Synergistic effect between organic borate esters and phosphorus-based additives on tribological performances as lubricant additives in mineral oil

Nowadays, looking for environment-friendly lubricant additives to replace phosphorus-based derivatives in automotive lubricating system is one of the main objectives. Organic borate ester is one kind of potential substitutes for phosphorus-based derivatives. In this paper, the interactions of organic borate esters and dioctylphosphite ester on tribological performances were studied. Results showed that they had a synergistic effect on antiwear performance and friction-reducing property as lubricant additives in mineral oil. The chemical nature of worn surface was explored by X-ray absorption near-edge structure spectroscopy. Borate ester could facilitate the decomposition of phosphorus-based additive and the formation of phosphate. What is more, boron-containing species could react with phosphate to form boron-phosphorus-based thin film on the surface. Boron mainly forms B2O3 on the surface. In the near surface of tribofilm, nitrogen-containing borate ester reduced the polyphosphate chain length due to the formation of ammonium cation. The polyphosphate chain length of the tribofilm generated from dioctylphosphite ester mixed with nitrogen-containing alkyl borate ester is shorter compared to that of the tribofilm generated from dioctylphosphite ester mixed with nitrogen-containing heterocyclic borate ester.