Solvent-free carbon sphere nanofluids towards intelligent lubrication regulation

By simply switching the electrical circuit installed on steel/steel contact, the tribological behaviors of nanofluids (NFs) can be regulated in real time, thereby achieving the desired performance of friction reduction and wear resistance. Herein, solvent-free carbon spherical nanofluids (C-NFs) were successfully prepared for intelligent lubrication regulation. C-NFs with excellent lubrication performance can immediately reduce the coefficient of friction (COF) despite applying a weak electric potential (1.5 V). Moreover, polyethylene glycol 400 (PEG400) containing 5.0 wt% C-NFs remained responsive to electrical stimulation under the intermittent voltage application with an average coefficient of friction (ACOF) reduction of 20.8% over PEG400. Such intelligent lubrication regulation of C-NFs under an external electric field (EEF) mainly depends on the orderly arranged double-electric adsorption film of ion canopy-adsorbed carbon spheres (CSs). The intermittent electrical application can continuously reinforce the adsorption film and its durability for real-time controlling the sliding interfaces. Electrical-stimulation-responsive intelligent lubricants provide a new technical support for realizing intelligent stepless control of devices.


Introduction
With the diversified development trends of mechanical assemblies for meeting different operating requirements, essential lubrication technology must confront the challenge on how to realize the active control of tribological behaviors via the as-used lubricating material itself [1,2].As an emerging lubricating material, intelligent lubricating material can respond to environmental changes, and then realize active regulation of the lubrication state on the friction interface, which will bring great changes to future lubrication technology [3][4][5][6].More pressing is that the development of artificial intelligence technology (such as industrial robots and intelligent control systems) requires active control of lubrication modes.Therefore, it is crucial to constructing "intelligent lubricating materials" with stimulus response for meeting the needs of intelligent equipment [7][8][9][10].At present, electrical stimulation is the most convenient way of intelligent lubrication control [11][12][13].
Nanofluids (NFs), as the suspension of nanometric metallic and non-metallic particles at less than 100 nm in the base fluid, were recognized as a kind of cutting-edge liquid with excellent thermal conductivity in thermal sciences.In addition, NF has the potential to be an innovative lubricant for high-tech industries due to good thermophysical properties and regulable 96 Friction 12(1): 95-109 (2024) | https://mc03.manuscriptcentral.com/frictionrheological behaviors [14].With the development of the field of tribology and lubrication technology, the demand for lubricants with excellent physicochemical properties, precise regulation, and long-term service life has increased [15].Solver-free NFs as a typical electro-stimulation-responsive lubricating material offer a desired platform for intelligent lubrication application.NFs have a core-shell structure, generally consisting of solid particles as the core and ion oligomer canopy as the shell [16,17].Guo et al. [18] successfully synthesized four kinds of SiO 2 NFs with different corona molecular structures for investigating the intelligent lubrication regulation of SiO 2 NFs.The experimental results demonstrated that even applying a fairly low potential of 1.5 V, SiO 2 NFs could immediately and significantly reduce the coefficient of friction (COF).The work proved that injecting NFs into the friction region could accelerate the growth of friction film.Zhang et al. [19] fabricated the solvent-free graphene liquids and exhibited the most favorable lubrication properties due to the spreading effect and synergistic effect of liquid and graphene films.Namely, NFs, as ideal lubricants, combine the advantages of both liquids and solid lubricants, in which solid lubricants as key terms are worth for more attention to developing such lubricating systems.
Carbon exists widely in the nature and is one of the basic elements of the material world.Carbon materials play an important role from scientific research to practical application [20,21].Since the development of carbon fiber [22], C 60 [23], carbon nanotubes [24], graphene, and other materials in the last century [25], carbon nanomaterials have been applied to all walks of life [26].The booming development of carbon nanomaterials also provides a broad space for the research in the field of tribology [27][28][29].Therefore, a good idea is proposed to improve the lubrication effect of carbon sphere (CS)-based core-shell materials and realize the active regulation of friction states [30][31][32].However, the researchers have rarely focused on the tribological properties of solvent-free carbon spherical nanofluids (C-NFs) and their electric response behaviors as lubricant additives.Therefore, it is necessary to carry out the tribological properties of C-NFs.
In this work, CSs with a double canopy structure were prepared for achieving outstanding adsorb ability, dispersion, and stability of C-NFs.To illustrate excellent lubrication properties and external electric field (EEF)-induced intelligent lubrication regulation of C-NFs, their tribological behaviors were investigated in detail under normal friction conditions and electrical stimulation.More importantly, intelligent lubrication regulation via switching the EEF was explored and discussed.

