Ceramics are used in personal armor systems to defeat small-caliber threats such as pistols, rifles, and machine guns. Ceramics are relatively light compared to more traditional armor made of metallic alloys. Properties that contribute to the excellent performance of ceramic armor include high hardness, low density, high elastic constants, and high compressive strength. But as stand-alone items, ceramics would not be particularly good because of their low tensile strength and sensitivity to small mechanical defects such as pores and cracks. Hence, they are used in combination with other materials such as polymers and metals as laminar composites, which enhances their excellent protection properties.
A hard body armor plate typically includes a layer of dense boron carbide or silicon carbide backed by a layer of metal, polymer, or a composite. The purpose of this combination is to convert the projectile’s kinetic energy into plastic “work” and stored elastic energy and to broaden the area of contact of between the plate and the body during an impact event. The laminar ceramic/polymer composite is encased in tightly woven canvas. Sometimes additional layers of canvas or other materials are enclosed within the wrapping, depending on the particular manufacturer.
This composite armor package defeats the incoming missile by several mechanisms. On initial impact, the missile is held up by the ceramic surface, which behaves as an elastic barrier. The time the missile is held up is known as the dwell time—the longer the dwell time, the more effective the protective system. During the dwell time, the bullet flattens, flows plastically, and erodes from its tip. At the same time a compressive elastic wave and a shear wave are generated at the point of impact; they propagate radially, reflecting from the back surface and propagating back into the material. The compressive wave converts into a tensile wave upon reflection and acts as an initiator of many cracks within the ceramic. The magnitude of the wave passing into the backup plate depends on the elastic impedance mismatch of the ceramic and the backup material: The closer the match, the less reflection there is of the elastic waves.
A plastic zone develops beneath the contact site. The high pressure under the tip of the bullet and the constraint of the surrounding ceramic material tend to suppress macroscopic fracture and permit plastic deformation to occur. Plastic processes in the ceramic include microcrack formation, amorphization, phase
transformation, and twinning and dislocation generation. These are very general mechanisms of plastic deformation that occur in most crystalline ceramic materials. Plasticity is enhanced at the contact point by further constraining the material by inducing compressive stress over the entire outside layer of the ceramic. This can be done by tighter wrapping of the cloth that encases the laminar composite. In addition to a plastic zone, a cone crack develops at the point of contact and propagates into the solid and eventually completely through the ceramic plate. The highly fractured zone of ceramic material, which generates from the rear surface of the ceramic plate, forms primarily within the boundary of the cone crack. This cone crack plays an important role in transferring momentum from the bullet to the backup plate. The cone is filled with plastically deformed and crushed ceramic material before the impact event is complete.
For the bullet to penetrate into the ceramic plate, ceramic material that is under the bullet flows around the bullet and sprays into the air on the impact side of the plate. As the ceramic material flows away from the front of the bullet, it breaks into small particles of ceramic (10-100 µm). These particles erode the bullet as the crushed ceramic flows past the bullet and sprays into the air on the impact side of the armor. In the most favorable scenario, the bullet is completely eroded away and—if within the design parameters of the insert—eliminated as a fatal threat to the person wearing the vest.
Finally, and for purposes of this committee, the incoming momentum of the bullet has to be transferred to the target. This is first done by momentum transfer to the cone of crushed and deformed ceramic. The force is picked up by the backup plate, which catches the moving ceramic cone. As the base of the cone is very much larger than the apex (1 mm vs. 25 mm radius), the pressure at the base is about 1,000 times less at the base than at the apex. The backup plate then deforms, further absorbing the impact force of the bullet. The final transfer of momentum is to the person wearing the protective vest. This absorption of force ends up in blunt trauma injury, sometimes severe enough to topple the person but not to kill him. It should be noted that the momentum transfer of a bullet is only a hundredth that of severe head contact in American football.