Many Tribology Group publications are Open Access thanks to funding from the EPSRC.

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  • 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
  • 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
    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
    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
    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
    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
  • 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, Pages: 154502-154502, ISSN: 0021-9606
  • Journal article
    Terzano M, Dini D, Rodriguez y Baena F, Spagnoli A, Oldfield Met al., 2020,

    An adaptive finite element model for steerable needles

    , Biomechanics and Modeling in Mechanobiology, Vol: 19, Pages: 1809-1825, ISSN: 1617-7940

    Penetration of a flexible and steerable needle into a soft target material is a complex problem to be modelled, involving several mechanical challenges. In the present paper, an adaptive finite element algorithm is developed to simulate the penetration of a steerable needle in brain-like gelatine material, where the penetration path is not predetermined. The geometry of the needle tip induces asymmetric tractions along the tool–substrate frictional interfaces, generating a bending action on the needle in addition to combined normal and shear loading in the region where fracture takes place during penetration. The fracture process is described by a cohesive zone model, and the direction of crack propagation is determined by the distribution of strain energy density in the tissue surrounding the tip. Simulation results of deep needle penetration for a programmable bevel-tip needle design, where steering can be controlled by changing the offset between interlocked needle segments, are mainly discussed in terms of penetration force versus displacement along with a detailed description of the needle tip trajectories. It is shown that such results are strongly dependent on the relative stiffness of needle and tissue and on the tip offset. The simulated relationship between programmable bevel offset and needle curvature is found to be approximately linear, confirming empirical results derived experimentally in a previous work. The proposed model enables a detailed analysis of the tool–tissue interactions during needle penetration, providing a reliable means to optimise the design of surgical catheters and aid pre-operative planning.

  • Journal article
    Rong M, Liu H, Scaraggi M, Bai Y, Bao L, Ma S, Ma Z, Cai M, Dini D, Zhou Fet al., 2020,

    High lubricity meets load capacity: cartilage mimicking bilayer structure by brushing up stiff hydrogels from subsurface

    , Advanced Functional Materials, Vol: 30, ISSN: 1616-301X

    Natural articular cartilage has ultralow friction even at high squeezing pressure. Biomimicking cartilage with soft materials has been and remains a grand challenge in the fields of materials science and engineering. Inspired by the unique structural features of the articular cartilage, as well as by its remarkable lubrication mechanisms dictated by the properties of the superficial layers, a novel archetype of cartilage‐mimicking bilayer material by robustly entangling thick hydrophilic polyelectrolyte brushes into the subsurface of a stiff hydrogel substrate is developed. The topmost soft polymer layer provides effective aqueous lubrication, whereas the stiffer hydrogel layer used as a substrate delivers the load‐bearing capacity. Their synergy is capable of attaining low friction coefficients (order 0.010) under heavily loaded conditions (order 10 MPa contact pressure) in water environment, a performance incredibly close to that of natural articular cartilage. The bioinspired material can maintain low friction even when subjected to 50k reciprocating cycles under high contact pressure, with almost no wear observed on the sliding track. These findings are theoretically explained and compounded by multiscale simulations used to shed light on the mechanisms responsible for this remarkable performance. This work opens innovative technology routes for developing cartilage‐mimicking ultralow friction soft materials.

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

    A lubrication replenishment theory for hydrogels

    , Soft Matter, ISSN: 1744-683X
  • Journal article
    Tan Z, Ewen J, Galvan S, Forte A, De Momi E, Rodriguez y Baena F, Dini Det al., 2020,

    What Does a Brain Feel Like?

    , Journal of Chemical Education, ISSN: 0021-9584

    We present a two-part hands-on science outreach demonstration utilizing composite hydrogels to produce realistic models of the human brain. The blends of poly(vinyl alcohol) and Phytagel closely match the mechanical properties of real brain tissue under conditions representative of surgical operations. The composite hydrogel is simple to prepare, biocompatible, and nontoxic, and the required materials are widely available and inexpensive. The first part of the demonstration gives participants the opportunity to feel how soft and deformable our brains are. The second part allows students to perform a mock brain surgery on a simulated tumor. The demonstration tools are suitable for public engagement activities as well as for various student training groups. The activities encompass concepts in polymer chemistry, materials science, and biology.

  • 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., 2020,

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

    , IEEE Transactions on Biomedical Engineering, 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
    Parkes M, Tallia F, Young G, Cann P, Jones J, Jeffers Jet al., 2020,

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

    , Materials Science and Engineering C: Materials for Biological Applications, 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
    Vidotto M, Pederzani M, Castellano A, Pieri V, Falini A, Dini D, De Momi Eet al., 2020,

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

    , Annals of Biomedical Engineering, 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
    Wen J, Dini D, Reddyhoff T, 2020,

    Design and optimization of a liquid ring thrust bearing

    , Tribology International, Vol: 149, ISSN: 0301-679X

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

  • Journal article
    Luiz JF, Spikes H, 2020,

    Tribofilm formation, friction and wear-reducing properties of some phosphorus-containing antiwear additives

    , Tribology Letters, Vol: 68, Pages: 1-24, ISSN: 1023-8883

    The film-forming, friction and wear properties of a range of model and commercial ashless P and P/S antiwear additives have been studied. A method has been developed for removing the tribofilms formed by such additives in order to effectively quantify mild wear. In general the P/S additives studied formed thinner tribofilms but gave lower wear than the S-free P ones. In extended wear tests, three P/S additives gave wear as low, or lower, than a primary zinc dialkyldithiophosphate (ZDDP). For almost all lubricants tested the wear rate measured in short tests was considerably higher than that in long tests due to the greater contribution of running-in wear in the former. This highlights the importance of basing antiwear additive choice on reasonably long tests, where running-in becomes only a small component of the wear measured. It has been found that for both P and P/S ashless additives the addition of oil-soluble metal compounds based on Ti and Ca boosts tribofilm formation and can lead to very thick films, comparable to those formed by ZDDP. However, this thick film formation tends to be accompanied by an increase in mixed friction and also does not appear to reduce wear but may even increase it.

