Carbon on both sides of the interface
Most composites pair a reinforcing fiber with a different kind of matrix: glass in plastic, carbon fiber in epoxy, ceramic fiber in ceramic. Carbon-carbon is unusual because both parts are carbon. Carbon fibers are embedded in a matrix that is also carbon, usually a graphite-like or partly graphitic form. The result is a material that keeps its stiffness and strength at temperatures where ordinary polymer composites would char and most metals would soften or melt.
For readers new to the subject, the general ideas behind composite materials, a strong reinforcement held in place by a matrix, apply here too. The special feature is the ability to work at extreme temperature.
Why carbon holds up when others do not
Carbon does not melt at ordinary pressure. Instead it sublimes, turning directly to vapor, at temperatures far beyond what most structural materials ever see. In an oxygen-free environment, carbon-based materials can therefore stay solid through conditions that destroy metals. In addition:
- The material is light compared with metals and many ceramics.
- Its thermal expansion is generally low, so it does not distort much as it heats, which helps resist thermal shock.
- It conducts heat reasonably well, which helps spread heat through a brake disc rather than leaving one hot spot.
- It tends to be damage tolerant for a carbon-based material, since the fibers can bridge cracks instead of letting them run freely.
The weak point is oxygen. In air, carbon begins to oxidize noticeably at elevated temperatures, often starting in the range of several hundred degrees Celsius, and it can be lost gradually as gas. This is the main reason carbon-carbon is used with protective measures or in short-duration exposure.
How it is made
The challenge is to fill the gaps between fibers with carbon. This is done in stages.
- Build a preform. Carbon fibers are arranged into a woven, felted or wound shape close to the final part.
- Add a carbon-yielding material. One route is to impregnate the preform with a resin or pitch that leaves carbon behind when heated. Another is chemical vapor infiltration, in which a hydrocarbon gas decomposes inside the porous preform and deposits carbon on the fibers. The deposition chemistry is related to chemical vapor deposition used for coatings.
- Convert by heating. The impregnated part is heated in an inert atmosphere so the resin or pitch converts to carbon. This step, pyrolysis, shrinks the matrix and leaves tiny voids.
- Repeat. The cycle of impregnating and heating is repeated several times, or the infiltration continues for a long time, to increase density.
- Optional high-temperature treatment. Heating further can make the carbon more graphite-like, changing its thermal and mechanical behavior.
Because of the repeated cycles and long furnace times, carbon-carbon is slow and expensive to make. That cost limits its use to places where nothing else will do.
Brakes
Aircraft brakes are the best-known application. When an aircraft lands or aborts a takeoff, the brakes convert a very large amount of kinetic energy into heat in a short time. Carbon-carbon discs are light, tolerate the heat, and generally hold their friction behavior across a wide temperature range, so they are widely used on larger aircraft. High-performance motorsport also uses carbon-based brakes for similar reasons.
Some real-world considerations are worth knowing. Carbon brakes often perform differently when cold, and their friction coefficient can vary with temperature and moisture. Wear is influenced by how they are used, and oxidation of the surfaces is managed with protective treatments.
Heat shields and hot structures
When a spacecraft reenters the atmosphere, its leading surfaces face extreme heating. Reinforced carbon-carbon was used on the leading edges and nose cap of the Space Shuttle orbiter, where the heat was too high for the tile materials used elsewhere. Rocket nozzle throats and parts of solid rocket motors have also used carbon-carbon, because the material tolerates erosion by hot exhaust better than most alternatives.
Because the structures are exposed to air during reentry, protective coatings are applied. A common approach is to convert the outer layer to silicon carbide and add further sealing layers to slow oxygen from reaching the carbon. Such coating systems are a close relative of thermal barrier coatings in how they protect a base material, although the chemistry differs.
Limits and alternatives
Carbon-carbon is not the answer to every hot problem.
- Oxidation protection can crack, and protected carbon can degrade if the coating is breached.
- The material is porous unless densified well, which can matter for strength and sealing.
- Cost and lead time are high.
- It is generally less suited for long exposure to oxidizing hot gas, where ceramic matrix composites or refractory metals are often considered.
Selecting a material for a given hot structure involves comparing all of these options, which fall under the broader heading of high-temperature materials.
Place in the carbon family
Carbon-carbon combines two forms from the carbon family: fibers whose structure is largely graphite-like but imperfect, and a matrix that ranges from disordered to graphitic. It shows how carbon can be used as a structural engineering material rather than only a lubricant or conductor. For the broader overview, see the pillar on carbon allotropes.