Current research projects
Adsorption of Organic Friction Modifiers to Oxide Surfaces from First Principles
Researcher: Dr Chiara Gattinoni
Supervisor: Professor Daniele Dini
Organic friction modifiers (OFMs) are amphiphilic molecules which, when added to a lubricant, give lower friction in mechanical systems. They are particularly effective in the boundary regime where solid surfaces can come into direct contact. Their ability to modify friction is generally attributed to the thickness and strength of the films that they form on the contact surfaces. Therefore, the adsorption properties of OFMs and their friction reduction behaviour are closely related. The main objective of this project is to obtain accurate information regarding the adsorption of organic friction modifiers (OFM) on iron oxide surfaces from first principles calculations.
Brain Mimicking Hydrogels
Researcher: Zhengchu Tan
Supervisor: Professor Daniele Dini
The characterisation of the mechanical response of real soft tissues, such as brain, liver and cartilage, is immensely important as it allows us to understand the way they respond under a variety of different loads. Hence, ways to reduce damage to living tissues during real life scenarios can be identified and developed. However, real tissue is difficult to obtain and test due to accessibility. Therefore, there is a huge advantage in developing an accurate synthetic tissue phantom that is easier to procure and produce. This has led to the popularity of hydrogels, which have been developed into tissue mimicking materials due to their biocompatibility and stiffness tunability.
This project focuses on the development of a composite hydrogel (CH) constituting of poly(vinyl) alcohol (PVA) and phytagel that is able to match the complex viscoelastic behaviour of brain. The CH can be tuned to achieve different stiffness and relaxation responses by varying the concentrations of each hydrogel component, which allows the material to mimic other soft tissues.
Consequently, a mechanically accurate tissue phantom material opens the doors to many applications in the study of mechanobiology and regenerative medicine. This project investigates cell viability of the CH substrate, therefore extending the range of applicability to explore a variety of different mechanical loads affects cells seeded on a CH substrate These experiments include impact tests, needle insertion tests and tribological tests to match the high strain rate behaviour of brain for the study of TBI, fracture behaviour of liver during surgery and tribological behaviour of cartilage, respectively.
Computational Fluid Dynamics (CFD) Modelling of Elastohydrodynamic Lubrication (EHL)
Researcher: Damon Lee
Rolling element bearings, gears and many other machine elements operate in the Elastohydrodynamic Lubrication (EHL) regime. In this regime, the lubricant creates a very thin protective film between the contacting elements, improving reliability as well as reducing friction. Therefore, understanding of EHL lubrication allows optimisation of these components in terms of reliability and efficiency, through predictions of EHL film thickness and EHL frictional losses.
Typical methods for predicting the EHL oil film behaviour are either empirical relationships or numerical solutions to simplified fluid flow equations (Reynolds equation), coupled with an approximation to the linear elasticity equations. These methods rely on a number of assumptions that may not always hold. This project utilises finite volume Computational Fluid Dynamics (CFD) and linear elasticity to model the EHL contact through a complete solution of Navier-Stokes equations in the fluid domain, coupled with the Navier Cauchy equations in the solid domain, as well as the heat equation in all domains. A cavitation model is also implemented. This provides for more accurate treatment of relevant physical principles and allows for inclusion of additional effects such as surface roughness, surface coatings or inlet shear heating for example. EHL film thickness and friction predictions are more accurate as the full continuum mechanics description of the system is solved, resolving all gradients. The modelling domain is larger than the immediate contact so that, for instance, the entire flow, viscosity and temperature fields can be studied at the entrance to the contact. The complete shear stress field is predicted, hence providing an accurate way of studying EHL friction.
In addition to improving the EHL modelling tools, the project will attempt to generate charts that indicate the contact conditions where the simplified EHL solutions may be used with sufficient accuracy and those where the full CFD solution may be needed. Investigation will also be made into suitable rheology models for grease as a lubricant.
Coupled Fluid-Mechanical Modelling of the Brain
Researcher: Andrea Bernardini
Supervisor: Professor Daniele Dini
The brain is composed of two main types of tissues, namely grey and white matter. In this project, we focus on the latter, which is where glioblastoma is localised (i.e. where the EDEN system will infuse medication). The white matter is comprised of several types of cells and components which make it possible to theorise it as an anisotropic mechanical entity. Segmentation of the components from SEM images enables the 3-D reconstruction of the representative units to be modelled and then analysed via FEA.
A coupled fluid-mechanical model of the brain will be developed by modelling the mechanical behaviour of such tissue and its interaction with the fluidic environment in which it is submerged. This will enable the prediction of diffusive phenomena and patterns in the enhanced drug delivery in order to optimise the surgical procedure and the point of cancer-drug dosing.
