Influences of the spacing between nitrogen atoms on tribological performance of dual active site phosphorus-nitrogen (P-N) ionic liquid

Four dual active site ionic liquids (ILs): N2NP0, N4NP0, N6NP0, N10NP0 and three single active site ILs: N2P0, N3P0, N8P0 were synthesized successfully, furthermore, anti-wear and extreme pressure properties of the additives were tested with 0.5 wt% concentration in PAO10 base oil. The results showed that N6NP0 with carbon chain length of six between the nitrogen atoms exhibits the best tribological performance (The wear scar diameter is 0.39 mm, the values of P B (the last non-seizure loads) and P D (weld points) were 135 Kgf and 200 Kgf, respectively), which is mainly attributed to the formed thicker tribo-film (thickness of 50–60 nm) on the friction surface. Through analyzing the tribological morphology, chemical composition and tribo-film microstructure of wear scars surface by SEM-EDS, 3D profile, AFM, XPS and FIB-TEM, the lubrication mechanism of ILs was revealed. Furthermore, density functional theory (DFT) calculations confirmed that the tribological performance of dual active site ionic liquid additives depend strongly on their molecular configuration. It was found that rational adjustment of the length of the ILs cationic active site linkage group results in a reduction of the steric hindrance of the entire molecule. This study will help to guide the design of ILs with good tribological performance and low phosphorus content.