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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials APPENDIX B Processing OVERVIEW The processing of materials is crucial for achieving quality and efficiency in any technological enterprise. Processing, like synthesis, deals with the control of atoms and molecules to produce useful materials. However, processing also includes control of structure at higher levels of aggregation and may sometimes have an engineering aspect. The way in which materials are transformed—that is, “processed”—to form bulk materials, components, devices, structures, and systems is a major factor in determining the success of efforts in areas as diverse as the electronics and construction industries. Competence in materials processing is essential for the conversion of new materials into successful products, and for the continued improvement of products made from currently available materials. Today’s needs and opportunities for research in the processing of materials call for contributions from, and close interactions between, universities, industry, and the national laboratories: An integrated and interdisciplinary approach to education in materials processing is required in order to take advantage of new opportunities. The role of materials processing as an integral part of manufacturing technology has not received adequate emphasis in academic curricula, probably because it draws upon several disciplines and is a complicated mix of science, engineering, and empiricism. The development of commercial materials processing technology is the special and indispensable province of industry. A strong industrial capability
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials for carrying out research relevant to materials processing will be essential for achieving high quality and cost effectiveness in all industrial sectors. The national and federal laboratories, with their extensive technical resources and special facilities, are well positioned to make major contributions to research in materials processing. Recent scientific and technological developments have provided a remarkably diverse range of challenges and opportunities for research in the processing of materials. Among these opportunities are the following: Process modeling and simulation: High-speed computing capabilities coupled with improved theoretical understanding provide opportunities for improvements in materials processing technologies. Ceramics: Improvements in the technologies for processing of ceramic materials will be required in order to fabricate the new high-temperature superconductors, as well as to exploit the potential of ceramics in a broad range of electronic and structural applications. Optoelectronic materials: Advances in an array of processing technologies will be necessary in order to realize the opportunities present in optoelectronics. These technologies include crystal growth, molecular beam epitaxy, and physical and chemical vapor deposition. Rapid solidification: The continued development of this processing technology should lead to a wide range of applications and products. Metals: The processing technologies of the metals industries require a major infusion of resources, with an emphasis on areas such as automation and recycling of materials. Polymers: Processing techniques such as reaction injection molding, melt spinning, and polymer forging offer significant potential for further development. Technological challenges in the processing of materials, such as those listed above, bring with them needs for improved understanding at more basic scientific levels. Areas in which new needs for fundamental research are emerging include the following: Interfaces: Much of materials processing consists of manipulating and controlling the interfaces that separate various components of complex substances. New experimental methods and characterization techniques should lead to significant advances in understanding interfaces. Nonequilibrium materials: Materials processing technologies generally deal with materials in states that are very far from thermodynamic equilibrium. Experimental and theoretical advances in understanding the thermodynamics and kinetics of nonequilibrium states are now within reach and should lead to improvements in processing technology.
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials MATERIALS PROCESSING AND ECONOMIC COMPETITIVENESS The compelling theme of national economic competitiveness leads directly to a strong focus on materials processing. Processing and fabrication of materials—that is, the transformation of raw materials or synthesized substances into components, products, devices, structures, or systems—is an essential activity throughout U.S. technology. This is true for industrial sectors as diverse in nature and size as construction, computers, steelmaking, aerospace, electronics, and transportation. The term materials processing refers to an enormously wide range of technologies. A very incomplete list includes refining of metals, rolling of sheet steel, shaping of metals by machining, thermomechanical processing of alloys, growth of gallium arsenide crystals, zone refining of silicon, pressing and sintering of ceramic powders, ion implantation in silicon, formation of artificially structured materials by molecular beam epitaxy, spinning of high-strength polymeric fibers, gradient doping of glass fibers, sol-gel production of fine ceramic powders, modification of concrete by addition of polymers, and lay-up of composite materials. Some of these processing technologies are quite new and may, in time, lead to major technological advances and industrial growth. Others are well embedded in established industries but require continual improvement to maintain competitive positions. The nation’s eroding competitive position in some industries stems largely from national weaknesses in materials processing technologies. The essential difficulty seems to be that, with increasing frequency, industries abroad are bringing the results of R&D, or of innovative design, to production and thence to the marketplace in considerably less time than is the present practice in this country. In too many instances the new products produced elsewhere are more innovative in design and more reliable in function, and are produced at a lower cost than in the United States. The Japanese in particular have developed a formidable capability in manufacturing and the related materials processing technologies in a diverse set of industries, ranging from steel-making, automobiles, and shipbuilding to dynamic random access chips and video cassette recorders. Other nations, such as South Korea and Brazil, are now following the same path. For about four decades, the United States has been accustomed to holding a dominant position in virtually every important industrial sector. This situation was one of many inheritances from the Second World War, and it could not have been expected to continue indefinitely. Technological developments in transportation and communications have accelerated the trend to a globalization of technology and trade. In the young and rapidly evolving international economy the proper balance between the technological strengths of the U.S. and foreign economies is not yet evident.
