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Adsorption and Tribochemical Factors Affecting the Lubrication of Silicon-Based Materials by (Fluorinated) Ionic Liquids Andrea Arcifa, Antonella Rossi, and Nicholas D. Spencer J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b13028 • Publication Date (Web): 06 Mar 2017 Downloaded from http://pubs.acs.org on March 14, 2017

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The Journal of Physical Chemistry

Adsorption and Tribochemical Factors Affecting the Lubrication of Silicon-based Materials by (Fluorinated) Ionic Liquids Andrea Arcifa a, Antonella Rossi a, b, Nicholas D. Spencer a * a Lab. for Surface Science and Technology, Dept. of Materials, ETH Zurich, CH-8093 Zurich, Switzerland b Dipartimento di Scienze Chimiche e Geologiche, Università di Cagliari, 09042 Cagliari, Italy Corresponding Author *To whom correspondence should be addressed. Phone: +41-44-632-5850; fax: +41-44-6331027; e-mail: [email protected]

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ABSTRACT. 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(trifluoromethyl-sulfonyl)imide ([EMIM] TFSI, [HMIM]

TFSI,

[DDMIM]

tris(perfluoroethyl)trifluorophosphate

TFSI),

and

1-ethyl-3-methyl

([EMIM]

FAP)

were

selected

Elastohydrodynamic predictive formulas for minimum film thickness (h0

imidazolium as

EHL)

lubricants.

were used to

estimate the value of the critical h0 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 (σ ~1.1 nm), the differences in the critical value of h0 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 1hexyl-3-imidazolium-based bis(trifluoromethyl-sulfonyl)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 with other imidazolium-based ILs tests as lubricants for the same tribopair. The new results revealed some further important details on the


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relationship between the chemical structure of the IL and the wear mechanism of the investigated tribopair.

1. Introduction The field of ionic-liquid (IL)-mediated lubrication was initiated by Ye et al. 1 in 2001. In their study,

the

authors

reported

on

the

performance

of

1-alkyl-3-methyl

imidazolium

tetrafluoroborate ILs as lubricants for several materials and compared the results with conventional oils. They found that the ILs exhibited remarkably good lubrication behavior. Since ILs also exhibit a combination of physical properties of general interest in tribology, these initial findings triggered the interest of various research groups in the field of IL-mediated lubrication during the last two decades. In the pioneering work of Ye et al., as in many other studies published in the following years by the same and other research groups2,3,4 , it was suggested that the good tribological performance of ILs were related to two properties attributed to ILs: first of all, ions would be expected to adsorb on polar surfaces, forming robust interfacial layers; secondly, tribochemical reactions of ILs with the counterparts would result in the formation of a chemically altered layer at the sliding interface (tribolayers). In other words, ILs would act as boundary lubricants, exhibiting both the properties of oiliness and extreme-pressure additives, commonly used in oil formulations5. Tribological studies on series of homologous ILs suggested that longer alkyl chains attached to the ions (typically, to the cationic unit) generally result in an improvement in performance4. This correlation has been interpreted to be a result of cohesive interactions between the chains, which again resembles the properties of oiliness additives6. Nevertheless, it is well known that the analysis of friction and wear data alone, especially when considering tribological tests carried out


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under a single set of operating conditions, is generally not sufficient to draw robust conclusions about the molecular mechanisms underlying the observed tribological behavior. Notably, with regard to above-mentioned trend observed with homologous series of ILs, Minami pointed out that the observed trend of friction and wear with the length of alkyl chain might also be explained by the increase of viscosity along the series of homologous ILs4. In other words, it is not always clear from the literature data whether tribotests were carried out in a regime of boundary lubrication or, instead, in a regime of mixed lubrication. In this regard, it is worth mentioning that wear might alter the regime of lubrication during sliding by affecting the geometry and topography of the interface. Outside the area of macrotribology, widespread interest in understanding the structure and dynamic of interfacial and confined IL layers has emerged during recent years7,8. Experiments and simulations suggested that ILs near the solid surface commonly exhibit oscillatory density profiles7. Various factors are known to influence the layering of liquids at the interface with solids. The mere proximity of two rigid walls confining a simple liquid is known to promote layering in simple liquids; importantly, the effect does not necessarily require attractive liquidliquid or liquid-wall interactions9. Nonetheless, the chemical and physical properties of real surfaces and their interactions with the compounds, as well as the interactions between ions, are certainly important in affecting the properties of the interface. These interactions might be particularly relevant when considering liquids composed of ions7. Recent simulations

10, 11

have suggested that the combination of surface charge density and

steric properties of the ions could be particularly important in affecting the extension and composition of the structural layers, as well as the orientation of the ions within the interfacial


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film. Experiments carried out by atomic force microscopy (AFM) and lateral force microscopy (LFM) have generally supported this view12,13,14,15. These results indicate that the properties of the boundary layer formed at the solid/IL interface might deviate from that observed for traditional oiliness additives. In this context, the effect of small amounts of contaminants must be carefully considered, particularly with regard to water, which can be absorbed by the IL when exposed to humid air16,17,18,19,20,21. Of particular significance is a recent publication of Cheng et al.19, whose results show a striking dependence of the structure at the IL/mica interface on the presence of water. In particular, in agreement with the above-mentioned experiments and simulations, the surface charge density appears to play a decisive role in layering. In mica, charging would be essentially triggered by minor water dissolution in the IL: in a highly dry state, the low surface charging would result in mostly disordered layers and, possibly, an enrichment of anions in the layer closer to the surface. These recent finding are in contrast with previous publications, and highlight the current difficulties in understanding the interfacial structures at IL/solid interfaces. Coming to the other process of relevance in IL-mediated lubrication, i.e. surface damage and tribochemistry, understanding of the mechanisms responsible for surface alteration of the sliding surface is also rather fragmented. While the chemical analysis of tribostressed samples reported in the majority of tribological studies has shown the occurrence of reaction between IL and the substrate4, the details of the tribochemical mechanisms and their actual role in tribological behavior are often not clear. In general, it is presumed that the reaction products form protective tribolayers on the worn surface that are able to prevent the occurrence of severe forms of wear. Nonetheless, high wear and friction have also been observed in IL-mediated lubrication. For example, Jiménez et al.22 observed that [EMIM] BF4 exhibits poor performance when used as


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lubricant for a steel/aluminum tribopair and interpreted this as a result of tribo-corrosion, which would also be associated with the decomposition of the ions. The variability of the results reported in the literature clearly demonstrates that any generalizations concerning the type and effect of tribochemical reactions of IL-mediated lubrication should be treated with caution. The variation of materials and testing conditions can lead to substantially different behavior for the same lubricant. The present study is aimed at providing some clarification regarding the type, role and relevance of processes occurring in IL-mediated boundary lubrication for a tribopair consisting of a silica pin sliding against a (100)-oriented silicon wafer. Two sets of experiments are described. The first type of test is designed to investigate the onset of surface damage as a function of speed for various ILs. At high speed, a regime leading to negligible wear is observed—interpreted as elastohydrodynamic (EHL) lubrication. Upon decreasing the speed, asperity contact between the solid surfaces leads to a transition in tribological behavior. A comparison between the estimated minimum EHL lubricant-film thickness (h0 EHL) and the composite root-mean-square roughness of the counterparts (σ) indicate that this transition is observed for a value of λ ratio (h0 EHL/ σ) of ~ 1 – 3. Differences in the critical value of λ measured for the various ILs examined in this work, are discussed on the basis of models of the IL/solid interface presented in the recent literature19,23,24,. In addition, the change of friction trends observed while decreasing speed reveal some important aspects of the wear mechanism occurring in the presence of the different ILs used as lubricants. The second part focuses on the tribological behavior observed with a SiO2/Si tribopair lubricated with selected imidazolium-based bis(trifluoromethyl-sulfonyl)imide ILs. Test conditions, such as the surface damage occurring at the beginning of the test, were selected such


