The Intricate Manufacturing Process of Carbon-Carbon Composites
Introduction
Carbon-carbon composites (C/C) are advanced materials known for their high-temperature resistance, low thermal expansion, and thermal cycling stability. Their manufacturing process consists of three main stages, which contribute to the material’s unique properties and high cost. In this article, we will delve into the details of the manufacturing process of carbon-carbon composites, the factors affecting their properties, and the reasons for their high cost.
The Three Stages of Carbon-Carbon Composite Manufacturing
Material Lay-Up
The first stage of manufacturing carbon-carbon composites involves laying up the material in its intended final shape. Carbon filament and/or cloth are surrounded by an organic binder, such as plastic or pitch. To enhance the material’s properties, coke or other fine carbon aggregates are often added to the binder mixture. These aggregates help improve the material’s density and reduce the formation of voids during subsequent processing stages.
Pyrolysis
The second stage involves heating the lay-up to transform the binder into relatively pure carbon through a process known as pyrolysis. During this stage, the binder loses volume, leading to the formation of voids within the composite material. The addition of aggregates in the first stage reduces this problem, but it does not completely eliminate it. The pyrolysis process plays a vital role in determining the final properties of the carbon-carbon composite, as it influences the material’s density, thermal conductivity, and strength.
Voids Filling and Graphitization
In the third and final stage, the voids formed during pyrolysis are gradually filled by forcing a carbon-forming gas, such as acetylene, through the material at high temperatures. This process can take several days, during which the carbon forms larger graphite crystals. The long heat treatment process is a major factor contributing to the material’s high cost.
The finished product can be manufactured during this stage, depending on the specific requirements of the application. The gray “Reinforced Carbon-Carbon (RCC)” panels used on the Space Shuttle’s wing leading edges and nose cone, for example, cost NASA $100,000 per square foot to produce. Much of this cost can be attributed to the advanced geometry and research costs associated with these panels.
Properties of Carbon-Carbon Composites
Carbon-carbon composites are hard materials with high resistance to thermal expansion, temperature gradients, and thermal cycling. These properties depend on the way the fiber scaffold is laid up and the quality/density of the matrix filler. Carbon-carbon materials retain their properties even at temperatures exceeding 2000 °C, making them suitable for use in extreme conditions. To prevent oxidation at these high temperatures, protective coatings can be applied to the material.
Carbon-carbon composites have a density ranging between 1.6 to 1.98 g/cm3, which can be tailored based on the desired properties and applications. This versatility in density allows for the optimization of the material’s performance in various industries.
Conclusion
The intricate manufacturing process of carbon-carbon composites is responsible for their unique properties, making them ideal for high-temperature applications where thermal stability and resistance to thermal cycling are crucial. Despite their high cost, carbon-carbon composites have found widespread use in industries such as aerospace and Formula One racing due to their exceptional performance in extreme conditions. Understanding the manufacturing process of these materials and the factors that influence their properties is crucial for further development and optimization of carbon-carbon composites for various applications.
