Comparative Study on Tribological Behavior of Graphene/Polyimide and Carbon Fibers/Polyimide Composites: A Review

Recently, graphene and carbon fibers have enticed extensive consideration in many scientific fields, they are considered by many scientists to be one of the most promising materials in the 21 st century. Due to the uniqueness of their properties was attracted to be used to reinforce polyimide. Impressive graphene properties coupled with those excellent of polyimide composites produced composites materials with good tribological properties, different contents of graphene and its derivatives tend to improve polyimide composites properties particularly friction and wear. Furthermore, nanofillers decorated graphene derivatives showed also an effect on the tribological properties of composites. Carbon fibers coupled with polyimide composites also reviewed and showed a significance in the improvement of the properties of composites materials. The results nanofillers reinforced carbon fibers/polyimide exhibit enhanced tribological properties, which can be applied in various fields. Therefore, this survey article gives an enormous review study of the tribological properties under various conditions of graphene/polyimide and carbon fibers/polyimide composites. Besides the effects of nanofillers size on tribological properties, preparation, and research challenges were also reviewed.


Introduction
Polymer materials with lightweight, high strength, good self-lubricity, and low friction have gradually replaced some metal materials in the area of tribology and have broad application prospects. Among high-molecular-weight anti-friction World Journal of Engineering and Technology other materials, have been studied to formulate materials with excellent properties. Furthermore, the composition and microstructure of material have a great influence on the tribological characteristics of materials. Metals, ceramic, polymers, etc. are used for tribological applications. For metal friction materials, the combination of various elements can improve friction and wear properties [16] [17] [18]. Dyachkova  indicated that the addition of Ni/NbSe 2 significantly improves the tribological properties of copper matrix composites. When the content of Ni/NbSe 2 added to the copper matrix composite is 15 wt%, the friction coefficient (0.16) and wear rate (4.1 × 10 −5 mm 3 /N•m) are very low compared with other composites [21].
The friction and wear of polymer and its composites reinforced by various materials have been improved significantly. Polyphenylene sulfide (PPS) containing nanoparticles such as alumina, TiO 2 , ZnO, CuO, and SiC, and so on, makes the tribological properties of the composites change significantly [22] [23]. Polyetheretherketone composites coupled with various kinds of SiC, ZrO 2 showed an impact on friction and wear of material composite [24]. Epoxy composites enhanced with various particles such as Al 2 O 3 and SiC changed the tribological properties of epoxy nanocomposites [25]. Therefore this review has emphasized the study of graphene and carbon fibers with different contents and conditions that emerged into polyimide and formulated materials with incredible properties.

Graphene
Graphene as a covalently bonded monolayer of carbon atoms with a hexagonal structure and can be transformed as zero dimension (fullerenes), 1D (Carbon nanotubes) and 3D (Graphite) (Figure 1) make the thinnest material in the world at present, and also makes it one of the world's best in properties [26].
Graphene not only has ultra-high carrier mobility, thermal conductivity, mechanical modulus (1 TPa), breaking strength (125 GPa), but also has high transparency, superior gas barrier, and high specific surface area. These characteristics make graphene have great potential application prospects and market value in transportation, electronic devices, energy storage, biomedical and other fields.
Graphene has excellent electrical, thermal, and mechanical properties, which can be used as an ideal nanofiller for high strength conductive composites. The addition N. Daniel [28] [29].
of graphene can give the composite different functions, which not only shows excellent mechanical and electrical properties but also has excellent processing performance, which provides a broader application space for composite mate-

Carbon Fibers
Carbon fiber is a new type of material with excellent processing and mechanical properties. The tensile strength and modulus of carbon fiber are about 2 -7 GPa and 200 -700 GPa respectively. The density is about 1.5 -2.0 g/cm 3 , which is related to the carbonization temperature besides the precursor structure. The carbon fiber coefficient of thermal expansion is different from other fibers. This makes carbon fiber have the highest specific strength and specific modulus among all high-performance fibers. Besides, carbon fiber has the advantages of high-temperature resistance, low-temperature resistance, good thermal conductivity, and corrosion resistance. Because of its low density, it is particularly suitable for aerospace and automotive applications [30] [31] [32].

