When selecting a resin, metal, ceramic, or carbon-matrix composite for a given application, three criteria are generally used: the carbon composite must have the desired physical and mechanical properties; it must be capable of being processed or manufactured into the desired shape; and it must be economical to produce.
For most carbon composites, the primary consideration for a given application concerns the properties of the reinforcing fibers. The mechanical properties (strength and modulus) of carbon fibers are also related to the physical properties (thermal and electrical conductivity and coefficient of thermal expansion).
Because of the interrelationships that exist between the microstructure and the elastic and physical properties, the choice of a fiber based on one property usually determines the value of the other properties. For instance, the microstructure of very high-modulus fibers usually consists of long fibrils that are almost perfectly aligned parallel to the fiber axis. As a result, the transverse modulus will be relatively low, the thermal and electrical conductivity will be high in the longitudinal direction, and the thermal expansion coefficient will be small or negative.
A composite matrix usually serves to protect the reinforcement fibers from damage or reaction with the environment, to provide some measure of support in compression, to provide adequate matrix-dominated properties, and to provide a continuity of material. This last property is important in electrical and thermal applications and is particularly important in mechanical applications since load must be transferred to the fibers through the matrix. In this respect, a load can be transferred to the fibers across a chemically or physically bonded interface or across a mechanically interlocked one formed by the matrix shrinking onto and thereby gripping the fiber surface.
The properties of the matrix dominate the properties of carbon composite in any direction in which fibers are not aligned. Such properties include the transverse tensile strength and modulus, interlaminar shear strength, thermal expansion coefficient, and the electrical conductivity. cfccarbon.com
Composite materials reinforced with continuous fibers are complex materials that, for a given weight, exhibit specific mechanical properties almost always superior to those exhibited by conventional metals and alloys. Three basic composite types can be conveniently discussed in terms of maximum temperature of use: reinforced polymers, which can be used at intermediate temperatures; and reinforced carbons and ceramics, which can be used up to very high temperatures.
In order for the mechanical properties of a continuous fiber-reinforced composite to be superior to an unreinforced material, the modulus and strength of the fiber reinforcement must be greater than the matrix. In addition, there must be chemical, physical, or mechanical bonds formed between the fibers and matrix that are strong enough to transfer load between individual fibers and between fiber layers.
In estimating the mechanical properties of specific composites, assumptions are made with respect to the contribution of the matrix. For instance, the elastic modulus of typical polymers is usually so small that the contribution to the mechanical properties of the composite is ignored. Conversely, the modulus of metals is much larger; thus, the stiffness and strength of the composite will reflect a significant contribution to the tensile modulus. The modulus of polycrystalline carbon is small; however, the basal planes of carbon within the matrix of a composite can become highly oriented with respect to the fiber axis, showing a large increase in modulus. The modulus of carbon is very large in the direction of its basal plane, but very small in the out-of-plane direction; thus, any contribution to the modulus of the composite will depend on the alignment of the basal planes. In the previous section of this chapter, we have shown that a carbonaceous matrix highly oriented with respect to the axis of the fibers is most often produced from a pitch precursor. A range of structures (isotropic and anisotropic) can be produced from CVI and resin. However, basal planes alignment can be more easily achieved in CVI.