  • Journal article
    Spikes HA, 2020,

    Triboelectrochemistry: influence of applied electrical potentials on friction and wear of lubricated contacts

    , Tribology Letters, Vol: 68, Pages: 1-27, ISSN: 1023-8883

    Research on the effects of applied electrical potential on friction and wear, a topic sometimes termed “Triboelectrochemistry”, has been reviewed. Historically, most such research has focussed on aqueous lubricants, whose relatively high electrical conductivities enable use of three-electrode electrochemical kinetic techniques, in which the electrode potential at a single electrode | fluid interface is controlled relative to a suitable reference electrode. This has led to identification of several different mechanisms by which applied electrode potentials can influence friction and wear. Of these, the most practically important are: (i) promotion of adsorption/desorption of polar additives on tribological surfaces by controlling the latters’ surface charges; (ii) stimulation or suppression of redox reactions involving either oxygen or lubricant additives at tribological surfaces. In recent years, there has been growing interest in the effects of applied electrical potentials on rubbing contacts lubricated by non-aqueous lubricants, such as ester- and hydrocarbon-based oils. Two different approaches have been used to study this. In one, a DC potential difference in the mV to V range is applied directly across a thin film, lubricated contact to form a pair of electrode | fluid interfaces. This has been found to promote some additive reactions and to influence friction and wear. However, little systematic exploration has been reported of the underlying processes and generally the electrode potentials at the interfaces have not been well defined. The second approach is to increase the conductivity of non-aqueous lubricants by adding secondary electrolytes and/or using micro/nanoscale electrodes, to enable the use of three-electrode electrochemical methods at single metal | fluid interfaces, with reference and counter electrodes. A recent development has been the introduction of ionic liquids as both base fluids and lubricant additives. These have relat

  • Journal article
    Hu S, Reddyhoff T, Puhan D, Vladescu S-C, Shi X, Dini D, Peng Zet al., 2020,

    Droplet manipulation of hierarchical steel surfaces using femtosecond laser fabrication

    , Applied Surface Science, Vol: 521, Pages: 146474-146474, ISSN: 0169-4332
  • Journal article
    Hu S, Cao X, Reddyhoff T, Puhan D, Vladescu S-C, Wang J, Shi X, Peng Z, deMello AJ, Dini Det al., 2020,

    Liquid repellency enhancement through flexible microstructures

    , Science Advances, Vol: 6, Pages: 1-7, ISSN: 2375-2548

    Artificial liquid-repellent surfaces have attracted substantial scientific and industrial attention with a focus on creating functional topological features; however, the role of the underlying structures has been overlooked. Recent developments in micro-nanofabrication allow us now to construct a skin-muscle type system combining interfacial liquid repellence atop a mechanically functional structure. Specifically, we design surfaces comprising bioinspired, mushroom-like repelling heads and spring-like flexible supports, which are realized by three-dimensional direct laser lithography. The flexible supports elevate liquid repellency by resisting droplet impalement and reducing contact time. This, previously unknown, use of spring-like flexible supports to enhance liquid repellency provides an excellent level of control over droplet manipulation. Moreover, this extends repellent microstructure research from statics to dynamics and is envisioned to yield functionalities and possibilities by linking functional surfaces and mechanical metamaterials.

  • Journal article
    Xu Y, Dini D, 2020,

    Capturing the hardness of coating systems across the scales

    , Surface and Coatings Technology, Vol: 394, ISSN: 0257-8972

    A two-dimensional multi-scale modelling approach that concurrently couples discrete dislocation plasticity and crystal plasticity finite element has been applied to study the hardness variation of coating systems across different scales, covering nano- to micro-indentation. The difference in indentation size sensitivity between film and substrate gives rise to three regimes of hardness, one typically dictated by the intrinsic coating indentation size effect, which is regulated by dislocations activity, and the other two linked to the continuum response of the coating and the substrate. We propose a new hardness formula that incorporates physics-based indentation size effects of thin films into established continuum hardness transition formulae. This formula is shown to substantially improve the hardness prediction of coating systems, particularly when relative indentation depth is at the nanometre scale.

  • Journal article
    Heyes DM, Dini D, Smith ER, 2020,

    Statistical Analysis and Molecular Dynamics Simulations of the Thermal Conductivity of Lennard–Jones Solids Including Their Pressure and Temperature Dependencies

    , Physica Status Solidi B: Basic Solid State Physics, Pages: 2000344-2000344, ISSN: 0370-1972

    Aspects of the thermal conductivity, λ, of a Lennard–Jones (LJ) solid along an isotherm and the sublimation line are studied using equilibrium molecular dynamics (MD) simulations. A reformulation of the Green–Kubo time correlation function expression for λ in the form of a probability distribution function (PDF) of single trajectory contributions (STC) exhibits the same characteristic statistical trends as found previously for liquids, even at high pressures and low temperatures. The analysis reveals that for short periods of time the thermal conductivity can be negative. This feature is evident along the sublimation line isobar and a low‐temperature isotherm going to high densities. Along the isobar and isotherm lines, λ is to a good approximation a power law in temperature and density, respectively. This behavior is used in a more general thermodynamics‐based analysis description of the state point dependence of the thermal conductivity. The heat flux autocorrelation function increasingly develops a damped oscillatory appearance as pressure increases or temperature decreases, consistent with the phonon formulation of thermal conductivity.

