Mechanochemistry in Lubrication

Lead Research Organisation: Imperial College London
Department Name: Dept of Mechanical Engineering

Abstract

Improvements in lubricant technology are needed to reduce friction in machines and thus save energy and control global warming. Lubricants consist of a mineral or synthetic oil in which are dissolved up to ten or so chemical additives. The most important of these additives are friction and wear-reducing agents. These react with rubbing metal surfaces to form thin protective films that, as their names suggest, give low friction and wear. These films form only when surfaces rub together so they are often called "tribofilms".

Until recently we had very little idea of what caused tribofilms to form - was it the high temperature or pressure in rubbing contacts, or the metals becoming activated in some way by rubbing? This ignorance made it almost impossible to design additives except by trial and error or to build models their behaviour. However earlier this year it was shown conclusively that the most widely-used antiwear additive reacts in rubbing contacts because of the high shear forces present. These forces stretch the bonds in the molecules until they break, which leads to chemical reaction to form a tribofilm. This concept, of applied forces driving chemical reactions, is quite well known in modern chemistry and is called mechanochemistry. But this is the first time it has been shown indubitably to control tribofilm formation in the field of lubrication. It is very important insight since it points the way to us being able to predict how particular additive molecular structures will behave in rubbing contacts and thus design better additives to give lower friction and less wear.

The current project will explore the full significance of mechanochemistry to lubricant design and use. It will test which types of lubricant additive reaction are driven by shear forces and develop quantitative relations between reaction rate, applied shear force and temperature so as to enable modelling to proceed. It will look at a range of model antiwear additives with different but related structures to identify which bonds break to precipitate tribofilm formation - thereby enabling molecular structure to be optimised. It will also follow the reaction sequence that results from initial bond breaking to tribofilm formation by looking into rubbing contacts (with one transparent surface transparent) using chemical spectroscopy. All of this will be done in specially-designed test equipment that is able to reach the very high contact shear forces normally present in solid-solid rubbing contact conditions and that drive the chemical reactions involved.

The overall goal is to understand, for the first time and through the use of advanced experimental and modelling techniques, how lubricant additives react in rubbing contacts to form low friction and low wear films, and so to enable new and more energy-saving lubricants to be designed in future.

Planned Impact

Economic Impact

This research will, for the first time, show at a molecular level how lubricant additives react at a in rubbing contacts to form low friction and low wear films. This will have immediate impact on the ability of lubricant and lubricant additive companies to design new and improved products. Currently new additive molecular designs and lubricant formulations are identified based largely on heritage knowledge and trial and error. The proposed research will transform this to a more scientific approach involving virtual testing prior followed by informed experiment.
The UK has a strong and successful presence in the lubricant and lubricant additive industrial sectors. There are two major UK-based lubricant producers as well as smaller ones. The lubricant additive sector in particular is very highly dependent on technical research and innovation as reflected in their spending on R&D. These companies will thus benefit directly from the proposed research as exemplified by the fact that a major additive and a major lubricant company have agreed to contribute both in cash and materials to the project.
The economic impact will, however, extend beyond lubricant-related industries. Currently there is strong legislative pressure on equipment manufacturers to improve the energy efficiency of their products. One important route to improved fuel efficiency is via liquid and grease lubricants that give reduced friction. Thus machine component and vehicle manufacturers will be beneficiaries of greater understanding of lubricant additive behaviour, improved low friction lubricants and reliable models of thin film lubrication. This benefit is exemplified by the fact that the world's leading rolling bearing company has agreed to contribute financially towards to the project.


Societal Impact

An urgent challenge in mechanical engineering is to increase the efficiency of machine components and so reduce energy consumption. At a national level this is needed to meet CO2 emission limits, to help cope with rising fuel costs and to reduce dependence on imported energy supplies. In global terms, increased machine use in countries such as China and India as these develop can only be mitigated by large increases in machine efficiency. Such efficiency increases can be achieved by reducing friction and it has been estimated that in passenger car vehicles, trucks and buses this has the future potential to reduce CO2 emissions by 3% of the world's total. In practical terms such friction reduction requires the parallel introduction of low viscosity lubricants and improved friction and wear-reducing additives. Such additives can only be developed and optimised from improved understanding of how they work and how their structure controls their performance, as will be provided by the current proposed research. Thus the proposed research will impact beneficially on the quest to reduce energy consumption and thereby control global warming.
A second very important societal issue is the control and reduction of NOx and particulate emissions, in particular from transport vehicles and especially in urban environments. The EU has strong legislation in place to ensure such reductions but current limits are constrained by the effectiveness of vehicle after-treatment systems. One problem is that these are susceptible to contaminants from lubricating oils, especially from the breakdown of antiwear and friction modifier additives that enter the exhaust gas via the combustion chamber. There is currently great interest in developing less harmful antiwear and friction-reducing additives. By providing understanding of the relationship between additive molecular structure and performance the proposed project should greatly assist in this quest; in particular by suggesting new molecules "outside the box" of existing lubricant additive structures rather than relying, as at present, on incremental progress based on existing experience.

