synthesis and tribological properties of graphene: a review · synthesis and tribological...

Jurnal Tribologi 13 (2017) 36-71

Received 13 Jan 2017; received in revised form 5 March 2017; accepted 20 April 2017.

To cite this article: Parveen and Wani (2017). Synthesis and tribological properties of graphene: A review. Jurnal

Tribologi 13, pp.36-71.

Synthesis and tribological properties of graphene: A review

Parveen Kumar, M.F. Wani*

Tribology Laboratory, Department of Mechanical Engineering, National Institute of

Technology, Srinagar, Jammu & Kashmir, India. *Corresponding author: [email protected]

HIGHLIGHTS

Synthesis methods of graphene.

Tribological behaviour of graphene as self-lubricating coating, as solid lubricant and as

reinforcement.

ABSTRACT

The target of this review is to explore the fundamental tribological behaviour of graphene, the first existing

two-dimensional (2-D) material and evaluate its performance as a self-lubricating material. The importance

and potential impact of this new class of material was recognised by the whole scientific community when

the Noble prize was awarded to Geim and Konstantin Novoselov for their discovery and development of

graphene in 2010. Graphene is the strongest material, chemically and thermally stable, gas-impermeable,

and atomically-thin. The fundamental tribological behaviour of graphene and other 2-D materials under

sliding conditions is recently being studied. Mainly, the wear of graphene has hardly been investigated. In

this paper, the latest developments in tribological applications of graphene, and preparation methods are

reviewed. It is shown that various graphene coatings, graphene as lubricant additive and as reinforcement in

metal matrix can be successfully employed to decrease friction and wear in tribological applications. A

comprehensive review is provided with the aim to analyse such properties of graphene. Moreover, the

application of graphene in the field of tribology for reducing friction and wear for better lubrication will be

explored.

Keywords:

| Graphene | Tribology | Friction | Wear and lubrication |

© 2017 Malaysian Tribology Society (MYTRIBOS). All rights reserved.


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1.0 INTRODUCTION

Energy resources are depleting at a very fast rate throughout world and designers

and engineers are under tremendous pressure to explore all available alternatives to

develop innovative and creative technologies to conserve energy, materials and preserve

environment. Poor friction and wear properties of material culminate into high energy

consumption, high wear rate and more emission to environment. Therefore, it is

inevitable not only to reduce friction but also increase the wear resistance of materials

used for design and fabrication of components and mechanical systems (Wani et al.,

2010). One of the oldest and most effective method of reducing friction and wear

performance of mechanical systems and components is accomplished by application of

mineral oil based lubricant at the interface in a tribosystem (Willing, 2001; Farhanah et

al., 2016). But due to the inherent toxicity and non-biodegradable nature of mineral oil-

based lubricants, the need of developing eco-friendly self-lubricating materials is

significantly increasing day by day. In order to enhance the performance and efficiency

of lubricant, various additives, such as antifriction, antiwear and EP additives are mixed

with the mineral oil. Liquid lubricants reduce friction by preventing sliding contact

interfaces from severe or more frequent metal-to-metal contacts or by forming a low-

shear, high durability boundary film on rubbing surfaces (Mercurio et al., 2004, Bartz,

1998, Nuraliza et al., 2016). For example, depending on the sliding speed and other

operating conditions, engine oils can effectively separate the contacting surfaces of

tribopairs and, thereby, prevent the direct metal-to metal contacts which results low

friction and wear. If and when there is metal-to-metal contact, the additives in these oils

form a low shear, highly protective boundary film to provide additional safety. In

addition, reducing friction and wear means higher fuel savings and longer durability of

tribosystems. Liquid lubricants are delivered to sliding interfaces through a pump or by

other kinds of oil delivery mechanisms, while solid lubricants have to be applied as thin

films using chemical vapor deposition (CVD) and/or physical vapor deposition (PVD)

methods. Due to their finite thickness, solid lubricant films wear out eventually and lose

their effectiveness. They are also very sensitive to the environments; in fact, some of

them, like MoS2, will not lubricate or last long if oxygen and/or water molecules are

present; indeed, graphite and boric acid will not function without humidity in the

surrounding air. Therefore, the most desirable properties of solid lubricant are:

environmentally insensitive, highly durable, and easy to deliver to the contact interfaces

(Beerschwinger et al., 1994, Kim et al., 2009, Kim et al., 2012, Woo et al., 2011, Kim et

al., 2009, Holmberg et al., 2012, Penkov et al., 2012, Penkov et al., 2013). Recently, it

has been established that graphene possesses excellent mechanical properties such as,

high strength, hardness, fracture toughness, and non-toxic/eco-friendly. Besides the use

of graphene as surface coatings or solid lubricants, graphene sheets are also considered to

be green lubricant additives as graphene contains C, H and O instead of N, S, P and


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heavy metal elements. In addition, graphene has no toxic particles. As such, graphene has

attracted the attention of scientist, engineers and designers in various applications

throughout the world (Geim et al., 2007, Novoselov et al., 2011, Stankovich et al., 2006,

Wintterlin et al., 2009, Ramanathan et al., 2008, Park et al., 2009, Avouris et al., 2012,

Taghioskoui, 2009, Singh et al., 2011, Peng et al., 2008, Schwarz et al., 1997, Yu et al.,

2000, Young et al., 2012). In view of superior physical, electrical, optical and mechanical

properties, graphene have recently been used in the field of tribology (Smolyanitsky et

al., 2012, Prasai et al., 2012, Chen et al., 2011, Spread, 1962, Shioyama et al., 2003, Li et

al., 2005). In addition, researchers have also used various methods for synthesis of

graphene to further improve its mechanical and tribological properties. It is therefore

necessary to understand the process of fabrication of graphene fully and also carry out

detailed literature survey to understand its friction and wear characteristics for reaping the

benefits in the field of tribology, as additive or as solid lubricant reinforcement in metals

and non-metals.

In the present review, various procedures for synthesis of graphene and detailed

discussions about tribological properties of graphene as used in self-lubricating coatings,

reinforcement in MMC’s and as lubricant additive is presented. Also, the operating

conditions at which graphene shows excellent tribological performance are evaluated

from the literature and presented in a useful manner. In this context, it is expected that the

present review will prove useful to researchers working in the field.

2.0 DIFFERENT METHODS OF GRAPHENE SYNTHESIS

Since the invention of graphene through mechanical exfoliation of pyrolytic

graphite, many approaches from several disciplines were used to fabricate it. Ones may

be close to “perfect”, mainly for research purposes but extremely expensive. Others may

not be that “perfect” but cheap enough to make it real and carry it to the industry. The

classification of various synthesis processes on the basis of quality and cost is shown in

Figure 1 (Ghaffarzadeh, 2016). Figure 1 shows that graphene produced through different

techniques has different qualities and characteristics. Various manufacturing techniques

include micro-cleavage, chemical vapor deposition, liquid-phase exfoliation, and

oxidisation-reduction were divided on the basis of graphene quality, cost, and scalability.

It has been observed that there is a trade-off between cost and scalability, on the one

hand, and graphene quality, on the other hand. This implies that certain techniques will

be better suited for high-volume applications with relaxed performance requirements,

while others serve applications demanding high-performance levels. As is evident from

Figure 1 that Scotch Tape has higher quality and cost whereas Thin Graphite has lower

quality as well lower cost. It is clear from Figure 1 that Chemical Vapor Deposition

(CVD) process (e.g. on copper, nickel, ruthenium…) meet both requirements in an


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optimal way. However, different synthesis methods led to “different qualities and

characteristics” of the graphene produced.

Figure 1: Plot of the main graphene synthesis methods regarding quality and cost (Y axis)

and scalability (X axis) (Ghaffarzadeh, 2016)

2.1 Mechanical Exfoliation

Mechanical exfoliation was developed by Geim and Novoselov in 2004 and

awarded with Noble prize in late 2010. It was the primary technique used to isolate one

monolayer of graphite. Its straight forward mechanism is predicated on repetition peeling

of extremely oriented graphite. Geim et al. (Novoselov et al., 2004) pressed patterned

highly oriented pyrolytic graphite (HOPG) sq. meshes on a photograph resist spun over a

glass substrate followed by repeated peeling using scotch tape and so released the flakes

thus obtained in acetone. Some flakes got deposited on the SiO2/Si wafer when dipped

within the acetone dispersion. By using this technique, atomically thin graphene sheets

were obtained. This technique was simplified to just peeling off one or a few/couple of

sheets of graphene using scotch tape and depositing them on SiO2 (300 nm)/Si substrates.

