Surfactant assisted and in situ formed micro liquid metal as excellent lubricant additive in polyimide coating
Introduction
Since polyimide (PI) was primarily commercialized by DuPont in 1960s, it has been widely used in aerospace, insulating paint, flexible electronics, automotive industry and new energy because of its excellent temperature resistance, high insulation, good anti-corrosive and mechanical properties [1,2]. However, the high friction coefficient and wear of pure PI limit its wider application in the industry [3,4].
Reinforcing PI with solid or liquid fillers is a common method to improve their tribological performance [5,6]. Solid fillers, such as nano graphene and glass fibers, can effectively enhance the tribological properties of polymers, but partial agglomeration and broken fibers easily damage the friction antithesis [7,8]. The addition of lubricating oil and ionic liquids in PI is able to greatly improve the tribological properties, however, the filling mode, leakage, dispersion and thermal stability of fillers face challenges in practice [6,[9], [10], [11]]. And these problems also limit the application of liquid fillers. Moreover, many traditional fillers cannot be filled into PI coatings since the thickness of coatings is normally tens of micrometers. The filler should particularly have high thermal stability to stand the high temperature during thermal curing of PI coating. Hence, we need to find a kind of filler with multi-functional performance and easy preparation method for PI composite coating.
The liquid metals (LM) are metal alloys that are liquid state at room temperature, such as EGaIn (75.5 wt% gallium and 25 wt% indium), Galinstan (68.5 wt% gallium, 21.5 wt% indium and 10 wt% tin) [[12], [13], [14]]. It can occur more frequently in many important fields due to its high thermal conductivity and electrical conductivity, high stability, low toxicity, low viscosity and self-assembly [15,16]. The scientists and engineers paid much attention to improve thermal and electrical conductivity of polymers by filling LM in the early applied process [17,18]. Recently, the uniqueness of LM were gradually discovered in lubricating performance [19]. Gallium based liquid metals showed great lubricant performance at high load (1500 N) due to boundary lubrication and the formation of Ga protective tribo-film, which exceed many kinds of oil and ionic liquid [20]. On the other hand, the anti-wear performance was improved by adding gallium based liquid metals to pure greases [21]. This reveals the great potential of liquid metals in the lubrication filed.
For effective and wide use of LM, microfluidic technology, grinding and ultrasonic shearing are usually used to prepare LM microsphere particles [22,23]. However, microfluidic technology produces capsules that are too large to be filled into the coatings and the particle size effect of grinding machine is not uniform. Therefore, ultrasonic shearing is the simplest and most effective method to dispose LM. Very recently, LM nanoparticles modified by a self-assembled alkylthiolate monolayer were successfully synthesized and dispersed in PAO10 base oil. The wear rate of LM filled in PAO10 was just 7.1% of pure PAO10 [24]. It is envisaged that microscale and even nanoscale LM particles could be in situ formed in the PI's precursor solution, and after the evaporation of solvent a well dispersed PI-LM composite coating would be fabricated.
In order to achieve the micronization and uniform dispersion of the LM in PI coatings, we directly added Ga–In based LM and DDM as surfactant into the 4, 4′-diaminodiphenyl ether solution, followed by ultrasonic shearing, the blending with Bisphenol A dianhydride, coating on the steel and thermal curing. The tribological properties of PI-LM coatings were carefully studied under dry sliding condition. In addition, we also studied the morphology, thermal and mechanical properties of the PI-LM coating, and analyzed the lubrication mechanism of PI-LM.
Section snippets
Materials
Bisphenol A dianhydride (BPADA) was purchased from Shandong Xiya Chemical Co., Ltd. 4, 4′-diaminodiphenyl ether (ODA) was purchased from Shanghai Macklin Biochemical Co., Ltd. 1-dodecanethiol (DDM) and N, N′-dimethylformamidel (DMF) were obtained from Sinopharm Chemical Reagent Co., Ltd. Gallium (Ga, 99.99%, melting point: 29.8 °C, density: 5.904 g/mL at 25 °C) and indium powders (In, 99.99%, melting point: 156.6 °C, density: 7.300 g/mL at 25 °C, size≥0.0750 mm) were purchased from Shanghai
Morphology
SEM images of the fractured cross section of PI and PI-LM coatings are shown in Fig. 2(a‒h). At low LM loadings of 2%, 5% and 8%, the LM microspheres can be clearly seen with a diameter of around 1–2 μm. As the loading of LM reaches 10%, the LM particles become larger (Fig. 2(e)). As the LM content further increases to 15%, the LM aggregates instead of forming microspheres (Fig. 2(f)). These agglomerations may destroy the internal structure of the matrix, and result in a serious deterioration
Conclusions
In this study, the polyimide/liquid metal micreosperes composite coating was prepared by ultrasonic blending and dispersion followed by thermal imidization process, with DDM as surfactant to promote the uniform dispersion of LM to form microspheres. The LM microspheres were used as lubricating additives in the PI coatings, showing great improvement in tribological properties. Specifically, the friction coefficient of PI-8%LM was reduced by 31.6%, and the wear volume was just 10% of pure PI
CRediT authorship contribution statement
Lisong Dong: Investigation, Methodology, Conceptualization, Writing – original draft. Jian Wu: Methodology, Formal analysis, Writing – original draft. Danyang Cao: Data curation, Methodology. Xin Feng: Resources, Supervision. Jiahua Zhu: Resources. Xiaohua Lu: Resources. Liwen Mu: Conceptualization, Writing – review & editing, Resources, Supervision.
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 National Natural Science Foundation of China [Nos. 21808102, 21838004, 91934302 and 21908093] and the Swedish Research Council for Environment, Agricultural Sciences, and Spatial Planning [Formas, grant number 2019-01162]. China Postdoctoral Science Foundation funded project (No. 2020M671461) and Postgraduate Research & Practice Innovation Program of Jiangsu Province (Grant No. SJCX20_0356) are also acknowledged. We gratefully acknowledge the financial support of the
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