Preparation, characterization, and tribological properties of silica-nanoparticle-reinforced B-N-co-doped reduced graphene oxide as a multifunctional additive for enhanced lubrication

Microwave-synthesized SiO2-reinforced B-N-co-doped reduced graphene oxide (SiO2-B-N-GO) nanocomposites were characterized by X-ray photon spectroscopy (XPS), X-ray diffraction (XRD), infrared (IR) spectroscopy, and transmission electron microscopy/energy dispersive X-ray (TEM/EDX) analysis. The tribological properties of the SiO2-B-N-GO prepared as anti-wear additives for enhanced lubrication were studied using a four-ball tester. The experiment results indicated that SiO2-B-N-GO exhibits excellent load-carrying, anti-wear, and anti-friction properties in a base oil, especially at the optimal concentration of additives at 0.15 wt%. The wear scar diameter decreased from 0.70 to 0.37 mm and the coefficient of friction was reduced from 0.092 to 0.070, which reductions are attributed to the formation of B-N and graphene layer tribofilms of several tens of nanometers in thickness that prevented direct contact between metals.


Introduction
Lubrication associated with nanotribology has aroused great public concern, and the lubricant industry must improve the performances of lubricants considering both efficiency and the environment [1−4]. Various lubricant-containing nanoparticles, such as metallic TiO 2 , CuO, SiO 2 , and graphene, are used to reduce the friction and wear between metals because a lubricating film composed of nanoparticles forms on the friction pair during sliding [2−4].
Graphene and graphene oxide have attracted much attention owing to their excellent electrical, thermal, and mechanical properties [5−9]. Graphene has long been shown to be an excellent solid lubricant with significant tribological properties due to its ability to undergo easy shear [7,8]. Recent studies of graphene and graphene oxides have attempted to investigate the friction and wear behavior between metals with their use, but their application has not yet been fully studied in the field of lubrication [10,11]. The addition of grapheme has been shown to slow down the oxidation and corrosion on the surface of a friction pair [12]. Berman et al. [5] characterized the graphene tribological behavior by atomic force microscopy, and the results demonstrated that the friction coefficient decreases significantly and the surface morphology was improved under a load of 5 N. Moreover, intermittent graphene has been applied to ensure a reduction in the friction and wear between metals during an entire testing process [13]. The synergetic enhancement effect has been achieved using graphene-based composites, and the tribological properties of graphene have been improved, thereby allowing for the full utilization of graphene in tribological applications [14]. In another study of the tribological behavior of graphenebased composites, boron-and nitrogen-based additives were used. A boron nitride protective film was formed on the friction surface between the metals and showed excellent synergistic effects, thereby reducing the friction and wear [15]. In the field of tribology, silica nanoparticles have been widely studied as effective antifriction agents over recent years. Therefore, in order to study the significant reduction in friction and wear under high loads, SiO 2 , B-N-GO, and SiO 2reinforced B-N-GO nanoparticles are prepared and analyzed using X-ray photon spectroscopy (XPS), X-ray diffraction (XRD), infrared (IR) spectroscopy, and transmission electron microscopy/energy dispersive X-ray (TEM/EDX). The tribological behaviors of these prepared nanoparticles acting as multifunction additives in a base oil are then studied using a four-ball tester and the worn surfaces were observed by XPS. The wear and friction mechanisms of the nanoparticles used as anti-wear additives between the metal friction pairs are proposed.

Preparation of silica-nanoparticle-reinforced B-N-co-doped reduced graphene oxide (SiO 2 -B-N-GO) lubricant
TEOS was added to a solution of ethanol and aqueous ammonia, and this was gently stirred for 3 h at 40 °C . Silica (SiO 2 ) nanoparticles were prepared by the Stöber method according to previous research [16], as shown in Fig. 1. B-N-GO was prepared by the microwaveassisted method [17]. Graphene oxide (GO, 250 mg) and SiO 2 were dispersed in ethanol (50 mL), followed by the addition of aboric acid solution (1 mg/mL) and

Sample characterization
The morphologies of the prepared nanoparticles (SiO 2 , B-N-GO, and SiO 2 -B-N-GO) were observed through TEM with EDX analysis. XRD using Cu K1 radiation was employed to characterize the crystal structures of the samples. The molecular structures of SiO 2 , B-N-GO, and SiO 2 -B-N-GO were observed using Fourier transform infrared (FT-IR, Bruker VERTEX 80V) spectra in the range from 4,000 to 500 cm -1 . XPS (Sigma Probe, produced by Thermo VG Scientific) was used to characterize the synthesized SiO 2 -B-N-GO.

