Imidazolium-based ionic liquids used as additives in the nanolubrication of silicon surfaces

Abstract

In recent years, with the development of micro/nanoelectromechanical systems (MEMS/NEMS), the demand for efficient lubricants of silicon surfaces intensified. Although the use of ionic liquids (ILs) as additives to base oils in the lubrication of steel/steel or other types of metal/ metal tribological pairs has been investigated, the number of studies involving Si is very low. In this work, we tested imidazolium-based ILs as additives to the base oil polyethylene glycol (PEG) to lubricate Si surfaces. The friction coefficients were measured in a nanotribometer. The viscosity of the PEG + IL mixtures as well as their contact angles on the Si surface were measured. The topography and chemical composition of the substrates surfaces were determined with atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), respectively. Due to the hygroscopic properties of PEG, the first step was to assess the effect of the presence of water. Then, a series of ILs based on the cations 1-ethyl-3-methylimidazolium [EMIM], 1-butyl-3-methylimidazolium [BMIM], 1-ethyl-3-vinylimidazolium [EVIM], 1-(2-hydroxyethyl)-3-methylimidazolium [C2OHMIM] and 1-allyl-3-methylimidazolium [AMIM] combined with the anions dicyanamide [DCA], trifluoromethanesulfonate [TfO], and ethylsulfate [EtSO4] were added to dry PEG. All additives (2 wt %) led to a decrease in friction coefficient as well as an increase in viscosity (with the exception of [AMIM][TfO]) and improved the Si wettability. The additives based on the anion [EtSO4] exhibited the most promising tribological behavior, which was attributed to the strong interaction with the Si surface ensuring the formation of a stable surface layer, which hinders the contact between the sliding surfaces.

Recent highlights in nanoscale and mesoscale friction

“…Sliding in a confined geometry has been investigated with the surface-force balance and an impressive evidence of layering effects on friction was demonstrated [ 114 ]. RTILs are being also investigated as additives in liquid lubrication [ 115 ].…”

Lubrication Mechanism of Phosphonium Phosphate Ionic Liquid in Nanoscale Single‐Asperity Sliding Contacts

“…components, and micro-electromechanical systems. Since the first report in 2001 proposing the use of ILs for lubrication, [14] a number of studies have been published on the lubrication mechanism of ILs, most of which focused on imidazolium-, [22,25] phosphonium-, [26,27] or phosphate-based ILs. [28] Studies at the nanoscale have ascribed the lubrication mechanism of ILs to their adsorption and confinement between smooth surfaces, in absence of any mechano-chemical reactions.…”

Picolinium‐Based Hydrophobic Ionic Liquids as Additives to PEG200 to Lubricate Steel‐Silicon Contacts

“…[31] Effect of water content [TfO]) was studied in a previous work. [29] It was demonstrated that water saturated PEG200 led to a slight increase of the friction coefficient (CoF), while the CoF values obtained with the IL mixtures were independent of the water content. In the present work, the CoF of the mixture [C 6 -4-pic][TfO] + PEG200 was measured as a function of the water content (F = 30 mN, v = 4, 8, 12, 16 and 20 mm�s À 1 ), from the dry state to the saturation level achieved after several days exposure at ambient atmosphere, and the results are shown in Figure 3.…”

“…These results show that CoF is independent of the water content, in agreement with the previous report. [29] In the literature, the results concerning the effect of water on the lubrication performance of ILs are contradictory. Arcifa et al [32] found that tests done with hydrophobic imidazolium-based ILs using silica/silicon pairs, at a small applied load (0.5 N), in humid air led to an increase of wear and friction, which was attributed to the disruption of lubricant film.…”

“…Imidazolium, guanidinium, and pyridinium-based cations were coupled with the anions methylsulfate [MeSO 4 ], ethyl sulfate [EtSO 4 ], and methanesulfonate [MeSO 3 ] leading to excellent tribological properties. [29] This behavior was attributed to the strong interaction between sulfur and the Si surface ensuring the formation of a stable surface layer that hinders the contact between the sliding surfaces. The objective of the present work is twofold: 1) to test a more sustainable IL synthetic method based on microwave irradiation; 2) to study the tribological behavior of three new ILs based on picolinium, as additives to model lubricant Figure 1 illustrates the chemical structures of the synthesized ILs.…”