  • Journal article
    Eder SJ, Rodriguez Ripoll M, Cihak-Bayr U, Dini D, Gachot Cet al., 2020,

    Unraveling and mapping the mechanisms for near-surface microstructure evolution in CuNi alloys under sliding

    , ACS Applied Materials & Interfaces, Vol: 12, Pages: 32197-32208, ISSN: 1944-8244

    The origin of friction and wear in polycrystalline materials is intimately connected with their microstructural response to interfacial stresses. Although many mechanisms that govern microstructure evolution in sliding contacts are generally understood, it is still a challenge to ascertain which mechanisms matter under what conditions, which limits the development of tailor-made microstructures for reducing friction and wear. Here, we shed light on the circumstances that promote plastic deformation and surface damage by studying several FCC CuNi alloys subjected to sliding with molecular dynamics simulations featuring tens of millions of atoms. By analyzing the depth- and time-dependent evolution of the grain size, twinning, shear, and the stresses in the aggregate, we derive a deformation mechanism map for CuNi alloys. We verify the predictions of this map against focused ion beam images of the near-surface regions of CuNi alloys that were experimentally subjected to similar loading conditions. Our results may serve as a tool for finding optimum material compositions within a specified operating range.

  • Journal article
    Shen L, Denner F, Morgan N, Van Wachem B, Dini Det al., 2020,

    Transient structures in rupturing thin-films: Marangoni-induced symmetry-breaking pattern formation in viscous fluids

    , Science Advances, Vol: 6, ISSN: 2375-2548

    In the minutes immediately preceeding the rupture of a soap bubble,distinctive and repeatable patterns can be observed. These quasi-stabletransient structures are associated with the instabilities of the complexMarangoni flows on the curved thin film in the presence of a surfactantsolution. Here, we report a generalised Cahn-Hilliard-Swift-Hohenberg modelderived using asymptotic theory which describes the quasi-elastic wrinklingpattern formation and the consequent coarsening dynamics in a curvedsurfactant-laden thin film. By testing the theory against experiments on soapbubbles, we find quantitative agreement with the analytical predictions of thenucleation and the early coarsening phases associated with the patterns. Ourfindings provide fundamental physical understanding that can be used to(de-)stabilise thin films in the presence of surfactants and have importantimplications for both natural and industrial contexts, such as the productionof thin coating films, foams, emulsions and sprays.

  • Journal article
    Lu J, Reddyhoff T, Dini D, 2020,

    A study of thermal effects in EHL rheology and friction using infrared microscopy

    , Tribology International, Vol: 146, Pages: 106179-106179, ISSN: 0301-679X
  • Journal article
    Rogers SR, Bowden D, Unnikrishnan R, Scenini F, Preuss M, Stewart D, Dini D, Dye Det al., 2020,

    The interaction of galling and oxidation in 316L stainless steel

    , Wear, Vol: 450-451, Pages: 203234-203234, ISSN: 0043-1648
  • Journal article
    Heyes DM, Dini D, Smith ER, 2020,

    Single trajectory transport coefficients and the energy landscape by molecular dynamics simulations

    , JOURNAL OF CHEMICAL PHYSICS, Vol: 152, ISSN: 0021-9606
  • Journal article
    Ewen JP, Ayestarán Latorre C, Gattinoni C, Khajeh A, Moore JD, Remias J, Martini A, Dini Det al., 2020,

    Substituent effects on the thermal decomposition of phosphate esters on ferrous surfaces

    , The Journal of Physical Chemistry C, Vol: 124, Pages: 9852-9865, ISSN: 1932-7447

    Phosphate esters have a wide range of industrial applications, for example in tribology where they are used as vapour phase lubricants and antiwear additives. An atomic-level understanding of phosphate ester tribofilm formation mechanisms is required to improve their tribological performance. A process of particular interest is the thermal decomposition of phosphate esters on steel surfaces, since this initiates polyphosphate film formation. In this study, reactive force field (ReaxFF) molecular dynamics (MD) simulations are used to study the thermal decomposition of phosphate esters with different substituents on several ferrous surfaces. The ReaxFF parameterisation was validated for a representative system using density functional theory (DFT) calculations. During the MD simulations on Fe3O4(001) and α-Fe(110), chemisorption interactions between the phosphate esters and the surfaces occur even at room temperature, and the number of molecule-surface bonds increases as the temperature increases from 300 to 1000 K. Conversely, on hydroxylated, amorphous Fe3O4, most of the molecules are physisorbed and some desorption occurs at high temperature. Thermal decomposition rates were much higher on Fe3O4(001) and particularly α-Fe(110) compared to hydroxylated, amorphous Fe3O4. This suggests that water passivates ferrous surfaces and inhibits phosphate ester chemisorption, decomposition, and ultimately polyphosphate film formation. For the alkyl phosphates, thermal decomposition proceeds mainly through C-O and C-H cleavage on Fe3O4(001). Aryl phosphates show much higher thermal stability, and decomposition on Fe3O4(001) only occurs through P-O and C-H cleavage, which require very high temperature. The onset temperature for C-O cleavage on Fe3O4(001) increases as: tertiary alkyl < secondary alkyl < primary linear alkyl ≈ primary branched alkyl < aryl. This order is consistent with experimental observations for the thermal stability of antiwear addi

