Functionalized graphene-oxide nanosheets with amino groups facilitate macroscale superlubricity

Graphene-oxide (GO) has been recognized as an excellent lubrication material owing to its two-dimensional structure and weak interlayer interactions. However, the functional groups of GO that can contribute to anti-friction, anti-wear, and superlubricity are yet to be elucidated. Hence, further improvement in GO-family materials in tribology and superlubricity fields is impeded. In this study, macroscale superlubricity with a coefficient of friction of less than 0.01 is achieved by exploiting the high adhesive force between amino groups within aminated GO (GO-NH2) nanosheets and SiO2. It was observed that GO-NH2 nanosheets form a robust adsorption layer on the worn surfaces owing to the high adsorption of amino groups. This robust GO-NH2 adsorption layer not only protects the contact surfaces and contributes to low wear, but also causes the shearing plane to transform constantly from solid asperities (high friction) into GO-NH2 interlayers (weak interlayer interactions), resulting in superlubricity. A SiO2-containing boundary layer formed by tribochemical reactions and a liquid film are conducive to low friction. Such macroscale liquid superlubricity provides further insights into the effect of functional groups within functionalized GO materials and a basis for designing functionalized GO materials with excellent tribological performances.


Introduction
The friction and wear of mechanical component surfaces incur significant economic losses annually, i.e., approximately 6% of the gross national product of developed nations [1,2]. To alleviate the significant effects of friction and wear and to achieve mechanical components with higher energy efficiencies and better durability, researchers have developed efficient lubricants that exhibit excellent anti-friction and antiwear properties [3][4][5], thereby providing a promising lubricating state for operating machines. Superlubricity has been proposed to describe the lubricating state, in which the sliding coefficient of friction (COF) is on the order of 0.001 [6,7]. Owing to the extremely low friction of superlubricity, the reduction in energy dissipation and durability induced by friction and wear can be alleviated significantly. Therefore, researchers have extensively investigated superlubricity. Hitherto, superlubricity has been achieved using both solid and liquid materials. Typical solid superlubricity materials include molybdenum disulfide [8], graphite sheets [9,10], graphene [11][12][13], and diamond-like carbon (DLC) films [14], all of which can achieve superlubricity at the microscale or nanoscale. Macroscale solid superlubricity can be achieved at DLC/graphene interfaces, wherein graphene nano-scrolls are formed around nanodiamond particles, and provide an extremely low shearing stress [15]. Liquid superlubricity has been achieved using various additive-containing liquid materials at specific interfaces, such as castor oil at nitinol 60 alloy/steel interfaces [16], nanodiamond-and silica nanoparticlecontaining water solutions [17,18], salt and ionic liquid solutions at ceramic interfaces or poly(vinylphosphonic acid) coatings [19][20][21], polyalkylene glycol [22], polyethylene glycol (PEG) at bearing steel interfaces [23], acid solutions at ceramic interfaces [24,25], and two-dimensional additives in liquid lubricants [26]. Graphene oxide (GO), as a prevailing material, exhibits excellent properties in many fields, and demonstrates excellent anti-friction and anti-wear properties as an additive in liquid lubrication [3]. GO functionalization is considered to be effective for improving its dispersibility and tribological properties for practical applications. Researchers have proven that various ionic liquids and isopropyl triisostearyl can significantly enhance the dispersibility, anti-friction, and anti-wear properties of GO [27][28][29][30]. The improved tribological properties were primarily attributed to the formation of a dense adsorption film of functionalized GO sheets at the interfaces. Our group recently achieved liquid superlubricity using a GO-containing dihydric alcohol aqueous solution at the macroscale [31]. Thereafter, GO was utilized as an additive to achieve superlubricity on DLC films [32]; and to extend the contact pressure to 600 MPa during superlubricity period [33]. These studies demonstrate the significant potential of GOfamily materials for achieving macroscale superlubricity. GO generally contains many functional groups, such as carboxyl, hydroxyl, epoxy, and carbonyl groups, which can significantly regulate its properties and performance [34]. However, the functional groups that can contribute to anti-friction, anti-wear, and superlubricity are yet to be elucidated. Hence, further improvement in GO-family materials in tribology and superlubricity fields is impeded.
