Probing the nanofriction of non-halogenated phosphonium-based ionic liquid additives in glycol ether oil on titanium surface

The nanofrictional behavior of non-halogentated phosphonium-based ionic liquids (ILs) mixed with diethylene glycol dibutyl ether in the molar ratios of 1:10 and 1:70 was investigated on the titanium (Ti) substrate using atomic force microscopy (AFM). A significant reduction is observed in the friction coefficient μ for the IL-oil mixtures with a higher IL concentration (1:10, μ ∼ 0.05), compared to that for the lower concentration 1:70 (μ ∼ 0.1). AFM approaching force-distance curves and number density profiles for IL-oil mixtures with a higher concentration revealed that the IL preferred to accumulate at the surface forming IL-rich layered structures. The ordered IL-rich layers formed on the titanium surface facilitated the reduction of the nanoscale friction by preventing direct surface-to-surface contact. However, the ordered IL layers disappeared in the case of lower concentration, resulting in an incomplete boundary layers, because the ions were displaced by molecules of the oil during sliding and revealed to be less efficient in friction reduction.


Introduction
Friction is one of the major causes that results in energy and material losses in many technical applications, including nano-fluidic technology, micro/nano electromechanical systems, etc. Implementing lubrication technologies, such as solid or liquid lubricants, enable the decrease of friction and wear between the sliding contacts, thus potentially reducing the energy losses by 18%-40% [1]. The low friction in the elastohydrodynamic regime, high thermal conductance, as well as the long-term endurance, enable the liquid lubricants more advantageous than solid lubricants [2]. While conventional liquid lubricants have insurmountable shortcomings [3], e.g., volatility, degradability, and narrow liquid range. Such oil lubricants adhere weakly to solid surfaces, and can be easily squeezed out of the contacts during sliding, resulting in direct surface-to-surface contacts, thus providing a high friction [4].
Ionic liquids (ILs) consisting of cations and anions, are liquid-state salts at room temperature, possessing high thermal stability, non-flammability and ionic conductivity, molecular designability, non-volatility, and so on [5][6][7][8]. Typically, ILs offer an ability to resist 'squeeze out' because of their strong interactions with solid surfaces, including van der Waals and electrostatic forces, as well as hydrogen-bonding interactions [5,[9][10][11][12][13][14]. The main limitation of ILs is their relatively higher price compared to conventional lubricants. However, this problem can be tackled by mixing ILs with conventional lubricating base oils to reduce the cost and improve their lubrication performance together with physicochemical properties [15,16].
It has been found that the lubricant employing ILs as additives in base oils boosts the lubricating efficiency and reduces the cost to increase their potential applicability at industrial scales [4,8,[17][18][19][20]. A phosphonium-based IL adding into the hydrocarbon oil with an IL concentration of 5 wt%, has demonstrated a significant reduction in the friction coefficient in steel-cast iron contacts [21]. It is observed that at the nanoscale, only 2.0 mol% of an IL in hexadecane lubricates silica by forming robust boundary layers and the friction forces are significantly reduced than the neat hexadecane [15]. In another study, it was found that the friction is reduced effectively with the addition of 1 wt% of quaternary ammonium IL to poly-α-olefin oil, while a further increase of the IL from 1 wt% to 3 wt% did not make any significant change in the friction-reducing characteristic [14].
Obviously, the tribological performance may not be necessarily improved by a higher concentration of ILs [8]. For example, 8 wt% imidazolium-based ILs in glycerol showed a higher friction coefficient at steel-steel contacts than that of 0.63 wt% [22]. While in the rapeseed oil lubrication, the friction coefficient and wear volume increased as the IL concentration increased from 1 wt% to 3 wt% [23]. Our recent work even demonstrated a 'negative' load-dependent nanofriction in IL-glycol ether mixtures with an IL concentration of 75 wt% on the titanium surface, i.e., the friction decreases with increasing load, not following the Amontons' law [24].
Furthermore, lubricant formulations can be specific to different tribological material systems. A lubricant might be able to improve tribological performance in one system, while the same lubricant may not be as effective in protecting other materials against the friction and wear [25]. Therefore, it led to an immediate controversy over the mixed IL-oil lubricants on the improvement of the tribological performance, which is necessary to be clarified. And there is an urge to understand the underlying mechanisms of ILs-based lubricants in various tribological materials and to improve the lubrication performance of non-ferrous materials, which remains a challenge for the scientific community.
As is known, the halogenated ILs exhibit excellent lubricity [26][27][28], mainly with fluorine as the halogen element for the anion, e.g., tetrafluoroborate and hexafluorophosphate. When the halogenated ILs are used for lubrication, the formed metal fluorides on the sliding surfaces act as the boundary lubricating layer [26,29]. However, the most studied halogen containing ILs as oil additives may release toxic and corrosive products including hydrofluoric acid (HF) into the surrounding environment, causing corrosion and toxicity to the contacts [30]. This can be avoided via a proper molecular design of ILs, e.g., the employment of halogen-free ILs in lubrication [31]. Thus, in this work, a series of halogen-free phosphoniumbased ILs are studied as IL additives into diethylene glycol dibutyl ether base oil, with molar ratios of 1:10 and 1:70.
A systematic nano-frictional behavior of the IL-oil mixtures were investigated on the titanium (Ti) substrate. Ti was employed herein as the substrate to be lubricated, because Ti has wide applications, e.g., spacecraft triboelement [32], due to its high melting point and strength-to-weight ratio, as well as excellent corrosion resistance [33][34][35][36]. However, the Ti surface is one of the most difficult metal to lubricate and usually lead to high friction and wear, limiting its application in mechanical systems [35,37,38]. This study is an attempt to unveil the underlying mechanisms of ILs-based lubricants in Ti contacts.

