Regulating the Nb2C nanosheets with different degrees of oxidation in water lubricated sliding toward an excellent tribological performance

Novel two-dimensional (2D) Nb2C nanosheets were successfully prepared through a simple lultrasonic and magnetic stirring treatment from the original accordion-like powder. To further study their water-lubrication properties and deal with common oxidation problems, Nb2C nanosheets with different oxidation degrees were prepared and achieved long-term stability in deionized water. Scanning electron microscope (SEM), transmission electron microscope (TEM), scanning probe microscope (SPM), X-ray powder diffraction (XRD), Raman, and X-ray photoelectron spectrometer (XPS) experiments were utilized to characterize the structure, morphology, and dispersion of Nb2C nanosheets with different degrees of oxidation. The tribological behaviors of Nb2C with different degrees of oxidation as additives for water lubrication were characterized using a UMT-3 friction testing machine. The wear scars formed on the 316 steel surface were measured using three-dimensional (3D) laser scanning confocal microscopy. The tribological results showed that a moderately oxidized Nb2C nanosheet, which owned the composition of Nb2C/Nb2O5/C, displayed excellent tribological performance, with the friction coefficient (COF) decreasing by 90.3% and a decrease in the wear rate by 73.1% compared with pure water. Combining the TEM and Raman spectra, it was shown that Nb2O5 nanoparticles filled in the worn zone, and the layered Nb2C and C were adsorbed into the surface of the friction pair to form a protective lubricating film. This combined action resulted in an excellent lubricating performance.


Introduction
In the mechanical or energy industry, friction reduction, and wear resistance have continuously achieved ideal tribological characteristics [1]. Considering that countless losses are caused by friction and wear [2,3], the most direct and effective method is to add a lubricant between friction pairs to avoid direct contact between them [4]. Water-based lubricants are widely used in the fields of mechanical lubrication, cutting fluids, and hydraulic fluids owing to their excellent cooling and flame retardant properties as well as their higher safety and lower pollution than oil-based lubricants [5][6][7]. However, traditional pure water lubricants have gradually failed to meet the requirements of particular applications. In view of this, researchers often choose to add nanoadditives to the water lubricating fluid to enhance its tribological performance. Furthermore, two-dimensional (2D) layered materials, such as graphene-based materials [8][9][10][11][12][13][14], hexagonal boron nitride, and black phosphorus, are the most common additives, and many scholars have conducted extensive studies around them. Ci et al. [15] explored the tribological behaviors from graphene to fluorographene and found that fluorinated reduced graphene oxide nanosheets (F-rGO) with different degrees of fluorination display excellent tribological properties, which is due to the nanosheets with a small size easily entering the contact surface, and the large interlayer spacing makes the sample show a significant shear capability. Wang et al. [16] evaluated the tribological characteristics of black phosphorus nanosheets as water-and oil-based lubricant additives and found that it exhibited an outstanding resistance to extreme pressure and bearing capacity at high loads. This demonstrated that black phosphorus, as a new additive, displayed an excellent lubrication performance. Moreover, MXene is also a novel 2D layered material, but its lubrication performance has rarely been explored.
MXene was first prepared by the Naguib team in 2011 [17]. The parent phase of MXene is a ternary layered metallized ceramic MAX. The MAX phases have the formula M n+1 AX n (n = 1-3), where M is an early transition metal, A is an element traditionally from groups [13][14][15][16], and X is carbon or nitrogen [18]. For the MAX phase materials, because the M-X bond strength is greater than the M-A bond, people selectively etched M-A bonds to obtain 2D MXene materials with large specific surface areas, good electrical conductivities, high hydrophilicities, elastic moduli, and high carrier mobilities [19][20][21].
