Metal Information Center

Carbon-Graphite Materials

March 29, 2007 – 6:14 am

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

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.

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.