Fundamental aspects of biomechanics, tribology, materials science and the biological environment in which joint replacements function have been outlined in earlier chapters. The fundamentals of tribology and corrosion are outlined and the significance of their synergy is emphasised. Application of these basic principles to joint replacements is complex, both in terms of modelling and simulation. Only steady-state elastohydrodynamic analysis of total hip joint replacements was available initially, but realistic, more comprehensive analyses for time-dependent situations are rapidly enhancing our understanding of tribological actions in such implants. The analysis is complex, since speeds, loads and lubricant properties all vary throughout each cycle. An introduction to corrosion is presented, together with the background to corrosion in metal-on-metal total hip replacements. Protective films formed naturally on metallic surfaces of implant components and tribo-films formed in the presence of proteins can both be disturbed by asperity contacts and wear debris. The total loss of material from metallic bearing components is clearly a combination of mechanical surface wear and tribo-corrosion, with indications that the latter can account for 0%–40% of the total material loss.

It is almost 50 years since theoretical work on elastohydrodynamic lubrication commenced in the Department of Mechanical Engineering of The University of Leeds. Details of the development of numerical solutions to the line contact problem during the 6-year period (1956–1962) that the authors worked together on the problem will be outlined. The computing aids available during the 18-month period involved in generating the first solution consisted of two hand-operated mechanical calculating machines, with the first digital computer at Leeds being installed in 1959. The general research environment during the period will be recalled and a number of significant events recorded. It is appropriate to record at this Symposium aspect of these initial developments in a subject that has dominated research in tribology throughout the latter part of the 20th century and into the early years of the 21st century. The excitement of being involved in taking some of the first steps in a field described by the late Professor F.T. Barwell (1970) as ‘The major event in the development of lubrication science since Reynolds's own paper’, will be recalled.

Following the high clinical failure rates of metal-on-metal total hip replacements much work has been undertaken to investigate their poor performance. So called adverse loading scenarios such as acetabular inclination and microseparation have been attributed to indicators for failure of the implants. The ISO hip simulation standards (ISO 14242:1) still rely on gravimetric and ex situ analysis, considering only the total wear during articulation. Live in situ sensing can provide valuable insight into the degradation mechanisms of metallic interfaces under such scenarios. Clinical 28 mm diameter metal-on-metal components were articulated in a full-ISO hip simulator. The bearings were subjected to increasing angles of acetabular inclination and retroversion over short-term periods of articulation. Corrosive degradation was monitored during sliding by means of an in situ three-electrode cell. Changing acetabular inclination from 30° to 50° resulted in greater cathodic shifts in OCP upon the initiation of sliding; from −50 mV to as much as −150 mV. Under anodic polarisation (0 mV vs. Ag/AgCl) the resultant currents at the initiation of sliding also increased significantly with inclination; from approximately 4–10 µA to over 120 µA. Increased retroversion of 20° also resulted in increased anodic currents of 55–60 µA. Changing the nature of articulation demonstrated increased corrosive material loss compared to a standard ISO 14242 profile. The sole use of gravimetric assessment to determine a wear rate for hip replacement bearings under simulation can therefore neglect important degradation mechanisms, such as tribocorrosive loss in devices with metal sliding interfaces.

Oral lubrication deals with one of the most intricate examples of biotribology, where surfaces under sliding conditions span from the hardest enamel to soft oral tissues in human physiology. Complexity further arises with surfaces being covered by an endogenous biolubricant saliva before exogenous food particles can wet, stick, or slip at the surfaces. In this review, we present a description of soft oral surfaces, comparing them with the recent approaches that have been used to study oral lubrication using in vitro to ex vivo setups. Specifically, lubrication behaviors of saliva and soft microgels are discussed highlighting instances of hydration lubrication. We have structured this information creating a strong link between theoretical concepts and oral lubrication, which has thus far remained elusive in literature. Finally, we highlighted some of the several challenges remaining in this field and discussing how emerging technologies in material science might help overcoming them.

Thermo-mixed-hydrodynamics of compression rings and big-end bearings are presented. Frictional losses under normal engine operating conditions for a gasoline engine and those with cylinder deactivation (CDA) are predicted. With CDA, the combustion chamber pressure increases in the active cylinders, whilst some residual pressure remains in the deactivated ones. For the former, the increased in-cylinder temperatures reduce viscous friction, whilst reducing the load carrying capacity, promoting increased boundary interactions. In deactivated cylinders, lower contact temperatures yield increased viscous friction. Overall, a 5% improvement in expended fuel is expected with the application of CDA. However, 10% of these gains are expended due to increased friction. The study demonstrates the need to consider total system effects when introducing new technologies such as CDA.

