Tribological properties and action mechanism of a highly hydrolytically stable N-containing heterocyclic borate ester

Purpose - The phosphorus and zinc contained in zinc dialkyl dithiophosphate (ZDDP) caused severe environment pollution and catalyst poison. Thus, the phosphorus-free additive, such as borate esters, has become one of studying hot topics in the area of oil additive. However, the stability of hydrolysis greatly limited the use of borate esters. The purpose of this paper is to improve the stability of hydrolysis by synthesizing a new kind of N-containing heterocyclic borate ester (MTTDB) as a lubricant additive. Design/methodology/approach - The tribological properties of novel borate ester (MTTDB) as an additive in the base oil were studied by a four-ball machine. The element composition and chemical state of the tribofilm were investigated by scanning electron microscopy, energy dispersive spectrometer and X-ray photoelectron spectroscopy. Findings - The results showed that the base oil lubricated by MTTDB exhibited high hydrolytic stability, good anti-wear property and excellent extreme pressure performance. When 2.5 per cent MTTDB was added into the 100N base oil, the smallest wear scar diameter (0.46 mm) was obtained. Furthermore, the decomposed borate ester, organic sulfide adsorbed on the worn surface was detected, and S element reacted with the steel surface and generated FeSO4, both of which contributed to the formation of the tribofilm. Originality/value - Based on N-containing heterocyclic compounds, for instance, thiadiazole derivatives, introducing nitrogen and sulfur elements into borate ester, a new kind of N-containing heterocyclic borate ester (MTTDB) exhibited excellent property in hydrolysis stability, friction-reducing, anti-wear and extreme pressure. This synthesized method would be helpful for the borate ester used as additive in engine oil, gear oil and other industrial lubricants.