Five-Stage Selection Procedure of Ionic Liquids for Lubrication of Steel–Steel Contacts in Space Mechanisms

An experimental approach using the stage-gate process is presented to efficiently select and assess ionic liquids (ILs) for their usability in space mechanisms, thus, for their potential to replace commonly used liquid lubricants based on perfluorinated polyethers and multiply alkylated cyclopentanes. This methodology was based on a five-stage selection procedure comprising the determination of rheological properties, outgassing properties, corrosion-inhibiting capabilities, screening of friction and wear performance in vacuum, which was completed by tribometrical lifetime assessments. Five ILs were benchmarked against Fomblin^® Z25 as reference at the end of each stage and selected for the next stage depending on the performance. One IL of the type pyrrolidinium bis(trifluoromethylsulfonyl)amide outperformed Fomblin^® Z25 in all stages except pour point. Thus, only in the case of fluidity at very low temperature showed Fomblin^® Z25 a better performance. Additives slightly improved corrosion inhibition of this IL but showed adverse effects on friction and wear in comparison to the neat IL. In lifetime experiments, the IL resulted in a lifetime extension of at least factor 23 and 31 compared to the reference. Even with the use of additives in this IL, the lifetime extension was still by a factor of 6 to 15 compared to Fomblin^® Z25.