# Single-Crystal HfB2 Nanorod-Induced Synergy in HfB2–SiC Ultrahigh-Temperature Ceramics: Enhancement of Mechanical and Ablation Resistance

## TL;DR

This paper shows how adding single-crystal HfB2 microrods improves the strength and heat resistance of a ceramic material used in hypersonic vehicles.

## Contribution

The study introduces a novel method of using single-crystal HfB2 microrods to enhance mechanical and ablation resistance in HfB2–SiC composites.

## Key findings

- Adding 6 wt.% single-crystal HfB2 microrods increased hardness by 4.1% and fracture toughness by 37.6%.
- The material showed minimal weight gain after oxidation at 1,500 °C and low ablation rates at 2,000 °C.
- The improved performance is attributed to the low reactivity of specific HfB2 crystal planes confirmed by first-principles calculations.

## Abstract

Achieving a synergistic improvement in the toughness and oxidation resistance of boride ultrahigh-temperature ceramic composites remains a challenge for advancing new-generation hypersonic vehicles. In this study, HfB2–SiC composites were synthesized by integrating self-made single-crystal HfB2 microrods with commercial powders via spark plasma sintering, which exhibited enhancement of mechanical and ablation resistance. Compared to the sample without addition, the incorporation of 6 wt.% single-crystal HfB2 microrods into the HfB2–SiC composites resulted in a 4.1% increase in hardness and a 37.6% improvement in fracture toughness, reaching 15.45 ± 0.89 GPa and 7.58 ± 0.66 MPa·m1/2, respectively. After static oxidation at 1,500 °C in air for 300 min, the ceramic block supplemented with 3 wt.% HfB2 microrods exhibited a minimal weight gain (0.018 mg/cm3). Upon exposure to a plasma flame at 2,000 °C for a period of 60 s, the material demonstrated a mass ablation rate of −0.013 mg/s and a linear ablation rate of 0.25 μm/s. Based on the experimental results, the excellent oxidation and ablation resistance might be related to the naturally low reactivity of the exposed 101¯0 crystal planes present on HfB2 microrods, which aligns with the findings from first-principles calculations. This approach improves the comprehensive performance of the material by leveraging the inherent strengths of single-crystal HfB2 microrods and provides a promising design concept for HfB2 composites, which lays both theoretical and material groundwork for the development of a new generation of hypersonic vehicles.