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Appendix E Processing Techniques and Available Classes of Armor Ceramics This appendix covers additional material and details rel- lently bonded polycrystalline ceramics, the required dense evant to Chapter 5 of this report. These pages address several microstructure is hard to achieve using solid state sintering; topics related to processes used in the manufacturing of ce- therefore, additives are used to form a small amount of liq- ramics for armors and include discussions on potential armor uid phase between the particles at the sintering temperature. materials such as functionally graded materials, biomimetics, The liquid phase provides a high-diffusivity path to trans- foams, smart sensors, and phononic band gap materials. port matter into the pores and facilitates densification. For Some of the key manufacturing processes, together with example, hard-to-sinter silicon carbide (SiC) is processed their advantages and disadvantages, are listed in Table 5-1 as a liquid-phase sintered ceramic. The price of SiC tiles of Chapter 5. Table E-1 presents the relevant properties of manufactured by means of a pressureless sintered process is materials listed in Table 5-1 of Chapter 5. in the range of $40/lb to $50/lb. Solid- and liquid-state sintering processes are widely employed to densify refractory ceramics, but at much higher PRESSURELESS SINTERING temperatures than required by the hot-pressing (HP) tech- nique. Sintering of SiC was first performed by Prochazka,2 The pressureless sintering process offers cost reduction through net shape processing using innovative powders and using boron and carbon as sintering aids to reduce the interfacial energy of the grains (boron),3 and by reacting processing methods to obtain full density without the ap- plication of pressure. The goal of densification is to create the carbon with residual silica (carbon) present on the SiC particle surface.4,5 Sintering of b-SiC is more difficult than strong bonds in the material and eliminate porosity so that sintering of α–SiC because of the b to α phase transforma- theoretical densities, along with homogeneous microstruc- tures, can be achieved for the sintered bodies. Residual tion at 1900°C to 2000°C, which generates voids between porosity, along with the shape and size distributions of the grains owing to the difference in the growth morphology of b and α grains.6 Several additives, such as Al–C, Al2O3–C, pores and grains, influences the characteristics of dynamic performance. The low-cost alumina armor plates manufac- and Al2O3–Y2O3, have been tested as sintering aids for SiC tured by means of uniaxial pressing, slip casting, and sinter- powder to enhance the sintering rate and to reduce grain ing are used in vehicle armor applications in large volumes. The typical cost of these plates runs about $2.50/lb to $10/ lb for the finished tile. Solid state sintering is achieved by heating the “green” compact to the temperature that is in 2Prochazka, S. 1974. Hot pressed silicon carbide. U.S. Patent 3,853,566, the range between approximately 50 and 80 percent of the filed December 21, 1972, and issued December 10, 1974, to General Electric melting temperature.1 Company. Schenectady, NY. 3Maddrell, E. 1987. Pressureless sintering of silicon carbide. Journal of At these temperatures the powder does not melt, but Materials Science Letters 6(4): 486-488. fusing of adjacent powder particles and reduction in the 4van Rijswijk, W., and D. Shanefield. 1990. Effects of carbon as a sinter- overall porosity occur by atomic diffusion in the solid state. ing aid in silicon carbide. Journal of the American Ceramic Society 73(1): Solid state sintering is typically used for pure, single-phase 148-149. polycrystalline materials, such as α–Al2O3. For many cova- 5Hamminger, R. 1989. Carbon inclusions in sintered silicon carbide. Journal of the American Ceramic Society 72(9): 1741-1744. 6Williams, R., B. Juterbock, C. Peters, T. Whalen, and A. Heuer. 1984. Forming and sintering behavior of B- and C-doped α- and b-SiC. Journal 1Rahaman, M. 2007. Sintering of Ceramics. Boca Raton, Fla.: Taylor and Francis Group. of the American Ceramics Society 67(4): C62-C64. 125
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126 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS TABLE E-1 Summary of Properties of Various Ceramics for Personnel Armor Application Young’s Flex Fracture Density Grain Modulus Strength Toughness Hardness r Σ Size E K Fracture (HK-Knoop hardness, HK 2 kg Areal (MPa-m1/2) (kg/mm2) Material Designation (g/cc) (μ) (GPa) (MPa) Mode HV-Vickers hardness) Density Al2O3 CAP-3 3.90 — 370 379 4-5 — 1,440 (HK 1 kg) 1,292 20.2 B 4C Ceralloy-546 4E 2.50 10-15 460 410 2.5 TG 3,200 (HV 0.3 kg) 2,066 13.0 Norbide 2.51 10-15 440 425 3.1 TG 2,800 (HK 0.1 kg) 1,997 13.0 SiC SiC-N 3.22 2-5 453 486 4.0 IG, TG — 1,905 16.7 Ceralloy 146-3E 3.20 — 450 634 4.3 — 2,300 (HV 0.3 kg) — 16.6 Hexoloy 3.13 3-50 410 380 4.6 TG 2,800 (HK 0.1 kg) 1,924 16.2 Purebide 5000 3.10 3-50 420 455 — TG — 1,922 16.1 SC-DS 3.15 3-50 410 480 3-4 — 2,800 (HK 1 kg) — 16.4 MCT SSS 3.12 3-50 424 351 4.0 TG — 1,969 16.2 MCT LPS 3.24 1-3 425 372 5.7 IG — 1,873 16.8 Ekasic-T 3.25 1-3 453 612 6.4 IG — 1,928 16.8 SiC (RB) SSC-702 3.02 45 359 260 4.0 TG 1,757 (HK 0.5 kg) — 15.7 SSC-802 3.03 45 380 260 4.0 TG — 1,332 15.7 SSC-902 3.12 45 407 260 4.0 TG — 1,536 16.2 SiC/B4C (RB) RBBC-751 2.56 45 390 271 5.0 TG — 1,626 13.3 +Ductile Si TiB2 Ceralloy 225 4.5 — 540 265 5.5 — — 1,849 23.4 NOTE: Areal density in pounds per square foot (PSF): weight of a 12 × 12 × 1 in. panel in pounds; TG, transgranular fracture; IG, intergranular fracture. SOURCES: CAP-3, SC DS: CoorsTek; Ceralloy, Ekasic-T: Ceradyne; Norbide, Hexoloy: Saint-Gobain; Purbide: Morgan AM&T; SiCN: Cercom (BAE); SSC, RBBC, BSC, SSS and LPS: M Cubed Technologies (MCT). Properties for other manufacturers’ materials are from their respective Web sites except for 2 kg Knoop hardness, grain size, and fracture mode. growth. Aluminum (Al)7 and alumina8 with carbon promote that the presence of B2O3 coatings on B4C particles inhibits densification and facilitates grain coarsening.12 The boria can silicon carbide sintering by means of a solid state mechanism be removed by heat treatment in a hydrogen environment, at a temperature over 2000°C, while alumina and yttria lead which then permits direct contact between B4C–B4C grains, to a high-density sintered sample by means of a liquid-phase mechanism at temperatures below 2000°C.9 facilitating densification. As a result, the B4C powders with a particle size of approximately 1 μ can then be sintered to Boron carbide (B4C) is mainly produced by the HP 96 percent of theoretical density and with hardness values method. The cost of a B4C tile is in the range of $75/lb to similar to hot-pressed samples. Methods used to produce $85/lb. The pressureless sintering processes of B4C and densification of B4C by solid state sintering techniques10 pressureless sintered B4C have been developed at the Geor- gia Institute of Technology and commercialized at Verco are slow, and it is difficult to reach high density due to low Materials,13 as well as by larger armor producers such as self-diffusion. Sintering aids such as SiC, Si, Al2O3, Mg, and Saint-Gobain. Armor-grade material of B4C with a zero Fe have been used to increase the density by means of liquid- phase sintering;11 however, the mechanical performance of porosity state can be produced using pressureless sintering combined with hot isostatic pressing. liquid-phase-sintered B4C is inferior. It has been established Both SiC and B4C are harder materials with lower densities than alumina, yet alumina has been widely used in 7Stutz, D., S. Prochazka, and J. Lorenz. 