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H~gl-Technology Ceramics ALBERT R. C. WESTWOOD AND IAN P. SKALNY The excitement in the field of ceramics these days is referred to in Japan as "ceramic fever." It relates to the intriguing prospects of having this class of solids finally live up to its potential in terms of strength at high temperatures, resistance to environmental degradation, and low cost. Of course, these are not traditional ceramicsthose beautiful, usually rather fragile examples of the potter's art made from naturally occurring substances such as clays, talc, and feldspars (Figure 1~.~ Early materials technologists were able to increase the durability of such products some- what by applying glazes, substances formulated both to improve ap- pearance and to have a coefficient of thermal expansion smaller than that of the substrate ceramic. On cooling, the substrate placed the glaze into compression, making it more difficult for minor scratches to develop into catastrophic cracks. Unfortunately, traditional ceramics suffer from two almost insurmountable disadvantages first, the inevitable pres- ence of voids and microcracks at phase boundaries that serve as crack nuclei and, second, the absence of any means of stopping a crack once it gets started. The "high-technology ceramics" (HTCs) now beginning to emerge from R&D laboratories are designed to reduce or circumvent both of these failings. But to develop these, ceramists literally have had to start again from the beginning. Instead of using natural but impure and ir- reproducible starting materials, they are now using chemically pure sub- stances, such as alumina, silica, carbon, and nitrogen. Their approach 117

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18 ADVANCES IN STRUCTURAL MATERIALS FIGURE 1 Grecian vase, circa seventh century B.C., prior to restorations Reprinted with permission. is similar to that used to make semiconductor materials for electronic applications, with great emphasis on control of composition and struc- ture. Because the strength of a ceramic solid is inversely related to the square root of the size (d) of its largest flaw, and since d usually depends directly on the size of its component "grains," the modern ceramist prefers to build up ceramic shapes from chemically homogeneous par- ticles, typically 1 micrometer (lam) or less in diameter and produced by a variety of essentially chemical routes. Such particles are compacted and then heated to sinter them together into a ceramic solid, but at much lower temperatures than those used in the past. There are two reasons for this. First, reduced temperatures and times are sufficient because the diffusion distances needed to accomplish densification are smaller. Second, it is necessary to avoid thermally induced growth of the small particles because, as noted above, structures containing larger grains are weaker. The approach described may be contrasted with that for traditional ceramics in which substantial masses of constituents are reacted at rel- atively high temperatures and the resulting clinker is ground down to

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HIGH-TECHNOLOGY CERAMICS 119 inhomogeneous particles of perhaps 5 to 20 Em in diameter that are then sintered back together, again at relatively high temperatures. Grinding is avoided in the new approach not only because it is inefficient, costly, and time-consuming, but also because it introduces impurities from the grinding media. Ideally, comminution, or grinding, is now used only to break up loosely bound clusters of fine particles or to add strain energy to them to accelerate subsequent sintering reactions. Developments leading to the generation of HTCs, or, as the Japanese prefer to call them, "fine ceramics," are occurring in three areas: the production of powders, of shapes, and of the important property of toughness. Some comments on progress in each of these areas and on possible applications for these new ceramics are presented next. POWDER PRODUCTION Various approaches are being pursued to produce chemically ho- mogeneous, submicron-dimensioned particles of ceramics such as TiB2, ZrO2, SiC, or of tough ceramic alloys such as partially stabilized ZrO2 (PSZ). The routes followed involve either liquid- or vapor-phase chemistry. In the first case, this usually involves production of a colloidal suspension and subsequent removal of the solvent. Unfortunately, if simple drying by heat or evaporation is used, coarse crystals or agglomerates often result. Accordingly, other techniques, such as spray- or freeze-drying, are generally used. An alternative technique that has attracted some attention is the solgel approach. This usually involves three steps: (1) producing a concentrated dispersion of a metallic salt in a dilute acid (the sol); (2) adjusting the pH, adding a gelling agent, and evaporating the liquid to produce a gel; and (3) calcining the gel under carefully controlled atmospheric conditions to produce fine particles of the req- uisite ceramic. This approach has proved especially useful for oxide- based ceramics, e.g., Al203, ZrO2, and ferrites. Another popular method for making colloidal dispersions is via the hydrolysis of metal alkoxides, these being the products of reaction be- tween alcohols and metal oxides. One advantage of this approach is that the alkoxides and hence the product oxides can be purified by dis- tillation. Also, the precipitated hydroxides tend to be uniform, spherical, submicron particles. Excellent packing densities can be obtained by the use of conventional colloid chemistry procedures. An example of TiO2 ceramic solid produced by calcining ethoxide- derived precursor particles at 1050C is shown in Figure 2(b).2 The solid is >99 percent dense, and the grain size is ~1.2 ~m. The precursor

