Nano- and Macroscale Study of the Lubrication of Titania Using Pure and Diluted Ionic Liquids

Abstract

Titanium is a strong, corrosion-resistant light—weight metal which is poised to replace steel in automobiles, aircraft, and watercraft. However, the titanium oxide (titania) layer that forms on the surface of titanium in air is notoriously difficult to lubricate with conventional lubricants, which restricts its use in moving parts such as bearings. Ionic liquids (ILs) are potentially excellent lubricants for titania but the relationship between IL molecular structure and lubricity for titania remains poorly understood. Here, three-ball-on-disk macrotribology and atomic force microscopy (AFM) nanotribology measurements reveal the lubricity of four IL lubricants: trioctyl(2-ethylhexyl)phosphonium bis(2-ethylhexyl)phosphate (P 8,8,8,6(2) BEHP), trihexyl(tetradecyl)phosphonium bis(2-ethylhexyl)phosphate (P 6,6,6,14 BEHP), trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate (P 6,6,6,14 ( i C 8 ) 2 PO 2 ), and trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide (P 6,6,6,14 TFSI). The macrotribology measurements demonstrated that friction decreased in P 6,6,6,14 TFSI by four times (μ = 0.13) compared to in hexadecane, even at 60°C and loads up to 10 N. On the other hand, P 8,8,8,6(2) BEHP reduced friction most effectively in the AFM nanotribology measurements. The results were interpreted in terms of the lubrication regime. The lower viscosity of P 6,6,6,14 TFSI coupled with its good boundary lubrication made it the most effective IL for the macrotribology measurements, which were in the mixed lubrication regime. Conversely, the cation structure of P 8,8,8,6(2) BEHP allowed it to adsorb strongly to the surface and minimized energy dissipation in the nanotribology measurements, although its high bulk viscosity inhibited its performance in the mixed regime. These results reinforce the importance of carefully selecting IL lubricants based on the lubrication regime of the sliding surfaces.

Ultrathin ionic liquid films on metal surfaces: adsorption, growth, stability and exchange phenomena

“…IL interfaces are particularly important for interface-controlled applications such as sensor [4,[16][17][18][19], lubrication [20][21][22], separation [4,19,[23][24][25][26][27][28][29], electrochemistry [4,19,[30][31][32][33][34][35][36][37][38][39] and electronics [40][41][42] technologies. ILs further stimulated completely new concepts for catalysis [1,4,[43][44][45][46][47][48][49][50][51][52][53][54][55][56][57].…”

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

“…[1] These surface-active molecules achieve their lubricating function by adsorbing and/or reacting with sliding surfaces, which creates sacrificial, low-shear-strength layers (also called "tribofilms") that prevent hard/hard contact between the sliding counterparts. [2][3][4][5] Among the additives that are used to reduce wear (so-called "anti-wear"), zinc dialkyldithiophosphates (ZDDPs) have been successfully employed since 1940s. [6,7] The most generally accepted view of ZDDP anti-wear action on steel involves the surface reaction of ZDDP to form patchy, glassy films composed of short-chain iron/zinc poly-(thio)phosphates in the bulk, with an outer layer of long-chain zinc poly-(thio)phosphates and a sulfurrich layer near the metal surface.…”

“…[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. [4,29,30] The confined ions resist being "squeezed out" when surfaces are compressed, thus resulting in a film that prevents direct contact between the two surfaces. The properties and dynamic behavior of the adsorbed films are dependent on surface chemistry, charges and applied potentials, molecular structures of the ILs, as well as environmental conditions.…”

“…The properties and dynamic behavior of the adsorbed films are dependent on surface chemistry, charges and applied potentials, molecular structures of the ILs, as well as environmental conditions. [4,[31][32][33] While these nanotribological studies have provided significant insights into IL-mediated lubrication for tribological contacts that involve no chemical reactions between ILs and solid surfaces, studies at the macroscale, which were performed at much higher applied normal pressures and temperatures, provided evidence for the mechano-chemical reaction between IL and metallic surfaces. [34,35] Among the ILs that have been shown to react on solid surfaces under the combined action of normal pressures and shear stresses, phosphonium phosphate ILs (PP-ILs) have been explored extensively due to their good miscibility with hydrocarbon fluids.…”

Atomic Force and Scanning Tunneling Microscopy of Ordered Ionic Liquid Wetting Layers from 110 K up to Room Temperature

“… 5 − 8 The large number of possible cations and anions with an enormous range of combinations allows for an excellent tunability of the properties of the ILs and enables tailoring them for specific applications. 9 , 10 Besides the fundamental interest in the interface properties of liquids, the gas/IL and IL/support interfaces are of specific importance for lubrication, 11 − 13 separation, 14 − 18 electrochemistry, 19 − 29 and catalysis 30 − 40 applications. In catalysis, ILs even stimulated the new concepts “supported ionic liquid phase” (SILP) 36 , 38 , 39 catalysis and “solid catalyst with ionic liquid layer” (SCILL).…”