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

    Flexibility-Patterned Liquid-Repelling Surfaces.

    , ACS Appl Mater Interfaces

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

    THEORETICAL DEMONSTRATION OF A CAPACITIVE ROTOR FOR GENERATION OF ALTERNATING CURRENT FROM MECHANICAL MOTION

    , Nature communications
  • 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
    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
    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
    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
    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
  • 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
    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
    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
    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
    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
    Reddyhoff T, Ewen J, Deshpande P, Frogley M, Welch M, Montgomery Wet al., 2021,

    Macroscale superlubricity and polymorphism of long-chain n-alcohols

    , ACS Applied Materials and Interfaces, Vol: 13, Pages: 9239-9251, ISSN: 1944-8244

    Simple n-alcohols, such as 1-dodecanol, show anomalous film-forming and friction behaviors under elastohydrodynamic lubrication (EHL) conditions, as found inside bearings and gears. Using tribometer, diamond anvil cell (DAC), and differential scanning calorimetry (DSC) experiments, we show that liquid 1-dodecanol undergoes a pressure-induced solidification when entrained into EHL contacts. Different solid polymorphs are formed inside the contact depending on the temperature and pressure conditions. Surprisingly, at a moderate temperature and pressure, 1-dodecanol forms a polymorph that exhibits robust macroscale superlubricity. The DAC and DSC experiments show that superlubricity is facilitated by the formation of lamellar, hydrogen-bonded structures of hexagonally close-packed molecules, which promote interlayer sliding. This novel superlubricity mechanism is similar to that proposed for the two-dimensional materials commonly employed as solid lubricants, but it also enables the practical advantages of liquid lubricants to be maintained. When the pressure is increased, 1-dodecanol undergoes a polymorphic transformation into a phase that gives a higher friction. The DAC and DSC experiments indicate that the high-friction polymorph is an orthorhombic crystal. The polymorphic transformation pressure coincides with the onset of a dimple formation in the EHL films, revealing that the anomalous film shapes are caused by the formation of rigid orthorhombic crystals inside the contact. This is the first demonstration of a macroscale superlubricity in an EHL contact lubricated by a nonaqueous liquid that arises from bulk effects rather than tribochemical transformations at the surfaces. Since the superlubricity observed here results from phase transformations, it is continuously self-replenishing and is insensitive to surface chemistry and topology. This discovery creates the possibility of implementing superlubricity in a wide range of machine components, which would resul

  • Journal article
    Gao H, Ewen J, Hartkamp R, Mueser M, Dini Det al., 2021,

    Scale-dependent friction-coverage relations and non-local dissipation in surfactant monolayers

    , Langmuir: the ACS journal of surfaces and colloids, Vol: 37, Pages: 2406-2418, ISSN: 0743-7463

    Surfactant molecules, known as organic friction modifiers (OFMs), are routinely added to lubricants to reduce friction and wear between sliding surfaces. In macroscale experiments, friction generally decreases as the coverage of OFM molecules on the sliding surfaces increases; however, recent nanoscale experiments with sharp atomic force microscopy (AFM) tips have shown increasing friction. To elucidate the origin of these opposite trends, we use nonequilibrium molecular dynamics (NEMD) simulations and study kinetic friction between OFM monolayers and an indenting nanoscale asperity. For this purpose, we investigate various coverages of stearamide OFMs on iron oxide surfaces and silica AFM tips with different radii of curvature. We show that the differences between the friction–coverage relations from macroscale and nanoscale experiments are due to molecular plowing in the latter. For our small tip radii, the friction coefficient and indentation depth both have a nonmonotonic dependence on OFM surface coverage, with maxima occurring at intermediate coverage. We rationalize the nonmonotonic relations through a competition of two effects (confinement and packing density) that varying the surface coverage has on the effective stiffness of the OFM monolayers. We also show that kinetic friction is not very sensitive to the sliding velocity in the range studied, indicating that it originates from instabilities. Indeed, we find that friction predominately originates from plowing of the monolayers by the leading edge of the tip, where gauche defects are created, while thermal dissipation is mostly localized in molecules toward the trailing edge of the tip, where the chains return to a more extended conformation.