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120 ADVANCES IN STRUCTURAL MATERIALS ~ . FIGURE 2 (a) TiO2 powder, about 0.35 ,um in diameter, produced by the alkoxide route. (b) TiO2 ceramic of >99 percent theoretical density made from powder above. Average grain size is ~1.2 ,um.2 Repnnted with permission. particles from which this solid was produced are shown in Figure 2(a). They are ~0.35 ,um in diameter. The sintering temperature required in this case was some 300C below that conventionally used for TiO2. Lead zirconium titanate (PZT) made via a butoxide route likewise can be sintered at 950C instead of the conventional 1300C, and alkoxide- produced ZrO2 can be sintered at 1200C to 1300C instead of the normal 1500C to 1700C.

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HIGH-TECHNOLOGY CERAMICS FIGURE 3 SiC particles, ~0.5 Am in diameter, produced by reacting SiO vapor and C particles at 1600C.6 121 Alternate routes lie in reactions that occur in the vapor phase. Various energy sources, including plasmas and lasers, have been used to promote such reactions. In the latter case researchers at MIT and elsewhere are making fine particles of silicon (Si), SiC, and Si3N4 from gaseous reac- tants such as SiH4, NH3, and C2H4, using a CO2 laser. The powders produced typically are <0.1 Em in diameter.3 In another vapor-phase route, water vapor is reacted with aerosol droplets of alkoxides to pro- duce either pure or mixed oxide powders of less than 1 Em diameter.4 While technically elegant, the cost-effectiveness of such approaches has yet to be established, and sometimes it can be useful to revisit an old approach with the perspective of improved mechanistic understand- ing. To illustrate: It is now recognized that the mechanism by which SiO2 and carbon interact at high temperatures to produce SiC involves the intermediate production of gaseous SiO.5 This substance adsorbs onto the carbon particle and reacts with it to produce SiC and CO. If the carbon particles are small the reaction can be controlled in such manner that the size and shape of the SiC particles are essentially de- termined by that of their carbon precursory SiC particles ~1 Em in diameter have now been produced in this manner at temperatures as low as 1600C, some 500C below the traditional processing temperature (see Figure 3~.6 No subsequent grinding is necessary. This approach, when scaled up, should permit the production of submicron-sized SiC for sale at $5 to $10 per pound.

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122 ADVANCES IN STRUCTURAL MA TERIALS PRODUCING CERAMIC SHAPES Two principal steps forming and consolidation are involved in making a ceramic shape, and improved processes are being developed for both. The trend in forming is toward utilization of an approach standard in the plastics industry but actually developed for ceramic shape production in the 1930s, namely, injection molding. In this process the ceramic powder, together with a low-melting-point binder, is heated to 150C and then injected into a mold under a pressure of about 5,000 pounds per square inch (psi). The part so produced is then heated to ~200C to evaporate most of the binder, the product being a weakly bonded "green" (unsintered) shape. Sufficiently complete removal of the binder to then permit the eventual production of a component of close to theoretical density can take several days, although NGK in Japan has recently reported reducing this time to about one day. Conventionally, the green shape is then heated at some elevated tem- perature in a controlled atmosphere to sinter the ceramic particles to- gether and to produce the final part. During the past few years, however, the technique of hot isostatic pressing (called HIPing) has been intro- duced to further density the sintered part and so minimize the number of fracture-initiating voids remaining. This technique can produce com- plex ceramic shapes of <2 percent porosity and with pore sizes suffi- ciently small that room-temperature tensile strengths of 50 to 100 kpsi (thousand pounds per square inch) are achievable. Unfortunately, the equipment required for "HIPing" is rather ex- pensive an industrial unit might cost $1 million or so. The process cycle also is slow. Moreover, ceramics such as SiC are intrinsically dif- ficult to sinter, requiring the use of sintering aids the presence of which can be detrimental to subsequent high-temperature performance. Ac- cordingly, alternative approaches are now being investigated, among which are high-pressure (200 atmosphere) gas sintering,7 shock pro- cessing, and "rapid omnidirectional compaction," or "ROCing." Shock processing, using explosives, is a collective term for three pos- sible procedures: (1) shock loading to enhance sinterability, (2) shock compaction (preferably without binding agents) followed by conven- tional sintering, and (3) "one-shot" compaction and sintering.8 Because of the substantial pressures (to 1 megabar [Mbari), high local temper- atures, and short reaction times involved, shock processing permits the production of nonequilibrium phases and the use of novel combinations of materials to provide preferred properties, e.g., toughness, low elec- trical conductivity, or enhanced catalytic efficiency. In just the past year, shock compaction has been used to produce high-density test pieces from Si3N4, Al2O3, ZrO2, TiC, and TiB2, the latter material exhibiting

