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High Performance Synthetic Fibers for Composites (1992)

Chapter: 3 Fiber-Forming Processes: Current and Potential Methods

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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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Suggested Citation:"3 Fiber-Forming Processes: Current and Potential Methods." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
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FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 49 3 Fiber-Forming Processes: Current and Potential Methods INTRODUCTION This chapter addresses the processes by which high-performance fibers are formed, with emphasis on those technologies that have been most broadly successful. The technologies include • Polymer fibers. • Pyrolytic conversion of precursor fibers. • Chemical conversion of precursor fibers. • Fibers produced by chemical vapor deposition. • Single crystal fibers. Also included is the important technology associated with the coating of fibers, a technology that allows chemical and physical tailoring of the fiber surface. The processes for converting bulk materials into fibers, while specific to the desired end product, have a series of elements in common. These include • Conversion of the room-temperature solid to a low-viscosity (up to 104 poise) melt or solution. • Passing the filtered solution through a plate of holes (spinnerette) to form fibers. • Solidifying the fiber over a distance of centimeters to meters under conditions of controlled temperature, stress, and mass transfer. If the process begins with a stable melt, the process is termed "melt spinning." Solution spinning with high- volatility solvents that may be flashed off during solidification is termed "dry spinning." Conversion of solutions in low-volatility (high boiling point) solvents to fiber, requiring filament coagulation by exchanging solvent for nonsolvent, is termed ''wet spinning.'' These processes are illustrated in Figure 3.1.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 50 Figure 3.1 General processing steps for converting bulk materials to fibers.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 51 Important variations on this theme include removal of the spinnerette from the wet spinning coagulation bath (allowing separate control of spinning and coagulation temperatures) necessary for the spinning of many lyotropic polymer systems (aramid, polyphenylene polybisbenzoxazole [PBO]) (dry jet wet spinning); and the spinning of low-concentration solution to minimize chain entanglements in the resulting fiber (gel spinning). Depending on the conditions chosen, filaments may be molecularly oriented or disoriented as spun. Additional orientation may be imparted through "drawing" processes where the molecules comprising the fibers are elongated at a rate faster than their relaxation time. Heat may be applied to yarns to increase morphological stability through controlled crystallization and/or molecular relaxation processes. Finally, chemistry through the application of heat, pressure (vacuum), or reactants may be utilized as a separate step, or in conjunction with the process steps, to convert "precursor" fibers to the desired final composition. In the ensuing portions of this chapter, processing technologies that are key to the formation of important high-performance fibers critically analyzed. PROCESSES TO FORM POLYMERIC ORGANIC FIBERS Introduction The technical concepts behind the processing of ultrahigh-strength, (200-500 Ksi), ultrahigh-modulus (greater than 7 Msi) polymeric organic fibers differ from those for inorganic and metallic fibers and are related to the one-dimensional nature of polymeric molecular chains. To make a fiber that takes maximum advantage of the strength of the interatomic forces (covalent bonding) in the polymer, the molecules must be extended and oriented parallel to the axis of the fiber. The commercial processes for high-strength, high-modulus fibers are based on two physical concepts: melt or dry jet wet spinning from a nematic liquid crystalline phase in which the already rod-like molecules are uniaxially ordered, and melt or gel spinning and drawing of conventional, random-coil polymers under conditions that permit extremely high elongational forces (high draw ratios) to mechanically elongate and orient the component molecules (cf. Figure 3.1). The resulting highly oriented morphologies yield moduli that approach the theoretical. However, the relatively high strengths achieved are far (10x) from theory, offering potential for further development. These fibers are highly anisotropic, with lateral tensile properties up to two orders of magnitude lower than axial. This results in some inherent performance losses, most notably in compressive properties.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 52 Technical Description Spinning of Liquid Crystalline Solutions The first commercial high-performance organic fiber, developed by the duPont Company from poly (paraphenylene) terephthalamide, under the Kevlar trademark, was spun from a nematic (liquid crystalline) solution. In a nematic phase the individual rod-like molecules are oriented parallel to their nearest neighbors. The parallel orientation exists over a correlation volume of micrometer dimensions. The elongational force field during the spinning process orients these volumes in the direction of flow. When the initially extruded fiber is fixed by extraction of the solvent, the molecules solidify in a form that places the strong molecular chains parallel to the fiber. In addition to the aramids, rigid, para-oriented, aromatic hetrocyclic polymers (e.g., poly(benzobisoxazole) [PBO's]), are of greatest important. The cost of polymeric fibers is dependent on the cost of the starting materials and the complexity of the polymerization and conversion processes. Monomers capable of polymerization to required molecular weights must be extremely pure. The necessary steps to achieve this can add significantly to the cost and greatly complicate scale-up to commercial levels. PBO is synthesized from expensive monomers in corrosive acid solvents such as methyl sufonic or polyphosphonic acid. A related polymer with similar expensive monomers is the corresponding benzthiazole. The commercial aramid fiber from poly(paraphenylene terephthalamide) (PPD-T) is synthesized in amide solvents such as N-methylpyrollidone by the condensation of p-phenylene diamine and terephthaloyl chloride. The finished polymer is washed to remove the solvent, and the equipment required for the mixing and isolation makes this an expensive process. Both aramids and the aromatic heterocyclics are redissolved and converted to highly oriented fiber through dry jet wet spinning (cf. Figure 3.1). Aramid and PBO fibers are characterized by use temperatures up to about 250°C and by chemical inertness. The H-bonding potential to the aramids leads to moisture sensitivity with consequent property reductions under humid conditions. Spinning of Liquid Crystalline Melts The majority of polymers in this class are aromatic copolyesters that have been previously commercialized as molding resins. Their advantage is the inherently lower cost of melt spinning, which avoids dissolution and solvent recovery steps. Melt spinning has the potential for much higher spinning rates, with a consequent decrease in capital investment per output. The first commercial product to make its appearance is the fiber Vectran, by Hoechst Celanese and the Kuraray Company. This product's advantages include very high cut resistance (up to eight times that of the aramids) and excellent hydrolytic stability. Tensile properties at room temperature are similar to those of Kevlar® fibers but decrease with

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 53 increasing temperature. Use temperatures are intermediate between Kevlar® and polyethylene, 150° to 200°C depending on composition. Gel Spinning High molecular weight linear polyethylene is gel spun commercially (see Introduction) with the resulting fiber being drawn to very high (20-100) draw ratios. The elongational forces cause the molecular chains to be extended and highly oriented in the fiber direction, resulting in high levels of tensile modulus and tensile strength. The solvent recovery aspect of the process and the controlled low-speed drawing increase the cost of the fiber to a level comparable to other high-strength fibers, despite the low polymer cost. Performance problems inherent in highly oriented polyethylene fibers include low-use temperature (<100°C) and creep. "Normal" Solution Spinning Spinning of a normal isotropic (nonliquid crystalline) solution with subsequent drawing of the fiber to induce orientation can be accomplished with certain rigid molecules. Teijin has commercialized an aramid (Technora) that is a polymer of paraphenylene diamine, 4,4"-diamino diphenylether, and terephthalic acid. The fiber, after removal of the amide (n-methylpyrollidone)/LiCl solvent, is drawn at 485°C. Technical Future A major class of polymers with demonstrated fiber potential but no commercial product yet is the polyimides. Two Japanese companies (Ube and Toray) have announced the manufacture of fibers based on polyimides with tenacities of about 20 gpd. It is unlikely that new spinning processes will be applied to high modulus fiber production. However, the postheating processes that have been shown to improve fiber properties are poorly understood. Better understanding of these processes could lead to improved fibers. The cost of the additional step will remain a factor. Research on the orientation of molecules in elongational and shear fields could lead to incremental but significant improvements in tenacity and modulus. Research on fiber physics, which could lead to commercially important discoveries, is not being done at an appropriate level by industry and only at a very few university laboratories. Rheological and morphological studies have the ultimate potential of finding processes to form high-strength fibers from commodity polymers other than polyethlyene. High-strength nylon or conventional polyester (polyethylene terephthalate) fibers made by a melt process could be the first high-strength fibers with prices compatible with commercial ground transportation applications and land-and marine-based applications. Improved low-cost routes to monomers for liquid crystalline melts, or different low-cost monomers for the same polymers, could have significant commercial impact.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 54 No process variable (other than, perhaps, molecular cross linkage) is likely to significantly improve the tendency toward compressive failure in these highly oriented materials. Process parameters are also unable to affect use temperatures, which will remain in a range well below those of ceramics. The application of blend concepts has been exploited to a relatively small extent. The field of engineering resins has demonstrated the potential of blending to result in order-of-magnitude property changes. The commercially announced fibers from liquid crystalline melts illustrate the strong Japanese effort toward development of fibers discovered in the United States. This reflects the unwillingness of American industry to commercially develop new materials with a limited current market. Although industry is doing little fundamental research on organic fibers, development in general will be handled by industry more effectively than for the other fibers discussed in this report because the organics have a broader commercial market. This indicates that limited government resources should be applied to the more specialized inorganics. FIBER FORMATION BY PYROLYTIC CONVERSION OF PRECURSOR FIBERS Introduction The use of precursors that can be pyrolyzed to form continuous inorganic filaments has provided a route to the manufacture of synthetic inorganic fibers of many different compositions. The precursor materials include polymers, concentrated salt solutions that may behave like polymeric materials, polymer-modified solutions and slurries, and sol-gel systems. Polymeric precursors are used for the fabrication of continuous nonoxide filaments such as carbon, graphite, and silicon carbide (sic). Oxide fibers are produced from all of the precursor types listed above. Since the rheological properties necessary for spinning continuous filaments at high speed are provided by the polymeric-type material present in the fiber-forming precursor composition, spinability of the final fiber composition in its fused form is not a necessary property, as it is in the forming of traditional glass filaments. Therefore, materials such as aluminum oxide (A12O3) or zirconium oxide (ZrO2) and many others may be prepared in fiber form, even though the properties of their liquid phases would not normally permit fiber spinning at any practical rate. Similarly, fibers of materials not generally considered to melt, such as carbon (c) and SiC, are also prepared by this technology. Polymeric materials may embody in their composition all of the precursor components of the final solid inorganic, such as SiC or C, or they may be present principally for their contribution to fiberizing properties. In the latter case, the polymer may be completely fugitive, while the actual fiber components exist in the precursor formulation as decomposable salt compounds or as colloids added as particles or compatible sols such as aqua sols.1 Thus, formation of the inorganic fiber precursor (or organic in the case of carbon fibers) involves spinning melted polymeric

