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Application of Boron Carbide Ceramics in Defence and Military
Boron carbide is a new type of non oxide ceramic material, because of its high melting point, high hardness, low density, good thermal stability, strong chemical resistance and neutron absorption ability, it is used in the field of national defense and military industry, and plays an important role in the maintenance of national security.
Preparation of boron carbide ceramics
Hot pressing sintering
Because of the pressure at high temperature, the particles rearrange and produce plastic flow, which leads to grain boundary slip and strain induced twinning, creep and volume diffusion. The results show that boron carbide ceramics with high density and high strength can be obtained by the combination of these mechanisms. Hot pressing sintering is carried out in inert atmosphere or vacuum. Generally, the hot pressing temperature is 2200 ℃ ~ 2300 ℃, the pressure is 20MPa ~ 40MPa, and the holding time is 0.5h ~ 2H. However, boron carbide is a compound with strong covalent bond. It is difficult to obtain high density and high performance products without adding boron carbide. In order to reduce the sintering temperature and improve the properties of boron carbide, sintering additives must be added to promote sintering. Because of the poor thermal shock resistance of boron carbide, it is necessary to slow down the temperature, so hot pressing sintering can only prepare products with simple shape.
Hot isostatic pressing sintering
The densification of boron carbide can be achieved without additives by hip sintering, and the fine-grained microstructure and high bending strength can be obtained. Hot isostatic pressing (hip) is to use inert gases such as N2, AR as the medium to transfer pressure, and put the boron carbide powder compacts or the powder packed into the high-pressure container to make the powder withstand high temperature and balanced pressure, reduce the sintering temperature and avoid grain growth, so as to obtain high-density boron carbide ceramic materials. Compared with the conventional hot pressing method, it can make the material receive the same direction pressure, so the microstructure of the ceramic is uniform. The disadvantage is that the equipment cost is higher and the size of the workpiece to be processed is limited.
It is difficult to densify pure boron carbide by pressureless sintering. The main factors affecting the density of boron carbide ceramics are sintering temperature and powder size. When sintered at 2300 ℃ under normal pressure, the relative density of the products is generally less than 80%, and abnormal grain growth and surface melting are easy to occur. Increasing the specific surface area and reducing the average particle size of boron carbide powder can improve the sintering density to a certain extent. Therefore, in order to reduce the sintering temperature and improve the comprehensive properties of boron carbide products, sintering additives, such as boron, carbon and aluminum, should be added to promote the sintering of boron carbide.
Spark plasma sintering (SPS)
Spark plasma sintering (SPS) developed in recent years is a new rapid sintering process, which can realize the rapid and efficient sintering of materials at low temperature. In the SPS sintering process, the discharge plasma generated instantly when the electrode was connected with a high DC current, which made the particles in the sintering system generate Joule heat uniformly, and the surface of the particles was activated. SPS can make full use of the internal heating and the dispersion distribution of discharge points to realize uniform heating. In addition, SPS can realize the effective discharge between particles, resulting in local high temperature, partial melting of particle surface and peeling off of surface oxide film. With this new sintering technology, boron carbide ceramics can be sintered with high density without sintering additives, and the structure of the products is uniform and the chemical composition is controllable.
Application of boron carbide ceramics in bulletproof field
The high melting point, high hardness and low density of boron carbide ceramics make it an ideal substitute for bulletproof materials. At present, boron carbide bulletproof materials have been widely used in bulletproof vests, bulletproof armor, armed helicopters and other protection fields.
At present, according to the bulletproof materials, the national defense bulletproof vests can be divided into three types: hard, soft and combination of soft and hard. Among them, the hard bullet proof vests are mainly composed of metal, ceramic and other composite materials. Because of its high hardness, the bullet is difficult to penetrate, so as to achieve the purpose of bulletproof. Bulletproof clothing is regarded as an important equipment for human protection. The interceptor, the first boron carbide bulletproof vest for personal protection, was developed in the early 1960s. By 2012, 68000 sets of interceptor body armor had been put into the battlefield. The United States and Israel have also produced bulletproof vests with boron carbide ceramic chips embedded in Kevlar fabrics.
Armored ceramic materials are mainly used in bulletproof armored vehicles. The single-phase ceramics used for armor protection mainly include alumina, boron carbide and silicon carbide. However, in practical applications, they usually appear in the form of composite armor. Armored ceramic materials are widely used in additional roof, hatch cover, exhaust plate, turret seat ring, bulletproof glass, pivot frame and other armored components as well as the lower body of tank vehicles; it is also used to manufacture trunk plate, side plate, vehicle door and driver’s seat. Among the main tanks, the current German Panther – Ⅱ, the British Challenger series, Israel’s mecava, the US Abrams, ee-t1, Osorio, and the former Soviet Union’s T-72 are equipped with carbide ceramic composite armor at the top, bottom and around.
The properties of boron carbide ceramics are especially suitable for armed helicopters and other aircrafts, which improves the traditional design and can be used to resist the shell attack from the ground. For example, boron carbide and Kevlar composite armor have been widely used in Black Hawk helicopter passenger seats in the United States; in the 1960s, boron carbide ceramics were assembled to the cockpit floor, side wall bulletproof plate and pilot seat of armed helicopters.
Application of boron carbide ceramics in nuclear industry
Neutron absorbing materials
In the reactor core assembly, the neutron absorbing material (control rod, regulating rod, accident rod and safety rod) is the important functional element next to the fuel element. Because boron carbide has high neutron absorption cross section, wide absorption energy spectrum, low price and rich raw material source, it is easy to dispose of waste material. Therefore, boron carbide is an important neutron absorbing material.
With the development of various nuclear reactors, the requirements of reactor shielding materials and other shielding systems are higher and higher. Now many shielding materials have been difficult to meet the requirements of their use, mainly in the shielding effect of shielding materials and other properties such as mechanical properties, heat resistance, radiation resistance and so on. Boron carbide can be used as a shielding material for nuclear reaction. Compared with pure elements B and CD, boron carbide is widely used in nuclear industry because of its low cost, no radioisotope production, low secondary ray energy, corrosion resistance and good thermal stability.
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