Collaborative Research: Pushing Molecules Around: Identifying and Understanding the Elementary Steps in Tribochemical Reactions

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Lubrication of moving parts is essential to reduce friction and wear of mechanical components. Typical lubricants are hydrocarbon compounds with friction-reducing additives. Both improving the efficiency of the lubrication process and its useful lifetime can reduce energy consumption, reduce maintenance costs, and reduce the amount of waste lubricant. Lubricants can, however, wear out through both the heating during use which can breakdown the lubricant structure and by chemical reactions occurring at the bearing surfaces. This award supports research into the basic mechanisms leading to this chemical breakdown of essential lubricants through a collaborative effort involving both experiment investigation of the process and its simulation. Fundamental understanding of these surface-related chemical reactions, occurring under conditions typical of their use, is lacking. The determination of these chemical reactions can be used to design new more resilient lubricants where these detrimental reactions are reduced or eliminated leading to improved lubricants. The results of the research will therefore benefit many sectors of the US economy and technology dependent on lubrication. Increased lifetime of mechanical systems and reduced energy consumption would result from improved lubrication technologies. The University of California-Merced and the University of Wisconsin-Milwaukee actively engage undergraduates into these research activities, with UC Merced possessing a high percentage of Hispanic and first generation students. Experiments will be carried out on simple model systems comprising clean copper and gold lubricated by gas-phase dialkyl disulfides, which, although simple, nevertheless retain the key aspects of realistic lubricants. The reaction pathways and their kinetics will be measured using in-situ techniques, by monitoring the gas-phase products produced by sliding in ultrahigh vacuum (UHV), and by measuring the evolution in friction force. These experiments will be complemented by surface analyses of the rubbed region in the same apparatus. The experimentally-measured kinetics will be modeled using molecular dynamics (MD) simulations using reactive potentials that can reveal the key ingredients contributing to increased reaction rates at a sliding interface. The insights obtained from the MD simulations and UHV experiments will be used to develop accurate analytical models. These integrated experiments, simulations and analytical models will lead to new fundamental understanding of the elementary steps that occur in tribochemical reactions that ultimately underlie lubricant function. The students within the program will participate in both aspects, calculation and experiment, through exchanges between the institutions. These exchanges will provide unique training and create a tight linkage between the computational and experimental studies.