Effect of copper oxide@boron nitride nanosheet hybrid nanocomposite on tribological properties of paraffin liquid

Due to the significant friction-reduction and anti-wear properties of nano particle, nano copper oxide (CuO) particle decorating on boron nitride nanosheets (BNNS) nanocomposites are fabricated with polydopamine (PDA) as the linking agent. The structural and morphological characteristics of the CuO@BNNS composite are characterized by scanning electron microscopy (SEM), X-ray diffraction patterns, fourier transform infrared spectroscopy and thermogravimetric analysis. The synergistic effect of CuO and BNNS on extreme pressure of lubrication oil are investigated. A four-ball tribometer is adopted to investigate the tribological behaviors of the as-prepared oil with different additives. Optical microscope and SEM are used to analyze the topography of worn surface. Energy dispersive spectroscopy is adopted to investigate the element distribution on worn surface. The results demonstrate that the CuO@BNNS nanoparticle could effectively improve the friction-reduction and anti-wear properties of the paraffin liquid, compared to paraffin liquid containing modified CuO (f-CuO) or PDA-BNNS respectively. The worn surface of steel ball presents smooth morphology for oil containing CuO@BNNS, contrast to oil containing f-CuO or PDA-BNNS respectively. With the tribo-film formation on the worn surface, the elements distribute uniformly on the worn surface for oil containing CuO@BNNS. Compared to lower load, the effect of load-carrying capacity and easy-shear property of CuO@BNNS outstand under 392 N, which results in the smooth worn surface compared to oil containing other additives. CuO@BNNS hybrid nanoparticle was prepared with a simple and mild method. CuO@BNNS presents superior anti-friction and anti-wear properties due to the synergistic effect of nanoparticle. The result obtained in the paper promotes the application of h-BN based nanocomposites in tribology.