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials Although the nature, diversity, and scale of materials processing technologies differ markedly among industrial sectors, there is a universal need to improve technological competence and efficiency in all aspects of materials synthesis, processing, and fabrication. Increased competence and sophistication in manufacturing and the accompanying materials processing technologies are necessary to produce high-quality and reliable products in an efficient and cost-effective manner. Competence in materials processing is also essential to the efficient incorporation into technology of advanced materials such as structural ceramics, composite materials, and high-temperature superconductors. HISTORICAL BACKGROUND Materials processing is probably the oldest of human technological activities, extending back in time to far before the advent of recorded civilization. For many centuries, materials processing was an empirical activity. Progress depended upon careful observation, extensive trial and error, and, no doubt, shrewd insight. There was no guidance, however, from what would now be regarded as soundly based scientific understanding. Nevertheless, by this empirical approach, a substantial variety of materials—metals, alloys, ceramics, glasses, concrete, and so on—were developed in useful compositions and shapes for both functional and artistic purposes. It was not until the seventeenth and eighteenth centuries, when a sound understanding of both chemistry and mechanics began to emerge, that materials processing under-went a transition from a craft to a science-based technology. Iron processing and steelmaking were among the first areas to benefit from the new knowledge. In present times, the infrastructure of our society has come to depend strongly on the existence of highly diversified and sophisticated materials technologies. The technological foundations of sectors such as agriculture, transportation, manufacturing, communications, construction, and national security are dependent upon the ability to process materials into a variety of compositions and shapes and to do this with accurate control of properties. An enormous proliferation of diverse materials—from stone and bronze, to steel and ceramics, and, most recently, to composites and high-temperature superconductors—has been driven by the demands of our technological infrastructure. The associated technologies for processing these materials are equally diverse. The growth in the materials base has been explosive in the decades following the Second World War, and there is every sign that this growth will continue well into the twenty-first century. THE PRESENT SITUATION: SOME EXAMPLES The processing of materials is an enabling activity, one that is essential to the practical realization of the ideas generated by scientists, engineers,
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials and designers in all fields, and one that can greatly expand the potential for performance of materials. In many instances, materials processing is a critical step in the development of an entire technology. Electronics is an outstanding example of an industry that depended from its beginning on the invention of processing techniques—in this case, the growth and purification of semiconductor crystals. In other instances, new materials processing techniques may lead to materials sub-stitutions in existing technologies with striking improvements in performance and reductions in cost. Examples of the latter situation are the substitution of optical glass fiber for copper wire in long-distance communication systems and the replacement of some metallic components in aircraft and automobiles by composite materials. In all cases, materials processing draws upon the results of research and design and also relies upon empirical investigations and extensive testing programs. Semiconductors The birth and rapid growth of the electronics and computer industries required an extensive series of interacting developments in basic research, materials processing, and design. An early and key step was the invention of the transistor in the late 1940s. This has been celebrated, and justly so, as a triumph of basic research. The enabling fundamental studies were strongly knowledge driven, although they were carried out in the context of a search for radically new communications technologies. In the course of the basic studies, both experimental and theoretical, wholly new patterns of electronic behavior were discovered and interpreted. The invention of the transistor was the first step, albeit the most important, in a long series of discoveries and technical developments that were necessary to transform the discovery of electronic amplification in solids into the modern electronics and computer industries. Many of those still dynamic and evolving technologies are materials processing techniques that were developed specifically to exploit the new scientific discoveries for practical purposes. These were application- or market-driven developments, in contrast to the invention of the transistor, which was knowledge driven. An excellent example of application-driven research is the development of the zone refining method for purifying silicon, which followed shortly after the invention of the transistor. This research was application driven in the sense that silicon and germanium semiconductors required impurity levels that were far below those obtainable by existing technologies at that time. In zone refining, purification is achieved by the passage of a thin molten zone, which removes the impurities, through the otherwise solid material. This invention was essential to the successful production of homogeneous high-purity crystals at reasonable cost.
