Fabrication And Joining Of B₄C-TiB₂-SiC-TiC Composite Ceramics

As an important structural ceramic,boron carbide（B₄C）has the characteristics of low density,high hardness,high strength and good neutron absorption ability.It has a wide application prospect in bulletproof armor and nuclear industry.Sintering to produce high density B₄C ceramics usually requires a very high sintering temperature（above 2000℃）.The addition of the second phase is an effective way to improve the sintering properties and comprehensive properties of B₄C.Ti B₂,TiC and SiC have the characteristics of low density and high strength.As the second phase,they can improve the electrical conductivity,machining properties,oxidation resistance and comprehensive mechanical properties of B₄C.In addition,in practical applications,it is necessary to achieve the preparation of complex shape components through material connection.Therefore,it is of great value and significance to improve the comprehensive mechanical properties of B₄C ceramics and develop ceramic bonding technology to promote its application.In this paper,B₄C-Ti₃SiC₂-TiH₂ powders were used as raw materials,and the second phase of Ti B₂,SiC and TiC were introduced into B₄C ceramics based on in situ reaction.The effects of the content of the second phase on the microstructure and mechanical properties of the composite ceramics were studied by means of XRD,SEM,EDS and TEM,and the sintering reaction thermodynamics,strengthening,toughening mechanism of the composite ceramics were discussed.The B₄C composite ceramics were brazed by Ag-Cu-Ti solder and Ag-Cu-Ti solder containing B₄C.The microstructure and mechanical properties of the joint were characterized and analyzed.The main conclusions are as follows:1.Adding Ti₃SiC₂ and TiH₂ sintering additives can improve the sintering property of B₄C ceramics,improve the density and comprehensive mechanical properties,and realize the synergistic strengthening and toughening of ceramics.Under the conditions of 1750℃,30 MPa and 10 min,within the range of experimental conditions,the relative density,Vickers hardness and fracture toughness of the composite ceramics increased first and then decreased with the increase of the content of sintering additives,and the bending strength of the composite ceramics increased continuously.When the volume fraction of the second phase is 25vol.%,the composite ceramics have the best comprehensive properties,with the relative density of 97.2%,Vickers hardness of 28.1 GPa and bending strength of 548.4 MPa.Compared with B₄C ceramics,the hardness and bending strength are increased by 222.9%and 171.1%respectively.The improvement of comprehensive mechanical properties of ceramics is mainly attributed to the fine and uniform grain distribution in the second phase,which realizes fine grain dispersion strengthening,crack deflection,crack bridging and residual stress toughening.2.The bonding of B₄C-Ti B₂-SiC-TiC composite ceramics with Ag-26.7Cu-4.5Ti as interlayer was realized at the brazing temperature of 830～890℃and holding time of 5～20min.The brazing joint is mainly composed of interfacial reaction layer,Ag-based solid solution and Cu-rich phase.The reaction layer is composed of Ti B₂ whiskers and TiC nanoparticles.The bending strength at room temperature increases firstly and then decreases with the increase of brazing temperature and holding time.When the brazing temperature is 870℃,the joint strength is the highest,which is 291.3 MPa.When B₄C particles are added to the filler metal,the residual stress of the joint can be relieved and the bending strength of the joint can be further improved.The joint strength increased first and then decreased with the increase of the mass fraction of B₄C particles.When B₄C particles are added as 1wt.%,the joint strength reaches 314.2 MPa,which is 7.8%higher than that of pure Ag-Cu-Ti joint.