To analyze the effects of functional groups on the lubricating behavior of GO materials, carboxylated GO (GO-COOH) and hydroxylated GO (GO-OH) were used as additives in aqueous dihydric alcohol solutions. Carboxyl and hydroxyl groups were selected because they are the most typical functional groups in GO. In addition, both the carboxyl and hydroxyl groups are negatively charged functional groups; therefore, the positively charged amino group (aminated GO (GO-NH 2 )) was selected for comparison. Meanwhile, tribo-pair Si 3 N 4 /sapphire was used because Si 3 N 4 is a typical and important material used in many fields, including the automobile industry, bearings, high-temperature materials, orthopedic applications, metal cutting tools, and electronics, whereas sapphire is a typical material used in micro electromechanical system and electro-optical equipment [35]. In general, most of these applications require low friction and wear at the interfaces. Friction experiment results show that the amino groups contributed the most to anti-friction and anti-wear at the Si 3 N 4 /sapphire interfaces, and that liquid superlubricity can be achieved at the macroscale. The underlying superlubricity and anti-wear mechanisms of GO-NH 2 containing dihydric alcohol aqueous solutions are discussed herein.

Materials
Three types of functionalized GO nanosheet powders, i.e., GO-NH 2 , GO-COOH, and GO-OH, were provided by the Aladdin Industrial Co., Ltd., China. Four types of dihydric alcohols were introduced as base liquids: ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG), and polyethylene glycol with an average molar mass of 200 g/mol (PEG200), all of which were provided by the Aladdin Industrial Co., Ltd., China, and exhibited 99% purity. All chemical reagents were used without pretreatment. Aqueous solutions of dihydric alcohol were prepared by mixing the aforementioned dihydric alcohols (1 g) with pure water (5 g), and they were denoted as EG(aq), DEG(aq), TEG(aq), and PEG(aq), respectively. Functionalized GO-containing dihydric alcohol aqueous solutions were prepared by mixing the dihydric alcohols (1 g) with a functionalized GO aqueous solution (5 g), where the functionalized GO aqueous solution was prepared by fully dispersing functionalized GO nanosheets (10 mg) in pure water (5 g) via sonication at room temperature. The concentration of functionalized GO nanosheets in the functionalized GO-containing dihydric alcohol aqueous solution was approximately 0.17 wt%.
The size distribution of the functionalized GO www.Springer.com/journal/40544 | Friction nanosheets in the aqueous solution was measured using a mastersizer (APA+AWM 2,000, Malvern, UK). The height profile and morphology of the functionalized GO nanosheets were measured using the atomic force microscope (AFM; Icon, Bruker, Germany) and the high-resolution transmission electron microscope (HRTEM; 2100F, JEM, Japan). The defects and vibrational features of the functionalized GO nanosheets were characterized using the Raman spectroscope (HR-800, Horiba, Japan). The chemical groups within the functionalized GO nanosheets were characterized via the X-ray photoelectron spectroscope (XPS; PHI Quantera II, Ulvac-Phi Inc., Japan) and the Fourier transform infrared spectroscope (FTIR; Nicolet IS50, Thermo Scientific, USA).

Tribological experiments
Friction experiments were conducted using a universal microtribometer (UMT-3, Bruker, Germany). The contact model was a fixed Si 3 N 4 ball (Ø 4 mm, surface roughness (Ra) ≈ 10 nm) loaded onto a rotational sapphire disc (Ra ≈ 1 nm). All fresh ball and disc specimens were washed thoroughly in ethanol and pure water (10 min each) and then dried under air flow. Normal loads from 1 to 4 N were set, which corresponded to the contact pressures of 1.2-1.9 GPa at the Si 3 N 4 /sapphire interfaces. The sliding velocity of the sapphire disc relative to the fixed Si 3 N 4 ball ranged from 0.0125 to 0.25 m/s. All experiments were conducted in an atmospheric environment at room temperature and a relative humidity of 10%-30%. For each experiment, the dosage of the prepared solution was 20 μL, and the COFs were recorded automatically with an accuracy of ±0.001. To reduce the experimental error, the tribometer platform was adjusted until the COFs obtained under clockwise and counterclockwise rotations were identical. Each experiment was performed in triplicate to ensure repeatability.

