Lubrication of Si-Based Tribopairs with a Hydrophobic Ionic Liquid

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Lubrication of Si-Based Tribopairs with a Hydrophobic Ionic Liquid: The Multiscale Influence of Water Andrea Arcifa, Antonella Rossi, Shivaprakash N. Ramakrishna, Rosa M. Espinosa-Marzal, Alexis Sheehan, and Nicholas D. Spencer J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01671 • Publication Date (Web): 16 Mar 2018 Downloaded from http://pubs.acs.org on March 17, 2018

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The Journal of Physical Chemistry

Lubrication of Si-Based Tribopairs with a Hydrophobic Ionic Liquid: The Multiscale Influence of Water A. Arcifa1, A. Rossi1,2, Shivaprakash N. Ramakrishna1, Rosa Espinosa–Marzal3, Alexis Sheehan3, and N. D. Spencer1* 1

Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, CH-8093 Zurich, Switzerland

2

Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Cagliari, Cittadella Universitaria di Monserrato, I – 09100 Cagliari, Italy 3

University of Illinois at Urbana-Champaign, Urbana 61801, Illinois, USA.

* Corresponding author, email: [email protected]

ABSTRACT

This work aims to elucidate the role of water on the tribological behavior of silicon-based surfaces lubricated with a hydrophobic ionic liquid (IL), by means of a multi-technique, multi-scale approach. At the nanoscale, the presence of water at the interface was found to 1 ACS Paragon Plus Environment

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promote adhesion between a sharp silicon tip and a silicon substrate, when submerged in the IL. In line with this finding, in the case of samples that had been exposed to humid air, lateral force microscopy at low loads revealed a significant contribution of adhesion to friction. Under dry conditions, a low-to-high-friction-regime transition is observed at low loads, which is reminiscent of the behavior already observed at the nanoscale in previous studies on IL-mediated lubrication. The comparison of friction-vs-load curves from tests carried out under both humid and dry conditions suggests that a similar mechanism of energy dissipation, presumably involving solid-solid contact between sliding counterparts, is established when applied loads are sufficiently high. The macroscopic behavior of a fused silica pin sliding against a Si (100) substrate in a ball-on-disk configuration was investigated over a wide range of sliding speeds.

Wear was evaluated by means of both optical

microscopy and profilometry. Changes in the surface chemistry and near-surface structure of the contact area following tribotesting were characterized by both Raman and X-ray photoelectron spectroscopies. Macrotribological tests show that, for sufficiently low sliding speeds, the water adsorbed at the solid/IL interface promotes a tribochemical form of wear. However, at high sliding speeds, a regime of wear characterized by extended damage in the form of plastic deformation and fracture dominates, regardless of the presence of water in the IL. Under these conditions, the prevailing mechanism of friction is likely to be related to the welding and rupture of asperity/asperity junctions, and a direct comparison of LFM results might be not possible. In contrast, when in the presence of humid air and at low sliding speed, the absence of plastic deformation in the near-surface region suggests that pressures within the asperity-asperity contacts are in the range of those existing in the LFM experiments described here.

Introduction

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Interfacial properties of room-temperature ionic liquids (ILs) have been intensively investigated during recent years1,2. A thorough understanding of the fundamental principles governing these properties is necessary for the application of ILs in a variety of fields, including heterogeneous catalysis3, electrochemical applications4,5 and lubrication6, 7, 8, 9. The surface forces apparatus (SFA) and the atomic force microscope (AFM) have been widely employed to probe the structure and dynamics of interfacial or confined IL films. The tendency of ILs to exhibit oscillatory forces under confinement was already evidenced in the pioneering work of Horn et al10. Later, the occurrence of layering of the confined ions was confirmed for several other ILs by means of SFA and AFM studies. The dynamics and frictional response of the liquid film as a function of the chemical composition of the ions11,12, surface composition13 and topography14 of the solid surface, environmental conditions15,12,16, and externally applied electrical potential17 have been investigated. An important aspect to take into consideration when examining the interfacial properties of ILs is the tendency of virtually any molten salt to absorb water when exposed to air18. The effect of this ubiquitous component of the atmosphere on the properties of ILs was recognized early in the field of electrochemistry19, but has received rather limited attention in IL-mediated lubrication, both on the nano- and macroscales. In our laboratories, interfacial force measurements by SFA and colloidal-probe lateral force microscopy (CP-LFM) have demonstrated that even small amounts of water can significantly alter the frictional response and dynamics of the confined IL12,15 . The effect of water uptake from humid air by the IL on its interfacial structure and on adhesion and tribological response has been recently shown in experiments 16, 20, 21, 22, 23 and molecular-dynamics simulations 24. Friction at the macroscale, while being ultimately related to dissipation mechanisms occurring at the molecular level, often encompasses a number of phenomena that may not be readily mirrored in AFM and SFA experiments, such as (elasto-) hydrodynamic



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contributions, third-body effects, as well as chemical and topographical changes of the rubbing interface (wear) during sliding. The interplay between these phenomena, typically occurring over a broad range of lengthscales, greatly complicates the quantitative interpretation of macrotribological behavior. On the other hand, the demand for technological advances in the field of macrotribogy is very high, and reflects the current need for more sustainable and energy-efficient processes in industry. In this context, ILs have been proposed as novel lubricants since the beginning of the current century25. More recently, ILs have been combined with water-based solutions26 and water/oil emulsions27, and have displayed promising performance for the lubrication of self-mated steel. It was also shown that the reactivity of water plays a significant role in affecting the tribological behavior of steels. Given the mechano-chemical sensitivity of ceramics to water, its presence in an IL-based lubricant is expected to have an impact on this class of materials. However, this phenomenon has been scarcely investigated in the past. The pioneering work of Philips and Zabinski28 highlighted the effect of water in combination with fluorinated ILs on the mechanism of wear of self-mated Si3N4, particularly with regard to the duration of the running-in processes. The lubrication of a mixed ceramic/steel contact lubricated with IL/water solutions was investigated by Espinosa et al29, who also found that the presence of water significantly affects the running-in process. In recent years we have carried out a number of combined tribological and spectroscopic investigations on IL-mediated lubrication of silicon-based materials30,31. A significant result emerging from our analysis is that the water dissolving in a hydrophilic IL (1-ethyl-3-methyl ethylsulfate) as a result of exposure to humid air can trigger a tribochemical wear mechanism31. In this case, damage is localized at the surface and involves the continuous chemical reaction and removal of the uppermost layer of material, which results in a


