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Tribochemistry Studied by Surface-Analytical Tools
Contribution from C. Kajdas, Poland
Major C. Kajdas contribution to the Project relates to tribochemistry of biodegradable and environmentally friendly lubricants/additives composed of only Carbon, Oxygen, Hydrogen (CHO) and/or Nitrogen (CHON or CHN) compounds. Chemical reactions initiated in the direct friction contact zone relate to the tribochemistry which deals with the chemical changes of both solid mating elements and lubricant molecules due to the influence of boundary and/or mixed lubrication operating conditions. Tribochemical reactions differ significantly from those initiated by heat only. Thus, one important aim of this Project part is to better understand mechanisms of tribochemical reactions of hydrocarbon base oils and CHO/CHNO tribological additives with different solids. This specific approach clearly focused on the environment protection also serches for answers of several important questions. How the present knowledge of tribochemistry can be applied in elucidation of detailed mechanisms by which CHO/CHNO boundary lubricant additives reduce wear? How the boundary friction process can affect base oils, particularly model base oils, for example, n-hexadecane in comparison with tribochemistry of esters, carboxylic acids, alcohols, lactams, etc. What is the part of the Negative-Ion-Radical Action Mechanism (NIRAM) in initiating tribochemical reactions and what are their specific reactive intermediates? One of the questions dealt with in the literature is concerned with the magnitude of the surface temperature rise during the friction process. According to the NIRAM approach, friction in a tribological process can lead not only to high surface temperatures (flash temperature) but also to the emission of low-energy electrons (1 - 4 eV). The two types of phenomena are not entirely unrelated since high temperatures cause termionic emission. Accordingly, thermal electrons can be considered as the bridging gap between surface “hot spots” and the exoemission process. Figure below presents initiation process of tribochemical reactions caused by friction. Low-energy electron interaction with lubricant molecules produce reactive intermediates that, in turn, react with the solid activated sites froming specific organometallic films.
Most of experiments will be carried out ussing a pin-on-on-disk tribometer under various conditions with a given set of base fluids and CHO/CHNO additives. Some experiments will also be performed at the standard HFRR test rig conditions. The chemical state of the wear track or wear scar surface and near-surface region will be then analyzed with techniques such as FTIRMA, XPS and ToF-SIMS, in order to make a correlation between tribological conditions, lubricant chemistry, surface reactions, and ultimate tribological performace. To better understand the overall reaction process SEM/EDS analysis will also be performed. Emphasis of that research is on generation results that should facilitate the prediction of tribochemical reactions for relevant systems.
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