Fabrication And Anti-oxidation Properties Analysis Of Ta-modified ZrB₂-based Ultra-high Temperature Ceramics

Hypersonic vehicles are subjected to extreme thermal environments such as high temperature,vibration,and thermal load during flight,which puts forward new requirements for the design and preparation of materials related to their thermal protection systems.Zr B₂ has the advantages of moderate thermal expansion coefficient,low resistivity,high thermal conductivity,high elastic modulus,high hardness,excellent flexural strength and oxidation resistance,etc.,and has a wide range of applications in high temperature fields.Ta B₂ has a similar crystal structure to Zr B₂,and also has excellent antioxidant properties and mechanical properties.When sintered at high temperature,Zr B₂ and Ta B₂ easily form a boride solid solution(Zr_0.5Ta_0.5)B₂,which has better oxidation resistance and mechanical properties than single-phase ceramics.Therefore,Ta-modified Zr B₂-based ultra-high temperature ceramics can be used to prepare composite materials with better properties by forming boride solid solution(Zr_0.5Ta_0.5)B₂.The specific research contents of this paper are summarized as follows:（1）Using the thermodynamic calculation software HSC6.0,the Gibbs free energy of the chemical reaction that may occur in the Zr Si₂-Ta Si₂-B₄C-C mixed powder at high temperature was calculated,and the possibility of generating Zr B₂,Ta B₂ and Si C was determined.（2）Zr B₂-Ta B₂-Si C and(Zr_0.5Ta_0.5)B₂-Si C were prepared at 1200℃and 1500℃by in-situ reaction hot pressing sintering of Zr Si₂-Ta Si₂-B₄C-C mixed powder,respectively.In the XRD pattern,obvious diffraction peak shift phenomenon can be seen.The relative density of Zr B₂-Ta B₂-Si C is 87.78%,the surface is relatively rough,the pores are slightly more,and the boundaries of each phase are not obvious;The relative density of(Zr_0.5Ta_0.5)B₂-Si C formed boride solid solution is 92.50%,the surface is smoother,the pores are less,and the boundaries of each phase are obvious.（3）The hardness and fracture toughness of Zr B₂-Ta B₂-Si C are 12.59 GPa and 3.66MPa·m^1/2,respectively,and the hardness and fracture toughness of(Zr_0.5Ta_0.5)B₂-Si C are15.11 GPa and 5.89 MPa·m^1/2,respectively.The hardness and fracture toughness of the boride solid solution increased by 20.02%and 60.93%,respectively,which greatly improved the mechanical properties of the composite.（4）The oxidation resistance of(Zr_0.5Ta_0.5)B₂-Si C and Zr B₂-15 vol.%Zr Si₂ were tested in the temperature range of 200～1600℃,and the weight gain after oxidation was 0.34%and1.12%,respectively.Compared with Zr B₂ ceramics,the surface of(Zr_0.5Ta_0.5)B₂-Si C after oxidation is smooth and smooth,and the amount of oxides is less,but the quality is basically stable after a period of increase,and the oxidation protection effect is excellent.After oxidation,a Zr-Ta-Si-O multiphase oxide glass layer is formed on the surface,which is composed of Zr-Si-O and Ta-Si-O oxide layers,including Ta₂O₅,Ta_0.8O₂,Ta_0.97O₂,Si O₂,Zr O₂,Zr Si O₄,etc.Compared with Zr-Si-O and Ta-Si-O oxide layers,the protection against oxidation is greatly improved.