Reciprocating ball-on-disc (52100-vs.-52100 steel) tribological tests were performed at 353 ± 3 K in the mixed-lubrication regime with neat [N1118][BOB], [N1118][BScB], and [N2226][BOB]. [HMIM][TFSI] was used as the reference IL, due to its extensive use in tribological studies [27]. Figure 2a displays the evolution of coefficient of friction over time represented as number of cycles, while the corresponding steady-state coefficient of friction values are presented in Fig. 2b. In the case of [N1118][BOB] and [N1118][BScB], a steady-state coefficient of friction of 0.114 ± 0.005 and 0.120 ± 0.003 was achieved after a running in period of 50 cycles and 190 cycles, respectively. While these values are significantly lower than the friction response achieved with neat [HMIM][TFSI], tribological experiments performed with [N2226][BOB] resulted in a steady-state coefficient of friction slightly higher than the one of neat [HMIM][TFSI] after a running-in period of ~ 160 cycles. These results indicate that the substitution of the [N1118] cation with the more symmetrical [N2226] has a significant impact on the friction-reducing ability of tetraalkylammonium orthoborate ILs. This finding agrees well with previous reports indicating that increasing the length of the alkyl chains in the cations improves the lubricating properties of ILs at the macroscale [88, 95, 96]. However, the specific wear rate of steel discs after tribological experiments (Fig. 2c) shows that the anti-wear performance does not strongly correlate with the chemical architecture of the ammonium orthoborate ILs. Notably, the specific wear rate of 52100 steel discs used in experiments performed with neat [HMIM][TFSI] is significantly lower than those observed with discs lubricated with any of the ammonium orthoborate ILs.
The analysis of the morphology of the wear tracks provided insights into the origin of this difference in specific wear rates (Fig. 3). While a primarily abrasive wear mechanism was observed in the case of the experiments performed with ammonium orthoborate ILs, an abrasive/corrosive wear mechanism is observed in neat [HMIM][TFSI], as indicated by the presence of pits and discoloration within the wear track. This is likely due to the formation of corrosive halogen halides as a result of the reaction between the halogenated [HMIM][TFSI] and atmospheric moisture [49, 50].
To gain insights into the chemical processes occurring at sliding interfaces and dictating the difference in friction response among the tetraalkylammonium orthoborate ILs synthesized in the present work, ex situ X-ray photoelectron spectroscopy (XPS) analyses were performed.
Before analyzing the steel discs used in tribological experiments, XPS measurements were performed on as-synthesized ILs as well as 52100 steel discs after polishing and aging (i.e., after storing the discs in a desiccator for at least 3 days under reduced humidity to allow for the growth of native oxides). The high-resolution carbon 1s spectrum (Fig. 4a) acquired on 52100 steel discs showed the presence of multiple components at 283.3 ± 0.1 eV (assigned to carbides [97, 98]), 285.0 ± 0.1 eV (assigned to aliphatic carbon [5, 97, 98]), and 286.5 ± 0.1 eV, 287.7 ± 0.1 eV, and 288.7 ± 0.2 eV (assigned to carbon bound to oxygen [5, 97, 98]). The asymmetric oxygen 1s signal (Fig. 4b) includes components at 529.9 ± 0.1 eV, 531.1 ± 0.1 eV, 531.9 ± 0.1 eV, and 533.0 ± 0.1 eV, which were respectively assigned to iron oxide [5, 99–102], iron hydroxide [5, 99–101], iron carbonate [5, 99–104], and adsorbed water [5, 100–102]. The high-resolution spectrum of iron 2p (Fig. 4e) exhibited two peaks (2p3/2 and 2p1/2) due to spin-orbit coupling. Curve synthesis, which was carried out only on the 2p3/2 peak, revealed the presence of the characteristic contributions of metallic iron at 706.5 ± 0.1 eV, iron (II) oxide at 709.2 ± 0.1 eV (together with its shake-up satellite at 714.7 eV), iron (III) oxide at 710.5 ± 0.1 eV, and iron oxide-hydroxide at 712.2 ± 0.2 eV [5, 100–102, 105–107].
XPS analyses were also performed on the as-synthesized ILs to obtain reference binding energies for the characteristic signals of the ions. Figure 4 displays the spectra acquired on [N2226][BOB]. The carbon 1s spectrum shows three components at 285.0 ± 0.1 eV (assigned to aliphatic carbon [5, 97, 98]), 286.6 ± 0.1 eV (assigned to carbon bound to nitrogen [98]), and 289.6 ± 0.2 eV (assigned to carbon bound to oxygen [5, 97, 98]), while the oxygen 1s exhibited two main peaks at 532.5 ± 0.1 eV and 533.5 ± 0.1 eV (assigned to oxygen bound to carbon in [BOB]) together with a component at 534.1 ± 0.1 eV due to water absorbed in the ILs. The characteristic signals of the cation and anion, namely the nitrogen 1s and boron 1s, were respectively detected at 402.3 ± 0.1 eV (in agreement with the nitrogen 1s binding energy of other quaternary ammonium-containing ILs [108–110]) and 193.4 ± 0.1 eV (close to the boron 1s binding energy of tetrafluoroborate-containing ILs [108]). A small contribution to the boron 1s signal was detected at 192.2 ± 0.2 eV, which can be assigned to species generated by the X-ray beam damage of the IL. The composition of [N2226][BOB] computed on the basis of XPS results agrees well (within the experimental uncertainty) to the expected, nominal composition. While similar results were obtained with for [N1118][BOB], it has to be highlighted that changing the chemical architecture of the anion (from [BOB] to [BScB]) only resulted in a shift towards lower binding energy of the characteristic boron 1s signal of the anion (from 193.4 ± 0.1 eV to 192.9 ± 0.1 eV) without any significant change of the position of the nitrogen 1s signal of the cation. This result indicates that the introduction of aromatic rings in the anion increases the negative charge density on the boron atom without any significant charge-transfer from anion to cation, which is most likely due to the presence of alkyl chains around the cationic center that effectively shields it from the anion, in agreement with the previous work by of Blundell and Licence [108].
