Tribological properties of polyimide-graphene composite coatings at elevated temperatures
Introduction
The interest of the replacement of metal parts by polymer has been increased over the past few years and it is continuously growing. Because the self-lubricating polymer avoids the needs of oil or grease lubricants and it is an emergency to maintain pollutants free environment [1]. Also, the polymer can be used in the vacuum and high-temperature conditions where the conventional lubricants evaporate [2] and the use of ionic liquid will be expensive in this situation [3]. These exceptional features of the polymer can be utilized as bearing and rubbing materials where it can maintain high speed/load and deliver consistent operation over an extensive range of temperatures [4]. Therefore, using an appropriate polymer in particular applications could maintain all of the above advantages. However, most of the polymers cannot be used when the service temperatures approach 120 °C which is encountered in the industry [5]. Therefore, different fillers are added to the polymers to enhance their service temperature, mechanical, chemical as well as tribological properties. For example, the commercially available high-performance polymers like Polyetheretherketone (PEEK) has been tested up to 210 °C with the incorporation of different weight percentage (wt%) TiO2. The study showed that the friction coefficient and wear of pure PEEK increases with increasing temperature while its composites have low friction but high wear at high temperature. Compared to pure PEEK, the PEEK composites were worsened due to the abrasive effect of the induced particle in the transfer film. Here they also used polybenzimidazole (PBI) and polyparaphenylene (PPP) polymer for comparison [6]. Moreover, pure PEEK, pure PBI and their composites (50:50) has been experimented up to 280 °C for tribological application [7]. Various weight percentages of graphene were incorporated into Epoxy polymer, which could be used in high temperatures like 200 °C. This result revealed that the Epoxy/graphene composites improved the tribological performance at elevated temperatures due to the formation of tribo-film in the contact interfaces [8]. Moreover, the 10 wt % of carbon fiber and 15 wt % graphite flake were added in PI and can be utilized in the tribological field of temperature 260 °C [9,10]. Friction and wear behaviors of the mesoporous silica-reinforced PI nanocomposite were examined at maximum temperature up to 300 °C in dry sliding to imitate the practical conditions [11]. So, it is noticed that the maximum tested temperature of high-performance polymers such as PEEK, PBI in the tribological field is 280 °C whereas the highest temperature for epoxy, PPP and PI are 200 °C, 210 °C and 300 °C respectively.
Polyimides (PIs) are the high class of plastic materials suitable for high-temperature applications because of their significant mechanical, thermal and chemical properties. Therefore it is widely used as composites in aerospace, aircraft wire, electronics, and cable coatings [12,13]. Besides these advantages, the pure PI still has some shortcomings such as poor adhesion, high friction and wear which limit its application [14,15]. Hence, various micro and nanoscale fillers or solid lubricants such as carbon fiber, graphite, CNTs, PTFE, are used by the researchers to improve the tribological properties of PI [9,[16], [17], [18], [19]].
Graphene (GP) is a 2-D hexagonal honeycomb lattice structure with excellent physical [20], thermal [21], and self-lubricating [22] properties, a potential filler for the addition in the PI and have been used for the improvement of tribological properties. The GP is properly dispersed in the PI by the method of edge-aminated with l-phenylalanine (PheG) [23] and aniline trimer [24]. The tribological study result of the PI/GP composites showed that the proper dispersion of GP improved the wear-resistant and lubrication property of PI. Furthermore, 2 wt % of modified PI/GP nanocomposite revealed 20 times increase in wear resistance and 12 % reduction of friction than pure PI attributed to the increase in mechanical properties [25], suppression and protective effect of modified GP in the production of wear debris, and the friction force respectively [26]. Also, exfoliated graphene oxide (GO) enhanced the thermal as well as friction and wear properties of thermosetting PI, owing to the formation of transfer film and the increasing of hardness [27,28]. Moreover, PI/GO demonstrated better tribological performance under lubricated conditions because of the excellent lubricating effect of seawater [29]. Flurographene is also incorporated in the PI to enhance the lubricating properties of the graphene in the polymer composite [[30], [31], [32]]. However, all the researchers were confined to their research of GP contents polymer composites in dry and lubricated conditions. The excellent mechanical properties of PI at elevated temperature render it used in extreme sliding conditions such as aerospace and high speed/load-bearing surfaces. To the best of the author’s knowledge, no one has done such a tribological test of the PI/GP composite coatings in unlubricated conditions at elevated temperatures.
In this paper, PI/GP composites have been prepared by the in situ polymerization system and coatings made on the steel substrate. A reciprocating type ball on disc SRV-III machine was used for the measurement of the tribological properties of the PI composites coatings under dry conditions at room temperature (RT) and elevated temperatures ranging from 50 °C to 200 °C. Furthermore, the effect of graphene as a filler on the mechanical properties, thermal stabilities, friction reduction and wear-resistant in PI/GP composites have been deliberated. This work will broaden the potential application of PI at high-temperature applications.
Section snippets
Materials
The polyimide precursors 4, 4 oxydianiline (ODA) (97 % purity, Mm: 200.24 g/mol); N, N-dimethylacetamide (DMAc) (Mm: 87.12 g/mol) and solvent pyromellitic dianhydride (PMDA) (97 % purity Mm: 218.12 g/mol) were purchased from Sigma-Aldrich. Also, 5 % graphene (GP) slurry in N-methyl-2-pyrrolidone (NMP) solvent with an average of 1−5 μm diameter was brought from Nanjing XFNANO Materials Tech Co., Ltd, China.
Preparation of PI and PI/GP composites
In a small transparent measuring beaker, 2 ml of N, N-dimethylacetamide (DMAc) aprotic
Thermal and mechanical characteristics
Fig. 3 shows the thermal stability of the PI and PI/GP composite coatings. All the TG curves, from 34 °C to 100 °C somewhat reduces the weight because of the evaporation of moisture. After that, all the lines follow the same pattern but the neat PI curve has a slightly higher value but constant trend than composite coatings. It means that the neat PI from 100 °C to 550 °C does not reduce the weight at all where the PI composites losses its weight a little but follows the steady curve till
Conclusions
The tribological properties of the GP incorporated PI composite coating that has been conducted at elevated temperatures ranging from RT to 200 °C. The gradual addition of GP reduces friction and increases the wear-resistant properties both at RT and elevated temperatures. The PI/3 G P shows 19∼29 % low friction and 35∼78 % higher wear-resistant than pure PI at different temperatures because of the proper dispersion and inclusion of graphene into the polymer matrix. The reinforcement of GP into
CRediT authorship contribution statement
Amit Roy: Writing - original draft, Investigation. Liwen Mu: Funding acquisition, Writing - review & editing, Supervision. Yijun Shi: Resources, Funding acquisition.
Acknowledgements
This work was supported by the Swedish Kempe Scholarship Project [grant numbers SMK-1740, JCK-1903.1], the Swedish Research Council for Environment, Agricultural Sciences, and Spatial Planning [Formas, grant number 2016-01098], Swedish Energy Agency foundation [Energimyndigheten, grant number 2017-008200], the support of the Swedish Mistra Foundation [grant number MI16.23], Swedish Governmental Agency for Innovation Systems [Vinnova, grant number 2018-04274] and National Natural Science
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