ZrB₂ is considered as a crucial candidate material for thermal protection systems in hypersonic vehicles due to its excellent high melting point and ablation resistance. However, the brittleness and poor thermal shock resistance severely limit practical application. To enhance the performance, the introduction of continuous carbon fiber-reinforced ZrB₂ (i.e., C_f/C-ZrB₂) composites becomes a research hotspot. Needle-punched carbon fiber preforms exhibit a promising reinforcement potential due to their flexible structural design and low preparation cost. To address the drawback of long processing cycles in conventional preparation methods, this study employed injection-assisted vacuum impregnation (IVI) to fabricate C_f/C-ZrB₂ composites. The as-fabricated composites subsequently underwent carbon densification treatment via chemical vapor infiltration (CVI). The anisotropic characteristics of mechanical and thermal conductivity properties were investigated from multiple fiber orientation angles before and after the CVI treatment.

ZrB₂ powder, phenolic resin, and acetone were mixed to prepare a ceramic-resin slurry of 20% solid content. This slurry was injected into a needle-punched carbon fiber preform with the sizes of 20 mm×20 mm×20 mm. Subsequently, two rounds of vacuum impregnation, drying, and pyrolysis at 800 ℃ were performed to obtain the C_f/C-ZrB₂ composite. Carbon densification treatment was then conducted via chemical vapor infiltration (CVI) with CH₄ as a carbon source and Ar as a carrier gas at 1000 ℃ for 144 h, yielding the CVI-C_f/C-ZrB₂ composite material. The macroscopic microstructure and 3D morphology of the composites were characterized by a model VHX-700 white light ultra-deep microscope (Keyence Co., Japan). The detailed microstructural features were analyzed by a model SU8230 cold field emission scanning electron microscope (Hitachi Co., Japan). To analyze the anisotropy of the material properties, the samples were cut at angles of 0°, 30°, 45°, 60° and 90° relative to the plane XY of the carbon fiber felt. The compressive strength was measured for the cuboid shaped samples with dimensions of 10 mm (width)×10 mm (length)×15 mm (height) by a model 34TM-30 screw-driven test frame (Instron Co., USA). The thermal diffusivity was determined by a model LFA 467 laser flash instrument (Netzsch Co., Germany) in a temperature range from room temperature to 900 ℃ on the samples with 10 mm (width)×10 mm (length)×1 mm (height).

The C_f/C-ZrB₂ composites prepared by IVI exhibit a non-uniform internal pore distribution, with a pore volume of (10⁴-10⁶ μm³), concentrated primarily in fiber net felt and interlaminar region. After the CVI treatment, approximately 17.16% of pores are filled with pyrolytic carbon, reducing pore volume and increasing material density from 1.8 g/cm³ to 2.1 g/cm³. The microstructural analysis reveals that pyrolytic carbon grows in spherical crown-shaped configurations, gradually forming continuous layered coatings that fill material pores. Regarding the compressive properties, the samples at 0° and 90° orientations exhibit higher Young's moduli ((2.52±0.99) GPa and (3.80±1.03) GPa, respectively) and compressive strengths ((41.62±9.60) MPa and (41.50±5.18) MPa). The samples at 60° demonstrate the lowest modulus ((0.41±0.08) GPa) and strength ((8.47±1.44) MPa). The intermediate-angle samples show a reduced performance due to shear stress concentration at inter-layer pores. For the CVI carbon densification, the mechanical properties significantly improve in all orientations. The samples at 90° achieve Young's modulus of (10.79±2.21) GPa and compressive strength of (155.98±39.22) MPa, while the samples at 0° reach (6.76±0.82) GPa and (130.09±13.19) MPa. Although the intermediate angles show a substantial improvement, they maintain relatively low values due to a incomplete densification of inter-layer pores. The results of thermal conductivity test indicate that prior to CVI, the samples at 90° display a thermal diffusivity of 1.40 mm²/s and a thermal conductivity of 1.57 W/(m·K) at 25 ℃, significantly exceeding those of the samples at 0° Post-CVI, pyrolytic carbon enhances thermal pathways. The conductivity of the samples at 90° increases to 3.48 W/(m·K) and 0° to 2.37 W/(m·K), with a reduced anisotropy ratio. The samples at 30°-60° orientations also show significantly an enhanced thermal diffusivity, with a heat transfer capability approaching that of the 90° direction.

The C_f/C-ZrB₂ composites fabricated via IVI exhibited a non-uniform pore distribution. After CVI, pores were refined and partially filled, increasing material density and structural densification. The compressive properties showed a significant anisotropy. The orientations at 0° and 90° demonstrated an optimal mechanical performance, while intermediate angles suffered a substantial degradation due to the shear stress dominance and pore concentration. The CVI treatment could enhance the properties across all the orientations, with a maximum improvement at 90°. The thermal conductivity also exhibited an anisotropy, revealing a superior heat transfer efficiency along fiber-dominant directions. For the CVI process, the thermal diffusivity and conductivity both increased, accompanied by a significant reduction in thermal anisotropy ratio, as pyrolytic carbon filling improved heat transfer pathways.