However, an accordion-like MXene material is too thick to be stably dispersed in an aqueous solution [22], and its large thickness is not conducive to get themselves into the friction pair, and thus it is necessary to further exfoliate it into nanosheets. The most common method to solve this problem is to add an intercalation agent to make the separation between adjacent layers to finally obtain few or even single-layer MXene materials. Thus, MXene materials can be better dispersed in water [23]. However, oxidation is a common problem in a MXene material [24]. In this study, we prepared Nb 2 C nanosheets with different degrees of oxidation to explore its influence on the dispersion and structure of Nb 2 C, and whether it was beneficial as a water lubricating additive.
Herein, we adopted the MXene material Nb 2 C as raw material and the organic base tetrabutylammonium hydroxide (TBAOH) as the intercalation agent, supplemented by simple magnetic stirring, centrifugation, and ultrasound to reduce the number of layers. The microstructure of Nb 2 C was then regulated by controlling its degree of oxidation; thus, the original Nb 2 C, moderately oxidized Nb 2 C and completely oxidized Nb 2 C nanosheets were obtained. Meanwhile, a small amount of surfactant benzalkonium chloride was added to make a relatively stable and dispersed lubricant additive in pure water. The structural evolution process of Nb 2 C nanosheets with different degrees of oxidation and its tribological behavior when used as a water-based lubricant additive were investigated. The results showed that the moderately oxidized Nb 2 C nanosheets, which had a Nb 2 C/Nb 2 O 5 /C composite material system, displayed extremely high stability and anti-wear ability when used as a water lubrication additive.

Materials
An accordion-like structured powder of Nb 2 C was purchased from Beike Nano Materials Technology Co., Ltd., China, and a tetrabutylammonium hydroxide aqueous solution (TBAOH, 50 wt%) was supplied by Energy Chemical Co., Ltd. In addition, benzalkonium chloride (80 wt%) was purchased from Aladdin Bio-Chem Technology Co., Ltd., and ascorbic acid was obtained from Ruji Biotechnology Development Co., Ltd. Other reagents were purchased from Aladdin Industrial Co. and used directly.

Intercalation thinning of Nb 2 C sheets
The accordion-like original Nb 2 C powder and deionized water were configured into a 100-mL aqueous solution at a ratio of 1 mg/mL. According to the method of successfully intercalating and 400 stripping the MXene material V 2 C in Ref. [21], we selected TBAOH as the intercalation agent and a 10 mL/50 wt% TBAOH aqueous solution was added.
To prevent oxidation, 20 mg of ascorbic acid was added to the solution. At this time, the entire system was in a reducing atmosphere, and the solution was magnetically stirred in an ice water bath for 12 h to make TBAOH play a role as an intercalation agent within a sufficient amount of time. The mixture obtained was first adjusted to pH7 with hydrochloric acid, then centrifuged at 3,000 rpm for 10 min, and the supernatant was discarded. The above operations were repeated twice with deionized water and anhydrous ethanol. Finally, the residual liquid was treated with a freeze-drying method to obtain a powder.

Structure regulation of Nb 2 C nanosheets
The powder obtained above was divided into three parts: The first part was prepared as a solution directly with deionized water in the proportion of 0.5 mg/mL, and we could then obtain a type of black Nb 2 C solution. The second part was also prepared as an aqueous solution in a proportion of 0.5 mg/mL. On this basis, the powder was magnetically stirred in a 60 ℃ water bath for 6 h, and we obtained a clear and transparent yellow moderately oxidized Nb 2 C aqueous solution (MO-Nb 2 C). In addition, the third part used the same ratio, but the solution was magnetically stirred at room temperature for 7 days to obtain a completely oxidized Nb 2 C aqueous solution (CO-Nb 2 C). Moreover, a small amount of surfactant (benzalkonium chloride) was added to these aqueous solutions to improve its stability, and the dosage of surfactant was 0.5 mL/100 mL.

