Preparation And Characterization Of Boron Carbide Multiphase Ceramics

Boron carbide（B₄C）has the characteristics of low density,high melting point,high hardness,excellent thermoelectric properties and good neutron absorption capacity,is widely used in the fields of nuclear industry,abrasives,military electronics and engineering structural parts,etc.However,due to the strong covalent bond of boron carbide,B₄C ceramics have the disadvantages of difficult sintering,difficult densification and low fracture toughness,which seriously limits the application of B₄C ceramics in engineering and military fields.In order to solve the above problems,the sintering property of B₄C was improved by introducing the second phase.Aluminum（Al）or carbon nanotubes（CNTs）were introduced into the B₄C matrix to prepare the boron carbide multiphase ceramics.A series of properties,such as relative density,phase composition,microscopic morphology,bending strength,fracture toughness and Vickers hardness were characterized and analyzed.Firstly,the thermodynamic analysis of the Al-B₄C system was carried out,and then the chemical reaction process of Al and B₄C was studied by X-ray diffraction and differential thermal analysis.The results showed that Al₃BC and Al B₂were produced at a lower temperature,between 650～672℃.Al B₁₂C₂was produced in the high temperature range of 1358～1397℃.At the same time,when the temperature exceeds 1105℃,the brittle phase Al₄C₃will be formed.Then,using B₄C powder and Al with low melting point and high toughness as raw materials,three ratios were designed,in which the volume fraction of Al was 5%,10%and 20%,respectively.The 20 vol.%Al powder was sintered by reactation-hot pressing at1200℃,1300℃,1400℃and 1600℃.Then,the powders with three ratios were sintered by reaction-hot pressing at 1400℃.The effects of sintering temperature and Al content on the mechanical properties of Al/B₄C composites were investigated.Under certain conditions of other processes,the higher the sintering temperature,the better the properties of composites,and the performance of 20 vol.%Al/B₄C composites is the best.Among them,the 20 vol.%Al/B4C composite prepared at 1600℃conditions has the best comprehensive properties,with a density of 90.12%,a Vicky hardness of 5.06 GPa,a bending strength of 253.9 MPa and a fracture toughness of 4.35 MPa·m^1/2,which is 253%higher than that of pure B₄C ceramics prepared at the same conditions.The toughening mechanism is:（1）the second phase particles（Al and its secondary phase）make the crack deflect and bifurcate;（2）residual stress toughening caused by mismatch of thermal expansion coefficient of each phase;（3）crack energy release by microcrack.Finally,B₄C powder and CNTs with excellent performance were used as raw materials to design the ratio of 5 raw materials,in which the volume fraction of CNTs was10%,20%,30%,40%and 50%,respectively.After ultrasonic dispersion and magnetic stirring,the raw material is prepared by pumping and filtration.CNTs/B₄C composites were prepared by discharge plasma sintering at 1600℃under 30 MPa pressure and holding for 10min.The results show that the density and hardness of CNTs/B₄C composites decrease with the addition of CNTs,while the bending strength and fracture toughness first increase and then decrease.The fracture toughness of 30 vol.%CNTs/B₄C composites reaches the maximum value of 1.53 MPa·m^1/2,which is 24%higher than that of pure B₄C ceramics under the same conditions.The toughening mechanism of CNTs is crack deflection,fiber bridging and fiber drawing.