Study On Preparation And Properties Of ZrB₂-Based Ceramics Based On Different Solid-State Thermal Reduction

Among the compounds of ultra-high temperature ceramics,ZrB₂-based ceramics have excellent properties such as high melting point,high bending strength and high thermal conductivity.ZrB₂ based powders could be prepared by different solid-state reduction methods.Depending on the method and raw materials used,the particle size and purity of the ZrB₂ products varied,leading to the variations on the densification behaviour and mechanical properties.In the present work,four methods,including the conventional borothermal reduction,borothermal reduction combined with the water-washing,borothermal reduction combined with solid-solution and boro/carbothermal reduction,were used to prepared ZrB₂-based powders.On this basis,spark-plasma sintering was performed to fabricate ZrB₂ and ZrB₂-SiC ceramics.The influences of the starting ZrB₂ powder on the densification,microstructure,mechanical property and thermal conductivity of ZrB₂-based ceramics were investigated.Starting from ZrO₂,B,TiO₂,B₄C and graphite,four types of ZrB₂ powders were prepared by the conventional borothermal reduction,borothermal reduction combined with the water-washing,borothermal reduction combined with solid-solution and boro/carbothermal reduction.The results revealed that no obvious changes in the lattice parameter were observed among the four types of ZrB₂ product,in comparison with the one in the ICSD database.The finest particle size was found in ZrB₂ product prepared by borothermal reduction combined with the water-washing?0.17?m?and the largest particle was observed in conventional borothermal reduction?1.39?m?.The highest oxygen content was found in borothermal reduction combined with the water-washing?1.98 wt%?and the lowest was found in boro/carbothermal reduction?0.56 wt%?.The densification behaviour,microstructure,mechanical properties and thermal conductivity of ZrB₂ products without the addition of the secondary phase were investigated by SPS at 2000^oC for 10 min.The relative density was measured to be68.7%,94.7%,89.9%and 93.3%for the of ZrB₂ ceramics originated from the conventional borothermal reduction,borothermal reduction combined with the water-washing,borothermal reduction combined with solid-solution and boro/carbothermal reduction.The overall mechanical property was found in ZrB₂ceramics derived from borothermal reduction combined with the water-washing method:Vickers'hardness and fracture toughness reached the values of 14.0±0.5GPa and 4.94±0.19 MPa·m^1/2 with the highest thermal conductivity of 81.5 W/m·K.ZrB₂ ceramics prepared by borothermal reduction combined with solid-solution showed the hardness of 13.0±1.0 GPa,toughness of 4.17±0.15 MPa·m^1/2 and thermal conductivity of 72.6 W/m·K.Boro/carbothermal reduction exhibited the hardness of 12.2±0.9 GPa,fracture toughness of 4.09±0.23 MPa·m^1/2 and thermal conductivity of 66.4 W/m·K.ZrB₂-SiC ceramics were fabricated by starting from the above-mentioned self-synthesized ZrB₂ powders and SiC as the secondary phase and spark plasma sintering at 2000^oC for 10 min.The relative density was measured to be 92.1%,98.6%,94.9%and 97.2%for ZrB₂-SiC derived from the conventional borothermal reduction,borothermal reduction combined with the water-washing,borothermal reduction combined with solid-solution and boro/carbothermal reduction.Similar to ZrB₂ ceramics without SiC,ZrB₂-SiC from borothermal reduction combined with the water-washing demonstrated the excellent property combination:hardness was measured to be 22.4±1.7 GPa and toughness was 5.48±0.74MPa·m^1/2.The comparatively lower hardness and the close toughness values were found in ZrB₂-SiC ceramics from borothermal reduction combined with solid-solution(13.1±1.6 GPa,5.67±0.50MPa·m^1/2)and boro/carbothermal reduction(14.6±1.7 GPa,5.95±0.74MPa·m^1/2).The highest thermal conductivity was evidenced in ZrB₂-SiC from the conventional borothermal reduction?70.9 W/m·K?.