Materials
The synthetic procedure of C-NFs is shown in Fig. 1.Firstly, 0.5 g of CSs was added to H 2 SO 4 (98%) : HNO 3 (65%) solution (volume ratio = 3:1) and stirred at 70 °C for 10 h for oxidation treatment for forming a large number of hydroxyl (-OH) groups on the surface of CSs.The CS was washed repeatedly using deionized water until pH = 7 and dried at 70 °C.Then, the oxidized CS was re-dispersed into deionized water.After 30 min of ultrasound, 5 mL dimethyloctyl [3-(trimethoxylsiliconyl)propyl] ammonium chloride  (DC5700, 65% methanol solution) was added.The mixture was ultrasonically dispersed and aged at room temperature for 24 h with intermittent shaking.After the reaction, the product was washed with ethanol and deionized water for three times and dried overnight at 70 °C in a vacuum drying oven to obtain covalently grafted CS-DC5700.Then, the CS-DC5700 was dispersed in deionized water by ultrasound.5 mL nonylphenol sodium polyoxyethylene sulfonate (NPES; C 9 H 19 -C 6 H 4 -O(CH 2 CH 2 O) 10 SO 3− Na + ) was dropped and stirred at 70 °C for 24 h.During this process, ion exchange was carried out.The product was repeatedly cleaned with deionized water and acetone for three times.Finally, after vacuum drying at 70 °C for 24 h, CS-DC5700-NPES was obtained, namely, C-NFs with a core-shell structure.

Characterization
The morphologies of the as-obtained samples and wear surfaces on the friction pairs were studied by a field-emission scanning electron microscope (SEM; FEI, Inspect F50, USA) with an energy dispersive X-ray spectroscopy (EDS; acceleration voltage: 20 kV) detector.The thermogravimetric analysis (TGA; NETZSCH STA449F3, Germany) was used to investigate the thermodynamic behavior of the as-prepared sample.The functional groups of the samples were examined in the wavenumber range of 500-4,000 cm −1 using a Fourier transform infrared (FTIR) spectroscopy instrument (Nicolet iS50, USA).The X-ray diffraction (XRD) analysis was used to study the crystal structures of the as-prepared core-shell materials.