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HIGH-TECHNOLOGY CERAMICS 123 ROCing Process ~ ~ MOLD Cold-compacted "green" piece. Cast "Fluid Die," e.g., a proprietary glass, around _ green piece. Remove mold. Preheat to forging temperature. 1 al RAM e= POT ._. - ///////// ~ Put die in pot on press table. Ram "fluidizes" die, and pressure is applied isostatically to part. Cool. Strip solidified die. Finished part. FIGURE 4 Elements of Rapid Omnidirectional Compaction (ROC) process. Is Reprinted . . . wit ~ permission. the respectable fracture toughness of ~9 MPami'2 (megapascals square- root meter).9 Another process used to date only with metals but exhibiting promise for the future production of near-net shapes in ceramics makes use of a "fluid"-containing die to transfer the pressure generated by a forging press isostatically to the part. This process, termed ROCing, is illustrated in Figure 4.4 The "fluid" used can be mild steel, a Cu-10 percent Ni alloy, or a variety of proprietary glasses, the appropriate medium being one that is very plastic or molten at the forging temperature. Pressures

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124 ADVANCES IN STRUCTURAL MA TERIALS of order 100 to 150 kpsi can be applied to the green compact, these being several times that typical for a HIPing operation. The availability of such high pressures should permit the use of reduced forging tem- peratures, producing finer-grained, stronger products. After cooling, the solidified die medium must be machined, melted, or fractured away from the shape. Even so, this technique appears to provide several advantages over HIPing, not the least of which is the use of conventional, readily available forging presses, and the brief in- press cycle time, one stroke of the press being sufficient. In comparison, a HIPing cycle usually takes several hours. THE DEVELOPMENT OF TOUGH CERAMICS Over the past few years, materials scientists have sought to circumvent the intrinsic fragility of ceramics by reducing the size and concentration of preexisting flaws through the use of ultrafine particles and compacting processes capable of producing components of near-theoretical density, as just discussed, and by introducing into the ceramics a variety of synthetic crack-retarding entities, such as phase-transforming particles, fibers, and distributions of cracks (see Figure 5~. The critical factors determining the relative efficiency of particulate (b) ~ ,Ll_\O ~ of )( \ \ ~Ld i_: ~ (c) \ FIGURE 5 Crack-retarding entities used to produce toughness in ceramics; (a) phase-transforming particles, e.g., ZrO2 (tetragonal) > ZrO2 (monoclinic); (b) fibers with weak fiber-matrix interfaces; and (c) other cracks. -

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HIGH-TECHNOLOGY CERAMICS 125 '. ~ ~ ::-. FIGURE 6 Partially stabilized ZrO2 (a) unstressed, showing coherent tetragonal precip- itate particles; (b) stressed by indenter, revealing transformed monoclinic particles near indentation (arrows) and untransformed tetragonal particles elsewhere. 12 Reprinted with . . permission. crack-retarding entities are the size, concentration, and spacing between the dispersed particles or fibers, and the differences in mechanical prop- erties between them and the matrix. ii Essentially, the smaller and closer the particles and the bigger the differences in properties the better. The toughening of otherwise brittle inorganic solids by the addition of fibers has long been practiced, e.g., in asbestos cement. But it is now recognized that to obtain any substantial increase in the toughness of ceramics, fiber diameter and spacing should be <10 to 50 ~m. Homo- geneity of distribution also is very important. The most interesting current exploitation of these principles occurs in the developing class of "transformation-toughened" ceramics. In these solids dispersed small particles of some metastable phase are trans- formed crystallographically when the strain field of a crack passes through them. Some of the energy of the crack is absorbed thereby. If the particles also increase in volume, they can apply a compressive stress to the crack tip, reducing its effective driving force. Further crack- retarding interactions occur at the particle-matrix interface and within the particle itself. The best-known example of this behavior occurs in partially stabilized ZrO2 (PSZ). In this case, a two-phase ZrO2 is produced by partially stabilizing the tetragonal ZrO2 phase by additions of up to 10 percent Y2O3, MgO, or other oxides. A typical structure is shown in Figure 6.~2 Note the relatively high volume, crystallographic orientation, and small size (0.5 to 2.0 Am) of the tetragonal phase. When a crack cuts through this material, the tetragonal phase transforms locally into a monoclinic