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 55 precursors, viscous polymeric solutions, or mixtures of polymers, salt solutions, and colloids. Regardless of the precursor used, the principal common steps for the fabrication of fibers with polymeric precursor systems are as shown in Figure 3.2. Figure 3.2 Simplified flowsheet for pecursor pyrloysis processes. An especially important feature of the precursor pyrolysis process, in addition to its versatility in the preparation of fibers over a wide range of compositions, is the low temperatures (350°C or lower) used for precursor fiber spinning. Despite the straightforward flowsheet of Figure 3.2, it is important to realize that precise control of conditions in each step is required for the fabrication of filaments with optimum properties. Since the pyrolysis step may involve the decomposition and removal of up to 50 percent or more of the polymeric precursor fiber, it becomes obvious that careful control at this point is extremely critical to ensure that fiber defects are minimized or that the fiber even survives this part of the process. Control at this point will include factors such as the atmosphere, temperature, and rate of temperature increase, it being necessary to control rates of decomposition and removal of volatile products as well as rates of reaction of residual precursor components of the final fiber composition. The specific details for the fabrication of fibers vary according to the composition of the final inorganic fiber and precursors used for their preparation. The major portion of development work on inorganic fibers via the pyrolytic conversion of precursor fibers has been done since 1960. The review by Bracke et al.2 provides a useful summary of the state of the art on inorganic fibers in patents and other publications up to 1984. More complete details on processing, current status, performance, and needs in the area of high-performance synthetic fibers are provided in the following sections on nonoxide and oxide fibers. Carbon Fibers Even though high-performance carbon fibers were first introduced in the 1960s, the physical properties of these reinforcing fibers have improved dramatically over the past decade. This can be attributed to three developments: significant improvements in both quality and performance of the precursor fiber, a substantial increase in worldwide production capacity and the process improvements resulting from this gain in production experience, and a strong push on the part of the United States and Western European aerospace industry for high- performance carbon-based reinforcing fibers.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 56 There are three types of precursors used on a commercial basis to manufacture carbon fibers: rayon, polyacrylonitrile (PAN), and pitch. Rayon, the raw material used for the first high-performance carbon fibers, was the dominant precursor material in the 1960s and early 1970s. However, because of its low carbon yield (20-30 percent), high processing cost, and limited physical properties, today rayon precursor is used for less than 1 percent of commercial carbon fibers. At the present time, North American Rayon is the U.S. supplier of rayon precursor, and Amoco Performance Products is the only U.S. supplier of rayon-based carbon fiber. Use of rayon- based fiber is primarily limited to C-C ablative shields. Since PAN and pitch are used as the precursor for over 99 percent of today's commercial carbon fibers, this discussion will concentrate on the strengths and limitations of the carbon fibers produced from these feedstocks. Noncontinuous carbon fibers have also been made on a developmental basis from a radically different process that does not use polymeric precursors. This process, often referred to as vapor grown carbon fiber, involves the combination of a hydrocarbon vapor with a catalyst source and hydrogen. A discussion of its process is included in the section on whisker-like materials. PAN-based Carbon Fibers Technical Description and Present Status. PAN-based carbon fibers are derived from polyacrylonitrile or acrylic copolymers. Normally, a solution of the polymer is either wet or dry spun into PAN precursor fibers that are ultimately converted into carbon fibers. Figure 3.3 gives a flow diagram for the process, which is well documented in the literature.1 The precursor fibers used by the major PAN-based carbon fiber suppliers differ significantly from the acrylic fibers used for textile acrylic apparel and industrial applications. PAN precursor fibers have fewer filaments per tow stage, a higher level of purity, smaller filament diameter, and higher acrylonitrile (AN) content (i.e., normally greater than 90 percent AN) than fibers used in textile applications. Also, the polymer composition and molecular weight must be modified to produce the desired carbon fiber properties. Because of these differences, the cost of acrylic precursor fibers is three to five times greater than that of acrylic fibers used in textile applications. Stabilization of PAN involves heating the fiber in air to temperatures ranging from 200° to 300°C for approximately 1 hour. The stabilization treatment is followed by carbonization in an inert atmosphere at temperatures greater than 1200°C. Orientation of the graphite-like crystal structure, and thus the fiber modulus, can be further increased by heat treatment (termed "graphitization") at temperatures up to 3000°C. The continuous carbon or graphite fiber is then surface treated and coated with a sizing agent prior to winding the continuous filaments on bobbins. The surface treatment is an oxidation of the fiber surface to promote adhesion to the matrix resin in the composite, and the size promotes handleability and wettability of the fiber with the matrix resin. Carbon fiber strength is primarily controlled by the defect level, and modulus is controlled by crystalline orientation and degree of crystallinity. Thus, by varying those process parameters that can influence orientation and

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 57 Figure 3.3 PAN Based Process

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 58 crystallinity (e.g., precursor purity, stretch ratios, overall process tension levels, carbonization furnace temperature,) in the manufacture of both the precursor and the carbon fiber, the resulting carbon fibers can have a broad range of properties. Generally, carbon fibers are classified into three major groupings (see Table 3.1): standard or aerospace grade, intermediate modulus (I.M.), and high modulus (H.M.). To date, most sales and aerospace qualifications have been with the standard-grade carbon fiber, and this is likely to continue as long as the present applications continue in production. The newer aerospace (and some recreation) applications, however, are primarily involved with the I.M. and H.M. Fibers. TABLE 3.1 Carbon Fiber Classifications Grade Product Modulus % Strain to Failure Development Activity Standard 32-35 1.6 Lower Cost 32-35 2.0 Increased Strain to Failure Intermediate (I.M.) 40-50 2.0 Improved Composite Balance of Properties (Compression, Toughness) Increased Strain to Failure High Modulus (H.M.) 55-85 1.0 Increased Strain to Failure Carbon fibers possess the highest specific modulus (modulus/density) of all commercially available reinforcing fibers (several times that of conventional metals). However, since commercial PAN-based carbon fibers were first introduced 20 years ago, their physical properties have improved dramatically. In fact, over the past 7 to 8 years improvements in precursor as well as carbon fiber technology have increased the tensile strength of this class of fibers from approximately 3.45 GPa (500 Ksi) to 6.9 GPa (1000 Ksi). Today, specific strengths (tensile strength/density) achievable with PAN-based carbon fibers are among the highest of all commercially available reinforcing fibers. They also have a very good balance of properties, particularly compressive strength. Approaches toward improved compression performance, however, are an area where extensive research is being directed since polymeric-matrix composites (PMCs) made with the new higher-strength, higher

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 59 modulus fibers show corresponding improvements in tensile and flex properties but little improvement in compression. A wide variety of fibers are now commercially available that range in tensile modulus from 207 GPa (30 Msi) to 586 GPa (85 Msi) with elongation from less than 1 percent to over 2 percent. Finally, a substantial amount of progress has been made in the ability to convert carbon fibers into intermediate product forms. The material can be braided, woven, knitted, and converted into unidirectional tapes that ultimately can be processed into composites. These materials, however, are inherently more difficult to handle than conventional textile fibers and conventional processing speeds are not practical with them. Very specialized techniques are therefore continually being developed to improve processing efficiencies. Future Potential. Even though PAN-based carbon-fibers have been commercially available for over 20 years, the manufacturing technology, including both precursor and carbon fiber formation, is still very low on the technology growth curve, and there is great potential for improvements in product and process areas. Technological developments for PAN-based carbon fibers will center on the development and implementation of domestic precursor; lower-cost manufacturing technology (e.g., reduced capital as well as operating costs); new high-strength, intermediate, and high-modulus fibers; fibers with an improved overall balance of mechanical properties (e.g., compression); improved composite performance through optimization of the fiber and matrix interface; tailored physical properties (i.e., independent control of electrical and mechanical properties); and improved thermal oxidative stability. Precursor Technology. Currently, all commercial production of PAN-based precursor fibers is based on solution spinning of PAN polymer. Typically, a dilute solution of acrylic polymer is extruded into a coagulation bath (wet spinning) or a hot-gas environment (dry spinning). The use of large amounts of solvents is a fundamental factor in the production process, which results in environmental as well as product design limitations. As indicated previously, all of the major U.S. carbon fiber producers have installed or will be installing domestic precursor capacity principally to meet the Department of Defense's precursor directive. Most of these companies are simply installing the standard wet or dry spinning technology that is the domestic equivalent of the technology used overseas to produce their precursor. However, one company, BASF Structural Materials, Inc., has developed and is now commercializing a unique new technology that utilizes melt-assisted extrusion as the basis for the spinning process.3 This process eliminates the need for conventional solvent-recovery systems, reducing the required wastewater treatment. In short, this new process can easily produce the current standard high-quality precursor fiber, but it also has the versatility to modify fiber composition and even fiber cross-sectional shape, offering the potential to create a new family of carbon fibers with unique properties. Carbon Fiber Technology. The development of processes capable of yielding higher-strength, higher- modulus, PAN-based carbon fibers is a critical research goal. Most of this research is centered around routes to eliminate defects from the fiber (the strength is flaw controlled), to obtain

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 60 higher orientation during processing, and to obtain a more uniform radial structure. Another major research initiative is in improving the carbon fiber surface chemistry in order to optimize the interface between the fiber and the resin, the weak link in the composite. Fiber surface chemistry can be modified via a number of methods, including overall process modifications, improved fiber coatings, enhanced surface oxidation techniques, and grafting reactive species onto the fiber surface. Increased composite compression performance from the newer higher-strength, higher-modulus fibers is another goal that may be realized through additional process developments. The key to this problem is having a fundamental knowledge of the compression failure mode(s) and then altering the carbon fiber process/product to avoid premature failure. Some of the approaches being evaluated include modified precursor morphology, improved surface treatment techniques, and use of noncircular-shaped carbon fibers (available via the melt- assisted precursor technology). The second major thrust of carbon fiber process research (after higher-performance fibers) is lower-cost fibers. The primary factors controlling the current cost of PAN-based carbon fibers are the low carbon fiber yields (approximately 50 percent) inherent in the use of acrylic polymers, acrylic precursor cost, and the high capital intensity and low productivity of the process. The first factor represents one of the single most important components in reducing costs; however, there is very little that can be done about it short of developing an alternate precursor polymer. Some work in this area has been done, particularly in Japan, but a new high carbon content, nonpitch, non-PAN polymer will be many years in development. Use of a lower-cost precursor (e.g., very high filament counts possibly approaching textile deniers) coupled with improved stabilization and carbonization processes are the key steps in the development of a lower-cost carbon fiber. Such a fiber would likely have substantially reduced mechanical properties, but they may be quite acceptable for some industrial and civil engineering applications. Both stabilization and carbonization are very energy intensive with low productivity. Development of an improved heat transfer process where stabilization can occur in seconds or minutes rather than hours is the key to next-generation stabilization. Such processes have been demonstrated in the laboratory, but none have yet been commercialized on a large scale. As a result of the above efforts, it is projected that commercial carbon fibers with tensile strength well in excess of 6.9 GPa (1000 Ksi) and 2 percent elongation will be readily available. A common rule of thumb is that the theoretical strength should be about 10 percent of the modulus of a single crystal (approximately 1034 GPa in the case of graphite). Thus, carbon fiber in its current strength level range is about 15 to 20 times below the theoretical strength limit. Ultimately, how high the strength will rise is an open question, depending, as previously indicated, on the effectiveness of approaches to eliminate fracture-initiating sites. Efforts will continue toward the development of higher strain-to-failure fibers at all modulus levels. Improvements will also be made in fiber/resin systems such that composites with compressive strengths in excess of 2.8 GPa (400 Ksi) will probably be obtainable. Continual improvements in manufacturing technology for high-to-medium performance fibers will be coupled with substantially

