Abstract:

High-entropy boride ceramics (HEBs) consisting of four or more principle metallic elements rapidly develop in recent years due to their outstanding unique physical properties and excellent elevated temperature properties, showing extraordinary promise as potential thermal protection materials applied in extreme environments. However, on the basis of unclear role of each element on their oxidation reaction, HEBs are generally difficult to densify because of their low self-diffusion coefficients and possible sluggish diffusion effect, resulting in limited mechanical properties and low oxidation resistance. In this work, a novel type of HEBs, (Ti_0.25Zr_0.25Hf_0.25Ta_0.25)B₂-B₄C composites, were prepared by boro/carbothermal reduction method combined with hot-pressing sintering at 1900 ℃. The effect of B₄C at the volume fractions ranging from 10% to 30% on the mechanical properties and oxidation resistance of the composites was systematically investigated. Microstructure analyses indicate that homogenously distributed B₄C can suppress grain growth of the HEBs matrix and promote toughening mechanisms such as crack deflection and crack branching, consequently resulting in strengthening and toughening composites. When the volume fraction of B₄C is 20%, the as-prepared composite shows a high relative density (96.1%) and good mechanical properties with Vickers hardness of (24.6±1.1) GPa, flexural strength of (570.0±27.6) MPa and fracture toughness of (5.58±0.36) MPa·m^1/2. In addition, exploration on the oxidation resistance of (Ti_0.25Zr_0.25Hf_0.25Ta_0.25)B₂-B₄C composites at temperatures ranging from 800 ℃ to 1400 ℃ shows that excellent oxidation resistance occurs at the chosen temperatures due to the formation of a dense and continuous oxidation scale, which acts as a barrier layer preventing oxygen inward diffusion. The main compositions of the oxide scale are TiO_x, (Zr, Hf)O₂ oxides and B₂O₃ at 800 ℃, while multicomponent oxidation products of (Zr, Hf, Ta)O_x, (Zr, Hf)O₂ and TiTaO₄ are formed in the oxide scale at 1100 ℃. As the temperature increased to 1400 ℃, thickness of the oxide layer significantly increases due to their volatilization of B₂O₃, while continuous B₂O₃ glassy phase plays a crucial role in the oxidation process of HEBs. When the B₄C volume fraction not less than 20%, TiTa₂O₇ and TiO₂ which were embedded in B₂O₃ glass, could effectively insulate inward oxygen and interfacial oxide thickness and enhance oxidation resistance of the composites. In summary, the primary work can be used as a reference to the researches relating to optimizing mechanical properties and oxidation resistance for HEBs.

Key words: high-entropy boride ceramic, B₄C, mechanical property, oxidation resistance