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Carbon - Graphite Materials
Written by AZoM
Chemical Formula
Topics Covered
Introduction
Carbon-graphites offer
the design engineer a unique family of mechanical materials. Manufactured
entirely from carbon and including high temperature carbonaceous bonding,
these materials combine the strength, hardness and wear resistance of carbon
with the corrosion resistance and self lubricating properties of graphite.
The precisely controlled inherent porosity of carbon-graphites can be filled
with a variety of impregnants to enhance chemical, mechanical and
tribological properties.
Types of Carbon
The terms ‘carbon’ and
‘graphite’ are often used interchangeably. This is unfortunate since each
form of the element carbon offers specific properties that can be used to
benefit different types of applications.
Amorphous Carbon
Amorphous carbon is a
very hard, strong compound. The crystals exhibit a turbostratic disorder
which makes the material extremely resistant to wear. The strength and wear
resistance properties of this material make it of interest in some
applications. However, these strengths can also be a weakness -carbon
generates high friction when rubbed against another surface.
Graphite, on the other
hand, is softer and relatively weak because of the crystalline order and
closer spacing between the monoplanes and stacks. A graphite structure can be
compared to a deck of cards with individual layers able to easily slide off
the deck. This phenomenon gives the material a self lubricating ability which
is matched by no other material. External lubricants are simply not
necessary.
Carbon-Graphites
It is possible to
combine amorphous carbon and graphite to take full advantage of the strengths
and weaknesses of each of these two types of carbon, table 1. The proper
mixture of the two materials is strong and hard and has low friction. At the
same time, this composite has excellent corrosion resistance and is capable
of operating at temperatures in excess of 315°C for extended periods of time,
depending on the specific grade. The ability to create materials that have
these properties is the basis of the manufactured mechanical carbon materials
that perform well in difficult tribological situations such as pumps.
Table 1. Properties
of typical carbon and graphite materials.
density (g.cm-3)
Compressive
strength (MNm-2)
strength (MNm-2)
of elasticity (GNm-2)
conductivity (W/m.°C)
limit in air (°C)
Processing Carbon-Graphites
Carbon-graphites are
created by combining the two forms of carbon with coal tar pitch. The coal
tar pitch acts as a temporary binder that holds the two structures together
during the compression moulding process in which near net shapes are formed.
Following the moulding operation, the parts are sintered at temperatures high
enough to carbonise the coal tar pitch. The result is a structure that is
completely carbon bound and contains both carbon and graphite. This structure
is extremely strong in compression and will not creep under load. The
carbonisation of the temporary binder leaves holes in the structure - on a
micro scale the sintered body is a black sponge.
The formation of holes
during the processing of a carbon-graphite composite has various advantages.
The holes can be filled with resins, metals, carbon, or inorganic salts,
depending on the planned use of the material, table 2. These fillers serve to
improve the strength, thermal conductivity and tribological characteristics
of the material. Additionally, carbon-graphite can be sintered to an even
higher temperature to convert the entire structure to graphite to provide especially
good performance in very high temperature, high speed applications.
duty to 260°C in water, coolants, fuels, oils. Light chemical solutions,
food and drug.
water, steam, light hydrocarbons.
pressure service to 20.7MNm-2.
corrosive environments.
formers (fluorides etc)
dry environments, vacuum or cryogenic.
inhibitors (phosphates etc)
temperature and/or high speed (to 538°C and 240ms-1).
Carbon-Graphites vs Traditional Lubricants
Carbon-graphites are
used in a wide range of applications where traditional lubricating methods
are not appropriate. For example, a typical oil lubricated bearing struggles
at temperatures below -40°C because of the high viscosity of the oil. Above
200°C oils carbonise, making them abrasive and ineffective. Graphite bearings
are capable of extended use at temperatures above 600°C.
Chemically Aggressive Environments
Chemically aggressive
applications represent another application niche for carbon-graphite. For
instance, sterilisation processes tend to leach the oil from the structure of
an oil lubricated bearing. Also, solvents and radiation can break down
lubricating oils, and low pressures can cause the oil to vaporise.