Tribological property testing and characterization
The dispersion stability of the as-prepared core-shell nanomaterials was characterized by the sedimentation method.The settlement method with the simple and intuitive advantages fits with the actual experimental situation.The core-shell nanomaterials into base oil at a certain proportion were added and fully dispersed via using ultrasound.Then the hybrid oils were placed on the horizontal table for a period of time to observe its settlement.Polyethylene glycol 400 (PEG400) as base oil was used for evaluating the tribological properties of C-NFs as the lubricating additive.All the friction experiments in this work were conducted by a reciprocating friction and wear testing machines.Steel balls with a diameter of 10 mm (surface roughness (R a ) = 53 nm) were selected for the upper sample, and GCr15 bearing steel blocks with a hardness of 62 HRC (24 mm × 7.9 mm) were selected for the substrate.The surfaces of substrates before friction tests were polished (R a = 28 nm) unless otherwise specified.To ensure that solid additives were evenly dispersed in the base oil, the C-NFs-containing lubricating oil was dispersed by ultrasound for 2 h.The experiment was carried out at room temperature with a relative humidity of ~55%.Before tribological testing, the balls and disks are carefully cleaned with ethanol to ensure that the contact surfaces are free of contaminants.The specific load, frequency, time, and other parameters in each experiment are determined according to the experimental design of each system.All experiments are repeated three times to ensure the accuracy of data.
To fully study the influence of the dosage on the tribological properties, C-NFs were added into PEG400 at the dosages of 0.5, 1.0, 5.0 and 10.0 wt%, and their tribological behaviors were compared with those of pure CS.C-NFs were used as a lubricant to study the self-lubrication performance.Meanwhile, to explore the influence of electrical stimulation on the tribological properties of pure C-NFs and the C-NFs hybrid oil, the upper and lower samples were connected in series with a power supply by wire, and the lower samples were subjected to electrical stimulation.Experimental simulation is shown in Fig. 2. When electrical stimulation is needed, the power supply is switched on.Subsequently, the change of COF is observed, and the time from stimulation to response is | https://mc03.manuscriptcentral.com/frictionjudged to evaluate the response effect.The parameters of the reciprocating sliding friction experiment were set as a frequency of 1 Hz, displacement amplitude of 1 mm, applied load of 60 N, and duration of 30 min.The Raman spectra of wear tracks were obtained by the Raman spectrometer (Thermo Fisher Scientific, DXR, USA) with 532 nm laser excitation.The surface mapping microscopy profilometer (Bruker, ContourGT, Germany) was used to examine the profiles and wear volumes (W V ) of worn surfaces.The X-ray photoelectron spectrometer (Thermo Scientific, ESCALAB 250Xi, USA; Al Kα as an excitation source) was used to analyze the tribo-chemical reaction of the worn surface.

Characterization of C-NFs
The synthesis of NFs requires a particle size small enough to serve as the core nanoparticle and the presence of surface functional groups (such as -OH) to facilitate the introduction of active groups.Therefore, the CS used in this study was commercially sourced and operated according to the synthetic procedure of C-NFs.Figures 3(a) and 3(b) show the transmission electron microscopy (TEM) images of C-NFs.The as-prepared C-NFs with CS as the core has a particle size of 10-20 nm, and a large number of oligomer canopy is wrapped around CS.These CSs have no contact with each other and have good dispersion and uniformity.Under high-resolution observation of a single particle, it can be found that the core CS has a graphite crystal structure with a lattice spacing of 0.26 nm corresponding to the (100) plane of graphite.Figure 3(c) shows the TEM image of CS; pure CS has poor dispersion and agglomerates into large particles, which is not conducive to lubrication.
Figure 3(d) shows the XRD pattern of pure CS for ensuring the crystal structure.The characteristic diffraction peak at 2θ = 26° corresponds to the (002) plane of graphite.In addition, the characteristic diffraction peak at 2θ = 54° corresponds to the (004) peak of graphite, which is the second-order diffraction peak of the (002) peak.Moreover, the obvious (004) peak indicates a higher degree of graphitization of CS.A higher degree of graphitization can make CS play a higher carrying role in the friction process [33].
The composition of C-NFs was studied by the TGA according to the difference in decomposition temperatures of small molecular solvents, oligomers, and inorganic particles.no small molecular solvents in the sample.In the high-temperature range, the weight of C-NFs is reduced by about 43%.The small molecular fragments of the shell are decomposed by oxidation.A large number of organic molecules were gradually degraded until they were all decomposed and carbonized, indicating a high proportion of C-NFs shells.In fact, the oligomer canopy is a special ionic liquid (IL), which can be used as the "solvent" of CS core, which provides an important guarantee for it to reflect the fluidity of the liquid.
The   PEG400.The results show that the oligomer ion canopy can improve the dispersion stability of CSs, which will improve the lubrication effect of carbon materials.
Under the control of the EEF, effective adsorption of CS on the contact interface could regulate the lubrication behaviors.Hence, the electrical conductivity of C-NFs and hybrid lubricants can reflect the control ability of electrical stimulation on tribological behaviors.Table 1 gives the conductivity of C-NFs (1,748.21μS/cm), which is close to that of conventional IL.In fact, the oligomer shell of C-NFs is also a special IL in nature.The conductivity of PEG400 was only 2.13 μS/cm, while the addition of C-NFs into PEG400 obviously enhances the conductivity that increases with the increase of adding amount.The enhanced conductivity of hybrid oil provides the possibility to adjust and control the lubrication state under electrical stimulation towards intelligent lubrication.

