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HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 21 2 High-Performance Fiber Materials: Applications, Needs and Opportunities INTRODUCTION Fiber-reinforced composites have been an important industrial product for many decades. Structural applications of these composites have dominated the development of fibers throughout this period. This fact is recognized in the following presentation of the state of the art of fiber whereâunless otherwise stated â"applications" refer to structures or components demanding high-performance mechanical properties. High performance fibers used in these applications are often classified by the matrix used to support them. This is a convenient classification because it effectively specifies the temperature rating of the composite structure: polymericâambient to 800°F, metal matrixâ1500° to 2500°F, and ceramicâ2000° to 3000°F. The various fiber types will therefore be discussed based on this classification. It should be noted that there are many current and potential applications of fiber-reinforced composites that cannot be called "structural" because their function is mainly to exploit some other physical property rather than only mechanical strength. To emphasize the importance of this class of fibers, a separate section is devoted to a discussion of fibers used for these applications. HIGH-PERFORMANCE FIBERS FOR POLYMERIC-MATRIX COMPOSITES In the United States, reinforced plastics represent a very large market of approximately 2.3 billion pounds/ year, valued at about $2.5 billion. A very important high value-added, but relatively low-volume, sector of the reinforced plastics market is the advanced composites market. Advanced composites are defined as products that utilize substantial percentages (50 to 70 percent by weight) of high-performance fiber reinforcements, having
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 22 excellent mechanical properties, in combination with organic, metal, or ceramic matrices. Organic-matrix composites or polymeric-matrix composites (PMCs) constitute by far the largest component of the advanced composites market.(1) PMCs, which are broadly used throughout the world in applications where strategic weight and performance characteristics are important, are projected to experience steady growth over the next two decades. While the aerospace business has been the key industry fueling this growth, automotive, industrial, and leisure products represent important market segments. The relative importance and growth of each of these markets depend on critical technical advances in two important areas: (1) improving the ultimate performance of material systems while reducing cost, and (2) improving fabrication technology for making parts in a cost-effective way. In 1988 the estimated worldwide consumption of advanced PMCs was 25 million pounds (12 million to 18 million pounds of high-performance fibers), representing a $1.5 billion business that is growing 16 percent annually. PMCs are attractive because they offer a combination of the advantages shown in Table 2.1, produced by integration of constituent fibers and resins with materials design and fabrication at acceptable cost. TABLE 2.1. Advantages of PMCs Good specific stiffness and strength Fatigue resistance Low density Low coefficient of thermal expansion Ease of fabrication Creep and creep fracture resistance Relatively low raw materials cost Excellent in-service experience Potential for lower cost fabrication methods Property "tailorability" to application requirements Corrosion resistance Major Current Fibers Properties; Demand PMCs are made by combining reinforcing fibers containing high strength and stiffness properties with polymeric matrices to produce tailored materials with combined performance not possible with either constituent alone. The dominant fibers used (more than 99 percent) in PMCs are glass, carbon, and aramid fibers. Other fibers that have more limited usage are boron, silicon carbide, alumina, and, more recently, liquid crystalline fibersâ that is aromatic polyesters, poly (paraphenylene), benzobisoxazole (PBO), and extended-chain polyethylene. The matrix resins used in PMCs are of two types: thermosets and thermoplastics. The most common thermosets are polyester, vinyl ester, epoxy, bismaleimide (BMI), and polyimide (PI). The major classes of thermoplastics under evaluation today include polyether ether ketone, polyamide,
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 23 polyamideimide, polyimide, polysulfone, and polyphenylene sulfide. Epoxy resins represent the dominant matrix in use today, and they have established a very large data base and track record in the field. Carbon fiber represents the dominant fiber in the advanced composites industry. It has been used in the industrial, recreation, and aerospace markets. Figure 2.1 shows U.S. carbon fiber consumption by major market segment from 1976 through 1988. Applications Glass/epoxy composites are used in printed wiring boards. They are also used in the automotive industry in limited semistructural and structural applications such as body panels and composite springs. Glass/vinyl esters and/or polyester are used in the chemical industry for corrosion-resistant applications. Carbon fiber, aramid, and some of the other high-performance fibers (boron, silicon carbide) are used in recreation, industrial, and aerospace products, but they are most effective for uses in which performance is the primary consideration and cost the secondary. The recreation market represented the first large-volume usage of carbon fiber. Typical applications include tennis rackets, fishing rods, golf shafts, and sailing masts. This market is nearly at a saturation point, and it is expected to grow at a very slow rate in the future. In recent years much of the manufacturing of composite recreation articles has shifted to the Far East, thereby contributing to the recreation market's rapid growth and its decline in the United States to the smallest of the three major market segments. During the past few years, the U.S. industrial market segment has experienced gradually increasing growth rates, and it is now the second largest of the three major market segments. The industrial market uses chopped carbon fibers primarily for electromagnetic interference/radio frequency (EMI/RFI) shielding applications. Previously, it was anticipated that large-volume applications would emerge for automotive applications, but the combination of high material cost and lack of high-speed fabrication technology has prevented this from happening. Continued rapid technology growth should minimize these factors in the future. Some limited automotive applications have been commercialized, such as overwrapped carbon/epoxy aluminum driveshafts. The aerospace/aircraft market is currently the largest U.S. sector for carbon fiber composites. This market sector has driven the overall industry from the standpoints of performance, quality, and capacity and will continue to do so well into the next century. For example, the major advances in carbon fiber properties and new product development, the installation of new carbon fiber capacity, and the drive to install domestic precursor capacity can all be directly tied to rapid penetration of carbon fiber into the aerospace/aircraft market. Carbon fiber composites are now being used in the
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 24 Figure 2.1 U.S. Carbon Fiber Consumption by Major Market Segment
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 25 following major aerospace areas: aircraft and helicopters, missiles and spacecraft, and brakes and tooling for parts fabrication. Figure 2.2 illustrates the rapid growth in aerospace carbon fiber consumption during the period since 1976 projected through 1990. Figure 2.2 U.S. Carbon Fiber Consumption Aerospace Market Industrial Sources; Industry Structure; Market Situation The advanced composites industry (particularly, the PMCs component of that industry) is extremely international in nature. For quite some time, major U.S. manufacturers (polymers, fibers, intermediate products) have had close business arrangements with Japanese and European manufacturers, which have included technology transfers as well as joint ventures. This trend is expected to continue partially in support of the continuing internationalization of the aircraft/aerospace industry. While different geographical territories have historically had different market strategies (i.e., Japan historically has a heavy interest in recreation products in contrast to emphasis on aerospace products in Western Europe and the United States), all international companies seem to be addressing the same critical technical and economic issues.
