Ultra-high-temperature ceramic (UHTC) nanocomposites based on zirconium diboride (ZrB₂) and silicon carbide (SiC) or hafnium carbide (HfC) are critical for aerospace applications, particularly in extreme environments such as hypersonic flight and re-entry vehicles. These materials must withstand temperatures exceeding 2000°C while maintaining structural integrity, oxidation resistance, and ablation performance. The incorporation of nanoscale additives, such as carbon nanofibers (CNFs), graphene platelets, or other nanostructured reinforcements, enhances mechanical properties, fracture toughness, and thermal stability. Spark plasma sintering (SPS) has emerged as a leading fabrication technique for these nanocomposites, enabling dense, homogeneous microstructures with tailored properties.
**Spark Plasma Sintering of ZrB₂/SiC and HfC-Based Nanocomposites**
Spark plasma sintering is a rapid consolidation method that utilizes pulsed direct current and uniaxial pressure to achieve near-theoretical density at lower temperatures and shorter dwell times compared to conventional hot pressing. For ZrB₂-SiC systems, SPS typically operates at 1800–2000°C under pressures of 30–50 MPa, with heating rates of 100–300°C/min. The process minimizes grain growth while ensuring effective densification, critical for preserving nanoscale features. The addition of SiC (10–30 vol%) improves sinterability by forming liquid phases that enhance particle rearrangement. HfC-based composites, though requiring higher sintering temperatures (2000–2200°C), benefit similarly from SPS, achieving densities above 98% with submicron grain sizes.
Nanoscale additives play a pivotal role in microstructure refinement. Carbon nanofibers, when incorporated at 1–5 wt%, disperse uniformly within the matrix, acting as bridging agents that impede crack propagation. The high aspect ratio of CNFs increases the fracture toughness of ZrB₂/SiC composites from ~3.5 MPa·m¹/² to over 5.5 MPa·m¹/², as measured by Vickers indentation. Similarly, graphene nanoplatelets (GNPs) at 0.5–2 wt% improve thermal conductivity and reduce brittleness by promoting energy dissipation through pull-out and crack deflection mechanisms.
**Oxidation Resistance Mechanisms**
At ultra-high temperatures, ZrB₂ and HfC oxidize to form protective scales. ZrB₂-SiC composites develop a multi-layered oxide layer consisting of a porous ZrO₂ top layer and a viscous borosilicate glass sub-layer (SiO₂-B₂O₃) at 1200–1600°C. The glassy phase seals microcracks and impedes oxygen diffusion, reducing linear oxidation rates to 0.1–0.5 mg/cm²·h in static air. HfC exhibits superior oxidation resistance above 1800°C due to the formation of a stable HfO₂ scale with a higher melting point (~2800°C) than ZrO₂ (~2700°C). However, the absence of a self-healing glassy phase necessitates SiC or Si3N4 additions to enhance durability.
Nanoscale SiC particles (50–200 nm) improve oxidation resistance by accelerating the formation of a continuous SiO₂ layer at lower temperatures. The finer dispersion reduces oxygen permeability and delays the onset of catastrophic oxidation. Carbon nanofibers, while beneficial for toughness, may oxidize above 500°C, leaving behind nanopores that can compromise the oxide layer’s integrity. To mitigate this, hybrid reinforcements such as SiC-coated CNFs or in-situ formed carbides (e.g., TaC) are employed to balance mechanical and oxidation performance.
**Ablation Performance Under Extreme Conditions**
Ablation resistance is critical for thermal protection systems (TPS) in re-entry vehicles. ZrB₂-SiC composites exhibit mass ablation rates of 0.01–0.05 mm/s under plasma arc jet testing at 2500–3000°C, outperforming monolithic ZrB₂ by an order of magnitude. The ablation mechanism involves active oxidation of SiC to gaseous SiO, which reacts with ZrO₂ to form a refractory ZrSiO4 (zircon) layer. This layer, along with the borosilicate glass, acts as a thermal barrier, reducing heat flux and erosion rates.
HfC-based composites demonstrate even lower ablation rates (0.005–0.02 mm/s) due to the higher stability of HfO₂ and its lower thermal conductivity (2.5 W/m·K vs. ZrO₂’s 3.5 W/m·K). The incorporation of 5–10 vol% TaC or WC further enhances ablation resistance by forming complex oxides (Hf-Ta-O or Hf-W-O) that increase viscosity and reduce volatilization. Nanoscale TaC additions (50–100 nm) are particularly effective, as they homogenize the oxide scale and suppress crack nucleation.
**Role of Nanoscale Additives in Toughness Enhancement**
The brittleness of UHTCs remains a limiting factor for aerospace applications. Carbon nanofibers, with their high tensile strength (3–7 GPa) and modulus (200–600 GPa), provide bridging and pull-out toughening. In ZrB₂-20 vol% SiC-3 wt% CNF composites, crack deflection along CNF/matrix interfaces increases fracture energy by 40–60%. Similarly, graphene platelets (0.5–1.5 wt%) enhance toughness by promoting interfacial sliding and nano-scale crack branching.
Mechanical properties of selected nanocomposites:
| Composition | Flexural Strength (MPa) | Fracture Toughness (MPa·m¹/²) |
|----------------------------|-------------------------|-------------------------------|
| ZrB₂-20SiC | 550–650 | 3.2–3.8 |
| ZrB₂-20SiC-3CNF | 700–800 | 5.0–5.6 |
| HfC-15SiC-2GNP | 600–700 | 4.5–5.0 |
**Challenges and Future Directions**
Despite advancements, challenges persist in scalable production, interfacial bonding, and long-term stability under cyclic thermal loads. Agglomeration of nanoscale additives remains a concern, necessitating advanced dispersion techniques such as ultrasonic mixing or in-situ synthesis. Future research may explore hierarchical architectures, such as nanofiber-reinforced matrices with micro-scale ductile phases, to further optimize toughness and oxidation resistance. Computational modeling of interfacial reactions and thermal stress distribution will aid in designing next-generation UHTC nanocomposites for aerospace applications.
In summary, ZrB₂/SiC and HfC-based nanocomposites fabricated via spark plasma sintering exhibit exceptional ultra-high-temperature performance, with nanoscale additives playing a critical role in enhancing toughness and oxidation resistance. These materials are poised to enable advanced aerospace systems operating in the most demanding thermal environments.