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that they led to a sufficient degree of roughening of the counterpart (higher than few nanometers) so as to allow the study of the boundary lubrication regime over an extended range of speed. Following the approach developed in previous studies, the results are discussed on the basis of the chemical, structural and topographical analysis of the worn surfaces. A comparison with the results previously obtained with other imidazolium-based ILs highlights some important aspects concerning the surface damage mechanisms in IL-lubricated SiO2/Si tribopairs. All experiments discussed here were carried out with dried ILs in a nitrogen atmosphere, so as to exclude the effects of small amounts of water adsorbed at the interface. It is anticipated that environmental humidity can significantly alter both the interfacial structure and tribochemistry of these ILs. These effects will be discussed in a future publication. 2 Materials and methods 2.1 Ionic liquid In this study, 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide, [EMIM] TFSI, 1-hexyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide, [HMIM] TFSI, 1-dodecyl-3methyl imidazolium bis(trifluoromethylsulfonyl)imide, [DDMIM] TFSI (IoLiTeC GmbH) and 1ethyl-3-methyl

and

1-ethyl-3-methyl

imidazolium

tris(perfluoroethyl)trifluorophosphate,

[EMIM] FAP (Merck), were tested as lubricants for tribopairs consisting of fused silica pins sliding against a Silicon (100) disks. The ILs were dehydrated in a vacuum of ~10-2 mbar as described in previous publications25 and stored in a nitrogen-filled glovebox. 1H-NMR and 19-NMR spectra of the compounds are reported in the Supporting Information (Figures S1, S2, S3, S4). The literature values for the viscosities of the dry liquids are reported in Table 1. It must be noted that not all values of the pressure-viscosity coefficients (α), used for the estimation of the


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EHL film thickness, were available and some assumptions had to be made. In particular, Paredes et al.26 reported α values for 1-ethyl-3-methyl imidazolium and 1-decyl-3-methyl imidazolium TFSI at 303.15 K of 10.7 and 11.1 GPa-1, respectively. Small variations are observed by increasing the temperature to 313.15 K. Given the apparently small effect on the estimated film thickness, a value of 11 GPa-1 is taken for all the TFSI ILs used in this work. No literature values for [EMIM] FAP have been found; in this work a value of 15 GPa-1 for this IL is assumed on the basis of the results reported by Fernández et al.27 for the structurally similar 1-butyl-3-methylimidazolium FAP. Table 1: ILs used in this work and their relevant rheological parameters. Ionic liquid

Viscosity at 298.15 K (mPa.s)

Pressure-viscosity coefficient α (GPa-1) *

[EMIM] TFSI

33 (a)

11

[HMIM] TFSI

70 (a)

11

[DDMIM] TFSI

154 (a)

11

[EMIM] FAP

61 (b)

15

(a) Values from Tariq et al.28 (b) Values from Nazet et al.29 * Values adapted from literature according to the assumptions reported in the text.


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2.2 Tribopair All the tribological tests described in this work were carried out using pins consisting of 2mm-diameter fused silica balls (Corning 7980, J. Hauser GmbH & Co. Solms, Germany). Wafers consisting of p-type (100)-oriented silicon wafers (Silicon Materials, Kaufering, Germany, size 5 x 10 mm2) were employed as disks. The mechanical and topographical properties of relevance for this study are reported in Table 2. Table 2: Materials used for pin-on-disk tests and their mechanical properties Material

Young’s Modulus

Poisson’s ratio

Roughness RMS

Fused SiO2 (Corning 7980)

72.7 GPa(a)

0.16(a)

1.1(0.1) nm

(100)-oriented Si

130 GPa(b)

0.27(b)

< 0.3 nm

(a) Values from the supplier (b) Values from Hopcroft et al.30

Prior to the tests, silicon wafers were sonicated for 5 minutes each in toluene and isopropanol, while the fused-silica-glass spheres were rinsed with isopropanol. The two surfaces were O2plasma treated for two minutes shortly before the test with the aim of removing organic contamination and obtaining reproducible starting conditions for the tribological experiments. The samples were thoroughly rinsed with analytical-grade ethanol prior both Raman and XPS analyses

2.2 Tribological tests Tribological tests were carried out using an UMT-2 tribometer (Bruker Nano Inc. Campbell, USA) operating in pin-on-disk mode. A load cell with a maximum capacity of 5 N and resolution


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of 0.0049 N (manufacturer’s data) was used. Two rotary motors were employed in this study: a model S20HE (rotational speed 0.001 to 30 rpm) and a model S23LE (rotational speed 0.1 to 3000 rpm). The types of tests performed in this study are described in the following. Ramp tests. Multi-step tribological experiments were carried out in order to investigate the onset of surface damage (i.e. the transition from fluid film lubrication to mixed and boundary lubrication) for SiO2/Si contact lubricated with different ILs as a function of the speed. The ramp tests consisted of a sequence of steps having a progressively decreasing speed. The load was kept at a constant value of 4.5 N (corresponding to an initial maximum Hertzian pressure of 1330 MPa) for the entire duration of the tests. Importantly, the pin was lowered onto the disk while the disk was already in motion at the highest speed, in order to avoid the occurrence of damage during the initial acceleration of the disk. For [EMIM] FAP, [EMIM] TFSI and [HMIM] TFSI, the S23LE motor was used. In the case of tests carried out with the [DDMIM] TFSI the S20HE motor was preferred, in order to investigate its particular behavior at low speeds. The duration of the steps of each ramp test was fixed at 30 seconds for speeds higher than 240 mm min-1, while, for lower speeds, the duration was adjusted such that each step consisted of a minimum of seven turns. Prior to any test, a volume of 150 µL of IL was placed on the disk. A Teflon ring (internal diameter: 16 mm) was pressed against the wafer to avoid lubricant starvation. In the case of [DDMIM] TFSI, the speed was decreased stepwise from 600 to 6 mm min-1, while in the case of [EMIM] TFSI, tests were carried out in the range of speed of 10000 to 500 mm min-1. For both [EMIM] FAP and [HMIM] TFSI, the speed was decreased stepwise in the range 10000 to 60 mm min-1. Constant-speed tests. In order to evaluate friction and wear as a function of speed, tests were carried out at a constant speed of 50, 500 or 5000 mm/min and for a total duration of 400 turns.


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A constant load of 4.5 N was applied. The pin was placed in contact with the disk prior to the start of rotation, so as to promote initial surface damage. Since this method was found to be unreliable at the highest speed, in this case, prior to the actual test, four rotations at 50 mm min-1 and at the same load were carried out, yielding the required initial damage to prevent full fluidfilm lubrication. The amount of lubricant at the beginning of the constant-speed test and the type of disk holder used in these set of tests were the same as those described above for the ramp tests 2.3 Chemical analysis of the tribostressed surfaces: X-ray photoelectron spectroscopy (XPS) The X-ray photoelectron spectra presented in this work were acquired with a PHI Quantera SXM (ULVAC-PHI, Chanhassen, MN, USA). Analyses were carried out with a monochromatic AlKα (1486.6 eV) source, using a beam diameter of 20 µm, the analyzer working in constantanalyzer-energy (CAE) mode. For the high-resolution spectra, the pass energy and the step size were 69 eV and 0.125 eV, respectively, (full-width at half-maximum (fwhm) of the peak height for Ag 3d5/2 = 0.7 eV). Survey spectra were acquired with a pass energy of 280 eV and a step size of 1 eV. In both cases, the emission angle was 45°. The spectrometer was calibrated according to ISO 15472:2010, with an accuracy of ± 0.1 eV. The analysis were carried out while using a low-voltage argon ion gun / electron neutralizer. Following the procedure described in our previous work25, in order to minimize the effects of differential charging31, the tribological samples were mounted on the sample holder by using an insulating double-sided adhesive tape thus insulating the sample from the ground. The flood gun conditions were: cathode voltage was − 5 V (with respect to instrumental ground), the emission