1985. Sintering and microstruc - personnel and vehicle armor systems because of its lower ture formation of b–silicon carbide. Journal of the American Ceramics Society 68(9): 479-482. 8Sakai, T., H. Watanabe, and T. Aikawa. 1987. Effects of carbon on phase transformation of b–SiC with Al2O3. Journal of Materials Science Letters 12Cho, N., Z. Bao, and R. Speyer. 2005. Density and hardness-optimised 6(7): 865-866. pressureless sintered and post-hot isostatic pressed B4C. Journal of Materials 9Omori, M., and H. Takei. 1988. Preparation of pressureless-sintered Research 20 (8): 2110-2116. 13Campbell, J., M. Klusewitz, J. LaSalvia, E. Chin, R. Speyer, N. Cho, SiC–Y2O3–Al2O3. Journal of Materials Science 23(10): 3744-3749. 10Thévenot, F. 1990. Boron carbide—A comprehensive review. Journal N. Vanier, H. Cheng-Hung, E. Abbott, P. Votruba-Drzal, W. Coblenz, and of the European Ceramic Society 6(4): 205-225. T. Marcheaux. 2008. Novel processing of boron carbide (B4C): Plasma 11H. Kim, H-W., Y-H. Koh, and H-E. Kim. 2000. Densification and synthesized nano powders and pressureless sintering forming of complex mechanical properties of B4C with Al2O3 as a sintering aid. Journal of the shapes. ADM002187. Proceedings of the Army Science Conference (26th). American Ceramic Society 83(11): 2863-2865. Accessed April 1, 2011.
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127 APPENDIX E Compared to lower-density, pressureless sintered prod- raw material cost and ease of fabrication. Large-size alumina ucts, the hot-pressing process is a slow batch process that panels up to 400 mm × 550 mm are currently produced by means of pressureless sintering14 for use in lightweight armor typically yields a close-to-full-density product with superior ballistic properties. Hot-pressed SiC and B4C provide supe- vehicles and police car door protection. Morgan is one is the rior ballistic properties but are manufactured at high cost main producers of armor-grade alumina made by the pres- and in limited volumes through a batch process. The U.S. sureless sintering technique. Army Manufacturing Technology Program has developed an HP apparatus with multiple heating and cooling chambers HOT PRESSING and central hot pressing chamber to reduce the cost of hot pressed 4 in. × 4 in. tiles from $135/lb to $85/lb.16 The goal Hot pressing is often the procedure of choice for the is to increase production volumes, thus reducing the cost to manufacture of opaque ceramics, since it can produce fully $35/lb, similar to pressureless sintered material. dense ceramics at reasonably moderate temperatures and Due to processing differences, hot-pressed and pressure- pressures. However, HP can only produce limited shapes less sintered materials have different microstructures; see, such as flat plates or those with a small curvature. A current for example, Figure E-1. Hot-pressed material is typically Army program is developing HP to fabricate SiC tiles at lower cost.15 fully dense with fine microstructures, whereas pressureless sintered materials have large grains with texture. However, Traditional hot pressing is a batch process in which the recent dynamic magnetic compaction (DMC) work has “green” powder compacts are formed by means of a suitable shown the promise of obtaining fine grain structures similar pressing method and then loaded into a hot-pressing die. to those of HP material by combining DMC and pressure- Some armor manufacturers use tape casting or extrusion to less sintering. Such a process needs to be further developed. build up green B4C armor shapes for hot pressing. The die and powder are ramped up to the sintering temperature and pressure is applied to the die. To meet the required high sin- CURRENT-ASSISTED SINTERING tering temperatures (>2000°C), heated dies made of graphite Nano-grain-size ceramic powders are currently being or other high-strength inert materials are used in special explored by various laboratories, including the U.S. Army hot-pressing furnaces. As the ceramic part size increases, Research Laboratory, to obtain better mechanical proper- the load requirements increase, making the HP equipment ties through microstructural modifications. However, when large. HP is associated with small production volumes, and ceramic powders are hot-pressed or sintered at very high typically large billets are produced to be cut into individual temperatures for extended times, grain growth takes place. armor tiles. Often, multiple parts, separated by spacers, Processing methods such as dynamic compaction or spark are pressed together to increase production rate. Once the plasma sintering (SPS) techniques can be used to retain the ceramic is hot-pressed, it is cooled and then machined to its small grain size of nanograined powders. final dimensions by diamond tools using slow machining One method of accelerating the sintering process of steps and grinding rates. difficult-to-sinter armor ceramics involves the use of electri- Materials with different final densities and mechanical cal current. The name most often used for such field-assisted properties are produced by varying the nature of powder sintering is SPS, but the process is also known as plasma additives and hot-pressing conditions such as pressure, pressure sintering, pulsed electric current, and electric- temperature, and time. Typically, in hot-pressing SiC armor material, powders of α–SiC are mixed with suitable sinter- pulse-assisted consolidation. Significant advantages exist for using current-assisted sintering over that of hot-pressing, ing aids (boron and carbon, for example); additional carbon hot isostatic pressing, or pressureless sintering; the most im- is added to remove the silica passivation layers from the portant advantage is lower sintering temperature and reduced SiC particles. Temperature steps are adjusted based on the holding time, which results in marked comparative improve- specific mix of powders and additives, and during the last ments in mechanical properties.17 For example, attaining a high-temperature cycle, pressure is applied to achieve maxi- heating rate of 600°C/min, typically used in SPS of ceramics, mum densification. Different final density and mechanical could help retain the homogeneous grain size distribution properties are reached by varying the hot-pressing condi- tions and the powder additives used. Additives and precise processing conditions are kept as proprietary information by different manufacturers. 16Campbell, J., J. LaSalvia, W. Roy, E. Chin, R. Palicka, and D. Ashkin. 2008. New Low-Cost Manufacturing Methods to Produce Silicon Carbide 14Medvedovski, E. 2010. Ballistic performance of armour ceramics: (SiC) for Lightweight Armor Systems. ADA504013. Proceedings of the Influence of design and structure Part 2. Ceramics International 36(7): Army Science Conference (26th). Accessed April 4, 2011. 17Munir, Z., U. Anselmi-Tamburini, and M. Ohyanagi. 2006. The effect 2117-2127. 15Protection materials—Research to acquisition. Briefing by E. Chin, of electric field and pressure on the synthesis and consolidation of materi - U.S. Army Research, Development and Engineering Command, to the als: A review of the spark plasma sintering method. Journal of Materials committee, July 23, 2010. Science 41(3): 763-777.