Tribological properties
Figure 5 shows the COFs and W V of PEG400, pure C-NFs, and C-NFs and CS hybrid lubricants without applying an EEF.The COFs and W V of pure PEG400 as base oil are higher than those of hybrid lubricants.The introduction of C-NFs as PEG400 additive at a concentration of 1.0-10.0wt% continuously decreases the COF during the whole friction process.The enhancement effect of C-NFs on the lubrication of PEG400 base oil was demonstrated.The average  Figure 6 shows the three-dimensional (3D) morphologies with two-dimensional (2D) contours of the wear tracks.The substrate lubricated by PEG400 presents the deepest abrasion depth, reaching about 1.2 μm.The abrasion depth under 5.0 wt% C-NFs lubrication is the lowest and only about 0.19 μm.The variation trend on wear depth from the hybrid lubricants with different additive concentrations is consistent with that of wear loss.Although the wear depth of pure C-NFs lubrication is about 0.21 μm, the wear width is about 250 μm, resulting in the higher

Study on electrical response performance
Because the oligomer canopy of C-NFs as ionic compounds has excellent electrical conductivity, the applied EEF can inspire more nanoparticles being adsorbed to the contact interface.Therefore, the controlling tribological behaviors and realizing intelligent lubrication via adjusting the density and thickness of the adsorbed film were carried out.Figure 7 shows the COF curves of intermittent electrical stimulation during the friction experiments.For C-NFs lubrication, the lubrication state of the substrate can quickly enter the low friction under a +1.5 V applied voltage, and the COF decreases from  | https://mc03.manuscriptcentral.com/friction0.086 to 0.072 within 3 s.Given the factors such as power supply overheating, the low friction holds the maximum time of 11 s.The change of the lubrication state can be achieved multiple times for maintaining the same drop.In contrast, electrical stimulation of 5.0 wt% C-NFs hybrid lubricant also has a certain response effect, but the COF decreased relatively less (Fig. 7(b)).When the friction test was carried out for 20 s, the COF decreased from 0.097 to 0.092 under a +1.5 V applied voltage.For the normal friction experiment without electrical stimulation, the COF keeps rising and even reaches 0.12 after 80 s.By comparison, ensuring that intermittent electrical stimulation of 5.0 wt% C-NFs hybrid lubricant has a small decrease in the COF (5.1%), it could effectively keep the lower COF than that under the normal friction experiment.This phenomenon may be possibly because the EEF stimulates a large number of positive and negative charges on the friction interfaces, thereby improving the adsorption capacity of CS and further strengthening the organic-inorganic hybrid lubricating film.Experiments show that intermittent electrical stimulation can continuously consolidate the adsorption film and display a long-term friction-reducing effect.
In general, the bumps on the sliding pair will have electron escaping, making these tiny bumps positively charged.Because of electrostatic interaction, the negatively charged ionic oligomer shell of C-NFs is easy to be adsorbed on the positively charged sites.The CSs are assembled in turn according to the principle of electrical neutrality.The CSs are dragged to the friction interface by the ionic shell molecules, and a special double-electric layer structure including the nanoparticles and the ionic oligomer shell has been established.In this case, the CSs on the interface can give full play to the rolling bearing effect.In fact, the oligomer canopy is a special IL, which can be used as the solvent of the CS core, providing an important guarantee for its ability to reflect the fluidity of the liquid.Oligomer ion canopies can improve the dispersion stability of CSs, which will improve the lubrication effect of carbon materials.Meanwhile, some scholars [18] pointed out that the longer the chain of the ionic oligomer shell, the stronger the corresponding adsorption capacity and lubrication performance.The strong ion adsorption and the change of molecular structure are the basic mechanisms that cause the lubrication of long-chain ionic oligomers to be superior to that of short-chain ionic oligomers.The lubricity of C-NFs shows obvious electric response when the electric field is applied.The EEF significantly improves the boundary lubrication effect, accelerating the adsorption of ions in the ionic oligomer shell to the friction surface.Therefore, the counter ions can gather, further strengthening the established double-electric layer.When the EEF disappears, the COF recovers immediately, indicating that the electrical response is reversible.The electric field enhances the adsorption of corona and canopy ions on the interface, and helps to improve the stability of the boundary layer.However, when the EEF is removed, the bearing capacity of the boundary film decreases again, thus restoring the COF.By controlling the opening and closing of the circuit, the high and low switching of the COF can be controlled in real time, which meets the demand of people for active regulation of the friction and wear state.The function of C-NFs is no longer limited to the lubrication of friction pairs, which During the friction process, ion characteristics can promote the adsorption of NFs on the positively charged metal surface, forming a unique inorganicorganic adsorption membrane.After applying the EEF, more anions are easily adsorbed on the friction surface, and then the covalently bonded silane cation with CS-DC5700 is successfully assembled according to the principle of electrical neutrality.The EEF accelerated the adsorption of NFs molecules on the friction surface, established an enhanced doubleelectric layer containing organic shell molecules and CS-5700 on the interface, and showed higher resistance to extreme pressure.The above results show that the synthesized C-NFs have excellent antifriction and anti-wear properties, and have high application potential in intelligent lubrication.In other words, C-NFs can be used as lubricants and additives for motion systems with controllable friction.Modern society constantly pursues the advancement of lubricating materials to improve energy efficiency and durability.In particular, the development of task specific lubrication materials in response to external stimuli, such as light, electricity, and heat, is of particular significance for achieving intelligent lubrication and greater energy efficiency in the field of intelligent and automatic control devices.

