Effect of sulfur on the hydrolytic stability and tribological properties of N-containing borate ester

Two borate esters, one with benzotriazole (BTB) and another with 2-mercaptobenzothiazole (BMB) were synthesized and used as additives in mineral oil. BMB showed better hydrolysis stability, probably because S atoms in BMB provided negative charges to the N to enhance B←N coordination. Moreover, BMB was superior in terms of improving PB value, anti-wear and friction-reducing properties of mineral oil. XPS spectra revealed that h-BN, B2O3 and FexOy were detected in BTB-tribofilm, while additional FeSO4, FeS and B2S3 were found in BMB-tribofilm. The possible triple effects of S atoms in BMB can be summarized as: enhancing the B←N coordination; improving the adsorption ability of BMB on the metal surface; and generating tribochemical reaction products with tribological effects.