Low-temperature PIP offers an efficient route for fabricating UHTCMCs, but the structural evolution of such materials during ultrahigh-temperature exposure remains unclear. In this work, C/HfC–SiC composites fabricated from an oxygen-free precursor at 800 °C were investigated, with counterparts prepared from a conventional precursor at 1600 °C as references. Their structural evolution and mechanical-property changes were systematically compared under inert ultrahigh-temperature and low-pressure oxidation conditions. The low-temperature-fabricated composite showed good structural stability at 2000 °C, retaining a flexural strength of 355 MPa and a flexural modulus of 59.4 GPa after 30 min oxidation, higher than those of the reference. At 2200 °C, matrix denitrification, phase separation, and structural rearrangement caused degradation, although the composite still retained a flexural strength of 243 MPa. These results indicate that low-temperature-PIP-derived C/HfC–SiC composites have potential for ultrahigh-temperature thermostructural applications.

Hafnium-based ultrahigh-temperature ceramic precursors that combine low-temperature formability with a high ceramic yield remain a key research focus in the advanced ceramics domain. This study presents an oxygen-free B-Hf-based precursor capable of producing fibers with a ceramic yield of 49.3 wt. % and enabling the fabrication of ultrahigh-temperature ceramics following pyrolysis. The reaction mechanism of the precursor, ceramic synthesis under an argon atmosphere, ceramic transformation, and microstructural evolution were systematically investigated. The results indicate that accurately identifying reactive sites and implementing molecular modification can simultaneously enhance the formability of precursors and their ceramic yields. While evolving ceramics at high temperatures, boron atoms can suppress the growth of grains. At 2400 degrees C, the grain size of HfB2 remains 32.4 nm. This study proposes a new strategy for designing hafnium-based precursors with low-temperature formability and high ceramic yield while providing deeper insights into the evolution of oxygen-free hafnium-based ceramics at high temperature. (c) 2026 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

Oxygen-free Hf-N-C ultra-high temperature ceramic (UHTC) precursors serve as critical raw materials for manufacturing high-performance UHTC matrix composites and HfC ceramic fibers. However, the currently observed extreme hygroscopicity of Hf-N-C precursors severely limits their practical applications. In this work, we strategically suppress the hygroscopicity of precursors through targeted reduction of N-H bond content in the molecular architecture. The PHNC-G3 precursor system, employing N,N′-dimethylethylenediamine as bridging agent and diallylamine as end-capper, demonstrates optimal performance with moisture absorption reduced from 34% to 14% under humid conditions. Ceramization studies further reveal that this enhanced moisture resistance effectively suppresses HfO2 formation during pyrolysis. The resulting Hf2ON2 intermediate phase significantly lowers the carbothermal reduction temperature, enabling complete transformation to HfC at 1400 °C.

To address the high processing temperature, long fabrication cycle, and insufficient densification associated with the conventional PIP route for preparing UHTCMCs, this work employed a novel liquid oxygen-free Hf-based precursor to densify continuous fiber-reinforced C/HfC-SiC composites at 800 degrees C after 13 PIP cycles. This approach avoided the 1600 degrees C heat treatment required in the conventional process and reduced the total thermal-processing time by approximately 56%. The composites combined a low density of 2.19 g/cm3 while maintaining excellent mechanical properties, with flexural and tensile strengths of 432 MPa and 224 MPa, respectively. These properties were attributed to the formation of an intact matrix structure and a favorable interfacial bonding state at low temperature enabled by the oxygen-free precursor. Meanwhile, the composites retained good ablation resistance under a 2500 degrees C oxyacetylene flame, with linear and mass ablation rates of 7.1 & times; 10-3 mm/s and 4.9 & times; 10-3 g/s, respectively, after 90 s of ablation. These results demonstrate that the PIP route based on an oxygen-free precursor provides a mild and efficient strategy for the fabrication of highperformance C/HfC-SiC composites.