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progressive smoothening of the contact area. The situation is reminiscent of the mechanism observed in the case of silicon-based materials sliding in humid air32 or water33. Similar evidence of tribochemical processes in silicon-based materials lubricated with IL/water mixtures was presented in the above-mentioned work of Phillips and Zabinski28, and more recently by Xie et al.34. In the present investigation, we focus on the effect of water on the tribological behavior of SiOx-based surfaces lubricated with a hydrophobic IL, 1-methyl-3-butyl-imidazolium bis(trifluoromethyl-sulfonyl) imide, on both the nano- and macroscales.

Experimental 2.1 Ionic liquids As-received 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide [EMIM] TFSI (IoLiTeC GmbH, Heilbronn Deutschland) was dried by placing about 1 cm3 of IL under a rough vacuum of ~ 10-2 mbar for three days and later storing it in a nitrogen-filled glovebox (< 10 ppm H2O an O2). ILs used for macrotribological tests carried out in the presence of humid air were stored for one week at a relative humidity of 45% (296 ± 2 K) inside a sealed vessel. The water content after exposure to this environment was measured by coulometric Karl Fischer titration and found to be 0.46 wt.%. This value is comparable to that reported by Cheng et al.23 for the same IL at equilibrium at 43% RH and 296 K.

2.2 Adhesion and friction-force measurements A MFP3D AFM (Asylum Research, Oxford Instruments, Santa Barbara, USA) was used for carrying out adhesion and friction-force measurements. P-type (100)-oriented silicon wafers used as substrates (Silicon Materials, Kaufering, Germany) were sonicated for 5 minutes each, in toluene and isopropanol, after which they were O2-plasma treated for 2 minutes just before the measurements. The AFM cantilevers 5 ACS Paragon Plus Environment


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(ContAl-G, from Budget Sensors, Sofia, Bulgaria) were rinsed with ethanol and cleaned by means of UV-ozone for 30 minutes. Measurements were carried out with the tip fully immersed in a droplet of [EMIM] TFSI deposited on the substrate. A homemade setup attached to the AFM controlled the humidity during the tests by allowing either humid air or nitrogen to flow through the device. In this study, measurements were carried out at 296 ±2 K and either 45±3% or 5±3% RH. In order to obtain the lower value of humidity, the inlet of the device was connected to a dry nitrogen tank. In the following we refer to the atmosphere achieved by purging the device with nitrogen as “dry conditions”. Prior to LFM measurements, the AFM was purged for about 30 minutes with either humid air or nitrogen; during the waiting time, normal force-separation curves were acquired to evaluate the change of adhesion with time; tests were started when the value of the pull-off force remained constant. The normal spring constant of the cantilever used for pull-off and LFM measurements was calibrated by means of the thermal-noise method35 and it was 0.149 N/m for the experiments shown here. The friction force was measured by acquiring friction loops while scanning the cantilever across the sample surface at a constant speed of 200 nm s-1 and over a stroke of 1 µm. The measured signal on the AFM photodetector was then converted into a lateral force by using the ‘wedge’ method for lateral force calibration36. The absolute friction force was obtained by averaging 5 friction loops (forward and backward traces). The friction force was recorded by varying the applied load between 10-70 nN.

2.3 Macrotribological tests 2.3.1 Tribopairs and lubricant preparation


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All the macrotribological tests described in this work were carried out using pins consisting of 2-mm-diameter fused silica balls (Corning 7980, J. Hauser GmbH & Co. Solms, Germany). Silicon wafers of the type described in Section 2.2 were employed as disks. The Young’s moduli, Poisson’s ratios and the roughness values of the materials used for this study are reported in Table 1.

Table 1: Characteristics of the materials used for pin-on-disk tests 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.37

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 O2-plasma treated for two minutes shortly before the test, with the aim of removing organic contamination.

2.3.2 Tribological tests Tribological tests were carried out using a 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 of 0.0049 N (manufacturer’s data) was used. Tests were carried out at a constant speed of 50 and 5000 mm/min, for a total duration of 400 turns and with a constant load of 4.5 N.



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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 any loss of lubricant during the test at high rotational speeds. The same instrumental parameters were adopted for two different environmental conditions: nitrogen atmosphere (water and oxygen content below 10 ppm) and humid air (45%-55% RH; T = 296 ± 2 K). The nitrogen atmosphere was achieved by placing the tribometer inside a glove box filled with nitrogen (MBraun Labmaster 100, Garching, Germany). When comparing the results of experiments carried out in the presence of humid air with those carried out in a nitrogen atmosphere, it is assumed that water, rather than other air components—such as oxygen—plays a major role in modifying the observed tribological behavior of a silica/silicon tribopair lubricated with [EMIM] TFSI. Silicon, silica and siliconbased ceramics are known to react more rapidly with water than with oxygen under sliding38, which supports the assumption made here.

2.4 Analysis of the worn surfaces 2.4.1 Optical microscopy and profilometry 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 of the worn pin. Wear data of disks presented in this work were obtained by evaluating the wear volume of the tracks with a 3D optical profiler (Sensofar PLu Neox, Sensofar-Tech, SL., Terrassa, Spain) operating in white-light interferometry (ePSI) mode, with a 50x objective.

2.4.2 Raman microscopy


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A WITec Confocal Raman Microscope 200 (WITec, Ulm, Germany) with a laser light source (λ = 532.14 nm) and a lateral resolution better than 1 µm was used to characterize the structure of the worn surface of the silicon disks after tribotesting. The following conditions were applied for the measurements: 1.6 mW of power on the sample 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.