  • Journal article
    Ajdari N, Tempelaere C, Masouleh MI, Abel R, Delfosse D, Emery R, Dini D, Hansen Uet al., 2020,

    Hemiarthroplasties: the choice of prosthetic material causes different levels of damage in the articular cartilage

    , Journal of Shoulder and Elbow Surgery, Vol: 29, Pages: 1019-1029, ISSN: 1058-2746

    Background Hemiarthroplasty has clear advantages over alternative procedures and is used in 20% of all shoulder joint replacements. Because of cartilage wear, the clinical outcome of hemiarthroplasty is unreliable and controversial. This paper suggests that the optimal choice of prosthetic material may reduce cartilage degeneration and improve the reliability of the procedure. The specific objectives were to assess 3 materials and assess how the severity of arthritis might affect the choice of prosthetic material. Methods A CoCr alloy, an AL2O3 ceramic, and a polycarbonate urethane polymer (PCU) were mechanically tested against 5 levels of human osteoarthritic cartilage (from intact to severely arthritic, n = 45). A high friction coefficient, a decrease in Young's modulus, an increase in permeability, a decrease in relaxation time, an increase in surface roughness, and a disrupted appearance of the cartilage after testing were used as measures of cartilage damage. The biomaterial that caused minimal cartilage damage was defined as superior. Results The CoCr caused the most damage. This was followed by the AL2O3 ceramic, whereas the PCU caused the least amount of damage. Although the degree of arthritis had an effect on the results, it did not change the trend that CoCr performed worst and PCU the best. Discussion and Conclusion This study indicates that ceramic implants may be a better choice than metals, and the articulating surface should be as smooth as possible. Although our results indicate that the degree of arthritis should not affect the choice of prosthetic material, this suggestion needs to be further investigated.

  • Journal article
    Smith E, Trevelyan D, Ramos-Fernandez E, Sufian A, O'Sullivan C, Dini Det al., 2020,

    CPL library - a minimal framework for coupled particle and continuum simulation

    , Computer Physics Communications, Vol: 250, Pages: 1-11, ISSN: 0010-4655

    We present an open-source library for coupling particle codes, such as molecular dynamics (MD) or the discrete element method (DEM), and grid based computational fluid dynamics (CFD). The application is focused on domain decomposition coupling, where a particle and continuum software model different parts of a single simulation domain with information exchange. This focus allows a simple library to be developed, with core mapping and communication handled by just four functions. Emphasis is on scaling on supercomputers, a tested cross-language library, deployment with containers and well-documented simple examples. Building on this core, a template is provided to facilitate the user development of common features for coupling, such as averaging routines and functions to apply constraint forces. The interface code for LAMMPS and OpenFOAM is provided to both include molecular detail in a continuum solver and model fluids flowing through a granular system. Two novel development features are highlighted which will be useful in the development of the next generation of multi-scale software: (i) The division of coupled code into a smaller blocks with testing over a range of processor topologies. (ii) The use of coupled mocking to facilitate coverage of various parts of the code and allow rapid prototyping. These two features aim to help users develop coupled models in a test-driven manner and focus on the physics of the problem instead of just software development. All presented code is open-source with detailed documentation on the dedicated website ( permitting useful aspects to be evaluated and adopted in other projects.

  • Journal article
    Gurrutxaga Lerma B, Verschueren J, Sutton A, Dini Det al., 2020,

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

    , International Materials Reviews, 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
    Wen J, Reddyhoff T, Hu S, Puhan D, Dini Det al., 2020,

    Exploiting air cushion effects to optimise a superhydrophobic/hydrophilic patterned liquid ring sealed air bearing

    , Tribology International, Vol: 144, Pages: 106129-106129, ISSN: 0301-679X
  • Journal article
    Menga N, Dini D, Carbone G, 2020,

    Tuning the periodic V-peeling behavior of elastic tapes applied to thin compliant substrates

    , International Journal of Mechanical Sciences, Vol: 170, ISSN: 0020-7403

    In this paper, we investigate the periodic peeling behavior of opposing symmetric peeling fronts involving an elastic tape peeled off from a deformable substrate of finite thickness, backed onto a rigid foundation.We treat the problem by means of an energetic formulation, and we found that, depending on the values of the initial detached length l, substrate thickness h, and peeling periodicity λ, the translational invariance of the peeling process is lost and restored, as the elastic interaction between the peeling fronts is limited by the substrate thickness. Indeed, given h and λ, a critical value of the detached length can be found, which is able to prevent unstable peeling of the tape under a fixed applied load, thus resulting in enhanced adhesion strength, with respect to the classical Kendall’s solution for peeling from a rigid substrate. On the other hand, given the geometrical system configuration (i.e. the detached length l) the load necessary to trigger the peeling can be minimized by conveniently tuning the ratio h/λ. This feature might be of interest for the development of innovative designs for future biomedical devices, such as Transdermal Drug Delivery Systems or wound dressing, requiring low peel adhesion for safe successive removals.

