Organophosphates have been the dominant additives for anti-wear in high-end aerospace lubricants for over half a century. Tricresyl phosphate (TCP) and similar aryl and alkyl phosphates reduce the amount of wear of ferrous materials in mechanical contacts, especially at elevated temperatures, high contact pressures, and high sliding speeds by forming thin layers of Fe-, O-, and C-containing compounds that are thought to preferentially shear and protect the underlying surfaces from material removal. These tribo-films consist of multilayered structures up to 10’s of nanometers thick that contain various iron phosphates, polyphosphate, and carbon. However, the thermally- and mechanically-activated reactions mechanisms that lead to their formation continues to be debated. To elucidate initial reactions steps, we have conducted experiments to measure gaseous reaction products and corresponding simulations to compare our observations to products predicted from the hypothetical mechanisms proposed in literature. Thermal Programmed Reaction Spectroscopy (TPRS) and Gas Chromatography Mass Spectroscopy (GCMS) measurements of powders of Fe, Fe2O3, and Fe3O4 that have been exposed to TCP demonstrated that cresol is the primary gaseous product released during thermally-activated reactions under both air and oxygen/water-starved environments. Further, reactivity was observed to depend on the particular TCP isomer, reaction substrate, and atmospheric content. Reactive molecular dynamics simulations generally supported these experimental trends and provided further insight into the important role of the carbon functional groups in the reactions with the iron surfaces.

Tricresyl phosphate reactions with ferrous surfaces: Effect of surface composition, molecular isomer, and atmospheric content

2019 Fall National Meeting

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