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5 Lightweight Protective Materials: Ceramics, Polymers, and Metals OVERVIEW AND INTRODUCTION The history of improving protection while reducing the weight of armor has been a remarkable materials success sto- ry. Over the last half-century, new choices of materials such as ceramics, polymers, and polymer fibers and lower density metals have significantly decreased the weight of the armor needed for the protection of personnel and vehicles. Figure 1-2 in Chapter 1 illustrates the revolutionary reductions in the areal density of vehicle armor as advanced materials have become available, starting with rolled homogeneous armor and advancing to complex composite systems. There have been similar advances in lightweight materials for personnel protection as well. As described in Chapter 2, armor systems are designed and fabricated using suitable combinations of ceramics, metals, polymers, fibers, and composites to meet specific threat requirements. The choice of materials, as well as their geometry and the means by which they are assembled, is a key factor in armor design. Each material FIGURE 5-1 Schematic presentation of the cross section of an component serves a specific purpose not only in defeating armor tile typically used for armored vehicles showing the complex- the kinetic energy of projectiles or mitigating a blast but also ity of the armor architecture. Different classes of materials, such as in maintaining the structural armor’s integrity. dense and porous ceramics, fiber composites, thermoplastic poly - To provide a basic understanding of current armor mate- mers, and adhesives are used for the tile assembly. DEA, diethanol - rials and to anticipate areas where there could be revolution- amine. SOURCE: James W. McCauley, Chief Scientist, Weapons and Materials Research Directorate, Army Research Laboratory ary improvements in armor materials, this chapter examines (ARL) fellow, ARL, “Armor Materials 101-501: Focus on Funda- the synthesis and processing of each of the main types of mental Issues Associated with Armor Ceramics ‘Kinetic energy materials, with particular emphasis on the resultant material passive armor,’” presentation to the committee on March 9, 2010. structure from the atomic to the macro scale. Potential new compositions and the tailoring of microstructures to discover material behaviors that could dramatically enhance armor bers, environmental coatings, polymer binders, and adhesive performance are highlighted, as are the challenges involved joints. The complex tile architecture presented in Figure 5-1 in achieving such advances. uses several materials and different assembly methods for The schematic in Figure 5-1 depicts a notional armor those materials such that the layers perform their protective structure,1 consisting of both dense and porous ceramics, fi- functions during the projectile impact. This chapter will ex- amine how achieving improved material behavior but also 1James W. McCauley, Weapons and Materials Research Directorate, minimizing manufacturing cost requires a deep scientific Army Research Laboratory (ARL) fellow, ARL, “Armor materials 101-501: and engineering understanding of the desirable structures Focus on fundamental issues associated with armor ceramics ‘kinetic energy and compositions of advanced protective materials as well passive armor,’” presentation to the committee, March 9, 2010. 69

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70 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS as how to make and process them. That said, as explained it is almost mandatory for the candidate material to also in Chapter 3, the requisite material properties that are to be possess a residual plastic behavior greater than the HEL, optimized cannot be measured by the usual quasi-static mea- because the greatest velocity threats typically induce stresses sures of mechanical behavior. However, even at lower strain that are higher than the HEL of materials that are commonly rates, conducting mechanical tests at small scale—that is, at available. Properties such as hardness and modulus are the microstructural level, on the order of nanometers or mi- determined by the chemical and phase compositions and crons—will likely shed light on the deformation mechanisms microstructure of the material. Besides composition, many under known loading states and can provide information that ceramic material properties can be influenced by the relative is very useful for parallel modeling efforts, keeping in mind amounts of the various possible phases/polytypes, average that the ultimate goal is real-time measurements of many grain size, grain-size distribution, and grain morphologies, properties on ballistic timescales. as well as minor-phase content. As shown in Chapter 4, the behavior of an assembly in One of the most important aspects of ceramic materials the face of a particular threat is not the simple sum of the that makes them suitable for ballistic protection is the strong behaviors of its component parts. Thus, an integrated experi- covalent bonding between lightweight atoms located in the mental and modeling approach that allows clear variation of first quarter of the periodic table of elements. The elements crystal and material microstructures and subsequent high- include beryllium, boron, carbon, oxygen, magnesium, rate dynamic characterization of the material behavior by aluminum, and silicon. Indeed, the most developed and best itself and as part of an armor system may enable the develop- explored armor ceramics are Al2O3 (aluminum oxide, or ment of ever lighter and more effective protection materials. alumina), B4C (boron carbide), and SiC (silicon carbide). A more rapid development of materials and their suc- However, these three materials are but a small portion of cessful insertion into armor necessitates attention to such the ceramics that could be used for armor application. For basic issues as the reduction of voids and impurities along example, novel boron icosahedra containing higher borides, with attention to the challenges of advanced designs and ternary B–C–Si and B–C–N systems, and homologous creating and synthesizing new material compositions, new Al(Mg)–B–C(N) compounds have yet to be explored. phases, and preferred microstructures. This chapter discusses Because ceramics are relatively brittle materials, they the main issues surrounding several important classes of pro- are sensitive to flaws, and flaws adversely affect materials tection materials. The accompanying set of appendixes goes performance. If flaws are prevalent, it is often difficult or into considerable detail—especially on the synthesis and almost impossible to assess the intrinsic properties and be- processing of ceramics, cermets, and polymers—because haviors of materials. Thus, it is critical to be able to process these classes of materials have the best potential for signifi- ceramics to near-theoretical maximum density, eliminating cant improvements if the interrelationships can be elucidated most of the void-type defects in order to explore the fun- between synthesis, processing methods, and the resultant damental behavior. Such defects are often responsible for structures, along with the corresponding high-rate measure- ceramic armor failure from the shock wave of a ballistic ment of material behavior. For the reader to appreciate the impact, which causes cracks to nucleate at the defect sites issues, the selected materials are introduced at the atomic, and then grow and coalesce, causing massive failure. As noted by Lankford,3 the ceramic would never fail (in penetra- molecular, micro, and macro scales before describing the synthesis and processing methods. Finally, areas of potential tion) if it could be constrained such that it would undergo innovation that may bring transformational changes in the plastic flow. Of course the presence of defects will keep the design and performance of armor materials are described, ceramic from reaching the stress levels necessary to activate along with the challenges to be overcome. plasticity mechanisms, and simple, practical improvement in performance can be realized by employing nondestructive evaluation analysis to reveal the larger defects in the mate- CERAMIC ARMOR MATERIALS rial. Better compaction technology and sintering techniques High-temperature refractory ceramic materials offer should result in a more uniform and higher density com- a unique combination of physical and mechanical prop- ponent. Upgrades in powder quality (purity, uniformity of erties that in turn can offer favorable protection against particles) and improvements in the formulation of sintering high-velocity armor-piercing bullets (see Chapter 2). Ce- aids can also help eliminate voids and porosity and retain ramics feature high hardness, high elastic modulus, low homogeneous microstructure. Highly nonuniform grain-size density, sufficient flexure, and good compressive strengths, distributions and the presence of grain boundary phases due but relatively low fracture toughness. The Hugoniot elastic to poor compositional quality of the starting powders can limit (HEL)—the maximum uniaxial dynamic stress that a also adversely affect performance. Agglomerated particles material can withstand elastically—represents the nominal potential of a ceramic as an armor-grade material.2 However, 2Fanchini, 3Lankford G., J.W. McCauley, and M. Chhowalla. 2006. Behavior of Jr., J. 2004. The role of dynamic material properties in the disordered boron carbide under stress. Physical Review Letters 97(6): performance of ceramic armor. International Journal of Applied Ceramic Article number 035502. Technology 1(3): 205-210.

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71 LIGHTWEIGHT PROTECTIVE MATERIALS: CERAMICS, POLYMERS, AND METALS due to poor mixing of sintering aids in the powders4,5 and Alumina nanoceramics that can reach the theoretical extraneous carbon additions or poor mixing of the carbon maximum density present an opportunity to probe the effects reduce the grain growth of nearby SiC grains and leave large of microstructure on material behavior in a cost-effective carbon inclusions inside the fine SiC matrix.6 material. While B4C and SiC ceramics require temperatures Early in the Vietnam conflict, the Department of Defense of 2150°C to 2200°C and, typically, applied pressure to carry (DoD) approached the Los Alamos National Laboratory and out sintering to achieve to full density, alumina can be eas- the Lawrence Livermore National Laboratory with a request ily sintered into complex shapes to full density at 1500°C to for lightweight body armor for ground troops. John Taylor at 1600°C by pressureless sintering. Indeed, Al2O3 nanopow- Los Alamos and Mark Wilkins at Lawrence Livermore began ders can be sintered at 1100°C to 1200°C to full density while retaining their nanograin microstructure.10,11 Krell’s investigating ceramics for protection against small arms fire. Coors Ceramics was asked to fabricate an alumina molded work on Al2O3 indicated a Hall-Petch relationship, whereby decreasing the grain size yielded an increase in hardness.12 body panel, but ground troops in the jungles of Vietnam Chen et al.13 suggested the importance of effective plasticity found it too heavy and would only wear the armor on guard duty at a fixed post. Later, Wilkins et al.7 demonstrated a rela- on the ballistic behavior of alumina. tionship between hardness, compressive strength, and ballis- Of the other ceramics named above, SiC and B4C are the tic performance and showed that bulk properties alone were leading opaque ceramic materials for next-generation body not a sufficient basis for the design of armor. They argued and vehicle armor systems. Their favorable characteristics that some trade-off between the various properties would relative to alumina (Al2O3) are lighter weight, higher hard- be necessary to derive benefits from other key properties ness, and higher stiffness. such as fracture toughness and plasticity.8,9 Their early work A central tenet of materials science and engineering is eliminated most silicate-based ceramics from consideration that composition, crystal structure, and microstructure influ- owing to their low hardness and the fact that mullite and other ence the mechanical behavior of the material. According to McCauley,14 alumina ceramics containing silicate seemed to fail under lesser ballistic attack than did high-purity alumina. Wilkins . . . the fundamental factors that affect the intrinsic material et al. further focused their research on other oxides such as characteristics [are] related to crystal physics, i.e., elastic aluminum magnesium spinel; carbides such as silicon and properties and anisotropy, phase transformation, and de- boron carbides; borides such as titanium diboride; and a formation mechanisms along with the development of new few nitrides, including aluminum nitride. Alumina emerged materials and transformational processing methods [that as today’s most widely used ceramic armor, combining can] yield large 25-40 percent improvements in ceramic good mechanical behavior with relatively low cost. Because performance. alumina is manufactured in quantities of millions of pounds throughout the world, it is much less expensive than either A recent case in point is the great improvement in the SiC or, especially, B4C. The densities of B4C (2.52 g/cm3) mechanical performance of B4C-SiC layered particulate and SiC (3.29 g/cm3) are considerably less than that of Al2O3 ceramics achieved by Orlovskaya et al. by introducing high (3.98 g/cm3). However, because of its easy sinterability and compressive thermal residual stresses to their outer surface the lower cost of the raw powders, alumina is still preferred layer.15 for use in vehicle applications, where the extra weight can be tolerated, while the lighter B4C and SiC ceramics are now used in body armor. 10Krell, A. 1996. The influence of shaping method on the grain size dependence of strength in dense submicrometre alumina. Journal of the 4Bakas, M., V.A. Greenhut, D.E. Niesz, J. Adams, and J. McCauley. European Ceramic Society 16(11): 1189-1200. 11Bakas, M., V.A. Greenhut, D.E. Niesz, J. Adams, and J. McCauley. 2003. Relationship between defects and dynamic failure in silicon carbide. Ceramic Engineering and Science Proceedings 24(3): 351-358. 2003. Relationship between defects and dynamic failure in silicon carbide. 5Bakas, M., V.A. Greenhut, D.E. Niesz, J. Adams, and J. McCauley. 2008. Ceramic Engineering and Science Proceedings 24(3): 351-358. 12Krell, A., P. Blank, H.W. Ma, T. Hutzler, and M. Nebelung. Processing Relationship between defects and dynamic failure in silicon carbide. Chap- ter 52 in 27th Annual Cocoa Beach Conference on Advanced Ceramics and of high-density submicrometer Al2O3 for new applications. Journal of the Composites: A: Ceramic Engineering and Science Proceedings, Volume 24, American Ceramic Society 86(4): 546-553. 13Chen, M.W., J.W. McCauley, D.P. Dandekar, and N.K. Bourne. 2006. Issue 3. W.M. Kriven and H.-T. Lin, eds. Hoboken, N.J.: John Wiley & Sons. 6Raczka, M., G. Gorny, L. Stobierski, and K. Rozniatowski. 2001. Effect Dynamic plasticity and failure of high-purity alumina under shock loading. of carbon content on the microstructure and properties of silicon carbide- Nature Materials 5(8): 614-618. 14James W. McCauley, Weapons and Materials Research Directorate, based sinters. Materials Characterization 46(2-3): 245-249. 7Wilkins, M.L., C.F. Cline and C.A. Honodel. 1969. Light Armor, Army Research Laboratory (ARL) fellow, ARL, “Armor materials 101-501: UCRL-71817, July 23. Livermore, Calif.: Lawrence Radiation Laboratory, Focus on fundamental issues associated with armor ceramics ‘kinetic energy University of California. passive armor,’” presentation to the committee, March 9, 2010. 8Ibid. 15Orlovskaya, N., M. Lugovy, V. Subbotin, O. Radchenko, J. Adams, M. 9Wilkins, M.L., R.L. Landingham, and C.A. Honodel. 1971. Fifth Prog - Chheda, J. Shih, J. Sankar, and S. Yarmolenko. 2005. Robust design and ress Report of Light Armor Program, UCRL-50980, January. Livermore, manufacturing of ceramic laminates with controlled thermal residual stress - Calif.: Lawrence Radiation Laboratory, University of California. es for enhanced toughness. Journal of Materials Science 40(20): 5483-5490.

