In situ nanoscale evaluation of pressure-induced changes in structural morphology of phosphonium phosphate ionic liquid at single-asperity contacts

Abstract

In this work, we perform atomic force microscopy (AFM) experiments to evaluate in situ the dependence of the structural morphology of trihexyltetradecylphosphonium bis(2-ethylhexyl) phosphate ([P6,6,6,14][DEHP]) ionic liquid (IL) on applied pressure.

Linking Molecular Structure and Lubrication Mechanisms in Tetraalkylammonium Orthoborate Ionic Liquids

“…Under the contact conditions used in the present study, tetraalkylammonium orthoborate ILs with asymmetric cations (i.e., [N 1118 ]) do not form sacri cial tribo lms via shear-induced mechanochemical reactions. This supports the conclusions of previous studies that suggested the antiwear mechanisms of ILs to arise from their pressure-induced morphological change, which results in the formation of a solid-like layered structure at the contact interface with low shear strength and di cult to displace [27,28,[73][74][75][76][77]. However, in the case of IL with a more symmetric cation (i.e., [N 2226 ][BOB]), the higher friction response is proposed to originate from the inability of this IL to create a lubricious, solidlike layer due to the reduced van der Waals interactions between the alkyl chains.…”

“…Surface force apparatus (SFA) and colloidal atomic force microscopy (cAFM) studies demonstrated that, in the absence of mechanochemical reactions (at low normal pressures, i.e., < 100 MPa), ILs form layered ionic structures when nanocon ned between model, smooth surfaces [27,28,[73][74][75][76], while also highlighting a strong dependence of the properties of the interfacial layers on the molecular architecture of the ILs, the water content in ILs, the chemistry of the solid surfaces, and the applied electrical potential [26,27]. To evaluate the response of phosphorus-based ILs at higher contact pressure (> 500 GPa), Li et al recently performed in situ AFM experiments and provided evidence that the lubrication mechanism of phosphonium phosphate ILs (PP-ILs) strongly depends on the applied normal pressure [77,78]. At an applied pressure below 5.5 ± 0.3 GPa, a lubricious, quasi-solid interfacial layer forms as a result of the pressure-induced morphological change of con ned ILs [77], while at normal pressures between 5.5 ± 0.3 GPa and 7.3 ± 0.4 GPa the progressive removal of the oxide layer from the steel surface leads to the adsorption of phosphate ions on metallic iron surface, which was proposed to generate a densely-packed, boundary layer that reduces nanoscale friction [78].…”

“…To evaluate the response of phosphorus-based ILs at higher contact pressure (> 500 GPa), Li et al recently performed in situ AFM experiments and provided evidence that the lubrication mechanism of phosphonium phosphate ILs (PP-ILs) strongly depends on the applied normal pressure [77,78]. At an applied pressure below 5.5 ± 0.3 GPa, a lubricious, quasi-solid interfacial layer forms as a result of the pressure-induced morphological change of con ned ILs [77], while at normal pressures between 5.5 ± 0.3 GPa and 7.3 ± 0.4 GPa the progressive removal of the oxide layer from the steel surface leads to the adsorption of phosphate ions on metallic iron surface, which was proposed to generate a densely-packed, boundary layer that reduces nanoscale friction [78]. Macroscale tribological experiments in the boundary-lubrication regime, however, provided evidence that ILs can tribochemically react on metallic surfaces [24,31,32,44,66,67,[79][80][81][82][83][84][85]].…”

Ionic Liquid‐Mediated Scanning Probe Electro‐Oxidative Lithography as a Novel Tool for Engineering Functional Oxide Micro‐ and Nano‐Architectures

“…On the other hand, the SAM on a polished steel surface resembles a well‐ordered “brush‐like” structure as a result of the low surface roughness as well as the crystalline subsurface structure. [ 80,83 ] This highly ordered SAM‐substrate complex exhibits higher surface polarization compared to the disoriented SAMs on the amorphous IL‐o‐SPL oxide films, which accounts for the distinction in surface potential between the two areas as shown by KPFM and LEEM. Regarding the chemistry of the SAMs, P 2p XANES spectra of the IL‐o‐SPL films of different tip voltages and the IL‐NO region do not exhibit significant differences in peak shape or position (Figure S4, Supporting Information), thus indicating that the adsorbed IL maintains the same chemical structure after the IL‐o‐SPL process.…”