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Transfer Report LOW AND ZERO SAPS ANTIWEAR ADDITIVES FOR ENGINE OILS by Juliane F. L. Benedet A Thesis submitted to Imperial College London in fulfilment of the degree of Doctor of Philosophy and the Diploma of Imperial College. November 2012 Tribology Section Department of Mechanical Engineering Imperial College of Science, Technology and Medicine London PREFACE This thesis is a description of work carried out in the Tribology Section of the Department of Mechanical Engineering, Imperial College of Science, Technology and Medicine, London, under the supervision of Professor Hugh A. Spikes. Except where acknowledged, this material is original work and no part of it has been submitted for a degree at this or any other university. ABSTRACT Almost all modern engine lubricants use the additive zinc dialkyldithiophosphate (ZDDP) to provide antiwear and extreme pressure protection. However existing and proposed emissions regulations include constraints in the concentration of ZDDP or other sulphated ash-, phosphorus- and sulphur- (SAPS) containing additives in engine oils, as well as limits to the permissible phosphorus loss from the oil in running engines. The deleterious effects of SAPS on exhaust aftertreatment systems from ZDDP decomposition has lead to a great interest in identifying alternative low and zero SAPS antiwear additives that can partially of fully replace ZDDP in the next generation of engine oils to extend the life of exhaust after-treatment systems. The aim of the work described in this thesis is to explore under the same test conditions, the film-forming, friction and wear-reducing properties of a very wide range of low and zero SAPS antiwear additives as possible replacements for ZDDP in engine oils, and, where additive types are effective, to investigate their mechanism of action. Some of the alternative low and zero SAPS antiwear additives investigated show wear-reducing performance comparable to the ZDDP used as a benchmark in this research. The most promising alternatives to be used as supplements/replacements for ZDDP are suggested and discussed. One characteristic of ZDDP is that it can form quite thick films on surfaces during rubbing. It is found that many different types of additive can also form thick films, though not generally as thick as ZDDP. However it has been found that there is no significant correlation between the ability of an additive to form a thick boundary film and its ability to control wear. Thick boundary film formation does, however, correlate positively with an increase in friction in contacts operating at intermediate entrainment speeds, so thick film formation should not be taken as a necessary or desirable feature of antiwear additives. It is also found that antiwear additives can give a very wide range of boundary friction coefficient values, depending on the molecular structure of the additive. There thus appears to be scope for optimising the structure of antiwear additives to provide reduced boundary friction. ACKNOWLEDGEMENTS I owe my deepest gratitude to my PhD supervisor, Professor Hugh Spikes, for his invaluable support and encouragement throughout this journey. I am also grateful to Riaz Mufti, Jonathan Green, Gordon Lamb and David Atkinson at Castrol Ltd. for the support and motivation throughout this project. I would also like to thank all colleagues from the Tribology section of the Mechanical Engineering Department at Imperial College, London, for their help, in special to Tom Reddyhoff and Chrissy Stevens. Also thanks to Rich Baker, Matt Smeeth, Clive Hamer and John Hutchinson at PCS Instruments, David Scurr at Nottingham University, Mihaela Gorgoi at the Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Lucia Zuin and Yongfeng Hu at the Canadian Light Source, Maura Crobu at ETH Zurich and Richard Chater at Imperial College London, And finally, a very special thanks goes to my husband, Ramiro, for his undying support. I could not have done this without you. TAPS. Table of Contents 5 TABLE OF CONTENTS PREFACE ............................................................................................................................................................. 2 ABSTRACT ........................................................................................................................................................... 3 ACKNOWLEDGEMENTS ......................................................................................................................................... 4 TABLE OF CONTENTS ........................................................................................................................................... 5 LIST OF FIGURES................................................................................................................................................ 10 LIST OF TABLES ................................................................................................................................................. 20 1. INTRODUCTION ............................................................................................................................................... 22 2. THE IMPACT OF SAPS ON EXHAUST AFTERTREATMENT SYSTEMS ................................................................... 26 2.1 Exhaust Aftertreatment Systems ........................................................................................................... 27 2.1.1 Three-Way Catalyst ....................................................................................................................... 28 2.1.2 NOx Storage Catalyst .................................................................................................................... 29 2.1.3 Diesel Oxidation Catalyst ............................................................................................................... 30 2.1.4 Diesel Particulate Filter .................................................................................................................. 31 2.1.5 Selective Catalytic Reduction Catalyst ........................................................................................... 32 2.2 The Impact of SAPS on Exhaust Aftertreatment Systems ..................................................................... 34 2.3 Specifications ......................................................................................................................................... 35 3. LUBRICATION ................................................................................................................................................. 37 3.1 Lubrication Regimes .............................................................................................................................. 38 3.1.1 Boundary Lubrication ..................................................................................................................... 39 3.1.2 Mixed Lubrication ........................................................................................................................... 40 3.1.3 Fluid Film Lubrication (Hydrodynamic and Elastohydrodynamic lubrication) ................................. 41 3.2 Lubricants .............................................................................................................................................. 43 3.2.1 Lubricant Formulation Challenges ................................................................................................. 44 3.3 Wear ...................................................................................................................................................... 45 3.3.1 Archard’s Wear Coefficient ............................................................................................................ 47 3.4 Antiwear Additives ................................................................................................................................. 48 3.4.1 Zinc Dialkyldithiophosphate (ZDDP) .............................................................................................. 49 3.4.2 Low and Zero SAPS Antiwear Additives ........................................................................................ 49 Table of Contents 6 4. EXPERIMENTAL TECHNIQUES .......................................................................................................................... 51 4.1 MTM-SLIM Method ................................................................................................................................ 52 4.1.1 Description ..................................................................................................................................... 52 4.1.2 Experimental Procedure ................................................................................................................ 57 4.2 AFM Method .......................................................................................................................................... 59 4.2.1 Description ..................................................................................................................................... 59 4.2.2 Experimental Procedure ................................................................................................................ 62 4.3 MTM-Reciprocating Method .................................................................................................................. 63 4.3.1 Description ....................................................................................................................................
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