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72 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS FIGURE 5-2 Rhombohedral unit cell structure of B4C showing B11C icosahedra and the diagonal chain of C–B–C atoms. Boron atoms are represented as red spheres and carbon atoms as white spheres. Crystalline Ceramics: Phase Behavior, Grain Size or percent C. None of the unit cells of the interstitial compounds Morphology, and Grain Boundary Phases can be defined precisely. Instead, the materials are made up of composition-dependent, statistically distributed, and Chemical composition, crystalline structure, and stabil- nearly isomorphous elementary cells, whose commonality is ity under elevated temperatures and under stress play an the 12-atom slightly distorted icosahedra at each cell vertex important role in determining both the quasi-static properties and the mostly 3-atom linear chains on the main diagonal of these materials and their dynamic deformation and failure parallel to the crystallographic c-axis. The unit cells thus behavior. An examination of B4C and SiC will give readers a comprise B12 and B11C icosahedra, while the chains com- sense of the complexity of the atomic bonding and crystalline prise C–B–C, C–B–B, or B– –B (the symbol – indicates an unit cells in these simple binary ceramics and will introduce atom vacancy) since the similarly sized C and B atoms read- them to intrinsic crystal defects such as stacking faults, ily substitute for each other. The general structure formula twins, and grain boundaries, which they need to know about is (B12)n(B11C)1-n(CBC)p(CBB)q(B– –B)1-p-q.17 The second to understand some aspects of the ballistic performance of constituent—for example, C, Al, or O—occupies sites on these two important protection materials. the diagonal chain (see the unit cell shown in Figure 5-2).18 For the approximately stoichiometric B4C material, the ico- Crystal Structure of Boron Carbide sahedra are B11C and the chains are C–B–C. Boron carbide (13.3 mol percent C) melts congruently at 2490°C and forms Because it is not possible to precisely control the stoi- a eutectic mixture with carbon at 2375°C–2400°C at a com- chiometry of boron carbide in commercially synthesized position of 29 mol percent C (see the B–C phase diagram, powders, it is important to understand how composition Figure 5-3).19 The extremely rigid framework arises from influences the atomic structure and the corresponding micro- the covalently bonded icosahedra and the chain units of co- structure and properties. Boron carbide can be considered as a prototype of the interstitial compounds of rhombohedral boron, which include B12C, B12C2Al, B12S, B12O2, B12As2, B12P2, B3Si, and B4Si. Interestingly, the stoichiometric compound B4C does not exist, and the denomination “boron carbide” refers to the whole homogeneity range extending 17Werheit, H., H.W. Rotter, S. Shalamberidze, A. Leithe-Jasper, and from B4.3C at the carbon-rich limit to B~11C at the boron-rich T. Tanaka. 2010. Gap-state related photoluminescence in boron carbide. limit,16 a range of 8.8 mol percent to approximately 20 mol Physica Status Solidi B 1-5. Available online at http://onlinelibrary.wiley. com/doi/10.1002/pssb.201046342/pdf. Last accessed March 31, 2011. 16Kuck, 18Emin, D. 1988. Structure and single-phase regime of boron carbides. S., and H. Werheit. 2000. Boron Compounds. Pp. 1-491 in Non- Tetrahedrally Bonded Binary Compounds II, Landolt-Börnstein: Numerical Physical Review B 38(9): 6041-6055. 19Thevenot, F. 1990. Boron carbide: A comprehensive review. Journal of Data and Functional Relationships in Science and Technology, New Series, subvolume 41. D. O. Madelung, ed. New York, N.Y.: Springer. the European Ceramic Society 6(4): 205-225.

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73 LIGHTWEIGHT PROTECTIVE MATERIALS: CERAMICS, POLYMERS, AND METALS pound.24,25 Moreover, the details of the phase boundaries and relative amounts of the polytypes have not yet been firmly established. Boron Carbide Amorphization The maximum contact pressure generated by a projectile incident on a ceramic depends on the velocity, bulk modulus, density, and yield strength of the projectile.26 The impact can also lead to a rapid increase in the local temperature. When the pressure and/or the temperature exceeds a critical thresh- old, amorphization (the transition from the crystalline phase to the amorphous phase) or other phase transformations (crystal A to crystal B) can occur in certain materials. Bo- ron carbide possesses the highest HEL of ceramic materials (~17-20 GPa), surpassing all of its denser competitors such as silicon carbide and alumina by a factor of 2.27,28,29 High FIGURE 5-3 The boron-carbon phase diagram over the range HEL would suggest that boron carbide could outperform 0-36 at % carbon. The cross-hatched region is commonly referred other armor materials. However, when the impact pressures to as “B4C.” Different phase diagrams for the B–C system were reported in the past and there is no currently agreed upon reference exceed 20 GPa, an abrupt drop in shear strength occurs, phase diagram that can be reliably used to determine the correct leading to a much lower dynamic performance for B4C than stoichiometry and equilibrium phases. that expected from its hardness and HEL.30,31 The loss in performance of B4C under high-velocity impact is currently believed to be related to the formation of amorphous bands valently bonded atoms and is responsible for the material’s inside the crystalline grains and a related weakening of the refractory nature and extreme hardness.20 bonds. These amorphous bands were discovered using high- The average structure, measured by x-ray diffraction resolution transmission electron microscopy (TEM) to ana- pattern or by nuclear magnetic resonance, varies as the boron lyze fragments of B4C ballistic tiles that had previously been content is varied.21 The theoretical density increases linearly subjected to supercritical impact velocities and pressures (in with increasing carbon content, extending from 2.465 g/ excess of 20-23 GPa). TEM images revealed the formation cm3 for B10.4C to 2.52 g/cm3 for B4C. Kwei et al.22 showed of 2-3-nm-wide intragranular amorphous bands that occur theoretically that the central boron atom in the C–B–C chain parallel to specific crystallographic planes and contiguously is relatively loosely held and that these locations can form with apparent cleaved fracture surfaces (see Figure 5-4). vacancies along the three-atom chain, leading to a decrease At subcritical impact velocities, the amorphous bands were in thermal conductivity.23 Aselage et al. found a significant never observed; instead, a relatively high concentration of drop in elastic modulus when the carbon concentration fell below 13.3 percent, reflecting a change in stiffness of the most compressible structural unit, the icosahedra (when B11C →B12). Very little is known about (1) the relative ratio of B12, 24Emin, D. 1988. Structure and single-phase regime of boron carbides. B11C, and C–B–C, C–B–B, B– –B structural units in boron Physical Review B 38(9): 6041-6055. 25Thevenot, F. 1990. Boron carbide: A comprehensive review. Journal of carbide or (2) the rates of growth of the different crystal the European Ceramic Society 6(4): 205-225. structures and their mutual transformations in the solid state 26Lundberg, P., R. Renstrom, and L. Westerling. 2002. Transition between as a function of pressure, temperature, and time. interface defeat and penetration for a given combination of projectile and The current working-phase diagram (Figure 5-3) for ceramic material. Ceramic Transactions 134: 173-181. boron carbide shows that it is not a so-called line com- 27Bourne, N.K. 2002. Shock–induced brittle failure of boron carbide. Proceedings of the Royal Society A: Mathematical, Physical & Engineering Sciences 458(2024): 1999-2006. 20Schwetz, K.A. 1999. Boron carbide, boron nitride, and metal boride. In 28Johnson, G.R., and T.J. Holmquist. 1999. Response of boron carbide Ullmann’s Encyclopedia of Industrial Chemistry, Sixth Edition (electronic subjected to large strains, high strain rates, and high pressures. Journal of release). T. Kellersohn, ed. Weinheim, Germany: Wiley-VCH Verlag. Applied Physics 85(12): 8060-8073. 21Werheit, H., H.W. Rotter, S. Shalamberidze, A. Leithe-Jasper, and 29Thevenot, F. 1990. Boron carbide: A comprehensive review. Journal of T. Tanaka. 2010. Gap-state related photoluminescence in boron carbide. the European Ceramic Society 6(4): 205-225. 30Bourne, N.K. 2002. Shock–induced brittle failure of boron carbide. Physica Status Solidi B 1-5. Available online at http://onlinelibrary.wiley. com/doi/10.1002/pssb.201046342/pdf. Last accessed March 31, 2011. Proceedings of the Royal Society A: Mathematical, Physical & Engineering 22Kwei, G.H., and B. Morosin. 1996. Structures of the boron-rich boron Sciences 458(2024): 1999-2006. 31Dandekar, D.P. 2001. Shock Response of Boron Carbide, ARL- carbides from neutron powder diffraction: Implications for the nature of the inter-icosahedral chains. Journal of Physical Chemistry 100(19): 8031-8039. TR-2456. Available online at http://www.arl.army.mil/arlreports/2001/ 23Ibid. ARL-TR-2456.pdf. Last accessed April 7, 2011.

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74 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS FIGURE 5-4 A boron carbide ballistic target that comminuted during impact (left) and a high-resolution TEM image of a fragment produced by a ballistic test at an impact pressure of 23.3 GPa (right).Figure 5-4.eps either side of the band correspond to the [-101] direction The lattice images on of crystalline B4C, and the loss of lattice fringes in the band indicates localized amorphization in a band within the grain. SOURCE: Chen, bitmap M., J. McCauley, and K. Hemker. 2003. Shock-induced localized amorphization in boron carbide. Science 299(5612): 1563-1566. stacking faults and microtwins was observed, suggestive of Yan et al. indicated a significant decrease in volume of the plastic deformation of the material under shock loading.32 B4C unit cell owing to the bending of the C–B–C chain at a Understanding the pressure dependence of boron car- destabilization pressure of 19 GPa for uniaxial compression, bide phases would shed light on the issue of the pressure- consistent with the HEL of 15-20 GPa. At higher pressures, induced, crystal-to-amorphous transformation. Yan et al.33 the C–B–C chain bends until the central B atom bonds with used in situ Raman spectroscopy to monitor the quasihydro- neighboring B atoms in the surrounding icosahedra, forming static and nonhydrostatic compression of a boron carbide a stable higher energy structure. It has been suggested that single crystal up to 50 GPa, followed by depressurization the release of this energy during depressurization is respon - to ambient pressure. Under quasihydrostatic compression, sible for breaking the covalent bonds and for the collapse of Raman analysis did not detect any signs of amorphization the B4C structure, with the formation of a local amorphous region.36 A computational study of the phase stability of during either loading or unloading, and the material remained a perfect single crystal without any visible surface relief various boron carbide polytypes at elevated pressures was conducted by Fanchini et al.37 under increasing purely hy- features or cracks. However, under highly nonhydrostatic compressive conditions (i.e., uniaxial compression), the drostatic pressure at room temperature. The results indicated results were significantly different, the pressurized sample that the energetic barrier for pressure-induced amorphization having broken into a number of small fragments. In situ Ra- of boron carbide is lowest for the B12(C–C–C) polytype, man spectroscopy detected the formation of the amorphous which was found to be unstable at 6-7 GPa during hydrostatic phase, indicating that a nonhydrostatic high-pressure state loading; however, no such collapse has been observed experi- mentally up to 40 GPa (see Yan et al.38 and the references can make boron carbide unstable. This compressive stress transformation has been in - therein). Further, the model of chain bending under uniaxial vestigated by simulating molecular dynamics.34,35 Work by 36Yan, X.Q., Z. Tang, L. Zhang, J.J. Guo, C.Q. Jin, Y. Zhang, T. Goto, 32Chen, M.W., J.W. McCauley, and K.J. Hemker. 2003. Shock-induced J.W. McCauley, and M.W. Chen. 2009. Depressurization amorphization localized amorphization in boron carbide. Science 299(5612): 1563-1566. of single-crystal boron carbide. Physical Review Letters 102(7): Article 33Yan, X.Q., Z. Tang, L. Zhang, J.J. Guo, C.Q. Jin, Y. Zhang, T. Goto, number 075505. 37Fanchini, G., J.W. McCauley, and M. Chhowalla. 2006. Behavior J.W. McCauley, and M.W. Chen. 2009. Depressurization amorphization of single-crystal boron carbide. Physical Review Letters 102(7): Article of disordered boron carbide under stress. Physical Review Letters 97(6): number 075505. Article number 035502. 34Ibid. 38Yan, X.Q., Z. Tang, L. Zhang, J.J. Guo, C.Q. Jin, Y. Zhang, T. Goto, 35Fanchini, G., J.W. McCauley, and M. Chhowalla. 2006. Behavior J.W. McCauley, and M.W. Chen. 2009. Depressurization amorphization of disordered boron carbide under stress. Physical Review Letters 97(6): of single-crystal boron carbide. Physical Review Letters 102(7): Article Article number 035502. number 075505.