Surface characterization
After the friction experiment, the contact temperature was measured immediately using an infrared thermometer (Decemtmm-1520, Delixi Electric, China). The Ra and worn scar diameter (WSD) of the balls and discs were measured using a white light interferometer (Nexview, ZYGO Lamda, USA). The topographies of the worn areas were measured using the scanning electron microscope (SEM; S-4800, Hitachi, Japan). As the balls and discs were insulators, their worn areas were coated with a thin film of platinum to improve the image contrast prior to SEM observation. The worn areas were analyzed via XPS to investigate the chemical characteristics of the tribo-layer formed during the friction experiment. The adhesive forces between the functionalized GO nanosheets and SiO 2 probe were measured using AFM. Cross-sectional images of the worn area lubricated with GO-NH 2 containing DEG(aq) were obtained using HRTEM. The sample for HRTEM was prepared as follows: First, an approximately 20 nm thickness Cr layer was coated onto the worn surfaces. Second, an approximately 100 nm thickness Pt layer was coated on top of the Cr layer as a protective layer. Finally, HRTEM samples measuring 10 μm in length were cut off and prepared using a focused ion beam (Lyra3, Tescan, Czech Republic).

Material characterization
The  Fig. 1(g). The representative D band at 1,350 cm −1 represents the defects, vacancies, and distortions within the functionalized GO nanosheets, and the G band at 1,590 cm −1 represents the stretching of C-C bonds within the functionalized GO nanosheets [36,37]. Additionally, a typical 2D band at 2,690 cm −1 and a D + G band at 2,935 cm -1 were observed [31,38]. The FTIR spectra of the functionalized GO nanosheets are shown in Fig. 1(h). The characteristic transmittance bands associated with the carbon ring can be detected at approximately 1,625 cm −1 [39]. For the GO-NH 2 nanosheets, the transmittance bands detected at approximately 1,375 cm −1 (C-N) [40] and 3,325 cm −1 (N-H) [39] proved the presence of amino groups in GO-NH 2 . In terms of GO-OH, the transmittance bands at approximately 3,400 cm −1 (O-H) indicate the presence of hydroxyl groups within GO-OH [40]. For GO-COOH, the transmittance bands at approximately 1,720 cm −1 (C=O) indicate the presence of carboxyl groups within GO-COOH [40]. The XPS spectra of the functionalized GO nanosheets show that the concentrations of carbon and oxygen within the functionalized GO samples were between 67-69 at% and 30-31 at%, respectively. The nitrogen concentration in the GO-NH 2 sample was 3.02% (Table S1 in the ESM). Furthermore, the C 1s and N 1s peaks of the three functionalized GO nanosheets were deconvoluted into several chemical shifts. The C-C bond (284.8 eV in the C 1s peak) detected in all functionalized GO samples indicates the non-oxygen carbon of C-C or C-H, indicating a graphene ring structure [39]. The C-NH 2 bond (286.1 eV in C 1s peak), NH-C(=O)-NH bond (288.9 eV in C 1s peak), C-N bond (402.1 eV in N 1s peak), and N-H bond (400.1 eV in N 1s peak) [19,39] detected in the GO-NH 2 spectra (Figs. 1(i) and 1(j)) indicate the presence of the amino groups in GO-NH 2 . The C-OH bond (286.3 eV in the C 1s peak) [40] detected in the GO-OH spectrum ( Fig. 1(k)) indicates the presence of hydroxyl groups. The O−C=O bond (288.8 eV in the C 1s peak) [39] detected in the GO-COOH spectrum ( Fig. 1(l)) indicates the presence of carboxyl groups.