Characterization of physical and chemical properties
A field emission SEM (FEG 250) was used to observe the micromorphology of the Nb 2 C samples. The microstructure of MO-Nb 2 C was recorded using TEM (Talos F200x), and the thickness of Nb 2 C was evaluated using SPM (Dimension 3100). The structural change of Nb 2 C was obtained using XRD (D8 Advance, Bruker), and the XRD scans were measured from 5° to 55° during a 34 min period. In addition, Nb 2 C nanosheets with different degrees of oxidation were analyzed using a Raman microscope (RENISHAW, Renishaw) with a 532-nm laser to observe the structure and electronic characteristics. XPS (Axis Ultra DLD) was utilized to explore the chemical binding energy and elements of Nb 2 C nanosheets with different degrees of oxidation.

Tribology test and analysis
Nb 2 C nanosheets with different degrees of oxidation were configured into three groups of water-based lubricants according to the mass/volume ratios of 1.0, 0.75, 0.5, and 0.25 mg/mL of pure water. Their tribological behaviors at room temperature were studied using a UMT-3 friction testing machine. Commercially available alumina (Al 2 O 3 ) balls with a diameter of 6 mm were slid reciprocally against 316 steel under a point contact load of 5 N. The sliding frequency was 5 Hz, the test time was 30 min, and the length of the friction track was 5 mm. Each test was repeated at least three times under the same condition using the average friction coefficient (COF) to ensure repeatability. At the same time, an α-step IQ surface contour meter (ASTQ) was used to measure the depth of the wear scar. The wear depth after the friction test was characterized using a surface profiler, and the wear rate (W s ) was calculated from the wear depth, which is given by the following: where V is the total friction volume, F is the normal load, and L is the total distance. Meanwhile, the three-dimensional (3D) morphology of the wear scars on the 316 steel and the wear scar of the alumina (Al 2 O 3 ) ball were examined using a 3D laser scanning confocal microscope (LSCM, Keyence model VK-X200K), and the Raman spectrum was obtained using a Renishaw inVia Reflex excited by a 532 nm laser. The focused ion beam (FIB, Aurigua) was used to cut the 316 steel for taking TEM images to observe the composition of the friction film and the element distribution in the cross-section of the wear scar.

Structural transformation of Nb 2 C nanosheets with different degrees of oxidation
The dispersity of the Nb 2 C sheets with different degrees of oxidation in deionized water is shown in Figs. 1(a)-1(c). From the digital photos, it can be seen that the color of the Nb 2 C nanosheets with different degrees of oxidation was obviously different. The aqueous solution of Nb 2 C was black, the moderately oxidized Nb 2 C showed a clear and transparent yellow color, and the color of the completely oxidized Nb 2 C was milky white. It is vividly shown in Fig. 1(b) that there was no precipitation for the Nb 2 C nanosheets with a relatively low degree of oxidation added benzalkonium chloride at 15 days, indicating that the surfactants could keep them stable for at least 15 days, but the CO-Nb 2 C could not remain stable. This phenomenon indicates that the stability of the system decreased in the case of excessive oxidation. However, at 30 days, even in the presence of surfactants, Nb 2 C almost completely precipitated, whereas MO-Nb 2 C still showed good stability. Meanwhile, it can also be seen in Fig. 1(d) that MO-Nb 2 C had the best stability that its absolute zeta potential could reach approximately 50 mV. By contrast, the zeta potential of CO-Nb 2 C was low and irregular, which also proved that the stability of CO-Nb 2 C was poor. The working life is one of the most important performance indexes of liquid lubricants, and thus the long dispersion time can be considered as the reliability index of oxidized Nb 2 C material as an additive of an aqueous solution, which is also an important prerequisite for the long-term operation of water-based lubricants.
The SEM images of the original accordion-like Nb 2 C powder are shown in Fig. 1(e), and scanning probe microscope (SPM) images of Nb 2 C nanosheets with different oxidation degrees are shown in Figs. 1(f)-1(h). By comparing the images of Nb 2 C powder before and after treatment with an intercalation agent (TBAOH), it was observed that the thickness of Nb 2 C was indeed reduced. It can be clearly observed in Fig. 1(g) that there were many circular flakes around the lamellar structure material, and in Fig.  1(h), there were only a few white nanoparticles left. The reason for this phenomenon is that with the progress of oxidation, Nb 2 C would gradually change into its oxide Nb 2 O 5 , and Nb 2 O 5 fell off from the original Nb 2 C, resulting in white nanoparticles, and when the oxidation degree was too high, the lamellar structure of Nb 2 C would gradually disappear, and only the oxidation products Nb 2 O 5 were left [25].