Carbon-graphite materials are inherently stable and chemically resistant,
making them ideally suitable for these types of application, figure 1.
Inside of a pump. The four vanes on the rotor are made of carbon-graphite,
and are consequently self lubricating, temperature resistant and
impermeable to gases and liquids.
High Load Applications
Lubricants are
inappropriate in other applications for a variety of reasons. High loads can
squeeze the lubricant from a surface. Without the hydrodynamic layer of
lubricant, failure is imminent if the material cannot provide self
lubrication. In some applications, such as those involving food handling,
lubricants can contaminate the surrounding environment. Specific grades of
carbon-graphites are approved for use in food handling applications.
Design Factors
Carbon-graphite
bearings are used in both wet and dry operating conditions. Carbon-graphite
allows the designer to specify the bearing close to the boundary lubricated
condition without the risk of seizure. Permissible loads and running speeds
depend on the allowable wear rate. Shaft materials and surface finishes are
important factors in the wear rates of carbon-graphite materials. As a rule
of thumb, the harder and more polished the surface, the lower the wear. Where
possible, aluminium and bronze should be avoided for use as shaft materials.
Carbon-graphite
materials are also widely used as rotating shaft and face seal materials,
figure 2. They perform well when running against metal and ceramic
counterfaces. Seals are manufactured from solid rings, split rings and
segmented rings for use in both liquid and dry-running applications in the
aerospace, nuclear, petrochemical and general marine industries.
2. Carbon-graphite seals are
self lubricating, resistant to chemical corrosion, and capable of running
at temperatures up to 538°C.
Carbon-Graphite Seals
Seal materials require
high strength and a relatively high modulus of elasticity to withstand
deformation at the interface. Carbon-graphite seal materials provide the
strength and rigidity which are especially important in high pressure, zero
leakage mechanical end-face seals. High thermal conductivity is essential in
removing heat from the interface.
Seal wear is a result
of adhesive wear, chemical wear, erosive wear and sometimes radioactive wear.
Carbon-graphite is inert to most chemical reagents so it survives where other
materials fail. However, chemical wear is evidenced in certain strong
oxidising environments or where the additives are attacked by specific
oxidising reagents.
Impregnation of Carbon-Graphite Seals
Impregnation of carbon
seals can be done with a variety of materials to control permeability. In
addition to thermoset resins, other types of impregnants include
thermoplastics, metals, and inorganic salts or glasses. The temperature limit
of the impregnant places an upper limit on the operating temperature of the
carbon parts.
Metals such as
antimony, silver, copper, nickel, and babbitt can improve the strength,
thermal conductivity, and tribological characteristics of the materials.
Impregnants made of inorganic salts usually phosphate or borate - and glasses
are used in high temperature applications. Carbons impregnated with soluble
salts must be handled carefully to avoid exudation, especially under humid
conditions, but loss of impregnant rarely affects any physical property of a
seal other than permeability.
Blistering
Blistering is a
critical concern with carbon seal materials. Strangely, the reason why blistering
occasionally occurs is not clear. One of the most popular explanations is
that a certain amount of fluid becomes absorbed in the carbon substrate and
expands due to frictional heat, creating a subsurface pressure and eventual
crater in the seal face. Another theory is that softer mating materials can
tend to tear pieces from the carbon-graphite in the presence of heavy
hydrocarbons. Blistering is most often found in applications involving
hydrocarbons or cyclical temperature service such as air conditioning
compressors. In some cases, the use of silicon carbide as a mating surface
will reduce or even eliminate a blistering problem, possibly because of its
high thermal conductivity and hardness.
Carbon-Graphite Seals in Aerospace
Applications
Carbon-graphite and
graphites are excellent materials for aircraft turbine engine mainshaft
seals. The mainshaft in a turbine engine rotates at very high speeds and
operates in an environment of changing high temperature conditions. Mainshaft
bearing compartment seals are used to protect rotor support bearings from hot
gases flowing through the engine and to prevent the loss of lubricant in the
bearing compartments.