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128 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS FIGURE E-1 Silicon carbide sample microstructures showing grains in (left) hot-pressing, (center) dynamic magnetic compaction followed by pressureless sintering, and (right) uniaxial pressing followed by pressureless sintering. SOURCE: Chelluri, B., and E.A. Knoth. 2008. SiC armor tiles via magnetic compaction and pressureless sintering. Presentation to the 32nd International Conference and Exposition on Advanced Ceramics and Composites, January 26-February 1, Daytona Beach, Florida. along with the small average grain size of B4C.18,19 The over the past decade.21,22,23,24 In reaction bonding, which spall strengths20 of SPS-processed B4C and SiC ceramics uses silicon-based matrixes, the pressureless infiltration of are improved over those from hot-pressing or pressureless a powder preform is achieved by good wetting and a highly exothermic reaction between liquid silicon and carbon.25 The sintering techniques. SPS is still under development, espe- cially for sintering larger parts. Density variations are still process is known variously as reaction bonding, reaction observed in the case of difficult-to-sinter ceramics because sintering, self-bonding, or melt infiltration. of current flow along the highest-conductivity graphite die M Cubed Technologies Inc., a developer of the reac- walls, although low or no current density is detected inside tion bonding process, uses a process that includes the the part. As a result of such a temperature gradient, material following steps: (1) mixing of B4C (or SiC) powder and a density varies with its location relative to the die walls. The binder to make a slurry; (2) shaping of the slurry by various widespread adoption of SPS in the past decade was pos- techniques, such as casting, injection molding, pressing, sible because of the availability of commercially built SPS and others; (3) drying and carbonizing of the binder; (4) systems. Currently the two major players are SPS SYNTEX green machining; (5) infiltration (reaction bonding) with Inc., Japan, and FCT Systeme GmbH, Germany. Recently, molten Si (or alloy) above 1410°C in an inert or vacuum the U.S. firm Thermal Technology LLC also started selling atmosphere; and (6) solidification and cooling. During the field-assisted sintering furnaces. However, to the best of the infiltration step, carbon in the preform reacts with molten committee’s knowledge, there are no commercially produced 21Waggoner, W., B. Rossing, M. Richmond, M. Aghajanian, and A. Mc- ceramic armor tiles using SPS, suggesting a potential oppor- Cormick. 2003. Silicon carbide composites and methods for making same. tunity for improved processing of dense ceramics. U.S. Patent 6,503,572, filed July 21, 2000, and issued January 7, 2003, to M Cubed Technologies, Monroe, Conn. 22Aghajanian, M., B. Morgan, J. Singh, J. Mears, and R. Wolffe. 2002. REACTION-BONDED CERAMICS A new family of reaction bonded ceramics for armor applications. Ceramic Transactions 134: 527-539. Reaction-bonded SiC and reaction-bonded boron car- 23Aghajanian, M., McCormick, B. Morgan, and A. Liszkiewicz, Jr. 2005. bide have been successfully used for armor applications Boron carbide composite bodies, and methods for making same. U.S. Patent 6,862,970, filed November 20, 2001, and issued March 8, 2005, to M Cubed Technologies. Monroe, Conn. 24Karandikar, P., M. Aghajanian, and B. Morgan. 2003. Complex, NET- 18Hayun, S., S. Kalabukhov, V. Ezersky, M. Dariel, and N. Frage. 2010. shape composite components for structural, lithography, mirror and armor Microstructural characterization of spark plasma sintered boron carbide applications. Pp. 561-566 in 27th Annual Cocoa Beach Conference on ceramics. Ceramics International 36(2): 451-457. Advanced Ceramics and Composites: B. Ceramic Engineering and Sci- 19Hayun, S., V. Paris, M.P. Dariel, N. Frage, and E. Zaretzky. 2009. ence Proceedings 24(4). W. Kriven and H-T. Lin, eds. Hoboken, N.J.: John Static and dynamic mechanical properties of boron carbide processed by Wiley & Sons. 25Karandikar, P., S. Wong, G. Evans, and M. Aghajanian. 2010. Micro - spark plasma sintering. Journal of the European Ceramics Society 29(16): 3395-3400. structural development and phase changes in reaction bonded boron carbide. 20Paris, V., N. Frage, M., Dariel, and E. Zaretsky. 2010. The spall strength Pp. 5-22 in Advances in Ceramic Armor VI: Ceramic Engineering and Sci- of silicon carbide and boron carbide ceramics processed by spark plasma ence Proceedings 31(5). Swab, J., S. Mathur, and T. Ohji, eds. Hoboken, sintering. International Journal of Impact Engineering 37(11): 1092-1099. N.J.: John Wiley & Sons.
