Mechanochemistry in Lubrication

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.

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.