Publications

10 25 50
 
Description Lubricants contain chemical additives that are included in the formulation to react with rubbing surfaces and form "tribofilms". These films can reduce friction and protect the surfaces from damage such as wear, seizure and fatigue. They are becoming increasingly important due to the need to make machines more energy-efficient and thus reduce greenhouse gas emissions. For many years, there was debate about what caused these additives to react with surfaces during rubbing; was it the high temperatures reached as surfaces rub together; catalysis by the metal surfaces themselves; or electrons emitted during metal formation? From about 2015, based on the growth of a new field of research in chemistry known as "mechanochemistry", it started to be suggested that lubricant additive reactions might simply be driven by the very large mechanical forces present in rubbing contacts. These forces might stretch and bend molecular bonds until they break to form reactive intermediates.

Our project aimed to show that the chemical reactions that form tribofilms are, indeed, largely driven by mechanical forces. We have proved categorically that the reactions of antiwear additives and one important class of friction modifier additive are driven by mechanical forces. By studying different molecular structures, we have been able to work out how these reactions occur and to quantify the effects of the various processes involved at the molecular scale. In parallel, we have developed and applied molecular modelling tools to study the impact of mechanical forces on lubricant additive reactions using high performance computing.

The insight that lubricant additives react in rubbing contacts in response to applied forces, coupled with the structure-performance relationships identified, should help lubricant and additive companies in the design of new additives to give optimal friction and wear performance. This is particularly pertinent to new applications, such as wind turbines and electric vehicle transmissions, where specifications are yet to be fully agreed. The demonstration that these behaviours can be modelled, together with the molecular modelling tools developed, will enable rapid screening by additive companies of new additive types, spanning a much wider range that would be possible if all of them had to be synthesised and experimentally tested. The ability to predict tribofilm formation rates is also a key contribution to the development of more accurate larger-scale models of the friction and wear of machine components that are being developed both in academia and by lubricant and additive companies.

An unplanned outcome of the research is the finding that even fuels and lubricants without any additives can form very thin carbon-based films on rubbing surfaces on steel surfaces. This is because the very high forces present in the contact can break strong carbon-hydrogen and carbon-carbon bonds in the lubricant molecules and allow the fragments to reform with a graphene-like structure. These carbon films give very low friction and wear. They are not seen in air, which oxidises any carbon intermediates, but only in an inert gas atmosphere such as nitrogen. This means that some lubricated contacts that operate in a nitrogen atmosphere perform better than those that operate in air. This has promising potential applications for aircraft and spacecraft.

For this work, we have had to develop new instruments that are able to rub surfaces together at very high pressure and also to chemically analyse the rubbing surfaces in situ within a test rig. We have also modified several types of test equipment that rub surfaces together so that these can operate in controlled atmospheres. From a computational perspective, we have pushed molecular simulations to new boundaries such that they are useful tools in mechanochemistry research.
Exploitation Route The insights and findings of this project enable the whole process by which lubricants form protective films to be modelled realistically based on proven reaction kinetics. This make it possible in future to develop new and better additives largely using computers - requiring much less expensive and time-consuming experimental synthesis and testing to be carried out.

