Ball-Milling of Hexagonal Boron Nitride Microflakes in Ammonia

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Ball-Milling of Hexagonal Boron Nitride Microflakes in Ammonia Fluoride Solution Gives Fluorinated Nanosheets that Serve as Effective Water-Dispersible Lubricant Additives Yongqing Bai, Jing Zhang, Yongfu Wang, Zhongyue Cao, Lulu An, Bin Zhang, Yuanlie Yu, Junyan Zhang, and Chun-Ming Wang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00502 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 22, 2019

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ACS Applied Nano Materials

Ball-Milling of Hexagonal Boron Nitride Microflakes in Ammonia Fluoride Solution Gives Fluorinated Nanosheets that Serve as Effective Water-Dispersible Lubricant Additives Yongqing Bai,

†, ‡, #

Jing Zhang,

†, §, #

Yongfu Wang,

†

Zhongyue Cao,

†

Lulu An,

†

Bin Zhang,

†

Yuanlie Yu, *, †, ‡ Junyan Zhang, *, †, ‡ Chunming Wang*, § †

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,

Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China. ‡

Center of Materials Science and Optoelectronics Engineering ， University of

Chinese Academy of Sciences, Beijing 100049, People’s Republic of China. § School

of Physics, State Key Laboratory of Crystal Materials, Shandong University,

Jinan 250100, People’s Republic of China. #

The authors have the equal contribution to this work.

*Corresponding

author

email:

[email protected],

[email protected],

[email protected].

ABSTRACT: Herein, we demonstrate that hexagonal boron nitride nanosheets (h-BNNSs) can be used as water dispersible lubricant additive through simple fluoride modification. The fluorination of h-BNNSs (F-BNNSs) is carried out via a facile ball milling of commercial h-BN microflakes in ammonia fluoride solution. The as-obtained F-BNNSs exhibit excellent antifriction and antiwear performance as water dispersible lubricant additive with low concentration of less than 1.0 mg/mL, and their friction coefficients can be lower than 0.08, achieving low friction. This low

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friction originates because F-BNNSs at a sliding interface roll up to form nanoscrolls induced by the fluoride doping curling effect and subsequent shearing force. These resulted

nanoscrolls

can

slide

against

the

substrate,

achieving

a

rolling/incommensurate contact, thus substantially reducing friction coefficients. The overall lubricant mechanism is further elucidated based on the first principles simulations. This finding indicates the potential of achieving low even ultralow friction of h-BNNSs as water dispersible lubricant additives through composition and structure designing. Keywords: Fluoride modification, hexagonal boron nitride, ball milling, nanoscrolls, rolling/incommensurate contact, low friction.

1. INTRODUCTION Friction and wear are inevitable and important phenomena in human’s daily life, for example, sliding, rolling or rotating contact interface in every manmade, natural or biological system can generate friction. High friction and wear will lead to severe mechanical component failure and massive energy loss. According to the statistics, friction and wear result in more than 30% of energy consumption and 80% of mechanical part failure

1-2.

Therefore, it is great significance to improve the

efficiency, reliability and durability of mechanical components by reducing friction and wear. Currently, liquid lubricant lubrication is one of the most commonly used solving methods. However, most of the liquid lubricants are oils, which will cause severe environmental and ecological damage when directly discharged or spilled, because the oils are flammable and can decompose to other harmful chemicals,


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further polluting natural environment and threatening aqueous habitats and human health

3-4.

Apart from this, the dissipation or spillage of the oil is also a waste of

energy, especially the shortage of energy has becoming the worldwide problem currently. Therefore, with the increasing concerns of environmental and energy problems, exploring new alternative green liquid lubricants have become the focus of researches. Low cost, good biocompatibility and environmental friendliness make water to be the best alternative liquid lubricant. But, low viscosity, severe chemical corrosion and high friction and wear have greatly restricted the use of water as lubricant. Hence, developing high-performance water dispersible lubricant additives, such as graphene, graphene oxide, carbon nanotube, diamond particle and fullerenes 5-14,

becomes an attractive way to achieve widely use of water as lubricants. Hexagonal boron nitride nanosheets (h-BNNSs), one of typical two-dimensional

materials, possess high thermal conductivity, outstanding mechanical strength and good lubricant properties as graphene

15-17.