  • Journal article
    Heyes DM, Dini D, Smith ER, 2021,

    Viscuit and the fluctuation theorem investigation of shear viscosity by molecular dynamics simulations: the information and the noise

    , Journal of Chemical Physics, Vol: 154, ISSN: 0021-9606

    The shear viscosity, η, of model liquids and solids is investigated within the framework of the viscuit and Fluctuation Theorem (FT) probability distribution function (PDF) theories, following Heyes et al. [J. Chem. Phys. 152, 194504 (2020)] using equilibrium molecular dynamics (MD) simulations on Lennard-Jones and Weeks–Chandler–Andersen model systems. The viscosity can be obtained in equilibrium MD simulation from the first moment of the viscuit PDF, which is shown for finite simulation lengths to give a less noisy plateau region than the Green–Kubo method. Two other formulas for the shear viscosity in terms of the viscuit and PDF analysis are also derived. A separation of the time-dependent average negative and positive viscuits extrapolated from the noise dominated region to zero time provides another route to η. The third method involves the relative number of positive and negative viscuits and their PDF standard deviations on the two sides for an equilibrium system. For the FT and finite shear rates, accurate analytic expressions for the relative number of positive to negative block average shear stresses is derived assuming a shifted Gaussian PDF, which is shown to agree well with non-equilibrium molecular dynamics simulations. A similar treatment of the positive and negative block average contributions to the viscosity is also shown to match the simulation data very well.

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

    High-accuracy incident signal reconstruction for in-situ ultrasonic measurement of oil film thickness

    , Mechanical Systems and Signal Processing, Vol: 156, Pages: 107669-107669, ISSN: 0888-3270
  • Journal article
    Garcia Gonzalez C, Ueda M, Spikes H, Wong Jet al., 2021,

    Temperature dependence of Molybdenum dialkyl dithiocarbamate (MoDTC) tribofilms via time-resolved Raman spectroscopy

    , Scientific Reports, Vol: 11, Pages: 3621-3621, ISSN: 2045-2322

    Molybdenum dialkyl dithiocarbamate (MoDTC) is a friction reducing additive commonly used in lubricants. MoDTC works by forming a low-friction molybdenum disulphide (MoS<sub>2</sub>) film (tribofilm) on rubbed surfaces. MoDTC-induced MoS<sub>2</sub> tribofilms have been studied extensively ex-situ; however, there is no consensus on the chemical mechanism of its formation process. By combining Raman spectroscopy with a tribometer, effects of temperature and shear stress on MoS<sub>2</sub> tribofilm formation in steel-steel contacts were examined. Time-resolved Raman spectra of the tribofilm were acquired, together with the instantaneous friction coefficient. The tribofilm is constantly being formed and removed mechanically during rubbing. Increasing shear stress promotes MoS<sub>2</sub> formation. The nature of the tribofilm is temperature-dependent, with high-temperature tribofilms giving a higher friction than lower temperature films. Below a critical temperature T<sub>c</sub>, a small amount of MoS<sub>2</sub> gives significant friction reduction. Above T<sub>c,</sub> a patchy film with more MoS<sub>2</sub>, together with a substantial amount of amorphous carbon attributed to base oil degradation, forms. The composition of this tribofilm evolves during rubbing and a temporal correlation is found between carbon signal intensity and friction. Our results highlight the mechanochemical nature of tribofilm formation process and the role of oil degradation in the effectiveness of friction modifier MoDTC.

  • Conference paper
    Bartolo MK, Accardi MA, Dini D, Amis Aet al., 2021,

    A MACHINE-LEARNING APPROACH FOR MEASURING ARTICULAR CARTILAGE DAMAGE IN THE KNEE

    , International Society for Technology in Arthroplasty (ISTA) Meeting, New Early-Career Webinar Series (NEWS)
  • Journal article
    Ewen J, Spikes H, Dini D, 2021,

    Contributions of molecular dynamics simulations to elastohydrodynamic lubrication

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

    The prediction of friction under elastohydrodynamic lubrication (EHL) conditions remains one of the most important and controversial areas of tribology. This is mostly because the pressure and shear rate conditions inside EHL contacts are particularly severe, which complicates experimental design. Over the last decade, molecular dynamics (MD) simulation has played an increasingly significant role in our fundamental understanding of molecular behaviour under EHL conditions. In recent years, MD simulation has shown quantitative agreement with friction and viscosity results obtained experimentally, meaning that they can, either in isolation or through the use of multiscale coupling methods, begin to be used to test and inform macroscale models for EHL problems. This is particularly useful under conditions that are relevant inside machine components, but are difficult to obtain experimentally without uncontrollable shear heating.

