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  • Journal article
    Bahshwan M, Gee M, Nunn J, Myant CW, Reddyhoff Tet al., 2022,

    In situ observation of anisotropic tribological contact evolution in 316L steel formed by selective laser melting

    , Wear, Vol: 490-491, Pages: 1-12, ISSN: 0043-1648

    A consensus on the tribological performance of components by additive-versus conventional manufacturing has not been achieved; mainly because the tribological test set-ups thus far were not suited for investigating the underlying microstructure's influence on the tribological properties. As a result, utilization of additive manufacturing techniques, such as selective laser melting (SLM), for tribological applications remains questionable. Here, we investigate the anisotropic tribological response of SLM 316L stainless steel via in situ SEM reciprocating micro-scratch testing to highlight the microstructure's role. As-built 316L SLM specimens were compared against annealed wire-drawn 316L. We found that: (i) microgeometric conformity was the main driver for achieving steady-state friction, (ii) the anisotropic friction of the additively manufactured components is limited to the break-in and is caused by the lack of conformity, (iii) the cohesive bonds, whose strength is proportional to frictional forces, are stronger in the additively manufactured specimens likely due to the dislocation-dense, cellular structures, (iv) low Taylor-factor grains with large dimension stimulate microcutting in the form of long, thin sheets with serrated edges. These findings uncover some microstructurally driven tribological complexities when comparing additive to conventional manufacturing.

  • Journal article
    Ueda M, Wainwright B, Spikes H, Kadiric Aet al., 2022,

    The effect of friction on micropitting

    , Wear, Vol: 488-489, Pages: 1-13, ISSN: 0043-1648

    Micropitting is a type of surface fatigue damage that occurs in rolling-sliding contacts operating under thin oil film conditions. It is caused by stress fluctuations, brought about by surface asperity interactions, which lead to initiation and propagation of numerous surface fatigue cracks and subsequent loss of material. Despite its increasing importance to gear and bearing reliability, the mechanisms of micropitting are poorly understood. This is particularly the case concerning the effects of friction on micropitting which are difficult to study under controlled conditions. This is because it is difficult to isolate the friction effects from other influential factors, in particular from the build-up of any anti-wear tribofilm and its subsequent effect on the running-in of counterface roughness that is known to strongly affect micropitting through its influence on severity of asperity stresses. This paper presents new data on the impact of friction on micropitting obtained using a new test methodology. Micropitting tests were conducted using a ball-on-disc MTM rig with the additional functionality to continuously monitor the growth of tribofilm during the test. Friction was varied by using custom-made oils containing different concentrations of MoDTC. Crucially, the effect of friction was isolated from the effect of counterface roughness running-in by introducing the MoDTC blend only after the running-in period was completed with a ZDDP solution alone. This approach eliminates the influence of MoDTC on ZDDP anti-wear tribofilm growth in early stages and hence ensures the same running-in takes place in each test. This gives similar asperity pressure history, regardless of the amount of MoDTC present.Resultsshow that friction has a very significant impact on micropitting; for example, the extent of micropitting was reduced by a factor of 10 when friction coefficient was reduced from about 0.1 to 0.04. Lower friction results in fewer surface cracks which grow at a s

  • Journal article
    Puhan D, Jiang S, Wong J, 2022,

    Effect of carbon fiber inclusions on polymeric transfer film formation on steel

    , Composites Science and Technology, Vol: 217, Pages: 1-10, ISSN: 0266-3538

    High performance polymers (HPPs) with good tribological properties are commonly used in dry contacts, wheretheir tribological performance often depends on properties of polymeric transfer materials (transfer layers) oncounterfaces. Most HPPs suffer from high temperature degradation due to frictional heating, leading to excessivedeformation and wear. Incorporating thermally conducting fillers increases their thermal conductivity andmechanical strength. The impact of these fillers on formation and properties of transfer layers, however, isunclear. In this work the effect of short carbon fiber fillers (CFs) on the nature of the transfer layers andtribological performance of polyetheretherketone (PEEK) and polyetheretherketone-polybenzimidizole (PBP)against steel were investigated at temperature up to 300 ◦C. Transfer layers of CF reinforced PEEK and PBPcontain CF-related materials, resulting in a reduction of friction as compared to neat PEEK and PBP, especiallyaround the glass transition temperature of PEEK (Tg− PEEK) when the transfer layer is relatively thick. While theinclusion of CFs increases the bulk thermal conductivity of polymer composites, the average contact temperatureis not affected. Rather, local hot spots are generated. As a result, their transfer layers may have formed morereadily and have undergone more severe degradation than those from neat polymer. At 300 ◦C, the PBP + CFtransfer layer is thin possibly due to abrasion by CFs dislodged from the matrix. The improvement in the wearresistance due to CF inclusion is observed with PBP up to 300 ◦C due to its improved mechanical strength. PEEK+ CF however suffers higher wear than PEEK below Tg− PEEK. Above Tg− PEEK, a thick transfer layer is formed andthe wear of PEEK + CF reduces.

  • Journal article
    Fellows AP, Puhan D, Wong J, Casford M, Davies Pet al., 2022,

    Probing the nanoscale heterogeneous mixing in a high-performance polymer blend

    , Polymers, Vol: 14, ISSN: 2073-4360

    The blend of polyetheretherketone (PEEK) and polybenzimidazole (PBI) produces a high-performance blend (PPB) that is a potential replacement material in several industries due to its high temperature stability and desirable tribological properties. Understanding the nanoscale structure and interface of the two domains of the blend is critical for elucidating the origin of these desirable properties. Whilst achieving the physical characterisation of the domain structures is relatively uncomplicated, the elucidation of structures at the interface presents a significant experimental challenge. In this work, we combine atomic force microscopy (AFM) with an IR laser (AFM-IR) and thermal cantilever probes (nanoTA) to gain insights into the chemical heterogeneity and extent of mixing within the blend structure for the first time. The AFM-IR and nanoTA measurements show that domains in the blend are compositionally different from those of the pure PEEK and PBI polymers, with significant variations observed in a transition region several microns wide in proximity to domain boundary. This strongly points to physical mixing of the two components on a molecular scale at the interface. The versatility intrinsic to the combined methodology employed in this work provides nano- and microscale chemical information that can be used to understand the link between properties of different length scales across a wide range of materials.

  • Journal article
    Jamal A, Bernardini A, Dini D, 2022,

    Microscale characterisation of the time-dependent mechanical behaviour of brain white matter

  • Journal article
    Charalambides M, Bikos D, Samaras G, Cann P, Masen M, Hartmann C, German J, Vieira J, Hardalupas Iet al., 2022,

    Effect of structure on the mechanical and physical properties of chocolate considering time scale phenomena occuring during oral processing

    , Food Structure, Vol: 31, Pages: 1-14, ISSN: 2213-3291

    Micro-aeration has been employed by the chocolate industry as a texture and flavour modifier. However, the impact of micro-aeration on oral processing is still not well understood. This study quantifies the mechanical, thermal and tribological behaviour of chocolate materials of different porosity levels. These material properties were then linked to sensory data considering the temporal phenomena of the oral process. In-vivo mastication tests were utilised to define the level of fragmentation of chocolate and coupled with heat transfer numerical models to simulate the melting during oral processing. Micro-aeration affects all material properties resulting in lower fracture stresses, rapid melting and a lower friction coefficient. The sensory results showed that micro-aeration creates a perception of a softer, less sticky chocolate which melts fast inside the mouth, without compromising the sweetness perception. This research adopts an innovative multidisciplinary approach to the physics of chocolates, bringing together the fields of solid mechanics, heat transfer, tribology, and sensory analysis and employing engineering experimental and numerical approaches to provide a link between chocolate structure, material properties and sensory perception. The outcome can contribute a powerful design tool for controlling the perception of sensory attributes for specific chocolate composition.

