© 2018 Sayali S. Satam ALL RIGHTS RESERVED OPTIMIZATION OF WET FRICTION SYSTEMS BASED ON RHEOLOGICAL, ADSORPTION, LUBRICANT AND FRICTION MATERIAL CHARACTERIZATION A Dissertation Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy Sayali S. Satam May 2018 OPTIMIZATION OF WET FRICTION SYSTEMS BASED ON RHEOLOGICAL, ADSORPTION, LUBRICANT AND FRICTION MATERIAL CHARACTERIZATION Sayali S. Satam Dissertation Approved: Accepted: Advisor Department Chair Dr. Erol Sancaktar Dr. Sadhan C. Jana Committee Member Dean of College Dr. Sadhan C. Jana Dr. Eric J. Amis Committee Member Dean of the Graduate School Dr. Xiong Gong Dr. Chand K. Midha Committee Member Date Dr. Mesfin Tsige Committee Member Dr. Gary L. Doll Committee Member Dr. Rashid Farahati ii ABSTRACT Improved friction characteristics and reduced wear are desired in most of the contacting surface systems. With this motivation, our work is focused on two different applications. First application involves improving friction characteristics of automobile wet clutch system by exploiting ‘lubricant additive-friction material’ interaction and material modification aspects. The second part is focused on reducing friction and wear in boundary lubrication using multiwalled carbon nanotube (MWCNTs) as a lubricant additive. Wet clutch is an integral part of a transmission system in automobiles. Positive slope of the friction coefficient versus sliding speed curve along with a high dynamic friction coefficient value indicate ideal friction characteristics for smooth clutch engagement between the friction material (FM – also called “friction paper”) and the reaction plate (steel) in the presence of automatic transmission fluid (ATF). The first part of our work involved adsorption analysis of ATF additives on friction material components (filler and fiber) using DSC and UV/VIS techniques. Adsorption behavior was further correlated with rheological and friction phenomena. Shear stress and strain rate obtained from rheological testing were correlated with friction and sliding speed, respectively, as obtained from friction characteristics testing. It was observed that ATF causes shear thickening while base oil causes shear thinning behavior when mixed with FM filler. Higher rate of increase of shear stress with shear rate (i.e., enhanced shear thickening) is iii expected lead to higher friction coefficient with sliding speed. Following up on this hypothesis, the filler component of the friction paper was found to show shear thickening behavior and improved friction characteristics, as compared to its fiber component. Further concentrating on the filler component, we used three different fillers (diatomaceous earth and a proprietary clay) to top-coat the friction paper. It was observed that filler coating increases direct contact between the steel and the filler during clutch engagement, thus improving the friction characteristics due to the shear thickening effect. It was further found that the filler with the highest rate of increase in shear stress with shear rate in the presence of ATF, showed the best friction characteristics as compared to other two fillers as well as the non-coated FM. Due to our consideration of filler coating the friction paper, which may reduce its permeability, effect of oil permeability of filler coated FM surface on friction characteristics and FM durability were also studied. FM surface porosity was varied using parameters such as fiber/filler ratio in base layer FM composition, FM density and coating thickness. FM base layer composition was optimized comparing aramid and aramid/cotton compositions at different densities. In the last part of our work, phosphonium ionic liquid was used as an additive for MWCNT dispersion in non-polar lubricant. Ionic liquid adsorption on MWCNT walls was studied using DSC, FTIR and rheological techniques. Ionic liquid stabilized dispersion of nanotubes in lubricant was confirmed from UV/VIS and TEM. Addition of MWCNTs in lubricant resulted in decreased friction coefficient and wear in boundary lubrication for steel-steel contact as compared to base oil. iv DEDICATION This dissertation is dedicated to my parents and brother for their unconditional love, support and encouragement to pursue my dreams. v ACKNOWLEDGEMENTS I would like to express my deep sense of thanks and gratitude to my advisor Dr. Erol Sancaktar for all the encouragement and freedom he gave me in pursuing new ideas and opportunities during this journey. This work could not have been possible without the constant support and guidance from Dr. Sancaktar. I owe my sincere gratitude to Dr. Rashid Farahati for giving me the opportunity to work at LuK USA LLC for my dissertation work. He has been a great mentor who always shared his tremendous knowledge in this field of study and also gave me freedom to work in the industrial setting. I would like to thank our other industry collaborators Dr. Timothy Newcomb and Mr. Christopher Prengaman from Lubrizol Corporation for their support and guidance in this work. I would also like to thank my committee members Dr. Sadhan Jana, Dr. Xiong Gong, Dr. Mesfin Tsige and Dr. Gary Doll for their insightful suggestions and guidance. Special thanks to Dr. Gary Doll for guidance in the tribology experiments. I am also grateful to Dr. Toshikazu Miyoshi for his time and help with NMR experiments. A special thanks to all my friends in Akron for all the beautiful moments we have shared together. Thanks to them for being with me in the ups and downs of my journey and for their constant support and love. I am also thankful to my group members and colleagues vi at The University of Akron and LuK USA LLC for their help, support and enlightening discussions. At the end, I would like to thank my dear family and friends for their unconditional love and support. I know I always have them to count on when times are rough. I would not have made it this far without them. Thank you. vii TABLE OF CONTENTS Page LIST OF TABLES ........................................................................................................... xii LIST OF FIGURES ........................................................................................................ xiii CHAPTER I. INTRODUCTION .................................................................................................. 1 II. BACKGROUND ................................................................................................... 5 2.1. Tribology..................................................................................................... 5 2.1.1. History of Tribology ....................................................................... 5 2.1.2. Lubrication Regimes and The Stribeck Curve ................................ 6 2.2. Wet Clutch System ..................................................................................... 8 2.2.1. Friction Materials History ............................................................... 9 2.2.2. Friction Material Ingredients ........................................................ 10 2.2.2.1. Fibers ............................................................................. 10 2.2.2.2. Fillers ............................................................................. 11 2.2.2.3. Friction Modifiers ......................................................... 11 2.2.2.4. Binders .......................................................................... 12 2.2.3. Automatic Transmission Fluid (ATF) .......................................... 13 2.2.3.1. Friction Modifiers ......................................................... 14 2.2.3.2. Detergents...................................................................... 15 2.2.3.3. Dispersants .................................................................... 15 2.2.4. Wet Clutch Tribology ................................................................... 16 viii 2.2.4.1. SAE No. 2 Friction Tester ............................................. 16 2.2.4.2. Friction Characteristics ................................................. 17 2.2.4.3. Tribo-film Formation and Additive Adsorption ............ 20 2.2.5. Factors Affecting Friction Characteristics .................................... 21 2.2.5.1. Friction Material Parameters ......................................... 21 2.2.5.2. ATF Parameters ............................................................. 23 2.2.5.3. Process Parameters ........................................................ 27 2.3. Solid Nanomaterials as Lubricant Additives ............................................ 29 2.3.1. Carbon Nanotubes as Lubricant Additives ................................... 31 2.3.2. Carbon Nanotube Dispersion ........................................................ 34 2.3.3. Ionic Liquids as Additives for Lubricants..................................... 35 2.3.4. Interaction between CNTs and Ionic Liquid ................................. 37 2.3.5. CNTs and Ionic Liquid Lubricant Systems .................................. 39 III. CORRELATION BETWEEN ADSORPTION, RHELOGICAL AND FRICTION CHARACTERISTICS OF WET CLUTCH SYSTEM COMPONENTS ............ 40 3.1. Introduction ............................................................................................... 40 3.1. Experimental ............................................................................................. 42 3.1.1. Materials ......................................................................................