  • Journal article
    Poole B, Barzdajn B, Dini D, Stewart D, Dunne FPEet al., 2020,

    The roles of adhesion, internal heat generation and elevated temperatures in normally loaded, sliding rough surfaces

    , International Journal of Solids and Structures, Vol: 185-186, Pages: 14-28, ISSN: 0020-7683

    The thermal effects of plastic and frictional heat generation and elevated temperature were examined along with the role of adhesion in the context of galling wear, using a representative crystal plasticity, normally loaded, sliding surface model. Galling frequency behaviour was predicted for 316L steel. Deformation of the surfaces was dominated by the surface geometry, with no significant effect due to variations in frictional models. Plastic and frictional heating were found to have a minimal effect on the deformation of the surface, with the rapid conduction of heat preventing any highly localised heating. There was no corresponding effect on the predicted galling frequency response.Isothermal, elevated temperature conditions caused a decrease in galling resistance, driven by the temperature sensitivity of the critical resolved shear stress. The extent of deformation, as quantified by the area of plastically deformed material and plastic reach, increased with temperature. Comparisons were made with literature results for several surface amplitude and wavelength conditions. Model results compared favourably with those in the literature. However, the reduction in predicted galling resistance with elevated temperature for a fixed surface was not as severe as observations in the literature, suggesting other mechanisms (e.g. phase transformations, surface coatings and oxides) are likely important.

  • Journal article
    Zhang J, Spikes H, 2020,

    Measurement of EHD friction at very high contact pressures

    , Tribology Letters, Vol: 68, Pages: 1-12, ISSN: 1023-8883

    EHD friction curves have been measured up to very high pressure (pmean = 5 GPa, pmax = 7.5 GPa) using a newly developed, rolling-sliding, ball on disc machine, the ETM. Six base fluids have been studied, spanning the API base oil categories Group I to Group V. At high pressures, thermal effects become substantial even at quite modest slide-roll ratios, and these must be considered when analysing friction measurements in terms of the underlying rheological properties of the oils. By comparing measurements from steel/steel and WC/WC ball and disc combinations with very different thermal conductivities, the use of thermal correction to derive isothermal friction curves has been validated. At relatively low pressures (mean pressure = 1 GPa), there are substantial differences between the EHD friction properties of the various API Group base oils, but as pressure is raised these diminish and the EHD friction coefficients of all the Groups approach a similar maximum value at a given temperature. EHD friction continues to be quite strongly temperature dependent even at very high pressure. As pressure is increased, EHD friction curves become progressively steeper, so that friction coefficients at very low slide-roll ratios (1 to 2% SRR) become several times greater at high than at low pressure. This has important practical implications for the efficiency of rolling element bearings at high pressures since these components normally operate in this SRR range. There is no evidence of any of the base oils reaching a limiting shear stress over the whole pressure and temperature range studied. Instead, shear stress continues to increase with log(strain rate) in accord with the Eyring-activated flow model up to very high pressures.

  • Journal article
    Kim HM, Spikes H, 2020,

    Correlation of elastohydrodynamic friction with molecular structure of highly refined hydrocarbon base oils

    , Tribology Letters, Vol: 68, Pages: 1-14, ISSN: 1023-8883

    The molecular compositions of a range of low viscosity hydrocarbon base oils spanning API Groups II to IV have been quantified using 13C NMR and correlated with base oil elastohydrodynamic (EHD) friction. A strong correlation has been found between the proportions of paraffin, linear and branched carbons and EHD friction, with a high proportion of linear and paraffinic carbon atoms contributing to low-EHD friction but branched carbons contributing to high-EHD friction. Correlation equations have been developed to predict EHD friction based on base oil composition. At very high temperature and low pressure, this correlation breaks down as the lubricant in the contact does not reach sufficiently high shear stress for shear thinning to occur. For Group IV polyalphaolefin, the correlation must be extended to account for the very high proportion of linear carbons originating from linear alkene oligomerization. The correlations developed in this study can be used to guide the design of low-EHD friction base oils.

  • Journal article
    Fry B, Moody G, Spikes HA, Wong JSSet al., 2020,

    Adsorption of organic friction modifier additives

    , Langmuir, Vol: 36, Pages: 1147-1155, ISSN: 0743-7463

    Organic friction modifier additives (OFMs) are surfactant molecules added to engine oils to reduce friction in the boundary lubrication regime. They are thought to work by forming an absorbed layer which provides low friction. This paper studied the relationship between the adsorption of OFMs and their friction and wear reducing properties in a rubbing contact formed by a stationary glass ball and a rotating silicon disk under the boundary lubrication regime. The effect of molecular structure was investigated by using OFMs of various tail saturation and head group chemistry. OFM tested were oleic acid, octadecylamine, oleylamine and glycerol monooleate. The thickness of an OFM adsorbed layer in hexadecane, examined in-situ by spectroscopic ellipsometry and quartz crystal microbalance (QCM), depends on the molecular structure and the concentration of the OFM. As expected, saturated, linear chain gives the thickest film. A critical OFM layer thickness of about 0.6 nm is necessary to achieve low initial and maximum friction. The thicker OFM layers are accompanied by narrower wear tracks, which are rougher than the wider, smoother wear tracks formed with thinner OFM layers. The interplay between the thickness of the OFM layer and wear track surface roughness results in all OFM layers having similar steady friction. This shows that the apparent effect of OFM depends on the stage of rubbing test: initially on friction; and then subsequently on surface damage. Despite OFMs and the base oil having similar refractive indices, ellipsometry was found to be a suitable technique for examining the adsorption of OFM additives from an oil based solution, and showed reasonable correlation with QCM results.

