It is still impossible to predict between tribological interfaces what chemical reactions will occur and their rate. This presents a major barrier to being able to predict friction when reactions are occurring at the interface. Tribo- or mechano-activation processes, which are often not in equilibrium, are a subject of much debate. Crucially, frictional changes in sliding interfaces depend on the creation of reactions films, their removal, subsequent entrainment of species and wear. In order to dig deep into the reaction kinetics in an experimental framework, so that parameters such as x_tribo (Bulgaravich)⁶ can be validated, there is a need to be able to follow surface reactions in real-time. Synchrotron-based real-time tribometry will facilitate this for some specific systems in this PG. Project 1 will build on Proof-of-Concept work (Dorgham/Morina/Neville)⁷ to develop a methodology for measuring tribofilm chemical changes in real-time and in ambient (non-vacuum) conditions in collaboration with the beamline scientists (Mosselmans/Ignatyev) at Diamond. This project will optimise the in-situ tribometry for analysis of tribofilms formed from next generation low Sulphated Ash Phosphorous and Sulphur (SAPS) oils. The scientific questions to be answered relate to the kinetics of tribofilm build-up and the influence of organic oil additives on the S- and P-containing elements of the film and how water as a contaminant in oils affects the S and P-containing film chemistry and kinetics.

The tribology of hip joints has been studied since the early implants of Charnley, and Dowson (Leeds) played a pioneering role in the development of lubrication models for their bearing surfaces. More recently, Fisher et al⁸ has measured the wear resistance of current and new materials using realistic simulation conditions. Much of the early work, including that in Leeds, ignored the fact that implanted joints are in an environment where the lubricating fluid is saline, is at 37°C and can react with the nascent surfaces created from asperity contact. This is significant because (i) it means that corrosion (electrochemical charge transfer – tribocorrosion) occurs and changes the local environment and (ii) the released ions and subsequent reaction products have a large influence on the friction and wear evolution on the surface. Similarly, there are questions relating to the reactivity of ceramic systems and an analogous role that chemistry plays in the mechanical transformations. Although ceramics are generally considered to be inert, there is mounting evidence that chemical and structural changes induced by frictional contact result in important changes to the surface that can then influence subsequent performance. Early work showed that plastic deformation via dislocation flow at the surface of alumina ceramics is responsible for the time dependent wear transition and demonstrated that the surface grains behaved quite differently to that of the bulk. Moreover, it showed that a tribolayer forms on the surface of alumina much in the same way that is observed on metals. This layer is intimately integrated with the surface and must therefore have a strong influence on friction. Later, it was demonstrated the key role that oxygen vacancies play in the tribology of alumina ceramics. Remarkably, the lubricant (e.g. serum) can extract oxygen from the alumina, leaving a vacancy structure at the alumina surface. Also, some accentuation of degradation in polyethylene surfaces has been attributed to the synergy between wear and corrosion. Interestingly, the presence of Cr in Biolox Delta™ significantly reduces this oxygen loss, which may well be a hitherto unknown reason why that the material performs so well. These observations are new and suggest a radical new way of exploring the friction and wear of ceramics. Developing further understanding will require a combination of high resolution characterization techniques, both structural and spectroscopic. Based on work from Leeds and Sheffield we can challenge the notion that “bedding–in wear” is a purely mechanical process. In this challenge we specifically want to investigate the thin C-rich reaction film that is formed as a result of the surface/environment (protein/amino acids) interactions at an asperity level. The work of Hesketh et al⁹, and more recently Bryant et al¹⁰, has demonstrated that the “tribofilms” formed at metallic contacts have got components of the Co-material and the proteins but are not a simple stoichiometric compound mimicking the protein composition. The role of the denaturing process and how this links to the simultaneous release of Co ions into solution requires attention. In a similar manner there are questions relating to the reactivity of ceramic systems and an analogous role that chemistry plays in the mechanical transformations seen. In fact some accentuation of degradation in polyethylene surfaces has been attributed to the synergy between wear and corrosion. In this challenge we bring together expertise in tribocorrosion/tribochemistry of metals and ceramics (Neville/Rainforth) with expertise in C-base materials (Brydson) and organic molecules (Perrault/Armes). Understanding the surface structural changes is central to explaining the materials performance. We have extensive expertise in site-specific Focused Ion Beam (FIB) microscopy and subsequent high resolution TEM to explore the physical structure of surfaces. We will make use of the new cryo-FIB system (EP/P0122X/1). Of particular importance is the use of high-resolution Electron Energy Loss (EEL) data from key locations within the structure (e.g. surface reaction layers). The near-edge structure (ELNES) of an EEL spectrum can be used to determine local bonding and oxidation state of the atoms.