Date of Award
Doctor of Philosophy
Wilfred T Tysoe
Mark L Dietz, Alan W Schwabacher, Arsenio A Pacheco, Jorg C Woehl, Peter V Kotvis
Tribology is the study of friction and wear and of the energy dissipated when two surfaces slide against each other. A recent DOE report shows that improved lubricants could save ~103 exaJoules (1 exaJoule = 10^18 joules) per year, equivalent to 20% of the world’s total energy consumption. The focus of this work is to understand the surface chemistry and tribological reactions of model sulfur-, carbon-, and phosphorus-based lubricant additives to understand the mechanisms by which they operate. Previous work has shown that lubricant additives react by a novel mechanochemical process in which the force acting on the adsorbate accelerates the rate of reaction and can lead to new metastable materials.
The tribochemical reactions are studied using a tungsten carbide pin sliding against a planar substrate in ultrahigh vacuum where the reaction is followed by detecting gas-phase products and analyzing the rubbed region using Auger spectroscopy.
The tribochemistry of dimethyl disulfide (DMDS) on copper has been studied previously. Here, reaction is initiated by the decomposition of adsorbed methyl thiolate species to form small gas-phase hydrocarbons, followed by a process in which the surface sulfur penetrates the bulk of the copper to produce a metastable copper sulfide film. This work aims at further understanding the second step in this process, namely, the surface-to-bulk transport of sulfur into copper by studying the effect of coverage and the crystallinity of the copper substrate on the transport kinetics. These kinetic studies were supplemented by electron microscopy if the subsurface region of samples that had been prepared by fast-ion beam methods.
These studies explored the effect of binding to the substrate. As a consequence, work was also carried out to investigate how binding of the reactive molecule to the tungsten carbide pin might influence the tribochemistry. This work is carried out using unsaturated carboxylic and saturated carboxylic acids, where the carboxylate group has been shown to anchor strongly to the copper substrate, and where the C=C species are expected to bind to the tungsten carbide counterface and thus influence the reactivity. The adsorbed carboxylic acids were found to decompose during sliding by rapid cleavage of the bond between the carboxylate and the hydrocarbon groups. Differences in the surface chemistry was found, but was traced to difference in the reactivity of the hydrocarbon.
In addition to the sulfides investigated above, phosphorus-containing molecules, notably phosphite esters, have been found to be good lubricants for iron. The surface tribochemistry model lubricant additives, triethyl phosphite and trimethyl phosphite were studied on iron oxide (Fe3O4) by both experimental and theoretical approach, where the molecules were found to decompose by sequential removal of alkoxide species to form friction-reducing iron phosphate films.
Rana, Resham, "Understanding Mechanochemical Processes on Surfaces Measured in Ultrahigh Vacuum" (2020). Theses and Dissertations. 2584.