There is a concerted effort throughout the world to find other applications for C/Cs. One of the two major obstacles that is preventing the full utilization of C/Cs is the lack of a method for protecting it from oxidizing in air at elevated temperatures. The second obstacle is that there is limited fundamental knowledge on how to develop the full capabilities of C/Cs for meeting special engineering requirements where specific properties are required. But such improvements frequently mean a further refinement and optimization of processing conditions through a better understanding of how they must be adjusted to obtain the desired physical properties on an economical basis. This understanding is derived from information that can only be acquired by conducting additional research. Significant investments must be made in time, talent, and equipment, all of which are in short supply. Thus, it behooves U.S. investigators to understand what research activities are being conducted abroad so as not to duplicate the European or Asian efforts and to use this information to complement the overall research efforts in the Office of Naval Research (ONR), the Department of Defense (DOD), and the U.S. scientific community. Research Activities in Asia Fiber Development for Use in Prcforms. Generally, all the organizations that were visited used fibers in their composites that were primarily obtained from Japan or the United States. Scveral industrial-sponsored fiber production facilities were visited in Japan because it is believed that their research and development activities are an important part of the available research data that also come from the universities and government laboratories. Numerous research and related technology activities in the foreign countries are concerned with the development of carbonaceous and graphitic fibers from pitch-based precursors and to a lesser extent fibers that come from PAN. The exact reasons that come from PAN. The exact reasons for this emphasis are not specifically known. One possibility is that sufficient information is known about the PAN system and how to improve the quality of the fibers. Another possible factor that may limit general research efforts on PAN is this fiber is being successfully used throughout the world, so a majority of the research is sponsored by the producers. Naturally, any of their research findings are being withheld from the public; thus, other researchers are reluctant to enter this area because the proposed research may have already been done. Consequently, only a limited amount of additional research is being funded either by industry or the government for making further improvements to the physical properties of the fiber or by universities for investigating new concepts. In contrast, the pitch system is not as widely used in the world market because it is more expensive to produce compared to PAN-based fibers. Therefore, a real incentive exists for conducting research to understand the fundamental reasons for improving the processing procedures and lowering costs or to upgrade the properties of the fiber so that it can become more cost effective. At this time, the information that is derived from such research remains largely in the public domain because no clear breakthrough is imminent that might give one industrial producer an advantage over the others. In fact, a number of industrial firms in Europe and Asia are giving small grants to universities for them to conduct exploratory research with the hope that new approaches and solutions can be found for the problems that are now identified. The purpose of the research for different pitch systems is to understand the mechanisms that cause the formation of the various types of microstructures and physical properties that are in the carbonaceous and graphitic fibers. For example, the high modulus and strength values are due to the well aligned layer planes of graphite. These may be planar or crenelated and have radial, concentric, or other orientations as viewed in the transverse sections of the fibers. Their different microtextures depend on the source of the fiber precursor, which is usually a coal or petroleum pitch, and the processing conditions. The sequence of formation of these microtextures is complex due to the large number of different chemical compositions and how these interact with each other as the heat treatment proceeds. But understanding this phenomenon is important if specific types of microstructures are to be produced in the fibers. So numerous investigations are underway for the purpose of determining the compositions of the different pitches and how they are affected by different pyrolysis and extrusion conditions (Koa Oil, Japan). Parameters that determine the ultimate microstructure of the fibers include the molecular weight, rheology, and viscosity characteristics of these pitches, which influence the flow characteristics of the pitch as it passes through the orifices during the spinning operation. Extensive studies are also being undertaken to understand the nature of the microtextures that are developed during the spinning operation. By carefully identifying the types and distributions of the defects that are produced during the spinning, the processing conditions can be altered to minimize the defects to the point that the specific moduli and strengths can exceed that of PAN fibers, which is a highly unusual situation (Shinshu Univ., Japan). Upon