Tribological behavior of sulfonitrided tribofilm generated from non-corrosive dimercaptobenzothiadiazole-based additives as effective boundary lubrication layer

Abstract

Lubricant additives are essential to protect mechanical devices. Although sulfur (S)-based dialkylpentasulfide (RC2540) exhibits acceptable extreme pressure (EP) performance, a high amount of S causes severe corrosion and poorer anti-wear (AW) performance. Herein, dimercaptobenzothiadiazole-based additives (DMTDs) consist of four and five S atoms, namely 2-(benzyldisulfanyl)-5-(benzylsulfanyl)-1,3,4-thiadiazole (4SBZ), 2-(octyldisulfanyl)-5-(octylsulfanyl)-1,3,4-thiadiazole (4SC8), 2,5-bis(benzyldisulfanyl)-1,3,4-thiadiazole (5SBZ), and 2,5-bis(octyldisulfanyl)-1,3,4-thiadiazole (5SC8) were synthesized. The synthesized DMTDs are non-corrosive (Grade 1a-1b) compared to RC2540 (Grade 4c). The 10 wt% 5SBZ demonstrated exceptional EP and AW performance than RC2540. The 10 wt% 5SBZ has improved weld load and reduced wear scar area by 5.0 folds and 94.1% compared to naphthenic base oil. The exceptional tribological properties of 5SBZ are attributed to the strong non-covalent interaction between the benzene ring and metal surfaces, high S content, and synergistic effect between S and N. Alternatively, the DMTDs exhibit better AW performance compared to RC2540. The XPS depth profiling demonstrated the formation of sulfonitrided tribofilm after lubrication with synthesized DMTDs. This tribofilm contributed to an excellent boundary lubrication layer for wear protection. The lubrication mechanism was discussed and proposed. This work provides promising DMTDs as EP and AW additives for industrial applications.