Lubrication Performance of Ionic Liquids as Lubricants for Space Mechanisms under High Vacuum and Low Temperature

Abstract

The objective of this study is to evaluate the boundary lubrication performance of ionic liquids under high vacuum and low temperature by taking the cosmic space environment into consideration, as a screening stage prior to evaluating lubrication performance in actual space mechanisms. The boundary lubrication performance of ionic liquids was evaluated at room temperature with a reciprocating linear motion tribometer, and at low temperature (from −80°C to room temperature) with a unidirectional rotation tribometer. Low-temperature rheometry was also carried out. Ionic liquids showed a supercooling state and crystallization. This crystallization was prevented by mixing different ionic liquids together. The equimolar mixture of 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl) amide (EMI-TFSA), 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl) amide (BMI-TFSA), and 1-hexyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl) amide (HMI-TFSA) showed no crystallization in our experiment. The antiwear performance of this sample oil mixture was similar to that of MAC and much better than that of PFPE at low temperatures. This mixture prevented metal contact at −80°C, most likely due to its high viscosity and high adsorption of molecules.

Ionic Liquids as Performance Ingredients in Space Lubricants

“…As many of the properties that are attractive for solvents are also attractive for lubricants, tribologists have suggested the use of ionic liquids as lubricants since 2001 [ 2 ]. The concept of ILs as lubricants has been reviewed thoroughly over the years [ 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 ], and ILs as space lubricant have been an active field of research since the start of IL lubricant research [ 2 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 ]. Although ILs have gained a strong foothold as lubricant additives in terrestrial applications [ 22 ], their potential as space grade lubricants has not been truly realized yet [ 18 , 19 ].…”

Effect of a Long Alkyl Chain in Protic Ionic Liquids Molecules on Tribological Properties at High Temperature

“…These ILs have been shown to exhibit superior antiwear and friction-reducing properties. Because triboreaction film has been detected on rubbed steel surfaces via X-ray photoelectron spectroscopy (XPS), ILs are thought to break down upon reaction with steel surfaces [2][3][4][5][6][7][8][9][10][11][12]. This behavior of ILs is similar to that of extreme pressure (EP) additives, which typically contain sulfur, phosphorus, or halogen atoms.…”

Nano- and Macroscale Study of the Lubrication of Titania Using Pure and Diluted Ionic Liquids

“…ILs are pure salts that are liquid at low temperatures. Their exceptional physical properties including high thermal stability and negligible vapor pressures mean that they can be used in high temperature or low pressure environments such as outer space (Kobayashi et al, 2015; Nancarrow and Mohammed, 2017). Furthermore, ILs interact strongly with themselves and surfaces via electrostatic, van der Waals, h-bonding and solvophobic interactions (Jiang et al, 2018).…”