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EXECUTIVE SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS 1 EXECUTIVE SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS High-performance fibers are unique because they are an enabling technology for so many present and future high-technology products. If the United States is to remain a leader in technology-based products, support for the development of these new fibers is vital. Loss of leadership in the production of these fibers could lead to the loss of our international dominance in aircraft production and further weaken our competitiveness in the automotive market. Indeed, loss of our leadership role in these fibers could have a markedly deleterious effect on the U.S. economy. The strengthening of materials by incorporating fibers as the primary load-bearing or fracture-inhibiting components in otherwise weak matrices occurs in nature (e.g., wood, bone). Man has made use of this principle since ancient times (e.g., in making of bricks reinforced with straw). The advanced composites industry is the modern expression of this technology, wherein exceptionally strong and stiff (high-performance) organic or inorganic fibers are embedded in a variety of matricesâpolymer, ceramic, or metallicâto produce lightweight products with structural properties vastly superior to those of the separate components. Because of their low density and high strength or modulus, advanced composites have become critical components for modern aircraft and aerospace vehicles, and they play an essential role in assuring U.S. preeminence in these industries. The same characteristics have contributed to their application to other fields, from automotive components to electronics to recreation products. With lowered production costs, a vast expansion of their applications to many other areas is possible, making them of even greater economic importance. Present defense applications put a premium on high-strength and high-temperature performance and current government-sponsored research on high-performance fibers is directed toward improving their mechanical and thermal properties either through the development of new fibers or through new methods of synthesizing or processing existing reinforcing fibers. However, improvements in these fibers (performance per dollar) can be expected to have a significant effect on civilian end products as well. Future developments, leading to cost- effective parts production driven by improved or new types of synthetic fibers, improved manufacturability, and reduction in manufacturing costs, will create entirely new practical applications for fiber-based materials (e.g., in building construction and automotive applications). The
EXECUTIVE SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS 2 market for a low-price, high-performance fiber could be extremely large, rivaling that of the present synthetic fiber industry. Thus, it is probable that domestically produced high-performance fibers will be every bit as vital to the economic health of the United States as they are to its security. Industrial interest in all aspects of advanced composites, including research and development on high- performance fibers, is international. Certain aspects of U.S. industrial and government practices, particularly the short time frame and lack of continuity of research and development (R&D) support, make the American fiber industry vulnerable to the strong competition it faces from Europe and Japan. CONCLUSIONS AND RECOMMENDATIONS Conclusion 1. High-performance fibers are the backbone of the advanced composites industry, which is a critical U.S. industry because of its strategic defense applications and its role in the creation of new products for the domestic and international markets. The United States is in danger of losing its preeminent position in this industry unless steps are taken to strengthen the technology, facilitate its implementation, and broaden the industrial base for high-performance fibers. Because advanced fibers are of such long-term importance to our national security and economy, they require long-range programs to develop sound technical principles and continuing substantive technical progress. However, some of the federal government's major systems development programs, whose ultimate success depends on advanced composites, are attempting to schedule the creation and optimization of advanced fibers on an inappropriate engineering time schedule. Recommendations. Government and industry should each address the problem of maintaining fiber R&D as a continuous, uninterrupted activity and should establish appropriate means for funding this mode of fiber development. Government-funded development of high-performance fibers should not be relegated to a subtask of a systems development project with an engineering time schedule. Closer and more frequent communication on issues of technology, development, and supply should be established between the government, private companies, trade associations, and universities. Conclusion 2. High-performance fiber development has been guided mainly by aerospace application demands for lighter, stronger, stiffer, and higher-temperature fibers. These goals, rather than low cost, have been the primary factors dictating the choice of manufacturing processes. There are, however, important cost-sensitive, potentially high-volume, military and civilian applications of high-performance fibers (marine, submarine, land transport, and electronic) that do not require the ultimate mechanical strength and temperature resistance. Many applications can tolerate considerable
EXECUTIVE SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS 3 compromises in these characteristics, particularly if the resulting product permits exploitation of desirable chemical, electrical, magnetic, and thermal properties. Realization of these large-volume applications will require substantial reductions in the cost of fiber manufacturing for both government and commercial purposes. Recommendation. A broader view should be taken of "high performance" to include properties of fibers other than mechanical ones. Effort should be directed toward large potential applications for high-performance fibers for which the highest attainable mechanical properties and temperature resistance are not essential and can be traded off for lower-cost products and/or products that exploit other useful physical and chemical properties. This effort justifies consideration of R&D directed toward achieving lower manufacturing costs for high- performance fibers for both government and commercial purposes. Conclusion 3. Improvement of high performance synthetic fibers requires a combination of the materials science approach with the chemical and chemical engineering approaches. Recommendation. Research on fibers should emphasize fundamental research directed at four specific interrelated technical issues for which this combined approach is essential. ⢠Fiber formation processes and mechanisms ⢠The effect of fiber processing on microstructure ⢠The relationship of microstructure to fiber properties ⢠Fiber-matrix interfacial interactions during fabrication and in service. Conclusion 4. The universities' contribution toward the advancement of fiber science and technology is hampered by outdated equipment for teaching and research; the highly interdisciplinary nature of fiber R&D, which places extraordinary demands on the curriculum; and the lack of substantive high-performance fiber programs. Recommendations. Increased industry and university interaction should be promoted in order to bring the facilities and personnel of both sectors to bear on the problems of equipment accessibility and interdisciplinary instruction. Specifically, ⢠A major equipment-oriented funding initiative should be undertaken, both by government and industry, to update fiber-related experimental facilities at U.S. universities. ⢠Curriculum changes and industrial participation in educational efforts should be implemented to deal with the interdisciplinary demands of the fiber industry.