Moreover, different from graphene,

h-BNNSs also exhibit good biocompatibility, excellent electrical insulation, remarkable oxidation resistance and high chemical stability, making them an ideal candidate for water base lubricant additive 18-21. However, compared to graphene, the interlayers among h-BNNSs have extremely strong polarities besides weak Van der Waals forces owing to the difference in the electronegativity of boron and nitrogen atoms

21.

This phenomenon can suppress the interlayer slippage and weaken the

lubricant properties of h-BNNSs as lubricant additives. As a result, the lubricant property of h-BNNSs simply based on the interlayer slipping is hardly superior to



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graphene 22-24. Besides, due to strong chemical stability，the dispersibility of h-BNNSs in solvent is extremely poor 25, which will result in agglomeration and highly hinder their antifriction effect. Therefore, finding an effective way to adjust the interaction between adjacent h-BNNSs layers and improve h-BNNSs dispersibility in water has become the top priority issue to use them as water dispersible lubricant additives. Additionally, recent studies have shown the introduction of rolling/incommensurate contact is also an effective method to reduce friction and wear

2, 26,

implying that the

nanostructure transformation of h-BNNSs during friction process may also be a possible way to achieve low friction. Based on the above discussion, fluoride modified h-BNNSs (F-BNNSs) are selected as water-based additives to achieve low friction. In one respect, fluoride modification can reduce the surface polarity of h-BNNSs and inhibits their agglomeration in solvents 27-28. On the other hand, fluoride modification can promote boron nitride bonding mode to convert from sp2 to sp3, thus providing a preliminary transformation of boron nitride nanostructures 29. Similar to the development trend of graphene, the preparation processes of h-BNNSs are also gradually diversified30-37. Among them, the ball milling preparation process is one of the commonly used ones. In 2011, Li et al20. prepared high-quality h-BNNSs by ball milling for the first time and preliminarily clarified the exfoliting mechanism. Subsequent the ball milling process for preparation of h-BNNSs has being widely used. Lei et al38. prepared h-BNNSs modified with ammonium ion by using urea-assisted ball milling and achieved the preparation of boron nitride ultra-light aerogel and freestanding


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membranes. Lee et al39. prepared hydroxyl-modified h-BNNSs using sodium hydroxide assisted ball milling achieving excellent dispersion of h-BNNSs in various solutions. Recently, Ding et al40. also provided a high-yield process for preparing h-BNNSs by 2-furoic acid assisted ball milling. The as-produced h-BNNSs possess an extremely high production yield (~98%) and exhibited outstanding aqueous compatibility with a concentration of 35 mg/mL-1, which can be stored in the form of a concentrated slurry for months without the risk of re-stacking. Herein, F-BNNSs are fabricated via a facile ball milling of commercial h-BN microflakes in ammonia fluoride solution to explore their antifriction properties as water dispersible lubricant additive in current research. Furthermore, the overall lubricant mechanism is proposed based on the characterization and analysis of the wear tracks, debris as well as the first principles simulations. This research indicates the potential of achieving low even ultralow friction of h-BNNSs as water dispersible lubricant additives through composition and structure designing.

2. EXPERIMENTAL SECTION 2.1. Preparation of F-BNNSs

An optimized ball milling (QM-QX-2, MITR, China) process was employed to produce F-BNNSs using commercial h-BN powders with average size of 30 um and ammonium fluoride as precursors. Typically, 1.0 g of h-BN powders, 1.0 g of ammonium fluoride crystals and 50.0 mL of water were firstly mixed together in a