The discovery that protective, friction and wear-reducing carbon films can form on rubbed surfaces in inert gas conditions may promote the use of nitrogen blankets in high temperature lubricated systems such as aircraft and EV transmissions, where lubricant oxidation is an issue. To investigate this further will require further work on additive-containing lubricants.
Sectors Aerospace, Defence and Marine,Chemicals,Energy,Transport

 
Description The role that mechanical forces can have in driving chemical reactions, i.e. mechanochemistry, has only been widely recognised in the last decade. However, it has very rapidly become an important and growing area of research and application in the fields of chemistry and biochemistry; for example, in synthesising new chemicals, and understanding how stem cells become differentiated as a foetus develops. Clearly, very strong mechanical forces are present in lubricated rubbing contacts, and over the last decade it has become increasingly evident that these forces are important drivers of most (if not all) tribochemical reactions. This project aimed to confirm this conclusively and to quantify, at a molecular level, the impact of mechanochemistry on lubricant additive behaviour. The first task was to develop a test rig that provided very large and controlled mechanical forces in rolling-sliding contacts, to enable sufficiently high stresses for mechanochemical processes to take place using industrially important substrates, i.e. steel and diamond-like-carbon. This involved the development of a new tribometer, the ETM, with PCS Instruments. Using a range of additives specially synthesised for the project by Afton Chemical, this tribometer was then used to show unambiguously that shear stress is a key driver of the formation of protective tribofilms by antiwear additives and some friction modifiers. The main impacts of this were (i) proof of the mechanochemical origin of the tribofilm formation reactions of the various lubricant additives; (ii) correction of a previous, erroneous belief that pressure (as opposed to shear stress) was the most important driver; (iii) proven kinetic equations for additive reaction rates as a function of shear stress and temperature for use in macroscale mixed lubrication modelling; (iv) development of a new high pressure test rig that has now been marketed by PCS and is being used internationally by companies and academics to develop lubricant formulations to prevent scuffing and reduce friction and wear; and (v) improved understanding of molecular structure-performance relationships for lubricant additives under mechanochemical conditions. The second phase of the study involved the development of a method of monitoring the chemical composition of tribofilms on surfaces during rubbing. A new Raman spectrometer was designed and built that could be coupled into tribometers, including the ETM. This was used to quantify the change in the kinetics of friction modifier film formation due to mechanochemical effects. It has also been used in a study with BASF, that resulted from this project, of the mechanochemical processes involved in ball milling. It is currently being employed to demonstrate the mechanochemical formation of carbon-based films under inert atmospheric conditions. Its main impact to date is to provide a method of monitoring the chemical evolution of additive films on surfaces during rubbing, or in response to changes of conditions, and thereby to consolidate our understanding of mechanochemistry and lubricant behaviour. In parallel with the above, the project also applied a range of molecular modelling tools to study the impact of mechanical forces on lubricant additives, focussing on the additive types that have also been investigated experimentally. This has revealed the impact of molecular structure on the mechanochemical processes and has largely been validated by the experimental work. The key impact of this is that it can now be used to explore new chemical structures in silico, without the need for extensive synthesis and experimental testing. This could enable the realisation of high-throughput screening and even autonomous molecular design of new additive structures with optimized performance. It also enabled the Tribology Group to develop world-leading expertise in reactive force field (ReaxFF) molecular dynamics simulations to tribology problems, which have since been applied in a range of other projects with Shell and Baker Hughes. The main intended focus of this project was to study the response of antiwear and friction-reducing lubricant additives to the forces present in rubbing contacts. However, towards the end of the project we also discovered a new and potentially very significant mechanochemical process, the formation from lubricants of very thin, protective carbon films on rubbing steel surfaces. These graphene-like films are only formed under inert gas conditions, such as a nitrogen atmosphere. This finding is changing a general assumption of the tribology community, which is that lubricants are generally less effective in inert gas conditions. The impact of this work is not yet clear but since nitrogen gas is now readily available using lightweight filter systems, this opens the possibility of operating some high temperature lubricated components, such as those found in aircraft and spacecraft, in an inert atmosphere, with very large potential increases in energy efficiency and useful lubricant life. We are still in the process of disseminating this concept. It is important to note that this project has played a major role in enabling appreciation of the significance of mechanochemistry become embedded in the outlook of tribology professionals, both in academia and industry. During the life of this project, a community of academic tribologists specialising in mechanochemical effects has nucleated, based largely in continental Europe and the US, and this project has ensured that the UK has been powerfully represented at the resulting meetings and debates. This community has forged strong links with mechanochemical research within chemistry as a whole; for example the PI has presented and participated at an American Chemical Society (ACS) meeting on mechanochemistry. Because our Tribology Group at Imperial College has particularly strong links with industry, we have played the major role in informing lubricant and additive companies (e.g. Shell, BP, ExxonMobil, Afton Chemical, Lubrizol, and Croda) about the importance of mechanochemistry in lubricant design and this is starting to modify some of their paradigms. A very important feature of this project is that by combining, almost uniquely, both experimental and chemical modelling activities it has enabled experimental validation of the modelling tools used and these tools are now being effectively transferred to industry for additive development. Based on the above, the main impact of our work to date has been to provide to the tribology community as a whole with definitive proof that the mechanical forces present in rubbing contacts drive the formation of protective chemical films - generally known as tribofilms. This has changed the mindset of both academic researchers and industries concerned with lubricant design and performance. This change will almost certainly result in the development of improved lubricants and lubrication practices, with consequent energy saving and reduction of emissions.
First Year Of Impact 2021
Sector Chemicals,Transport
Impact Types Economic