The graphene produced by mechanical exfoliation is of the highest quality (with least

defects) but this process is restricted as a result of low productivity.

Zhang et al. (2005) obtained 10–100 nm thick graphene sheets using Graphite

Island connected to a tip of micro machined Si cantilever to scan over SiO2/Si surface.

Folding and tearing of the sheets arise due to the formation of sp3-like line defects within

the sp2 graphitic network, occurring preferentially along the symmetry axes of graphite.

The flakes thus obtained vary significantly in size and thickness and the size varies from

nanometers to several tens of micrometers for single-layer graphene, depending on the

preparation of the used wafer. Single-layer graphene has an absorption rate of 2% and

possible to see it under a light microscope on SiO2/Si, due to interference effects.

However, it is difficult to obtain larger amounts of graphene by this method, not even


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taking into account the lack of controllability. The complexity of this method is basically

low, nevertheless, the graphene flakes need to be found on the substrate surface, which is

labour intensive. The quality of the prepared graphene is very high with almost no defects

(Novoselov et al., 2004, Zhang et al., 2005).

2.2 Chemical Exfoliation

Another possibility is to obtain graphene via wet chemical routes like chemical

exfoliation that which consists in the intercalation of a reactant among the graphene

sheets of graphite that softens the Van der Waals interactions. This is often achieved by

immersing graphite in an acidic solution (nitric or sulfuric acid). The soften interlayer

interactions may be broken then, by means of 2 steps: 1st step is a thermal method and

2nd eventually ultra-sonication to disperse them. The result consists of graphene

chemical compound sheets suspended in a very colloidal solution, which is deposited on

a substrate (Figure 2) (Parvez et al., 2014). For the ultimate objective of getting pure

graphene flakes, the chemical compound must be removed in a reducing atmosphere

using alkaline solutions, or applying hydrogen plasma, or reducing hydrazine vapors

(Figure 3), or through heat treatments (What is Graphene, 2016). Graphene flakes

partially oxidized are obtained because, unfortunately, the reduction processes are not

very efficient (a highest C/O ratio of ~17, what means a 5.5 wt. % of oxygen content

(Freire et al., 2014). Another disadvantage is that within the chemical exfoliation method,

the sp2 like graphene bonds are partially degraded to sp2-sp3 structures. The method

involving chemical exfoliation permits a correct management of the dimensions of the

graphene sheets. For example, the longer the sonication process/method, the smaller the

graphene. The graphene size range can be completed within the molecular approach,

wherever the production of polycyclic aromatic molecules (HBC, hexabenzocoronene)

reaches the dimensions comparable to the smaller graphene sheets obtained by different

approaches, whereas giving an endless path to mesoscopic and even macroscopic

dimensions (Soldano et al., 2010).

The main advantage of the chemical exfoliation is the high output that makes it

economically competitive and conjointly convenient to manipulate. Chemical exfoliation

is a very well-known method used for the production of composite materials, powder

coatings, inks and biological applications. This process is versatile because it is a low-

cost solution-phase method. It is scalable and would be capable of depositing graphene

on a wide variety of substrates, which is not possible using other processes. Furthermore,

this method can be extended to produce graphene-based composites and films, which are

the key requirements for special applications, such as thin-film transistors, transparent

conductive electrodes, and so on.


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Figure 2: Solvothermal-assisted exfoliation and dispersion of graphene sheets: (a) pristine

expandable graphite, (b) expanded graphite, (c) insertion of acid into the interlayers of

the expanded graphite, (d) exfoliated graphene sheets dispersed, and (e) optical images of

four samples obtained under different conditions (Parvez et al., 2014)

Figure 3: Schematic illustration of the possibilities to obtain graphene, graphene oxide,

graphite, and graphite oxide from each other (What is Graphene, 2016)

2.3 Epitaxial Growth

An alternative growth technique on a substrate is the epitaxial growth on

crystalline carbide wafers. By heating C-SiC at high temperatures in a very controlled

atmosphere of Argon (or even vacuum) the Si close to the surface is sublimated, not the

carbon atoms. For high enough temperatures (about 1300°C) the carbon reorganizes and

therefore the graphitization is achieved (Berger et al., 2004). The obtained result is in the

form of a very thin layer of graphite which under appropriate conditions produces

graphene monolayers (Figure 4) (Hass et al., 2008). From the various polytypes of SiC,

most of the studies are targeted on the hexagonal ones, being the foremost normally used

forms. Significant variations in graphene growth are found for the two polar faces of SiC:

SiC (0001) is Si-terminated and the SiC (000-1) is C-terminated. The high surface

roughness of graphene obtained from the Si-face or the lower sublimation temperature

from the C-face features limit the performance of graphene in determined applications

(Graphene Platform Supplies the World’s largest Single Layer Single Crystal Graphene


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Samples, 2016). Epitaxial growth is promising as a result of its simply deployable to the

semiconductor industry, different electronic devices, and transistors. On the other hand,

the standard of graphene under low-temperature processes still need to be improved as

compared to different technologies like CVD processes or mechanical exfoliation on

transition metals. However, the final extension of graphene is merely restricted by the

substrate (SiC wafer) area.

The main disadvantage of this method is that the graphene is not perfectly

homogeneous, due to defects or grain boundaries. Its quality is not as good as that of

exfoliated graphene, except the graphene would be grown on a perfect single crystal.

However, the size of the homogeneous graphene layer is limited by the size of the crystal

used. The possibility to produce large amounts of graphene by epitaxial growth is not as

good as by liquid-phase exfoliation, though the controllability to gain reproducible results

is given. Also, the complexity of these methods is comparatively low.

Figure 4: Illustration of an epitaxial growth on a SiC substrate after the sublimation of

silicon, carbon remains on the surface where it would become graphene later (Hass et al.,

2008)

2.4 Chemical Vapor Deposition (CVD)

Chemical vapor deposition is a well-known process in which a substrate is

exposed to gaseous compounds. In a CVD method, the graphene growth on a surface is

due to thermal decomposition of molecules of a hydrocarbon gas (propane, acetylene, and

methane) catalyzed by a metal surface or attributable to the segregation/precipitation of

carbon atoms from the bulk metal (Mattevi et al., 2011). Presently, transition metals are

widely used as catalysts in the production process of different carbon allotropes like

nanotubes. Therefore, it is not surprising /shocking that the transition metals (Cu, Ni, Re,

Ru, Ir, Co, Pt, and Pd) are the main focus of analysis for the production of graphene.

Transition metals are significantly appealing for getting large-area high-quality graphene

and for developing a method able to be integrated to the existent semiconductor industry.

The main disadvantage of the CVD methodology is the need for a step where the

graphene is transferred from the metal on to an additional appropriate substrate. CVD is a


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synthesis process in which the chemical constituents react in the vapor phase near or on a

heated substrate to form a solid deposit. The CVD technology combines several scientific

and engineering disciplines including thermodynamics, plasma physics, kinetics, fluid

dynamics, and of course chemistry. The number of chemical reactions used in CVD is

considerable and include thermal decomposition (pyrolysis), reduction, hydrolysis,

disproportionation, oxidation, carburization, and nitridation. They can be used either

singly or in combination. Figure 5 shows the process of chemical vapor deposition of

graphene on copper from methane, ethane, and propane: Evidence for bilayer selectivity

(Wassei et al., 2012).

It is used in electronic applications because CVD produces high-quality graphene.

In the creation of transistors or photovoltaic cells, the graphene must be correctly aligned

to most effectively transport the electrons across the band gaps. There are many

disadvantages of CVD that prevent it from becoming the preferred method of graphene

synthesis in the future. CVD process requires the usage of much specialized equipments,

harsh chemicals, materials, and high amounts of energy that makes it difficult to cost

effectively mass produce. For graphene to be used effectively in the market, its cost must

not increase the price of electronics unreasonably. Additionally, as the size of the

graphene sheet increases, the scope for error also increases because there is more scope

for the graphene to be miss deposited.