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials A third major innovation was the design concept that culminated in the invention of the chip, that is, the circuit on silicon. The chip became the focus of integrated circuit technology, which required many new processes, such as patterning, photolithography, and metallization. It is evident from this brief description that the birth of the electronics industry required major contributions from fundamental knowledge, applied research, and innovative design. The further explosive development of the electronics and computer industries has been paced by continuous improvements in the interactive technologies for silicon processing, circuit design, and manufacture of integrated circuits. For example, it has continued to be necessary to improve crystal growth procedures in order to provide larger substrates for production of chips. New techniques have also been developed for doping silicon with the appropriate electrically active solutes, first by diffusion processes and then, more recently, by ion implantation. These are but a few of the many developments in materials processing technology that have been required for the rapid growth of the electronics industry. An important but often overlooked aspect of advances in materials processing is the development of new instrumentation for both processing and characterization. Examples from the electronics and computer industries include crystal growers, optical steppers, deposition systems, photolithographic and electron beam machines, and Rutherford backscattering systems for elemental analysis. Effective, state-of-the-art instrumentation is essential for improvements in the materials processing component of any industry. Optical Glass Fibers Just 20 years ago it was realized that glass fiber, as a replacement for copper wire, offers a considerable advantage as a communication medium at optical frequencies. Both improved performance and reduced costs were predicted. Following a period of intensive development of methods for processing glass fibers, the conversion of most long-distance communication in the United States to optical frequencies is now complete. An optical fiber cable has been laid across the Atlantic Ocean; conversion of the long-distance network in Europe from copper to glass is under way; and it is expected that distribution networks and short-distance links in the United States will be converted to optical fiber in the next decade. It is a matter of perspective whether this is regarded as merely the result of a materials substitution or as the development of a wholly new technology for the communications industry. It is certainly a major development in terms of improved performance and reduced costs. The production of hundreds of thousands of miles of glass fibers of uniform properties suitable for optical transmission of information required development of a new materials processing technology. This technology included
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials a sound choice of material composition, vapor deposition techniques for the elimination of optically absorbing impurities, careful chemical and mechanical design to eliminate optical leakage, and appropriate fiber-drawing techniques. The important phenomena for optical transmission are scattering loss, absorption loss, and dispersion. In the new silica glass fibers, absorption loss has been eliminated by reducing the concentration of optically absorbing impurities to below 1 ppb. Propagation losses for fused silica fibers are now at an intrinsic minimum of approximately 0.1 dB/km, caused by Rayleigh scattering from density fluctuations. To achieve even lower losses, which would allow greater distances between repeater stations, wholly different glass compositions must be considered. The development of optical glass fiber was application driven. It was not derived from new basic research, as was the case for the transistor, but rather from a careful engineering, systems, and economic analysis. The development of the new glass fiber processing technologies was carried out in industrial laboratories, with relatively little input from universities or the national laboratories. Rapid Solidification Technology Rapid solidification is an emerging technology. Its roots are in the universities and national laboratories as well as in industry, but most processing development currently is being carried out in industrial laboratories. There has been good interaction between the communities. It is probable that in the case of rapid solidification basic scientific progress has been accelerated because of the excitement generated by the potential for commercialization. Rapid solidification is an advanced example of nonequilibrium processing of materials from the liquid state and therefore is the latest in a long line of metallurgical developments that probably started with the accidental melting and casting of native copper many centuries ago. Although there were several experimental and theoretical precursors to the technology of rapid solidification, the critical experiment was probably the demonstration in a university laboratory some 26 years ago that metallic glasses—noncrystalline solid solutions of metallic components—could be produced by extremely rapid quenching from the melt. This powerful demonstration of the utility of rapid cooling in producing unexpected nonequilibrium structures was shortly followed up in many university and industrial laboratories. It opened a new realm in the study of materials, microstructures, and properties. In conventional or ingot processing, the cooling rates from the liquid state are of the order of 1 K/s or less, and may be as small as 10–3 K/s. The cooling rates in rapid solidification processing, however, may vary from