  • Journal article
    Zhang J, Ewen JP, Ueda M, Wong JSS, Spikes HAet al., 2020,

    Mechanochemistry of zinc dialkyldithiophosphate on steel surfaces under elastohydrodynamic lubrication conditions

    , ACS Applied Materials & Interfaces, Vol: 12, Pages: 6662-6676, ISSN: 1944-8244

    Zinc dialkyldithiophosphate (ZDDP) is added to engine lubricants to reduce wear and ensure reliable operation. ZDDP reacts under rubbing conditions to form protective zinc/iron phosphate tribofilms on steel surfaces. Recently, it has been demonstrated that this process can be promoted by applied stresses in lubricated contacts, as well as temperature, and is thus mechanochemical in origin. In this study, a tribology test rig capable of applying very high loads has been developed to generate ZDDP tribofilms under full-film elastohydrodynamic lubrication (EHL) conditions in steel/steel ball-on-disk contacts. This provides a well-defined temperature and stress environment with negligible direct asperity contact in which to study mechanochemical processes. ZDDPs with branched primary and secondary alkyl substituents have been studied in three base oils, two with high EHL friction and one with low EHL friction. In the high EHL friction base oils, the tribofilm growth rate increases exponentially with shear stress and temperature for both ZDDPs, as predicted by a stress augmented thermal activation model. Conversely, under otherwise identical conditions, negligible ZDDP tribofilm formation takes place in the low EHL friction base oil. This confirms that the ZDDP reaction is driven by macroscopic shear stress rather than hydrostatic pressure. The secondary ZDDP forms tribofilms considerably faster than the primary ZDDP under equivalent conditions, suggesting that the initial decomposition reaction is the rate determining step for tribofilm formation. The rate of tribofilm growth is independent of ZDDP concentration over the range studied, indicating that this process follows zero-order kinetics. Under full-film EHL conditions, ZDDP tribofilm formation is promoted by macroscopic shear stress applied through the base oil molecules, which induces asymmetric stress on adsorbed ZDDP molecules to promote their decomposition and initiate rapid phosphate polymerisation.

  • Book chapter
    Ewen J, Ramos Fernandez E, Smith E, Dini Det al., 2020,

    Nonequilibrium Molecular Dynamics Simulations of Tribological Systems

    , Modeling and Simulation of Tribological Problems in Technology, Editors: Paggi, Hills, Publisher: Springer Nature, Pages: 95-130, ISBN: 978-3-030-20376-4
  • Book chapter
    Putignano C, Dini D, 2020,

    Contact Mechanics of Rubber and Soft Matter

    , Modeling and Simulation of Tribological Problems in Technology, Editors: Paggi, Hills, Publisher: Springer Nature, ISBN: 978-3-030-20376-4
  • Journal article
    Campen S, Moorhouse SJ, Wong JSS, 2020,

    Mechanism of an asphaltene inhibitor in different depositing environments: Influence of colloid stability

    , Journal of Petroleum Science and Engineering, Vol: 184, ISSN: 0920-4105

    Additives are used to reduce unwanted carbonaceous deposits of asphaltenes on surfaces during petroleum production from natural oil and gas reservoirs. The working mechanism of formulated additive packages can be multifaceted. Additives may be effective in the bulk fluid by preventing asphaltenes aggregation, as well as at the surface by preventing asphaltenes adhesion. In this paper, we investigate the numerous different mechanisms by which an asphaltene inhibitor can interfere with the formation of carbonaceous deposits using a combination of techniques including dynamic light scattering to determine particle size distribution, quartz crystal microbalance with dissipation monitoring to examine deposition behaviour and atomic force microscopy to probe deposit morphology. The tested inhibitor prevents deposition of asphaltenes in toluene, where asphaltenes exist as a stable colloidal dispersion of nanoaggregates, by forming barrier-type films that inhibit asphaltenes adhesion and displacing adsorbed thin films of asphaltenes. However, inhibitor performance in heptane-toluene, where asphaltenes are destabilised, depends on the degree of destabilisation. At low heptane volume fraction, inhibitor slows the rate of deposition and deposition rate decreases with increasing inhibitor concentration. However, at high heptane volume fraction, inhibitor can increase the deposition rate, particularly when used in high concentration. At high heptane volume fraction, inhibitor addition alters the morphology of the deposit from that consisting of large flocculent aggregates to that consisting of smaller, submicrometer aggregates. This is consistent with the finding that inhibitor acts as an anti-agglomerant and prevents the formation of large aggregates in the bulk liquid. This paper shows that the impact of inhibitor addition depends on the environmental conditions encountered and the degree of destabilisation of the asphaltenes. Where inhibitor addition alters the nature of depo

  • Journal article
    Knight C, O'Sullivan C, Dini D, Van Wachem Bet al., 2020,

    Computing drag and interactions between fluid and polydisperse particles in saturated granular materials

    , Computers and Geotechnics, Vol: 117, Pages: 1-16, ISSN: 0266-352X

    Fundamental numerical studies of seepage induced geotechnical instabilities and filtration processes depends on accurate prediction of the forces imparted on the soil grains by the permeating fluid. Hitherto coupled Discrete Element Method (DEM) simulations documented in geomechanics have most often simulated the fluid flow using computational fluid dynamics (CFD) models employing fluid cells that contain a number of particles. Empirical drag models are used to predict the fluid-particle interaction forces using the flow Reynolds number and fluid cell porosity. Experimental verification of the forces predicted by these models at the particle-scale is non-trivial. This contribution uses a high resolution immersed boundary method to model the fluid flow within individual voids in polydisperse samples of spheres to accurately determine the fluid-particle interaction forces. The existing drag models are shown to poorly capture the forces on individual particles in the samples for flow with low Reynolds number values. An alternative approach is proposed in which a radical Voronoi tesselation is applied to estimate a local solids volume fraction for each particle; this local solids fraction can be adopted in combination with existing expressions to estimate the drag force. This tessellation-based approach gives a more accurate prediction of the fluid particle interaction forces.

