Publications

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>

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

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, Vol: 257, Pages: 1-14, 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

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

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, Vol: 97, Pages: 4078-4083, 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

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

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

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

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

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

Cite

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

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, ISSN: 0301-679X

Infrared microscopy is used to obtain through-thickness oil temperature measurements from EHL contacts between different surface materials (steel, silicon nitride and zirconia) for the lubricants Santotrac 50 and PAO4. The measurement technique was first adapted to overcome focussing issues due to the partially transparent zirconia surface. Results were used to infer in-contact rheological behaviour of the lubricants. Santotrac 50 shows significant shear localisation under all conditions with the position of the shear heating zone being highly affected by the contact surfaces' thermal properties. For PAO4, the shear profile depends on the contact surfaces’ thermal properties with moderate to high surface conductivities favouring uniform shearing, whereas highly insulating surfaces (zirconia) cause shear localisation at the surface for both lubricants. These results are used to interpret friction measurements and show how the thermal properties of surfaces can be used to control rheology and friction. This paper is prefaced by a review of thermal EHL theory upon which our analysis is based.

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 (cpl-library.org) permitting useful aspects to be evaluated and adopted in other projects.

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, ISSN: 0301-679X

A thrust bearing consisting of an air cushion formed within a liquid ring has been developed, which takes advantage of the Laplace pressure induced by the liquid/air surface tension. As forces induced by Laplace pressure and surface tension scales down much more slowly than gravity and inertial forces, such a bearing has great potential when scaled down to the micro-scale. The liquid ring between the rotor and the stator of the bearing is anchored there by alternating hydrophilic and superhydrophobic patterns. An important discovery is that the performance of this bearing is greatly enhanced by the sealed cushion of air within the ring. This air cushion and thin liquid ring arrangement mean that the solid/solid contact of the bearing is replaced by solid/air and solid/liquid contact which significantly reduces the friction and wear. The factors which affects the performance of the bearing have been studied both experimentally and numerically providing results that can be used to optimise the design of this new type of bearing.

Patent

Southby M, Morgan N, Kontou A, Spikes Het al., 2020,

Lubricating composition

, EP3615641A1

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.

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.

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

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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

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Journal article

Eder SJ, Ripoll MR, Cihak-Bayr U, Dini D, Gachot Cet al., 2020,

MD simulations of FCC alloys under dry sliding yield a mechanism map for near-surface microstructural development

, Tribologie und Schmierungstechnik, Vol: 67, Pages: 38-39, ISSN: 0724-3472

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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

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

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

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

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

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

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.

Journal article

Kanazawa Y, De Laurentis N, Kadiric A, 2019,

Studies of friction in grease lubricated rolling bearings using ball-on-disc and full bearing tests

, Tribology Transactions, Vol: 63, Pages: 77-89, ISSN: 1040-2004

This article evaluates the frictional performance of different bearing grease formulations in full rolling bearings and a ball-on-disc rig and subsequently assesses whether the ball-on-disc test results can be used to predict the grease performance in actual bearings. A selection of custom-made greases with systematically varied formulations as well as their base oils were tested. Bearing torque was measured in two different cylindrical roller thrust bearings and a thrust ball bearing. The same lubricants were tested with ball-on-disc tribometers, a mini traction machine (MTM) to measure friction and an optical elastohydrodynamic (EHD) rig to measure film thickness. Both lithium complex and diurea greases were observed to produce lower friction than their base oils within the low speed, low nominal lambda ratio region, whereas the greases and oils had the same friction at high nominal lambda ratio values. These relative trends were the same in full bearing and single-contact MTM tests. The reduction in friction was seen to be related to the level of film thickness enhancement provided by greases at lower speeds, which leads to an increase in the effective lambda ratio and hence reduced friction. By extracting the sliding torque component from the overall measured bearing torque, a plot of the friction coefficient against the effective lambda ratio was produced encompassing all bearing and single-contact tests and all lubricants and test conditions. This plot was seen to follow a general shape of a master Stribeck curve, indicating that the numerical values of the friction coefficient from ball-on-disc and full bearing tests overlap and can be related to each other using this approach over the range of conditions employed here. Thus, single-contact ball-on-disc tests can provide a fast and economical way of establishing the frictional performance of bearing greases in full bearings in terms of both relative performance rankings and quantitative values of bearing fric

Patent

Reddyhoff T, 2019,

LUBRICANT COMPOSITIONS

, WO2019193368

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Journal article

Gouda K, Rycerz P, Kadiric A, Morales-Espejel Get al., 2019,

Assessing the effectiveness of data-driven time-domain condition indicators in predicting the progression of surface distress under rolling contact

, Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, Vol: 233, Pages: 1523-1540, ISSN: 1350-6501