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 61 larger carbon fiber capacities to yield declining pricing (constant dollars). Development of a lower-cost, lower- performance fiber ($8-$10/1b) with significant cost savings over conventional fibers is possible in the 1990s using a PAN-based precursor; a truly low-cost fiber would probably require a non-PAN precursor and is not likely before the turn of the century. Finally, all major U.S. carbon fiber suppliers will install domestic precursor capacity. This massive undertaking will require a significant technical effort and expense directed toward product requalifications and new product introductions. A positive outgrowth of this effort will be a natural evolution of the overall carbon fiber technology toward fulfilling specific U.S. market needs. Pitch-based Carbon Fibers Technical Description and Present Status. The production of high-modulus, pitch-based carbon fibers begins by heat treating a petroleum or coal tar pitch feedstock to produce a liquid crystal precursor, termed ''mesophase.'' The liquid crystal material is melt spun into a precursor fiber that is converted into a carbon fiber in a process somewhat analogous to that used for PAN-based carbon fiber. The high degree of molecular orientation of the as-spun fiber allows it, unlike PAN, to develop a truly graphitic crystalline structure during the carbonization/ graphitization step. The fundamentals of the process are well documented in the literature,4,5 and an overall schematic of the process is shown in Figure 3.4. Potential advantages for pitch-based carbon fibers include economics and unique properties that result from their high degree of molecular orientation. Since the starting carbon content for pitch precursor is significantly higher than PAN precursor (approximately 93 percent versus 68 percent), the theoretical yield for pitch fibers is very substantially higher than that of PAN fibers. Also, acrylonitrile, which is the raw material for PAN-based carbon fibers, costs almost twice as much as the raw pitch used to produce pitch-based carbon fibers. These differences should make pitch-based fibers much less expensive than PAN-based carbon fibers. However, today's selling prices for these two varieties of carbon fibers are nearly identical. The likely cause for this is the limited manufacturing experience in pitch-based fibers. However, in the future it is probable that at least some portion of this potential cost advantage will be realized. Pitch-based carbon fibers exhibit the highest specific modulus (tensile modulus/density) among the commercially available reinforcing fibers. Presently, pitch-based carbon fibers are available with moduli as high as 965 GPa (140 Msi). Their graphite structure also allows pitch-based fibers to possess excellent axial conductivities (both electrical and thermal) and very low axial coefficient of thermal expansion. Commercial pitch-based fibers are available with thermal conductivities that are three to four times that of copper. Because of this, pitch-based carbon fibers are preferred for space applications where stiffness, thermal expansion, or conductivity are critical. However, the extended graphite structure of present commercial pitch-based fibers also makes them more sensitive to fiber surface defects and structural flaws. This increased sensitivity causes the tensile strength of these fibers to be 40 to 50 percent lower than that of PAN-based carbon fibers. Also, it

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 62 Figure 3.4 Pitch Based Process

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 63 results in a compressive strength for pitch-based carbon fibers that is only one-third that of PAN-based fibers. Future Potential. In the area of pitch-based carbon fibers, the technical programs will be directed toward higher tensile strength products, fibers with improved compressive properties, improved routes to mesophase production, fibers with improved conductivity, and fibers that have improved processability. As indicated earlier, there is a substantial amount of interest in pitch-based fibers for their excellent conductivity, which offers opportunities in military, industrial electronic, and structural applications. Research in this area will continue, and fibers with even higher thermal conductivities will most certainly be developed. It should be noted that only pitch-based carbon fibers (and vapor-grown carbon fibers) have the graphite crystal structure needed to develop high thermal conductivity; this property creates a unique market for this fiber. Improved mesophase production techniques are a key to both improved process economics as well as improved products. The other key is reducing the flaw sensitivity of the fiber. Recent research shows that by modifying the microstructure of the pitch-based fiber during the spinning process, without significantly altering the crystalline orientation, it is possible to improve both the fiber tensile strength (reduced flaw sensitivity) and the compression strength.6 In just the past 2 years pitch-based fibers with new microstructures have been introduced that have tensile strengths as high as 3.9 GPa (570 Ksi), compared with 2.4 GPa (350 Ksi) for the best previously available pitch-based fibers. In the future it is anticipated that fibers with tensile strengths in excess of 7.0 GPa (1000 Ksi) will be produced commercially and that compressive strength can be increased significantly over present values. Even though commercial pitch-based fibers have reached 93 percent of the modulus of perfect graphite, their tensile strength is less than 4 percent of that predicted by theory. Obviously, considerable improvement in the tensile strength of pitch-based carbon fibers is possible. Currently, as in the case of PAN-based carbon fibers, noncircular pitch-based carbon fibers are being evaluated as a route to higher-performance fibers that yield composites with improved properties.6,7 As mentioned previously, a key component in the development of a low-cost pitch product is improving the economics of the mesophase preparation step. Currently, the purification, preparation, and spinning operations are very expensive. If, in the future, a lower-performance, high-filament-count, pitch-based carbon fiber can be made for $11 or less per kilogram, industrial, automotive, and civil engineering applications would create a substantial market for this variety of carbon fiber. The Japanese have been very active in the development of advanced pitch technology and could have the lead in the development of such a process/product. Other Precursors for Carbon Fibers Today, PAN and pitch are the principal precursors used to produce commercial carbon fibers. Even though the carbon content of PAN is 68 percent, the actual carbon yield after carbonization is only about 54 percent. This means that 46 percent of the mass of the precursor fiber is lost during its conversion to carbon fiber. Pitch contains over 90 percent carbon, but

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 64 the isotropic pitch must be converted to a mesophase if a high-performance fiber is to be produced. During this conversion process a significant fraction of the starting pitch is often lost. Obviously, research continues on other precursors for carbon fibers. In the 1960s, Japanese researchers began studying polyvinylchloride (PVC) as a possible precursor. Unfortunately, the physical properties of the fibers produced from PVC precursor have been disappointing, and the net conversion of PVC to fiber has been less than 30 percent. However, it is possible that other precursors, or precursor preparation processes may be developed which significantly increase conversion. Polymers such as polyphenylene have a carbon content of 96 percent. If high molecular weight polymers such as polyphenylene can be produced commercially, they may replace PAN and possibly pitch as carbon fiber precursors and significantly alter the process economics. Supercritical extraction is typical of pitch-separation techniques which could significantly change the economics of pitch-based carbon fiber processes. Such techniques could permit the preparation of a mesophase precursor with the regular molecular character of a synthetic polymer. This could not only increase conversion but also greatly ease the melt spinning process, reducing the cost of pitch-based carbon fibers. Observations and Conclusions • Carbon fibers, as a class, are known for their specific strength and modulus properties. • Carbon fiber is likely to be the dominant high-performance reinforcing fiber in future ambient structural applications. • Carbon fiber will probably be the first high-volume, high-performance fiber produced at a relatively low price. • Even though both pitch and PAN-based carbon fibers are commercially produced, they are not mature products. Thus, it is likely that in the future their cost will decrease and their physical properties will significantly increase. A critical ingredient in these developments will be improved precursor technology. • Considerable improvement is needed in the balance of properties possessed by both pitch and PAN-based carbon fibers (particularly compression strength). This will be vital if carbon fibers are to make further penetration in military applications (primary structures). • In the near future, pitch fibers will likely dominate applications requiring high modulus and high thermal conductivity and, perhaps, increased resistance to oxidation. On the other hand, the small crystallite size typical of PAN-based carbon fibers will probably lead them to dominate in intermediate-modulus, high- strength applications.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 65 • Pitch-based carbon fibers have the long-range potential to yield a substantially lower cost process than PAN-based fibers due to higher theoretical yields and lower carbonization costs. However, the approaches to reduced manufacturing costs for both fibers have some common features: improved precursor operating/capital costs, improved heat transfer processing operations for the conversion of precursor to carbon fiber, and use of larger precursor tows. • The family of pitch and PAN-based carbon fibers encompasses a broad range of thermal and electrical conductivity. This set of conductivity properties is useful for a number of applications in which electromagnetic properties are at least as important as the mechanical properties of the reinforcing fiber. Currently, the mechanical and electromagnetic properties cannot be independently modified. Further research should be conducted in this area in order to be able to produce carbon fibers with tailored mechanical and electromagnetic properties. SILICON CARBIDE AND SILICON NITRIDE FIBERS Introduction The ability to transform silicon-based organometallic polymers to silicon-based ceramic fibers was recognized in the mid-1970s.8 About the same time the desirability of a high-modulus, high-strength, thermo- oxidatively stable, low-conductivity fiber for a variety of aerospace, defense, and consumer applications became apparent. For structural applications above about 1100°C, carbon-fiber-based composites (without special protective treatment) fail due to poor oxidation stability; most bulk ceramics, including the common oxide ceramics, fail due to poor mechanical stability. Nonoxide ceramics (e.g., silicon carbide [SiC] and silicon nitride [Si3N4]), are attractive candidates but suffer from low toughness and susceptibility to thermal shock. A method of overcoming these deficiencies is to reinforce the bulk ceramic of choice with thermally and oxidatively stable ceramic fibers. While many matrix systems are available, the technology for converting ceramics to fibers exists only for a small number of systems. Preparation of plastic-matrix fiber-reinforced composites with mechanical properties like those produced by carbon fiber, but with lower electrical conductivity, is also of interest. These considerations have led to a focus on the production of continuous SiC and Si3N4 fibers over the past decade. Several of these products are now commercially available through Japanese suppliers, such as Nippon Carbon Company and Ube. Technical Description To a first approximation, the technology for the production of Si ceramics from polymeric precusors is analogous to the general process shown in Figure 3.1. As shown in Figure 3.5, after polymerization the polymer is melt spun (other possible fiber-forming processes have not been systematically investigated). Next the resulting fiber is cured through a thermochemical cross-linking step ("preoxidation"), and the stabilized fiber is then fired to

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 66 form the desired ceramic. Three basic differences from carbon fiber processing, however, dominate the Si ceramic fiber technology. They are: • The insensitivity of final fiber properties to orientation steps during processing • Detrimental effects of oxidative crosslinking on ceramic fiber properties. In carbon fiber production, oxygen containing moieties less thermodynamically stable than the desired carbon structure are expelled from the structure during pyrolysis. In Si-based ceramics SiO2 is the thermodynamically most stable form and the compound least desirable in the final ceramic structure. • Carbon fiber is normally produced from high molecular weight, mechanically sound, environmentally stable precursor fibers amenable to fiber-handling technology. The precursors for silicon-based nonoxide ceramic fibers are brittle and environmentally unstable, leading to major difficulties in handling and storage. This results in fibers of relatively short lengths and a high degree of variability. Over the past few years it has been shown that the strength of SiC and Si3N4 fibers is a very strong function of the flaw content of the final ceramic and that this flaw content is, to a very large extent, a function of impurities and lack of homogeneity in the starting polymer. It was further shown that strength loss at elevated temperatures and in various chemical environments is, to a significant degree, caused by exacerbation of existing flaws rather than by creation of new ones. It was also established that all Si-based ceramic fibers derived from polymeric precursors are classically brittle materials and that they all fit a universal curve of strength (at room or elevated temperature) versus reciprocal root of flaw size, as shown in Figure 3.6. While linear, high molecular weight, Si-containing polymers of excellent spinability are known, these are not useful as precursors for ceramic fibers because, under the condition needed to cure the fibers, thermodynamically stable Si ring systems are formed and fiber integrity is lost. To overcome the ring-formation tendency, precursor polymers of low molecular weight and high degree of branching are used. Some of the more commonly used precursor structures are shown in Figure 3.5. All of these polymers contain non-stochiometric amounts of Si, C, nitrogen, and oxygen (based on the desired final ceramic), and are difficult to characterize. All of these factors make continuous spinning of infinite-length fibers and yarns both difficult and expensive. As already stated, to maximize ceramic fiber properties, the cure step must minimize introduction of oxygen and, because of fiber instability and brittleness, be kinetically compatible with spinning speeds as much as possible.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 67 Figure 3.5 Production of Si Ceramic Fibers from Polymeric Precursors.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 68 Figure 3.6 Variation of tensile strength with flaw size.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 69 An advantage of the HPZ system developed by the Defense Advanced Research Projects Agency/Air Force Materials Laboratory (Dow Corning, Celanese) is the ability to chemically cure the precursor fiber at high speeds. Pyrolysis of cured fiber to form the ceramic is straightforward, with final properties dependent on time, temperature, and chemical environment. Typically, fibers are obtained with moduli in the range of 25 to 30 Msi, strengths in the range of 300 ksi, and densities of about 2.5 g/cm3. While useful, the modulus of these fibers is considerably less than that of the dense ceramic, and the chemical and thermal behavior of the fibers is also reduced from that typical of SiC or Si3N4. Chemically, no Si ceramic produced from a polymeric precusor is stoichiometrically correct, and these materials are better described as SiC-like or Si3N4-like. Physically it has been shown that in all cases the structure of the fiber is essentially amorphous or consists of very small SiC or Si3N4 crystals sitting in an amorphous matrix. This similarity of physical structure is the rationale for the observed "universal" strength response and low modulus of these fibers. The porosity of these structures is difficult to characterize, but it probably plays a role in the low level of observed properties. Technical Future The production of SiC-like or Si3N4 fibers from polymeric precursors is well established and reasonably well understood. Technical directions that could lead to significant property improvements are identifiable and include: • Improved stability and higher molecular weight precursor polymers. • Polymers with reduced flaw concentrations as spun, in conjunction with improved process control throughout. • Approaches to stoichiometrically correct ceramic chemistry to improve chemical stability. • Approaches to ceramics with larger crystal size to create structures of improved physical stability. • Densification of existing structures to remove residual porosity. • Development of effective diffusion boundary coatings for ceramic fibers to block reactive gases from entering the fiber structure and exacerbating the existing flaw populations. The present limitation to increased use of Si-based ceramic fibers is their very high cost. Unless it can be shown that improved properties will increase market acceptability at current prices, it is unlikely that major improvements in fiber technology will occur without major government support. Lower-cost fiber is a function of both raw material availability and process complexity. Improvements in these regards, while possible, are unlikely to be achieved without external pressures being brought to bear on potential

