High-Temperature Limits of (Hf₀.₂₅Zr₀.₂₅Ti₀.₂₅Ta₀.₂₅)C: Understanding Oxide Layer Instability in High-Entropy Carbides

There is currently an urgent need to develop materials capable of withstanding the extreme thermal and oxidative conditions encountered during hypersonic flight or atmospheric re-entry. Ultra-high temperature ceramics (UHTCs) especially transition metal carbides such as HfC, ZrC, TiC, and TaC, are known for their exceptional melting points and impressive mechanical strength at elevated temperatures. Yet, despite these favorable properties, a major challenge remains: their limited resistance to oxidation in harsh environments. To address this issue, researchers have often turned to incorporating silicon carbide (SiC) into UHTC systems. SiC can passively oxidize to form a dense SiO₂ layer that temporarily shields the underlying material. However, this solution has its own limitations. Above 2000 °C, SiO₂ tends to volatilize due to the formation of gaseous SiO, which makes the protective layer increasingly unreliable in sustained high-temperature applications. As a result, SiC-containing systems struggle to provide consistent performance in the kinds of extreme operating environments envisioned for next-generation aerospace platforms.  This has led to growing interest in high-entropy carbides (HECs), which offer an alternative strategy grounded in compositional complexity. By combining multiple carbide-forming elements—typically in near-equimolar ratios. This thermodynamic principle helps stabilize single-phase solid solutions and results in materials that often exhibit enhanced hardness, thermal stability, and chemical resistance. Notably, some of the earliest HEC systems, made by mixing HfC, ZrC, TaC, NbC, and TiC, have already shown promising resistance to oxidation compared to conventional binary carbides. However, most prior studies have been limited to relatively moderate conditions, often below 2200°C and under controlled or low heat flux settings. These parameters fall short of replicating the truly aggressive environments seen by thermal protection systems on hypersonic vehicles or re-entry capsules. To this account, a recent study published in the Journal of the European Ceramic Society and conducted by PhD candidate Shiyan Chen and led by Professors Zhaoke Chen and Xiang Xiong at Central South University, with contributions from Weilong Song and Yi Zeng, the team investigated how to push HECs to their operational extremes. They focused on a quaternary composition—(Hf₀.₂₅Zr₀.₂₅Ti₀.₂₅Ta₀.₂₅)C—that deliberately excluded niobium, since niobium-based oxides tend to melt at lower temperatures and would likely compromise high-temperature performance.

The researchers first synthesized dense bulk samples using a carefully controlled sintering route. High-purity powders of the four binary carbides were mixed in equal molar ratios and ball-milled thoroughly to ensure homogeneity at the microscopic level. After drying, the powder blend was hot-pressed at 2100°C under a vacuum and subjected to 50 MPa of uniaxial pressure. The outcome was a nearly fully dense ceramic, with phase analysis via X-ray diffraction confirming the formation of a single-phase face-centered cubic structure—no signs of unreacted binary phases remained. SEM images revealed a dense microstructure with minimal porosity, and energy-dispersive spectroscopy showed that the elemental distribution was uniform throughout the material. These well-characterized samples served as the foundation for the subsequent high-temperature ablation studies. The ablation experiments were carried out using a high-intensity plasma flame gun, which generated a harsh thermal and oxidative environment, mimicking the conditions experienced by materials used in hypersonic flight or atmospheric re-entry. Each sample was exposed to the flame for 90 seconds, at surface temperatures of 2070°C, 2280°C, and 2600°C, respectively. These were labeled as HEC-1, HEC-2, and HEC-3. A non-contact optical pyrometer monitored the temperature in real time, while the heat flux during testing ranged from 5.3 to 6.2 MW/m². The results showed a striking dependence on temperature: HEC-1 experienced minimal surface damage, maintaining both structural integrity and a relatively low mass loss rate (0.23 mg/cm²·s), with a linear ablation rate of just 4.6 µm/s. However, this behavior deteriorated rapidly as the temperature increased. At 2280°C, the rates rose sharply, and by 2600°C (HEC-3), the material’s protective oxide layer had essentially failed—both the mass and linear ablation rates were more than tenfold higher than at 2070°C which suggest severe surface erosion.