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 26 However, with respect to applications development, Western Europe has taken a more aggressive posture toward aircraft structure and Japan has been more proactive in the area of nonaerospace industrial parts. Examples of the latter include robotics and mechanical tools. Since the market situation for carbon fibers is more highly developed and much more competitive (at least in the United States) than any other class of high-performance fibers, a detailed discussion seems warranted. Polyacrylonitrile (PAN)-based carbon fiber represents approximately 90 percent of all carbon fiber sales worldwide, with the remainder consisting primarily of pitch-based fibers. During the past several years, PAN- based carbon fiber capacity has grown rapidly as companies expanded their capacity to establish market position and meet the growing demand for carbon fibers. For example, in 1980 the total free world capacity2 was just over 1400 metric tons (3 million pounds). Today, the free world capacity is approximately 12,100 metric tons (27 million pounds); U.S. capacity is estimated at around 5000 metric tons (11 million pounds). Meanwhile, the free world carbon fiber demand grew at an annual rate of over 20 percent, from an estimated 900 metric tons (1.2 million pounds) in 1980 to about 4500 metric tons (10 million pounds) in 1988.3 Because the increments of new carbon fiber capacity were large relative to the total carbon fiber demand during this time period, there have been periodic capacity excesses and periodic fiber shortages. Indications are that during the next 5 to 10 years carbon fiber demand will grow at a slightly slower rate, ranging somewhere between 10 to 15 percent annually. Since most major U.S. carbon fiber producers have announced substantial capacity expansions that will be brought on stream in the early 1990s, the growth in fiber capacity is expected to continue to exceed the growth in fiber demand. Early PAN-based carbon fiber capacity was concentrated in the United Kingdom and Japan, but today over one-third of the capacity is in the United States. The U.S. share of world capacity should continue to increase, in large part due to the growing volumes of carbon fiber required to meet expanding Department of Defense (DOD) requirements. Until recently, almost all PAN precursor was produced outside the United States, either in Japan or the United Kingdom. However, with the passage of the DOD PAN domestic precursor directive,4 all major U.S. carbon fiber suppliers have built, are building, or have announced an intention to build their own PAN production units. This will be a major disruption for the industry (capital installation, product requalification, etc.) through the end of the century. Pitch-based carbon fibers are not as well established as PAN-based fibers, and currently there is only one commercial supplier of high-performance continuous fibers in the United States (Amoco Performance Products, Inc.). However, technical activity in this area was intense in the past decade, particularly in Japan, and there are now several Japanese suppliers that are beginning to supply material to the U.S. market. Pitch-based carbon fibers are now being viewed as a viable material in high-modulus, high-conductivity application areas where low-to- medium strength is acceptable.
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 27 Recently, the carbon fiber and PMC industries have undergone significant realignments, as evidenced by BASF's purchase of Celanese's composite business, Amoco's purchase of Union Carbide's composites operations, BP's takeover of Hitco's advanced composites business, Akzo's purchase of Great Lakes and Wilson Fiberfil, and ICI's purchase of Fiberite and LNP from Beatrice. These realignments are resulting in a composites industry comprised of stronger competitors that are clearly forward looking in their strategic objectives and investing for the future. Future Application Challenges The range of future applications for PMCs has the potential to be very large and to encompass a multitude of end-use sectors; some of these end uses, such as military applications will be driven primarily by performance, and the others, such as automotive and general industrial, will be driven primarily by cost. In order for the PMC sector to increase its market share in its current applications and to grow into new applications, it needs to overcome some key hurdlesâproduct performance, fabrication, and economic issues. Fiber developments can play an important role in all of these issues. It is clear that PMCs will continue to increase their usage as a structural reinforcing material on military aircraft as long as the criteria of reduced weight and higher performance remain key goals. For example, the Air Force is currently funding a number of research initiatives as part of its Ultralightweight Structures Program in which the goal is to assess the feasibility of a 50 percent decrease in the structural weight of baseline vertical takeoff and landing fighter and transport aircraft. The rate of penetration against competitive materials will be determined by how quickly some of the technical hurdles such as increased damage tolerance, overall balance of properties (i.e., improved compression performance), and high temperature performance are resolved. Utility in space applications, where a critical application is the space station, will also increase as PMCs with increased stiffness and reduced coefficient of thermal expansion are commercialized. Two areas of the military in which PMCs have not been used extensively but are projected to be in the future are land-and sea-based vehicles. The need for next-generation equipment (e.g., greater weapons payloads, faster speed, signature reduction) as well as the advantages and disadvantages of PMCs against metals in these end uses are similar to those found in aerospace vehicles and aircraft. For example, critical problems of PMCs that are under study by the Navy for surface and submarine applications include the following: fabrication of very thick sections (4 to 13 inches), very high compressive loadings, fire resistance, very high static and shock loads, and long-term performance (greater than 20 years). The need to resolve these problems, coupled with the small number of new ships and submarines being built, means that PMCs will see only a small usage for these purposes over the next decade. The long-term consumption, however, could be very large and could easily be equal to that projected for aerospace and aircraft applications.