Materials Selection and Suitability
Loads, speeds,
temperatures, mating materials, cost constraints and projected volume are
critical factors to be kept in mind when selecting materials. Scores of base
carbon materials are available with hundreds of modifications that can be
customised for specific designs and environments. General service carbon-graphites
are usable up to 260°C while special grades are available that provide
resistance up to 538°C. Special carbons and impregnants are used for seal
applications in the 260-538°C temperature range.
These materials are
chemically inert, temperature resistant, lightweight, resilient,
dimensionally stable, and impermeable to gases and liquids. They can be
moulded to size or machined to close tolerances, impregnated, plated,
vulcanised to rubber, and cemented or shrunk into housings or retainers - a
truly versatile set of materials.
Primary author: Joe Boylan
Source: Materials World, Vol. 4, no. 12 pp. 707-8, December
For more information on Materials World please visit .
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This site uses cookies. By continuing to browse the site you are agreeing to our use of cookies.giant covalent structures
GIANT COVALENT STRUCTURES
This page decribes the structures of giant covalent substances like diamond, graphite and silicon dioxide (silicon(IV) oxide), and relates those structures to the physical properties of the substances.
The structure of diamond
The giant covalent structure of diamond
Carbon has an electronic arrangement of 2,4.
In diamond, each carbon shares electrons with four other carbon atoms - forming four single bonds.
In the diagram some carbon atoms only seem to be forming two bonds (or even one bond), but that's not really the case.
We are only showing a small bit of the whole structure.
This is a giant covalent structure - it continues on and on in three dimensions.
It is not a molecule, because the number of atoms joined up in a real diamond is completely variable - depending on the size of the crystal.
&We quoted the electronic structure of carbon as 2,4.
That simple view is perfectly adequate to explain the bonding in diamond.
If you are interested in a more modern view, you could read the page on
in the organic section of this site.
In the case of diamond, each carbon is bonded to 4 other carbons rather than hydrogens, but that makes no essential difference.
How to draw the structure of diamond
Don't try to be too clever by trying to draw too much of the structure!
Learn to draw the diagram given above.
Do it in the following stages:
Practise until you can do a reasonable free-hand sketch in about 30 seconds.
The physical properties of diamond
has a very high melting point (almost 4000&C).
Very strong carbon-carbon covalent bonds have to be broken throughout the structure before melting occurs.
is very hard.
This is again due to the need to break very strong covalent bonds operating in 3-dimensions.
doesn't conduct electricity.
All the electrons are held tightly between the atoms, and aren't free to move.
is insoluble in water and organic solvents.
There are no possible attractions which could occur between solvent molecules and carbon atoms which could outweigh the attractions between the covalently bound carbon atoms.
The structure of graphite
The giant covalent structure of graphite
Graphite has a layer structure which is quite difficult to draw convincingly in three dimensions.
The diagram below shows the arrangement of the atoms in each layer, and the way the layers are spaced.
Notice that you can't really draw the side view of the layers to the same scale as the atoms in the layer without one or other part of the diagram being either very spread out or very squashed.
In that case, it is important to give some idea of the distances involved.
The distance between the layers is about 2.5 times the distance between the atoms within each layer.
The layers, of course, extend over huge numbers of atoms - not just the few shown above.
You might argue that carbon has to form 4 bonds because of its 4 unpaired electrons, whereas in this diagram it only seems to be forming 3 bonds to the neighbouring carbons.
This diagram is something of a simplification, and shows the arrangement of atoms rather than the bonding.
The bonding in graphite
Each carbon atom uses three of its electrons to form simple bonds to its three close neighbours.
That leaves a fourth electron in the bonding level.
These "spare" electrons in each carbon atom become delocalised over the whole of the sheet of atoms in one layer.
They are no longer associated directly with any particular atom or pair of atoms, but are free to wander throughout the whole sheet.
If you are interested (beyond A'level):
&The bonding in graphite is like a vastly extended version of the .
