**1. Introduction**

In space environments, large frictional heat is difficult to dissipate from sliding surfaces. Additionally, a protective oxide film is difficult to form on contact surfaces during the rubbing process. These issues harm the adhesive and the adhesive wear can lead to failure of moving parts in certain conditions. In addition, the liquid lubricants are easily evaporated under high vacuum and high temperature conditions [1], which made the liquid lubricant hard to maintain on the contact area in order to provide an effective long term lubrication performance. The liquid lubricants are limited under extremely harsh working conditions such as high vacuum or high and/or low temperature [2]. Therefore, the reliability of solid or liquid lubricants to improve the lubrication performance of the moving parts is of grea<sup>t</sup> significance for long-life spacecraft. Polymer composite coatings have many advantageous behaviors, such as being able to operate with good oxidation resistance, being able to withstand acid and alkali due to corrosion resistance, and having chemical stability and anti-friction lubricity. As such, polymer composite coatings are often employed in the machinery of bearings, gears, and other moving parts in the space vehicle such as satellite antenna drive system and solar panel equipment.

The major setbacks of polymer composite coatings are usually their high friction coefficient and poor wear resistance. Such poor tribological performance of polymers can be improved by adequately incorporating nano or micro particles and fibers into the polymer matrix. Our previous work [3]

showed the possible improvement of polymer tribological behaviors by appropriately embedding graphite and MoS2 particles. Jitendra [4] improved the friction and wear behavior of polymer composite by adding talc and graphite powders and showed that the friction coefficient and wear rate of SU-8 composites decreased as comprehensive usage of talc and graphite. Wan [5] utilized Ag nanoparticles to improve tribological properties of polyimide/epoxy resin-polytetrafluoroethylene (denoted as PI/EP-PTFE) coating, and confirmed the friction reduction and the enhancement of wear resistance of PI/EP-PTFE coating by the addition of Ag nanoparticles. A large number of available research studies indicate that the type and size of fillers, as well as the test conditions, had strongly influenced tribological performances of polymer composites [6,7]. However, these fillers usually aggregate with a relatively large size and are poorly dispersed in the polymer matrix, which is unfavorable for enhancing the mechanical properties of polymer coatings [8]. Some polymer coatings also have disadvantages of low hardness and high temperature creep. Due to this, the thermostability and tribological properties are clearly deteriorate in extreme environments.

Graphene is a two-dimensional crystal consisting of carbon atoms that has excellent mechanical, thermal, and tribology properties. Additionally, the nano size with properties of larger surface areas and rich oxygen containing functional groups can enhance the adhesive between the fillers and the chains in the polymer matrix [9], which is beneficial for improving the thermostability and wear resistance of the polymer composite. Meanwhile, graphene can form a self-lubrication transfer film on the contact interfaces during the friction process, which endows the graphene/polymer composite with a low and stable friction coefficient and wear rate [10–12].

When Liu [13] added graphene into the polyimide, the thermal stability and the hardness of the polyimide were significantly improved. With the addition of graphene content, the adhesive strength of the transfer film was increased. Compared with the neat polyimide, when the graphene content reached 3.0 wt %, the friction coefficient and wear rate decreased 21.3% and 26.3%, respectively. Ren [14] synthesized functionalized graphene and employed it as filler to improve the anti-wear property and load-carrying capacity of fabric/phenolic composites. It was found that the 2.0 wt % graphene filled fabric/phenolic composite exhibited excellent tribological properties. Lahiri [15] reinforced the tribological behavior of ultrahigh molecular weight polyethylene by coupled with graphene platelet. The wear resistance was enhanced more than four times as increasing graphene content from 0.1 to 1.0 wt %. Recently, Masood [16] added graphene and PTFE to improve the tribological response of nylon-based composites, and an optimal graphene content (0.5% in weight) could synergistically improve the friction coefficient and wear rate of Nylon 66. Numerous studies showed that graphene had excellent lubrication and anti-wear performance, and graphene was widely used as filler to improve the mechanical property and tribological performance of the polymer composites [17–19]. The enhancement on the wear property of the composite was mainly due to the self-lubrication of graphene and the easily-formed transfer film on the counterpart surface. However, the tribological mechanisms of graphene/polymer coating are not well understood and thus require further investigation. Thermo-stability and the tribological properties are important properties of polymers. The full exploration of lubrication behaviors of polymer coating and the understanding of their tribological applications under high temperatures are very meaningful and significant.