  • Journal article
    Hu S, Cao X, Reddyhoff T, Puhan D, Vladescu S-C, Wang Q, Shi X, Peng Z, deMello AJ, Dini Det al., 2019,

    Self-compensating liquid repellent surfaces with stratified morphology

    , ACS Applied Materials and Interfaces, Vol: 12, Pages: 4174-4182, ISSN: 1944-8244

    Artificial liquid repellent surfaces have recently attracted vast scientific attention; however, achieving mechanical robustness remains a formidable challenge before industrialization can be realized. To this end, inspired by plateaus in geological landscapes, a self-compensating strategy is developed to pave the way for the synthesis of durable repellent surfaces. This self-compensating surface comprises tall hydrophobic structural elements, which can repel liquid droplets. When these elements are damaged, they expose shorter structural elements that also suspend the droplets and thus preserve interfacial repellency. An example of this plateau-inspired stratified surface was created by 3D direct laser lithography micro-nano fabrication. Even after being subjected to serious frictional damage, it maintained static repellency to water with a contact angle above 147 and was simultaneously able to endure high pressures arising from droplet impacts. Extending the scope of nature-inspired functional surfaces from conventional biomimetics to geological landscapes, this works demonstrates that the plateau-inspired self-compensating strategy can provide an unprecedented level of robustness in terms of sustained liquid repellency.

  • Journal article
    Fry B, Moody G, Spikes H, Wong Jet al., 2019,

    Effect of surface cleaning on performance of organic friction modifiers

    , Tribology Transactions, Vol: 63, Pages: 305-313, ISSN: 1040-2004

    The performance of surface active additives, such as friction modifiers, depends on their interactions with surfaces. Their effectiveness thus hinges upon the surface conditions. In this work, the effect of cleaning methods of test substrates on the friction reduction capabilities of different organic friction modifier (OFM) additives was investigated. 52100 steel discs and balls were the test specimens. They were cleaned in five different ways. The cleaned surfaces were characterised by using ellipsometry and atomic force microscopy. The tribological performance of stearic acid (STA), octadecylamine (ODA) and octadecanol (ODO) on these surfaces were then tested. As-received steel surfaces were covered with contaminants which may impede the formation of OFM surface layer. Cleaning these surfaces with solvents cannot completely removed these contaminants, with residue layers remain. Cleaning with oxygen or argon plasma results in cleaner surfaces as compared to those cleaned by solvents only. The impact of the choice of cleaning methods on friction depends on the strength of the interaction between the OFM and the steel surface, which determines the ability of an OFM to displace surface contaminations. Cleaner surfaces result in lower initial friction for STA and ODA. Steady state friction is also affected, but to a smaller extent. It may be because most containments remained in the wear track are mechanically removed during rubbing.

  • Journal article
    Ciniero A, Fatti G, Righi MC, Dini D, Reddyhoff Tet al., 2019,

    A combined experimental and theoretical study on the mechanisms behind tribocharging phenomenon and the influence of triboemission

    , Tribology Online, Vol: 14, Pages: 367-374, ISSN: 1881-218X

    This work describes recent research into the mechanisms behind tribocharging and the influence of triboemission. The term tribocharging is a type of contact-induced electrification and refers to the transfer of charge between rubbing components. The term triboemission, on the other hand, refers to emission of electrons, ions and photons generated when surfaces are rubbed together. The understanding of tribocharging is of wide interest for several industrial applications and in particular the combination of tribocharging and triboemission may be important in lubricated contacts in the formation of boundary lubricant films. We report the use of a unique vacuum measurement system that enables to measure surface charge variations while simultaneously recording triboemission events during the sliding of a diamond tip on silica specimens. Results show for the first time that tribocharging and triboemission behavior are linked and depend on the surface wear. The contribution of contact-induced electrification to the charging of the surface is then described by means of density functional theory (DFT). Results give insight into the transfer of charge from the SiO2 amorphous surface (silica) to the C(111) surface (diamond ) and into the variation of charging during simulated sliding contact.

  • Journal article
    Profito FJ, Zachariadis DC, Dini D, 2019,

    Partitioned fluid-structure interaction techniques applied to the mixed-elastohydrodynamic solution of dynamically loaded connecting-rod big-end bearings

    , Tribology International, Vol: 140, ISSN: 0301-679X

    The present contribution proposes different partitioned techniques, which are commonly used in fluid-structure interaction (FSI) applications, in the context of tribological simulations of the transient mixed-elastohydrodynamic problem of dynamically loaded connecting-rod bearings. With the premise that the FSI framework developed is general, in the current work the fluid flow effects have been considered through the averaged Reynolds equation by Patir & Cheng and the mass-conserving Elrod-Adams cavitation model. The multiphysics simulation framework developed has been used to simulate the connecting-rod big-end bearings of both heavy-duty diesel and high-speed motorcycle engines. In the latter case, the influence of polymer concentration in VM-containing oils with similar HTHS150 values on the bearing power loss is investigated and discussed in details.

