*2.2. Characterization*

Micro-hardness of the polymer coating was performed by using a TMVS-1 hardness tester (TIMES Group, Beijing, China). For each specimen, five measurements were conducted under load of 50 gf and lasting 15 s. Thermal gravimetric analysis (TG) of the polymer coatings filled with different contents of graphene were evaluated using a TG apparatus (Netzsch Sta 409 PC/PG, Netzsch, Selb, Germany). 3.5 mg specimen was heated from 25 to 750 ◦C at a heating rate of 10 ◦C/min in nitrogen atmosphere. Thermal conductivity of the polymer composite was carried out using a LFA 447 Nanoflash (Netzsch, Selb, Germany), according to ASTM E1461 standard [21]. The sample was prepared with a size of 10 mm diameter and 1 mm thickness. Before each experiment, a thin graphite layer was coated on two sides to increase emission/absorption behavior. The test was performed at a room temperature of 25 ◦C, the thermal diffusivity values (cm2/s) of the composites were recorded.

To evaluate the tribological properties of polymer coating from room temperature to 200 ◦C, the disc specimen was heated and carried out on UMT-2 tribometer (CETR Corporation Ltd., Campbell, CA, USA). Linearly reciprocating ball-on-disk sliding tests were performed according to ASTM G133-05 [22], the lower specimen was the polymer coating and the upper specimen was a GCr15 ball with diameter of 9.5 mm and hardness of 62 HRC. Before the tests, the ball specimens were ultrasonically cleaned in deionized water for 5 min and the disc specimens were cleaned with alcohol wipes, and then dried in hot air. The test conditions were as follows: applied load of 4 N, reciprocating sliding frequency of 6 Hz, with a liner stroke of 6 mm, and sliding time of 60 min. The friction tests were carried out under 25, 100, 150, and 200 ◦C, respectively, and each test was repeated three times. After the friction tests, the wear scars were observed by a scanning electron microscope (SEM, JSM-6460, JEOL, Tokyo, Japan).

Cross-sectional profile of the wear scar was measured using a surface profilometer (TR3000, TIMES Group, Beijing, China), and the wear rate was calculated as the ratio of its wear volume and the corresponding sliding distance and applied load. From the profile curves, this allowed estimation of the sectional area of the wear trace and the average wear width. The average wear width was used to calculate the average worn volume *V*, and the corresponding wear rate *k* was calculated in the following equation [23].

$$k = \frac{V}{P \times S} = \frac{\pi D \left[ (\arcsin \frac{L}{2r}) r^2 - \frac{L \sqrt{4r^2 - L^2}}{4} \right]}{P \times S} \tag{1}$$

where *k* is the wear rate (mm3/N·m), *V* is the wear volume (mm3), *r* is the ball radius (3 mm), *D* is the diameter of wear track (10 mm), *L* is the width of wear scar (mm), *S* is the sliding distance (m), and *P* is the applied load (N).

#### **3. Results and Discussion**

#### *3.1. Characterization of the Composite Coating*

The surface morphology of graphene is characterized using a JSM-6460 scanning electron microscope (SEM), as shown in Figure 1a. It shows that the lateral dimensions of the graphene are layered with a nano-scale thickness. However, some graphene particles agglomerate with each other, and form a corrugated structure. The fracture surface of the composite coating filled with graphene is shown in Figure 1b. This shows that the graphene is tightly combined in the epoxy matrix without loosening even after the fracture. The graphene has a larger specific surface area, which assures strong anchoring and increases the inter-facial contact between the graphene and polymer. It indicates that the presence of well-dispersed graphene in the polymer composite can lead to desirable bearing properties and improve the tribological performance.

**Figure 1.** SEM images of (**a**) graphene and (**b**) the fracture surface of polymer coating (4.0 wt % graphene).

Figure 2 shows the micro-hardness of polymer coating depending on various graphene contents. The hardness is gradually increased when the content of graphene increases from 0.0 to 4.0 wt %. When the graphene content is higher than 2.0 wt %, the micro-hardness increases slightly with the increase of graphene content. The lowest hardness of the neat epoxy coating is 20.4 Hv, while the hardness is 37 Hv as the graphene content increases to 4.0 wt %. The increase in graphene

content caused hardness to rise, this result is consistent with previous works [24]. The addition of a coupling agen<sup>t</sup> can effectively enhance the adhesive strength between epoxy matrix and fillers [25]. Homogeneous dispersion of fillers improves the hardness of the polymer coating. The improvement in hardness is mainly due to the formation of a three-dimensional network by the uniform and staggered distribution of graphene in a polymer matrix. When increasing graphene content in the polymer matrix, it is beneficial to enhance the wear resistance of the composite.