heating the fibers to graphitization temperatures, the anisotropic phase will be highly oriented whereas the isotropic phase will show a poor degree of orientation. The different combinations of these phases within fibers result in variations of the fiber microstructure, which is important in determining its mechanical properties (Nippon Oil, Japan). Graphite fibers, from either acrylic and rayon precursors, can have mechanical properties of 600 kpsi strength and modulus and strains of >90 Mpsi and >2%, respectively (Toho Rayon Co., Japan). With other precursors, such as pitch, some of these properties can be altered to obtain even larger values. But fibers become more expensive as their properties increase in value. Expense is the main reason that PAN fibers are used more than pitch fibers. Their cost must be reduced by 50% to 100% before they will be competitive with PAN fibers. Studies are also being conducted to find alternative and cheaper precursors for PANbased fibers (Feng Chia Univ., Taiwan; National Physical Laboratory, India). Some experimental types of fibers are being made with hollow centers, star shapes, etc. in the laboratory to determine if the larger surface area fibers will bond better to the matrix or perhaps be stronger. At this time the processing conditions are being determined to obtain uniform cross sections so no data are available (Chungnam National Univ., Korea). The general assessment of the status of the R&D on fibers is that there are numerous organizations that are making significant and sometimes unique contributions toward a better understanding of the complex phenomena that take place during the formation and processing of these fibers. Without question, Japan is ahead of the world in developing and producing carbonaceous fibers and will probably maintain this position in the future because of the large technical and financial investments that are being made in this area. Other countries, like Taiwan, Korea, and India, are working towards producing their own fibers for internal consumption and possibly for export. Weaving of Preforms. The preforms that were seen during the site visits in Asia usually contained yarns that are woven into 2D or 3D orientations. The 2D cloth materials are usually impregnated with a polymer and processed into preforms by conventional procedures, either as layups by layers of cloth or tape winding. In the case of 3D preforms, a unique method has been developed, over the past 10 years, of weaving by an interlocking stitch different shapes, i.e., cubes, rectangles, tubes, cylinders, Ls, or Ts (Research Institute for Polymers and Textiles, Japan). The yarn spacing can be varied from 0.7 mm (0.028 inch) to 3.2 mm (0.128 inch) and the preforms can be woven with high modulus carbon as well as with silica yarns. This method has now been licensed to a Japanese industrial firm that has scaled up and refined the method. This company wants to continue to reduce the cost of weaving by another 25% to 50% (Shikibo, Ltd., Japan). This method of weaving is different because the z orientation yarns are endless as they exit from the preform, loop back, and reenter it. The refinements to the method of weaving allow yarns to be oriented at 0°, 45°, and 90° and to be added or subtracted from the weaving pattern. These capabilities permit many different shapes to be woven such as a turbine wheel, where the curved blades are an integral part of the hub. The finest weave I was shown was a 12-inch cube of 3D where the yarns were spaced 0.5 mm apart (Chung-Shan Institute of Science and Technology, Taiwan). This is a refined weave that is comparable or slightly better than any preforms made in the United States or Europe. The general assessment about weaving is that in Japan, and probably elsewhere like Taiwan and China, organizations exist with capabilities of weaving any configurations that are required by the design. There was no opportunity to determine what types of research are being conducted in these organizations for designing improved weaves through computer analysis. Methods for Densifying Preforms. The next step in the fabrication procedure for making C/Cs is to densify the woven preforms by impregnation methods that use either liquids or gases to fill the voids that exist between the fibers and yarns. Impregnation is followed by heat treatment to form a carbonaceous matrix around each fiber and in-between the yarns. Usually the impregnation and heat treatment cycles must be repeated a number of times to attain the desired densities of the C/Cs. The physical property of each preform is determined by the fiber type, the architecture of the yarns, and the interaction of the fiber with the matrix. This latter parameter, the matrix, is controlled by the microstructure which, in turn, is a function of the processing conditions such as whether the preforms are permeated with liquid or gases (Toho Rayon Co.). It is important to understand what mechanisms are occurring during the heat treatment step that control the formation of the necessary microstructures in order to acquire the desired properties of the C/Cs and also to fully utilize the mechanical properties of the fibers (Nippon Oil Co.). Today, even after more than 25 years of making C/Cs, this utilization factor is usually less than 65% of the strength the C/C should have based on the strengths and volumes of fibers that are contained in the C/C. Furthermore, this value is far less than