Tribological performance and thermal conductivity of graphene–Fe₃O₄/poly(phenol-formaldehyde resin) hybrid reinforced carbon fiber composites

Abstract

Poly(phenol-formaldehyde resin)/carbon fiber composites with different ratios of graphene–Fe₃O₄ were manufactured through a molding press process. Fe₃O₄ was introduced into the graphene to avoid recombination of the layers. The effects of the amount of added graphene–Fe₃O₄ on the morphology, thermal conductivity, friction performance and thermal properties of the composites were investigated via scanning electron microscopy, thermal constants analysis, tribology and thermogravimetric tests. Scanning electron microscopy confirmed the enhanced friction surface of the composites and the combination of the resin with the fibers. The thermal conductivity measurements showed that the best results were achieved with 2.0 wt% graphene–Fe₃O_4. The friction coefficient and wear rate also decreased with the addition of graphene–Fe₃O₄ based on the tribological results.

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1. Introduction

Friction material composites are widely used in the automotive, rail and aerospace industries as brake materials because of their excellent properties, including a moderate friction coefficient, low wear and reliable strength under different operating conditions.^1,2 Friction materials generally fall into one of four different classes, namely, binders, reinforcements, fillers and friction modifiers. According to the specific demands of the application, the performance of the composites can be tuned by varying the relative amounts of the four components. Poly(phenol-formaldehyde resin) (PF), used as a binder, is one of the main friction matrices as a result of its low cost, thermal stability, structural integrity and good resistance.^3–5 Carbon fibers (CFs), glass fibers and aramid fibers are used as reinforcement fibers in friction materials. Compared with other fibers, CFs have a superior performance, with extremely high strength, heat resistance and a high modulus.⁶ Resin-based CF friction materials are extensively utilized in the automobile, machinery and aerospace industries.^7–10 Fillers, such as graphite, BaSO₄ and CaCO₃, have a crucial role in tuning the friction performance of composites, although the amounts used are small relative to those of the binder and reinforcements. Friedrich et al.¹¹ have reported that various fillers reduced the sliding wear of polymer composites, whereas Park et al.¹² mixed carbon black and PEEK powder in composites to enhance their wear characteristics.

Much research has been carried out on graphene because of its remarkably high thermal conductivity, large specific surface area and superior electronic conduction.^13–17 Functionalized graphene has been introduced into polymers to meet the requirements of particular applications. Graphene has been reported to improve the friction, wear properties and thermal conductivity of materials such as epoxy,¹⁸ phenol polymers,^19,20 polystyrene,²¹ polyimide,^22,23polypropylene²⁴ and poly(ether ether ketone).²⁵ Liu et al.²⁶ showed that surface-functionalized reduced graphene oxide in bismaleimide composites has a high tribological performance and good thermal stability. Cao and coworkers²⁷ reported the improved lithium storage and magnetic properties of graphene–Fe₃O₄ composites. We have previously reported that graphene additives help to improve the thermal conductivity and friction performance of PEEK.²⁸ However, the tribological properties of functionalized graphene/CF composites have rarely been studied. One of the challenges of such a process is the dispersion of particles into the polymer matrix. Zheng and coworkers²⁹ reported multifunctional polyetherimide/graphene@Fe₃O₄ composite foams that could form a shield against electromagnetic pollution. We speculated that magnetic particles used as an additive could form a barrier between the graphene layers and avoid recombination of the graphene layers. We report here a facile strategy for the fabrication of functionalized graphene–Fe₃O₄ nanoparticles (G@Fe₃O₄) and an investigation of the thermal conductivity of the composites. The introduction of Fe₃O₄–graphene into PF/CF composites showed not only improved dispersion, but also a better tribological performance and thermal properties.

2. Materials and methods

2.1. Materials

Phenol (99%) and formaldehyde solutions (37%) were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd, China. The CFs (T300, d_25°C = 1.7 g cm⁻³) were supplied by Sinosteel Engineering & Technology Co. Ltd (China). Natural graphite and Fe(NO₃)₃ were supplied by Shanghai National Medicine Group Chemical Reagent Co. Ltd, China. The CF fabric was knitted using an automatic sampling loom (Evergreen, CCI TECH Inc., Taiwan).

2.2. Synthesis of functionalized graphene–Fe₃O₄ nanoparticles (G@Fe₃O₄)

GO was prepared from natural graphite powder using the Hummers method.³⁰ The graphene–Fe₃O₄ nanoparticles were prepared by hydrothermal synthesis. The weight ratio in the reaction was 0.1 g GO to 1.0 g Fe(NO₃)₃ to 1.5 g ascorbic acid. These reagents were ultrasonically dispersed in 100 ml of deionized water for 60 min. Hydrazine hydrate was then added to the aqueous solution with magnetic stirring for 60 min. After stirring, the homogeneous solution was transferred into a Teflon-lined stainless-steel autoclave and heated at 180 °C for 8 h in a vacuum drying oven, followed by cooling naturally to room temperature. The resulting black precipitate was rinsed with deionized water and absolute alcohol and freeze-dried before further use.