Introduction
In recent years, scientist has focused on the uses of hybrid nanocomposites as lubricant additives, such as composites based on graphene, MoS 2 , WS 2 and h-BN, which can play a great role on friction-reduction and anti-wear properties [1][2][3][4]. In modern equipment and manufacturing industry field, to some extent, production cost and service life of equipment are restricted by lubricant system. As the critical component in lubricant oil, additive plays an important role on the tribological performance under the complex conditions. It is worthwhile exploring the tribological properties of different combinations with various nano particles. In the past few years, lots of researches have carried out on preparation of two-dimensional (2D) layered materials for the friction-reduction and anti-wear property in fluid lubricants [5][6][7]. The excellent tribological properties can be attributed to the weak Vander Waals force among layers and the formation of a robust tribo-film on friction pairs.
However, as the elevation of the synthesis tactics for nanocomposites, many researches have demonstrated hybrid nano-material can effectively enhance the lubrication properties, due to the synergistic effect [3,4,[19][20][21][22][23][24][25]. The decoration of Fe 3 O 4 on h-BN can promote the dispersion of Fe 3 O 4 in oil. And this can effectively improve the lubricant effect of nano particle and facilitate the formation of tribo-film on the worn surface [21]. During the friction, Fe 3 O 4 on h-BN surface can act a part in roll lubrication and formation of tribo-film as molecular bear and active substance. Sahoo [23] investigated the tribological properties of h-BN modified with GO through covalent interaction. Similar results showed that the covalent modification endowed h-BN superior dispersion in oil, which resulted in the excellent tribological performance. Apart from nano particle, the synergetic effect between ionic liquid and BN as oil additive is also explored by Lopez's group [24]. Clearly, the cooperation with nano particle is an effective way to enhance the tribological performance of h-BN based composites. The synergetic effect between h-BN and nanoparticle can be achieved through the combination of nano particles with different dimensions.
Nano Cu or CuO has been adopted as lubricant additive in many studies. Researches have demonstrated that Cu or CuO on lamellar structure can markedly enhance the wear-resistance of lubricant oil or self-lubricant composites [2,[25][26][27][28][29][30]. Due to the good dispersion ability of Cu@WS 2 in polyalkylene glycol (PAG), the PAG oil showed better friction-reduction and anti-wear properties than WS 2 and Cu nanoparticles. The uniform distribution of Cu on the WS 2 endowed the good adsorption on the friction pairs, which promoted the tribo-chemical reaction and formation of tribo-film [2]. And this is also be confirmed in the lubricant oil system containing Cu@GO [26,27]. Similarly, the polyimide composite reinforced with CuO@g-C 3 N 4 presents superior wear-resistance capacity due to the combination with CuO@g-C 3 N 4 hybrid composite [31]. As the combination with CuO, the anti-wear property of mineral oil containing CuO and h-BN as additives is greatly improved. Result showed that with the sliding process, the steel-steel contact at the beginning was protected by the formation of cooper tribo-film on the friction pairs [7,28]. With the collaboration of CuO nano particle, tribological properties of nano diamond also performs well as oil additive [7]. For recent years, the effect of exfoliated h-BN nano sheets (BNNS) decorating with CuO were examined at catalytic degradation [32] and electrochemical detection [33]. The CuO@BNNS composite presents better catalytic activity for bisphenol A removal and electrochemical detection for L-cysteine. However, there are few studies on effect of combinated CuO@BNNS on tribological properties as lubricant oil.
During the traditional preparation tactics of CuO@h-BN, the calcination or hydro-thermal will inevitably result in the agglomeration of the nano h-BN, which is not desired for tribological application. Therefore, an intermediate that connect layered material and nano particle is necessary. As a neurotransmitter secreted by human being, dopamine plays an important role in the preparation of biomimetic materials. Apart from that, polydopamine (PDA) can spread on the surface of h-BN during polymerization due to the weak electrostatic interaction with the heteroatom [34]. Then the polydopamine uncovered on the h-BN surface becomes the gathering center of meatal ion or compound. And this will strength the interaction between CuO and h-BN. However, the tribological properties of BNNS decorated with CuO has not been repored. Based on the above result, in this paper, CuO@ BNNS composite was fabricated with a simple and mild method firstly. Then, suspension abilities of the obtained lubricant oil containing CuO, BNNS and CuO@BNNS were explored. In Sect. 3, the tribological properties of asobtained paraffin liquid lubricant oil with different additives were investigated on a four-ball tribometer. After that, the element distribution of hybrid composite was investigated. Then the worn surface and lubricant mechanism were analyzed and discussed in the last section.

Preparation of h-BN nano sheet (BNNS)
BNNS was prepared by ultrasonic exfoliation of pristine h-BN powder with the help of ultrasonic disrupter. In the experiment, 2 g of pristine h-BN was added in 100 mL isopropanol, followed by sonication at the power of 550 W for 6 h at 10 ℃. Then, the emulsion was settled down for 2 h. After stratification of emulsion, the upper is centrifugated at 4000 r/min. Then the upper system become slightly transparent and precipitate at the bottom are multilayer h-BN. The upper system is poured into suitable amount of water and frozen to dry.

Preparation of PDA-BNNS
The synthesis of PDA-BNNS was performed with dopamine as described in reference [34]. Following dopamine hydrochloride added to the tris solution, BNNS powder was added quickly to the mixed solution with stirring (10 min). The final mixture was ultrasonically treated for 3 h and equilibrated at 60 ℃ for 24 h. The mixture was filtered with microfiltration membrane and washed with deionized water several times. Finally, the grey powder was frozen to dry.

Preparation CuO@BNNS hybrid nanocomposites
The CuO@BNNS hybrid was prepared with the similar method in the reference [34]. 0.3 g PDA-BNNS was added into to 50 mL of DMF utilizing ultrasonic treatment for 2 h. After that 0.1 g f-CuO (modified with KH550) was added. Finally, the suspension stirs for another 6 h at 100 ℃. The mixture was filtrated and washed with deionized water and anhydrous ethanol several times, and the products were treated with filtration and then frozen to dry. The whole preparation process was illustrated in Fig. 1.