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126 ADVANCES IN STRUCTURAL MATERIALS LAS-SIC composite 24 cat ~ 20 us Lo an I Cal To ~ 4 Unidirectional ~ composite ~ _ L~- 16 I2 8 Cross-pied (0190) \` ~ composite , _ ~ - _ {~/ . Monolithic Corning / 9608 LAS O , ,`~- ~ ~ r , ~ I 100 " 700 800 900 1000 1100 1200 TEMPERATURE C FIGURE 7 Relationship between fracture toughness and tempera- ture for a lithium-alumino-silicate glass 50 percent SiC fiber com- posite.~6 Reprinted with permission. structure, and toughening occurs by the mechanisms described above. Polycrystalline PSZ can exhibit room-temperature strengths of 50 to 150 kilograms per square millimeter (kg/mm2) and fracture toughness of about 6 to 10 MPami'2. PSZ can also provide strengths of up to 100 kpsi at 1500C- when steel is already molten. i~ Noncubic ZrO2 particles can be incorporated in other ceramic substances also, e.g., alumina, again with useful results. In this case the best data reported to date are for A12O3 containing 10 to 15 percent ZrO2, values of fracture toughnesses ranging up to 15 MPami'2, and strengths to 175 kpsi.~3 Other materials besides ZrO2 undergo expansive phase transforma- tions, e.g., protoenstatitei4 and Ca2SiO4, and it may be that a variety of lighter and less expensive toughening particles than ZrO2 will be available for exploitation in the future. The alternative to particle toughening is fiber toughening, and ex- tremely useful progress is beginning to be made in this area, too. Most of the composites prepared to date utilize fine (<10-~m diameter) graph- ite fibers, or SiC fibers made by the pyrolysis of organic precursors, and silicate glass matrices.~5 Data for fiber-reinforced ceramics are limited at present, but their potential may be surmised from recent data on SiC

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HIGH-TECHNOLOGY CERAMICS 127 fiber-reinforced glass ceramics. In this case the matrix was a lithium aluminosilicate glass, and test specimens were prepared with the SiC filament reinforcements present in both unidirectional and cross-ply orientations. In both instances the volume fraction was ~50 percent. Figure 7 presents the fracture toughness data obtained. It ranges from ~17 MPami'2 at room temperature to a remarkable 25 MPami'2 at 1000C, at which temperature the matrix begins to soften appreciably. For pur- poses of comparison, the Charpy notch impact strength of this material is about 50 times greater than that of hot pressed Si3N4. In the future we are likely to see the development of ceramics con- taining several alloying components, processed using complex thermal and mechanical treatments to produce structures equivalent to those in advanced metallic alloys, and exhibiting a dense distribution of crack- arresting, fiberlike entities throughout. A start in this direction has already been made. A number of workers are producing interesting lamellar-type structures by the unidirectional solidification of oxide, carbide, or boride eutectic compositions. i7 To date, their toughness has been somewhat disappointing, typically ~6 MPami'2 or less, but this may be because the interlamellar interfaces are too strong and so do not produce the desired multitude of crack-retarding microcracks ahead of the propagating major crack. Perhaps the addition of surface-active species that would segregate to and embrittle these interfaces would help. STRUCTURAL APPLICATIONS FOR HIGH-TECHNOLOGY CERAMICS Whereas electronic applications for HTCs are already in the market- place, e.g., alumina substrates and zirconia sensors, products developed to improve on the structural performance of metals in arduous envi- ronments are just beginning to be introduced and are still undergoing vigorous technical development. Certainly the most publicized future application for advanced ceramics is the auto engine.~~20 However, development of an "all-ceramic" en- gine of the conventional piston-gasoline variety is not considered likely, both because of design problems and because really substantial gains in fuel efficiency cannot be anticipated. After all, there are metal-engined autos available today providing >50 miles per gallon (mpg). It seems more likely that over the next few years parts of conventional engines will be produced in ceramics, with different ceramics being chosen for different parts to meet specific operating requirements. Sub- sequently, new types of engines will be introduced, these being designed