 
Description Ball Milling Processes
Amount £220,000 (GBP)
Organisation BASF 
Sector Private
Country Germany
Start 02/2020 
End 01/2021
 
Description Controlling Friction Through Molecular Engineering
Amount £500,000 (GBP)
Organisation Royal Academy of Engineering 
Sector Charity/Non Profit
Country United Kingdom
Start 09/2020 
End 09/2025
 
Description INFUSE: Interface with the Future - Underpinning Science to Support the Energy transition
Amount £4,191,430 (GBP)
Funding ID EP/V038044/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 03/2021 
End 02/2026
 
Title Atmophere-controlled HFRR 
Description A new research tool to investigate lubricity and wear resistance of lubricants in controlled atmosphere, i.e. inert gas such as nitrogen and argon, controlled oxygen level... 
Type Of Material Improvements to research infrastructure 
Year Produced 2021 
Provided To Others? Yes  
Impact Due to mechanochemical reaction of base oil at asperity contacts, a carbon-based boundary film can form on rubbing surfaces to generate low friction and wear. This film is either unable to form or is oxidised in higher oxygen content atmospheres, and thus leads to high friction and wear. 
 
Title Extreme traction machine 
Description A very high pressure machine to measure fluid friction at very high pressure and to study lubricant behaviour at up to 7 GPa was commissioned and has been validated 
Type Of Material Improvements to research infrastructure 
Year Produced 2018 
Provided To Others? Yes  
Impact EHD traction data at < 3 GPa needed for predicting performance of rolling bearings and traction drives. Demonstration that stress derived antiwear additive reaction in steel/steel contacts 
 
Title In-lubro MTM Raman 
Description A new research tool to directly interrogate the chemical composition of the tribofilm on a rubbed surface formed and immersed in lubricant 
Type Of Material Improvements to research infrastructure 
Year Produced 2021 
Provided To Others? Yes  
Impact Elcidation of the kinetics of MoDTC film formation and fricion reduction and the competing formation of carbon-based and Mo-based film material. 
 
Description Baker Hughes - PhD project 
Organisation Baker Hughes
Country United States 
Sector Private 
PI Contribution Using molecular simulations to improve understanding of diamond-rock friction, wear, and mechanochemistry
Collaborator Contribution PhD funding and tribometer experiments on rock surfaces
Impact Collaborative paper on diamond-rock friction: https://pubs.acs.org/doi/abs/10.1021/acs.jpcc.1c02857
Start Year 2019
 
Description Ball Milling Processes 
Organisation BASF
Department Advanced Materials and Systems Research
Country Germany 
Sector Private 
PI Contribution Experimental study of mechanochemistry related to ball milling.
Collaborator Contribution Funding for PDRA and samples
Impact Report submitted to BASF. Multi-disciplinary including chemistry, mechanical engineering and materials
Start Year 2020
 
Description PCS instruments - support for mechanochemistry 
Organisation PCS Instruments
Country United Kingdom 
Sector Private 
PI Contribution We have commissioned and used a new test rig that PCS developed to our request
Collaborator Contribution Design , manufacture and free y of new test rig to our request Provision of a novel test rig able to operate in controlled atmosphere
Impact New research on tribology presented at six international conferences
Start Year 2017
 
Description SKF - support for mechanochemistry 
Organisation SKF
Department S.K.F. Engineering & Research Services B.V
Country Netherlands 
Sector Private 
PI Contribution Research on mechanochemstry
Collaborator Contribution £20000 cash plus use of SKF test equipment
Impact Presentations at 6 international conferences
Start Year 2017
 