Each carbon atom undergoes sp2 hybridisation, and then the unhybridised p orbitals on each carbon atom overlap sideways to give a massive pi system above and below the plane of the sheet of atoms.
The important thing is that the delocalised electrons are free to move anywhere within the sheet - each electron is no longer fixed to a particular carbon atom.
There is, however, no direct contact between the delocalised electrons in one sheet and those in the neighbouring sheets.
The atoms within a sheet are held together by strong covalent bonds - stronger, in fact, than in diamond because of the additional bonding caused by the delocalised electrons.
So what holds the sheets together?
In graphite you have the ultimate example of van der Waals dispersion forces. As the delocalised electrons move around in the sheet, very large temporary dipoles can be set up which will induce opposite dipoles in the sheets above and below - and so on throughout the whole graphite crystal.
&If you aren't sure about
follow this link before you go on.
Use the BACK button on your browser to return to this page.
The physical properties of graphite
has a high melting point, similar to that of diamond.
In order to melt graphite, it isn't enough to loosen one sheet from another.
You have to break the covalent bonding throughout the whole structure.
has a soft, slippery feel, and is used in pencils and as a dry lubricant for things like locks.
You can think of graphite rather like a pack of cards - each card is strong, but the cards will slide over each other, or even fall off the pack altogether.
When you use a pencil, sheets are rubbed off and stick to the paper.
has a lower density than diamond.
This is because of the relatively large amount of space that is "wasted" between the sheets.
is insoluble in water and organic solvents - for the same reason that diamond is insoluble.
Attractions between solvent molecules and carbon atoms will never be strong enough to overcome the strong covalent bonds in graphite.
conducts electricity.
The delocalised electrons are free to move throughout the sheets.
If a piece of graphite is connected into a circuit, electrons can fall off one end of the sheet and be replaced with new ones at the other end.
&The logic of this is that a piece of graphite ought only to conduct electricity in 2-dimensions because electrons can only move around in the sheets - and not from one sheet to its neighbours.
In practice, a real piece of graphite isn't a perfect crystal, but a host of small crystals stuck together at all sorts of angles.
Electrons will be able to find a route through the large piece of graphite in all directions by moving from one small crystal to the next.
The structure of silicon dioxide, SiO2
Silicon dioxide is also known as silicon(IV) oxide.
The giant covalent structure of silicon dioxide
There are three different crystal forms of silicon dioxide.
The easiest one to remember and draw is based on the diamond structure.
Crystalline silicon has the same structure as diamond.
To turn it into silicon dioxide, all you need to do is to modify the silicon structure by including some oxygen atoms.
Notice that each silicon atom is bridged to its neighbours by an oxygen atom.
Don't forget that this is just a tiny part of a giant structure extending on all 3 dimensions.
&If you want to be fussy, the Si-O-Si bond angles are wrong in this diagram.
In reality the "bridge" from one silicon atom to its neighbour isn't in a straight line, but via a "V" shape (similar to the shape around the oxygen atom in a water molecule).
It's extremely difficult to draw that convincingly and tidily in a diagram involving this number of atoms.
The simplification is perfectly acceptable.
The physical properties of silicon dioxide
Silicon dioxide
has a high melting point - varying depending on what the particular structure is (remember that the structure given is only one of three possible structures), but around 1700&C.
Very strong silicon-oxygen covalent bonds have to be broken throughout the structure before melting occurs.
This is due to the need to break the very strong covalent bonds.
doesn't conduct electricity.
There aren't any delocalised electrons.
All the electrons are held tightly between the atoms, and aren't free to move.
is insoluble in water and organic solvents.
There are no possible attractions which could occur between solvent molecules and the silicon or oxygen atoms which could overcome the covalent bonds in the giant structure.
Questions to test your understanding
If this is the first set of questions you have done, please read the
before you start.
You will need to use the BACK BUTTON on your browser to come back here afterwards.
Where would you like to go now?
& Jim Clark 2000 (modified October 2012)