The Preparation And Properties Of SiC_?w?/B₄C Composites

Boron carbide ceramics have been widely applied in the military,metallurgic,mechanical and nuclear industry fields,owing to a series of excellent performance such as high hardness,high melting point,low density,high wearing resistance and high neutron absorption ability.However,their widespread application has been seriously limited due to their poor sinterability and low fracture toughness.Drawing on the principle of "RBSC",carbon black was added to B4C preform and dense SiC(w)/B4C composites were prepared by vacuum liquid Si infiltration reactive sintering at a low temperature(1450?1600?).Based on the research of the preparation process of B4C-C perform,the influence of different carbon black content?different molding pressure and different particle ratio of B4C powders on microstructure and mechanical properties of SiC/B4C composites were investigated.In addition,SiCw/B4C composites with good fracture toughness were prepared by using SiCw and nano-SiCp to improve fracture toughness of B4C ceramics.The effect of SiCw content on the mechanical properties and microstructure of the composites were studied,and phase composition,fractography surface and the toughening mechanism of SiCw and nano-SiCp were investigated.The main conclusions are as follows:(1)By using the method of particle ratio,the performance of SiC/B4C composites can be improved for the reasons that the packing density of powder can be improved and the porosity of B4C-C perform can be reduced.The experiments showed that density,hardness,bending strength increased first but decreased finally and fracture toughness increased as the B4C fine particles content increasing.When the content of B4C fine particles reaches 33wt.%,density,hardness and bending strength reach their maximum values.The values of density,hardness and bending strength are 2.58g/cm3,3179kg/mm2 and 337MPa respectively.Compare to the B4C ceramic with no fine particles,the bending strength increased 51.8%.When the content of B4C fine particles reaches 43wt.%,the fracture toughness reaches the maximum value 4.62MPa·m1/2.(2)The adding of SiCw increased the fracture toughness of B4C ceramic,but reduced its'bending strength.The fracture toughness and bending strength of SiCw/B4C composites increased and reduced respectively as the SiCw content increasing.When the SiCw content reaches 24wt.%,the fracture toughness reaches the maximum value 4.88MPa·m1/2 and the bending strength reduces to the least value 243MPa.Compare to the B4C ceramic with no SiCw,the fracture toughness and the bending strength increased 9%and reduced 25%respectively.(3)XRD shows that the phase compositions of both the SiC/B4C and SiCw/B4C composites are SiC,B4C,Si and B12(C,Si,B)3,without C.(4)The fracture pattern of SiC/B4C composites is the mixture of intergranular and transgranular fractures.With the increasing of B4C fine particles,the fracture toughness of SiC/B4C composites increased for the reasons that the crack propagation path extended and the energy required for the crack propagation increased.The main reason of the improvement of fracture toughness is the pullout of SiCw.The phenomenon that the bending strength decreases as the increasing of SiCw is attributed to the dissolve of SiCw,the unmatched thermal expansion coefficient between SiC and B4C and the weakening of the interfacial bonding strength.(5)No matter SiCw or nano-SiCp can improve the fracture toughness of B4C ceramics.At last,the synergistic toughening efifect of SiCw and nano-SiCp depends on the comprehen-sive results of particle toughening of nano-SiCp and whisker toughening of SiCw.