TWI Frequently asked questions
Availability of carbon fibres in the late 1950s led to development of improved materials now known as carbon-carbon (C/C) composites. These composites are a family of materials which consist of a carbon (or graphite) matrixreinforced with carbon (or graphite) fibres. Thus the attractive properties of carbon are combined with the high strength, versatility and toughness of composites. The C/C family is unique in that it is the only elementalcomposite.
Carbon-carbon composites range from simple unidirectional fibre reinforced structures to complex woven 3-dimensional structures. The variety of carbon fibres and multidirectional weaving techniques now available allow tailoring ofC/C composites to meet complex design requirements. By selection of fibre-type, lay-up (or fibre-weave), matrix and composite heat treatment, the properties can be suited to different applications.
In the early 1960s, carbon-carbon composites were developed for elevated temperature aerospace applications such as nose cones and rocket nozzles. These composites can be tailored to higher strengths and stiffness than otherengineering metallic alloys and, unlike metals, can maintain these properties to high temperatures as indicated in Figure 1.
Fig. 1. Strength-to-weight ratio as a function of temperature, for a number of advanced materials
However, the fabrication costs for these materials are high and this has limited their use to primarily aerospace and military applications at present. Therefore, reduction in costs will be achieved by an improvement in the carbonyield. An increase in pressure from ambient, where a 50% carbon yield is achieved, to 100MPa improves the carbon yield to 90%. This is the basis of high pressure carbonisation and pyrolysis, a technique being investigated by TWI andothers.