  • Journal article
    Vidotto M, Pederzani M, Castellano A, Pieri V, Falini A, Dini D, De Momi Eet al., 2021,

    Integrating diffusion tensor imaging and neurite orientation dispersion and density imaging to improve the predictive capabilities of CED models

    , Annals of Biomedical Engineering, Vol: 49, Pages: 689-702, ISSN: 0090-6964

    This paper aims to develop a comprehensive and subject-specific model to predict the drug reach in Convection-Enhanced Delivery (CED) interventions. To this end, we make use of an advance diffusion imaging technique, namely the Neurite Orientation Dispersion and Density Imaging (NODDI), to incorporate a more precise description of the brain microstructure into predictive computational models. The NODDI dataset is used to obtain a voxel-based quantification of the extracellular space volume fraction that we relate to the white matter (WM) permeability. Since the WM can be considered as a transversally isotropic porous medium, two equations, respectively for permeability parallel and perpendicular to the axons, are derived from a numerical analysis on a simplified geometrical model that reproduces flow through fibre bundles. This is followed by the simulation of the injection of a drug in a WM area of the brain and direct comparison of the outcomes of our results with a state-of-the-art model, which uses conventional diffusion tensor imaging. We demonstrate the relevance of the work by showing the impact of our newly derived permeability tensor on the predicted drug distribution, which differs significantly from the alternative model in terms of distribution shape, concentration profile and infusion linear penetration length.

  • Journal article
    Parkes M, Tallia F, Young G, Cann P, Jones J, Jeffers Jet al., 2021,

    Tribological evaluation of a novel hybrid for repair of articular cartilage defects

    , Materials Science and Engineering C: Materials for Biological Applications, Vol: 119, Pages: 1-10, ISSN: 0928-4931

    The friction and wear properties of silica/poly(tetrahydrofuran)/poly(ε-caprolactone) (SiO2/PTHF/PCL-diCOOH) hybrid materials that are proposed as cartilage tissue engineering materials were investigated against living articular cartilage. A testing rig was designed to allow testing against fresh bovine cartilage. The friction force and wear were compared for five compositions of the hybrid biomaterial articulating against freshly harvested bovine cartilage in diluted bovine calf serum. Under a non-migrating contact, the friction force increased and hence shear force applied to the opposing articular cartilage also increased, resulting in minor damage to the cartilage surface. This worse case testing scenario was used to discriminate between material formulations and revealed the increase in friction and damaged area was lowest for the hybrid containing the most silica. Further friction and wear tests on one hybrid formulation with an elastic modulus closest to that of cartilage were then conducted in a custom incubator system. This demonstrated that over a five day period the friction force, cell viability and glucosaminoglycan (GAG) release into the lubricant were similar between a cartilage-cartilage interface and the hybrid-cartilage interface, supporting the use of these materials for cartilage repair. These results demonstrate how tribology testing can play a part in the development of new materials for chondral tissue engineering.

  • Journal article
    Ruebeling F, Xu Y, Richter G, Dini D, Gumbsch P, Greiner Cet al., 2021,

    Normal load and counter body size influence the initiation of microstructural discontinuities in copper during sliding

    , ACS Applied Materials and Interfaces, Vol: 13, Pages: 4750-4760, ISSN: 1944-8244

    Near the interface of two contacting metallic bodies in relative motion, the microstructure changes. This modified microstructure leads to changes in material properties and thereby influences the tribological behavior of the entire contact. Tribological properties such as the friction coefficient and wear rate are controlled by the microstructure, while the elementary mechanisms for microstructural changes are not sufficiently understood. In this paper, the influence of the normal load and the size of the counter body on the initiation of a tribologically induced microstructure in copper after a single sliding pass is revealed. A systematic variation in the normal load and sphere diameter resulted in maximum Hertzian contact pressures between 530 MPa and 1953 MPa. Scanning electron microscopy, focused ion beam, and transmission electron microscopy were used to probe the subsurface deformation. Irrespective of the normal load and the sphere diameter, a sharp line-like feature consisting of dislocations, the so-called dislocation trace line, was identified in the subsurface area at depths between 100 nm and 400 nm. For normal loads below 6.75 N, dislocation features are formed below this line. For higher normal loads, the microstructure evolution directly underneath the surface is mainly confined to the area between the sample surface and the dislocation trace line, which itself is located at increasing depth. Transmission Kikuchi diffraction and transmission electron microscopy demonstrate that the misorientation is predominantly concentrated at the dislocation trace line. The results disclose a material rotation around axes roughly parallel to the transverse direction. This study demonstrates the generality of the trace line phenomena over a wide range of loads and contact pressures and the complexity of subsurface processes under a sliding contact and provides the basis for modeling the early stages in the microstructure evolution.

