Tribological study and mechanism of B–N and B–S–N triazine borate esters as lubricant additives in mineral oil

●Preparing B–N and B–S–N triazine borate esters as lubricant additives.●The as-prepared additives have high hydrolytic stability.●B–S–N triazine borate ester showed excellent EP property while B–N triazine borate ester hardly owned EP property.●Tribological mechanisms were investigated by using XANES spectroscopy and discussed using the hard and soft acid base (HSBA) principle.

The tribological study of N-containing heterocyclic borate esters as lubricating additives had been the research hotspot. In this work, B–N and B–S–N triazine borate esters were synthesized and their antiwear/extreme pressure (AW/EP) properties were studied. Results showed the synthetical additives had good AW performance. However, B–S–N triazine borate ester showed excellent EP property while B–N triazine borate ester hardly owned EP property. The hydrolytic stability of borate ester additives was improved by the formation of coordination of nitrogen to boron. The XANES spectroscopy analysis showed that there was a layer of borate–oxygen–iron inorganics in the tribofilms. The existence of iron sulfate and iron sulfide guaranteed good AW/EP properties of B–S–N triazine borate ester additive in mineral oil.