The microstructure of the MO-Nb 2 C was observed using TEM. From the EDS images shown in Figs. 2(a)-2(d), it can be seen that the elements Nb, C, and O were distributed uniformly on the surface of MO-Nb 2 C,  indicating that the oxidation reaction occurred on all parts of the surface. In addition, the HRTEM images of MO-Nb 2 C are shown in Figs. 2(e)-2(f). It is worth noting that there were only a few lattice fringes in MO-Nb 2 C, and the electron diffraction image also showed that MO-Nb 2 C was an amorphous or polycrystalline structure, which was caused by the transformation of Nb 2 C into Nb 2 O 5 owing to the proceedings of the oxidation, and Nb 2 O 5 would fall off from the original structure [26]. Thus, the original Nb 2 C with a crystalline structure was transformed into amorphous carbon. Meanwhile, as indicated in the HRTEM image in Fig. 2(g), the oxidation phenomenon was consistent with the white circles in the SPM image, from which we can see obvious lattice stripes, which also proved that the thin and round objects were indeed crystalloid Nb 2 O 5 , which was the oxidation product of Nb 2 C. Moreover, the amorphous ring coexisting with bright diffraction spots in the electron diffraction pattern also proved the existence of the structure of Nb 2 C/Nb 2 O 5 /C in MO-Nb 2 C.
To study the crystallographic phase and compositional variation of Nb 2 C nanosheets before and after oxidation, X-ray diffraction (XRD) was used. As shown in Fig. 3(a), the peak centered at approximately 2θ = 9.1° and 22.6° could be ascribed to the characteristic peaks of Nb 2 C (002) and Nb 2 O 5 (001), respectively [27], and the appearance of the characteristic peak at 9.1° showed that Nb 2 C was well prepared from Nb 2 AlC. This indicates that the oxidized product of Nb 2 C was Nb 2 O 5 . Moreover, when the degree of oxidation increased, the characteristic peak strength of Nb 2 C decreased significantly and even disappeared for CO-Nb 2 C, whereas the characteristic peak strength of Nb 2 O 5 increased at the same time, indicating that the formation of a hybrid material containing Nb 2 C, Nb 2 O 5 , and Nb 2 C gradually transformed into Nb 2 O 5 with an increase in the degree of oxidation. Meanwhile, the strength of the peak centered at approximately 2θ = 38.9° of the parent material Nb 2 AlC decreased, and we speculated that when the degree of oxidation increased, the content of Nb 2 AlC decreased.
Raman spectroscopy plays an important role in studying the structural transformation of the materials. To more accurately determine the chemical components of Nb 2 C, the Raman spectra of Nb 2 C with different degrees of oxidation were obtained, as shown in Fig. 3(b). The bands appearing at 233 cm -1 (bending modes of Nb-O-Nb linkages) and 688 cm -1 (Nb-O stretching mode) were the characteristic bands of Nb 2 O 5 [28]. The presence of these two peaks can be seen in all stages of oxidation. This indicates that the oxidation occurred easily when Nb 2 C nanosheets were thinned, and the strength of the peak at approximately 688 cm -1 increased with an increase in the degree of oxidation. Meanwhile, the peaks appearing at 1,381 and 1,589 cm -1 were assigned to the D and G bands, respectively. The D band represents the presence of defects in the carbon-based body and the G band is presented owing to the in-plane vibration of sp 2 carbon atoms. This indicates that a hybrid material Nb 2 C/Nb 2 O 5 /C was produced [29,30].