Description Shell Global Solutions UK Renewal 
Organisation Shell Global Solutions International BV
Department Shell Global Solutions UK
Country Netherlands 
Sector Private 
PI Contribution Renewal of Shell GS UTC in Fuels and Lubricants at Imperial College 2013
Collaborator Contribution Renewal of Shell GS UTC in Fuels and Lubricants at Imperial College 2018
Impact Establishment of Shell GS UTC in Fuels and Lubricants at Imperial College 2013, seven PhD projects and five PDRA projects Renewal in 2108 for a further four years - four PhD students and one PDRA project to date
Start Year 2017
 
Description afton chemicals- support for mechanochemistry 
Organisation Afton Chemical
Country United States 
Sector Private 
PI Contribution We are carrying out research on new chemicals that Afton have synthesised to our request
Collaborator Contribution Afton Chemicals have synthesised new molecules to our design to help us understand structure-property relations of lubricant additives at a fundamental level
Impact Presentations and posters at five international conferences
Start Year 2017
 
Description ITC_20190918 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact Multiple presentation in one of the largest tribology conference, showcasing our research to both practioners, students and researchers. All presentations drew large crowds and lively discussions follows
Year(s) Of Engagement Activity 2019
 
Description Mechanochemical Behaviour of ZDDP, presentation to WTC 2017 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Study participants or study members
Results and Impact Presentation to scientists and engineers at the 6th World Tribology Congress (WTC), Beijing, 2017
Year(s) Of Engagement Activity 2017
 
Description Mechanochemical film formation by ZDDP lecture at Asiatrib Conference 2018 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Study participants or study members
Results and Impact Lecture to 6th Asia International Conference on Tribology, Kuching, September 2018. Described importance of mechanochemistry in determining ZDDP behaviour.
Year(s) Of Engagement Activity 2018
 
Description Mechanochemistry in high shear stress, full film EHD conditions 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Study participants or study members
Results and Impact Lecture at STLE conference in Minneapolis, May 2018 to mainly US but also international researchers/business attendees
Year(s) Of Engagement Activity 2018
 
Description Organised Web Seminar Series on Tribology 2020 and 2021 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact Organised several webinars with world-leading experts in tribology. Streamed on YouTube to an audience in the thousands (https://www.youtube.com/channel/UCN0JlzEr-iPAcWNb2JBnwMw). Funding and support obtained from IMechE, RSC, IOP, and CECAM.
Year(s) Of Engagement Activity 2020,2021
URL https://wesstribo.com/
 
Description Poster presentation to 2018 STLE Tribology Frontiers Conference 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Study participants or study members
Results and Impact Poster presentation at 2018 STLE Tribology Frontiers Conference
Year(s) Of Engagement Activity 2018
 
Description STLE conference_Nashville_20190520 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact Multiple presentations were done in STLE meeting (one of the largest professional lubrication engineering conference). They are have attracted large crowds and smaller groups discussion were held on various topics of our research.
Year(s) Of Engagement Activity 2019
 
Description Stress-augmented thermal activation - a new paradigm in Tribology 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Industry/Business
Results and Impact Invited lecture at New Challenges in Tribology meeting organised by IET, Birmingham March 2018
Year(s) Of Engagement Activity 2018
URL https://pcs-instruments.com/iet-tribology-event-success-2018/
 
Description Talk on mechanochemical carbon film formation at 24th Wear of Materials conference 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Presented new research showing for the first time that when surfaces are rubbed together in a hydrocarbon-based lubricant in a nitrogen atmosphere, a friction and wear-reducing carbon film is formed on the surfaces very rapidly by a mechanochemical process. This overturns a long-held (90 year old) model of lubricated oxidational wear and opens the possibility, coupled with nitrogen filters, of operating high temperature lubricated components such as aircraft transmissions in nitrogen gas rather than air.
Year(s) Of Engagement Activity 2021
URL https://www.wearofmaterialsconference.com
 
Description presentation of research and general discussion on this at Shell Research Centre, Houston USA 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Industry/Business
Results and Impact 40 to 50 Shell technologists attended talk by me on how mechanochemistry underpins lubricant additive behaviour. Great interest and disucssion
Year(s) Of Engagement Activity 2018