Chlorination reactions enable the efficient fabrication of carbide-derived carbon (CDC) coatings on silicon carbide (SiC) fibers. In this study, we used this method to deposit CDC coatings on near-stoichiometric SiC fiber surfaces and systematically examined how the CDC coatings influence their high-temperature resistance and oxidation behavior. The CDC-coated SiC fibers, with an average coating thickness of approximately 755 nm, exhibited a higher strength retention ratio than the pristine SiC fibers after heat treatment in an Ar atmosphere. After treatment at 1900 degrees C in an Ar atmosphere, the CDC-coated fibers retained similar to 24 % of their original strength. The CDC-coated SiC fibers maintained its strength primarily due to (1) the gradual densification of CDC coatings during thermal exposure, (2) the expansion of the effective load-bearing area of the fibers, and (3) the mitigation of structural defects caused by SiC grain growth through the bridging effect by the CDC coating. In air above 275 degrees C, CDC coatings undergo oxidative volatilization, converting to gaseous products and degrading. The oxidation of the CDC coating is initially dominated by gas diffusion, with the reaction rate following a parabolic relationship with oxidation time and an activation energy (E-a,E-d) of 67.7 kJ & centerdot;mol(-1). At later stages, surface reactions become dominant, during which the reaction rate exhibits a linear relationship with oxidation time and the activation energy (E-a,E-r) is calculated as 93.1 & centerdot;kJ mol(-1).

Hafnium (Hf)-containing polymers have garnered increasing attention as single-source precursors for ultrahightemperature ceramics (UHTCs), but it still remains significant challenges in regulating the highly active Hf atoms. Herein, we designed and synthesised a novel polymeric Hf-B-C-N precursor (PHBCN) by the dehalogenative substitution reactions using B(NHCH3)3, ClCH2CN, and HfCl4 as raw materials. A molecular collaborative strategy was described through the coordination effects of amidinate ligands and the conjugation effects of BN3 aromatic systems to modulate the electronic environment of Hf centers and promote the water/oxygen resistance of PHBCN. The pyrolysis of PHBCN at 1600 degrees C produced HfC/HfB2 nanocomposite ceramics with Hf content of 75.71 wt% and the phase evolution of polymer-derived ceramics was systematically investigated. Moreover, the hydrolytic-oxidative stability and proper rheological properties endowed PHBCN precursor with good spinnability, which would provide a new idea for advancing Hf-based UHTC fibers.

HfC-SiC ceramics are promising for ultrahigh-temperature applications, yet their structural degradation due to SiC decomposition at 2500 degrees C remains underexplored. In this study, HfC-SiC nanocomposites with a uniform nanoscale microstructure were successfully fabricated via the polymer-derived ceramics route using a liquid, non-oxygen PHNCS precursor. Upon heat treatment at 2500 degrees C, significant SiC decomposition occurs, and the volatilization of silicon promotes the deposition of residual carbon as graphitic layers on HfC particles, forming a HfC@C structure. Commercial SiC exhibits much less decomposition than the HfC-SiC nanocomposites. This may be attributed to nano-sized HfC particles that precipitate on the SiC surface above 1900 degrees C, which act as defects and accelerate SiC decomposition at extreme temperatures. These findings elucidate the structural degradation mechanisms of HfC-SiC ceramics at ultrahigh temperatures and provide valuable insights for defining their upper service temperature and guiding the design of optimized multiphase architectures.

The excellent properties of near-stoichiometric silicon carbide (SiC) fibers make them promising materials for applications in the aerospace and nuclear industries. However, the presence of a small amount of oxide phase and free Si within the fiber seriously affects its high-temperature resistance. In this study, different near-stoichiometric SiC fibers were prepared via sintering in nitrogen and boron atmospheres. The effects of the sintering atmosphere on the composition, structure, and properties of near-stoichiometric SiC fibers were studied using mechanical property analysis, elemental analysis, X-ray photoelectron spectroscopy, Auger electron spectroscopy, and high-temperature resistance in Ar and air. The results showed that sintering in a boron atmosphere resulted in the introduction of a small amount of boron on the SiC fiber surface, while the internal composition and structure of the fibers were not significantly affected by the sintering atmosphere. When treated at high temperatures in Ar, the presence of boron prevented the formation of large SiC grains on the fiber surface, resulted in a denser fiber structure, and improved its high-temperature resistance in an Ar atmosphere without reducing its oxidation resistance.

Silicon carbide (SiC) fiber is a high-performance ceramic fiber mainly prepared by the polymer-derived ceramics (PDC) method. However, balancing the precursor's ceramic yield and spinnability is challenging in the preparation process. Herein, we proposed a new strategy, chemical bonding with PCS to form a high cross-linked network structure, utilizing the flexibility and high aspect ratio within MWCNTs. As a result, based on the spinnable PCS (57.58 %), only the incorporation of 0.05 wt% MWCNTs increase the ceramic yield by 21.66 %- 70.05 % and 1 wt% MWCNTs achieved as high as 81.44 % when pyrolyzed at 900 degrees C. Moreover, the hybrid precursors showed good spinnability, except for the 1%-MWCNTs/PCS. In addition, the presence of chemical bonding and cross-linking structures has been thoroughly proven, and the mechanism behind the increase in ceramic yield has been further explored. MWCNTs occupied cross-linking sites and formed cross-linked structures to effectively reduce weight loss during thermal cross-linking in the 300-500 degrees C temperature range, thereby increasing ceramic yield. The increment of spinnable PCS in the ceramic yields can effectively reduce the preparation cost and enhance the performance of the SiC fibers using the PCS method.