plastic beaker. Then the mixture was sonicated in an ultrasonic bath for 15 minutes to make it thoroughly mixed. After ultrasonication, the mixture and zirconia balls (diameter: 10/8/5 mm = 50/50/50 g) were added into a 250 mL zirconia vial with a ball-to-powder weight ratio of 150: 1. The speed of rotation of ball milling was 300 rpm, and the ball milling process was continued for 10 hours. When the ball milling process was terminated, the product was washed with deionized water repeatedly until the pH of the supernatant was close to neutral. Finally, the products centrifuged between 3000-6000 rpm was extracted and redispersed in water for further friction and wear performance evaluation. 2.2. Chemical and structural characterization The structure of F-BNNSs and raw h-BN powder were examined using X-ray diffraction (XRD) (X’PERT PRO, 40 kV/20 mA, Cu Kα radiation, λ=1.5406 nm). The morphology of the h-BN powder, F-BNNSs, wear debris and wear scar were characterized by field emission scanning electron microscopy (FESEM, JSM-6701F), transmission electron microscope (TEM, FEI Tecnai-G2-F30) and atomic force microscope (AFM, Smart-SPM) respectively. The chemical states of F-BNNSs were investigated by X-ray photoelectron spectroscopy (XPS, PHI-570), and fourier transform infrared spectroscopy (FTIR, Nexus 870). The wear scar and its cross-sectional profiles were detected through a Micro-XAM 3D surface profiler (GBS Smart WLI, GER). FTIR of all samples was recorded using Nexus 870 with a resolution of 4 cm-1.


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2.3. Friction and wear performance measurements The tribological experiments were performed using a ball-on-disk standard tribometer (UMT-2 CETR) operating in bidirectional modes as shown in Figure S1. In standard tribometer experiments, Si3N4 balls (Ф = 6 mm) were used as sliding materials against the silicon wafers (100). Before tests, all balls and silicon wafers were cleaned in alcohol in an ultrasonic bath for 15 min and dried with nitrogen gas. 2.4. Calculation details: To further explain the mechanism of low friction, the first principles calculations were carried out using DMol3 of Material Studio based on density functional theory (DFT).41-42 Because the interaction force between inter-planar is weak van der Waals force, so it was not considered in this report. The calculations were performed in a Fast-Fourier-Transform grid, and the models were fully optimized in the given symmetry using the generalized gradient approximation (GGA)43 treated by the Perdew−Burke−Ernzerhof exchange-correlation potential with long-range dispersion correction via Grimme’s scheme.44 In additional an all-electron double numerical atomic orbital augmented by d-polarization functions (DNPs) was used as the basis set. In the first place, a crystal structure of h-BN was built. The number of the space group was 194 and lattice parameters were a = 2.504 Å, b = 2.504 Å, and c = 6.652 Å. 45-46

The fractional coordinates of the boron and nitrogen atoms were (0.3333, 0.6667,

0.25) and (0, 0, 0.25), respectively. Then, geometry optimization was taken for the crystal structure to obtain the most stable structure in the local area. In the process of geometry optimization, the self-consistent field (SCF) procedure was used with a



Page 8 of 42

convergence threshold of 10−6 au on the energy and electron density. And, the direct inversion of the iterative subspace technique developed by Pulay was used with a subspace size of 6 to speed up SCF convergence on these systems.47 Geometry optimizations were performed with convergence thresholds of 0.002 au/Å on the gradient, 0.005 Å on displacements, and 10−5 au on the energy, respectively. The real-space global cutoff radius was set as 4.10 Å; the unit cells for the BN sheet were set as 4 × 4; and the vacuum between sheets was chosen with 15 Å to avoid interactions between periodic images. After the convergence test, the Brillouin zone was sampled by using the Monkhorst−Pack scheme with 9 × 9 × 1 k-points.

3. RESULTS AND DISCUSSION F-BNNSs were fabricated via one-step ball milling of commercial h-BN powders in ammonium fluoride solution as demonstrated in Figure 1. Ammonium fluoride solution was selected as the exfoliation reagent due to its contribution of F- and NH4+ which are advantageous for the exfoliation and modification of h-BN

29.