Sample preparation
The  Fig. 1.
The samples were prepared by dissolving these ILs in the DEGDBE oil at varying molar ratios, i.e., 1:10, 1:70. The wetting IL-oil mixtures and neat oil were deposited onto Ti surfaces. As for the case with a higher IL concentration, i.e., the molar ratio of 1:1, we had already studied that in our previous work [24].

Force measurements
Friction force measurements were performed as what we did previously [24]. In brief, these forces were measured on a Dimension Icon atomic force microscopy (AFM, Bruker) in contact mode at ambient conditions. Si3N4 cantilever tips (DNP-10, a tip with tip radius of 20 nm) were employed with a scan rate of 2 Hz and scan size of 5 μm × 5 μm. The tips used in friction measurements were calibrated using the thermal tune method. The lateral force (in V) was transformed into a true friction force (in N) following Liu's method [40]. Detailed experiment processes for friction force measurements were addressed in Electronic Supplementary Material (ESM).
The force-distance curves were captured with AFM glass colloidal probe (20 μm in dimension), which were performed as that in our recent work [41]. The borosilicate glass microspheres were stick to NSC35 tipless Cr-Au coated cantilevers (Cantilever A) via epoxy glue (BDMA:DDSA:CY212 = 1:10:10 in volume ratio).

Surface characterization
The surface morphologies of the bare Ti substrate, neat oil coated Ti, and IL-oil mixture coated Ti were observed by AFM tapping mode. Attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR, Nicolet IS-10, Thermo Fisher Scientific) and X-ray photoelectron energy spectra (XPS, Thermo VG Scientific X-ray photoelectron spectrometer with a monochromatic Al Kα X-ray) were used to characterize the molecular structural information of the IL-oil mixtures, neat ILs, and oil on Ti.

