Fabrication And Characteristics Of B₄C Multiphase Ceramics

Has been recognized as one of the important and promising engineeringceramics, boron carbide (B₄C) is widely used in light bombproof armor,wearable and lubricate materials, cutting tools, radiation protection materials,and prime materials for reactivity control and shield of nuclear power reactorsdue to its excellent physical and chemical properties, for instance, low density(ρ_v=2.52g/cm³), super hardness, high elasticity modulus, good wear andcorrosion resistance, and neutron absorption ability. However, it is difficult toachieve high sintered density of boron carbide (B₄C) ceramics which is mostlycomposed of strong covalent bond, because of its low plasticity, lowself-diffusivity coefficient, high resistance to grain boundary sliding, and lowthermodynamic surface energy. Even though boron carbide (B₄C) is sintered at2300℃which is near the melting point, the sintered density could hardly achieve80%. In the process, substance transfer is hard to occur, but the phenomenonsuch as abnormal grain size and surface melting is easy to appear. The sinteringmechanism comprised both body diffusion and boundary diffusion. Otherwise,the exhaustive application of boron carbide, to its fullest potential, has beenlimited by a number of aspects, the key being: its low fracture toughness, and thelow stability towards metal, etc.The present work is initiated to prepare the Ultrafine boron carbide,examine the activated sintering of boron carbide (B₄C), with the addition ofAl₂O₃ which is selected as the sintering aid. The light B₄C- Al₂O₃ composites areprepared by hot pressing processing. The mechanical properties are measured byVickers indentor and three-point flexural test. To observe microstructure andphase constitute of B₄C- Al₂O₃ composites, the scanning electron microscopy(SEM) and X-ray diffraction (XRD) pattern are used. The enforcementmechanism of fracture toughness is discussed in this present work. Theconclusions indicate that:1. The ultrafine boron carbide powders were prepared by high energy ballmilling method. The influences of ball milling parameter (ball to powdermass ratio, ball mill time) on boron carbide powders were investigated. Theultrafine powders can be obtained when the ball to powder mass ratio is 30:1,and rotation speed is 250r/min,the time of ball milling more than 36 hours.The mean size of powders were 1.2μm. 2. The sintering temperature of boron carbide (B₄C) was obviously lowered bythe addition of Al₂O₃, the sintered density was improved obviously. Thedensification of B₄C was enhanced because of Al₂O₃ molten in the process ofsintering when the sintering temperature higher then 1600℃. 5wt.% was theproper content of Al₂O₃. The effects on densification were not obvious withfurther Al₂O₃ addition.3. The Vickers hardness of B₄C- Al₂O₃ composites could increase to 34GPawith 5wt% Al₂O₃ addition, but the hardness decreased slightly with furtherAl₂O₃ addition. Improved sintered density contributed to enhancing hardnesswhen the Al₂O₃ content was lower than 5wt.%. With further Al₂O₃ addition,the effects of Al₂O₃ addition on improvement were not obvious any more.Otherwise, hardness of Al₂O₃ is lower than B₄C, more Al₂O₃ addition couldlowered hardness of B₄C-Al₂O₃ composites.4. The flexural strength and fracture toughness of B₄C- Al₂O₃ composites weremuch higher than that of pure B₄C ceramics. As the content of Al₂O₃ increase,higher flexural strength and fracture toughness could be obtained. When thecontent of Al₂O₃ is 5wt.%, flexural strength and fracture toughness couldachieve 480MPa and 4.8MPa·m^1/2, which were 200MPa and 2.8MPa·m^1/2higher than that of pure B₄C ceramics, respectively. However, the flexuralstrength and fracture toughness decreased slightly with further Al₂O₃addition.5. The observation of microstructure showed that there was much more pores inthe pure B₄C simples which were sintered at 1850℃for 1 hour than othersimples. The size of gains had not grown, it is difficult to improve thesintered density of B₄C ceramics. Compared to pure B₄C ceramics, thesimples with Al₂O₃ addition had lower porosity and higher sintered density.When the content of Al₂O₃ got to 10wt.%, the corresponding simple could besintered to near fully density.