Analysis of lubrication mechanism
The morphology of the worn surface can reveal the friction mechanism during the friction process to a certain extent.Figures 8(a)-8(d) show the SEM images of wear tracks under PEG400, PEG400 with 5.0 wt% CS, C-NFs, and PEG400 with 5.0 wt% C-NFs lubrication.The worn surface under PEG400 lubrication is full of furrows, and gives the widest wear track (287 μm), indicating serious wear.The worn surface lubricated by 5.0 wt% CS hybrid PEG400 shows obviously deeper furrows, because the spherical carbon particles with poor dispersion have some difficulty in reducing friction and wear instead of displaying serious abrasive wear.The worn surface lubricated by C-NFs has shallow furrows and small width (251 μm), but there are a lot of spalling pits or corrosion pits, which is the reason for good friction reduction of C-NFs but less wear resistance.Compared with the worn surfaces from four lubricants, the worn surface from 5.0 wt% C-NFs hybrid lubricant presents few furrows or pits without peeling, illustrating that C-NFs hybrid lubricant displays good synergy from the rolling effect of CS and the friction-reducing advantage of fluid, so as to avoid the serious wear [35].
The tribological behaviors of lubricant are closely related to the formation ability of tribo-film.Chemical analysis on the wear tracks can determine the main friction-formed components, so great significance is to explore the lubrication mechanism.Figure 9(a) shows the EDS spectra of each point on the wear tracks in Fig. 8. Fe and C elements exist on all the wear tracks, and there are high peaks of O and S elements on the worn surface lubricated by C-NFs, where S element comes from the ionic shell of C-NFs.Obvious Raman bands at 1,358 and 1,553 cm −1 on the wear tracks of 5.0 wt% C-NFs lubricant correspond to the disordered and defection-induced vibration of carbon (D peak) and sp 2 hybrid carbon atom vibration (G peak) [36], respectively.However, there is no iron oxide peak.The hybrid lubricant has a good synergistic effect and can effectively prevent oxidation wear.The characteristic peaks of wear track under C-NFs  For further analyzing the chemical states of typical elements on the worn surfaces, the X-ray photoelectron spectroscopy (XPS) was carried out to explore the more detailed lubrication mechanism on C-NFs hybrid lubricant, as shown in Fig. 10.The wear track from PEG400 presents O 1s peaks at 531.6 and 529.1 eV and Fe 2p peaks at 710.4-724.2eV, confirming the presence of iron oxides (like FeO and Fe 2 O 3 ) verifying severe oxidative wear [37].For the worn surface lubricated by C-NFs hybrid lubricant, the peak of Fe 2p at 713.87 eV is probably due to the presence of FeSO 4 tribo-product, indicating the occurrence of tribo-chemical reaction between the ionic shell of spherical CS and metal matrix during the friction process.For the worn surface under C-NFs hybrid PEG400 lubrication, the C 1s peak at 284.5 eV is attributed to glycol structure (HO(CH 2 CH 2 O) n H) from base oil, while the peaks at 285.2, 287.1, and 288.7 eV are attributed to the C-C/C=C, C-O, and C=O groups, respectively.In addition, the Si 2p peaks at 102.5 and 100.5 eV are attributed to the SiO 2 and tetra-alkyl silicon produced by ion shell during the friction process, respectively [38].The wide N 1s peak at 400.3 eV belongs to CO-NH from the ion canopy [39], respectively, which is a strong proof on the adsorption of C-NFs to sliding surfaces.These results reveal that C-NFs can be adsorbed to friction surfaces, and occur the tribo-chemical reaction to a certain extent, thereby displaying the synergistic effects of a dense protective layer composed of a liquid-adsorbed film, embedded CS, and tribo-reaction film for reducing friction and wear.