Figure 5: A schematic of one and bilayer graphene growth with ethane and/or propane

feedstock gas. (a) Top view is shown with a space-filling model and (b) side view is

shown with a ball-and-stick model. Copper atoms are shown as orange spheres, carbon in

black (first layer) and in blue (second layer), and hydrogen in gray (Wassei et al., 2012)

2.5 Chemical Synthesis Methods

The bottom-up chemical synthesis routes have the potential for large-scale

production of graphene at an affordable cost and may lead to a hypothesis shift in this

field. A number of patents related to graphene chemical synthesis methods have been


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filed (JP2009062241A, WO2009029984A1, KR2010108106A, US20110201739A1,

WO2011017338A2, US20120068124A1, and CN101462719A) (Sivudu et al., 2012).

Facile synthesis of graphene by Shankman (US20140227211 A1) has been a major

breakthrough for low-cost, facile and large-scale production of graphene from a carbon

source. The carbon source is preferably a sugar containing a 6-membered ring structure,

although many other carbonaceous materials may be subjected to dehydration, pyrolysis,

or oxidation and used (Shankman et al., 2015). This discovery has provided an impetus

for the large-scale production of inexpensive graphene, and therefore has the potential to

provide commercialization opportunities for real-world applications. Mainly some others

used methods for synthesis of GO are given as section 2.5.1 – 2.5.4.

2.5.1 Hummer Method

Graphene oxide can also be synthesized by Hummers method through oxidation

of graphite (Hummers et al., 1958, Paulchamy et al., 2015). The synthesis of graphene

oxide can be achieved through following steps:

1. Graphite flakes (2 g) and NaNO3 (2 g) are mixed in 50 mL of H2SO4 (98%)

in a 1000 mL volumetric flask kept under at ice bath (0-5°C) with continuous

stirring.

2. The mixture is stirred for 2 hrs at this temperature and potassium

permanganate (6 g) is added to the suspension very slowly. The rate of

addition is carefully controlled to keep the reaction temperature lower than

15°C.

3. The ice bath is then removed, and the mixture is stirred at 35°C until it

became pasty brownish and kept under stirring for 2 days.

4. It is then diluted with slow addition of 100 ml water. The reaction

temperature is rapidly increased to 98°C with effervescence till the colour

changes to brown colour.

5. Further this solution is diluted by adding additional 200 ml of water with

continuous stirring.

6. The solution is finally treated with 10 ml H2O2 to terminate the reaction by

appearance of yellow colour.

7. For purification, the mixture is washed by rinsing and centrifugation with

10% HCl and then deionized (DI) water several times.

8. After filtration and drying under vacuum at room temperature, the graphene

oxide (GO) is obtained as a powder.


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2.5.2 Modified Hummer’s Method of Synthesis

This modified method of synthesis involves both oxidation and exfoliation of

graphite sheets due to thermal treatment of solution. The stepwise synthesis method is

given as follows (W. S. Hummers et al., 1958, Paulchamy et al., 2015, S. D. Perera et al.,

2012):

1. Graphite flakes (2 g) and NaNO3 (2 g) is mixed in 90 mL of H2SO4 (98%) in a

1000 ml volumetric flask kept under at ice bath (0-5°C) with continuous

stirring.

2. The mixture is stirred for 4 hrs at this temperature and potassium

permanganate (12 g) is added to the suspension very slowly. The rate of

addition is carefully controlled to keep the reaction temperature lower than

15°C.

3. The mixture is diluted with very slow addition of 184 ml water and kept under

stirring for 2 hrs. The ice bath is then removed, and the mixture is stirred at

35°C for 2 hrs.

4. The above mixture is kept in a reflux system at 98°C for 10-15 min. After 10

min, change the temperature to 30°C which gives brown coloured solution.

5. Again after 10 min, change it to 25°C, and maintain the temperature for 2 hrs.

6. The solution is finally treated with 40 ml H2O2 by which colour changes to

bright yellow.

7. 200 ml of water is taken in two separate beakers and equal amount of solution

prepared is added and continuous stirred for 1 hr.

8. It is then kept without stirring for 3-4 hrs, where the GO particles settles at the

bottom and remaining water is poured to filter.

9. The resulting mixture is washed repeatedly by centrifugation with 10% HCl

and then with deionized (DI) water several times until it forms gel like

substance (pH- neutral).

10. After centrifugation the gel like substance is vacuum dried at 60°C for more

than 6 hrs.

The main drawback of this method is the use of strong oxidizing agents and small

lateral size of graphene nanosheets.

2.5.3 Improved Tour Method

The stepwise synthesis process is given as follows (Marcano et al., 2010):

1. Graphite powder (3 g) and Potassium permanganate (KMnO4) (2 g) are mixed

in 360 mL of Sulphuric acid (H2SO4) (98%) and 40 mL of Phosphoric acid

(H3PO4) kept under at 5°C with continuous stirring.


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2. The mixture is stirred for 12 hrs at 5°C temperature and then cool down the

solution

3. Ice water and 4 ml of hydrogen peroxide (H2O2) is added to the suspension

very slowly and stirred continuously. The addition rate of H2O2 is carefully

controlled to keep the reaction temperature lower than 15°C. Mango colour of

the solution is obtained at the starting.

4. The mixture is stirred about 1 hour at 5-10°C till the colour changes to brown

colour.

5. For purification, the mixture is washed with 200 mL of deionized water. It is

then filtered by the mixture of 20% of Hydrochloric acid (HCl) and 80% of

water (H2O) and finally filtered by 70 ml of ethanol.

6. After filtration and drying at room temperature, the graphene oxide (GO)

flakes are obtained as powder.

7. GO flakes are converted into few layer sheets by ball milling for a duration of

15 hours.

Improved method provides a greater amount of hydrophilic oxidized graphene

material as compared to other methods. Moreover, even though the GO produced by

improved method is more oxidized than that prepared by Hummers’ method, when both

are reduced in the same chamber with hydrazine, chemically converted graphene (CCG)

produced from this new method is equivalent in its electrical conductivity. In contrast to

Hummers’ method, the new method does not generate toxic gas and the temperature is

easily controlled. This improved synthesis of GO may be important for large-scale

production of GO as well as the construction of devices composed of the subsequent

CCG. Figure 6 shows the comparison between Hummers, Modified Hummers and

Improved methods of synthesis.

Figure 6: Representation of the procedures followed starting with graphite flakes (GF).

Under-oxidized hydrophobic carbon material recovered during the purification of IGO,

HGO, and HGO+. The increased efficiency of the IGO method is indicated by the very

small amount of under-oxidized material produced (Marcano et al., 2010)


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2.5.4 Hydrothermal Method

This method was proposed by Tang et al. (2012) and approaches to fabricate GO

aiming to develop a facile, safer and controllable technique. Glucose, sugar, and fructose

with deionized water solution are used as the sole reagent, utilizing the bottom- up

assembly techniques to grow GO with controllable number of layers from monolayer to

multilayer. The concentrations range from 0.075 to 0.8 M. 40 mL source solution is

poured into a 50 mL Teflin-lined autoclave. The growth temperature ranges from 160 to

220 oC with a growth period of 70 to 660 min. After that the autoclave is cooled to room

temperature, the graphene oxide nanosheet is rinsed with deionized water and then

transferred onto a desirable substrate for thermal annealing. The detailed preparation of

the graphene oxide nanosheets is schematically illustrated in Figure 7 (Tang et al., 2012).

This synthesis method is eco-friendly, facile, cheap and is capable of scaling up for mass

production.

Figure 7: Schematic illustration of preparing a graphene oxide nanosheet (Tang L et al.,

2012)

From the above various synthesis methods, chemical reduction of GO by

Improved Tour and Hydrothermal methods seems to be the most promising route because

it enables large-scale production of functionalized nano-graphene at low-cost. Moreover,

through chemical functionalization the unique physical and electrical properties of

graphene can be exploited for a wide range of mechanical and electronic applications.

However, GO synthesized by Hummer’s or modified Hummer’s methods require use of

strong and hazardous oxidizers such as sulphuric acid, potassium permanganate etc.