Dispersion stability
The dispersion stabilities of the SiO 2 , B-N-GO, and SiO 2 -B-N-GO nanomaterials in the base oil were studied by the absorbance method. First, these prepared nanomaterials were ultrasonically dispersed in the base oil for 2 h, and the optimal concentration of these nanomaterials in the base oil was 0.15 wt% based on the results of the tribological tests. These solutions were then further diluted by 10 times. The absorbances of these solutions were evaluated at different time intervals using an UV-2600 ultravioletvisible (UV-vis) spectrophotometer.  In the spectrum of B-N-GO, the absorption peak at 3,396 cm -1 is broad and strong, which is due to -OH stretching vibrations, and the C=O stretching of -COOH groups from B-N-GO can be observed at 1,734 cm -1 . The absorption peaks at 1,127 cm -1 and 1,438 cm -1 result from the C-O-C and C-O vibrations, respectively. For SiO 2 , the peaks at 1,596 cm -1 and 1,445 cm -1 are attributed to the stretching and bending vibrations of -OH, respectively, which are caused by water absorbing on the surface of the nanoparticles. The peak at 1,595 cm -1 is the typical vibrational absorption peak of SiO 2 . The SiO 2 -B-N-GO nanocomposites had additional peaks at 1,445 cm -1 (Si-O-Si) and 1,595 cm -1 (Si-O-C), and there was no obvious absorption peak at 875 cm -1 compared with B-N-GO. The intensity of the peak at 3,407 cm -1 did not increase significantly. These results indicate that the SiO 2 was generated after the reaction of SiO 2 -B-N-GO, and the processing did not consume the oxygen-containing functional groups.

Additive optimization
Figure 6(a) shows the variation of the maximum nonseizure load (P B ) for the base oil with an increasing additive concentration. It can be seen that all of the additives can improve the P B values of the base oil dramatically, and it is also illustrated that these nanomaterial additives play a large role in the loadcarrying capacities of these materials. With the addition of 0.05 wt% SiO 2 , the P B value of the base oil is enhanced by 43%. In addition, the P B value of the synthesized SiO 2 -B-N-GO is the highest among all additives at the same concentration, which indicates that   Based on the maximum no-seizure load and WSD, a new performance indicator, the extreme pressure anti-wear coefficient, ω, is proposed to analyze the tribological properties of lubricants using the following equation [22]: where P B is the maximum non-seizure load and WSD is the mean wear scar diameter. All of the tribological parameters are listed in Table 1.
As shown in Table 1, the values of ω for these nanomaterials are maximized at the optimized concentration of 0.15 wt%. These results indicate that these nanomaterials exhibit excellent load-carrying and anti-wear properties in the base oil, especially at the optimal concentration of 0.15 wt%.   The excellent anti-wear and anti-friction behaviors of the SiO 2 -B-N-GO in the base oil is due to the formation of tribofilms ( Fig. 10(b)) between metals. These tribofilms combine the synergistic effects provided by the nano-SiO 2 acting as nano-bearings and the flocculated structure of B-N-GO [23]. , which implies that the additives play an anti-wear role in the metal-to-metal sliding process. The WSD value increases as the load increases, and this is mainly because the oil film thickness between the metals decreases or even breaks, resulting in direct metal contact [24]. The thin oil film cannot support a load beyond 686 N in the case of SiO 2 and B-N-GO additives in the base oil. However, SiO2-B-N-GO can sustain this load and the WSD values are larger. The SiO 2 -reinforced B-N-GO nanomaterial can further improve the load-carrying capacities of B-N-GO and SiO 2 , which is attributed to the synergistic effect of the adsorbed B-N-GO on the sliding surfaces forming a protective tribochemical film [15] and the SiO 2 nanoparticles acting as nano-bearings between the metals [25]. Moreover, the maximum load-carrying capacities of SiO 2 -B-N-GO providing enhanced lubrication was found to reach up to 882 N. Figure 8 shows the SEM morphologies of the worn surfaces with different lubrications. When lubricated with only the base oil, the WSD of the worn surface is very large (0.70 mm) and there is severe surface destruction, which results from the plastic deformation of the worn surface. However, a smoothening of the surfaces is observed with the addition of the synthesized nanomaterials, which is in agreement with the aforementioned order of the anti-wear properties. The WSD value is considerably reduced when lubrication  with the SiO 2 -reinforced B-N-GO is used, as shown in Fig. 8(d). Some adhesiveness structures can be seen on the wear tracks of the surfaces lubricated by B-N-GO and SiO 2 -reinforced B-N-GO because the graphene sheets are adsorbed on the worn surface during sliding.