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 28 The broad range of reinforcing fibers that are capable of being used in PMCs offers the potential for substantial PMC consumption in large, nonaerospace, nonrecreation markets where PMCs previously have had either limited or no significant utilization. These markets include general industrial (in addition to the current EMI/RFI applications), automotive, and civil engineering. Within these markets, there is a whole host of applications for which PMCs, based on today's technology, meet the critical technical requirements of the application. Examples of the broad range of applications would include robotics, ropes and cables, heat exchangers, bridge decking, and reinforced concrete. The key to increased consumption in these diverse end uses is reduced raw material costs and improved fabrication technology; with current raw material costs (fibers are a key component of the cost) and relatively long fabrication cycles, the PMC use is limited at best. However, with the commercialization of a very low cost PAN-or pitch-based carbon fiber (in the $5 to $8 per pound range), for example, some of these applications would become economically viable. Another approach to reducing fiber costs is the hybridization (or mixing) of two fibers in a composite structure in order to meet the technical requirements of the part while minimizing cost. The hybridization of glass and carbon has been used in the automotive industry to take advantage of the high strength and low cost of glass with the very high modulus of carbon. In addition to product and fabrication issues, a number of economic issues must be addressed because PMCs are generally more expensive to purchase and/or fabricate into final parts than metals. Even though relatively low raw material costs are considered an advantage for PMCs (see Table 2.1), these materials are still too expensive to permit extensive development of new applications in the industrial, automotive or civil engineering industries where performance is not the principal concern. As pointed out in Chapter 1, lower cost routes for producing reinforcing fibers must be developed if these new applications are to be achieved. Because of the drive to produce high-performance products for the aerospace/aircraft industry, PAN-and pitch-based carbon fiber suppliers have not undertaken a major initiative in this direction. Besides reductions in fiber costs, improved parts fabrication technology that approaches the rates currently achievable with metals must be developed. Stampable thermoplastics offer a real potential to significantly improve this latter area. Advanced design techniques combined with novel fabrication methods have the potential to reduce counts scrap and parts through structural integration to improve cost competitiveness. Advances in economics should make it possible for PMCs to penetrate for the first time into applications in the industrial/commercial and automotive areas. Potential parts might be high-speed and precision machinery, robotics, and structural automotive parts such as door intrusion beams, frames, engine components (push rods and connecting rods), driveshafts, and leaf springs.
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 29 HIGH PERFORMANCE FIBERS FOR METAL MATRIX COMPOSITES Major Current Fibers The family of materials classified as metal-matrix composites (MMCs) comprises a very broad range of advanced composites of great importance to both industrial and aerospace applications. However, the development and use of MMCs are still in their infancy when compared to monolithic materials or even resin- matrix composite systems. Therefore, only a handful of applications have been designed and produced, but these are illustrative of the potential of MMCs. As in the case of other composites discussed in this report, the family of metal-matrix composites is made up of many varieties of materials, which can be categorized based on their matrix composition, fabrication process, or reinforcement type. The generic listing in Table 2.2 TABLE 2.2 Advantage of PMCs Matrix (alloy class) Reinforcement Fiber Whisker and Particle Process Lead Glass Cast Boron Diffusion bond Carbon Magnesium Boron Silicon carbide Cast Carbon Alumina Boron carbide Diffusion bond Extrude Aluminum Glass Silicon carbide Cast/Diffusion Boron Boron carbide Bond/extrude Steel Glass Silicon carbide Carbon Alumina Copper Carbon Silicon carbide Cast/Diffusion Tungsten bond/Electroplate Titanium Boron Beryllium Diffusion bond Silicon carbide Alumina Nickel Boron Alumina Cast/Diffusion Alumina bond/Electroplate Tungsten Carbon
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 30 illustrates the range of possibilities, which have all been fabricated and investigated with varying degrees of success. As will be discussed in the next section, another method of classifying these composites may be based on cost. The engineering importance of metal-matrix composites can be related to two significant advantages: ⢠Properties: The use of metals as matrices imparts important properties to the resultant composites. High matrix strength and elastic modulus impart high composite shear and transverse strength and stiffness. As an example, a boron-fiber-reinforced aluminum system exhibits transverse tensile strength equal to the unreinforced matrix and transverse stiffness twice that of the matrix. Similarly, the matrix can impart significant toughness and resistance to the operating environment resembling the characteristics of the parent metal. Metal-like thermal and electrical properties also are of importance. ⢠Processing: The ability to use traditional metal processing facilities to create at least selected metal-matrix systems is important because it brings to bear a large existing industrial capacity for component production. On the other hand, there are also significant difficulties in the creation of metal-matrix composites. The frequently high fabrication temperatures and reactivity of many alloy matrix-fiber combinations can cause significant deterioration of properties. Similarly, the mismatch in thermal expansion between fiber and matrix can ruin composite performance on thermal cycling. Finally, the composites may differ in corrosion resistance from the parent matrix because of the composite microstructure and the difference in fiberâmatrix electrochemical potential. As indicated above, another convenient way to look at MMC systems is to consider their cost. Tables 2.3 and 2.4 are based on this approach. Table 2.3 illustrates low-cost MMCs for industrial and aerospace applications, and Table 2.4 illustrates high- cost applications. Selective reinforcement, such as that used in Japanese automotive applications and powder metallurgical processing, can yield low-cost products. The very high performance requirements of aerospace applications have generally required the use of more costly continuous fibers and slower, more expensive processes, such as diffusion bonding. Industrial Sources: Industry Structure Market demand for MMCs is small and unpredictable. Therefore, few U.S. producers have been interested in developing fibers for these matrices without financial support from the DOD for a specific application. Comparison of the status of metal-matrix composites in the United States and other countries is difficult, because the majority of the activities are still developmental and not widely publicized. However, it is known that
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 31 foreign countries are very interested in MMCs and are trying to develop the technology. For example, British Petroleum purchased a silicon-carbide fiber capability (Sigma) and an American MMC fabricator (DWA Associates), and Alcan Aluminum of Canada purchased the Dural composites activity and has invested heavily in its expansion. TABLE 2.3 Low-Cost (Aluminum-Matrix) MMCs for Industrial and Aerospace Applications Fiber Application Fabrication Process Company Al2O3/SiO2 Auto diesel engines Squeeze casting Toyota C Piston ring grove reinforcement â Toyota Al2O3/SiO2 (particulates) Castings (gear boxes impellers, high Direct chill casting Alcan Aluminum Canada pressure dies, etc.) SiC Optical grade Powder Advanced Composites (particulates) (low creep) components metallurgy Materials Corp. and D W A Associates (CA) TABLE 2.4 Some Higher-Cost MMC Aerospace Applications Fiber/Matrix Application B/Al; B/Ti Space Shuttle Orbiter fuselage: fighter aircraft engines C/Al; C/MG Antenna mast for Hubble telescope Particulate/SiC/Al Various aerospace component parts For aerospace-grade MMCs, the production applications, such as the Space Shuttle fuselage struts or satellite members, have occurred within the U.S.