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129 APPENDIX E form stable silicides38 may also serve to reduce the amount Si, forming SiC around the original ceramic particles and bonding them together—hence the term “reaction bonding.” of residual silicon. Typically the final product—that is, reaction-bonded boron carbide composite—consists of the original boron carbide FUNCTIONALLY GRADED MATERIALS particles, a newly formed ternary B–Si–C carbide, SiC, and some residual silicon. Because residual silicon adversely A functionally graded material (FGM) is a two-compo- affects the mechanical properties of the composite,26,27,28,29 nent composite system with a defined compositional gradient its amount, which is related to the free carbon present in across its section; the system is structured in such a way as compacted preform and also to the initial porosity,30 should to preserve the inherent properties of each component. In be decreased. The porosity of the preform at the outset may a metal/ceramic FGM structure, for example, the gradual be somewhat reduced by sintering31 or by using multimodal transition between an impact-resistant outer ceramic layer powder mixtures.32 Adding titanium or iron or compounds bonded to a tough metal backing can be advantageously ap- plied in armor protection design.39 FGMs such as titanium/ that react with the boron carbide and release more free carbon33,34,35,36,37 or adding elements that react with silicon to titanium boride composites have the potential to reduce or eliminate the need for thermal protection in extreme environ- ments such as those encountered by aerospace vehicles. They 26Hayun, S., D. Rittel, N. Frage, M.P. Dariel. 2010. Static and dynamic are also ideal for minimizing thermomechanical mismatch in mechanical properties of infiltrated B4C-Si composites. Materials Science metal/ceramic bonding.40,41,42,43 FGMs are also of interest for Engineering A 487(1-2): 405-409. 27Aghajanian, M., B. Morgan, J. Singh, J. Mears, and R. Wolffe. 2001. other defense-related applications, including as heat shields A new family of reaction bonded ceramics for armor applications. Pp. 527- for rockets, fusion energy devices, and thermal barrier coat- 555 in Ceramic Armor Materials by Design. Ceramic Transactions 134. J. ings for turbine blades. For armor protection applications, McCauley, J., and A. Crowson, eds. Baltimore, Md.: American Ceramic FGMs with bulk dimensions on the order of millimeters to Society. 28Chhillar P., M. Aghajanian, D. Marchant, R. Haber, and M. Sennett. centimeters are needed, but commercially viable processes 2009. The effect of Si content on the properties of B4C-SiC-Si composites. to make such structures are still in development. These Pp. 161-167 in Advances in Ceramic Armor III: Ceramic and Engineering processes are hindered by the high cost and specialized na- Science Proceedings 28(5). Franks, L., J. Salem, and D. Zhu, eds. Hoboken, ture—they often involve segregation approaches employing N.J.: John Wiley & Sons. sedimentation forming, slip casting, centrifugal casting, and 29Hayun, S., A. Weizmann, M. Dariel, and N. Frage. 2009. The effect of thixotropic casting.44,45,46 particle size distribution on the microstructure and the mechanical properties of boron carbide-based reaction-bonded composites. International Journal Experimental and theoretical work recently revealed of Applied Ceramic Technology 6(4): 492-500. that controlled gradients in mechanical properties can guide 30Hayun, S., N. Frage, M. Dariel, E. Zaretsky, and Y. Ashuah. 2006. the design of surfaces that are resistant to contact defor- Dynamic response of B4C-SiC ceramic composites. Ceramic Transactions mation and damage; such properties cannot be realized in 178: 147-156. 31Hayun, S., N. Frage, and M. Dariel. 2006. The morphology of ceramic conventional homogeneous materials. Wear-resistant, nano- phases in BxC-SiC-Si infiltrated composites. Journal of Solid State Chem - istry 179(9): 2875-2879. 32Hayun, S., A. Weizmann, M. Dariel, and N. Frage. 2009. The effect of 38Messner, R., and Y-M. Chiang. 2008. Processing of reaction-bonded particle size distribution on the microstructure and the mechanical properties silicon carbide without residual silicon phase. Pp. 1053-1059 in Proceed - of boron carbide-based reaction-bonded composites. International Journal ings of the 12th Annual Conference on Composites and Advanced Ceramic of Applied Ceramic Technology 6(4):492-500. Materials, Part 1 of 2: Ceramic Engineering and Science Proceedings 9(7/8). 33Hayun, S., N. Frage, H. Dilman, V. Tourbabin, and M. Dariel. 2006. J. Wachtman, ed. Hoboken, N.J.: John Wiley & Sons. 39Hirai, T. 1996. Functional gradient materials. Pp. 293-341 in Processing Synthesis of dense B4C-SiC-TIB2 composites. Ceramic Transactions 78: 37-44. of Ceramics, Part 2. Cahn, R., and R. Brook, eds. New York, N.Y.: VCH. 34Mizrahi, I., A. Raviv, H. Dilman, M. Aizenshtein, M. Dariel, and 40Ibid. 41Chin, E. 1999. Army focused research team on functionally graded N. Frage. 2007. The effect of Fe addition on processing and mechanical properties of reaction infiltrated boron carbide-based composites. Journal armor composites. Materials Science and Engineering A259(2): 155-161. 42Suresh, S. 2001. Graded materials for resistance to contact deformation of Materials Science 42(16): 6923-6928. 35Sigl, L., H. Thaler, and K-A. Schwetz. 1994. Elemental carbon-contain- and damage. Science 292(5526): 2447-2451. 43Gooch, W., B. Chen, M. Burkins, R. Palicka, J. Rubin, and R. Ravi - ing boron carbide-titanium diboride composites, and their manufacture and use. [Verbundwerkstoff auf Basis von Borcarbid, Titanborid und Elemen- chandran. 1999. Development and ballistic testing of a functionally gradient tarem Kihlenstoff sowie Verhahren zu ihrer Herstellung.] European Patent ceramic/metal appliqué. Materials Science Forum 308-311:614-621. 44Kleponis, D., A. Mihalcin, and G. Filbey, Jr. 2005. Material design para- 628525, filed June 11, 1993, and issued June 9, 1994, to Elektroschmelzwerk Kempten GmbH, Munich, Germany. digms for optimal functional gradient armors. Army Research Lab Weap - 36Sigl, L., H. Thaler, and K-A. Schwetz. 1996. Composite materials based ons and Materials Directorate. Available at http://handle.dtic.mil/100.2/ on boron carbide, titanium diboride and elemental carbon and processes ADA436346. Accessed April 4, 2011. 45Clougherty, E. 1974. Graded impact resistant structure of titanium for preparation of same. U.S. Patent 5,543,337, filed May 24, 1994, and issued August 6, 1996, to Elektroschmelzwerk Kemptem GmbH. Munich, diboride in titanium. U.S. Patent 3,802,850, filed November 13, 1972, and Germany. issued April 9, 1974, to Man-Labs, Incorporated, Cambridge, Mass. 37Sigl, L. 1998. Processing and mechanical properties of boron carbide 46Johnson, G., T. Holmquist, and S. Beissel. 2003. Response of aluminum sintered with TiC. Journal of the European Ceramic Society 18(1):1521- nitride (including a phase change) to large strains, high strain rates, and high 1529. pressures. Journal of Applied Physics 94(3), 1639-1647.
