Research On Process And Properties Of Reaction Sintered Boron Carbide

Boron carbide（B₄C）is an important engineering material,owing to its excellent properties,such as high hardness,high melting point,low density,low coefficient of expansion,stable chemical properties and so on.However,because of its typical covalent bond and small diffusion coefficient even at a high temperature,B₄C is very difficult to be sintered.B₄C ceramic materials were prepared by reaction sintering process in this paper.The influence of carbon density,forming pressure and the second phase（TiC,TiO₂）on the properties of ceramics was studied.The influence of process parameters on microstructure,mechanical properties and fracture behavior of ceramic materials was studied by means of XRD,SEM,EDS and other test methods,such as micro indentation and three point bending.The main conclusions are as follows:（1）With the increase of molding pressure,the relative density of green body and the density of sintered body increase.When the molding pressure is 300MPa,and the carbon density of the green body is 0.5g/cm³,the maximum value of the bending strength of B₄C ceramic materials prepared by the reaction sintering method is243MPa.（2）The phase formation of B₄C ceramic materials added with TiC has become B4C,SiC,Si and TiC.With the increase of Ti C content,the open porosity of B4C ceramic materials increased.The hardness and bending strength of the ceramic material is first increased and then decreased.When the TiC content reached 1wt%,the Vickers-hardness of the ceramic material is the maximum value（2948HV0.5）.When the content of 5wt%reached TiC,the maximum bending strength value（265MPa）of the ceramic material is obtained.Fracture morphology analysis shows that with the increase of TiC content,the fracture mode of the material is from mainly transcrystalline rupture to intergranular fracture.（3）The phase formation of B₄C ceramic materials added with TiO₂ has become B₄C,SiC,Si and TiB₂.With the increase of TiO₂ particle content,the content of free Si in sintered body decreased,the content of B₄C and SiC remained unchanged,and the content of TiB₂ increased.With the increase of TiO₂ content,the open porosity of the B₄C ceramic material is increasing,and the hardness of the ceramic material is firstly increased and then decreased.When the content of 8wt%reached TiO₂,the maximum value of the hardness was 84.When the content of TiO₂ was more than8wt%,the Rockwell hardness of the reaction sintered B₄C ceramic material decreased.The bending strength of the reaction sintered B₄C ceramic material is firstly increased and then decreased with the increase of TiO₂ content.When the content of 8wt%reached TiO₂,the maximum bending strength was 269MPa.The crack morphology of the content of 8wt%TiO₂ reaction sintered.B₄C ceramic materials was analyzed by SEM.There are transcrystalline rupture and intergranular fracture in ceramic materials.TiB₂,SiC,B₄C,and free Si coefficient of thermal expansion does not match,making the spread to the Ti B₂,SiC,B₄C,and free Si interface,cracks tend to intergranular fracture,making the material toughness enhancement.