Molecular Design of Self-Adsorbed Sulfur- and Phosphorus-Free Organic Borate Additives and Their Tribological Properties

To address hydrolysis susceptibility and high additive concentrations in borate ester lubricants, this study designs molecules with sterically hindered aryl groups, B → N coordination bonds, and stable six-membered rings. These features reduce boron nucleophilicity, extending hydrolytic stability to 27 days versus 24 h for tributyl borate. In POE 100 oil (2 wt %), the optimized additive P2–12 improves performance: the noseizure load (PB) increases by 67%, the weld point (PD) by 26%, and the wear scar diameter (WSD) decreases by 37%, with a stable COF under 100 kg. DFT indicates load-dependent film evolution: at low loads, electron-rich benzene rings form a chemisorbed film that reduces friction and inhibits corrosion; at high loads, the film converts to a robust tribofilm of BN, graphitized carbon, and polymeric carbon that prevents wear. This work presents a molecular design strategy for organoborate esters with improved hydrolytic stability, anticorrosion properties, and surface adsorption.