Christoph Gabler

How anion and cation species influence the tribology of a green lubricant based on ionic liquids

A group of halide-free ionic liquids involving two different anions (methyl sulfate and methyl sulfonate) and four types of cations (short-chain tetraalkylammonium, dialkylpyrrolidinium, choline, and methoxycholine) were investigated as 2.5 wt% additives in glycerol as a model base fluid, yielding highly biodegradable polar lubricants for study of ionic liquid interaction with a substrate. The results were compared to the behavior of conventional bis(trifluoromethylsulfonyl)imide ([Tf2N]) ionic liquids with identical counter-ions. The neat ionic liquids (100 wt%) were tested in identical manner and compared to the behavior when they operate as additives. Tribotests were performed in a ball-on-disc configuration under boundary conditions, by lubricating steel–steel couples at room temperature and at 100°C. Wear reduction was achieved for all temperatures, and the results were strongly anion-dominated, with good results for methyl sulfates and the [Tf2N] references. Particularly for higher temperatures, ionic liquids were also able to reduce friction by a substantial amount, with a clear order between the individual anions, and the lowest values were again obtained for methyl sulfates. Cationic influence on the test results was found to be subordinate for both temperatures. It could be recognized that at elevated temperatures, the newly formulated lubricants containing an ionic liquid as an additive behaved similarly to neat ionic liquids in terms of friction and wear reduction. Using X-ray photoelectron spectroscopy analyses, the formation of a beneficial iron sulfide film was detected, with the sulfur originating from the sulfate of the ionic liquid, presumably as a result of a redox reaction with metallic iron. For this mechanism, a hypothesis for possible reaction pathways was developed.

Corrosion properties of ammonium based ionic liquids evaluated by SEM-EDX, XPS and ICP-OES

Two ammonium-based ionic liquids (IL) with the potential to be used as lubricants were investigated with respect to their corrosion ability. With the intention to reduce the potential negative environmental impact of lubricant ILs, (2-hydroxyethyl)-trimethyl-ammonium (choline) was chosen as the cation and compared with butyl-trimethyl-ammonium. Two commonly used metals were immersed in the selected ILs to simulate severe conditions. For each material, a corrosion inhibitor was selected and added to the respective IL to confer corrosion protection. The corrosive behavior of the ILs was determined by specially designed small-scale experiments and monitoring of the metal content in the ILs by inductively coupled plasma optical emission spectrometry (ICP-OES), showing preferable properties of the choline IL. After the corrosion experiments, the morphology and the elemental composition of the corroded metal surfaces exposed to ILs were studied by scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry (EDX) and X-ray photoelectron spectroscopy (XPS). Besides ICP-OES, surface analysis to determine the chemical composition on the surface and to identify corrosion products contributed to the qualitative evaluation of the effectiveness of the corrosion inhibitors. It was also possible to show that the corrosion properties of ILs can be significantly improved by the addition of selected corrosion inhibitors.

Imaging of a Tribolayer Formed from Ionic Liquids by Laser Desorption/Ionization-Reflectron Time-of-Flight Mass Spectrometry

For the first time, imaging using laser desorption/ionization (LDI) reflectron time-of-flight (RTOF) mass spectrometry (MS) was demonstrated to be a powerful tool for an offline monitoring of tribometrical experiments directly from disc specimen applying selected ammonium-, phosphonium-, and sulfonium-based ionic liquids (IL) with bis(trifluoromethylsulfonyl)imide as counterion for lubrication. The direct measurement of IL tribolayers by LDI-MS allowed the visualization of the lubricants in the form of the distribution of their intact cations and the anion in and outside the wear scar after the tribometrical experiment with a low degree of in-source generated fragmentation. Besides, also, an oxidation product formed during a tribometrical experiment was detected and located exclusively in the wear track. Comparative data of identical wear tracks were obtained by X-ray photoelectron spectroscopy (XPS) imaging not only enabling the determination of elemental distributions of the IL across the area imaged but also corroborating the mass spectrometry imaging (MSI) data, thus generating multimodal images. Merging data from MSI and XPS imaging exhibited that areas, where iron-fluorine bonds were detected in the wear track, are corresponding to data from LDI-MS imaging showing absence of IL cations and anions.