Superlubricity of black phosphorus as lubricant additive - ACS Applied

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Superlubricity of black phosphorus as lubricant additive Wei WANG, Guoxin Xie, and Jianbin Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14730 • Publication Date (Web): 13 Nov 2018 Downloaded from http://pubs.acs.org on November 16, 2018

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ACS Applied Materials & Interfaces

Superlubricity of black phosphorus as lubricant additive Wei WANG 1,2 , Guoxin XIE 1*, Jianbin LUO 1* 1 2

State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China

School of Metallurgy Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China; *Corresponding

author: [email protected] (Jianbin LUO), [email protected] (Guoxin XIE)

Abstract: Superlubricity is defined as a sliding regime in which friction, or the resistance to sliding of two relatively moving surfaces, almost vanishes. From a practical point of view, the development and use of new materials that can enable superlubricity (coefficient of friction, COF99.999%) are provided by Aladdin Ltd. (Shanghai, China). BP modified by sodium hydroxide (BP-OH) powders was synthesized by HEBM technique. BP powders (1g) and NaOH powders (4g) as the starting materials were used. The ratio of ball and the powder is 30:1 by weight. The diameters of the ball are Ø10 mm and Ø5 mm. The HEBM process was conducted under an Ar atmosphere at ambient temperature for total 20 hours with the rotation speed of 800 rpm and every 120 min milling time with 30 min interval. After ball-milling, the jar was cooled sufficiently at room temperature in a glovebox filled with Ar and vented, then the capsule lid was opened carafully in air and violent sparkling was observed (caution!). BP-OH powders were dispersed in aqueous solution, and then ultrasonicated using an ultrasonic bath of 400 W for 48 h, followed by centrifugation at 4000 rpm for 10 min. The supernatant of BP-OH dispersion is identified as few-layer BP-OH nanosheets. Friction measurements of few-layer BP-OH nanosheets as water-based lubricant additives: Macroscopic friction measurements were performed using a ball-on-plate tribo-tester (UMT-5, CETR Corporation Ltd, USA). The upper sample of the friction pair was a silicon nitride (Si3N4) ball of Ø4 mm or Ø10 mm. The surface roughness Sq of these balls was ~53.476 nm. The lower sample of the friction pair was a silicon dioxide (SiO2) plate having a surface roughness Sq ~0.04 µm.

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Lubricants were introduced into the gap between the ball and the plate prior to the friction tests. The loads applied to the plate were in the range 0.5~4 N and the corresponding maximum pressures were 489~977 MPa. The rotational speeds of the plate were in the range 0~900 rpm with a track radius of 3 mm. The corresponding sliding velocities were in the range 0~282 mm/s. Unless otherwise stated, the contact pressure was 836 MPa (3 N), and the sliding velocity was 56 mm/s. All the tests were performed at ambient conditions with a temperature of 25°C and a relative humidity of 45~55%. Characterization: X-ray diffract meter (XRD; Shimadzu XRD-6000, CuKα source), scanning electron microscope (SEM) and transmission electron microscopy (TEM, JEM-2010HT and JEM-2010FEF) were used for evaluating the phase structure, morphology and chemical composition. The stability of the BP nanosheets was evaluated by thermogravimetric analysis (TGA, Thermo plus EV02 TG8121; Rigaku Corp.) in Ar gas atmosphere with a heating rate of 10 °C/min. A Raman spectrophotometer (Lab-Ram HR-800; Horiba Jobin Yvon, Inc.) was used to identify the structural units in the BP powder. X-ray photoelectron spectroscopy (XPS) analysis (K-Alpha; Thermo Fisher Scientific Inc.) was used by using monochromatic Al-Kα radiation (1486.6 eV) as the X-ray source. An optical microscope (Olympus BX60), SEM (QUANTA 200 FEG), and a white light interferometer (MICROXAM-3D) were evaluated for the surface state of the samples after friction test. XPS and FTIR techniques were used to analyze the chemical properties of the surfaces before and after the friction tests. 3. Results and Discussion Fig. 1 illustrates the preparation process of BP-OH as water-based dispersion. The initial BP powders were prepared by the reported method by literature 13. The XRD patterns, Raman spectra and TEM images of BP powders at ball milling (BM) for 20h are shown in Figure 2 (a)-(c). After BM, red phosphorus for 20 h [Figure 2 (a)], the diffraction peaks of RP disappear and instead distinct diffraction peaks appear at 2θ=25°, 35° and 56°, being consistent with those of standard orthorhombic BP (JCPDS No.76-1957). It suggests a phase transformation of RP to orthorhombic


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BP in the BM process. Furthermore, from the results of the Raman spectra analysis, the similar phenomenon was happened, the diffraction peaks of RP were not found, while the characters of BP are presented [Figure 2(b)]. The TEM and high-resolution TEM (HRTEM) analyses were carried out to investigate the morphology and the crystalline quality of the as-synthesized BP powders. Figure 2(c) reveals the morphology of BP powders collected as the precipitate at 3000 rpm. The TEM images show that the obtained samples consist of nanosheets with a lateral size of a few micrometers. The selected area electron diffraction (SAED) of the sample shows that the obtained BP powders exhibit an orthorhombic crystal structure. The clear lattice fringes with an interlayer spacing of 0.5 nm are assigned to the (020) plane, and the layer number of the ﬂakes is about 7~9 layers. The lattice fringes with an interlayer spacing of 0.3 nm corresponding to the (110) plane of the BP crystal 14. The modified BP nanosheets by NaOH, i.e., BP-OH nanosheets, were investigated by XPS, as shown in Figure 2(d). There are mainly three different elements in the full spectrum: P, O and Na. As compared with the XPS spectra of the BP powders, a new prominent peak of Na 1s at 1073.2 eV appears after NaOH modification. The details of the O 1s, P 2p and Na 1s peaks are shown in Figure S1, and two components for P-O and Na-O (metal oxide) at 530.3 eV and 535.2 eV can be observed. This, in conjunction with the FTIR results in Figure 2(e), is indicative of the hydroxylation of BP in the presence of alkali. In combination with Figure S1(c) and Fig. 2(f), it indicates that BP powders are also presented in the mixed powders after HEBM. Except these, the residue of NaOH were also founded. This results are corresponding to the XPS results. From the XRD results [Fig. 2(f)], three small peaks where the new sharp diffraction peaks at 2θ = 23.06°, 46.56°, and 68.57° emerge were presented in this Figure. It means that the new metal phosphate Na3P may be presented in the mixed powders. In the mixed