Tribological experiments
To investigate the anti-friction properties of various functional groups, the lubricating behavior of functionalized GO-containing EG(aq) was investigated ( Fig. 2(a)). Because a valid tribo-layer could not be www.Springer.com/journal/40544 | Friction formed during the wearing-in period, the COF of EG(aq) remained relatively high at 0.04 throughout the test. By contrast, the COF decreased significantly with the addition of functionalized GO nanosheets, indicating that the functionalized GO significantly reduced friction upon the completion of the wearing-in period. The COF of the GO-OH containing EG(aq) decreased continuously during the wearing-in period, followed by an increase to a value of 0.011; subsequently, it remained at that value until the end of the experiment. Similarly, the COF of the GO-COOH containing EG(aq) decreased continuously during the wearing-in period for approximately 600 s, and finally stabilized at approximately 0.009, which is indicative of a superlubricity state, but only on the edge of the superlubricity level. In particular, the COF of the GO-NH 2 containing EG(aq) decreased continuously to a value less than 0.01 upon the completion of the wearing-in period. Thereafter, the COF decreased further, and reached a minimum value of 0.004, followed by a short period of growth before finally stabilized at 0.006. These results indicate that the GO-NH 2 nanosheets possessed good anti-friction properties and facilitate superlubricity. Subsequently, the lubricating behavior of the solutions was investigated by increasing the viscosity of dihydric alcohol (Figs. 2(b)-2(d) and Fig. S2(a) in the ESM). The COF during the stable lubricating period increased continuously, whereas the viscosity of the dihydric alcohol increased from 17 to 53 mPa·s (from EG to PEG200). With the addition of the functionalized GO nanosheets, the stable COF decreased in the following sequence: COF(GO-NH 2 ) < COF(GO-COOH) < COF(GO-OH). Notably, only the GO-NH 2 containing solution achieved superlubricity when DEG(aq) was used as the base liquid. Therefore, GO-NH 2 containing aqueous DEG was used in subsequent experiments. The optimized parameters ( Fig. 2    | https://mc03.manuscriptcentral.com/friction would absorb heat, and hence reduce the surface temperature. The Ra and WSD of the worn surfaces after the friction experiments were observed using a white-light interferometry microscope (Fig. S3 in the ESM). The results show that the Ra for both the Si 3 N 4 ball and sapphire disc were similar after the friction experiment, i.e., approximately 10 and 3 nm for the Si 3 N 4 ball and sapphire disc, respectively. The WSDs of the worn area on the ball were approximately 138, 100, 141, and 184 μm for the DEG(aq), GO-NH 2 containing DEG(aq), GO-OH containing DEG(aq), and GO-COOH containing DEG(aq), respectively. It is clear that the worn area lubricated with GO-NH 2 containing DEG(aq) was the smallest among those of the selected functionalized GO materials, indicating that GO-NH 2 possesses the best anti-wear property. The average contact pressure during the stable lubricating period was estimated by dividing the normal load (3 N) by the worn area of the ball (assuming the worn area was a regular circle). The values obtained were 200, 382, 192, and 113 MPa for DEG(aq), GO-NH 2 containing DEG(aq), GO-OH containing DEG(aq), and GO-COOH containing DEG(aq), respectively. This implies that GO-NH 2 possessed the best load-bearing capacity among the selected functionalized GO materials.
The results above indicate that the GO-NH 2 nanosheets possessed the best load-bearing capacity, anti-wear, and anti-friction properties among the selected functionalized GO materials. However, it is unclear whether the basic GO (used to generate functionalized GO) rendered GO-NH 2 the best material among the selected functionalized GO materials. In other words, it is yet to be clarified whether the amino group within the GO-NH 2 nanosheets can benefit the tribological properties of other GO materials. To prove whether the amino groups contributed to the improvement in the tribological properties and achievement of superlubricity, an experiment was designed (Fig. 3). First, GO nanosheets were used to reduce the COF of DEG(aq) from 0.031 to 0.017, and the corresponding contact pressure was 265 MPa, with a WSD of approximately 120 μm (Fig. S4(a) in the ESM). Second, GO was mixed with the three types of functionalized GO nanosheets at a ratio of 1:1 to prepare GO + GO-NH 2 , GO + GO-OH, and GO + GO-COOH powders. Third, the mixed powders were added to DEG(aq). Friction experiments involving these mixed additives were conducted, and the results are shown in Fig. 3. Both the GO + GO-OH and GO + GO-COOH powders further reduced the COF to approximately 0.011, but not as low as the superlubricity level. Notably, the GO + GO-NH 2 nanosheets reduced the COF to approximately 0.008, indicating the achievement of superlubricity, and the corresponding contact pressure is 340 MPa with a WSD of approximately 106 μm (Fig. S4(b) in the ESM). With the addition of GO-NH 2 nanosheets, not only was liquid superlubricity achieved, but also the contact pressure increased as the WSD decreased. These results prove that the amino groups contributed to the improvement in the load-bearing, anti-wear, and anti-friction properties, as well as to the achievement of liquid superlubricity at the macroscale.

