Composites Communications 28 (2021) 100922

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Composites Communications
journal homepage: www.elsevier.com/locate/coco

Tribological properties of bismaleimide-based self-lubricating composite

enhanced by MoS2 quantum dots/graphene hybrid
Chao Liu a, *, Xi Li b, Yang Lin b, Xin Xue b, Qiming Yuan b, Wenbo Zhang b, Yan Bao c,
Jianzhong Ma c, **
a
Shaanxi Collaborative Innovation Center of Industrial Auxiliary Chemistry and Technology, Shaanxi University of Science &Technology, Xi’an, 710021, China
b
College of Chemistry and Chemical Engineering, Shaanxi University of Science & Technology, Xi’an, 710021, China
c
College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science & Technology, Xi’an, 710021, PR China

A R T I C L E I N F O A B S T R A C T

Keywords: In order to promote the formation of high-quality graphene-based self-lubricating transfer film. The reduced
Molybdenum disulfide quantum dots graphene oxide (RGO) deposited with molybdenum disulfide quantum dots (MoS2QDs) were successfully pre­
Graphene pared. The as-prepared MoS2QDs/RGO hybrid was used for the first time to improve the tribological properties of
Tribological properties
polymer composites. Compared with the neat bismaleimide (BMI) and RGO filled composites, the MoS2QDs/
Self-lubricating transfer film
RGO/BMI showed the best tribological properties, and the frictional coefficient and volume wear rate could be
decreased to 0.15 and 8.9 × 10− 6 mm3/(N⋅m), respectively. This is mainly due to the MoS2QDs loaded on the
graphene surface are beneficial to prevent the graphene nanosheet stacking, and the introduction of MoS2QDs
can make the self-lubricating transfer film formed more uniform, smooth and robust during the friction process.

1. Introduction good dispersion in the polymer matrix [8,9]. Many researches have
proved that the QDs are expected to be excellent lubricating fillers and
Polymeric composites have been widely applied in practical appli­ have potential application value in the field of tribology [10–13].
cations. Nevertheless, their high frictional coefficient and poor wear In this work, the MoS2QDs/RGO hybrid was successfully prepared by
resistance of them greatly limit their application in tribology. Graphene loading zero-dimensional MoS2 quantum dots on two-dimensional
possesses huge specific surface areas, high mechanical strength, and reduced graphene oxide (RGO) nanosheets. Subsequently, the
excellent self-lubricity [1]. The mechanical and tribological properties MoS2QDs/RGO/BMI composites were prepared by introducing the
of the polymer composites can be significantly enhanced by adding only MoS2QDs/RGO hybrid into the bismaleimide (BMI) resin by the casting
tiny amount (≤1.0 wt%) of graphene [2–5]. However, graphene nano­ method. The mechanical and tribological properties of BMI composites
sheets are easy to stack and assiduous to disperse well in the polymer were characterized. Meanwhile, the friction and wear mechanism of
matrix, which extremely limiting its reinforcing and lubricating effect. MoS2QDs/RGO hybrid structure on the tribological performances of BMI
Molybdenum disulphide (MoS2) also shows excellent self-lubricating were systematically researched. Moreover, the results showed that the
performance, it can cause tribochemical reaction with the metal fric­ tribological properties of the BMI composites could be significantly
tion pair, forming a robust self-lubricating transfer film on the surface of improved by introducing MoS2QDs/RGO hybrid. The MoS2QDs were
the friction pair [6,7]. However, no matter graphene or MoS2 nano­ decorated on the RGO can not only prevent the aggregation of graphene
sheets, which are difficult to enter the micro concave region of the metal nanosheets but also can mend the micro concave region of the metal
friction pair due to their large size. Therefore, it is difficult to form friction pair more easily, contributing to the quick formation of high-
high-quality self-lubricating transfer film. quality self-lubricating transfer film.
Quantum dots (QDs) possess small size, high special surface area and
abundant edge atoms, which make them can conveniently enter the
micro concave region on the surface of the metal friction pair and show

* Corresponding author.
** Corresponding author.
E-mail address: lc1010158@163.com (C. Liu).

https://doi.org/10.1016/j.coco.2021.100922
Received 12 July 2021; Received in revised form 1 September 2021; Accepted 2 September 2021
Available online 11 September 2021
2452-2139/© 2021 Elsevier Ltd. All rights reserved.
C. Liu et al. Composites Communications 28 (2021) 100922

2. Experimental filler to prepare the MoS2QDs/RGO/BMI composite material according

to our previous research [15]. The preparation process of the
2.1. Materials MoS2QDs/RGO hybrid and its BMI composite is shown in Fig. 1.