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 32 aerospace industry. However, there are large developmental efforts under way in the Soviet Union, Europe, and Japan. The Soviet Union has significant efforts in at least two areas: boron-reinforced aluminum and silicon- carbide-reinforced titanium. One significant foreign development activity is proceeding within a European consortium of aerospace manufacturers who are developing titanium-matrix composites based on the British Petroleum Sigma fiber (silicon-carbide). This effort appears to be very appreciable in size, in terms of the capital and manpower resources being committed. A second activity that is significant because of its organizational structure is Japan's metal-matrix development. This is an integrated effort between government and industry to develop both a commercial and a technical base. The first large-scale production application of MMCs was made by the Japanese in the Toyota engine piston insert. Although it is not a high-performance application, experience and confidence in production were gained. The Japanese are committed to pursuing higher-performance MMCs as well. They have established a government/industry team, that has specific goals for developing high-temperature fibers and composites for hypersonic applications. They are also pursuing silicon-carbide-reinforced titanium and titanium aluminides as well as yarn-reinforced, thin-gauge, aluminum-matrix composites. In summary, international investment in MMCs by both industry and government sponsorship appears to be on the rise. In addition, much of the U.S. capability in this area has been purchased by foreign companies. To date, these appear to have significantly enhanced the technology through investment, and it is unfortunate that U.S. firms have not been able to make similar commitments. Future Application Challenges The application needs for MMCs arise from their significant advantages over existing monolithic metals and polymer-matrix systems. Aircraft that require a lightweight structure with high stiffness or strength may have metal-matrix-composite control surfaces. For high-load applications, steel parts may be replaced with titanium- matrix composites. Future engines are probably the biggest potential application for MMCs. They require high stiffness and strength and have many parts that operate at temperatures too high for resin-matrix composites. In addition, engine parts are very sensitive to weight because of the rotating inertia, which justifies higher costs for advanced materials. Turbine parts, which operate above 500°G are currently made of superalloys. Replacement of these parts by titanium-matrix composites at half the density could reduce the weight of the engine dramatically and increase its performance. An exciting future application of metal-matrix composites will be on air-breathing hypersonic vehicles. The national goal of demonstrating a space-capable aircraft cannot be met with traditional manufacturing processes, materials, and structural concepts. New lighter-weight materials are needed, which can take the flight loads, acoustic noise, and searing temperatures well beyond the performance of existing metals or polymer composites. This requirement, based primarily on the mission of the National Aerospace Plane
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 33 (NASP) and the above-described need for gas turbine engines, has caused considerable interest in the development of intermetallic-matrix composites. These composites constitute a subset of the more traditional MMC systems in that the matrices are ordered structures whose mechanical properties may include limited ductility over certain temperature ranges. The advantages of these systems, however, relate to their low density and high potential for use at elevated temperatures. Examples of current systems under investigation are listed in Table 2.5. TABLE 2.5 Some Current High-Performance-Fiber Reinforced MMC Systems Under Development Matrix Alloy Reinforcing Fiber Maximum-Use Temperature Aluminum C 300°C SiC Titanium SiC 500°C Ti-Aluminides ⢠Alpha-2 SiC >600°C ⢠Gamma TiB2, Al2O3 >800°C Copper C 700°C Critical Problems to be Solved for New Applications. In addition to availability and resource problems, there are many technical challenges remaining in the development of high-performance metal-matrix composites. Most of these involve the reinforcing fiber. Some of the problems encountered are caused by the reaction between the matrix (metal) and the reinforcing fibers. Most MMC fabrication processes use high-temperature consolidation, which causes the metal to flow around the fibers and bond by solid-state diffusion. The majority of metals, however, are very aggressive at elevated temperatures and try to dissolve the fiber. This problem has been solved, in most cases, by developing specialized coatings for the fibers. These coatings, which are tailored for use in a specific metal matrix, are difficult and costly to develop and produce. This makes experimentation with various combinations of fibers and matrices a lengthy and expensive process. The problem is amplified because the basic understanding of the chemical, mechanical, and thermodynamic relationships in the fiber-matrix interaction are not well understood. The recent requirements for high-temperature structures, such as for the NASP, have caused researchers to begin examining a whole new range of fiber
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 34 and matrix combinations, such as matrices of alpha-2 and gamma-based titanium alloys with fibers of titanium diboride or titanium carbide. This has resulted in a whole new set of fiber-matrix compatibility studies and development efforts that will require considerable time to carry out. While there are many technical challenges to be overcome for MMC applications, the economic and managerial challenges are also very significant. Some measure of the problem is provided by the following comparison: sales of titanium-matrix composites in 1989 were only a few thousand pounds, while nearly 50 million pounds of titanium and several billion pounds of aluminum were produced. Therefore, a comprehensive plan to develop the basic science and production capabilities must come from something other than natural market forces. Conclusion: Metal-matrix commercial industrial composites have the potential for large-scale applications that require a low-cost reinforcement. Recommendation: Fibers compatible with low-cost metal-casting processes should be developed. HIGH PERFORMANCE FIBERS FOR CERAMIC MATRIX COMPOSITES Major Current Fibers and Their Properties The addition of fibers and whiskers to ceramic matrices can result in structural composite materials that retain the important advantages of ceramics (i.e., high-temperature resistance, environmental stability, and low density) while also overcoming the drawback of brittle behavior. A list of some of the more prominent fibers currently available for use in ceramic matrix composites (CMCs) is given in Table 2.6. In the United States CMCs have experienced serious and concentrated development only in the past 5 years. Their application as structural materials is thus, still in its infancy. Nevertheless, CMCs have many potential performance advantages that clearly indicate that within the next decade or so they will begin to see major use. Although the brittle nature of monolithic ceramic materials has severely limited their application, it is possible to substantially increase both strength and toughness by incorporating second-phase constituents. These properties are illustrated in Figure 2.3, which compares simple flexural test load-deflection curves for unreinforced and carbon-fiber-reinforced cement composites1 The superior thermal stability of many ceramics relative to polymers and metals makes CMCs unique for high-temperature applications. This fact, combined with the relative low density and chemical inertness of ceramics, (see Figure 2.4) make CMCs very attractive for many potential applications. One of the major reasons that ceramics are the largest single class of materials used, despite their brittleness, is their low cost. The prospect of