  • Journal article
    Dou P, Jia Y, Zheng P, Wu T, Yu M, Reddyhoff T, Peng Zet al., 2022,

    Review of ultrasonic-based technology for oil film thickness measurement in lubrication

    , Tribology International, Vol: 165, ISSN: 0301-679X

    Lubricant film thickness is the most informative variable that reflects lubrication conditions and transmission efficiency in the mechanical equipment, therefore its measurement is highly important. Despite a large number of theoretical models that have been developed to describe the lubricant film, complexities and uncertainties in a real tribo-pair contact still hinder the implementation of accurate and robust methods of in-situ film thickness measurements. Recently, ultrasonic-based measurement has been widely studied, showing promising potential owing to its non-destructive characteristics, high sensitivity, and limited physical modifications. This paper comprehensively reviews basic principles of ultrasonic-based oil film measurement; summarizes progress on calculation models and associated signal processing methods; exhibits in-lab demonstrations and in-situ applications; and discusses key technical issues and possible solutions.

  • Journal article
    Zhang J, Campen S, Wong J, Spikes Het al., 2022,

    Oxidational wear in lubricated contacts – or is it?

    , Tribology International, Vol: 165, Pages: 1-9, ISSN: 0301-679X

    This study examines the influence of inert gas atmosphere on the wear behaviour of rubbing steel-on-steel contacts lubricated by two hydrocarbon base fluids, isooctane and hexadecane. It is found that for both fluids, wear and mean friction in nitrogen and argon atmospheres are considerably lower than in dry air. As the oxygen content in nitrogen is increased, mean friction and wear both increase, to level out above about 10% oxygen (an O2 partial pressure of 10 kPa). Raman analysis of rubbed surfaces shows the presence of a carbon film on surfaces rubbed in inert gas and at low O2 levels. This film is not observed at high O2 levels.These findings indicate that the prevailing model of oxidational wear in lubricated contacts, that states that wear is greater in air than in inert gas because of corrosion by oxygen, is largely incorrect. Instead, the deleterious effect of oxygen on lubricated wear is primarily due to it preventing the formation of a lubricious, carbon-based boundary film that is generated from hydrocarbon base fluids on rubbing steel surfaces in inert gas conditions.The ability of organic fuels and lubricants to form carbon-based films on rubbing steel surfaces in inert atmospheres may provide a mechanism for reducing friction and wear of fuel- and oil-lubricated machine components. The study also provides a platform from which to design lubricant formulations for use in inert atmospheres.

  • Journal article
    Ayestaran Latorre C, Moore J, Remias J, Spikes H, Dini D, Ewen Jet al., 2021,

    Mechanochemistry of phosphate esters confined between sliding iron surfaces

    , Communications Chemistry, Vol: 4, Pages: 1-11, ISSN: 2399-3669

    The molecular structure of lubricant additives controls not only their adsorption and dissociation behaviour at the nanoscale, but also their ability to reduce friction and wear at the macroscale. Here, we show using nonequilibrium molecular dynamics simulations with a reactive force field that tri(s-butyl)phosphate dissociates much faster than tri(n-butyl)phosphate when heated and compressed between sliding iron surfaces. For both molecules, dissociative chemisorption proceeds through cleavage of carbon−oxygen bonds. The dissociation rate increases exponentially with temperature and stress. When the rate−temperature−stress data are fitted with the Bell model, both molecules have similar activation energies and activation volumes and the higher reactivity of tri(s-butyl)phosphate is due to a larger pre-exponential factor. These observations are consistent with experiments using the antiwear additive zinc dialkyldithiophosphate. This study represents a crucial step towards the virtual screening of lubricant additives with different substituents to optimise tribological performance.

  • Journal article
    Yu M, Evangelou S, Dini D, 2021,

    Parallel active link suspension: full car application with frequency-dependent multi-objective control strategies

    , IEEE Transactions on Control Systems Technology, ISSN: 1063-6536

    In this article, a recently proposed at basic level novel suspension for road vehicles, the parallel active link suspension (PALS), is investigated in the realistic scenario of a sport utility vehicle (SUV) full car. The involved rocker-pushrod assembly is generally optimized to maximize the PALS capability in improving the suspension performance. To fully release the PALS functions of dealing with both low- and high-frequency road cases, a PID control scheme is first employed for the chassis attitude stabilization, focusing on the minimization of both the roll and pitch angles; based on a derived linear equivalent model of the PALS-retrofitted full car, an H∞ control scheme is designed to enhance the ride comfort and road holding; moreover, a frequency-dependent multiobjective control strategy that combines the developed PID and H∞ control is proposed to enable: 1) chassis attitude stabilization at 0-1 Hz; 2) vehicle vibration attenuation at 1-8 Hz; and 3) control effort penalization (for energy saving) above 10 Hz. With a group of ISO-defined road events tested, numerical simulation results demonstrate that, compared to the conventional passive suspension, the PALS has a promising potential in full-car application, with up to 70% reduction of the chassis vertical acceleration in speed bumps and chassis leveling capability of dealing with up to 4.3-m/s² lateral acceleration.

  • Journal article
    Navarro Acero P, Mohr S, Bernabei M, Fernández C, Dominguez B, Ewen Jet al., 2021,

    Molecular simulations of surfactant adsorption on iron oxide from hydrocarbon solvents

    , Langmuir: the ACS journal of surfaces and colloids, Vol: 37, Pages: 14582-14596, ISSN: 0743-7463

    The performance of organic friction modifiers (OFMs) depends on their ability to adsorb onto surfaces and form protective monolayers. Understanding the relationship between OFM concentration in the base oil and the resulting surface coverage is important for improving lubricant formulations. Here, we use molecular dynamics (MD) simulations to study the adsorption of three OFMs─stearic acid (SA), glycerol monoostearate (GMS), and glycerol monooleate (GMO)─onto a hematite surface from two hydrocarbon solvents─n-hexadecane and poly(α-olefin) (PAO). We calculate the potential of mean force of the adsorption process using the adaptive biasing force algorithm, and the adsorption strength increases in the order SA < GMS < GMO. We estimate the minimum area occupied by OFM molecules on the surface using annealing MD simulations and obtained a similar hard-disk area for GMS and GMO but a lower value for SA. Using the MD results, we determine the adsorption isotherms using the molecular thermodynamic theory (MTT), which agree well with one previous experimental data set for SA on hematite. For two other experimental data sets for SA, lateral interactions between surfactant molecules need to be accounted for within the MTT framework. SA forms monolayers with lower surface coverage than GMO and GMS at low concentrations but also has the highest plateau coverage. We validate the adsorption energies from the MD simulations using high-frequency reciprocating rig friction experiments with different concentrations of the OFMs in PAO. For OFMs with saturated tailgroups (SA and GMS), we obtain good agreement between the simulations and the experiments. The results deviate for OFMs containing Z-unsaturated tailgroups (GMO) due to the additional steric hindrance, which is not accounted for in the current simulation framework. This study demonstrates that MD simulations, alongside MTT, are an accurate and efficient tool to predict adsorption isotherms at solid–liquid int

  • Journal article
    Dou P, Wu T, Luo Z, Yang P, Peng Z, Yu M, Reddyhoff Tet al., 2021,

    A finite-element-aided ultrasonic method for measuring central oil film thickness in a roller-raceway tribo-pair

    , Friction, ISSN: 2223-7690

    Roller bearings support heavy loads by riding on an ultra-thin oil film (between the roller and theraceway), the thickness of which is critical as it reflects the lubrication performance. Ultrasonicinterfacial reflection, which facilitates a non-destructive measurement of oil film thickness, has beenwidely studied. However, insufficient spatial resolution around the rolling line contact zone remainsa barrier, despite the employment of miniature piezoelectric transducers. In this paper, a finiteelement-aided method is utilized to simulate wave propagation through a three-layered structureof roller-oil-raceway under elastohydrodynamic lubrication (EHL), with nonlinear characteristicsof i) the deformed curvature of the cylindrical roller and ii) the non-uniform distribution of fluid bulkmodulus along the circumference of the oil layer taken into account. A load- and speed-dependentlook-up table is then developed to establish an accurate relationship between the overall reflectioncoefficient (directly measured by an embedded ultrasonic transducer) and the objective variable ofcentral oil film thickness. Moreover, the proposed finite-element-aided method is verifiedexperimentally in a roller-raceway test rig, with the ultrasonic measured oil film thicknessessentially corresponding to the calculated values by EHL theory.

