Research On Self-propagating High-temperaure Synthesis And Sintering Properties Of Fine B₄C Powder

Boron carbide (B4C) ceramics has been recognized as one of the important and promising engineering ceramics due to its is widely used in light bombproof armor, wearable and lubricate materials, cutting tools, radiation protection materials, and prime materials for reactivity control and shield of nuclear powder reactors because of its excellent physical and chemical properties, for instance, low density, super hardness, high elasticity modulus, good wear and corrosion resistance, and neutron absorption ability. However, because of the high proportion covalent bond, low plasticity, high resistance to grain boundary sliding, boron carbide is difficult to be sintered. When boron carbide is sintered at2300℃in normal atmosphere, the density only can get to80%, furthermore the mechanical properties are not perfect, so it cannot satisfied the engineering demands.In this paper, B4C powder was prepared by self-propagating high-temperature synthesis with two systems:B2O3-Mg-CB, B2O3-Mg-CA (Critic Acid). And boron carbide ceramic was sintered by hot-pressing sintering based on the fine B4C powder synthesized.For B2O3-Mg-CB system, the adiabatic temperature of the system and the Gibbs free energy of every possible reaction were calculated. It is shown that the adiabatic temperature of the system is up to2750K and the whole reaction for the formation of B4C powder is given in the equation and may proceed via two steps:B2O3+3Mg=2B+3MgO,4B+C=B4C.The effects of compact pressure, mixture ratio and diluents on combustion reaction mechanism, combustion product micro structure and powder characteristic were investigated systematically. Results showed that the influence of Mg content on purity of B4C powders was very important. Adding diluents can effectively reduce the system combustion temperature, but the longer high-temperature reaction time, the greater the grain. At the condition of excessive20wt.%Mg,5wt.%B2O3and200MPa compact pressure, the purity of B4C powders is97%, and the mean grain size is less than200nm.For B2O3-Mg-CA system, the organic compounds were used as carbon source to synthesis B4C powder. The effects of process parameters on the SHS process and combustion products were investigated. The research shown that the reactants have lots lose, need to increase the ratio to improve the purity. At the condition of ratio 12:39:1, the mean grain size of150nm powders were obtained.Based the B4C ultra-fine powders synthesized by B2O3-Mg-CB system, hot-pressing (HP) is used to prepare boron carbide ceramics. Studies shows that reducing the grain size, increasing the specific surface area, can promote the sintering process, enhance the density, improve the mechanical properties.