Graphite is said to be known as one of the forms of carbon present in usually crystalline form. This too has various types and varieties in which graphite can be exhibited. However, recently it has come to light that the anti-friction coatings have a vast portion of graphite applications which enable them to flourish in the industries. 

The properties that graphite exhibits are phenomenal and speed up their productivity and functions. Graphite is also present in the form of carbon nanotubes and a major portion of the anti-frictional applications are exhibited by these carbon nanotubes.

Introduction

Graphite is carbon’s crystalline form in which the carbon atoms are arranged in a hexagonal structure. This is graphite’s natural form and appearance. Archaically, the word plumbago is used to refer to graphite. Under standard conditions, graphite is carbon’s most stable form. Graphite converts to diamond under high temperatures and pressures. Graphite is a good electricity and heat conductor and is utilized in lubricants and pencils. Due to the high conductivity of graphite, graphite is useful in electronic products like solar panels, batteries, and electrodes.

Types and Varieties

Natural graphite is of different types, each taking place in different ore deposit types. Some of them are;

• Flake graphite (Graphite’s crystalline small flakes) occurs as flat, isolated, plate-like particles. If it is not broken, then it has hexagonal edges however the edges of flake graphite can be angular or irregular if it is broken.
• If the flake graphite is very fine, then sometimes it is called amorphous graphite.
• In fractures or fissure veins, vein graphite (lump graphite) is used. Lump graphite occurs as massive plate intergrowths of acicular or fibrous crystalline aggregates. In origin, it is possibly hydrothermal.
• Pyrolytic graphite of high order refers to graphite in which there is an angular spread between the graphite sheets that are less than 1°.
• Sometimes, graphite fiber is used for referring to the carbon-fiber-reinforced polymer or carbon fibers.

Occurrence

Graphite occurs in meteorites (with silicate and troilite minerals), igneous rocks, and metamorphic rocks because of sedimentary carbon compounds reduction during metamorphism. Tourmaline, micas, calcite, and quartz are the associated minerals with graphite. In tonnage order, China, Mexico, Canada, Brazil, and Madagascar are the mined graphite's principal export sources. In meteoritic iron, there are small graphitic crystals that are known as cliftonite. Distinctive isotopic compositions are possessed by some microscopic grains which tell that those grains were produced before the Solar system. The Solar System is predated by 12 known mineral types and they are one of those 12. One can also detect them in the molecular clouds. These minerals were made when less or medium-sized stars expelled their outer enveloped or supernovae exploded. In the universe's list of oldest minerals, graphite comes third or second.

Properties of Graphite on Anti-friction Coatings

Structure

The chemical bond's type determines the form of a solid. Such different forms are called allotropes. Graphite and diamond are two of the most common allotropes. Buckminsterfullerene is the less common allotrope. The bonds are sp3 orbital hybrids in diamond and tetrahedra is formed by the atoms with four of the closest neighbors whereas the bonds are sp2 orbital hybrids in graphite and the atoms are formed in planes with each atom bound to the three nearest neighbors that are 120 degrees apart. The individual layers are known as graphene. The carbon atoms are organized in each layer in a honeycomb lattice with 0.142 nm bond length, and 0.335 nm of the distance between the planes. There is a covalent bond between the atoms in the plane. There was a total of 4 bonding sites, out of which only 3 were satisfied.Graphite is an electrical conductor as its fourth electron can migrate freely in the plane. Graphite’s layers can slide past each other or be separated easily because of the weak Van der Waals bonds between the layers. Perpendicular to the layers, electrical conductivity is about 1000 times less.

Other Properties

Since phonons move slowly from one plane to another but can propagate along the tightly bound plains quickly, that’s why graphite’s thermal and acoustic characteristics are highly anisotropic. The broad utilization of graphite as refractories and electrodes in high-temperature material processing applications is because of the high thermal and electrical conductivity of graphite along with the high thermal stability. Although, at 700 C and above temperature, graphite willingly oxidizes for forming CO2 in oxygen-containing atmospheres.Electrical conductor:

