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Molecular Dynamics Simulations of Lubricants and Additives

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Molecular Dynamics Simulations of Lubricants and Additives 1 Molecular Dynamics Simulations of Lubricants and Additives by James Patrick Ewen A thesis submitted to Imperial College London for the degree of Doctor of Philosophy and for the Diploma of Imperial College (D.I.C.) 2017 Tribology Group Department of Mechanical Engineering Imperial College London 2 Declaration of Originality This thesis describes work carried out in the Department of Mechanical Engineering at Imperial College London. I declare, to the best of my knowledge, that the work presented in this thesis is my own, except where acknowledged. James Ewen 12/09/2017 3 Copyright Declaration The copyright of this thesis rests with the author and is made available under a Creative Commons Attribution Non-Commercial No Derivatives licence. Researchers are free to copy, distribute or transmit the thesis on the condition that they attribute it, that they do not use it for commercial purposes and that they do not alter, transform or build upon it. For any reuse or redistribution, researchers must make clear to others the licence terms of this work 4 Abstract In many areas of science and technology, behaviour at the smallest scales has been shown to drive the performance of macroscopic systems. Such relationships are particularly pertinent in tribology, where key phenomena (e.g. friction and flow of lubricants) ultimately depend on atomic-scale interactions. Nonequilibrium molecular dynamics simulations can probe these scales and give unique insights into the tribological behaviour of complex molecular systems. In this thesis, several industrially important tribological systems are studied through nonequilibrium molecular dynamics simulations. Firstly, in order to ensure reliable results, potential models are compared in terms of their ability to reproduce realistic viscous behaviour of a model lubricant. These accurate models are then used to study the atomic-scale behaviour of various organic friction modifier additives under boundary lubrication conditions. The effect of molecular structure and surface coverage along with sliding velocity, pressure and surface roughness are investigated. The friction and wear reduction mechanisms of promising carbon nanoparticle additives are also examined. Finally, the effect of base oil molecular structure on friction and flow behaviour in the elastohydrodynamic lubrication regime is studied. The work has contributed to a more complete understanding of the atomic-scale behaviour of lubricants and additives and, in combination with experiments, has helped to explain several important macroscopic phenomena. 5 Acknowledgements I would like to thank the following people without whom this work would not have been possible. Firstly, my academic supervisors Prof. Daniele Dini and Prof. Hugh Spikes whose invaluable guidance, motivation, insights, time and support have made this work possible. My thanks are also expressed to all of the members of the Tribology Group at Imperial College. I have greatly enjoyed my time in the group and this is chiefly because I have been surrounded by such a diverse and fun group of people. I would also like to thank Chrissy Stevens for her assistance throughout the course of this project. I would also like to thank my collaborators, particularly Chiara Gattinoni from ETH Zurich who taught me the basics of molecular dynamics and was always happy to help with any more advanced problems. Also Prof. David Heyes whose unrivalled knowledge of the fundamentals of molecular dynamics simulation made my introduction to the subject far easier. Sebastian Echeverri of SKF and Sridhar Kumar Kannam and Prof. Billy Todd of Swinburne University of Technology whom I hope to continue to work with well into the future. Thanks are also expressed to my sponsors the Engineering and Physical Science Research Council (EPSRC) and Shell, who have provided the funding that made this research possible. I am greatful to Neal Morgan of Shell UK for his supervision throughout my PhD and Foram Thakkar, Indranil Rudra, Abhinav Verma, and Abhiroop Guha of Shell India, who kindly hosted me for three months which was invaluable both academically and for my personal development. Finally, and most importantly, I would like to thank my friends and family, and my partner India Martin for supporting me throughout the course of my postgraduate studies. 6 Contents Declaration of Originality ........................................................................................................................ 2 Copyright Declaration ............................................................................................................................. 3 Abstract ................................................................................................................................................... 4 Acknowledgements ................................................................................................................................. 5 List of Tables ......................................................................................................................................... 10 List of Figures ........................................................................................................................................ 11 Abbreviations, Acronyms and Terminology .......................................................................................... 17 Chapter 1. Introduction ........................................................................................................................ 19 1.1. Motivation ............................................................................................................................. 19 1.2. Chief Targets of the Project .................................................................................................. 20 1.3. Layout of the Thesis .............................................................................................................. 21 Chapter 2. A Review of Classical Molecular Dynamics Simulations of Lubricants and Additives ......... 23 2.1. Lubricant Composition and Function ......................................................................................... 23 2.1.1. Regimes of Lubrication ....................................................................................................... 23 2.1.2. Lubricant Formulations ....................................................................................................... 25 2.2. Simulation Techniques ............................................................................................................... 30 2.2.1. Simulation Scales ................................................................................................................ 31 2.2.2. Molecular Simulation .......................................................................................................... 32 2.3. History of NEMD Simulations ..................................................................................................... 36 2.4. Methodological Developments .................................................................................................. 39 2.4.1. Temperature and Pressure Control .................................................................................... 40 2.4.2. Classical Force-fields ........................................................................................................... 41 2.4.3. New Simulation Algorithms ................................................................................................ 42 2.4. NEMD Simulations of Lubricants ............................................................................................... 44 2.4.1. Bulk NEMD Simulations ...................................................................................................... 44 2.4.2. Confined NEMD Simulations ............................................................................................... 48 2.4.3. Summary ............................................................................................................................. 56 2.5. NEMD Simulations of Lubricant Additives ................................................................................. 57 2.5.1. Organic Friction Modifiers .................................................................................................. 58 2.5.2. Nanoparticle Friction Modifiers .......................................................................................... 63 2.5.3. Summary ............................................................................................................................. 67 2.6. Conclusions ................................................................................................................................ 68 Chapter 3. Benchmarking Classical Force-Fields for Density and Viscosity Prediction of n-Hexadecane .............................................................................................................................................................. 70 7 3.1. Introduction ............................................................................................................................... 70 3.1.1. Classical Force-Fields ........................................................................................................... 71 3.1.2. Non-Bonded Interactions .................................................................................................... 71 3.1.3. Bonded Interactions ...........................................................................................................
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