Carbon-carbon composites rank first among ceramic composite materials with a spectrum of properties and applications in various sectors. These composites are made of fibers in various directions and carbonaceous polymers and hydrocarbons as matrix precursors. Their density and properties depends on the type and volume fraction of reinforcement, matrix precursors used and end heat treatment temperature. Composites made with thermosetting resins as matrix precursors possess low densities (1.55-1.75 g/cm3) and well-distributed microporosity whereas those made with pitch as the matrix precursor, after densification exhibit densities of 1.8-2.0 g/cm3 with some mesopores, and those made by the CVD technique with hydrocarbon gases, possess intermediate densities and matrices with close porosities. The former (resin-based) composites exhibit high flexural strength, low toughness and low thermal conductivity, whereas the latter (pitch- and CVD- based) can be made with very high thermal conductivity (400-700 w/m.k) in the fiber direction. Carbon-carbon composite are used in a variety of sectors requiring high mechanical properties at elevated temperatures, good friction properties for brake pads in high speed vehicles or high thermal conductivity for thermal management applications. However, for extended life applications, these composites need to be protected against oxidation either through matrix modification with Si, Zr, Hf etc. or by multilayer oxidation protection coating consisting of SiC, silica, zircon etc.
Carbon is a truly remarkable element existing as four allotropes, viz. diamond, graphite, carbines and fullerenes, each having significant scientific and technological importance. Its most abundant allotrope, graphite, can take many forms with respect to microstructure, amorphous to highly crystalline structure, high dense with density 2.2 g/cm3 to highly porous with density 0.5 g/cm3 and different shapes. These types of graphite are called synthetic carbons and technical terms, engineered carbons. Examples are cokes, graphite electrodes, mechanical carbons, glassy carbons, carbon black, porous carbons, activated carbons, carbon fibers and composites etc.. Solid carbon are preferred for structural applications under extreme environmental conditions of temperature or corrosion etc. This is mainly because, theoretically, carbon materials with covalently bonded atoms possess very high specific strengths and retain this strength as high temperatures in the temperature range over 1500C. However, the normal bulk synthetic graphite exhibits less than 2% of the theoretical strength. Therefore, for long there has been a quest by scientists to explore and achieve the maximum possible strength in carbon materials. This, coupled with the requirement of high performance reinforcing fibers for composite materials, had led to the development of carbon fibers.
Over the past three decades, carbon fibers have proved to be the main reinforcement for advanced composites for a wide range of applications. Carbon fibers are a few micron thick, light weight, very strong and stiff black synthetic fibers with long aromatic molecular chains comprising mainly carbon. These fibers are capable of maintaining their structure and properties under extreme condictions of temperature and pressure, fluids etc. and therefore can be used with all types of matrices, polymer, ceramic and metal, employing different composite processing techniques. Interest in carbon fibers as reinforcement for composites for structural applications started with the demand from the aeronautical sector for light weight strong and stiff material. Subsequently its application extended to civilian sector, especially in sports gods and biomedical sectors. Though major consumption of carbon fibers is for polymer matrix composites, these are the only choice materials for special application high temperature composites with specific high thermal and thermo-mechanical properties. This kept alive the interest in carbon and carbon fiber research. Presently a large variety of carbon fiber derived from PAN and pitch precursors with varying properties are available, some in the open market and some for restricted sale.
Carbon fiber reinforced carbon matrix composites or the so called carbon-carbon (C/C) composites have densities in the range 1.6-2.0 g/cm3, much lower than those of metals and ceramics and hence make lower component weight an important consideration for aero-vehicales. Some of the most important and useful properties of carbon-carbon composites are light weight, high strength at high temperature in non-oxidising atmospheres, low coefficient of thermal expansion, high thermal conductivity, high thermal shock resistance and low recession in high pressure ablation environments. The mechanical strength of C/C increases with temperature, in contrast to the strength of metal and ceramics, which decrease with increasing temperature. These extraordinary properties of carbon/carbon composites have made these materials extremely useful right for aerospace and defence applications such as brake discs, rocket nozzles, leading edges of re-entry vehicles, furnace heating, thermal management components in space vehicles etc. to those for common man as biomedical implants, glass and high temperature glass and ceramic industry etc. A well-know example for the practical application of C/C composites is in the American Space shuttle, wherein the fuselage nose and the leading edge of the wings are manufactured from C/C composites and have withstood a total of 100 missions under the extreme re-entry condition. A national programme has been launched in India for indigenous development of carbon/carbon composite and related technologies. These composites have been successfully developed and used as nose-tip for the Agni missile and as brake pads for LCA.