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75 LIGHTWEIGHT PROTECTIVE MATERIALS: CERAMICS, POLYMERS, AND METALS compression proposed by Yan et al.39 assumes transformation at the pressures used for the manufacturing of the ceramics. to another crystal structure in the loading stage, whereas in Additionally, pressures corresponding to those encountered situ Raman analysis does not show any sign of such a crystal- in ballistic and blast events should be explored to understand crystal transformation. the nonequilibrium phase aspects of armor ceramics. One way to avoid amorphorization may be to avoid Finding 5-1c. Time-temperature-transformation and time- forming the B12(C–C–C) polytype, which occurs as a mi- nority phase during normal processing and sintering. This pressure-transformation diagrams need to be drawn using may be accomplished by doping. Al and Si are both able to advanced instrumentation to provide a basic understanding substitute for C in B4C. These dopants occupy sites in the of the kinetics of structural transformations of ceramic ma- diagonal chain in the rhombohedral B4C structure. Moreover, terials, in particular boron carbide. it is known that Si addition strongly promotes the sp3-C con- tent in amorphous carbon materials, which may prevent the Crystalline Structure of Silicon Carbide segregation of C into two-dimensional graphitic (sp2) layers. Hence, the notion of significant Si doping to inhibit amor- Types and Characteristics phorization depends on the ability to synthesize a material with stable B12SiC2 polytypes, avoiding B12 (C–C–C) forma- SiC is a simple 1:1 compound of two atoms that both prefer sp3 bonding. Owing to the similarity of the tetrahedral tion. Unfortunately, the solubility of Si in boron carbide is quite low (~2.5 at% Si). There are some studies on the B–C– bonding, SiC has a surprisingly wide variety of polytypes. Si system that have explored higher Si concentrations (>20 Whereas many materials are polytypic to a limited extent (e.g., α-Al2O3, g-Al2O3), the polytypism of SiC is extensive, at%) with the goal of developing useful SiC–B4C composite with over 200 polytypes having been observed.40,41,42 The materials for potential armor use. Thermodynamic calcula- tions suggest the difference between the Gibbs free energy basic unit is a tetrahedron of Si4C or, equivalently, C4Si; of the B11C1-g, p-Sig, p(C–B–C) polytype and that of the most these are joined at the corners to other tetrahedra. The struc - energetically favored minority polytype B12(C–Sig–C1-g–C) ture can be seen as invariant in the basal plane; the various increases with increased Si content. This suggests that if a polytypes are distinguished by the stacking sequence in the solid solution of B4C with Si or Al could be made, it might direction normal (c-axis) to the basal planes. An essentially prove resistant to high-pressure amorphization, which could infinite number of stacking sequences can be achieved by improve the ballistic performance of this important ceramic altering the number of layers before repeating the sequence. armor material. Clearly, further experimental and theoreti- A number of notations have been developed; the most com- cal work is required to more fully understand the structural mon notation, Ramsdell’s, labels the polytypes as nL, where changes in boron carbide under impact loading. n is a number indicating the periodicity in the stacking of the Amorphization has limited the effectiveness of boron tetrahedra layers along the c-axis and L is a letter indicating carbide to high-velocity threats. Modification of the crystal the general crystal symmetry. For example, 3C is indicative structure via the ternary alloying chemistry of boron carbide of cubic symmetry with a three-layer repeat. This is in fact the only cubic polytype for SiC and is designated as b-SiC. may inhibit amorphization. This would provide an armor material that is 25 percent less dense than SiC and 40 percent The most common polytypes—2H, 4H, and 6H—all have less dense than Al2O3. hexagonal symmetry. There is one common rhombohedral polytype, 15R, and countless other less common and more exotic combinations like 33R or 1,200R. All of the noncubic Findings polytypes, although different, are grouped together and con- sidered as α-SiC. Five common polytypes of SiC are shown Finding 5-1a. Additional ceramic compositions and struc- tures merit investigation as potential new armor materials. in Figure 5-5. For the currently available armor ceramics, the difficulties in While it is often simple to qualitatively discern the pres- powder synthesis, availability, and processing of the powders ence of a particular polytype in an x-ray diffraction pattern by into dense ceramics mean that many opportunities for per- finding certain characteristic peaks, overlapping peaks make formance improvements remain unexplored, including the it not nearly as straightforward to quantitatively determine addition of alloying elements and variations in nanostructure all of the polytypes present in samples. Many researchers and microstructure. 40Shaffer, P.T.B. 1969. A review of the structure of silicon carbide. Finding 5-1b. There is a need for a fundamental understand- Acta Crystallographica Section B: Structural Crystallography and Crystal ing of the equilibrium phases and crystal structures of armor Chemistry 25(3): 477-488. ceramics and for the construction of accurate equilibrium- 41Mrotek, S.R. 1998. Microstructural Control of Silicon Carbide via Liq - phase diagrams for the B–C system at ambient pressure and uid Phase Sintering, Ph.D. Dissertation. Newark, N.J.: Rutgers University. 42Kaza, A. 2006. Effect of Gas Phase Composition in Pores During the Initial Stages of Sintering. Ph.D. Dissertation, Newark, N.J.: Rutgers 39Ibid. University.

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76 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS α grains can act as detrimental flaws and decrease other me- chanical properties.47,48 Impurities and intentional additives to SiC play an im- portant role in the development and transformation of poly- types. As far back as 1948, Lundqvist49 had observed that different polytypes were often associated with SiC crystals of varying colors in certain powders: 6H were green, 15R were yellow, and 4H grains or samples with mixtures of polytypes appeared black. Through careful x-ray examination of over 200 powders from a variety of locations, accompanied by spectrochemical analysis, large variations in aluminum con- tent and smaller variations in iron content were observed. At very low Al contents, the 6H polytype appeared to be favored, whereas 0.05-0.06 wt percent Al promoted the for- mation of 15R, with a transition to 4H above 0.10 wt percent Al. Lundqvist also observed inclusions in the grains, most of which were unreacted graphite, noting few inclusions in the clearest to light green samples. In the darker and black samples, large inclusions, found to be compounds of alumi- num and iron, were often present along with changes in the FIGURE 5-5 Schematics of the stacking sequence of layers of Si–C nearby crystal structure. In present practice, a wide range tetrahedra in various SiC polytypes. of other impurity elements and sintering-aid additions also exert considerable influence over the temperature at which the polytype transformations occur and the exact sequence of the transformations.50,51,52,53,54 As mentioned, densifying SiC at temperatures above measure only the α and b contents of their powder and often 1900°C will cause any b grains to transform into various α fail to be any more specific about the relative amounts of 2H, polytypes, accompanied by rapid anisotropic grain growth.55 4H, 6H, and others because considerable effort would be However, if the initial material is instead an α powder, sinter- required. During densification at high temperature, a given ing at or above 1900°C will result in a fine, equiaxed α micro- polytype can transform into a more stable one, and this can be structure. Careful control over powder purity, sintering aids, accompanied by desirable or undesirable grain growth, along with changes in porosity, which influence various properties and ballistic performance.43,44,45,46 For example, it is com- 47Zhan, G.D., M. Mitomo, H. Tanaka, and Y.-W. Kim. 2000. Effect of mon to improve the fracture toughness of SiC by exploiting annealing conditions on microstructural development and phase transfor- mation in silicon carbide. Journal of the American Ceramic Society 83(6): the anisotropic grain growth that occurs when polytypes 1369-1374. transform. The high sintering temperatures required for 48Zhan, G.-D., R.-J. Xie, M. Mitomo, Y.-K. Kim, and N.P. Padture. 2001. densifying SiC promote the transformation of b grains to α Effect of beta-to-alpha phase transformation on the microstructural develop- grains, which can become large, elongated platelet grains. By ment and mechanical properties of fine-grained silicon carbide ceramics. purposefully seeding an α-SiC powder with b grains before Journal of the American Ceramic Society 84(5): 945-950. 49Lundqvist, D. 1948. On the Crystal Structure of Silicon Carbide and Its sintering, microstructures with improved fracture toughness Content of Impurities. Acta Chemica Scandinavica 2: 177-191. can be designed by taking advantage of the increased crack 50Rixecker, G., K. Biswas, A. Rosinus, S. Sharma, I. Wiedmann, and F. paths around the elongated α grains. In other cases, the large Aldinger. 2002. Fracture properties of SiC ceramics with oxynitride ad- ditives. Journal of the European Ceramic Society 22(14-15): 2669-2675. 51Biswas, K., G. Rixecker, and F. Aldinger. 2003. Improved high temper- ature properties of SiC-ceramics sintered with Lu2O3-containing additives. Journal of the European Ceramic Society 23(7): 1099-1104. 52Kim, J., A. Rosenflanz, and I.W. Chen. 2000. Microstructure control 43Shaffer, P.T.B. 1969. A review of the structure of silicon carbide. of in-situ-toughened α-SiAlON ceramics. Journal of the American Ceramic Acta Crystallographica Section B: Structural Crystallography and Crystal Society 83(7): 1819-1821. Chemistry 25(3): 477-488. 53Kim, Y.-W., Y.-S. Chun, T. Nishimura, M. Mitomo, and Y.-H. Lee. 44Pezoldt, J. 1995. Are polytype transitions possible during boron diffu - 2007. High-temperature strength of silicon carbide ceramics sintered with sion? Materials Science and Engineering B 29(1-3): 99-104. rare-earth oxide and aluminum nitride. Acta Materialia 55(2): 727-736. 45Jepps, N.W., and T.F. Page. 1981. The 6H→ 3C reverse transforma- 54Kim, W., Y.-W. Kim, and D.-H. Cho. 1998. Texture and fracture tough - tion in silicon carbide compacts. Journal of the American Ceramic Society ness anisotropy in silicon carbide. Journal of the American Ceramic Society 64(12): C-177-178. 81(6): 1669-1672. 46Irmscher, K., M. Albrecht, M. Rossberg, H.-J. Rost, D. Siche, and G. 55Pezoldt, J. 1995. Are polytype transitions possible during boron diffu - Wagner. 2006. Formation and properties of stacking faults in nitrogen-doped sion? Materials Science and Engineering B 29(1-3): 99-104. 4H-SiC. Physica B: Condensed Matter: 338-341.

