Tribochemical Mechanisms of Trimethyl and Triethyl Phosphite on Oxidized Iron in Ultrahigh Vacuum
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The surface chemistry of model lubricant additives, trimethyl phosphite (TMPi) and triethyl phosphite (TEPi), is studied on oxidized iron in ultrahigh vacuum (UHV) and the results compared with the gas-phase lubrication of TEPi on oxidized iron in a UHV tribometer. Oxide films are grown on a polycrystalline iron substrate and characterized by X-ray photoelectron spectroscopy (XPS) and assigned to the formation of an Fe3O4 film. Measurements of the friction coefficient and contact resistance of the oxide films show that the oxide film remains intact while rubbing in UHV at a normal load of 0.29 N. It is found that phosphite esters adsorb on the oxide by electron donation to the phosphorus atom, with a binding energy that increases in the order tributyl phosphite > TMPi > TEPi, and correlates well with the location of the vacant lowest-occupied molecular orbital (LUMO) energy. The phosphite esters decompose via sequential P‒O bond scission to form adsorbed alkoxy species, which then react on the surface either by hydrogen addition to form the corresponding alcohol, or by hydrogen abstraction to yield an aldehyde. XPS studies of the surface shows that essentially no carbon remains after the TMPi has reacted, while a small amount of carbon is present when TEPi has decomposed. On heating, the phosphite esters convert to phosphate species. Gas-phase lubrication experiments in the presence of 1 × 10−7 Torr of gas-phase TEPi reveal that the friction coefficient is significantly reduced, where the friction reduction is found to increase with increasing reaction temperature. The friction reduction correlates well with the proportion of phosphate product formed in the film and indicates that the formation of phosphate tribofilm is primarily responsible for reducing friction. However, friction is also reduced for a reaction at ~ 300 K, lower than the temperature at which the adsorbed phosphite ester reacts, suggesting that its rate of decomposition could be accelerated by interfacial shear.
KeywordsTrimethyl phosphite Triethyl phosphite Temperature-programmed desorption X-ray photoelectron spectroscopy Ultrahigh vacuum tribometer Tribochemistry
We thank the National Science Foundation for support of this work under Grant Number CMMI-1265742.
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