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128 ADVANCES IN STRUCTURAL MATERIALS FIGURE 8 Si3N4 automobile turbocharger rotors made by Toshiba.2i Reprinted with permission. to take specific advantage of the particular attributes of ceramics. The ceramic gas turbine and so-called ceramic adiabatic diesel are the current prime targets. Over the next five years or so, then, we can expect to see the intro- duction of ceramic piston caps, cylinder and exhaust manifold liners, valve heads, and turbocharger rotors (see Figure 8),2i and so forth, because these do not require any substantial redesign of conventional engines, and the control of microstructural flaws is relatively easy. Such components should provide modest improvements in efficiency and du- rability. Subsequently, with the advent of new types of auto and espe- cially truck engines in the 1990s, we can anticipate the advantages of increased fuel economy by virtue of superior combustion efficiency (be- cause of higher operating temperatures); elimination of cooling (no radiators, water pumps, and so forth); probable elimination of oil lu- brication; and reduced emissions. In principle, at least, the cost of such engines should be less than that of current gasoline types. To demonstrate the feasibility of some of these concepts, the U.S. Army Tank Command (TACOM) and Cummins Engine Company re- cently teamed up to build an uncooled diesel engine for an army truck (see Figure 9~.22 This truck has operated well so far, providing 9 mpg as compared with the standard 6 to 7 mpg. By mid-1984, the team hopes to demonstrate an oilless version of this engine, with an intrinsic thermal efficiency of ~54 percent as compared with values in the low 30s for conventional gasoline engines. The ceramics now being developed for auto engine applications in-

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HIGH-TECHNOLOGY CERAMICS 129 elude SiC, Si3N4 (for turborotors, valves, piston caps, and so forth), PSZ (for combustion-chamber components), and aluminum silicate (for regenerator cores). However, the auto engines of the mid-199Os will probably utilize more sophisticated and complex ceramics than these, most likely alloys with toughness and durability optimized by precise thermomechanical treatments and with surfaces processed ("glazed") to minimize the potential consequences of small flaws introduced by abrasion or erosion. Companies known to be active in the development of ceramic-containing auto engines include GM, Ford, Cummins, Gar- rett, Volkswagen, Toyota, Isuzu, Nissan, Saab, and Rolls-Royce. Another area of application is cutting tools. Tungsten carbide was introduced to this application in the 1930s; cemented TiC tools came next, followed by Al2O3 in the l950s, and industrial diamond and cubic boron nitrogen (BN) in the 1960s, the objective always being improved tool life and enhanced rates of metal removal. Recently, however, and as a spin-off from their research on ceramics for gas turbine engines, Ford Motor Company has demonstrated that Si3N4 has excellent cutting characteristics, doubling the productivity of conventional cutting tools in the machining of cast-iron auto parts, e.g., wheel drums and clutch components. The Ford material (S-8) contains 8 percent Y203 and is hot pressed.23 GTE has disclosed similar data for its Quantum 5000 Si3N4-based material, which contains Y2O3, Al203, and 30 percent TiC as a dispersed phase.23 They have found that the number of brake drums that can be - - FIGURE 9 Army truck used in Cummins-TACOM tests of uncooled, ceramic diesel engined Reprinted with permission.

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130 ADVANCES IN STRUCTURAL MATERIALS FIGURE 10 A variety of prostheses made from alumina by Kyocera.24 Reprinted with permission. produced per tool at cutting rates of ~25 meters per second (m/s) in- creased from ~10 per conventional tool to ~100 for the new Si3N4- based tools. Other companies, especially in the United Kingdom, are finding that the Sialons (Si3N4-Al203 alloys) also provide excellent tool performance. Japanese companies, e.g., Toray, are now using A1203 and PSZ to produce household scissors, nonmagnetic blades for slitting videotape, and surgical saws. They have found that these materials are durable and capable of retaining a very sharp edge. Other emerging applications for structural ceramics include (1) com- ponents required to resist abrasion, erosion, and corrosion, e.g., seals, valves, nozzles, and bearings for the chemical, petrochemical, and min- eral-processing industries; (2) armor, especially lightweight body armor utilizing EN backed with Kevlar; and (3) orthopedics. Because of their resistance to corrosion by body fluids and their capability of being formed with surface characteristics closely simulating those of natural bone, ceramics are finding increasing use in surgical applications. Among the many interesting developments in this area are Kyocera's use of single- crystal sapphire to produce a range of products from hip prostheses to dental implants (see Figure 10),24 and Corning's introduction of potas- sium-magnesium-silicate ceramic crowns that are cemented directly to the remaining tooth structure and do not require any metal bridgework.3 Of course, before high-technology ceramics can truly be considered

