Synergistic effects of single-crystal HfB2 nanorods: Simultaneous enhancement of mechanical properties and ablation resistance

Background

Ultra-high temperature ceramics (UHTCs), with their exceptional high-temperature stability, oxidation resistance, and ablation resistance, have become key materials for the thermal protection systems of hypersonic vehicles. However, ceramic materials constructed from traditional polycrystalline boride powders exhibit inherent defects under extreme service environments: grain boundaries, acting as preferential active regions for oxidation reactions and rapid diffusion channels for oxygen atoms, tend to trigger localized oxidation that spreads inward, ultimately leading to material structural damage and functional failure. To address this bottleneck, single-crystal HfB₂ powders, by maximizing the reduction of grain boundary defects, can effectively retard the internal oxidation process of materials and enhance their overall oxidation and ablation resistance. Meanwhile, their unique high aspect ratio structure can activate toughening mechanisms such as crack deflection and bridging in composite materials, achieving the simultaneous optimization of material properties. Therefore, developing controllable preparation technologies for single-crystal HfB₂ ceramic powders holds important scientific significance and application value for the synergistic improvement of the comprehensive properties of ceramic matrix composites.

Research Progress

The research team developed single-crystal HfB₂ with a nanorod structure as the toughening phase for ceramic matrix composites, successfully fabricating HfB₂-SiC composites that simultaneously possess excellent mechanical properties and oxidation/ablation resistance. Compared with the sample without the toughening phase, the composite incorporating 6 wt.% HfB₂ nanorods exhibits a 4.1% increase in hardness and a 37.6% improvement in fracture toughness, respectively. The adopted "fiber-first, then composite" process breaks through the uncontrollable limitations of traditional in-situ growth on fiber morphology and distribution, providing greater flexibility in component regulation and structural optimization for the strengthening and toughening design of UHTCs.

First-principles calculations indicate that the <100>crystal planes have lower reactivity compared to the (0001) crystal plane. Furthermore, the<100> crystal planes tend to align parallel to the sample surface, an orientation that can effectively impede the penetration of oxygen atoms inside the material. Validated by plasma flame ablation experiments, the composite containing 3 wt.% HfB₂ micro-rods demonstrates excellent ablation resistance: after ablation at 2000 °C for 60 s, its mass ablation rate is only -0.013 mg/s and linear ablation rate is 0.25 μm/s. This result confirms the synergistic effect of single-crystal HfB₂ nanorods in enhancing both the mechanical properties and oxidation/ablation resistance of materials.

Future Prospects

The method proposed in this study fully leverages the intrinsic characteristics of single-crystal HfB₂ nanorods, providing a new approach to solving key issues such as high brittleness and insufficient ablation resistance of traditional UHTCs. It also opens up a promising research direction for the design of HfB₂-based composites, thereby laying a theoretical and material foundation for the development of next-generation hypersonic vehicles. Subsequent research can focus on the systematic optimization of ceramic micro-rod compositions and the design of material microstructures to further promote the preparation and comprehensive performance enhancement of new-system ceramic matrix composites.

Sources: https://spj.science.org/doi/10.34133/research.0963