© 2019 Alireza Saatchi ALL RIGHTS RESERVED THE EFFECT OF GREASE COMPOSITION ON FRETTING WEAR A Dissertation Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy Alireza Saatchi May 2019 THE EFFECT OF GREASE COMPOSITION ON FRETTING WEAR Alireza Saatchi Dissertation Approved: Accepted: _____________________________ _____________________________ Advisor Department Chair Dr. Gary Doll Dr. H. Michael Cheung _____________________________ _____________________________ Committee Member Dean of the College Dr. Qixin Zhou Dr. Donald P. Visco, Jr. _____________________________ _____________________________ Committee Member Dean of the Graduate School Dr. Rajeev Gupta Dr. Chand Midha _____________________________ _____________________________ Committee Member Date Dr. Curtis Clemons _____________________________ Committee Member Dr. Gopal Nadkarni ii ABSTRACT This research studies the effects of composition and oil release mechanisms of greases on its performance properties, especially against a certain type of tribological damage called fretting. Grease is a complex lubricant that consists mainly of a base-oil and a thickener. Whereas the base oil is the primary lubricous component of the grease, the thickener gives the grease its consistency, or its ability to “stay put” wherever it is applied. Most thickeners are in the form of fibers dispersed colloidally in oil, entangled and connected together to form a three dimensional structure that traps the oil and prevents it from flowing freely. The oil needs to separate from the mixture and insert itself into the tribological contact in order to perform its function of preventing friction and wear. Therefore, the separation of oil from the grease, which is loosely referred to as the grease bleed, is an essential step in the protection mechanism of the grease. The “bleed rate” is a standard specification of the grease determined by a test in which the grease is put in a cone sieve under high temperature and the relative amount of oil which is “bled” out is measured. Despite the widespread application of greases, their oil release mechanism is not well understood. The theories that have been developed for oil lubrication have been applied to grease lubricated contacts, which fail to accurately account for the experimental observations. Therefore the grease bleed mechanism is the major topic in this study. We will focus on the oil release mechanism of the grease under fretting contact. Fretting is defined as a small oscillatory displacement of bodies in sliding or rolling contact and is often observed when a source of vibration or cyclic stress is present, causing excessive wear or premature fatigue failures in different applications and it is considered as a plague of the modern industry. In many of these applications, grease is iii employed as the lubricant. Another key motivation for this research is the fact that the term fretting in the literature and industrial standards fail to provide distinction between fretting under rolling and sliding contact. This is despite the fact that rolling and sliding fretting have been seen in numerous occasions to be two entirely different phenomena, especially as far as the performance of the grease is concerned, causing a great deal of confusion for both grease manufacturers and end-users. Finally, another motivation for this research is the investigation of non-soap thickened greases that unlike the more traditional soap thickened greases are not so well studied or understood especially their oil release mechanism which is for the most part, somewhat of a mystery. A good example of that is the calcium sulfonate grease that is among the newer grease types in the market and has exceptional properties. In grease making, this grease acts very differently than others in the way that it requires a lot more thickener to achieve the desired consistency. This is the reason for the extremely low bleed rate of the calcium sulfonate grease, however to the best of our knowledge, no one in the past had been able to explain the exceptionally good performance of this grease despite its nominal bleed rate being practically zero. This brings us back to the discussion of the grease bleed and points out the discrepancies in its definition. How can we define bleed as the oil release mechanism of the grease being responsible for its protection properties for the case of calcium sulfonate grease, where there is exceptional protective properties but zero bleed rate? Therefore, it seems to be necessary to redefine the bleed itself. To this end, three different grease types with soap and non-soap thickeners have been tested for their fretting wear performance. The soap thickener chosen was the lithium complex, which is one the most common grease thickener types of all times. We also tested non-soap, iv calcium sulfonate and polyurea thickened greases for comparison. Each grease was mixed with its own base oil to obtain a range of bleed rates and then the mixtures were tested for their