The Pennsylvania State University The Graduate School MECHANOCHEMISTRY OF ADSORBED MOLECULES AT TRIBOLOGICAL INTERFACES A Dissertation in Chemical Engineering by Xin He 2019 Xin He Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy December 2019 ii The dissertation of Xin He was reviewed and approved* by the following: Seong H. Kim Professor of Chemical Engineering and Materials Science Dissertation Advisor Chair of Committee Graduate Program Coordinator Adri C.T. van Duin Professor of Mechanical Engineering Professor of Chemical Engineering Professor of Engineering Science and Mechanics Themis Matsoukas Professor of Chemical Engineering Xueyi Zhang John J. and Jean M. Brennan Clean Energy Early Career Assistant Professor of Chemical Engineering *Signatures are on file in the Graduate School iii Abstract Mechanochemistry refers to reactions activated by mechanical force or stress. It is ubiquitous in engineering systems as well as daily life; but poorly understood because the reactions are taken place at solid tribological interfaces. The main challenges in mechanochemistry research are to identify the reactants and products at sliding interfaces, which can be overcome via vapor phase lubrication (VPL). The main questions in this work are the role of shear force or stress in mechanochemical reaction and how surface chemistry properties affect the reaction rate. This study investigated the molecular structure dependence of precursor molecules during mechanochemical reactions. Several monomers were studied to reveal the molecular structure dependence at tribological interfaces, including -pinene (C10H16), pinane (C10H18) and n-decane (C10H18), which are all hydrocarbon precursors containing 10 carbon atoms. The friction coefficient of these molecules was around ~0.2 and they can be polymerized by mechanical shear. The molecules with ring structure (-pinene and pinane) were found to produce more products compared with linear molecule n-decane. The modified Arrhenius-type equation is used to study the relationship between tribopolymerization yield and applied load; as well investigate how critical activation volume (V*) is affected with the structure of monomers. The estimated V* of -pinene and pinane, which possess higher internal strain, showed the lowest activation volume, while the value is higher for n-decane (10%), which possesses low internal strain. The experimental results were then compared with molecular dynamic (MD) simulations with a ReaxFF reactive force field to reveal the physical meaning of activation volume. The tribo- polymerizable model reactants, allyl alcohol and -pinene were studied. The results suggested that the precursor molecules first chemisorb on the surfaces through surface oxygen. During the sliding iv process, the precursors undergo partial distortion from its equilibrium conformation, corresponding to the critical activation volume for mechanochemical reactions. The activated intermediates polymerize through the formation of C-O-C ether bond, rather than direct C-C covalent bond. The effect of surface chemistry and the surrounding gas environment on mechanochemical polymerization reactions are also revealed. Mechanochemical reactions of -pinene are studied on eight substrate materials – chemically reactive group (palladium, nickel, copper, stainless steel) and chemically inert group (gold, silicon oxide, aluminum oxide, DLC) The more reactive surfaces appear to be capable of chemisorbing -pinene molecules even without the mechanical shear process. In the case of reactive surfaces, the critical activation volumes are relatively lower compared to that only capable of physisorption. Such chemisorption process can be significantly enhanced through the oxidative gas environment, in turn increasing tribochemical reactivity. When the water is introduced to the allyl alcohol system at silicon oxide tribological interfaces, it enhances the tribo-polymerization activity, in turn increasing the yield rate of tribo-polymers. The estimated activation energy is found lower with the presence of water molecules. The tribochemistry is widely-investigated in anti-wear tribofilm formation. Environmentally-friendly ionic liquids (ILs) are being developed as ashless additives for hydraulic fluids. Candidate ILs, at a treat rate of 0.5 wt.%, were blended into a non-polar mineral base oil, a hydrophilic Polyalkylene glycol (PAG), and an oil-soluble PAG (OSP). Compared with a commercial primary zinc dithiophosphate (ZDDP), the ILs showed lower friction coefficient and wear volume. This attributes to the formation of a protective layer on the contact surface as revealed by characterization of wear scar morphology and composition. In addition to the superior v lubricating performance, these ILs have advantages of higher thermal stability and lower toxicity than commercial hydraulic fluid additives. vi Table of Contents List of Figures ................................................................................................................................. x List of Tables ............................................................................................................................... xxi List of Equations ......................................................................................................................... xxii Acknowledgements .................................................................................................................... xxiv Chapter 1 ......................................................................................................................................... 1 Introduction and Motivation ........................................................................................................... 1 Chapter 2 ......................................................................................................................................... 6 Experiment details and data processing .......................................................................................... 6 Vapor phase lubrication tests ...................................................................................................... 6 Sample preparation ................................................................................................................... 11 Calculation of flash temperature ............................................................................................... 12 Tribo-polymer yield calculation ............................................................................................... 13 Mechanical behavior tests ......................................................................................................... 16 Adsorption isotherm measurement methods and principles ..................................................... 19 Chemical Characterization ........................................................................................................ 21 Molecular Dynamics simulation ............................................................................................... 22 Chapter 3 ....................................................................................................................................... 23 Tribochemical synthesis of nano-lubricant films from adsorbed molecules at sliding solid interface: Tribo-polymers from α-pinene, pinane, and n-decane .................................................. 23 Overview ................................................................................................................................... 23 Introduction ............................................................................................................................... 24 Experimental Details ................................................................................................................. 26 Result and Discussion ............................................................................................................... 28 Conclusion ................................................................................................................................ 44 Supporting Information ............................................................................................................. 46 Chapter 4 ....................................................................................................................................... 51 vii Mechanochemistry of physisorbed molecules at tribological interfaces: ..................................... 51 Molecular structure dependence of tribochemical polymerization ............................................... 51 Overview ................................................................................................................................... 51 Introduction ............................................................................................................................... 53 Experiment Details.................................................................................................................... 55 Results and Discussion ............................................................................................................. 57 Conclusion ................................................................................................................................ 69 Supporting Information ............................................................................................................. 70 Chapter 5 ......................................................................................................................................