Until now, some questions about medium-entropy carbide ceramic and the corresponding multi-phase carbide ceramic with the same cation and proportion remain unclear. Regarding oxidation behavior, do both have a synergistic oxidation ability and what role does entropy stabilization play in medium-entropy carbides? In this work, the oxidation behaviors of HfC-ZrC-TiC multi-phase carbide (HZT-MPC) and (Hf_1/3Zr_1/3Ti_1/3)C medium-entropy carbide (HZT-MEC) powders were investigated. After thermogravimetry (TG) oxidation, the TG curve of HZT-MPC had a bimodal distribution. The “preferential oxidation” of HfC/ZrC occurred within HZT-MPC, followed by the formation of multi-phase oxides (HfO₂, ZrO₂ and TiO₂). Uneven compositional distribution slowed down their solid solution reactions to form Ti-doped (Hf, Zr)O₂ and (Hf, Zr)TiO₄. While the TG curve of HZT-MEC had a single-peak characteristic. Uniform compositional distribution at the atomic scale promoted the rapid interdiffusion of oxides, forming Ti-doped (Hf, Zr)O₂ and (Hf, Zr)TiO₄ without ZrO₂, HfO₂ and TiO₂ after TG oxidation. Additionally, HZT-MEC had a higher onset oxidation temperature (470 °C) compared to HZT-MPC (430 °C), and the TG single peak of HZT-MEC was between the TG bimodal peaks of HZT-MPC. Therefore, HZT-MEC showed superior oxidation resistance compared to HZT-MPC, which was attributed to the entropy stabilization effect of HZT-MEC suppressed the “preferential oxidation” of HfC/ZrC and the “delayed oxidation” of TiC, promoting the synergistic oxidation ability of multi-principal elements.

Multicomponent Ti‐containing ultra‐high temperature ceramics (UHTCs) have emerged as more promising ablation‐resistant materials than typical UHTCs for applications above 2000 °C. However, the underlying mechanism of Ti improving the ablation performance is still obscure. Here, (Hf,Zr,Ti)B₂ coatings are fabricated by supersonic atmospheric plasma spraying, and the effects of Ti content on the ablation performance under an oxyacetylene flame are investigated. The (Hf_0.45Zr_0.45Ti_0.10)B₂ coating shows superior ablation resistance and cycling reliability at ≈2200°C. A functionally graded oxide scale comprising an outer dense layer and an underlying fine granular layer formed. The former is a better oxygen barrier owing to fewer cracks and the latter has high strain tolerance due to finer grain size. The uniform dissolving of ≈4 mol% Ti in the inner layer results in grain refinement via sluggish diffusion and thus stress release. For the outer layer, Ti segregation at the nanoscale leads to a metastable cubic (Hf,Zr,Ti)O₂ and local severe lattice distortion, inhibiting the propagation of cracks. Ti ions’ unique dissolving in the oxide scale enables a strong oxygen diffusion barrier with high strain tolerance, which is responsible for superior performance. This study provides new insights into the ablation behavior of Ti‐containing multicomponent UHTCs.

In recent years, high-entropy metal-carbides (HECs) have attracted significant attention due to their exceptional physical and chemical properties. The combination of the excellent performance exhibited by bulk HECs ceramics and the distinctive geometric characteristics has paved the way for the emergence of one-dimensional (1D) HECs as a novel material with unique development potential. Herein, we successfully fabricated a novel (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C nanowire derived via Fe-assisted single-sourced precursor pyrolysis. Prior to the synthesis of the nanowires, the composition and microstructure of (Ti, Zr, Hf, Nb, Ta)-containing precursor (PHECs) were analyzed, and divinylbenzene (DVB) was used to accelerate the conversion process of the precursor and contribute to the formation of HECs, which also provided partial carbon source for the nanowire growth. Additionally, multi-branched, single-branched and single-branched bending nanowires were synthesized by adjusting the ratio of PHECs to DVB. The obtained single-branched (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C nanowires possessed smooth surfaces with an average diameter of 130~150 nm and a length of several tens of micrometers, which were single-crystal structure and typically grew along [11(_)1] direction. And the growth of (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C nanowires was in agreement with top-type vapor-liquid-solid mechanism. This work not only successfully achieved the fabrication of HECs nanowires by a catalyst-assisted polymer pyrolysis, but also provided a comprehensive analysis of the factors affecting their yield and morphology, highlighting the potential application of these attractive nano-materials.

Ultra-high temperature ceramic nanowires have offered increasing potential for thermal structural components. Herein, a novel single-crystal Hf_0.5Ta_0.5C solid solution nanowires were synthesized and incorporated with HfC coating, constructing a robust structure with Hf_0.5Ta_0.5C solid solution nanowires uniformly distributed and interconnected within the coating. The novel Hf_0.5Ta_0.5C solid solution nanowires could effectively hinder crack propagation through crack tip pinning and crack deflection. This mechanism substantially enhanced the elastic modulus and fracture toughness of HfC coating by 53.29 % and 59.67 %, respectively. The toughened HfC coating displayed superior fracture toughness and good interfacial binding strength with substrate to resist sever oxidation and scouring. Additionally, the high thermal conductivity of toughened HfC coating could promote the heat transmission that the coating sustained. Thus, in comparison to pure HfC coating, the toughened HfC coating displayed smaller mass and linear ablation rates of -0.35 mg·s^-1 and -0.46 μm·s^-1, which decreased by 39.66% and 36.98%, respectively. Our work not only simultaneously enhances the mechanical property and ablation resistance of HfC coated carbon/carbon composites, but also provides a novel prospect for advanced ultra-high temperature ceramic nanowires under extreme condition.

Polymer-derived ultra-high-temperature ceramic (UHTC) nanocomposites have attracted growing attention due to the increasing demands for advanced thermal structure components in aerospace. Herein, hafnium carbide (HfC) whiskers are successfully fabricated in carbon fiber preforms via the polymer-derived ceramic (PDC) method. A novel carbon nanotube (CNT) template growth mechanism combined with the PDC method is proposed in this work, which is different from the conventional vapor–liquid–solid (VLS) mechanism that is commonly used for polymer-derived nanostructured ceramics. The CNTs are synthesized and proved to be the templates for fabricating the HfC whiskers, which are generated by the released low-molecular-weight gas such as CO, CO₂, and CH₄ during the pyrolysis of a Hf-containing precursor. The formed products are composed of inner single crystal HfC whiskers that are measured to be several tens of micrometers in length and 100–200 nm in diameter and outer HfC/HfO₂ particles. Our work not only proposes a new strategy to prepare the HfC whiskers, but also puts forward a new thinking of the efficient utilization of a UHTC polymer precursor.