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TaxHf1–xC are promising candidates for many applications under harsh environments due to their unique properties. Up until now, low‐temperature densification is still a big challenge for these ceramics. Moreover, the effect of Ta:Hf ratio on microstructure development and mechanical behavior of the ceramics is not clear. In this work, highly dense TaxHf1−xC–SiC ceramics (x = 0.2, 0.4, 0.6, 0.8) were fabricated at 1700°C by a novel reactive hot‐pressing processing with 8 wt% Si as sintering aid, which present excellent mechanical properties. Fracture toughness of the ceramics is higher than 6.3 MPa·m1/2, with the highest toughness achieved in Ta0.6Hf0.4C–SiC (8.5 MPa·m1/2). With the increase of Ta content, hardness of the ceramics tends to increase, while the bending strength tends to decrease. The highest bending strength is achieved in Ta0.2Hf0.8C–SiC (637 MPa), and the highest hardness is achieved in Ta0.8Hf0.2C–SiC (17.6 GPa). This work lays the foundation for the composition design of TaxHf1−xC‐based ceramics and composites. [ABSTRACT FROM AUTHOR]

The oxide layer formed by ultra‐high melt point oxides (ZrO2, HfO2) and SiO2 glassy melt is the key to the application of traditional thermal structural materials in extremely high‐temperature environment. However, the negative effect of ZrO2 and HfO2 phase transitions on the stability of oxide layer and rapid volatilization of low viscosity SiO2 melt limit its application in aerospace. In this study, the ablation behavior of Cf/(CrZrHfNbTa)C‒SiC high‐entropy composite was explored systematically via an air plasma ablation test, under a heat flux of 5 MW/m2 at temperatures up to 2450°C. The composite presents an outstanding ablation resistance, with linear and mass ablation rates of 0.9 µm/s and 1.82 mg/s, respectively. This impressive ablation resistance is attributed to the highly stable oxide protective layer formed in situ on the ablation surface, which comprises a solid skeleton of (Zr, Hf)6(Nb, Ta)2O17 combined with spherical particles and SiO2 glassy melt. The irregular particles provide a solid skeleton in the oxides protective layer, which increased stability of the oxide layer. Moreover, the spherical particles have a crystal structure similar to that of Ta2O5 and are uniformly distributed in SiO2 glassy melt, which hinder the flow of SiO2 glassy melt and enhance its viscosity to a certain degree. And it reduces the volatilization of SiO2. In summary, the stable oxide layer was formed by irregular particles oxide and the SiO2 glassy melt with certain viscosity, thereby resulting in the impressive ablation resistance of the composite. This study fills a gap in ablation research on the (CrZrHfNbTa)C system. [ABSTRACT FROM AUTHOR]

Ultrahigh temperature ceramic matrix composites (UHTCMCs) are critical for the development of high Mach reusable hypersonic vehicles. Although various materials are utilized as the thermal components of hypersonic vehicles, it is still challenging to meet the ultrahigh temperature ablation‐resistant and reusability. Herein, the Y2O3 reinforced Cf/ZrB2‐SiC composites are designed, which demonstrates near‐zero damage under long‐term ablation at temperatures up to 2500 °C for ten cycles. Notably, the linear ablation rate of the composites (0.33 µm s−1) is over 24 times better than that of the conventional Cf/C‐ZrC at 2500 °C (8.0 µm s−1). Moreover, the long‐term multi‐cycle ablation mechanisms of the composites are investigated with the assistance of DFT calculations. Especially, the size effect and the content of the Zr‐based crystals in the oxide layer fundamentally affect the stability of the oxide layer and the ablation properties. The ideal component and structure of the oxide layer for multi‐cycle ablation condition are put forward, which can be obtained by controlling the Y2O3/ZrB2 mole ratio and establishing Y‐Si‐O – t‐Zr0.9Y0.1O1.95 core‐shell nano structure. This work proposes a new strategy for improving the long‐term multi‐cycle ablation resistance of UHTCMCs. [ABSTRACT FROM AUTHOR]