2.3. Manufacture of PF/G@Fe₃O₄/CF composites

Different mass fractions (0–4.0 wt% loading) of G@Fe₃O₄ nanoparticles were dispersed in an ethanol solution containing PF with ultrasonication for 30 min. The mixture was stored under vacuum at 60 °C to remove the ethanol solvent. About 10.0 g of the obtained PF/G@Fe₃O₄ was used to presoak one layer of CF fabric. The composites were prepared using a mold press at 165 °C with 0.5 MPa pressure. The weight, size and thickness of each piece of CF fabric with PF/G@Fe₃O₄ were regulated, thus the PF/G@Fe₃O₄/CF composites had a uniform distribution of CFs and matrix. The PF/G@Fe₃O₄/CF fabric was dried in a vacuum oven at 30 °C for 30 min before the manufacturing procedure to reduce the amount of air bubbles. The synthetic approach for the preparation of G@Fe₃O₄ and the manufacture of the G@Fe₃O₄/CF composites is shown in Fig. 1.

Fig. 1 Schematic representation of the synthetic procedure for the preparation of G@Fe₃O₄ and the manufacture of the composites.

2.4. Characterization

X-ray diffraction analysis. X-ray diffraction (XRD) analyses of the samples were carried out using a Shimadzu-6000 X-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å) operated at 40 kV and 50 mA.

Thermogravimetric analysis. The thermal stability of the composites was determined by thermogravimetric analysis (TGA) (Mettler Toledo TGA 2). The samples were heated from 30 to 900 °C at a heating rate of 10 K min⁻¹. Experiments were carried out on samples with an average mass of 10.0 mg and the samples were cut longitudinally from the center of the composites.

Morphological analysis. Scanning electron microscopy (SEM) (JSM-5600, JEOL, Japan) was used to investigate the worn surfaces and the fracture surfaces of the composites after coating the samples with a thin layer of gold.

Thermal conductivity measurements. The thermal conductivity of each composite was measured using a thermal constants analyzer (Hot Disk 2200, Hot Disk, Sweden) based on the transient plane source method. The samples were 60 × 60 × 1 mm in size and the sensor was placed between two equivalent slabs of the sample. The sensor supplied a heat pulse of 50 mW for 10 s and the experimental data were recorded.

Friction and wear tests. A universal tribo-tester (MVF-1A, Jinan Heng xu Testing Machine Technology Co. Ltd, China) was used to evaluate the friction behavior of the composites, which was of the rotating end-face friction type. The contact area was 125 mm² and the slide speed was 0.278 m s⁻¹ under a load of 100 N under ambient conditions for 60 min. Before each test, the samples and the plain carbon steel ring were cleaned with ethanol. The specific wear rate was calculated using the equation:
where Δm is the mass loss, ρ is the density of the composite, F_n is the applied load and L is the sliding distance.

3. Results and discussion

3.1. Crystalline structure

X-ray diffraction was used to determine the chemical composition and crystallographic structure of the materials (Fig. 2). In the original graphite sample, the characteristic diffraction peak (002) appeared at around 2θ = 26.3° (Fig. 2, line a). After oxidation, this diffraction peak shifted to around 10.3° (Fig. 2, line b), indicating oxidized graphite with oxygen-containing functional groups on the graphite sheets, i.e. the formation of GO, similar to previously reported results.³² Fig. 2 (line c) shows the diffraction peak for Fe₃O_4. The XRD pattern of the as-prepared G@Fe₃O₄ nanocomposites (Fig. 2, line d) matched the structure for magnetite (Fe₃O₄). The crystallographic structure confirmed that we had obtained the desired nanoparticles.

Fig. 2 X-ray diffraction patterns of samples: (a) natural graphite, (b) GO, (c) Fe₃O₄ and (d) G@Fe₃O₄ nanoparticles.

3.2. Morphological analysis

SEM was used to characterize the prepared G@Fe₃O₄. Fig. 3a shows the 2D structure of graphene with its characteristic crumpling, which indicated that the graphene morphology was well preserved during modification with Fe₃O₄. The SEM image also suggests that magnetite nanobeads appeared on the graphene surface after the hydrothermal reaction, as reported previously.³¹ To assess the dispersion of the G@Fe₃O₄ nanoparticles in the fabricated composites, the surface of the PF/2.0 wt% G@Fe₃O₄/CF composites were examined by SEM. Fig. 3b shows an SEM image of the PF/2.0 wt% G@Fe₃O₄/CF composites, which shows that the PF matrix permeated into the CFs and uniformly coated them. The circle in Fig. 3c shows the conglobation of G@Fe₃O₄ particles in PF, indicating that the addition of 4 wt% or more of G@Fe₃O₄ particles might lead to aggregation.

