Atomic Force Microscopy to Evaluate the Effect of Concentration And
Total Page：16
File Type：pdf, Size：1020Kb
Atomic Force Microscopy to evaluate the effect of concentration and reciprocal position on ZDDP tribofilm topography and an assessment of its ability to determine tribofilm durability Rachel Frances Bingley Cunningham Submitted in accordance with the requirements for the degree of Doctor of Philosophy The University of Leeds Institute of Functional Surfaces School of Mechanical Engineering Faculty of Engineering June 2020 i The candidate confirms that the work submitted is her own and that appropriate credit has been given where reference has been made to the work of others. This copy has been supplied on the understanding that it is copyrighted material and that no quotation from the thesis may be published without proper acknowledgement. The right of Rachel Frances Bingley Cunningham to be identified as the Author of this work has been asserted by Rachel Frances Bingley Cunningham in accordance with the Copyright, Designs and Patents Act 1998 ii iii Acknowledgements During my research I have received support and input from numerous places. I would like to take the opportunity to thank some of these people now. Firstly, I would like to thank Prof. Anne Neville, for guiding me through what was sometimes a confusing transition to tribological research. I am aware that I was often difficult to wrangle, and I appreciate the determined effort with which you kept me on track. I would also like to thank my other supervisors, Prof. A. Morina and Dr. C. Wang, both of whom challenged and engaged me, even when the research seemed to be making no real progress. I’d like to thank the entire CDT community, my friends at the university, and particularly Kimberley Matthews, who absorbed a number of the administrative headaches which usually eat up researchers’ time, allowing me to make progress. Thank you, Kim! My thanks also go to my colleagues at ESR Technology, whose support and encouragement enabled me to face the final uphill battle to completion. Thank you to my parents, sister and all our family. You have tried hard to keep me sane, and managed to convince me that throwing the towel in was not an acceptable response to extreme stress, and that I wouldn’t be able to live with myself if I didn’t complete the “darn book”. You were, as usual, completely right. Last, but definitely not least, I would like to thank my wonderful husband, Oscar, for his ongoing and unfailing support. He has kept me going, particularly when the going got extremely tough, and he never let me face the terror of the write up alone. He was by my side; feeding me, keeping me in clean clothes, comforting me in moments of extreme self- doubt, acting as a sounding board (particularly when de-bugging annoying MATLAB codes!) and generally keeping our lives moving whilst I panicked my way to the end of the road. Without you this PhD would not have happened. Thank you Darling! iv v Abstract ZDDP has been used in most vehicles since the early 1950s; however, it has been proven to reduce the efficiency of catalytic converters. The incoming Euro 6 legislation introduces a stricter restriction on harmful emissions, increasing the importance of achieving maximum efficiency from catalytic converters. Therefore, it is necessary to remove ZDDP from cars. However, ZDDP is an effective anti-wear additive, without which modern cars, with tight design tolerances, are unable to function. Previous work has shown that the morphology of ZDDP tribofilms can relate to their anti-wear efficacy; however, no previous work has undertaken a robust characterisation, considering both the topography and durability, of ZDDP tribofilms. This research activity has shown that the concentration of ZDDP within a base oil affects the resultant pad topography in the tribofilm, with higher concentrations resulting in smaller but more evenly distributed pads. It has also shown that for lower concentrations the position along a reciprocating stroke influences the topography, with regions that experience lower lambda ratios displaying topography similar to that seen for higher concentrations. This effect is understood to occur as a result of increased shearing of the ZDDP resulting in the adherence of more, but shorter, polyphosphate chains to the surface and a hypothesis for how this occurs is presented. This thesis also demonstrates the potential of AFM for use in assessing the durability of ZDDP tribofilms. It has been shown that the AFM can be used to conduct a semi- quantitative assessment, with the potential for further improvements to be developed. It has also been shown that there can be an effect of the ZDDP concentration used to form a ZDDP tribofilm on its durability. The upper layer of the ZDDP tribofilm can be easily manipulated by the AFM tip, and two hypotheses are presented to explain this phenomenon. vi vii Table of Contents Acknowledgements ....................................................................................................... iii Abstract ......................................................................................................................... v Table of Contents ......................................................................................................... vii Table of Figures ............................................................................................................. xi Table of Tables .............................................................................................................xix List of Abbreviations .................................................................................................... xx Chapter 1: Introduction ............................................................................................. 1 1.1 Rationale for Research ............................................................................................. 1 1.2 Aims and Objectives ................................................................................................. 4 1.2.1 Phase 1 ............................................................................................................. 4 1.2.2 Phase 2 ............................................................................................................. 5 1.3 Report Outline.......................................................................................................... 6 Chapter 2: Fundamentals of Tribology ....................................................................... 7 2.1 What is Tribology? ................................................................................................... 7 2.2 The Mechanics of Friction ........................................................................................ 8 2.2.1 Adhesion .......................................................................................................... 8 2.2.2 Ploughing ....................................................................................................... 10 2.3 Wear ....................................................................................................................... 11 2.3.1 Adhesive ......................................................................................................... 11 2.3.2 Abrasive .......................................................................................................... 11 2.3.3 Erosive ............................................................................................................ 12 2.3.4 Corrosive ........................................................................................................ 12 2.3.5 Fatigue ............................................................................................................ 13 2.3.6 Archard’s Wear Law ....................................................................................... 13 2.4 Hertzian Contact Mechanics .................................................................................. 14 2.5 Lubrication ............................................................................................................. 16 2.5.1 Lambda Ratio and Stribeck Curves................................................................. 16 2.5.2 Boundary Lubrication ..................................................................................... 18 2.5.3 Mixed Lubrication .......................................................................................... 19 2.5.4 Elastohydrodynamic Lubrication ................................................................... 20 2.5.5 Hydrodynamic Lubrication ............................................................................. 21 2.5.6 Additives......................................................................................................... 22 2.5.6.1 Anti-Wear Additives ................................................................................... 22 2.5.6.2 Zinc DialkylDithioPhosphate ...................................................................... 23 viii 2.6 Types of tribometer ............................................................................................... 25 2.6.1 Unidirectional Tribometers ............................................................................ 26 2.6.2 Reciprocal Motion Tribometers ..................................................................... 27 Chapter 3: Surface Parameters ................................................................................ 30 3.1 ISO 25178 - 2 .......................................................................................................... 31 3.2 Arithmetic