LUBRICANT-DERIVED TRIBOCHEMICAL FILMS - AN INFRARED SPECTROSCOPIC STUDY

Lubricant-derived tribochemical films were prepared in a cam/tappet tribometer using fully formulated engine oils containing zinc dialkydithiophosphate. A direct acting bucket tappet geometry with a removable friction disc in the tappet was employed. The friction discs could easily be removed for analysis after the completion of the friction experiments. Reflection-absorption infra-red was used to characterize the films, together with XPS and Auger spectroscopies. The results were consistent with an inorganic, amorphous phosphate film with cations derived from the lubricant additives. Building on these results, additional experiments were undertaken to determine the nature of the films better. Model materials for the films were prepared by synthesizing phosphate glasses and by thermally and thermo-oxidatively decomposing zinc diisopropyl dithiophosphate. The results indicated that the tribochemical films were not phosphate glasses as expected, but rather predominantly amorphous orthophosphates and pyrophosphates formed by thermooxidative decomposition of zinc dialkyldithiophosphate.