Molecular simulations
To uncover the fundamental mechanism of Ti surfaces lubricated with IL-oil mixtures at molecular level, molecular dynamics (MD) simulation was performed to investigate the number density distribution of [P6, 6,6,14][BScB]-oil mixtures with a molar ratio of 1:10. As is known, the Ti surface is usually covered by native dense titanium dioxide layers [42], we thus employed the rutile in MD simulations to mimic the substrate that was used in AFM experiments. The tip is almost a flat surface at the microscale, forming a slit with the substrate surface. Thus, a slit pore model (pore width of 10.7 nm) composed of a bilayer graphene tip and rutile (110) substrate (thickness = 0.903 nm) was used. Simulation details are described in ESM.

Interactions of IL-oil mixtures with the Ti substrate
The local surface topographic images of the pristine bare Ti, neat DEGDBE oil on Ti, and IL-oil mixtures on Ti, were characterized using AFM to track the oil or the IL-oil film on the Ti surface in Fig. 2. The pristine bare Ti substrate exhibits an irregular rough morphology with cracks/scratches and particles  [44,45], and O-C [46] bonds in the [BScB] -anion. In the middle panel of Fig. 3, P 2p1/2 and P 2p3/2 peaks are identified at ~130.0 and 131.8 eV, respectively [47], and a distinctive peak observed at ~132.5 eV is assigned to the P-C bond in the [P6,6,6,14] + cation. The overlap of B 1s and P 2s photopeaks was observed in the right panel of Fig. 3, which "occurs heavily in oxide-based samples containing both boron and phosphorus" [48]. The B 1s core-level spectra exhibit a peak at 189.9 eV, due to B-O4 linkage [49]    FTIR spectra were employed to examine the interactive strength of the oil, neat ILs, and IL-oil mixtures with the underlying Ti substrates. Figure 4 shows FTIR spectra of the bare  Fig. S2 in the ESM. Figures 4 and S2 in the ESM exhibit stretching vibrations in the frequency range from 2,800 to 3,000 cm -1 , which are caused by the aliphatic C-H groups [50] in the cations of ILs or the oil. The peak at approximately 1,110 cm -1 in the spectrum of the neat DEGDBE oil is attributed to C-O-C bands [51]. The vibrations around 1,685 cm -1 in the neat bulk IL [P6, 6,6,14][BScB] are due to C=O stretching, while the vibrations around 1,610 cm -1 to the aromatic ring [50,52]. The vibrational signatures at 1,604 and 1,270 cm -1 are assigned to O-C=O stretching mode from the [BScB] -anion [53]. The band presenting at

Nanoscale friction of IL-oil mixtures on the Ti substrate
The nanoscale friction of the bare Ti substrate, the neat DEGDBE oil, and the ILs-oil mixtures on Ti surface is investigated using AFM. The relationship of the friction force (FF) with the normal load (FN) is obtained with Si3N4 AFM tips, as shown in Fig. 5. The friction coefficient, μ, is defined as the gradient of the friction-load curve based on Amontons' law, FF = μFN + F0. We observed in Fig. 5 that the line describing the linear friction-load relation does not extrapolate back to the origin but toward F0 as FN = 0, where F0 is the friction at zero load attributed to the contribution of the adhesion force [54][55][56]. This The raw friction data has been listed in Fig. S4 and Table S1 in the ESM. adhesion is mainly from van der Waals, electrostatic, and capillary forces [57]. The capillary forces can be eliminated as the tip is immersed in the samples of IL-oil mixtures or neat oil coated on Ti, and the non-meniscus adhesion is significantly dependent on the liquid media [58]. As exhibited in Fig. 5, as the liquid varies, the friction force extrapolated at zero load was found to be different, indicating different contributions of the adhesion to the nanoscale friction. The observed negative value of F0, i.e., the negative intercept of the friction force versus normal load curves, means the attractive adhesion force at zero load [59]. However, the determination of adhesion forces does not depend on the lateral movement [60]. The pulloff adhesion force could be measured directly by separating the probe from the sample surface (Fig. S3 in the ESM).
In Fig. 5, as expected, the friction coefficient is the highest for the bare Ti substrate (μ = 0.23), and the neat oil (μ = 0.14) reduces its magnitude by approximately 50%. For the ILs-oil mixtures, the friction coefficient decreases with increasing IL concentration (Fig. 5(b)). The friction coefficients for the bare Ti substrate, neat oil on Ti, and ILs-oil mixtures (molar ratios of 1:70, 1:10) on Ti are compared in Table 1.
Especially, the friction coefficient in the ILs-oil mixtures with a molar ratio of 1:70 (μ ~ 0.10) is similar to or slightly lower than that of the neat DEGDBE oil. The higher IL concentration with a molar ratio of 1:10, enables a significant reduction in the friction coefficient (μ ~ 0.052-0.063), most likely due to the tightly packed IL boundary layers on the Ti substrate (Fig. 8). In comparison, the boundary layer in the case of 1:70 remains incomplete during sliding because of the lower IL concentration, and the ions of the ILs could be displaced by the oil during the sliding motion, resulting in the similarity of the friction coefficients in 1:70 IL-oil mixture and neat oil.