Discussion
To reveal the underlying mechanisms associated with the excellent tribological properties achieved using the www.Springer.com/journal/40544 | Friction GO-NH 2 nanosheets, the topographies of the worn areas were compared using SEM (Figs. 4(a)-4(h)). The worn area of the ball lubricated with the GO-NH 2 containing DEG(aq) was relatively rough, and contains many small pits and abrasive particles, whereas the worn areas of the other solutions were relatively smooth. All the worn areas on the discs were relatively smooth, and only a few furrows were observed. Notably, the small pits within the worn area of the ball lubricated with GO-NH 2 containing DEG(aq) may serve as surface textures to improve the load-bearing capacity of the liquid film and enhance the antifriction and anti-wear properties [41]. Therefore, the particular worn area topography caused by GO-NH 2 may be one of the factors that result in the smallest COF under the highest contact pressure.
To analyze the possible anti-friction, anti-wear, and superlubricity mechanisms of the amino groups, the existing state of matter in the worn areas lubricated with GO-NH 2 was investigated. XPS analysis was performed to investigate the potential tribochemical reactions occurring during the friction process (Figs. 4(i)-4(n)). In the worn areas of the ball and disc, a typical C 1s peak at 284.8 eV corresponding to C-C and C-H bonds [39] was observed, which was attributable to GO-NH 2 nanosheet adsorption. The C 1s peaks (ball) at 286.1 and 288.7 eV were assigned to C-NH 2 and NH-C(=O)-NH bonds, respectively [39], which were identical to those detected in the original GO-NH 2 nanosheets (Fig. 1(i)). The C 1s peaks (disc) at 285.7 and 288.4 eV were assigned to C-N and N-C=O bonds, respectively [40,42]. The binding energies detected between the ball and the disc were different slightly. This may be caused by the slightly different surface charge properties of the Si 3 N 4 ball and sapphire disc, which resulted in a slightly different chemical environment for the adsorption of the GO-NH 2 nanosheets. Even though the C-NH 2 , NH-C(=O)-NH, and N-C=O bonds overlapped with the C-O and C=O bonds (obtained from worn areas lubricated with other solutions), which appeared at approximately 286.1 and 288.7 eV (Fig. S5 in the ESM), respectively, the C-N peak at 285.7 eV was only detected in the worn areas lubricated with GO-NH 2 containing DEG(aq), thereby proving the adsorption of GO-NH 2 nanosheets onto the contact surfaces. The N 1s peak at 400.0 eV for both the ball and disc was assigned to the C-NH 2 bond [39], further proving the adsorption of GO-NH 2 nanosheets onto the worn areas. In the Si 2p spectrum of the Si 3 N 4 ball, the peak at 102.3 eV assigned to the Si-O bond indicated the presence of SiO 2 [33], which is an excellent anti-friction material derived from the classical tribochemical reactions between Si 3 N 4 and water [43]. In the Al 2p spectrum of the sapphire disc, the only peak at 74.1 eV was assigned to Al 2 O 3 [44], proving that no reaction occurred on the sapphire surface. This is consistent with the findings of our previous study, which indicates that GO can protect the sapphire surface from tribochemical reactions [33]. These XPS results imply that, during the friction process, a SiO 2 containing boundary layer and a GO-NH 2 adsorption layer were formed on the Si 3 N 4 ball, and that only a GO-NH 2 adsorption layer was formed on the sapphire disc.
In the XPS spectra of the worn areas lubricated with other functionalized GO-containing DEG(aq) (Fig. S5 in the ESM), a SiO 2 containing boundary layer was detected. This implies that the formation of the boundary layer was not a key factor facilitating the achievement of superlubricity. Meanwhile, the adhesive forces against the SiO 2 probe were measured via AFM. The values obtained were approximately 17.8, 3.8, and 7.0 nN for the GO-NH 2 , GO-OH, and GO-COOH nanosheets, respectively (Figs. 5(a)-5(c)). Because the main difference between the selected functionalized GO nanosheets was the functional groups, it was deduced that the greater adhesive force between the GO-NH 2 nanosheet and SiO 2 surface may be derived from the electrostatic adsorptive interaction of the positively charged amino groups and the negatively charged SiO 2 surface. It was proven that different charges between lubricant molecules and surfaces resulted in the formation of a dense adsorption film at the interfaces because of electrostatic adsorptive interactions [27,28]. Therefore, the electrostatic adsorptive interaction may result in a greater adhesive force between GO-NH 2 and the SiO 2 surface, resulting in a more robust and denser www.Springer.com/journal/40544 | Friction adsorption layer of GO-NH 2 on the ball to reduce friction and wear. For the sapphire disc, the electrostatic adsorptive interaction derived from the positively charged amino groups and negatively charged sapphire surfaces [45] may render the GO-NH 2 adsorption layer more robust and denser than the other functionalized GO materials on the sapphire disc. Therefore, the high adsorption owing to the greater adhesive force between GO-NH 2 and SiO 2 was one of the main factors that contributed to the better tribological properties and superlubricity.