Graphite is beneficial in such applications as an electrode of a lamp because it is an electrical conductor. Electricity can be conducted by graphite because of the huge electrondelocalization between the layers of carbon (known as aromaticity). These valence electrons can conduct electricity as they can move freely. Although, primarily, the electricity is conducted between the plane of the layers. Graphite is capable of being utilized in carbon microphones as a pressure sensor due to powdered graphite’s conductive characteristics.Graphite powder:

Due to their dry lubricating and self-lubricating characteristics, Graphite and graphite powder are of high worth in industrial applications. It is believed by scientists that the lubricating characteristics of the graphite are because of the loose interlamellar coupling between the sheets in the structure. Although, graphite degrades in a vacuum as a lubricant because of the hypoxic conditions. This led to a hypothesis that the fluids (water and air) presence within the layers is the reason for lubrication. Naturally, the absorption of water and air occurs from the environment. Studies refuted this hypothesis by proving that water and air are not absorbed. The lubricating characteristics of graphite also include superlubricity according to recent studies.Graphite's usage is restricted because it promotes galvanic corrosion between different metals because of its electrical conductivity and its facilitation in pitting corrosion in the stainless steel. In moisture’s presence, it is also corrosive to aluminum. Due to this reason, its utilization as a lubricant is banned by US Air Force in aluminum aircraft and they also discouraged the lubricant’s usage in the aluminum-containing automatic weapons. Corrosion can be facilitated even by marks of graphite pencil on the aluminum parts. Hexagonal boron nitride is another high-temperature lubricant, and its molecular structure is the same as graphite. Because of the extremely similar characteristics of hexagonal boron nitride, it is also known as white graphite.Crystallographic components:

Graphite loses its characteristics of lubrication when a large amount of crystallographic defects binds these planes together and turns graphite into pyrolytic graphite. Graphite floats above a strong magnet in mid-air as it is diamagnetic and highly anisotropic. It is isotropic turbostratic if it is made at 1000-1300 C in a fluidized bed. Isotropic turbostratic is not diamagnetic and is known as pyrolytic carbon. It is also utilized in blood-contacting devices, for instance, mechanical heart valves. Despite being confused together often, it is clear that pyrolytic carbon and pyrolytic graphite are extremely different materials. Because of crystalline and natural graphite’s inconsistent mechanical characteristics, brittleness, and shear-planes, the crystallite and natural graphites are not utilized much in pure form as structural materials.

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Applications of Graphite on Anti-friction Coatings

The characteristics of graphite, for instance, low thermal expansion coefficient and solid lubricating, are possessed by the copper–graphite composites along with the characteristics of copper, for instance, remarkable electrical and thermal conductivities. In many applications, copper-graphite composites are broadly utilized as bearing materials, they are also utilized broadly as brushes.

Moustafa et al. studied copper-graphite compositions’ friction coefficient and wear mechanism. Such composites are made up of powder metallurgy from the graphite particles that were Cu-uncoated and Cu-coated. In Cu-uncoated, graphite had a volume of 0-20%. According to them, at severe, mild, and low wear regimes, pure copper’s wear mechanisms can be seizure-wear, delamination, and oxidative-dominated mechanisms, respectively. Although, the same wear mechanisms, for instance, sub-surface delamination, high strained delamination, and oxidative induced delamination, were exhibited by both Cu-uncoated and Cu-coated graphite composites. According to them, at constant load, the higher the content of graphite is in either uncoated Cu- or coated Cu-graphite composite, the lower is the coefficient of friction.Rohatgi et al. stated that the friction coefficient of metal matrix composites approaches the friction coefficient of the pure graphite and doesn't depend on the content of graphite when metal matrix composites' content of graphite is more than about 20 vol%. At constant load, copper graphite composites' friction coefficient was affected by the composition according to Kovacik et al. and he also researched the effect in 0-50 vol% graphite range, for determining the critical content of graphite because after that content limit is passed, the composites’ coefficient of friction stays constant and independent of composition.

When the graphite’s concentration increased, at first the wear rate and coefficient of friction of uncoated and coated composites decreased but after graphite’s particular concentration threshold is reached, then the composites’ coefficient of friction does not depend on the composition and becomes equal to the used graphite material’s friction coefficient however there comes a decrease in their wear rate too. In coated and uncoated graphite particle composites, the concentration threshold will be 25 vol% and 12 vol%. There is no established relationship between metal matrix composites’ wear resistance, processing parameters, and microstructural factors because of the complication of wear mechanisms. According to recent researches, their friction coefficient is strongly affected by the size of the particles, graphite particles’ spatial distribution, and volume fraction of the particles.