Layering structured ILs at the interfaces
Typical approaching force-distance curves are obtained with AFM colloidal probes, to further examine the boundary layers of IL-oil mixtures at the Ti substrates, as shown in Fig. 6. Here we employed glass colloidal probes (dimension of ~ 20 μm, the inset in Fig. 6(a)) to replace sharp AFM tips, because "the larger well- defined interaction area afforded by the colloidal probe increases measurement sensitivity and reduces the possibility that an isolated surface asperity produces unrepresentative force data" [38]. Ti. Apparently, the film thickness of IL-oil mixtures is larger in 1:10 than that in 1:70. In addition, the thickness of the neat oil on Ti is 18.5 nm, as observed in Fig. S3(b) in the ESM.
In Fig. 6, the major features for all ILs-oil mixtures with a molar ratio of 1:10, consist of two steps at discrete separations, corresponding to the two interfacial layers, in which, the force required to penetrate a layer is referred to as the push-through force. As is known from the previous work, the magnitude of the pushthrough force indicates the degree of order in a layered structure, where a higher force reflects a stronger order [38]. Thus, the observed stronger push-through force closer to the surface of the substrate (blue arrow), is indicative of a more ordered layer. While the probe encounters a less pronounced layer at a greater distance to the underlying substrate (grey arrow), and it pushes against this layer with a weaker pushthrough force.
However, no step-like force behavior was observed in the IL-oil mixtures with 1:70 molar ratio during approach, where the probe first contacts the IL-oil mixture surface and the attractive force gradually increases on continued approach. As the tip eventually encounters the hard Ti substrate, a drastic collapse is observed in the force at zero tip-sample distance.
www.Springer.com/journal/40544 | Friction The disappeared ordered layer in the case of 1:70, is probably ascribed to the lower IL concentration, resulting in an incomplete boundary layer on the Ti surface.
Noting that the values of the forces in the approaching force curves are observed to be attractive in Fig. 6. According to the explanations in Atkin's group, the attractive force is probably due to 1) the repulsions being completely masked by the attractive dispersion forces [61]; and 2) the surface-induced IL nanostructures [38]. As for the latter explanation, Atkin's group had suggested that "the bulk sponge structure presenting in many ILs is transformed to a more layered structure by the underlying surface" [62]. This surface-induced nanostructural transformation in ILs is analogous to surface-induced phase transition from sponge to lamellar in aqueous surfactant [63,64], where an attractive force was caused by the capillary condensation of the lamellar phase. A similar effect of aqueous surfactant was contended to be in operation for the attractive forces observed in the approaching force curves in ILs [38].
The number density profile [65][66][67] of the [P6, 6,6,14][BScB]-oil mixture (1:10) along the direction perpendicular to the substrate was calculated by molecular dynamics (MD) simulation to reveal the spatial distribution of the cation, anion, and molecules of the DEGDBE oil in a slit pore (pore width of 10.7 nm) composed of graphene and rutile. Since the Ti surface is usually covered by a native dense titanium dioxide (TiO2) layer [42], we employed rutile-TiO2 as the model substrate, and the AFM tip is modeled by a bilayer graphene. The spatial distribution of ions and oil would provide a better understanding of the observed layering structure of the IL-oil mixture with a molar ratio of 1:10 at the Ti substrate.
The pronounced layered spatial distributions for both [P6,6,6,14]  and [BScB]  are found in Fig. 7. Tighter packing density of ions was observed near the surface of slit pore, referring as the dense layer (tLD), with less prominent ion density peaks at the position further away from the surface (loose layer, tLL). The formed ordered IL-rich layers (tLD + tLL), would prevent the direct substrate-to-substrate contact, facilitating the reduction of the nanofriction [68].
In the center of the slit pore, which is at a greater distance to the surface of the slit pore, no obvious ion density peak was found, while a broad density peak of the oil appears in this regime. Thus, the existing fewer ions, together with the dispersed oil, formed a third oil-rich regime, similar to the bulk oil phase (tB, Fig. 8). The observed three distinct layers agreed well with a recent simulation findings, in which, the slits confined IL exhibits an apparent triple layered spatial distribution [69].
The number density profile suggests that the ions of ILs prefer to accumulate at the surface to form a layered structure, as illustrated in Fig. 8 (right panel). The thickness of the IL-rich layered structure (tL) consists of dense (tLD) and loose (tLL) layers. The dense layer is the ordered IL layers adjacent to the underlying surface, while the latter one is the upper IL with decayed ordering at a greater distance from the surface. The IL-rich layered structure confirms the observation of a more ordered layer closer to the surface in forcedistance profiles as shown in Fig. 6.
The magnitude of the nanofriction coefficient could be determined largely by the layered structure (tL) formed at interfaces. We have previously found a monotonical increase of μ with decreasing tL [41]. Because the layering assembled IL closer to the underlying surface is more ordered, resulting in a stronger interaction for the same value of contact area and thus a higher friction. As for the lower IL concentration (1:70), the formed layering structured IL is incomplete (left panel in Fig. 8), so the displacement of ions by the oil would occur, resulting in a higher friction coefficient.
It is worth noting that the upper IL layers (tLL) is  [24]. The observed negative friction-load dependence where the friction decreases with the increasing normal load, was attributed to the structural reorientation of the ILs. In addition, the magnitude of the measured friction in the case of 1:1 was much higher than that in the case of dilute solutions with molar ratios of 1:70 and 1:10.