The structure of wear debris is an important information for analyzing the friction mechanism of the friction pairs and lubrication mechanism of core-shell nanoparticles.Figure 11 shows the TEM images of wear debris during the friction process.C-NFs were gathered into a relatively large sphere structure during the process of friction; this may be due to the friction-induced electronic spillover of metal surface, making the ion canopy of C-NFs attract each other to form micelles.The spherical micelles, as the mixture of ion shell and CS, can spread out under the action of friction force for forming a dense protective film.Figure 11 also shows some lamellar or rod-like structures mixed in the wear debris.Judging from the crystal plane spacing from the electron diffraction diagram, the mixture of Fe 2 O 3 and CS is ensured, which is consistent with the results of Raman spectra on the wear track.The analyses of wear debris confirm that C-NFs, as a lubricant additive, can effectively maintain the basic structure and fully play the role of filling repair, rolling, and load-bearing effect.Meanwhile, the charged ions can adsorb and wrap the metal debris to prevent serious abrasive wear.The lubrication mechanism of C-NFs under electric field stimulation is shown in Fig. 12. Reference [40] shows that applying the EEF or changing the direction of the electric field can regulate the adsorption capacity of IL on the metal surface, so the IL shell of C-NFs meets the prerequisite conditions for realizing intelligent lubrication.The lubrication mechanism of C-NFs can be explained from two aspects.Under the lubrication without an EEF, the moving metal friction pair will lead to electron overflow and form a positive charge on the friction surface.The presence of a positive charge can attract the ionic oligomers to move towards the sliding surface, and the anionic ions drive the CS to quickly adsorb to the wear surface for forming an adsorption film.The adsorption film composed of CS and ionic oligomers can play the  | https://mc03.manuscriptcentral.com/frictionrole of filling repair, rolling, and load-bearing effect, thereby reducing friction and wear.Under the EEF, the amount of positive charge on the substrate surface will be greatly increased.In a short time, more CS will be adsorbed to the sliding surface driven by the ion canopy, and the anions and cations of hybrid lubricant will be orderly arranged under the action of the electric field.A double-electric adsorption film on the sliding surface was formed via charge attraction, which provides a higher quality lubricating film with easilyshearing for further reducing the friction and wear.Applying an EEF can regulate the tribological behaviors of C-NFs, and control the real-time lubrication state via switching low/high friction for achieving the goal of intelligent lubrication.Besides, the intermittent electrical stimulation can continuously adjust and strengthen the adsorption film, and the durability of the adsorption film could improve the service life of lubricant and get better lubrication protection.

