A number of techniques designed to provide some level of oxidation protection have been examined. For instance, the composites (or matrix) can be purified to remove catalytic oxidants (refs. 8 and 9), or they can be treated to passivate reactive sites (refs. 10 through 13). Such inhibited composites appear useful, especially for low-temperature applications. For high-temperature applications, elements or compounds can be added to serve as oxygen getters, providing constituents for protective glassy films, or providing materials that might diffuse and block oxygen transport through cracks in surface films (refs. 14 through 17). Nevertheless, despite all the research activity directed to inhibit oxidation of the matrix, development of an external coating system capable of protecting the total composite (ref. 18) is considered essential. This coating system is particularly important if CC materials are to be used in long-term high-temperature oxidative environments.

Carbon Fibers

Efficient, high-performance materials such as those used in aeronautics, astronautics, and other segments of the transportation industry must be strong and stiff and must also exhibit a minimum weight. It is well known that the potential ultimate tensile strength of a material is proportional to the Young's modulus; thus, the specific modulus (Young's modulus/density) is the best single measure of the potential mechanical performance of a material. This property is considered useful in the assessment of fibers because it represents a first consideration in selecting a carbon fiber with respect to its electrical conductivity, its coefficient of thermal expansion, and its strength and stiffness. A summary of the major carbon fiber varieties available now is given in tables 1 and 2* (ref. 19).

Table 1. Comparison of Properties of Pitch and Pan Fibers

In contrast to most fibrous materials, carbon fibers can be fabricated to exhibit a range of Young's modulus values between 30 Msi and 100 Msi (206 GPa and 689 GPa) and strength values from 255 ksi to 600 ksi (1759 MPa to 4140 MPa); in some cases, strengths of 1000 ksi (6896 MPa) have been obtained for very small diameter fibers. The higher modulus fibers possess higher thermal conductivity, higher density and carbon assay, and lower thermal expansion coefficients than the lower modulus fibers. Within the polyacrylonitrile (PAN) based carbon fiber family, fibers exist which exhibit high tensile strengths (600 ksi (4140 MPa)) but medium Young's modulus values (35 Msi (241 GPa)), and which exhibit high modulus values (55 Msi (379 GPa)) but lower strengths (250 ksi (1724 MPa)). See references 20 through 22.

Continuous carbon fibers are fabricated using a process that essentially consists of three steps: spinning, stabilization (oxidation), and carbonization (heating to 1000°C to 1600°C in an inert atmosphere, usually nitrogen). Carbonization involves the pyrolysis of the stabilized fibers to increase the carbon content. Thus, this step involves the elimination of heteroatoms and other molecules such as H2, HCN, NH3, O, CO, N2, H, CH3, and S. A fourth step, graphitization (heating to about 2500°C in an inert atmosphere) is sometimes performed if very high modulus, high thermal conductivity, or low thermal expansion fibers are required.

In the early 1960's, rayon was a popular carbon fiber precursor, and it is presently used in the nose cap and exhaust nozzle of various aerospace vehicles, e.g., the Space Shuttle (ref. 18). However, large quantities of carbon fibers are no longer routinely produced using this material because it must be stretched at a high temperature in order to produce fibers with optimum properties. This requirement, when combined with the low carbon yield of about 35 percent, results in a process that is not commercially viable (ref. 20).

Rayon can be replaced by a precursor based on PAN. High-quality carbon fibers are cheaper to produce from PAN because stretching only needs to be carried out at the low temperatures typically used during stabilization, i.e., between 200°C to 280°C. In addition a carbon yield of about 65 percent is exhibited.

Fibers produced from pitch should be the cheapest carbon fibers since pitch fibers require no stretching and can give a carbon yield of about 85 percent. Pitch fibers can be processed from an isotropic pitch or from a material that has been extensively treated to produce mesophase. Although isotropic pitch-based fibers are inexpensive, they are restricted for thermal insulation because of their mechanical properties. More competitive properties are produced from the more expensive mesophase feedstocks. Even in this case, however, the superior modulus values of the resulting carbon fibers are not matched by correspondingly high strengths (refs. 23 through 26). Very high modulus fibers (>75 Msi (517 GPa)) can be more easily produced from pitch than from PAN-based fibers. Thus, they tend to be used where a high modulus or the associated properties of high thermal and electrical conductivity or negative thermal expansion coefficient is desired. However, except for this specialized use, pitch fibers tend to be uncompetitive; PAN fibers are, therefore, the carbon fiber most used as a structural reinforcement (ref. 20).