Moreover, the reduced graphene film is prepared by subjecting GO to chemical

treatments using highly toxic and unstable hydrazine, which requires utmost care. In view

of this, a number of green approaches for the synthesis of graphene and GO are being

developed. In addition, the current status of environmentally friendly approaches for the

mass production of graphene from GO have been comprehensively addressed. So, the

Improved Tour and Hydrothermal methods do not generate toxic gas and the temperature


48

is easily controlled. These synthesis methods are eco-friendly, facile, cheap and are

capable of scaling up for mass production.

2.6 Characterization Tools for Graphene

Various characterization tools for the analysis of graphene which include XRD,

FTIR, SEM and Raman spectroscopy are discussed in detail in (Hummers et al., 1958,

Paulchamy et al., 2015, Perera et al., 2012, Marcano et al., 2010, Tang et al., 2012).

Raman spectroscopy is a very important tool to characterize the structure of

graphene, by giving information regarding interactions of phonons and electrons in the

material while measuring the wavelength and intensity of light scattered off the

molecules. Raman spectra of graphite and a few layers of graphene are shown in Figure 8

(a). Typically, Carbon allotropes present three distinguished peaks G (1580 cm-1), D

(1350 cm-1), and 2D (2700 cm-1) (Dimitrios et al., 2013). The G peak ( 1580 cm-1)

(Dimitrios et al., 2013, Ferrari et al., 2006) is the crystallization peak of graphene and

arise due to the plane in the vibration of the sp2 carbon atoms. The G peak is present in

all carbon materials but the peak intensities and widths can vary. The D peak ( 1360 cm-

1) is related to defects in the layer and can, therefore, be used to observe damage in the

graphene structure (Dimitrios et al., 2013). The absence of the D peak implies a high-

quality single layer defect-free graphene. In the G, also called 2D band ( 2650 cm-1),

intensity decrease with increasing numbers of graphene layers (J.S. Park et al., 2009).

This 2D peak is the second order of the D peak. The 2D peak in bulk graphite consists of

two components. 2D1 and 2D2 measuring 1/4 and 1/2 the height of the G peak,

respectively. An increment in layers leads to a decrease of intensities of the lower

frequency 2D1 peaks (Figure 8 (b)) (Ferrari et al., 2006).

The ratio of the intensity of the D peak divided by the intensity of the G peak

(I(D)/I(G)) gives a measure of the level of defects in a graphene structure (Dimitrios et

al., 2013, Ferrari et al., 2006). The intensity ratio of the peak (I(2D)/I(2G)) is almost 4 for

single layer graphene and decreases when the number of layers increases (Dimitrios et

al., 2013, Ferrari et al., 2006). Raman spectroscopy can also provide information about

the presence of mechanical strain at the molecular level (Georgia et al., 2009). Georgia

Tsoukleri (Georgia et al., 2009) reported that in tension; the 2D peak decreases with

strain and the peak tends to be shifted to the left. In contrast, the peak is shifted to the

right side in compression and presents higher frequency than tension.


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Figure 8: (a) Graphite and graphene few layered Raman spectra (Dimitrios et al., 2013)

(b) 2D band changes with increment on layers (Ferrari et al., 2006)

3.0 TRIBOLOGICAL BEHAVIOUR OF GRAPHENE

Graphene has also played an important role in friction and wear reduction in

various tribological and mechanical applications. Therefore, the detailed behaviour of

graphene, used as an anti-friction and anti-wear additive in micro, nano, and self-

lubricating composite material at room and high temperature is discussed in the following

sections.

3.1 The Friction and Wear of Graphene at Microscale

Several researchers have made an attempt to apply graphene protective coatings

as a solid lubricant at the macro-scale (Penkov et al., 2014, Diana Berman et al., 2014).

Kim et al. (2011) introduced the frictional behaviour of graphene in tribology. The

frictional behaviour of CVD-grown graphene transferred to SiO2/Si substrates was

estimated under the normal loads up to 70 µN. It has been found that the friction

coefficient was affected by various parameters, as well as the material of the initial

substrate. The coatings grew on Nickle exhibit better frictional behaviour compared to

the coatings grown on copper. Moreover, it has been found that the transfer of graphene

from the particular substrate like Copper or Nickle considerably enhanced the friction

coefficient (approximately by two times). Finally, it has been suggested that graphene

samples that are few nanometers thick will be used as a possible solid lubricant at the

micro and nanoscale. It has been found that the tribological characteristics of graphene

coatings depend on the adhesion between the coating and the particular substrate. When


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the adhesion was high like as for the non-transferred protective coating on Nickle (Ni)

then the friction coefficient was considerably lower compared to the coatings transferred

onto Silicon dioxide (as shown in Figure 9) (Kim et al., 2011). The friction and wear

characteristics of multilayer graphene (MLG) improved when the graphene was tightly

joined to the particular substrate and exhibited nanoscale variations in thickness.

Several studies have investigated the frictional characteristics of graphene using a

micro-scale scratch test. In contrast to nano-scale scratch tests conducted with an AFM

tip, the contact pressure in the micro-scale scratch test can be significantly higher (Lee et

al., 2009, Lee C et al., 2009, Lin et al., 2011, Filleter et al., 2010, Li et al., 2010, Lee et

al., 2010, Cho et al., 2013). Shin et al. (2011) investigated the friction coefficient of

epitaxial and exfoliated graphene with few layers under normal load up to 0.5 µN using

an AFM tip radius of one metre. The author found that the number of layers of graphene

did not strongly depend on the friction coefficient. The friction coefficient for one, two

and three layers of graphene was found around 0.03. The authors additionally

investigated the result of structural defects within the graphene coatings on the

tribological properties (Shin et al., 2011). It has been found that the presence of defects

enhanced the friction by two times for both epitaxial and exfoliated graphene. By oxygen

plasma treatment the formation of defects was simulated.

Figure 9: Coefficient of friction for CVD-grown graphene transferred to SiO2/Si

substrates (Kim et al., 2011)

Won et al. (2013) have also confirmed the influence of defects occurred by the

friction on the tribological properties of graphene. The author reported that the friction

and wear mechanisms of CVD-grown graphene on copper substrates under a load of 20

µN and 1mm diameter chrome steel balls were used as a counter surface. It has been

found that for a given set of deposition parameters and the number of defects, the average

number of layers within the graphene coatings was dependent upon the growth time. For


51

an extended growth time, the number of layers increased and therefore the graphene

structure improved due to a major decrease in the defect concentration. It has been found

that by varying the number of layers from three to seven did not have an effect on the

friction coefficient, however thicker and less defective protecting coatings has

considerably higher sturdiness. The author investigated the failure mechanism and it has

been found that after a few number of sliding cycles, the formation of a disordered layer

occurred within the sliding track, that led to a major rise within the friction coefficient

(from 0.18 to 0.5).

Analysis on the tribological properties of epitaxial graphene is necessary for

understanding the fundamental mechanisms of friction and wear. However, from an

experimental point of view, the behaviour of graphene transferred to ordinarily used

substrates, like metals or plastics, should be examined. Yan et al. (2011) investigated the

friction of graphene transferred onto compound substrates under loads of up to 40 µN. It

has been found that the friction coefficient was considerably influenced by the applied

normal force. The results from the literature showed that the graphene was partially worn

by repeated tests, which can contribute to the gradual increase determined within the

frictional force with test number.

Marchetto et al. (2012) studied the friction and wear of single layer graphene

under conditions of multi-asperity contacts, which are typical for comparatively rough

substrates. Single layer graphene grown on SiC substrates was investigated and has been

found that intact graphene coverage exhibits lower friction coefficient. It has been

determined that the microscopic asperities burst the graphene layer throughout the

primary few sliding cycles, even at low loads. In consideration of the outstanding

tribological and mechanical properties of graphene, it would be expected that solely

continuous graphene protective coatings could be used at macro scale as a solid lubricant.

However, many studies have reported that the continuity of the graphene is not a

necessary condition. Berman et al. demonstrated that multilayer graphene flakes (MLGF)

may be successfully used as a solid lubricant for chrome steel (Berman et al., 2013,

Berman et al., 2013).