Coupling Continuum and Discrete Methods for Fluid Simulation
Researcher: Dr Edward R. Smith
Supervisor: Professor Daniele Dini, Professor David Heyes
The field of simulation can be broadly split into continuum and discrete methods. Continuum methods are the backbone of engineering simulation, solving differential equations to model flow with computational fluid dynamics (CFD) or deformation of a solid with finite element analysis (FEA). Discrete methods, which include molecular dynamics (MD), the discrete element method (DEM) and other mesh free methods, model the particles as they evolve and interact.
The aim of this project is to develop techniques to couple these two approaches, allowing us to simulate cases which are not possible with either method alone. For tribology, this will allow the modelling of molecular detail near interfaces, parameterisation of viscosity for complex molecules or modelling of the conditions caused by extremes of pressure and shear.
The work is split into computational and theoretical developments. The computational strand involves active support of the open-source software CPL library. The theoretical developments include the application of a mathematical framework based on the control volume to unify the continuum and discrete formulation in an exactly conservative coupling.
Degraded Oil Performance
Researcher: Sarah Bellingham
Supervisor: Dr Amir Kadiric
EDEN2020 (An Enhanced Delivery Ecosystem for Neurosurgery)
Due to an aging population and the spiralling cost of brain disease in Europe and beyond, EDEN2020 aims to develop the gold standard for one-stop diagnosis and minimally invasive treatment in neurosurgery. Supported by a clear business case, it will exploit the unique track record of leading research institutions and key industrial players in the field of surgical robotics to overcome the current technological barriers that stand in the way of real clinical impact.
Effect of Dry Sliding on High Performance Polymers
Researcher: Annelise Jean-Fulcrand
Moving parts often require constant lubrication to ensure reliable and efficient operation of the equipment. However, lubricant properties are highly dependent on operational condition, especially at high temperature. The lubricant viscosity decreases rapidly with the increase in temperature leading to poor lubricant efficiency and failure of the equipment. At elevated temperature, oxidation can occur and induces the degradation of the lubricant. This can cause system failure. Over the years, polymers have been widely used for tribological applications in order to replace metal and ceramic components due to their chemical resistance and self-lubricating properties. Polymers with self-lubricating properties form a transfer film during friction that can act as a lubricant film. The efficiency of the transfer film is determined by the material composition, its adhesion to the countersurface and its thermal and oxidative stability.
The aim of this PhD is to investigate the role of the transfer film of high performance polymer and understand how it reduces friction and wear of the material. For this project high performance polymers and polymer blends will be investigated. These polymers have high strength and chemical resistance, and have a high glass transition temperature. These properties make them potentially suitable for high temperature tribological applications. In order to determine if any of the polymers could be a good candidates for tribological applications friction and wear are measured. No previous study exists on the tribological behaviour for these types of blends. This research focuses on the mechanisms of transfer film formation through an understanding of:
- chemical composition of the transfer film
- the type of interaction between transfer film and countersurface
- the evolution of film thickness, film composition and adhesion over time
- the impact of the environment and operating conditions on the transfer film performance
- the effect of different polymers blending ratios on the transfer film tribological properties
Finite Element Method (FEM) Modelling of the Skin-Surface Interface
Supervisor: Dr Marc Masen, Professor Daniele Dini
This project involves numerically modelling of the skin-surface interface in order to better understand the sensations of touch and perception. This involves developing finite element method (FEM) models of multi-layered soft tissue to observe the stress/strain fields in the vicinity of mechanoreceptors.
Friction of 3D Printed Materials
Friction Reduction and Optimisation of Tribological Interactions via Microtexturing and Superhydrophobic Surfaces
Researcher: Jun Wen
Supervisor: Dr Tom Reddyhoff, Prof. Daniele Dini
The aim for this PhD project is to reduce friction in hydrodynamic bearings using surface treatments. The application will be targeted first is micro-scale bearings to explore the lubrication of Micro-Electro-Mechanical-Systems (MEMS). Due to the small size and the functions they can perform, MEMS devices have the potential to significantly impact our way of life, but are currently limited by severe problems of friction and wear that occur at the micro-scale.
To address this problem, the way of entrapping air pockets on the bearing surfaces by surface modification will be used. An important and highly novel aspect of the project will be to use silicon fabrication techniques, including Deep Reactive Ion Etching (DRIE), to produce surface features which will act to anchor the gaseous regions in place. Then, the models of bubble interface interactions will be combined with the hydrodynamic bearing lubrication model in order to validate and optimize the experimental results. This will also help to explore the possibility of bubble/cavitation induced friction reduction in other application such as macro-scale hydrodynamic bearings.
Fundamentals of Dislocations in Motion
Researcher: Jonas Verschueren
Our understanding of dislocation mobility - quantifying the relationship between the force on a dislocation and its resulting velocity - is largely based on experiment. However, the validity of mobility laws extracted from this work breaks down for fast travelling dislocations moving with speeds comparable to the speed of sound in the medium. In the last 20 years, large-scale non-equilibrium molecular dynamics simulations have been used to simulate qualitative mobility laws for fast travelling dislocations. However they have contributed little to our fundamental understanding of dislocation mobility in this regime. Ultimately, a physically motivated theory of dislocation mobility in the pure-glide regime in good quantitative agreement with existing simulation data is the aim of this project. This could shed light on the phenomenology associated with these fast travelling dislocations. Debate on this topic has been ongoing for over half a century and is problematic given that in this regime, the usual approximations by which elasticity theory is linearised are violated and the quasi-static approximation no longer holds.