Molybdenum disulfide (MoS2) powder (~1.5 μm) was produced by 3. Results and discussion
Aladdin Reagent Co. Ltd. N-methyl-2-pyrrolidone (NMP) was phrased
from Shanghai Macklin Biochemical reagent Co. Ltd. Natural graphite 3.1. Microstructure of MoS2QDs/RGO hybrid
flakes (325 mesh) was obtained from Hubei Xinrunde chemical Co., Ltd.
The 4, 4′ -Bismaleimidodiphenylmathane (BDM) and Diallyl Bisphenol A The morphologies of samples were characterized by TEM (Fig. 2). As
(DBA) were provided by Wuhan Zhisheng Science &Technology Co., shown in Fig. 2a, the RGO nanosheets are covered with wrinkles and
Ltd. Ethanol was phrased from Tianjin Fuyu Fine Chemical Co., Ltd. stacked. Fig. 2b reveals that the particle size of MoS2QDs is about 1–5
Acetone was phrased from Tianjin Chemical Reagent II Factory. nm. The high-resolution TEM image of the MoS2QDs displays high
crystallinity with a lattice parameter is 0.25 nm, which is consistent with
2.2. Preparation of MoS2QDs and GO the (103) face of MoS2 crystals (Insert image in Fig. 2b) [16]. As for the
MoS2QDs/RGO, there are many black dots on the graphene nanosheets
MoS2QDs were prepared according to top-down method. 1.0 g raw surface (Insert image in Fig. 2c). This phenomenon is mainly due to the
material of MoS2 powder was dispersed in 150 mL of NMP solution and uniform deposition of MoS2QDs on their surface [17]. Moreover, the
sonicated with an ultrasonic processor for 3 h. The dispersion was uniform distribution of C, O, Mo and S elements mapping images is
centrifuged at 3000 rpm for 15 min. The suspension was collected and indicated that the MoS2QDs are uniformly deposited on the graphene
placed in a Teflon-lined autoclave and kept at 160 ◦ C for 12 h. After that, surface (Fig. 2e–h). The XRD and XPS characterization results further
the light yellow supernatant was centrifuged at 8000 rpm for 40 min. confirm the successful preparation of MoS2QDs and MoS2QDs/RGO (See
Finally, the MoS2QDs solution was stored at 4 ◦ C for further use. The Fig. S2, S3 and S4).
yield of MoS2QDs is greater than 20%, and it shows good dispersion in
organic solvents (Fig. S1). Graphene oxide (GO) was prepared from
3.2. Tribological properties of composites
natural graphite flakes using an improved Hummers’ method [14].
As presented in Fig. 3a and b, the frictional coefficients and volume
2.3. Preparation of MoS2QDs/RGO hybrid wear rate of RGO/BMI and MoS2QDs/RGO/BMI composites decrease
significantly with the increase of the fillers contents, and reach the
1.5 g as-prepared MoS2QDs, 0.3 g GO and 6 mL N2H4•2H2O were minimum of 0.25 and 0.15 at 0.6 wt% fillers content, respectively (See
mixed with a 200 mL mixture solution of water and ethanol (55:45, v:v) Fig. 3a). The frictional coefficient of 0.6 wt% MoS2QDs/RGO/BMI
under ultrasonic for 1 h. The mixture solution was then further stirring composite is reduced by 56% compared to that of neat BMI (0.34). It is
at room temperature for 24 h to get the MoS2QDs/RGO hybrid. The worth noting that the frictional coefficients of MoS2QDs/RGO/BMI
product was washed with deionised water several times and then dried changing with rotational time are more smoothly than those of RGO/
at 60 ◦ C for 4 h in a vacuum environment. BMI composites (See Fig. S8). Moreover, the volume wear rates of RGO/
BMI and MoS2QDs/RGO/BMI composites also decrease to the lower
2.4. Fabrication of MoS2QDs/RGO/BMI composite values of 15.3 × 10− 6 and 8.9 × 10− 6 mm3/(N⋅m) (See Fig. 3b).
Compared with neat BMI (48 × 10− 6 mm3/(N⋅m)), which are reduced by
The MoS2QDs/RGO was introduced into BMI resin as a lubricating 68.1% and 81.5%, respectively. However, with the addition of more

Fig. 1. Schematic diagram of the fabrication process of MoS2QDs/RGO hybrid and MoS2QDs/RGO/BMI composite.

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Fig. 2. TEM images of RGO (a), MoS2QDs (b) and MoS2QDs/RGO (c), respectively, insets of (b) are the HRTEM and particle size distribution images of MoS2QDs, the
HAADF image of MoS2QDs/RGO (d) and the corresponding EDX area mapping (e ~ h).