Higher Mechanical Properties

Carbon nanotubes and fibers are used in preference to the graphite particles due to their higher mechanical characteristics. Carbon fibers possess good wear resistance and good lubrication effect along with high modulus and high strength. The matrix was reinforced by the carbon fiber and it also enhanced the composite’s tribological characteristics as a solid lubricant. Thus, as compared to the wear resistance of pure metals, the wear resistance of the carbon fiber-reinforced composites is higher. Moreover, the investigations of graphite’s effect on high temperature wear proved that composites can be protected from high-temperature wear by adding graphite particles to Cu–SiC composite.Some of the methods like centrifugal casting and powder metallurgy are utilized for the production of these composites. Graphite particles’ agglomeration is the major problem in these methods. Graphite particles agglomerate into graphite clusters due to graphite’s lower density as compared to copper. Moreover, one other reason for graphite particles’ agglomeration is being greasy. Agglomeration of the graphite particles can be reduced by using copper to coat graphite particles by the electro-less process. Porosity is present in the final product in both casting processes and powder metallurgy.

Brake Linings

For heavier vehicles (non automotive), fine flake and natural amorphous graphite are utilized in brake shoes or brake linings, and they turned important when they were considered as a substitute for asbestos. For quite some time, this usage has been significant but now the market share of graphite is starting to get reduced by non-asbestos organic (NAO) compositions. In 2005, brake lines consumed 6510 tonnes of US natural graphite consumption according to the USGS.

Graphite and Carbon-bearing Materials

When the mixtures of coal tar and petroleum coke are being pressed and heat-treated with natural graphite’s small amount, the graphitic carbon-bearing materials are formed. Under low specific holdings, in air, and at 480 C or more temperature, they are utilized as the bearing materials for operating without lubrication. The graphitic carbon-bearing materials impregnated with resins or metals are also made. Graphite basically makes up carbon nanotubes and that's why they exhibit applications in regard to anti-friction coatings. Following are the applications that they exhibit.

Oil-based Lubricant

One of the promising proposed concepts is the idea of nanoparticle-assisted lubrication. Solid particles are already contained by many lubrication oils. In the case of debris or soot contaminants, they are either added deliberately or accidentally present. Dichalcogenides and their derivatives like WS2 and MoS2 are examined for the latter. Operating conditions, for instance, high humidity and high temperature, massively restricted the solid lubricants from being used according to an application point of view.

The lubricant’s oxidation may get activated from the heat that arises from the process of friction, leading to the lubricant’s malfunction. Nanocomposite lubricants were made by Zhang et al. when he electrodeposited molybdenum disulfide on the vertically aligned carbon nanotubes. Amazingly, when nanocomposite film was rubbed on dry sliding at 300 C against alumina ball, the friction coefficient that will be achieved will be as low as 0.04. This depends on Carbon nanotubes’ very high thermal conductivity, as it dissipates the generated energy instantly inside the matrix, hindering the friction heat’s local accumulation, and thus delaying the process of oxidation.

Commercial Machine Oil

CNT composites have a positive tribological role which can be seen as there can be further reduction in the friction coefficient by around 15 % when these composites are involved in the commercial machine oil. During the friction process, gradual exfoliation of the particles’ external sheets occurs, leading to metal dichalcogenides’ lubrication effect. Then there is a subsequent transfer of lubrication effect onto the reciprocating surfaces, thus forming a tribofilm that has a nano-scaled thickness. According to this material transfer mechanism, the lifetime of this lubricant is finite, usually short. Attempts have been made on using carbon nanotubes as novel lubricant additives as an alternative.

Carbon nanotubes can easily agglomerate together to form rope/bundles as they have a great propensity for agglomeration and they have high surface energy. Due to this reason, they have poor solubility in the liquid media. 2-(1-butylimidazolium-3-yl) ethyl methacrylate and 1-butyl-3-methyl imidazolium hexafluorophosphate have been used for functionalizing multi-walled carbon nanotubes so this barrier can be eliminated. Both the wear volume and the friction coefficient decrease significantly when they are added to the base lubricant. The micro-gap in the rubbing surfaces are filled by the functionalized multi-walled carbon nanotubes during friction and they also deposit there. A self-assembly lubricating thin film is then formed which protects and prevents the surface of the specimen from extreme wear.