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials 102 K/s to as high as 108 K/s. These rates are so high that thermally driven diffusive rearrangement of atoms during cooling is impeded or wholly prevented. Highly nonequilibrium structures are then produced, often with unique properties. At sufficiently high rates, many alloys can be produced with an amorphous, or glasslike, atomic structure. At somewhat slower but still quite high rates, very fine grained crystalline structures are obtained, often in composition regimes that are not accessible to conventional processing. Very high cooling rates require rapid extraction of heat. Therefore at least one dimension of the solidified material must be small so that the whole sample can be in close thermal contact with a cold substrate. Consequently, rapid solidification technology cannot be used at present to produce large, monolithic objects. Rapid solidification technology has led to materials with new and useful combinations of magnetic properties. Attempts now under way to exploit their unique soft magnetic properties should lead to applications in electronics, power distribution, motors, and sensors. New permanent magnets recently produced by rapid solidification should be useful in building compact, powerful, electric motors. Direct ribbon casting is a version of rapid solidification that has much promise for producing thin sheets of materials with unusual combinations of properties. Among the present applications where this technology has led to improved performance and lower costs is the production of brazing filler alloys, solder alloys for electronic packaging, and thin stainless steel sheet. Rapid solidification also has been used to produce fine-grained and homogeneous crystalline—as opposed to amorphous—materials with much-improved properties and performance. The materials that have responded well to this processing technology include high-strength aluminum and magnesium alloys, tool steels of high toughness, and nickel-based superalloys. Rapid solidification recently played a key role in the remarkable discovery of the so-called quasi-crystalline phases. These phases were first produced accidentally during rapid solidification of aluminum-manganese alloys. The scientific interest in these phases arises from the fact that they display long-range order—they are not amorphous or glassy—but the symmetry of the order is not consistent with the heretofore accepted rules defining the allowable symmetries of crystals. The discovery of quasi-crystals has led to an ongoing reexamination of the basic principles of crystallography, a science that now will have to be reformulated in a more general framework. It is not known at present whether these new phases will have interesting and useful properties, but this entirely new phenomenon clearly calls for intense investigation. It is surely interesting and instructive that a study of structure and properties through rapid solidification processing should lead to a major discovery in crystallography.
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials Steelmaking The production of iron and steel is materials processing on a wholly different scale from that of the production of the high-technology materials discussed in the previous examples. The annual production of most high-technology materials is in the range of thousands of pounds per year, whereas the current annual output of U.S. steel mills is about 70 million tons. Furthermore, the production rate in the steel industry is extraordinary in comparison with that in other materials industries. Such volumes and rates require process technologies that are, in important ways, as complicated and sophisticated as those that are found in the processing of high-technology materials. For example, with a modern basic oxygen furnace, approximately 300 tons of steel are produced in about 30 minutes from molten iron containing 4 percent (by weight) carbon. Twenty minutes later, this steel is decarburized with vacuum degassing to less than 0.003 percent by weight carbon. In another 40 minutes, the steel is continuously cast into 9-in.-thick slabs. Such a complex materials processing technology requires a fundamental and detailed understanding of a whole series of complicated materials phenomena, including interactions in gas-metal, metal-slag, and metal-refractory systems. An equally deep understanding of fluid flow and heat transfer in complicated geometries over a wide range of temperatures is required. Process control is maintained by an array of computers operating on programs developed from detailed simulation and modeling of each step in the process. An expert system decides if the slabs produced meet the designated specifications. Further processing of the steel requires equally detailed modeling and process control technology. Still, continual and extensive process development will be required in the future to produce steel at a competitive quality and price. Steel remains the most versatile of the materials of the technological infrastructure. In contrast to most ceramics and composite materials at their present stage of development, steels are tough and their properties are both predictable and reproducible to a high degree of precision. Steels can be processed to exhibit a wider range of useful combinations of mechanical properties than any other material. Steels are unique in their sensitivity and range of response to variations in process technology, including composition, mechanical deformation, heat treatment, and thermomechanical processing. In recent years, however, the in-house capability of the U.S. steel industry for developing new technology has steadily eroded. This erosion is closely tied to the problems of competition and profitability that the industry has experienced. Some U.S. steel companies have improved their positions through the purchase of Japanese materials processing technologies, but that tactic is at best a catch-up game, one that cannot lead by itself to a position of technological strength. It is true that the purchase of technology from abroad