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

    Analysis of an actuated frictional interface for improved dynamic performance

    , Pages: 227-230, ISSN: 2191-5644

    Friction in assembled structures is of great interest due to its ability to reduce the vibration amplitude of critical components. The nonlinear behaviour of a structure depends on a variety of physical parameters. Among these parameters, the contact pressure distribution and the contact area have shown to be critical for the behaviour of the joint and the responses of assembled structures. In most application cases the impact of the interface geometry is not considered as a design parameter, although some attempts have been reported to shape the interface geometry for a specific dynamic response. Taking this idea of designing an interface geometry for a better dynamic performance a step further, the concept presented here propose an actively controlled interface geometry and contact pressure distribution, to change the joint behaviour during a vibration cycle. The concept consists of a device capable of manipulating the shape and pressure of a flexible membrane in contact with a rigid punch, subjected to a normal load and a tangential excitation, via a row of piezoelectric actuators.

  • Journal article
    Ayestaran Latorre C, Ewen J, Dini D, Righi MCet al., 2021,

    Ab initio insights into the interaction mechanisms between boron, nitrogen and oxygen doped diamond surfaces and water molecules

    , Carbon, Vol: 171, Pages: 575-584, ISSN: 0008-6223

    Diamond and diamond-like carbon coatings are used in many applications ranging from biomedicine to tribology. A wide range of dopants have been tested to modify the hydrophilicity of these surfaces, since this is central to their biocompatibility and tribological performance in aqueous environments. Despite the large number of experimental investigations, an atomistic understanding of the effects of different dopants on carbon film hydrophilicity is still lacking. In this study, we employ ab initio calculations to elucidate the effects of B, N, and O dopants in several mechanisms that could modify interactions with water molecules and thus hydrophilicity. These include the adsorption of intact water molecules on the surfaces, minimum energy pathways for water dissociation, and subsequent interactions of hydrogenated and hydroxylated surfaces with water molecules. We find that all of the dopants considered enhance hydrophilicity, but they do so through different means. Most notably, B dopants can spontaneously chemisorb intact water molecules and increase its interactions in H-bond networks.

  • Journal article
    Eder SJ, Grützmacher PG, Rodríguez Ripoll M, Dini D, Gachot Cet al., 2020,

    Effect of temperature on the deformation behavior of copper nickel alloys under sliding.

    , Materials (Basel), Vol: 14, Pages: 1-16, ISSN: 1996-1944

    The microstructural evolution in the near-surface regions of a dry sliding interface has considerable influence on its tribological behavior and is driven mainly by mechanical energy and heat. In this work, we use large-scale molecular dynamics simulations to study the effect of temperature on the deformation response of FCC CuNi alloys of several compositions under various normal pressures. The microstructural evolution below the surface, marked by mechanisms spanning grain refinement, grain coarsening, twinning, and shear layer formation, is discussed in depth. The observed results are complemented by a rigorous analysis of the dislocation activity near the sliding interface. Moreover, we define key quantities corresponding to deformation mechanisms and analyze the time-independent differences between 300 K and 600 K for all simulated compositions and normal pressures. Raising the Ni content or reducing the temperature increases the energy barrier to activate dislocation activity or promote plasticity overall, thus increasing the threshold stress required for the transition to the next deformation regime. Repeated distillation of our quantitative analysis and successive elimination of spatial and time dimensions from the data allows us to produce a 3D map of the dominating deformation mechanism regimes for CuNi alloys as a function of composition, normal pressure, and homologous temperature.