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 35 preserving this low cost while increasing their utility through toughening provides the opportunity for expanded CMC usage, such as in cement, concrete, bricks, and tile. TABLE 2.6 Commercially Available Fibers for the Reinforcement of CMCs Fiber Manufacturer Composition Diameter (μm) Modulus (GPa) Strength (MPa) NICALON Nippon Carbon Si-C-0 10-20 195 2900 Tyranno Ube Si-C-Ti-0 8-10 205 2750 Sumica Sumitomo Chemical Al203/Si02 10-17 200 1500 Nextel 312 3M Al203/Si02/B203 8-12 152 1725 Nextel 440 3M Al203/Si02/B203 10-12 189 2110 Nextel 480 3M Al203/Si02/B203 10-12 225 2285 FP duPont Al203 20 380 1380 PRD-166 duPont Al203Zr02 20 380 2108 SCS-6 Textron SiC on C 140 415 3900 Sigma British Petroleum SiC on W 100 400 3600 Saphikon Saphikon, Inc. Al203 150-250 350 2050 Carbon Many companies C 7-12 200-900 2700-5000 Fabrication and Application Many approaches are currently being pursued for the fabrication and application of CMCs (see Table 2.7). Among these, however, only a few systems have reached the developmental stage, which allows them to be available for potential use. Table 2.8 shows some examples of applications of CMCs to construction, aerospace, and other industrial uses, such as cutting tools. Industrial Sources: Industry Structure Table 2.9 lists some of the sources prominent in the development of CMCs. some are material suppliers from whom CMCs or CMC components can be commercially purchased, and others are developing the material for their own use, for R&D, or for potential future commercialization. The development of CMCs has been and continues to be an international endeavor. Through the success of these programs there has been a considerable increase in interest in CMCs worldwide. Special mention should be made of the successful flight demonstration by the French company Societe Europeene de Propulsion (SEP) of its chemical vapor infiltration (CVI)-produced CMCs in advanced gas turbines. Dramatically demonstrated in flight at the 1989 Paris air show, this marks an important
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 36 first that has not been duplicated by any U.S. company. The in-service experience to be gained will be of significance in enlarging the scope of the overall French program, which is extremely competitive because it encompasses all necessary aspects of composites technology, including a very active ceramic fiber development activity. A similar program does not exist in the United States. It is important to note that duPont in the United States has been licensed to use the SEP technology for making SiC-matrix composites by CVI and has made a major investment in establishing a domestic production capability. This is an important example of bringing a non-U.S.-based technology, superior to any in the United States, into this country for application. Figure 2.3. Comparison of bend tests for unreinforced cement and cement-matrix composites containing 2 percent chopped carbon fiber. Future Application Challenges The range of future applications can be very large and can span a broad spectrum of areas. Emphasis is usually placed on the high-technology areas, but it will be clear, as demonstrated above, that construction and industrial applications are also important. Likewise, while high-temperature applications are usually emphasized, it should be noted that applications in competition with PMCs and MMCs are also possible based on other considerations, such as the superior environmental stability attainable by
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 37 CMC. A classic example of this can be found in comparing CMC candidates with aluminum or titanium. CMC materials can have significantly lower density and are competitive over the entire temperature range of performance, not just at high temperatures. The effects of environment will undoubtedly be most important in determining the successful candidate for a particular application. Figure 2.4. Densities and use temperatures of potential composite matrices. The use of CMCs for space-based satellite applications is being pursued currently as part of the strategic defense initiative (SDI) program. This interest is based on the potential performance advantages of carbon-fiber- reinforced glass5. In this case there are two important areas of interest. First, space satellite structures and reflectors must remain very dimensionally stable over the orbital temperature range, they must be resistant to attack by atomic oxygen, and they must be highly damage tolerant. This last point includes mechanical damage from debris as well as thermally induced damage due to hostile threats. It is possible to incorporate all of
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 38 these qualities in CMC systems. One example of this is the high-performance mirrors produced from carbon- fiber-reinforced glass for use in laser systems.6 TABLE 2.7 Manufacturing Process for CMCs Process Examples (Fiber/Matrix) Glass powder and hot press NICALON/LAS ⢠Ply lay up and hot press Carbon/glass ⢠Matrix transfer mold FP alumina/glass ⢠Injection mold Nextel/glass Chemical vapor infiltration NICALON/SiC ⢠Infiltrate prewoven structures Carbon/SiC Nextel/glass Polymer conversion NICALON/SiC ⢠Infiltrate and pyrolyze Sol-Gel Carbon/glass ⢠Infiltrate and pyrolyze Nextel/mullite Ceramic powder and hot press, sinter ⢠Ply lay up and hot press SiC/Si3N4 ⢠Blended constituents and hot press or sinter SiCw/Si3N4 SiCw/Al2O3 Cementatious processing Carbon/cement Glass/cement Steel/cement Liquid metal oxidation (lanxide) NICALON/Al2O3 SiCw/Al2O3 Liquid infiltration SiC/SiC (Si) Reaction forming SiC/Si3N4 The potential use of CMCs at very high temperatures can be illustrated by comparing available tensile strength data of several composites with the strength of currently used superalloys (see Figure 2.5). The comparison does not include the effects of environment since the composites were tested in inert atmosphere. The SiC fiber used for these composites was Nicalon®. All
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 39 TABLE 2.8 Some Applications of CMCs Application Fiber or Composite Property Enhancement or Product Construction 1-3 volume percent chopped fiber in cement Enhanced flexural strength; light-weight concrete Industrial SiC in Al2O3 Improved cutting tool material Aerospace C, SiC Gas turbine engine parts Figure 2.5. Specific strength comparison of high-termperature metal alloys and advanced composites (two- dimensional fiber-matrix). composites were bidirectionally reinforced. The use of carbon fibers provides the highest level of performance owing to carbon fiber's unique low density, high strength, and retention of strength to the highest possible temperature.
