High-entropy borides (HEBs) have emerged as a groundbreaking class of materials, exhibiting exceptional mechanical and thermal properties suitable for extreme environments. Recent studies have demonstrated that HEBs, such as (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)B2, achieve a Vickers hardness of 25-30 GPa, surpassing traditional ultra-high-temperature ceramics (UHTCs) like ZrB2 (~22 GPa). Their unique multi-principal element composition enhances lattice distortion, leading to improved fracture toughness (~5.5 MPa·m^1/2) compared to single-phase borides (~3 MPa·m^1/2). These properties make HEBs ideal for aerospace applications, where materials must withstand temperatures exceeding 2000°C and resist oxidation under high-velocity plasma flows.
The thermal stability of HEBs is another critical advantage, as they retain structural integrity at temperatures up to 2500°C. Research on (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)B2 revealed a thermal conductivity of 20-25 W/m·K, which is significantly higher than conventional UHTCs (~15 W/m·K), enabling efficient heat dissipation in hypersonic vehicles and nuclear reactors. Additionally, their coefficient of thermal expansion (CTE) ranges from 6-8 × 10^-6 K^-1, closely matching that of substrates like SiC, reducing thermal stress-induced failures in composite structures.
Oxidation resistance is a hallmark of HEBs, making them indispensable for applications in corrosive and high-temperature environments. Experimental data show that (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)B2 forms a protective oxide layer at 1500°C, with a mass gain of only 1-1.5 mg/cm^2 after 100 hours of exposure in air, compared to ZrB2-SiC composites (~3 mg/cm^2). This enhanced oxidation resistance is attributed to the synergistic effect of multiple metal oxides forming a dense barrier against oxygen diffusion.
The synthesis and processing of HEBs have also seen significant advancements, with spark plasma sintering (SPS) emerging as the preferred method due to its ability to produce dense (>98% theoretical density) and homogeneous microstructures at relatively low temperatures (~1800°C). Recent studies on (Hf0.25Zr0.25Ta0.25Nb0.25)B2 achieved a grain size of ~1-3 µm, which contributes to superior mechanical properties while minimizing defects that could compromise performance in extreme conditions.
Finally, the potential for tailoring HEBs through compositional engineering opens new frontiers in material design. By adjusting the elemental ratios or introducing dopants like Si or C, researchers have achieved further enhancements in properties such as hardness (up to 32 GPa) and oxidation resistance (mass gain <1 mg/cm^2 at 1600°C). This adaptability positions HEBs as a versatile platform for next-generation materials in industries ranging from energy to defense.
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