Densification Of Boron Carbide Ceramics By Spark Plasma Sintering

Boron carbide is a well-known structural ceramics with excellent physical and chemical properties,including high melting point,high hardness,low density,good wear resistance,excellent high temperature stability,chemical stability and good neutron absorption capacity.It is widely used in bullet-proof materials,refractories,wear-resistant materials,self-lubricating materials,acid-alkali corrosion resistant materials,cutting and grinding tools,atomic reactor control and shielding materials.However,its low selfdiffusion coefficient— resulting from strong covalent bonding between atoms,low plasticity,high resistance to grain boundary sliding and low surface tension– makes sintering the powder a difficult task.In view of this situation,the densification of boron carbide ceramics was studied.Boron carbide ceramics were prepared by spark plasma sintering using nano-boron carbide powder as raw material and 5.77 wt% zirconia as sintering aids.The effects of sintering temperatures,applied pressures,heating rates and soaking times on the density of boron carbide prepared by spark plasma sintering were investigated.Moreover,the mechanism of zirconia in the preparation of dense boron carbide ceramics by spark plasma sintering was explored.The main conclusions are as follows:(1)The optimum sintering temperature,applied pressure,holding time and heating rate of dense boron carbide ceramics prepared by spark plasma sintering are 1800 ?,80 MPa,12 min and 100?/min,respectively.Sintering temperature and applied pressure are the main factors affecting the densification of boron carbide ceramics.When sintering temperature is lower than 1800 ?,increasing sintering temperature can obviously improve the density of boron carbide ceramics.Once sintering temperature reaches 1850 ? or above,the samples of boron carbide will be over-sintered due to the presence of sintering aids,resulting in the decrease of the density of boron carbide.Pressure can obviously increase the relative density of boron carbide samples,but the effect of applied pressure on the density of boron carbide is not so obvious when boron carbide reaches a certain density.The soaking time and heating rate are also factors affecting the densification of boron carbide ceramics.With the increase of heating rate,the pulse current increases,and the heat generated by Joule effect increases,which is beneficial to the densification of boron carbide ceramics.Prolonging the holding time is conducive to discharging the voids in the samples,thus forming a dense zone in some areas and improving the density of boron carbide ceramics.(2)The densification process of boron carbide ceramics can be divided into three stages.The main densification of boron carbide ceramics with zirconia as sintering aids takes place in the first and second stages,while the densification of boron carbide ceramics without zirconia mainly takes place in the second stage.This is mainly due to the solidstate reaction between boron carbide and zirconia to form zirconium boride in situ,which promotes the bonding between boron carbide particles and thus promotes the densification of boron carbide ceramics.