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials was one component of the Japanese drive to technological leadership. However, the Japanese steel industry also put a high priority on developing an in-house capability for R&D and, as a consequence, was able immediately to absorb the new technology purchased from abroad and to effect further improvements in a timely and efficient manner. An important but not often noticed consequence of the decline of the U.S. steel industry is that, in the area of materials processing, there has been a marked decrease in interactions between the industry and the universities. This situation has developed steadily over the last 15 to 20 years as R&D in the industry has declined. Up to the mid-1960s, there were many flourishing interactions through technical societies and a myriad of personal contacts. A steady stream of students went from university graduate programs, usually in metallurgy, to stimulating and technically productive careers in the steel industry. However, career opportunities in the industry for materials scientists with graduate degrees are now sharply diminished, and consequently so are the university graduate programs that relate to steelmaking technology and, indeed, to the metals industry in general. This situation will not be turned around easily. High-Modulus Polymer Fibers The recent development and commercialization of high-modulus polyethylene (PE) fibers well illustrates the interplay between industrial and academic laboratories, as well as the transfer of knowledge and technology between laboratories in the United States and abroad. High-modulus and high-strength polymer fibers require that the molecules be aligned along the axis of the fiber. It was first noticed in a European university laboratory that, upon stirring a PE solution, fibrous crystals formed on the stirrer. This was in contrast to the usual appearance of folded chain crystals in solution. These fibrous crystals could be processed to form fibers with a high modulus. It was then shown in a second European university laboratory that PE could be drawn to a high-modulus fiber in the solid state. Special drawing conditions—slow drawing rates at temperatures above those at which molecular segments could move through the crystals—were required to produce high-modulus fibers. In that processing regime, the modulus of the fibers increased with draw ratio. Molecular chain entanglements are an important consideration in that they are responsible for holding the fibers together laterally. The resulting modulus may be as large as one-fourth the modulus of diamond. The solid-state process was developed further in an American university laboratory, where a hydrostatic extrusion technique was developed to produce high-modulus PE fibers. Very high modulus fibers were produced by ap-
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials propriate combinations of temperature and extrusion ratio. The extrusion technique was somewhat faster than the drawing method. The importance of the entanglements was emphasized by the next development, which occurred in a European industrial laboratory. The density of entanglements and the morphology were optimized by converting the polymer solution to a gel before solid-state processing. Gelling sets the molecular topology of the chain entanglements. High-modulus fibers are produced when the gel is drawn to a high ratio. The gel-and-draw technique has been commercialized by an American corporation, following an extensive in-house development program. A new polymer fiber with distinctive properties has appeared in the marketplace. The starting material is cheap, the process technology is relatively inexpensive, and the modulus and strength are superior. The fundamental information developed early in academic laboratories was essential to the industrial research, which led to successful development and commercialization of the fibers. OPPORTUNITIES FOR THE FUTURE The final section of this appendix is devoted to some selected areas in which research in materials processing might have a significant impact in the not-too-distant future. Computers, Modeling, and Simulation The rapid development of fast and versatile computers provides opportunities for significant improvements in materials processing technologies. On-line computational control of process parameters can lead to major improvements in product quality and performance as well as to increased efficiency and reduced costs. The union of computers and processing technology already has appeared in some industrial sectors, and greater effort is anticipated in the future. Processing technologies as different as steelmaking, crystal growth, fabrication of integrated circuits, near-net-shape forming, and rapid solidification might benefit greatly from full use of this capability. To take advantage of this opportunity, major advances will be required in modeling and simulation, characterization of materials behavior during processing, and sensor technology to monitor process inputs and material response. Materials processing typically involves unusual and sometimes extreme conditions of temperature, pressure, strain rate, flow velocity, or other material or system parameters. The phenomena that occur during materials processing are generally complex and highly nonlinear; they must be modeled with considerable precision in order for the numerical results to be meaningful.
