Study On Preparation And Oxidation Behavior Of Y And Yb Elements Co-doped ZrB₂-based Ceramics

ZrB2-based ceramics have the advantages of high melting point,high strength and excellent physical and chemical properties.It is considered to be the most promising candidate material for the nose cone,wing leading edge and thermal protection system of hypersonic vehicles.Oxidation resistance is the key performance to judge whether the material can be used in high-temperature aerobic environment.ZrB2-SiC ceramics have excellent oxidation resistance in high temperature and oxygen-rich environments.However,under high temperature and low oxygen partial pressures,the protective glass layer on the surface of ZrB2-SiC will be damaged,resulting in the degradation of the oxidation resistance of the materials.In this study,Y and Yb elements are introduced in order to form dense and stabilized ZrO2 oxidation layer on the surface of the ceramics,which will improve the oxidation resistance.Firstly,(Y0.5Yb0.5)B6 rare earth boride powders were prepared by boron/carbon thermal reduction method.The effects of synthesis conditions and raw material ratios on the morphology and purity of the powders were investigated.(Zr,Y,Yb)B2 ceramics and(Zr,Y,Yb)B2-SiC ceramics co-doped with elements Y and Yb were prepared by pressureless sintering.The microstructure characteristics and the static oxidation behavior under different oxygen partial pressures were analyzed.The effect of(Y0.5Yb0.5)B6 on the sintering densification and oxidation resistance properties of ZrB2-based ceramics was discussed.Finally,(Zr,Y,Yb)B2/ZrB2-SiC laminated ceramics were designed and prepared to explore the effect of laminated structure on the oxidation resistance of the materials.It is shown that(Y0.5Yb0.5)B6 rare earth hexaborides with high purity can be prepared by a one-pot method combined with boron/carbon thermal reduction.(Y0.5Yb0.5)B6 powders synthesized at 1650℃ for 2 h have a purity of 94.60%and an average particle size of 0.53±0.14 μm when B4C is excess by 6.25%.(Y0.5Yb0.5)B6 is stable at 2000℃ without significant decomposition.During sintering of(Zr,Y,Yb)B2 ceramics,Y and Yb atoms dissolve into ZrB2 lattices with the formation of(Zr,Y,Yb)B2 solid solution.The added(Y0.5Yb0.5)B6 can eliminate the oxide impurities on the surface of the starting powders,resulting in improving the density and refining the grains.Furthermore,the oxygen partial pressure has little effect on the oxidation behavior of(Zr,Y,Yb)B2 ceramics at 1650℃.The presence of Y and Yb elements can stabilize the crystal type of ZrO2 and improve the sintering activity of ZrO2,thus a dense ZrO2 layer is formed on the surface of the ceramics.However,this dense oxide layer inhibits the volatilization of B2O3 oxidation product,which makes B2O3 gas accumulate in the oxide layer and discharge through the bubble blasting,resulting in the damage and spalling of the oxide film.The(Zr,Y,Yb)B2-SiC ceramics obtained by pressureless sintering have a high relative density of 99.25%.However,the introduction of(Y0.5Yb0.5)B6 promotes the grain growth of each phase,which is related to the presence of the liquid phase at the grain boundaries.Static oxidation experiments show that the oxide layer thickness of(Zr,Y,Yb)B2-SiC ceramics increases significantly,and the oxide layer structure consists of SiO2 layer containing agglomerated ZrO2 particles,ZrO2-SiO2 layer,porous ZrO2 layer and SiC depletion layer.The presence of Y and Yb increases the ZrO2 solid content in the glass layer and reduces the viscosity of the glass phase.As a result,it promotes oxygen diffusion and degrades the oxidation resistance.Meanwhile,the kinetics of ZrB2-SiC ceramics before and after doping are similar.They exhibite parabolic behavior in the air environment,while linear oxidation occurs in the low oxygen environment.In addition,the interface of(Zr,Y,Yb)B2/ZrB2-SiC laminated ceramics is tightly bonded without defects such as microcracks.The oxide layer of(Zr,Y,Yb)B2/ZrB2-SiC laminated ceramics is composed of ZrO2-SiO2 layer,porous ZrO2 layer and dense ZrO2 layer after oxidation at 1650℃ in the air atmosphere.The dense Zr02 layer and the SiO2 filled in the ZrO2 skeleton improve the overall resistance of the oxide layer to oxygen attack,resulting in the oxidation kinetics of the laminated ceramics in the air environment controlled by the diffusion process,and the oxidation resistance is better than that of each monolayer ceramics.The research work in this thesis shows that the introduction of elements Y and Yb is not conducive to the enhancement of antioxidant properties of monolithic bulk ZrB2-based ceramics,but the material can be made to have both excellent sintering activity and high-temperature antioxidant properties through the layer design,which will provide guidance for the selection design and performance enhancement of ultra-high-temperature ceramics.