  • Book chapter
    Boidi G, Grützmacher P, Varga M, da Silva MR, Gachot C, Dini D, Profito F, Machado Iet al., 2021,

    Tribological Performance of Random Sinter Pores vs. Deterministic Laser Surface Textures: An Experimental and Machine Learning Approach

    , Tribology, Editors: Pintaude, Cousseau
  • Journal article
    Xu Y, Balint DS, Dini D, 2021,

    On the Origin of Plastic Deformation and Surface Evolution in Nano-Fretting: A Discrete Dislocation Plasticity Analysis

    , MATERIALS, Vol: 14
  • Journal article
    Amis A, Bartolo MK, Accardi M, Williams A, Newman S, Provaggi E, Dini D, Athwal Ket al., 2021,

    Strength of interference screw fixation of meniscus prosthesis matches native meniscus attachments

    , Knee Surgery Sports Traumatology Arthroscopy, ISSN: 0942-2056

    PurposeMeniscal surgery is one of the most common orthopaedic surgical interventions. Total meniscus replacements have been proposed as a solution for patients with irreparable meniscal injuries. Reliable fixation is crucial for the success and functionality of such implants. The aim of this study was to characterise an interference screw fixation system developed for a novel fibre-matrix-reinforced synthetic total meniscus replacement in an ovine cadaveric model.MethodsTextile straps were tested in tension to failure (n = 15) and in cyclic tension (70–220 N) for 1000 cycles (n = 5). The textile strap-interference screw fixation system was tested in 4.5 mm-diameter single anterior and double posterior tunnels in North of England Mule ovine tibias aged > 2 years using titanium alloy (Ti6Al4Va) and polyether-ether-ketone (PEEK) screws (n ≥ 5). Straps were preconditioned, dynamically loaded for 1000 cycles in tension (70–220 N), the fixation slippage under cyclic loading was measured, and then pulled to failure.ResultsStrap stiffness was at least 12 times that recorded for human meniscal roots. Strap creep strain at the maximum load (220 N) was 0.005 following 1000 cycles. For all tunnels, pull-out failure resulted from textile strap slippage or bone fracture rather than strap rupture, which demonstrated that the textile strap was comparatively stronger than the interference screw fixation system. Pull-out load (anterior 544 ± 119 N; posterior 889 ± 157 N) was comparable to human meniscal root strength. Fixation slippage was within the acceptable range for anterior cruciate ligament graft reconstruction (anterior 1.9 ± 0.7 mm; posterior 1.9 ± 0.5 mm).ConclusionThese findings show that the textile attachment-interference screw fixation system provides reliable fixation for a novel ovine meniscus implant, supporting

  • Journal article
    Yu M, Reddyhoff T, Dini D, Holmes A, O'Sullivan Cet al., 2021,

    Using ultrasonic reflection resonance to probe stress wave velocity in assemblies of spherical particles

    , IEEE Sensors Journal, Vol: 21, Pages: 22489-22498, ISSN: 1530-437X

    A high-sensitivity method to measure acousticwave speed in soils by analyzing the reflected ultrasonic signalfrom a resonating layered interface is proposed here.Specifically, an ultrasonic transducer which can be used to bothtransmit and receive signals is installed on a low-high acousticimpedance layered structure of hard PVC and steel, which in turnis placed in contact with the soil deposit of interest. The acousticimpedance of the soil (the product of density and wave velocity)is deduced from analysis of the waves reflected back to thetransducer. A system configuration design is enabled bydeveloping an analytical model that correlates the objectivewave speed with the measurable reflection coefficient spectrum.The physical viability of this testing approach is demonstratedby means of a one-dimensional compression device that probesthe stress-dependence of compression wave velocity of differentsizes of glass ballotini particles. Provided the ratio of thewavelength of the generated wave to the soil particle size issufficiently large the data generated are in agreement with dataobtained using conventional time-of-flight measurements. Inprinciple, this high-sensitivity approach avoids the need for thewave to travel a long distance between multiple transmitterreceiver sensors as is typically the case in geophysical testingof soil. Therefore it is particularly suited to in-situ observation ofsoil properties in a highly compact setup, where only a single transducer is required. Furthermore, high spatialresolution of local measurements can be achieved, and the data are unaffected by wave attenuation as transmitted insoil.

  • Journal article
    Miyazaki M, Nakano K, Tadokoro C, Vladescu S-C, Reddyhoff T, Sasaki S, Tsujii Yet al., 2021,

    Enhancing durability of concentrated polymer brushes using microgrooved substrates

    , WEAR, Vol: 482, ISSN: 0043-1648
  • Journal article
    Boidi G, Grützmacher PG, Kadiric A, Profito FJ, Machado IF, Gachot C, Dini Det al., 2021,

    Fast laser surface texturing of spherical samples to improve the frictional performance of elasto-hydrodynamic lubricated contacts

    , Friction, Vol: 9, Pages: 1227-1241, ISSN: 2223-7704

    Textured surfaces offer the potential to promote friction and wear reduction by increasing the hydrodynamic pressure, fluid uptake, or acting as oil or debris reservoirs. However, texturing techniques often require additional manufacturing steps and costs, thus frequently being not economically feasible for real engineering applications. This experimental study aims at applying a fast laser texturing technique on curved surfaces for obtaining superior tribological performances. A femtosecond pulsed laser (Ti:Sapphire) and direct laser interference patterning (with a solid-state Nd:YAG laser) were used for manufacturing dimple and groove patterns on curved steel surfaces (ball samples). Tribological tests were carried out under elasto-hydrodynamic lubricated contact conditions varying slide-roll ratio using a ball-on-disk configuration. Furthermore, a specific interferometry technique for rough surfaces was used to measure the film thickness of smooth and textured surfaces. Smooth steel samples were used to obtain data for the reference surface. The results showed that dimples promoted friction reduction (up to 20%) compared to the reference smooth specimens, whereas grooves generally caused less beneficial or detrimental effects. In addition, dimples promoted the formation of full film lubrication conditions at lower speeds. This study demonstrates how fast texturing techniques could potentially be used for improving the tribological performance of bearings as well as other mechanical components utilised in several engineering applications.

  • Journal article
    Wang X, Bao L, Wen J, Dini D, Zhang J, Sun L, Yang W, Zhou F, Liu Wet al., 2021,

    Anomalous boundary behavior in non-newtonian fluids at amphiphobic surfaces

    , Tribology International, Pages: 107261-107261, ISSN: 0301-679X

    In this work, the effect of amphiphobic surfaces on the rheological behavior and boundary slip of the shear thickening fluids (STFs) was investigated. The experimental results suggested the viscosities were diminished, shear thickening was delayed and weakened, and an ultrahigh drag reduction was obtained. Furthermore, slip length was observed to vary with shear rate. Dissipative particle dynamics (DPD) simulations were adopted to further investigate these specific rheology and slip behavior. The simulation results conformed with experiments and established a linear relationship between the slip length and viscosity. We consider this study could be a conducive practical reference for the investigation of boundary slip in complex fluids and possibly a crucial protocol for analyzing STFs’ manipulation.

