Preparation Of B₄C-HfB₂-SiC Based Composites By Arc-melting

B₄C-HfB₂-SiC based composites may possess the excellent combination properties of B₄C,HfB₂ and SiC,such as extraordinary mechanical,electrical and thermal properties,and may be applied as a new generation of ultra-high temperature structural materials,e.g.,cutting tools,plasma-arc electrodes,furnace elements,gas turbine system,hypersonic vehicles,rocket nozzles and sharp leading edges of propulsion system.In the present study,the B₄C-HfB₂-SiC based composites were prepared by arc melting.The eutectic composition,eutectic temperature,microstructure,the crystal orientation among phases,mechanical,electrical and thermal properties were studied.The effect of the composition and microstructure on the properties was investigated.B₄C-HfB₂ binary composites were prepared by arc melting using B₄C and HfB₂as raw materials.At the composition of 70B₄C-30HfB₂?mol%?,the uniform and fine eutectic microstructure in large area was obtained.The lamellar and regular eutectic microstructure was composed of B₄C matrix with HfB₂ phase about 1?m in thickness.The Vickers micro-hardness of B₄C-HfB₂ composites significantly increased with increasing B₄C content and showed the maximum value of 31.2 GPa at the eutectic composition.The fracture toughness of B₄C-HfB₂ composites had the maximum value of 5.9 MPa m^1/2 around the eutectic composition.Both the electrical conductivity and thermal conductivity of B₄C-HfB₂ composite increased with the increase of HfB₂ content and decreased nearly concomitantly with increasing the temperature.The electrical conductivity and thermal conductivity of B₄C-HfB₂ eutectic composite were 7.43×10⁴ to 8.94×10⁴ Sm^-1 in the range of298-800 K and 16 to 18 WK^-1m^-1 in the range of 298-973 K,respectively.The thermal expansion coefficient of B₄C-HfB₂ composites showed the highest value of7.18×10^-6 K^-1 in the range of 293-1273 K at the eutectic composition.SiC shows high hardness?28 GPa?,good thermal shock resistance and high-temperature strength,as well as high-temperature oxidation resistance.Moreover,SiC is also effective to significantly improve the mechanical properties because it could prevent an excessive growth of the diboride matrix.Therefore,B₄C-HfB₂-SiC ternary eutectic composite was also investigated.The ternary eutectic composition of 45B₄C–15HfB₂–40SiC?mol%?was first identified with a lamellar eutectic texture.HfB₂ about 500 nm in thickness and SiC about 700 nm in thickness were uniformly dispersed in B₄C matrix of the eutectic composite.B₄C-HfB₂-SiC composites showed the highest Vickers micro-hardness of 35.7 GPa and highest fracture toughness of 6.5 MPa m^1/2 at the eutectic composition,which were both higher than that of B₄C-HfB₂ eutectic composites.The electrical conductivity of B₄C-HfB₂-SiC composites increased with the increase of HfB₂ content and nearly keep constant(0.94×10⁴-1.09×10⁴ Sm^-1)from298 to 800 K at the eutectic composition.The thermal conductivity of B₄C-HfB₂-SiC composites increased with the increase of HfB₂ content and the decrease of B₄C content and decreased with increasing the temperature.B₄C-HfB₂-SiC eutectic composite had the minimum thermal conductivity,which decreased from 32 to 23WK^-1m^-1 with increasing the temperature from 373 to 973 K.The thermal expansion coefficient of B₄C-HfB₂-SiC composite increased with the increase of HfB₂ content and the decrease of SiC content and showed the maximum value of 6.42×10^-6 K^-1 in the range of 293-1272 K.