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current was 20 mA. In this work, the aliphatic component of the C1s signal was used as an internal reference (285.0 eV). The spectra of the pure TFSI-based ILs deposited on aluminum holders were acquired without charge compensation. The method described by Villa-Garcia et al. for calibration of the energy scale is used in this work32; details on charge correction and fitting of the XPS spectra of ILs are presented in the Supporting Information. 2.4 Raman microscopy A WITec Confocal Raman Microscope 200 (WITec, Ulm, Germany) with a laser light source (λ = 532.14 nm) and with a lateral resolution better than 1 µm was used to characterize the structure of the worn surface of the silicon disks after tribotesting. The measurements were performed setting the Raman power on the sample to 1.6 mW and using a 100x objective having 0.8 N.A. (numerical aperture). The Raman spectrometer was equipped with a diffraction grating having a groove density of 900 grooves/mm. 2.5 Optical microscopy and profilometry A Sensofar PLu Neox (Sensofar-Tech, SL., Terrassa, Spain) 3D optical profiler was used for characterizing the surface of the tribologically stressed samples at the end of the tests. Data were acquired by using SensoSCAN software (v.3.1.1.1, Sensofar-Tech, SL., Terrassa, Spain) and processed by SensoMAP software (v.5.1.1. Digital Surf, Besancon, France). 3D images of worn disks were collected in white-light-interferometry (ePSI mode), using a 50x objective. An AX10 Imager M1m (Carl Zeiss, Oberkochen, Germany) with objectives ranging from 5x to 40x and equipped with a CCD camera was used for acquiring optical images. 3 Results


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3.1 Friction trends observed in multi-step tests at decreasing speed: onset of lubricant film failure and evolution of surface damage Regardless of the IL used as lubricant, the coefficient of friction (CoF) trend observed during the first step of the ramp test (i.e., at the highest speed) was always characterized by a stable CoF value. Beyond instrumental noise, the only source of variability consisted of small periodic oscillations having the same frequency as the rotation of the disk. This behavior is typically observed in pin-on-disk experiments, when negligible wear occurs during testing33,34; it derives from a displacement of the normal to the disk with respect to the rotation axes. For small displacements, the amplitude of the signal is approximately equal to the tilt angle, in radians, of the disk34. In our case, a good agreement between the tilt angle (~ 0.05°) and the observed displacement was observed. Over three replicas, the average values of the CoF measured at 104 mm/min for [EMIM] TFSI and [HMIM] TFSI were 0.0844(0.0001) and 0.0896(0.0001), respectively. At the same speed, a higher value, 0.1032(0.0004), was observed for [EMIM] FAP. For [DDMIM] TFSI, the highest speed investigated in the ramp test was 600 mm/min. Under these conditions, the CoF measured over three replicas was the lowest: 0.075(0.005). The behavior observed at the beginning of the tests was essentially retained during the following steps, with small changes in the average value of CoF as the speed decreased. At a certain point, however, spikes of high friction could be observed that were superimposed on the steady behavior described above (Figures 1-3). These features are interpreted as the onset of filmfailure and wear. It has to be noted that the measurements discussed in this work do not allow us to establish whether asperity-asperity contacts may have already occurred before the detection of spikes in the friction traces. Nonetheless, these latter features are most probably associated with


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significant interaction and alteration of the contact surface. The evolution of the CoF trends observed in subsequent steps was found to depend on the specific IL. Essentially, three types of transitions in the tribological behavior were observed and are described in the following. Film failure and lubrication-regime transition for a SiO2/Si tribopair lubricated by [EMIM] TFSI or [HMIM] TFSI. Figure 1a shows a selection of the friction traces taken from two ramp tests carried out in the presence of [EMIM] TFSI. In the example reported in Figure 1a, a striking change in friction was observed during the 600 mm/min step. At this speed, film failure was detected and the CoF increased to a value higher than 0.3 within a few turns. Interestingly, the occurrence of highfriction spikes clearly follows the frequency of rotation of the disk. The intensity and width of the spikes increased from one turn to the next and this process eventually led to a regime of high CoF.


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Figure 1. (a) CoF trace extracted from a ramp test carried out with a [EMIM] TFSI- lubricated SiO2/Si sliding tribopair. The ramp test consists of a series of steps with progressively lower speeds (details are reported in the experimental section). All the steps were carried out at a constant load of 4.5 N, at room temperature and in the presence of a nitrogen atmosphere. (b) CoF trace during a test at a constant speed of 5000 mm min-1 and a load of 4.5 N, following the ramp test illustrated in (a). Very similar behavior was observed in the case of [HMIM] TFSI, although in this case the transition occurred at lower speed (Figure S5). During the subsequent steps at lower speed, the high-friction regime was essentially retained. The high friction regime observed after film failure was retained when increasing the speed again at the end of the ramp test (Figure 1b). In addition,


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a large amount of debris and severe wear of the counterparts was observed at the end of the test, which resembles the behavior observed in the constant-speed tests described later. Film failure and lubrication-regime transition for a SiO2/Si tribopair lubricated by [DDMIM] TFSI. Figure 2a shows a selection of friction traces observed for a ramp test carried out using [DDMIM] TFSI as lubricant.

Figure 2 . (a) CoF traces extracted from a ramp test carried out with [DDMIM] TFSI - lubricated SiO2/Si sliding tribopair. The ramp test consists of a series of steps with a progressively lower speed (details are reported in the experimental section). All the steps were carried out at a constant load of 4.5 N, at room temperature and in the presence of a nitrogen atmosphere. (b)


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CoF trace during a test at a constant speed of 500 mm min-1 and a load of 4.5 N, following the ramp test illustrated in (a). Inset of figure 2b: CoF trend registered during the first turns. Similarly to the case of the other two TFSI-based ILs, at a sufficiently low sliding speed it was possible to detect the appearance of periodic spikes in the friction trends, whose height and width were found to progressively increase with the sliding distance. Nevertheless, in the case of [DDMIM] TFSI the process was extremely gradual compared with the above-described cases: a complete transition to a high-friction regime was not observed, even at the end of the test. Following the ramp, a further 400 turns step at constant speed of 500 mm min-1 was carried out, resulting in the friction trace displayed in Figure 2b. Periodic spikes of higher friction were again observed throughout the constant-speed test. The progressive growth of these features resulted in a slow increase of friction during the 400 turns, up to a value of ~ 0.15. At the end of the test, the pin was checked at the microscope, revealing the presence of wear debris, although the amount of surface damage was significantly less than that observed with [EMIM] or [HMIM] TFSI (Figure S6) Transition in tribological behavior with decreasing speed for a SiO2/Si tribopair lubricated by [EMIM] FAP Figure 3a shows a selection of friction traces observed for a multi-step decreasing speed ramp test carried out using [EMIM] FAP as lubricant.


