Effects of phosphorus-free antioxidants on oxidation stability and high-temperature tribological properties of lubricants

Oxidation stability plays an important role on the engine oil service performance. In this paper, the phosphorus-free antioxidants of diphenylamine, hindered phenol and dibutyldithiocarbamate, combined with zinc dialkyldithiophosphate were added as antioxidants in the base oil and the fully formulated 5W-30 oil, and the oxidation stabilities were evaluated by pressurized differential scanning calorimetry and Romaszewski oil bench oxidation standard tests. Meanwhile, the tribological properties of the fresh and aged oils were evaluated by a SRV tribo-meter. The results indicated that (i) an optimal ternary complex antioxidant of dibutyldithiocarbamate: diphenylamine: hindered phenol (ratio of 2:1:2) displayed good antioxidation property, and (ii) the fully formulated 5W-30 oil containing optimized ratio phosphorus-free antioxidants had better tribological properties than the commercial SN 5W-30 oil.