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77 LIGHTWEIGHT PROTECTIVE MATERIALS: CERAMICS, POLYMERS, AND METALS and processing is clearly required in order to systematically fault glide by purposeful alloying may provide an opportu- modify the microstructure and polytypes in silicon carbide. nity to enhance plasticity and energy absorption. Stacking Faults Availability of Ceramic Powders In addition to the various long repeat sequences that Synthesis and processing of armor ceramics begins constitute a particular polytype, a localized change in stack- with ceramic powders, which are compacted and processed ing sequence within any specific grain is a type of stacking using a variety of techniques. The important issue of pow- fault. For example, a specific grain of 6H could contain local der availability is discussed in this subsection. Appendix D regions where the stacking sequence has changed to 4H for a further characterizes the current understanding of powder few layers and then back to 6H. An understanding of stacking production for the protection materials of interest, including faults and their connection with plastic deformation behavior SiC, B4C, Al2O3, AlN, AlON, and spinel, and suggests op- has come about in the study of metals over many years. The portunities to improve the situation. process of slip on a close-packed plane can produce the same It is difficult, if not impossible, to fabricate high-quality shift in stacking sequence for a number of layers in a crystal; ceramic components without having control of the powders this shifted region is known as a deformation stacking fault comprising them. The U.S.-based companies that supply but is structurally identical to a growth stacking fault. many strategic ceramic components have seen a loss of The stacking fault can be described as an extended domestic powder suppliers over the past two decades. More- dislocation that is bounded by two partial dislocations. Like over, many critical armor systems rely on unique, highly all imperfections, the stacking fault has an energy associated specified powders for the hard ceramics. Applications rang- with its creation that can differ greatly between materials. ing from armor for personnel or vehicles to high-intensity Materials with low stacking-fault energy readily form many mirrors to missile radomes to rocket nozzles rely on powders stacking faults and have large separations between the coming from India, China, and Russia. bounding partial dislocations. Materials with high stacking- There is no powder manufacturer in the United States fault energy require more energy for their creation and capable of producing the armor-grade ceramic powders therefore form fewer and narrower, smaller faults. Silicon needed by armor manufacturers. Nearly all oxide and car- carbide has low stacking-fault energy, and it is not uncom- bide powders on the market have been engineered to satisfy mon to find many growth stacking faults present throughout the requirements of applications other than armor. As a the crystals. Fragments from ballistic impact experiments do consequence, ceramic armor manufacturers and university indeed show a considerable amount of stacking faults and researchers are forced to employ powders that are almost twins,56,57 suggesting that materials with low stacking-fault certainly not optimal for armor applications. Beyond imped- energy twin readily under shock loading also, because the ing research and development generally and, particularly, the presence of large numbers of stacking faults provides loca- development of better protection materials, the precarious- tions at which twins form easily.58 ness of domestic supply poses a risk for DoD should a need There are a very large number of crystal structures for arise for surge production of ceramic armor materials. SiC differing by the stacking sequence of tetrahedral Si4C The consequence of this eroded domestic supply base or C4Si units, and the identification and characterization of has been the inability of component manufacturers to design the polytypes is laborious. The phase content depends on powders for a specific application. Instead, domestic produc- variations in chemical impurities and sample process history. ers sort or modify highly variable commodity powders of Because well-defined SiC single crystals are available non-U.S. origin to impart the requisite “uniqueness” for an from the electronics industry, an improved understanding of application. This is a problem for a host of powders: those for the deformation of a particular polytype can be conducted. opaque armor (SiC, B4C, AlN) and those for transparent ar- Additionally, the effect of the amount of each polytype and mor (MgO-Al2O3 [spinel] and AlON). In many cases, lower its spatial and size distribution within model polycrystalline cost, less highly specified end uses, such as abrasive grain, materials merits investigation, especially the effect on high- have given rise to a proliferation of new powder suppliers rate behavior. Reducing the activation energy for stacking in the emerging nations. In most cases, the foreign supply chain links many small powder producers with a handful of brokers, virtually eliminating the production of tailored 56Shih, C.J., M.A. Meyers, V.F. Nesterenko, and S.J. Chen. 2000. Dam- powders and lowering quality. age evolution in dynamic deformation of silicon carbide. Acta Materialia Furnace reactors were once large-scale operations; now, 48(9): 2399-2420. small producers can introduce highly variable product into 57Chen, M.W., J.W. McCauley, D.P. Dandekar, and N.K. Bourne. 2006. Dynamic plasticity and failure of high-purity alumina under shock loading. a distribution stream. Precursor raw materials are also a Nature Materials 5(8): 614–618. problem. For example, for silicon carbide and boron carbide, 58Murr, L.E. 1987. Metallurgical effects of shock and high-strain-rate carbon used to be obtained from high-grade, petroleum- loading. Pp. 1-45 in Materials at High Strain Rates. T.Z. Blazynski, ed. New derived coke. However, in China it is not uncommon to York, N.Y.: Elsevier Science.

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78 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS to assist ceramic producers in researching or producing new see anthracite coal or low-purity petroleum coke used. The prototype powders. consequence is an end product whose chemistry is highly variable. Component suppliers are now faced with how to Finding 5-2d. Although the availability of high-quality ce- make a consistent product meeting today’s armor specifica- ramic powders for protection materials is critical to national tions. Improving ceramic performance can no longer entail defense, there is currently no domestic source of ceramic simply changing the initial powder since the production and powders to meet DoD needs. supply of powders are no longer within domestic control. By losing control of powder processing, U.S. armor makers have reached a point at which variability in powders PROCESSING AND FABRICATION TECHNIQUES FOR is expected, tolerated, and, in many cases, ignored. While ARMOR CERAMICS processing treatments have been developed to improve the A variety of fabrication techniques have been employed overall uniformity of powders, this results in dense com- in the processing of armor ceramic materials. There are two ponents whose microscale variability reflects the intrinsic broad classes of forming operations: (1) cold methods— variability of the parent powder. From a simple business or slip casting, extrusion, and die pressing—and (2) high- logistical point of view, manufacturers can no longer assure temperature pressureless and pressure-assisted sintering that the powders used in highly specified components will methods—hot pressing, hot isostatic pressing, and spark meet strict testing requirements. plasma sintering (see Box 5-1). The Defense Production Act Title III program gives Since armor materials are mostly strongly covalently DoD special authority to issue purchase commitments, loan bonded solids, high-temperature densification, often with guarantees, capital investment, or research and development pressure-assisted techniques, is required. The goal of den- investment to provide an assured domestic supply for criti- sification is to optimize bonding and eliminate porosity cal materials. The business case analysis to support a Title in the compacted powder so that full theoretical densities, III program in ceramic armor materials is beyond the scope along with homogeneous microstructures, can be achieved of this study. The committee recommends DoD undertake in the final sintered materials. Near-net-shape fabrication such an analysis to determine whether domestic production that minimizes machining and finishing operations is also of ceramic armor precursor materials would be a good can- desired for cost savings. didate for Title III. Finding 5-2a. The goal for future armor systems is not only “Green” Compaction to maintain current performance but to dramatically increase The starting point in ceramic forming is the compaction it as well. As such, it is critical that the United States regain of powders. Die pressing is the predominant forming method and maintain control of the armor raw material supply chain. for symmetrical shapes such as hexagonal and square tiles. There is a need for a strategic powder production infrastruc- High-pressure compaction methods can be divided into ture within the United States to bring about the next genera- tion of opaque and transparent armors. This will not only permit a consistent and reliable supply but also allow for the design of powders whose intrinsic properties are optimized for armor applications. Finding 5-2b. Powder processing affects the intrinsic prop- erties of many armor ceramics. There is little work on how the powders can be designed and manipulated at the atomic, BOX 5-1 nano, and micro levels in ways to maximize their potential Processing of Ceramic Powders as raw armor materials. Hot-pressed SiC and B4C powders yield uniform full-density products Finding 5-2c. There are no powders produced specifically with homogeneous microstructures and good ballistic performance. for armor applications. The oxide and carbide powders that Near-net-shape ceramic processing is of great interest, although pow- are commercially available have been designed for other ders with additives and sintering aids compacted by means of lower applications. Most powder processes are energy-intensive cost conventional pressing methods into “green,” or unfired, form processes with large carbon emission footprints, and U.S. en- and then pressureless-sintered (that is, at atmospheric pressure) yield vironmental regulation costs have reduced the competiveness materials with nonuniform density distribution and microstructure. of U.S. producers, with foreign powder producers benefiting Their ballistic performance is inferior for higher threat levels compared from low-cost but environmentally questionable operations. to that of hot-pressed material. There is no domestic feedback on powder characterization

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79 LIGHTWEIGHT PROTECTIVE MATERIALS: CERAMICS, POLYMERS, AND METALS static and dynamic techniques.59 With static compaction, a factory environment. The DMC rapid consolidation tech- a constant pressure is applied onto a sample for a certain nique developed by IAP Research, Inc., is based on a magnet- period of time, typically a few seconds. Dynamic compac- ic pulse that launches a pressure wave that travels at 100-300 tion uses a pressure pulse with a pulse duration of less than m/s through the powders, giving rise to stress gradients; the technique is designed so as to absorb reflected waves.62 The a few milliseconds, resulting in a pressure wave that travels through the sample. In both static and dynamic compaction stress gradients cause particle motion, particle deformation, the pressure can be applied in a uniaxial, biaxial, radial, or and particle fracture, especially in brittle powder materials isostatic/isodynamic mode. at high pressure; accordingly, they bring about a higher de- gree of consolidation than static pressing.63 Very high green The choice of a process for compacting powders for the fabrication of ceramics depends on the complexity of densities of the compacts can be realized—in fact, they ap- the shape of the ceramic part.60 The most widely used form- proach theoretical densities even before sintering. Because ing method for armor production is uniaxial die pressing, DMC samples have higher compact densities they can be whereby uniaxial pressure is applied to the powder placed sintered at lower temperatures or for shorter periods of time in a die between two rigid punches. Binder and/or lubricants to obtain close-to-full-density materials. The dynamically are added to the powders to reduce the friction and facilitate pressed samples exhibit rather homogeneous microstructures extracting the formed part from the die. This formed part, after pressureless sintering and properties similar to those often termed a “green” compact because it is unfired, is sub- of hot-pressed material. In addition, dynamic processing sequently heat treated (“sintered”) to densify it. techniques allow retention of special powder microstructures The typical density of the parts achieved after uniaxial (including nano grain size) after sintering owing to the short die pressing is 50-55 percent of the theoretical density. Den- sintering time and lower sintering temperatures. In light of sity gradients occur depending on the part’s shape, aspect its advantages, dynamic compaction needs to be seriously ratio, and size. These gradients are a likely source of voids investigated for armor production methods. and undesirable porosity in sintered armor tiles. Other de- fects in laminar character can appear oriented normal to the Sintering pressing axis. After die pressing, the part will have shrunk by 20 to 40 percent or so, and the final part dimensions are Appendix E characterizes commonly used ceramic sin- achieved by machining and grinding. tering processes and discusses issues surrounding their ap- Uniaxial die pressing is widely used for the low-cost plication to opaque armor materials. The advantages and dis- mass production of simple parts. In certain cases, cold iso - advantages of these processes are summarized in Table 5-1. static pressing (CIP) is used to further increase the density The effect of specially designed powder microstruc- (up to 73 percent) after die compaction. CIP is conducted as tures, such as nano grain sizes, on the controlled fracture to wet bag isostatic pressing in pressure vessels, and parts can enhance ballistic performance is being investigated by the be produced as large as a few meters in height and a meter Army Research Laboratory (ARL) and other laboratories. or more in diameter, with large parts having substantially Ceramic manufacturers are also exploring ways to improve higher costs. Hydrostatic pressures of 100-700 MPa can be performance through modifications to the front surface of a achieved with suitable CIP systems.61 ceramic armor plate—in one case by molding multiple nodes Dynamic compaction approaches are potential alterna- with conical or rounded shapes. By modifying the impact tives for making near-net-shape parts with very high “green” angle of the projectile, the ballistic performance of ceramics densities (up to 95-100 percent). Dynamic compaction could be improved. Ceramic nodes, spheres, or hollow ce- depends on the way the pressure waves needed to densify ramic spheres give the structure a multiplicity of surfaces for a multiplicity of crack initiation sites.64 These nodes cause the sample are generated and how the reflected waves are absorbed. One of the best-known methods is compaction us- part of the energy of the projectile to initiate a multiplicity of ing explosives. However, this method would be problematic cracks at the node surface; however, spherical nodes arrest as an industrial manufacturing process in a factory environ- cracks. Other candidates for exploring the improvement of ment. The alternative dynamic magnetic compaction (DMC) performance include novel alloying and doping methods. technique uses magnetic pulse pressures and is suitable for 59Jak, 62Chelluri, Michiel J.G. 2004. Dynamic compaction of nano-structured ce - B., E. Knoth, E. Schumacher, and L.P. Franks. 2010. Method ramics. Nanocomposites. Volume 10 in Electronics Materials Science & for Producing SiC armor tiles of higher performance at lower cost. Pp. Technology. Springer. 199-205 in Advances in Ceramic Armor VI: Ceramic Engineering and 60Tressler, R.E. 2004. An assessment of low cost manufacturing tech - Science Proceedings, Volume 31, Issue 5. J.J. Swab, ed. Hoboken, N.J.: nology for advanced structural ceramics and its impact on ceramic armor. John Wiley & Sons. 63National Research Council. 1983. Dynamic Compaction of Metal and Pp. 451-462 in Progress in Ceramic Armor. New York, N.Y.: John Wiley & Sons. Ceramic Powders. Washington, D.C.: National Academy Press. 61Nishimura, T., K. Jinbo, Y. Matsuo, and S. Kimura. 1990. Forming 64Medvedovski, E. 2010. Ballistic performance of armor ceramics: of ceramic powders by cyclic-CIP: Effect of bias pressure. Journal of the Influence of design and structure. Part 2. Ceramics International 36(7): Ceramic Society of Japan 98(7): 735-738. 2117-2127.