The authors also performed microscopic investigations of the ablated surfaces which helped clarify what was happening at the material’s surface. For the lower-temperature sample, HEC-1, they observed a relatively intact oxide network. This protective layer was made up of high-melting-point HfO₂ and ZrO₂, which acted as a robust skeleton, interspersed with Ti- and Ta-based oxides that served as fillers and binders. The oxide scale was dense and tightly adhered to the substrate, providing effective protection against the plasma jet. However, for HEC-3, this oxide system broke down. SEM images revealed widespread cracking, surface porosity, and areas where the oxide had delaminated. Cross-sectional imaging further confirmed that the protective layer became thinner and more porous as the ablation temperature increased—evidence that the oxide’s structural resilience had been compromised. Next, the team used focused ion beam sectioning and transmission electron microscopy to uncover a complex, multilayered structure within the oxide scale and found in the lower-temperature cases, the layers consisted of well-defined Ti-rich, Hf/Zr-rich, and Ta-rich domains, organized in a way that helped the oxide function as a barrier. But this organized structure deteriorated at the highest temperature. The Ti- and Ta-based oxides, which have relatively low melting points, either volatilized or were removed by the high-velocity plasma. This left behind a fragile, Hf/Zr-rich oxide skeleton that lacked cohesion. Without the presence of lower-melting-point oxides to bind and seal the surface, the skeleton disintegrated and peeled off. This exposed fresh carbide to the environment, setting off a repetitive oxidation cycle where newly formed oxides were stripped away almost as quickly as they formed—a damaging loop that accelerated material loss. The damage, however, extended deeper than just the surface. Electron probe microanalysis of the interface between the oxide scale and the untouched carbide revealed the development of a chemically distinct transition layer. This layer consisted of crystalline oxycarbides that had formed from an initially amorphous Hf-Zr-Ti-Ta-C-O phase. These oxycarbides were compositionally segregated: Ti-rich phases exhibited characteristics similar to rutile, Hf/Zr-rich domains adopted a monoclinic structure, and the Ta-rich areas remained nanocrystalline and less ordered. This gradient of crystallinity reflected the oxidation hierarchy of the individual elements, with Hf and Zr oxidizing first, followed by Ti and finally Ta.

What makes the findings from Central South University scientists so compelling is that it’s not the core material that breaks down first, but the oxide scale that forms on the surface. Once the protective oxides—particularly those based on Ti and Ta—begin to evaporate or are stripped away by plasma flow, the integrity of the whole system unravels. Even the more refractory HfO₂ and ZrO₂, which form the structural backbone of the oxide layer, can’t hold the line without support from these lower-melting binders. In that sense, the failure is not abrupt, but a slow and cascading collapse that begins when the oxides can no longer self-repair. Moreover, designing a UHTC with a high melting point is no longer sufficient. What really matters is how the material behaves once oxidation begins, how the oxides interact and evolve, and how well they hold together under stress. This study makes a strong case for shifting design strategies toward engineering the oxide phase, not just the carbide matrix. Parameters like oxide viscosity, evaporation rate, and thermal compatibility suddenly become as important as the entropy-driven stability of the base ceramic. Equally important is the detailed characterization of the transitional phases observed near the oxidation front. The identification of an amorphous-to-crystalline transition zone—marked by nanoscale elemental segregation and these regions act as temporary buffers that slow oxygen ingress and delay surface degradation, albeit only for a while. We believe understanding these transitional structures opens up new possibilities for surface engineering. Imagine coatings or compositional gradients that are tailored not only to resist oxidation but to preserve these transient, self-limiting layers for longer durations.