Preparation of lubricant oil samples
A certain amount of the obtained modified CuO, PDA-BNNS and CuO@BNNS was poured into paraffin liquid. Then the mixture stirs violently. And then oil mixtures are mixed well under with sonic water bath for 2 h under 25 ℃ to form the homogenous oil system before tribo-test. The weight concentrations of additives in the oils were 0.05 wt% compared to paraffin liquid.

Tribological property test
The tribological properties of the lubricant oils were conducted on a four-ball tribo-test machine MMW1 (Jinan Shidai assay instrument machine Co., Ltd). Due to put focus on the effect of nanocomposites on extreme pressure property, the tribological test condition was set as following. The rotation speed was 1200 r/min, the load was 100, 200, 300 and 392 N, the test duration time is 60 min. The balls used in the test was made of GCr15 bearing steel with a hardness of 62 HRC and a diameter of 12.7 mm. Each sample was repeated at least for 3 times. Before ever test, the steel ball should be clean with petroleum ether and acetone.

Characterization
Fourier transform infrared spectra (FT-IR) was performance on FT-IR-650(Guangdong, China) spectrometer in wavenumber range of 400-4000 cm −1 . The X-ray diffraction (XRD) pattern was performance on a Bruker D2 phaser machine (30 kV, 10 mA) with a 2θ angle ranging from 5° to 60° and a scan rate of 5° min −1 with Cu Ka radiation (k = 0.154 nm) (Bruker, Germany). The topography and composition of nano composites was probed using a SU8020 scanning electron microscopy (SEM) with 3 kV acceleration voltage (Hitachi, Japan). Thermogravimetric analysis (TGA) was conducted on a Netzsch-TG209F3(Netzsch, Germany) thermogravimetric analyzer under nitrogen atmosphere from 50 to 900 ℃ with a heating range of 10 ℃.min −1 . The worn wear scars were measured with Olympus optical DSX-CB microscope and Zeiss Ultra-55 SEM with 20 kV acceleration voltage (Olympus, Japan).

TGA analysis
TGA was used to confirm the modification of pristine h-BN, f-CuO, PDA-BNNS and CuO@BNNS. As plotted in Fig. 3, there is no distinctive weight loss of pristine h-BN and BNNS under N 2 atmosphere, which correspond to its good heat-resistance properties. However, PDA-BNNS and CuO@ BNNS present weight loss about 5% and 9% under 900 ℃. According to the TGA curve of PDA-BNNS, the weight loss of 5% can be ascribed to the degradation of PDA. In contrast, the mass loss of CuO@BNNS is higher than PDA-BNNS, due to the decomposition of coupling agent KH550 grafted on nano f-CuO at about 300 ℃. Thus, the modification of CuO nano particle with KH550 and the successful preparation of hybridization of CuO@BNNS are confirmed, combined with the FT-IR spectrum.

XRD analysis
X-ray diffraction (XRD) curves of the pristine h-BN, f-CuO, PDA-BNNS and CuO@BNNS composites are plotted in Fig. 4. In order to analysis the variation, the intensity of the Y value is normalized. As depicted in Fig. 4

SEM analysis
Morphological characteristics of the prepared hybrid composite were characterized by SEM. As depicted in Fig. 5, the bulk CuO is consist of many nanoparticles with diameter obout 40 nm. And the morphology of h-BN displays lumpy and multi-layer stacking structure. Compared to h-BN drawn in Fig. 5c, d, BNNS shows few-layered structure in Fig. 5f. This can be ascribed to the effect of exfoliation during ultrasonic treatment. And the exfoliation happens along (002) face, based on the XRD analysis. According to images of CuO@BNNS in Fig. 5g, h, the rough surface of BNNS exactly illustrated the nanoparticle decoration on BNNS. To further identify the homogeneity of nanocomposite, element distribution of composite analysis was carried out through EDS mapping image corresponding to the area in red rectangle, as presented in Fig. 6. The element dispersion results show that copper, oxygen, boron and nitrogen elements evenly distributed in the composite, which demonstrated that the homogenous nano CuO@BNNS composites are obtained.