Tribochemistry
According to American Society for Testing Material (ASTM) D5183-05, Fig. 9 shows the XPS spectra of B 1s, C 1s, N 1s, O 1s, Si 2p, and Fe 2p for the sample lubricated with SiO 2 -B-N-GO. Figure 9(a) shows the C1s spectra, which is deconvoluted into three peaks at 287.9, 286.3, and 284.8 eV, corresponding to C=O, N-sp 3 C, and sp 2 C-sp 2 C moieties, respectively [19]. The binding energies between B 1s ( Fig. 9(b)) and N 1s ( Fig. 9(c)) are indicated at 189.8 and 398.6 eV, respectively, which prove the existence of boron nitride among the tribopairs [26,27]. Nevertheless, a prominent shift was not found in the peak of Si 2p, revealing that the SiO 2 nanoparticles were tribosintered onto the steel surface ( Fig. 9(d)) [28]. The Fe on the surface has been oxidized to Fe 2 O 3 under the testing conditions, which can be seen from the binding energies of 711.8 eV for Fe 2p and 530.6 eV for O 1s (Figs. 9(e) and 9(f)) [29]. Therefore, SiO 2 -B-N-GO nanomaterials provide remarkable tribological benefits via the formation of in-situ protective tribofilms composed of boron nitride and graphene layers that are deposited on the steel-steel interfaces. The SiO 2 nanoparticles form a uniform layer and act as nano-bearings, which prevents the direct contact of the metals, thereby decreasing the friction and wear.

Proposed mechanism
The use of SiO 2 , B-N-GO, and SiO 2 -B-N-GO nanocomposites as multifunctional additives in a base oil has shown that they all possess excellent tribological properties based on the above tribological and surface characterization results. Compared with SiO 2 and B-N-GO, however, SiO 2 -B-N-GO demonstrated better tribological properties. The reasons behind the enhanced tribological properties of SiO 2 -B-N-GO can be summarized as follows. Initially, the nanomaterial evenly adheres between the friction pairs. During friction, SiO 2 -B-N-GO reacts with the steel ball and forms a protective tribofilm of several tens of nanometers in thickness ( Fig. 10(b)) to reduce the asperity-asperity adhesion under extreme conditions ( Fig. 10(a)). The WSD value is therefore much smaller when SiO 2 -B- N-GO is used. The formation of this in-situ protective tribofilm is related to the sliding time, and as time progresses, the friction film forms more easily [30].
In addition, the SiO 2 nanoparticles deposited on the surface of B-N-GO further prevent agglomeration [30]. Spherical SiO 2 nanoparticles act as nano-bearings between the B-N-GO layers and friction pairs, thus decreasing both the friction and wear [25]. Therefore, the excellent tribological behaviors of SiO 2 -reinforced B-N-GO result from the synergistic effects of the formation of an in-situ BN protective tribofilm of several tens of nanometers in thickness on the worn surface and the SiO 2 particles acting as nano-bearings during sliding.

Conclusions
SiO 2 nanoparticles (30−50 nm) were prepared using the Stöber method, B-N-co-doped GO (B-N-GO), and SiO 2 -reinforced B-N-GO (SiO 2 -B-N-GO) nanoparticles were successfully obtained by microwave-synthesis. These materials were characterized by TEM/EDX, XRD, IR spectroscopy, and XPS. The tribological behaviors of these materials were studied using a four-ball tester with the optimum concentration (0.15 wt%) of these nanoparticles in the base oil. The results indicated that SiO 2 -reinforced B-N-doped GO exhibited a significant reduction in both the WSD (from 0.70 to 0.37 mm) and COF (from 0.092 to 0.070) values. These reductions are attributed to the in-situ formation of a tribofilm that is several tens of nanometers thick and consists of graphitic carbon, boron nitride, and tribosintered SiO 2 nanomaterials that prevent metal-metal contact.
The above results reveal that the prepared SiO 2 -B-N-GO nanocomposites are potential candidates to be developed as effect anti-wear lubricant additives for enhanced lubrication under boundary lubricating conditions.
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