesicles of different structures. The vesicles are subjected to shear both in bulk and between solvophobic walls. In bulk shear, the mechanisms of cargo release are similar for all vesicles, with cargo travelling through vesicle membrane with no preferential release location. When sheared between walls, high cargo release rate is only observed with ABC vesicle after it touches the wall. For ABC vesicle, the critical condition for high cargo release rate is the formation of wall-polymersome interface after which the effect of shear rate in promoting cargo release is secondary. High release rate is achieved by the formation of solvophilic pathway allowing cargo to travel from the vesicle cavity to the vesicle exterior. The results in this paper show that well controlled target cargo release using polymersomes can be achieved with polymers of suitable design and can potentially be very useful for engineering applications. As an example, polymersomes can be used as carriers for surface active friction reducing additives which are only released at rubbing surfaces where the additives are needed most.

  • Journal article
    Morales-Espejel GE, Rycerz P, Kadiric A, 2018,

    Prediction of micropitting damage in gear teeth contacts considering the concurrent effects of surface fatigue and mild wear

    , Wear, Vol: 398-399, Pages: 99-115, ISSN: 0043-1648

    © 2017 The Authors The present paper studies the occurrence of micropitting damage in gear teeth contacts. An existing general micropitting model, which accounts for mixed lubrication conditions, stress history, and fatigue damage accumulation, is adapted here to deal with transient contact conditions that exist during meshing of gear teeth. The model considers the concurrent effects of surface fatigue and mild wear on the evolution of tooth surface roughness and therefore captures the complexities of damage accumulation on tooth flanks in a more realistic manner than hitherto possible. Applicability of the model to gear contact conditions is first confirmed by comparing its predictions to relevant experiments carried out on a triple-disc contact fatigue rig. Application of the model to a pair of meshing spur gears shows that under low specific oil film thickness conditions, the continuous competition between surface fatigue and mild wear determines the overall level as well as the distribution of micropitting damage along the tooth flanks. The outcome of this competition in terms of the final damage level is dependent on contact sliding speed, pressure and specific film thickness. In general, with no surface wear, micropitting damage increases with decreasing film thickness as may be expected, but when some wear is present micropitting damage may reduce as film thickness is lowered to the point where wear takes over and removes the asperity peaks and hence reduces asperity interactions. Similarly, when wear is negligible, increased sliding can increase the level of micropitting by increasing the number of asperity stress cycles, but when wear is present, an increase in sliding may lead to a reduction in micropitting due to faster removal of asperity peaks. The results suggest that an ideal situation in terms of surface damage prevention is that in which some mild wear at the start of gear pair operation adequately wears-in the tooth surfaces, thus reducing sub

  • Journal article
    Masen M, Cann PME, 2018,

    Friction measurements with molten chocolate

    , Tribology Letters, Vol: 66, ISSN: 1023-8883

    A novel test is reported which allows the measurement of the friction of molten chocolate in a model tongue–palate rubbing contact. Friction was measured over a rubbing period of 150 s for a range of commercial samples with different cocoa content (85–5% w/w). Most of the friction curves had a characteristic pattern: initially a rapid increase occurs as the high-viscosity chocolate melt is sheared in the contact region followed by friction drop as the film breaks down. The exceptions were the very high (85%) and very low (~ 5%) cocoa content samples which gave fairly constant friction traces over the test time. Differences were observed in the initial maximum and final friction coefficients depending on chocolate composition. Generally, the initial maximum friction increased with increasing cocoa content. At the end of the test, the rubbed films on the lower slide were examined by optical microscopy and infrared micro-reflection spectroscopy. In the rubbed track, the chocolate structure was severely degraded and predominately composed of lipid droplets, which was confirmed by the IR spectra. The new test provides a method to distinguish between the friction behaviour of different chocolate formulations in a rubbing low-pressure contact. It also allows us to identify changes in the degraded chocolate film that can be linked to the friction profile. Further development of the test method is required to improve simulation of the tongue–palate contact including the effect of saliva and this will be the next stage of the research.

  • Journal article
    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.

  • Conference paper
    Ferretti A, Giacopini M, Mastrandrea L, Dini Det al., 2018,

    Investigation of the Influence of Different Asperity Contact Models on the Elastohydrodynamic Analysis of a Conrod Small-End/Piston Pin Coupling

    , WCX World Congress Experience

    © 2018 SAE International. All Rights Reserved. Bearings represent one of the main responsible of friction losses in internal combustion engines and their lubrication performance has a crucial influence on the operating condition of the engine. In particular, the conrod small-end bearing is one of the most critical engine parts from a tribological point of view since limited contact surfaces have to sustain high inertial and combustion forces. In this contribution an analysis is performed of the tribological behaviour of the lubricated contact between the piston pin and the conrod small-end of a high performance motorbike engine. An algorithm is employed based on a complementarity formulation of the cavitation problem. A comparison between two different approaches to simulate the asperity contact problem is performed, the former based on the standard Greenwood-Tripp theory and the latter based on a complementarity formulation of the asperity contact problem. A model validation is performed by comparing the results with those obtained adopting the commercial software AVL Excite Power Unit. Similar results are obtained from both the approaches, if a proper calibration of the model input data is performed. However, a remarkable sensitivity is highlighted of the results obtained using the Greenwood/Tripp model to the adjustment parameters. The realistic (engineering) difficulty in defining and identifying the roughness data and their purely statistical nature returns results that may be afflicted by a dose of uncertainty. Considering that results of such simulations usually offer guidelines for a correct design of the coupling, further investigations are suggested to identify a relationship between simply available roughness data and model input, starting from a direct experimental measurements of real roughness profiles.

  • Journal article
    Parkes M, Sayer K, Goldhofer M, Cann P, Walter WL, Jeffers Jet al., 2017,

    Zirconia phase transformation in retrieved, wear simulated, and artificially aged ceramic femoral heads

    , JOURNAL OF ORTHOPAEDIC RESEARCH, Vol: 35, Pages: 2781-2789, ISSN: 0736-0266
  • Journal article
    Menga N, Carbone G, Dini D, 2017,

    Do uniform tangential interfacial stresses enhance adhesion?

    , Journal of the Mechanics and Physics of Solids, Vol: 112, Pages: 145-156, ISSN: 0022-5096

    We present theoretical arguments, based on linear elasticity and thermodynamics, to show that interfacial tangential stresses in sliding adhesive soft contacts may lead to a significant increase of the effective energy of adhesion. A sizable expansion of the contact area is predicted in conditions corresponding to such scenario. These results are easily explained and are valid under the assumptions that: (i) sliding at the interface does not lead to any loss of adhesive interaction and (ii) spatial fluctuations of frictional stresses can be considered negligible. Our results are seemingly supported by existing experiments, and show that frictional stresses may lead to an increase of the effective energy of adhesion depending on which conditions are established at the interface of contacting bodies in the presence of adhesive forces.

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