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Ballistic Boron Carbide Ceramics: Why do leading companies in the industry compete to participate?
2022-08-26 14:14:07
Boron carbide ceramics are important wear-resistant and high hardness structural ceramic materials in new ceramics. As both boron and carbon are non-metallic elements, and their atomic radii are close, their combination mode is different from that of ordinary interstitial compounds. Therefore, boron carbide ceramics have many unique excellent properties, such as high melting point, ultra-high hardness, low density, wear resistance and corrosion resistance. It is increasingly showing its broad development and application prospects in national defense, nuclear energy, aerospace, machinery, wear resistance technology and other fields.
1. Characteristics of boron carbide ceramics
Boron carbide was first discovered in 1858, but it was not until 1934 that a compound with the stoichiometric formula B4C was proposed and recognized.
Boron carbide is a kind of superhard material whose hardness is only inferior to diamond and cubic boron nitride, and the hardness is up to 3000 kg/mm; Low density, only 2.52g/cm, 1/3 of steel; High elastic modulus, 450GPa; High melting point, about 2447 ℃; Its thermal expansion coefficient is low and its thermal conductivity is high. In addition, boron carbide has good chemical stability, acid resistance, alkali resistance and corrosion resistance. It does not react with acid, alkali and most inorganic compound liquids at room temperature. It only has slow corrosion in hydrofluoric acid sulfuric acid and hydrofluoric acid nitric acid mixed solutions, and does not wet with most molten metals. Boron carbide also has a good ability to absorb neutrons, which other ceramic materials do not have.
The densification sintering of pure boron carbide is extremely difficult. This is because the number of covalent bonds is 93.94%, higher than other structural ceramics, such as SiC (88%), Si3N4 (70%), etc. Therefore, the elimination of pores in boron carbide and the mass transfer mechanism of grain boundary and volume diffusion need to be above 2000 ℃. For example, ordinary B4C powder can only reach 80%~87% relative density when sintered under normal pressure at 2250~2300 ℃. When sintered at such a high temperature, the grains will coarsen and grow rapidly, which is not conducive to the elimination of pores, and will result in a large number of residual pores, limiting the density of materials. Therefore, the sintering of boron carbide must adopt effective additives or pressure sintering.
Application of boron carbide ceramics
As mentioned above, B4C ceramics have low density (lower density than SiC and Si3N4 ceramics), high hardness (higher hardness than SiC and Si3N4 ceramics), high elastic modulus, corrosion resistance, wear resistance, good neutron absorption performance, and good high-temperature semiconductor characteristics, which are widely used in national defense, nuclear energy, wear resistance technology and other fields.
3.1 Bullet proof armor field
As B4C ceramics have the characteristics of light weight, super high hardness and high elastic modulus, it is the best material for bulletproof vests, bulletproof helmets and bulletproof armor. Boron carbide ceramics have been used as ceramic armor materials since the 1960s. Compared with other bulletproof materials (such as SiC and Al2O3), B4C ceramics are lighter and harder, which is particularly suitable for armed helicopters and other aircraft as bulletproof armor materials, and can effectively resist artillery shells. Therefore, B4C ceramics are generally only used in some special occasions with high requirements for protective performance, such as the crew seat of the V22 Osprey rotary wing aircraft of the US military. In addition, the enhanced body armor (EBA) used by the British army also uses boron carbide ceramics, which can defend 12.7mm steel core armor piercing projectiles.
Boron carbide ceramics produced by BAE Advanced Ceramics Branch of British BAE Systems have been used as the US Army's "interceptor" bulletproof vest. In 1997, the United States Army determined the research project of ceramic armor process in the manufacturing technology plan, which mainly studied how to mass produce boron carbide (B4C) small arms protective flashboard armor required by the "interceptor" body armor system at low cost. The units participating in this project include the US Marine Corps, the US Army Soldier and Biochemical Command, Professional Defense Systems, Simula Jishan, Cercom, CoorsTek and Ceradyne. By 2012, 68000 sets of "interceptor" bulletproof vests will be put into the battlefield.
3.2 Wear resistance technology field
In the field of wear resistance technology and engineering, various nozzles are prepared with the high hardness of B4C, desander nozzles for hull rust removal and nozzles for high-pressure water jet cutting. B4C nozzle has the longest service life under severe service conditions, dozens of times longer than Al2O3 nozzle, and much longer than SiC and WC nozzles. It is the best choice for sand blasting industry because of its high cost performance. The excellent chemical stability of B4C makes it applicable to the nozzles of mud and liquid grinders; Mortar, grinding rod and similar grinding devices made of B4C are the first choice in chemical analysis, because they can avoid wear pollution during grinding.
Boron carbide can be used as an abrasive material in the polishing, finishing or grinding process of other hard materials such as cemented carbide and engineering ceramics, replacing the diamond abrasive used previously, which can greatly reduce the cost of the grinding process. For example, boron carbide devices are used as journal bearings for pneumatic slide valves, hot extrusion dies, and cooling systems in atomic power plants; It is used as a corrosion resistant and wear-resistant component in ceramic gas turbine.
3.3 Nuclear field
In the reactor core assembly, neutron absorbing materials (control rods, control rods, accident rods, safety rods, shielding rods) are important functional elements second only to fuel elements. As boron carbide has high neutron absorption cross section, wide absorption energy spectrum, low price and rich raw materials, there is no strong secondary radiation of rays after absorbing neutrons, so it is easy for waste disposal. Therefore, boron carbide is an important neutron absorbing material.
3.4 Thermocouple
Using the thermoelectricity of B4C, the thermocouples measuring 2200 ℃ were sintered in Japan and Germany for high temperature measurement and control. Its high thermoelectricity and stability enable it to be used reliably for a long time to repeatedly measure temperature. The boron carbide/graphite thermocouple consists of a graphite tube, a boron carbide rod, and a boron nitride bushing between the two. In inert gas and vacuum, the operating temperature is up to 2200 ℃. Between 600 ℃ and 2200 ℃, the linear relationship between potential difference and temperature is good.
3.5 Other applications
As B4C is stable to molten iron and has good thermal conductivity, it can be used as a continuous casting mold in the mechanical industry; The B4C can be used for the flow transmitter tip of liquid rocket engine fuel by virtue of its strong acid corrosion resistance and wear resistance; B4C is also an excellent gas bearing material in long-life gyroscopes. Gyroscopes are extremely important sensors in the inertial navigation and inertial guidance systems of aircraft, ships, space bridges and other carriers.
3 Deficiencies of boron carbide ceramics
With the development of sintering technology, the excellence of boron carbide ceramics is becoming more and more outstanding. Its products have special use value for the working environment such as high temperature, high speed, strong corrosive media that are often encountered in modern technology, and have potential applications in many fields. Although boron carbide ceramics have been widely used, they still have high cost, low fracture toughness, high sintering temperature (about 2300 ℃), poor oxidation resistance (oxidation starts at 600 ℃ in air, and rapid oxidation occurs at 900 ℃), poor stability to metals (except for Ag, Cu, Sn, Zn, etc., which react with almost all metals to form metal borides), and machining difficulties, These shortcomings restrict the large-scale application of boron carbide ceramic block materials. Therefore, in the future, it is still necessary to adjust and control the structure and performance of B4C ceramics according to the requirements of the actual use environment, and constantly improve and develop new sintering technology, so as to continuously reduce its manufacturing cost, improve its reliability and strength and toughness, and make boron carbide ceramics, an excellent engineering material, more widely used.