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88 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS of composite fabrics with higher elongation in weft yarns investigated the effects of projectile shape, including ogival, and lower elongation-to-break in warp yarns was greater conical, hemispherical, and flat-headed, on the ballistic than that of fabrics made from a single material, which may performance of single-ply Twaron fabrics; they observed the sequence hemispherical>flat-headed>ogival≥conical be due to the lesser influence of yarn crimp. By consider- ing yarn crimp in modeling, Tan et al.112 obtained more when projectile velocity is 100-600 m/s. Conical and ogival accurate results. The number of fabric plies also affects the projectiles caused the least yarn pullout, which suggests that ballistic performance (note that typically there may be 20-50 they were able to wedge through the fabrics. plies). Shockey et al.113 observed increased specific energy The velocity of the projectile will also affect the per- absorbed for multi-ply targets owing to the friction forces formance of fabrics. In low-velocity impact, the transverse between layers. The influence of interply distance on ballistic wave has a longer time to propagate and more fabric area performance has also been investigated.114,115 The influence is involved, which leads to higher energy absorption. Also, of projectile geometry also becomes less important with the yarn pullout becomes the predominant failure mode. At increased number of plies.116,117,118 A three-dimensional wo- high-velocity impact, some types of fibers become stiffer and ven structure was studied in a fabric composite119 designed stronger owing to their viscoelastic properties, and primary bond failure becomes the predominant failure mechanism.124 to provide greater through-thickness direction reinforcement than in conventional two-dimensional woven fabrics; this structure showed higher ballistic performance and led to Fabric Boundary Conditions fewer penetrated layers under impact. When fabrics are impacted by a projectile, the size of the target and gripping conditions are important. For instance, Projectile Characteristics and Fabric Damage a longer yarn can absorb more deformational energy than a The geometry of a projectile will strongly affect its shorter one before failure; thus a larger target area will lead penetration ability. A sharp-edged or pointed projectile per- to higher energy dissipation. However, this is not true when forates the fabric more easily than a blunt-faced projectile, the velocity of the projectile is very high compared to the shearing yarns across their thickness direction and leading velocity of the shock wave in the fibers since then only a to a smaller specific energy absorbed.120,121,122 Tan et al.123 small portion of the target can dissipate the kinetic energy of the projectile. The boundary conditions of the target also play an important role. Shockey et al.125 observed that a two- 112Tan, V.B.C., V.P.W. Shim, and X. Zeng. 2005. Modelling crimp in edge gripped fabric absorbs more energy than a four-edge woven fabrics subjected to ballistic impact. International Journal of Impact gripped fabric, and fabrics with free boundaries absorb the Engineering 32(1-4): 561-574. least energy. Chitrangad126 observed that when pre-tension 113Shockey, D.A., D.C. Elrich, and J.W. Simmons. 2001. Improved Bar- is applied on aramid fabrics, their ballistic performance is riers to Turbine Engine Fragments: Interim Report III, DOT/FAA/AR-99/8, improved. Zeng et al.127 observed that for four-edge gripped III. Menlo Park, Calif.: SRI International. 114Cunniff, P.M. 1992. An analysis of the system effects in woven fabrics fabrics, energy absorbed is improved if the yarns are oriented under ballistic impact. Textile Research Journal 62(9): 495-509. at 45° relative to the edge. 115Lim, C.T., V.B.C. Tan, and C.H. Cheong. 2002. Perforation of high- strength double-ply fabric system by varying shaped projectiles. Interna- tional Journal of Impact Engineering 27(6): 577-591. Friction 116Ibid. 117Montgomery, T.G., P.L. Grady, and C. Tomasino. 1982. Effects of Frictional effects between a projectile and a fabric are projectile geometry on the performance of ballistic fabrics. Textile Research observed at low-velocity impact but diminish at a higher Journal 52(7): 442-450. velocity.128 A quantitative study on Kevlar yarn friction 118Prosser, R A., S.H. Cohen, and R.A. Segars. 2000. Heat as a factor in the penetration of cloth ballistic panels by 0.22 caliber projectiles. Textile Research Journal 70(8): 709-722. 119Grogan, J., S.A. Tekalur, A. Shukla, A. Bogdanovich, and R.A. Cof - 124Shim, V.P.W., V.B.C Tan, and T.E Tay. 1995. Modelling deformation felt. 2007. Ballistic resistance of 2D and 3D woven sandwich composites. and damage characteristics of woven fabric under small projectile impact. Journal of Sandwich Structures & Materials 9(3): 283-302. International Journal of Impact Engineering 16(4): 585-605. 120Lim, C.T., V.B.C. Tan, and C.H. Cheong. 2002. Perforation of high- 125Shockey, D.A., J.W. Simons, and D.C. Erlich. 1999. Improved Barri - strength double-ply fabric system by varying shaped projectiles. Interna- ers to Turbine Engine Fragments: Interim Report I, DOT/FAA/AR-99/8, I. tional Journal of Impact Engineering 27(6): 577-591. Menlo Park, Calif.: SRI International. 121Montgomery, T.G., P.L. Grady, and C. Tomasino. 1982. Effects of 126Chitrangad, I., 1993. Hybrid Ballistic Fabric. U.S. Patent 5,187,003. projectile geometry on the performance of ballistic fabrics. Textile Research Available online at http://www.freepatentsonline.com/5187003.pdf. Last Journal 52(7): 442-450. accessed April 12, 2011. 122Prosser, R.A., S.H Cohen, and R.A. Segars. 2000. Heat as a factor in 127Zeng, X.S., V.P.W. Shim, and V.B.C. Tan. 2005. Influence of bound - the penetration of cloth ballistic panels by 0.22 caliber projectiles. Textile ary conditions on the ballistic performance of high-strength fabric targets. Research Journal 70: 709-722. International Journal of Impact Engineering 32(1-4): 631-642. 123Tan, V.B.C., C.T. Lim, and C.H. Cheong. 2003. Perforation of high- 128Tan, V.B.C., C.T. Lim, and C.H. Cheong. 2003. Perforation of high- strength fabric by projectiles of different geometry. International Journal of strength fabric by projectiles of different geometry. International Journal of Impact Engineering 28(2): 207-222. Impact Engineering 28(2): 207-222.

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89 LIGHTWEIGHT PROTECTIVE MATERIALS: CERAMICS, POLYMERS, AND METALS Finding 5-7a. Environmental factors can lead to degrada- was conducted by Briscoe et al.:129 The yarn pullout force increases with an increase in interyarn friction, and the in- tion of fiber and fabric properties and hence ballistic perfor- crease in effective yarn modulus is attributed to the increase mance, particularly when exposed to extreme temperatures, in interfilament friction. Fabrics with high friction and lower ultraviolet radiation, cyclical deformation, and humidity effective modulus can dissipate more energy than those with over long times. Reliable methods need to be developed lower friction. Duan et al.130 modeled the effects of interyarn for predicting the effect of these factors on the mechanical friction and found that it accounts for only a small portion properties at high strain rates over the useful life of fibers of energy dissipation during impact. Friction does help and fabrics. maintain the integrity of local fabrics in the impact region by allowing more yarns to be involved in the impact, and it Conclusion increases energy absorption by increasing yarn strain and kinetic energy. Dischler131 applied a thin polymeric film on The ideal microstructure for fibers is known and has Kevlar (20-ply), which increased the coefficient of friction been experimentally approached. Further emphasis on pro- from 0.19 to 0.27; he observed a 19 percent improvement in cessing to eliminate molecular-level irregularities in chain ballistic performance in stopping a flechette. packing and to reduce residual solvents should provide severalfold improvements in fiber properties. Environmental Degradation Finding 5-7b. A combination of high-rate experimental Environmental factors such as temperature, moisture, measurements and computational modeling and simulation is residual spinning solvents, and UV radiation may cause needed to more deeply understand the dynamic deformation high-performance fabrics to degrade, reducing their ballistic and failure mechanisms of ballistic fabrics and to provide performance over time. In particular, Zylon (PBO) ballistic insight into the most desirable high-level organization of fabrics exhibited loss of performance when exposed to UV fibers into yarns and yarns into plies and fabrics. In situ im- radiation or moisture.132 See Appendix F for details on the aging of impact events and post-test assessment of fibers and environmental effects on fibers. In addition, the effect of fabrics need to be undertaken to reveal damage and failure cyclical deformation/fatigue on the ballistic performance mechanisms and to improve multi-hit performance. of fibers, fabrics, and composites needs to be investigated. Ultimately, high-performance polymer fibers are used as METALS AND METAL-MATRIX COMPOSITES fabrics or as fabric panels, which are reinforced with resin in helmets. Laboratory-scale work has enabled fiber micro- Metals have been the defining armor materials for more structures to increasingly approach the ideal structure, and than 2,000 years, and steel has been the armor material of corresponding single-fiber properties (strength, modulus, choice for most of the world’s armed forces. Steel technol- strain-to-failure) have recently reached impressive levels. ogy is sophisticated, cheap, and has a very large installed Prospects for further improvements appear promising: They industrial base. The modern army is a very heavy user of steel are expected to attain the fully extended and aligned state, as a protection material, particularly in the form of rolled which optimizes fiber tensile properties. Ballistic and blast homogeneous armor steel, also known as rolled homoge- performance of fabrics depends, however, on a host of pa- neous armor (RHA). The substitution of lighter (nonferrous) rameters beyond single-fiber tensile properties, including metals for steel has always been of interest to armies, and yarn friction, yarn pullout, and others. More sophisticated such substitution became increasingly important in the early modeling and simulation efforts that examine important in- 20th century as armed forces became more mechanized. In fluences such as environmental factors need to be performed. the current security climate, the global reach of our nation is intimately tied to its ability to rapidly deploy mechanized armored forces. This continues to drive the development of lighter armored systems and thus the use of lighter metals 129Briscoe, B.J., and F. Motamedi. 1992. The ballistic impact charac - for armor. teristics of aramid fabrics: The influence of interface friction. Wear 158 The largest fraction of protection materials in currently (1-2): 229-247. deployed vehicular fleets is metallic, primarily in the form 130Duan, Y., M. Keefe, T.A. Bogetti, and B.A. Cheeseman. 2005. Modeling friction effects on the ballistic impact behavior of a single-ply of steel and aluminum alloys and particularly in the rolled high-strength fabric. International Journal of Impact Engineering 31(8): condition. Some of the reasons for the large role of met- 996-1012. als include the fact that they are relatively cheap to make, 131Dischler, L. 2001. Bullet Resistant Fabric and Method of Manufacture. easily weldable, and able to play dual roles as structural U.S. Patent 6,248,676. Available online at http://www.google.com/patents/ materials and as armor materials. Because these materials about?id=nGsIAAAAEBAJ&dq=Martin-Electronics&ie=ISO-8859-1. Last accessed April 12, 2011. have a significant commercial market, a large industrial base 132Holmes, G.A., K. Rice, and C.R. Snyder. 2006. Ballistic fibers: A re - has grown up, along with downward pressures on the costs view of the thermal, ultraviolet and hydrolytic stability of the benzoxazole associated with extraction, processing, and metalworking. ring structure. Journal of Materials Science 41(13): 4105-4116.