Hydrodynamic Lubrication Modelling using MD-CFD Hybrid Methods
Researcher: Eduardo Ramos Fernandez
The field of nanotribology has remained slightly detached from mainstream macro-scale tribology, focusing primarily on specialized nano-scale applications. At the macro- and mesoscopic levels, continuum models are often able to correctly model fluids. However, at smaller scales, continuum models do not consider the atomic nature of matter and can sometimes fail to capture the essential physics. In such cases, explicit molecular models must be employed, for example to model a liquid-solid interface.
The development of a truly multi-scale approach, which spans nano- to macro-scales, is a decisive step forward in understanding engineering tribological interfaces. Hybrid methods, where atomistic simulations such as molecular dynamics (MD) and continuum computational fluid dynamics (CFD) inter-operate, offer a solution that combines the strengths of both paradigms. The aim of this project is to model contact-lubrication problems with a multi-scale simulation methodology taking advantage of an in-house coupling software (CPL_library) that has been developed in the group in the past years.
Implementing Lubrication in Micro-Electro-Mechanical Systems (MEMS)
Researcher: Peng Wang
Supervisor: Dr Tom Reddyhoff
Micro-electro-mechanical systems (MEMS) are tiny (sub-millimetre) machines, which have arisen from advances in semiconductor fabrication. Due to their low cost, high tolerances, and ability to combine sensors and actuators with microprocessors, MEMS have the potential to profoundly affect our way of life. However, high friction and wear problems mean that current commercial MEMS designs are confined to non-, or very low sliding devices.
This project aims to demonstrate how low viscosity liquids combined with friction modifier additives are an effective means of the lubricating MEMS devices. This has so far only been achieved in lab-based tests; therefore the current aim is to implement this type of lubrication in an actual MEMS device. To do this, the project is using semiconductor fabrication techniques to build micro-hydrodynamic bearings which will be incorporated and tested in a MEMS turbine energy harvester.
In addition to the goal of producing a MEMS turbine that runs on hydrodynamic micro-bearings, a number of more fundamental avenues of research, involving tribology and silicon MEMS, are being explored. These include a feasibility study into the development of sliding MEMS with textured surfaces.
This project is a collaboration with the Optical and Semiconductor Devices Group at Imperial College.
Influence of Catalyst Binder Chemistry on the Microstructure of Polycrystalline Diamond During Liquid Phase Sintering
Researcher: Branislav Dzepina
Supervisor: Professor Daniele Dini
Sponsor: Element Six
The main problem of polycrystalline diamond cutters (PDCs) used in oil and gas drilling is the brittle nature of the diamond cutting face. Premature fracture during a drilling operation results in ineffective rock cutting. Repair of the fractured cutters requires complete removal of the drill head and string. The subsequent down-time imposes a great monetary burden to the driller. It is thus important to be able to manipulate the behaviour of the cutter to prevent or delay the onset of long cracks which lead to catastrophic brittle failure.
One possible way to affect the properties of the diamond cutter is through manipulation of the microstructure. To this end, the project proposes the development of a Monte Carlo model to simulate the evolution of the microstructure during high-pressure high-temperature (HPHT) liquid phase sintering. In order to enable inputs for the model, molecular dynamics simulation and HPHT experimentation will be conducted. It is anticipated that the proposed simulation will not only identify new mechanisms for the diamond sintering model, but also allow microstructural prediction given key input variables.
This project forms part of the Diamond Science and Technology CDT.
Influence of Fuel and Lubricant Additives on Friction and Wear in Engines
Influence of Grease Composition on Friction in Elastohydrodynamic (EHD) Contacts
Researcher: Dr Nicola De Laurentis
Supervisor: Dr Amir Kadiric
The aim of this project is to examine the relationship between bearing grease composition and rolling-sliding friction in lubricated contacts. The friction coefficient and lubricating film thickness of a series of commercially available bearing greases and their bled oils will be measured in laboratory tribometers. Test greases will be selected to cover a wide spectrum of thickener and base oil types, and base oil viscosities. The trends in measured friction coefficients will be analysed in relation to grease composition in an attempt to establish the relative influence of individual grease components on friction.
Infrared Microscopy to Study In-Contact Friction Behaviour
Researcher: Jia Lu
The overall aim of this PhD project is apply infrared microscopy to a range of sliding interfaces in order to increase our understanding of the in-contact mechanisms that give rise to heat generation.