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130 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS crystalline surface coatings, with grain sizes as small as a few serves to toughen by many of the same mechanisms that oc- tens of nanometers, can be synthesized by means of electro- cur in natural nacre. The entire array is fastened onto the steel deposition, thermal spray, sputter deposition, metal-organic plates of tanks and similar vehicles using Velcro. The armor chemical vapor deposition, and eletrophoretic deposition.47 has been implemented onto various ground and air vehicles, Many of these processes can create surface layers wherein including 1,000 High Mobility Multipurpose Wheeled Ve- the grain sizes are smoothly graded from the surface to the hicles (Humvees) for the U.S. Marines. bulk, bringing about controlled gradients in strength and Another example of a natural material for biologically fracture toughness. Similarly, improved resistance to contact inspired design is the skins of certain fish. Both the scales and damage can be achieved by tailoring gradients in porosity dermis of fish skin are highly pliant, lightweight, and resist penetration—all of this in an ultrathin structure.51 Although below the contact surfaces. Current materials synthesis and processing capabilities, engineered gradations in proper- fish scales have received very little attention, it is recognized ties—from nanometer to macroscopic length scales—appear that bony scales are difficult to penetrate and dissipate en- ergy quite well.52 Scales form a physical barrier that deters promising for the design of improved fracture-, damage-, and wear-resistant structures and surfaces and for armor protec- attacks by predators; indeed, they were likely the inspiration tion applications. for scale armor not only in ancient times but also in modern times.53 While its hierarchical organization is important for the overall mechanical performance of fish skin,54 the contri- BIOMIMETIC MATERIALS butions at the different length scales are rarely investigated. Natural materials that are mechanically robust often It is not known, for instance, how adjacent scales interact to have hierarchical designs. Abalone nacre, rat teeth, fish thwart penetration, but this mechanism should be understood scales, wood, and spider silk exhibit highly complex hierar- if its performance in the new generation of ultralight pliant chical structures, multifunctionality, and even self-healing armor systems is to be replicated. capabilities48 and thus are appealing to mimic for use in Biomorphic ceramics using natural products such as advanced armor design. The remarkable mechanical perfor- wood and cellulose-fiber paper and cardboard have also been of interest for their potential use as armor materials.55 In mance of certain natural materials stems from their complex ordered microstructure, organized over several length scales, particular, wood-based biomorphic SiC (bioSiC) is a promis- even though the materials are often made of relatively weak ing material for armor. The fabrication of bioSiC entails the constituents. Nacre, which is found in a number of mollusk rapid mineralization of wood, during which the wood is car- shells, combines stiffness and strength along with a high bonized and then infiltrated with either Si vapor or Si melt. level of toughness. Mimicking of the abalone structure was The formed SiC replicates the wood microstructure, and the first attempted in the 1980s, when a laminated structure of diversity of the wood texture results in a large and varied Al–B4C was produced.49 A significant increase in fracture selection of bioSiC ceramics. Novel biomorphic SiC ceram- toughness (up to 16 MPa-m 1/2) was achieved; however, ics have been successfully developed at DLR, Germany’s national research center for aeronautics and space.56,57 At Al4C3 was formed during the processing of the laminates, limiting the useful armor application of the produced lami- DLR, wood-based preforms are converted to SiSiC materials nates owing to low hardness and strength and the high brittle- using the liquid silicon infiltration process. In this process, a ness of the Al4C3 phase. Another example of mimicking green body or preform based on low-cost raw materials—for abalone nacre, but on a macroscale, was conducted a decade ago by Foster-Miller50—LAST (Light Applique Segmented 51Vernerey, F., and F. Barthelat. 2010. On the mechanics of fishscale Tile) armor plates were produced using Coors alumina and structures. International Journal of Solids and Structures 47(17): 2268-2275. SiC or B4C hexagonal tiles covered in a thermoset laminate 52Bruet, B., J. Song, M. Boyce, and C. Ortiz. 2008. Materials design of Kevlar and held together with a Velcro-type adhesive. A principles of ancient fish armour. Nature Materials 7: 748-756. 53Vernerey, F., and F. Barthelat. 2010. On the mechanics of fishscale nacre-like armor structure such as this absorbs energy and structures. International Journal of Solids and Structures 47(17): 2268-2275. 54Fratzl, P., and R. Weinkamer. 2007. Nature’s hierarchical materials. 47Suresh, S. 2001. Graded materials for resistance to contact deformation Progress in Materials Science 52(177): 1263-1334. 55Medvedovski, E. 2010. Ballistic performance of armour ceramics: and damage. Science Magazine 292(5526):2447-2451. Available online at http://www.sciencemag.org/content/292/5526/2447.full. Accessed October, Influence of design and structure. Part 1. Ceramics International 36(7): 11, 2010. 21032115. 48Meyers, M., P-U. Chen, A. Lin, and Y. Seki. 2008. Biological materi - 56Heidenreich, B., M. Gahr, E. Strassburger, and E. Lutz. 2010. Biomor- als: Structure and mechanical properties. Progress in Materials Science phic SiSiC-materials for lightweight armour. Pp. 21-33 in Proceedings of 53(1): 1-206. 30th International Conference on Advanced Ceramics & Composites 2010. 49Sarikaya, M., and I.A. Aksay. 1992. Nacre of abalone shell: A natural Hoboken, N.J.: John Wiley & Sons. 57Heidenreich, B., M. Crippa, H. Voggenreiter, H. Gedon, M. Nordmann, multifunctional nanolaminated ceramic polymer composite material. Pp. 1-25 in Structure, Cellular Synthesis and Assembly of Biopolymers (Re- and E. Strassburger. 2010. Development of biomorphic SiSiC- and C/ sults and Problems in Cell Differentiation) 19(1). S. Case, ed. Amsterdam: SiSiC- materials for lightweight armour. Advances in Ceramics Armor VI: Springer-Verlag. Ceramic Engineering and Science Proceedings (31). Hoboken, N.J.: John 50QinetiQ. www.foster-miller.com. Accessed October 13, 2010. Wiley & Sons.
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131 APPENDIX E consists of b-SiC formed as a result of the interaction of example, wood fibers and phenolic resin—is manufactured by means of warm pressing. One preform used for manu- Si and C, a significant quantity of residual Si (up to 30 vol facturing biomorphic SiSiC is medium-density fiberboard, percent), and a very small amount of carbon (up to 3 vol which is widely used in the furniture industry. The preform percent). The typical structure of bioSiC is more homoge- is made by pressing fine fibers of needle wood with binders neous than conventional reaction-bonded SiC and is defined based on formaldehyde or phenolic resins in a mass produc- by the particular wooden preform. The typical size of SiC tion process, making very large panels—typically 1.22 m × grains is 5 to 20 μ, but owing to the relatively high content of residual Si, the density is about 2.8 g/cm3. BioSiC ceramics, 2.44 m (4 ft × 8 ft) up to 2.8 m × 6.5 m (9.2 ft × 21.3 ft)—at a cost of about $1.75/kg. After pyrolysis, the porous C-preform such as one manufactured using inexpensive preforms from medium-density fiberboard66 can be produced inexpensively is siliconized in a vacuum at temperatures above 1450°C; next, capillary forces allow molten silicon to infiltrate the for armor systems consisting of large, single-piece compo- open pores of the C-preform. The resulting reaction with nents. Biomorphic siliconized silicon carbide (SiSiC) has the carbon forms SiSiCx.58 The final composition—that is, demonstrated good potential for use in lightweight ceramic the content of SiC, Si, and C—is heavily influenced by the armor systems. Although manufacturing defects and exces- porosity and microstructure of the C-preform and can be sive residual silicon in bioSiC reduce ballistic performance, varied widely by using tailored green bodies. Because practi- especially in multi-hit situations, appropriate armor system cally no change in geometry occurs during siliconization or design—that is, with the right selection of ceramic thickness in reproducible contraction rates during pyrolysis, even large and type and backing thickness—allows the materials to and complicated shaped parts can be manufactured using a withstand even armor-piercing rounds. In order to increase cost-effective, near-net-shape technique. resistance to multiple hits, novel materials based on the The processing technology for bioSiC makes the pro- combination of biomorphic SiSiC and C/C-SiC ceramics duction of complex shapes relatively easy; production is have been developed, with carbon fibers integrated into bio- much cheaper, because the bioSiC forms at much lower morphic SiSiC to increase ductility and damage tolerance. temperatures than those required for SiC sintering or hot- There is special interest in replicating dragline silk, pressing techniques. Biomorphic SiC shows excellent ther- the extremely strong silk that forms the framing threads of momechanical performance along with structural stability spider webs. The comparative properties of selected silk and over a wide range of temperatures.59,60,61,62,63,64,65 BioSiC manmade fibers are presented in Table E-2. Mimicking the structure of materials found in nature might provide insight into the creation of armor materials 58Gahr, M., J. Schmidt, W. Krenkel, A. Hofenauer, and O. Treusch. 