Polyimide/Graphene Oxide Composites Preparation
There are many preparation methods to prepare polymer/graphene oxide nanocomposites such as solution mixing, melt mixing, and in-situ polymerization.
Different kinds of polymer graphene composites can be prepared by using the above three methods. Graphene and its derivatives, as two-dimensional nanofillers, have significant effects on the thermal, flame retardant, electrical, mechanical, and tribological properties of polymer composites [33]. Polyimide (PI) matrix can be prepared by different methods; the polymer/graphene composites obtained by in-situ polymerization have stronger interfacial interaction, there-N. Daniel, H. Liu World Journal of Engineering and Technology fore, its comprehensive performance has been significantly improved. Liu, H. et al. developed a PI matrix using in-situ polymerization. PI resin was analyzed using 4-phenylethynylphthalic anhydride (PEPE), 4,4'-Oxydianiline (ODA), and 4-phenylethynylphthalic anhydride (ODPA) monomers mixture with methanol (MeOH) and 1-Methyl-2-pyrolidinone (NMP). In preparation for the PI/GO integration, GO was introduced to the resin solution at different weight tests and subjected to 3 hours of ultra-sonication to ensure uniform dissolution [34]. Choi et al. prepared PI/GO nanocomposites using the In-situ polymerization method by combining different monomers such as 2,6-Diaminoanthracene (AnDA), 5(4H)-tetrone (DAn), dimethylacetamide (DMAc), and so on. Firstly ( Figure 2) GO powder using sonication was placed into DMAc; and AnDA was added to modify the GO. Finally, a mixture of DAn, AnDA, and AnDA-GO in DMAc was stirred to produce poly (amic acid)-GO (PAA-GO), which was then chemically imidized to obtain PI-GO [35].

Carbon Fibers/Polyimide Composites Preparation
Fiber-based polymeric composites are made into a standard polymer production process. These processes include molding, extrusion, injection molding, resin transfer molding, and vacuum transfer, just to name a few. This well-developed process of production techniques has already shown that it produces high-quality composite materials [36].

Effect of Particle Size on Wear of Composites
Nanoparticle fillers are a method to improve the wear resistance of polymers.
Nanoparticles introduced into the matrix improve the toughness, leading to reduced wear by sub-surface fatigue. The nanoparticle has lower angularity, it reduces abrasiveness; improve reinforcement of the polymer matrix, due to enhanced mechanical properties of nanoparticles, such as strength, modulus, and toughness; higher specific surface areas and thus, improved adhesion. A lot of studies have been conducted based on nanotechnology. Particle filled polymer composites have significant characteristics in the development of wear-resistant materials. Nanoparticle polymer composite is a new type of material, with a very low filling amount the material properties showed to be improved. The high specific surface area is the most attractive characteristic of nanoparticles, because it produces a large number of interfacial phases in the composites, thus forming  were greatly affected by the particle size as well as particle content. The wear rate of a polyester matrix reinforced by ultrafine biochar with a particle size of 45 nm reduced to about 56.36% than cured pure resin. The friction coefficient of the same sample is 6.42% reduction as compared with the cured resin [38]. Olea Mejia et al. studied the wear mechanism of polymer with different metals. Wear mechanisms of low-density polyethylene (LDPE), depending on the type and size of the metal particles used. The wear rate of LDPE was significantly reduced by the addition of Al (micron and nano) particles. The outcomes indicated that the wear rate of Ag with either micro-and nano-particle filled and Ni nanocomposites are better than that of nanocomposites. By comparing, they concluded that nanoparticles as well dispersed reduced wear than that of micro-particles [39]. Chauhan et al. explored the effect of particle size on the friction and wear properties of vinyl ester composites. Three different sizes of cenosphere particles (2 μm, 900 nm, and 400 nm diameter) were used. Comparatively submicron size of cenosphere particles greatly plays a contribution to wear resistance improvement than microparticles. The findings revealed that 6 wt% submicron particle content tends to reduce the wear rate, friction coefficient also decrease proportionally to the decrease of particle size content of the composites.
In addition, under a high normal load of 70 N and a sliding speed of 5.7 m/s, submicron hollow microspheres (400 nm) are the most effective antifriction fillers. Under normal load, a decrease of particle size consequently decreases the specific wear rate ( Figure 3). The specific wear rate of the composite filled with submicron (400 nm) hollow microspheres was the lowest. Both vinyl particles and submicron particles filled with nanoparticles can enhance the wear resistance and mechanical properties of the composites. Compared with micron particles, submicron particles are more adequate in enhancing wear resistance [40].
Bernd Wetzel et al. studied the effect of particle size on the mechanical and tri- Figure 3. Variation of specific wear rate with filler size of (a) normal load applied, 10 N and 1.9 m/s and (b) normal load applied 70 N, and 5.7 m/s [40]. World Journal of Engineering and Technology bological properties of polymer composites. Toughness, wear, friction, and stiffness properties of the composites are affected by the dispersion state, particle size, shape, and content. Different micro and nano-ceramic particles (calcium silicate, CaSiO 3 , 4 -15 μm, alumina, Al 2 O 3 , 13 nm) were introduced into the epoxy resin matrix as reinforcing fillers. In general, ceramic nanoparticles can lower the wear rate matrix of epoxy resin. However, this largely depends on the content of fillers and morphology of particles. The findings indicated that the volume fraction of nanoparticles is beneficial to the agglomeration tendency, and its tribological properties were the best [41].