  • Journal article
    Jobanputra R, Boyle C, Dini D, Masen Met al., 2020,

    Modelling the effects of age-related morphological and mechanical skin changes on the stimulation of tactile mechanoreceptors

    , Journal of The Mechanical Behavior of Biomedical Materials, Vol: 112, Pages: 1-10, ISSN: 1751-6161

    Our sense of fine touch deteriorates as we age, a phenomenon typically associated with neurological changes to the skin. However, geometric and material changes to the skin may also play an important role on tactile perception and have not been studied in detail. Here, a finite element model is utilised to assess the extent to which age-related structural changes to the skin influence the tactile stimuli experienced by the mechanoreceptors. A numerical, hyperelastic, four-layered skin model was developed to simulate sliding of the finger against a rigid surface. The strain, deviatoric stress and strain energy density were recorded at the sites of the Merkel and Meissner receptors, whilst parameters of the model were systematically varied to simulate age-related geometric and material skin changes. The simulations comprise changes in skin layer stiffness, flattening of the dermal-epidermal junction and thinning of the dermis. It was found that the stiffness of the skin layers has a substantial effect on the stimulus magnitudes recorded at mechanoreceptors. Additionally, reducing the thickness of the dermis has a substantial effect on the Merkel disc whilst the Meissner corpuscle is particularly affected by flattening of the dermal epidermal junction. In order to represent aged skin, a model comprising a combination of ageing manifestations revealed a decrease in stimulus magnitudes at both mechanoreceptor sites. The result from the combined model differed from the sum of effects of the individually tested ageing manifestations, indicating that the individual effects of ageing cannot be linearly superimposed. Each manifestation of ageing results in a decreased stimulation intensity at the Meissner Corpuscle site, suggesting that ageing reduces the proportion of stimuli meeting the receptor amplitude detection threshold. This model therefore offers an additional biomechanical explanation for tactile perceptive degradation amongst the elderly. Applications of the develo

  • Journal article
    Fatti G, Righi M, Dini D, Ciniero Aet al., 2020,

    Ab initio study of polytetrafluoroethylene defluorination for tribocharging applications

    , ACS Applied Polymer Materials, Vol: 2, Pages: 5129-5134, ISSN: 2637-6105

    Polytetrafluoroethylene (PTFE) is one of the most efficient polymers for green energy-harvesting devices like triboelectric nanogenerators because of its high capability of acquiring and retaining negative charge. Despite its extensive use, the relation between PTFE triboelectric behavior and its electronic properties has never been analyzed. To shed light on this important feature, we have studied the electronic properties of PTFE low-index surfaces in the high-pressure phase by means of density functional theory. We start by showing that adding either a positive or a negative charge on pristine surfaces is energetically unfavorable. We then demonstrate the role played by surface defects. When a surface fluorine vacancy is introduced, the analysis of the band structure reveals that the defect generates a trap state that enables the surface to acquire and retain negative charge.

  • Journal article
    Bahshwan M, Myant CW, Reddyhoff T, Pham MSet al., 2020,

    The role of microstructure on wear mechanisms and anisotropy of additively manufactured 316L stainless steel in dry sliding

    , Materials and Design, Vol: 196, ISSN: 0264-1275

    Wear control, which relies on understanding the mechanisms of wear, is crucial in preserving the life of mechanical components and reducing costs. Additive manufacturing (AM) techniques can produce parts with tailored microstructure, however, little has been done to understand how this impacts the mechanisms of wear. Here we study the impact of initial grain arrangement and crystal orientation on the wear mechanisms of austenitic stainless steel (SS) in dry sliding contact. Specifically, the anisotropic sliding wear behavior of as-built, AM-ed 316L SS is compared against annealed, wire-drawn counterparts. We describe, in-detail, how the sliding wear mechanisms of delamination, abrasion, oxidation, and plastic deformation are attributed to the initial surface microstructure under different loading conditions using a number of techniques. This new understanding sheds light on how different AM-induced microstructures affect wear, thereby allowing for better utilization of this technology to develop components with enhanced wear properties.