Temperature above 2000°C and additional pressure is generally required to achieve the full densification of TaxHf1−xC‐based ceramics. This work proposed a novel method to fabricate dense Ta0.2Hf0.8C ceramics at relatively low temperature. Using a small amount of Si as a sintering aid, Ta0.2Hf0.8C was densified at 1700°C by reactive hot‐pressing (RHP), with SiC formed in situ. Microstructure evolution mechanisms of the ceramics during RHP were investigated. The effect of silicon content on the densification and mechanical properties of the ceramics was revealed. It is indicated that the apparent porosity of the Ta0.2Hf0.8C–SiC ceramics was as low as 0.5%, whereas bending strength and fracture toughness of the ceramics were as high as ∼637 MPa and 6.7 MPa m1/2, respectively, when the silicon content was 8 wt.%. This work provides a new idea for the low‐temperature densification of TaxHf1−xC and other ultrahigh temperature ceramics with high performance. [ABSTRACT FROM AUTHOR]

In this study, a Cf/Ta4HfC5‐SiC ultra‐high‐temperature ceramic matrix composite exhibiting a homogeneous phase distribution was successfully fabricated via precursor infiltration and pyrolysis processing. Initially, the pyrolysis and solid solution mechanisms exhibited by the Ta4HfC5 precursor were investigated and characterized through TG‐MS and XRD analysis. The as‐fabricated Cf/Ta4HfC5‐SiC composite exhibited a density and open porosity of 2.84 g/cm3 and 10.62 vol%, respectively. It also exhibited outstanding mechanical properties, with a flexural strength of 339 ± 20 MPa and fracture toughness of 11.56 ± 0.77 MPa·m1/2. The Cf/Ta4HfC5‐SiC composite demonstrated strong ablation resistance under a heat flux of 5 MW/m2 at ~2400℃, with corresponding linear and mass recession rates of 5.33 μm/s and 6.18 mg/s, respectively. The combination of strong mechanical properties and ablation resistance provides a solid basis for the use of the Cf/Ta4HfC5‐SiC composite in a new generation of ultra‐high‐temperature materials. [ABSTRACT FROM AUTHOR]

In this work, Cf/SiBCN composites are fabricated by an improved precursor infiltration and pyrolysis (PIP) approach. Ablation behavior of the Cf/SiBCN composites is investigated in plasma ablation flame at a heat flux of 4.02 MW m−2, which provides a quasi‐real hypersonic service environment at a temperature up to 2200°C. After ablation, the ablated surface is covered with oxidation products in the form of oxide layer, fibrous residues, or bubbles, which effectively isolates the sample surface from the plasma flame and inhibits the scouring of high‐speed flame to the composites. As a result, the Cf/SiBCN composites present an excellent ablation‐resistant property, with linear and mass recession rates as low as 0.0030 mm s−1 and 0.0539 mg mm−2 s−1, respectively. It is also revealed that the material at ablation center undergoes crystallization and oxidation processes during ablation, while the ablation behavior at transition area and ablation fringe only contains oxidation process due to the local temperature difference. Si3N4 and SiC grains are precipitated from amorphous SiBCN matrix during the crystallization process, and the oxidation process mainly involves the oxidation of carbon fiber and SiBCN matrix, etc. [ABSTRACT FROM AUTHOR]

Cf/SiC‐ZrC composites with different amounts and distributions of ZrC were fabricated by polymer impregnation and pyrolysis. The effects of the ZrC amount and distribution on the microstructural, mechanical, and ablation properties of Cf/SiC‐ZrC composites were investigated. Cf/SiC‐ZrC composites obtained by the alternating infiltration of ZrC organic precursors and polycarbosilane groups exhibit good tensile strength (240 ± 17.7 MPa) because the ZrC and SiC matrix can mix evenly. However, Cf/SiC‐ZrC composites using only ZrC organic precursor infiltration show a low tensile strength (191 ± 16.6 MPa) because more defects can be introduced into the composites. Ablation characterization by a 30 kW plasma wind tunnel for 60 seconds showed that the Cf/SiC‐ZrC composites with the highest amount of ZrC matrix (67.8 wt.%) possessed the lowest linear erosion rate of 4 μm/s because liquid SiO2 could fill the porous ZrO2 to form a homogenous protective layer. Nevertheless, the Cf/SiC‐ZrC composites with a relatively high ZrC amount (55.3 wt.%) exhibited a poorer ablation performance compared to that of Cf/SiC‐ZrC composites with a low ZrC amount (38.7 wt.%). [ABSTRACT FROM AUTHOR]

