Research On Preparation And Properties Of Boron Carbide Composite Ceramics By Spark Plasma Sintering

Boron carbide is one of the important engineering ceramic materials with super high hardness and low density,as well as excellent neutron absorption,wear resistance,acid and alkali corrosion resistance.However,the difficulty to be densified and the poor fracture toughness have limited its application.Sintering temperature of the boron carbide ceramics can be reduced by adding sintering additives,meanwhile achieving the effect of toughening.In this paper,0.5?m boron carbide powder,?-Al₂O₃ and Y₂O₃ nano powders were used as raw materials.B4C-Ni and B4C-Ni-Si composite ceramics were prepared with Ni or Ni-Si as sintering aids and B4C-Al2O3 composite ceramics were prepared with Y2O3 as sintering aids by SPS sintering.The sintering process,phase composition,microstructure,Vickers hardness and bending strength of the specimens were analyzed.The results are as follows:?1?The main sintering mechanism of B4C composite ceramics prepared by using Ni or Ni-Si as sintering aids is liquid phase sintering.The densification rate of SPS sintering can be as high as 3?10^-3/s.At the final stage of sintering,under the influence of pressure and liquid phase,the main densification mechanism of B4C composite ceramics is plastic flow?creep?.Under 1700?and 60MPa,the composites with 1.0vol.%Ni achieve the highest density of97.76%,and the hardness,fracture toughness and bending strength is 25.50GPa,4.66MPa·m^1/2,392.40MPa respectively.The relative density of B4C-Ni-Si composite ceramics prepared with Ni-Si as sintering aids decreased slightly compared with B4C-Ni.The relative density,hardness,fracture toughness and bending strength of NSV25 is 96.90%,16.56GPa,5.00MPa·m^1/2,and 431.6MPa respectively.?2?Nearly full dense B4C-Al2O3 composite ceramics can be prepared with high densification rate(4×10^-3/s)using Y2O3 as sintering aid by SPS sintering,under 1350-1500°C.The addition of Y2O3 can reduce the densification activation energy of B4C-Al2O3 composite ceramics,and can effectively reduce the densification temperature through reacting with Al2O3to form liquid phase?YAG?.The densification sintering temperature decreases from 1500 to1350°C as the increases Y2O3 content.The densification mechanism of B4C-Al2O3 composite ceramics at final stage of sintering is plastic flow.B4C-Al2O3 composite ceramics containing a small amount of Y2O3 have a high relative density and good mechanical properties.The relative density,hardness,fracture toughness and bending strength of BAY2 is 98.60%,23.75GPa,4.89MPa·m^1/2 and 464.96MPa respectively.?3?The toughening mechanism of B4C-Al2O3 composite ceramics is particle toughening and microcrack toughening.The presence of large particles of B4C and Al2O3,and the microcracks caused by the difference in thermal expansion coefficient between the two phases,can absorb the crack propagation energy and cause crack deflection,branching,bridging,thus can achieve the effect of toughening.The B4C-Al2O3 composite ceramics with a relative high bending strength?>450MPa?,since the domination fracture mode is transgranular fracture due to the tightly bounded between internal grains,and the fracture surface of grains is relatively rough.