Synthesis and tribological properties of graphene: A review · Synthesis and tribological...
Transcript of 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
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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
Jurnal Tribologi 13 (2017) 36-71
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).
Jurnal Tribologi 13 (2017) 36-71
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.
Jurnal Tribologi 13 (2017) 36-71
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
Jurnal Tribologi 13 (2017) 36-71
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.
Jurnal Tribologi 13 (2017) 36-71
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.
Jurnal Tribologi 13 (2017) 36-71
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
Jurnal Tribologi 13 (2017) 36-71
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.
Jurnal Tribologi 13 (2017) 36-71
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%
Jurnal Tribologi 13 (2017) 36-71
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
Jurnal Tribologi 13 (2017) 36-71
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
Jurnal Tribologi 13 (2017) 36-71
61
3.5 The Friction and Wear of Graphene at Nanoscale
The friction and wear behaviour of graphene at the nano- scale 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
Jurnal Tribologi 13 (2017) 36-71
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
Jurnal Tribologi 13 (2017) 36-71
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
Jurnal Tribologi 13 (2017) 36-71
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.
Jurnal Tribologi 13 (2017) 36-71
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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