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90 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS of model is typically sufficient to describe the constitutive response of the metal at high strain rates. The parameters that define the overall behavior include the modulus, the yield strength in uniaxial tension or compression, the strain hardening, the rate sensitivity, the ultimate tensile strength in uniaxial tension, and the strain-to-failure in a uniaxial tensile test. Desirable Attributes of Metals as Protective Materials There is general agreement on some of the key features of good metal protection materials: high-strength, good duc- tility, some strain hardening, and some increase in strength with an increasing rate of deformation (“rate-sensitivity”). Other characteristics that are desirable include good form- FIGURE 5-10 Stress-strain curve for RHA steel deformed in Figure 5-10.eps ability so that the material can be formed into structures of compression at a high strain rate. The oscillations in the curve bitmap the appropriate shapes, good long-term performance in the are an artifact of the experimental technique and do not represent material behavior. The elastic response is also not captured accu - operating environment (e.g., corrosion and fatigue resis- rately in such experiments. SOURCE: Zhang, H., J. Ye, S.P. Joshi, tance), and weldability for ease of joining. J.M. Schoenung, E.S.C. Chin, G.A. Gazonas, and K.T. Ramesh. A commonly asked question about potential substitutes 2007. Superlightweight nanoengineered aluminum for strength for RHA is whether their yield strength is on the order of under impact. Advanced Engineering Materials 9(5): 355-359. 1 GPa. This strength-driven approach can be misleading, Copyright Wiley-VCH Verlag GmbH &Co KGaA. Reproduced because in addition to a protection material’s basic constitu- with permission. tive behavior, its dynamic failure processes have a major influence on its performance in the face of a specific threat. The primary failure mechanisms consist of void growth un- der largely tensile conditions (typically defined by the spall strength133) and adiabatic shear localization134 under condi- tions of superimposed pressure and shear (both mechanisms These nontechnical factors surrounding the dual use potential are described in Chapter 3). The resistance of a metal to and the economics of processing, metalworking, and join- dynamic failure is essentially its resistance to spall and its ing are likely to continue to make metallic materials strong susceptibility to adiabatic shear localization (the conven- candidates for major components of robust and affordable tional properties of ductile fracture, while also relevant, are armor systems. While the development of increasingly in- relatively well understood). The spall strength of a metal can tense threats makes it less likely that an all-metal structure be effectively bounded (at the lower end) using analyses such for armor will prevail, metals will probably continue to play as those of Wu et al.,135 which incorporate the constitutive re- an important role in cost-effective armor packages that can sponse, and the approaches of Molinari and Wright,136 which represent an optimal solution to an array of potential threats. account for the internal defect distribution. The susceptibility RHA continues to be the benchmark with respect to to adiabatic shear localization is dependent primarily on the which most protection materials are judged: A typical objec- rate of thermal softening of the material and the strain-rate tive is framed in relative terms—for example, “at least the sensitivity.137 Thus the hardening and softening mechanisms performance of RHA at a lower areal density.” Although the within the material must be considered in addition to the performance of an RHA-based armor system is measured mechanisms that simply raise the initial yield strength. For in terms of a specific threat, the fundamental stress-strain instance, many high-strength metals have very low rate response is a good initial benchmark for materials design. sensitivity and may therefore be susceptible to adiabatic A compressive stress-strain curve for RHA at high strain rates is shown in Figure 5-10. The dynamic strength is well over 1 GPa, and there is a small but distinct strain-hardening 133Antoun, T., L. Seaman, D.R. Curran, G.I. Kanel, S.V. Razorenov, and A.V. Utkin. 2003. Spall Fracture. New York, N.Y.: Springer. domain. A collection of such experiments over a range of 134Wright, T.W. 2002. The Physics and Mathematics of Adiabatic Shear strain rates provides an estimate of the strain-rate sensitivity Bands. New York, N.Y.: Cambridge University Press. of the flow stress, and similar experiments performed over a 135Wu, X.Y., K.T. Ramesh, and T.W. Wright. 2003. The effects of thermal range of temperatures provides an estimate for the thermal softening and heat conduction on the dynamic growth of voids. International softening of the flow stress. The constitutive behavior of Journal of Solids and Structures 40(17): 4461-4478. 136Molinari, A., and T.W. Wright. 2005. A physical model for nucleation metals such as steels and aluminum alloys can be relatively and early growth of voids in ductile materials under dynamic loading. Jour- easily incorporated within a J2-flow type plasticity theory nal of the Mechanics and Physics of Solids 53(7): 1476-1504. by associating the stress and strain mentioned above with 137Wright, T.W. 2002. The Physics and Mathematics of Adiabatic Shear the equivalent stresses and the equivalent strains. This kind Bands. New York, N.Y.: Cambridge University Press.

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91 LIGHTWEIGHT PROTECTIVE MATERIALS: CERAMICS, POLYMERS, AND METALS Aluminum and Aluminum Alloys shear localization. The degree to which each of these failure mechanisms is important for a specific material depends Aluminum and aluminum alloys were developed early on the specific geometry of the armor structure as well as in the twentieth century, and beginning around the time of the specific threat; this means a detailed understanding of World War II, they were pressed into service, beginning the connection between the microstructure and the failure with armor for aircraft, to reduce weight. The introduction mechanisms in the metal is important in the design of new in the late 1950s of the T113 (later M113) vehicle type built metallic protection materials. of an aluminum alloy was followed by the deployment of The mechanical properties of metals can be changed significant quantities of aluminum alloys in the armored fleet. substantially by controlling the microstructure by chemical While pure aluminum is very soft, conventional aluminum and thermomechanical means. Typical strengthening mech- alloys can have yield strengths that easily compete with those anisms include solid solution hardening, precipitation and of the simpler steels. Specific approaches such as solid solu- dispersoid hardening, and grain boundary strengthening. In tion strengthening and age-hardening have been developed addition, many metals and metal alloys can be strengthened to strengthen Al alloys. substantially by increasing the internal dislocation density The trade-offs between weight, structural performance, through processes such as work hardening. This is one of ballistic performance, ease of production, and ease of main- the advantages of metallic materials: that the processing tenance, including resistance to corrosion, play a significant routes associated with metalworking can often be optimized role in the choice of alloy for vehicular applications. Most to increase the strength and the ductility. An example of of these aluminum alloys are used as rolled plate, and work- such a useful work-hardening route is rolling, which is typi- hardening alloys such as the 5000 series (5083 being the cally used to produce plate geometries. Rolled metal can prime example) have some advantages. Aluminum alloys be much stronger than the metal before rolling, and indeed used as armor in Army vehicles include 2024, 2519, 5083, the largest tonnages of metallic armor materials are rolled 5059, 6061, 7039, and 7075. Promising new commercial alloys (such as RHA). Materials with submicron structural alloys include 2139 Al, a commercial alloy with significant features are known to have higher yield strengths. Indeed, strength (around 600 MPa at high strain rates) and reason- controlling not only grain size but also feature size—for able ductility. example, in metals and bicontinuous composites like ce - There is significant potential for the development of ramic/polymer or metal/ceramic—can improve mechanical novel aluminum-based materials with very high strengths behavior.138,139 through alloying approaches and by the development of nanostructured systems and aluminum-based composites. Finding 5-8a. Although metal alloys have been in use for many years, only a small fraction of the alloys in use have Finding 5-8b. There is a substantial potential for the de- actually been characterized at the high strain rates relevant velopment of new and improved aluminum alloys that can to ballistic problems. As a result, much of the modeling and substitute for steel in military vehicles. Efforts to increase simulation that is performed using these alloys has become both strength and ductility of aluminum alloys at high strain heuristic rather than based on fundamental experimental rates are likely to bring significant benefits. data. This makes it very difficult to design with these alloys when new threats are presented. A sustained effort to develop Magnesium and Magnesium Alloys a database of high-strain-rate material properties for metallic materials would benefit armor designers. Magnesium has a remarkably low density of 1,700 kg/m3 (in comparison, Al is 2,800 kg/m3, Ti is 4,950 kg/ Nonferrous Metal Alternatives m3, and steels are 7,800 kg/m3). Its density approach es that of polymers. Magnesium and magnesium alloys are Appendix H provides a more detailed review of the main thus among the lightest structural metals, and they are nonferrous metals that may (and sometimes do) compete becoming increasingly important in the automotive and substantially with steel: titanium and titanium alloys, alu- hand-tool industries. The rapid growth in the commercial minum and aluminum alloys, magnesium and magnesium use of magnesium is intimately tied to the increasing cost alloys, and metal-matrix composites. A brief discussion on of energy. Their low density makes these materials very aluminum and magnesium and their alloys follows. attractive for defense applications, but magnesium alloys have historically had relatively low strengths (~250-300 MPa) in comparison to aluminum alloys. There has also been lingering, albeit somewhat exaggerated, concern about the flammability of magnesium and about the relative ease 138Kraft, O., P.A. Gruber, R. Monig, and D. Weygand. 2010. Plasticity in with which these alloys corrode in severe environments. confined dimensions. Annual Review of Materials Research 40: 293-317. 139Lee, J.-H., L. Wang, S. Kooi, M.C. Boyce, and E.L. Thomas. 2010 However, these concerns are relatively easily mitigated by Enhanced energy dissipation in periodic epoxy nanoframes. Nano Letters proper design and appropriate protocols for maintenance. 10(7): 2592-2597.

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92 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS A substantial effort has begun over the last decade to polymers, including thermoplastics and thermosets. The role generate high-strength magnesium alloys using a variety of of the adhesive is to hold the armor together before and after approaches, including solid solution strengthening and pre- impact and to both deform and delaminate to absorb energy. cipitation strengthening. Commercial magnesium alloys that These functions require an intermediate level of bonding can substitute for some aluminum alloys include AZ31140 and between the adhesive and the hard components (glass, ce- ZK60, and several alloys containing rare earths show prom- ramic, metal)—neither too weak an interface nor too strong. ise. Most of the innovation in this area is occurring abroad, Adhesives must absorb little to no light, because any absorp- particularly in China and Japan, and this may represent a tion will lower the overall transmission of the transparent long-term risk for the United States. A recent workshop at composite material. This applies not only to the visible Johns Hopkins on the potential of magnesium and magne- wavelengths of light but also to the near-infrared as some sium alloys as protection materials highlighted a variety of applications are used with night vision goggles and sensors. opportunities. One of the more promising strengthening ap- As well as giving an overview of commonly used adhesives, proaches appears to be the development of ultra-fine-grained this section also reports on design criteria for composites and or nanostructured magnesium alloys through severe plastic their testing, both experimental and computational. deformation. A major research effort to gain a fundamental understanding of the strengthening mechanisms in magne- General Considerations for the Selection of an Adhesive sium alloys is likely to be very fruitful, and the opportunities Interlayer presented by the metal’s low density should not be missed. Many material properties are important for the adhe- Finding 5-8c. A fundamental research effort to improve sives that are used as interlayers in armor and transparent magnesium alloys could have a big impact on the weight armor applications. These include the strength of the ad- of the armored vehicular fleet (a nascent effort exists at this hesive bonds across the various interfaces, which is highly time, driven by the ARL). There is a need to intensify re- dependent on chemistry but also on surface roughness, search into other lightweight protective metals as well, which environmental stability, mechanical impedances, mechani- would help maintain the U.S. infrastructure for critical ma- cal properties over a very large range of strain rates, and terials associated with protection systems for the warfighter. transparency in the visible and near-infrared spectrum to The outstanding performance of metals, the ease of fabricat- name but a few. Figure 5-11 shows a cross-sectional view ing and joining them, and the well-established industrial base of a typical ballistic-resistant glass composite made up of a ensure that these materials will be significant components of ceramic strike face, an outer region of plies of thick glass and protection material systems for the foreseeable future. As the alternating adhesive interlayers plus a transition section of a efforts in aluminum have demonstrated, there is a substantial thick plastic (e.g., polyurethane) and an absorbing section, potential for dramatic improvements even within the classes usually made of polycarbonate. of metal alloys currently available. Important Issues Surrounding Adhesives for Lightweight ADHESIVES FOR ARMOR AND FOR TRANSPARENT Armor Applications ARMOR In automotive safety glass the strength of adhesion Adhesive interlayers are key components of both bal- between the adhesive interlayer and the polymer, glass, and ceramic layers has been tailored with good effect.142,143,144 listic glass and composite armor. Understanding, testing, and modeling of the adhesive interlayers in composite Interlayers with bonds that can delaminate from the sub- and armor are crucial for their future design and improvement. superstrates may better absorb a projectile’s kinetic energy. Studies of nontransparent armor141 as well as transparent At the same time, however, they must retain enough integrity armor indicate that the adhesives significantly influence the ballistic behavior of the composite structure. While results and material information are generally kept secret, published information on the subject provides insight into which ad- hesives are commonly used, the state of their development, 142Fock, K., H.D. Hermann, K. Fabian, and J. Ebigt. 1987. Reduction in and their modeling capabilities. Most interlayer materials are the Adhesion to Glass of Thermoplastic, Plasticized Polyvinylbutyral Mold- ing Compositions. U.S. Patent 4,663,235.Available online at http://www. patents.com/us-4663235.html. Last accessed April 12, 2011. 140Mukai, T., M. Yamanoi, H. Watanabe, and K. Higashi. 2001. Ductility 143Hermann, H.D., K. Fabian, and J. Ebigt. 1985. Polyvinylbutyral enhancement in AZ31 magnesium alloy by controlling its grain structure. Films Which Contain Plasticizer and Have a Reduced Adhesive Power on Scripta Materialia 45(1): 89-94. Glass. U.S. Patent 4,533,601. Available online at http://www.patents.com/ 141Zaera, R., S. Sánchez-Sáeza, J.L. Pérez-Castellanos, and C. Navarro. us-4533601.html. Last accessed April 12, 2011. 144Garrison, W.E. 1969. Glass Laminate. U.S. Patent 3,434,915. Available 2000. Modelling of the adhesive layer in mixed ceramic/metal armours subjected to impact. Composites Part A: Applied Science and Manufactur- online at http://www.freepatentsonline.com/3434915.pdf. Last accessed ing 31: 823-833. April 12, 2011.