Literature shows that the friction coefficient will decrease by six times by

employing a low concentration of graphene flakes. This low friction behaviour was found

to persist for thousands of sliding cycles, even if the graphene layers shaped on the

sliding surfaces were not continuous or endlessly replenished and they were made of a

few sheets of graphene. The wear rates of the steel test pairs were also decreased by the

maximum amount as 2 orders of magnitude, despite the terribly irregular and thin nature

of the graphene layers. A potential rationalization for the low friction and wear reduction

behaviour is that graphene, as a 2- dimensional material, shears easily due to the weak

Vander Waal forces at the sliding contact interface (Berman et al., 2013). A similar type

of behaviour was reported by Pu et al. (2011) and Liang et al. (2013) and they

investigated the tribological properties of chemicals reduced graphene flakes (CRGF).


52

The benefits of graphene may be synergistically enhanced by using polymer

composite coatings, combined with diamond-like coating (DLC) and graphene flakes.

The structures may be deposited by numerous methods, like ion-beam deposition (Zhang

et al., 2012) and sputtering (Penkov et al., 2013). An outline of the frictional behaviour of

graphene at the micro-scale taken from literature is given in Table 1.

Table 1: Summary of Experimental Results of Graphene on Micro-Tribology (Kim et al.,

2011; Barman D et al., 2013) Deposition Parameter Friction Test Parameters

Gro

wth

subst

rate

Met

hod

Addit

ional

trea

tmen

t

Subst

rate

Ra

(nm

)

Counte

r

surf

ace

Conta

ct

pre

ssure

/

load

Condit

ions

CO

F

Yea

r

Ni

CVD - SiO2

1 Fused silica lens 5-10 μm Air, RT

0.15

2011 Cu 0.20

Ni Ni 0.05

- Exfol.

-

1 Diamond tip 0.05 μm Air, RT

0.03

2011 O2 plasma Si 0.07

- 0.03

O2 plasma SiC 0.07

SiC CVD - PET 5 SiO2 sphere 20 μN

Air, RT 0.08

40 μN 0.18 2011

- Solution - Si - Steel ball 100-400

μN Air, RT 0.12 2011

Ni Sputterin

g - Ni - Si3N4 ball 2 N Air, RT 0.06 2012

Cu CVD - Cu - Steel ball 220 MPa /

20 μN

Air, 24°C

/ 45% 0.2 2013

- Solution - Steel

440C - 1-5 N Air, RT 0.15 2013

- Solution - Steel

440C 15 440C ball 1-5 N H2 0.15 2013

3.2 Tribological Behaviour of Graphene as Reinforcement at Room

Temperature

Tai Z et al. (2012) studied the behaviour of ultra-high relative molecular mass

polythene (UHMWPE)/graphene oxide (GO) composite. The composite fabricated

through an optimised toluene-assisted mixing followed by hot-pressing. The tribological

and mechanical properties of pure UHMWPE and the GO/UHMWPE composites are

examined by using a micro-hardness tester and a high speed reciprocating tribometer.

The results show that, when the content of GO nanosheets is up to 1 wt%, then the wear

resistance and hardness of the composites are improved significantly, while the COF will

increase rapidly.


53

Min et al. (2014) investigated the graphene oxide (GO)/polyimide (PI)

nanocomposite films by situ polymerization and their tribological behaviours under dry

friction, seawater lubrication and pure water lubrication were comparatively investigated.

The GO/PI exhibited showed better results beneath seawater-lubricated condition than

other conditions, because of the excellent lubricating effect of seawater. The

reinforcement of GO greatly improved the thermal stability of the composites. The tensile

modulus and tensile stress of the nanocomposite matrix significantly improved by adding

graphene oxide (GO). The incorporation of GO under seawater lubrication can greatly

improve the wear and tear resistance of PI. Specifically, the best wear resistance obtained

with 0.5 wt% GO of the reinforced PI composite

Zhu Q et al. (2014) described that the dry sliding tribological tests of Ni3Al matrix

composites (NMCs) with 0.5 wt% graphene nanoplatelets (GNPs) sliding against

different counter face balls. The experiment carried out with an applied load of 10 N and

a sliding velocity of 0.234 m s-1. When NMCs sliding against GCr15 steel, a consistent

and thick friction layer is formed, leading to a lower friction coefficient. And when

NMCs sliding against Si3N4 and Al2O3, the formation and stability of the friction layers

are restricted within the severe wear regime, and therefore the NMCs exhibit higher

friction coefficients and wear rates. The friction layer will facilitate shear and reduce the

wear rate with the refinement and slippage of laminated sheets of GNPs throughout the

sliding process.

Yao J et al. (2014) explore the combined lubrication of WS2 and multilayer

graphene (MLG). The prepared sample of NiAl–5 wt% WS2 (NB)-1.5 wt% MLG

exhibited excellent tribological properties. The MLG play the role of reinforcement

particles and improved loading carrying ability. The addition of a combination of MLG

and WS2 offered a chance to prepare NASC that possessed superior antifriction and the

wear resistance.

Gonzalez CF et al. (2015) studied the dry sliding behaviour of a graphene/alumina

(Al2O3) composite against alumina in an open environment. The experiments were done

on the reciprocating tribometer with an applied load of 20 N, a sliding distance of up to

10 km and a sliding speed of 0.06 ms-1. Under the testing conditions, the graphene/

alumina (ceramic) composite showed a 10% lower friction coefficient and approx. half

the wear rate than the monolithic alumina. It has been also found that this behaviour is

related to the presence of graphene nanoplatelets (GNPs) adhered to the particular surface

of friction and GNPs form a self-lubricating layer that provides enough lubrication so as

to decrease both friction coefficient and wear rate, as compared to the Al2O3/ Al2O3

combination. These adhered graphene platelets act as a self-lubricating layer on the

contact surface between the composite and the Al2O3 ball that acts as counterpart

material.

Yazdani B et al. (2015) investigated the tribological performance of the hot-

pressed pure alumina and its composites containing numerous hybrid contents of


54

graphene nanoplatelets (GNPs) and carbon nanotubes (CNTs) under different loading

conditions by using the ball-on-disc type tribometer. Benchmarked against the pure

alumina, the composite reinforced with 0.5wt% graphene nano-platelets which reduced

the COF up to 23% and enhance the wear rate by 70%. The hybrid reinforcement

consisting of 0.3 wt. % GNPs and 1 wt. % carbon nanotubes (CNTs) and remaining

Al2O3 resulted in even better performance, with an 86% reduction in the wear rate.

GNPs played the vital role in the formation of a tribo-film on the worn surface by

exfoliation.

Llorente J et al. (2016) described the friction and wear behaviour of

graphene/silicon carbide (SiC) composites under the dry sliding conditions and using

Si3N4 (silicon nitride) balls as counter bodies and investigated as a function of the

graphene source and the graphene nanoplatelets (GNPs) content. GNPs composites

showed an improvement in the wear resistance as compared to monolithic silicon carbide,

with maximum enhancements of 70 vol. % for the material containing up to 20 vol. % of

graphene nanoplatelets. The friction performance depends on the GNPs content and

sliding distance. Under dry sliding conditions, the wear resistance of SiC ceramics

considerably enhances with the addition of GNPs. This response is joined to the

flexibility of the graphene fillers to be pulled out and exfoliated and making a wear

protective graphene-based tribo-film. The selection of the foremost appropriate material

for mechanical applications and tribological can depend on their specific operating needs,

20 vol. % graphene nanoplatelets composite clearly showing the best wear resistant

performance which is 70% more as compared to the monolithic SiC, whereas 5 vol. %

reduced graphene oxides composites exhibit an excellent fracture toughness.

Kalin M et al. (2015) investigated that the effect of the morphology of well-

reputed solid lubricant nanoparticles and of the material type of poly-ether-ether-ketone

(PEEK) composites on the mechanical and tribological characteristics. In the PEEK

matrix different type of nanoparticles were added, like graphene nanopowder, WS2

needle-like (WS2N), WS2 fullerene-like (WS2F) and carbon nanotubes (CNT). The results

obtained under dry sliding tribological conditions show that the material and therefore the

morphology of the nanoparticles have an important effect on the friction coefficient and

the wear, primarily by affecting their macroscopic hardness, also the thickness and

therefore the surface coverage of their transfer films. The carbon-based particles

deteriorated the wear and tear behaviour by 20 wt% (CNT) and the maximum amount of

three times in the case of the GNP.