In Situ Polymerization

Carbon nanotubes can be incorporated into polymer matrix by using one of the most famous methods, known as In situ polymerization. Until now, this method has been successful in fabricating polyacrylonitrile/ methylmethacrylate/MWCNT co- polymer composites, poly- styrene/acrylonitrile/MWCNTs composites, and poly (methyl methacrylate)/ styrene/MWCNTs copolymer composites. In all of these studies, one thing remains common and that is a serious concentration of carbon nanotube with respect to the composites’ tribological performance.

When the concentration of carbon nanotube is lower than the critical concentration, there comes a substantial decrease in the wear rate and friction coefficient. When this value is exceeded by the concentration of carbon nanotube, then there comes an increase in both wear rate and friction coefficient.

Detrimental Effects

Carbon nanotubes' agglomeration is the reason for the harmful effects of excessive carbon nanotubes. Multi-walled carbon nanotubes were functionalized by Huang et al. with maleic anhydride using Friedel-Crafts acrylation. Then the in situ solution polymerization was done to incorporate the resulting multi-walled carbon nanotubes into poly (methyl methacrylate). With a 1 % increase in a load of multi-walled carbon nanotube, the obtained composite's weight loss also increased. If the concentration of CNT is increased further to 5 wt. %, it leads to a little decrease in weight loss. According to the authors, a grinding wheel can cause unsustainable microscale damage and can scratch the composites easily at this concentration as that microscale damage can’t be sustained by the nanoscale of MWCNTs at this concentration.

Polyphenylene Sulfide Composites

In multi-walled carbon nanotube reinforced polyphenylene sulfide composites, the change in friction coefficient against the concentration of MWCNT was tiny. After 0.2 vol % multiwalled carbon nanotubes were added, the specific wear rate decreased but with more increase in the concentration of MWCNT, the wear rate became larger. The author commented on this data that carbon nanotubes may lack the lubricating capability however MoS2 and graphite don't lack that capability. Authors have explained it by saying that the by-products in pristine multi-walled carbon nanotubes may offset carbon nanotubes’ desirable effect.

The critical concentration of carbon nanotube varies from system to system and the presented results are sometimes controversial. However, only through trial and error, one can find the true magnitude. Another challenge is developing a theoretical model which can accurately predict the true magnitude. A descending trend is displayed by the friction coefficient of carbon composites/multi-walled carbon nanotubes and an ascending trend is displayed by the wear volume when the load is increased from 50 N to 250 N. The friction coefficient’s scale changed to 0.13-0.32 from 0.16-0.42 when there was an increase in the sliding speed to 0.84 m/s from 0.42 m/s. This change showed the friction-reduction trend, but a reverse trend was showed by the wear volume as it increased greatly.

Numerous Sliding Applications

Liquid media are used in various sliding applications as a working fluid or as coolants. The material’s tribological characteristics may also get affected by these media. As compared to dry sliding, water’s introduction increases the wear rate and decreases polyamide composites’ friction coefficient. The direct zone of contact between the sliding counterparts was reduced by water as it was a good lubricant for the composite. Water is also utilized as a cooling fluid for dissipating the frictional heat.

As compared to the friction coefficient in dry sliding, the friction coefficient in water was lower. However, water can distill into the composite's amorphous region, leading to a reduction in strength and hardness, making the PA6 fibrils easily detachable, causing it less wear-resistant. Moreover, a higher wear rate can also result due to inhibition of the PA6 transfer layers formation on the sliding counterpart, and the same trend was seen in MWCNTs/polytetrafluoroethylene/polyimide composite.

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Conclusion

Graphite applications in anti-friction coatings are extremely huge as the properties that it exhibits are excellently phenomenal and capable of enduring all these applications. A majority of its portion lies in the slide bearing applications as they are the best example of anti-friction components. Hence, graphite is proved to be the best element in regard to carrying out anti-friction applications as per several types of research and studies.

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References

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