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in addition to its two well-defined allotropic forms (diamond and graphite), can take any number of quasicrystalline forms ranging continuously from turbostratic (amorphous, glassy carbon) to a highly crystalline graphite (fig. 3).
Figure 2. Tightly bonded, hexagonally arranged carbon layers (ref: 1) held together by weak van der Waals forces.
The anisotropy of the graphite single crystal encompasses many structural forms of carbon. It ranges in the degree of preferred orientation of the crystallites and influences the porosity, among other variables. A broad range of properties is the result of this anisotropy, which is available in carbon material. In CC composites, this range of properties can extend to both constituents. Coupled with a variety of
processing techniques that can be used in the fabrication of CC composites, great flexibility exists in the design of and the resultant properties to be obtained from CC composites.
The wide range of properties of carbon materials can be shown when comparing the tensile moduli of commercially manufactured carbon fibers that range from 27.6 GPa (4 x 106 psi) to 690 GPa (100 x 106 psi). In fabrication, the fibers can be used in either continuous or discontinuous form. The directionality of the filaments can be varied ranging from unidirectional lay-ups to multidirectional weaves. The fiber volume used constitutes another variable. The higher the volume fraction of a specific high-strength fiber in a matrix, the greater the strength of the composite. The matrix can be formed via two basic approaches: (1) through the carbonization of an organic solid or liquid, such as a resin or pitch, and (2) through the chemical vapor deposition (CVD) of carbon from a hydrocarbon. A range of carbon structures can be obtained by either approach. Finally, heat treatment of the composite material at graphitization temperatures offers additional variability to the properties that can be obtained. Typically, there is an optimum graphitization temperature at which the highest strength can be obtained for a given composite composition of fiber and matrix (refs. 4 and 5).
The properties of carbon fibers can vary over a wide range depending on the organic precursor and processing conditions used. At present, graphite fibers are produced from three precursor materials: rayon, polyacrylonitrile (PAN), and petroleum pitch. Fibers having a low modulus (27.6 GPa (4 x 10 psi)) are formed using a rayon precursor material that may be chemically pretreated by a sequence of heating steps. First, the fiber is heated to ^ 400°C to allow cellulose to pyrolyzeJ Carbonization § is completed more rapidly at >1000°C. Upon completion of carbonization, the fiber is graphitized" by heating to >2000°C; the fiber is now, for all practical purposes, 100 percent carbon. High-modulus carbon fibers from rayon precursors are obtained by the additional process of stretching the carbon fibers at the final heat treatment temperature. High-modulus (344 GPa (50 x 106 psi)), high-strength (2.07 GPa (300 x 103 psi)) carbon fibers are typically made from PAN or, in some cases, mesophase pitch precursors. These fibers are processed similarly in a three-stage operation (fig. 4, ref. 6). The PAN fibers are initially stretched from 500 percent to 1300 percent and then stabilized (crosslinked) in an oxygen atmosphere at 200°C to 280°C (under tension). Carbonization of the fibers is conducted between 1000°C and 1600°C. Finally, graphitization is accomplished at >2500°C. Mesophase pitch fibers undergo the same processing procedure as PAN fibers but do not require an expensive stretching process during heat treatment to maintain preferred alignment of crystallites (fig. 4, ref. 6). Control of fiber shape has resulted in improved fiber strength (4.1 GPa (600000 psi)), see ref. 7, when produced from melt-spun, mesophase petroleum pitch (fig. 5, ref. 7). Round fibers using the same method had a strength of 2.1 GPa (300 x 103 psi), as shown in reference 4. Of the shapes studied, the c-shape and hollow fibers were found to be superior in strength to round solid and trilobal cross sections (refs. 4 and 7).
T Decomposition or chemical change by thermal conversion of organic materials to carbon and graphite.
"Continued heating of organic material to >1000°C to initiate ordering of the carbon structures produced by pyrolysis.
"Continued heating of carbonized organic materials to the 2000 C to 3000°C range to produce a 100-percent graphite-ordered crystal structure.
Carbon Fibers in Carbon Matrix
Addition of a matrix to carbon fiber, either through the carbonization of an organic precursor or by the deposition of pyrolytic carbon, is conducted at 800°C to 1500°C. Subsequent heat treatment of the composite material may involve temperatures to 3000°C.
Discontinuous Fiber Composites
Fabrication of discontinuous fiber composites uses short carbon fibers combined with either a pyrolytic carbon or pyrolyzed organic matrix. This approach to CC composites generally does not have true fiber reinforcement as an objective. Rather, discontinuous fiber substrates have been used to: (1) increase fabrication capability of large-scale structures. (2) achieve a more nearly isotropic material. (3) increase the composite interlaminar tensile strength, and (4) along with continuous filament substrates, obtain a stronger composite by providing additional nucleation sites that serve to reduce composite porosity.
Figure 5. Melt spinning apparatus used to produce noncircular carbon fibers (ref. 7).
The fabrication techniques most widely applied are a carbonized, rayon felt substrate with a pyrolytic carbon matrix, and short, chopped fibers in a pitchbased matrix. Felt is produced through the mechanical carding of viscous rayon fibers to produce a continuous web of fibers. The webs are folded one on top of another to produce a ban. The batts are then cut, stacked, and needled to produce the required felt. The rayon felt is subjected to a controlled carbonization cycle in an inert atmosphere or vacuum; the maximum temperature determines such factors as shrinkage, weight loss, and chemical composition of the felt. A maximum carbonization temperature of 1200°C is a nominal standard; the length of the carbonization cycle and rate of temperature rise are dictated by the thickness of the felt. Carbon content in the fibers is «98 percent. Carbon-carbon composites have also been fabricated from short carbon fibers using isotropic casting, flocking lay-up, spray lay-up, and pulp-molding techniques (fig. 6, refs. 8 to 10). The rationale for using these short fibers is to reduce composite properties of anisotropy, specifically, the effect that relatively long fibers used in other discontinuous fiber substrates produce fiber alignment during processing resulting in anisotropic composite properties (ref. 9).
Continuous Fiber Composites
Continuous filament substrates use either the properties of high-strength filaments or achieve a high degree of preferred orientation on the macroscale of the matrix. The fabrication complexity involving continuous-filament substrates is
Figure 6. Models of fiber arrangements for four short-fiber fabrication techniques: (a) flocking lay-up; (b) pulp molding; (c) isotropic casting; and (d) spray lay-up (ref. 5).
determined by two parameters: the directionality of the filaments and the amount of layer interlocking achieved in the substrate. The plies and filament winding of unidirectional tapes can be used to achieve a highly oriented substrate, usually with no interlocking between layers. Woven fabrics are used to form a two-dimensional laminate with no interlocking between layers. Helical filament winding, which is directional, results in continuous, adjacent layer interlocking. Multilayer locking is achieved through complex weaving patterns or yarn placement resulting in "multidirectional" substrates (fig. 7).
Chemical Vapor Deposition
The deposition of carbon on the filament substructures just discussed is accomplished either by pyrolyzing an organic matrix or through CVD. The CVD of carbon from a hydrocarbon gas within a substrate is a complex process. Various techniques have been applied to infiltrate various fiber substrates including isothermal thermal gradient (ref. 11), pressure gradient (ref. 12), and pressure pulsation (ref. 13). The first two have been the most extensively used. The isothermal technique is illustrated in figure 8. The substrate is radiantly heated by an inductively heated susceptor so that the gas and substrate are maintained at a uniform temperature. Infiltration is normally accomplished at 1100°C and at