Three Main Characteristics of Carbon-carbon Composites

What do the space shuttle, aircraft brakes, rocket nozzles and hip prostheses have in common? These examples demonstrate the versatility of carbon-carbon composites in a wide variety of extreme situations where their unique combination of mechanical, thermal, electrical, microstructural, and chemical properties have opened up new possibilities. Carbon-carbon composites are becoming more widely used in today's engineering applications, and are considered by some to be the ultimate development in carbon science. There are three characteristics of carbon-carbon composites that make it superior to many other material selections available.

One of the most important characteristic of CC composites is the superior strength it has and the ability to maintain its strength at high temperatures. Carbon fibers are responsible for the excellent strength seen in carbon-carbon composites. The most important properties of carbon-carbon composites are their thermal properties. C-C composites have very low thermal expansion coefficients and they have high thermal conductivity. The main reason carbon-carbon composites are used as a material, is its ability to maintain these strength levels at temperatures in excess of 2000C . At this elevated temperature, the strength is close to the same as at room temperature. A third characteristic of carbon-carbon composites is the inertness to many chemical agents such as strong acids, alkalis, and reducing agents. These chemical traits make CC composites an excellent choice for surgical implants and prostheses. They are also resistant to thermal shock due to rapid and extreme changes in temperature.

Other properties include low-weight, high abrasion resistance, high electrical conductivity, non-brittle failure, and resistance to biological rejection and chemical corrosion. Carbon-carbon composites are very workable, and can be formed into many different shapes. PAN fibers have been around as textiles for nearly 50 years, they are more commonly known as acrylics. Acrylics are basically a carbon atoms, surrounded by cyanides. Making carbon fiber involves heat treating PAN fibers to remove the cyanides, this leaves the carbon fiber, which is stronger than steel, and lighter.The conversion of PAN to carbon fibers is made in 4 continuous stages: oxidation, carbonisation, surface treatment, and sizingOxidation involves heating the fibers to around 300 deg C in air. The polymer changes from a ladder to a stable ring structure, and the fiber changes color from white though brown to black.

Carbonisation involves heating the fibers up to 3000 . C in an inert atmosphere, the fibers are now nearly 100 % carbon. The temperature will determine the grade of fiber produced.

Surface treatment forms chemical bonds to the carbon surface, this gives a better attachment to the resin system of the compositeSizing is a neutral finishing agent (usually epoxy) to protect the fibers during further processing. The main drawback of carbon-carbon composites is that they oxidize easily at temperatures between 600-700.C, especially in the presence of oxygen. A protective coating (usually silicon carbide) must be applied to prevent high-temperature oxidation. Carbon-carbon composites are currently very expensive and complicated to produce, this limits their use mostly to aerospace and defense applications.Carbon fiber was called a miracle material when it was first introduced. This miracle material is now a part of our everyday lives. Carbon fiber can be found almost anywhere. It is used for fishing rods, golf clubs, tennis rackets, and other such goods.

In the aerospace and aircraft industry, it is used in aircraft, rockets, and satellites. In the engineering field, it is used in office automation equipment, such as personal computers, electrical and electronics parts, mechanical parts, and medical instruments.Carbon fibers are used mainly in the aerospace industry where higher specific strength, higher specific moduli and low density are required. Carbon fibers were developed to meet this demand. The superior properties of carbon fiber to steel and other metals allow for increased fuel savings or a greater payload. Carbon fibers are used extensively in both military and civil aircraft structures. Carbon fiber is being used for an ever-increasing number of new applications. Carbon fiber is used to reinforce or repair concrete structures.

Another example is its use in clean energy fields, including wind-power generation and compressed natural gas tanks for vehicles. This field is expected to grow quickly to meet the need for environmental protection. The potential areas for the application of carbon fiber are nearly unlimited. It will continue to find a place in the fields of marine development, space development, and many others.

References

1. http://me.mit.edu/2.01/Taxonomy/Characteristics/Ceramics/MainCeramics.html
2. http://www.isc.tamu.edu/%7Eclint/html/CCcomposite.html