2.4.3 X-ray photoelectron spectroscopy (XPS) The X-ray photoelectron spectra presented in this work were acquired with a PHI QuanteraSXM (ULVAC-PHI, Chanhassen, MN, USA). Analyses were carried out with a monochromatic AlKα (1486.6 eV) source, using a beam diameter of 9 µm or 20 µm, the analyzer working in constant-analyzer-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 halfmaximum (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 analyses were carried out while using a low-voltage, argon-ion gun / electron neutralizer. Following the procedure described in our previous work39, in order to minimize the effects of differential charging, the samples were mounted on the sample holder by using double-sided adhesive tape—thus insulating the sample from ground. The flood gun conditions were −5 V cathode voltage (with respect to instrumental ground), and an emission current of 20 mA. In this work, the aliphatic component of the C1s signal was used as an internal reference (285.0 eV).



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Results 3.1 Macrotribological measurements 3.1.1 Pin-on-disk tribometry Figure 1 shows the CoF trends observed for a SiO2/Si tribopair at 4.5 N of load lubricated with [EMIM] TFSI at a sliding speed of 50 and 5000 mm min-1, in the presence of humid air and a nitrogen atmosphere.

Figure 1: CoF versus number of cycles during tribological tests at a sliding speed of 50 and 5000 mm min-1 (a and b, respectively) in the presence of [EMIM] TFSI. All tests were carried out at a constant load of 4.5 N, at room temperature, in the presence of either a nitrogen atmosphere (red traces) or humid air (blue traces).

The trends observed in the presence of anhydrous ILs and a nitrogen atmosphere (Figure 1, red traces) have already been discussed by the authors39. For both sliding speeds, the CoF was found to be close to 0.24, with relatively small deviations during the test. For friction traces obtained in the presence of humidity, similar behavior was observed at high speed. In contrast, a significant increase in the CoF value was measured at a speed of 50 mm min-1. In this case the friction trace consisted of large oscillations with an average CoF value of 0.43 (0.2). 10 ACS Paragon Plus Environment

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3.1.2 Analysis of wear and appearance of the contact area after tribotesting Figure 2 shows the wear coefficients of silicon disks and fused silica pins lubricated with [EMIM] TFSI, measured at the end of 400 turns at a speed of 50 or 5000 mm min-1. Results obtained in the presence of humid air are compared with those obtained in the presence of a nitrogen atmosphere39.

Figure 2: Wear coefficients of disks (a) and pins (b) measured after tribological tests at different speeds (normal load: 4.5, total duration: 400 turns) carried out in the presence of [EMIM] TFSI and either humid air or a nitrogen atmosphere. Results in nitrogen refer to previously published work39.

An increase of sliding speed led to higher wear coefficients, regardless of the environmental conditions. In addition, tests carried out with the anhydrous lubricant resulted in consistently higher values of wear coefficients measured after 400 turns. Despite the qualitatively similar trend of wear coefficient with speed for tests carried out in the two environments, a visual inspection of the worn areas suggests the occurrence of significantly different modes of surface damage in the two cases (Figure 3). 11 ACS Paragon Plus Environment


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Figure 3: Appearance of the wear scars at the end of the tribological tests carried out at: 50 mm min-1 (a and c), 5000 mm min-1 (b and d) and in the presence of a nitrogen atmosphere (a and b, from39) or humid air (c and d). Normal load: 4.5, total duration: 400 turns.

In particular, the wear scar obtained at low speed in the presence of humid air (Figure 3c) deviates from all the other conditions, as it appears smooth and delineated by a circular contour. A closer look at Figure 3.c reveals the presence of large fractures in the form of “crescents”, possibly indicating that the contact stresses generated during the first part of the tests were large enough to generate macroscopic fractures40. In contrast, under dry conditions, multiple fractures were observed at the edge of the contact and the scar appears rough and irregular. This difference in the morphology suggests that, at the lowest speed, 50 mm min-1, the presence of humidity triggers an alternative mode of surface damage for a SiO2/Si tribopair lubricated with [EMIM] TFSI. This hypothesis is supported by the results of the surface chemical and structural analysis described in the following paragraphs (3.1.3 – 3.1.4).


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3.1.3 Raman microscopy of the disks In our previous publication39, Raman spectra of silicon discs lubricated with [EMIM] TFSI in the presence of nitrogen atmosphere evidenced the occurrence of structural changes in the near-surface area of the wear tracks. Metastable phases of silicon (Si-III/Si-XII) and amorphous silicon were also detected, indicating the occurrence of plastic flow41, 42 as a mode of mechanical damage during tribotesting.

In addition, signals likely attributed to the

presence of nanocrystalline silicon could also be detected when probing areas covered by wear particles. In the present study, similar results were observed for tests carried out at high speed in the presence of humid air. As an example, in Figure 4, several representative Raman spectra of a silicon disk lubricated with [EMIM] TFSI at a speed of 5000 mm min-1 are displayed. For a comparison, a spectrum of a pristine silicon wafer is also shown.

(a)

(b) (4)

2 3 4

1

(3) Intensity (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 4: (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 and in humid air. (b) Four representative Raman spectra acquired at the points indicated by arrows in the micrograph; the spectrum of pristine (100) oriented Si wafer is also presented. Test conditions: sliding speed: 5000 mm min-1, normal load: 4.5 N, duration: 400 turns.

Amorphization was the major type of structural change detected in the contact area, although signals characteristics of metastable phases (Si-III/Si-XII) could be detected at several points (Raman spectrum #2, Figure 4); signals attributable to nano-crystalline silicon were also observed (Raman spectrum #4, Figure 4). The spectra of samples tribostressed at a speed of 50 mm min-1 in the presence of humid air were significantly different from those resulting from tests carried out at higher speeds, or in the presence of a nitrogen atmosphere. The micrograph in Figure 5 shows a portion of the contact area of a silicon disk tribostressed under [EMIM] TFSI at 50 mm min-1; two representative Raman spectra are reported in Figure 5.b.

(b)

(a)

(2)

1

Intensity (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

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2 (1) Wavenumber (cm-1)


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Figure 5: (a) Micrograph of an area of the wear track on a silicon disk produced after sliding against a fused silica pin under [EMIM] TFSI in humid air. (b) Two representative Raman spectra acquired at the points indicated by arrows in the micrograph. Test conditions: sliding speed: 5000 mm min-1, normal load: 4.5 N, duration: 400 turns.

