Thermo-oxidative stability and corrosion properties of ammonium based ionic liquids

Ionic liquids (ILs) as a novel and potential type of lubricants possess partly superior properties over traditional classes of lubricants. Their extremely low vapor pressure, generally high thermal stability and non-flammability suggest them for high performance applications. However, their tendency towards corrosiveness is neglected in general. The selection of three ILs was based on the aim to achieve relevant lubricant properties, in particular high oxidation stability and low corrosiveness, as well as high environmental benignity. The cations were in all cases ammonium based with and without functionalization. Bis(trifluoromethylsulfonyl)imide (NTF2) was chosen as anion and has been replaced for one IL by methanesulfonate. Artificial aging was carried out to obtain knowledge about the lubricant long-term performance under both oxidative and humid conditions while being in contact with CuSn8P and 100Cr6 commonly used in tribology. For the evaluation of IL corrosion potential, the metal content in the IL was monitored by ICP-OES, metal specimens were examined optically and by SEM-EDX analysis. To find out the IL suitability for the long-term applications the thermo-oxidative stability of the IL has been analyzed by several mass spectrometric techniques. In this study, NTF2 based IL – regardless of the cationic moiety – showed superior performance over methanesulfonate based IL under all conditions. In the case of 100Cr6, dry conditions lead to the lowest corrosion whereas CuSn8P caused the lowest corrosion under humid conditions. The degradation process based on thermally induced transmethylation of the IL investigated occurred only at the cationic moiety of methanesulfonate based IL This is based on mass spectrometric investigations and indicates a contribution to the enhanced corrosiveness by means of the IL reduced stability.