Dispersibility and tribological performance of oil
The lubricating oil is fabricated with 0.05% additives and 99.5% amount of paraffin liquid in ultrasound bath. Preventing from agglomeration of additives, the temperature of ultrasound bath was controlled below 20 ℃. Before the tribological test, all lubricating suspensions based on paraffin liquid with 0.05% additives were sonicated for 30 min. As depicted in Fig. 7, the additives can disperse well in liquid paraffin after sonication. And there is no obvious sedimentation in 3 days. The obvious sedimentation occurs after 5 days. And all the oil become transparent after 7 days. The tribological tests were conducted according to the conditons mentioned in 2.6 (tribological property test). Figure 8 presents the average coefficient of friction and diameter of wear scar under different conditions. And Fig. 9 depicts the friction coefficient curves with the increasing of sliding time. As shown in Fig. 8, the friction coefficients climb up with the increase of load for each kind of lubricant oil system. Meantime, the oil containing CuO@BNNS presents the lowest friction coefficient, varying from 0.017 to 0.050. And friction coefficient displays more stable with small fluctuations as increasing sliding time for oil with CuO@BNNS, as presented in Fig. 9. By contrast, paraffin liquid without any additive shows the highest friction coefficient, varying from 0.038 to 0.083. In addition, friction coefficient of oil containing PDA-BNNS shows lower value than paraffin liquid, followed by that of f-CuO. Figure 8 presents the average diameter of wear scar on the steel ball of different systems for reflection of wear rate. As expected, oil containing additive can work on friction coefficient reduction and anti-wear property. Oil containing CuO@BNNS exhibits the lowest average friction coefficient and diameter of wear spot on the steel ball, followed by f-CuO and PDA-BNNS. Statistically, compared with paraffin liquid, the average friction coefficient of oil containing CuO@BNNS, f-CuO and PDA-BNNS were reduced by 53%, 31% and 7% respectively under applied load of 100 N, reduced by 43%, 23% and 12% respectively under applied load of 200 N, reduced by 43%, 29% and 27% respectively under applied load of 300 N and reduced by 39%, 23% and 23% respectively under applied load of 392 N. And the lubricant abilities of PAD-BNNS can result in the few-layered structure exfoliated by sonification and functionalization of BNNS nanosheets.
As illustrated in Fig. 8, the diameter of wear spot becomes larger, as the increase of load, for different lubricant oils. The scar diameter of paraffin liquid shows the highest diameter value under different load condition, followed by PDA-BNNS and f-CuO according to the decreasing order. In summary, contrast to paraffin liquid, oil containing CuO@BNNS presents the best antiwear properties with the smallest scar diameter on the steel ball. Meanwhile, the optical graphs of wear scar are presented in Fig. 1s (additional data are given in Online Resource ESM_1). On the whole, as the load increasing, the worn surfaces gradually become rougher. And the wear scars diameter become larger. For paraffin liquid, the clear rises and falls can be found on the ball under the condition of load 392 N, compared to the morphologies under other lower load. For oil containing PDA-BNNS or f-CuO, the worn surface appears lack of smoothness especially under higher load. The distinctive grooves in Fig. 1s can be easily distinguished under loads of 200 N, 300 N and 392 N, combined with the higher resolution image in Fig. 10. By contrast, the smooth worn surface of oil containing CuO@ BNNS demonstrates the better anti-wear property. It is clear that worn surface of paraffin liquid presents lots of grooves and deep scrapes on the ball, as shown in Fig. 10. By comparison, oils containing additives present relatively smooth worn surface, especially for oil containing CuO@ BNNS. From the worn surface in Fig. 10d, the worn surface of oil containing CuO@BNNS presents more smoother   Fig. 11, it is clear that the worn surface of oil containing CuO@BNNS present smaller and more smooth morphology, which is corresponding to the excellent friction reduction and antiwear effect of CuO@BNNS.