Tribological Properties under Drying Sliding Condition
Polyimide composite has good self-lubricating properties and can be used in dry condition, when the content of FG is 0.5%, the specific wear rate of 0.5FG/PI decreased by 51.2% less than pure PI [48]. Different graphene derivatives showed to enhance the tribological properties as reinforced polyimide composites.

Tribological Properties under Water-Lubricated Condition
Water-lubricated tribology is a new branch of tribology, which is being established and developed. In the seawater environment, the pump, open hydraulic transmission system, and blades of the ship will be exposed to the seawater during operation. Thus, due to oil leakage and high corrosiveness of seawater, the liquid lubricant cannot provide sufficient lubrication effect. To expand the service life of mechanical parts and lowering the maintenance cost of parts, mechanical parts are demanded to have good tribological properties and excellent seawater corrosion resistance. Therefore, it is very significant to research and develop new self-lubricating materials for seawater. Compared with metals and ceramics, polymer matrix composites showed the advantages of good corrosion resistance, strong environmental adaptability, and excellent tribological properties, which can meet the requirements of the seawater environment [49] [50] [51]. Polyimide has good thermal stability, excellent mechanical and electrical properties, high chemical inertness, as well as high energy radiation resistance. However, has worse tribological properties, it cannot be widely used in lubricating materials. Go has been widely concerned in the study of lubrication and mechanical properties. The introduction of PI shows excellent tribological properties under different conditions. Min et al. Examined the friction coefficient and wear rate of PI/GO nanocomposite under seawater lubrication condition. In particular, the reinforced PI composite with 0.5 wt% GO had shown the best wear resistance. Consequently, GO addition changed dramatically thermal stability, tensile stress, and tensile modulus of the composites materials. With incredible properties, PI/GO formed showed a potential use as a candidate for tribological applications in water lubrication. Figure 4 showed the film of composites comparative for dry friction and seawater lubrication. Under different lubricating conditions ( Figure 5)    great change in friction coefficient as well as wear rate for composites materials were decreased by 69% and 45% respectively. Finally, various amounts of GNS and PTCDA filled into PI produced a material with anti-wear and friction reduction aspects [45].

Tribological Properties under Oil-Lubricated Condition
It has been proved that lubricating oil helps to form a lubricating film, thereby

Friction and Wear of the Polyimide/Carbon Fibers Composites
To improve the tribological behavior of the pure polyimide, the reinforced phases, lubricating additions, and/or nanoparticles were added to improve the wear rate and friction coefficient. Introducing carbon fibers into PI composites increases the compressive strength, loading capacity, mechanical strength, and wear resistance of composites.