Fig. 3 (a) SEM image of G@Fe₃O₄. SEM images of the surface of the PF/CF composites: (b) 2 wt% G@Fe₃O₄ and (c) 4.0 wt% G@Fe₃O₄.

3.3. Thermogravimetric analysis

TGA measurements were carried out to evaluate the effect of G@Fe₃O₄ on the stability of the PF/CF composites; the corresponding thermogravimetric curves are shown in Fig. 4. The initial degradation temperature (T_i) and the second loss temperature (T_s) for the PF/CF and PF/G@Fe₃O₄/CF composites are given in Table 1. Fig. 4 shows the effects of the G@Fe₃O₄ content on the decomposition temperature of the PF/CF composites. The initial decomposition temperature of the composites increased with the G@Fe₃O₄ content, in agreement with the well-known fact that inorganic particles improve the heat resistance of composite materials reinforced with G@Fe₃O₄. The T_i for the PF/CF composites was 140.2 °C, whereas the T_i for the PF/2.0 wt% G@Fe₃O₄/CF composites increased to 171.9 °C. However, there was no obvious change in the second loss decomposition temperature. The impact properties of the PF/G@Fe₃O₄/CF composites might be a result of the decomposition of the characteristic structure of PF. Fig. 4 shows that the amount of residue at 800 °C increased with the amount of the G@Fe₃O₄ dopant. For example, the residual amount of pristine PF/CF was around 77%, while the residual amounts of PF/0.5 wt% G@Fe₃O₄/CF and PF/4.0 wt% G@Fe₃O₄/CF were 82 and 84%, respectively, which was consistent with our design. The decomposition stage of PF/1.0 wt% G@Fe₃O₄/CF and PF/2 wt% G@Fe₃O₄/CF showed obvious differences from those of PF/0.5 wt% G@Fe₃O₄/CF and PF/4.0 wt% G@Fe₃O₄/CF, although the residual amounts were fairly similar. It is reasonable to assume that the increase in thermal stability can be ascribed to strong interactions between the PF matrix and the G@Fe₃O₄. The strong interaction between the filler and the matrix hindered the segmental motion of the polymer chains and restricted the thermal mobility of the PF chains near the G@Fe₃O₄ surface. In addition, the significantly enhanced thermal conductivity of the composites can increase their thermal stability. In accordance with the morphology studies, redundant G@Fe₃O₄ did not have a major effect on improving the thermal stability as a result of the inhomogeneity of the aggregated G@Fe₃O₄.

Fig. 4 TGA thermogram of PF/CF composites with different G@Fe₃O₄ contents.

Table 1 Thermal properties of PF/G@Fe₃O₄/CF composites^a

Sample	Ti (°C)	Ts (°C)
a Ti and Ts are the initial decomposition temperature and the second decomposition stage temperature, respectively.
PF/CF	140.2	354.5
PF/0.5 wt% G@Fe3O4/CF	146.3	357.0
PF/1.0 wt% G@Fe3O4/CF	163.3	364.9
PF/2.0 wt% G@Fe3O4/CF	171.9	362.0
PF/4.0 wt% G@Fe3O4/CF	179.6	349.9

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3.4. Friction behavior

The evolution of the friction coefficients of the composites was examined by a tribological test. Fig. 5a shows the evolution of the friction coefficients of the PF/G@Fe₃O₄/CF composites as a function of the sliding time. The incorporation of G@Fe₃O₄ obviously reduced the friction coefficient of the composites. It was shown that, when increasing the G@Fe₃O₄ content from 0 to 4.0 wt%, the friction coefficient first gradually decreased and then increased when the content of dopant was >2.0 wt%. Fig. 5b shows the mean friction coefficients of the composites with different contents of G@Fe₃O₄ under the same applied conditions. The mean friction coefficient of the composites was decreased by the incorporation of the G@Fe₃O₄ nanoparticles. The friction coefficient reached a minimum at 2.0 wt% G@Fe₃O₄.

Fig. 5 (a) Evolution of friction coefficients of PF/CF and PF/G@Fe₃O₄/CF composites; (b) mean friction coefficients of composites as a function of G@Fe₃O₄ content.