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93 LIGHTWEIGHT PROTECTIVE MATERIALS: CERAMICS, POLYMERS, AND METALS FIGURE 5-11 Composite stack of transparent layers: a ceramic strike face (C), adhesive interlayers (Ad), glass (G), polyurethane (PU), and polycarbonate (PC). Thermal Expansion Coefficient to minimize flying debris (spall).145 Additionally, the adhe- sion of the interlayer affects multi-hit performance; retaining The coefficient of thermal expansion (CTE) of the ad- and/or confining the hard ceramic and/or glass is necessary hesive interlayer is important for applications that will see a and so must be considered when designing interlayers. wide range of temperatures. Transparent armors are usually constructed with both high CTE (plastics such as polycar- Ultraviolet Radiation/Humidity/Temperature (Environmental) bonate) and low-CTE (ceramic and glass) materials. When Stability directly bonded and exposed to a change in temperature, ma- terials with much different CTEs will change in dimension Service temperatures for armor can vary widely depend- by different amounts, resulting in stresses and deformation, ing on where in the world it is deployed. Additionally, envi- including shape change and/or delamination. The CTE of ronmental degradation as a result of ultraviolet (UV) radia- the adhesive interlayer, along with its mechanical properties, tion or oxidation of the polymeric interlayer can affect both can mitigate the effect of CTE mismatch in these systems. the transparency and the adhesive strength of the interlayer. Index of Refraction Strain-Rate Dependence Reflection from interfaces reduces the amount of light Most adhesive interlayers are for protection against bal- transmitted through a composite material. Ideally the re- listic threats. Thus, the properties of the adhesive at relevant fractive index of the interlayer material should be chosen strain rates must be known. Polymeric materials, of which according to the relationship nadhesive = √(nmat1 × nmat2) with most of the interlayers are made, typically have mechanical the thickness equal to a quarter wavelength so as to minimize properties that depend greatly on strain rate and pressure. reflection.146 Mechanical Impedance Cost Waves traveling through composite armor can be re- Generally the cost of the adhesive is insignificant com- flected or transmitted depending on the impedance mismatch pared to that of armor materials such as AlON and sapphire. between consecutive layers. Control of the mechanical im- pedance of the interlayer is therefore important in the design Additional Functionality of the armor. The impedance of many polymers is only about 0.05 to 0.005 of that of ceramics, so that most (90 percent or The implantation of metal wires or conductive materials more) of the incident energy is reflected from the ceramic- for resistive heating (for defogging, for instance) relies on the interlayer interface. ability of the soft interlayer material to act as a host matrix. Implementation of heads-up displays or other electronics 145Hou, S., and H. Reis. 2009. Adhesive bond evaluation in laminated safety glass using guided wave attenuation measurements. Pp. 33-44 in Advances in Ceramic Armor IV: Ceramic Engineering and Science Pro- 146Hecht, E. 2001. Optics, 4th edition. Old Tappan, N.J.: Addison-Wesley ceedings, Volume 29, Issue 6. L.P. Franks, ed. Hoboken, N.J.: John Wiley & Sons. Longman.

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94 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS epoxies have shown improved performance.150 Epoxies for are sometimes required, which places additional demands on the adhesive. use as adhesives in nontransparent composite armor have also been studied.151 Glass Transition Temperature and Mechanical Loss Peaks Other Materials The transition temperature is key for processing and for providing a flexible material at the service temperature. Thermoplastic poly(ethylene vinyl acetate), and low- Polymeric materials can absorb energy due to their many temperature flowing glass or glass ceramics have been molecular motions. Such motions are temperature and fre- used as an interlayer for bonding alumina and sapphire or other high-temperature materials.152 Other hybrid materials quency dependent and the associated mechanical loss peaks can be tailored for energy absorption in high-rate events. specifically engineered for combining the adhesive and the rear panel are available from some manufacturers (e.g., FAE- NAC, a transparent plastic composite from Saint-Gobain Types of Adhesive Interlayers Sully153,154). Thermoplastic Polyvinyl Butyral Testing, Simulation, and Modeling of Adhesives Developed in the late 1930s and commonly used in a utomotive glass applications, thermoplastic polyvinyl Adhesives are generally tested as part of a composite. butyral (PVB), which is generally plasticized, has been the It is the combined properties of the system that matter, and workhorse of polymeric adhesive interlayers. Examples are the interplay between the various hard components of the Saflex (Solutia, Inc.), Butacite (DuPont), Trosifol (Kuraray armor is transmitted via the adhesive interlayers. Chapter 2 of Advances in Ceramic Armor155 provides a good overview Europe), S-LEC (Sekisui Chemical), and KB (GlasNova- tions). Positive features of PVB include good optical trans- of destructive testing methods for adhesives. The simplest parency when bonded to glass, controllable adhesion to composite armor can be considered to be a ceramic plate glass, resistance to elongation when struck with a projectile, adhesively bonded to a metal plate. An obvious question and good UV stability.147 beyond the choice of material for the adhesive concerns the impedance and thickness of the adhesive. As for deformation and failure of the ceramic, an interlayer with a higher imped- Thermoplastic Polyurethanes ance is better since less of the energy would be reflected back Thermoplastic polyurethanes (TPUs) come in two from it as a tensile wave, and a thinner interlayer likewise is broad categories, aliphatic or aromatic, depending on the better. This is because more of the incident energy is more precursor from which they are synthesized. Examples are quickly transmitted to the metal layer, and when a compres- Dureflex (Bayer Material Science), IM800 (and others from sive wave reflected from the interlayer-metal interface arrives Inter Materials), Deerfield 4700 (and others from Deerfield at the ceramic, it helps to prevent bending and subsequent Urethane), and Huntsman 399. Aliphatic TPUs are generally cracking of the ceramic. As for the metal, however, a thicker preferred for transparent armor applications because of their interlayer is better since it would spread the deformation superior clarity compared to aromatic TPUs. TPUs are some- times preferred to PVB since they do not contain plasticizer, which can chemically attack other polymers such as acrylics 150 Uram, Jr., J.R. 1984. Moisture-Resistant Transparent Mercaptan Com - and polycarbonate.148 TPUs are typically extruded and rolled positions. U.S. Patent 4,555,450. Available online at http://www.patents. in sheet form. The composite is formed by layering the ma- com/us-4555450.html. Last accessed April 13, 2011. 151 Zaera, R., S. Sánchez-Sáeza, J.L. Pérez-Castellanosa, and C. Navarro. terials, which are then sealed in a bag that is then evacuated 2000. Modelling of the adhesive layer in mixed ceramic/metal armours of air and autoclaved to consolidate the layers. subjected to impact. Composites Part A 31(8): 823-833. 152 Patel, P.J., G.A. Gilde, P.G. Dehmer, and J.W. McCauley. 2000. Trans - parent armor. The AMPTIAC Newsletter 4(3): 1-2. Thermosets 153 Saint-Gobain Sully. Undated. Technical Datas FAENAC® Film. Available online at http://www.saint-gobain-sully.com/GB/quality/tech/ Other cross-linkable polyurethanes may be used for FICHE%20TECHNIQUE%20FILM%20FAENAC%20A.pdf. Last ac- adhesive interlayer materials. One such example uses a cessed April 2011. poly(urethane urea) elastomer.149 Blends of mercaptans with 154 Jones, C.D., J.B. Rioux, J.W. Locher, E.S. Carlson, K.R. Farrell, B.C. Furchner, V. Pluen, and M. Mandelartz. 2009. Transparent Ceramic 147Freeguard, G.F., and D. Marshall. 1980. Bullet-resistant glass: A re - Composite Armor. U.S. Patent 7,584,689. Available online at http://www. view of product and process technology. Composites 11(1): 25-32. freepatentsonline.com/7584689.pdf. Last accessed April 13, 2011. 148 See http://www.bayerfilms.com/tpu/content.php?p=security-glaze for 155 Sun, X., K.C. Lai, T. Gorsich, and D.W. Templeton. 2009. Optimiz - more information. ing transparent armor design subject to projectile impact conditions. Pp. 149 Sarva, S.S., and A.J. Hsieh. 2009. The effect of microstructure on the 15-22 in Advances in Ceramic Armor IV: Ceramic Engineering and Sci- rate-dependent stress-strain behavior of poly(urethane urea) elastomers. ence Proceedings, Volume 29, Issue 6. L.P. Franks, ed. Hoboken, N.J.: Polymer 50(13): 3007-3015. John Wiley & Sons.

