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