Research On Preparation And Properties Of ZrB₂-based Ceramics Based On Modulation Of ZrO₂ Particle Size And Borothermal Reduction

In the family of ultra-high temperature ceramics,zirconium boride（ZrB₂）based ceramics have become one of the important candidate materials in the aerospace field due to their low density,high melting point,high thermal conductivity and good chemical inertness,and have received extensive attention from researchers.The performance of ZrB₂-based ceramics is significantly affected by the quality of ZrB₂ powder,and the quality of ZrB₂powder is closely related to the ZrO₂ raw material and synthesis process.Therefore,this thesis has carried out research on ZrB₂ powder synthesis and ZrB2 ceramic preparation by adjusting the particle size of ZrO₂ and the borothermal reduction process.On this basis,Si C whiskers were introduced as the second phase to further improve the density and fracture toughness of ZrB₂ ceramics,in order to prepare high-density and high-toughness ZrB₂-based ceramics.The specific research content and results are as follows:First of all,six kinds of ZrB₂ powders were synthesized by using two particle sizes of ZrO₂（55 nm and 113 nm）as raw materials,and using three borothermal reduction processes（traditional borothermal reduction,borothermal reduction with water-washing process and borothermal reduction combined with in-situ solid solution）.The influence of ZrO₂ particle size and borothermal reduction process on the morphology,particle size and oxygen content of ZrB₂ powder were studied.The results show that in the three borothermal reduction,the particle size of the ZrO₂ powder significantly affects the particle size of the ZrB₂ powder which reducing the particle size of ZrO₂contributes to the refinement of the ZrB₂ powder.However,irrespective of the particle size of the ZrO₂ powder,the ZrB₂ powder synthesized by the traditional borothermal reduction is the coarsest,and the ZrB₂ powder synthesized by the borothermal reduction with water-washing process is the finest.The oxygen content of ZrB₂ powder synthesized by traditional borothermal reduction method and borothermal reduction combined with in-situ solid solution process is low and similar,while the oxygen content of ZrB₂ powder synthesized by borothermal reduction combined with water-washing process is relatively high.Then,using the above-mentioned six kinds of ZrB₂ powders as raw materials and spark plasma sintering（SPS）,ZrB₂ ceramics were prepared at 2000℃,and the effects of particle size and oxygen content of ZrB₂ powders on the relative density,microstructure and mechanical properties of ZrB₂ ceramics were studied.The results show that the two ZrB₂ceramics prepared by the traditional borothermal reduction method have similar densities（95.5-96.4%）,coarse grains（～9μm）,poor Vickers hardness and fracture toughness.Based on the finer ZrB₂ powder with higher oxygen content synthesized by the 55nm ZrO₂ raw material and water-washing process,ZrB₂ ceramics has the worst relative density（90.5%）,but a higher fracture toughness(～3.71 MPa·m^1/2).Based the ZrB₂ powder with high sintering activity by the 55nm ZrO₂ raw material and in-situ solid solution process,ZrB₂ceramics has the full densification（99.6%）and the highest Vickers hardness（～16.0 GPa）,but the low fracture toughness(～2.13 MPa·m^1/2).Finally,in order to improve the density and fracture toughness of ZrB₂ ceramics,20vol%Si C whiskers（Si C_w）were added to prepare ZrB₂-Si C_w composite ceramics by SPS at2000℃.The results show that compared with single-phase ZrB₂ ceramics,the relative density of ZrB₂-Si C_w ceramics has been significantly improved.The densification microstructure and mechanical properties of ZrB₂-Si C_w composite ceramics prepared by two kinds of ZrB₂ powders synthesized in the same process only had very small difference,but ZrB₂-Si C_w ceramics prepared from ZrB₂ powders by different processes show the greater differences.The relative density of ZrB₂-Si C_w ceramics prepared by two ZrB₂powders synthesized by traditional borothermal reduction is higher than 99%,the microstructure is significantly refined by Si C_w grain boundary pinning,and the Vickers hardness（～18 GPa）and fracture toughness are close(～4.9 MPa·m^1/2).The higher oxygen content of ZrB₂ powder synthesized based on the water-washing process,leads to degradation of Si C whiskers,and poor mechanical properties of ZrB₂-Si C_w ceramics.Due to the finer particle size and lower oxygen content of two kinds of ZrB₂ synthesized based on the solid solution process,the Si C whisker size is smaller and the morphology is more complete after sintering,and ZrB₂-Si C_w ceramic has a refined microstructure and higher fracture toughness(～5.28 MPa·m^1/2 and～5.83 MPa·m^1/2).