  • Journal article
    Menga N, Carbone G, Dini D, 2019,

    Corrigendum to “Do uniform tangential interfacial stresses enhance adhesion?” [Journal of the Mechanics and Physics of Solids 112 (2018) 145–156]

    , Journal of the Mechanics and Physics of Solids, Vol: 133, Pages: 103744-103744, ISSN: 0022-5096
  • Journal article
    Dawczyk J, Russo J, Spikes H, 2019,

    Ethoxylated amine friction modifiers and ZDDP

    , Tribology Letters, Vol: 67, ISSN: 1023-8883

    The influence of a series of Ethomeens (ethoxylated alkylamine organic friction modifiers) on the durability and friction of tribofilms formed by a commercial blend of primary and secondary ZDDP in sliding/rolling contact has been studied. When pre-formed ZDDP tribofilms are rubbed in Ethomeen solution, boundary friction is reduced and some of the ZDDP film is removed. Ethomeens having just two ethoxy groups give lower boundary friction on ZDDP than those with 15 ethoxy groups, but result in much greater removal of the tribofilm itself. Based on XANES analysis, the film removed by both types of Ethomeen consists primarily of nanocrystalline orthophosphate. The level of boundary friction and its dependence on sliding speed, coupled with the dimensions of the molecules, suggests that the Ethomeens with two ethoxy groups may form quite closely packed vertical monolayers on ZDDP tribofilm surfaces, but that those with fifteen ethoxy groups cannot be close packed; yet they still reduce boundary friction significantly. The study shows that selection of an appropriate aminic friction modifier for use with ZDDP is a balance between its ability to reduce friction and its potentially harmful effect on a ZDDP tribofilm.

  • Journal article
    Ueda M, Kadiric A, Spikes H, 2019,

    On the crystallinity and durability of ZDDP tribofilm

    , Tribology Letters, Vol: 67, Pages: 1-13, ISSN: 1023-8883

    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, mixed lubrication conditions, where the risk of surface damage is increased. Consequently, the performance and durability of the tribofilms formed by antiwear additives, and in particular zinc dialkyldithiophosphate (ZDDP), the main antiwear oil additive used in engine oils, has become an increasingly important issue. In this paper, it is confirmed that ZDDP tribofilms are initially relatively easily removed by rubbing but that they become more durable during prolonged rubbing. FIB-TEM analyses at different stages of tribofilm formation show that during the early stages of rubbing only the tribofilm close to the steel substrate is nanocrystalline, while the outer region is amorphous and easily removed. However, after prolonged rubbing all regions of the tribofilm become nanocrystalline and able to withstand rubbing in base oil without being removed. XPS analysis shows that after extended rubbing the outermost polyphosphate structures change from longer-chain structures such as metaphosphate and polyphosphate to shorter-chain structures including orthophosphate. This depolymerization of ZDDP tribofilm from long- to short-chain phosphate and consequent nanocrystallization are driven by heat and shear stress. EDX analysis shows that this conversion is promoted by diffusion of Fe cation into the bulk of the tribofilm. The finding that ZDDP tribofilms evolve during rubbing from a weaker amorphous structure to a more durable nanocrystalline one has important implications in terms of the behaviour of ZDDPs at low concentrations, on non-metallic surfaces and at very high contact pressures, as well as for the development of ZDDP tribofilm, friction and wear models.

  • Journal article
    Heyes DM, Dini D, Costigliola L, Dyre JCet al., 2019,

    Transport coefficients of the Lennard-Jones fluid close to the freezing line.

    , J Chem Phys, Vol: 151, Pages: 204502-204502

    Molecular dynamics simulations have been carried out along four Lennard-Jones (LJ) fluid isomorphs close to the freezing line, covering a temperature, T, in the range of 0.8-350 and a number density, ρ, in the range of 1.1-3.0 in LJ units. Analysis of the transport coefficients is via the Green-Kubo time correlation function method. The radial distribution function, percolation threshold connectivity distance, self-diffusion coefficient, and shear viscosity are shown to be invariant along an isomorph to a very good approximation when scaled with Rosenfeld's macroscopic units, although there are some small departures for T ≃ 1 and lower temperatures. The thermal conductivity is shown for the first time also to be isomorph invariant. In contrast, the Einstein and moment-based frequencies, and especially the bulk viscosity, ηb, show poor isomorphic collapse at low T but not surprisingly tend to an "inverse power" potential limiting value in the high T limit. In the case of the bulk viscosity, the significant departures from invariance arise from oscillations in the pressure autocorrelation function at intermediate times, which scale for inverse power potential systems but not for the LJ case, at least in part, as the pressure and bulk elastic moduli are not isomorph invariant.

  • Journal article
    Costa H, Spikes H, 2019,

    Interactions of ethanol with friction modifiers in model engine lubricants

    , Lubricants, Vol: 7, ISSN: 2075-4442

    When employed as an engine fuel, ethanol can accumulate in the lubricant during use. Previous work has shown that ethanol contamination affects friction and elastohydrodynamic lubrication (EHL) film formation, and also the growth and stability of anti-wear tribofilms. The present work uses spacer-layer ultrathin interferometry and MTM tests to investigate how ethanol (both hydrated and anhydrous) interacts with friction modifiers in model lubricants. Small proportions (5 wt %) of ethanol were added to solutions of friction modifiers (one MoDTC and three organic friction modifiers) in a Group I base oil. For the three organic friction modifiers, the presence of ethanol promoted the formation of thick viscous boundary films so that very low friction coefficients were measured at low entrainment speeds. For the MoDTC additive, the presence of ethanol prevented the formation of a low friction film at low speeds at 70 °C, but this effect disappeared at 100 °C, probably due to ethanol evaporation.

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