To further investigate the state of the GO-NH 2 adsorption layer, the cross-sectional structure and thickness of the GO-NH 2 adsorption layers on both the ball and disc were investigated using HRTEM (Figs. 5(d) and 5(e), respectively). The GO-NH 2 adsorption layer on both the ball and disc exhibited multilayered structures with a thickness of approximately 10 nm. The adsorption layer of GO-NH 2 was slightly thicker than that of the original GO-NH 2 nanosheets (approximately 2-3 nm, see Fig. 1(d)), indicating that the GO-NH 2 nanosheets in DEG(aq) may converge and form a thicker adsorption layer on the worn surfaces subjected to the friction process [46]. The GO-NH 2 adsorption layer was not continuous, and its lateral dimensions were smaller than those of the original GO-NH 2 nanosheets, which may because the original nanosheets were ruptured into debris via the rubbing motion. The GO-NH 2 adsorption layer at the interfaces protected the contact surfaces against solid asperity collisions, and caused the shearing plane to constantly transform from solid interfaces to GO-NH 2 interlayers. Because GO family materials possessed weak interlayer interactions, the adsorption layer of GO-NH 2 can offer extremely low friction to superlubricity [47]. Based on the XPS, AFM, and HRTEM results of the GO-NH 2 nanosheets, the excellent anti-friction and anti-wear properties, as well as the achievement of superlubricity, were primarily ascribed to the formation of a robust GO-NH 2 adsorption layer between the contact interfaces.
As shown by the results in Fig. 2(d), the relationship between the average COF and viscosity of the dihydric alcohol was linear; hence, a liquid film was formed at the Si 3 N 4 /sapphire interface. To determine the lubrication regime during the stable lubricating period with the GO-NH 2 containing DEG(aq), the liquid film thickness (h) was estimated using the Hamrock-Dowson theory, as described in detail in our previous paper [31], and the relevant data are listed in Table S2 in the ESM. The calculation results show that the h during the stable lubricating period was approximately 18.9 nm. The ratio (λ) of the h value to the equivalent Ra values of the two worn areas was utilized to distinguish the lubrication regime, which included boundary lubrication, mixed lubrication, and elastichydrodynamic lubrication. The equation used was , where σ is the Ra (10 nm for the Si 3 N 4 ball, and 3 nm for the sapphire disc), and λ was estimated to 1.81 for the GO-NH 2 containing DEG(aq). The calculation indicates that the lubrication regime during a stable lubrication period involving lubrication with GO-NH 2 containing DEG(aq) was the mixed lubrication regime.
To further confirm that the GO-NH 2 adsorption layer, boundary layer, and liquid film contributed to the extremely low friction, two additional experiments were performed. In the first experiment, a normal experiment was conducted using GO-NH 2 containing DEG(aq), and the experiment was paused after a stable lubrication period (the coefficient of friction (μ) ≈ 0.0091) was achieved. Subsequently, the worn track was cleaned with ethanol to remove the GO-NH 2 adsorption layer. Thereafter, the experiment was continued on the same worn track by introducing pure DEG. The corresponding results ( Fig. S6(a) in the ESM) show a COF of 0.021-0.014 during the pure DEG period (lasting 1,800 s), which was higher than the COF of GO-NH 2 containing DEG(aq). This result confirms that without the GO-NH 2 adsorption layer, the liquid film and boundary layer cannot yield extremely low friction; hence, the GO-NH 2 adsorption layer is necessary to achieve extremely low friction and superlubricity. In the second experiment, a stable lubricating period (μ ≈ 0.0088) was achieved, as in the first experiment. Thereafter, the residual solution between the tribo-pairs was absorbed using dry tissue until no residual liquid remained, which could erase the liquid film. Subsequently, the experiment was continued on the same worn track without introducing any liquid (dry sliding). The corresponding results ( Fig. S6(b) in the ESM) show that the COF during the dry sliding period was 0.025-0.015 and remained stable for 300 s. The GO-NH 2 adsorption and boundary layers were destroyed, resulting in an abrupt increase in the COF to 0.32. This result confirms that during the initial 300 s, the GO-NH 2 adsorption and boundary layers resulted in low friction at the liquid lubrication level. The results of these two experiments indicate that the GO-NH 2 adsorption layer, boundary layer, and liquid film contributed to the extremely low friction and superlubricity.