Epoxy resin is widely used for composite coating, and the polymer coating with good adhesion performance can be conveniently prepared on the surface by a spray process. However, under a poor heat dissipation condition, huge frictional heat is hard to dissipate in time, which influences the lifetime of the polymer coating. In this work, in order to enhance the high temperature tribological performance of polymer coating, graphene was prepared and incorporated into epoxy resin matrix. The main objective of this study was to investigate the influences of graphene content and test temperature on tribological properties of the polymer coating, as well as to discuss their corresponding lubrication and wear mechanisms.

#### **2. Materials and Methods**

#### *2.1. Preparation of Polymer Composite Coating*

The detailed process of synthesis graphene was as follows. Graphene oxide was first prepared by the oxidation–deoxidization method according to a modified Hummer's method [20]. To fabricate graphene oxide:(i) 120 mL sulfuric acid with 98% concentration was added into the beaker and cooled in the ice bath, then 5 g flake graphite (300 mesh) and 2.5 g NaNO3 were slowly added in the sulfuric acid solution, and then 15 g KMnO4 was gradually joined in the solution, the magnetic agitation was applied during the reaction for 1.5 h; (ii) the beaker was heated to 35 ◦C in a warm bath, meanwhile 200 ml distilled water was slowly add into the mixture and heated 1 h; (iii) the mixture was heated to 95 ◦C, the reaction was lasted for 30 min to ge<sup>t</sup> tan precipitation, which was the graphene oxide (GO). Finally, graphene oxide was ultrasonic cleaning in deionized water, and dried in vacuum oven.

For graphene: (i) 6.5 g GO (graphene oxide) was added into 1000 mL deionized water and later ultrasonic dispersed for 2 h to obtain a stable graphene oxide solution; (ii) pre-measured 60 mL ammonia (acted as a complexing agent) and 65 mL 85% hydrazine hydrate (acted as a reducing agent) were added dropwise to the graphene oxide solution, respectively. The obtained solution was heated at 80 ◦C for 2 h, and then the reaction system was cooled down to room temperature after the reaction completed; (iii) the obtained graphene was washed five times with deionized water by centrifugation at 3000 rpm over 15 min, then the solution was frozen in the refrigerator for 72 h until the graphene was freeze dried.

GCr15 steel with a diameter of 30 mm and a thickness of 5 mm was applied as a substrate specimen, and the disc specimen had hardness of HRC 60–63. The specimens were polished with 100 grain size sand paper to increase the surface roughness. The disc specimens to be coated with polymer coating were cleaned in an ultrasonic bath with acetone for 10 min successively three times so as to ensure the proper removal of residual pollutants. Then the specimens were dried at 100 ◦C before spraying the coating. The polymer coating was achieved by spraying epoxy matrix filled with graphene, the epoxy matrix was high purity E51 epoxy resin with an epoxide equivalent weight of 210–250 g/eq. 650 type low molecular polyamine (with amine value of 180–220 mg KOH/g) was employed as the curing agen<sup>t</sup> for the epoxy monomer.

The graphene (content increases from 0.0% to 4.0% in weight) were slowly added into the E51 epoxy resin at 70 ◦C and uniformly stirred. Then the curing agen<sup>t</sup> was introduced into the mixture at 60 ◦C, and the solution was continuously mixed for 10 min and diluted with ethyl alcohol. The prepared graphene–epoxy solution was then sprayed on the GCr15 substrate, and it was followed by curing at room temperature for 24 h. Finally, the composite coating with a thickness 30 μm and surface roughness *R*a of 0.5 μm was prepared.