Conclusions
In this work, oil-soluble solvent-free C-NFs with a core-shell structure were successfully prepared via functionalized CSs using DC5700 and NPES as the ion canopy.The ion canopy makes the CS have a strong adsorption capacity, which tends to form a high-quality lubrication film on the sliding surface and strengthen the lubrication function.More importantly, the tribological behavior of highly conductive NFs can be regulated by electrical stimulation.C-NFs with a double canopy structure as PEG400 additive displays excellent friction-reducing effect not only under normal friction condition, but also under the EEF.Applying the EEF of +1.5 V could greatly adjust the electric charge and form a double-electric adsorption film on the sliding surface via charge attraction, thereby providing a high-quality lubricating film with easilyshearing.In a word, excellent lubrication function and intelligent regulation of C-NFs as a lubricant or lubricating additive mainly depend on the synergy of the liquid-adsorbed film, embedded CS, and tribo-reaction film.
Figure 3(e) shows the TGA curve of C-NFs under the flow of N 2 at 30-600 °C.In the low-temperature range (30-200 °C), the mass loss is very small, indicating that there are almost

Fig. 3
Fig. 3 (a, b) TEM images of C-NFs, (c) TEM image of pure CS, (d) XRD pattern of pure CS, (e) TGA pattern of C-NFs, and (f) FTIR spectra of CS and C-NFs.
FTIR spectrum was used to further clarify the chemical structure of C-NFs (Fig. 3(f)).Compared with that of the original CS, the curve of C-NFs showed new peaks at 1,119, 1,237, 1,464, 2,851, and 2,920 cm −1 .There are absorption peaks at 3,428, 1,735, and 1,403 cm −1 , corresponding to the O-H stretching vibration of -OH group, C=O stretching vibration, and C-O stretching vibration of carboxyl (-COOH) group [19, 34], respectively.The presence of -OH and -COOH groups on CS surface provides active sites for the preparation of C-NFs.DC5700C-grafted CS shows the absorption peaks at 2,920, 2,851, and 1,119 cm −1 , corresponding to the C-H tensile vibration of -CH 3 , C-H tensile vibration of -CH 2 -, and C-O-Si tensile vibration, respectively.The successful grafting of NPES shows the absorption peaks of benzene skeleton vibration at 1,628 and 1,464 cm −1 and absorption peaks of -SO 3− at 1,237 cm −1 .The above results prove that DC5700 and NPES were successfully grafted onto CS surfaces.The dispersion stability of nanoparticles in PEG400 is crucial for tribological properties.
Figure 4 shows the physical diagrams of C-NFs and the standing dispersion diagrams of 1.0 wt% C-NFs and 1.0 wt% CS in PEG400.The physical substance of C-NFs is a black viscous liquid with fluidity at room temperature.When the sample bottle was inverted, C-NFs could flow down the wall of the bottle, which meets the basic definition of NFs.1.0 wt% C-NFs and 1.0 wt% CS were added into PEG400 to modulate the composite lubricating oil.The results of the standing test show that the dispersion stability of C-NFs in PEG400 is better than that of CS (Figs. 4(b) and 4(c)).After 7 d of standing the hybrid PEG400, the pure CS have completely settled, while NFs are still suspended in