Based on the analysis above, a possible superlubricity model for a GO-NH 2 containing solution was proposed (Fig. 6). The friction experiment results show that the lubricating process comprise two phases: the wearing-in and superlubricity phases. During the wearing-in phase, the wearing of solid asperities resulted in contact area development, contact pressure reduction, and the formation of a unique topography. The unique topography comprising many small pits may serve as a surface texture to improve the loadbearing capacity of the liquid film and enhance the anti-friction and anti-wear properties. Tribochemical reactions induced the formation of a SiO 2 containing boundary layer which reduced friction. Therefore, the pressure reduction, as well as the liquid film and boundary layer, contributed to the friction reduction and low wear. The completion of the wearing-in period was proceeded by the friction process, i.e., the superlubricity phase. In this phase, the boundary layer and liquid film can offer low friction but not as low as the superlubricity level, as demonstrated in the experiment above. In addition to the boundary layer and liquid film, the greater adhesive force between the amino groups and SiO 2 allowed the GO-NH 2 nanosheets to be adsorbed onto the SiO 2 containing boundary layer more effectively compared with other functionalized GO nanosheets under friction, resulting in a more robust and denser GO-NH 2 adsorption layer on the Si 3 N 4 ball. For the sapphire disc, a GO-NH 2 adsorption layer was formed under friction, which may result from the electrostatic adsorptive interaction derived from the positively charged amino groups and the negatively charged sapphire surfaces [27,28,45]. The robust GO-NH 2 adsorption layers between the contact surfaces can shield solid asperities and protect the contact surfaces. Subsequently, the GO-NH 2 adsorption layers caused the shearing plane to constantly transform from solid interfaces into the GO-NH 2 interlayers. The weak interlayer interactions of the GO-family materials further decreased the friction, and facilitated the achievement of superlubricity. It was concluded that the amino groups within the GO-NH 2 nanosheets contributed primarily to the achievement of liquid superlubricity.
This study demonstrates that by designing functionalized GO materials with various functional groups, the anti-friction, anti-wear, and other tribological Fig. 6 Superlubricity model for GO-NH 2 containing solution at macroscale: (a) contact areas at low magnification; (b) anti-friction derived from SiO 2 containing boundary layer and liquid film at high magnification; (c) anti-friction and anti-wear derived from GO-NH 2 adsorption layer (due to amino groups) and shearing between GO-NH 2 interlayers. www.Springer.com/journal/40544 | Friction properties of the functionalized GO materials can be manipulated. Consequently, an ideal lubricating state can be achieved under various conditions using functionalized GO materials, including actual operating conditions in industrial fields. Although other aspects (such as various tribo-pair materials, thermal stability and influence, and high speed and load) must be considered for industrial lubrication and engineering applications, this study facilitates the achievement of liquid superlubricity using functionalized GO materials, and proves the potential of functionalized GO materials as lubricating additives for industrial lubrication.

Conclusions
In this study, the mechanism underlying the achievement of a liquid superlubricity state using GO-NH 2 nanosheets is revealed qualitatively. Liquid superlubricity is achieved primarily through the high adhesive force between amino groups within GO-NH 2 nanosheets and SiO 2 , as well as the consequent formation of a robust and dense GO-NH 2 adsorption layer, which protects the contact surfaces, and results in the shearing plane transforming constantly from solid asperities into GO-family material interlayers with weak interlayer interactions. Hence, the enhancement in the anti-friction and anti-wear properties of the solutions and the realization of liquid superlubricity are demonstrated. Additionally, the SiO 2 containing boundary layer and dihydric alcohol liquid film contribute to the friction reduction and wear resistance. This study demonstrates that the amino groups within the functionalized GO nanosheets possess better anti-friction and anti-wear properties than the carboxyl and hydroxyl groups when mixed with dihydric alcohols to lubricate Si 3 N 4 /sapphire tribopairs. This study reveals the effect of functional groups within functionalized GO materials, and provides a basis for designing functionalized GO materials with excellent tribological performances.