2004. with superior ballistic properties. However, the task of pro- Dense SiSiC ceramics derived from different wood-based composites: ducing manmade materials with similar microstructures and processing, microstructure and properties. P. 425 in Proceedings of the 5th performance is challenging precisely because the structures International Conference on High Temperature Ceramic Matrix Composites. Westerville, Ohio: The American Ceramic Society. are so complex. 59Martinez-Fernández, J., F. Valera-Feria, and M. Singh. 2000. High tem- perature compressive mechanical behavior of biomorphic silicon carbide MACHINING, GRINDING, AND POLISHING CERAMICS ceramics. Scripta Materialia 43(9): 813-818. 60Martínez-Fernández J., F. Valera-Feria, Rodríguez, A., and M. Singh. The machining, grinding, and polishing of ceramics are 2000. Microstructure and thermomechanical characterization of biomorphic silicon carbide-based ceramics. Pp. 733-740 in Environment Conscious expensive processes. For example, the final shape of the ar- Materials: Ecomaterials. 39th Annual Conference of Metallurgists. Ottawa, mor product from a flat, hot-pressed part is created by grind- Canada: Canadian Institute of Mining. ing with diamond wheels. The pressureless sintered process, 61Singh, M., J. Martínez-Fernández, A., and de Arellano-López. 2003. with its much larger shrinkage (20 to 30 percent), requires Environmentally conscious ceramics (ecoceramics) from natural wood pre - considerably more grinding to achieve final tolerances. cursors. Current Opinion in Solid State and Materials Science 7(3): 247-254. 62de Arellano-López, A, J. Martínez-Fernández, P. González, C. Domín - Therefore, grinding and finishing costs make the final cost guez, V. Fernández-Quero, and M. Singh. 2004. Biomorphic SiC: A new of hot-pressed parts higher than that of pressureless sintered engineering ceramic material. International Journal of Applied Ceramic parts. Part geometry and concentricity or parallelism also Technology 1(1): 56-67. affect the final cost. For example, improving the tolerance of 63Varela-Feria, F., J. Martínez-Fernández, A. de Arellano-López, and M. the outside diameter from 0.020 in. to 0.010 in. can double Singh. 2002. Low density biomorphic silicon carbide: Microstructure and mechanical properties. Journal of the European Ceramic Society 22(14-15): the cost of a piece. Typically, a ground part has a surface 2719-2725. finish tolerance of 16 μin. or better. A better finish of 4 μin. 64Varela-Feria, F., J. Ramírez-Rico, A. de Arellano-López, J. Martínez- can be obtained using lapping and honing, but will cost more. Fernández, and M. Singh. 2008. Reaction-formation mechanisms and microstructure evolution of biomorphic SiC. Journal of Materials Science 43(3): 933-941. 65Bautista, M., A. de Arellano-López, J. Martínez-Fernández, A. Bravo- 66Heidenreich, B., M. Gahr, and E. Medvedovski. 2005. Biomorphic reac- Léon, and J. López-Cepero. 2009. Optimization of the fabrication process tion bonded silicon carbide ceramics for armor applications. Pp. 45-53 in for medium density fiberboard (MDF)-based biomimetic SiC. International Proceedings of the 107th Annual Meeting of the American Ceramic Society. Journal of Refractory Metals and Hard Materials 27(2): 431-437. Hoboken, N.J.: Wiley-Blackwell.
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132 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS TABLE E-2 Tensile Mechanical Properties of Spider Silks and Other Materials Toughness (MJ-m–3) Material Stiffness (GPa) Strength (GPa) Strain to Failure Natural fibers Araneus major Ampullate (MA) silk 10 1.1 0.27 160 A. viscid silk 0.003 0.5 2.7 150 40-130a Nephila clavipes silk 11-13 0.88-0.97 0.17-0.18 208b N. edulis silk Bombyx mori cocoon silk 7 0.6 0.18 70 35-55c B. mori silk (w/sericin) 5-12 0.5 0.19 30-70d B. mori silk (w/o sericin) 15-17 0.61-0.69 0.4-0.16 Synthetic materials Nylon fiber 5 0.95 0.18 80 Kevlar 49 fiber 130 3.6 0.027 50 Carbon fiber 300 4 0.013 25 High-tensile steel 200 1.5 0.008 6 aGosline, J., M. DeMont, and M. Denny. 1986. The structure and properties of spider silk. Endeavour 10(1): 37-43; Zemlin, J. 1968. A study of the mechani - cal behavior of spider silks. Technical Report 69-29-CM (AD 684333). U.S. Army Natick Laboratory, Natick, Mass.; Cunniff, P., S. Fossey, M. Auerbach, J. Song, D. Kaplan, W. Adams, R. Eby, D. Mahoney, and D. Vezie. 1994. Mechanical and thermal properties of dragline silk from the spider Nephila clavipes. Polymers for Advanced Technologies 5(8): 401-410. bVollrath, F., B. Madsen, and Z. Shao. 2001. The effect of spinning conditions on the mechanics of a spider’s dragline silk. Proceedings of the Royal Society 268(1483): 2339-2346. cPérez-Rigueiro, J., C. Viney, J. Llorca, and M. Elices. 1998. Silkworm silk as an engineering material. Journal of Applied Polymer Science 70(12): 2439-2447. dPérez-Rigueiro, J., C. Viney, J., Llorca, and M. Elices. 2000. Mechanical properties of single-brin silkworm silk. Journal of Applied Polymer Science 75(10): 1270-1277. FOAMS thicknesses, using 20 mm fragment simulating projectiles (FSP). A test performed with an impact velocity of 1,067 m/s Foam is a complex assemblage of dispersed voids or on a baseline setup, followed by a test on the baseline with pores separated by a film. The reason for using foams to a 12.7-mm-thick piece of aluminum foam incorporated into absorb shock wave energy is that as the shock wave passes the material stack, revealed that the rise time of the stress into the foam, the individual cells collapse; it is through this wave increased from 1 ms for the baseline sample to 2 ms deformation that energy is absorbed. Foams can be made for the sample with foam. The use of foam also delayed the from any number of materials and may be open- or closed- time for the stress wave to reach the stress gauge by about cell, but it is the metal foams, particularly aluminum, and 14.6 ms. The maximum stress reached in both cases was the polymeric foams, particularly polyurethane, that are em - about 6.25 GPa. The air-filled cellular structure of the foam ployed most frequently in shock wave research.67,68,69 Two is not conducive to wave propagation because the waves are important directions for future research are (1) constructing only transmitted along the cell walls, which, owing to their foams from a wider variety of materials and (2) developing random orientations, tend to disperse the wave. For a foam methods for greater control over foam microstructure. Such thickness of 12.7 mm, a stress of 0.825 GPa was recorded in model foams will help computational efforts on porous the ceramic tile, while a thickness of 30.48 mm completely structures. eliminated the stress recorded in the tile. In this case, the The use of foams as impact barriers was demonstrated foam was not fully compacted by the FSP and thus acted as by Gama et al.,70 who performed impact tests on layered an excellent wave barrier. composite armor systems with various foam positions and The location of the foam is also important, demon- strating the need not only to consider the inherent material 67Hanssen, A., L. Enstock, and M. Langseth. 2002. Close-range blast properties in isolation but also to consider them as part of loading of aluminum foam panels. International Journal of Impact Engi - the overall armor system. neering 27(6): 593-618. 68Ramachandra, S., P. Sudheer Kumar, and U. Ramamurty. 2003. Impact energy absorption in an Al foam at low velocities. Scripta Materialia 49(8): TRANSPARENT CERAMICS AND EMBEDDED DAMAGE 741-745. SENSORS 69Gama, B., T. Bogetti, B. Fink, C. Yu, T. Dennis Claar, H. Eifert, and J. Gillespie. 2001. Aluminum foam integral armor: A new dimension in armor Transparent armor ceramics must provide good trans- design. Composite Structures 52(3-4): 381-395. parency in the visible (0.4-0.7 μ) and mid-infrared (1-5 μ) 70Ibid.