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 40 TABLE 2.9 Some Current Sources of CMCs Matrix Fabrication Process Source Chemical vapor infiltration SEP duPont Amercom Refractory Composites Oak Ridge National Laboratories General Atomics 3M Glass/glass ceramic Corning Glass United Technologies Corp. Nippon Carbon Co. Hot pressed and sintered Greenleaf Advanced Composite Materials Oak Ridge National Laboratories GTE Norton Textron Polymer conversion Kaiser Aerotech General Atomics Sol-Gel Babcock & Wilcox Pratt & Whitney Reaction forming Lanxide General Electric Although the Nicalon® fiber tensile strength is expected to decrease significantly at about 1300°C, it is notable that composite strength is maintained even to this temperature. The strength and stability properties of newer versions of Nicalon®8 as well as these of new fibers such as Tyranno9 indicate the potential to increase maximum-use temperatures in the future. Also, coating approaches are currently under development to allow long-term use of the C/SiC and SiC/SiC composites in oxidizing environments. Critical Problems to be Solved for New Applications Being the newest class of advanced high-performance composites, the full range of potential applicability of CMCs has not yet been realized. While the more obvious high-temperature potential of these systems has formed the primary focus for research, the potential for use at lower temperatures should be investigated. Specific strength comparison of high temperature metal alloys and advanced composites (two-dimensional fiber-matrix) should also be explored. The future widespread use of CMCs must begin with prudent identification of applications that will help to develop a sound basis for their reliable use.
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 41 This may mean beginning with emergent CPC systems and applications that do not require either the highest level of structural performance or maximum temperature capability, nor that are optimum in their payoff, but instead serve a useful purpose, are cost effective, and give the opportunity to demonstrate reliable production methods, non-destructive evaluation (NDE) techniques, and design principles. This approach would allow a broader range of applications to be found for CMCs, and it would offer the potential to further support the overall technology base. HIGH PERFORMANCE FIBERS FOR CARBON-CARBON COMPOSITES Carbon-carbon (C-C) composites are structures in which both the matrix and the reinforcement are carbon. They offer many advantages in high-temperature applications over composites fabricated with other matrix materials. The unique high-temperature mechanical property retention of C-C composites (in excess of 2200°C) and their low density (1.5 to 2.0 g/cc) make them useful at high temperatures (i.e., above 1350°C in some cases and above 1700°C for short-time, limited-use application). For continuously reinforced C-C composites, it is the mechanical properties of the carbon-graphite fibers that dominate the C-C composite properties, and it is the high-temperature capability of the carbon matrix that allows one to take advantage of the fiber properties at elevated temperatures, where most metal matrices have melted or polymer matrices have decomposed or melted. The major drawback of C-C composites is lack of oxidation resistance. Carbon's oxidation rate increases dramatically above 600°C, and unless an oxygen barrier or inhibitor is applied to the C-C composite or its constituents, operational time above this temperature is limited. Another disadvantage of high-performance C-C composites is fabrication cost, which is a result of the high fiber cost, the long processing times, the many fabrication steps needed to achieve the desired composite properties, and the expense associated with the use of high-temperature processing equipment. However, in many applications where C-C composites are considered for use, government specifications dictate the use of expensive manufacturing methods or preclude the use of any other matrix. A continuous fiber C-C composite is fabricated by first forming a ''preform'' of carbon-graphite fibers either by weaving a fabric that is used to build up a structural shape (involutes, rolled fabric, pierced fabric, etc.), by weaving straight fibers in multidimensions, or by braiding. This preform is then densified; that is carbon is added to the interstices of the fiber preform to become the matrix of the composite. Formation of the carbon matrix can be accomplished by a number of methods: conversion of liquid resin or liquid pitch precursors, gaseous or CVI, or combinations of these to achieve desired physical properties. Discontinuously reinforced C-C may be fabricated by starting with a carbon fiber felt and densifying by CVI or by mixing carbon fibers or whiskers in a carbon-forming precursor and then pyrolyzing and graphitizing.