Interphase between the fibers and matrix plays a key role on the properties of fiber reinforced composites. In this work, the effect of interphase on mechanical properties and microstructures of 3D Cf/SiBCN composites at elevated temperatures was investigated. When PyC interphase is used, flexural strength and elastic modulus of the Cf/SiBCN composites decrease seriously at 1600°C (92 ± 15 MPa, 12 ± 2 GPa), compared with the properties at room temperature (371 ± 31 MPa, 31 ± 2 GPa). While, the flexural strength and elastic modulus of Cf/SiBCN composites with PyC/SiC multilayered interphase at 1600°C are as high as 330 ± 7 MPa and 30 ± 2 GPa, respectively, which are 97% and 73% of the values at room temperature (341 ± 20 MPa, 41 ± 2 GPa). To clarify the effect mechanism of the interphase on mechanical properties of the Cf/SiBCN composites at elevated temperature, interfacial bonding strength (IFBS) and microstructures of the composites were investigated in detail. It reveals that the PyC/SiC multilayered interphase can retard the SiBCN matrix degradation at elevated temperature, leading to the high strength retention of the composites at 1600°C. [ABSTRACT FROM AUTHOR]

Abstract: 3D Cf/SiBCN composites were fabricated by an efficient polymer impregnation and pyrolysis (PIP) method using liquid poly(methylvinyl)borosilazanes as precursor. Mechanical properties and microstructure evolution of the prepared 3D Cf/SiBCN composites at elevated temperatures in the range of 1500‐1700°C were investigated. As temperature increased from room temperature (371 ± 31 MPa, 31 ± 2 GPa) to 1500°C (316 ± 29 MPa, 27 ± 3 GPa), strength and elastic modulus of the composite decreased slightly, which degraded seriously as temperature further increased to 1600°C (92 ± 15 MPa, 12 ± 2 GPa) and 1700°C (84 ± 12 MPa, 11 ± 2GPa). To clarify the conversion of failure mechanisms, interfacial shear strength (IFSS) and microstructure evolution of the 3D Cf/SiBCN composites at different temperatures were investigated in detail. It reveals that the declines of the strength and changes of the IFSS of the composites are strongly related to the defects and SiC nano‐crystals formed in the composites at elevated temperatures. [ABSTRACT FROM AUTHOR]

Abstract: Si‐B‐C ceramics were prepared through reaction sintering, and the influence of Al2O3 addition on the high‐temperature (1100‐1300°C) oxidation behavior of the material under air atmosphere was studied. The erosion behavior and mechanism are determined from the measurement of weigh changes, microstructure observations, and characterization of the generated oxides on postexposure specimens. Results show that Al2O3 is enriched in the oxidized layer, inhibiting the volatilization of B2O3 and impeding the crystallization ability of oxide (cristobalite). Narrower erosion layer and less weigh change are observed with Al2O3. Low‐frequency Raman results reveals that with the increase in Al2O3, the bending vibrations of the BO4 units and B‐O‐B stretching of the metaborate ring relative intensity are enhanced. Furthermore, high‐frequency Raman results shows that the relative proportion of high‐dimensional vibration modes Q3 and Q4 which result in a higher viscosity of melt and a greater resistance of oxygen diffusion are positively correlated with Al2O3. [ABSTRACT FROM AUTHOR]

Layer-structured interphase, existing between carbon fiber and ultrahigh-temperature ceramics ( UHTCs) matrix, is an indispensable component for carbon fiber reinforced UHTCs matrix composites (Cf/ UHTCs). For Cf/ UHTCs fabricated by reactive melt infiltration ( RMI), the interphase inevitably suffers degradation due to the interaction with the reactive melt. Here, Cf/SiC-ZrC-ZrB2 composite was fabricated by reactive infiltration of ZrSi2 melt into sol-gel prepared Cf/B4C-C preform. (PyC-SiC)2 interphase was deposited on the fiber to investigate the degradation mechanism under ZrSi2 melt. It was revealed that the degraded interphase exhibited typical features of Zr aggregation and SiC residuals. Moreover, the Zr species diffused across the interphase and formed nanosized ZrC phase inside the fiber. A hybrid mechanisms of chemical reaction and physical melting were proposed to reveal the degradation mechanism according to characterization results and heat conduction calculations. Based on the degradation mechanism, a potential solution to mitigate interphase degradation is also put forward. [ABSTRACT FROM AUTHOR]