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133 APPENDIX E optical ranges and protection against fragmentation from have potential for use in new dome construction requiring ballistic impacts, with multi-hit capability and minimal dis- substantial durability and high transparency across the opera- tortion around the impacted regions.71,72 tional bandwidth for infrared-guided missile sensing.79,80,81 As mentioned in Chapter 5, transparent armor systems Special glasses and glass-ceramics such as lithium disilicate are typically constructed of multiple layers of armor-grade or aluminum-lithium-based crystallized or partially crystal- ceramic plates, separated by transparent polymer (for ex- lized structures also offer advantages as host materials for ample, polycarbonate) interlayers, and are bonded together laser use, since their refractive indexes and strain-optical with a transparent adhesive. The polymer phase mitigates coefficients can be readily controlled through changes in chemical composition.82 the stresses generated by thermal expansion mismatches and inhibits crack propagation from ceramic to polymer. Poly- meric materials such as transparent nylons, polyurethanes, Damage-Reactive Sensors for Armor and acrylics have also been explored as separators, but they have not been widely used in armor protection owing to less- Combat vehicles could be outfitted with smart ceramic than-optimal optical and durability characteristics. Transpar- sensors built into the protective armor material. Such sen- ent alumina (Al2O3) and magnesia (MgO) are two commonly sor could detect and report on structural damage in real used transparent ceramic armor materials. The composite time. Structural damage caused by a wide range of ballistic system formed by these two materials provides good protec- impacts can be expected to affect armor structures under tion against high-velocity ballistic projectiles.73,74 Silicon battlefield conditions. Changes in the armor’s structural nitride (Si3N4), a nonoxide ceramic, has also been employed condition can be detected by tiny piezoelectric transducers, for use in radomes because of its good transit of microwave or sensors, built into the protective armor plate material. energy and its superior mechanical strength. Piezoelectric sensors are usually ferroelectric, perovskite These materials can be produced as transparent poly- structure materials—for example, lead zirconate/lead tita- crystalline ceramic parts, often with complex geometries, by nate, barium titanate, and others—that have been suitably using standard ceramic-forming techniques such as pressing, doped and electrically poled to optimize their piezoelectric (hot) isostatic pressing, and slip casting.75 response characteristics.83,84 Nanocomposite ceramic materials of yttria (Y2O3) and Given the above relationships, the piezoelectric trans- magnesia (MgO) have been explored for use in transparent ducers can be designed both to generate and to receive volt- armor protection. The materials exhibit an average grain size age responses when coupled with ultrasonic waves that are of approximately 200 nm, and near-theoretical transmission generated to pass through the material. To determine the best in the 3 to 5 μ infrared band range. These complex ceramic response characteristics for particular environmental condi- n anocomposites reportedly offer improved mechanical tions, the ultrasonic signals may vary over a wide frequency properties such as superplastic flow and metal-like machin- range (1.0 kHz to 200 kHz). A generated shock wave through ability. However, mechanical failure modes and armor pro- the plate picks up the reflections of sound waves and con- tection characteristics must still be fully evaluated for these verts them into electrical voltages from which, with suitable nanocomposite materials systems.76,77,78 These materials amplification, one can determine their spectra and whether the plate is cracked or damaged. 71Patel, P., G. Gilde, P. Dehmer, and J. McCauley. 2000. Transparent ceramics for armor and EM window applications. P. 1 in Proceedings of the International Society for Optics and Photonics 4102(1). 72Harris, D. 2009. Materials for infrared windows and domes: Proper- 79Huang, Z., X. Sun, Z. Xiu, S. Chen, and C-T. Tsai. 2004. Precipitation ties and performance. Bellingham, Wash.: International Society of Optical synthesis and sintering of yttria nanopowders. Materials Letters 58(15): Engineers. 2137-2142. 73Villalobos, G., J. Sanghera, and I. Aggarwal. 2005. Transparent ce - 80Jeong, J., S. Park, D. Moon, and W. Kim. 2010. Synthesis of Y O nano- 23 ramics: Magnesium aluminate spinel. Naval Research Laboratory Optical powders by precipitation method using various precipitants and preparation Sciences Division. of high stability dispersion for backlight unit (BLU). Journal of Industrial 74Hogan, P., R. Stefanik, C. Willingham, and R. Gentilman. 2004. Engineering Chemistry 16(2): 243-250. 81Nihara, K., and T. Sekino. 1993. New nanocomposite structural ceram - Transparent yttria for IR windows and domes—Past and present. Raytheon Integrated Defense Systems. ics. P. 405 in Materials Research Society Symposium Proceedings held at 75Ibid. the Nanophase and Nanocomposite Materials Symposium. Warrendale, Pa.: 76Bisson, J-F., Lu Jianren, K. Takaichi, Yan Feng, M. Tokurakawa, A. Materials Research Society. 82Hartmann, P., R. Jedamzik, S. Reichel, and B. Schreder. 2010. Optical Shirakawa, A. Kaminskii, H. Yagi, T. Yanagitani, and K-I. Ueda. 2004. Nanotechnology is stirring up solid-state laser fabrication technology. glass and glass ceramic historical aspects and recent developments: A Schott Recent Research Developments in Applied Physics 7(Part II): 475-469. view. Applied Optics 49(16): D157-D176. 77Wen, L., X. Sun, S. Chen, and C-I. Tsai. 2003. Synthesis of nanocrystal- 83Meitzler, T., G. Smith, M. Charbeneau, E. Sohn, M. Bienkowski, I. line yttria powder and fabrication of transparent YAG ceramics. Journal of Wong, and A. Meitzler. 2008. Crack detection in armor plates using ultra- the European Ceramics Society 24(9): 2681-2688. sonic techniques. Materials Evaluation 66(6): 555-559. 78Wen, L., X. Sun, Q. Lu, G. Xu, and X. Hu. 2006. Synthesis of yttria 84Song, J., and G. Washington. 2000. Plate vibration modes identification nanopowders for transparent yttria ceramics. Optical Materials 29(2-3): by using piezoelectric sensors. Pp. 867-878 in Smart Structures and Materi - 239-245. als 2000. International Society for Optics and Photonics 3985.