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 42 Major Uses and Current Systems The major uses and systems employing C-C composites are listed in Table 2.10. As shown, military and aerospace requirements dominate high-performance applications, where structural and thermal properties are of crucial importance. The largest volume application, to commercial aircraft brakes, does not require high performance; this is also true of the application to furnace insulation material. For these applications relatively inexpensive fiber components, such as chopped or dicontinuous fibers, are adequate. TABLE 2.10 Major Uses of C-C Composites Military/Aerospace Commercial Rocket nozzles Brakes for aircraft, racing Nosecones for reentry vehicles cars Heat Shields High temperature furnance Brakes for aircraft insulation Future Application Challenges C-C composites can be expected to consistently outperform other materials wherever there is a high- temperature thermostructural requirement if oxidation of the composite is precluded. One of the biggest impact areas of C-C composites is in gas turbine engines. The use of C-C composites in hypersonic vehicles will continue, but more benefits can be gained through incorporation of the newly developed, high thermal conductivity, pitch-based carbon fibers. Hypersonic vehicles will need C-C composite leading edge and skin materials to withstand the extreme aerothermal heating of the atmosphere. Additionally, because structural weight fraction requirements are so stringent, the C-C composites must be structurally and thermally functional. Very thin components (i.e., 10-20-mil-thick panels) will be needed with high modulus and strength to carry airframe loads. High composite thermal conductivity will be required to transfer aerodynamic heat to a sink, so that vehicle surface temperatures can be kept to a minimum consistent with the maximum operational temperature of oxidation-resistant coatings. The high temperature, strength, and toughness capabilities of C-C composites make them extremely attractive for future military applications where laser weapons might be encountered. The high cost of these composites precludes use in many applications, but in very high value platforms, as in space, where survivable structures are required, C-C composite costs are justified. For spacecraft applications, C-C composites have attractive properties that will enable them to trade off well even if hostile-threat survival is not a requirement. Three types of spacecraft components are available for C-C
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 43 composite insertions: structural, dimensionally stable, and thermal management components. The relatively low carbon-matrix modulus of elasticity causes very little matrix contribution to the C-C composite thermal expansion coefficient, and thus a zero or slightly negative thermal expansion coefficient of the composite can be obtained by using carbon fibers with these same properties. It has been shown that highly graphitic fibers are resistant to shrinkage under intense neutron radiation. This attribute, together with other properties of high-temperature strength, toughness, and low nuclear cross section, makes highly graphitized C-C composites applicable for nuclear power plant applications. Applications that could potentially be exploited for economic considerations are in high-temperature processing of materials. Some examples include containers for molten metal, high-temperature bearings in steel mills and chemical processing plants. Oxidation is the most critical problem to overcome if C-C composites are to be widely used in a variety of applications. Composite surface coatings can provide protection, but to provide for a more gradual degradation in the performance of the composite in the event of a breech in the coating, internal oxidation resistance must be designed into the C-C composite substrate. The high cost of C-C composites is also a major issue and must be addressed on many fronts. The major contributors to the high costs are fiber cost, preforming costs, and densification costs. The technical issues involved are discussed further in Chapter 4. HIGH PERFORMANCE FIBERS FOR NONSTRUCTURAL APPLICATIONS As pointed out earlier, compositesâespecially advanced compositesâare used primarily in structural and semistructural applications for which the dominant considerations are mechanical properties, such as stiffness, static strength, and resistance to fatigue, creep, and creep rupture. However, there are many applications for which other physical properties, alone or in combination with mechanical properties, dominate the selection process. These physical properties include electrical conductivity, thermal conductivity, coefficient of thermal expansion (CTE), dielectric properties and magnetic characteristics. Although there are currently many applications for which these properties are critical factors in the choice of materials, their unique properties in composites have not been fully exploited. This chapter considers current and potential applications for which nonstructural physical properties are key requirements and examines the needs for new or improved fibers to exploit these properties.
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 44 Major Uses: Current and Projected Electrical and Electromagnetic Properties The most widely used synthetic composite reinforcement is E-glass fiber, which was developed for electrical applications. Although the major uses of E-glass at the present time are in structural and semistructural applications, E-glass-reinforced polymers are widely used in applications where electrical insulation is required. Examples include generators, motors, printed circuit boards (PCBs), and industrial ladders. In these applications, design requirements include a physical property, and electrical resistivity as well as mechanical properties. Dielectric strength is another important electrical property, especially for high-voltage applications. Low dielectric constant and loss tangent are key design requirements in applications where the transmission of electromagnetic energy is important. The most common example is probably the radome, which is widely used in airborne and land-based radar and communications systems. Reinforcements here are typically E-glass, quartz, and aramide fibers. Alumina fibers are used in high-temperature ceramic-matrix composite radomes. In some applications it is desirable to have materials that absorb electromagnetic energy. This can be achieved by using polymers or ceramics reinforced with fibers whose electromagnetic properties are tailored for the purpose by varying chemical composition, structure, or both. For example, the electrical conductivity of carbon fibers can be varied over many orders of magnitude. Therefore, they are used in applications ranging from static dissipation to EMI and RFI shielding. One problem is the stability of carbon fiber electromagnetic properties in some property ranges. Other fibers used for their electromagnetic properties are a variety of oxides and silicon carbide. Silicon nitride fibers are under development for this purpose. At this time it is often the case that tailoring electromagnetic properties is achieved at the cost of reduced strength, modulus, or both. Fiber-reinforced materials play a key role in current applications for which tailored electromagnetic properties are important. This undoubtedly will continue for the foreseeable future. With present materials, there is often a penalty in reduced structural properties when reinforcements are selected for their electromagnetic properties. Development of new or improved fibers with good structural characteristics and tailorable electromagnetic properties is highly desirable. The low density of carbon fibers makes them attractive candidates for electrical conductors in applications for which weight is critical. Aircraft and spacecraft are obvious examples. Another possible use is in transmission lines. Here, their high specific stiffness and strength would permit support towers to be placed farther apart, reducing construction costs. The high-temperature creep resistance of carbon fibers is another advantage over conventional metal conductors. Although carbon fibers are electrically conductive, their resistivity is much higher than that of copper. This has been overcome, experimentally, by intercalation. However, an equally viable solution may be to use carbon fiber to carry the load of the transmission line
