Tribological properties of polyimide-graphene composite coatings at elevated temperatures

Highlights

• Tribological properties of PI/GP composite coating at high temperatures.

• GP could improve thermal stabilities of PI composites.

• GP reduces friction and wear of PI coating at RT and elevated temperatures.

• Worn surface SEM investigation reveals wear mechanism at different temperatures.

Abstract

This paper focuses on developing a polyamic acid (PAA) of polyimide (PI) and PI filled with graphene (PI/GP) on 100CR6 ESU hardened steel. The tribological testing result of the pure PI and PI/GP against 100Cr6 steel ball at room temperature (RT) and elevated temperatures (50 °C, 100 °C, 150 °C, 200 °C) exhibits that the addition of GP clearly enhances the friction reduction and wear-resistant properties of PI, due to the excellent lubricating properties of GP. The 3 wt % of GP content in the PI is the optimum loading for the lowest friction and wear at RT and elevated temperatures. The SEM analysis of the worn surfaces showed that the high wear-resistant of the PI/GP composite is due to the generation of the loose particles from the coating surfaces which could be embedded on the friction pairs and rolled over there as a lubrication film. Also, at 60 % weight residue the PI/3 G P showed the high decomposition temperature.

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.