the >90% that exists for the polymeric matrix types of carbon fiber containing composites. This difference of utilization factors means there is a good possibility for improving the properties of C/Cs provided the proper yarn architectures and types of matrix microstructures can be identified and produced. Therefore, numerous research activities are being undertaken to achieve better performance of C/Cs (Nippon Steel, Japan; Beijing Research Institute of Materials, China). The liquid impregnation method is one of the two major procedures for the densification of C/Cs. The liquids used are normally derived from organic precursors like a phenolic or from pitch that is derived from coal or oil. In general, pitch is preferred because a larger variety of microstructures can be derived from it. The conversion of pitch to carbon takes place by a process of pyrolysis where there is evaporation of low molecular weight species as the temperature is progressively raised. At higher temperatures, on the order of 350 to 425 °C, cracking reactions occur followed by evaporation of volatile fragments that mainly result from the thermal scission of aliphatic side-chains to polycondensed aromatic ring structures. The polynuclear aromatic radicals produced are quite reactive and combine to form planar aromatic ring structures of even higher molecular weight and aspect ratio. These will have an influence on the viscosity and rheology of the pitch which, in turn, will determine how well the pitch enters and impregnates the woven preforms. As pyrolysis is continued, an alignment of these planar ring molecular structures will occur and the liquid or mesophase state of the pitch system will develop. It is at this stage of the transformation of pitch to carbon that the ultimate microstructure of the matrix is determined (Shaanxi Nonmetallic Material and Technology Institute, China). It is for this reason that a great deal of research effort is being directed toward understanding the nature of these pitch systems and the influence on the mechanical properties of C/Cs (Toyohashi Institute of Technology, Japan). Numerous research investigations are concerned with the chemical composition of these mesophase systems and how chemical additives influence the polymerization, carbonization, and graphitization steps and control the grain size and orientation of the matrix microstructure (Hokkaido Univ., Japan; Univ. of Kyushu, Japan; National Physical Laboratory; GIRI Kyushu, Japan). Microstructure, as it is used in this report, means the inclusion of the pores and cracks that exist in C/Cs. Research has shown that controlling the size and distribution of the pores can have a profound influence on the mechanical properties of C/Cs. The concentration of pores can be altered by properly heat treating C/Cs. This capability is useful for it can be used to increase the toughness values of the C/Cs (Toyohashi Institute of Technology). Cracks can be distributed either within the matrices or at the interfaces between the matrix and the surfaces of the fibers or yarns. A significant amount of research is being undertaken to understand the phenomenology of bonding between fibers and matrices as it is important to know how to properly adjust the degree of bonding to fully utilize the strengths of the fibers (Tokyo Institute of Technology, Japan). The densification of preforms to a density of 1.9 g/cc by liquid impregnation is usually considered to be accomplished by good processing methods and the use of a high pressure autoclave. These kinds of billets were seen in Japan (Nippon Steel), Taiwan (Chung-Shan Institute of Science and (Chung-Shan Institute of Science and Technology), and China (Beijing Research Institute of Materials and Technology). In the latter case, this institute has various size autoclaves for processing preforms. The largest is 18 inches in diameter and >36 inches long and is comparable to the maximum size that is processed in the United States. However, in India, preforms are being processed to nearly 1.9 g/cc density with six or seven impregnation cycles without the use of a high pressure autoclave! To my knowledge, this is an achievement that has not been done before and represents a real breakthrough because the cost of processing C/Cs will be reduced by the elimination of a very expensive autoclave. Equally important, there is no size restriction on the sizes of the preforms that can be processed due to the size of the autoclave (National Physical Laboratory). The second major procedure for the impregnation of C/Cs is to deposit carbon by the chemical vapor deposition (CVD) method onto the walls of pores of the preforms. This method is the best for its ability to fill pores that are too small for liquid pitches to enter. Consequently, C/Cs that are densified in this manner have slightly higher mechanical strengths than those processed by liquid pitch. However, one of the major difficulties in using the CVD method is that the deposited carbon is not deposited uniformly through the preform if it is too thick. Therefore, special attention needs to be given to the processing conditions by understanding the phenomenology of gas diffusion and carbon/graphite deposition on the various types of pore distributions that exist in preforms (Nippon Steel). The time to fully densify large preforms can be hundreds of hours, which is very costly unless large quantities of materials are being processed at the same