Study Of Microstructure And Mechanical Properties Of Reaction Bonded Boron Carbide Composites

In recent decades, based on molten infiltration, a new technology to fabricate B4C composite has been developed, which with a series of advantages, such as lower cost and near net shape sintering, etc. Nevertheless, the dissolution and reaction of boron carbide with Si during the infiltration, lead the content of boron carbide decreased drastically. It limits the exploiting and applications of B4C composites. Specifically, this work, is to study the influence of infiltration temperature and carbon coating on the phase compositions, microstructure, volume density and mechanical properties of RBBC (reaction bonded boron carbide) composites, to study the grain growth mechanisms of boron carbide in liquid Si and the reaction mechanisms of boron carbide with Si. This work is thus believed to have important theoretical significance and practical application value in exploiting the potential in applications of B4C composites.The main results are:(1) Dense RBBC composites were fabricated by infiltrating silicon into green boron carbide preforms with carbon black under vacuum at different infiltration temperatures ranging from 1450 to 1650℃, for a dwell time of 1 h. Phase composition of the composite are:B4C, B12(C,Si,B)3, SiC and Si. With the increase of infiltration temperatures, the vol% of boron carbide reduced, while that of SiC increased; the size of boron carbide particles increased; the morphology of SiC phases developed from discontinuous and cloud-like SiC to continuous and integrated SiC zones; both the amount of boron carbide particles with increased grain size and reaction formed integrated SiC zones increased. The mechanical properties of RBBC ceramics were improved initially and then deteriorated with the increase of infiltration temperatures. The optimum open-porosity, volume density, Vickers-hardness, flexural strength and fracture toughness of the obtained RBBC ceramics fabricated at 1600℃ were 0.09%,2.54 g/cm3,19 GPa,344 MPa and 3.8 MPa-m1/2, respectively. (2) Carbon coated boron carbide particles were fabricated successfully using phenolic resin as carbon sources by mechanical stirring, ultrasonic dispersion and carbonization. The coating carbon layers can decrease the grain size increase and dissolution of boron carbide particles in liquid Si effectively. Lots of SiC nano-particles generated in the RBBC composites with phenolic resin as coating sources. The Vickers-hardness, flexural strength and fracture toughness of the obtained RBBC composites fabricated with phenolic resin as coating sources were 23 GPa,442 MPa and 4.9 MPa·m1/2, which increased of 15%, 35% and 36%, respectively, compared with those of RBBC composites fabricated with black carbon as coating sources. The decrease of grain size increase of boron carbide particles and the generation of SiC nano-particles can be responsible for the increase of mechanical properties of RBBC composites. (3) The shape of boron carbide grains evolved from irregular to faceted, and the grain size of boron carbide grains increased too, with the increase of infiltration temperature and time. When the infiltration temperature is lower than 1750 ℃, irregular boron carbide grains showed a unimodal distribution, indicating normal grain growth controlled by diffusion. The growth activation energy of irregular boron carbide grains is 156 kJ·mol-1. In contrast, when the infiltration temperature is higher than 1750℃, faceted boron carbide grains showed a bimodal distribution, indicating abnormal grain growth controlled by two-dimensional nucleation. The coalescence is expected to accelerate abnormal grain growth. (4) Reaction layer was formed in boron carbide matrix by liquid Si infiltrating into boron carbide matrix and reacting with it, when the reaction temperature is higher than Si molting point. With the increase of reaction temperatures, the size of reaction layers increased, and the morphology of SiC phases developed from irregular to rod-like. Phase compositions of the reaction layers are:SiC, SiB6, B12(C,Si,B)3 and Si, when the reaction temperature is 1700-2200℃. With the increase of reaction times, the size of reaction layers increased too, and the morphology of SiC phases developed from irregular to rod-like, and further to dendritic. The reaction mechanism of boron carbide with liquid Si is dissolution-precipitation mechanism. The reaction process of boron carbide with liquid Si is that:first, Si reacted with C origined from boron carbide with results of SiC and Bi2(C,Si,B)3 generated; second, B12(C,Si,B)3 dissolved in liquid Si and re-precipitated as SiC, SiB6 and B12(C,Si,B)3 from the supersaturated liquid Si during cooling.