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Figure 3. (a) CoF traces extracted from a ramp test carried out with [EMIM] FAP- lubricated SiO2/Si sliding tribopair. The ramp test consists of a series of steps with a progressively lower speed (details are reported in the experimental section). All the steps were carried out at a constant load of 4.5 N, at room temperature and in the presence of a nitrogen atmosphere. (b) CoF trace during a test at a constant speed of 500 mm min-1 and a load of 4.5 N, following the ramp test illustrated in (a). As for the tribotests carried out with TFSI-based ILs, also in the case of [EMIM] FAP the stepwise decrease of speed resulted in film failure. In this case, however, the intensity of spikes


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in the CoF trend did not show a progressive increase throughout the sequential steps. Instead, while some evidence of film failure was already detectable at a speed of 1200 - 3000 mm min-1 (with some variability for the different replicates, as illustrated later) the intensity of the periodic spikes was found to decrease starting from 600 mm min-1, essentially disappearing at 360 mm min-1. A further decrease of speed eventually led to complete film failure, resulting in an irregular friction trend. Very similar behavior was observed for independent replicates of the experiment. Following the 60 mm min-1 step, a further run at a constant speed of 5000 mm min-1 was carried out (Figure 3b). In this case, after an initial decrease of friction, the CoF reached a steady value of 0.103 (0.001) (standard deviation over three independent tests). Comparing the friction trend at 5000 mm min-1 before and after film failure, it appears that, within the experimental uncertainty, the CoF value is the same in the two cases. This behavior is very different compared with that observed for TFSI-based ILs, clearly illustrating a significant difference in the wear mechanism in the two cases, as discussed below. 3.2 Tribological and spectroscopic investigation of severe wear in [EMIM] and [HMIM] TFSI- lubricated SiO2/Si 3.2.1 Tribological testing shows a comparison of the CoF traces observed for tests carried out using [EMIM] TFSI and [HMIM] TFSI at constant speeds of 50, 500, and 5000 mm min-1. All tests were carried out at a constant load of 4.5 N in the presence of a nitrogen atmosphere. As discussed in the experimental section, it was necessary to induce an initial small damage for tests carried out at high speed in order to avoid fluid film lubrication.


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Figure 4. CoF versus number of cycles during tribological tests at a sliding speed of 50 (a, b), 500 (c, d), 5000 (e, f) mm min-1 and in the presence of [EMIM] TFSI (a, c, e) or [HMIM] TFSI (b, d, f). All the tests are carried out at a constant load of 4.5 N, at room temperature and in the presence of a nitrogen atmosphere. Each graph shows two independent replicates of the same type of test. The data presented in Figure 4 indicate that the friction trends obtained at different speeds exhibit remarkable similarity. In particular, the average value at the end of the test was rather similar (~ 0.24) for all conditions and irrespective of the particular ionic liquid. During the first few turns of each test, friction was generally higher than the value observed through the rest of test. The duration (number of turns) of this transient was found to increase with the speed. The effect was more pronounced in the case of [EMIM] TFSI. In addition, regardless of the lubricant,


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the trends observed at 50 mm min-1 exhibited some short transients during the test, characterized by high-friction spikes. 3.2.2 Optical microscopy and profilometry For all the experiments described in this section, at the end of the test, wear debris could be detected by eye, suspended in the liquid and deposited on the disk and in the wear track. The wear of pin and size track measured at the end of the 400 turns tests (sliding distance of 10.05 m) were quantified by measuring the wear coefficient of pins and disk (Figure 5)

Figure 5. Wear coefficients measured after tribological tests at different speeds (normal load: 4.5 N, radius: 4 mm, total duration: 400 turns) carried out in the presence of [EMIM] TFSI or [HMIM] TFSI: (a) wear coefficient of fused silica pin; (b) wear coefficient of silicon disk. The values of the columns are averaged over two independent tests; the associated error bars correspond to the interval between the twice-measured values.


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A slight dependence of wear on speed was observed for both the ILs: a positive correlation between speed and wear coefficient was observed. Only for the case of [HMIM] TFSI-lubricated pin, no significant variation was observed between 500 and 5000 mm min-1. Optical micrographs of the tribostressed pins are reported in Figure 6. An irregularly shaped scar, with debris collecting on the damaged surface, was observed for all the tested conditions. The contours of the scars seem to suggest the occurrence of brittle fracture. Rougher edges were observed at slower speeds. The analysis profiles taken in the direction orthogonal to the track reveal a similar trend concerning the shape of the scratches produced during sliding as a function of sliding speed: as is observable in the case of [EMIM] TFSI lubricated contacts (Figure 6a), a rougher profile is observed at the slowest speed. Similar results were observed for [HMIM] TFSI (data not shown).

Figure 6. (a) Profiles of wear tracks on silicon disks sliding against a fused silica pin in the presence of [EMIM] TFSI; the profiles were taken perpendicularly to the sliding direction. The


22

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three graphs refer to tests carried out at different speeds (from top to bottom: 50, 500, 5000 mm min-1. (b - g) Wear scar on fused silica pin at the end of the tribological tests at speed of 50 (b, c), 500 (d, e), 5000 (f, g) mm min-1 and in the presence of either [EMIM] TFSI (b, d, f) or [HMIM] TFSI (c, e, g). For all the pictures: normal load: 4.5 N, radius: 4 mm, total duration: 400 turns, nitrogen atmosphere. 3.2.3 Structural characterization of worn disks by Raman spectroscopy The Raman spectra of four representative points of a silicon disk tribostressed at a sliding speed of 5000 mm min-1 and in the presence of [EMIM] TFSI are reported in Figure 7b. The corresponding spots irradiated by the laser beam are indicated in Figure 7a. For a comparison, a spectrum of a pristine silicon wafer is also shown.

(a)

(b) (4)

(3)

Intesity (A.U.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(2)

(1) (Si - I)

Wavenumber (cm-1)

Figure 7. (a) Micrograph of an area of the wear track on a silicon disk produced after sliding against a fused silica pin in the presence of [EMIM] TFSI. Sliding speed: 5000 mm min-1, normal load: 4.5 N, radius: 4 mm, total duration: 400 turns, nitrogen atmosphere. (b) Four


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representative Raman spectra acquired at the points indicated by arrows in the micrographs; the spectrum of a pristine (100) oriented Si wafer is also presented (Si-I). A comparison with Raman spectra reported in the literature35 indicates that the spectral features observed in Figure 7b can be attributed to the presence of various phases of elemental silicon. In particular, the Raman spectrum of the thermodynamically stable phase of elemental silicon at ambient pressure (Si-I, following the notation used by Domnich and Gogotsi35) is dominated by a single line at 520 cm-1. This line is clearly visible in the spectrum (1) of Figure 7b, and at a large number of points. At other points the signal was only poorly detectable or absent, indicating a conversion of Si-I to other phases down to a depth greater than the penetration depth dp of the laser light used in the analysis, which was estimated to be about 700 nm for a wavelength of 532 nm (dp is defined as the depth corresponding to 99% of absorption of the scattered light) in the case of Si-I. The various peaks observed in spectrum (2) indicate the presence of other crystalline phases of silicon. In particular, the frequencies of the main peaks are comparable with some of the signals expected for the Si-III and Si-XII phases. Spectrum (3) shows features that might suggest the presence of nano-crystalline silicon35. Nonetheless, it must be noted that this attribution is somewhat ambiguous, because the Si-IV metastable phase, which may also be formed during unloading of the metallic Si-II phase, would produce a band located in the same wavenumber range35,36. The broad bands observed in Figure 7b (4) can be attributed to the presence of amorphous silicon (α-Si) within the entire volume sampled by Raman microscopy37. The two maxima at about 160 cm-1 and 470 cm-1 are indeed characteristic of the presence of α-Si. The position and