Infection, erosion of mucosal and uro-epithelial layers, tissue trauma and encrustation associated with catheterisation is still an issue faced by health-care professionals and patients – effecting patients subject to spinal cord injuries, chemotherapy and incontinence to name a few. Over the past decade efforts have been made to optimise catheter surfaces in an attempt to reduce the occurrence of trauma related complications during insertion. Organic and inorganic materials have been mooted as potential methods of reducing bio-film formation and increasing lubricity. The use of charged species has been further hypothesised as a potential method to reduce the occurrence of bio-film formation, reducing the need for therapeutic intervention. This study investigates the feasibility of functionalising silicone surfaces with charged ionic polymer brush technologies with the view to reduce urethral trauma and infection for indwelling and self-catheterisation devices. A simple three step synthesis route has been proposed, complimented by surface analysis and tribological assessment of the surfaces. The effects of initial monomer content on the functional outcomes of the surfaces has been investigated. Functionalisation of surfaces was seen to significantly reduce the hydrophobicity of the surfaces. A significant reduction in the coefficient of friction from μ = 0.4 to 0.005 for un-functionalised and functionalised surfaces, respectively, was seen. The speed dependence and effects of lubricant chemistry on the coefficient of friction have also been investigated.

In this article an arbitrary Lagrangian–Eulerian (ALE) formulation for modelling cavitation in elastohydrodynamic lubrication (EHL) is derived and applied to line contact geometry. The method is developed in order to locate the position of cavitation onset along the length of the contacting region which gives the transition from liquid to vapour in the fluid. The ALE is implemented by introducing a spatial frame of reference in which the solution is required and a material frame of reference in which the governing equations are solved. The spatial frame is moved from the material frame according to the error in the Neumann pressure gradient constraint required at the cavitation location when Dirichlet constraints are imposed for pressure in the liquid phase. Results are calculated under both steady-state and transient operating conditions using a multigrid solver. The solutions obtained are compared to established literature and conventional approaches to modelling cavitation which show that the ALE formulation is an alternative, straightforward and accurate means of implementing such conditions in EHL. This is achieved without the penalties associated with the numerical modelling of Heaviside functions or free boundaries.

The degradation of Metal-on-Metal (MoM) Total Hip Replacements (THRs) is a complex mix of tribological, corrosive phenomena and their synergistic processes. Previous links between the corrosion of these devices and their sliding conditions over a cycle have been observed in simulator studies instrumented with a three-electrode electrochemical cell. This study further quantifies that link; demonstrating clear repeating periodicity in the anodic current transients of a 28mm diameter MoM bearing under a standard ISO-14242 walking profile. A simplified 2D model and an expression of the Hamrock-Dowson equation was utilised to estimate the Theoretical Minimum Film Thickness (hmin) over a cycle, which was shown to match closely to the measured anodic current in both shape and magnitude.

The lubrication regime in which artificial hip joints operate adds complexity to the prediction of wear, as the joint operates in both the full fluid film regime-specifically the elastohydrodynamic lubrication (EHL) regime-and the mixed or boundary lubrication regimes, where contact between the bearing surfaces results in wear. In this work, a wear model is developed which considers lubrication for the first time via a transient EHL model of metal-on-metal hip replacements. This is a framework to investigate how the change in film thickness influences the wear, which is important to further investigation of the complex wear procedure, including tribocorrosion, in the lubricated hip implants. The wear model applied here is based on the work of Sharif et al. who adapted the Archard wear law by making the wear rate a function of a relative film thickness nominalized by surface roughness for examining wear of industrial gears. In this work, the gait cycle employed in hip simulator tests is computationally investigated and wear is predicted for two sizes of metal-on-metal total hip replacements. The wear results qualitatively predict the typical wear curve obtained from experimental hip simulator tests, with an initial "running-in period" before a lower wear rate is reached. The shape of the wear scar has been simulated on both the acetabular cup and the femoral head bearing surfaces. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 497-506, 2017.

Despite extensive research into alternative methods, the internal combustion engine is expected to remain as the primary source of vehicular propulsion for the foreseeable future. There are still significant opportunities for improving fuel efficiency, thus directly reducing the harmful emissions. Consequently, mitigation of thermal and frictional losses has gradually become a priority. The piston-cylinder system accounts for the major share of all the losses as well as emissions. Therefore, the need for an integrated approach, particularly of a predictive nature is essential. This paper addresses this issue, particularly the role of cylinder liner temperature, which affects both thermal and frictional performance of the piston cylinder system. The study focuses on the top compression ring whose critical sealing function Makes it a major source of frictional power loss and a critical component in guarding against further blow-by of harmful gasses. Such an integrated approach has not hitherto been reported in literature. The study shows that the cylinder liner temperature is critical in mitigating power loss as well as reducing Hydrocarbon (HC) and Nitrogen Oxide (NOx) emissions from the compression ring - cylinder liner conjunction. The results imply the existence of an optimum range for liner working temperature, independent of engine speed (at least in the studied cases) to minimise frictional losses. Combined with the study of NOx and HC emissions, the control of liner temperature can help to mitigate frictional power loss and reduce emissions. (C) 2017 The Author(s). Published by Elsevier Ltd.