The micrograph reveals a patchy layer of finely dispersed debris covering an otherwise homogeneous surface. Cracks are also visible, and presumably originated from the high tensile stress acting at the sliding interface. All the spectra from the contact area exclusively exhibited the signal characteristic of Si-I; the only effect of the debris observed in the Raman spectrum (Figure 5, spectra ♯2) was an increase in the Raman background.

3.1.4 X-ray photoelectron spectroscopy High-resolution spectra of silicon disks lubricated with [EMIM] TFSI in the presence of humid air are presented in Figure 6. The spectra from the non-contact area of a silicon disk tribostressed at 5000 mm min-1 are also presented for comparison. A detailed description of the chemical attribution of the XP-signals is presented in the Supplementary Information.



(c)

Int ensit y ( A.U.)

Int ensit y ( A.U.)

(b)

Int ensit y ( A.U.)

(a)

108

104

100

96

540

Bi nd i n g Energ y ( eV)

696

537

534

531

528

525

296


(d)

688

684

680


Figure 6:

240

288

284

280

276

(f )

Int ensit y ( A.U.)

Int ensit y ( A.U.) 692

292


(e)

Int ensit y ( 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

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236

232

228

224


408

404

400

396

392


High-resolution XP-spectra of silicon disks tribostressed in the presence of

[EMIM] TFSI in the presence of humid air: (a) Si2p, (b) O1s, (c) C1s, (d) F1s, (e) S2s, (f) N1s. The spectra of samples tested at a sliding speed of 5000 mm min-1 (upper row) and 50 mm min-1 (middle row) are shown, together with those of the non-contact area (lower row).

The XP-spectra of samples tribostressed at the higher speed in the presence of humid air (Figure 6, upper row) closely resemble those obtained at the same speed and in the presence of a nitrogen atmosphere, presented in our previous work39. This substantiates the outcomes of Raman analysis, optical microscopy and tribometry: a similar mechanism of friction and wear seems to be established for tests carried out at high speed under the two environmental conditions.


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In particular, the clearly detectable signals attributable to the presence of silicon directly bonded to fluorine (686.6 eV), sulfur (227-0 eV), nitrogen (398.2 eV) and carbon (283.8 eV), as well as the intense signal of SiO2-x (0 < x < 2) at 100.3 – 101.8 eV, indicate that elemental silicon directly reacted with ions and also that shear-induced mechanical mixing occurred as a result of sliding. The analysis of the Si 2p signal of the elemental silicon also reveals a certain degree of structural disorder, as evidenced by the broader width of this signal in the contact area, as compared with that of the signal of a pristine silicon wafer (0.9 vs 0.6 eV). In contrast, spectra acquired following tests carried out in the presence of humid air and at 50 mm min-1 exhibit marked differences from those obtained under the same conditions but in the presence of a nitrogen atmosphere. This result, together with those of other techniques, reported in the previous section, suggests that the presence of humidity triggers a significantly different mechanism of interaction between the two counterparts for the specific case of tests carried out at a sufficiently low speed and in the presence of [EMIM] TFSI. The XP-spectra reveal a substantial lack of extended structural damage—notably, down to a level of few nanometers—and the formation of mostly oxidized compounds of silicon. As exemplified in the following section, these signals are attributable to the debris formed as a result of tribochemical wear.

Chemical analysis of the third body for tests carried out at a sliding speed of 50 mm min-1 in the presence of humid air. Visual inspection of wear tracks of disks tribostressed at 50 mm min-1 in the presence of humid air revealed the presence of debris partially covering certain areas of the scar. As observed in Figure 5, the debris consists of aggregates of finely divided powder. The size of the deposit is below the resolution of the XP-spectrometer43, thus it was not possible to probe exclusively debris, or debris-free areas. Nonetheless, optical and X-ray-induced secondary-



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electron images (SXI) presented in Figure 7 show that the patchy third body preferentially collects along certain channels in the wear track. These areas appear as bright stripes in the SXI. In Figure 7.b, the Si2p spectra acquired with a beam size of 9 µm on a debris-covered and a (mostly) debris-free area are presented and compared with the spectrum of the non-contact area. The large amount of Si4+ detected on the debris-covered area suggests that the material mostly derives from the hydrolysis or oxidation of the sliding counterparts, supporting the hypothesis that a tribochemical form of wear prevails in this specific case. The decrease in the intensity of the Si4+ component detected on debris-free areas also points to the occurrence of a surface-localized mechanism of wear, consisting of the continuous formation and removal of chemically altered material at the surface30.

Figure 7: (a) X-ray induced secondary electron image (SXI) of the wear track of a Si disk lubricated with [EMIM] TFSI in the presence of humid air (test conditions: total duration 400 turns; sliding speed 50 mm min-1; applied load 4.5 N); the SXI, in the upper part of the picture, is stitched to a portion of an optical micrograph taken from approximately the same 18 ACS Paragon Plus Environment

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area of the disk. (b) High-resolution Si2p XP-spectra of three points on the same samples, as indicated in the SXI. The white spots have a diameter representative of the nominal size of the spot (diameter: 9 µm). The unfilled circle delimited by a dashed line represents a point (nominal diameter: 20 µm) whose spectra are reported in Figure 6.

3.2 Adhesion and Lateral Force Microscopy (LFM) measurements. Figure 8 shows an example of a recorded force-vs-Z-sensor-distance (distance Z between sample surface and the relative position of the cantilever) curve in [EMIM] TFSI under dry and humid conditions (5±3% and 45±3% RH, respectively). The pull-off force under dry conditions was smaller than 0.4 nN, indicating negligible adhesion between the silicon tip and the silicon surface. A pull-off force of 11±5 mN (averaged over 20 force-separation curves) was obtained in humid air.

Figure 8: Force-vs-Z-sensor-distance curves obtained for silicon tip vs silicon substrate under [EMIM] TFSI under: a) dry (RH 5±3%) and b) humid (RH 45±3%) conditions. The scan velocity was 200 nm/s. Z-sensor distance: distance Z between sample surface and the relative position of the cantilever.



