Spinning mesophase pitch aligns the liquid crystal plates that subsequently evolve into planar arrays of carbon atoms during pyrolysis (ref. 7). The PAN fibers are stretched before or during oxidative cross-linking. This stretching produces axial orientation of the polymer structure, which is later converted to carbon (ref. 8). Increasing heat treatment temperatures allows for developing higher degrees of preferred orientation. Commercial carbon fibers that are twisted or untwisted are available in a variety of yarn sizes ranging from 1000 to over 10 000 filaments per yarn. Because most high-performance fibers are used in resin matrix composites, all but a few special types are given an oxidative surface treatment to improve resin wetting and bonding. Manufacturers normally apply a protective organic coating, or sizing, to the fibers as the final step in processing in order to ease handling and avoid fiber damage during weaving. Carbon fibers are generally classified according to their elastic moduli, which correlate directly with the degree of alignment. The four basic classifications include low, intermediate, high, and ultrahigh modulus. Table 1 shows the properties of commercial fibers representative of the four classifications that have been reported by the manufacturers. Decreases in the thermal expansion coefficient and increases in the fiber density with an increasing elastic modulus are consequences of the development of more highly aligned and compact atomic structures. The reduction of fiber tensile strength with increasing elastic modulus in the PAN-precursor fibers is due to defects that develop between an increasingly aligned skin and less oriented core (ref. 8). Pitch-precursor fibers are generally not as strong as PAN-derived fibers at the intermediate and high-modulus levels. This is due primarily to inclusions present in the natural product that are incorporated into the fibers and act as strength-limiting flaws (ref. 7). In the past several years, PAN-precursor fibers exhibiting unprecedented strengths at the intermediate and high-modulus levels have become available. Recent data from the manufacturers show tensile strengths of 5000 MPa to 5650 MPa at modulus values of approximately 275 GPa and strengths of 4100 MPa to 5500 MPa at modulus values of more than 340 GPa. Because all of these fibers have diameters between 5 μm and 6 μm, it is logical to assume that the strength improvements are the result of more complete alignment of the fibers, with smaller defects due to a more uniform skin-to-core orientation. Carbon fibers are sold as yarns wound on spools. Configuring and consolidating the yarns into preforms or directly into useful shapes can be accomplished by a wide variety of textile processes, including fabric weaving, filament winding, braiding, and multiaxial weaving. Carbon fibers are generally not as flexible as common textile fibers, so care must be taken to avoid fiber damage, especially of highmodulus fibers, when using processes such as the machine weaving of fabric. Nevertheless, successful fabric weaving of ultrahigh-modulus pitch precursor fibers has been accomplished without apparent damage to the fibers (ref. 9). Carbon fibers are generally resistant to attack by moisture, weak acids, bases, and solvents at room temperature (ref. 10). The oxidation of carbon fibers at elevated temperatures has been the subject of moderate interest and study for approximately 20 years (refs. 11 to 13). Generally, resistance to oxidation in the 350°C to 450°C range can be improved by controlling impurities that catalyze the oxidation and by stabilizing the fibers with higher temperature heat treatments during processing. The higher heat treatments produce denser, more highly aligned fibers with less active surfaces. Improved oxidation resistance at temperatures above 450°C has been achieved only by coating the fibers with oxidation-resistant materials. Fiber Coating Methods Most of the carbon fiber coating development over the past 20 years was conducted to enable the fabrication of high-quality metal-matrix composites. The purpose of the coatings was to provide a metal matrix or to protect the fibers from chemical reaction and enhance wetting by molten metals. The challenge was to develop appropriate methods, without bonding the fibers together, for the continuous deposit of thin, adherent, dense coatings on the thousands of individual micron-sized fibers that make up the carbon yarns. Early methods that proved useful were chemical vapor deposition (CVD) and electroplating (ref. 14). Physical vapor deposition (PVD) processes and liquid precursor coating are also shown to be effective methods (refs. 15 and 16). Table 2 summarizes the methods used for coating carbon fibers which are described in the open literature (refs. 14 to 29). Table 2. Summary of Carbon Fiber Coatings and Coating Methods This process involves the decomposition and reaction of gases in a chamber in order to deposit solid inorganic coatings. Reaction rates and the chemical and physical characteristics of the coatings are controlled by the concentrations of the reactants, temperature, gas flow rate, and total pressure. Reviews discussing the basic principles of CVD and current CVD technology are given by Hastie (ref. 30) and Sherman (ref. 31). The coating of carbon fibers presents the special problem of producing uniformly thin coatings on thousands of individual micron-sized fibers within a yarn. Coatings thicker than several tenths of a micron or coatings that bond fibers together severely reduce the flexibility of the yarn. Winding or weaving such a yarn can result in both coating and fiber damage. Achieving thin uniform coatings by CVD on all of the fibers within a yarn, without bonding the fibers together, requires a uniform temperature across the yarn and a diffusion of the gaseous reactants and products into and out of the yarn. This diffusion must be rapid compared with the rate of the overall chemical reaction (refs. 18 and 21). Reduced reactant concentrations, a low total pressure, and mechanical vibration of the yarn are methods of limiting the reaction rate, enhancing diffusion, and keeping the fibers separated. Lowering the temperature also produces the desired effect of a decrease in the reaction rate relative to the diffusion rate. However, low temperatures may compromise the temperature uniformity across the yarn and can result in porous, poorly adherent coatings. A method to ensure thermal uniformity and to improve the quality of CVD coatings made at low pressures and temperatures is to perform the process in a radiofrequency (RF) or microwaveinduced plasma (refs. 32 and 33). Plasma-assisted CVD (PACVD) is currently an important technique for the fabrication of thin films in the microelectronics industry and has been applied to the continuous coating of carbon fibers (ref. 20). Equipment for the continuous CVD coating of carbon fiber yarns is described in the literature (refs. 15 and 18). Figure 1 shows a schematic and a photograph of a CVD apparatus at General Atomics which is currently being used for fiber coating. The schematic shows the apparatus for conventional CVD, where an induction-heated graphite susceptor tube is used to heat the yarn. The photograph shows the apparatus modified for PACVD operation. In the PACVD mode, a stable glow discharge is established by RF coupling to the process gas mixture at reduced pressures. A scanning electron micrograph of carbon fibers coated with silicon nitride by PACVD at General Atomics is shown in figure 2. Physical Vapor Deposition The three PVD processes are sputtering, ion plating, and evaporation. All involve vaporizing the coating material in a chamber to produce a flux of atoms or molecules which condenses on the material to be coated. These are essentially line-of-site processes in which little penetration of multifiber yarns is normally achieved. For this reason PVD fiber coating is conducted most often by spreading the yarns into layers only several fibers thick. In sputtering, a low-pressure inert gas glow discharge is produced by establishing a large electrical potential between a cathode of the coating material and an anode. The substrate to be coated can be the anode or can be situated adjacent to the cathode and anode. Positive inert gas ions bombard the cathode target, dislodging groups of atoms to form a vapor that deposits on the substrate. In evaporation and ion plating, the vapor is produced by directly heating the coating material at low pressures. Ion plating is different from evaporation because the vapor passes through a gaseous glow discharge that ionizes some of the atoms. The glow discharge is created by introducing a gas such as argon at low pressures and negatively biasing the material to be coated. Thus, positive ions are accelerated toward the substrate receiving the coating. All three PVD processes can also be conducted in a reactive mode. This means that the vaporized material can be made to chemically react with an active gas that |