For the sake of quantitatively analyzing the degree of oxidation Nb 2 C, X-ray photoelectron spectroscopy (XPS) was utilized. The X-ray photoelectron spectra of Nb 2 C with different degrees of oxidation in the Nb 3d, C 1s, and O 1s regions were fitted by the corresponding components and are shown in Fig. 4. From the Nb 3d spectrum, two strong bands centered at binding energies of approximately 207.8 and 210.7 eV were in good agreement with the binding energy of Nb 2 O 5 [31], indicating that oxidation occurred in all sets of samples, which was consistent with the Raman spectra. In addition, some small peaks could be observed in Nb 3d, which can be assigned to Nb I , Nb II , or Nb IV [32], Nb-O [33], and Nb 3+ -O [34]. The appearance of these peaks also verified the occurrence of the oxidation reaction in Nb2C. In the C 1s spectrum, a weak band at the binding energy of approximately 282.9 eV can be observed in Nb2C and MO-Nb 2 C, which implies the existence of Nb-C, although it did not appear in the spectrum of CO-Nb 2 C [34]. This phenomenon proved that Nb2C would gradually transform into Nb 2 O 5 under excessive oxidation. Further, three main peaks of the C 1s spectrum centered at binding energies of approximately 286.1, 287.1, and 289.2 eV, could be assigned to the C-C bond [35], CH x , and C-O=O [36], respectively. The main peaks of O 1s corresponded to Nb 2 O 5 and C-Nb-OH x [37]. The distribution of elements Nb, C, and O in Nb 2 C with different degrees of oxidation is shown in Table 1. It can be observed that the element content was consistent with the corresponding strength. The Nb/O ratios of Nb 2 C, MO-Nb 2 C, and CO-Nb 2 C were 0.994, 0.311, and 0.216, respectively. These results indicate that MO-Nb 2 C was transformed into a hybrid material mainly composed of Nb 2 C, Nb 2 O 5 , and C.

Tribological behaviors of Nb 2 C nanosheets with different degrees of oxidation as water lubricant additive
The tribological behaviors of Nb 2 C nanosheets with different degrees of oxidation dispersed in pure water are shown in Fig. 5. To determine the optimal amount of added MXene materials with different degrees of oxidation, we tested an aqueous solution with different Nb 2 C mass fractions under the same friction condition (10 N, 2 Hz). Figures 5(a)-5(f) show the COF curves and mean COF of Nb2C nanosheets with different degrees of oxidation dispersed in pure water with different concentrations. The results showed that the average COF was the lowest when the mass volume ratio of the additive was 0.5 mg/mL for Nb 2 C, which was only 0.08. This was 81.2% lower than that of pure water (the average COF of pure water is 0.44). For the MO-Nb 2 C, the change in the average COF was in a single trend, and the optimal value of 0.04 was obtained at a concentration of 0.25 mg/mL (the minimum value of the initial experimental group). To explore the optimal solution, we supplemented a set of 0.1 mg/mL data, and the  experimental results showed that the lowest value of COF was indeed obtained at 0.25 mg/mL, which also meant that the average COF of the MO-Nb 2 C was 90.3% lower than that of pure water. Moreover, for the CO-Nb 2 C sample, the average COF was the lowest when the mass volume ratio of the additive was 0.5 mg/mL, which was 0.12, i.e., 72.7% lower than that of pure water. This means that the Nb 2 C material still displayed good tribological properties even in the case of excessive oxidation. The above results showed that, with the addition of Nb 2 C nanosheets with different degrees of oxidation, the COF of pure water could be effectively reduced.