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HIGH-TECHNOLOGY CERAMICS 131 as "structural materials" and not merely as components of structures, the technology must be developed to join them into integral and self- reliant systems. Work on this problem is underway, and studies at Stan- ford Research Institute have shown that it is possible to "braze" Si3N4 using silicon oxynitride glass compositions similar to those found in the grain boundary phases of this material. Alumina-based ceramic adhe- sives also are beginning to appear.3 Many innovative developments are expected to have occurred in the field of high-technology ceramics by the year 2000. By then, multicom- ponent, self-reinforced ceramic alloys, heat-treated to optimize prop- erties, protected by compressive surface layers that are perhaps applied by ion bombardment or laser glazing (a new approach to a traditional process), and joined by lasers, electron beams, or novel cements will become respected members of the engineer's portfolio of useful struc- tural materials. NOTES 1. M.I. Finley, Horizon, 9:51, 1967. 2. E.A. Barringer and ELK. Bowen, J. Am. Ceram. Soc., 65:C-199, 1982. 3. Reported by R.D. McIntyre, Mater. Eng., p. 19, June 1983. 4. E. Matijevic, Acc. Chem. Res., 14:22, 1981. 5. E.P. Bond, reported in P. Kennedy and B. North, Proc. Brit. Ceram. Soc., No. 33:1, May 1983. 6. D.C. Nagle, L. Struble, and K. Bridger, Martin Marietta Laboratories, Baltimore, Md. Unpublished work, 1982. 7. Reported by J.R. Hartley, Auto. Ind., 162:56, 1982. 8. R.A. Graham et al., Shock Activated Sintering, Rep. No. SAND-82-2335C, Sandia National Laboratories, Albuquerque, N.Mex., 1982. 9. V.D. Linse and J.H. Adair, Proc. A.P.I. Conf. on Shock Waves in Condensed Matter, Santa Fe, N.Mex., p. 14, 1983. 10. Courtesy Powder Technology Center, Division of Kelsey Hayes Corporation, Traverse City, Mich. 11. R.W. Rice, Chemtech, p. 230, April 1983. 12. D.L. Porter and A.H. Heuer, J. Am. Ceram. Soc., 60:183, 1977; 62:298, 1979. 13. N. Claussen, J. Am. Ceram. Soc., 59:49, 1976. 14. D.A. Anderson, Martin Marietta Laboratories, Baltimore, Md. Unpublished work, 1982. 15. R.A.J. Sambell, D.H. Bowen, and D.C. Phillips, J. Mater. Sci., 7:663, 1972. 16. J.J. Brennan and K.M. Prewo, J. Mater. Sci., 17:2371, 1982. 17. V.S. Stubican and R.C. Bradt, Annul Rev. Mater. Sci., 11:267, 1981. 18. J.W. Dizard, Fortune, p. 76, July 25, 1983. 19. A.F. Mclean, Ceram. Bull., 61:861, 1982. 20. D.J. Godfrey, Mater. Des., 4:759, 1983. 21. "Technical Information Si3N4 Applied Examples," No. 7, Toshiba Corp., Tokyo, Japan, 1983.

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132 ADVANCES IN STRUCTURAL MA TERIALS 22. Reported by S. Robb, Ceram. Bull., 62:756, 1983. 23. Reported by S. Robb, Ceram. Bull., 62:206, 1983. 24. Challenging the Future, Publication No. Z-lOlE-l, Kyocera Corp., Kyoto, Japan, p. 20, 1982. ACKNOWLEDGMENTS It is a pleasure to acknowledge the contributions to this paper made by Roy W. Rice, Naval Research Laboratory, Washington, D.C.; H. Kent Bowen, Massachusetts Institute of Technology, Cambridge, Massachusetts; and a number of the authors' colleagues at Martin Marietta Laboratories, Baltimore, Maryland.