  • Journal article
    Vidotto M, Bernardini A, Trovatelli M, De Momi E, Dini Det al., 2021,

    On the microstructural origin of brain white matter hydraulic permeability

    , Proceedings of the National Academy of Sciences of USA, Vol: 118, ISSN: 0027-8424

    Brain microstructure plays a key role in driving the transport of drug molecules directly administered to the brain tissue, as in Convection-Enhanced Delivery procedures. The proposed research analyzes the hydraulic permeability of two white matter (WM) areas (corpus callosum and fornix) whose three-dimensional microstructure was reconstructed starting from the acquisition of electron microscopy images. We cut the two volumes with 20 equally spaced planes distributed along two perpendicular directions, and, on each plane, we computed the corresponding permeability vector. Then, we considered that the WM structure is mainly composed of elongated and parallel axons, and, using a principal component analysis, we defined two principal directions, parallel and perpendicular, with respect to the axons’ main direction. The latter were used to define a reference frame onto which the permeability vectors were projected to finally obtain the permeability along the parallel and perpendicular directions. The results show a statistically significant difference between parallel and perpendicular permeability, with a ratio of about two in both the WM structures analyzed, thus demonstrating their anisotropic behavior. Moreover, we find a significant difference between permeability in corpus callosum and fornix, which suggests that the WM heterogeneity should also be considered when modeling drug transport in the brain. Our findings, which demonstrate and quantify the anisotropic and heterogeneous character of the WM, represent a fundamental contribution not only for drug-delivery modeling, but also for shedding light on the interstitial transport mechanisms in the extracellular space.

  • Journal article
    Bhamra J, Ewen J, Ayestaran Latorre C, Bomidi J, Bird M, Dasgupta N, van Duin A, Dini Det al., 2021,

    Interfacial bonding controls friction in diamond–rock contacts

    , The Journal of Physical Chemistry C: Energy Conversion and Storage, Optical and Electronic Devices, Interfaces, Nanomaterials, and Hard Matter, Vol: 125, Pages: 18395-18408, ISSN: 1932-7447

    Understanding friction at diamond–rock interfaces is crucial to increase the energy efficiencyof drilling operations. Harder rocks usually are usually more difficult to drill; however, poorperformance is often observed for polycrystalline diamond compact (PDC) bits on soft calcitecontaining rocks, such as limestone. Using macroscale tribometer experiments with adiamond tip, we show that soft limestone rock (mostly calcite) gives much higher frictioncoefficients compared to hard granite (mostly quartz) in both humid air and aqueousenvironments. To uncover the physicochemical mechanisms that lead to higher kinetic frictionat the diamond–calcite interface, we employ nonequilibrium molecular dynamics simulations(NEMD) with newly developed Reactive Force Field (ReaxFF) parameters. In the NEMDsimulations, higher friction coefficients are observed for calcite than quartz when watermolecules are included at the diamond–rock interface. We show that the higher friction inwater-lubricated diamond–calcite than diamond–quartz interfaces is due to increasedinterfacial bonding in the former. For diamond–calcite, the interfacial bonds mostly formthrough chemisorbed water molecules trapped between the tip and the substrate, while mainlydirect tip-surface bonds form inside diamond–quartz contacts. For both rock types, the rate ofinterfacial bond formation increases exponentially with pressure, which is indicative of astress-augmented thermally activated process. The mean friction force is shown to be linearlydependant on the mean number of interfacial bonds during steady-state sliding. Theagreement between the friction behaviour observed in the NEMD simulations and tribometerexperiments suggests that interfacial bonding also controls diamond–rock friction at themacroscale. We anticipate that the improved fundamental understanding provided by thisstudy will assist in the development of bit materials and coatings to minimise friction byre

  • Journal article
    Gamaniel SS, Dini D, Biancofiore L, 2021,

    The effect of fluid viscoelasticity in lubricated contacts in the presence of cavitation

    , Tribology International, Vol: 160, ISSN: 0301-679X

    In this work we study the influence of fluid viscoelasticity on the performance of lubricated contacts in the presence of cavitation. Several studies of viscoelastic lubricants have been carried out, but none of them have considered the possibility of the presence of cavitation. To describe the effect of viscoelasticity, we use the Oldroyd-B model. By assuming that the product between ϵ, i.e. the ratio between vertical and horizontal length scales, and the Weissenberg number (Wi), i.e. the ratio between polymer relaxation time and flow time scale, is small, we can linearise the viscoelastic thin film equations, following the approach pioneered by "Tichy, J., 1996, Non-Newtonian lubrication with the convected Maxwell model." Consequently, the zeroth-order in ϵWi corresponds to a Reynolds equation modified to describe also the film cavitation through the mass-conserving Elrod-Adams model. We consider the flow of viscoelastic lubricants using: (i) a cosine profile representing a journal bearing unwrapped geometry, and (ii) a pocketed profile to model a textured surface in lubricated contacts. The introduction of viscoelasticity decreases the length of cavitated region in the cosine profile due to the increasing pressure distribution within the film. Consequently, the load carrying capacity increases with Wi by up to 50% in the most favorable condition, confirming the beneficial influence of the polymers in bearings. On the other hand for the pocketed profile, results show that the load can increase or decrease at higher Wi depending on the texture position in the contact. The squeeze flow problem between two plates is also modeled for viscoelastic lubricants considering an oscillating top surface. For this configuration a load reduction is observed with increasing Wi due to the additional time needed to reform the film at high Wi. Furthermore, if viscoelastic effects increase, the cavitation region widens until reaching a value of Wi for which a full-film ref

  • Journal article
    Yu M, Cheng C, Evangelou S, Dini Det al., 2021,

    Series active variable geometry suspension: full-car prototyping and road testing

    , IEEE-ASME Transactions on Mechatronics, ISSN: 1083-4435

    In this paper, afull-car prototype of the recently proposed mechatronic suspension, Series Active Variable Geometry Suspension (SAVGS), is developed for on-road driving experimental proof of concept, aiming to be adopted by suspension OEMs (original equipment manufacturers) as an alternative solution to fully active suspensions. Particularly, mechanical modifications are performed to both corners of the front double-wishbone suspensionof a production car, with active single-links attached to the upper-ends of the spring-damper units, while both corners of the rear suspension remain inthe original (passive) configurations.The mechanical modifications involve innovatively designed parts to enable the desired suspension performance improvements, while maintaining ride harshness at conventional levels.Areal-time embedded system is further developed to primarily implement:1) power supply, data acquisition and measurementsof the vehicle dynamics related variables, and 2) robust control application for the ride comfort and road holding enhancement, which is based on a derived linearized model of the full-car dynamics and a newly synthesizedH-infinity control scheme. Results obtained from on-road driving experiments are inessential agreement with numerical simulation results also produced. Overall, the full-car prototypeof SAVGS demonstrates promising suspension performance,with anaverage 3 dB attenuation (or equivalently 30% reduction) of the chassis vertical acceleration at aroundthe human-sensitive frequencies (2-5Hz),as compared to the original vehicle with the passive suspension system. More importantly, the prototype also indicatesthe practicality of the solution, as the SAVGS retrofit to a real car is achieved by simple mechanical modifications, compact actuator packaging, smallmass increment(21.5kg increase with respect to the original vehicle), limited power usage

  • Journal article
    Hu S, Reddyhoff T, Li J, Cao X, Shi X, Peng Z, deMello AJ, Dini Det al., 2021,

    Biomimetic water-repelling surfaces with robustly flexible structures

    , ACS Applied Materials and Interfaces, Vol: 13, Pages: 31310-31319, ISSN: 1944-8244

    Biomimetic liquid-repelling surfaces have been the subject of considerable scientific research and technological application. To design such surfaces, a flexibility-based oscillation strategy has been shown to resolve the problem of liquid-surface positioning encountered by the previous, rigidity-based asymmetry strategy; however, its usage is limited by weak mechanical robustness and confined repellency enhancement. Here, we design a flexible surface comprising mesoscale heads and microscale spring sets, in analogy to the mushroomlike geometry discovered on springtail cuticles, and then realize this through three-dimensional projection microstereolithography. Such a surface exhibits strong mechanical robustness against ubiquitous normal and shear compression and even endures tribological friction. Simultaneously, the surface elevates water repellency for impacting droplets by enhancing impalement resistance and reducing contact time, partially reaching an improvement of ∼80% via structural tilting movements. This is the first demonstration of flexible interfacial structures to robustly endure tribological friction as well as to promote water repellency, approaching real-world applications of water repelling. Also, a flexibility gradient is created on the surface to directionally manipulate droplets, paving the way for droplet transport.