powder, the Na3P crystal phase is not an important phase. When the mixed powders were putted into the ultrapure water, the results of the reaction of Na3P with the ultrapure water are NaOH and H2O. Thus, the Na3P crystal phase is the by-product of this reaction and have a minor role for achieving superlubricity. The TEM and HRTEM analyses of the BP-OH dispersion in Figure S2 show that BP-OH nanosheets (about 100 nm in size) only contain the elements of O, P and Na, and BP crystalline structure is still retained for the nanosheets.

Figure 1 Illustrations of the preparation process of BP-OH as water-based dispersion


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Figure 2 XRD patterns (a), Raman spectra (b) and TEM images (c) of BP powder with HEBM for 30h; (d) XPS full spectra of BP powders before and after NaOH modification; (e) FTIR spectra of BP, BP-OH and NaOH powders; (f) XRD patterns of BP after BM for 20 h and BM with NaOH for 48 h.

In order to evaluate the friction properties of the water-based lubricants with BP nanosheets as lubricate additive, the COFs of ultrapure water (pH: 7.2), BP solution (nanosheets without NaOH modification. pH: 6.5) and BP-OH solution (pH: 7.8) as the lubricants were measured. A 7 wt.% BP-OH solution with a volume of 50 µL was introduced into the gap between the ball and the plate prior to friction tests. As shown in Figure 3(a), the whole friction process could be divided into three stages: The first stage from 0 s to 1000 s is the running-in process in which the COF deceases rapidly from 0.35 to 0.08. The second stage from 1000 s to 3600 s is the process in which the COF reduces slowly from 0.08 to 0.01, entering the superlubricity state. The third stage from 3400 s to the end of the test (6500 s) is the process of stable superlubricity where the COF fluctuates between 0.0006 and 0.003. In contrast, no superlubricity phenomenon could be



observed in pure water or BP solutions as the lubricants. As shown in Figure 3(b), the COF of ultrapure water as the lubricant decreases in the initial 600 s period, followed by an increase from 0.25 to 0.50, and finally increases again due to volatilization of water. In the case of 0.06 wt. % BP dispersion, the COF decrease initially and then increase over time, and the lowest COF is merely 0.05. In the case of 3 wt. % NaOH aqueous solution as the lubricant, the lowest COF (0.027) merely keeps for 900 s, and then increase over time due to the evaporation of water. The 3 wt. % BP-OH dispersion which has same BP concentration with 0.06 wt. % BP dispersion realizes superlubricity. It means that BP nanosheets which were modified by NaOH have an important role to achieve superlubricity phenomenon. It is also found that the decrease of the initially-added lubricant volume from 50 µL to 20 µL could shorten the running-in period before reaching the superlubricity state (Figure S3). In the following section, the 20 µL lubricant volume is considered for more elaborate discussion.

Figure 3 (a) Variation of the COF over time with the 7 wt. % BP-OH solution as the lubricant. The inset is the magnification of the COF after the running-in process; (b) The comparative COFs of four different lubricants; (c) The variation of the COF of the 5 wt. % BP-OH solution as the lubricant with the sliding velocity after entering the superlubricity state; (d) The COFs of the 5 wt.% BP-OH solution as the lubricant at different contact pressures after entering the superlubricity state.


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Superlubricity can be observed for all the BP-OH solutions with different concentrations as the lubricants after the running-in period, as shown in Figure S4. The time of the running-in process decreases with the increasing concentration of BP-OH nanosheets from 1 wt.% to 7 wt.%, and the minimum COF reduces from 0.003 to 0.0006. The superlubricity behavior of the BP-OH solution as the lubricant can be observed for a wide range of sliding velocities, as shown in Figure 3(c). After entering the superlubricity state, the sliding velocity was changed from 56 mm/s to 282 mm/s, and the superlubricity state could still be maintained in this case. Although the COF increases to 0.4 as the velocity was decreased to 0.3 mm/s, superlubricity can be recovered immediately without an obvious running-in process when the velocity was increased back to 38 mm/s (Figure S5), which is the critical velocity for achieving superlubricity. The reversibility of the transition between superlubricity and high friction with the change of velocity has never been observed previously with the ball-on-plate friction tests. Furthermore, superlubricity could be maintained when the maximum contact pressure was changed. As shown in Figure 3(d), the contact pressure was changed from 489 MPa to 977 MPa after reaching the lowest COF (μ=0.0035), and the COF fluctuates between 0.0035 and 0.008. In previous research of our group about superlubricity, the superlubricity was achieved by H3PO4 solution, PEG solution, and so on. However, the superlubricity in my research is different from previous reports, has special benefits. These special benefits are summarized as follows. (1) A lowest COF value of 0.0006 ever measured by an engineering-orientated ball-on-plate tester was achieved, which is 1/5 of the existing results reported in available literature. (2) Comparable ultralow COF results (COF

Superlubricity of black phosphorus as lubricant additive - ACS Applied

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