Full Length ArticleEnhancing the tribological properties of boron nitride by bioinspired polydopamine modification
Graphic abstract
PDA modified BN can be obtained by annealing the bulk BN in air and the subsequent polymerization of DA, and the as-prepared BNO-PDA shows better tribological perfermances than the original BN as an additive in water lubrication.
Introduction
Hexagonal boron nitride (BN) presents superior performances in thermal conduction [1], electric insulation [2], oxidation resistance [3] and radiation shield [4] et al., and these properties make it have promising application prospects in physics, chemistry, material and biology. As a typical two-dimensional (2D) material, BN also shows excellent tribological properties due to the intrinsic interlayer slipping, which can effectively lower the sliding resistance of the friction pair [5], [6], [7], [8], [9]. Indeed, tribology is one of the most important applications of BN because the superb chemical inertness, high temperature resistance, and electric insulativity of this special material can satisfy many harsh conditions.
During the past three decades, the tribological properties of BN and its nanomaterials, including nanoparticles (BNNPs), nanotubes (BNNTs) and nanosheets (BNNSs), have been intensively studied. The tribological researches of BN based materials can be divided into four types according to the used methods: directly studying the sintered bulk BN [10]; sputtering BN onto the solid substrates to form nanofilms [11]; adding BN or its nanomaterials into metals [12], alloys [7], [13], [14], ceramics [15] and polymers [16], [17], [18] to form composites; using them as additives of liquid lubricants, such as water and oil. High temperature tests prove that the coefficient of friction (COF) of the bulk BN has a high point at 400 °C and then decreases with the increased temperature, indicating that the BN is very suitable to be served as high temperature lubricant [10]. Also, the introduction of CaB2O4 can further decrease the COF values of BN since the advanced crystalline. Incorporating the BN into 316L stainless steel [13], [14] and copper [12] can obtain lower COF values, but the wear rates are increased mainly because of the weakened hardness. However, the BNNSs enhanced Ni3Al compound [7] and Si3N4 [15] ceramic show both decreased COF values and wear rate through the strong bridging, strengthening and toughing interactions. It was also reported that the BNNTs can largely lower the wear rate of hydroxyapatite only with a slight increase of the COF [19].
Polymers are relatively soft and thus the BN based materials generally can simultaneously decrease their COF values and wear volumes due to the enhanced strength and the natural solid lubrication effect of BN [8], [16], [20], [21]. The surface modified BN can better improve the tribological properties of polymers than the unmodified ones because of the fortified interfacial bond strength [18].
One of the most important applications of BN in tribology is served as additives in lubricants. Researchers exfoliated the bulk BN into nanosheets or nanoparticles through ball-milling [22], [23], hydrothermal reaction [6] and other routes [24], [25], [26], and these BNNPs and BNNSs can reduce the friction and wear when adding into either oil or water. To improve the dispersity and stability of BN nanomaterials in lubricants, which are responsible for the stable tribological performances, the fluorine atom [5], alkyl chains [27], [28], alkylamine [29] and carbon quantum dots [30] are grafted onto the BNNSs or BNNPs. In the very recently, Hu and her coworkers [9] synthesized a gel based on functionalized BNNSs and PAO10, which displayed an outstanding tribological property even at a harsh condition of 800 N and 180 °C.
The above description demonstrated that the BN based materials have splendid anti-friction and anti-wear abilities and consequently have tremendous application potential in tribology. Nevertheless, in reality, the COF values of BNNSs is larger than those of graphene [11], and the multiwalled BNNTs exhibits ultrahigh interlayer friction compared to carbon nanotubes (CNTs) [31]. This phenomenon is ascribed to the strong ionic bonds, the so-called “lip-lip” stacking structure and the robust interlayer viscous friction in BN. Therefore, it is necessary to reduce the interlayer friction to further improve the overall performances of BN as lubricating materials, especially under some extreme conditions. In this aspect, heterogeneous atom doping and surface modification are two effective methods. Density functional theory (DFT) calculation indicates that the carbon doping can reduce the sliding friction between the two-layer BN layers by changing the BN’s intrinsic electric structures [32]. The above-mentioned organic chains and fluorine atom can also lower the sliding resistance in BN layers via the lowered interface energy.
In this work, we improved the tribological properties of BN as additives in water via the bioinspired polydopamine (PDA) modification. The BN was first oxidized by annealing in air at high temperature and then hydrolyzed in hot water to produced hydroxyls, which are beneficial to form covalent bonds between BN and PDA. Our results demonstrate that the PDA modified BN is superior than BN and the hydroxylated BN in anti-friction and anti-wear as an additive of water lubrication. This work shed a new light on exploring more excellent BN based lubricant additives. We believed that grafting specific organic molecules and polymers onto BN and/or its nanomaterials to form inorganic-organic composite structures can largely improve their tribological performances and stabilities.
Section snippets
Materials
Hexagonal boron nitride (BN) with a purity of 98% was purchased from Aladdin (China) Industrial Co., Ltd. The 3-hydroxytyramine hydrochloride (dopamine hydrochloride, 99.5%, coded as DA) and tris(hydroxymethyl)aminomethane (99.5%, coded as Tris) were obtained from Sigma–Aldrich. Other chemicals were analytical grades and used without further treatment.
Preparation of the hydroxylated BN and polydopamine modified BN
In this work, the hydroxylated BN (BNO) was prepared according to a previously reported method [33]. The bulk BN was first annealed in air at
Fabrication and characterizations of BN, BNO and BNO-PDA
In this work, the BN was firstly oxidized in air at 1000 °C for 1 h to form BNO, then the BNO reacted with DA to yield BNO-PDA, as depicted in Fig. 1. The grafted amount of PDA on the surface of BNO was controlled by the reaction time, and the products of 6 h, 24 h, and 48 h were respectively coded as BNO-PDA6, BNO-PDA24, and BNO-PDA48.
It is demonstrated in Fig. 1 that the interactions between BNO and PDA include π-π stacking and covalent bond. The π-π stacking is caused by the interactions
Conclusions
In summary, we successfully fabricated the BNO through oxidizing the bulk BN in air, and then the BNO-PDA was prepared by the self-polymerization of DA on the surface of BNO. After the hydroxylation and the PDA modification, the dispersity of the obtained BNO and BNO-PDA was largely improved. The tribological tests indicate that both the BNO and BNO-PDA present better tribological behaviors than the BN. However, compared to BNO, all the BNO-PDA samples show lower COF values when the testing
CRediT authorship contribution statement
Songfeng E: Conceptualization, Methodology, Writing - original draft. Xiangyuan Ye: Methodology, Investigation, Project administration. Meigui Wang: Data curation, Investigation. Jizhen Huang: Investigation, Formal analysis. Qin Ma: Methodology, Investigation. Zhanfan Jin: Methodology, Investigation. Doudou Ning: Methodology, Investigation. Zhaoqing Lu: Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Nature Science Foundation of China (Nos. 51805007 and 51803110), the National Key Research and Development Plan of China (Nos. 2017YFB0308300 and 2017YFB0308302), the State Key Laboratory of Pulp and Paper Engineering, China (No. 201333), the Natural Science Basic Research Program of Shaanxi Province, China (No. 2020JQ-722) and the Zhejiang Province Key Research and Development Project (2019C04008). The authors are grateful to Zezhou Zhu for the help on
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