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 It has been proved unambiguously for the first time that the formation of protective lubricant films by the main additives used to control wear in engine oils is driven by the very high stresses present in lubricated contacts. This enables useful models of protective film formation by these additives to be developed based on additive molecular structures. It also provides icrucial information to the many researchers who are trying to apply molecular modelling to antiwear additive behaviour

The ability to measure film formation in full fluid film conditions, where there is no parallel film removal process due to abrasion, provides accurate kinetics of film formation for the first time. Again this is crucial in model development

It has been found that the boundary friction of the main class of antiwear additives used in engine lubricants is critically dependent on the hydrocarbon groups present in the additives' molecules. This enables informed design of new, energy-efficient additives.

The development of a tribometer able to measure friction in very high pressure conditions has enabled the first detailed study of how fluid film friction in such contacts depends on base oil Group number and thus molecular structure. This has enables selection and design of base oils able to deliver energy efficient lubricants for transmission systems.
Exploitation Route The availability and commercialisation of the high pressure roling-sliding test rig developed in this project opens up many new possibilities to study and also to test phenomena such as EHD friction, scuffing, friction modifier behaviour at high pressures, as well as mechanochemistry.

Accurate models of antiwear film formation have been made possible from his study and are very likely to be developed by others

The insights concerning the mecahnochemical origins of additive behaviour are likely to be used by addiitve comanies to develop improved additives that "see" appliecontact d stresses effectively as needed.
Sectors Aerospace, Defence and Marine,Chemicals,Energy,Transport

 
Description Ball Milling Processes
Amount £22,000 (GBP)
Organisation BASF 
Sector Private
Country Germany
Start 02/2020 
End 01/2021
 
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 
 
Description Ball Milling Processes 
Organisation BASF
Department Advanced Materials and Systems Research
Country Germany 
Sector Private 
PI Contribution Study of mechanochemistry related to ball milling.
Collaborator Contribution Funding for PDRA and samples
Impact No outputs as yet. 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
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 Invited Mechanochemistry talk at RCS Chemical Challenges in Tribology symposium 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Study participants or study members
Results and Impact Talk to symposium on new developments in Tribology research organised by RCS - caused strong interest
Year(s) Of Engagement Activity 2017
 
Description Mechanochemical behaviour of ZDDP lecture to STLE Annual conference 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Industry/Business
Results and Impact Lecture to scientists and engineers at STLE Annual Meeting, Atlanta, 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.
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 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
 
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