  • Journal article
    Terzano M, Spagnoli A, Dini D, Forte AEet al., 2021,

    Fluid-solid interaction in the rate-dependent failure of brain tissue and biomimicking gels

    , Journal of The Mechanical Behavior of Biomedical Materials, Vol: 119, ISSN: 1751-6161

    Brain tissue is a heterogeneous material, constituted by a soft matrix filledwith cerebrospinal fluid. The interactions between, and the complexity of eachof these components are responsible for the non-linear rate-dependent behaviourthat characterizes what is one of the most complex tissue in nature. Here, weinvestigate the influence of the cutting rate on the fracture properties ofbrain, through wire cutting experiments. We also present a model for therate-dependent behaviour of fracture propagation in soft materials, whichcomprises the effects of fluid interaction through a poro-hyperelasticformulation. The method is developed in the framework of finite straincontinuum mechanics, implemented in a commercial finite element code, andapplied to the case of an edge-crack remotely loaded by a controlleddisplacement. Experimental and numerical results both show a toughening effectwith increasing rates, which is linked to the energy dissipated by thefluid-solid interactions in the process zone ahead of the crack.

  • Journal article
    Hu S, Cao X, Reddyhoff T, Shi X, Peng Z, deMello AJ, Dini Det al., 2021,

    Flexibility-patterned liquid-repelling surfaces

    , ACS Applied Materials and Interfaces, Vol: 13, Pages: 29092-29100, ISSN: 1944-8244

    Droplets impacting solid surfaces is ubiquitous in nature and of practical importance in numerous industrial applications. For liquid-repelling applications, rigidity-based asymmetric redistribution and flexibility-based structural oscillation strategies have been proven on artificial surfaces; however, these are limited by strict impacting positioning. Here, we show that the gap between these two strategies can be bridged by a flexibility-patterned design similar to a trampoline park. Such a flexibility-patterned design is realized by three-dimensional projection micro-stereolithography and is shown to enhance liquid repellency in terms of droplet impalement resistance and contact time reduction. This is the first demonstration of the synergistic effect obtained by a hybrid solution that exploits asymmetric redistribution and structural oscillation in liquid-repelling applications, paving the rigidity-flexibility cooperative way of wettability tuning. Also, the flexibility-patterned surface is applied to accelerate liquid evaporation.

  • Journal article
    Haimov E, Chapman A, Bresme F, Holmes A, Reddyhoff T, Urbakh M, Kornyshev Aet al., 2021,

    Theoretical demonstration of a capacitive rotor for generation of alternating current from mechanical motion

    , Nature Communications, Vol: 12, Pages: 3678-3678, ISSN: 2041-1723

    Innovative concepts and materials are enabling energy harvesters for slower motion, particularly for personal wearables or portable small-scale applications, hence contributing to a future sustainable economy. Here we propose a principle for a capacitive rotor device and analyze its operation. This device is based on a rotor containing many capacitors in parallel. The rotation of the rotor causes periodic capacitance changes and, when connected to a reservoir-of-charge capacitor, induces alternating current. The properties of this device depend on the lubricating liquid situated between the capacitor’s electrodes, be it a highly polar liquid, organic electrolyte, or ionic liquid – we consider all these scenarios. An advantage of the capacitive rotor is its scalability. Such a lightweight device, weighing tens of grams, can be implemented in a shoe sole, generating a significant power output of the order of Watts. Scaled up, such systems can be used in portable wind or water turbines.

  • Journal article
    Ueda M, Spikes H, Kadiric A, 2021,

    Influence of black oxide coating on micropitting and ZDDP tribofilm formation

    , Tribology Transactions, Pages: 1-21, ISSN: 1040-2004

    Micropitting is a type of surface fatigue damage that occurs in rolling-sliding contacts operating under thin oil film conditions. Application of black oxide (BO) coating to steel rubbing surfaces has been suggested as a potential approach to alleviate micropitting. This paper confirms that BO coatings can prevent micropitting and identifies the predominant mechanism by which this occurs.Micropitting tests were carried out using ZDDP solutions in a ball on disc tribometer. Micropitting was preferentially generated on the smooth balls and this was completely prevented by applying a BO coating to the rougher discs, regardless of whether the balls were coated or not. In contrast, when the rough discs were not BO-coated, micropitting was consistently generated on both BO-coated and uncoated balls. BO coating has about one quarter the hardness of the steel used and was found to be very rapidly removed from the surface asperity peaks at the onset of rubbing, despite the presence of ZDDP. This resulted in an almost immediate and very large reduction of the surface roughness of the discs and this prevented high asperity stresses that would normally initiate and propagate the surface fatigue cracks leading to micropitting. Parallel measurement showed that BO did not suppress tribofilm growth, so the ZDDP was able to protect against adhesive wear while not promoting micropitting. The insights presented here can help with the design of components and lubricants that are effective in controlling both sliding wear and micropitting.

  • Journal article
    Whitehouse S, Myant C, Cann PM, Stephens Aet al., 2021,

    Fluorescent imaging of razor cartridge/skin lubrication

  • Journal article
    Yang S, Zhang D, Wong J, Cai Met al., 2021,

    Interactions between ZDDP and an oil-soluble ionic liquid additive

    , Tribology International, Vol: 158, ISSN: 0301-679X

    Zinc dialkyldithiophosphates (ZDDP) and a P-based ionic liquid (IL) were added in a polyalphaolefin base oil (PAO). Their tribological performance in steel-steel contacts in the boundary (BL) and “starved” elastohydrodynamic (sEHL) lubrication conditions were investigated.In BL conditions, IL reduces friction at 40 °C. It reduces both friction and wear at 100 °C. ZDDP reduces wear but has relatively high friction at both temperatures. Synergy between IL and ZDDP is observed only at 100 °C, due to the generation of a smoother worn surface and potentially a thicker boundary film.In sEHL conditions at room temperature, ZDDP, IL and their mixture change the inlet condition and increase lubricant film thickness. No synergy between the two additives is observed.

  • Journal article
    Pagkalis K, Spikes H, Jelita Rydel J, Ingram M, Kadiric Aet al., 2021,

    The influence of steel composition on the formation and effectiveness of anti-wear films in tribological contacts

    , Tribology Letters, Vol: 69, Pages: 1-20, ISSN: 1023-8883

    The effectiveness of antiwear additives in laboratory tests is commonly evaluated using specimens made of AISI 52100 through-hardened bearing steel. However, many lubricated machine components are made of steels with significantly different material compositions, which raises an important practical question of whether the performance of antiwear additives with these other steel types is different from that established with AISI 52100. To help answer this question, this paper investigates the influence of steel composition on the formation and effectiveness of antiwear films. Four steels that are commonly used in tribological applications, namely AISI 52100 through-hardened bearing steel, 16MnCr5 case-carburised gear steel, M2 high speed steel and 440C stainless steel are tested in rolling-sliding, ball-on-disc contacts lubricated with three custom-made oils, one containing ZDDP and two containing different types of ashless antiwear additives. The relative effectiveness of their boundary films was assessed by measuring their thickness and associated wear and friction over 12 h of rubbing at two specimen roughness levels. For ZDDP it was found that the formation of antiwear film was not significantly influenced by steel composition or specimen surface roughness. A similar tribofilm thickness, final tribofilm roughness and friction was observed with all four steels. No measurable wear was observed. By contrast, for the ashless antiwear additives the thickness and effectiveness of their tribofilms was strongly influenced by steel composition, particularly at higher roughness levels. The exact trends in film thickness vs steel relationship depended on the specific chemistry of the ashless additive (ester-based or acid-based) but in general, relative to AISI 52100 steel, M2 steel promoted ashless tribofilm formation whilst 440C retarded ashless tribofilm formation. This behaviour is attributed to the presence of different alloying elements and the ability of the additives