Surface analysis
SEM pictures of worn surfaces after tribological test for these four kinds of oil are presented in Fig. 12. It is clear that worn surfaces of the four kinds of lubricant oil demonstrate different surface morphologies. As shown in Fig. 12a, there are remarkable lumpy transfer film and deep grooves on the surface for paraffin liquid. By contrast, the worn   In order to analyze the synergetic effect and lubricant mechanism of oil with CuO@BNNS, the distribution of different elements on the worn surface were detected. The selected area was marked with rectangle shape, as shown in Fig. 12 D. As depicted in Fig. 13, the EDS mapping result implies that the element distribution of copper and boron demonstrate uniform on the worn surface, which exactly accounts for the smooth worn surface. And this also indicates that the nano f-CuO and PDA-BNNS play an important role on the tribological properties, during sliding process.
It is well known that the nanoparticle dispersibility in oil plays a great role on tribological performance. During sliding progress, nanoparticle dispersed in oil penetrates into the friction interface, which can improve the lubrication and anti-wear performance of lubricant oil. To further explore the lubricant mechanism, the schematic diagram of friction process is depicted in Fig. 14. During this test, the well-dispersed CuO@BNNS can easily penetrate into the sliding interface as illustrated in Fig. 14. In the sliding interface, the combined nanocomposites can effectively act as lubricant additive. By comparison, the rough worn surface is obtained for pure oil, as presented in Fig. 12a. Due to the easy shear property of PDA-BNNS, a lower coefficient of friction displays in the test as displayed in Figs. 8 and 12. With the friction proceeding, the CuO@BNN in the oil will fill up the grooves on the sliding interface due to the physical adsorption [34]. And then the nanocomposites act effectively. The nano BNNS arrays under the shear force along the friction interface. Thus, the friction pairs are separated by nano composites from direct contact during the friction process. Meanwhile, nano f-CuO can act as tiny bearing proposed by wang' group between contact interfaces [35]. Therefore, CuO@BNNS plays a positive role on friction reduction and anti-wear, as illustrated in the SEM images. Simultaneously, the uniformly dispersed f-CuO nanoparticle can enhance the load carrying capacity of PDA-BNNS. In summary, the two aspect endow CuO@ BNNS the significantly synergistic effect on tribological performance between f-CuO and PDA-BNNS.

Conclusion
Compared to the study on the effect of CuO, h-BN or BNNS on tribological property respectively [12,13,[27][28][29], this paper aims at investigating the tribological performance of cooperation effect of f-CuO and PDA-BNNS used as lubrication oil additive. A synthesis route of CuO@BNNS was proposed, in this paper. From the tribological results, it is can be concluded that the synergistic effect happened during sliding, contrast to f-CuO or PDA-BNNS, respectively. The tribo-film present smooth morphology and uniform element distribution on the worn surface. And these conclusions can be drawn as following. Compared to h-BN, PDA-BNNS presents better dispersion property in paraffin oil, which plays important role on the tribological property. Oil containing with PDA-BNNS can effectively enhance the property of friction reduction. However, the worn surface still demonstrates deep grooves. For oil containing with f-CuO, the abrasion diameter and friction coefficient decreased compared to paraffin liquid. However, there exists distinctive aggregation on the worn surface. Due to the synergistic effect of f-CuO and PDA-BNNS, oil containing with CuO@BNNS presents outstanding anti-wear and friction reduction properties. The worn surface presents smooth morphology. Meantime, distribution of Cu, O, B and N on the worn surface are really uniform, which results in the above excellent tribological performance. And the lubrication mechanism was proposed to illustrate the excellent tribological process. In summary, the CuO@BNNS nanocomposite can be an appropriate candidate as an effective lubricating oil additive with good anti-friction and anti-wear properties in tribological applications. However, the dispersibility Data availability statements All data generated or analyzed during this study are included in this published article and its supplementary information files. The data used to support the findings of this study are available from the corresponding author upon request.

Conflict of interest
The authors declare no conflict of interest.

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Ethical approval This article does not contain any studies with human participants performed by any of the authors.
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