High modulus (HM) fibers are fabricated by heating to very high temperatures. These fibers cost more than low modulus fibers, exhibit smaller longitudinal coefficients of thermal expansion, have high thermal conductivities, and normally have lower surface reactivities. The fiber used in most engineering structures exhibits a low modulus (LM) of about 30 Msi (206 GPa).

If low modulus fibers are heated to a graphitization temperature of approximately 2500°C, primarily to increase the modulus, the degree of axial alignment and the size of the crystallites also increase. Conversely, the interplanar spacing decreases and the fiber shrinks. In order to avoid the dimensional changes, fibers used to produce CC are often, but not always, graphitized, by heating to about 2500°C, before being infiltrated with a matrix precursor.

Fiber types are obtained in a tow form and can contain a small number of fibers (1000) or a relatively large number of fibers (12000). Normally, the larger tows are cheaper to produce.

Fabrication Methods of CC Composites

The classical method for fabricating carbon materials involves combining solid particles of pure carbon (filler coke) with a precursor that can be carbonized to serve as a binder. In CC composites, carbon fibers are used as the primary carbon instead of the filler coke. Unfortunately, the mass loss and shrinkage of the matrix during carbonization result in a final material that exhibits considerable bulk porosity.

The objective of repeated infiltration-pyrolysis is to densify the initial porous skeleton. Densification is achieved by impregnation with liquid or gaseous carbon precursor compounds and subsequent pyrolysis. A key factor in the selection of a matrix carbon precursor involves the ability to fully densify the preform and to achieve a high char yield.

Three basic methods of fabricating carbon fiber reinforced carbon materials exist. The first two methods are based on thermally degrading a thermosetting resin or a thermoplastic pitch. The third method involves depositing carbon into a fibrous preform using CVI. As discussed in a following section, the choice of fabrication method depends to a major extent on the geometry of the part being processed. Thin sections are prime candidates for CVI processing; however, since this method tends to preferentially deposit in and on the surface layers, it is not suitable for the fabrication of thick sections. Thick sections therefore tend to be produced using resin or pitch infiltration. Thermosetting resins remain solid during carbonization; however, pitches soften and tend to flow from a preform at high temperatures; therefore, they require containment during the carbonization step. Complex shapes are difficult to fabricate using either CVI or pitch matrix. materials owing to the difficulty of maintaining the dry fiber preform shape during the initial infiltration. Hybrid densifications are sometimes practiced when a rigid structure is first made using the resin prepreg, autoclave molding process. In this process after carbonization, subsequent infiltrations are made with CVI or pitch.

A recent development allows the impregnation of thin-section carbonaceous preforms with 100 percent mesophase (ref. 27). Carbonization can then be achieved after an oxidation step has rendered the mesophase infusible.

Liquid Phase Infiltration

Two types of liquid impregnants are used to densify CC performs. The first includes pitch, which may be coal tar or petroleum based, and the other is derived from resins or polymers. Both are used because they have suitable viscosities and the carbon yields are high enough to provide high-density CC composites up to ~1.9 g/cm3.

Pitch Matrices

The basic fabrication method for producing pitch matrix CC composites is to use pressure to force pitch into an evacuated cavity that contains the dry fiber preform. The chemical composition of the pitch is believed to control the microstructure and reactivity of the resulting CC solid (ref. 28). It is known, for example, that while most pitches produce graphitic matrices, other pitches cannot be graphitized because some of the molecules are not capable of forming a hexagonal-type three-dimensional graphitic network. In other pitches, side chains exist, for example, alkyl side chains, which distort the crystal structure and render the resulting carbonized material somewhat reactive (ref. 28). Conversely, many pitches contain trace metal impurities that catalyze graphitization locally (ref. 29), while others are known to catalyze high-temperature oxidation reactions (refs. 30 through 32).

The infiltration-carbonization cycle can be performed at atmospheric pressure or at pressures up to 30000 pounds per square inch. At atmospheric pressure, the carbonization of the pitch matrix is carried out by heating to 1000°C under a very slight partial pressure of nitrogen. This carbonization process can be characterized by the viscosity changes experienced by the pitch. Depending upon the elemental composition and the thermal history, the matrix softens until oxygenated functional groups and their sulfur and nitrogen equivalents are released, subsequently resulting in minimum viscosity (ref. 33). Typical viscositytemperature relationships for pitches are shown in figure 1 (ref. 34). Essentially, heating isotropic pitch from room temperature results in the melting of solid pitch and in low viscosity. Continued increase of temperature causes relatively insignificant changes in viscosity although devolatilization of the pitch continues until, at a critical temperature (dependent on the elemental composition and thermal history), the viscosity of the pitch increases rapidly (ref. 35).

The kinetics of mesophase formation depend on the composition, reaction temperature. degree of agitation, and the removal rate of the lower molecular