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relative intensity of these bands change according to the degree of disorder of the amorphous structure. In the case reported here, it should be noted that a small contribution of α-Si is present in all the analyzed points of the wear track. The remaining spectra acquired in the area delimited by a red box in Figure 7a (one spectrum was acquired per µm2) are comparable with those reported here: they can be considered as a combination of spectra of different silicon structures. A comparison of these spectra reveals a significant degree of structural heterogeneity within the contact area. The dark regions in the optical micrograph, presumably associated with the presence of debris, were generally found to show features closely resembling those reported for nano-crystalline silicon, suggesting that the debris is at least partially composed of nano-sized SiI powders. Nonetheless, it should be borne in mind that the metastable Si-IV phase would also produce the same spectral features. It is rather difficult to discern other correlations between the optical and structural features. Overall, the Raman analysis of samples tested at 500 and 50 mm min-1 sliding speed also revealed a high variability of structures coexisting within the contact area for both lubricants. Although a quantification of the effect of speed on the structural transformation occurring in silicon during sliding was not attempted, it seemed to emerge that lower speeds were associated with a lower contribution of signals of metastable phases of silicon to the Raman spectra. 3.2.4 Surface-chemical characterization silicon disks after tribological testing Survey spectra: The XPS survey spectra of silicon disks tribostressed in the presence of [EMIM] TFSI or [HMIM] TFSI and at a speed of 50 mm min-1 are reported in Figure S7. The elements characteristic of the ionic liquid—nitrogen, fluorine and sulfur—were clearly observed in the spectra of the worn surface, while their signals were barely or not detectable in


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the non-contact area, indicating the removal of the physisorbed ionic liquid layer by rinsing with ethanol. The survey spectra of silicon samples after tribological testing at higher speeds (not shown) were largely similar to that reported in Figure S7 for both the contact and non-contact area.

Figure 8. Si2p region of the XP-spectra of silicon disks tribostressed in the presence of [EMIM] TFSI at a speed of: (a) 50, (b) 500, (c) 5000 mm min-1. (a, b, c). The Si2p spectrum of a pristine silicon wafer is reported in (d). For all the tribostressed samples: normal load: 4.5 N, radius: 4 mm, total duration: 400 turns, nitrogen atmosphere. Si2p signal. Figure 8 shows a comparison of the high-resolution (HR) spectra for the Si2p signal of disks lubricated with [EMIM] TFSI after tribological testing. A representative spectrum for each condition of speed is reported in the figure. Additionally, the Si2p spectrum of a pristine sample is reported for a comparison. The spectra of silicon disks tribostressed with [HMIM] TFSI, closely reassembling the one reported in Figure 8, are reported in the Supporting Information (Figure S8).


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The curve-fitting model used here has already been applied in previous publications dealing with the characterization of silicon disks lubricated with ILs25,38. In addition to the curves representative of elemental silicon and SiO2, three components representative of the silicon suboxides (Sin+, n = 1, 2, 3) are included in the model. Each signal was fitted constraining the intensity ratio of the two components at a value of 1:2 for the 2p1/2 and 2p3/2, respectively, while the distance between the two maxima was constrained at a value of 0.6 eV. Their positions on the binding-energy scale relative to elemental silicon were also constrained following Seah et al39. No constraints to the peak position of SiO2 doublet were applied.. As discussed in our previous works, other species having binding energies similar to the values of the oxides (Table S3, Supporting Information) are expected to contribute to the spectrum, although they are not explicitly considered in the model. The spectra of the non-contact areas mostly resembled that of pristine silicon wafers: the elemental-silicon Si2p3/2 peak (99.2(0.1) eV, FWHM = 0.6(0.1) eV) always showed the highest intensity among the signal of the various oxidation states. Most of the oxidized silicon of the non-contact consisted of SiO2 (103.4(0.1) eV, FWHM = 1.6(0.1) eV). Two main features can be recognized in all the Si 2p spectra of the tribostressed disks. Firstly, an appreciable increase of the signal in the region of the intermediate oxidation states of silicon was observed compared with the intensity of the signals of the undamaged wafer. Secondly, a broadening of the Si2p components of elemental silicon was detected as a result of surface damage, as evidenced by a comparison between the FWHM values of the worn and undamaged material (0.9 and 0.6 eV, respectively). A similar trend is observed for the SiO2 components: the values of FWHM of this signal for worn and undamaged surfaces are 2.0(0.1) and 1.5(0.1) eV, respectively. In addition, the binding energy (103.0(0.1) eV) was found to be shifted towards


27


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lower binding-energy values compared to those measured for the spectra from the non contactarea. It is worth comparing the features of the Si2p signal for different ILs and ILs/water mixtures tested so far by the authors for a SiO2/Si tribopair under comparable conditions. The Si2p spectra obtained with the two TFSI-based ILs reported here are similar to those obtained with [EMIM] EtSO4 in the presence of a nitrogen atmosphere38, while they are remarkably different from the results obtained with [EMIM] and [HMIM] FAP25, or with [EMIM] ETSO4/H2O mixtures38. This observation is believed to be a strong indication of the type of wear mechanism acting in the presence of different ILs, as discussed in the following paragraph. Figure 9 shows the high-resolution spectra for F1s, C1s, and O1s in the first row, and for S2s and N1s signals in the second row. The spectra were acquired from the worn surfaces of disks after tribological testing at a speed of 50 mm min-1 in the presence of [EMIM] TFSI. Spectra of the non-contact area are also shown for comparison, as well as those of the pristine lubricant.


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Figure 9. F1s (a), C1s (b), O1s (c), S2s (d), N1s (e) regions of the XP-spectra of pure [EMIM] TFSI and of silicon disks tribostressed in the presence of [EMIM] TFSI (applied load: 4.5 N, duration of the test: 400 turns, radius: 4 mm speed 50 mm min-1, nitrogen atmosphere). From top to bottom: pure ionic liquid, contact area; non-contact area. C1s signal. The signal of the non-contact area shows the typical features of adventitious carbon. The model used for fitting the signal has been employed in previous works on ILlubricated SiO2/Si pairs38: details are reported in the Supporting Information. As in our previous publications, a broadening of the C1s signal toward lower binding energy values was observed in


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the spectrum of the contact area, which might be associated to the formation of amorphous carbon, organic moieties bonded to silicon, or a combination of the two. A component has been added to the curve-fitting model of the C1s signal to take this feature into account. The position of the added component on the energy scale was found to be comparable with previous fittings (B.E. = 283.8(0.1) eV, FWHM = 1.4(0.1) eV). A comparison with the signal of the neat IL clearly indicates that no significant amounts of the intact ions are present after rinsing with ethanol, which is also confirmed by analysis of the other signals. S2s signal. The S2s signal was not detectable on the unworn surface, while inside the contact a low-intensity band was found at 227.0 (0.1) eV (FWHM = 2.8(0.3) eV, which is clearly different from the signal acquired on the pure IL. This indicates the occurrence of the chemical degradation of the TFSI anion, leading to reaction products comparable with those observed in the case of experiments carried out with the EtSO4 anion38. In fact, as discussed in the following paragraph, a similar assumption regarding the assignation of this XP-signal might be made in the two cases: the detection of sulfur in a reduced state might be attributable to its embedding in the near-surface region as a result of shear-induced intermixing N1s signal. The N1s spectra of the non-contact area showed very weak signals, possibly associated with the presence of a residue of degradation products of the imidazolium cations. The spectra of the contact area are quite comparable with that observed in a previous experiment with a SiO2/Si tribopair lubricated with imidazolium-containing ILs25,38: a broad band with a maximum at 398.2 (0.1) eV (FWHM = 1.9(0.1) eV) and a shoulder extending towards higher binding energy was detected. The main signal may indicate the presence of oxynitride compounds or nitrogen-containing amorphous carbon, while the shoulder is probably associated with the presence of nitrogen-containing organic compounds deriving from the degradation of