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials Improved process control requires a detailed knowledge of material and system parameters during processing. Thus improved real-time sensing capabilities are required to monitor parameters such as temperature, composition, and flow speed at many locations in the processing system. A sensor development program is under way in the steel industry at present, but a much larger effort and one directed also to other processing technologies is needed. Advanced numerical schemes for optimizing materials processing may ultimately be able to include not just the process parameters themselves but also criteria relating to performance and life prediction. Fully integrated simulations of this kind will require better understanding of the relations between processing and performance, and also will require improved methods for nondestructive evaluation and testing. High-Temperature Superconductors The unexpected discovery of high-temperature superconductors in some ceramic oxides will surely spark a major new effort in materials processing. In fact, this effort is already under way. At this writing, the knowledge base relevant to these new superconducting materials is growing rapidly, and there may well be further surprises in the months ahead. In any event, the processing of these brittle ceramic superconductors to form useful devices will be a high priority for the materials processing community. A wide variety of shapes and properties will be required if these new superconducting materials are to play their anticipated roles in microelectronics, power transmission, and transportation. Thin films, interconnects, wires, and cables are a few of the configurations that are of interest. Novel processing techniques may be needed, perhaps analogous to those developed earlier for the A15 superconductors, which, because they are intermetallic compounds, are also brittle materials. It will be necessary to integrate processing studies with structural determinations and property measurements. The high-temperature superconductors under intense study at present are complex oxides of the perovskite family. Their superconducting properties are strongly influenced by the oxygen concentration and the corresponding concentration and spatial distribution of oxygen vacancies. Processing studies of some subtlety will be required in order to establish the relations between processing parameters, oxygen concentration, oxygen defect characteristics, and optimum superconducting properties. Ceramic Processing Ceramics, in general, have been the subject of renewed scientific interest because of their potential advantages for use in microelectronic, structural,
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials and sensing applications. The new superconducting ceramics discussed above are just one part of this picture. The problems that will need to be solved in order to realize these advantages are largely in the area of processing. The unique properties of ceramics stem from the strong ionic (and, in some cases, partially covalent) bonds between the constituent atoms. This kind of bonding leads to unique combinations of properties, specifically: high strength, light weight, retention of strength and shape at high temperatures, and resistance to corrosion and erosion. The major use-limiting characteristic of ceramics is their brittleness. The strong bonding and complex crystal structures of ceramics cause them to be essentially unable to undergo plastic deformation by motion of dislocations. Consequently, stress concentrations at natural or artificial flaws cannot be relieved by dislocation plasticity, and catastrophic brittle fracture may ensue. This flaw sensitivity of ceramics is responsible for a severe lack of reliability and reproducibility in their mechanical properties. The synthesis and consolidation of powders is the usual processing technology for ceramics, although the use of techniques such as sputter deposition and chemical vapor deposition for thin-film ceramics is now an active area. Conventional melting methods, widely used for metals, glasses, and polymers, are generally not useful for bulk ceramics because their stability often leads to vaporization before melting, and substantial volume changes accompany melting when it does occur. Major developments in processing technology will be required in order to improve the toughness of ceramic materials. New methods of producing fine, pure, mono-size powders should lead to improved sintering characteristics and lower sintering temperatures. In ceramics such as zirconia, phase transformations can be exploited to improve toughness through microstructural manipulation. Improved toughness may also be achieved in ceramic-ceramic composites, where strong ceramic fibers are embedded in a brittle ceramic matrix. The strong fibers act as bridges across cracks in the brittle matrix, thereby increasing toughness. There are many processing technologies that can be explored with the goal of improving the toughness of ceramic materials. Artificially Structured Materials The recent development of new materials and materials systems structured on an atomic scale should have many important applications. For example, the processes used for producing artificially structured materials make it possible to combine optically active materials with electronic circuitry in ways that should lead to qualitatively new kinds of optoelectronic devices. Artificially structured materials can be produced by a variety of techniques including molecular beam epitaxy (MBE), liquid-phase epitaxy (LPE), chem-
