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Preparation technology of boron carbide ceramics
Boron carbide is a new type of non-oxidized ceramic material, which is widely used in energy, military, nuclear and bulletproof fields due to its high melting point, high hardness, low density, good thermal stability, strong resistance to chemical erosion and strong neutron absorption. Boron carbide, also known as black diamond, is the third hard material after diamond and cubic boron nitride, so it becomes an important member of the superhard material family.
At present, boron carbide bulletproof materials are mainly prepared by sintering method. However, boron carbide is a ceramic material with strong covalent bond, accounting for more than 90%. Moreover, due to poor plasticity of boron carbide, great resistance of product boundary movement, and small surface tension in solid state, it is determined that boron carbide is an extremely difficult ceramic material to be sintered. In the process of sintering pure boron carbide usually has some problems such as high sintering temperature, low density of ceramics after sintering and poor fracture toughness. In industry, non-pressure sintering, hot pressing sintering, hot isostatic sintering, discharge plasma sintering and other technologies are generally adopted. The mechanical properties of boron carbide are improved by improving the sintering process and adding sintering auxiliaries, which lays a foundation for further research on the sintering process of boron carbide.
It is very difficult to densify pure B4C without pressure sintering. The porosity defect and density are the key factors that affect the performance of boron carbide ceramics. Sintering temperature and powder particle size are important indexes that affect the density of boron carbide ceramics. The study shows that the most important conditions for densification of pure boron carbide are ultrafine powder with particle size ≤3 m and low oxygen content and temperature range between 2250 ℃ and 2350℃.
Non-pressure sintered boron carbide products have simple technology, low processing cost, and few requirements for sintering conditions. It is suitable for the production of products with complex shapes, and suitable for mass industrial production. It is a common sintering technology for the preparation of ceramics. However, due to the high sintering temperature, abnormal grain growth is easy to occur, which makes the sintering process difficult to control and the product performance is unstable.
Hot pressing sintering
Hot pressing is to improve the powder plasticity at high temperature, which has the advantages of low forming pressure, small deformation resistance, high product density, fine microstructure and so on. Therefore, hot pressing sintering process can be used to reduce the sintering temperature of boron carbide.
Compared with pure hot pressing, liquid phase sintering and hot pressing sintering are combined to reduce sintering temperature and increase density.
Usually, the conditions of hot pressing sintering are vacuum or inert atmosphere, pressure 20~40MPa, temperature 2200~2300℃, holding time 0.5~2h. Boron carbide is a compound with strong covalent bond. At high temperature, the sintering diffusion rate is slow and the material flow occurs less, which makes the densification process very difficult.
In order to reduce sintering temperature and surface energy and improve the comprehensive properties of boron carbide ceramics, additives must be added to promote boron carbide hot pressing sintering. The additives include sintering assistant agent or second reaction sintering, which can promote sintering, control grain growth, improve mechanical properties and obtain high density and high performance boron carbide ceramics under high temperature and pressure conditions. At present, the additives added mainly include elemental metals (Fe, Al, Ni, Ti, Cu, Cr, etc.), metal oxides (Al2O3, TiO2, etc.), transition metal carbides (CrC, VC, WC, TiC, etc.) and other additives (AlF3, MgF2, Be2C, Si, etc.).
Hot pressing sintering is characterized by complex process, high equipment requirements, high processing cost and low production efficiency, and it can only produce products with simple shapes.
Hot isostatic sintering
Hot isostatic pressing sintering boron carbide, the powder compact formation or loading package sets of powder into the high pressure vessel, without sintering additives, the inert gas as the transfer medium of stress (such as N2, Ar), make the powder by isotropic pressure, reduce the sintering temperature, can obtain fine grain microstructure, high bending strength and density of boron carbide ceramic materials.
Thermal isostatic sintering is characterized by uniform microstructure, good comprehensive performance, high equipment cost, and can only process parts with simple shapes.
Discharge plasma sintering
Discharge plasma sintering (SPS) is a new type of rapid sintering technology, which integrates plasma activation, resistance heating and hot pressing together, and has the characteristics of fast heating rate, short sintering time, uniform grain size, rapid cooling, applied pressure, high density of materials and controllable sintering atmosphere. The internal heating of powder can be effectively utilized and the dispersion distribution of discharge point can realize uniform heating.
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