As is well known, during the experimental water lubrication process, owing to the load application, the rough summits of the friction pair contact with each other, and a discontinuous fluid film at the microscale will appear in the later period of the friction test, which is called mixed friction or semidry friction state. The contact interface contains a solid contact area and a microscale liquid lubrication area. When a fluid film exists, the friction resistance is generally caused by the shear deformation of the rough peak in the solid-solid contact area and the shear action of the fluid in the liquid lubrication area. During the initial stage of the friction test, the friction contact surface gradually changes from a point contact at the microscale to a face contact, which often shows a significant reduction in the COF curve [38]. It can be seen from Figs. 5(a)-5(c) that this phenomenon only occurs in pure water and CO-Nb 2 C, which showed that when Nb 2 C nanosheets with relatively low oxidation degrees were used as water lubricating additives, the running in the aqueous solution could be effectively improved. Through comparison, it was found that for Nb 2 C and MO-Nb 2 C, when the friction stage was stable, the average COF also tended to be stable, which showed that the additive had a good dispersion in pure water. The large fluctuation of the COF curve at low concentrations was caused by the lack of www.Springer.com/journal/40544 | Friction effective components, which could no longer play the role of effective lubrication. It was found that the overall COF value of CO-Nb2C was higher than that of Nb2C and MO-Nb 2 C, which was because when the oxidation degree of the system was too high, a large number of Nb 2 C nanosheets were transformed into Nb 2 O 5 nanoparticles, and the amount of materials that could play the role of an interlayer slip decreased. The mean wear rate and mean COF results of Nb 2 C with different degrees of oxidation dispersed in pure water with different concentrations are shown in Figs. 5(d)-5(f). It can be seen that Nb2C can effectively reduce the wear rate of 316 steel under the water lubrication system, and the wear rate can be reduced up to 62.8% for Nb 2 C, 73.1% for MO-Nb 2 C, and 25.6% for CO-Nb 2 C, respectively. Moreover, the wear rates of the three samples were all higher than those of pure water at low concentrations. This was because at low concentrations, the contents of lamellar Nb2C and C, which could play an effective lubrication role, were insufficient to support the effective interlayer slip, and the friction film formed was incomplete, which could not provide sufficient lubrication for the friction pair, thus aggravating the wear rate. The above results demonstrated that the MO-Nb 2 C showed a better anti-wear property and friction reduction capacity, with an optimal concentration of 0.25 mg/mL. The 3D morphologies of the wear tracks of MO-Nb 2 C with different concentrations were investigated using a laser scanning microscope, as shown in Fig. 6. It could be observed that when the concentration was 0.25 mg/mL, the wear mark size and depth of the MO-Nb 2 C were much smaller than those of pure water under the same friction condition. However, at low concentrations, the additive content was insufficient, the formation of friction film at the boundary was discontinuous, and effective lubrication could not be achieved. At the same time, when the concentration was too high, the nanofillers agglomerated, which would promote the wear, and thus it can be seen from the wear scar that the wear degree approached or even exceeded that of pure water.

Tribological mechanism of Nb 2 C nanosheets with different degrees of oxidation
To explore the lubrication mechanism of Nb 2 C after oxidation as a water-based lubricant additive with excellent tribological performance, we carried out a series of characterizations on the surface of 316 stainless steel rubbed with a 0.25 mg/mL MO-Nb 2 C water solution. The SEM images of the worn scar are shown in Fig. 7(a). When it was rubbed in pure water, there were obvious furrows and cracks on the stainless steel surface, indicating that the wear mode was mainly adhesive wear accompanied by slight abrasive wear. However, the worn scar after  friction with a 0.25 mg/mL MO-Nb 2 C water solution was relatively flat, as shown in Fig. 7(b). Only a few minor furrows were produced during friction, and it was found that there were plenty of small white spots existing among the wear scars. We speculated that this was because the Nb 2 O 5 formed by Nb 2 C after oxidation filled the wear area, whereas the original lamellar Nb 2 C and the C produced by www.Springer.com/journal/40544 | Friction oxidation were adsorbed on the surface of the friction pair, forming a lubrication protective film.
