Carbon-Carbon Composites    

Carbon-carbon composites consist of highly-ordered graphite fibers embedded in a carbon matrix. C-C composites are made by gradually building up a carbon matrix on a fiber preform through a series of impregnation and pyrolysis steps or chemical vapor deposition. C-C composites tend to be stiffer, stronger and lighter than steel or other metals.

Processing carbon-carbon composites consists of building up of the carbon matrix around the graphite fibers. There are two common ways to create the matrix: through chemical vapor deposition and through the application of a resin.

Chemical vapor deposition (CVD) begins with a preform in the desired shape of the part, usually formed from several layers of woven carbon fabric. The preform is heated in a furnace pressurized with an organic gas, such as methane, acetylene or benzene. Under high heat and pressure, the gas decomposes and deposits a layer of carbon onto the carbon fibers. The gas must diffuse through the entire preform to make a uniform matrix, so the process is very slow, often requiring several weeks and several processing steps to make a single part.

In the second method a thermosetting resin such as epoxy or phenolic is applied under pressure to the preform, which is then pyrolized into carbon at high temperature. Alternatively, a preform can be built up from resin-impregnated carbon textiles (woven or non-woven) or yarns, then cured and pyrolized. Shrinkage in the resin during carbonization results in tiny cracks in the matrix and a reduction in density. The part must then be re-injected and pyrolized several times (up to a dozen cycles) to fill in the small cracks and to achieve the desired density. Densification can also be accomplished using CVD.

A limiting factor on the use of carbon-carbon composites is the manufacturing expense associated with these slow and complex conventional methods. In response, two less-expensive alternative methods for building up the carbon matrix have been developed. The first is a forced-flow/thermal gradient process developed at the Georgia Institute of Technology in Atlanta, and is a variation on CVD. This method deposits carbon matrix up to 30% faster than conventional methods, and allows thicker items to be produced. Carbon-bearing propylene, propane or methane is forced under pressure through the preform while it is heated in an oven at 1200°C. A temperature gradient in the material forces vapor to flow through the preform, ensuring the even formation of the matrix. Vapor infiltration and carbon deposition are faster with this method, so parts up to 1cm thick can be produced in as little as eight hours. Parts up to 2cm thick (with material properties comparable to CVD-produced parts) have also been produced.

Because the process itself ensures uniform vapor infiltration, it can be run under a wider range of operating conditions than CVD -- the process is less dependent on precise heating, pressure and timing conditions. In the future, this flexibility may even allow the addition of graphitization catalysts and oxidation preventers during production, thereby eliminating a separate treatment. T he second alternative method was developed by the Across Company of Japan, and is a variation on pre-impregnated or "prepreg" materials used to create a preform. Graphite yarns are coated with graphite precursor powders made from coke and pitch, and are then sealed in a flexible thermoplastic sleeve to protect the powder coating during handling and manufacture. The treated yarn can then be woven into sheets or chopped into short fibers and applied to a mold. The laid-up form is then hot-pressed to make the composite part. Yarns can also be processed into tubes, rods, cloth, thick textiles, unidirectional sheets and tapes. Better penetration of the matrix into fiber bundles ensures uniform properties in the composite and higher strength than conventional composites. Fewer densification steps are needed, so manufacturing time and costs are reduced.

General Properties

The most important class of properties of carbon-carbon composites is their thermal properties. C-C composites have very low thermal expansion coefficients, making them dimensionally stable at a wide range of temperatures, and they have high thermal conductivity. C-C composites retain mechanical properties even at temperatures (in non-oxidizing atmospheres) above 2000°C.

They are also highly resistant to thermal shock, or fracture due to rapid and extreme changes in temperature. The material properties of a carbon-carbon composite vary depending on the fiber fraction, fiber type selected, textile weave type and similar factors, and the individual properties of the fibers and matrix material.

Fiber properties depend on precursor material, production process, degree of graphitization and orientation, etc. The tensioning step in fiber formation is critical in making a fiber (and therefore a composite) with any useful strength at all. Matrix precursor material and manufacturing method have a significant impact on composite strength. Sufficient and uniform densification is necessary for a strong composite.

Generally, the elastic modulus is very high, from 15-20 GPa for composites made with a 3D fiber felt to 150-200 GPa for those made with unidirectional fiber sheet. Other properties include low-weight, high abrasion resistance, high electrical conductivity, low hygroscopicity, non-brittle failure, and resistance to biological rejection and chemical corrosion. Carbon-carbon composites are very workable, and can be formed into complex shapes.

Shortcomings

The chief drawback of carbon-carbon composites is that they oxidize readily at temperatures between 600-700°C, especially in the presence of atomic oxygen. A protective coating (usually silicon carbide) must be applied to prevent high-temperature oxidation, adding an additional manufacturing step and additional cost to the production process.

The high electrical conductivity of airborne graphite particles creates an unhealthy environment for electrical equipment near machining areas. Carbon-carbon composites are currently very expensive and complicated to produce, which limits their use mostly to aerospace and defense applications. However processing has been greatly enhanced from the "early' days and costs have been reduced significantly in recent years.