Xiao Y et al. (2015) investigated the effect of graphene additive in self-lubricating

composites at different loads on the tribological application. The friction and wear

behaviours of nickel aluminium (NiAl) self-lubricating matrix composite with graphene

(NSMG) were studied at different loads at room temperature, within the load range of 2-

16 N and the results showed that NSMG has excellent tribological performance at load of

16 N due to the formation of anti-friction tribo-film on the worn surface.


55

Tabandeh et al. (2016) investigate the tribological behaviour of Al matrix

composites reinforced by graphene nanoplatelets (GNPs) and pure Al, pin-on-disk

experiments were carried out on the prepared samples. During the tests, the influence of

traditional load, reinforcement, volume fraction and the sliding speed on the tribological

performance was studied. Results showed that the wear and tear rate of Al-1wt. % GNP

is enhanced with increasing normal loads. However, the COF of the Al-1wt. % GNP

reduced with increasing normal loads. It has been found that the graphene nanoplatelets

(GNPs) reinforced nanocomposites showed excellent tribological properties and

illustrated the ability of the self-lubricating nature of the composite throughout the

tribological experiment.

Belmonte M et al. (2016) studied the tribological properties of graphene

nanoplatelets (GNPs)/Si3N4 composites for the first time by employing a reciprocating

ball-on-plate type configuration under iso-octane lubrication. Graphene nanoplatelets are

excellent nanofillers for improving the tribological performance of ceramics. Under the

high contact pressures, GNPs are able to decrease the friction and, especially, to enhance

the wear resistance up to 56% due to the exfoliation of the GNPs that creates an adhered

protective coating/ tribo-film. The exfoliation of the nano-platelets (NPs) generates

graphene flakes, which become a part of an adhered lubricating tribo-film that effectively

limits wear volume.

The brief summary of the test parameters of graphene as reinforcement composite

in different materials are given in Table 2.


56

Table 2: Summary of experimental results on tribology of graphene/composites

Materials Mfg.

Method

Counter

Surface

Test

Motion

Contact

pressure/load

Contact

velocity

Sliding

Distance COF

Wear rate

(mm3/Nm)/

Wt. Loss

(μg)

Year

UHMWPE

+0.1wt%GO Toluene-

assisted

mixing &

hot

pressing

ZrO2

Ball Recipro 5N 9cm/s -

0.115 1.35*10-5

2012

+0.3wt%GO 0.116 1.53*10-5

+0.7wt%GO 0.116 1.1*10-5

+1.0wt%GO 0.120 1.1*10-5

+2.0wt%GO 0.115 1.05*10-5

+03.0wt%GO 0.115 1.03*10-5

Polyimide

+0.1GO

Situ

polymer-

ization

Steel

ball Recipro 3N 0.156m/s -

0.41 6*10-6

2014

Polyimide

+0.3GO 0.46 13.5*10-6

Polyimide

+0.5GO 0.54 8*10-6

Polyimide

+0.7GO 0.49 7.9*10-6

Polyimide

+1GO 0.55 6.5*10-6

Ni3Al SPS

Al2O3

ball

Rotary 10N 0.234m/s -

0.38-

.44 9.3-12*10-5

2014 Ni3N4

ball

0.32-

.36 4.8-6.3*10-5

GCr15

ball

0.29-

.31 2.0-3.1*10-5

NiAl +1.5wt%

MLG +5wt%

WS2

SPS

Al2O3

ball

Rotary 10N 0.2m/s -

0.35 0.40*10-5

2014 Ni3N4

ball 0.21 0.17*10-5

WC-Co

ball 0.25 0.11*10-5

Al2O3

+0.22wt%

GNP

SPS Al2O3

ball Recipro 20N 0.06m/s 10km 0.67 3.5*10-9 2015

Al2O3

+0.5wt% GNP

SPS Si3N4

ball Recipro

5N

10mm/s -

0.40 -

2015

15N 0.48 2.3 µg

25N 0.58 6 µg

35N 0.54 5 µg

Al2O3 +2wt%

GNP

5N

10mm/s -

0.49 -

15N 0.55 7 µg

25N 0.54 9 µg

35N 0.53 40 µg

Al2O3 +5wt%

GNP

5N

10mm/s -

0.45 -

15N 0.54 62 µg

25N 0.51 80 µg

35N 0.45 160 µg

SiC +5vol%

GO

SPS Si3N4

ball Recipro 5N 0.1m/s 360m

0.71 1.4*10-3

2015 SiC +15vol%

GO 0.75 1.3*10-3

SiC +20vol%

GO 0.96 0.5*10-3

PEEK +2wt%

GNP SPS

100Cr6

pin Recipro 1MPa 0.05m/s - 0.62 8.8*10-6 2015

NiAl +1.5wt%

MLG SPS

Ni3N4

ball Rotary

2N

0.2m/s -

0.6-

0.8 2.82*10-5

2015 6N 0.5-

0.6 3.57*10-5

12N 0.5 4.02*10-5

16N 0.42 5.24*10-5

Al+0.1wt%

GNP SPS

440C

steel

disk

Recipro

5N

100rpm 1.13km

0.36 0.008grms

2015 10N 0.34 0.01grms

15N 0.25 0.016grms

Al+1wt% SPS 440C Recipro 5N 50rpm 1.13km 0.339 0.011grms 2015


57

GNP steel

disk

100rpm

150rpm

0.33 0.0051grms

0.33 0.0120grms

10N

0.36 0.0180grms

0.33 0.015grms

0.33 0.0155grms

15N

0.258 0.022grms

0.239 0.02grms

0.239 0.027grms

Si3N4 +3wt%

GNP SPS

Si3N4

ball Recipro

5N

0.1m/s 360m

0.225 1.0*10-3

2016 100N 0.185 1.1*10-3

150N 0.169 1.3*10-3

200N 0.16 1.5*10-3

3.3 Tribological Behaviour of Graphene as Reinforcement at High Temperature

Xu Z et al. (2015) investigated the self-lubrication characteristics of multi-layer

graphene and the high-temperature tribological properties of graphene titanium

aluminium matrix composite (GTMC) for the first time from a 100 to 700°C using a

rotating ball-on-disk configuration at an applied load of 10N and a constant speed of 0.2

m s-1. During the experiment the temperature varied from 100 to 550°C, MLG presents

the superb lubricative properties due to its extremely protecting nature and low shear. The

development/transition of lubricative properties of MLG occurs between 550 and 600°C.

Above 600°C, MLG lost their self-lubrication characteristics due to the formation of the

oxide layer which improves the oxidation resistance of GTMC by restricting the grain

boundaries and inhibiting the inflow of oxygen through grain boundaries. In addition, the

work provides valuable data to acquire deep knowledge to understanding the graphene’s

lubricative properties, also enrich the tribological theories of a self-protective layer of the

graphene reinforced composites. The brief summary of the test parameters of graphene as

reinforcement composite in different materials at high temperature are given in Table 3.

3.4 Tribological Behaviour of Graphene as Lubricant Additive

Lin J et al. (2011) showed when graphene platelets were chemically modified in a

reflux reaction with oleic and stearic acids. The tribological behaviour of the modified

graphene platelets (MGP) based lubricant was investigated using a four-ball tester. The

result shows that the oil containing only 0.075 wt% of MGP clearly reduced the wear and

load-carrying capability of the material.

Senatore A et al. (2013) investigated the tribological properties of graphene oxide

nanosheets in mineral oil under boundary and mixed lubrication and elasto-hydrodynamic

regimes. The experiments are carried on a ball-on-disc setup tribometer and verify the

friction reduction due to the addition of nanosheets prepared by Modified Hummers

Method and dispersed in oil. The results clearly prove that graphene platelets based

lubricant form protective film to prevent the direct contact between mating steel surfaces

and improve the frictional behaviour of the base oil.


