To date, some questions about medium-entropy carbide ceramics and the corresponding multi-phase carbide ceramics with the same cations and proportions remain unclear. Regarding oxidation behavior, do both have synergistic oxidation abilities 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). The uneven compositional distribution slowed their solid solution reactions to form Ti-doped (Hf,Zr)O₂ and (Hf,Zr)TiO₄. The TG curve of HZT-MEC had a single peak. A 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 (T_o; 470 °C) than did 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 suppressing the “preferential oxidation” of HfC/ZrC and the “delayed oxidation” of TiC, promoting the synergistic oxidation ability of multiple principal elements.

Ultra-high-temperature ceramic nanowires have shown increasing potential for use as thermal structural components. Herein, novel single-crystal Hf_0.5Ta_0.5C solid solution nanowires were synthesized and incorporated with a HfC coating to construct 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 the HfC coating by 53.29% and 59.67%, respectively. The toughened HfC coating displayed superior fracture toughness and good interfacial binding strength with the substrate to resist severe oxidation and scouring. Additionally, the high thermal conductivity of the toughened HfC coating promoted heat transmission. Thus, in comparison to the 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 properties and ablation resistance of HfC-coated carbon/carbon (C/C) composites but also provides novel prospects for advanced ultrahigh-temperature ceramic nanowires under extreme conditions.

Recently, the preparation of ultra-high temperature HfC ceramic coating has gained significant attention, particularly through the application of the HfCl₄-CH₄-H₂-Ar system via Chemical Vapor Deposition (CVD), which has been found widely applied to C/C composites. Herein, an analysis of the reactions that occur in the initial stage of the CVD-HfC coating process is presented using Density Functional Theory (DFT) and Transition State Theory (TST) at the B3LYP/Lanl2DZ level. The results reveal that HfCl₄ can only cleave to produce hypochlorite, which will further react with methyl to synthesize intermediates to form HfC. According to the analysis of the energy barrier and reaction constant, HfCl preferentially reacts with methyl groups to form complex adsorptive intermediates at 1573 K. With a C—Hf bond production energy of 212.8 kcal/mol (1 kcal = 4.18 kJ), the reaction rate constant of HfCl + CH is calculated to be 2.15 × 10⁻¹⁸ cm³/s at 1573 K. Additionally, both the simulation and experimental results exhibit that the upward trend of reaction rate constants with temperature is also consistent with the deposition rate, indicating that the growth curve of the reaction rate constants tends to flatten out. The proposed reaction model of the precursor’s decomposition and reconstruction during deposition process has significant implication for the process guidance.

In recent years, high-entropy metal carbides (HECs) have attracted significant attention due to their exceptional physical and chemical properties. The combination of excellent performance exhibited by bulk HEC ceramics and distinctive geometric characteristics has paved the way for the emergence of one-dimensional (1D) HECs as novel materials with unique development potential. Herein, we successfully fabricated novel (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C nanowires 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 a 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 a single-crystal structure and typically grew along the [1 $\overline{1}$1] direction. Also, the growth of the (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 HEC 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.

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.