Fig. 3. The frictional coefficient (a) and volume wear rate (b) of RGO/BMI composites and MoS2QDs/RGO/BMI composites.

than 0.6 wt% fillers, the frictional coefficients and volume wear rate of 229.6 eV and 232.4 eV correspond to the Mo3d5/2, and Mo3d3/2 of MoS2
the BMI composites increase, while they are still lower than those of neat and the peaks at 233.2 eV and 235.8 eV are attributed to Mo6+3d3/2 of
BMI [18]. This is impute the fact that the increase of fillers contents MoO3. This confirms that fewer MoS2QDs are oxidized to MoO3 during
make them easier to form aggregates in the BMI matrix, which make the friction process [20]. The peak appears at 168.9 eV is attributed to
them hardy to form high-quality self-lubricating transfer film on the the S6+ state realized in FeSO4 (Fig. 4f) [21]. In addition, the binding
surface of the metal friction pair. Fig. S5, S6 and S7 also confirm that the energy of Fe 2p is composed of six peaks, which are belong to FeS2
introduction of MoS2QDs/RGO is beneficial to the improvement of (708.9 eV), FeS (712.5 eV), Fe3O4 (723.5 eV), Fe2O3 (710.9 eV), FeOOH
mechanical and hardness properties of composites. (711.8 eV), FeS (712.5eV) and FeSO4 (713.6 eV), respectively (Fig. 4g)
[22]. The above results reveal that tribochemical reactions occur during
3.3. Friction and wear mechanism analysis the friction process, leading to the formation of a robust transfer film
composed of MoS2, RGO and metallic compounds. Comprehensive the
As shown in Fig. 4a, severe ploughs with abundant stratification and above information, the excellent tribological properties of the
cracks appear on the worn surface of neat BMI, which suggests that the MoS2QDs/RGO/BMI composite can be attributed to the synergistic ef­
major wear mechanism of it is fatigue wear and adhesive wear. The fect of graphene and MoS2QDs (such as the excellent self-lubricating and
uneven area on the 0.6 wt% RGO/BMI composite worn surface are strengthening properties of graphene, the ball effects of MoS2QDs, and
related to the inhomogeneous dispersion of graphene and the occur­ the tribochemical reaction between the MoS2QDs and the metal friction
rence of graphene aggregates in the BMI matrix (Fig. 4b). Whereas, the pair, etc.) enable it to form a high-quality self-lubricating transfer film,
worn surface of 0.6 wt% MoS2QDs/RGO/BMI composites displays the thereby significantly improving the friction reduction and wear resis­
smoothest and has a large number of little scratches (Fig. 4c). The wear tance of the composites (See Fig. S10, S11 and S12) [23].
mechanism belongs to the slight abrasive wear [19]. This is attributed to
the MoS2QDs can repair the concave region of the friction pair and 4. Conclusion
promote the generation of high-quality self-lubricating transfer film.
Fig. 4d shows the XPS spectra of the transfer film corresponding to In this work, we have successfully prepared the MoS2QDs/RGO
0.6 wt%MoS2QDs/RGO/BMI. For the Mo3d signal (Fig. 4e), the peaks at hybrid lubrication nanoparticles through a facile sonication method.

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C. Liu et al. Composites Communications 28 (2021) 100922

Fig. 4. SEM images of worn surfaces of composites, and the XPS spectra of the transfer film.

The tribological properties of MoS2QDs/RGO/BMI composite are curation, Validation. Wenbo Zhang: Methodology, Software. Yan Bao:
significantly better than those of neat BMI and RGO/BMI composite. Supervision. Jianzhong Ma: Methodology, Supervision, Validation.
When the MoS2QDs/RGO addition is only 0.6 wt%, the frictional coef­
ficient and volume wear rate of its BMI composite are reduced to 0.15 Declaration of competing interest
and 8.9 × 10− 6 mm3/(N⋅m), which are reduced by 56% and 81.5%
compared to those of neat BMI, respectively. Even under harsh friction The authors declare that they have no known competing financial
conditions, the tribological properties of the MoS2QDs/RGO/BMI com­ interests or personal relationships that could have appeared to influence
posites are still better than those of neat BMI. Therefore, this study the work reported in this paper.
shows that the quantum dot/graphene composite lubrication nano­
particles are expected to show potential application value in the field of Acknowledgments
polymer-based friction materials.
This work was supported by the National Natural Science Foundation
CRediT authorship contribution statement of China (Grant Numbers 22008145, 22078188); the Project funded by
China Postdoctoral Science Foundation (Grant Number 2021M692861)
Chao Liu: Conceptualization, Methodology, Writing – review & and the Scientific Research Program Funded by Shaanxi Provincial Ed­
editing. Xi Li: Writing – original draft, preparation. Yang Lin: Investi­ ucation Department (Grant Number 21JY004).
gation. Xin Xue: Investigation, Validation. Qiming Yuan: Data

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