58

Fan X et al. (2014) investigated the graphene as a lubricant additive and improve

the performance of oil/grease products in efforts to accumulate fundamental information

of its mechanical and tribological properties. As compared to ionic liquid and graphite,

multilayer graphene as a bentone lubricating grease additive reduce the friction and wear,

however additionally greatly improves the thermal stability and load bearing capacities of

bentone lubricating grease.

Zhe C et al. (2015) studied the performance of a lubricant which depends on the

additives it involves. However, presently used most of the lubricant additives produce

severe pollution when they are burned and exhausted. Therefore, it is necessary to

develop a new generation of green lubricant additives. Graphene oxide consists of only

C, O and H and therefore it is considered to be eco-friendly. The author found that, with

the addition of GO sheets, both the COF and wear are reduced by increasing the

temperature. Moreover, GO sheets have better performance under higher sliding speed

and therefore the optimized concentration of GO sheets is set to be 0.5wt%. After the

experiment, GO layer is found on the wear scars through Raman spectroscopy.

Dou X et al. (2016) reported the use of crumpled graphene balls as a high-

performance additive that can significantly improve the lubrication properties of poly

alpha olefin (PAO) base oil. The tribological performance of crumpled graphene balls is

weakly dependent on their concentration in oil and readily exceeds that of other carbon

additives such as graphite, reduced graphene oxide, and carbon black. PAO base oil with

only 0.01–0.1 wt % of crumpled graphene balls outperforms a fully formulated

commercial lubricant in terms of friction and wear reduction.

Azman et al. (2016) investigated the effects of graphene nano-platelets (GNP) as

additives in palm oil trimethylolpropane ester blended in PAO. Lowest COF and wear

scar diameter were obtained with the addition of 0.05 wt% GNP in blended PAO and

also, reduced by 5 and 15%, respectively.

Meng Y et al. (Meng Y et al., 2016) investigated the tribological properties of the

as-synthesized nano-composites as lubricant additives in engine oil were by a four-ball

tribometer. The engine oil with 0.06~0.10 wt.% Sc-Ag/GN nano-composites displays

remarkable lubricating performance, superior to the pure engine oil, the engine oil

containing zinc dialkyl dithiophosphate (ZDDP), as well as the oil dispersed with the

single nano-material of graphene oxides (GOs) and nano-Ag particles alone. The

remarkable lubricating behaviour of Sc-Ag/GN is expected to be due to the synergistic

interactions of nano-Ag and graphene in the nanocomposite and the action of the formed

protective film on the contact balls.

Rasheed et al. (2016) investigated the performance of graphene-based nano-

lubricant using a 4-stroke IC engine test rig and thermo gravimetric analysis. The

addition of 0.01 wt% graphene to API 20W50 SN/ CF resulted in 23% increase in

thermal conductivity (k) at 80 °C, and 21% reduction in the COF. Moreover, 70%


59

enhancement in heat transfer rate of the engine is also achieved in the presence of

graphene.

The brief summary of the test parameters of graphene as lubricant additive are

given in Table 4.

Table 3: Summary of experimental results on tribology of graphene/composites for high

temperature

Mat

eria

ls

Man

ufa

cture

d

Met

hod

Counte

r

surf

ace

Tes

t M

oti

on

Conta

ct

pre

ssure

/load

Conta

ct v

eloci

ty

Tem

p

(oC

)

CO

F

Wea

r ra

te

(mm

3/N

m)

Yea

r

TiAl+3.

5wt%

MLG

SPS Ni3N4

Ball Rotary 10 N 0.2 m/s

100

200

300

400

500

550

600

700

0.360

0.350

0.375

0.375

0.370

0.375

0.510

0.525

0.91 × 10-4

0.95 × 10-4

1.00 × 10-4

1.01 × 10-4

1.40 × 10-4

1.20 × 10-4

2.30 × 10-4

2.20 × 10-4

2015


60

Table 4: Summary of experimental results on tribology of graphene as a lubricant

Lub

rica

nt

Mat

eria

ls

Co

un

ter

surf

ace

Tes

t M

oti

on

No

rmal

load

(N)

Fre

qu

ency

(Hz)

/sp

eed (

rpm

)

Str

ok

e

(mm

)

CO

F

Wea

r ra

te

(mm

3/N

m)/

wea

r vo

l. (

μm

3)

Yea

r

SN 350+

0.075

wt%.

MGP

ALSI

52100

steel

ALSI

52100

steel ball

Rotary at

75 ± 20C 147N 1200 rpm 0.121 0.25 2011

SN 150+

0.1wt%

GO

X155CrV

Mo12-1

steel disc

X45Cr13

steel ball

Rotary at

250C

Rotary at

500C

Rotary at

800C

60N

1 m/s

1.5 m/s

2 m/s

1 m/s

1.5 m/s

2 m/s

1 m/s

1.5 m/s

2 m/s

0.134

0.141

0.152

0.131

0.141

0.153

0.141

0.15

0.16

2013

PAO40 +

0.05 wt %

GO

ALSI

52100

steel

ALSI

52100

steel ball

Recipro.

100

200

300

400

500

30 2

0.121

0.119

0.125

0.126

0.130

2014

PAO40 +

0.5 wt %

GO

ALSI

52100

steel

ALSI

52100

steel ball

Recipro.

100

200

300

400

500

30 2

0.118

0.118

0.116

0.118

0.124

2014

PAO40 +

0.1 wt %

GO

ALSI

52100

steel

ALSI

52100

steel ball

Recipro.

100

200

300

400

500

30 2

0.117

0.114

0.112

0.113

0.116

4×105 μm3

7×105 μm3

10×105 μm3

12×105 μm3

21×105 μm3

2014

PAO40 +

1 wt %

GO

ALSI

52100

steel

ALSI

52100

steel ball

Recipro. 300

10

20

30

40

50

2

0.09

0.16

0.112

0.113

0.114

2×105 μm3

4×105 μm3

10×105 μm3

24×105 μm3

228×105 μm3

2014

HC base

oil + 0.5

wt% GO

ALSI

52100

steel

ALSI

52100

steel ball

Recipro. at

500C

50

100

150

50 2

0.13

0.11

0.105

2015

HC base

oil + 0.5

wt% GO

ALSI

52100

steel

ALSI

52100

steel ball

Recipro. at

1000C

50

100

150

50 2

0.17

0.15

0.14

2015

HC base

oil + 0.5

wt% GO

ALSI

52100

steel

ALSI

52100

steel ball

Recipro at

1500C

50

100

150

50 2

0.18

0.15

0.14

2015

HC base

oil + 0.5

wt% GO

ALSI

52100

steel

ALSI

52100

steel ball

Recipro. at

500C 100

10

20

30

40

50

2

0.122

0.12

0.119

0..18

0.117

2015

PAO4 +

0.01 wt %

r-GO

PAO4 +

0.1 wt %

r-GO

52100

steel disks

M50 steel

ball Rotary 10N 10 mm/s

0.132

0.131

10.5x10-5

(m3.pas/N.m)

8 x10-5

(m3.pas/N.m)

2016


61

3.5 The Friction and Wear of Graphene at Nanoscale

The friction and wear behaviour of graphene at the nanoscale is under

investigation. Several researchers have made an attempt to apply graphene protective

coatings as a solid lubricant at the nano-scale (Penkov et al., 2014, Diana Berman et al.,

2014). Primarily graphene is perceived as a nano-material. Therefore, it is not stunning

that almost all studies are dedicated to the tribology of graphene at the nano-level. AFM

is the most ordinarily used instrument to perform such studies. The investigation of the

friction coefficient of graphene was firstly reported by Lee et al. (Lee et al., 2009) during

the test frictional forces on graphene and graphite were obtained by employing a Si3N4

AFM tip. It was found that the behaviours of graphene and graphite are quite completely

different, although each material has a similar surface in terms of morphology and

chemical structure. The investigators confirmed that the resistance force between

associate AFM tip and graphene does not rely on the presence of a substrate and reduces