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134 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS Armor Damage Control Sensors mor designs to take advantage of the new ability to control mechanical waves in armor materials. The piezoelectric transducers can perform other func- Because there are three polarizations of elastic waves tions that contribute to armor and vehicle survivability. For in solid materials—longitudinal, transverse (shear) in plane, example, they can be made to act as antennas; to monitor and transverse out of plane—for a structure to possess a the temperature of armor structures, including that of body full band gap, it must prohibit the propagation of all types armor, and to detect and monitor projectile impacts on the of waves in all directions. A phononic crystal can create an armor surface. For the latter application, each projectile acoustic band gap through a combination of Bragg diffrac- striking the armor will create an electrically generated shock tion (destructive wave interference) and Mie resonances86 wave and differing amounts of electricity; a smart sensor as well as anticrossing of bands having the same mode sym- can integrate these effects to generate useful information. metry.87 Bragg scattering occurs when the wavelength of Complex mathematical algorithms can be used to analyze the phonon is approximately equal to the periodicity of the the amount of electricity generated by a bullet’s impact to structure, and Mie resonances occur when the diameter of determine what kind of round was used, since a small-caliber the scattering features is of the same order as the wavelength. projectile will generate less electricity than a large-caliber Some of the earliest references to phononic crystals are from projectile.85 This combination of detection and assessment Sigalas and Economou88,89 and Kushwaha et al.90,91 A rather of threat level in real time could be significant for develop- complete library of phononic crystal research may be found ing armor survival strategies. The following conclusions by consulting Vlasov and Dowling.92,93 may be drawn: The length scale of phononic crystals ranges from the macroscopic—meters for acoustic waves (kHz) to millime- 1. Infrared-transparent nanocomposite materials in the ters for the ultrasound typically used in medical imaging systems SiC/Al2O3, SiC/Si3N4, SiC/MgO, Al2O3/ (MHz), and down to the nanoregime—approximately100 ZrO2, and transparent Al2O3 offer greatly enhanced nm for waves in the gigahertz (GHz) regime. In general, ballistic needs for transmission in the 3 to 5 μ range, design rules for creating a gap are based on Bragg scattering significantly increased mechanical strength of (700 and the use of highly symmetric structures to minimize the MPa), and fracture toughness and creep resistance irreducible Brillouin zone over which the gap occurs. The of interest for next-generation armor use. materials parameters that are important are density, elastic 2. Piezoelectric transducers coupled with acoustic modulus, and Poisson ratio or, equivalently, density and the wave propagation and complex mathematical al- transverse and longitudinal speed of sound in the materials. gorithms can be used to analyze impact damage to Because the waves are scattered from interfaces that affect armor structures from ballistic projectiles. wave propagation implies that understanding the differences in mechanical impedance of the materials comprising the PHONONIC BAND GAP CONCEPTS FOR PROTECTIVE structure (impedance is the product of density and wave MATERIALS speed) is key to understanding how phononic crystals can control the propagation of mechanical waves. However, in Shortly after research on photonic band gaps began to show promise of controlling the flow of photons, the idea was extended to mechanical waves in periodic elastic struc- 86Mie theory, also called Lorenz-Mie theory, Lorenz-Mie-Debye theory, tures. Referred to as phononic crystals, such structures can and Mie scattering, is an analytical solution of Maxwell’s equations for the create what are called phononic, or acoustic, band gaps. A scattering of electromagnetic radiation by spherical particles. 87Kushwaha, M., A. Akjouj, B. Djafari-Rouhani, L. Dobrzynski, and J. phononic crystal prevents the propagation of elastic waves Vasseur. 1998. Acoustic spectral gaps and discrete transmission in slender if the frequencies of the waves fall within a band gap. The tubes. Solid State Communications 106(10): 659-663. normalized width of the band gap—the ratio of band-gap 88Sigalas, M., and E. Economou. 1992. Elastic and acoustic wave band width to the central frequency of the gap—is a measure of structure. Journal of Sound and Vibration 158(2): 377-382. 89Sigalas, M., and E. Economou. 1993. Band structure of elastic waves the performance of the particular phononic crystal design. In in two dimensional systems. Solid State Communications 86(3): 141-143. addition to preventing the propagation of waves, phononic 90Kushwaha, M., P. Halevi, L. Dobrzynski, and B. Djafari-Rouhani. 1993. crystals dictate the nature of the modes that are allowed to Acoustic band structure of periodic elastic composites. Physical Review propagate in the material; they can decrease the velocity Letters 71(13): 2022-2025. of the waves and even force their negative refraction. This 91Kushwaha, M., P. Halevi, G. Martinez, L. Dobrzynski, and B. Djafari- capability suggests numerous ideas that could someday be Rouhani. 1994. Theory of acoustic band structure of periodic elastic com - posites. Physical Review B 49(4): 2313-2322. developed to influence material fabrication and enhance ar- 92Yaslov, Y. Photonic band gap links. Available at http://www.pbglink. com. 85Sands, J., C. Fountzoulas, G. Gilde, and P. Patel. 2009. Modelling 93Dowling, J. 2008. Photonic and sonic band-gap bibliography. Avail - transparent ceramics to improve military armour. Special Issue on Transpar- able at http://phys.lsu.edu/~jdowling/pbgbib.html. Accessed on November ent Ceramics, Journal of the European Ceramics Society 29(2): 261-266. 6, 2010.
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135 APPENDIX E propagation of elastic waves is promising since in addition solids, in addition to simple reflection at an interface, polar- to creating a band gap, there is the possibility of creating a ization conversion always occurs; making it difficult to form set of band gaps that would significantly block multiple fre- complete gaps, and a design strategy for optimal constructs quencies. Moreover, phononic crystals permit the tailoring of is not yet available. The use of fluids (which support only the allowed modes and their wave speeds inside the material, longitudinal waves) makes it difficult to form gaps because such that the frequencies of various material loss regimes of the conversion of shear modes into longitudinal modes at may be matched with the density of states and frequencies the solid-fluid interface. of the allowed modes to provide enhanced energy absorption. The idea of using a structured material to influence the