Orthogonal 3-D constructions may also be fabricated by a method other than weaving continuous yarns through the length, width, and height of the preform. This method stacks layers of woven fabric according to a desired 2-D orientation sequence and pierces the layers of fabric by locating graphite yarns that provide the Z-direction reinforcement (refs. 12, 13, and 14). This technique permits higher in-plane fiber volume fractions than can be realized with 3-D orthogonal weaving. A drawback of this technique is that in-plane fibers are broken during the placement of the Z-yarn bundles. The 3-D CC composite whose properties are listed in table 1 was constructed by this method.

Design Summary

The foregoing discussion concludes a brief survey of carbon preform design. The range of possible substructures is as broad as for any other fiber-reinforced composite system. Carbon-carbon is primarily an aerospace material because of its advantageous properties, including its unparalleled ability to withstand hightemperature environments. Applications imposing the most complex thermal and mechanical loads are best served by CC composites having through-the-thickness reinforcements. These materials typically result in higher cost than laminates as a result of the higher weaving costs.

Because of the high costs of densification processing, multidirectional (3-D) CC composites are only used where their unique properties will serve specific

requirements such as in reentry missile nose tips, or where a long-term savings realized from using CC makes it cost-competitive with less expensive materials or fabrication approaches.

Carbon-Carbon Composite Densification

Densification processing, like the design of a woven carbon preform, offers many approaches and alternatives to reach the final composite product. Selection of the process is dependent on final application, preform architecture, and cost considerations.

Densification processing is typically carried out using either a gaseous (i.e., chemical vapor deposition) or liquid (i.e., resin or pitches) precursor material. Each of the processes entails numerous variations including treatment of the impregnants, temperatures, gas ratios, pressures, and process sequences. The commonality is that the objective of the process is to fill the voids and interfaces of the woven preforms with matrix material as rapidly and cost effectively as possible while meeting the requirements of the application.

The most widely used impregnants for densification are gases (methane) and resins or pitches. The resulting properties of the carbons these precursors form vary significantly, depending on the process parameters and the basic chemical nature of the impregnant.

In almost all instances, the objective of densification is to fill a void or volume formed by the woven structure. Void volumes typically range from 35 to 60 percent. The actual mass that is added to the composite will vary, depending on the selection of the impregnant. For example, pitch precursor materials such as petroleum pitch or coal tar pitch form high density matrices in excess of 2.1 gm/cc when heat treated to more than 2000°C. Alternately, thermosetting type resins such as phenolic or furfural-based resins result in lower specific gravities owing to their inability to form intermediate graphitic crystals. Rapid and cost-effective densification is a result of the volumetric yield the impregnant provides when fully processed. An untreated pitch precursor nominally provides 50 percent carbon yield when pyrolyzed using atmospheric pressure and can provide as high as 90 percent yield when pyrolyzed at pressures exceeding 100 atmospheres. However, the volumetric yield is low when considering the change in density from an impregnant precursor (1.2 gm/cc) to the carbon formed (2.1 g/cc) subsequent to pyrolysis and heat treatment. Therefore, repeated impregnation and pyrolysis are required to obtain a low volumetric porosity in the woven structure.

The following section discusses typical processes used to densify or fabricate various forms of CC composites. Again, note that variations to the processes discussed are applicable.

Laminates

Densification of 2-D CC reinforced laminates starts with prepregging of fabric or aligned yarns. Prepregging of woven fabric is readily accomplished on a largescale basis and is typically automated. For graphite fabric destined for CC, the resin impregnant is commonly a high char (carbon) yielding phenolic, although other resin systems may be used. The process consists essentially of passing the fabric through a controlled resin bath and depositing the desired amount of resin pickup, followed by curing or staging prepregged fabric through thermal treatment.

The process of making laminates from prepreg consists of cutting the prepreg material to the desired size, stacking the layers in a predetermined sequence of orientation, and molding. During molding, pressure is applied to the stack while heat is applied to cure the resin. The time-temperature-pressure requirements for molding depend upon the characteristics of the prepreg. For phenolic resins, molding may be conducted at 160°C to 180°C under a pressure of 200 psi to 300 psi for 30 min to 60 min. Molding is followed by a lengthy 24 hr to 100 hr postcure to temperatures higher than those of the molding temperature but well below those of the resin decomposition temperature. A postcure temperature of 200°C is typical for a phenolic resin. Following postcure, the temperature is raised to the 650°C to 800°C range to pyrolyze the resin, leaving a carbon matrix. Postcuring and pyrolysis are carried out under an inert atmosphere to prevent oxidation. After the resin has been pyrolyzed, the laminate is a CC composite. A stabilization or graphitization heat treatment follows to a temperature that will match or exceed the maximum temperature that the composite will experience in subsequent processing or in use.

The purpose of any subsequent processing would be to reduce porosity and improve mechanical properties. Repeated resin impregnation, pyrolysis, and graphitization; several cycles of pitch impregnation, pyrolysis, and graphitization; CVD densification; or a combination of these, may be done. Pitch-based processing will be described in the context of processing 3-D CC. Subsequent resin impregnation of 2-D CC panels typically is done by a resin transfer or submerged impregnation technique whereby the part, by itself or with others, is placed in a chamber that is then evacuated to remove gases from the pores of the composite. Following this, resin is admitted to the chamber, filling it. A pressure of 50 psi to 100 psi nitrogen is then applied to assist the penetration of the resin into the panel. The excess resin is then drained off, or the panel is removed from the resin bath and a low pressure (~50 psi nitrogen) resin cure is done. Because the fiber volume fraction has been fixed by the initial molding process, higher pressures are not required for this cycle. Postcure, pyrolysis, and graphitization follow the cure cycle. The sequence of impregnation and thermal processing may be repeated a number of times until the target density of the laminate is achieved. Figure 13 illustrates the manufacture of 2-D CC from prepregged fabric or unidirectional tape.