The Dynamics of Nanolubricants within a Tribological Contact

This proposal concerns research into the functioning mechanisms and effectiveness of nanoadditives (NAs) in lubricated contacts. This particular application is chosen here because NAs, such as molybdenum disulphide, fullerenes and dispersed PTFE (Teflon), are being increasingly considered for use in formulation of advanced lubricants. Lubricant nanoadditives (NAs) are broadly defined as nanoparticles (NPs) that are deliberately added to a lubricant to obtain desired properties. Greener, more efficient lubricants can only be developed through thorough molecular understanding on lubricating mechanisms under realistic working conditions. Currently, mechanisms of how NP interacts with lubricant contacts are not known. Previous research either infers behaviour from indirect methods, or only correlates macroscopic responses, such as friction & wear, to macroscopic control variables such as load, speed, etc. The proposed work is aimed at radically change the research landscape by introducing methods to directly observe NPs' behaviour, in the nano-scale, in and around the lubricated contact.

The proposed work will focus on the dynamics and behaviour of NPs around and within tribological contacts, and their effectiveness as NAs. The overall aim of the proposed research is to understand the interactions between NPs, base oil and the rubbing surfaces and how these influence NPs' lubricating properties and the rheology of a formulated lubricant. The effect of substrate properties and NP surface properties, NPs' size and the relative velocity of the two rubbing surfaces will also be explored. The proposed research constitutes a vital step towards exploring and understanding the lubricating mechanism of NPs. Various researchers have hypothesised different lubricating mechanisms when NPs is added in lubricant and the proposed work provide results that could be used to directly verify simulation works. The findings will contribute to improved understanding of the fundamental mechanisms of lubrication and the role of NPs in forming boundary films. The research will also contribute to improved strategies for wear and friction reduction both in lubricant formulations and engineering components design.

The study will develop advanced in-contact analytical techniques to allow in situ observation of the dynamics of NPs under confined conditions. A test apparatus for this purpose will be constructed, and advanced, single particle/molecule level fluorescence techniques will be applied to track NPs in-situ in very thin film lubricated contacts under engineering conditions. This will allow correlation of macroscopic observables, such as frictional force and wear damage, to nanoscopic/molecule level behaviour, such as self-assembly and adsorption-desorption kinetics of nanoadditives. It should be noted that the experimental approaches that will be devised during the research programme are generic and will have applications to several other engineering fields such as multiphase flow and microfluidics.

The proposed work is aimed at introducing fluorescence imaging techniques, as a means to directly observe nanoparticle (NP) behaviour, at the nanoscale, in and around lubricated contacts. This capability augments traditional tribological research methods, which mostly depend on observations of macroscopic properties, and indirect deduction of the underlying mechanisms. Direct observation of NPs will provide definitive information about the mechanisms of how nano-additives (NAs) interact with lubricants. For example, in a proposed experiment, this method is going to be used to measure the motion of NPs and correlate this with the flow properties of specific lubricants.

1. Academic Impact

There are two areas of potential impact. First is methodological. Both tribologists and researchers in other areas will benefit because the fluorescence technique is general, and can be used to study NPs beyond just lubricant systems. The specific experimental platform and associated software methods being developed are versatile and should be applicable to areas such as, and not limited to, fluid flow, nanofluidics, adsorption-desorption kinetics, and molecular self-assembly. In fact, they should be useful to any research where nanometer scale phenomena are of interest, and where surface interactions dominate.

Secondly, the proposed studies will provide detailed information on the behaviour of NPs and other additives in lubricants. This can be used to understand the microscopic mechanisms that cause the macroscopic phenomena of friction and wear. Thus researchers in tribology, especially those interested in elastohydrodynamic lubrication, boundary lubrication, and additive interactions, will find such knowledge useful. The experimental quantities measured in the proposed experiments, for example, diffusion coefficients of NPs, will provide direct inputs to theoretical models and simulations of NAs and boundary lubrication.

2. Economic and societal impacts

The lubricant and additive industries invest a great deal of money and effort into additive and lubricant research and development. They should directly benefit from this research, as should lubricant users such as the automobile and machine component industries.

The proposed work will provide scientific information, as well as a new method to study the fundamental lubrication mechanisms by which nanoadditives act. This information will help develop more efficient lubricants, both in terms of lower friction and also reduced component wear. It is especially important at present to reduce friction and save energy so as to limit consumption of fossil fuels and thus CO2 emissions.

At the same time, the personal care industry should also benefit from the proposed work. Many personal care products, such as hand creams, lipsticks and toothpastes contain particles that can be tracked and observed directly by fluorescence techniques. The success of these products will partly depend on how these particles interact with human skin/teeth. Better understanding of relevant NPs can lead to better product designs.

Heat transfer is a major issue in enabling smaller and more powerful processors in the semi-conductor industry. Nano-fluid, a fluid that contains special nanoparticles, has been shown to have high potential in facilitating heat-transfer. The properties of nano-fluids that allow efficient heat transfer can be studied with the experimental platform developed.

Finally, the tribology group at Imperial College London is already collaborating with leaders in a wide range of industries including SKF, Infineum, Ford, Tata Motors and Unilever. These existing relationships will allow any scientific and methodological results from this proposal to disseminate to industry and create meaningful impact in an efficient manner.