Few-layer

h-BNNSs will be exfoliated by the shearing force generated by the relative motion of zirconia balls under the assistance of F- and NH4+ during ball milling process. The Fwill interact with B-N bonds to form new B-F and N-F bonds to drive the transformation of BN bond from sp2 to sp3, which will cause B atoms extrude out from the basal plane of h-BN. Simultaneously, the ammonium radicals (NH4+) can easily to intercalate h-BN with buckling tendency, and finally achieve the exfoliating of h-BN under ball milling. After ball milling, washing and centrifuging, few-layer F-BNNSs can be collected. These as-obtained few-layer F-BNNSs can be steadily


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dispersed in water with different concentrations and the path of a green laser beam can be clearly seen through all the dispersions due to light scattering effect by few-layer F-BNNSs (Tyndall effect, Figure S2). More importantly, the yield of the exfoliated F-BNNSs in this proposed ammonium fluoride assisted ball milling process can reach up to about 1.5% and the as-obtained F-BNNSs suspensions have long-term dispersion stability in water which can be stored for more than 30 days (Figure S3).

Figure 1. Schematic representation for exfoliation and fluorination of h-BN. Figure 2a shows the XRD patterns of h-BN powders and the as-obtained F-BNNSs. Compared the XRD pattern of the as-obtained products with that of raw h-BN powders, the peaks originating from (100), (101), (102), and (110) planes are disappeared, revealing the decrease of the thickness of h-BN. In addition, the relative intensity of the peaks from (002) and (004) planes slightly decreases with inconspicuous broadening, indicating that the exfoliation and functionalization cause the preferential orientation of h-BNNSs and only have a slight impact on the in-plane structure of h-BN 48. Moreover, all the XRD peaks of F-BNNSs slightly move to low angle zone implying the slight boarding of layer spacing 49 Figure 2b shows the FTIR transmittance spectra of h-BN powders, F-BNNSs and NH4F. The FTIR spectra of F-BNNSs and h-BN powders show two strong and sharp absorption peaks at 1374



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and 811 cm-1 attributed to the in-plane B-N stretching mode and B-N-B out-of-plane bending mode, respectively 50-51. Additionally, two weak peaks at 1078 and 1270 cm-1 can be observed in the FTIR spectrum of F-BNNSs, which correspond to B-F vibrational features, indicating the existence of fluoride doped h-BNNSs 29. However, N-F bond vibrations at about 640 cm-1 cannot be found in the FTIR spectrum of F-BNNSs which should be overlapped with the B-N-B out-of-plane bending vibrations 52. Figure 2c-f show the XPS spectra of F-BNNSs and h-BN powders. Obviously, the peak of F can be observed from the survey XPS spectrum of F-BNNSs as shown in Figure 2c. Figure 2d shows the B1s fine spectrum of the as-obtained sample which can be fitted into three peaks at 190.3, 191.0 and 192.0 eV, respectively arising from B-N, B-F and B-O bonds

23, 52-53.

These bonding energies for B-N and B-F bonds are

consistent with those reported for F-BNNSs prepared by hydrothermal reaction

52-53.

The B-O bonds can be ascribed to hydroxyl groups which are introduced during the exfoliation process 54, or the contamination due to the exposure of the sample in air 55. Figure 2e exhibits the N1s fine spectrum of the as-obtained sample which can be deduced into two peaks at 397.9 and 398.7 eV. The main peaks centered at 397.9 eV can be assigned to N-B bonds, corresponding to the B-N bonds in B1s zone, while the other peak centered at 398.7 eV can be ascribed to N-F bonds 52-53. The fine spectrum of F1s was displayed in Figure 2f, which is located at 686.8 eV

29,

indicating the

existence of fluoride in the as-obtained sample. In addition, the appearance of C1s peaks in both survey XPS spectra (Figure 2c) are associated with the carbon coating


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used as a bonding reference for XPS analysis. Based on the above analysis, it can be concluded that fluorine has been successfully modified onto the surface/edge of h-BNNSs.