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the IL. It is worth noting that, in the case reported here, nitrogen is not only present in the cation but also in the anion, as clearly shown by the spectrum of the pure IL. Nonetheless, the presence of amidic nitrogen in the anion does not seem to significantly affect the composition of the tribolayer, as the signal is very similar to that observed in our previous works. F1s signal. A clear F1s signal was detected in the contact area, while the signal in the noncontact region was barely visible. A two-component model developed in our previous investigations on imidazolium FAP-based ILs25 was found to fit the data obtained with TFSIbased ILs effectively. Compared with results in the previous publication, only a minor shift (0.10.2 eV) of the components towards the lower-binding-energy side was observed here. Following the considerations expressed previously, the main component (B.E. = 686.6(0.1), FWHM = 1.6(0.1) eV) is assigned to fluorinated silicon (Si-F), the smaller one (B.E. = 687.8(0.1) eV, FWHM = 1.8(0.1) eV) being assigned to oxyfluoride. In some of the analyzed points on the worn surface, a small tail was observed on the higherbinding-energy side of the peak. In our previous study on FAP-based ILs, differential charging was proposed as a possible cause of the observed broadening. In addition to this, it might be possible that degradation products of the FAP ions might be present in the debris and be responsible for the observed tail. O1s signal. The O1s spectra of the non-contact area were also fitted with a single component (BE = 533.0(0.1 eV, FWHM = 1.6(0.1) eV). The binding energy of the O1s signal of the contact area, also fitted with a single component, was found to be quite close to those of the signal of the non-contact (BE = 532.9(0.1) eV), although being broader (1.8(0.1) eV).


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4 Discussion 4.1 Speed-dependent tests: estimation of lubricant film thickness and its relation with the onset of surface damage. The friction trends observed in the ramp tests are consistent with the occurrence of a transition from a fluid-film (elastohydrodynamic) to a mixed-boundary lubrication regime as the sliding speed reached sufficiently low speed. The thickness of the fluid film has to be compared with the roughness of the sliding counterparts: as the two values become comparable, asperity-asperity interactions are expected to occur. In order to substantiate this hypothesis, in the following the ratio of the minimum (theoretical) film thickness (h0) to the composite surface roughness of the two surfaces in contact is estimated5. The λ ratio is defined as the following:

=

ℎ

+

Given the non-conformal geometry of the pin-on-disk contact, an approach based on elastohydrodynamic theory is required. In this work the following equation is applied5 (see Supporting Information for details)

ℎ = 3.63 ′ ′

.

!" #′$.%&

'

(

).*+

!1 − . ). $


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For a constant load, a specific contact geometry, specific materials, and a given lubricant, the equation reduces to a relationship of the form: h0 EHL = f v 0.68 (v = sliding speed). Using the literature values for the mechanical properties of fused silica and silicon and the rheological parameters for the ILs presented in the experimental section, and assuming Newtonian behavior for the ILs under study the values obtained for the constant f are the following: 1.06, 1.76, 2.97, 1.86 10-2 mm-0.68min068 nm for [EMIM] TFSI, [HMIM] TFSI, [DDMIM] TFSI, [EMIM] FAP, respectively. In the following, it should be noted that the minimum film thickness predicted by EHL theory, h0 EHL, is obtained under the assumption that the liquid behaves as a continuum and deviations have to be expected when approaching a thickness comparable with the size of the boundary layer of ions. As a result, a difference in the critical h0 EHL at which the onset of surface damage is detected might be expected when comparing the results obtained with different ILs and might reveal a difference in the ability of the boundary layer to prevent contact between asperities, as discussed later. The graphs reported in Figures 10 and 11 are used to summarize the outcome of this study. The graphs show the measured average and the maximum values of CoF as a function of the estimated EHL. The following points should be considered: •

A dashed line connects the points representing the maximum values of CoF measured during each step of a ramp test.

•

The same approach is adopted for the points representing the average values. A marker further highlights each of these points.


33


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•

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To better highlight the difference between the average and maximum value of CoF for the same step, the area in between the “line of the maximum” values and the “line of the average” is shaded in color.

•

Three replicates are shown for each lubricant; each independent test can be identified by the different color of the filled area between the “line of the maximum” and the “line of the average”.

Figure 10. Average and maximum values of CoF measured for the various steps of a ramp of decreasing speed, as a function of the estimated minimum film thickness (h0 EHL) according to EHL calculations. Each marker connected by a dashed line represents the average CoF value of a step whose speed correspond to the value of h0 EHL reported on the abscissa (speed and h0 EHL are


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related by h0 EHL = f v0.68). For any given step, the maximum in CoF lies on the other dashed line bordering the same colored area, which serves as a guide to the eye to better identify each independent ramp. Three replicates are reported for the same set of conditions (denoted by different shades of the same color). The horizontal lines flanked by two arrows indicate the interval within which the onset of film failure was detected (i.e. spikes were first observed in the CoF trend). The vertical lines correspond to the condition h0

EHL

= Rq and h0

EHL

= 3 Rq,

respectively. The data obtained for [EMIM], [HMIM] and [DDMIM] TFSI are presented. Details of the ramp test are reported in the text.


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Figure 11. Average and maximum values of CoF measured for the various steps of a ramp of decreasing speed, as a function of the estimated minimum film thickness (h0 EHL) according to EHL calculations. The vertical lines correspond to the condition h0 EHL = Rq and h0 EHL = 3 Rq, respectively. Data obtained for a SiO2/Si tribopairs lubricated with [EMIM] FAP. The conventions used for the representation are described in the text. It should be noted that a clear limitation of this approach concerns the assumption that the detection of spikes in the CoF traces indicates the onset of asperity-asperity interactions, as already mentioned in the results section. Considering this possible limitation, it emerges from Figures 10 and 11 that the experimental values of h0 failure (the range of “critical” h0

EHL

EHL

corresponding of the onset of film

is highlighted by horizontal lines below each graph) are

relatively close to the range Rq – 3 Rq. It has been reported5 that the onset of asperity contact occurs close to λ ~ 3, which is in good agreement with the values obtained here. Interestingly, a progressive shift of the onset of film failure with the length of the alkyl chains attached to the cation is observed, which may indicate a contribution of the aliphatic chains to the boundarylubrication mechanism. The behavior exhibited by [EMIM] FAP is summarized in Figure 11. The onset of film failure with [EMIM] FAP was detected at a λ ratio of ~ 3, which is appreciably higher than the value observed with the TFSI salt having the same cation. This factor seems to indicate that not only the aliphatic chain, but also the type of anion plays a role in affecting the robustness of the interfacial layer. It must be emphasized here that the validity of these conclusions relies on the assumption that the applied EHL film-thickness equation retains its validity down to a few nanometers of liquid-film thickness, such that the observed difference in the critical λ ratio would effectively represent a boundary effect. Even more importantly, it is a significant


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assumption that the values of the rheological parameters for the ILs used in this work are correct. Indeed, a possible explanation of the results would be that [EMIM] FAP had a viscosity and/or a pressure-viscosity coefficient lower than that of [EMIM] TFSI, but this would be inconsistent with experimental results in the literature.27 Finally, a closer look at the trend reported in Figures 10 and 11 reveals the already discussed differences between ILs, in terms of the progression of surface damage after film failure. In summary, in the case of [EMIM] and [HMIM] TFSI, the onset of film failure is immediately followed by a sharp increase of the average value of friction. In contrast, in the case of [DDMIM] TFSI, the change of average friction is rather slow, which indicates that the presence of a sufficiently long aliphatic chain results in a substantial inhibition of the catastrophic form of wear observed with the shorter-chain cations. Finally, the presence of the FAP ion seems to induce a “self-healing” wear mechanism, which allows the recovery of fluid-film lubrication after the first interaction between asperities. This last observation corroborates the conclusions of our previous investigation on FAP-based lubrication of silicon-based surfaces5, supporting the idea that the FAP anion decomposes under the action of mechanical stress, promoting a smoothening of the contact area by a sacrificial-layer mechanism. 4.2 Possible factors affecting the strength of the interfacial layer The data summarized in the previous section reveal that both the length of the aliphatic chain attached to the imidazolium unit and the type of anion seem to affect the onset of film failure, which could be interpreted as an indicator of the ability of the interfacial IL layer to prevent contact between sliding surfaces. There is an extensive literature on adsorption of, and lubrication by, surfactants. These molecules are commonly used as boundary additives in oil formulations5, 6. In this context, the