PAO 10 +

0.01 wt %

GNP

+ 0.03

wt% GNP

+ 0.05

wt% GNP

+ 0.1 wt%

GNP

+ 0.2 wt%

GNP

+ 0.5 wt%

GNP

+ 1 wt%

GNP

+ 3 wt%

GNP

52100

steel ball

52100

steel ball Rotary 392 N 1200 rpm

0.08

0.081

0.073

0.087

0.081

0.080

0.086

0.089

2016

10W40 +

0.1 wt %

GO

10W40 +

0.1 wt %

Ag/GN

10W40 +

0.1 wt %

Sc-Ag/GN

GCr15

steel ball

GCr15

steel ball Rotary 343 N 1200 rpm

0.09

0.09

0.078

2016

SN/CF20

W50+addi

tive + G60

SN/CF20

W50+

G60

+ G12

+ G8

SJ/CF20W

50+additiv

e + G60

SJ/CF20W

50+ G60

+ G12

+ G8

Steel ball Steel ball Rotary 40 N 1200 rpm

0.011

0.014

0.013

0.014

0.017

0.018

0.017

0.017

2016


62

with thickness for samples with one to four layers. Because the correlation between the

frictional behaviour and also the number of graphene layers has become evident,

additional investigations were performed to clarify this development (Lee C et al., 2009,

Lin et al., 2011, Filleter et al., 2010). As an example, for graphene grown on silicon

carbide (SiC), the friction on a single-layer graphene was found to be twice greater than

that in a bilayer film. It was found that the friction does not arise from the distinction in

lateral contact stiffness, structural properties or contact potential. The friction force of

graphene was reduced because the number of layers increased. Furthermore, based on the

force-distance relationship, it was suggested that the distinction in friction between

graphene and graphite is related to weak Van der Waals forces.

However, very little literature is available on friction and wear of graphene. Lin et

al. (2011) reported the friction and wear of multilayer graphene (MLG) synthesis by

exfoliation. It has been found that graphene films exhibited a lower friction than bare

silicon (Si) surface once the applied loads ranged from 3 to 30 nN. Detectable wear of the

graphene occurred when the sliding of the surfaces is performed for a hundred cycles

under a 5-N applied load. Li et al. (2010) investigated that by increasing number of layers

the friction and wear reduced due to the adhesion force between the graphene and the

substrate. A typical schematic of associate AFM friction experiment is shown in Figure

10 (Filleter et al., 2010). The primary preparation to link the anticipated tribological as

well mechanical properties of graphene was discussed by Lee et al. (2010). The author

described the elastic properties associated frictional characteristics of graphene with

numerous numbers of layers victimization by an AFM. To calculate/measure the

mechanical properties of the material, graphene was suspended over the micron-sized

circular holes and indented by an AFM tip. The force-displacement curves provided by

the author describe the ultimate strength of graphene, which was found to be the highest

measured value of any material (1TPa). It was shown that the number of layers (ranging

from 1-3) failed to influence the intrinsic stiffness or strength of graphene. The authors

additionally investigated graphene on Silicon dioxide/Silicon substrates and graphene

freely suspended over wells which increase the friction because the number of graphene

layers increased. This increasing trend of friction with the decreasing thickness was

absent for graphene deposited on translucent substance (mica), however the graphene is

powerfully bonded to the substrate. These measurements combined with a model

projected by the researchers counsel that mechanical confinement to the substrate plays a

vital role within the frictional behaviour of graphene. Loosely bound or suspended

graphene sheets will pucker within the out-of-plane direction because of tip-graphene

adhesion. This behaviour will increase the contact area and also permits for additional

deformation of the graphene throughout sliding, resulting in higher friction. As a result of

thinner samples have a lower bending stiffness, frictional resistance and the puckering

effect are higher for thin layers of graphene. However, if the graphene is powerfully

bound to the substrate, the puckering effects are suppressed and no thickness dependence


63

ought to be ascertained (Filleter et al., 2010). Similar results were discussed by Cho et al.

(2013) and the author made a comparison of the friction of graphene on numerous

substrates, like h-BN, bulk-like graphene, SiO2 and mica, the result of adhesion level

between the graphene and also the underlying surface on friction (Sasaki et al., 2010). It

was found that the friction of graphene on associate atomically thin substrate, like h-BN

or bulk-like graphene is low and admires of bulk like graphene. In distinction, the friction

of graphene folded into bulk-like graphene which is identical to that of the single-layer

graphene on silicon dioxide (SiO2), despite the ultra-smoothness of bulk-like graphene.

Berman D et al. (Berman D et al., 2015) demonstrate that super lubricity can be achieved

at engineering scale when graphene is utilized in combination with nano-diamond

particles and diamond-like carbon (DLC). Macroscopic super lubricity originates because

graphene patches at a sliding interface wrap around nano-diamonds to form nano-scrolls

with reduced contact area that slide against the DLC surface, and significantly reduced

coefficient of friction (~0.004).

Figure 10: (a) Optical image of exfoliated graphene on SiO2/Si, showing the dimension

and thickness contrast of the flake; (b) a schematic of an AFM tip scanning over a flake

containing areas with different thicknesses (Filleter et al., 2010)

A brief overview of the frictional behaviour of graphene from the literature at the

nano-scale is given in Table 5.

This review highlighted the recent developments regarding the synthesis methods

and the use of graphene in enhancing the tribological behaviour at nano to macro-scales.

Further, recent studies dealing with the use of graphene as an additive for lubricating oils

and fabrication of graphene-based self-lubricating nano-composite materials are

presented. Graphene was shown to be very useful for friction and wear reduction not only

as an additive in oils but also in coatings and composite materials. The literature suggests

a decrease in friction with the increase in the number of layers of graphene as confirmed

by AFM. Micro-scale tribological studies showed that introduction of defects in graphene

increases friction. Tribological studies of graphene as lubricant additive clearly

confirmed how graphene reduces the friction and wear irrespective of atmospheric

conditions (humid or dry) and also acts as a perfect passivation layer. Macro-scale studies


64

demonstrated that graphene layers largely suppress the friction and wear of the sliding

interfaces.

It is evident from the literature review that graphene has been already used to

improve the tribological performance at nano, micro and macro level in various

applications at room temperature. However, it has not been widely explored for high

temperature applications like those in space industry, I.C. engines etc. Hence research

needs to be carried out on the high temperature tribological properties of graphene for its

efficient utilization in high temperature applications.

Table 5: Summary of experimental results on nano-tribology of graphene. Deposition Parameters Friction Test

Growth

Substrate Process Substrate

Ra

(nm)

Counter

Surface

Contact

pressure/load Conditions COF Year

Exfol Si 0.5 SiN 0.01~0.5 nN Air, RT 0.025 2009

Exfol SiO2 0.5 Si 1 nN Air, RT 0.6-0.15 2009

6H-SiC Thermal

decomp SiC Si 40 nN Air, RT 0.2 2010

Exfol

SiO2 0.5 Si

1 nN

Air, RT

2010 Si 0.5 Si Air, RT

Exfol SiO2 0.5 DLC 5 nN Air, RT 0.4-1.0 2010

Exfol Si 0.5 Si 3~30 nN Air, RT 0.02 2011

Solution Si 0.5 Si 1 nN Air, RT 0.10 2011

Solution Graphene

coated SiO2

DLC

coated

SiO2 ball

SiO2 ball

1 N

Dry Nitrogen

0.004

0.04

2015

CONCLUSION AND FUTURE SCOPE

Graphene has been gaining interest as a unique and attractive material to be

employed in numerous applications like electronics, mechanical and tribological systems.

According to recent works on graphene by various researchers, the tribological properties

of graphene were reviewed. Despite the ultra-thin nature of graphene sheets, they were

found effective in enhancing the friction and wear not only under micro-scale contact

loads but also in relatively high loads under the dry, lubricated, and as well as in high-

temperature conditions. Graphene was also found to be effective even on metallic

substrates with typical surface topography. Graphene nano-particles have contributed

directly to the latter tribofilm formation which is more dominant for the reduced

coefficient of friction when it is used as lubricant additive and as reinforcement. With the

growth of more economical graphene synthesis methods, which explore the application of

graphene in the tribology, the employment of graphene in tribological applications is

expected to grow continuously in the near future.


65

ACKNOWLEDGEMENTS

I would like to express sincere gratitude to my colleague Mr. Jebran Khan and

Mr. Jagtar Singh and also very special thanks to Mr. Dharmender Jangra for their

guidance and encouragement in carrying out this review.

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