Figure 2. (a) XRD spectra of original h-BN powders and the as-obtained F-BNNSs, (b) FTIR transmittance spectra of original h-BN powders, F-BNNSs and NH4F, (c) XPS survey spectrum of F-BNNSs and original h-BN powders, (d) Fitted B1s XPS spectrum of F-BNNSs, (e) Fitted N1s XPS spectrum of F-BNNSs, and (f) F1s XPS spectrum of F-BNNSs. Figure 3a and b show the FESEM images of original h-BN powders and F-BNNSs, respectively. The h-BN powders possess round structures with lateral size in the range of 5-30 um and thickness in the range of 1-3 um (Figure 3a). After ball milling, both the lateral size and thickness of the as-obtained F-BNNSs obviously reduced as demonstrated in Figure 3b. Figure 3c shows the low magnification TEM image of the as-obtained F-BNNSs, in which several thin nanosheets can be clearly seen. The edges of the F-BNNSs are obviously curled due to the modification of fluoride, coinciding with the phenomenon observed in previous research 29. The reason for the curling of F-BNNSs edges will be further discussed combining with the first principles simulations result later. The corresponding selected area electron diffraction (SAED) pattern inserted in Figure 3c exhibits a typical six-fold symmetry feature of h-BN, demonstrating that the as-obtained F-BNNSs are well-crystallized



and not damaged during the ball milling process. The high-resolution TEM (HRTEM) image (Figure 3d) collected from the curling edge of F-BNNSs marked in Figure 3c was adopted to further analyze the F-BNNSs structure proving that the thickness of F-BNNSs is about 2 nm with five layers. The lattice distance between adjacent fringes is about 0.34 nm (inset in Figure 3d), which is typical for (002) crystal plane of h-BN. This result also implies that the F-BNNSs are mainly peeled off along the (002) plane of bulk h-BN. Furthermore, the thickness of the F-BNNSs were also investigated by AFM analysis as shown in Figure 3e and f. Obviously, the flat nanosheets (Figure 3e) with thickness of about 1.42 nm (Figure 3f) can be found. This result is similar to that observed in the HRTEM image (Figure 3c).

Figure 3. (a) SEM image of original h-BN powders, (b) SEM image of F-BNNSs, (c) Low-magnification TEM image of F-BNNSs and corresponding SAED pattern, showing the hexagonal structure of h-BNNSs, (d) High-resolution TEM image of the region marked in (c) with a red square, consisting of five atomic layers, (e) AFM image, and (f) corresponding height profile of F-BNNSs. The friction and wear properties of F-BNNSs were investigated by using F-BNNSs as water dispersible lubricant additives. For comparison, the friction and wear performance tests were also conducted in both water and oil (PAO10 mineral oil). Figure 4a and b show the changes of the friction coefficients with the increase of


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friction time and average friction coefficients for different lubricants. Obviously, the water without F-BNNSs exhibits a high friction coefficient of about 0.245±0.021, and the friction coefficients are significantly reduced when very few F-BNNSs are added as lubricant additive as shown in Figure 4a, b and Figure S4. The lowest friction coefficient cab be reached at a concentration of 0.5 mg/mL with lowest friction coefficient of 0.08 ± 0.009 (Figure S4), which is even smaller than the friction coefficients of PAO10 (0.155±0.004). Figure 4c-d show the sectional profile of wear scars and corresponding wear rates with water, PAO10 and 0.5 mg/mL F-BNNSs solution as lubricant, respectively. Compared with the pure water, the sectional profile of wear scars of 0.5 mg/mL F-BNNSs solution remarkably reduced, even lower than that of PAO10 as lubricant as shown in Figure 4c and Figure S5. Moreover, the wear rate achieves a reduction in magnitude, decreasing to (2.70 ± 0.42)  10-6 mm3/Nm from (4.2 ± 0.36)  10-5 mm3/Nm of pure water, and this value is even lower than that ((5.2 ± 0.12)  10-6 mm3/Nm) of PAO10 as lubricant. In addition, more detailed properties of friction of F-BNNSs based on changes in frequency, load and dual materials are shown in Figure S6 and S7. And h-BNNSs without fluoride modification were also tested for comparative friction performance (Figure S8). These results demonstrate that F-BNNSs can effectively reduce the friction coefficients and wear rates as water lubricant additive.