EXECUTIVE SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS 4 Conclusion 5. Fiber-formation experiments that go beyond the feasibility stageâon into screening and end- use evaluationâinvolve the commitment of major capital resources, expertise, and man-hours. These resources are beyond the capabilities of most if not all universities and, in fact, beyond the capability of all but a few fiber producers and specialty consulting firms. This has obvious ramifications for the development of inorganic fibers of limited market appeal, which may be crucial to long-range national purposes; it has much less effect on the development of organic fibers, which have a broad commercial market. Recommendation. The federal government must protect American security and economic interests by explicitly underwriting critical fiber development and production, rather than by the frequently used practices of short-term buys or incorporation into systems procurement contracts. Government support emphasis should be placed on the development of inorganic fibers because of their limited industrial base. Conclusion 6. The advanced composites industry (and high-performance fibers manufacturing, in particular) is international in nature, with many U.S. fiber producers being foreign owned. As a result, substantial foreign capital has been invested in the United States, and there is potential for significant technology exchange on product and process development. This trend is expected to continue well into the twenty-first century. Recommendation. The regulations concerning technology export need to be reviewed with this international aspect in mind, so that the United States can take full advantage of worldwide composites technology development as well as improve its strategic position. Conclusion 7. Development of a new fiber in the quantities required for evaluation is expensive and time consuming. The same is true for qualification of a known fiber made by a new process. Recommendations. Rapid and simple screening tests and characterization methods, which can be used on small quantities of fiber to predict the performance of large-scale product lots, should be developed. Standardization of fiber characterization and testing should be the subject of a major initiative involving government, industry, and universities. In addition to the above general conclusions and recommendations, the following conclusions and recommendations apply to specific processing methods and/or specific fiber types. Conclusion 8. Chemical vapor deposition (CVD) and single-crystal growth are well-demonstrated routes to the fabrication of continuous high-performance synthetic fibers. Pyrolytic conversion of precursor fibers (PCPF) and chemical conversion of precursor fibers (CCPF) are versatile methods of synthesis capable of producing fibers that are difficult or impossible to make by other methods. Their full potential remains to be developed.