The first task is measuring the temperature distribution of the oil within an elastohydrodynamic contact. This involves using an infrared camera and microscope with a number of filters to record the radiation emitted from a contact between a metal ball and transparent sapphire disc. The radiation data obtained in this way is calibrated and processed using Planck’s Law and combined with film thickness measurements in order to separate the temperature of the two bounding surfaces from that of the film of oil.
Once achieved, results will then be used to test theories that predict oil temperature rheology.
In addition to, effect of surface coatings and lubricant additives on in-contact temperature and rheology will be studied using this technique.
Investigation of Tribocharging and Triboemission by Atomistic Simulations
Researcher: Dr Alessandra Ciniero
The aim of this project is to investigate the mechanisms by which phenomena known collectively as “triboemission” (i.e. the emission of photons electrons and charged particles due to rubbing) occur. This is important because triboemission may be responsible for certain tribochemical processes such as lubricant degradation. Ab intio molecular dynamics techniques will be used to model tribocharging and triboemission during sliding.
Linking Modelling and Experiments in Tribology
Lubrication and Fluid Load-Support in Hydrogels for Cartilage Substitutes
Researcher: Elze Porte
Sponsor: Imperial College London
This research focuses on gaining a better understanding of the lubrication mechanisms in articular cartilage. Currently, there is no thorough understanding of the relationship between the lubrication of the material, its fluid load support, and its mechanical properties.
Hydrogels have been suggested as promising substitute materials for cartilage because of their specific mechanical and tribological properties. This makes them suitable substitutes for use in lubrication experiments and, ultimately, as a potential cartilage replacement material in the surgical treatment of osteoarthritis
Fluid exudation from the bulk material into the loaded region is believed to provide the fluid load support and lubrication. To study the lubricating mechanisms of hydrogels as cartilage substitutes, contact and lubrication experiments are done on the newly developed Biotribometer (PCS Instruments, London UK). The obtained knowledge can be used to improve the existing hydrogel structures.
Machine-Learning Pattern Recognition Prototype for Liquid Foams
Researcher: Dr Li Shen
Supervisor: Professor Daniele Dini, Dr Tom Reddyhoff
This project is an EPSRC Impact Acceleration Account (IAA) entitled “Machine-Learning Pattern Recognition Prototype for liquid foams”. The IAA grant supports the development of a potentially marketable project to create impact away from a strictly academic setting. The project aims to develop a foaming rig and a Machine-Learning analysis algorithm that will enable a consistent quantitative analysis of the foaming process in beverages and oil lubricants.
Mechanisms of Transfer Film Formation at the Interface between High Performance Polymers and Steel
Researcher: Dr Debashis Puhan
Supervisor: Dr Janet Wong, Dr Tom Reddyhoff
This project aims to build fundamental understanding of the formation of transfer films athe the interface between high performance polymers (HPPs) and steel.
HPPs have a thermal resistance above 150 C which makes them suitable for use at high service temperatures. They have been replacing traditional metal components and continuing to do so in various applications in aerospace, chemicals, energy electronics and transportation sector due to several reasons such as durability, chemical resistance and mechanical properties.
Of particular interest to stakeholders is the potential energy saving attainable upon replacement of metal components that are in continuous relative motion with other metal components by HPPs, since these have the potential to reduce power loss due to their low weight.
Since the HPP components are in relative motion with surface of another component, the friction and wear properties become important. It is known that a low friction is desirable for increased energy saving. Thus, the tribology for these
polymeric components governs the efficiency and durability of the systems that are involved. Yet, the tribology of HPPs and, more importantly, how HPPs interact with metals in engineering conditions remains little known.
Tribology at the interface of the contacting surfaces is due to formation of an interfacial film termed as transfer film. The efficiency of the transfer film is determined by the material composition, its adhesion to the counter-surface and its thermal and oxidative stability. This film may or may not result in a friction reduction due to the impact of service temperature, environment and operating conditions. Therefore, often HPPs are used in the form of blends or composites to obtain a set of desired properties that includes tribological, mechanical and electrical for various
applications. Of particular interest include composite matrices, coatings, adhesives, fibres, films, membranes and active polymers for potential use in sectors such as aerospace, chemicals, energy electronics and transportation. The absence of prior knowledge of HPPs tribological performance make it impossible to assess if HPPs or their blends or composites are suitable for tribological applications. But friction between components varies with which makes it difficult to predict.
We aim to obtain an in-depth, molecular understanding on the formation and the properties of transfer films. We are interested in the effects of mechanical energy and the nature of the metal counterface on material transfer processes.
The principal focus of this research is on the molecular structure/processability/property relationships of HPPs on the mechanisms of transfer film formation, chemical composition of the transfer film, the evolution of film thickness, film morphology and composition over time, the impact of the environment and operating conditions on the transfer film performance, the effect of different polymers blends on the transfer film tribological properties.
