Liu Pingying, Su Xiaoli, Liu Xiqing, Wang Tao, Zhan Hongquan

(Jingdezhen Ceramic University, Jingdezhen 333403, Jiangxi, China)

Extended abstract:

[Significance] Hypersonic vehicles flying above Mach 5 expose sharp leading-edge (SLE) components to surface temperatures of ≥2000 ℃, combined with oxidative and erosive attack. These conditions exceed the capability of conventional metallic alloys. Hafnium carbide (HfC)-based ultra-high-temperature ceramics (UHTCs) are leading candidates for this application, owing to their exceptional melting point (~3890 ℃), low vapor pressure and the high-temperature stability of their oxidation product HfO₂. HfC also forms continuous solid solutions with ZrC, TaC and NbC and multiphase composites with SiC and HfB₂, providing broad compositions. However, the strong covalent Hf-C bond and extremely low atomic diffusion coefficient make densification exceedingly difficult. Conventional sintering methods, pressureless sintering (PLS), hot-pressing (HP) and spark plasma sintering (SPS), all require temperatures above 2000 ℃. The porous HfO₂ scale formed on monolithic HfC fails to impede oxygen diffusion under dynamic high-enthalpy flows, while intrinsic brittleness remains a critical barrier to structural reliability. These challenges collectively define the frontier of HfC-based UHTC research and motivate the compositional and architectural innovations reviewed herein.

[Progress] This review systematically covers recent advances in HfC-based UHTCs for hypersonic SLE applications, along an evolutionary trajectory from simple to complex material systems.

(1) Monolithic HfC UHTCs. Polymer-derived ceramic (PDC) technology enables liquid-phase processing and low pyrolysis temperatures (~1400–1600 ℃). A single-source precursor HfMQ resulted in a ceramic yield of 58.98 wt.% and complete conversion to nano-HfC@C core-shell structures at 1400 ℃. Residual free-carbon contamination, however, remains an unresolved issue that adversely affects oxidation resistance.

(2) Solid-solution-modified HfC UHTCs. Incorporating Group IV-V carbides exploit grain-boundary pinning, solid-solution strengthening and multi-oxide synergistic protection. In Hf1−xNbxC, optimum static oxidation resistance (x=0.2) arises from a dense Nb₂Hf₈O₂₁-bonded HfO₂ scale, whereas dynamic ablation resistance (4.2 MW·m⁻²) favors in the sample with x=0.1 via a self-supporting HfO₂ film, demonstrating strong service-condition dependence. The quaternary Zr_0.375Hf_0.375Nb_0.125Ta_0.125C (ZHNT) composition revealed a "scaffold-melt" dual-protection mechanism. Zr/Hf-rich high-melting-point oxides form a rigid outer scaffold, while Nb/Ta-derived low-melting-point melts seal inner pores, yielding the thinnest oxide layer in all compositions.

(3) HfC-SiC multiphase composites. A synergistic HfO₂ scaffold+SiO2 glass self-healing mechanism overcomes the dense-scale deficiency of monolithic HfC. With HfSi2 sintering aid, PLS achieved a relative density of 96.1% and a mass ablation rate (MAR) of -0.048 mg·cm⁻²·s−1 at 2700 ℃. In the Ta_xHf₁−xC-SiC system, reactive hot-pressing (RHP) at only 1700 ℃ delivered Ta_0.2Hf_0.8C-SiC with a flexural strength of 637 MPa, fracture toughness of 6.7 MPa·m1/2 and a linear ablation rate (LAR) of 0.15 μm·s⁻¹ at 6 MW·m⁻².

(4) HfB₂-HfC-SiC multiphase composites. B₂O₃-containing melts from HfB₂ provide intermediate-temperature self-healing, while HfC supplies skeletal support at ultra-high temperatures. Ta solid-solution design in (Hf,Ta)B₂-(Hf,Ta)C-SiC further optimizes ablation performance. At Hf/Ta=9:1, high-melting-point Hf₄Ta₂O₁₇ (m.p. 2244 ℃) acts as scaffold, while Ta₂O₅ (m.p. 1872 ℃) seals pores, realizing the “scaffold–melt” mechanism and hence achieving a LAR of −0.083 μm·s⁻¹.

(5) Carbon fiber-reinforced Cf/HfC-SiC composites. Continuous carbon fiber reinforcement overcomes UHTC brittleness via fiber pull-out, crack deflection and bridging. A PyC-SiC dual-layer interphase protects PyC from chemical attack by PIP-derived reactive species and shields fibers from matrix infiltration damage, increasing fracture toughness from 38.46 to 46.92 MPa·m1/2 and flexural strength from 208.0 to 240.3 MPa, versus a single-layer PyC design. LaB₆ modification generates high-viscosity La₂Hf₂O₇ and La₂Si₂O₇ melts, achieving a LAR of 4.75×10⁻³ mm·s⁻¹ and MAR of 1.39×10⁻³ g·s⁻¹ at 3100 ℃ over 240 s. Incorporating Ta₄HfC₅ introduces a three-component protection mechanism, HfO₂ skeletal support, Ta₂O₅ crack-sealing and dense Ta(Hf)C_xO_y oxygen-barrier layer, offering a new paradigm for precursor design.

[Conclusions and prospects] HfC-based UHTCs have advanced substantially from monolithic ceramics to fiber-reinforced complex architectures. PDC technology enables low-temperature processing, though free-carbon control remains critical. Multi-cation high-entropy solid-solution design unlocks cooperative protection beyond binary/ternary systems. HfC-SiC composites achieve near-ambient-pressure densification with excellent ablation resistance. PyC-SiC dual-layer interphase engineering resolves UHTC brittleness while preserving full toughening capability. Future work should prioritize multi-field coupled simulation and evaluation methods that better replicate real hypersonic re-entry conditions, scalable near-net-shape fabrication of complex SLE components and integration of first-principles calculations with AI/machine learning to guide multi-component phase and interface design for next-generation thermal protection systems.

Key words: ultra-high temperature ceramics; hafnium carbide; hypersonic vehicles; sharp leading edges; ablation-protection mechanism