Tribochemistry of Triphenyl Phosphorothionate (TPPT) by In Situ Attenuated Total Reflection (ATR/FT-IR) Tribometry

The surface reactivity of triphenyl phosphorothionate (TPPT) on air-oxidized iron surfaces under tribological conditions in the boundary-lubrication regime has been investigated by in situ attenuated total reflection (ATR) FT-IR tribometry and ex situ X-ray photoelectron spectroscopy (XPS). In situ ATR/FT-IR tribometry allowed the measurement of the friction force during the tribological tests and its correlation with ATR/FT-IR results, providing an insight into the chemical reactions taking place at the buried interface. The TPPT molecule, which did not behave as a friction-modifier additive, reacted at the air-oxidized iron surface upon tribotesting at 423 K to give pyrophosphate, organo-phosphate, and sulfate species. The oxygenated compounds produced by the oxidation of the base oil (poly-α-olefin, PAO) during the tribological tests, adsorbed onto the iron surface and reacted with it to form carbonates and carboxylates. During the tribotests, the iron coating was worn away and corroded by the aggressive species formed during the thermo-oxidative aging of PAO. This resulted in the availability of iron cations necessary for the production of orthophosphate compounds. The ATR/FT-IR results also suggested that the rate of tribofilm formation was lower than that of tribofilm removal. The ex situ XPS analysis corroborated the formation of short-chain iron polyphosphates, organo-phosphates, and iron sulfates, together with carbonates and carboxylates. Long-chain polyphosphates and sulfoxides were also generated in the boundary lubricant layers formed by TPPT at 423 K. The composition of the tribofilms formed by TPPT was found to differ significantly from that of the boundary layers generated by ZnDTP.