Preparation of 2,5-Dimercaptothiadiazole Mixed Phosphate Diamine Salt and Its Tribological Performance in Low-Viscosity Lubricating Oil

Abstract

Abstract. In this paper, 2,5-dimercaptothiadiazole mixed phosphate diamine salt (SNP), an antiwear additive, was formulated via stepwise temperature-controlled amination. Its structure and thermal stability were characterized using infrared (IR) spectroscopy, nuclear magnetic resonance (NMR), mass spectrometry (MS), and thermogravimetric analysis (TGA). Results confirmed that SNP contains a thiadiazole ring, phosphate ester, and amine functional groups, with its main thermal decomposition stage at 290.24–332.99 °C. The tribological performance of SNP in HVIP6 base oil was evaluated and compared with that of commercial phosphite, phosphate, and borate esters. Four-ball tests showed that 1% SNP yielded a wear scar diameter of 0.359 mm, 55.18% smaller than that of the base oil and significantly lower than those of phosphite ester (0.499 mm), phosphate ester (0.484 mm), and borate ester (0.810 mm). scanning electron microscopy and X-ray photoelectron spectroscopic analyses revealed that SNP dissociates into PO43− and NH4+ at the friction interface, forming a dense nanocrystalline FePO4 protective film; the positive charge of NH4+ enhances the orderliness of the FePO4 film. In contrast, phosphite and phosphate esters exhibited poor antiwear performance, primarily due to insufficient stability of the formed tribofilms. In contrast, borate esters barely generated effective protective films. The self-synthesized SNP consequently provides a new approach for the design of antiwear additives in low-viscosity lubricating oils.