Study On The Processing And Properties Of ZrB₂ Based Solid Solution And High Entropy Ceramic Composites

Zirconium diboride(ZrB2)is one of the most promising materials in the Ultra High Temperature Ceramics(UHTCs)family due to its excellent comprehensive properties such as high melting point,high strength,high hardness and good electrical and thermal conductivity.However,the application and development of single-phase ZrB2 are limited in the extremely ultra-high temperature environment due to its sintering difficulty,poor high temperature oxidation resistance and low toughness.To address these problems to a maximum extent and identify possible material candidates for Ultra high temperature application,researchers engaged in the study of solid solutions and took consideration of the field of high entropy ceramics.Solid solution could improve mechanical and chemical properties of ZrB2 based ceramics.Furthermore,high entropy ceramics created a new window on ceramics fabrication.Ceramics with five or more equal molar ratio phases were called high entropy ceramics.As a new type of ceramics,due to its high hardness,high modulus,good chemical stability and excellent electrical chemical properties,high entropy boride ceramics has a good application in aerospace and energy area.In this study,ZrB2 based solid solution and high entropy boride ceramics were fabricated.Additional stress field were used to reduce the sintering difficulty through hot pressing and oscillatory pressure sintering.Adding of second phase,synergistic effect of solid solution,high entropy effect,thermal steady properties of high entropy ceramic,and other methods were used to promote the oxidation resistance of ceramics.The main contents and results of the study are listed as following:(1)The(Zr,Hf)B2-SiC solid solution composites was prepared by mixture of HfB2 and ZrB2 using SiC as sintering aids through hot pressing sintering and reactive hot pressing.Phase,micro structure,mechanical properties,toughness mechanism and oxidation resistance were studied.The XRD scan results showed that ZrB2 and HfB2 form(Zr,Hf)B2 solid solution during sintering process,and its intensity peak locates at the position between ZrB2 and HfB2 peaks.Different raw material configuration may influence the formation of solid solution.The in-situ reaction brought about(Zr,Hf)solid solution in which grain size distributes uniformly and mechanical properties perform better.The 1900℃ sintered sample shows good properties,28.9±0.8 GPa of hardness,620±20 MPa of flexural strength,4.56±0.11 MPa·m1/2 of fracture toughness.Transgranular and intergranular fracture were both exhibited above the fracture surface.Crack path shows the extension were mainly occurred beyond the(Zr,Hf)B2 and SiC grain boundary and the inner crack mostly happened in(Zr,Hf)B2 particles.With the increase of oxidation temperature,the driving force of oxidation were promoted,which makes oxygen go deeper into the material and promote the oxidation reaction.Mass increment become bigger with the temperature goes up.SiC transforms to glass phase at a certain temperature,which prevents oxygen from entering the material.At the same oxidation temperature,with the increasing of SiC content,the weight gain of unit surface of material is lower when the oxidation temperature is higher.The low weight gain of the product indicates that the addition of SiC at higher temperature is beneficial to the improvement of the oxidation resistance of the material.(2)ZrB2,HfB2,TaB2,MoB2 and NbB2 powders were ball milled at equal molar ratio and were processed using hot press sintering and oscillatory pressure sintering,finally obtained the world’s first(Zr,Hf,Ta,Mo,Nb)B2high entropy composite ceramic.Formation mechanism of high entropy ceramic and the influence of high entropy effects to mechanical and thermal properties of ceramics were investigated.Five individual hexagonal phases could be seen in the mixed powders before sintering.Peaks of the five borides were disappeared after sintering.Only one single solid solution phase(Zr,Hf,Ta,Mo,Nb)B2 could be seen.Elements were uniformly distributed.Lattice parameter of the obtained high entropy ceramics was a=3.108 (?)and c=3.387 (?),which was close to the average value of the five borides.Single phase solid solution could be obtained through hot pressing in 1950℃,while the temperature is 1900℃ in oscillatory pressure sintering.Cause the driving force changed.Instead of steady pressure,oscillatory pressure could contribute to grain boundary sliding,grain rearrangement,and eliminate the porosity,which may reduce the sintering temperature of the ceramics.(Zr,Hf,Ta,Mo,Nb)B2 ceramic obtained at 1900℃ via oscillatory pressure sintering has a hardness of 24.0±1.0 GPa,and the fracture mode is mainly intergranular fracture.Due to the mass fluctuation and stress field change that is caused by the solid solution effect,(Zr,Hf,Ta,Mo,Nb)B2 high entropy ceramics exhibit low thermal conductivity,which is only 12.2 W/(m·K)at room temperature and one tenth of the corresponding value of ZrB2.(3)Five kinds of boride powder with the same molar ratio as mentioned above were mixed with SiC powder of various mass ratio and processed using oscillatory pressure sintering,finally obtaining the(Zr,Hf,Ta,Mo,Nb)B2-SiC composite ceramic.Influence of the second phase,to densification,microstructure,and mechanical properties of high entropy ceramics were investigated.XRD patterns shows the obtained ceramic was constituted by(Zr,Hf)Ta,Mo,Nb)B2 and SiC phase.Grains combined closely and few pores could be seen among the high entropy ceramics.Sintering properties promoted,densification increased,grain decreased with the SiC content increased,which improved the mechanical properties of the ceramic composites.When the SiC content is 15 wt.%,the maximum hardness is 29.2±1.2 GPa,flexural strength is 632.51±18.75 MPa,and fracture toughness is 4.65±0.72 MPa·m1/2.High entropy ceramic exhibits a good oxidation resistance.The weight gain per unit area oxidation under 1400 ℃,of(Zr,Hf,Ta,Mo,Nb)B2-SiC with 15 wt.%SiC,is only 3.26 mg/cm2.Adding SiC also promoted the thermal shock resistance.The critical thermal shock temperature difference is 757℃,when the SiC content is 15 wt.%.