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 70 manufacturers. To date, no new or growing market capable of catalyzing the necessary research has been identified. Observations and Conclusions The production of Si-based ceramic fibers from polymeric precursor is a demonstrated technology, and R&D programs can be identified that are highly likely to improve both fiber properties and fiber cost. Although the organometallic polymer approach to ceramic fibers could be extended to other ceramic systems, these R&D programs are expensive and time consuming and must involve, in addition to the identification of new materials and processes, the scale-up of proposed improvements to show that feasibility can be transformed into commercial reality. OXIDE FIBERS Introduction Ceramic oxide fibers can be prepared by any of the processes identified as polymeric, polymer-modified solutions or dispersions, fiber-forming salt solutions, or sol-gel systems, the latter sometimes being used as an all- inclusive term for all of these when applied to the preparation of nonvitreous ceramic fibers by pyrolytic conversion processes.9 Most of these fibers have been fabricated by processes that can be categorized generally under "pyrolytic conversion of precursor fibers." Although the information usually provided may disclose the general type of processing by which these fibers are prepared, for example, polymeric or sol-gel (sol is a colloidal dispersion in a liquid medium), details are generally not sufficient for their immediate duplication. The most extensive amount of published information on ceramic fiber processing is in patents, the reference of Bracke et al.2 having been cited in Chapter 1. A number of publications over the past several years have provided data on properties of ceramic oxide fibers currently available either commercially or on an experimental basis.1,9,10 Oxide fibers prepared from the precursor pyrolysis process and that are currently available or under development are listed in Table 3.2 . These fibers are, in principle, desirable for high-temperature applications where the potential for oxidation exists. Except for the NEXTEL Z-11 ceramic fiber of 3M, all of the oxide fibers in the table contain a major A12O3 component. The ZrO2-SiO2 fiber is not generally claimed to be of reinforcement grade since its modulus of elasticity is relatively low (76-90 GPa) (11-13 Msi).11 However, fabric made from this fiber does have outstanding resistance to flame penetration, and it may be useful for applications requiring such properties. Polymeric Precursors Preparation of oxide fibers from polymeric precursors is closely related to the processing of carbon or graphite fibers. However, polymerization of precursors for oxide fibers usually involves hydrolysis of the precursor. Wainer et al.12 (Horizons, Inc.) prepared polymer precursors for the oxides of

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 71 aluminum, yttrium, and the lanthanides, the group IV-A metals (titanium, zirconium, hafnium, and thorium) and the metals having atomic numbers 23-28 inclusive. Polymers were prepared from aqueous solutions of concentrated carboxylate salts by heat treatment in a closed vessel at 60° to 90°C. The resulting polymers had oxide yields (after calcination and oxidation) of up to TABLE 3.2 Oxide Fibers Identity Composition (wt. %) Approximate Diamater (μm) Manufacturer Fibre A1203 (100) 20 E.I. duPont de Nemours & Co., Inc. PRD-166a A1203 (80) 20 Zr02 (20) Saffilb A1203 (96) 3 Imperial Chemical Si02 Safimaxc A1203 (96) 3 Industries, plc, Ltd. (ICI) Si02 (4) Sumicaa A1203 (85) 17 Sumitomo Chemical Co., Si02 (15) Ltd. Alcenb A1203 (80) 2-3 Si02 (20) Denka continuous alumina A1203 (80) 10 Denka KK fiber Si02(20) Alumina continuous fibera A1203 (99.5) 10-12 Mitsui Mining Co., Ltd. Nextel 312a 3A1203/1B203/25i02 10-12 3M Co. (molecular ratios) Nextel 440a 3A12032Si02 (98) 10-12 3M Co. B203 (2) Nextel 480a 3A12032Si02 (98) (mullite) 10-12 3M Co. B203 (2) Nextel Z-11a 1Zr02/1Si02 11 3M Co. a Continuous filament form. b Staple. c Semicontinuous d Partially stabilized.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 72 90 percent (in the case of Zr02). Preparation of continuous, 6-μm-diameter monofilaments of Zr02 by extrusion and drawing of the polymer followed by controlled firing to temperatures up to 1250°C was described. The products of this work were generally characterized as being transparent (or almost transparent), flexible, and microcrystalline. The polymeric precursor approach has been successfully applied by Horikiri et al.13 in the preparation of alumina-silica fibers. This work is apparently the foundation for SUMICA, an alumina-silica fiber comprising 85 percent A1203 plus 15 percent Si02, which is manufactured by Sumitomo Chemical Co., Ltd. The precursors for A1203 and A1203-Si02 fibers comprised a spinning solution of an organic solvent, such as benzene or dioxane, a polyaluminoxane for A1203 fibers, and a mixture of the polyaluminoxane solution with a silicon- containing compound for A1203Si02 fibers. The solutions were spun into continuous filaments that were subjected to humid air for hydrolysis. Precursor fibers were then fired to temperatures between 600° and 1700°C for conversion to the oxide fibers. Spinnable Salt Solutions Oxide precursor fibers may be prepared by spinning solutions such as aqueous solutions of carboxylate salts (e.g., aluminum formoacetate).14,15 At the present time, no commercial continuous filament products are believed to be manufactured by this approach. Polymer-and Solution-Modified Sols and Slurries At the present time, the majority of continuous filament products are based on precursors comprising sols or slurries modified by salt solutions and/or organic polymers. Polymers maybe added solely to provide the theological properties necessary for fiberizing, but the presence of this thermally decomposed (i.e., fugitive) component may also affect the nucleation processes of the inorganic phase and thus affect the properties of the resultant fiber and the microstructure.16,18 The organic modifier may not only provide the necessary properties to permit spinning but also act as a precursor for an inorganic component of the fiber composition. For example, aqueous basic aluminum chloride19 and zirconium acetate solutions20 were blended with silica aquasols for the preparation of A1203-Si02 and ZrSi04 (zircon) fibers, respectively. In these cases the salt solutions provided fiber- forming properties and served as pyrolyzable precursors for metal oxide components in the final ceramic fibers. Continuous filaments of A1203-B203Si02 and Zr02-Si02 compositions with essentially unlimited length were also prepared in relatively early developments with this general approach.21,22 Continuous filament products of essentially pure A1203, or comprising a large percentage of A1203, have been reported as being prepared from modified slurries. Slurries differ from sols in that sols are generally considered as comprising colloidal particles predominantly less than 0.10 μm (100 nm) in size, while slurries comprise particles considerably larger. This difference in sizes of the suspended particles in sols and slurries is an important factor that affects many fiber properties, including optical transparency, smoothness, handleability, strength, microstructure, and performance at high temperatures. A modified slurry process is used to produce several commercially available fibers, including FP alumina (A1203) and PRD 166 (80