Condition monitoring of machine health via analysis of vibration, acoustic and other signals offers an important tool for reducing the machine downtime and maintenance costs. The key aspect in this process is the ability to relate features derived from the recorded sensor signals to the physical condition of the monitored asset in real time. This paper uses simple machine learning techniques to examine the ability of specific time-domain features obtained from vibration signals to predict the progression of surface distress in lubricated, rolling-sliding contacts, such as those found in rolling bearings and gears. Controlled experiments were performed on a triple-disc rolling contact fatigue rig using seeded-fault roller specimens where micropitting damage was generated and its progression directly observed over millions of contact cycles. Vibration signals were recorded throughout the experiments. Features known as condition indicators were then extracted from the recorded time-domain signals and their evolution related to the observed physical state of the associated specimens using simple machine learning techniques. Five time-domain condition indicators were examined, peak-to-peak, root-mean-square, kurtosis, crest factor and skewness, three of which were found not to be redundant. First, a classification model using KNN nearest neighbor was built with the three informative condition indicators as training data. The cross-validation results indicated that this classifier was able to predict the presence of micropitting damage with a relatively high precision and a low rate of false positives. Secondly, a k-means clustering analysis was performed to measure the significance of each condition indicator by leveraging patterns. The peak-to-peak condition indicator was found to be a good predictor for progression of micropitting damage. In addition, this indicator was able to distinguish between micropitting and pitting failure modes with a high success rate. Finally

Journal article

Ueda M, Spikes H, Kadiric A, 2019,

In-situ observations of the effect of the ZDDP tribofilm growth on micropitting

, Tribology International, Vol: 138, Pages: 342-352, ISSN: 0301-679X

The ongoing trend for using ever lower viscosities of lubricating oils, with the aim of improving the efficiency of mechanical systems, means that machine components are required to operate for longer periods under thin film, mixed lubrication conditions where the risk of surface damage is increased. For this reason, the role of zinc dialkyldithiophosphate (ZDDP) antiwear lubricant additive has become increasingly important in order to provide adequate surface protection. It is known that due to its exceptional effectiveness in reducing surface wear, ZDDP may promote micropitting by preventing adequate running-in of the contacting surfaces. However, the relationship between ZDDP tribofilm growth rate and the evolution of micropitting has not been directly demonstrated. To address this, we report the development of a novel technique using MTM-SLIM to obtain micropitting and observe ZDDP tribofilm growth in parallel throughout a test. This is then applied to investigate the effect of ZDDP concentration and type on micropitting.It is found that oils with higher ZDDP concentrations produce more micropitting but less surface wear and that, at a given concentration, a mixed primary-secondary ZDDP results in more severe micropitting than a primary ZDDP. Too rapid formation of a thick antiwear tribofilm early in the test serves to prevent adequate running-in of sliding parts, which subsequently leads to higher asperity stresses and more asperity stress cycles and consequently more micropitting. Therefore, any adverse effects of ZDDP on micropitting and surface fatigue in general are mechanical in nature and can be accounted for through ZDDP's influence on running-in and resulting asperity stress history. The observed correlation between antiwear film formation rate and micropitting should help in the design of oil formulations that extend component lifetime by controlling both wear and micropitting damage.

Journal article

Xu Y, Balint D, Dini D, 2019,

A new hardness formula incorporating the effect of source density on indentation response: a discrete dislocation plasticity analysis

, Surface and Coatings Technology, Vol: 374, Pages: 763-773, ISSN: 0257-8972

Planar discrete dislocation plasticity (DDP) calculations that simulate thin single crystal films bonded to a rigid substrate indented by a rigid wedge are performed for different values of film thickness and dislocation source density. As in prior studies, an indentation size effect (ISE) is observed when indentation depth is sufficiently small relative to the film thickness. Thedependence of the ISE on dislocation source density is quantified in this study, and a modified form of the scaling law for the dependence of hardness on indentation depth, first derived by Nix and Gao, is proposed, which is valid over the entire range of indentation depths and correlates the length scale parameter with the average dislocation source spacing. Nanoindentation experimental data from the literature are fitted using this formula, which further verifies the proposed scaling of indentation pressure on dislocation source density.

Journal article

Campen SM, Moorhouse SJ, Wong JSS, 2019,

Effect of aging on the removal of asphaltene deposits with aromatic solvent

, Langmuir, Vol: 35, Pages: 11995-12008, ISSN: 0743-7463

Surface-deposition of destabilised colloidal particles of asphaltenes poses a serious and costly problem during petroleum production. Remediation of asphaltene-fouled well-bore and surface facilities is often undertaken by flowing aromatic solvent to remove deposited films. However, little is known about the properties of deposited asphaltene films during their removal by solvent-rinsing. Here, we carry out quartz crystal microbalance with dissipation monitoring (QCM-D) experiments to investigate surface-deposition of destabilized colloidal particles of asphaltenes and their subsequent removal by solvent-rinsing. It is shown that the properties of deposited films during solvent removal depend on the history of the deposit. Newly formed deposit films are removed immediately without significant change in their mechanical properties during removal. However, deposits that remain on the surface for an extended time in a poor solvent (a low-asphaltene solubility solvent), “aged deposits”, are more difficult to remove and exhibit increased dissipation during the removal period, indicating that they swell and are softer. Liquid-cell atomic force microscopy (AFM) confirms that aged deposits swell when the quality of the solvent is subsequently improved by exchanging for a high-asphaltene solubility solvent. Deposit swelling is accompanied by a change in film morphology, from particulate to continuous. Stubborn deposits of aged asphaltene films, which remain after solvent-rinsing, may be partly removed by flowing dissolved asphaltenes in good solvent. Hence, reinjection of asphaltenes during remediation can aid deposit removal.

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MD simulations of FCC alloys under dry sliding yield a mechanism map for near-surface microstructural development

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