An alternative densification process, or a final one following a series of resin transfer and stabilization cycles, is chemical vapor deposition or CVD (ref. 15). In this process a carbon-carrying gas such as methane is diffused into the CC substrate and thermally decomposed to deposit carbon onto the interior and exterior surfaces of the composite. The process can be economically accomplished by processing a large number of parts in a single cycle. For example, CC brakes are CVD densified (ref. 2). The CVD-densified thin-walled CC tubes have exhibited improved strength and stiffness over otherwise identical tubes densified by the resin transfer method (ref. 16). The improvement has been attributed to small pore filling and a degree of interfilament bonding in the CVD-densified tubes.

Thin-Walled 3-D Composites

The resin transfer process is the preferred method of initially impregnating thinwalled braided construction and 3-D tapes. The process can be carried out with minimum pressure gradients across delicate unrigidized parts, thereby avoiding warpage or other damage to the parts. Subsequent to the first set of thermal processing cycles, the parts can be further densified by any of the available methods. The CVD process is especially effective for densifying thin-walled CC pieces because of the short diffusion distances presented by the thin-walled structures.

Thick-Walled 3-D Composites

Thick-walled through-the-thickness reinforced preforms can be densified by means of CVD, liquid resin, or pitch. Uniform densification of large, thick pieces by CVD is difficult. Limited penetration of the carbon-bearing gas prior to its decomposition usually results in matrix buildup near the surface of the part and high porosity in the middle. Liquid-resin impregnation normally requires a larger number of cycles than pitch densification to achieve the same density in the composite. Higher char and volumetric yields are obtainable from pitch by high-pressure carbonization.

Pitch impregnation is carried out at elevated temperature (~250°C) to reduce the pitch viscosity sufficiently for it to flow and penetrate the preform. A graphite frame with numerous openings to accommodate the passage of viscous liquid is employed to protect the preform and maintain its shape. The furnace chamber containing the part is evacuated and heated; hot pitch is then admitted to the chamber and penetrates the evacuated preform.

Subsequent pyrolysis or carbonization of the pitch is best done under pressure. The principal benefit is a higher carbon/volumetric yield, which translates into fewer impregnation cycles to achieve the final target density. The impregnated billet assembly is placed in a steel can, which is evacuated and sealed. The evacuated assembly is placed in a hot isostatic press and heated to ~650°C under pressure, typically 1000 atmospheres. The pressure-assisted pyrolysis process is commonly referred to as PIC (pressure-impregnation-carbonization). Although the terminology is not precise, it is widely used. The term “lo-PIC” refers to pitch impregnation followed by pressure-assisted carbonization under a relatively low pressure of 300 atmospheres, while a "hi-PIC" process is one in which a pressure of at least 700 atmospheres and usually 1000 atmospheres is used.

Following PIC, the part is removed from the can, cleaned, and graphitized. Pitch-impregnated billets usually are graphitized to temperatures in the range of 2400°C to 2500°C, and occasionally in excess of 2700°C. After five cycles of pitch impregnation, "hi-PIC", and graphitization, composite densities are in the range of 1.85 g/cm3 to 1.90 g/cm3. The pitch-based densification processing of CC is illustrated in figure 14.

As discussed previously, the selection of impregnant is made for reasons of processing efficiency and final composite properties.

Figure 15 illustrates the effects of pyrolysis pressure on the carbon or char yield of commonly used coal tar or petroleum pitch impregnants. These data are then used to determine the effective volumetric yield of the impregnant as shown in figure 16. Note that figure 16 is theoretical because the density of the impregnant under actual processing conditions is difficult to determine owing to its

own thermal expansion in a liquid state. For example, selected petroleum pitches have been shown to expand to nearly twice their volume between 100°C to 350°C under atmospheric pressure. Therefore, the density of the impregnant is not known for processing conditions, especially when high external pressures are used during pyrolysis.

Composite Properties

The properties of CC composites are dominated by the reinforcements used and their physical properties, which result from the processing selected and woven preform configuration. In almost all circumstances, physical strength characteristics of the fibers used are reduced as a result of their handling during weaving and the severe environment of densification. Therefore, prediction of composite characteristics typically uses selected test coupon values to establish basic characteristics.

Figure 17 indicates that CC composite properties consisting of tensile strength, modulus, strain, and property value are retained as a function of test temperature and fiber volume fraction. These typical composite data are developed based on PAN precursor fibers used in a 3-D orthogonal construction in which the fibers exhibit a tensile modulus of nominally 50 Msi and tensile strength of nominally 400 ksi prior to weaving and densification. The fiber values referred to for tensile

100

Figure 15. Char yield of pitch as function of applied isostatic pressure.

and modulus properties are in the direction of interest. For example, a 100-percent fiber volume references a unidirectional (i.e., 1-D) composite that has a fiber fraction of typically 50 percent. When considering a 3-D orthogonal CC composite, a Z-direction fiber volume of 40 percent is typical, with a resulting fiber fraction of under 20 percent when considering that total fiber fraction of the composite is less than 50 percent. Therefore, the predicted tensile strength would be nominally 50 ksi to 60 ksi. One of the unique characteristics of CC composites, similar to monolithic graphite, is the increase in strength properties with increasing temperature to approximately 4000°C.

The predictions of properties indicated in figure 17 are intended to act as a guideline. Significant variations may be obtained by both the selection of reinforcement and, to some extent, the densification process.

In summary, CC composites may be fabricated for a variety of constructions ranging from unidirectionally reinforced composites to n-directions of reinforcement. Selection of the woven configurations is mandated by final application and associated costs. Manufacturing processes such as braiding, filament winding, polar coordinate weaving, chopped fiber casting, involute, fabric weaving, and multidirectional weaving have all been successfully applied. The associated densification processes of CVD, resin/pitch impregnation, or combinations are again selected for the requirements of the applications. The basic methods of preform fabrication and densification have been described to indicate the techniques developed over the past 20 years. Numerous variations of these processes are applicable. Selection of the appropriate approaches (reinforcement, preform fabrication, and densification) is dependent on the desired composite properties and intended environment of the composite applications.