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials ical vapor deposition (CVD), vacuum evaporation, sputter deposition, ion beam deposition, and solid-phase epitaxy. Techniques for growing thin films epitaxially, such as MBE, have been used to produce artificially structured materials with levels of purity and structural perfection that seemed impossible only a few years ago. Layered semiconductor systems with layer thicknesses down to atomic dimensions and with atomically smooth interfaces are now grown. The GaAs-GaAlAs system has received the most attention to date, with structures of widely varying electronic properties produced by control of composition. Carrier mobilities in excess of 106 cm2/V-s have been achieved in systems with layer thicknesses of the order of 10–6 cm. An artificially structured material generally can be expected to exhibit novel and useful properties when the length scale of the structure is comparable to the characteristic length scale of the physical phenomenon of interest. Examples of interesting microscopic length scales include the de Broglie wavelengths of electrons, the wavelengths of phonons, the mean free paths of excitations, the range of correlations in disordered structures, characteristic diffusion distances, and the like. These distances can vary from a few atomic spacings to microns. The least explored area, and the one with the greatest potential interest for processing technology, lies between the atomic and the macroscopic sizes. To date, most interest in the field of artificially structured materials has been focused on semiconductors, but there are many opportunities for new and useful combinations involving metals, insulators, and even polymers. The processing technologies now available are capable of producing both equilibrium and novel nonequilibrium phases, including amorphous structures and extended solid solutions. The range of possibilities in this area is truly remarkable. Metals Industry The problems of the metals industry are generally well known. The technical infrastructure of the industry has eroded in recent years, to the extent that the technical knowledge that exists within the industry is not fully used, and the transfer of technology into the industry from outside sources is gravely impeded. As an example, the titanium and nickel industries lack the resources to implement innovative new processes such as electro-refining of titanium and advanced ingot production methods for nickel-based superalloys. The needs of the metals industry for research in materials processing center on the goal of becoming economically competitive in international markets. In particular, the steel, lead, and zinc industries need new processes that will allow them to take better advantage of their North American ore reserves;
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials the steel industry is trying to develop a strip casting technology that will circumvent continuous casting operations and hot strip mill complexes; and the lead and zinc industries need extraction processes that will enable them to recover the silver and other minority constituents of their ore deposits. The most pressing long-term need of the metals industry, however, is the development of processes for recovering metals from waste materials, and for upgrading vast quantities of currently unusable scrap to allow separation and economical recycling. With the exception of steel, there is a general shortage of high-quality raw materials available to the industry from within North America. Technology for efficiently recycling scrap emerges as the most important process area for R&D in the next 15 years. In the near term, secondary processing operations such as rolling and shaping need attention throughout the metals industry. Process simulation, modeling, sensing, and control improvements are needed to upgrade quality and reduce costs. For long-term improvements in product quality, truly innovative research is needed to develop predictive models of metallurgical phenomena and to simulate the properties and performance of finished products. This is a potentially important area for collaboration between industry, universities, and national laboratories. Specific processing areas of interest to the steel industry include the following: the roles of fluid dynamics, heat transfer, and diffusion in determining the mechanism of hot dip coating; optimization of batch anneal heating processes through the development of models that incorporate nitrogen pickup, energy input, and productivity; application of computer-based expert systems to process control; three-dimensional fluid flow analyses of gas distribution in batch annealing, steel flow inside a tundish or mold, and gas flow inside a blast furnace or a basic oxygen furnace; and solidification models for simulating near-net-shape casting. Polymer Processing The remarkable growth of the synthetic polymer industry over the last half-century has been due as much to the development of processing technology as to polymer chemistry. Materials processing will continue to be a key to the development of new polymeric systems with unusual properties. The properties of polymeric materials depend on organization at all levels—atomic, molecular, supermolecular, and macroscopic. Atomic organization is determined in the chemical reactions in which polymers are grown from small molecules. The organization of the polymer molecules themselves, that is, their state of folding or extension, is determined largely by processing. Molecular organization also depends on the chemistry of polymerization, which determines the molecular weights of individual molecules