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Values of friction force measured vs applied load with a silicon tip on the surface of a silicon wafer in [EMIM] TFSI, under dry (blue squares) and humid (red circles) conditions are depicted in Figure 9. The black triangles represent the reference friction force measured in water with the same tip.

Figure 9: Friction-force-vs-applied-load plot between a silicon tip and the surface of a silicon wafer under [EMIM] TFSI in dry (red circles) and humid (blue squares) conditions. The black markers (triangles) represent the friction-vs-applied-load curve measured with the same tip (after rinsing with ethanol and UV cleaning for 30 minutes) on a freshly cleaned silicon substrate immersed in water.

In the presence of a nitrogen atmosphere, within the range of applied load investigated in this study, the friction force exhibits an approximately linear dependency with the applied load, with a slope, µ, of 5.9. The linear fit of the friction vs applied load curve shows a positive intercept with the x-axis. As discussed later (Paragraph 4.2), the finding is consistent


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with the presence of two friction regimes as a function of applied load: a low-friction regime below ~ 10 nN, and a high-friction regime at higher applied loads. In equilibrium with humid air, the friction force shows high values at low applied loads, likely as a result of the significant adhesion between the tip and the surface (Figure 8.b). Above ~ 50 nN, the friction was found to be the same in the two environmental conditions, within experimental uncertainty. It is worth comparing the friction obtained at the lowest applied load (10 nN) and in the presence of water-containing [EMIM] TFSI with values reported in the literature for adhesive, nanometer-sized contacts at nearly zero applied loads. Li et al.

44

measured lateral

forces of ~ 190 nN for a silicon tip sliding against a silicon wafer in the presence of humid air at zero applied load. For a similar tribopair and environmental conditions, a friction force of ~ 110 nN was reported by Li et al.45. These values are comparable to that measured in the present investigation at the lowest applied load (10 nN) in humid conditions and in the presence of [EMIM] TFSI. The radius of the tip after the friction-force measurements with the IL inferred from reverse imaging of the tip is ~ 40 nm. Considering the range of load investigated in this study (10 – 70 nN), an estimation based on the JKR equation46 results in an average contact pressure of 1.2 – 2.5 GPa (See Supporting information). These values represent lower limits to the contact pressure acting during the test, as wear of the tip is likely to occur and so an increase of the radius of the tip is expected to take place with sliding. In order to verify the validity of the lateral-force-calibration procedure used in this study, a set of friction measurements was also carried out in milli-Q water. The same tip used for the measurements in the IL was rinsed with ethanol and UV cleaned for 30 minutes; the friction measurements were repeated on a freshly cleaned silicon substrate. The measured µ of 0.36 in water is in agreement with values from LFM studies of silica surfaces sliding in water at



moderately high contact pressures47, 48,

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49

. The outcome substantiates the validity of the very

high values of lateral force, and µ value, measured with the IL. Finally, in order to investigate the role of the initial conditions of the tip on friction in LFM measurements, we carried out an additional LFM experiment under humid conditions, using an as-received tip— i.e. a tip not subjected to ethanol rinsing and UV-ozone treatment prior to test. The results are reported in Figure S3. During the first loading ramp, friction was found to change in a rather complex fashion with load. Stable behavior was achieved after repeating the ramp two additional times. In contrast, when exposing the counterparts to oxidizing environments prior to the test (as in the case of the experiments reported in Figure 9), the “running-in” process was limited to the first few turns and subsequent loadingunloading ramps exhibited similar trends.

4. Discussion 4.1 Role of environmental humidity on the wear of a SiO2/Si tribopair lubricated with [EMIM] TFSI The data presented in this study demonstrate that, by replacing dry nitrogen with humid air (45-55%RH), significant changes in tribological behavior were found to occur only at low sliding speed (50 mm min-1). In particular, inspection of the micrographs of pins following tribological tests at 50 mm min-1 and in the presence of humid air revealed circular and smooth contours of the wear scar. Furthermore, no structural changes of the silicon disk in the near-surface region were detected by Raman spectroscopy. The results were supported by X-ray photoelectron spectroscopy, where the analysis of the Si2p signal of elemental silicon revealed no difference, in terms of FWHM, between the contact and non-contact— undamaged—areas from tests carried out at 50 mm min-1 in the presence of humid air, confirming the lack of significant structural changes to a depth of few nanometers31. In


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addition, the XP-spectra obtained from areas predominantly covered by debris indicated that they are composed mostly of Si4+ species, such as dioxide, hydroxides and oxynitrides/carbides/fluorides. The composition, structure and smoothness of the contact area suggest that a tribochemical mechanism is established at a sliding speed of 50 mm min-1 in the presence of humid air. In previous studies30,

39

, a similar mode of surface damage was observed in the presence of

FAP-based ILs, although in that case the tribochemical mechanism was intrinsically related to the reactivity of the IL and not to exposure to humid air. In contrast, exposure to humid air was found to be required for observing a tribochemical mechanism of wear in a SiO2/Si tribopair lubricated with [EMIM] TFSI. In this respect, it should be noted that this IL, despite being rather hydrophobic, does absorb water up to ~0.5 wt.% when exposed to 45-55 %RH. Most importantly, as discussed in the previous paragraph and in agreement with recent studies from different authors16, 23, there is a tendency for water to be enriched at the interface between [EMIM] TFSI and polar surfaces. In addition, tribochemical wear resulting from the interaction of water with the solid surface has been widely recognized as a principal mechanism of material removal from the surface of siliconbased materials32,

33

. In this respect, we have already explained the results observed for

SiO2/Si lubricated with an [EMIM] EtSO4/water mixture by proposing a similar type of mechanism31. Despite the fact that [EMIM] TFSI takes up a significantly lower amount of water than the hygroscopic [EMIM] EtSO4 when exposed to the same level of humidity, we propose that the above-mentioned enrichment of water at the interface might lead to the occurrence of a similar surface-localized form of wear, resulting in oxidation/hydroxylation of the sliding counterparts with concomitant smoothening due to preferential removal of reaction products.