fretting performance under different rolling and sliding conditions to find a correlation between the grease bleed rate and its protective properties. The results showed that the fretting performance of the grease under rolling contact strongly depends on the grease bleed rate whereas it did not affect the sliding fretting performance of the grease. Furthermore, there had been instances of scuffing which is the result of metal-to-metal contact due to failure in lubrication, which gave us a chance to study the starvation and scuffing resistance of the greases as well. There were some observations that were rather unexpected or somewhat hard to explain at first glance. Firstly, the polyurea grease, which showed a comparatively high bleed rate, performed worse than others and especially showed the lowest resistance against scuffing. On the other hand, the calcium sulfonate grease showed the best performance on all fronts, even though for most of the calcium sulfonate mixtures, the standard bleed rate was zero. The calcium sulfonate grease raised more questions as it showed an extremely non-linear bleed rate trend with the addition of oil. The bleed rate results were interpreted within a model that correlated the bleed with the grease thickener geometries. To obtain the geometry of the actual thickeners, the greases were tested using Dynamic/Static Light Scattering (DLS/SLS) techniques. In the proposed model the grease thickeners were treated as particles suspended in a matrix of oil instead of the traditional view, in which the thickener structure was assumed to be a sponge that holds the oil and releases it into the tribological contact. In the present model, the “sponge” view is reserved only for the “static bleed” which simulates the oil flow v behavior of the standard bleed test. Within the tribological contact, however, the “dynamic bleed” should be viewed as the separation of oil from the loose and mobile thickener particles. In other words, the thickener particles are connected with each other and hold the oil, resulting in the consistency of the grease. However, at the same time, the thickener particles do not constitute a rigid body and can move independently in the oil matrix and are also capable of separating from each other if their attraction forces are overcome or in instances where they are diluted. Another significant difference between this model than those of the past is that a region surrounding each thickener particle, called the effective media, is assumed to have wholly immobilized the oil. In older models, the bleed was inaccurately assumed to occur from the viscous flow of oil through thickeners. In the current model, it was shown that the static bleed is indeed the viscous flow of only the unbound oil in a porous structure made of not only the thickeners but the entire effective media. Further to clarifying the differences between rolling fretting and sliding fretting, in conclusion of this work, three different types of bleed mechanisms are discussed, static bleed, dynamic bleed under sliding and dynamic bleed under rolling contact. The difference between the two dynamic bleed mechanisms was attributed to the direction of the motion of the particles with respect to the sliding/rolling direction. In the light of this model, many aspects of the observed phenomena were also explained such as the unusual protection and bleed behavior of the calcium sulfonate grease which was related to the spherical geometry of its thickener micelles. vi ACKNOWLEDGEMENTS This dissertation is a summary of my Ph.D work which would not have been possible without the help, guidance, encouragements and well wishes of great mentors, colleagues, friends and family members. At this moment of accomplishment, I wish to express my sincere gratitude to my advisor, Professor Gary Doll for his constant support, guidance, mentorship and encouragement throughout the five years of my research work. I would also like to thank Dr. Paul Shiller for his invaluable suggestions, support and mentorship in every step of the development of this project. I thank Professors Qixin Zhou, Rajeev Gupta, Curtis Clemons and Gopal Nadkarni for serving as my committee members and providing valuable insights to improve this research. I would like to thank Dr. S. Ali Eghtesadi for his guidance and contributions in microstructural characterizations and his advisor Professor Tianbo Liu for the support of this project. I thank Dr. Soroush Heidari Pahlavian for most of the artwork in this dissertation and his guidance regarding fluid dynamic problems. I sincerely thank Dr. Gareth Fish from the Lubrizol Corporation for his mentorship and guidance as well as providing most of the baseline grease samples for this study. I thank Dr. Barbara Fowler and Richard Fowler for their help and guidance in this project and William Wenzel and Brett Bell for technical support. I, along with our entire research team, acknowledge the Timken Company for providing funding for this work and Drs.