Advancements in high-temperature stealth technology necessitate silicon carbide (SiC) fibers with hightemperature resistance, load-bearing capacity, and microwave absorption performance. Considering their ability to facilitate impedance matching between transmission and absorption layers, SiC fibers with continuously tunable resistivity are promising structural-functional integrated materials. Using electron beam-irradiated, crosslinked polycarbosilane fibers as precursors, we fabricated continuous SiC fibers through a stepwise process involving pre-oxidation, hydrogen heat treatment, and nitrogen sintering. We characterized their composition, structure, and properties using various characterization techniques, revealing a preliminary composition structural-electrical property relationship. The SiC fibers exhibited continuously tunable resistivity, tensile strength, Young's modulus, and diameter of 1.32-1.09 x 106 52 cm, 2.67-3.33 GPa, 226-304 GPa, and 10.46-11.03 mu m, respectively. After 1 h of heat treatment at 1400 degrees C in argon and 1200 degrees C in air, resistivity remained stable and tensile strength retention exceeded 1.25 GPa. These remarkable properties confirm their potential for hightemperature stealth applications.

Chlorination reactions can be used to efficiently fabricate carbide-derived carbon (CDC) coatings on SiC fibers. In this study, we leveraged this method to introduced CDC coatings on the surfaces of SiC fibers. By controlling the reaction temperature, time, and chlorine concentration, coatings of varying thicknesses were achieved, with the activation energies for the surface reactions of the two types of fibers being 115.10 kJ/mol and 174.41 kJ/mol, respectively. When the chlorine concentration was not saturated, the chlorination reaction was influenced by both surface reactions and chlorine gas diffusion. The reaction process and micro-mechanisms of the chlorination reaction were analyzed using scanning electron microscopy and focused ion beam-scanning transmission electron microscopy. The compositional structure of the fiber surface coatings was investigated through X-ray photoelectron spectroscopy and Raman spectroscopy. Subsequently, the influence of the CDC coatings on the mechanical properties of the SiC fibers was examined. Investigations focused on the porosity, surface roughness, and fracture morphology revealed that the increase in the size of surface roughness and surface defect dimensions post-chlorination are the primary causes for the reduced strength of SiC fibers.

Hafnium-containing ultra-high-temperature ceramics (UHTCs) are critical for aerospace structures exposed to extreme heat, oxidation and ablation. However, their fabrication by precursor infiltration pyrolysis (PIP) is hindered because existing non-oxygen multicomponent hafnium precursors are invariably intractable solids that trade processability for performance. A liquid precursor composed of non-oxygen polyhafniumnitrocarboborosilane was synthesised using tetrakis(diethylamino)hafnium, liquid polycarbosilane and borazine. Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance and viscosity measurement confirmed Si-N-Hf and B-N-Hf linkages. The synthesised precursor remained liquid at ambient temperature (eta=157.0 +/- 2.1 mPa.s) and afforded an argon-pyrolysed ceramic yield of 67.33 wt% at 1000 degrees C. The structure of the resulting ceramic was characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The as-pyrolysed material was predominantly amorphous, contained 0.77 +/- 0.05 wt% oxygen and showed no detectable hafnia phase at 1000 degrees C. Comprehensive XRD, SEM, TEM, Raman and elemental analysis revealed that further annealing to 2400 degrees C produced a dense nanocomposite exhibiting a total mass loss of 15.12 +/- 0.22 wt% and comprising HfCxN1-x, HfB2 and SiC crystallites (9.46-410.20 nm). In situ precipitation of HfCxN1-x occurred at 1600 degrees C, while segregation of free boroncontaining carbon (BCy(Nz)) and its subsequent reaction with HfCxN1-x above 2000 degrees C generated HfB2 and variable-crystallinity carbon, refining the multiphase structure. The resulting nanocomposite ceramic, with its unique chemistry and microstructure, offers considerable promise for aerospace applications.