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Page 38 of 50

role of the length of the aliphatic chain has been investigated since the pioneering work of Hardy and Doubleday40. It is generally believed that the main role of the adsorbed layers on polar surfaces is to limit the degree of contact between asperities, which would results in friction and wear reduction6. According to this view, longer aliphatic chains in oiliness additives (for example, fatty acids) would promote the stability of the adsorbed layer by increasing lateral cohesion and providing a greater separation between the surfaces. As mentioned in the introduction, in the field of ILmediated lubrication, several authors have also proposed that boundary-lubrication performance of ILs improves as the length of the aliphatic chain attached to the cation increases4. Nevertheless, there is a lack of clarity on this point in macrotribological studies of ILs. In addition, the current understanding of the structure of the interfacial layer in ILs is still developing and certainly not free from apparently contradictory results. Structural studies of IL/solid interfaces have frequently been carried out on silica surfaces23,41,42 . In the presence of water, the charge state of silica depends on acid-base equilibria involving surface silanol groups (Si-OH)43. In contrast, the mechanism of charging of silica in non-aqueous environments is still poorly understood. In particular, there seems to be disagreement in the literature concerning the charge density of silica surfaces in contact with ILs. In several AFM studies, silica-covered tips are often suggested to be (slightly) negatively charged13 and consequently the formation of an enriched cationic layer is assumed. Other authors have suggested that silica is essentially neutral in contact with ILs

24,44

. Finally, the recent works of Pfaffenhuber et al.45,46 on soggy-sand

electrolytes should be mentioned. Their zeta-potential measurements on silica particles dispersed in non-aqueous electrolytes, such as LiCF3SO3 / DMSO or LiCF3SO3 / PEG, have indicated that silica would preferentially adsorb anions. Cheng et al.19 recently used these findings to support


38

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their model for the interfacial structure of TFSI-based ILs in the absence of water. Notably, these authors suggested that the interfacial structure of long-chain ILs in dry environments and in contact with mica would result in an essentially disordered structure, which is in contrast with previous investigations47. This scenario, in silica-based surfaces, might be relevant from a tribological point of view, since the packing of aliphatic chain in an adsorption layer is expected to have an impact on the lubrication of the interfacial layer. In the present work, the comparison of [EMIM] TFSI, [HMIM] TFSI, [DDMIM] TFSI and [EMIM] FAP as lubricants for a SiO2/Si tribopair suggests that the onset of film failure is affected by the nature of both the anion and the cation. While the data reported in this work cannot provide any direct indication as to the structure of the confined layer, it is seems reasonable to assume that the values of the λ ratio at the onset of film failure might be indicative of differences in the structure of the confined liquid, particularly in terms of lateral cohesion and thickness of the boundary layer. Specifically, while the anion seems to have a major effect on the robustness of the interfacial layer, it would also seem that longer chains attached to the cation would provide further protection against direct asperity contact. It is possible that the same effect could at least be partially responsible for the ability of [DDMIM] TFSI to prevent catastrophic wear, compared with [HMIM] and [EMIM] TFSI. Nonetheless, it cannot be excluded that a tribochemical effect might also contribute to the lower friction observed with [DDMIM] TFSI. It has been observed that simple long-chain hydrocarbons can act as lubricants for silicon-based ceramics, leading to rather low values of coefficient of friction48,49. Although the origin of this phenomenon is still not clear, it has been suggested that the adsorption of hydrocarbons on silica surfaces may be related to the mechanically enhanced reactivity of acidic groups of the oxide surface, which would lead to


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chemical attachment of the aliphatic chains to the surface49. It is conceivable that, as the length of the aliphatic chain attached to the imidazolium unit becomes sufficiently long, as in the case of [DDMIM] TFSI, the reactivity of the IL would start to exhibit features similar to those of simple hydrocarbons. This aspect is not further explored here, but it certainly deserves further investigation. 4.3 Tribochemistry of silica surfaces lubricated by TFSI-ILs and comparison with previously investigated ILs The results of the ramp tests showed that the lubrication properties of very thin film of ILs are drastically altered as soon as any damage of the sliding surface occurs. This leads to the establishment of a new tribological regime, for which tribochemical reactions are expected to govern the evolution of mechanical and topographical properties of the interface. In addition, the entrapment of debris between sliding counterparts—the third body—is also expected to play a major role in the observed friction and wear50. Since the formation and composition of the debris are dictated by mechano-chemical processes and their presence affects wear and energy dissipation, the two aspects are intimately connected. The tribochemistry and wear mechanism of a SiO2/Si tribopair lubricated with 1-alky-3-methyl imidazolium FAP ILs has been already discussed in a previous publication25. The results of the ramp tests presented in section 2.1 largely confirm the hypothesis of our previous work. In the following, the tribological behavior observed in the presence of [EMIM] and [HMIM] TFSI is discussed. As mentioned in the previous section, the constant-speed tests used in this part of the study were carried out under conditions that lead to an initial damage of the surface, in order to avoid fluid-film lubrication. Under these conditions, substantial damage of the sliding counterparts was detected for all the tested speeds.


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Both Raman and XPS analysis of the worn surfaces showed the occurrence of large structural and compositional changes within the contact area, which strongly implies the presence of severe forms of mechanical damage during sliding. In particular, Raman analysis suggested the existence of amorphous silicon and several metastable phases. The observation of these structures on silicon wafers subjected to mechanical stress is commonly attributed to the rearrangement of the Si-II phase that is formed during loading. Since this high-pressure phase is ductile, the occurrence of stress-induced phase transitions in silicon might lead to plastic flow, as already reported in indentation51, scratching36 and tribological studies52. However, the observation of stress-induced phase transitions does not exclude that micro-fracture might also contribute to wear. Indeed, the visual and structural inhomogeneity revealed by optical and Raman microscopies suggest that a variety of contact geometries were created during tribotesting, leading to various forms of mechanical damage. The existence of structurally altered silicon is also demonstrated by XPS. In a previous publication38, we already noted that a broadening of the Si2p signal of elemental silicon was observed in wafers tribostressed in the presence of dry [EMIM] EtSO4 and suggested that this might be considered as a fingerprint of structural damage of the material. Here, to further support the hypothesis that the broadening of the elemental silicon peak can be related to the presence of a disordered structure, a simple experiment was carried out. A silicon wafer was subjected to sputtering with an argon ion beam with an applied voltage of 1 kV. As a result of the structural damage induced by the incident ions, a broadening of the Si2p signal was observed (Figure S9), which was found to be similar to the effect generated by mechanical damage. XPS analysis of the contact provided several further insights into the composition of the contact area of tribostressed silicon surfaces. A comparison with our previous studies is again


