Adsorption and Tribochemical Factors Affecting the Lubrication of Silicon-Based Materials by (Fluorinated) Ionic Liquids

In this study, the tribological behavior of a silica/silicon tribopair lubricated with anhydrous fluorinated ionic liquids was investigated by pin-on-disk tribometry. A first series of tests was designed to detect the onset of surface damage as a function of sliding speed. 1-ethyl-, 1-hexyl-, and 1-dodecyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide ([EMIM] TFSI, [HMIM] TFSI, and [DDMIM] TFSI, respectively), and 1-ethyl-3-methylimidazolium tris­(perfluoroethyl)­trifluorophosphate ([EMIM] FAP) were selected as lubricants. Elastohydrodynamic lubrication (EHL) predictive formulas for minimum film thickness (<i>h</i>_{0 EHL}) were used to estimate the value of the critical <i>h</i>_{0 EHL} at the transition to mixed lubrication, which was identified on the basis of the variations observed in friction upon decreasing the sliding speed. Taking into account the composite roughness of the counterparts (<i>R</i>_q ∼ 1.1 nm), the differences in the critical values of <i>h</i>_{0 EHL} measured for the various ILs are suggested to reflect changes in the interfacial structure of the confined ILs. In particular, the possible effects associated with the alkyl chain attached to the imidazolium cation or with the type of anion are discussed on the basis of models of the IL–solid interface presented in the recent literature. The second part of the investigation focuses on the tribological behavior observed with a SiO₂/Si tribopair lubricated with 1-ethyl- and 1-hexyl-3-methylimidazolium-based bis­(trifluoromethylsulfonyl)­imide ILs under boundary conditions. Tests were carried out under conditions that prevent fluid-film lubrication, so as to allow the study of the boundary-lubrication regime over an extended range of speed. The mechanism of wear was investigated on the basis of a chemical, structural, and topographical analysis of the worn surfaces. The prevailing mode of surface damage is described on the basis of a comparison with results previously obtained in tests of other imidazolium-based ILs as lubricants for the same tribopair. The new results revealed some further important details on the relationship between the chemical structure of the IL and the wear mechanism of the investigated tribopair