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 73 percent A1203 plus 20 percent partially stabilized Zr02) from duPont and an alumina fiber from Mitsui Mining Company. Microstructure Development Pyrolysis of precursor fibers to form oxide fibers overlaps with the early stages of crystallization. After removal of fugitive components, sintering continues. Densification, removal of pores, compound formation, and grain growth all occur as in the sintering of other more traditional ceramic articles. Similarly, the microstructure that results is affected by the precursors that are used. The use of polymeric, solution, or sol-gel precursors as well as the low molecular weight of the inorganic components make it possible to control the earliest stages of crystallization. Thus, the fine microstructures which can be obtained are not achievable with more traditional powder processes. Resultant fibers with fine-grained polycrystalline microstructures may possess very high strength, excellent flexibility, and optical transparency. Microstructures attainable are illustrated in the electron micrographs of Figures 3.7 and 3.8 of NEXTEL 440 ceramic fibers that have been converted to mullite (3A1203.2Si02).11 in this case, mullite grains are 20 to 60 nm in size. The development of microstructures in ceramic fibers prepared with sol-gel processes is discussed in more detail in other recent publications.1,23 As might be expected, microstructures of oxide fibers prepared from fiberizable slurry-type dispersions will be considerably coarser. Fiber FP, and PRD 166, both manufactured by duPont, are characterized by A1203 grains considered to be relatively large (e.g., 0.5μm, or 500 nm), as shown in the electron micrographs of Figures 3.9 and 3.10.19 The extreme differences in grain size of the NEXTEL mullite ceramic fibers and duPont FP alumina and PRD-166 alumina-zirconia fibers is obvious when one compares the micrographs of Figures 3.7 and 3.8 with those of Figures 3.9 and 3.10. The very fine microstructure of the NEXTEL ceramic fibers is largely responsible for its glass-like appearance and handling quality. Stacey10 has suggested that very fine microstructures are related to high strength, but for high-temperature uses it is important to increase grain sizes because grain-boundary creep is expected to be slower with larger crystallites. Similarly, microstructural stability (i.e., minimizing changes in grain size) is an important requirement where strength and modulus must be retained. Ceramic oxide fibers are, in principle, desirable for high-temperature applications where the potential for oxidation exists, and in this respect they are to be preferred over carbon or other nonoxide fibers. However, property data available on most ceramic oxide fibers has been obtained from testing at room temperature. Increasing amounts of data on tensile strength and modulus of elasticity are being obtained on fibers as they are being exposed to high-temperatures. Results have been reported in several recent publications.10,24,25 Thermal expansion values, vitally important to successful utilization in composites for high temperature applications, are also available. 26 A review of these data shows that ceramic fibers begin to lose strength and modulus of elasticity at temperatures above 800° to 1000°C.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 74 Figure 3.7. Scanning electron micrograph Figure 3.8. Transmission electron micrograph of fracture surface of mullite fiber of ion milled section of mullite fiber (50 000x). Copyright by the American ã (150 000x). Copyright © by the American Ceramic Society. Ceramic Society. Figure 3.9. SEM of Fiber FP Surface Figure 3.10. SEM of PRD-166 Surface Copyright © by the American Ceramic Copyright © by the American Ceramic Society. Society.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 75 Technological Future for Oxide Fibers A review of the properties of the oxide ceramic fibers that are available at the present time discloses that they cannot meet many of the projected requirements for reinforcement of high-temperature composites requiring stable strength and stiffness properties and resistance to creep at high temperatures, (e.g. > 1200°C), for extended periods of time. New fibers of selected compositions will be necessary to satisfy these needs. The controlled pyrolysis of precursor fibers has been shown to be a useful process for the fabrication of both oxide and nonoxide high-performance fibers for structural applications. It is a uniquely versatile process in that virtually any composition that can be compounded in a fiberizable precursor batch can be pyrolyzed to form continuous inorganic fibers. However, the preparation of fibers sufficiently handleable for use and possessing properties that would classify them as high-performance fibers requires sophisticated process control procedures tailored to each composition and adjusted for the precursor materials used. Survival of the filament form through pyrolysis does not necessarily guarantee a resultant high-quality fiber because it must undergo further heat treatment with associated solid-state reactions, crystal growth, and structural changes. Fundamental studies on progressive microstructural changes that take place during the pyrolysis process as well as during sintering and densification of the resulting inorganic fiber would facilitate development of advanced high-performance inorganic fibers not now available. Especially important is the development of methods for stabilization of microstructures so as to prevent or minimize property changes during high-temperature applications. CHEMICAL CONVERSION OF A PRECURSOR FIBER Introduction The Chemical Conversion Of A Precursor Fiber (Ccpf) Is A Versatile But Not Very Well Known Fiber- Making Technology Used By Researchers From The Early 1960s To The Mid-1970s To Develop A Number Of Interesting Refractory Fibrous Materials. As The Name Of The Process Implies, The Method Invokes The Conversion Of One Fiber Into Another By Reacting The Precursor Fiber With The Proper Reactants Under Precisely Controlled Conditions. Thus, The Difficult Task Of Forming The Refractory Fiber Directly Is Bypassed. Instead, Effort Is Focused On Dealing With A Chemical Problem To Accomplish Conversion Of An Existing Fiber Into Another. The Success Of This Method Depends On Two Important Factors: • Selection Of Available Fiber As Precursor For The Reaction. • Control Of Reaction Parameters To Facilitate The Reaction Between The Reactants And The Precursor Fiber.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 76 In practice, commercially available fiber (e.g., carbon fiber) or fiber that can be easily produced (e.g., boric oxide fiber) is used as a precursor. It should be noted that once a product fiber is produced, it can itself be used as precursor fiber for other products. For example, BN fiber is made from B203 fiber, and TiN fiber can be prepared by reacting BN fiber with a titanium chloride and hydrogen mixture. The versatility of this fiber-making approach lies in the numerous combinations of gaseous reactants and precursor fibers that can be used. This is illustrated by the examples shown in Table 3.3. TABLE 3.3 Refractory Fibers Prepared by Chemical Conversion of a Precursor Fiber Fiber Melting Point (°C) Precursor Reactants BN 3000 (Sublimes) B203 NH3F TiN 2950 BN TiCl4 + H2 NbN 2573 BN NbCl5 + H2 B4C 2450 C BCl3 + H2 Mo2C 2687 C MoCl5 + H2 NbC 3900 C NbCl5 + H2 NbCxN1-x — C NbCl5 + H2 + N2 Like any other processes, CCPF has its technical constraints, as follows: • The desired fiber composition must be thermodynamically achievable from the precursor and reactants. • The phase that forms initially in the conversion on the fiber surface must not be impervious to further diffusion of the gaseous reactants into the interior for continuous conversion. • Since the conversion is accomplished by a diffusion-controlled process, the reaction rate may be slow, but the fine diameter of precursor fiber will favor the conversion kinetics. Among the examples listed in Table 3.3, two fibers, boron nitride and boron carbide, are of particular importance and interest. Boron Nitride Fiber Preparation of boron nitride (BN) fiber from boric oxide precursor was first reported by Economy et al. in 1966.27 Subsequently, numerous articles were published that described the process, properties, and applications for this fiber.28,30 From a process standpoint, the fiber was made by the following steps:

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 77 • - Fiberization of B203 melt to form B203 fiber • - Nitriding of B203 fiber with NH3 • - Final stabilization to form BN fiber Stoichiometrically, the conversion can be expressed in a simple chemical reaction: The reaction mechanism, however, is actually quite complex. Microporous structure is developed from the initial reaction between boric oxide and ammonia. The formation of the microporosity permits diffusion of ammonia into the fiber core and escape of water from the interior through the reaction layer. This mechanism allows the conversion to proceed to completion. In the early stage of development, BN fiber was produced in a staple roving form. The roving was then converted into various textile forms such as staple yarn, mat, fabric, braided structure, and felt. These product forms made thorough characterization of the fiber properties possible, as follows: Mechanical properties: Typical fiber diameter: 4 to 6 microns Tensile strength: 345 to 862 MPa (50 to 125 ksi) Young's modulus: 27.6 to 68.9 GPa (4 to 10 msi) Elongation at break: 2 to 3 percent Thermal stability: No significant weight loss at 2500°C in inert atmosphere. Oxidation stability: Oxidation starts at ˜850°C; further oxidation leads to formation of boric oxide coating on the fiber surface, which acts as a glaze and affords protection up to 1300°C. Corrosion resistance: Excellent resistance against corrosive reagents as compared to carbon and glass fibers. Electrical properties: Very low dielectric constant of 4.0 and low dissipation factor. Thermal conductivity: High. BN fiber is indeed a multipurpose fiber. Its many properties make it uniquely suitable for numerous potential applications, such as military clothing, radomes, insulation in particle accelerator coils, thermal insulation, electrical applications, filter bags, chemical filtration, high-energy battery separators, and printed circuit boards. For BN fiber to be useful in the structural reinforcement area, its mechanical properties need improvement. In 1972 the method to produce high-strength, high-modulus BN continuous filament yarn was developed and

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 78 disclosed.31,32 Improvements in mechanical properties were achieved by hot stretching the partially nitrided fibers. BN fiber with a tensile strength of 2070 Mpa (300 ksi) and a modulus of 345 GPa (50 msi) was reported. In an unpublished work, test fibers exhibited a tensile strength of 3100 MPa (450 ksi) and a modulus of 480 GPa (70 msi). All these results showed the feasibility of producing BN fibers with superior mechanical properties suitable for structural applications. Further development work is required however, in order to produce high-modulus BN fibers in pilot quantity for application assessment purposes. Boron Carbide Fiber Preparation of boron carbide (B4C) fiber was achieved by the reaction of commercially available carbon fiber with boron trichloride in the presence of hydrogen at ˜800°C.33,35 The formation of boron carbide depends on a diffusion-controlled reaction in which boron trichloride and hydrogen react with the precursor carbon fiber; a layer of boron deposit is formed first, and it subsequently reacts with carbon to form boron carbide. Further conversion depends on the diffusion of boron through the large interstitial openings of the boron carbide structure. Like other diffusion-controlled reactions, the conversion rate depends on the fiber diameter. Fine-diameter precursor fiber favors the conversion kinetics. The diameter of the starting carbon fibers is about 8 microns. As the conversion proceeds, the fiber diameter increases according to the degree of conversion. The tensile strength and modulus of the product also increase, progressing with the enhancement of the degree of conversion. It is an advantage of this process that the properties of the product can be controlled by adjusting the degree of conversion during the reaction. The boron carbide fiber produced by this process often possesses crimps, which cause weakening of fiber properties. This shortcoming is eliminated by post reaction tensioning at an elevated temperature. At a temperature of ˜2100°C and a tensile stress of 20.7 to 27.6 MPa (3000 to 4000 psi), the boron carbide fibers are straightened, resulting in a drastic improvement in mechanical properties. Typically, boron carbide fibers display a modulus of 207 to 482 GPa (30 to 70 msi), and the tensile strengths for the straightened fiber are 2070 to 2760 MPa (300 to 400 ksi). Observations and Conclusions The technology of CCPF is a versatile fiber-making method. There are several practical and significant aspects that are unique to this approach. • The fiber-forming step is eliminated or greatly simplified, since commercially available fibers or fibers that can be readily produced are used as the precursors.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 79 • CCPF is a chemistry-driven process that primarily involves gas-solid heterogeneous reactions. It is important to recognize that in CCPF the reaction is not limited to the surface. The conversion develops inward to the core of the fiber via a mechanism unique to each fiber. • The fibers produced by CCPF are usually of fine diameter, which favors the kinetics of conversion. • The composition of the product fiber is governed by the precursor fiber and the reactants used in the conversion, offering many new possibilities for exploration. • Fiber properties can be further improved by post reaction treatment, such as crystallite orientation by hot stretching to improve modulus and strength in the case of BN and the removal fiber kinks also by hot stretching to improve strength in the case of B4C fiber. It must be pointed out that this technology is still in its early stage of development, the examples reported by early researchers having merely laid the groundwork for the technology. Efforts along the following lines may be fruitful: • Experimental work on various combinations of precursor fibers with a variety of reactants directed at making fibers of interest. The effort will help to define the practical limits of the approach. • Characterization and application development for the fibers made by this technology. Based on the results of application assessment, scale-up work should be done for selected fibers to provide a realistic basis for assessing costs. CHEMICAL VAPOR DEPOSITION Introduction Chemical vapor deposition (CVD) fibers are formed by the deposition on a monofilament substrate of a species generated by a vapor-phase reaction occurring adjacent to or at the substrate surface. In general, the CVD reaction is initiated as a result of the substrate being heated, usually to incandescence. For production fibers the substrate continuously moves through the reaction apparatus or reactor, thus increasing in diameter through the processing, with the CVD fiber product being spooled or taken up downstream of the reaction chamber. The formation of continuous fibers by the application of CVD technology has proven to be a reliable and routine approach to the production of consistent products such as boron and silicon carbide fibers. CVD processing has a wide range of applicability and flexibility in terms of chemical composition, and is limited only by the availability of volatile reactant molecules that incorporate the species to the deposited.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 80 Two CVD fibers are now in production in the United States at TEXTRON Specialty Materials in Lowell, Massachusetts: boron on a tungsten substrate and silicon carbide using a carbon monofilament substrate. Silicon carbide fiber on a tungsten substrate is produced in the United Kingdom by British Petroleum. Substantial production facilities for both boron and silicon carbide fibers exist in the USSR. New CVD fibers known to be currently under development include TiB2 and TiC on several substrates at TEXTRON Specialty Materials and at Amercom. Coatings involving TiB2 and TiC composites on silicon carbide fiber also are being investigated at these companies. Refractory Composites, Inc., is developing a process for depositing TiB2 on titanium wires by a plasma CVD process. Similarly, Professor Vladimir Hlavacek, of the State University of New York at Buffalo, is developing CVD fabrication methods for coating carbon and tungsten fibers with TiB2, TiC, and boron. A method for making potentially low cost carbon fibers via carbon CVD onto a carbon substrate grown in-situ also is under development. This process is being pursued mainly in two countries—in the United States by General Motors Research Laboratory and Hyperion International and in Japan by several companies and universities. Technical Description In view of their established position as actively used CVD fibers, it is instructive to provide some details on boron and the ''SCS'' series of silicon carbide fibers. Both fibers are continuous fibers produced by CVD on substrate wires that are pulled through glass reactor tubes. Boron fiber is currently produced in quantities of approximately 35,000 lb/yr, while silicon carbide (SiC) monofilament production is less than 2000 lb/yr in the United States. Boron Fibers Boron fiber is a continuous monofilament produced in two nominal diameters, 4 mil (100 μm) and 5.6 mil (140 μm). Properties of the fiber, listed in Table 3.4, are compared with typical properties of other fibers, including silicon carbide. Almost all of the boron fiber produced is used to form a boron-fiber epoxy-preimpregnated tape ("prepreg"), a product form in which a linear parallel array of fibers is coated with an epoxy resin and backed on one side with a light fiberglass fabric. The tacky prepreg is then sold to users for ply lay-ups and part fabrication. Figure 3.11 illustrates the boron fabrication process. A tungsten substrate wire, about 0.5 mil in diameter, is continuously drawn through a vertical glass reactor. Both ends of the reactor tube are sealed by a shallow pool of mercury, which acts as an electrical contact to the fiber. Electric power is applied across the ends of the reactor tube such that the tungsten becomes incandescent. A mixture of boron trichloride (BCl3) vapors and hydrogen (H2) gas is admitted to the reactor tube, and the reduction of the BCl3 to elemental boron by H2 takes place on the surface of the hot substrate. The fiber diameter increases as the substrate travels through the tube, and the diameter of the fiber emerging from the bottom electrode and mercury seal

