Tribological Behavior of Some Long-Chain Dimercaptothiadiazole Derivatives as Multifunctional Lubricant Additives in Vegetable Oil and Investigation of their Tribochemistry Using XANES

A series of novel long-chain dimercaptothiadiazole derivatives are prepared and used as antiwear (AW) and extreme pressure (EP) additives in vegetable oil, and their tribological performance is tested by using a four-ball tester. In order to understand the friction process further, X-ray absorption near edge structure spectroscopy is adopted to analyze the chemistry of tribofilms under AW/EP regime, and meanwhile thermal films are also considered for comparison. These derivatives are capable in improving the tribological characteristic of the base stock, and disubstituted derivatives are more effective than other derivatives. Though disubstituted derivatives and disubstituted polysulfur derivatives all fail in improving tribological performance at 0.1 wt%, they are still helpful at other additive concentrations. All these long-chain thiadiazole derivatives, in particular the disubstituted polysulfur ones, are good at improving the EP characteristic of the base colza oil. Thermal films generated from these derivatives are composed of ferrous sulfate and a small amount of adsorbed organic sulfide. During the heating process, these long-chain derivatives easily undergo thermal oxidation to generate high-valent sulfate on the metal surface. Ferrous sulfide is the main component of tribofilms generated by these derivatives, while ferrous sulfate in these films almost vanishes at 1.0 wt%. These long-chain derivatives tend to react with metal surface to generate low-valent sulfide under rubbing conditions. On the other hand, ferrous sulfide is also the main component of EP films generated by monosubstituted derivatives and disubstituted polysulfur derivatives at 1.0 wt%. But the EP films formed by disubstituted derivatives are composed of ferrous sulfide and ferrous sulfate.