Tribological Properties under Dry Sliding Condition
Carbon fiber (CF) has been used to reinforce PI under different conditions and showed marvelous improvements in properties. Chen et al. studied the tribological properties of short carbon as reinforced PI composites for different conditions. Under dry conditions, the results showed, compared with glass-fibers and quartz-fiber strengthened polyimide composites short-carbon-fiber has the best mechanical properties. As a good reinforcement material, CF coupled polyimide not only proved to have excellent tensile and modulus but also decrease friction coefficient and wear rate. CF composites materials promising to be considered for tribological application by replacing traditional materials [62]. Li  To improve the mechanical and Tribological Properties of polyimide (PI), hydroxylated carbon nanotubes (CNTs) were chemically added to the surface of carbon fiber (CF). The microstructure and fracture morphology of polyimide composites showed that CF-CNTs can be uniformly distributed in the polyimide matrix as a multi-scale reinforcement. A coupled CF-CNTs hybrid showed to affect different properties by increasing hardness, friction reduction, and better wear resistance.  and PI/CNFs (t) composites as compared to neat PI [70]. Qi et al. studied PI composites properties particularly tribological as coupled with different particles including PTFE, aramid fiber (AP), and carbon fiber (SCF). Reinforcement fillers and film formation effects on tribological behavior were also examined. Comparatively, each filler changes positively material properties compared to unfilled PI composite. AP seems to have under dry conditions good properties of tribology than SCF when every single one is coupled with PI composites. SCF reinforced PI revealed to exhibit excellent wear rate and good friction coefficient comparing to pure PI. Moreover, tribological mechanisms of reinforced fillers were connected to the interaction between fillers and matrix [71]. Su et al. studied the influences of SiC nanoparticles filled carbon fabric/Polyimide Composites on the tribological properties. The preparation of SiC nanoparticles coupled carbon fabric/Polyimide was conducted using dip-coating and hot press molding methods. Under dry condition, friction and wear, characteristics were studied using ring-on-block contact mode. SiC nanoparticles had a significant effect on the wear rate of the CF/PI composite but have a little effect on the friction coef-ficient. A different contents value of SiC nanoparticles were arranged into composites, 1 vol% SiC nanoparticles were incorporated into the CF/PI composite and reduction of the wear rate was found. When SiC nanoparticles increased to 5 vol% the lowest wear rate (0.96 × 10 −6 mm 3 /N•m) was obtained which is about a 70% reduction compared to the unfilled composite [72].

Tribological Properties under Water-Lubricated Condition
Carbon fibers also showed great changes when incorporated with PI composites under the water-lubricated condition for different properties of materials

Tribological Properties under Oil-Lubricated Condition
Oil-lubricated condition plays also great effects on the tribological properties of composites. Zhao et al. under oil lubrication, studied tribological characteristics as ZnS sub-micrometer particles and reinforcing fibers incorporated into the PI matrix. Compositions of PI-based materials ( Table 2) were used to evaluate friction and wear properties. Different fibers such as Short carbon fibers (SCF), short glass fibers (SGF) were decorated with ZnS sub-micrometer particle, and then introduced to PI composite. Figure 6 showed characteristics changes of friction coefficient and wear rate due to loading and sliding speed, meanwhile, the effect of fillers was investigated. Comparisons were made accordingly; SGF and SCF significantly improved the friction and wear of composites material. The combination of SGF and ZnS particles also reduced the friction coefficient. As a single filler, SGF showed a low wear rate compared to others. Moreover ZnS/SGF/PI proved to have excellent wear resistance comparing to any single filler. The hybrid composite ZnS/SGF/PI also showed to be promising for tribological application under oil lubrication conditions [76].

Research Challenges
Graphene and CF-based polymer composites have been receiving significant attention lately because of their interesting and useful characteristics, such as good mechanical properties, tribological, and electrical. Due to quality and good performance, they showed to be considered as a replacement for traditional materials. Nevertheless, challenges still exist, for examples: 1) Disparities in different material properties of commercial graphenes, such as size, structure, and purity which affect the expected results [77].
2) The processing and manufacturing technologies of graphene and carbon fiber-based polymer composites are insufficient in quality and commercial value [78] [79].
3) Fabrication of materials depends on different parameters such as the con-  11) The high loading of CF as introduced to polymer lead to the formation of weak and brittle composites [87].
12) The preparation of carbon fiber matrix composites is very difficult. Due to the rod-shaped characteristics of carbon fiber, a higher matrix volume fraction is needed to achieve compact packaging. Also, the pores in the matrix filled with CF will lead to the formation of voids, which will eventually affect the mechanical properties of the composites [82].

Conclusion
With comparative tribological properties under different conditions graphene/ polyimide and carbon fibers/polyimide were reviewed, and each composite exhibit tremendous properties. The special structure and excellent properties of graphene make it a great application value in the friction field. Different lubricants, lubricating films, and wear-resistant materials can be prepared. The modification of graphene and its derivatives prepared with various contents % introduced into polymer composites have become one of the hot research topics.
As polyimide itself has some defects, such as poor antistatic performance and high wear rate, limit its further application in high-performance fields. The addition of graphene into polyimide can give the composite different functional properties (not only excellent mechanical and electrical properties but also excellent tribological properties). Different contents values of derivatives graphene coupled polyimide play a great role to enhance the properties of composites.
Tribological properties were reviewed, by increasing contents values composites showed an excellent reduction in friction coefficient as well as wear rate under dry, water, and oil-lubricated conditions. Furthermore, other nanofillers were introduced as decorating graphene derivatives such as nano-CuO, nano-SiO 2,