Role of Boron in the Tribochemistry of Thermal Films Formed in the Presence of ZnDTP and Dispersant Additives

Thermally formed, additive-derived films are often a key precursor to tribologically protective films that are formed under the influence of mechanical stress. We have examined the tribological stability of model thermal films, tested in pure base oil under severe experimental conditions. Our aim was to exclude the self-healing mechanism of the protective film, which would be expected in a fully formulated lubricant. The thermal films were grown in the presence of various oil formulations containing combinations of ZnDTP and dispersants. The films were fully characterized by means of scanning electron microscopy and atomic force microscopy (for investigating the film morphology) in combination with X-ray photoelectron spectroscopy (for determining the chemical composition and thickness of the films). The mechanical stability of the films was evaluated via a series of tribological tests carried out applying two different contact pressures, 1.04 and 0.7 GPa, in base oil—a hydro-treated, heavy paraffinic fraction known as Yubase 4—at 100 °C. The tribological tests were performed using a ball-on-disc set-up and run until the failure of the protective film occurred. It was found that boron-containing thermal films displayed a much higher mechanical stability than their boron-free counterparts. It was also shown that the presence of boron species (8.4–11.1 at.% B) in the thermal films seems to be able to limit the formation of wear particles and prolong the lifetime of the protective coatings. Furthermore, it appears that very thin, boron-based thermal films can exhibit higher durability than films that are thicker but boron-free.