The Relationship Between the Chain Length and Tribological Properties of N/P Halogen-Free Ionic Liquid Lubricants

N/P halogen-free ionic liquids (NPILs: code as NP-11114, NP-11116, NP-11118) lubricants with different chain lengths were synthesized. The viscosity-temperature properties, thermal stability and lubricating properties of these ionic liquids were evaluated by using poly-α-olefin (PAO 10) and 1-octyl-3-methylimidazole hexafluorophosphate (L-P 108) as a reference. The effects of cation chain length changes of NPILs on their physicochemical properties and tribological properties were studied. The results show that the viscosity of NPILs is higher than PAO 10 and L-P 108, and the thermal decomposition temperature is lower than that of PAO 10 and L-P 108. The viscosity and thermal decomposition temperature of NPILs increase with the increasing chain length. As lubricants for steel/steel friction pairs, NPILs have lower friction reduction performance than L-P108 at room temperature, but the wear resistance of NP-11118 is better than L-P108; at high temperatures, the friction reduction and wear resistance of NPILs are better than L-P 108. NPILs have excellent antifriction and anti-wear properties both at normal temperature and high temperature, and tribological properties increase with increasing alkyl chain length. Scanning electron microscopy (SEM), elemental analysis and XPS results of the wear spot demonstrate that the excellent tribological properties of such ionic liquids are attributed to the tribochemical reaction of metal substrates and N, P in the ionic liquid structure to form tribo-film with excellent antifriction and antiwear properties.