Mechanochemical Behaviour of ZDDP
Researcher: Dr Jie Zhang (Jason)
Supervisor: Professor Hugh Spikes
It has recently been shown that tribofilm formation by the widely-used antiwear additive zinc dialkyl dithiophosphate (ZDDP) is driven by the applied shear stress present in rubbing contacts rather than by the energy dissipated in these contacts. This means that ZDDP reaction results from the stretching and breaking of molecular bonds under stress, i.e. mechanochemistry; an insight that enables relationships between molecular structure and reactivity to be developed. This project studies the impact of applied shear stress on ZDDP film formation under both full film and boundary lubrication conditions to support the principle that ZDDP reaction is controlled by mechanochemistry.
Mechanochemistry of Lubricant Additives
Sponsor: Afton Chemical, TSM-CDT
The aim of this project is to utilise molecular simulations to investigate the mechanochemical behaviour of lubricant additive molecules. Classical molecular dynamics simulations will be employed to study the stresses on additive molecules under shear. First-principles modelling frameworks will also be developed to accurately model lubricant reactivity in order to study their breakdown inside tribological contacts. By understanding the mechanochemical breakdown of current additives new and more effective additives can be designed from the molecular level.
Micro-Mechanical Model Development for Predicting the Effect of Microstructure on Bulk Behaviour of Aerated Soft Solids
Researcher: Georgios Samaras
Soft food systems are often used in confectionery products, such as emulsion or foam fillings, to provide consumers with a unique experience. Traditionally, the mechanical and rheological properties of these systems are studied in combination with sensory evaluation to describe their in-mouth flow properties. However, oral processing of food is very complex and several multi-scale mechanisms take place at the same time in the mouth. This is especially noticeable when dealing with more complex systems, e.g. when hard particles (inclusions) are included within a semi solid system, or products with different micro- and macroscopic structure, e.g. aerated systems. This hinders predictions of the in-mouth behaviour of the food. Knowledge of this behaviour is crucial as it is directly linked to sensory perception.
The aim of the project is to develop a multi-scale thermo-mechanical computational model for simulating the interaction between aerated fluid and solid structures. This includes first bite model for fracture and progressive damage of product into particles and subsequent shearing of particles in tongue/palate contact whilst they heat up, flow, mix with saliva and form cohesive bolus. Necessary rheological experiments, in compression and shear of boluses, will be developed to provide data for validating simulations.
Modelling Polycrystalline Diamond Cutting Tools Failure
Researcher: Mahdieh Tajabadi Ebrahimi
Element Six (E6) is the world's leading manufacturer of synthetic diamond for hard abrasive materials. Synthetic diamonds are used throughout many industrial applications such as cutting, grinding, drilling, and polishing. Polycrystalline diamond (PCD) is one of the E6 products that is formed by sintering diamond powders in the presence of the metallic catalyst. PCDs fail by cracking of diamond crystals under extreme conditions. The theory of fracture, indicates that the dislocations, impurities, and any imperfections present inside the diamond grains can affect their mechanical properties. PCDs contain various imperfections that might cause failure by producing nano-cracks in the system. The objective of the project is to shed light on the mechanisms that lead to these failures, involving different scale. Techniques at different length scale will be used to characterise possible mechanisms of nano-cracks initiation and propagation into macro-cracks inside the system.
Modelling the Sealing Behaviour of Windscreen Wipers
Researcher: Qian Wang (Alexis)
The interaction between rubber wiper blades and vehicles’ windscreens is of great significance in car industries. As the interplay between mechanical, physical and chemical properties of the mating surfaces under various lubricating conditions are complex, a comprehensive model is needed to predict the behaviour of the blades and sealing provided by the wiper.
To this end, this project focuses on simulating the in-contact fluid film behaviour and predicting the sealing performance. Both the material nonlinearity and geometric nonlinearity will be considered to closely mimic the behaviour of wiper blades. The FSI (Fluid Solid Interaction) solver will be used to capture the friction in the transition from boundary lubrication to hydrodynamic lubrication.
Molecular Behaviour at Surfaces and Interfaces
Researcher: Dr James Ewen
Recent advances in eperiments and simulations at the micro/nanoscale have demonstrated that behaviour at these scales often governs macroscopic tribological phenomena. In particular, molecular simulations can now be used to accurately model the behaviour of lubricant and additve molecules inside tribological contacts. In this project, moleular simulations will be used to give unique insights into various important open questions in tribology; from the mechanohemical dissociation of ZDDP molecules to the friction and flow of fluid molecules under EHL conditions.
Origins of Micropitting in Gears
Supervisor: Dr Amir Kadiric
Micropitting is a form of rolling contact fatigue associated with hardened gears and rolling element bearings. It consists of tiny pits on the scale of 10s of microns which can cover large portions of the face of the gear and raceway of the bearings. These pits can lead to loss of tooth profile and provide an initiation point of other types of rolling contact fatigue such as spalling. The mechanism by which micropits are formed is not well understood and there is a real need to produce models to predicate the useable life of components which may suffer from it.
