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.
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.
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
Film Formation and Friction in Grease-Lubricated Contacts with Focus on Low Speed Operation
Researcher: Yuta Kanazawa
Supervisor: Dr Amir Kadiric
This study presents the influence of grease formulation on friction and film thickness in non-conformal rolling-sliding contacts. Custom made greases are conducted in ball-on-disc tribometers under fully-flooded conditions and rolling bearings. Friction coefficient and film thickness are measured over a range of entrainment speeds, loads and temperatures. Specimens with variable surface roughnesses are used in order to cover a wide range of lambda ratios. To understand the fundamental behaviour of grease, additive-free greases (having similar ASTM worked penetration) and their base oils are used as test lubricants. The greases are lithium complex and diurea based in the same polyalphaolefins base oils. The base oils are also tested as a comparison.
The frictional behaviour of the greases is observed with following methods;
• Film thickness measurement in a single contact with EHD rig
• Friction coefficient measurement in a single contact with Mini Traction Machine (MTM)
• Film forming measurement in a full bearing with a bearing lubrication performance tester
• Friction torque measurement in a full bearing with a modified four ball machine
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.
Fuel-Delivered Friction Modifiers and Their Impact on Friction and Wear
Researcher: Joanna Dawczyk
Supervisor: Professor Hugh Spikes
Zinc dialkyldithiophosphates (ZDDPs) have been used as anti-wear additives for over 70 years. They are considered as the most efficient anti-wear additives. The tribofilm generated by the ZDDP is characterized by high boundary friction and it is known that organic friction modifiers can reduce this friction. Since both types of additives are employed together it is necessary to understand both the mechanism of the tribofilm formation and the mutual interaction between these additives. The scope of this PhD project is to study the interaction between the anti-wear film (generated by various types of ZDDP additives) and friction modifiers through the use of a range of lab analytical techniques including:
- Mini Traction Machine (MTM)
- Spacer Layer Imaging Method (SLIM)
- Atomic Force Microscopy (AFM)
- Focus Ion Beam Microscopy (FIB) followed by Ion Beam Erosion
- Scanning Auger Microscopy (SAM) followed by Ion Beam Erosion
- Scanning Electron Microscopy (SEM) followed by Ion Beam Erosion
- C13 Nuclear Magnetic Resonance (NMR) to determine the structure of Zinc/friction modifier complex
Fuel-Lubricant Interactions in the Combustion Chamber
Researcher: Jon Dench
The primary aim of this PhD is to develop a method to implement fluorescence spectroscopy, to a gasoline direct injection engine (GDI), to study the composition of the fuel and lubrication mixture in moving ring-pack area. This technique provides an excellent opportunity to determine not only the chemical properties of this mixture but possibly also its temperature and viscosity. In addition, the measurement of the liquid mixture film thickness is possible. Access will be made to the liner using optical fibres in a metal engine, thus ensuring typical engine operating conditions are achieved. Measurements with the fluorescence technique may be complimented with visualisation of the fuel spray in order to understand the physical mechanisms that determine the fuel-lubricant mixture composition on the liner.
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 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.
Influence of Steel Microstructure and Composition on the Formation and Effectiveness of Lubricant Boundary Layers
Researcher: Kostas Pagkalis
Supervisor: Dr Amir Kadiric
In the boundary lubrication regime, lubricant films are very thin so surface roughness asperities of the contacting bodies come into contact during rubbing. The load in boundary lubrication is supported mostly by contacting asperities as the local pressure exceeds what can be supported by the lubricant. Under such conditions, the protection for the contacting components can be provided by a boundary film, which is formed through the adsorption/reaction of various chemicals (additives) present in oil with the contacting solid surfaces. The formation of the boundary layers can obviously be affected by the contact conditions, but also by the composition and microstructure of the surface materials (in engineering components, this would usually be steel). Oil additives may form films preferentially with certain alloying elements and microstructures of steel. Although boundary lubrication has received significant attention in tribology research, this tends to be focused on the actual additives whereas the complete system i.e. the interaction between the steel composition and microstructure and the oil, has received little attention.
The current study will examine the influence of different steel compositions and microstructures on the formation and effectiveness of boundary films under concentrated contact and with a selection of different additives/lubricants. Different kinds of steel microstructures and compositions should be used to understand their influence with different additives and how they affect the boundary film formation. The aim of the project is to find which alloying elements with the existing lubricants are mostly beneficial to the boundary film formation.
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.
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.
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
Researcher: James Ewen
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.
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 Dynamics of Foaming and Antifoaming
Researcher: Li Shen
Foam dynamics can be summarised into four distinct stages, its formation, drainage, coarsening and eventual rupture. The aim of this project is to understand:
- The time-dependent dynamics of the foam structure subject to non-linear liquid drainage, rupture and the consequent structure rearrangement using multiphase numerical simulations
- The physical mechanisms involved in the formation of a large 3-dimensional foam structure due to rising bubbles (this comes from the industrial problem of foaming in lubricants)
- The coarsening phase of the foam structure exhibiting local fractal behaviour and macroscopic polyhedral packing (Weaire-Phelan structure) using both kinetic and topological models possibly leading to new theories and/or visualisations.
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.
Numerical and Experimental Fracture Mechanic Approach to Needle Insertion Crack Growth in Soft Tissue
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.
Origins of Micropitting in Rolling Element Bearings
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.