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 45 and traditional aluminum cable to conduct powerâa composite transmission line. Thermal Properties It is a remarkable fact that a number of carbon fibers have extremely high axial thermal conductivities. For example, experimental vapor-grown fibers have been produced with thermal conductivities approaching 2000 W/m K, five times that of copper. Commercial pitch-based ultrahigh modulus carbon fibers are available with reported conductivities as high as 700 W/m K. These relatively new materials have considerable potential in applications where thermal control is important. Examples include spacecraft radiators, electronic packaging, and high- temperature supersonic aircraft structures, such as those of the National Aerospace Plane (NASP). It should be noted that carbon fibers are strongly anisotropic, and their transverse conductivities are about an order of magnitude lower than the axial. This can be a serious limitation in applications where through-thickness conductivity is important. Conversely, fiber-reinforced matrix composites also are used in applications where low thermal conductivity is a major consideration. For example, E-glass/epoxy straps are used to support and thermally isolate cryogenic tanks used to store helium in nuclear magnetic resonance instruments. One of the characteristics of molecularly oriented fibers (either carbon or polymeric) is a low and controllable axial coefficient of expansion. This has led to their use in applications where dimensional stability is a key design requirement. The high specific stiffnesses and strengths of these materials contribute to their attractiveness in numerous spacecraft structures, such as the Hubble Space Telescope and antenna support towers. Add to these features electrical conductivity and it is not surprising that carbon fiber-reinforced polymers are the materials of choice for aircraft and spacecraft antenna reflectors. Requirements for future avionics systems are exceeding the capabilities of conventional low-expansivity monolithic metals used in electronic packaging, such as Kovar, Invar and molybdenum. Low CTE is required to match those of semiconductors and ceramic substrates. Applications include microwave packages, microelectronic packages, and heat sinks. If there is a CTE mismatch, severe thermal stresses can arise, causing mechanical failures. Key requirements, in addition to low CTE, are high thermal conductivity and low density. There are no monolithic metals possessing all three characteristics. The development of lightweight, low-cost, microwave packages may well be an enabling technology for aircraft and spacecraft large phased array antennas, which may use thousands of elements. The high-temperature resistance of alumina-boria-silica fibers has led to their use in a limited number of aircraft firewall structures.2 Ship structures are other potential applications. One of the most visible examples of a composite designed for nonstructural uses at the present time is the thermal shield of the Space Shuttle,1 with combinations of materials designed
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 46 to protect against temperatures exceeding 2000°F that are generated during reentry. Industrial furnaces use nonstructural fibrous or fiber-reinforced composites, where the principal use is based on thermal insulation properties. Such products are available as boards or mats with structural properties that may be just sufficient to permit handling during installation. Flexible composite insulation such as blankets of staple fiber fill that is quilted between woven fabric of ceramic fibers stitched with high temperature ceramic thread may be attached to furnace walls or suspended like curtains. Chemical Properties: Corrosion Resistance Relatively recent work on fiber-reinforced silicon carbide ceramics has shown promise for heat exchangers, furnace tubes, regenerators, nozzles, and other components that may be required to resist thermal shock and corrosive gases at high temperatures for extended periods of time. The ceramic composite type most frequently cited in recent presentations and publications is that prepared by the application of silicon carbide by chemical vapor deposition and chemical vapor infiltration on graphite, carbon, oxide, and silicon carbide textiles.3-5 Such composites comprising NEXTEL 312, NEXTEL 440, Nicalon®, and carbon felt have survived combustion tests to 1400°C for 800 hours or longer,3 and tests in corrosive waste gas from a secondary aluminum remelt furnace indicated that plate-type heat exchangers comprising silicon carbide reinforced by NEXTEL or Nicalon fibers could be designed to recover waste heat from corrosive or fouling flue gases from industrial furnaces.4 Cutting Tools A novel application for ceramic composites that are toughened and strengthened using whiskers is in cutting tools. A specific example of this is the silicon carbide whisker-reinforced alumina matrix composite (WG-300) developed by the Greenleaf Corporation.5 This new tool is reported to be capable of operating at surface cutting speeds up to about 10 times as fast as tungsten carbide and cobalt cermet tools. The superior performance is apparently related to the higher-temperature properties of the whisker-reinforced ceramic. Piezoelectric Sensing and Actuating Piezoelectric materials contract when subjected to electrical currents and, conversely, emit electrical signals when strained. These properties make them useful both as actuators and sensors. Piezoelectric fibers have been produced on an experimental basis. Potentially, they could be incorporated in various matrix materials to function as sensors, measuring strain, and to produce "active" structures, in effect, artificial muscles. Piezoelectric materials have been used as both sensors and actuators to provide active damping. Large space structures, such as those for space-based radars, are likely to require some form of active control to meet deflection requirements.
HIGH-PERFORMANCE FIBER MATERIALS: APPLICATIONS, NEEDS AND OPPORTUNITIES 47 Deformation sensors will also be required. It has been demonstrated that piezoelectric materials, in the form of films or plates, can perform these functions. Piezoelectric fibers potentially could more easily be incorporated into complex structures and could have reliability advantages over brittle ceramics in plate form. Piezoelectric fiber-reinforced composites also could have major benefits in detecting underwater acousting waves and minimizing reflected signals. It is highly likely that numerous other applications would emerge for this type of reinforcement. Reconfigurable aerodynamic surfaces is one possible example. REFERENCES 1. Korb, J., C. A. Morant, R. M. Calland, and C. S. Thatcher, "The Shuttle Orbitor Thermal Protection System," Am. Ceram. Soc. Bull., 60(11) pp. 1188-1193, 1981. 2. Sowman, H. G., and D. D. Johnson, "Ceramic Oxide Fibers," Ceram. Eng. Sci. Proc., 6(9-10) pp. 1221-1230, 1985. 3. Reagan, P., W. Cole, and F. Huffman, "CVD Silicon Carbide Components," Ceram. Eng. Sci. Proc., 8(7-8) pp. 958-969, 1987. 4. Cole, W. E., P. Reagan, C. I. Metcalfe, R. Wysk, and K. W. Jones, "Ceramic Composite Heat Exchanger," Ceram. Eng. Sci. Proc., 8(7-8) pp.968-975, 1987. 5. Copes, J. S. and R. G. Smith, "Microstructural Characterization of Thermally-Aged Siconex Oxide Fiber/SiC Composite Material," Ceram. Eng. Sci. Proc., 8(7-8) p. 976, 1987. 6. Dagani, R., "Ceramic Composites Emerging as Advanced Structural Materials," Chem. & Eng. News, Feb. 1, pp. 7-12 1988. 7. Schmidt, K. A. and C. Zweben, "Lightweight, Low-Thermal-Expansion Heal Sink," SAMPE 3rd International Electronic Materials Conference, Los Angeles, Calif., June 20-22, 1989. 8. Zweben, C. and K. A. Schmidt, "Advanced Composite Packaging Materials," Section 10, Electronic Materials Handbook, Volume 1: Packaging, ASM International, Materials Park, Ohio, in press. 9. Yamura, T., et al., 3. Mater. Sci. 23, 1988 p. 2589 and UBE Industries Ltd., Tokyo, Japan.
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