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 81 is a function of the fiber throughput rate, other reactor parameters being constant. The boron fiber is wound onto an 8-inch diameter take-up spool. TABLE 3.4 Comparative Properties of Reinforcing Fibers Fiber Density Average Tensile Strength ksi Modulus of Elasticity msi Approximate Cost $/lb Boron, 4 Mil 2.57 620 60 400 Boron, 5.6 Mil 2.49 520 60 400 Carbon 1.75 450 32 E-glass 2.54 490 10 2.5 Aramid 1.44 520 18 20 SiC, 5.6 Mil 3.0 570 80 2500 During the course of the deposition reaction, the tungsten substrate wire is reacted to form a mixture of tungsten borides, expanding in diameter to about 0.7 mil. The internal stress states in the fiber set up from this core expansion and simultaneous boron mantle deposition are key factors in determining the ultimate tensile strength of the fiber. The typical surface appearance of the boron fiber is that of a corncob structure (see Figure 3.12). The crystallite size, on the order of 20 A is so small that it is considered amorphous. The average tensile strength of high-quality boron fiber is the statistical result of many individual fiber tests; a typical histogram depicting these results is shown in Figure 3.13. Note that a comparatively low strength "tail" is present, related to various defects that can lead to premature failure of an individual fiber segment during a tensile test. Much higher test values for boron fibers of up to 106 psi can result from smoothing the surface by chemical etching or, especially, by removing thefiber core. However, neither practice is routine in boron filament processing. The costs of boron filament are mainly associated with the boron trichloride, the substrate, and capital equipment. While the cost of boron trichloride and substrate would decrease drastically if made in commodity quantities, it is unlikely that this will happen. Hence, while it is conceivable that boron filament could be produced for much less than $100 per pound, the present prices of several hundred dollars per pound are dictated by the current costs for boron trichloride and the substrate.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 82 Figure 3.11 Boron Filiment reactor. Figure 3.12 Photomicrographics of boron fiber (a) filament 100-μm.) core. 560 x. (b) Magnification of boron filament surface 110x. Figure 3.13 Histogram of boron Figure 3.14 Histogram of CVD fiber tensil strength. SiC fiber tensil strength.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 83 Silicon Carbide Fibers SiC continuous monofilament is produced in both 5.6-mil and 3-mil diameters by TEXTRON Specialty Materials, and in a 4-mil diameter by British Petroleum. A typical histogram of strength properties for 5.6-mil SiC is shown in Figure 3.14. Almost all of the SiC fiber now produced is used in development programs for reinforcing metal alloys of aluminum and titanium compositions. It has also been used successfully to reinforce organic and ceramic matrices. The 5.6-mil variations of silicon carbide fiber produced by TEXTRON are SCS-2, which is designed for reinforcing aluminum alloys; SCS-6, which is used to reinforce titanium alloys; SCS-8, which is used to reinforce an aluminum alloy structure when the composite must display higher tranverse properties than can be obtained by using SCS-2; and SCS-10, which has a surface coating for titanium alloys and is produced by a new process considerably less complex than that used for the other variations. Three-mil SCS-9, intended predominantly for National Aerospace Plane (NASP) applications, is also made using the simplified reactor scheme. The fiber is formed from the reaction of hydrogen with a mixture of chlorinated alkyl silanes at the surface of a resistively heated carbon substrate. One of the main attractions of SiC fiber is its potentially low cost, but it is currently priced at about 25 hundred dollars per pound. Projections for volume production levels that are roughly equivalent to present boron production rates indicate that a cost less than that for boron is achievable. The explanation for this cost reduction resides in the lower costs of the substrate and raw materials for SiC production, along with higher reaction and deposition rates than for boron. There is a major difference between using fibers to reinforce metallic structures and using them to reinforce organic or ceramic matrices. A high degree of compatibility at the fiber-metal interface is essential for the benefit of the reinforcement properties to be exhibited in the composite. However, if the degree of affinity between the fiber and matrix is too high, the reaction between the metal and the fiber at the interface will result in a severe degradation of fiber properties such that a strong composite will not result. In general, a chemical vapor deposition (CVD) fiber being used to reinforce a particular metal structure will probably not be a monolithic multipurpose fiber, but will more likely be a specialized material having a surface composition specifically selected for the particular matrix. For SiC fiber the significance of this is displayed in the SCS fiber series, which differ only in the gradations of surface composition. In all cases the bulk fiber consists of polycrystalline -SiC of much larger crystalline size than in the case with boron fibers. Thus, while the bulk of the fiber is the same, each of these fibers is produced with a different carbon-rich zone near the surface. The differences among the fibers reside in the respective values of the Si/C atomic ratio and the fineness of the grain structure as a function of fiber radius in this several-micrometer-thick surface zone.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 84 CVD Carbon Fibers In addition to the above methods for producing fibers by CVD onto a premanufactured substrate fiber, there are methods for producing fibers by insitu growth of the substrate fiber followed by a CVD step to produce a fiber having the desired composition. The predominant fiber that has been produced by this method is carbon. The driving force for development of this fiber has been the potential of lower manufacturing cost when compared to conventional carbon fiber manufacturing techniques. The use of these fibers is envisioned to be limited to secondary structural applications where discontinuous randomly oriented reinforcement can be employed. TECHNICAL FUTURE Boron fiber is a relatively mature product; boron-epoxy composites are in production use on F-14, F-15, and B-1B aircraft. Tens of thousands of pounds of boron-epoxy composites are also currently being used in sporting goods applications. Potentially large applications for boron in the future reside in boron-aluminum composites for electronic packaging materials for airborne applications, as repair materials for civilian and military aircraft, and for aeroshells for penetrating weapons for armor or earth structures. A production-scale application for silicon carbide fiber reinforced composites has yet to be demonstrated. However, successful tests of SCS-8 aluminum composites have been achieved in a development and test program for military aircraft sections, which has encouraged moves toward larger-scale engineering development. NASP applications could call for over 20,000 lb of SiC fiber in composites, as well. In general, the development of production CVD fibers has involved a high degree of engineering empiricism with regard to both processing methods and design of the fiber products themselves. This approach has resulted in complexity of processing methods and belated attention to important questions—for example, strength retention at elevated temperatures, matrix interactions during consolidation heat treatments, and internal stress structure in the fiber and its effect on composite performance under realistic stress and fabrication conditions. A basic science approach to these and other problems has, in general, been lacking. Observations and Conclusions • CVD fiber for advanced applications will probably not be a multipurpose fiber, but will more likely be a specialized material having a tailored surface composition specifically aimed at matrix comparability. • Application of an appropriate surface coating on existing fiber can markedly enhance its stability relative to either the matrix composition or to thermal and environmental conditions that the composite is subjected to during fabrication or service life.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 85 • A basic science approach to the CVD fiber-formation process, providing fundamental insights into decomposition kinetics, fiber-formation mechanisms, and surface reactions, can result in a better optimization of processes and products. Although it may not be inordinately difficult to form a new CVD fiber of a given composition initially, achievement of consistency in properties may require understanding and carefully controlling the parameters of the deposition process. Single-Crystal Fibers One of the main deficiencies of currently available ceramic fibers is creep at high temperatures. Above ˜1100°C the structure of polycrystalline A1203 and SiC fibers changes, causing slippage along grain boundaries, which greatly impairs the physical properties of the fiber.36 Many of the mechanisms responsible for high- temperature creep, such as grain growth and slippage at grain boundaries, can be eliminated through the use of single-crystal fibers. Single-crystal fibers can be produced by many different techniques, such as growth from solution, melt, and vapor phases. Production of such fibers at the present time is being performed primarily by three techniques: controlled drawing from the melt, vapor growth, and zone refining. Controlled Drawing from the Melt This process is a variation of the Czochralski37 method for producing single crystals. In this technique a single fiber is slowly drawn from a pool of molten ceramic material. Since the ceramic materials melt around 2000°C, extremely high temperatures are employed. One of the major problems in Czochralski growth is the presence of thermal convection in the melt pool, which causes local temperature fluctuations and increases the probability of nucleation of multiple crystals. The cooling rate of the fiber as it is drawn from the melt must also be carefully controlled to prevent the nucleation and growth of multiple crystals within the fiber. Thus, while controlled drawing from the melt using classical Czochralski techniques can be used for producing single-crystal fibers, the slow growth and the requirement for long-term stability make it likely that any fibers produced by this method will be extremely expensive. Most recently, a variation of the Czochralski technique called the micro-Czochralski (μ-CZ)38,39 has been reported that significantly increases the speed at which single-crystal fibers can be pulled from the melt. The μCZ method essentially minimizes the effects of convection in the melt by replacing the large melt crucible with a tiny heater that is wetted by the melt (Figure 3.15). The melt forms a film over the heater surface that is small in volume and which suppresses thermal convection. The single-crystal fibers are pulled from small microprotuberances on the surface of the heating element. Using this technique, LiNb03 single-crystal fibers have been pulled at rates of 0.4-1.0 mm/s. One of the most interesting aspects of this technique is the potential for pulling multiple fibers and hence increasing the throughput of the process and possibly decreasing fiber production costs.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 86 Vapor Growth In addition to the above method for producing single-crystal fibers, there are methods for producing fibers (whiskers) by insitu growth of the substrate fiber followed By a CVD step to produce a fiber having the desired composition and diameter. The predominant fiber that has been produced by this method is carbon.40,43 The driving force for development of this fiber has been the potential of a lower manufacturing cost44 when compared to conventional carbon fiber manufacturing techniques. The process is illustrated in Figure 3.16. A substrate fiber is produced y exposing a small (10-30 nm) metal particle ( -Fe) to a flowing mixture of a hydrocarbon gas (CH4, C6H6, etc.) in hydrogen at temperatures on the order of 900 to 1100°C. The metal particle becomes supersaturated with carbon and precipitates a cylindrical carbon tube typically on the order of ˜300 nm in diameter. This tiny fiber grows in length at rates of about 1-10 mm/min. The length of the fibers is a function of the exact process conditions, but fibers have Been grown to at least 30 cm. After these tiny-diameter fibers have been grown, their diameter is increased to macroscopic dimensions (about 1-10 μm) by CVD. The end product is an inextricably tangled mass of discontinuous carbon fibers. The individual fibers in this mass as prepared have Been shown to exhibit mechanical properties comparable to those of medium-strength PAN-based carbon fibers.44 After high-temperature heat treatment, the fibers develop Young's modulus on the order of 600-800 GPa comparable to those of high-temperature, heat- treated, pitch-based carbon fibers. Some vapor-grown fibers when heat treated to intermediate temperatures have exhibited a benign sword-in-sheath mode of failure in which the central core of the filament pulls out of the outer CVD sheath.45 Fibers that have failed in this manner have been shown to recover a fairly large fraction of their modulus and load-carrying capability provided that the central core has not been completely removed from the outer sheath. The heat-treated CVD carbon fibers also exhibit the highest levels of thermal conductivity that have been observed in carbon fibers,46 due to the high degree of perfection of the graphite crystal structure achieved in these fibers. Whiskers of SiC have been produced By the vapor-liquid-solid (VLS) process,47 which is somewhat similar to the vapor-phase carbon fiber process described previously. The VLS process is illustrated in Figure 3.17. A solid catalyst particle is placed in the growth reactor and exposed to the reactive gases H2, CH4, and Si0. The particle becomes saturated with Si and C and forms a molten (liquid) Ball that mediates the growth of the whisker through the liquid-solid interface. The crystal grows by precipitation from the supersaturated liquid. The growing crystal is fed from the vapor by reaction of the constituent molecules at the vapor-liquid interface. Beta-SiC whiskers having tensile strengths of over 8.4 GPa and Young's modulus of 581 GPa have been reported.48,49

