Friction: The Tribology Enigma

Friction plays a central role in life; in transport, in manufacturing, in process engineering, in medical devices and in everyday human activities. Friction has commanded the attention of Amontons, Coulomb and Da Vinci and their simplistic, empirical laws have been the cornerstone of friction theory. At the conceptual and theoretical levels the vast modern day friction literature has revealed the enormous complexity of even the simplest processes and the limitations of the early friction laws. Friction is intimately linked to both adhesion, contact geometry and wear and all require an appreciation of the highly non-equilibrium and non-linear processes occurring over multiple length scales. The challenge presented is that friction in realistic engineering contacts cannot be predicted. Understanding the physical and chemical processes at contacting interfaces is the only route to cracking the tribological enigma. The research gap addressed in this Programme Grant is linked to the development of accurate experimental and numerical simulations of friction. We appreciate that the search for a unified model for friction prediction is futile because friction is system dependent. However, the goal to predict friction is achievable. We have identified 4 key areas where there are current challenges in understanding the origins of friction because of different complexities as outlined below:
- Reactive surfaces; in many systems the frictional contact brings about chemical reactions that can only be described by non-equilibrium thermodynamics. We need accurate kinetic rate data for reactions which can only be provided by advanced in-situ chemical analysis
- Extreme interfaces; these can be described as any interfaces that are inducing high strain rate material deformation and combined with electrochemical or chemical reactions. Simulation and sensing are key to improving the understanding.
- Non-linear materials; in engineering and in biological systems we see the evolution of "soft" materials for tribological applications. Predicting friction in these systems relies on understanding the rheology/tribology interactions.
- Particles and 2nd phase materials; for materials processing or for understanding the transport of wear particles in a contact we need to understand particle-particle friction in complex contact conditions where fracture/deformation are occurring.

The Universities of Leeds and Sheffield have strong, sustained track records of engagement with users of their research and of commercialisation via licensing and spin-outs. The user groups are across sectors. As an example of the level of engagement by industry, in our shared Integrated Tribology CDT we have 18 industry sponsors all funding at £80K for the 4 year PhD.
We see the key objectives of our Pathways to Impact (PtI) being the following:
PtI1 - Efficient and timely dissemination of the low TRL research findings to ensure that exploitation of this by other funders (at higher TRL level) occurs; critical for the sustainability of the research area; our dissemination to industry strategy through the 6-monthly Network meetings will enable this to happen.
PtI2 - Exploiting the links between this major tribology programme and other related programmes; reaching out to other UK and overseas centres will be via our Innovation Fund (to create new links with UK Centres) and our International Network of collaborators.
PtI3 - Providing uniquely trained early career researchers who can go on to make substantial contributions in academia or industry; we have a strong programme for the PDRAs employed on the programme through Creativity@Home and employing experience from our other training networks (Marie Curie). In addition we have ECRs as Co-Is who will be exposed to the processes and challenges in a large programme of research
PtI4 - To demonstrate to the wider public that engineering science at this basic level will facilitate the advances of the future; we will engage with external agencies to capitalise on existing engineering outreach activities but will have our ECRs (Slatter and Bryant) implementing our outreach plan.
PtI 5 - Efficient commercialisation of research; we will use the Leeds and Sheffield infrastructure to ensure that we patent and commercialise as appropriate.
Advocacy; Tribology in 1966 was recognised by the then Minister of State for Education and Science, and recorded in Hansard , as a discipline that could have a massive impact on the economy. 50 years on and although the science and technology has evolved beyond recognition, tribology remains a lynch pin of efficient industry sectors. The Programme Grant therefore has to have an impact that will influence scientific policy in academia and industry.
To ensure advocacy we have the following in place (a) National academy committees; for UK through RAEng (Neville/Rainforth/Ghadiri are Fellows. Kapur/Neville sponsored Chairs) (b) Professional Institutions; we have membership in IMechE and IoP tribology committees (Morina (Vice Chair), Lewis (past Chair), Liskiewicz (Hon.Sec)). We have established links with IET UK Tribology through the current Chair (Dr Rich Baker) (c) Through published work; editorial roles for Neville, Dwyer-Joyce, Morina, Liskiewicz, Rainforth