Preparation and Tribological Behaviors of Sulfur- and Phosphorus-Free Organic Friction Modifier of Amide–Ester Type

With the increasingly demanding engine conditions and the implementation of “double carbon” policies, the demand for high-quality lubricants that are cost-effective and environmentally friendly is increasing. Additives, especially high-performance friction modifiers, play an important role in boosting lubricant efficiency and fuel economy, so their developments are at the forefront of lubrication technologies. In this study, 1,3-dioleoamide-2-propyloleate (DOAPO), which incorporates polar amide, ester, and nonpolar alkyl chains, was synthesized from 1,3-diamino-2-propanol to give an eco-friendly organic friction modifier. Nuclear magnetic resonance (NMR), high-resolution mass spectrometry (HR-MS), Fourier-transform infrared spectroscopy (FT-IR), and thermogravimetric analysis (TGA) were used to characterize the structure and thermal stability of DOAPO. Meanwhile, the storage stability and tribological behaviors of DOAPO in synthetic base oil were studied and compared with a commercial oleamide. The results show that DOAPO has better thermal stability and better storage stability in synthetic base oil. Additionally, 0.5 wt.% of DOAPO could shorten the running-in period and reduce the average friction coefficient (ave. COF) and wear scar diameter (ave. WSD) by 8.2% and 16.2%, respectively. The worn surface analysis and theoretical calculation results show that the ester bond in DOAPO breaks preferentially during friction, which can reduce the interfacial shear force and easily react with metal surfaces to form iron oxide films, thus demonstrating a better friction-reducing and anti-wear performance.