**Figure 2.** Effect of graphene content on the micro-hardness of the polymer composite coatings.

The thermal conductivities of the polymer composite measured by the laser flash method are plotted in Figure 3. It shows that the thermally conductive coefficient of the polymer composite is significantly enhanced to 2.36 W/m·K by adding 4.0 wt % graphene, which is 12 times higher than that of neat epoxy. The theoretical thermal conductivity of graphene is reported to be as high as 5000 W/m·K [26]. Therefore, it is reasonable to assume graphene is suitable for fabricating the epoxy nanocomposite with high thermal conductivity. It also indicates that there is an obvious increment for the thermal conductivity as an increase of graphene content. Because heat propagation in the polymer is mainly due to acoustic phonons, a uniform network in the polymer matrix may result in an increase in thermal conductivity in the composites [27]. With the increasing graphene content, the graphene particles connect to each other and the thermally conductive network is easily formed. For a high content of graphene, polymer composite possesses better interfacial compatibility, which favors phonon transport [28], and thus increases thermal conductivity. Similar conclusions in thermal conductivity of epoxy matrix was previously reported by [27,29].

**Figure 3.** Effect of graphene content on the thermal conductivity of the polymer composite coatings.

Thermal properties of polymer coating with various graphene content by thermal gravimetric (TG) analysis is shown in Figure 4. When the specimen is heated, there are two main stages of mass loss of polymer composite during the thermal gravimetric analysis. In the temperature range of 50–300 ◦C, all the TG curves of the specimens present almost the same change trend, the weight is slowly decreased with a rise in temperature, which is caused by the volatilization of trace amounts of water and organic matter in the matrix. When the temperature is 300–400 ◦C, the weight loss increases rapidly as the temperature rises. Decomposition tendencies with temperature are similar, this is due to thermal decomposition of main chain of epoxy. In the range of 400–700 ◦C, the decomposition speeds are slow, and the weight loss is significantly different. The decomposition rate of epoxy resin with 4.0 wt % graphene is slow, and the residue yields of degradation are highest at the temperature of 700 ◦C, which shows that the heat resistant properties of epoxy resin with 4.0 wt % graphene are superior. With the increase of graphene content, the heat resistant performance of polymer coating gradually improves. It indicates that the graphene is effective for enhancing the thermal stability of epoxy resin. The enhancement of thermal stability can be explained in terms of the dispersion of graphene and interfacial interaction with the epoxy matrix [30]. As such, the addition of graphene is helpful for improving the thermal stability of the polymer matrix in the high temperature stage.

**Figure 4.** TG curves of polymer composite coatings filled with various grapheme contents.

#### *3.2. Effect of Graphene Content on the Tribological Properties*

The effect of the graphene on the tribological properties of the reinforced polymer coating was investigated by determining the value of the friction coefficient and wear rate with various graphene contents (1.0–4.0 wt %, respectively).

Figure 5a shows the friction coefficient curves of the polymer coatings with various content of graphene at room temperature. The friction coefficient of neat epoxy coating is 0.41 at the start-up stage, and then quickly increases to 0.50 after running for 100 s. The increment rises slowly when the sliding continues, the friction coefficient rises to 0.60 at the end of the test. The friction coefficients of composite coatings are significantly reduced and more stable when adding graphene. For example, in the case of adding 1.0 wt % graphene, the friction coefficient of coating reduces to 0.25, which is 50% lower than that of neat epoxy coating. It also confirms that the friction coefficient gradually decreases as the content of graphene increases. Furthermore, when the graphene content is 4.0 wt %, the friction coefficient (with the lowest value of 0.11) is significantly lower than that of the other coatings. Figure 5b illustrates that the wear rate of the polymer coating varies with the content of graphene at room temperature. The wear rate of the neat epoxy coating is as high as 6.54 × 10−<sup>6</sup> mm3/N·m. When the graphene content is 1.0 wt %, its wear rate is reduced by 59% when compared with neat epoxy coating, this indicates an effective improvement for wear resistance of the polymer composite at a relatively low content. In addition, it can be seen that the wear rate gradually decreases as the graphene content increases.

The significant decrease in the friction coefficient and wear rate for polymer composite coating can be associated with the self-lubrication of graphene. When the embedded graphene in the polymer matrix is extruded and it forms solid stable transfer films on the relative sliding surfaces, which can prevent the sliding occurring between the rough surface of composite coatings and steel counterpart, it endows a self-lubricating characteristic of the polymer composite [31,32].

**Figure 5.** Tribology properties of polymer composite coatings: (**a**) friction coefficient and (**b**) wear rate.

#### *3.3. Effect of Elevated Temperature on the Tribological Properties*

Figure 6 shows the average friction coefficient of the polymer coating with various graphene contents slid against the GCr15 steel ball with temperatures varying from 25 to 200 ◦C.

It can be seen that the neat epoxy coating has the highest friction coefficient, and the friction coefficient first reduces and then increases as the test temperature rises. The friction coefficient is lowest (0.44) at 150 ◦C and then increases to 0.48 as the temperature rises to 200 ◦C. It is also found that the friction coefficient of the coating containing 1.0 wt % graphene is stable with a rise in test temperature, the average friction coefficient is in the range of 0.25–0.28, which is distinctly lower than the neat epoxy coating. In addition, the graphene content further increases to 3.0 wt %, the friction coefficient declines while the reduction rate slows down as the test temperature increases, the lowest friction coefficient is 0.07 at the test temperature of 150 ◦C. Furthermore, when the graphene content increases to 4.0 wt %, the friction coefficient is lower than the coating contents of 3.0 wt % graphene under the same test conditions. Generally, high graphene content benefits the high temperature tribological performance of polymer coating, whereas higher test temperature leads to a higher friction coefficient.