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useful for the discussion. The distribution of oxide species in silicon disks lubricated with TFSIbased ILs shows similarities to that reported in our previous study on dry [EMIM] EtSO4. The large amount of silicon in intermediate oxidation states may be indicative of the presence of debris within the contact, although it cannot be excluded that it may also originate from slidinginduced mechanical mixing. Another important aspect concerns the chemical state of fluorine in the tribolayer. The F1s signal of the tribostressed surface shows a great degree of similarity with that observed in our previous study on FAP-based ILs25. The signal was attributed to the presence of fluorine bound to silicon and its presence was thought to indicate a surface-localized mechanism of surface damage, leading to smoothening of the contact area and low wear. The data presented here show that similar products are formed in the presence of TFSI-based ILs, although extensive damage occurred in this case. This highlights a well-known limitation of ex-situ methods of analysis of tribologically stressed samples: the ex-situ information is not directly representative of the processes occurring during the test, but instead represents the final state of the system. In the case under discussion, the comparison of the results obtained with TFSI-based ILs with those based on FAP-based ILs suggests that the wear mechanism is mostly related to the specific reactivity of the anion with silica or silicon. In particular, comparing the molecular structure of FAP and TFSI anions, it might be supposed that the presence of P-F bonds in FAP may play a major role in inducing a surface-localized form of wear. In contrast, fluorinated carbon groups (CF3) do not seem to promote a similar form of surface damage, although the mechanically induced degradation at the SiO2/Si sliding interface ultimately leads to the formation of similar products. It should also be noted that, since the use of ex-situ analytical techniques cannot provide direct indications as to the specific reaction pathway involved in the wear mechanism,


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no conclusions concerning of the relationship between molecular structure and tribochemical reactions occurring in the contact area could be drawn from this study. When considering the high resolution XP-spectra presented in the present work, it might be possible that the presence of signals attributed to fluorides, as well as sulfides and nitrides, would be indicative of a certain degree of mechanical mixing occurring as a result of sliding. Evidence for shear-induced intermixing of the surface layer with the subsurface region has been already suggested by other authors. As an example, Furlong et al. found that rubbing a copper surface covered with a thiolate monolayer formed by DMDS adsorption causes transport of sulfur into the subsurface region53, 54. The possibility that, even in the case of silicon, the action of shear would result in the formation of mechanically mixed layer is currently under investigation in our laboratory. Clearly, the changes in the composition of the near-surface region might be related to the observed differences in wear mechanisms observed when using different ILs. In addition to the possibility that intermixing of the surface layer with the subsurface region may occur, leading to change in the properties of the counterparts, wear and friction in boundary conditions might be affected by the dynamic of the trapped debris (third body) 55,56. A final point to be discussed concerns the role of velocity on wear. The results reported here indicated that the wear coefficient shows a relatively small dependency on speed, between 50 and 5000 mm min-1. Sliding speed may be influencing the rate of particle circulation within the contact, which would in turn affect the action of the third body on wear, although no attempt is presented here to verify this hypothesis. Thermal effects may also have an impact on wear: increasing the speed at constant value of CoF results in higher dissipated power and higher flash temperatures. Thermal shocks can generate fractures and, consequently, increasing wear in brittle materials57. An attempt to


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estimate the temperature of the contact as a function of speed is reported in the Supporting Information. According to our calculation, the estimated increase of temperature would not be higher than ~ 16°C at the highest speed (5000 mm min-1). Based on this estimation, assuming that the model used here provide reliable, if not overestimated, predictions, friction heating would have negligible effects on wear. Of course, this outcome largely depends on the rather high thermal conductivity of silicon: if the mechanically mixed layer had conductivity closer to silica (which is a good insulator), it could be possible that significantly larger increase in temperature would be locally observed. 5 Conclusions In this work, two aspects of the tribological behavior of a SiO2/Si tribopair lubricated with fluorinated ILs were investigated under anhydrous conditions. Firstly, speed-dependent tribological tests were carried out to investigate the transition between fluid-film and mixed-boundary lubrication. The minimum thickness h0 of the liquid film was estimated by elastohydrodynamic considerations; the values found for the λ ratio (h0 EHL/σ) at the onset of surface damage suggest that both anions and cations form part of the adsorbed layer and contribute to hindering contact between asperities. The evolution of the friction traces as the speed further decreased provided insight into the surface-damage mechanisms taking place in the presence of the various ILs. In particular, both adsorption and sacrificial-layer mechanisms of boundary lubrication were observed, depending on the chemical structure of the IL used as lubricant. In the second part of this study, the wear mechanisms of [EMIM] and [HMIM] TFSI were investigated by carrying out constant-speed tests. The results of Raman and XPS analyses revealed that a mechanical form of wear dominates within the range of speed of 50 - 5000 mm


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min-1 for both ILs. In particular, the Si2p XP-spectra of samples tribostressed in the presence of TFSI-based ILs were comparable with those obtained from [EMIM] EtSO4-lubricated samples: in both cases, a broadening of the signal of elemental silicon and a significantly larger contribution of the components associated with silicon suboxides were observed in the spectra of the contact area, as compared with those of the non-contact area. A mechanical form of wear prevails in both cases. In contrast, spectra obtained with FAP-based ILs used as lubricants were found to be significantly different, as the above mentioned effects were not detected in the spectra of samples tribostressed with FAP-based ILs, confirming the potentiality of XPS in providing insight into the mechanism of surface damage of silicon. This work highlights the molecular mechanisms underlying the observed macroscale tribological behavior. ASSOCIATED CONTENT Supplementary Information A description of the equations used in this work is reported in the supplementary information. In particular, the terms appearing in the formulae used for estimating the EHL film thickness and the temperature of the sliding contact are presented. The parameters used for the curve fitting of the XP-spectra of the samples lubricated with TFSI-based ILs are reported in Table S1. The binding energies of the atoms of pure 1-alkyl-3methyl imidazolium TFSI ILs used in this work are reported in Table S2. A brief description of the method used for the calibration of the energy scale is attached to the table. A list of the compounds considered for the chemical-state analysis of the tribostressed silicon disks is reported in Table S3. The 1H and 19F NMR spectra of the 1alkyl-3methyl imidazolium TFSI and FAP ILs used in this work are reported in Figures S1-S4. A


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CoF trace extracted from a ramp test carried out with a [HMIM] TFSI- lubricated SiO2/Si sliding tribopair is reported in Figure S5. Micrographs of wear scars of fused silica pins after ramp tests carried out in the presence of [DDMIM] TFSI or EMIM FAP are illustrated in Figure S6. The XPS survey spectra of silicon disks tribostressed in the presence of [EMIM] TFSI or [HMIM] TFSI and at a speed of 50 mm min-1 are reported in Figure S7. The Si2p spectra of the contact area of silicon disks tribostressed in the presence of [HMIM] TFSI and in a nitrogen atmosphere (speed: 50, 500 or 5000 mm min-1) are presented in Figure S8. The spectra of a silicon wafer before and after 360 seconds of sputtering (1KeV, sputtering rate for silica: 0.029 nm/s) are compared in Figure S9. ACKNOWLEDGMENTS The authors wish to express their gratitude to Prof. Rosa Espinosa-Marzal for her comments on the manuscript. Andi Wyss is thanked for his introduction to the Raman spectrometer in the Laboratory for Nanometallurgy (ETH Zurich). REFERENCES 1. Ye, C.; Liu, W.; Chen, Y.; Yu, L. Room-temperature ionic liquids: A novel versatile lubricant. Chem. Commun. 2001, 2244-2245. 2. Bermúdez, M.-D.; Jiménez, A.-E.; Sanes, J.; Carrión, F.-J. Ionic liquids as advanced lubricant fluids. Molecules 2009, 14, 2888-2908. 3. Somers, A. E.; Howlett, P. C.; MacFarlane, D. R.; Forsyth, M. A review of ionic liquid lubricants. Lubricants 2013, 1, 3-21. 4. Minami, I. Ionic liquids in tribology. Molecules 2009, 14, 2286-2305. 5. Stachowiak, G.; Batchelor, A. W. Engineering tribology; Butterworth-Heinemann, 2013. 6. Bowden, F. P.; Tabor, D. The friction and lubrication of solids; Oxford university press, 2001. 7. Hayes, R.; Warr, G. G.; Atkin, R. Structure and nanostructure in ionic liquids. Chem. Rev. 2015, 115, 6357-6426.


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Ionic Liquids - ACS Publications

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