Space application represents a challenge for the lubrication of tribological components from two perspectives – performance and cleanliness. Due to the vacuum of space, and in particular in the presence of sensitive (e.g. optical) components, an extremely low volatility is required to limit evaporation or outgassing, which can lead to lubricant loss and contamination issues. Lubricants must remain functional over wide temperature ranges, chemically inert to withstand radiation and aggressive species, and compatible with a wide range of spacecraft materials including metals, plastics, and elastomers. 

The majority of fluids used for space applications is based either on perfluoropolyether (PFPE) or multiple alkylated cyclopentanes (MAC). PFPE lubricants have a long history in space mechanisms, they benefit from their robust chemical structure, which reflect in an extremely high chemical and thermal stability, and also from their extremely low vapor pressures. Their disadvantage is a limited tribological lifetime due to degradation reactions on ferrous surfaces (more general: in the presence of Lewis acids). Also, PFPE will be subject of a restriction proposal that addresses the risks to human health and the environment posed by the manufacture and use of per- and polyfluoroalkyl substances (PFASs) due to their very high persistence (so-called “forever” chemicals). MAC oils, on the other hand, are also known for low vapor pressures, good wear protection and a low tendency to creep. They display a high tribological durability, but are more susceptible to chemical attack and oxidation. Their stability can be improved by the use of additives, which however have a negative impact on outgassing. 

The unique properties of Ionic Liquids (IL) have been extensively documented, and their exceptional tribological performance has been showcased in numerous technical articles. They offer a wide range of achievable properties due to their significant variability in chemical structure. As molecular salts, ILs display negligible vapor pressures, which makes them attractive especially for vacuum applications that particularly suffer from evaporation, and where even traces of volatile material pose a contamination risk for sensitive equipment. 

However, identifying suitable candidates for vacuum lubricant base oils that meet all requirements for application in space means a challenge. The corrosive impact of many tribologically advantageous ILs on metallic materials is a major obstacle. Furthermore, a PFAS restriction might also restrict the use of ILs with fluoroorganic moieties, which have the widest range of use temperatures. Therefore, and based on the development work that has been conducted at Materiales since 2021, in this activity (ESA contract 4000141962_23_NL_AS_ov) Ionic Liquid base oil formulations for application in space were developed. While the development of a dedicated lubricant specification was part of the project, some fundamental target properties were defined in advance:

• Use temperature range from -20 °C (ideally -40 °C or lower) to +150 °C
• Low outgassing – fulfillment of ECSS-Q-ST-70-02C standard
• Non-corrosive to typically used metals and alloys
• Tribological lifetime (SOT level) significantly higher than for PFPE, ideally same or better than for MAC
• Not affected by upcoming restriction for PFAS 

An electrochemical corrosion test approach was to be developed in order to allow a fast indication and to accelerate development activities. A final oil formulation was to be selected for manufacturing and comprehensive testing of lubricant greases. Development target for the base oil formulations was TRL 3-4.