  • Journal article
    Stevenson H, Cann PM, 2021,

    Protein content of model synovial fluid and CoCrMo wear

    , Biotribology, Vol: 26, Pages: 1-13, ISSN: 2352-5738

    Wear of cobalt chromium molybdenum alloy in a reciprocating ball-on-plate test was measured for a series of model synovial fluid samples, where the effect of protein and phospholipid content was examined. The protein content (albumin and γ-globulin) was varied to replicate a range of healthy and diseased SF pathologies. The results showed reduced wear was strongly correlated with increasing protein content. The effect of phospholipid addition on wear was more complex. Limited evidence suggested phospholipids reduced wear for a high albumin/γ-globulin ratio (A/G) but increased wear for low A/G ratios. Post-test examination showed thick (~μm) insoluble “gel-like” films were deposited in, and around, the wear scar. Micro Infrared Reflection Absorption Spectroscopy analysis indicated the films were predominately denatured β-sheet proteins although in some cases lipids were also present. Similar films were found in tests with human synovial fluid samples. Scanning Electron Microscopy imaging showed an aggregated fibril “rope” structure typical of non-native β-sheet proteins. The gel film is a protein-rich viscous phase which is entrained intermittently to form a lubrication film which contributes to surface protection and reduction of wear. We also suggest the formation of gel deposits is comparable to the “boosted” lubrication model of proposed by Professor Duncan Dowson for articular cartilage. In the boosted model high-viscosity, concentrated protein films are formed in depressions on the cartilage surface. The tests indicate the chemistry of human synovial fluid, particularly the protein content, could affect CoCrMo wear and therefore the risk of implant failure.

  • Journal article
    Ueda M, Kadiric A, Spikes H, 2021,

    Wear of hydrogenated DLC in MoDTC-containing oils

    , Wear, Vol: 474-475, Pages: 1-10, ISSN: 0043-1648

    This paper describes a study of the effect on MoDTC-promoted a-C:H DLC wear of adding various surface-active additives used in engine lubricants, including ZDDP, an ashless EP additive, Ca detergents, dispersants, an OFM and a PAMA, to an MoDTC solution. Tribofilms formed on wear tracks on steel were analysed using SLIM, TEM, STEM-EDX, Raman spectroscopy and XPS. Relevant mechanisms by which these additives reduce the impact of MoDTC on DLC wear have also been suggested. DLC wear in PAO+Mo can be reduced by the presence of other surface-active additives in three ways. Firstly, asperity contact between DLC and steel can be mitigated by forming thick antiwear tribofilms. Secondly, other additives can increase the ratio of MoS2:MoO3, reducing the amount of wear-enhancing MoO3 in the tribofilm. Thirdly, the amount of MoDTC tribofilm including MoO3 can be reduced by the competitive adsorption of other surface-active additives. This study has practical implications for ways in which DLC surfaces can be protected by lubricant formulation.

  • Journal article
    Charalambides M, Bikos D, Masen M, Hardalupas I, Cann P, Samaras G, Hartmann C, Vieira Jet al., 2021,

    Effect of micro-aeration on the mechanical behaviour of chocolates and implications for oral processing

    , Food and Function, Vol: 12, Pages: 4864-4886, ISSN: 2042-6496

    Aeration in foods has been widely utilised in the food industry to develop novel foods with enhanced sensorial characteristics. Specifically, aeration at the micron-sized scale has a significant impact on the microstructure where micro-bubbles interact with the other microstructural features in chocolates. This study aims to determine the effect of micro-aeration on the mechanical properties of chocolate products, which are directly correlated with textural attributes such as hardness and crumbliness. Uniaxial compression tests were performed to determine the mechanical properties such as Poisson's ratio, Young's modulus and macroscopic yield strength together with fracture tests to estimate the fracture toughness. In vivo mastication tests were also conducted to investigate the link between the fracture properties and fragmentation during the first two chewing cycles. The uniaxial stress–strain data were used to calibrate a viscoplastic constitutive law. The results showed that micro-aeration significantly affects mechanical properties such as Young's modulus, yield and fracture stresses, as well as fracture toughness. In addition, it enhances the brittle nature of the chocolate, as evidenced by lower fracture stress but also lower fracture toughness leading to higher fragmentation, in agreement with observations in the in vivo mastication tests. As evidenced by the XRT images and the stress–strain measurements micro-aeration hinders the re-arrangement of the microscopic features inside the chocolate during the material's deformation. The work provides a new insight of the role of bubbles on the bulk behaviour of complex multiphase materials, such as chocolates, and defines the mechanical properties which are important input parameters for the development of oral processing simulations.

  • Journal article
    Boidi G, Profito FJ, Kadiric A, Machado IF, Dini Det al., 2021,

    The use of Powder Metallurgy for promoting friction reduction under sliding-rolling lubricated conditions

    , Tribology International, Vol: 157, ISSN: 0301-679X

    This work exploits the use of different sintering manufacturing techniques for obtaining superior performances in lubricated point contacts. Disc and ball specimens were manufactures varying porosity and pore characteristics. The effect of surface pores in sintered materials was evaluated based on the frictional behaviour under different sliding-rolling conditions and lubrication regimes. Furthermore, lubricant film thicknesses were measured using interferometric technique. Test results showed that the decrease of porosity generally improves tribological performance. Low porosity surfaces can promote friction reduction compared to non-porous reference materials in specific configurations and operating with similar specific lubricant thickness values. This work contributes to an improved understanding of how randomly distributed micro-irregularities could change lubrication conditions, potentially increasing the efficiency of lubricated mechanical systems.

  • Journal article
    Zimmer M, Vladescu S-C, Mattsson L, Fowell M, Reddyhoff Tet al., 2021,

    Shear-Area Variation: A Mechanism that Reduces Hydrodynamic Friction in Macro-Textured Piston Ring Liner Contacts

    , Tribology International, ISSN: 0301-679X
  • Journal article
    Ueda M, Kadiric A, Spikes H, 2021,

    Influence of steel surface composition on ZDDP tribofilm growth using Ion implantation

    , Tribology Letters, Vol: 69, ISSN: 1023-8883

    This paper examines the influence of steel surface composition on antiwear tribofilm formation by ion-implanting typical steel alloying elements, Ni, Mo, Cr, V and W, into AISI 52100 bearing steel surfaces. Such implantation changes the chemical composition of the steel surface but has relatively little effect on its mechanical properties or topography. The behaviour of zinc dialkyldithiophosphate (ZDDP) antiwear additive was studied. The study employs a ball on disc tribometer with ability to monitor tribofilm development and a range of analytical tools including STEM-EDX, XPS and FIB-TEM to analyse the formed tribofilms. It was found that Ni implantation promotes ZDDP tribofilm formation while Mo and Cr implantation deters tribofilm growth. V and W implantation do not significantly change tribofilm formation. Results on the influence of ZDDP concentration on tribofilm formation rate with different implanted metals suggest that one important mechanism by which steel composition influences tribofilm formation may be by controlling the extent of ZDDP adsorption. This study shows the importance of steel surface composition on ZDDP response and also demonstrates a powerful way to study and potentially improve the tribological performance of machine components via a combination of lubricant formulation and surface modification.

