Tribological Testing and Thermal Analysis of an Alkyl Sulfate Series of Ionic Liquids for Use as Aerospace Lubricants

Abstract

Due to their low vapor pressures, low melting points, high boiling points, high radiation resistance, and high thermal stability, room-temperature ionic liquids (ILs) appear to be suitable candidates as new aerospace lubricants for the upcoming return to the Moon and eventual Mars missions and for air and rotorcraft applications. In this study, three ILs with the same cation, 1-butyl-3-methylimidazolium, but different sulfate anions were tested using an ultra-high vacuum spiral orbit tribometer (SOT) and their thermal properties were determined by thermogravimetric analysis (TGA). Specifically, 1-butyl-3-methylimidazolium methyl sulfate, 1-butyl-3-methylimidazolium ethyl sulfate, and 1-butyl-3-methylimidazolium octyl sulfate were tested. The SOT experiments revealed that the lifetimes of the three ILs decreased with increasing alkyl substituent length on the sulfate anion. Infrared and Raman spectra were taken to detect unused ILs and graphitic degradation products, respectively, on worn parts. Post-run spectroscopic analysis indicated residual degraded, but still usable, ILs in all runs, coupled with varying amounts of amorphous graphitic material produced as the final degradation product of all ILs. SOT testing indicated that these ILs have lower friction coefficients and lifetimes greater than those of two commonly used perfluoropolyalkylether (PFPE) space lubricants. TGA showed that the methyl sulfate IL had the highest thermal stability in air and nitrogen. The vapor pressure of the methyl sulfate IL is as at least as low as Fomblin 815Z at 20°C.

KEY WORDS:

• Ionic Liquids
• Spiral Orbit Tribometry
• Vapor Pressure
• Thermogravimetric Analysis

ACKNOWLEDGEMENT

The authors thank Daniel A. Scheiman (ASRC Inc., NASA–Glenn Research Center, Cleveland, Ohio, USA) for the TGA profiles of the ILs used in this study.