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 87 Figure 3.15.Schematic drawing of the μ-CZ technique.Permission to print from Journal of Applied Physics,American Institute of Physics. Figure 3.16.Description of the production of vapour-grown fibres. (a)During the saturation phase an iron particle is loaded with carbon from the gas phase.(b)A carbon filament then precipitates and lengthens as more carbon is supplied by the gas phase.(c)Finally,the filament is thickened by vapour-deposited carbon.(G.Tibbets,C.P.Beets,M.Endo., SAMPE Journal 22,1986,p.30).Reprinted by permission of the Society for the Advancement of Material and Process Engineering.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 88 SiC whiskers are produced on a commercial scale via the pyrolysis of rice hulls, which inherently have a high Si content. The growth mechanism for SiC whiskers produced by this method has been shown to be a variant of the VLS process,50 thus illustrating that such whisker growth processes can be carried out on a small scale with economics51 that are acceptable for small speciality applications. Single Crystal Growth/Zone Refining One of the most versatile methods for producing single-crystal fibers is the laser-heated pedestal growth (LHPG) technique.52 This is a variation on the float-zone pedestal growth process.53 In the LHPG technique a source rod having the desired composition is heated by a laser focused onto one end of the rod, creating a molten zone from which the fiber is pulled (see Figure 3.18). The process does not employ a crucible, the position of the laser-heated zone being fixed in space while the source rod is translated into the laser beam at a rate necessary to conserve the melt volume. One of the principal advantages of this process is the ability of the laser to produce extremely sharp temperature gradients (˜1000°C/cm). The steep temperature gradient combined with the high surface-to-volume ratio of the fibers causes the fibers to cool very rapidly. This helps to suppress high- temperature, solid-state-phase transformations, which can be important for stabilizing metastable forms of the material for nonstructural applications. Another of the major advantages of this technique is the complete absence of hot furnace elements that can cause contamination of the fiber. ALTERNATIVE PROCESSES Processes emphasized earlier in this chapter have been found to be useful for the fabrication of high- performance fibers over a wide range of compositions. It is important to stress, however, that these processes represent only a select few chosen on the basis of demonstrated applicability. Other processes may also be useful in the development of new or improved fibers and for large-scale manufacturing. Fused composition processes such as in the traditional fabrication of glass filaments should not be forgotten. Commercial high-purity quartz yarn, used in dielectric composites, is made in France and the United States by drawing filaments from a melt fed by quartz precursor rods. Fusion and controlled spinning of some compositions, especially oxides, may be useful in the fabrication of some high-performance ceramic fibers. The "relic" process,2 wherein porous fugitive organic fibers are impregnated with inorganic solutions followed by drying and pyrolyzing so as to form ceramic fibers, should be recalled. Fibers made by this process are not generally considered useful for reinforcement of composites, but determined efforts to control the microstructure of the resultant fibers should not be automatically dismissed as an impossible task. Metallic fibers may have high strengths and high modulus of elasticity values. Very small metal fibers with diameters of about 10 μm or less have been prepared by drawing heated metal wires encased in a sacrificial sheath (e.g., glass) that is removed. 1 Metallic fibers, though handicapped in most cases by high

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 89 Figure 3.17. Illustration of the VLS process for SiC whisker growth. Permission by Journal of Materials Science, Chapman and Hall Ltd. Scientific, Technical & Medical Publishers. Figure 3.18. A Schematic diagram of the pedestal growth method. Permission to print by Materials Science and Engineering, Switzerland.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 90 density values, may provide useful properties for special applications. Other processes are expected to be discovered or exploited as the markets for high-performance synthetic fibers evolve. FIBER COATINGS Introduction As composite material systems become increasingly sophisticated to meet ever-increasing performance requirements, it has become necessary to develop more advanced methods for controlling the manner in which the reinforcing fibers interact with the matrix material. This is especially important in the ongoing efforts to develop high-strength, damage-tolerant ceramic-matrix composite materials54 that have higher-temperature capabilities, such as for use in high-temperature regions of turbine engines. Since there are a limited number of available types of fibers that have the requisite properties for high-temperature applications, it has become necessary to develop coatings and surface treatments that facilitate the use of these fibers in an ever-widening selection of matrix systems. The application of a coating to the fiber surface is one of the most versatile methods55,56 for controlling the fiber-matrix interaction. Other methods include providing additional elements to the matrix that will either form precipitates that segregate at or react with the fiber surface during fabrication and in situ modification of the fiber surface during fiber manufacturing. In this section fiber coatings will be emphasized. Coatings offer the possibility of tailoring the fiber-matrix interfacial properties so that optimum composite properties can be achieved. Fiber coatings can have a profound effect on the composite material at all stages of its existence from fabrication to in-service use. Fiber coatings perform the following general functions: • Control fiber-matrix bond strength. • Improve strength by reduction of surface stress concentrations. • Alter the wettability of fibers by the matrix. • Improve the chemical compatibility with the matrix. • Provide diffusion barriers. • Protect fibers from damage during consolidation and fabrication. • Protect the fiber and fiber-matrix interface from environmental degradation during service. Technical Description There are many different methods for applying coatings to fibers. The most frequently used techniques include electrodeposition, CVD, metallorganic coating, polymer precursor coating, and line-of-sight vacuum deposition techniques. Each of these coating methods is discussed in the next section.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 91 Electrodeposition Electrodeposition has proven to be one of the most convenient methods for applying metal coatings to fibers. Electroplating is used for depositing metals onto conducting substrates such as carbon fibers. In the electroplating process, continuous fibers are pulled through a plating solution while an electrical bias is applied between the fiber, which usually serves as the cathode and an anode structure in the plating bath. Figure 3.19 shows a schematic of a fiber electroplating process. During this process the fiber passes through several plating baths to build up the desired thickness of metal. The process provides good penetration of the fiber tow and usually results in uniform coating thickness. Figure 3.19 Schematic view of a continuous fiber electroplating process. Although electroplating is a mature science, there are still many unresolved problems, such as determination of the composition of the plating solution that gives the highest-quality coating. For example, many plating solutions contain additional chemicals that modify or enhance the interaction of the depositing metal with the fiber surface; the scientific basis of understanding is not well developed. Electroplating is practiced commercially by American Cyanamid for coating Ni and, until recently, Cu onto carbon fibers. The major disadvantages are the limitation to metal compositions, recovery of the unused metals salts, and disposal of the waste solutions. Methods have also been developed that permit electroplating on nonconducting fibers. These methods usually employ electroless plating to deposit a very thin conducting layer prior to entering the electroplating process. Electroless plating also has been used by itself for depositing metals on nonconducting fibers. Electroless plating is a chemical process that utilizes a surface-catalyzed reaction to deposit metal onto the fiber

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 92 surface. This method usually requires a pretreatment of the fiber to activate the surface before entering the plating solution. The major disadvantage of this process is the time required for the reactions to occur, which limits the throughput of the process. Chemical Vapor Deposition Chemical vapor deposition (CVD), previously discussed as a method of fiber synthesis, is also used for coating. It can be broadly defined as a materials synthesis method in which the constituents of a gaseous phase react with one another and with the substrate surface to produce a solid film having the desired composition on the substrate. It is most favorable that the reactions be heterogeneous, that is, occur at the substrate surface. This usually results in high-quality films that adhere well to the substrate. If the reactions are homogeneous and take place in the gas phase, the resulting films exhibit poor properties and adhesion as a result of the formation of solid particles in the gas that eventually contact the substrate surface producing a film. The heterogeneous reaction can be viewed as a sequence of events, any one of which can be rate limiting. The growth of a film via a heterogeneous process can be viewed as a sequence of the following processes:57 • Transport of the reactants to the substrate surface via diffusion. • Adsorption of the reactants on the substrate surface. • Surface processes—reaction, surface diffusion, diffusion in the substrate. • Desorption of volatile reaction products. • Transport of the products away from the substrate surface. A schematic view of these processes is shown in Figure 3.20. The different regimes in which each of these processes may become rate limiting will be controlled by the process variables—the partial pressures of the reactants, the substrate temperature, and the flow rate through the reactor. A diagram of a typical CVD reactor for continuously coating fibers is shown in Figure 3.21. The great flexibility and inherent simplicity make the CVD process one of the most widely used methods for applying coatings to fibers. The process is naturally suited to applying coatings in a continuous manner, and since the deposition is carried out in a regime in which the deposition is gas-phase diffusion limited, all of the fibers in a tow become uniformly coated. In addition, there are usually several sets of reactions that can be utilized for producing a particular coating composition. For example, -SiC coatings have been produced by CVD from many different gaseous mixtures such as SiH4, C3H8 and H2, or (CH3)3 SiCl and various hydrocarbons58 as well as others. Table 3.5 gives a brief listing of materials that have been deposited using CVD processes. For a discussion of many more compounds and metals that have also been deposited, the reader is directed to more extensive reviews.59 For each given set of reactants, the most crucial parameter is the substrate temperature because it determines the rate and the overall thermodynamic outcome of the reaction.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 93 Figure 3.20. Schematic Diagram of the CVD Process. Permission to print by the Electrochemical Society, Inc. Coating thicknesses from a few nanometers to a few hundred micrometers can be deposited; however, because deposition rates are slow, process throughput falls off for thick coatings. Typical deposition rates can range from ˜μm/hr to ˜100 μm/hr. In many instances of materials having slow deposition rates, it is necessary to have multiple deposition stations along the fiber line. Metallorganic Deposition Metallorganic deposition is an entirely nonvacuum technique for the preparation of thin films. In this process a liquid precursor that contains a metallorganic species dissolved in an organic solvent is deposited onto the substrate surface. The substrate is then subjected to drying and heating steps that result in removal of the solvent and decomposition of the organometallic compound to produce the desired coating. The process is shown schematically in Figure 3.22. Typical precursors are metal alkoxides, metal deconates, neodeconates, and preceramic oligomers or polymers. In the case of the metal alkoxides, after the metallorganic is deposited, it is subjected to a hydrolysis reaction to convert the coating to the oxide. The remaining heat treatments then serve to remove any remaining organic residues. This process usually produces coatings that are 50 to 200 nm thick after the heat treatment stage. The process is capable of applying graded compositions by using multiple dipping stations, each having a slightly different composition.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 94 Figure 3.21. Schematic of a continuous CVD fiber-coating line. TABLE 3.5 Materials Produced by CVD Processes CVD Material Reference A1203 60 B 61 B4C 62 HfC 63 SiC 64, 65 Si3N4 66 TiB2 67 TiC 68 ZrC 69

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 95 Figure 3.22. Metallorganic deposition process. The process has been used most frequently to apply oxide coatings such as SiO2, TiO2, and mixtures of various oxides. Metal deconates undergo a similar process, but the hydrolysis step is not necessary. Preceramic polymers such as polycarbosilane, which results in SiC, are applied in the same manner as the alkoxides, but as in the case of the metal deconates, the hydrolysis step is not needed. The major disadvantage of the preceramic polymer coating method is the lack of a wide variety of coating materials. Metallorganic deposition shares many common features with electrodeposition in that the process is continuous and all operations are carried out at atmospheric pressure. The major disadvantages of this coating method are that the coating thickness for a single pass is often too thin and thicker coatings put on using multiple passes often develop cracks at the interfaces between layers. Vacuum Deposition This category of fiber coatings includes sputtering, physical vapor deposition, e-beam evaporation, plasma- assisted CVD, and ion-plating techniques. With the exception of the plasma and ion plating, these processes are line-of-sight deposition techniques, making deposition of uniform film on a multifilament tow very difficult to achieve due to shadowing effects by other filaments in the tow. The film deposition rates are usually slower than with CVD methods, and hence the throughput of these processes is limited. Vacuum deposition techniques for continuous filaments are at an early state of understanding, and much more work in this area needs to be done.