EXECUTIVE SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS 5 Recommendation. Increased fundamental research on CVD and single-crystal processes should be encouraged for their potential to contribute to the development of high-performance fibers. Increased attention should also be given to PCPF and CCPF as fiber-preparation processes. Their technological base should be strengthened by conducting a systematic study of various combinations of precursor fibers with a wide variety of reactants and processing parameters to make fibers of practical interest and to define the limits of these approaches. Conclusion 9. Improved fibers are needed that are stable for long times at high temperatures. Recommendation. Such improvements may result from R&D on fibers with major anisotropic crystalline components (such as alumina, A1203; or mullite, 3A12O3 .2SiO2), if these components can be oriented with a major crystal axis aligned along the axial direction of the fiber. R&D with this in view should be encouraged. Conclusion 10. Ceramic-matrix composites can provide significant performance advantages at relatively low temperatures, starting above the 300°C limit of polymer-matrix composites. The high performance and availability of carbon fibers could be exploited to achieve this goal. Although in the past higher-temperature goals were set for carbon fiber development, a more modest 700°C goal would be of practical significance and should be achievable. Recommendations. R&D should be undertaken to develop advanced oxidation-resistant carbon fibers for use in ceramic-matrix composites at temperatures up to 700°C. R&D should be carried out on fiber-interphase-matrix combinations to achieve environmental stability for applications at temperatures up to 1200°C. There is a high probability for success in achieving this goal by emphasizing coating and modifying existing fibers (silicon carbide, silicon nitride, mullite, and aluminum oxide). Conclusion 11. Ceramic-matrix composites systems with temperature capabilities above 1200°C could provide great performance benefits to a wide range of significant aerospace programs. Recommendation. Research on ceramic fibers should be undertaken to develop fibers with enhanced stability for use at temperatures above 1200°C. No single fiber system is expected to be useful for all composite matrices or environments. The objective should be to develop a selection of several high-performance ceramic fiber choices. Attractive oxide possibilities include yttrium aluminum garnet, spinels, and zirconates, as well as single-crystal mullite and alumina. Among the carbides, those of hafnium, zirconium, titanium, and tantalum have some promise, in addition to further potential improvements of silicon carbide. In addition to silicon nitride, various other nitrides appear interesting.
EXECUTIVE SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS 6 Conclusion 12. High-temperature metal-and intermetallic-matrix composites have great potential for military and aerospace applications; however, the availability of compatible fibers is severely hindering their development. Recommendation. Fundamental studies on fiber-matrix compatibility in these systems should be expanded to include the study of fiber coatings and tailored interphase regions. The role of reaction products on the performance of both matrix and fiber also should be emphasized. Conclusion 13. By attention to four specific aspects of carbon-carbon (c-c) composites, dramatic performance gains can be realized by using composites in advanced turbine engines and hypersonic vehicle applications. ⢠Since it is highly improbable that C-C composites can be made oxidation resistant for long-life applications, coatings will be needed to decrease the oxidation rate of carbon fibers at high temperatures by allowing a gradual degradation in the performance of the composite system instead of a catastrophic failure from a surface breech. Increased oxidation resistance of carbon fibers also would improve performance in existing single-cycle short-time applications where ablation and/or erosion occurs, as well as for low-earth-orbit spacecraft applications, where atomic oxygen resistance of C-C composites is required. ⢠The low density of C-C composites combined with the inherent good thermal conductivity of highly graphitic carbon makes possible a very high specific thermal conductivity material for one-dimensional heat transfer applications. ⢠With increasing progress in composite strength through matrix improvement, carbon fiber compressive strength will become the controlling factor limiting composite strengthâan issue of particular importance for thin-walled C-C composite structures. ⢠The high processing temperatures used in fabricating C-C composites can affect the carbon fiber microstructure. Recommendations. ⢠Develop increased oxidation-resistant carbon fibers by investigations that include doping, intercalating, or fiber coatings. ⢠R&D to develop high thermal conductivity carbon fibers should be continued. Improvement in the perfection of highly graphitic fiber structures is a promising approach. ⢠R&D to increase carbon fiber compressive strength for high-tensile-strength, high-modulus fibers should be undertaken. Cross-section shape modification of the fibers is a promising approach.
EXECUTIVE SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS 7 ⢠Basic studies should be conducted to understand the microstructural changes that occur in carbon fibers during high-temperature C-C densification processing in order to avoid, or exploit, these processing effects on final composite properties. Conclusion 14. Advanced polymeric matrix composite (PMC) materials are state-of-the-art structural materials that have many other potential applications in addition to aerospace. It is anticipated that substantial improvements will continue to be made in both manufacturing cost and mechanical performance. To the degree that these improvements result in significant improvements in their cost/performance ratio, the application volumes could rise exponentially (e.g., naval submarines and surface ships, automotive [commercial and combat vehicles], light bridges, lightweight weapon systems). Given the broad range of potential fiber performance, improvements in the fibers already commercialized (e.g., high modulus organics, glass, carbon fiber) and those currently under development (e.g., polybenzobisthiazole [PBZT], polybenzobisoxazole [PBO]), it is likely that many of the critical future application requirements can be addressed with these fibers. Recommendation. A higher priority for future efforts on fibers for use in PMC's should be directed toward determining the performance limits of these commercial or developmental fibers rather than completely new fiber systems.
EXECUTIVE SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS 8
