The bulletproof mechanism of ceramics and metals is very different. Metals absorb the kinetic energy of projectiles due to plastic deformation, while ceramics absorb the kinetic energy of projectiles due to their rupture. Generally, the ceramic armor system is composed of a single piece of ceramic or ceramic-metal composite and covered with a nylon cloth layer combined with high tensile strength organic fibers (glass fiber is also possible).
Under the impact of the bullet (velocity>700～800m/s), the front side of the ceramic is broken and the remaining energy is absorbed by the soft reinforcement (such as nylon cloth layer) material on the back side. The reverse material must be able to support the fragments of the ceramic material and the bullet itself after the bullet impacts.
Bulletproof ceramics require many properties, such as density and porosity, hardness, fracture toughness, Young's modulus, sound velocity, mechanical strength, etc. None of the properties can have a direct and decisive relationship with the overall ballistic performance, so the fracture mechanism It is very complicated, crack formation is caused by many factors, and the time of occurrence is very short. The porosity of bulletproof ceramics should be as low as possible to improve the hardness and Young's modulus. For Al2O3 porcelain, the porosity should be close to zero, and the water absorption rate should not exceed 0.02%. The hardness of ceramics is very demanding, which should be higher than that of flying warheads. For Al2O3 ceramics, the hardness Hv should exceed 1220-1250. The speed of sound propagation in the ceramic indicates the ability of the ceramic to consume energy on the impact surface. A high speed of sound is desired, and a high speed of sound indirectly also indicates that the ceramic has good densification and low closed pores. According to actual experience, the sound velocity of Al2O3 porcelain should be greater than 10000m/s, preferably 10500-11500m/s. There are two types of bulletproof ceramics, namely monolithic ceramic structure and ceramic composite structure. Monolithic structure ceramics include oxide ceramics (mainly Al2O3 ceramics) and non-oxide ceramics (for example: SiC, Si3N4, AlN and TiB2, etc.) , And binary systems (for example: B4C-TiB2 based ceramics). Generally speaking, non-oxide ceramics have higher physical properties and relatively low density (except for TiB2-based ceramics), and are more advantageous than Al2O3 ceramics as bulletproof. However, these material manufacturing methods mostly use more expensive hot pressing, which is not easy to industrialize. However, it is also obvious that hot pressing can improve the mechanical properties of bulletproof ceramics. Ceramic matrix composites have high ballistic performance due to high mechanical properties, especially fracture toughness. After the impact of the projectile, ceramic matrix composites have better integrity than monolithic ceramics. Several composite ceramics are as follows: Al2O3/SiC(w), Al2O3/SiC(f), Al2O3/C(f), TiB2/B4C(p), TiB2/SiC(p) and cermet (such as LanxideTM) Its composition is Ni/(Al) carbides, such as Ni/TiC and Al/B4C(p). Most of these materials are hot-pressed, so they are more expensive. Although the bulletproof ceramic LanxideTM is not a hot pressing process, it requires special processes and equipment, so the price is relatively expensive.