To prove this, we employed the Raman spectrum to test the worn surface of stainless steel lubricated with 0.25 mg/mL MO-Nb 2 C. As shown in Fig. 7(c), the typical characteristic peaks of carbon and Nb 2 C appeared in the corresponding Raman spectrum, which confirmed that the original lamellar Nb 2 C and C formed in the oxidation of Nb 2 C deposited on the worn surface to form a protective lubricating film. The high strength characteristic peak at 688 cm -1 also indicated that Nb 2 O 5 was abundant in the wear scar. At the same time, to analyze the lubrication mechanism of samples from a more microscopic perspective, we conducted a TEM test on the crosssection of the wear scars lubricated by an aqueous solution of MO-Nb 2 C. As shown in Figs. 8(a)-8(c), the average thickness of the friction film formed in the cross-section of the worn scar of MO-Nb 2 C was approximately 75 nm, which was much thicker than the other two samples. In the area with serious wear, the thickness could reach more than 200 nm, indicating that the MO-Nb 2 C nanosheets showed a strong wear repairability, and it also proved that the structure of Nb 2 C/Nb 2 O 5 /C was better than that of a single Nb 2 C or excessive oxidized Nb 2 C. Moreover, to explore the chemical composition of the worn surface filled with MO-Nb 2 C nanosheets and reveal its lubrication mechanism, an element distribution mapping test was employed. It can clearly be seen that Nb and O existed on the friction film, indicating that Nb 2 O 5 can enter and fill the worn area well and C can also be found in the surface area of the wear scar, meaning the formation of a carbon-based lubricating protective film, which was consistent with the image of the Raman spectra.
Based on the above results, we proposed the tribological mechanism of MO-Nb 2 C nanosheets as a water-based lubricant additive with excellent  | https://mc03.manuscriptcentral.com/friction lubrication performance. During the recent oxidation treatment, the original Nb 2 C was transformed into the Nb 2 C/Nb 2 O 5 /C composite structure. Among them, layered C and Nb2C entered between the friction pair and adsorbed on the surface of the friction pair to form a lubrication protection film to prevent the friction pair from direct contact. The improvement of the tribological property of MO-Nb 2 C was due to the synergistic effect of Nb 2 C and C, rather than the contribution of pure C or Nb 2 C, which is a key point that resulted in the different friction properties of Nb 2 C, MO-Nb 2 C, and CO-Nb 2 C. At the same time, the oxidation product Nb 2 O 5 , with its small size, could enter and fill in the worn area, reducing the wear rate. Their synergistic effect endowed the MO-Nb 2 C with excellent tribological performance.
In addition, some illustrations of the excellent tribological properties of black phosphorus (BP) with the same 2D layered structure, as determined by previous researchers, also significantly aided this study [39][40][41][42]. The interaction between the nanoparticles and water contributed to the lubrication. The water layer retained by BP-OH nanosheets could provide an extremely low shear resistance, and the ambient degradation of BP also showed a positive effect. All of the above contents provide a novel way to explain the phenomenon in which Nb2C nanosheets with different degrees of oxidation all showed good tribological performances, and we will focus on these aspects in our future studies.

Conclusions
In this study, we added an intercalation agent (TBAOH) to an accordion-like Nb 2 C powder, with only a simple treatment, and obtained Nb 2 C nanosheets. At the same time, we prepared Nb 2 C with different degrees of oxidation, and prepared a long-term stable aqueous solution. On this basis, friction tests of Nb 2 C samples with different degrees of oxidation were carried out, and the results showed that the moderately oxidized Nb 2 C water solution displayed excellent tribological performances, with the COF decreasing by 90.3% and the wear rate decreasing by 73.1% compared with pure water. As the oxidation proceeded, the original Nb2C gradually changed into a structure of Nb 2 C/Nb 2 O 5 /C. The Nb 2 O 5 nanoparticles filled in the worn zone and the layered Nb 2 C and C adsorbed on the surface of the friction pair to form a lubricating film, and their combined action resulted in excellent lubrication. Meanwhile, we found that the lubrication property of Nb 2 C was always better than that of pure water, regardless of its degree of oxidation. Based on these conclusions, an excellent friction reduction and wear resistance of moderately oxidized Nb 2 C display significant potential as a nanoadditive in the application of water lubrication technology.