Ultra-high-temperature ceramic precursors are crucial for fabricating large, complex-shaped, high-performance ceramic matrix composites using a precursor impregnation pyrolysis (PIP) method. A liquid precursor composed of non-oxygen polyhafniumnitrocarbosilane was successfully synthesized from tetrakis(diethylamino)hafnium and self-made liquid polycarbosilane using a one-pot process. The precursor's composition, structure and properties were analysed using Fourier-transform infrared (FTIR) spectroscopy, nuclear magnetic resonance and viscosity testing, revealing the presence of Si-N-Hf structures. The synthesised precursor remained liquid at 25 degrees C with a tunable viscosity range of 38.8-136.1 mPas. The precursor's inorganic transformation was examined using thermogravimetry-FTIR-mass spectrometry, elemental analysis, and scanning electron microscopy (SEM), achieving a ceramic yield of 71.62 wt% under argon at 1000 degrees C. The resulting inorganic product was amorphous with low oxygen content (<= 2.84 wt%), no detectable HfO2 phase, and an adjustable Hf content of 16.28-42.90 wt%. The high-temperature evolution of the inorganic product from 1600 to 2400 degrees C in argon was studied using X-ray diffraction, Raman spectroscopy, SEM, and transmission electron microscopy. This product exhibited a thermal weight loss of 10.60-19.67 wt%. The product contained HfCxN1- x and SiC phases with grain sizes of 17.4-114.9 nm and an adjustable HfCxN1- x phase content of 21.6-69.4 wt%. This novel precursor, characterised by its non-oxygen composition, adjustable room-temperature viscosity, high ceramic yield, and substantial HfC content, demonstrates strong potential as an ideal raw material for the PIP method in preparing ultra-high-temperature ceramic composites, exhibiting a favourable outlook for aerospace applications.

Si–C–B–N fibers with suitable surface coating are benefit their application on reinforcing ceramic matrix composites with excellent mechanical properties. In this study, we reported an environmentally friendly and highly efficient technique to prepare in situ BN coating on Si–C–B–N fibers by annealing at above 1800°C in N 2 atmosphere. The BN coating showed a double‐layer structure: a rough layer coating with amounts of BN nanoparticles that formed via vapor–solid mechanism, and a BN(C) coating with thickness to above 380 nm that formed under the driving and chemical bonding by N 2 atmosphere. The Si–C–B–N fibers with in situ double‐layer coating could present strength retention up to 81.3% (1.60 GPa) after annealing at 1900°C in argon atmosphere, showing excellent thermal stability and significant potential for reinforcing high‐performance ceramic matrix composites.

Metallopolymers containing refractory metal in the main chain are highly desired but rarely reported as oxygen-free single-source precursors for preparing ultrahigh-temperature ceramics. Herein, a novel single-source precursor of poly-hafnium-borocarbosilane (PHBCNS) with an Hf-N-B backbone molecular structure was synthesized through the dehalogenation reaction between HfCl4 and B(NHCH3)(3). The polymeric Hf-N-B chain with large bond angle and conjugated molecular structure endowed the PHBCNS precursor to be relatively stable, dissolvable, and meltable. Subsequent precursor pyrolysis up to 1800 degrees C resulted in an HfC/HfB2 nanocomposite comprising an Hf content of 87.54 wt%, with the average grain size of HfC and HfB2 to be 34.9 and 74.1 nm, respectively. The ceramization mechanism and phase evolution from the PHBCNS precursor to HfC/HfB2 composite ceramics were thoroughly investigated. This work not only highlights a novel Hf-N-B backbone polymer with high metal content and good shaping properties but also provides fresh insights of designing single-source precursors for ultrahigh-temperature composite ceramics.

Two kinds of near-stoichiometric SiC fibers were prepared through the pyrolysis of oxygen-free cured polycarbosilane fibers at 1,000 degrees C within a controlled hydrogen atmosphere, followed by sintering at 1,500 degrees C under a nitrogen atmosphere by one-step and two-step firing processes. The compositions, structures, and properties of the obtained SiC fibers were characterized using elemental analysis, mechanical performance testing, X-ray diffraction analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. Silicon radicals were generated in the process of hydrogen firing, which readily reacted with oxygen to form SiO2 or SiCxOy phases on the fiber surface, resulting in the obtained fibers exhibited a gradient in compositional distribution across the radial direction, with higher oxygen content at their surfaces and higher carbon content at their cores. The strength of one-step-fired SiC fiber exceeded 1 GPa after treatment at 1,800 degrees C in argon and 1,500 degrees C in air for 1 h.