To better understand the mechanism behind micropitting, experiments are being undertaken on a PCS MicroPitting Rig (MPR). Preventative measures and ways to predict the life of gears after micropitting has occurred are also being investigated.
Premature Failures in Bearing Steels Associated with White Etching Cracks (WECs)
Researcher: Francesco Manieri
Supervisor: Dr Amir Kadiric
Premature failure of components is a significant problem in the energy and transport industries, particularly since energy requirements have become more ambitious and demanding. It is well known that components in gearboxes, especially at bearing location, tend to fail below the expected life. Generally, one tends to identify the problem of premature failure with a particular failure mode, i.e. white etching cracks (WECs) as they are very likely to appear in premature failures. A WEC is a crack accompanied by a microstructural change that appears white after etching. The aim of this project is to reproduce premature failures under controlled laboratory conditions, using a triple-contact rig, clarifying the relationship with WECs and identify their root causes.
Propagation of Surface Initiated Rolling Contact Fatigue Cracks in Bearing Steel
Researcher: Dr Pawel Rycerz
Supervisor: Dr Amir Kadiric, Professor Andrew Olver
Surface initiated rolling contact fatigue, leading to a surface failure known as pitting, is a life limiting failure mode in many modern machine elements, particularly rolling element bearings. Most research on rolling contact fatigue considers total life to pitting. Instead, this work studies the growth of rolling contact fatigue cracks before they develop into surface pits in an attempt to better understand crack propagation mechanisms.
A triple-contact disc machine will be used to perform pitting experiments on bearing steel samples under closely controlled contact conditions in mixed lubrication regime. Crack growth across the specimen surface will be monitored and crack propagation rates extracted. The morphology of the generated cracks will be observed by preparing sections of cracked specimens at the end of the test.
Scuffing in Non-Conformal Contacts
Short Crack Propagation of Surface Initiated Rolling Contact Fatigue Cracks in Bearing Steel
Researcher: Bjoern Kunzelman
Supervisor: Dr Amir Kadiric, Prof. Daniele Dini
Surface initiated rolling contact fatigue, leading to a surface failure known as pitting, is a life limiting failure mode in many modern machine elements, particularly rolling element bearings. Based on a prior study which investigated the crack propagation until pitting, this study focuses on short crack propagation. Short crack propagation induced by rolling contact fatigue is characterised by a relatively slow crack propagation and frequent crack arrests.
A triple-disc contact machine will be used to determine the major parameters which influence short crack growth (e.g. surface topography, fluid film properties, microstructure, etc.). Simple models shall then be derived and validated with the experiments.
Structure Breakdown During Oral Processing of Aerated Chocolates
Researcher: Dimitrios Bikos
Aerated chocolate products are popular consumer items associated with positive textural and sensory attributes. Among other microstructural features such as cocoa solids, sugar particles, etc. aerated chocolate consists of bubbles as well. Interaction between bubbles and particles occur during the structure build-up and structure breakdown process and therefore it needs to be investigated. Therefore, a multidisciplinary approach is taken to predict how microbubbles impact on the rheological, thermal and lubrication processes through progressive structure breakdown in the mouth. However, the effect of the aerated microstructure on the chocolate’s behavior during both industrial and oral processes is a very complex research field. During oral processing, food first fractures into particles, which interact with saliva to form a bolus, which is then swallowed. During this process, mechanical and thermal loads are applied whilst the effect of the contact with the oral cavity is also crucial in determining the consumer’s taste experience. Chocolate changes phase as it melts further complicating the behavior. However, the effect of aeration on industrial processing or manufacturing of chocolate cannot be neglected. Specifically, the effect of bubbles on the thermal properties of the chocolate is to be determined such that manufacturing can be controlled to ensure that chocolate will have the desirable textural and sensorial attributes.
Surface Deposition of Carbonaceous Materials
Researcher: Dr Sophie Campen
Supervisor: Dr Janet Wong
Fouling by carbonaceous deposits poses a serious and costly problem for the oil production industry. In upstream, midstream and downstream oil production, carbonaceous deposits frequently consist of asphaltene. Asphaltene is the densest, most polar fraction of crude oil and is generally stable in the reservoir. However, changes in environmental conditions, in particular pressure, but also temperature, shear rate and solvency of the crude oil base stock can lead to asphaltene being destabilised. Destabilisation of asphaltene, resulting in its precipitation from liquid crude is believed to be responsible for the formation of thick deposits that can completely plug the wellbore. Asphaltene is defined by its solubility: soluble in aromatic solvents like toluene, but insoluble in n-alkanes like heptane. Being a solubility class of compounds means that by definition, asphaltene is polydisperse. This makes cross-study comparisons challenging since crude oils from different sources possess different chemistries and hence display different deposition behaviours.
The aim of this project is to achieve a fundamental understanding of the mechanisms that govern asphaltene deposition. This will allow for better informed decisions on the most effective pathways to preventing fouling, for example through additive chemistry and smart surface coatings. Asphaltene deposition will be investigated experimentally using a quartz crystal microbalance. This technique allows us to measure in situ the deposited mass as a function of time.
