Fabrication Of B₄C/SiC Ceramic Matrix Composites By Reactive Melt Infiltration And Their Structure And Mechanical Properties

Boron carbide (B4C) ceramics are important structural materials. They have a lot of excellent properties such as high hardness, low density, high melting point, good corrosion and wear resistance, high chemical stability and neutron absorption capability, etc. They are now widely used as bulletproof materials, antiradiation materials, lubricate materials, wear resistant materials, etc. However, because of the strong covalent bond, boron carbide is difficult to sinter. When boron carbide is heated to2300℃in normal atmosphere, the density can only get to80%, resulting in low mechanical properties, which cannot meet fully engineering demands.The present work aims to fabricate B4C/SiC-based composites at low temperature by using reaction melt infiltration, in which Al-Si alloys and Si infiltrate and B4C-based preforms. Additives including extra carbon, SiC whisker product derived from rice husks, and metallic Mo powder were added in the preforms in order to further improve the mechanical properties of the composites. Optical microscopy. X-ray diffraction,, scanning electron microscopy and energy dispersive spectrometry were used to characterize the microstructure of the composites. Vickers hardness, three-point bending strength and the fracture toughness of the composites were measured. The relationship between microstructure and mechanical properties of the composites was analyzed.The results show that the Al-Si alloys with Si content ranging from36-80wt.%and pure Si have good wetting and infiltration ability to the preforms. Dense multi-carbides matrix composites are obtained by spontaneous reactive melt infiltration at temperatures slightly higher than the melting temperature of the Al-Si alloys and Si. The present result provides a new low cost technology in the fabrication and development and application of B4C and SiC ceramic matrix composites.The Al-Si alloys reacted with B4C and carbon during the infiltration process, forming newly SiC, AlB12C2and Al3B48C2in the composites. Most of B4C in the prefrom is consumed when the content of Si in the Al-Si is high. Considerable B4C maintain to the composite when the content of Si of the Al-Si alloys is low. The hardness of composites decreases, and the fracture toughness increases, with the increase of Al content in the Al-Si alloys. The flexural strength of the composite reaches maximum when the Al content in the Al-Si alloy is45wt.%. The composites without extra carbon addition have relatively small particle size, and limited unreacted carbon, they possess high strength, but low hardness. The composites infiltrated from B4C preforms with Al-55wt.%Si has favorable mechanical properties, possessing Vickers hardness, elastic modulus, fracture toughness and flexural strength of t17±3GPa,291±3GPa,5.4±0.5MPa·m1/2and328±8MPa, respectively.Composites composed of SiC, MoSi2, B12(C,Si,B)3and residual Si are obtained from melting reaction of preforms of B4C powder, pyrolyzed rice husks as well as metallic Mo powder by Si infiltrate. MoSi2phase of high melting point is formed by the reaction of Mo and Si in the melt infiltration process. The density and hardness of the composites increased with the increase of the amount of pyrolyzed rice husks in the preform, but the fracture toughness slight decreased accordingly. The addition of Mo is found improving the elastic modulus, hardness and fracture toughness of the composites. Themaximum values of elastic modulus, hardness and fracture toughness of the composites reach16.8±0.8GPa,297.8GPa and3.9±0.2MPa m1/2. respectively.