  • Journal article
    Samaras G, Bikos D, Vieira J, Hartmann C, Charalambides M, Hardalupas Y, Masen M, Cann Pet al., 2020,

    Measurement of molten chocolate friction under simulated tongue-palate kinematics: effect of cocoa solids content and aeration

    , Current Research in Food Science, Vol: 3, Pages: 304-313, ISSN: 2665-9271

    The perception of some food attributes is related to mechanical stimulation and friction experienced in the tongue-palate contact during mastication. This paper reports a new bench test to measure friction in the simulated tongue-palate contact. The test consists of a flat PDMS disk, representing the tongue loaded and reciprocating against a stationary lower glass surface representing the palate. The test was applied to molten chocolate samples with and without artificial saliva. Friction was measured over the first few rubbing cycles, simulating mechanical degradation of chocolate in the tongue-palate region. The effects of chocolate composition (cocoa solids content ranging between 28 ​wt% and 85 ​wt%) and structure (micro-aeration/non-aeration 0–15 ​vol%) were studied. The bench test clearly differentiates between the various chocolate samples. The coefficient of friction increases with cocoa solids percentage and decreases with increasing micro-aeration level. The presence of artificial saliva in the contact reduced the friction for all chocolate samples, however the relative ranking remained the same.

  • Journal article
    Yu M, Shen L, Mutasa T, Dou P, Wu T, Reddyhoff Tet al., 2020,

    Exact analytical solution to ultrasonic interfacial reflection enabling optimal oil film thickness measurement

    , Tribology International, Vol: 151, Pages: 1-10, ISSN: 0301-679X

    The ultrasonic reflection from a lubricated interface has been widely analyzed to measure fluid film thickness, with different algorithms being applied to overcome measurement accuracy and resolution issues. Existing algorithms use either the amplitude or the phase angle of the ultrasonic interfacial reflection. In this paper, a new algorithm (named the “exact model – complex”) that simultaneously utilizes both the amplitude and the phase of the complex ultrasonic reflection coefficient is proposed and mathematically derived. General procedures for theoretical analysis in terms of measurement accuracy and uncertainty are proposed and applied to the new algorithm, the beneficial features of which (as compared to other existing algorithms) can be summarized as: 1) a direct calculation, instead of an iterative approximation, 2) guaranteed maximum measurement accuracy, and 3) acceptable measurement uncertainty. None of the existing methods have showed this combination of benefits. Moreover, two groups of raw data from previous experimental studies are utilized to further validate the practical feasibility of the new algorithm. Overall, the proposed “exact model – complex” algorithm fully exploits the potential of ultrasonic reflection for oil film thickness measurement, with an accurate and a convenient calculation suited to practical implementation.

  • Journal article
    Fry B, Chui MY, Moody G, Wong Jet al., 2020,

    Interactions between organic friction modifier additives

    , Tribology International, Vol: 151, Pages: 1-8, ISSN: 0301-679X

    The interactions of different additives in engine oils can create synergistic or antagonistic effects. This paper studies how mixing different organic friction modifier additives affects friction reducing properties of lubricants in the boundary lubrication regime. Amines of different degree of saturation were mixed with either glycerol monooleate (GMO) or oleic acid in hexadecane. The model lubricants thus formed were characterised with Fourier-transform infrared spectroscopy. Friction tests in reciprocating motion using ball-on-disc steel-steel contacts were conducted to examine the tribological performance of these lubricants. Worn surfaces were examined using X-ray photoelectron spectroscopy. Oleic acid and oleylamine, a primary amine. Were found to form a partial ionic liquid, providing synergistic friction reduction. This positive interaction reduces with increasing degree of saturation of the amine. No synergistic effect was observed between GMO and oleylamine,suggesting that GMO does not hydrolyse into a carboxylic acid within a rubbing contact in the presence of amine. Keywords: Boundary Lubrication, Additives, Friction Abbreviations: organic friction modifier (OFM); glycerol monooleate (GMO); ionic liquid (IL); oleylamine (OA); diocylamine (DA); trihexylamine (TA); dimethylhexadecaylamine (DM16); high frequency reciprocating rig (HFRR); X-ray photoelectron spectroscopy (XPS).

  • Journal article
    Wen J, Zhang W-Y, Ren L-Z, Bao L-Y, Dini D, Xi H-D, Hu H-Bet al., 2020,