Fig. 6 shows the variation in specific wear rates of the composites at different G@Fe₃O₄ contents. The specific wear rates showed a similar trend to the friction coefficients. The wear resistance of the composites showed the lowest specific wear rate with the incorporation of 2.0 wt% G@Fe₃O₄, a decrease of 30.6% compared with the PF/CF composites. This can be explained by the surface of the composites forming a uniform transfer film in the presence of G@Fe₃O₄, which then carries the majority of the applied load on the sliding surface, resulting in excellent wear resistance. The downward trend in the friction coefficient and specific wear rate in the PF/CF composites might be attributed to the efficient removal of the heat generated in the sliding surface by G@Fe₃O₄, leading to an improvement in the friction reduction abilities of the composites. The inclusion of G@Fe₃O₄ also resulted in moderate decreases in the wear resistance of the composites. However, as shown in Fig. 5, the composites with 4.0 wt% G@Fe₃O₄ had poor friction coefficients, which is ascribed to a slight aggregation of the nanoparticles in the PF matrix, in agreement with the results of the morphological and thermal stability studies, which showed a slight aggregation of nanoparticles in the PF matrix.

Fig. 6 Wear rates of PF/G@Fe₃O₄/CF composites with different G@Fe₃O₄ contents.

To study the tribological mechanisms of G@Fe₃O₄/PF reinforced CF composites, the morphologies of the worn surfaces and sections of the composites were examined by SEM. Fig. 7 shows the morphologies of the worn surfaces of the PF/CF and PF/G@Fe₃O₄/CF composites after sliding under dry friction. The worn surface of the pure PF/CF composites was rough, displaying holes and plow marks, indicating abrasive wear. The PF resin was fragile, so cracks and breakaway features occurred on the surface of the specimen and formed brittle cleavage fracture surfaces.⁷ By contrast, the worn surfaces become relatively smooth with increasing amounts of G@Fe₃O₄ (Fig. 7a–d). The worn surfaces of the PF/2.0 wt% G@Fe₃O₄/CF composites (Fig. 7d) only showed mild scuffing and no obvious sign of plastic deformation. However, higher contents of nanoparticles resulted in poorer worn surfaces (Fig. 7e). This may be attributed to the agglomeration of particles with increasing G@Fe₃O₄ contents.

Fig. 7 SEM images of the worn and fracture surfaces of composites with different G@Fe₃O₄ contents: (a) 0 wt%; (b) 0.5 wt%; (c) 1.0 wt%; (d) 2.0 wt%; and (e) 4.0 wt% G@Fe₃O₄.

Fig. 7a′–e′ show the accumulation of wear debris around the fractured CFs. The amount of debris on the worn surfaces decreased with increasing amounts of G@Fe₃O₄ nanoparticles. With respect to the wear rate, the amount of debris corresponds to the wear resistance of the G@Fe₃O₄ additive in the reinforced CF composites. When 2.0 wt% G@Fe₃O₄ nanoparticles were added, the worn surface of composite was scuffed, but not fractured. These data imply that the addition of 2.0 wt% G@Fe₃O₄ nanoparticles gave excellent wear resistance.

3.5. Thermal conductivity measurements

Fig. 8 shows the relationship between the thermal conductivity of the PF/G@Fe₃O₄/CF composites and the amount of G@Fe₃O₄ added. The thermal conductivity of neat PF was about 0.21 W m⁻¹ K⁻¹. However, the thermal conductivity of the PF/CF composites was 1.54 W m⁻¹ K⁻¹. This improvement in thermal conductivity was mainly ascribed to the reinforcement of the CFs. The thermal conductivity of the composites increased with increasing G@Fe₃O₄ content and reached a maximum at 2.0 wt% G@Fe₃O₄. These results can be explained by regarding the CF as a heat conductive bridge, while the nanoparticles are well dispersed in the phenol resin matrix. The thermal conductivity of the PF/G@Fe₃O₄/CF composites was 11.34% (1.67 W m⁻¹ K⁻¹) higher than that of the PF/CF composites. The thermal conductivity was slightly decreased in the 4.0 wt% G@Fe₃O₄ as a result of the aggregation of nanoparticles, which also impaired the thermal stability, wear rate and friction coefficient of the composites.

Fig. 8 Thermal conductivity of PF/CF composites with different G@Fe₃O₄ contents.

4. Conclusions

A facile technique was designed to fabricate PF/G@Fe₃O₄/CF composites with different G@Fe₃O₄ contents. The PF/2.0 wt% G@Fe₃O₄/CF composites had the best thermal stability, conductivity and friction performance. The introduction of G@Fe₃O₄ into the PF improved the thermal conductivity and increased the phonon diffusion of the composites, which was attributed to the remarkably high thermal conductivity of G@Fe₃O₄. The excellent self-lubricating properties of the CFs and G@Fe₃O₄ decreased the friction coefficients and the specific wear rates of the composites. This new advanced multifunctional material is therefore suited to an extensive range of applications.

Acknowledgements

The authors acknowledge the Jilin Province Major Science and Technology Achievements Transformation Project of China (11ZDZH003) and Jilin Province Science and Technology Development Project of China (20130204031GX) for financial support.

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