**Figure 6.** Influence of temperature on the friction coefficient of the polymer composite coatings.

The wear rate of polymer coating with various graphene contents under a test temperature range from 25 to 200 ◦C, which is shown in Figure 7. It indicates that polymer coating without graphene has the highest wear rate, and the wear rate reduces and then increases with temperature increases, the minimum wear rate is 5.62 × 10−<sup>6</sup> mm3/N·<sup>m</sup> at 100 ◦C, and the wear rate reaches the highest value (9.75 × 10−<sup>6</sup> mm3/N·m) at 200 ◦C. When the graphene content increases to 1.0 wt %, the wear rate is significantly lower than neat epoxy coating. Particularly, the wear rate of the coating with a graphene content of 4.0 wt % is lowest in the four grapheme-containing polymer coatings. In the given contents, the wear resistance of the polymer coating is better under higher graphene content. Thus, the graphene improves the wear resistance of polymer coating under higher temperature. When graphene is intercalated in epoxy resin, and the organic-inorganic hybrid structure is formed, this prevents the heat flow to the epoxy resin and reduces the thermal decomposition of composite matrix, so the wear resistance is significantly enhanced at high temperature stages. Prior study also found that adding graphene in the epoxy matrix could enhance the bearing capacity and the fatigue strength of the polymer [15].

**Figure 7.** Wear rate of the polymer composite coatings under various test temperatures.

Neat epoxy polymer is viscoelastic material, the friction mainly depends on the adhesion of the epoxy and steel on the contact area. The relaxation of the branched chain of epoxy resin [33] is occurred under proper heat, and shear slip of the epoxy takes place under the rubbing process along the sliding direction and decreases friction. Therefore, friction coefficient of neat epoxy under 150 ◦C is lower than that under 25 ◦C. However, as the temperature continues to rise, the hardness declines and the real contact area also increases, which causes severe wear and results in further increase of the friction coefficient of neat polymer.

However, the friction coefficient of the polymer composite coating with the addition of graphene is slowly decreased or increased as the increase of test temperatures. This can be attributed to the embedded particles are free to roll, since the viscous polymer cannot hold them under higher temperature. The rolling particles could significantly reduce friction and temperature in the contact area [34,35]. Furthermore, the graphene also forms a transfer film on the counterpart surfaces. As a result, both the frictional coefficient and the wear rate of the polymer composite coating are effectively reduced.

#### *3.4. Analysis of Wear Morphology*

Figure 8 shows SEM micrographs of the worn surface of epoxy coating with various graphene contents. For the neat epoxy coating, a large amount of micro-cracking and material breaking off are observed on the wear track, which indicates a typical fatigue wear type of the neat epoxy coating under room temperature (Figure 8a). As graphene is added into the epoxy matrix, the damage of the

wear decreases because the contact stress is supported by the graphene. When the content of graphene is 1.0 wt %, the wear scratch on the surface gets smoother and shows no surface defects, and only a smooth and very shallow furrow. When the content of graphene is 3.0 and 4.0 wt %, the worn surfaces of polymer coating (Figure 8d,e) can be characterized similarly, which is a wear feature of the surface that exhibited plastic deformation.

When graphene content is increased, the wear damage of the polymer coating decreases, the reason is that a high filing ratio of graphene in the epoxy matrix can form a well-dispersed graphene-epoxy structure, which facilitates good load transfer to the matrix network, resulting in improvement of tribologilcal properties of the epoxy coating.

**Figure 8.** SEM images of worn surfaces of epoxy coating with the graphene content of (**a**) 0.0 wt %, (**b**) 1.0 wt %, (**c**) 2.0 wt %, (**d**) 3.0 wt %, and (**e**) 4.0 wt %, respectively, at room temperature.

The micrographs of their wear scars slid obtained under test temperature of 150 ◦C are compared to better understand the possible wear mechanisms of epoxy coating under high temperature. The SEM images of the worn surfaces are shown in Figure 9. The worn surface of the neat epoxy is composed of a loose debris layer, and a large number of micro cracks and peeling, which indicates that the neat epoxy coating experienced intense fatigue failure under high thermal stress (Figure 9a). When the graphene content increases to 1.0 wt %, it can be seen that wear debris are sheared by a micro convex body, and then roller compacted to form a transfer film on the worn surface, which can support the contact press. This also indicates that the continuous transfer film is hard to form under a low grapheme content. As shown in Figure 9c, for the graphene content of 3.0 wt %, the wear becomes relatively smooth and continuous transfer film is formed on the surface. The detection facilitates to deduce that in the sliding process, graphene can form a transfer film on the friction pair, and graphene in the polymer matrix can improve the thermal conduction performance, as well as avoid the initial failure of coating. Adding an appropriate amount of graphene can improve the wear resistance of polymer coating mainly because the graphene enhances the bearing strength of the coating. Therefore, the coating containing graphene can withstand a larger shear force. In addition, the micrographs of worn surface phenomena decrease with increasing graphene content.