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When considering the results of the tests at high speed (5000 mm min-1), the wear mechanism was found to be dominated by extended mechanical damage, regardless of the environmental conditions. Stress-induced phase transformations of the silicon disks were detected by Raman microscopy, suggesting that, in addition to brittle fracture, ductile deformation of silicon contributed to surface damage. X-ray photoelectron spectroscopy provided indications concerning the chemical transformations occurring at the sliding interface. In particular, signals attributable to silicon sub-oxides were observed, as well as others suggesting the presence of nitrogen, carbon and sulfur bonded to silicon, which might indicate the occurrence of mechanical mixing at the sliding interface. Comparing the results presented in this work and in our previous investigation, a transition from a tribochemical to a mechanical form of wear in SiO2/Si lubricated with [EMIM] TFSI is only observed when in the presence of humid air. The occurrence of mild-to-severe wear transitions has been widely investigated in the case of different tribopairs and tribological conditions50.

A classic example is the work of

Lancaster51, who investigated a transition in wear regimes occurring for the case of a brass pin rubbing against hardened steel in the presence of air. In particular, the author suggested that the transition from a mild to a severe form of wear would occur with increasing sliding speed, as the rate of exposure of fresh metal surfaces (which is proportional to the sliding speed) exceeds the rate of growth of the protective oxide film, whose function would be to avoid contact between metallic asperities and subsequent junction growth. Temperature was found to lead to an increase in the sliding speed at which this transition took place, due to the temperature dependence of the oxide-film growth. In the present work, the mild-wear regime observed at low sliding speed for a SiO2/Si tribopair lubricated with water-containing [EMIM] TFSI seems to derive from a rather different mechanism. On the basis of the results presented above, it is proposed that the


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mechanochemical (i.e. stress-induced) reaction of water enriched at the SiOx/IL with the counterparts would lead to the smoothening of the contact, which in turn limits the mechanical stresses at the interface. In this case, the transition from a tribochemical to a mechanical mode of wear with speed might be governed by the adsorption kinetics of water at the IL/silica interface from a very dilute solution of water in a viscous ionic liquid: after the generation of native surfaces as a result of sliding, the re-adsorption of water will require a certain time, allowing tribochemical wear to dominate only at sufficiently low speeds. For higher speeds, the amount of adsorbed water at the interface might not be sufficient: mechanical damage and roughening of the contact would dominate in this case. Another factor that may promote the observed transition from a tribochemical to a mechanical form of wear in a silica/silicon tribopair lubricated with [EMIM] TFSI is the increase in dissipated power at the sliding interface occurring with increasing sliding speed. In the case reported here, the resulting increase in local temperature may promote desorption of water from the contact area, which would hinder the tribochemical mechanism of wear from prevailing. An analogous mechanism is often evoked to explain the failure of organic friction modifiers—surfactants—in oil-lubricated metallic counterparts: a high temperature is expected to promote desorption of the boundary film, leading to severe damage in the form of adhesive wear and plastic deformation52. An attempt to estimate the flash temperature at the sliding interface for the system of interest has been already presented in our previous work39. The equation is reported in the Supporting Information. According to our calculation, the estimated increase of temperature would not be higher than ~ 16 K at 5000 mm min-1—assuming the contact consists of a perfect SiO2/Si interface. The data presented in this work cannot provide an indication as to whether such an increase of temperature at the sliding interface could promote a significant desorption of water. Nonetheless, it should be noted that the formation of a mechanically



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mixed layer, mainly consisting of SiOx, may lead to significantly higher temperature at the sliding interface, as the outcome of the employed flash-temperature model depends on the thermal conductivity of the materials.

4.2 Friction at the nanoscale In the absence of adhesion, the friction force, F, between two sliding surfaces at the nanoscale can usually be described by Amontons’ law, i.e. = μ , where L is the applied load and µ is a constant, referred to as the coefficient of friction53. On the other hand, in the presence of adhesion, the existence of attractive interactions alters the dependence of friction on load and Amontons’ law is not strictly valid anymore. Previous studies53 have shown that the friction force can be split up into two separate and additive contributions: an external load-dependent and an internal adhesion-dependent contribution.

Equation 1

Where: - is a shear-stress term, expressing the adhesive contributions to friction. - A is the area of the contact. - µ is the nanoscale coefficient of friction - L is the Load

For low loads and high adhesive forces, the friction force is dominated by the second term in Equation 1. On the other hand, for a single-asperity contact undergoing exclusively elastic deformation, the linear term in L eventually dominates at sufficiently high loads.


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It should also be noted that, even when the same materials and lubricant are employed, the

nanoscale coefficient of friction (µ) in Equation 1 does not generally equal the macroscale coefficient of friction54. In order to avoid confusion, in the present work the symbol µ will be exclusively used when referring to nanoscale contacts, and under the assumption that the friction force can be described according to Equation 1. When referring to macroscopic experiments, the abbreviation CoF is used instead to indicate the friction-to-load ratio. As for the results presented in this work, a negligible adhesion force was measured under dry conditions, while a pull-off force of 11±5 nN was measured under humid conditions. These findings confirm previous measurements of normal forces between silica and mica surfaces in the presence of water-containing [EMIM] TFSI16,23. In particular, Cheng et al.23 attributed the phenomenon to the action of capillary forces due to the meniscus between the wetting phase and the non-(or less-) wetting liquid55, here water and [EMIM] TFSI, respectively. The presence of water in the IL also has a remarkable effect on friction. Under dry conditions, the adhesion contribution to friction (second term of Equation 1) is expected to be small, as confirmed by the results shown in Figure 9 (red curve). Notably, a linear fit of the experimental data shows a non-zero intercept with the x-axis. This suggests the occurrence of a low-to-high friction-regime transition at ~ 10 nN, likely of the type already described by Espinosa-Marzal et al.12 for various ILs confined between a colloidal SiO2 sphere and a mica sheet. In the case of friction force measured with [EMIM] TFSI exposed to humid conditions (blue curve, Figure 9), a significant contribution of the adhesive term (Equation 1) to friction force was observed, which agrees well with the large increase in pull-off force. At the lowest applied load (10 nN), a lateral force of ~ 110 nN was measured. As mentioned in Section 3.2,



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this value is comparable to values reported in the literature for adhesive, nanometer-sized contacts at nearly zero applied loads sliding in the presence of humid air. In our LFM measurements we also noted that the initial conditions of the tip and substrate affected friction and adhesion during the initial stage of sliding, especially when the tip was used without any treatment prior to the test (Figure S3). This transient behavior seems to indicate the occurrence of wear, as structural or chemical changes of the interfaces triggered by direct contact between solid surfaces may lead to a progressive change in adhesion and friction for nano-sized contacts56.

