Carbon Reinforcements and Carbon/Carbon Composites

Today, carbon fibers are, by far, the main high strength, high modulus reinforcing material used in the fabrication of high performance composites. The need for these carbon fiber reinforced structural materials originated shortly after World War II, as military aircraft manufacturers in the USA found that existing materials were limiting the performance of their designs. Initially, boron fibers were developed to reinforce plastics, yielding the first “advanced structural composites” [1, 2]. Even though these fibers were extremely strong, stiff, and relatively light compared to existing metals, they had numerous drawbacks: (i) their large diameter, 100–150 μm, made the fibers difficult to handle; (ii) since the loop strength of the fibers was quite low, they could be used only as monofilaments; (iii) the outer shell of the fiber was boron and the core was a different material, creating thermal mismatch problems; (iv) most importantly, these boron fibers were extremely expensive.

Carbon/carbon composites are a generic class of synthetic, pure carbon materials consisting of carbon fibers reinforced in a carbon matrix. These composites, which were first synthesized in the late 1950s, were only used as an ablative material until the late 1960s [1–4] when space shuttle programmes required thermal protection materials with the following properties: light weight, high thermal shock resistance, a low coefficient of thermal expansion, maintenance of strength at high temperatures, high impact resistance, high stiffness etc. [2–3]. Graphite was known to have most of these properties with the exception of high strength and stiffness [4]. By that time PAN-based carbon fibers had been produced with much higher strength. Concurrently, fiber-reinforced plastic technology had also greatly advanced and new composite fabrication techniques were available [5]. Both of these factors gave impetus to the carbon/carbon technology and these composites emerged as the prospective high temperature structural material for selected critical components of advanced air force exit and re-entry systems. During the 1970s carbon/carbon composites were under extensive development in several laboratories in the US and Europe [6–11]. Research and development efforts were concentrated in all areas of carbon/carbon technology. They included improvements in the properties of constituent materials, i. e., carbon fibers and carbon matrix precursors, and design allowables, i. e., development of multidirection weaving technology and fabrication technology etc.

The most fundamental aspect of carbon/carbon technology is the substructure or preform which from design considerations may be referred to as fiber architecture. Fiber architecture is the backbone of composites. It not only imparts rigidity to the composites, but also in combination with fiber properties it determines the properties of the composites. Therefore, the choice of preform or fiber architecture is made on the basis of the intended application.

The high thermostability of the solid carbon is the basis for the high temperature applicability of all carbon materials. As can be seen from its phase diagram (Fig. 4.1) [1] carbon, or the thermodynamic equilibrium phase graphite, melts only under conditions of high pressure and temperature, i.e., in the order of 100 bars and 4000 K, respectively.

The use of carbon/carbon composites for thermostructural applications requires a basic knowledge of the structural characteristics of the composites that control their engineering properties. These include macroporosity, microstructure of the matrix including interbundle and interfilament matrix, microcracking and interfaces at different levels, fiber orientations, demaged fibers etc. By understanding details of the composites’ structure and mechanism of microstructural development, one can control the matrix microstructure and improve its mechanical properties, especially the fiber strength utilization. Microstructural features of carbon/carbon composites may be classified according to various scales of magnification. At the largest useful scale, the millimeter scale, microstructure relates to composite architecture. It comprises the distribution of fibers and matrix, especially interbundle matrix, voids, and pores. At the microscale, it reveals features such as interfilament matrix and at the nanoscale it relates to fiber/matrix interface. Figure 5.1 shows a broad classification of some of the characteristics. For 2D and MD carbon/carbon composites, Jortener [1] classified the former as minimechanical features and the latter as micro-mechanical features. The origin and characteristics of these depend on (i) the reinforcement configuration, (ii) type of carbon fiber used, (iii) matrix precursor, (iv) fabrication routes, and (v) processing conditions.

The vast interest in carbon/carbon composites is based on the unique mechanical and thermal properties of these materials, which in turn reflect the basic characteristics of carbon. Graphite, the crystalline form of carbon, can assume a variety of structures - fine to coarse mozaic, oriented, lamellar and columnar - with varying degrees of anisotropy. These structures can be recognized in bulk carbon and graphite materials even though reinforcing fibers also exhibit some of these structures. Since each structure is associated with characteristic properties [1], carbon/carbon composites with desired properties can be fabricated by choosing appropriate reinforcing fibers and controlling the microstructure of the matrix [2]. The available degree of freedom with respect to the fiber type and distribution and matrix microstructure enables the designer to select and obtain the proper type of materials that will withstand adverse environments.

Thermal transport properties of a solid material are generally controlled by two mechanisms: (i) electron charge cloud drift and (ii) lattice vibrations, known as phonons. In the latter case, thermal transport properties such as thermal conductivity or thermal diffusivity are directly proportional to the mean free path of phonons [1]. In a perfect crystal, phonon scattering takes place predominantly by four mechanisms; the crystal boundaries, the natural isotopic composition, the inharmonic interaction with other phonons, and the conduction of electrons [2].

Carbon-based materials are widely used in wear related applications, e.g., bearings, seals, and electrical brushes. The development of carbon/carbon composites with superior mechanical properties has opened up a new field of application for carbon materials in the wear domain, i.e., brake applications. The function of the brake is essentially controlled by its ability to absorb energy in the form of heat. Therefore, the thermal characteristics of candidate brake materials are of major importance. In particular, a potential high-performance brake material must have a high specific heat, a high melting point, and adequate mechanical properties at elevated temperature. Conventional monolithic graphite exhibits attractive thermal and physical characteristics. Since carbon materials obtained through different routes differ in the arrangement of hexagonal layers and, hence, in the interlayer cohesive energy, these materials accordingly vary in friction and wear characteristics. The friction and wear characteristics of different types of carbons have been studied in detail [1–4].

In contrast to the attractive thermal and mechanical properties of carbon/carbon composites discussed in previous chapters, a severe limitation with these composites is their high susceptibility to oxidation in an oxidizing environment at temperatures even below 500°C. The life span of carbon/carbon composites in a nonoxidizing environment is found to be same at 2500 °C as it is at 350 °C, whereas in the presence of air it has been found to decrease from 400 h to zero at 500 ^°C [1]. This quite often becomes a major barrier to the use of carbon/carbon composites as a structural material in high-temperature systems operating in oxidizing environments, such as turbine engines and ramjet components that are exposed to hot combustion gases, air breathing propulsion systems, and air frame components experiencing aerodynamic heating. Clearly, for wider applications of carbon/carbon composites as high temperature structural materials, some form of oxidation protection is essential. An understanding of the oxidation mechanism for carbons in general and that for carbon/carbon composites in particular helps in the development of protection systems for these composites.

As is evident from the previous chapters, carbon/carbon composites of the desired shape with properties required for particular application can be produced by meticulously choosing the type, architecture, and amount of carbon fiber and matrix precursor, and the processing conditions. Therefore, carbon/carbon composites are considered to be a class of materials with a wide spectrum of properties and applications. Moreover, the ongoing development of high-performance carbon fibers, high char yielding synthetic resins, and pitches continuously add to the spectrum of carbon/carbon properties and products. It is with these milestones, carbon/ carbon composites, which, were developed about 30 years ago to meet the anticipated needs of the emerging space program, are regarded nowadays as logical addition to the members of fine-grained carbons and graphites. Carbon/carbon composites offer a large potential as high-performance engineering material. Therefore, in addition to the special defense, aircraft, and spacecraft applications, a steady interest is also growing in civil market segments.