Tribological Properties of Amino Acid Ionic Liquids as Lubricants for Different Contacts

Three kinds of amino acid ionic liquids with different functional groups were synthesized, and the tribological properties of amino acid ionic liquids were evaluated as lubricants for the Si₃N₄/Si₃N₄ and steel/steel contacts by an Optimal SRV-Ⅳ oscillating friction and wear tester. Meanwhile, the glycerol was selected as reference lubricant. In addition, the morphology and chemical state of elements on the worn surfaces were analyzed by a JSM-5600LV scanning electron microscope and a PHI-5702 multifunctional X-ray photoelectron spectrometer. The results show that the three kinds of amino acids ionic liquids can be used as the lubricants for the two contacts and exhibited excellent tribological performance. It is concluded that the hydroxyl and carboxyl functional groups not only enhanced the physical adsorption effectively but also prompted the tribochemical reactions. Besides, the anti-friction and anti-wear properties of glutamic acid tetrabutylphosphorus ionic liquid with two carboxyl functional groups were better than those of the other two ionic liquids with a single carboxyl or hydroxyl functional group.