The XPS spectra acquired in the non-contact region of the steel discs used for tribological experiments in the presence of tetraalkylammonium orthoborate ILs exhibited carbon 1s, oxygen 1s, and iron 2p signals that were comparable to those collected on as-prepared steel substrates. In other words, no surface adsorption of ammonium or orthoborate ions was detected on steel. In contrast, the acquisition of XPS spectra inside the wear tracks showed evidence of shear-induced mechanochemical reactions. First of all, the characteristic boron 1s signal was detected at 191.9 ± 0.1 eV, which most closely corresponds to surface-adsorbed trivalent borate esters [12, 13, 111]. The presence of a boron-containing reaction product was corroborated by a new contribution in the iron 2p signal at 714.1 ± 0.2 eV, which is assigned to Fe3+---O2−-B3+ bonds [112–114]. Secondly, the most intense peak in the nitrogen 1s was detected at 399.8 ± 0.1 eV, which corresponds to surface-bound amines [98, 115]. A minor component at 402.3 ± 0.1 eV is also present and was assigned to adsorbed alkylammonium cations. In addition to these spectral changes relative to the non-contact region, a much more intense peak assigned to carbon bonded to oxygen (i.e., the CO3/COOX synthetic peak) was also present in both carbon 1s and oxygen 1s spectra collected within the worn regions.
The surface coverage of boron- and nitrogen-containing compounds inside the wear track was found to strongly depend on the chemical structure of the ILs used in the tribological experiments and be correlated with the lubricating properties of the ILs. Figure 5a (5b) displays the ratio between the intensity of the boron signal assigned to adsorbed trivalent borate esters (nitrogen signal assigned to adsorbed amines) and the total intensity of the iron signal as a function of the steady-state coefficient of friction. In the case of the experiments performed with ILs containing [N1118] cations, in which a very low steady-state coefficient of friction was achieved, a low surface coverage of boron and nitrogen was observed. In contrast, in the case of the IL that resulted in a higher steady-state friction coefficient, i.e., [N2226][BOB], a much higher surface coverage of boron and nitrogen was detected, indicating that a significantly higher fraction of cations and anions tribochemically reacted on steel surfaces.
Based on the results of tribological experiments and XPS measurements, the following model is proposed to explain the tribological response of steel/steel contacts in the presence of tetraalkylammonium orthoborate ILs (Fig. 6). Under the contact conditions used in the present study, tetraalkylammonium orthoborate ILs with asymmetric cations (i.e., [N1118]) do not form sacrificial tribofilms via shear-induced mechanochemical reactions. This supports the conclusions of previous studies that suggested the anti-wear 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 difficult to displace [27, 28, 73–77]. However, in the case of IL with a more symmetric cation (i.e., [N2226][BOB]), the higher friction response is proposed to originate from the inability of this IL to create a lubricious, solid-like layer due to the reduced van der Waals interactions between the alkyl chains. The resulting hard/hard contact induces, in the presence of absorbed water, the cleavage of B-O bonds to generate trivalent borate esters along with oxalic acid and hydroxide ions (Step I in Fig. 6). The tribochemically formed oxalic acid then adsorbs onto the steel surface, contributing to the detected increase in the intensity of the peak assigned to carbon-oxygen bonds in the carbon 1s and oxygen 1s XPS signals collected within the wear tracks. Furthermore, as reported by Kleijwegt et al., the generation of hydroxides can lead to the degradation of alkylammonium cations through either Hoffmann elimination or nucleophilic substitution reactions (Step II in Fig. 6) [116]. In the case of the Hofmann elimination reaction (mechanism “a”), the hydroxides perform a nucleophilic attack on the β-hydrogen, leading to the release of a long alkene, a tertiary amine (which can adsorb on the steel surface, as indicated by XPS analyses) and water. In the less prevalent nucleophilic substitution reaction (mechanism “b”), the hydroxide reacts with the α-carbon of the long alkyl chain, generating an alcohol and a tertiary amine, which both adsorb on steel, as indicated by XPS measurements. While a similar set of degradation mechanisms was proposed in the case of phosphonium orthoborate ILs [88], the XPS analyses performed in the present study demonstrate, for the first time, that orthoborate ILs do not form inorganic iron borate glasses.