When exposing the counterparts to oxidizing

environments prior to the test (Figure 9), only a small hysteresis was observed when comparing loading and unloading curves, possibly indicating additional changes in the structure, composition or geometry of the interface. A comparison between the crosssectional profiles of the tip after the friction-force measurements and a pristine tip suggests that wear might indeed occur to some extent during sliding (Figure S2), although the simplicity of this type of control measurement does not allow us to quantify the material loss as a function of external pressure and environmental conditions. Figure 9 also shows that the friction-vs-load trends obtained in the two environments overlap at loads higher than 50 nN. This suggests that the presence of water dissolved in the IL is not affecting the frictional behavior between the silicon tip and silicon surface above a sufficiently high contact pressure. The merging of the two curves might also suggest that the high applied pressure leads to removal of the water from the contact, resulting in direct contact between solid surfaces under both sets of environmental conditions. A notable finding in the LFM study concerns the value of µ. In the case of [EMIM] TFSI tested under dry conditions, where adhesion contributions can be neglected, the extraordinarily high value of ~ 6 was measured above an applied load of ~ 10 nN. On the other hand, below the range of loads investigated in this study, a regime characterized by a


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significantly lower µ is clearly established, which is more in line with the outcomes of LFM studies on ILs carried out under low-applied-pressure conditions. As an example, Werzer et al.57 investigated in detail the frictional behavior of a SiO2 colloidal probe sliding against mica in the presence of ethylammonium nitrate. Due to the geometry of the contact and to the range of applied loads, contact pressures of the order of few tens of MPa were applied and µ values of 10-2 – 10-1 were reported in that study. The authors interpreted their results as indicative of an activated sliding process, where energy dissipation arose from the movement of the colloidal probe over the potential energy landscape defined by the ions localized over the mica surface. Possibly, a similar description may be applicable for the low-friction regime observed below ~10 nN in the present investigation. For higher loads, direct contact between solid counterparts seems to occur during sliding, as suggested by the above-discussed indication of chemical/structural changes of the sliding counterparts. It is reasonable to assume that, under these conditions, covalent bonds across the counterparts may form, which then break as a result of sliding. This would not only result in wear by atomic attrition50, but also substantially contribute to friction. A comparison with the results from the literature for LFM experiments carried out with sharp tips is difficult, due to differences in either liquid or materials. Values of ~ 1 have been recently reported by Cooper58 et al for a sharp silicon tip sliding against steel in the presence of various ILs. The mechanical and chemical properties of a SiOx / MOx (with M = Fe or another metal present in the surface layer of the steel sample) pair are clearly expected to have a large impact on the measured friction, especially if friction derives from the rupture of chemical bonds at the sliding interface. On the basis of the materials employed in the present study, the conditions leading to the formation and rupture of interfacial siloxane bridges (SiO-Si) upon sliding might be crucial in affecting friction, as already demonstrated by Li et al.45, and later on by other authors59,60 , in the case of SiO2 noncontacts sliding in air. It is also



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expected that the chemical nature of the media to which the surface reactive sites are exposed—in this case, [EMIM] TFSI—would play a significant role in the chemical reactions supposedly responsible for friction. Clearly, at present, the interpretation remains speculative, and it will be the subject of future investigations.

4.3 Comments on the mechanism of friction at the macroscale and comparison with LFM results With the notable exception of SFA studies, extended contact between solids occurs between surfaces that are rarely molecularly smooth. The interaction between surfaces of stiff materials occurs through a small fraction of the “apparent” contact area, which can be envisioned as a collection of nanoscale contacts—asperities—between the two sliding surfaces. The topography, together with the mechanical properties of the two counterparts, determines the relationship between load and “real” contact area (Ar) and the stress distribution at the interface50. In particular, high values of roughness promote plastic deformation, even though its occurrence is also strongly affected by the ratio of Young’s modulus to the hardness of the materials. Broadly speaking, ceramics tend to deform elastically, unlike metals, although roughness modulates this tendency in both cases50. As for the description of friction in a regime of severe contact, in the following we refer to the simple model proposed by Bowden and Tabor50 in an attempt to interpret some of the aspects of friction observed in the experiments reported here. Despite its limitations, the model provides a general rationale for describing friction, both in the absence of lubricants and under boundary-lubrication conditions. In a purely plastic regime and for moderate loads, the contact area is proportional to the applied load according to L = H Ar (where H is the indentation hardness of the softer material). The model assumes that, under these extreme conditions, welding of interacting


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asperities occurs. Friction would thus arise from the rupture of multiple asperity-asperity junctions formed at the sliding interface. On the basis of the relationship between hardness and shear strength commonly observed in homogeneous solids, a contribution to CoF of ~ 0.2 is expected. An additional minor contribution is associated with ploughing. In short, homogenous materials sliding in a plastic regime are expected to exhibit values of friction of 0.2-0.3. The simple model neglects the phenomenon of “junction growth”61, i.e. the increase of contact area caused by shear—which is however mostly relevant for ductile metals under vacuum. The model can be easily extended to boundary–lubricated contacts: the protective interfacial layer formed by reaction products or adsorbed species is often thought to act by providing a lower-shear-strength plane, as compared with the material underneath, leading to a decrease in CoF. For multi-asperity contacts under a purely elastic regime, the real contact area is generally found to be proportional to load50. However, in contrast to the case of plastic deformation, the relationship between load and contact area stems from the close-to-Gaussian asperity-height distribution followed by common solid surfaces. As more asperity contacts take place at higher loads, Amontons’ behavior is still observed in a regime of elastic deformation62. It follows that micro-welding of asperities should not be considered as a universal mechanism of friction at the macroscale; especially in an elastic regime, it might be reasonable to assume that each asperity-asperity contact would resemble a nanoscale contact of the type found in LFM. In the present investigation, another primary aspect to take into consideration concerns the role of the wear mechanism on the evolution of the topography of rubbing counterparts. In our experiments with a SiO2/Si tribopair lubricated with [EMIM] TFSI, we could observe the establishment of two possible mechanisms of wear, each one leading to characteristic



Page 32 of 43

topographies of the contact as a result of sliding. The possible consequences on friction are discussed in the following.

