Tribological Mechanism of Environment-Friendly Amino Acid-Based Ionic Liquids as Water Lubrication Additives

The utilization of water-based lubricants in certain applied fields, as an alternative to oil-based lubricants that are prone to causing environmental pollution, represents one of the effective strategies for achieving green tribology. To tackle the issues of subpar lubrication properties and susceptibility to corrosion of friction interface for water-based lubrication, two amino acids-based ionic liquids, Lys-LS and Arg-LS, were synthesized in this manuscript by a simple proton exchange reaction using L-lysine or L-arginine and Lauroyl sarcosine, and were utilized as water-based lubricant additives. The thermal decomposition 5% temperatures of Lys-LS and Arg-LS reached 219.8 ℃ and 217.5 ℃, respectively, exhibiting exceptional thermal stability. The corrosion resistance properties of Lys-LS and Arg-LS aqueous solutions against first-grade gray cast iron sheets were assessed in accordance with the methodologies outlined in the national standard GB 6144-2010. Both the corrosion area and the number of corrosion stains were significantly reduced compared to deionized water when the two amino acids-based ionic liquids were incorporated at a mass fraction of ≥ 0.5%. The tribological properties of Lys-LS and Arg-LS additives were investigated using an SRV-V fretting friction and wear tester and automatic true-color confocal microscope. Compared with deionized water, the friction coefficient and wear volume of the water-based lubricants were reduced by about 70% and 85%, respectively, when Lys-LS and Arg-LS were supplemented at a concentration of 0.5%, providing a significant friction reduction and anti-wear performance. The micro-morphological analysis results of the wear spot surface revealed that the wear surface was smoother after lubricated by the aqueous solution containing ionic liquids additives, and at the same time, the adhesive wear and corrosive wear were effectively suppressed. The lubrication mechanisms of two ionic liquids additives were investigated in depth using the quartz crystal microbalance (QCM) and the X-ray photoelectron spectroscopy (XPS). The QCM tests indicated that the two amino acid-based ionic liquids could be effectively adsorbed onto the rubbing interface during the friction process to form a stable adsorption films. The XPS analysis results revealed that the tribochemical reaction occurred between the two additives and the surface of the friction pair, and a tribological protective films enriched with C, Fe, O and N elements were generated. The tribochemical reaction films and physical/chemical adsorption films formed by the ionic liquids at the interface worked synergistically to effectively prevent direct contact between the sliding pairs, endowing the water-based lubricant with excellent tribological properties. The two amino acids-based ionic liquids additives were characterized by simple preparation, non-toxicity and greenness, and were promising to be employed as critical additives for metalworking fluids and flame-retardant hydraulic fluids.