Tribological Study of Oil-Miscible Quaternary Ammonium Phosphites Ionic Liquids as Lubricant Additives in PAO

Two oil-miscible quaternary ammonium phosphites ionic liquids (POPA and PTPA) as lubricant additives were prepared with an environmentally friendly method. The tribological properties were evaluated on a four-ball tester, which suggests that both POPA and PTPA have better extreme pressure, antiwear and friction-reducing properties than that of ZDDP at all test conditions. Furthermore, the worn surfaces were investigated by X-ray absorption near-edge structure (XANES) spectroscopy. Both the P K-edge and L-edge XANES spectra show that the additives decompose and react to form polyphosphates under the boundary lubrication condition, which prevents the direct contact of asperities on the metal surface of the friction pairs. The polymerization degree of phosphate plays a key role in the tribological performances. The higher polymerization degree of polyphosphate leads to better antiwear and friction-reducing properties.