What is the bulletproof principle of ceramic materials
The basic principle of armor protection is to consume projectile energy, slow down the projectile and achieve harmlessness. Most traditional engineering materials, such as metal materials, absorb energy through plastic deformation of the structure, while ceramic materials absorb energy through the process of micro-crushing.
The energy absorption process of bulletproof ceramics can be roughly divided into three stages. (1) Initial impact stage: the projectile hits the ceramic surface, making the warhead dull, and absorbs energy during the process of crushing the ceramic surface to form small and hard fragments; (2) erosion stage: the dull projectile continues to erode the fragments , Forming a continuous ceramic fragment layer; (3) Deformation, cracking and fracture stage: Finally, tensile stress is generated in the ceramic to break the ceramic, and then the back plate is deformed, and the remaining energy is all absorbed by the deformation of the back plate material. When the projectile hits the ceramic, both the projectile and the ceramic are damaged.
What are the requirements of bulletproof ceramics for material performance
Because of the brittleness of the ceramic itself, it fractures rather than plastically deforms when it is impacted by the projectile. Under the action of tensile load, fracture first occurs in heterogeneous places such as pores and grain boundaries. Therefore, in order to minimize the microscopic stress concentration, armored ceramics should be high-quality ceramics with low porosity (up to 99% of the theoretical density value) and fine-grained structure.
Material performance and its influence on ballistic performance
There are many bulletproof ceramic materials, these are the most commonly used
Since the 21st century, bulletproof ceramics have developed rapidly and there are many types, including alumina, silicon carbide, boron carbide, silicon nitride, titanium boride, etc., among which alumina ceramics (Al2O3), silicon carbide ceramics (SiC), carbide Boron ceramics (B4C) are the most widely used. Alumina ceramics have the highest density, but relatively low hardness, lower processing thresholds, and lower prices. According to the purity, alumina ceramics are divided into 85/90/95/99 alumina ceramics, and the corresponding hardness and price also increase sequentially.
Performance comparison of different bulletproof ceramic materials
Silicon carbide ceramics have relatively low density and high hardness, and are structural ceramics with high cost performance. Therefore, they are also the most widely used bulletproof ceramics in China.
Boron carbide ceramics have the lowest density and the highest hardness among these types of ceramics, but at the same time they have high requirements on processing technology and require high temperature and high pressure sintering, so the cost is also the highest among these three types of ceramics.
Comparing these three more common bulletproof ceramic materials, alumina bulletproof ceramics have the lowest cost, but their ballistic performance is far inferior to silicon carbide and boron carbide. Therefore, at present, most of the domestic production units for bulletproof ceramics are silicon carbide and boron carbide. Aluminum ceramics are rare. However, single crystal alumina can be used to prepare transparent ceramics and is widely used as a transparent material with optical functions, and is used in military equipment such as individual bulletproof masks, missile detection windows, vehicle observation windows, and submarine periscopes.
Silicon carbide and boron carbide have become the two most popular bulletproof ceramic materials
Silicon carbide bulletproof ceramics
The covalent bond of silicon carbide is extremely strong, and it still has high-strength bonding at high temperature. This structural feature gives silicon carbide ceramics excellent strength, high hardness, wear resistance, corrosion resistance, high thermal conductivity, and good heat resistance. Vibration and other properties; at the same time, silicon carbide ceramics are moderately priced and cost-effective. It is one of the most promising high-performance armor protection materials. SiC ceramics has a broad development space in the field of armor protection, and its applications in the fields of individual equipment and special vehicles tend to be diversified. When used as a protective armor material, taking into account factors such as cost and special applications, small pieces of ceramic panels and composite backplanes are usually bonded to form a ceramic composite target plate to overcome the failure of the ceramic due to tensile stress and ensure that the projectiles During penetration, only single pieces are crushed without destroying the entire armor.
Boron carbide bulletproof ceramics
Boron carbide is currently known as a superhard material whose hardness is second only to diamond and cubic boron nitride. The hardness is as high as 3000 kg/mm2; the density is only 2.52 g/cm3, which is 1/3 of steel; the modulus of elasticity High, 450GPa; high melting point, about 2447 ℃; low thermal expansion coefficient, high thermal conductivity. In addition, boron carbide has good chemical stability, acid and alkali corrosion resistance, does not react with acid, alkali and most inorganic compound liquids at room temperature, and only slows down in the mixed liquid of hydrofluoric acid-sulfuric acid and hydrofluoric acid-nitric acid. Corrosion; and it does not wet or interact with most molten metals. Boron carbide also has a good ability to absorb neutrons, which is not available in other ceramic materials. The density of B4C is the lowest among several commonly used armor ceramics, and its higher elastic modulus makes it a good choice for military armor and space materials. The main problem of B4C is that it is expensive (about 10 times that of alumina) and brittle, which limits its wide application as a single-phase protective armor.
Preparation method of bulletproof ceramics
From the characteristics of the ceramic material preparation process, it can be seen that the current process development is more mature: reaction sintering, pressureless sintering and liquid phase sintering. These three sintering methods have lower production costs, simple preparation processes, and mass production. The possibility is higher. Hot pressing sintering and hot isostatic pressing sintering are relatively limited by the product size, the production cost is higher, and the maturity is lower. Ultra-high pressure sintering, microwave sintering, spark plasma sintering, and plasma beam melting are the lowest in maturity and are relatively novel preparation methods. However, they have high requirements for technology and equipment, and require high production costs to achieve mass production. The feasibility is low, and it is often used in the experimental exploration stage, which is of little significance to practical applications, and it is difficult to realize industrialization.
Preparation process of typical bulletproof ceramics
Upgrade of bulletproof ceramics
Although the bulletproof potential of silicon carbide and boron carbide is very large, the problems of poor fracture toughness and brittleness of single-phase ceramics cannot be ignored. The development of modern technology has put forward requirements for the functionality and economy of bulletproof ceramics: multi-function, high performance, light weight, low cost and safety. Therefore, in recent years, experts and scholars hope to realize the strengthening, toughening, weight reduction and economicalization of ceramics through micro-adjustment including multi-element ceramic system composite, functionally graded ceramics, layered structure design, etc., and such armors are compared with today's armors. The light weight improves the maneuverability of combat units.
Functionally graded ceramics show regular changes in component material properties through micro-design. For example, titanium boride and metal titanium, aluminum oxide, silicon carbide, boron carbide, silicon nitride and metal aluminum and other metal/ceramic composite systems, the performance changes gradually along the thickness position, that is, the transition from high hardness to high toughness ballistic ceramics is prepared .
Nanocomposite ceramics are composite ceramics formed by adding submicron or nanometer dispersed particles to a matrix ceramic. Such as SiC-Si3N4-Al2O3, B4C-SiC, etc., have a certain increase in the hardness, toughness and strength of ceramics. According to reports, Western countries are studying the sintering of nano-level powders to prepare ceramics with a grain size of several tens of nanometers to achieve material strength and toughness. Ballistic ceramics are expected to achieve a major breakthrough in this regard.
to sum up
Whether it is single-phase ceramics or multi-phase ceramics, the best bulletproof ceramic materials are still inseparable from the two materials of silicon carbide and boron carbide. Especially for boron carbide materials, with the development of sintering technology, the superiority of boron carbide ceramics is becoming more and more prominent, and the application in the field of bulletproof will be further developed.