Study On Synthesis,Densification,and Performance Of HfB₂ Powders Based On Different Thermal Reduction Methods

HfB₂ possessed a very high melting point and excellent mechanical properties,which was very potential in the aerospace field.The performance of HfB₂-based ceramics was closely related to the quality of HfB₂ powder,which was affected by raw materials and synthesis methods.To improve the densification and mechanical properties of the HfB₂-based ceramics,six different HfB₂ powder were synthesized by different thermal reduction methods in this paper.The powder characteristics,including microscopic morphology,particle size,impurity content,etc,were compared.Furtherly,the growth kinetics of the powder were deeply studied.The above-synthesized HfB₂powder were used as the raw materials,and 20 vol%Si C was introduced as the second phase,HfB₂ and HfB₂-Si C ceramics were prepared by SPS.The densification behavior,microstructure,and mechanical properties of the HfB₂-based ceramics were studied.First of all,six kinds of HfB₂ powder were synthesized by borothermal reduction,boro/carbothermal reduction,and borothermal reduction combined with solid solution(Ti B₂,VB₂,Nb B₂,Ta B₂),under the same experimental conditions.The results indicated that,from the perspective of powder particle size,the average particle size of HfB₂powder(2.04?m)synthesized by borothermal reduction was biggest,and the average particle size of HfB₂ powder(0.36?m)synthesized by borothermal reduction combined with solid solution Ta B₂ was smallest;From the perspective of powder impurity content,the oxygen content(1.40?1.51 wt%)of HfB₂ powder synthesized by borothermal reduction combined with solid solution was close to that synthesized by borothermal reduction.The oxygen content(1.29 wt%)of HfB₂ powder synthesized by boro/carbothermal reduction was relatively low,but a small amount of carbon impurities existed(0.46 wt%).Then,the HfB₂,Hf_0.95Ta_0.05B₂,and Hf_0.95Ti_0.05B₂ powder were selected for vacuum high-temperature heat treatment at 1700??2000?,and the grain growth behavior and growth kinetics of the powder were studied.The results indicated that,HfB₂maintained a slow growth rate during the heat treatment process.However,Hf_0.95Ta_0.05B₂and Hf_0.95Ti_0.05B₂ grew rapidly at 1900?or above.After 2000?,the grain size of HfB₂ increased from 2.04?m to 2.84?m,the grain size of Hf_0.95Ti_0.05B₂increased from 1.15?m to 2.79?m,and the grain size of Hf_0.95Ta_0.05B₂ increased from0.36?m to 1.70?m with the smallest grains.The grain growth exponent of the three powder was determined to be 3,and the dominant growth rate-controlling mechanism was regarded to be volume diffusion.By fitting the grain growth function,the average activation energy of HfB₂,Hf_0.95Ta_0.05B₂,and Hf_0.95Ti_0.05B₂ for grain growth at 1700?-2000?was calculated to be 191�34 k J mol^-1,678�73 k J mol^-1,and 321�61 k J mol^-1,respectively.Finally,six HfB₂ powder were used as raw materials,20 vol%Si C was used as the second phase,HfB₂ and HfB₂-Si C ceramics were sintered by SPS at 2000?for 10min.Effects of HfB₂ powder synthesized by different methods on the densification,microstructure,and mechanical properties of the HfB₂-based ceramics were studied.The results indicated that,the density of HfB₂ ceramics prepared by both boro/carbothermal reduction and borothermal reduction combined with solid solution Ta B₂(94.4%,98.8%)was significantly higher than that of HfB₂ ceramics prepared by borothermal reduction(71.2%)and borothermal combined with solid solution other elements(78.0%?79.5%).Due to relatively low density and coarse microstructure,the hardness(17.57�1.54 GPa)of the HfB₂ ceramic prepared by boro/carbothermal reduction was slightly low.The fracture toughness(4.51�0.67 MPa�m^1/2)was high,which was caused by the mixed fracture mode of transgranular and intergranular.Due to the high densification and fine microstructure,the hardness of HfB₂ ceramic(19.54�0.77 GPa)prepared by borothermal reduction combined with solid solution Ta B₂ was higher,and the fracture toughness(3.96�0.24 MPa�m^1/2)was lower,which was caused by the fracture mode of transgranular.After introduction of the second phase of Si C,six HfB₂-Si C ceramics had been completely dense(>98%),and the hardness was also greatly increased(>21 GPa).The hardness of HfB₂-Si C ceramics prepared by borothermal reduction and boro/carbothermal was similar(21?22 GPa).Due to the solid solution strengthening effect,HfB₂-Si C ceramic prepared by the borothermal reduction combined with solid solution performed higher hardness,the hardness of HfB₂-Si C ceramic prepared by borothermal reduction combined with solid solution Nb B₂ and Ta B₂ could be increased to 24 GPa.HfB₂-Si C ceramic prepared by the borothermal reduction method exhibited the highest fracture toughness(5.68�0.60MPa�m^1/2)due to the mixed fracture mode of transgranular and intergranular.HfB₂-Si C ceramics prepared by the borothermal reduction combined with solid solution had the lowest fracture toughness(4.03?4.44 MPa�m^1/2),which was caused by the strengthened grain boundaries and fracture mode of transgranular.This study provides selection principles for the synthesis routes of HfB₂ powder.By adapting different synthesis methods,the optimal manufacturing of different ceramics can be achieved.When preparing HfB₂ ceramics,boro/carbothermal method and borothermal reduction combined with solid solution Ta B₂ method should be used to synthesize HfB₂ powder;while preparing HfB₂-Si C ceramics,borothermal reduction method should be used to synthesize HfB₂ powder.