Effects of sodium ions on the tribological and antioxidation properties of borates

A borate ester (BMBS) and its sodium salts (BMBSNa) were synthesized and used as multifunctional lubricant additives in oleic acid. Results showed that BMBSNa possessed better anti-oxidation and tribological properties than BMBS. Worn surfaces analysis revealed that BMBSNa was more favorable to forming boundary film containing h-BN and FeS. Theoretical calculation suggested that BMBSNa had a higher chemical reactivity and better metal adsorption ability than BMBS. Fukui indices showed that sodium ions favor free radical reaction. Radial distribution functions (RDF) indicated that sodium ions help the adsorption of BMBSNa on the iron surface. It can be concluded that sodium ions benefit the oxidation resistance of base oil, and contribute to forming good boundary lubrication film to achieve good tribological properties.