Transmission efficiency is the main objective in the development of vehicular differential systems, comprising hypoid gear pairs. The overall aim is to contribute to improved vehicle fuel efficiency and thus levels of harmful emissions for modern desired eco-drive axles. Detailed predictive analysis plays an important role in this quest, particularly under realistic operating conditions, comprising high contact loads and shear rates. Under these conditions, the hypoid gear pairs are subject to mixed non-Newtonian thermo-elastohydrodynamic conditions, which is the approach undertaken in this paper. Such an approach for hypoid gear pair has not hitherto been reported in the literature.

This paper explores the contact of soft elastic and poroelastic bodies rotating under load and inspects the differences in load carrying capacity between the two types of material. Both materials have been widely used to describe the behaviour of biological systems such as articular cartilage in mammalian joints, however, we demonstrate here that there are fundamental differences between the responses generated in which the poroelastic response has an additional fluid contribution to the solid structural response. In order to produce the same load carrying capacity it is shown that the poroelastic material must deform more than an incompressible soft elastic material with the same stiffness and that the solid load generated by a poroelastic material is the same as a soft elastic material with a zero Poisson's ratio.

28mm Metal-on-Metal (MoM) and Metal-on-Ceramic (MoC) Total Hip Replacements were articulated to 1 million cycles under both Standard Gait and Microseparation conditions. The hip simulator was fully instrumented with a three-electrode electrochemical cell to facilitate monitoring of corrosive degradation. The estimated volume loss from corrosion at the bearing surface was seen to increase by nearly an order of magnitude for both devices, representing as much as 17% of total degradation. Anodic current transients also displayed near order of magnitude increases in the peak current for both bearing couples. An adverse loading scenario could cause as much as an order of magnitude increase in the metallic ions released into the joint capsule as well as an increased volume of wear debris.

Corrosion is not routinely considered in the assessment of the degradation or the lifetime of total hip replacement bearing surfaces. Biomechanical simulations are becoming ever more complex and are taking into account motion cycles that represent activities beyond a simple walking gait at 1 Hz, marking a departure from the standard ISO BS 14242. However, the degradation is still very often referred to as wear, even though the material loss occurs due to a combination of tribological and corrosion processes and their interactions. This article evaluates how, by incorporating real-time corrosion measurements in total hip replacement simulations, pre-clinical evaluations and research studies can both yield much more information and accelerate the process towards improved implants.

A modelling framework has recently been developed which considers tribochemistry in deterministic contact mechanics simulations in boundary lubrication. One of the capabilities of the model is predicting the evolution of surface roughness with respect to the effect of tribochemistry. The surface roughness affects the behaviour of tribologically loaded contacts and is therefore of great importance for designers of machine elements in order to predict various surface damage modes (e.g. micropitting or scuffing) and to design more efficient tribosystems. The contact model considers plastic deformation of the surfaces and employs a modified localized version of Archard’s wear equation at the asperity scale that accounts for the thickness of the tribofilm. The evolution of surface topography was calculated based on the model for a rolling/sliding contact and the predictions were validated against experimental results. The experiments were carried out using a Micropitting Rig (MPR) and the topography measurements were conducted using White Light Interferometry. Numerically, it is shown that growth of the ZDDP tribofilm on the contacting asperities affects the topography evolution of the surfaces. Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS) have been employed to confirm experimentally the presence of the tribofilm and its chemistry. The effects of the contact load and surface hardnesses on the evolution of surface topography have also been examined in the present work.

Water has long been recognised as a contaminant in lubricants. It can affect wear performance, especially in bearing systems, in complex ways. Water can also induce corrosion, which in turn can enhance wear. The individual parts of any tribocorrosion system are related to a complex mix of system parameters such as lubricant and additives, relative humidity and temperature. The effect of different water concentrations and different temperatures has been studied experimentally in this work. A modification to Archard׳s wear coefficient was applied with respect to the experimental measurements. The new wear model considering the effect of water was implemented into the previously-reported numerical model to develop a new semi-deterministic numerical wear model adapted to the tribo-corrosion system in this work.

A newly developed tribochemical model based on thermodynamics of interfaces and kinetics of tribochemical reactions is implemented in a contact mechanics simulation and the results are validated against experimental results. The model considers both mechanical and thermal activation of tribochemical reactions instead of former thermal activation theories. The model considers tribofilm removal and is able to capture the tribofilm behaviour during the experiment. The aim of this work is to implement tribochemistry into deterministic modelling of boundary lubrication and study the effect of tribofilms in reducing friction or wear. A new contact mechanics model considering normal and tangential forces in boundary lubrication is developed for two real rough steel surfaces. The model is developed for real tribological systems and is flexible to different laboratory experiments. Tribochemistry (e.g. tribofilm formation and removal) and also mechanical properties are considered in this model. The amount of wear is calculated using a modified Archard’s wear equation accounting for local tribofilm thickness and its mechanical properties. This model can be used for monitoring the tribofilm growth on rough surfaces and also the real time surface roughness as well as changes in the λ ratio. This model enables the observation of in-situ tribofilm thickness and surface coverage and helps in better understanding the real mechanisms of wear.