|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-graphite can be filled with a variety of impregnates 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 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.
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 carb.. 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.
Carbon-graphite 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. 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.
|Carbon has two natural crystalline allotropic forms: graphite and diamond. Each has its own distinct crystal structure and properties.
Graphite derives its name from the Greek word “graphein”, to write. The material is generally greyish-black, opaque and has a lustrous black sheen. It is unique in that it has properties of both a metal and a non-metal. It is flexible but not elastic, has a high thermal and electrical conductivity, and is highly refractory and chemically inert. Graphite has a low adsorption of X-rays and neutrons making it a particularly useful material in nuclear applications.
A fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and cylindrical ones are called carbon nanotubes or buckytubes. Fullerenes are similar in structure to graphite, which is composed of stacked graphene sheets of linked hexagonal rings; but they may also contain pentagonal (or sometimes heptagonal) rings.
The first fullerene to be discovered, and the family’s namesake, was buckminsterfullerene C60, made in 1985 by Robert Curl, Harold Kroto and Richard Smalley. The name was homage to Richard Buckminster Fuller, whose geodesic domes it resembles. Fullerenes have since been found to occur (if rarely) in nature.
The discovery of fullerenes greatly expanded the number of known carbon allotropes, which until recently were limited to graphite, diamond, and amorphous carbon such as soot and charcoal. Buckyballs and buckytubes have been the subject of intense research, both for their unique chemistry and for their technological applications, especially in materials science, electronics, and nanotechnology.
nanotubes are cylindrical fullerenes. These tubes of carbon are usually only a few nanometres wide, but they can range from less than a micrometer to several millimetres in length. They often have closed ends, but can be open-ended as well. There are also cases in which the tube reduces in diameter before closing off. Their unique molecular structure results in extraordinary macroscopic properties, including high tensile strength, high electrical conductivity, high ductility, high resistance to heat, and relative chemical inactivity (as it is cylindrical and “planar” — that is, it has no “exposed” atoms that can be easily displaced). One proposed use of carbon nanotubes is in paper batteries, developed in 2007 by researchers at Rensselaer Polytechnic Institute. Another proposed use in the field of space technologies and science fiction is to produce high-tensile carbon cables required by a space elevator.