Preparation And Toughening Method Of The Reaction Bonded SiC And B₄C Composites

Silicon carbide and boron carbide materials have been widely applied in the field of light armor, because of their excellent performances such as low density, high hardness, high modulus and wear resistance. However, due to the strong covalent bond and the low diffusion rate, the silicon carbide and boron carbide are difficult to be fully densified at a low sintering temperature.In this paper, the reaction bonded SiC and B₄C composites were prepared by reactive sintering at a low temperature（1500℃-1600℃）. The reaction sintering process has the advantages of simple process, low sintering temperature, rapid sintering speed and small changes in size. But the free silicon has a great impact on the mechanical properties of the composites.Therefore, based on the reaction sintering process, the toughening of reactive sintering silicon carbide and boron carbide composite materials were explored. In order to toughen the reaction bonded silicon carbide, boron carbide materials, aluminum nitride particles and short cut carbon fiber were added to the matrix of silicon carbide and boron carbide respectively. The microstructure structure was characterized by XRD, OM and SEM, and the mechanical properties of the composites were measured. The main conclusions are as follows:（1）The metal silicon used in the experiment can infiltrate into silicon carbide body and boron carbide body at the temperature of the melting point. The melting silicon and green body has good wettability. Because of the completely reacted between carbon and liquid Si, the presence of carbon in the material was not found by the XRD test. The photos of the metallographic and fracture morphology show that the density of the material is high, and there is almost no stoma. The silicon carbide and boron carbide composites, with complete reaction and high density, can be obtained by the reaction sintering process.（2） The reaction bonded silicon carbide and boron carbide composites were toughened by the addition of aluminum nitride particles. The mechanical properties of SiC/AlN are consistent with the B₄C/AlN composites, such as the opening porosity, bending strength and hardness. When the mass fraction of aluminum nitride is 6 %, the maximum bending strength of SiC/AlN composite is 256 MPa, and the maximum hardness value is 2660 HV. The maximum bending strength and hardness of B₄C/AlN composites are 324 MPa and 2857 HV, respectively, when the mass fraction of nitrogen is 4 %. The microstructure and fracture morphology of the two kinds of composite materials are more compact, and the fracture mode is the combination of intergranular fracture and cleavage fracture.（3） The results of the short cut carbon fiber reinforced reaction bonded silicon carbide and boron carbide composites were shown: With the increase of carbon fiber content, the density of the material decreases, and the bending strength and hardness of silicon carbide and boron carbide show a downward trend. Carbon fiber can react with the melting silicon and increase the binding force of the matrix. Due to the pull out of the fiber and the deflection of the crack, the fracture toughness of the composite material is increased. Among them, when the carbon fiber content is 7 %, the fracture toughness of C_f/SiC composites reached the maximum value, the maximum value is 4.89 MPa·m^1/2. The maximum fracture toughness of C_f/B₄C composites was 4.29 MPa·m^1/2, which is 10 % higher than the control group at 3.89 MPa·m^1/2.