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 96 Future Fiber-Coating Technology As the demand for structural materials that can operate at higher temperatures increases, it is imperative that a thorough understanding of the role of fiber coatings be developed. Given that there is a limited set of existing fibers that can be utilized at elevated temperatures, and that these fibers are usually unstable either relative to the matrix material or the environment during fabrication of the composite or in-service use, it becomes necessary to consider the use of coatings as a means of moderating the interaction of the matrix with the fiber. In the introduction to this chapter it was pointed out the fiber-coating matrix is a complex thermochemical- thermomechanical system and that a substantial amount of analysis is required to determine the optimum composition that a coating or series of coatings should have in order to keep the coating from interacting too strongly with either the fiber or the matrix. Currently, the best models available are thermodynamic chemical equilibrium calculations used to predict whether two compositions might react with one another under a given set of conditions to form a more stable series of products. These calculations usually do not take into consideration the kinetics of the reactions. It also needs to be pointed out that in many instances the coatings applied by the methods discussed are not ideal in that they may not be stoichiometric, or crystalline, and that if they are crystalline may contain defects or impurities that will play an important role in how the coating reacts with the fiber and surrounding matrix. When coatings are applied at temperatures well below that of the melting point of the coating material, as is the case for most of the coating methods discussed here, the coating can be in a metastable state. On subsequent exposure to high temperatures, it will evolve toward the thermodynamic equilibrium state, possibly producing precipitates and/or reacting with the fiber or matrix. In general, there is a need for better understanding of the coating process, so that coatings result that will have the desired stoichiometries and structure. This requires development of more useful in situ diagnostic probes for monitoring the coating process to ensure uniformity of coating properties. It is highly likely that ternary and quaternary coating systems will need to be developed, and this will require a closer relationship between synthetic chemistry for the design of organometallic molecules and the film growth community for developing new methods for coating fibers with compositionally complex systems. There is also a need for techniques to evaluate the effect of the coating on the interfacial bonding properties in the composite that relate to the structure and chemistry of the interface. Considerations Involved in Scale-up of Fiber Processing The processing of bulk materials into fibers is a broad-based technology that is relatively well understood in principle. However, scale-up of fiber processing is more difficult than that of other materials because of the simultaneous heat, momentum, and mass transfers, combined with chemical reactions and unique, complex, flow- induced orientation effects. Fiber

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 97 process development may conveniently be divided into three levels of detail and complexity, namely: feasibility, screening, and end-use evaluation. Demonstration of the feasibility of converting a given bulk material to fiber involves converting a minimal amount of the bulk material to a short length of fiber and a gross evaluation of the mechanical, thermal, and chemical properties of the filament so produced. To do this with almost any material is relatively simple and may be accomplished with as little as a few grams. Typical methods might include ''hand pulling'' of a fiber from a melt or solution or the use of a single-filament microspinning machine. Fibers so produced lack physical and structural uniformity and give only the broadest insight into the spinnability and property set of the fiber obtained. Screening involves tens to hundreds of grams of the fiber-forming precursor material (per experiment), which is spun into either a single filament or yarn with a small number of filaments using small-scale spinning equipment. This allows broad control of the key spinning variables such as temperature, stress, and mass transfer —parameters that control the structure and therefore the properties of the fiber produced. By allowing the systematic study of process-structure-property relationships, screening permits the initial evaluation of the fiber's probable utility. In screening experiments the purity and homogeneity of the precursor material start to play a major role and allow an initial characterization of the range of properties available from the material in fiber form. For example, polyethylene that is conventionally melt spun and drawn yields a modulus of around 100 g/d and a strength of perhaps 8 g/d. The same polyethylene gel spun and drawn to much higher draw ratios might yield a modulus of 2500 g/d and a strength of 30 g/d. The mere pulling of a polyethylene melt into a long thin length of material would not have given any indication that property variation over a range of several orders of magnitude was possible. The importance of purity of the material can be illustrated by the following example. Assume a melt- spinnable material being extruded into a 20 dpf fiber at the relatively slow spinning speed of 500 m/min. If the material contains one impurity particle per 100 g capable of upsetting the spinline, a single filament spinning might run for 25 min. yielding 45,000 m of yarn for evaluation and the impression of stable spinning. If the more realistic case of 10-filament spinning is attempted, the spinline might be expected to breakdown after only about 5 min. If a 100-filament process simulation is attempted, less than 1 min of stable spinning is possible. Similarly, a parameter control upset (i.e., an anomaly in temperature or quench rate) will interupt the spinning process and/or profoundly influence the fibers' properties. The difficulties associated with control are exacerbated as the fiber forming process becomes more complicated, and the small-scale simulation of highly complex processes, such as the production of ceramic fiber from polymeric precursors, may be essentially impossible to evaluate on a very small (100-1000 g) scale. True end-use evaluation of process development requires large amounts (tens to hundreds of kilograms) and highly controlled, relatively large scale equipment. The equipment must be capable of 24-hour operation because only fiber data generated under steady-state conditions are meaningful. Often,

FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 98 major property and processability improvements are noted in large-scale fiber evaluation because of the superior parameter control possible in large batch operation. In the case of a large-scale operation, feed material purity and homogeneity requirements are obviously high for the program to be initiated, much less successfully completed. Experiments beyond the screening stage are, therefore, nontrivial and involve the commitment of major capital resources, expertise, and man-hours. OBSERVATIONS AND CONCLUSIONS ABOUT FIBER FABRICATION AND PROCESSING • Technologies for the fabrication of a wide range of high-performance fibers are known and have been used to prepare fibers of many types with great diversity in composition and properties. • The high cost of manufacturing remains a common problem from the standpoint of economics and utilization of many high-performance synthetic fibers. • Pyrolysis and chemical conversion of precursor fibers, CVD, and single-crystal growth processes are promising routes to the fabrication of continuous high-performance synthetic inorganic fibers. • The control of microstructure during fabrication and applications is a vital consideration for all classes of high-performance synthetic fibers for structural uses. • New processes and increased understanding of the processes used now will be important in the development of reinforcement fibers for future composite applications with more demanding requirements. • Continued efforts on fiber coating processes and fundamental studies on interfacial effects between fibers and matrices are needed. REFERENCES 1. Sowman, H. G., "A New Era in Ceramic Fibers via Sol-Gel Technology", Am. Ceram. Soc. Bulletin, Vol 67 [12], pp 1911-1916, 1988. 2. Bracke, P., H. Schurmans and J. Verhoest, Inorganic Fibres and Composite Materials, Pergamon Press, 1984. 3. Watt, W., "Chemistry and Physics of the Conversion of Polyacrylonitrile Fibers into High Modulus Carbon Fibers," Handbook of Composites, Vol. 1 Strong Fibers, 327-387, Eds. W. Watt and B. V. Perov, Elsevier Science Publishers B.V., North-Holland, 1985. 4. Daumit, Gene P., Y. S. Ko, C. R. Slater, J. G. Venner, D. W. Wilson, C., C. Young, and H. Zabaleta, "MSP-A Domestic Precursor For Current And Future Generation Carbon Fibers," 20th International SAMPE Technical Conference, 20, 414, 1988.

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FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 101 38. Ohnishi, N., and T. Yao, Jpn, J. Appl. Phys. 28, L278, 1989. 39. Kurosaka, A., M. Aoyagi, H. Tominaga, O. Fukuda and H. Osanai, Appl. Phys. Lett. 55 390, 1989. 40. Koyama, T., M. Endo and S. Murayama, 14th Japan Congress on Materials Research 96, 1971. 41. Tibbetts, G. G., Appl. Phys. Lett. 42, 666, 1983. 42. Tibbetts, G. G., J. Crystal Growth 66, 632, 1984. 43. Tibbetts, G. G., and C. P. Beetz, J Phys. D: Appl. Phys. 20, 1987. 44. Tibbetts, G. G., C. P. Beetz and M. Endo, SAMPE J. 22, 30, 1986. 45. Tibbetts, G. G., C. P. Beetz, Jr., and Ch. H. Olk, Extended Abstracts, Graphite Intercalation Compounds, Materials Research Society, 1267, 1986. 46. Heremans, J., and C. P. Beetz, Phys. Rev. B 32, 1981, 1985. 47. Milewski, J. V., F. D. Gac, J. J. Petrovic and S. R. Skaggs, J. Marl Sci. 20, 1160, 1985. 48. Petrovic, J. J., J. V. Milewski, P. L. Rohr, and F. D. Gac. J. Mat'l Sci. 20, 1985, 1167. 49 Petrovic, J. J. and R. C. Hoover. J. Mat'l Sci. 22, p. 517, 1987. 50 Karpman, M. and J. Clark, Composites 18, p. 121, 1987. 51 Nutt, S. R., J. Amer. Ceram, Soc. 71, p. 149, 1988. 52 Feigelson, R. S., Mat'l Sci. and Eng. B1. p. 67, 1988. 53 Burrus, C. A. and J. Stone, Appl. Phys, Lett, 26, p. 318, 1975. 54 Hillig, W. B., "Prospects for Ultra-High Temperature Ceramic Composites," Tailoring Multiphase and Composite Ceramics, Eds. R. E. Tressler, G. L. Messing, C. G. Pantano and R. E. Newnham, Plenum Publishing Corp., p. 697, 1986. 55. Cranmer, Do C., "Fiber Coatings and Characterization," Amer. Ceram. Soc. Bull., 68, 415, 1989. 56. Kerans, R. J., R. S. Hay, N. J. Pagano and T. A. Parthasarathy, "The Role of the Fiber-Matrix Interface in Ceramic Composites," Ceramic Bull. 68, 529, 1989. 57. Kern, W., and V. S. Ban, "Chemical Vapor Deposition of Inorganic Thin Films," Thin Film Processes, Eds. J. L. Vossen and W. Kern, Academic Press, Inc., New York, p. 258, 1978.

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High performance synthetic fibers are key components of composite materials—a class of materials vital for U.S. military technology and for the civilian economy. This book addresses the major research and development opportunities for present and future structural composite applications and identifies steps that could be taken to accelerate the commercialization of this critical fiber technology in the United States.

The book stresses the need for redesigning university curricula to reflect the interdisciplinary nature of fiber science and technology. It also urges much greater government and industry cooperation in support of academic instruction and research and development in fiber-related disciplines.

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