time, such as brake disks. Not many fabricators are using the CVD method because of the limitations of density gradients and long processing times unless large quantities or sizes of preforms are used. The general assessment of impregnation processing technology is that significant capabilities exist in Asia and India for the densification of all types and sizes of C/Cs. Furthermore, these capabilities are being guided by research that is concerned with the nature of the liquid or gaseous precursors, the phenomenology that controls the type and distribution of the matrix microstructure, the use of optimum processing conditions, and how the matrix interacts with the fibers/yarns that it surrounds. Characterization of C/Cs and Optimization of Properties. The physical properties of C/Cs are determined by the type and the architecture of its fibers and how these are bonded together by the various kinds of matrices that exist in each type of preform. To be able to select the optimum combinations of processing conditions, characterization methods must be available to properly evaluate the performance of the constituent parts of the C/Cs as they are being subjected to thermal-mechanical stresses. The usual practice of characterizing C/Cs is to measure their mechanical properties and correlate any variations of these with changes of the processing conditions. Sometimes, the microstructure of test samples will be examined by optical or SEM methods after the samples have been fractured. Now more detailed fractographic experiments and theoretical analysis are being conducted to understand and model the fracturing behavior of these C/C materials (Toyohashi Institute of Technology; Tokyo Institute of Technology). Combinations of methods are being used for conducting evaluations of the microstructure of fibers at the molecular size of magnification (Australian National Univ.). This is important information if the physical properties of the fibers are to be improved by the elimination of defects in their microstructure. The various experimental methods that are used include SEMS, TEMs, optical microscope, xray, Raman spectroscopy, and electrical and thermal diffusion characteristics. Information from these various sources can be added by computer-aided analysis, resulting in an image of the sizes and distributions of the defects in a fiber. Variations of these defect distributions are compared to the processing conditions in order to select the optimum conditions for minimizing the defect problem (Shinshu Univ.). The benefit of C/Cs is its ability to have high strengths at temperatures over 2,000 °C. But such characterization measurements are difficult to make, so only a few organizations are conducting such research (Tokyo Institute of Technology). The optimum properties of C/Cs can be obtained more quickly and economically by understanding the microstructural factors that control the fracturing behavior. This is best achieved by using a combination of experimental data and theoretical analysis to select an optimum type matrix microstructure. But at this time only a small amount of research is being conducted for this purpose as has been indicated from the above information. Capabilities found in the organizations visited for conducting nondestructive evaluations (NDE) of C/Cs consist of the usual x ray and ultrasonic procedures that are used in most U.S. production organizations. No information was obtained concerning advanced technology such as computer-aided tomography, thermal mapping, or other advanced techniques. This is a surprising situation especially for large and advanced organizations in Japan. Perhaps these organizations do not believe these advanced techniques are cost effective. The general assessment of this last step in the fabrication of C/Cs is that there are important research programs being undertaken to determine the contributions of the constituents of C/Cs to their physical properties. Consequently, there is a capability for selecting optimum processing procedures for obtaining the desired engineering properties. It appears that the NDE capabilities in these countries are of a good quality to meet the needs for production purposes. But more advanced NDE technology and related research do not appear to exist. Protection of C/Cs Against Oxidation. Protecting C/Cs against oxidation at elevated temperatures is a major factor in maintaining their mechanical properties. Although research about oxidation and its suppression is not a prime objective of this study, such information is included in this report because of its importance to the future applicability of C/Cs. At least half of the 32 organizations that were visited are concerned with the phenomenology of or developing methods for inhibiting oxidation effects on C/Cs. Silicon carbide (SiC) is the material normally used to coat C/Cs to protect them from oxidation and it has been used for almost 20 years on the shuttle. One of the major problems with this system of protection is the differential thermal expansion effects between SiC and C/Cs that cause cracks to develop in the coating as it is thermally cycled, thereby reducing its ability to protect the C/Cs. Therefore, current research and developmental efforts are attempting to overcome the cracking problem by applying more than one layer of coating to the C/CS (GIRI Kyushu). Another approach is to form layers on the substrate that have various thermal expansion coefficients (Kobe Steel, Japan). The general assessment is that there is a lot of research and development effort being directed to finding a method for protecting C/Cs against the effects of oxidation. This opinion is based on a limited amount of information and therefore additional information should be obtain to be sure this assessment is representative. Factors That Enhance Asian Research Activities Collectively in Asia, the capabilities now exist through proper research efforts for acquiring the necessary information to develop advanced C/Cs. In the future it is expected these collective capabilities will have a very significant influence on the utilization and production of C/Cs in the United States and throughout the world. This expectation is made on the basis that in time these Asian countries, especially Japan, will be able to concentrate and coordinate their research and development efforts more effectively either on a national or international basis. In addition to Japan, a number of countries, such as Taiwan or India, would like to export their materials. The major question is how well can the capabilities of these countries can be integrated into effective research and development programs? A certain amount of duplication of effort naturally occurs between the different countries, but this is partially mitigated because there is usually a difference of interest and expertise even in the same research areas. Other nontechnical factors are being used to enhance the collective research capabilities of these countries, including: Communication links that are well established and are frequently used between investigators, organizations, and societies on national and international levels. For example, there are strong communication links within Japan and between the French and the Japanese in the carbon and ceramic areas. complicated as having visiting professors or graduate students. Sometimes these links for collaboration are established years earlier because many researchers in Asia did their graduate work at a foreign university. • Collection of research information is a most important factor, especially with the wealth of C/C data that exists throughout the world. Methods for collecting information vary. One of the more efficient ways is to invite a foreign expert to stay for several weeks in your country with all expenses paid for by the host country. In recent years many countries have increased their support of foreign travel because it has been found to be a very cost effective method of obtaining research information. The general assessment of this area is that these Asian countries are more effectively using the nontechnical factors (communication, coordination, collaboration, and collection) to augment and enhance their research capabilities than the United States. National coordination of research Collaboration between different researchers and organizations on a national or international basis. There is a large source of information available to the United States because all of the 32 organizations, including China, that were visited want to collaborate with us. The level of collaborative participation is dependent on the needs of the researchers and therefore is determined on a case-by-case basis. This may be as simple as the exchange of data or as Impact of Asian Efforts on U.S. Technology and Leadership The "relative" effectiveness of U.S. research efforts on C/Cs and related technology will diminish because of the increased efforts in Asia. Unfortunately, this problem is being compounded because the industrial and governmental sponsorship of U.S. research has been declining for the past 10 years. Unless there is a reversal of this trend "soon," a critical point will be reached where there is not enough research being performed in the United States to have a significant influence on the future development of advanced C/Cs. A current example of this situation is the National Aerospace Plane (NASP) program. It was forced, by time constraints, to use C/C materials that were developed many years ago based on research information that was obtained long before that. The total elapsed time may be as much as 15 to 20 years. • The reduction of ONR and DOD funds for C/C research will have the additional effect of cutting back on the training of graduate students at universities in the field of carbon. • The U.S. ability to be a leader of carbon research will decrease, so it cannot be the first to identify new opportunities or alternative approaches for developing advanced C/Cs and related technology. A prime example of this type of situation is a shift of the leadership in the area of carbon fibers to Japan during the last 5 years. The potential loss of U.S. leadership in the development of advanced C/Cs will also limit its capability to design and fabricate new and high technology equipment or systems for domestic and governmental purposes or for world consumption. Long term (<10 years) • It is expected the collective research efforts in Japan and the other Asian countries will improve and surpass in time the research activities in the United Stateds if the present financial support trend continues. Continuous cutbacks, for the reasons cited above, of research funds from ONR and DOD will mean fewer researchers at the universities will be interested in investing their careers or facilities in an area that appears to have no future growth. Consequently, fewer graduate students will |