Funded by BP, this study forms part of larger project within the International Centre for Advanced Materials and involves collaboration with partner members at Imperial College London, the University of Manchester, the University of Cambridge, and the University of Illinois at Urbana-Champaign.
Synovial Fluid Lubrication and Wear of Artificial Joints
Researcher: Harriet Stevenson
In 2015 there were over 180,000 primary hip and knee joint replacement procedures recorded in the United Kingdom. These devices are used to relive pain and restore function in degenerated joints caused by disease, trauma or genetic condition. Artificial joints are essentially tribological devices as the bearing surfaces articulate under load. As such they are susceptible to the usual tribology issues of high friction, wear, corrosion and fatigue and these problems can contribute to failure and revision.
Implant procedures are currently carried out for hips, knees, shoulder, elbows, ankles and spinal disks; the most common of which are hips and knees totalling 48 % and 49 % of all replacements recorded in the United Kingdom respectively. Whilst most implants remain fully functional nearly 10 % of hips required revision surgery in 2015. Prostheses are increasingly being implanted into younger patients and therefore the life expectancy and performance requirements are on the rise.
The aims of this study are to understand the fundamental lubrication mechanisms of synovial fluid (SF) and to characterise how friction and implant wear are related to SF chemistry. There is a limited amount of published work on the effects of SF chemistry on implant wear and most of this is limited to UHMWPE rather than CoCrMo with model or Bovine Calf Serum (BCS) fluids. One important aspect of this work is to include human SF in the research programme. There are very few studies on lubrication and wear with human SF, which is a significant omission to our understanding of the problem. At the start of the PhD project an opportunity arose to obtain human SF through collaboration with Dr Mathew Jaggard (Muscleoskeletal Research Laboratory). Bench testing of human SF and comparing the results to model formulations will contribute to our fundamental knowledge of the effect of chemistry on wear and the validity of using 25 % BCS as a reference fluid. The image shows metallic and organic deposits around a ball wear scar.
The Effect of Shear Stress on Lubricant Behaviour
Researcher: Stephen Jeffreys
The aim of the project is to investigate the effect of shear stress on lubricant behaviour, particularly in high-pressure high-shear environments such as those found in elastohydrodynamic (EHD) contacts. Here it is critical to gain an understanding of the rheological properties at a molecular level, considering the local structure of the lubricant. Given the severity of operating conditions lubricants can reveal unusual phenomena where the Newtonian assumption may be inadequate. An inaccurate description of the flow limits our understanding of lubricant rheology which affects the ability to theorize novel ways of controlling friction. This impacts the overall goal to manipulate the tribological performance of engineering systems and improve efficiency.
The Effects of Surface Texture in Reciprocating Bearings
Researcher: Dr Sorin-Cristian Vladescu
Supervisor: Dr Tom Reddyhoff
Sponsor: Ford Motor Company
The research project is conducted in collaboration with the Ford Motor Company, and evaluates the how textured surfaces, produced using Laser Surface Texturing (LST), can improve the tribological performances of an internal combustion engine components. Particular focus is on the reciprocating contact between the cylinder liner and piston rings, since this accounts for the approximately 4% of the overall fuel energy used.
To achieve this goal the project involves simultaneously measuring friction force and film thickness in a reciprocating contact using a test rig designed specifically for this purpose. A range of pocket configurations on the ring-liner pairing are investigated, in order to identify an optimum texture pattern suitable for actual piston ring conditions and also to shed light on the mechanisms that are occurring.
The Self-Assembly of Organic Friction Modifier Solutions
Researcher: Ben Fry
Organic friction modifiers (OFMs) are used to reduce friction in the boundary regime. This happens through self-assembled monolayers (SAMs) of the OFMs onto the surface. This project looks at the formation and properties of the SAMs in idealised systems and relate them to friction data to get a better understanding of the mechanism of the OFMs friction reducing properties.
The main techniques to observe the structure and growth of these monolayer are atomic force microscopy (AFM) and spectroscopic ellipsometry. With these combined techniques, a picture of how the monolayer is formed on the surface from a dilute solution can be created.
Tribofilm Properties of ZDDP-Containing Oils
Researcher: Mao Ueda
Supervisor: Professor Hugh Spikes
Zinc dialkyl dithiophosphate (ZDDP) is widely used as an anti-wear additive in engine oils. The tribofilms formed by ZDDP have been extensively investigated using friction and wear tests as well as surface analysis. However, the influence of ZDDP film properties on film durability and ultimately tribological performance remains unclear. The aim of this project is to uncover these relationships, as well as to investigate the effect of co-additives on ZDDP performance.