  • Journal article
    Putignano C, Burris D, Moore A, Dini Det al., 2021,

    Cartilage rehydration: the sliding-induced hydrodynamic triggering mechanism

    , Acta Biomaterialia, Vol: 125, Pages: 90-99, ISSN: 1742-7061

    Loading-induced cartilage exudation causes loss of fluid from the tissue, joint space thinning and, in a long term prospective, the insurgence of osteoarthritis. Fortunately, experiments show that joints recover interstitial fluid and thicken during articulation after static loading, thus reversing the exudation process. Here, we provide the first original theoretical explanation to this crucial phenomenon, by implementing a numerical model capable of accounting for the multiscale porous lubrication occurring in joints. We prove that sliding-induced rehydration occurs because of hydrodynamic reasons and is specifically related to a wedge effect at the contact inlet. Furthermore, numerically predicted rehydration rates are consistent with experimentally measured rates and corroborate the robustness of the model here proposed. The paper provides key information, in terms of fundamental lubrication multiscale mechanisms, to understand the rehydration of cartilage and, more generally, of any biological tissue exhibiting a significant porosity: such a theoretical framework is, thus, crucial to inform the design of new effective cartilage-mimicking biomaterials.

  • Journal article
    Gurrutxaga Lerma B, Verschueren J, Sutton A, Dini Det al., 2021,

    The mechanics and physics of high-speed dislocations: a critical review

    , International Materials Reviews, Vol: 66, Pages: 215-255, ISSN: 0950-6608

    High speed dislocations have long been identified as the dominant feature governing the plastic response of crystalline materials subjected to high strain rates, controlling deformation and failure in industrial processes such as machining, laser shock peening, punching, drilling, crashworthiness, foreign object damage, etc. Despite decades of study, the role high speed dislocations have on the materials response remains elusive. This article reviews both experimental and theoretical efforts made to address this issue in a systematic way. The lack of experimental evidence and direct observation of high speed dislocations means that most work on the matter is rooted on theory and simulations. This article offers a critical review of the competing theoretical accounts of high speed mechanisms, their underlying hypothesis, insights, and shortcomings, with particular focus on elastic continuum and atomistic levels. The article closes with an overview of the current state of the art and suggestions for key developments in future research.

  • Journal article
    Jamal A, Mongelli M, Vidotto M, Madekurozwa M, Bernardini A, Overby D, De Momi E, Rodriguez y Baena F, Sherwood J, Dini Det al., 2021,

    Infusion mechanisms in brain white matter and its dependence of microstructure: an experimental study of hydraulic permeability

    , IEEE Transactions on Biomedical Engineering, Vol: 68, Pages: 1229-1237, ISSN: 0018-9294

    Objective: Hydraulic permeability is a topic of deep interest in biological materials because of its important role in a range of drug delivery-based therapies. The strong dependence of permeability on the geometry and topology of pore structure and the lack of detailed knowledge of these parameters in the case of brain tissue makes the study more challenging. Although theoretical models have been developed for hydraulic permeability, there is limited consensus on the validity of existing experimental evidence to complement these models. In the present study, we measure the permeability of white matter (WM) of fresh ovine brain tissue considering the localised heterogeneities in the medium using an infusion based experimental set up, iPerfusion. We measure the flow across different parts of the WM in response to applied pressures for a sample of specific dimensions and calculate the permeability from directly measured parameters. Furthermore, we directly probe the effect of anisotropy of the tissue on permeability by considering the directionality of tissue on the obtained values. Additionally, we investigate whether WM hydraulic permeability changes with post-mortem time. To our knowledge, this is the first report of experimental measurements of the localised WM permeability, showing the effect of axon directionality on permeability. This work provides a significant contribution to the successful development of intra-tumoural infusion-based technologies, such as convection-enhanced delivery (CED), which are based on the delivery of drugs directly by injection under positive pressure into the brain.

  • Journal article
    Dine A, Bentley E, PoulmarcK L, Dini D, Forte A, Tan Zet al., 2021,

    A dual nozzle 3D printing system for super soft composite hydrogels

    , HardwareX, Vol: 9, ISSN: 2468-0672

    Due to their inability to sustain their own weight, 3D printing materials as soft as human tissues is challenging. Hereby we describe the development of an extrusion additive manufacturing (AM) machine able to 3D print super soft hydrogels with micro-scale precision. By designing and integrating new subsystems into a conventional extrusion-based 3D printer, we obtained hardware that encompasses a range of new capabilities. In particular, we integrated a heated dual nozzle extrusion system and a cooling platform in the new system. In addition, we altered the electronics and software of the 3D printer to ensure fully automatized procedures are delivered by the 3D printing device, and super-soft tissue mimicking parts are produced. With regards to the electronics, we added new devices to control the temperature of the extrusion system. As for the software, the firmware of the conventional 3D printer was changed and modified to allow for the flow rate control of the ink, thus eliminating overflows in sections of the printing path where the direction/speed changes sharply.

  • Journal article
    Yu M, Zhang J, Joedicke A, Reddyhoff Tet al., 2021,

    Experimental investigation into the effects of diesel dilution on engine lubrication

    , Tribology International, Vol: 156, Pages: 1-9, ISSN: 0301-679X

    The dilution of lubricant due to contamination with diesel fuel is an increasingly prevalent, potentially importantand poorly understood issue. Thisstudy addressestwo fundamental questions: 1) How doesthe change in lubricantrheology due to diesel dilution affect engine lubrication? 2) How is the chemical performance of lubricantcomponents (base oil and performance additives) impacted by diesel dilution under different lubrication regimes(boundary/full film, hydrodynamic/elastohydrodynamic). This is achieved by testing three lubricant samples: 1)neat fully formulated 0W-30 engine oil, 2) fully formulated 0W-30 oil diluted with diesel at a concentration of15%, denoted “0W-30D”, and 3) neat, fully-formulated 0W-16, with the same base oil components andperformance additives as the 0W-30, but blended to give a viscosity equal to that of the diluted an equivalent“0W-30D”. Tribometer tests, including 1) low pressure, low shear viscosity, 2) Ultra-high Shear Viscosity (USV),3) elastohydrodynamic film thickness, 4) Stribeck friction and 5) boundary friction and wear, are then conducted.To further emulate engine lubrication conditions, Stribeck curve measurements are performed on the threelubricants using a journal bearing test rig, fitted with a connecting-rod and commercial diesel engine shells.Results suggest that diesel dilution only slightly affects chemical additive performance (with friction modifiersbeing more inhibited than anti-wear additives) but does reduce both viscosity and film thickness. However, caremust be taken in using viscometrics to predict dilution behaviour because 1) the pressure viscosity coefficient isalso affected by diesel dilution which has implications for elastohydrodynamically lubrication contacts, 2) shearthinning means that viscosity modifier additives effects lose their functions at high shear rates; whereas dieselcontamination affects viscosity behaviour throughout the whole shear rate range.

  • Journal article
    Vlădescu S-C, Bozorgi S, Hu S, Baier SK, Myant C, Carpenter G, Reddyhoff Tet al., 2021,

    Effects of beverage carbonation on lubrication mechanisms and mouthfeel

    , Journal of Colloid and Interface Science, Vol: 586, Pages: 142-151, ISSN: 0021-9797

    The perception of carbonation is an important factor in beverage consumption which must be understood in order to develop healthier products. Herein, we study the effects of carbonated water on oral lubrication mechanisms involved in beverage mouthfeel and hence taste perception. Friction was measured in a compliant PDMS-glass contact simulating the tongue-palate interface (under representative speeds and loads), while fluorescence microscopy was used to visualise both the flow of liquid and oral mucosal pellicle coverage.When carbonated water is entrained into the contact, CO2 cavities form at the inlet, which limit flow and thus reduce the hydrodynamic pressure. Under mixed lubrication conditions, when the fluid film thickness is comparable to the surface roughness, this pressure reduction results in significant increases in friction (>300% greater than under non-carbonated water conditions). Carbonated water is also shown to be more effective than non-carbonated water at debonding the highly lubricious, oral mucosal pellicle, which again results in a significant increase in friction. Both these transient mechanisms of starvation and salivary pellicle removal will modulate the flow of tastants to taste buds and are suggested to be important in the experience of taste and refreshment. For example this may be one reason why flat colas taste sweeter.