In this study, nanosized Hf(C,N,O) ceramics were successfully prepared from a novel precursor synthesised by combining HfCl4 with ethylenediamine and dimethylformamide. Subsequently, the carbothermal reduction of these Hf(C,N,O) ceramics into hafnium carbide was investigated. The Hf(C,N,O) ceramics comprised Hf2ON2 and HfO2 nanocrystals and amorphous carbon. Upon carbothermal reduction, conversion began at 1300 degrees C, when HfC first appeared, and continued to completion at 1500 degrees C, resulting in irregularly shaped crystallites measuring 50-150 nm. Upon increasing the dwelling time, the oxides were completely converted into carbides at 1400 degrees C. Furthermore, nitrogen was introduced into the reaction to catalyse the conversion of oxides into carbides considering the beneficial gas-solid reaction between CO and Hf2ON2. We expect that the ceramics prepared in this study will be suitable for the fabrication of high-performance composite ceramics, with properties superior to those of current materials.

The near stoichiometric SiC fiber has been reported to play significant roles in the application of aeroengine field. An in-depth understanding on the degradation mechanism of the fiber during its corrosion in air and under a simulated aeroengine environment (P-H2O:P-O2:P-Ar = 14:8:78 kPa) will shine a light on the performance evaluations of the near stoichiometric SiC fiber-based materials as well as the development of their potential applications. In this study, X-ray diffraction, scanning electron microscope, and FIB-TEM were utilized to analyze the mechanical properties and microstructure of the fiber. After oxidation in dry air and Ar-H2O-O-2 for 1 h, respectively, the fiber strength retention rate has been found to decrease with the increased oxidation temperature. The raise in oxidation temperature also led to the increase of the thickness and the crystallization rate of the oxide scale. The most different oxidation behaviors of SiC being treated under the simulated environment than in air are the lower oxidation activation energy and the higher crystallization activation energy for cristobalite. Water vapor can promote the oxidation reaction and inhibit the crystallization of cristobalite in the oxide scale. Few significant differences have been observed otherwise in the oxidation process and oxidation chromatography crystallization mechanism of fibers being treated under different conditions. The increase of oxide layer thickness and the formation of cristobalite impair the structural integrity and compactness of the oxide scale and thus lead to the deterioration of the mechanical properties of SiC fibers. Therefore, it is proposed that oxidation resistance of SiC fiber can be improved by insulating the reaction between the oxidizing agents and the SiC fiber or by increasing the crystallization temperature of cristobalite in the oxidation process and reducing the crystallization rate.

Continuous SiC fibres have received widespread attention in the field of aero-engines due to their excellent mechanical properties and oxidation resistance. Free carbon is one of the main factors affecting the oxidation behaviour of SiC fibres, but its role and influence are still unclear. The oxidation behaviour of two types of SiC fibres with similar oxygen contents and crystallite sizes but different carbon-silicon ratios was investigated in this study. Free carbon did not significantly influence strength retention, but positively affected elastic modulus retention. The oxidised carbon-rich SiC fibres exhibited a thinner, smoother, and denser oxide scale. The oxidation rate of the carbon-rich fibres was lower than that of near-stoichiometric fibres after oxidising at 1200 degrees C. The oxidation activation energies of both were 170.44 +/- 15.80 and 234.45 +/- 24.94 kJ/mol, respec-tively. Free carbon can inhibit oxidation scale crystals at the beginning of oxidation while promoting crystal-lisation scale as oxidation intensifies. The oxidation scale crystals of carbon-rich fibres are mainly controlled by the interface, whereas those of near-stoichiometric fibres are mainly controlled by diffusion. A stress expansion zone was formed at the interface of the oxidised SiC fibres. Free carbon can enhance the internal stress of the stress expansion zone, resulting in lower residual compressive stress on the surface of carbon-rich SiC fibres. These results are significant for improving the oxidation resistance of SiC fibres by retaining free carbon.

聚合物先驱体转化法是制备超高温陶瓷的重要方法之一,它立足于元素有机聚合物的分子结构设计与可控构筑,实现对转化所得陶瓷微观结构与宏观性能的调控.具有单一分子结构与良好线性度的难熔金属聚合物,在转化制备超高温陶瓷尤其是连续超高温陶瓷纤维方面具有显著优势.系统总结了超高温陶瓷性能对难熔金属聚合物组元与分子结构的设计要求,综述了国内外难熔金属聚合物转化制备超高温陶瓷的最新研究进展与趋势,尤其是较为全面地总结了难熔金属聚合物的分子链构筑与调控方法,以期促进超高温陶瓷研发,推动超高温陶瓷在航空航天、核能等领域的应用.