    Controlling the number of vortices and torque in Taylor-Couette flow

    , Journal of Fluid Mechanics, Vol: 901, ISSN: 0022-1120

    We present an experimental study on controlling the number of vortices and the torque in a Taylor–Couette flow of water for Reynolds numbers from 660 to 1320. Different flow states are achieved in the annulus of width d between the inner rotating and outer stationary cylinders through manipulating the initial height of the water annulus. We show that the torque exerted on the inner cylinder of the Taylor–Couette system can be reduced by up to 20 % by controlling the flow at a state where fewer than the nominal number of vortices develop between the cylinders. This flow state is achieved by starting the system with an initial water annulus height h0 (which nominally corresponds to h0/d vortices), then gradually adding water into the annulus while the inner cylinder keeps rotating. During this filling process the flow topology is so persistent that the number of vortices does not increase; instead, the vortices are greatly stretched in the axial (vertical) direction. We show that this state with stretched vortices is sustainable until the vortices are stretched to around 2.05 times their nominal size. Our experiments reveal that by manipulating the initial height of the liquid annulus we are able to generate different flow states and demonstrate how the different flow states manifest themselves in global momentum transport.

  • Journal article
    Kondratyuk N, Pisarev V, Ewen J, 2020,

    Probing the high-pressure viscosity of hydrocarbon mixtures using molecular dynamics simulations

    , Journal of Chemical Physics, Vol: 153, ISSN: 0021-9606

    Computational predictions of the high-pressure viscosity of hydrocarbon mixtures could help to accelerate the development of fuels and lubricants with improved performance. In this study, we use molecular dynamics simulations to study the viscosity and density of methylcyclohexane, 1-methylnaphthalene, and their binary mixtures at 323 K and pressures of up to 500 MPa. The simulation results are in excellent agreement with previous experiments available up to 100 MPa for both pure compounds (200 MPa for 1-methylnaphthalene) and the binary mixtures. For 1-methylnaphthalene, the viscosity initially increases slower-than-exponential with pressure before it reaches an inflection point and then increases faster-than-exponential. The inflection point (300 MPa) occurs at a pressure slightly below the one at which 1-methylnaphthalene is expected to enter the supercooled phase (400 MPa). For methylcyclohexane, the increase in viscosity with pressure is slower-than-exponential over the entire pressure range studied. The binary mixtures show intermediate pressure–viscosity responses between the two pure cases. The applicability of equations commonly used to describe the pressure dependence of viscosity, as well as the viscosity of binary mixtures, is evaluated against the computational predictions.

  • Journal article
    Ueda M, Kadiric A, Spikes H, 2020,

    ZDDP tribofilm formation on non-ferrous surfaces

    , Tribology Online, Vol: 15, Pages: 318-331, ISSN: 1881-218X

    The current trend for using lower viscosity lubricants with the aim of improving fuel economy of mechanical systems means that machine components are required to operate for longer periods in thin oil film, boundary and mixed lubrication conditions, where the risk of surface damage is increased. In addition, non-ferrous materials are increasingly being introduced in machine components to reduce wear and increase efficiency. Thus, understanding of the ZDDP antiwear tribofilm formation on both ferrous and non-ferrous surfaces is increasingly important in order to formulate lubricants that give desired antiwear performance with both types of materials. In this paper the effect of ferrous and non-ferrous rubbing materials, namely, steel, Si3N4, WC, SiC and a-C:H DLC coating, on ZDDP tribofilm formation was investigated. Among non-ferrous materials, it was found that ZDDP tribofilms were formed on Si3N4 and WC in the boundary lubrication regime, but almost no tribofilms were formed on SiC and a-C:H DLC. In addition, although tribofilms formed on some non-ferrous surfaces, they were easily removed under boundary lubrication by direct asperity contact because of their weak adhesion to the substrate. This tribofilm removal makes quantification of ZDDP tribofilm formation rate on non-ferrous surfaces under boundary lubrication conditions difficult. By contrast, under high shear stress EHL conditions, thick tribofilms formed without film removal with the tribofilm thickness being the greatest for steel, followed by Si3N4 and then WC, with no tribofilm formation observed on SiC and DLC. QCM results suggest that ZDDP tribofilm formation might be considerably affected by the extent to which ZDDP adsorbs on the substrate surface. The chemical properties of tribofilms are discussed and a possible mechanism by which ZDDP forms tribofilm on non-ferrous surfaces is suggested. This study has practical implications for ways in which non-ferrous surfaces can be protected from wear via l

  • Journal article
    Porte E, Cann P, Masen M, 2020,

    A lubrication replenishment theory for hydrogels

    , Soft Matter, Vol: 16, Pages: 10290-10300, ISSN: 1744-683X

    <p>For soft porous materials, limited contact motion results in a non-replenished lubricant state with high friction.</p>

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