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95 LIGHTWEIGHT PROTECTIVE MATERIALS: CERAMICS, POLYMERS, AND METALS over a larger volume of the metal.156,157 Clearly, detailed characteristics and properties is the design of a buffer interface capable of accommodating the dissimilarities.162,163 modeling and simulation can provide optimized solutions for armor design. Studies elucidating the influence of defects in The fundamentals that need attention when joining plate materials158 have been conducted. The ARL is actively surfaces are surface roughness and surface contamination. modeling transparent composite armor.159,160 When two surfaces are brought into contact, the true area of contact is less than the apparent area of contact owing to Finding 5-9. There is need for an improved understanding inherent surface roughness and the nonplanarity on an atomic of the dynamic behavior of the adhesive by itself and of the scale of any surface. This inherent roughness is overcome adhesive placed between dissimilar hard materials as part of by deformation, diffusion of surfaces (direct bonding), or an armor system. infiltration of bonding filler between the two surfaces (indi- rect bonding). In armor systems, materials such as ceramics (for example, alumina, silicon carbide, or boron carbide) JOINING are bonded to metals (steels, aluminum, titanium, or mag- Armor systems use different classes of materials—ce- nesium), which in turn are joined to fibers, woven fibers, ramics, metals, polymers, and composites—to meet defined or polymer structures, mostly using indirect or mechanical ballistic threat requirements. The quality of the joints be- bonding processes. tween dissimilar materials therefore plays an important role The selection of a technique for manufacturing a particu- in the final armor performance, since these linkage sites lar component will be based on a number of factors: have to withstand the dynamic loads under ballistic and blast conditions. With the increased use of diverse advanced ma- • Types of materials to be joined, terials, the number of armor joints is increasing and greater • Desired component function—for example, strength, demands are being placed on them for better performance. • Operational temperature, Current methods of combining dissimilar armor material • Applied mechanical stresses (static and dynamic) on are somewhat empirical and based on experience with other the joint, products so that systematic research efforts are needed for • Required level of joint airtightness, understanding the dependence of ballistic performance on • Component design, and bond characteristics. • Cost. Joining different materials is often not an easy task.161 In general, bonding methods are chosen based on the particular Mechanical joints typically have poor joint strength materials to be combined, their geometrical configuration, and (10-50 MPa) and create areas of stress in ceramics, limiting design flexibility. Their use in armor applications164 is thus the performance requirements. In joining different materials careful attention has to be paid to minimize mismatches in restricted except where such conditions can be tolerated. In properties and structural discontinuities. The key to a success- both indirect and direct bonding, charge or mass transfer can occur between surfaces.165 In indirect joining, an intermedi- ful joint between dissimilar materials with different bonding ate layer of filler alloy is used for bonding different surfaces. Examples of indirect bonding include soldering, brazing, adhesive bonding, and other processes that provide contact between the surfaces through the intervening filler materials. In contrast, direct bonding uses no fillers, and the bonding 156 Ibid. occurs by means of the solid-state processes that depend on 157 Zaera, R., S. Sánchez-Sáeza, J.L. Pérez-Castellanosa, and C. Navarro. deformation and diffusion between surfaces. 2000. Modelling of the adhesive layer in mixed ceramic/metal armours subjected to impact. Composites Part A 31(8): 823-833. Bonding via solid-state diffusion requires the applica- 158 Fountzoulas, C.G., J.M. Sands, G.A. Gilde, and P.J. Patel. 2009. tion of heat and long exposure times, while deformation Applying modeling tools to predict performance of transparent ceramic requires relative sliding of surfaces with substantial applied laminate armors. Pp. 45-53 in Advances in Ceramic Armor IV: Ceramic stresses. Thus, while solid-state bonding methods yield Engineering and Science Proceedings, Volume 29, Issue 6. L.P. Franks, ed. Hoboken, N.J.: John Wiley & Sons. 159 Fountzoulas, C.G., B.A. Cheeseman, P.G. Dehmer, and J.M. Sands. 162 2009. A Computational Study of Laminate Transparent Armor Impacted Paiva, O.C., and M.A. Barbosa. Brazing parameters determine the by FSP, ARL-RP-249, June. Available online at http://www.arl.army.mil/ degradation and mechanical behaviour of alumina/titanium brazed joints. arlreports/2009/ARL-RP-249.pdf. Last accessed April 13, 2011. Journal of Materials Science 35(5): 1165-1175. 160 MacAloney, N., A. Bujanda, R. Jensen, and N. Goulbourne. 2007. 163 Howe, J.M. Bonding, structure, and properties of metal/ceramic Viscoelastic Characterization of Aliphatic Polyurethane Interlayers, ARL- interfaces: Part 2 interface fracture behaviour and property measurement. TR-4296, October. Available online at http://www.dtic.mil/cgi-bin/GetTR International Materials Reviews 38(5): 257-271. 164 Klomp, J.T., and G. de With. 1993. Strong metal-ceramic joints. Doc?AD=ADA474714&Location=U2&doc=GetTRDoc.pdf. Last accessed April 13, 2011. Materials and Manufacturing Processes 8(2): 129-157. 161 do Nascimento, R.M., A.E. Martinelli, and A.J.A. Buschinelli. 2003. 165Martinelli, A.E. 1995. Diffusion Bonding of Silicon Carbide and Sili - Review article: Recent advances in metal-ceramic brazing. Cerâmica con Nitride to Molybdenum. Ph.D. dissertation. Montral, Canada: McGill 49(312): 178-198. University.

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96 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS strong joints, they may not be suitable for most of the armor cooling owing to the different CTEs of metals and ceramics. applications because dissimilar materials have different Intermediate layers to alleviate expansion coefficient mis- temperature tolerances and deformation characteristics. The matches are being continuously developed, and the quality role of temperature in joining dissimilar materials for a given of the bond depends on filler layers that mediate the joining set of surfaces is also an important operational parameter in with minimum stress buildup. selecting bonding media. The vast majority of joining pro- Active soldering is an emerging technology similar to cesses involve heating surfaces that, upon cooling, develop active brazing but performed at lower temperatures (<450°C) residual stresses owing to mismatches in elastic modulus to reduce mismatch stresses during heating and cooling cy- and CTE. cles. Here, reactive elemental titanium is added to the solder More often, the ceramic-metal joint in armor applica- alloy as it is to a brazing alloy to enable direct wetting and tions is achieved through indirect bonding processes. Adhe- bonding. The lower joining temperatures offered by active sive joining is widely used. Even though adhesive joining soldering minimize thermal stresses while yielding reason- with epoxy is executed under ambient conditions and is able elastic impedance matching. Solder joint strengths are applicable to most materials, the resulting bond strength is similar to those of epoxy joints. The tensile strength of an relatively weak relative to brazing or soldering. Further, the epoxy joint for bonding a hot-pressed SiC surface to an an- low modulus of epoxy joints leads to a large elastic imped- nealed Ti-6Al-4V surface is greater (73 MPa) than that of an ance mismatch with ceramic and metal, which could lead to active solder joint (43 MPa), while the elastic impedance of poor ballistic performance.166 Very few adhesive materials a solder joint is 10 times better than that of an epoxy joint, exist with impedance close to that of metals and ceramics, thus approaching the elastic impedance of ceramic and metal because wave velocity depends on the elastic modulus and surfaces. density of the material. The class of adhesives whose imped- Finding 5-10a. Reliable methods for manufacturing dis- ance most closely matches that of ceramics and metals is high-temperature ceramic adhesives. However, such ceramic similar materials are in a nascent stage. Systematic studies to adhesives are not as strong as polymer glues, and they are understand the relationships between ballistic performance, often used as matching layers in mechanically bonded sys- bond adherence, key filler material characteristics, and elastic tems. By combining ceramic adhesives with polymeric and impedance matching are needed to enable the manufacture other glues, performance could be considerably improved. of armor systems containing dissimilar advanced materials. Multilayer adhesives with better impedance match have Finding 5-10b. Investment is needed in research and devel- demonstrated167 improved multi-hit ballistic performance and structural integrity. opment in active brazing and soldering materials, adhesives, Bonding options such as brazing and soldering typically and processing methods for joining armor material to pro- result in higher modulus interfaces and thereby decrease duce joints with minimal thermal mismatch stresses during (compared to adhesives) the elastic impedance mismatch the heating and cooling cycle of the bonding method. with ceramic and metal substrates. Brazing or soldering ce- ramics to metal relies on wetting the ceramic surface with a OTHER ISSUES IN LIGHTWEIGHT MATERIALS suitable metal or alloy that will react with both the metal and the ceramic to form a joint. However, heating the surfaces to Nondestructive Evaluation Techniques high temperatures develops residual stresses on cooling due to mismatches in elastic modulus and CTE. The heating tem- Nondestructive evaluation (NDE) techniques have been peratures for braze alloys are above 450°C and for soldering employed for the characterization of armor and armor mate- rials for several decades.169 These techniques are preferred below 450°C. Achieving a superior brazed or soldered bond while minimizing residual stresses is important. over destructive ones since they leave the material intact and Mizuhara et al.168 developed an active brazing method ready for use. NDE tests of entire lots of materials can iden- in which the active component, such as titanium, is incorpo- tify specific pieces that do not meet the appropriate criteria rated into silver-copper eutectic brazing alloys to enhance the without having to rely on statistical interpolations of destruc- wetting of ceramic and metal surfaces. This one-step vacuum tive test results carried out on a few select samples. Different brazing process wets most armor materials (ceramics, tita- evaluation techniques are applied to garner different kinds of nium alloys, and steels) and forms a superior metallurgical information from the armor material. NDE is applied at vari- bond. However, the high processing temperature required ous stages in the testing of armor to assess the performance for “active” brazing results in a large buildup of stress upon capabilities of armor materials, to ensure the integrity of assembled arrays of tiles, and to understand how materials become damaged when introduced to various threats. 166James, B. 2004. Practical issues in ceramic armor design. Pp. 33-44 in Progress in Ceramic Armor. New York, N.Y.: John Wiley & Sons. 167Ibid. 168Mizuhara, H., and K. Mally. 1985. Ceramic to metal joining with active 169See the American Society for Nondestructive Testing Web site at brazing filler metal. Welding Journal 64(10): 27-32. http://www.asnt.org/.

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97 LIGHTWEIGHT PROTECTIVE MATERIALS: CERAMICS, POLYMERS, AND METALS increased thickness; however, as the composite becomes NDE testing focuses on determining whether materials thicker, the marginal protective gain from increasing the for compiled armor assemblies will perform adequately when thickness is less,172,173 while the rate at which the weight they are used in the field. This can be as basic as a simple go/ increases is maintained. no-go test or as complicated as a three-dimensional repre- sentation of internal flaws and density gradients. A variety of nondestructive methods has historically been used to rapidly OVERALL FINDINGS locate and identify anomalous internal flaws in dense armor The overall findings of Chapter 5 are summarized be- materials. These methods, which include resonant ultrasound low and are addressed by the recommendations presented spectroscopy, high-frequency ultrasound scans, infrared ther- in Chapter 6. mography, and microfocus x-ray computed tomography, are discussed in Appendix I. Finding 5-11. A sustained effort is needed to develop a da- tabase of high-strain-rate material behavior for the ceramic, Fiber-Reinforced Polymer Matrix Composites polymeric, and metallic materials in use today and to expand the database as new materials are developed. Polymer matrix composites (PMCs), discussed further in Appendix J, consist of a polymer resin reinforced with Finding 5-12. The intrinsic properties of opaque and trans- fibers. One application is the combat helmet. PMCs can be parent ceramics and ceramic powders are underrealized in subdivided into two categories based on whether the fiber armor systems. There is a need for an atomic, nano, and reinforcement is continuous or discontinuous. PMCs with micron-size understanding of how powders and processing discontinuous fibers (less than 100 mm long) are made with can be designed and manipulated to realize the benefits of thermoplastic or thermosetting resins, whereas PMCs with dense and porous ceramic armor. continuous fibers usually employ thermosetting resins. The most common design for PMCs is a laminate Finding 5-13. A need exists to build a production infrastruc- structure made of woven fabrics held together by a polymer ture for strategic ceramic powders within the United States resin. Fabrics are incorporated to take advantage of their high for the next generation of opaque and transparent ceramic strength and stiffness and to improve energy absorption and armor. distribute the kinetic energy laterally. Owing to their highly engineered structures, PMCs are lightweight with high spe- Finding 5-14. Current opportunities include the develop- cific strength and high specific stiffness. ment of finer diameter and more ideal polymeric and carbon Common reinforcement materials are carbon, glass, ara- fibers with a two- to fivefold improvement in specific tensile mid, and polyethylene fibers. PMCs can be manufactured by strength over the current state-of-the-art fibers. Such im- wet and hand lay-up, molding (compression, injection, and provements would lead to significant reduction in the weight transfer), vacuum bag molding, infusion molding, vacuum- assisted resin transfer molding, prepreg170 molding, and of body armor. other common techniques. Unlike the usual structural com- Finding 5-15. Since polymers are often parts of systems posites, which typically contain up to about 60 vol percent (e.g., fabrics, matrixes, and reinforcing elements in com- fibers, ballistic PMCs contain a higher volume fraction of posites), a fundamental understanding of how to model the fibers or fabrics, up to about 80 vol percent, although the deformation mechanisms and failure processes of polymers effect of this variation in structure on the ballistic protection is critical to the successful large-scale modeling of complex properties of PMCs has not been thoroughly investigated. multicomponent armor systems. Because PMCs respond to ballistic impact in ways that depend on their particular structure, they are different from Finding 5-16. Advances are needed in test methods for other protective materials. Unlike fabric materials, the PMC determining the high-strain-rate (103 to 106 s–1) properties material responds only in the neighborhood of the impact of fibers, polymers, and ceramics and their dynamic failure position; thus the response is completely governed by the processes. Results could be used to develop a comprehensive local behavior of the material and unaffected by the bound- database of strain-rate behavior for such materials. ary conditions. Additionally, the penetration mechanism is dependent on the thickness of the composite. For thin com- Finding 5-17. The very low density of magnesium, including posites, the deformation across the thickness direction does not vary with depth, while for thick composites it does.171 magnesium alloy fibers, could lead to the development of very lightweight alternatives to traditional metallic materials Ballistic performance initially increases linearly with the 170Semifinished fiber products preimpregnated with epoxy resin (pre- 172Ibid. pregs). 171Naik, N.K., and A.V. Doshi. 2008. Ballistic impact behaviour of thick 173Faur-Csukat,G. 2006. A study on the ballistic performance of com- composites: Parametric studies. Composite Structures 82(3): 447-464. posites. Macromolecular Symposia 239(1): 217-226.

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98 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS Finding 5-18. The development of bonding materials (ad- in protection material systems. A better basic understanding of the strengthening mechanisms in magnesium, especially hesives, brazes, and solders) whose elastic impedances and the development of ultra-fine-grained magnesium alloys thermal expansion coefficients match those of the materials through severe plastic deformation, could be highly benefi- to be bonded will improve the ballistic and blast performance cial. Magnesium-based fibers are also worthy of exploration. of laminated armor.