  • Conference paper
    Lasen M, Sun Y, Schwingshackl CW, Dini Det al., 2021,

    Analysis of an Actuated Frictional Interface for Improved Dynamic Performance

    , Nonlinear Structures & Systems, Publisher: Springer
  • Journal article
    Xu Y, Ruebeling F, Balint DS, Greiner C, Dini Det al., 2021,

    On the origin of microstructural discontinuities in sliding contacts: a discrete dislocation plasticity analysis

    , International Journal of Plasticity, Vol: 138, Pages: 1-15, ISSN: 0749-6419

    Two-dimensional discrete dislocation plasticity (DDP) calculations that simulate single crystal films bonded to a rigid substrate under sliding by a rigid sinusoid-shaped asperity are performed with various contact sizes. The contact between the thin film and the asperity is established by a preceding indentation and modelled using a cohesive zone method (CZM), whose behavior is governed by a traction-displacement relation. The emergence of microstructural changes observed in sliding tests has been interpreted as a localized lattice rotation band produced by the activity of dislocations underneath the contact. The depth of the lattice rotation band is predicted to be well commensurate with that observed in the corresponding tests. Furthermore, the dimension and magnitude of the lattice rotation band have been linked to the sliding distance and contact size. This research reveals the underpinning mechanisms for the microstructural changes observed in sliding tests by explicitly modelling the dislocation patterns and highly localized plastic deformation of materials under various indentation and sliding scenarios.

  • Journal article
    Jiang S, Yuan C, Wong J, 2021,

    Effectiveness of glycerol-monooleate in high-performance polymer tribo-systems

    , Tribology International, Vol: 155, Pages: 1-9, ISSN: 0301-679X

    High performance polymers possessing superior mechanical properties may replace metal components and improve machine efficiency. Successful replacement, however, relies on the compatibility of these polymers with current engineering systems, including lubricants and their additives. This study examines the compatibility of two high performance polymers, polyetheretherketone (PEEK) and polyamide-imide (PAI), with glycerol monooleate (GMO), an organic friction modifier (OFM) commonly used in steel-steel rubbing contacts. Friction tests were conducted with a ball-on-disc geometry in reciprocating motion at 100 °C in polyalphaolefin (PAO) base oil. GMO reduces friction in polymer-steel and polymer-polymer contacts. When steel is involved, the use of GMO and oleic acid (OA) give similar friction coefficients. Since OA is believed to be a hydrolyzed product of GMO in steel-steel contacts, our results show that the interaction of OA with steel controls friction in polymer-steel contacts when GMO is the additive. Results from FTIR and Raman spectroscopies show that steel surfaces contain little to no polymeric materials, nor iron oxides after rubbing against polymers in GMO- and OA-containing PAO. This supports OFM layers are formed on steel surfaces. These OFM layers prevent polymer transfer layer formation and possibly protect steel surfaces from oxidation. Our results show that using OFM that interacts strongly with steel can, contrary to dry friction, eliminate the need of polymeric transfer film on steel for achieving low friction in polymer-steel contacts.

  • Journal article
    Kontou A, Taylor RI, Spikes HA, 2021,

    Effects of dispersant and ZDDP additives on fretting wear

    , Tribology Letters, Vol: 69, Pages: 1-13, ISSN: 1023-8883

    This paper examines the effect of dispersant and anti-wear additives on fretting wear in lubricated bearing steel contacts. Reciprocating sliding ball-on-flat fretting tests with a stroke length of 50 μm have been carried out on steel-to-steel contacts in both dry and lubricated conditions. Wear and friction coefficient have been measured, and surface characterisation has been carried out using optical techniques to investigate fretting wear. The presence of base oil reduces fretting wear markedly compared to dry conditions, but fretting damage is still observed at low reciprocation frequencies. As frequency is increased, there is a transition from oxidative to adhesive/scuffing damage. The anti-wear additive ZDDP is effective in forming a tribofilm on the surfaces and reducing visible oxidation and wear. A succinimide dispersant also reduces the accumulation of solid debris but does not alleviate wear damage. The combination of both ZDDP anti-wear additive and dispersant in base oil appears to provide significant protection against fretting wear.

  • Journal article
    Shi Y, Xiong D, Li J, Li L, Liu Q, Dini Det al., 2021,

    Tribological rehydration and its role on frictional behavior of PVA/GO hydrogels for cartilage replacement under migrating and stationary contact conditions

    , Tribology Letters, Vol: 69, Pages: 1-14, ISSN: 1023-8883

    Graphene oxide (GO) was incorporated into polyvinyl alcohol (PVA) hydrogel to improve its mechanical and tribological performances for potential articular cartilage replacement application. The compressive mechanical properties, creep resistance, and dynamic mechanical properties of PVA/GO hydrogels with varied GO content were studied. The frictional behavior of PVA/GO hydrogels under stationary and migrating contact configurations during reciprocal and unidirectional sliding movements were investigated. The effects of load, sliding speed, diameter of counterface, and counterface materials on the frictional coefficient of PVA/GO hydrogels were discussed. PVA/0.10wt%GO hydrogel show higher compressive modulus and creep resistance, but moderate friction coefficient. The friction coefficient of PVA/GO hydrogel under stationary and migratory contact configurations greatly depends on interstitial fluid pressurization and tribological rehydration. The friction behavior of PVA/GO hydrogels shows load, speed, and counterface diameter dependence similar to those observed in natural articular cartilage. A low friction coefficient (~ 0.03) was obtained from PVA/0.10wt%GO hydrogel natural cartilage counter pair.

  • Journal article
    Zhang J, Ueda M, Campen S, Spikes Het al., 2021,

    Boundary friction of ZDDP tribofilms

    , Tribology Letters, Vol: 69, Pages: 1-17, ISSN: 1023-8883

    The frictional properties of ZDDP tribofilms at low entrainment speeds in boundary lubrication conditions have been studied in both rolling/sliding and pure sliding contacts. It has been found that the boundary friction coefficients of these tribofilms depend on the alkyl structure of the ZDDPs. For primary ZDDPs, those with linear alkyl chains give lower friction those with branched alkyl chain ZDDPs, and a cyclohexylmethyl-based ZDDP gives markedly higher friction than non-cyclic ones. Depending on alkyl structure, boundary friction coefficient in rolling-sliding conditions can range from 0.09 to 0.14. These differences persist over long duration tests lasting up to 120 h. For secondary ZDDPs, boundary friction appears to depend less strongly on alkyl structure and in rolling-sliding conditions stabilises at ca 0.115 for the three ZDDPs studied. Experiments in which the ZDDP-containing lubricant is changed after tribofilm formation by a different ZDDP solution or a base oil indicate that the characteristic friction of the initial ZDDP tribofilm is lost almost as soon as rubbing commences in the new lubricant. The boundary friction rapidly stabilises at the characteristic boundary friction of the replacement ZDDP, or in the case of base oil, a value of ca 0.115 which is believed to represent the shear strength of the bare polyphosphate surface. The single exception is when a solution containing a cyclohexylethyl-based ZDDP is replaced by base oil, where the boundary friction coefficient remains at the high value characteristic of this ZDDP despite the fact that rubbing in base oil removes about 20 nm of the tribofilm. XPS analysis of the residual tribofilm reveals that this originates from presence of a considerable proportion of C-O bonds at the exposed tribofilm surface, indicating that not all of the alkoxy groups are lost from the polyphosphate during tribofilm formation. Very slow speed rubbing tests at low temperature show that the ZDDP solutions give boundar

  • Journal article
    Menga N, Carbone G, Dini D, 2021,

    Exploring the effect of geometric coupling on friction and energy dissipation in rough contacts of elastic and viscoelastic coatings

    , Journal of the Mechanics and Physics of Solids, Vol: 148, Pages: 1-12, ISSN: 0022-5096

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