Liquid Lubrication for Space Mechanisms: The Emerging Role of Ionic Liquids

Abstract Lubrication in space mechanisms is governed by specific requirements arising from extreme conditions such as vacuum, radiation, thermal gradients, and the impossibility of in situ maintenance. This article firstly traces the development of space-grade lubricants, from early investigations with silicones and mineral oils to the current use of heritage lubricants such as perfluoropolyethers (PFPEs) and multiply alkylated cyclopentanes (MACs). While PFPEs have become the reference fluids thanks to their low vapor pressure and high thermal stability, increasing environmental restrictions on fluorinated compounds drive the search for sustainable alternatives. MACs are a modern alternative to PFPEs with improved boundary lubrication properties. However, these lubricants suffer from technical limitations such as dewettability and initial seizure-like high friction values. In this context, ionic liquids (ILs) have emerged as promising candidates due to their negligible volatility, high chemical, and thermal stability. The second part of the review compares the performance of these lubricant families, addressing challenges such as evaporation losses, tribochemical degradation, or atomic oxygen resistance, and concludes with recent advances on ILs as both primary lubricants and functional additives.