Friction 12 ( 1 )
: 95-109 (2024) | https://mc03.manuscriptcentral.com/frictioncoefficient of friction (ACOF) values decrease from 0.123 (PEG400) to 0.115 (0.5 wt% C-NFs hybrid oil), 0.116 (1.0 wt% C-NFs hybrid oil), 0.111 (5.0 wt% C-NFs hybrid oil), 0.107 (10.0 wt% C-NFs hybrid oil), and 0.091 (pure C-NFs).Moreover, the W V values decrease from 43,621 μm 3 (PEG400) to 30,922 μm 3 (0.5 wt% C-NFs hybrid oil), 27,655 μm 3 (1.0 wt% C-NFs hybrid oil), 22,146 μm 3 (5.0wt% C-NFs hybrid oil), 33,134 μm 3 (10.0wt% C-NFs hybrid oil), and 34,336 μm 3 (pure C-NFs).By comparative analysis on the W V , the addition of 5.0 wt% C-NFs into PEG400 gives the smallest W V , which is 49% lower than that of PEG400.Combining with the ACOF (about 9.8% lower than that of PEG400) indicates that the optimal concentration of C-NFs is 5.0 wt% in the base oil.Compared with other lubricants, pure C-NFs have a better friction reduction effect, whose ACOF is reduced by 26.0%.The COF of pure C-NFs decreases rapidly within the first 50 s of friction, indicating that the nanoparticles in C-NFs can quickly adsorb to the surface of the friction pair for forming a protective film.Then, the COF gradually rises and stabilizes at about 0.09 during 50-400 s friction, which is still far lower than that of C-NFs hybrid PEG400.This indicates that the protective film formed by pure C-NFs could self-adjust in the process of friction, and gradually form a stable friction film to reduce friction.However, the wear reduction of pure C-NFs is not ideal, only 21% lower than that of PEG400, probably because the IL canopy of C-NFs leads to oxidation wear of friction surface, thus presenting the less ideal wear resistance.Figures5(c) and 5(d) ensure good lubrication properties of C-NFs at a concentration of 5.0 wt% in PEG400.The anti-wear effect of 5.0 wt% C-NFs is obviously better than that of 5.0 wt% CS, because the poor dispersion stability of CS tends to agglomerate into large particles, which are difficult to enter the friction interface for playing a role of barrier and repairing effect.

Friction 12 ( 1 )
: 95-109 (2024) 101 www.Springer.com/journal/40544| Friction W V .Compared with pure PEG400, both pure C-NFs and C-NFs hybrid lubrication oils can effectively reduce wear, which may be related to the ionic properties of NFs and the synergistic effect of CS.

Figure 9 (
b) shows the Raman spectra of wear tracks under PEG400, 5.0 wt% C-NFs lubricant, and pure C-NFs lubrication.The worn surface from PEG400 lubrication only has a single peak at 670 cm −1 attributed to friction-generated Fe 3 O 4 , indicating oxidative wear.

Friction 12 ( 1 )
: 95-109 (2024) | https://mc03.manuscriptcentral.com/frictionlubrication at 670 cm −1 belong to Fe 3 O 4 , and give the D and G bands from carbon nanoparticles.However, compared with the wear track of 5.0 wt% C-NFs lubricant, the position of D peak was shifted to the left, indicating that CS of C-NFs has more fully involved in the formation process of tribo-film.Because a large number of ions have caused a certain amount of corrosion wear, the product of iron oxide was detected.The above analysis results illustrate that C-NFs can effectively adsorb onto the sliding metal surfaces and form a strong lubricating film to reduce friction.Moreover, attention should also be paid to preventing corrosion and wear caused by the high conductivity of C-NFs.

Fig. 9
Fig. 9 (a) EDS spectra of each point in Fig. 8 and (b) Raman spectra of wear tracks after friction experiments.

Table 1
Conductivity of samples.