Origins of Ostheoarthritis
Researcher: Maryam Imani Masouleh
Supervisor: Professor Daniele Dini
Osteoarthritis (OA) is a common form of arthritis that causes joint degradation and affects up to 15% of the adult population. It is characterized by chronic and irreversible degeneration of articular cartilage (AC). Hemiarthoplasty is a surgical procedure, where the diseased (OA) cartilage on one side of the joint is replaced with an implant, while the other side remains intact. A key factor in determining the longevity of the implant is the friction properties of the material used as a counter-surface in contact with AC and their effect on the mechanical characteristics of the tissue.
The main aim of this study is to analyse the mechanical and frictional response of different shoulder humeral component materials against the natural glenoid. This has been developed as a two stage process. Initially, friction and wear properties of four different grades of human osteoarthritic AC were measured using pin-on-disc technique against three major types of implant materials used in hemiarthoplasty including Cobalt-Chromium alloy (Co-Cr), Ceramic (Al2O3) and Polycarbonate-urethane (PCU) polymer. The second stage of this study focuses on creating a model more anatomically realistic of the hemi-replaced shoulder joint and assesses the cartilage mechanical behaviour. A custom made joint simulator has been built and will be used to investigate the response of shoulder joints under representative loads. The glenoid will be tested against different humeral component materials to understand the friction/wear response of the cartilage. The correlation between mechanically/enzymatically damaged and healthy cartilage will be investigated. Histological analysis will be performed on the tissue to observe any structural changes due to wear. The results from this study can aid the surgeons to choose the best possible material for hemiarthoplasty according to the disease state of the patient.
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
Surface Crack Behaviour under Moving Contact Load: An XFEM Study
Researcher: Dr Yilun Xu
Supervisor: Dr Amir Kadiric
Sponsor: SKF, Marie Curie
Three-dimensional extended finite element method (XFEM) calculations that simulate penny shape surface-break crack behaviour under a rolling contact are conducted. The interaction between the surface-break crack and the moving contact is investigated under various working scenarios, including coefficient of friction, relative position of the crack cut and the contact size, etc. Stress intensity factors (SIFs) are evaluated along the crack front based on the stress field, which are further applied to estimate the propagation rate of the crack under a certain scenario. Numerical results illustrate the most vulnerable position of contact with respect to the crack that leads to a fast crack propagation. Besides, this research sheds light on the dependence of SIFs values along a crack front upon the parameters that define a rolling contact scenario. Relative slips between crack surfaces are also investigated under various rolling contact scenarios in this project.
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 Formation of ZDDP-Containing Oils
Researcher: Yasunori Shimizu
Supervisor: Professor Hugh Spikes
Sponsor: Idemitsu Kosan
Zinc dialkyl dithiophosphate (ZDDP) is widely used as an anti-wear additive in engine oil. Many researchers have studied its efficiency and reaction mechanisms by combining wear tests and surface analysis. The MTM-SLIM (Mini Traction Machine – Space Layer Imaging) is a useful method to investigate anti-wear performance and monitor ZDDP tribofilm growth in situ. These tests are mostly carried out in mixed sliding-rolling conditions, generally between 50 %SRR (slide roll ratio) and 100 %SRR. However engine oils are also required to show good anti-wear performance at higher SRRs such as in cam–tappet pairings and especially in pure sliding such as in the piston ring and cylinder liner assembly.
In this project, ZDDP tribofilm formations at higher SRRs, up to 230 %SRR, are being investigated and also a new approach using MTM-SLIM has been developed to monitor ZDDP tribofilm growth process in pure sliding conditions using a stationary steel ball on reciprocating or unidirectional rotating steel disc. Results show that ZDDP film forming behaviour in pure sliding conditions differs significantly from that one in mixed sliding-rolling conditions. In addition, since non-ferrous materials such as aluminium alloys are currently adopted on contact surfaces of engine parts, ZDDP tribofilm formation on Al alloys is also being investigated using this new technique.
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 Chocolate 1
Researcher: Dimitrios Bikos
Aerated chocolate products are popular consumer items associated with positive textural and sensorial attributes. At the same time, aeration can lead to associated reduced energy content which is important for fighting current obesity trends. However, the effect of the aerated microstructure on the chocolate’s behaviour during both industrial and oral processes is a very complex research area.
This project will determine the effect of aeration on the mechanical and thermal properties of chocolate. Tribology experiments and combined techniques will be developed to characterise structure breakdown. The results will also highlight the influence of food formulation on friction behaviour and structure breakdown.
Tribology of Chocolate 2
Tribology of Polyurea Greases
Researcher: Rory McAllister
The automotive industry is currently transitioning to electric vehicles, which require smaller, faster bearings than are typically found in an internal combustion engine. As a result, these bearings are at risk of overheating which can cause the grease that lubricates the bearing to thin, oxidise and degrade and eventually cause the bearing to fail.
This project aims to investigate the high-temperature performance of novel polyurea grease formulations provided by Shell. High-temperature rolling contact tests will be performed and techniques such as infrared spectroscopy will be used to analyse the degradation and tribological performance of the greases.
Tribology of Rolling Element Bearings
Researcher: Dr He Liang (Holly)
Supervisor: Dr Amir Kadiric
Sponsor: SKF, European Commision
This project uses a custom-made, model ball bearing rig to directly observe and measure lubricant films in the rolling track as well as EHL films in the ball-ring contacts at contact pressures and rotational speeds commensurate with those present in a real rolling bearing. Glass ring is used as the outer bearing race allowing full optical access to the EHL contact. Lubricant films in the rolling track and contact inlet are measured using fluorescence technique, while optical interferometry is utilized to measure thin EHL films in ball-ring contacts. The results are presented to illustrate the influence of multiple factors including entrainment speed and oil fill level on oil films in and around the contact.
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 Origin of 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.