Synergistic Enhancement of the Lubrication Performance of Zinc Dialkyldithiophosphate by Coexistence with Ionic Liquid

Abstract

Ionic liquids (ILs) are being considered as novel lubricant additives to improve the friction and anti-wear characteristics of sliding surfaces. Because ILs consist of only cations and anions, their physical and chemical properties can be easily tailored by modifying their combination. These features enable to design of specific ILs for a given tribological system. However, there are few reports on the effect of ILs used as lubricant additives on the friction and anti-wear characteristics of sliding surfaces. Moreover, it is necessary to investigate the synergism between ILs and other lubricant additives to achieve high practical efficiency. Therefore, in this study, two ILs with different cations but similar anions were used in combination with an anti-wear additive, zinc dialkyldithiophosphate (ZDDP), and the resultant wear and friction characteristics of the worn surfaces were analyzed. We found that the tribological performance under steel/ steel sliding conditions was dependent on the chemical composition of the ILs. In addition, mixed ZDDP and IL solutions exhibited lower friction and wear when compared to ZDDP alone. It is considered that the excellent lubrication and frictional performance observed with the mixed lubricant solutions is due to the formation of tribofilms comprising of ZDDP and IL on the sliding surface.

Effects of Relative Humidity on Lubricating Properties of Ionic Liquids

“…Iron phosphate is known to exhibit good lubricating properties as it forms a stable surface protective film. It is likely that this is the reason for the desirable results obtained for [BMIM][PF 6 ] at 15% relative humidity [7][8][9][10]. This tribolayer was also present at 50% relative humidity.…”

“…Therefore, lubricants are being developed to reduce the friction losses in these systems. Lubricants can be categorized as solid lubricants [2][3][4], semisolid lubricants [5], and liquid lubricants [6][7][8]. Liquid lubricants under boundary lubrication reduce friction and wear by forming an adsorption and reaction layer at the friction interface.…”

“…Liquid lubricants under boundary lubrication reduce friction and wear by forming an adsorption and reaction layer at the friction interface. In recent years, liquid lubricants that form a strong adsorption and reaction layer have been developed [7][8][9][10][11][12]. However, their lubricating properties are not significantly better.…”

Advances in Tribology Driven by Surface Science

“…Their application as lubricants was reported in a 2001 study by Liu et al [46]. Ionic liquids are expected to be used as lubricating base oils under special environments such as high temperatures [47] and in vacuum [48] due to their low vapor pressure, high thermal stability, and wide variety of structures, and their recent use as additives has also attracted attention [49]. When research began on ionic liquids as lubricating oils, most contained fluorine [50].…”