Conclusions
The interactions and nanofriction behavior of nonhalogenated phosphonium-based ILs as additives to the oil diethylene glycol dibutyl ether with molar ratios of 1:10 and 1:70 were investigated on titanium surface using AFM. The μ for the ILs-oil mixtures with a molar ratio of 1:70 is ~ 0.1, which is similar or slightly lower as compared to the neat DEGDBE oil. On the contrary, the higher IL concentration with a molar ratio of 1:10 significantly reduced the μ (~ 0.05). It is found that the higher IL concentration leads to tightly packed boundary layers on the Ti substrate, while the lower ILs concentration results in incomplete boundary layers and the ions of ILs are displaced by the molecules of oil during sliding. The approaching force-distance curves and number density profiles for the higher ILs concentration revealed a tighter packing density of ions near the surface (dense layer, tLD). The less prominent ion density peak further away from the surface confirmed a loose layer, tLL. The formed ordered IL-rich layers (tLD + tLL), prevented the direct substrate-to-substrate contact, facilitating the reduction of the nanofriction. On the other hand, the ordered IL layer disappeared in the case of a lower IL concentration, which results in an incomplete boundary layer and a less effective friction reduction on Ti substrate. It is observed that the upper loose IL layer is less stable, through which, the escape of the ions might occur, and the existing oil on the top at a greater distance to the substrate could block this escape. Thus, a further increase of the IL would not necessarily bring a smaller friction coefficient, because it results in a smaller fraction of the oil which cannot ensure the block of the ion escape. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.
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