Figure 4. (a) The dependency of friction coefficients under different lubricant conditions on the friction time, (b) The average friction coefficients with the respect to different lubricant conditions. (c) Wear scar profile under different lubricant conditions, and (d) wear rates under different lubricant conditions. Each bar is represented by an average value with standard deviation for 5 tests. For further study the lubricant mechanism of low friction of F-BNNSs, the wear scar and wear debris of F-BNNSs with concentration of 0.5 mg/mL were studied. Figure 5a shows the FESEM image of a sliding surface, in which a wear scar with width of about 180 um can be observed. Figure 5b and c are the magnified FESEM images marked in Figure 5a and b with red square. A large amount of rod-like boron nitride nano-structures (RBNs) can be observed inside the wear scar as shown in Figure 5c marked with red arrow. At the same time, the nanosheets that have not yet been curled are shown in Figure 5c with blue arrow. This indicates that the edge curling F-BNNSs have been curled into a RBNs during the friction process.


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Combining with the previous research

22,

we can draw a more in-depth conclusion

that the formation of these RBNs plays a decisive role in achieving low friction of F-BNNSs.

Figure 5. (a) The SEM image of sliding surface with 0.5 mg/mL F-BNNSs as dispersible water lubricant additive, (b) and (c) are the enlarged region in 5a and 5b marked with red square. The arrows in c represent different degrees of curling of F-BNNSs. The red arrows represent the completion of the curl to form RBNs and the blue arrows indicate that the curl has just begun, and the curling has not yet completed to form RBNs. To further explore the lubricant mechanism of F-BNNSs as water lubricant additive, the wear debris was evaluated by the TEM and HRTEM analysis as shown in Figure 6. A large amount of RBNs can also be observed in the low magnification TEM image (Figure 6a). The length of these RBNs varies from tens of nanometers to a few hundred nanometers and the width ranges from several nanometers to tens of nanometers. The enlarged TEM image of Figure 6b (marked with red square in Figure 6a) shows that the RBNs have regular cylindrical structure, and the corresponding SAED pattern (inset in Figure 6b) in the same region presenting a mix of ring and dot



pattern. This indicates that both regular and irregular structures are contained in the as-formed RBNs. The HRTEM (Figure 6c, collected from red square in figure 6b) image demonstrates that the RBNs are formed by the curling of several layers of h-BNNSs (marked with red line), confirming that the RBNs are folded up by the few layers h-BNNSs, that can be named as boron nitride nanoscrolls (BNNSRs). In addition, it has also provided an evidence to support the SAED pattern that regularly layered structure of BNNSRs results in the appearance of diffraction points while lattice mismatch induced by curling leads to the formation of diffraction ring. More wear debris collected from different friction stages have also been studied in detail as shown in Figure 6d-f. Consistent with the above discussion, curling and rolling process at different evolution stage have been vividly found as shown in Figure 6d to f. As shown in Figure 6d, the F-BNNSs with wrinkled edge begins to curl under shearing force, then partly rolling up (Figure 6e) and eventually forms BNNSRs (Figure 6f).

Figure 6. (a) Low-magnification TEM image of wear debris, (b) the enlarged region


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marked with red square in a and the corresponding SAED pattern in the same region, (c) the HRTEM of the region marked in b with red square (the red line marked the nano-rolled structure rolled up by several layers of nanosheets), and (d) to (f) the low-magnification TEM images of wear debris collected from different friction process showing curling edges, medium scroll and nearly completed scroll of F-BNNSs, respectively. To completely elucidate the overall lubricant mechanism, the first principles simulations was performed. Figure 7a shows the structure optimization diagram of the h-BNNSs and the as-obtained F-BNNSs. For h-BNNSs (Figure 7ai), the optimal distance between two surfaces is 3.1Å and their adjacent layers are parallel. After the introduction of the F on the upper surfaces (7aii-aiv), the different degrees of bending have occurred. When the doping ratio reaches 6.25 at% (7aii), the surface shows the significant wrinkling and curling. With the increasing of the doping ratio (12.5at%, 7aiii), the tortuosity and the inter-layer spacing further increase, which indicates that a higher doping ratio is more conducive to the generation of bending. As the doping ratio increases further (18.75 at%, 7aiv), the degree of curling becomes more pronounced. However, compared to the doping ratio of 12.5 at%, the layer spacing of 18.75 at% is reduced slightly, which may be caused by severe structure distortion of the h-BNNSs caused by excessively high doping ratio. The doping ratio in the actual experimental doping is low generally (

Ball-Milling of Hexagonal Boron Nitride Microflakes in Ammonia

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