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials and the ways in which they are branched and cross-linked. At the supermolecular level, polymers interact to form crystalline, liquid crystalline, or amorphous arrays, or mixtures of those forms. Supermolecular organization also emerges from the ways in which different molecules behave when mixed with each other to form, for example, solutions or dispersions. Macrostructure is determined in shaping and finishing operations. Spinning of fibers, extrusion of films, and various molding operations determine macrostructure and often have a profound influence on the lower levels of organization in the bulk material. The importance of both polymer chemistry and processing on organization and, consequently, on aggregate properties is well illustrated by the new polyethylene fiber. It is prepared by drawing the ultrahigh-molecular-weight polyethylene from dilute (but supersaturated) solutions. The individual molecules are predominantly extended, rather than folded as is the case with polyethylene processed by conventional spinning or molding techniques. Success in making strong fibers by gel spinning has led to efforts to produce strong films by shear spinning, in this case to produce a material with most of the molecules extended in the plane of the fiber. Important last stages of reaction chemistry often occur during processing. Much effort is now directed toward controlling and using, rather than avoiding, these late chemical changes. In reaction injection molding (RIM), chemicals are mixed immediately before extrusion as films or fibers, or injection into a mold, with the consequence that polymerization occurs concurrently with the formation of the shaped product. This allows the use of polymers that are not easily processed if they are prepolymerized and can also improve process economics. The control procedures needed with a RIM process are very stringent. Good modeling of the reaction kinetics in a moving stream in which temperature, composition, and viscosity are changing rapidly is a tremendous challenge, but, if well done, such models would be enormously useful for production. Self-organizing polymers, such as those that form liquid crystals in melts or in solutions, are of increasing interest industrially. Objects made by solidifying liquid crystals tend to be strong and stiff. It is expected that they will be important structural and optical materials, but optimization of their properties will require a detailed understanding and control of processing technology. A new area of polymer processing is based on a classic metallurgical procedure. It has been found that mechanical working (forging) of polymers at elevated temperatures followed by controlled cooling can increase molecular orientation and therefore increase strength markedly. This emerging field promises to be an arena of intense international competition. A long-term goal is a process for extrusion of thick polymeric objects with high degrees of internal order and uniform properties. If successfully de-
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials veloped, such a process might produce thick sheets, blocks, and shaped objects with strengths now seen only in fibers and films. Processing of composites involves all of the problems encountered in processing monolithic materials along with a number of new ones. In particular, the organization of reinforcing material in a polymeric matrix may be both critical and difficult to accomplish by economical practices. Fiber-reinforced composites are a good example. The term reinforced plastics often refers to resin-based materials made stiffer by the inclusion of strong, high-modulus fibers, which raise the modulus and strength of the composite by load transfer from the matrix to the fiber. Chopped fibers are widely used because the processing conditions are then similar to those used for monolithic plastics. A continuous filament is much more effective in reinforcement if it can be arranged properly in the matrix. The simplest process for manufacturing continuous fibers is hand layup. The fibers are arranged in a mold, and a thermosetting resin is poured in. In forming the very high quality laminates used in the aerospace industry, the mixture of fiber plus resin is then cured under pressure. An open-mold process is reasonable for production of large parts that do not require optimum properties, e.g., boat hulls or building panels. In either case, the process is labor intensive and not suitable for economic mass production. The expanded use of composites in applications such as automobile components is contingent upon major improvements in processing technology. Various processes, including injection molding, are suitable for continuous operation, but the reinforcing materials are restricted to short fibers or particles. Continuous processes that use continuous fibers are under development but require much more effort. In the winding of filaments, continuous strands are impregnated with resin and wound onto a mandrel. The pattern of the “fabric” can be designed to produce strength tailored for the desired application. “Pultrusion” is another continuous process and combines pulling and extrusion. The fibers, in the form of strands, mats, or tapes, are pulled through a resin, through a preformer, and then into a die, where the final product is cured. The electrical properties of composites also depend on the organization of the matrix and the included material. Polymer-based composites are often filled with metals or ceramics whose electrical properties—conductivity, piezoelectric coefficients, dielectric constant, and so on—differ greatly from those of the matrix. The aggregate electrical properties of the composite are complicated functions of the organization of the included or filler material. Important parameters include the interparticle spacing, the connectivity, and the chemical nature of the interfaces. The theory of these structure-property relationships is primitive, and so is the science of controlling the organization of the electroactive particles by controlled processing.
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials The science that underlies polymer processing often lags behind technological developments. It would be most useful for further progress if good dynamic process models could be developed based upon chemical kinetics, material properties, and mass transport. The systems are complicated, usually inhomogeneous, solids, viscous liquids, or suspensions of solids in viscous liquids. These systems seem difficult to analyze quantitatively, but the potential rewards for developing a systematic understanding of polymer processing are great.
Representative terms from entire chapter: