Densification And Microstructure Characteristics Of ZrB₂ Ceramics

Flying faster is an inevitable trend in the development of future aircraft.In the past 20 years,all military and aerospace powers globally have joined in the technological research of hypersonic aircraft.However,for the development of hypersonic aircraft,its biggest challenge comes from its external thermal environment.The faster the aircraft flies,the more intense the friction between its nose cone and the leading edge of the wing and the atmosphere,resulting in extremely high temperatures.Therefore,it is urgent to develop some ultra-high temperature materials with high-temperature resistance and excellent performance.ZrB₂ ceramics have high melting point,high thermal conductivity,low density,moderate thermal expansion coefficient,good oxidation resistance and thermal shock resistance,etc.It is considered to be an attractive material in the field of ultra-high temperature thermal protection.but its unique crystal structure,strong covalent bonds and low self-diffusion coefficient make it difficult to sinter and dense,and the low fracture toughness also limits the its application.High-entropy diboride ceramics also have the advantages of higher hardness,better oxidation resistance,and easier sintering and densification,While inheriting the advantages of single-component boride ceramics.In this work,firstly,high-entropy boride powders（HEB5 and HEB9）were innovatively selected as sintering aids.Comparative study on the effects of different additions of HEB5 and HEB9 powders on the density,phase composition,microstructure,mechanical properties,and thermal conductivity of ZrB₂ ceramics.And discuss the mechanism of HEB powder to promote the densification of ZrB₂ceramics.Then,for the poor fracture toughness of ZrB₂ ceramics,using Zr C and HEB9 as compound additives,control the addition amount of HEB9 unchanged,change the addition amount of Zr C,the effects of different contents of Zr C on the development and mechanical properties of ZrB₂ platelet-like grains were studied.The formation conditions of ZrB₂ plate-like grains and the roles of Zr C and HEB9 in them are also discussed.The main results of this paper are as follows:First,high-purity single-phase ZrB₂,HEB5,HEB9,and Zr C powders were synthesized by the advanced powder mixing process of my research group.Then,0,5,10,20wt.%HEB5 and HEB9powders were added to the ZrB₂ powders,respectively,and ZrB₂-HEB5 and ZrB₂-HEB9 series ceramic samples were prepared by mixing and spark plasma sintering at 1800℃/30MPa/10min.The results show that both HEB5 and HEB9 can promote the densification of ZrB₂ ceramics.When the addition amount is the same,the densification effect of HEB9 is more significant than that of HEB5.The density of the sample with 5 wt.%HEB9 was 98.87%.Different additions of HEB5 and HEB9can form a single phase with ZrB₂ ceramics.With the addition of HEB5 and HEB9 increasing,the average grain size of the samples first decreased and then increased.When the addition amount is10wt.%,the average grain size of both is the minimum,and a large number of platelet-like grains appeared in the Z-5HEB9 sample.The distribution of elements in ZrB₂-HEB9 series samples is basically uniform,and there are point and local element enrichment in ZrB₂-HEB5 series samples.Second,a comparative study on mechanical properties and thermal conductivity of ZrB₂-HEB5and ZrB₂-HEB9 series samples.The results show that both the Young’s modulus and the relative density of the material increase with the addition of HEB5 and HEB9,and the change trends of the two are basically the same.For ZrB₂-HEB9 series samples,the Vickers hardness and fracture toughness of the material has a good linear relationship with the increase of HEB9 addition,which can be approximated by a function fitting.For ZrB₂-HEB5 series samples,the Vickers hardness of the material increases first and then remains unchanged,and the fracture toughness increases first and then decreases.The Vickers hardness（14.64±0.71GPa）and fracture toughness(4.57±0.99MPa·m^1/2)of the samples with the addition of 10wt.%HEB5 both reached larger values.The thermal conductivity of the samples decreased to varying degrees with the addition of HEB5and HEB9.The addition of HEB5 has little effect on the thermal conductivity of ZrB₂,which is 3.3-29%lower than that of ZrB₂,while the addition of HEB9 has a greater effect on the thermal conductivity of ZrB₂,and the thermal conductivity decreases in the range of 37.65-64.61%.Surprisingly,the samples with the addition of 5 wt.%HEB5 had a little negative effect on their thermal conductivity while improving their Vickers hardness and fracture toughness.Third,with Zr C and HEB9 as composite additives,the addition amount of HEB9 is controlled to be 20wt.%unchanged,and the Zr C addition amount is changed to 0,1,2.5,5wt.%.Z-HEB9-Zr C series ceramics were prepared by mixing powder and spark plasma sintering at1800℃/30MPa/10min.The study found that when the addition amount of Zr C was 1 wt.%,the average grain size of the sample was refined to half of the original grain size,and a few short platelet-like grains appeared.The more Zr C added,the more slender grains,the aspect ratio of the sample with 5 wt.%Zr C added is 0.54 larger than that of the sample with 2.5 wt.%Zr C added.In terms of mechanical properties,the Young’s modulus,Vickers hardness and fracture toughness of Z-HEB9-1Zr C samples were improved.The fracture toughness of the Z-HEB9-2.5Zr C sample is91.19%higher than that of the sample without Zr C addition,this is due to the presence of a large number of plate-like grains inside the material,forming a unique interlocking structure.