Friction in a regime dominated by mechanical wear The absence of water dissolved in the IL and high sliding speeds were found to promote a “mechanical” form of wear in our macroscopic friction experiments: although no attempt to estimate the stress distribution over contact area is given in this work, ex situ spectroscopic investigations and optical microscopy clearly revealed the occurrence of substantial plastic deformation of silicon within the contact area. A coefficient of friction of ~0.24 was measured, which is in line with the prediction of the simple model of Bowden and Tabor briefly described in the previous paragraph (i.e. friction by microwelding of interacting asperities). In order to apply this model here, it should be assumed that at least one of the two materials exhibits ductile behavior. Silicon, despite being commonly regarded as a brittle material, can undergo a stress-induced transition to a metallic and ductile phase, whose occurrence has indeed been demonstrated by Raman and XPS results in this work. However, it should be noted that values close to 0.2-0.3 are also commonly observed for brittle counterparts, such as mica or silica, when damage and formation of trapped debris occurs under sliding53. This implies that several mechanisms of friction, observed under a regime of plastic deformation, may result in similar values of CoF, and thus our analysis cannot be conclusive. Nonetheless, whether brittle or ductile behavior is controlling the friction, the results presented here indicate that [EMIM] TFSI, despite modifying the composition of the sliding interface, does not significantly decrease the friction of a SiO2/Si pair in a regime dominated by plastic deformation.


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Friction in a regime dominated by tribochemical wear In the presence of water dissolved in the IL, and at sufficiently low speed (50 mm min-1), a “tribochemical” form of wear was found to prevail in our macroscopic friction experiments. The results of spectroscopic analysis showed no evidence of ductile flow in tribostressed silicon. Evidence of macro-fracture was collected by optical microscopy, but it almost certainly occurred during the first turns of sliding. On the other hand, the smooth appearance of both counterparts suggests that plastic deformation played a minor role at steady state. These outcomes point to the establishment of a regime dominated by elastic contacts. Under these conditions, it should be possible to consider each asperity-asperity contact as a nanoscale contact similar to that established in lateral force microscopy. Defining the macroscopic coefficient of friction in terms of the results obtained by lateral force microscopy would require a detailed description of the multi-asperity contact, which is not attempted in this work. Nonetheless, we suspect that the high friction measured at the macroscale in a prevalently elastic regime might be an expression of the rather high friction observed in LFM experiments carried out under similar conditions of humidity.

5. Conclusions This work describes the role of humidity on the macro- and nanotribological behavior of SiOx-based pairs lubricated with [EMIM] TFSI. In line with previous investigations, surfaceforce measurements obtained by AFM have indicated that the presence of small amounts of water in the IL promotes a significant increase in the adhesion between the counterparts. In addition, we found that the resulting adhesion contribution to friction dominates only at low contact pressures: at high loads, the external-load contribution to friction dominates, regardless of the presence of water.



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The very high value for the nanoscale friction coefficient, µ, measured in dry conditions is tentatively attributed to an interfacial chemical-bond-induced friction mechanism, although further investigations are required to shed light on the nanotribological behavior described in this paper. The analysis of the macrotribological behavior of a SiO2/Si pair lubricated with the IL also supports the hypothesis that a water-enriched layer is present at the SiOx surface. For sufficiently low sliding speeds, the water adsorbed at the solid/IL interface promotes a tribochemical form of wear consisting of the oxidation/hydroxylation of the asperities, and subsequent smoothening of the contact. The rather high CoF measured under these conditions might also be related to the high adhesion observed at the nanoscale when in the presence of water-containing [EMIM] TFSI. In this sense, further investigation of the relationship between the topography of the counterparts, and thus the stress distribution of the contact, might be helpful to bridge the information at the nano- and macro scales. At a sufficiently high sliding speed, a transition to a regime of wear characterized by extended damage in the form of plastic deformation and fracture was observed, regardless of the presence of water in the IL. It is supposed that the kinetics of water adsorption at the sliding interface, which is likely affected by a local increase in temperature at the sliding interface, controls the phenomenon.

Supporting information In Section S1, the X-ray photoelectron spectroscopy (XPS) survey spectra of tribostressed silicon discs lubricated with [EMIM] TFSI in the presence of humid air are presented (Figure S1). A description of the high-resolution spectra reported in Figure 6 is also presented. In Section S2, additional data and calculations concerning lateral force (LFM) experiments are presented. In particular: 34 ACS Paragon Plus Environment

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•

A cross-section of AFM tips used in LFM experiments obtained by reverse-tip

imaging is presented (Figure S2) •

Values of friction force measured vs applied load with a silicon tip on the surface of a silicon wafer under [EMIM] TFSI and in humid conditions are illustrated in Figure S3. The tip used for the experiment was used as received (i.e. without any cleaning/activation treatment before the test).

•

The JKR model is used to estimate the area and pressure of the contact between the AFM tip and substrate.

In section S3, an estimation of the local temperature increase in the contact between a silica sphere and a silicon wafer lubricated with [EMIM] TFSI and in humid conditions is presented.

Acknowledgements The authors wish to express their gratitude to Mr. Giovanni Cossu for the technical support in designing and building the humidity-control setup for the atomic force microscope. Ms. Sandra Häberli is acknowledged for technical support in performing tribological tests. The authors thank Prof. R. Spolenak for providing access to the Raman microscopy instrument. A. R. thanks Fondazione Banco di Sardegna and Regione Autonoma della Sardegna Progetti Biennali di Ateneo Annualità 2016, Fondazione Sardegna CUP F72F16003070002 for financial support.

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