Tribological Modelling of Water-Lubricated Bearings for Nuclear Reactors
Researcher: Ruby McCarron
Cobalt alloys are widely used in engineering applications requiring resistance at high temperatures to both mechanical and electrochemical (corrosion) wear. Water lubricated rolling element bearings are a working component in nuclear reactors. The alloys which make the race and ball parts of these components are Cobalt (Co) based alloys, Haynes 25 and Stellite 20, each composed of approximately 50% Co.
In the reactor environment, Co is irradiated producing the isotope Cobalt 60 (Co-60). Co-60 is a highly penetrative gamma emitter with a relatively long half-life. As rolling wear manifests in the bearings, wear debris containing this radioactive isotope is transported in the reactor loop and deposited at various locations. This leads to Occupational Radiation Exposure to operational and maintenance personnel.
This project aims to investigate a viable Co-free replacement for one or both of these alloys. The alternative alloys must exhibit the same wear resisting performance as the Stellite and Haynes alloys in reactor conditions. To demonstrate the success of the alternative(s), a like for like comparison should be made with the wear behavior of the existing alloys. Current work focuses on sliding wear behavior of Haynes 25 and Stellite 20 using a ball on disc tribometer in an autoclave, simulating reactor conditions.
These tests will be repeated with a Co-free, Stellite 20 alternative which has been elected by Rolls-Royce as a possible replacement. The data from these tests will be used to measure, characterise and compare the wear behavior of the materials.
Rolling wear is also of interest to this project and the relevant materials will be tested in a rolling bearing arrangement, with experimental results once again compared.
Finite Element Analysis is being carried out in line with these experiments, simulating the contact interaction and predicting the material wear using experimentally obtained data.
Tribology of Calcium Complex Greases
Researcher: Rory McAllister
Electrification of the automotive sector is putting even more emphasis on low-friction bearing lubricants, and the burgeoning battery market has driven up the price of lithium, a raw material in >70% of lubricating greases. There is, therefore, a need to replace the standard lithium grease thickener with a cheaper, equally effective alternative.
This project aims to investigate the performance of novel calcium complex grease formulations provided by Shell. Rolling contact tests will be performed with both lubricant degradation and inlet starvation controlled to emulate realistic bearing conditions. Techniques such as infrared spectroscopy will be used to analyse the rolled tracks in order to understand the mechanisms of grease lubrication.
Understanding and Prevention of False Brinelling Failure Mode in Rolling Element Bearings
Researcher: Rachel Januszewski
Supervisor: Dr Amir Kadiric
False brinelling is a type of surface damage that most commonly occurs in non-conformal, nominally stationary contacts that are subjected to externally generated vibration. All machine elements that rely on non-conformal, rolling-sliding contacts in their operation can suffer from false brinelling, but it is most commonly observed in rolling element bearings, especially in stand-by equipment stored near running machines and in the transport of automotive vehicles by rail or sea.
False brinelling is a specific type of a more general contact damage mechanism of fretting, often referred to as fretting corrosion. The underlying mechanisms causing fretting and false brinelling are thought to be similar, but false brinelling in rolling bearings has an added complication that the oscillatory motion is not pure sliding but also involves rolling of rolling elements on bearing raceways.
The aim of the proposed research is firstly, to gain a better understanding of the factors that drive the onset and progression of false-brinelling damage and secondly, to provide potential preventative measures, be it through the improvements in bearing design or lubricant formulation.
Understanding the Effectiveness of Corrosion Inhibitors in Metal Working Fluids
Researcher: Dr Asad Jamal
Metalworking fluids (MWFs) are multicomponent fluids designed to fulfil simultaneously three functional properties i.e. provide lubrication, eliminate the effect of friction and heat and remove metal particles during metalworking processes. They are covering a broad range of additives, from straight oils (petroleum oils) over water-based fluids (soluble oils and semi-synthetic fluids) to synthetic MWFs. Depending on the presence of functional groups, some of the additives in MWFs are surface active and thus can be used as corrosion inhibitors (CIs) and also as friction modifiers (FMs). Though it is commonly believed that their effectiveness depend on their ability of forming homogeneous surface films however an in depth understanding of the basic mechanism is still a matter of debate.
The objective of this work is to explore the mechanism leading to corrosion inhibition ability of commonly used CIs in aqueous medium. The project is focused on establishing the relationship between structure, adsorption and film formation, corrosion inhibition ability of CIs in aqueous solutions. Additionally, the effectiveness of CIs as FMs are aimed to be examined.
Understanding Viscosity Modifier Additive Performance
Researcher: Eliane Gendreau
Viscosity modifiers are polymers added to lubricants to control viscosity. Effective viscosity modifiers are crucial in the automotive industry to facilitate the use of base oils with lower viscosity which can reduce hydrodynamic losses and thus increase fuel efficiency. The aim of the project is to investigate how the architecture of polymeric viscosity modifiers affects lubricant rheology. Techniques such as fluorescence spectroscopy will be used to study the effect of polymer architecture on thier temperature and shear responses inside hydrodynamic contacts.