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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]

In this work, an efficient processing route was developed to fabricate C f /(Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C-SiC composites via slurry infiltration lamination (SIL) combined with precursor infiltration and pyrolysis (PIP). The (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C powder was initially synthesized at 1900 °C. Then the 2D C f /(Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C-SiC composites were obtained via SIL and PIP at temperatures no higher than 1100 °C. The as-fabricated composites show a density of 2.52 g/cm3 and an open porosity of 15.2 vol%, with a bending strength of 255 ± 15 MPa. The 2D C f /(Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C-SiC composites also present excellent ablation resistance, with the linear and mass recession rates of 0.33 µm/s and 0.60 mg/s after ablation at 2000 °C/300 s. This work provides a route for the efficient fabrication of C f /(Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C-SiC composites, and also inspires new strategies to develop high-entropy ceramic matrix composites with high performance. [ABSTRACT FROM AUTHOR]

Pitch-based short carbon fibres reinforced C sf /ZrB 2 -SiC composites were fabricated by direct ink writing of short carbon fibres, followed by slurry impregnation and reactive melt infiltration. Ablation behaviour of the C sf /ZrB 2 -SiC composite was studied by air plasma test. It is indicated that the skeleton of the oriented short carbon fibres provides heat diffusion channels. Consequently, temperatures at the ablation surface are as low as ∼1730 oC and ∼2000 oC respectively at 4 MW/m2 and 5 MW/m2. The composite presents outstanding ablation-resistant performance with a linear recession rate of ∼ − 0.04 µm/s and mass recession rate of ∼ − 3.40 mg/s at 4 MW/m2, ∼ − 0.17 µm/s and ∼ 3.58 mg/s at 5 MW/m2. It is revealed that the fibres area and matrix area of the composite present different ablation mechanisms. The fibres area is ablated severely, while the matrix area presents excellent ablation-resistance with continuous ZrO 2 -SiO 2 protective layer. [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 work, ablation behavior and mechanisms of 3D-C f /Ta 0.8 Hf 0.2 C-SiC composite were studied via air plasma test at temperatures up to 2500 °C. At temperatures below 2000 °C, a continuous oxides layer composed of o -Ta 2 O 5(H) – t -Ta 2 O 5 – Hf 6 Ta 2 O 17(H) – SiO 2 is formed on the ablation surface, which turns to o -Ta 2 O 5(H) – SiO 2 above 2200 °C. During ablation, o -Ta 2 O 5(H) precipitates from the glassy SiO 2 and grows up following Oswald ripening. Although the volatilization of SiO 2 aggravates with ablation temperature increase and ablation time extension, o -Ta 2 O 5(H) – SiO 2 melt can still serve as effective self-healing similar to SiO 2 glass. Accordingly, the multiphase oxides layer formed on the ablation surface provides a stable protective effect for the internal composite under all the tested ablation conditions. As a consequence, the 3D-C f /Ta 0.8 Hf 0.2 C-SiC composite presents outstanding ablation-resistant performance even at 2500 °C for 300 s, with a linear recession rate of ∼ 5.7 µm/s and a mass recession rate of 2.91 mg/s. [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]

• C f /(Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C-SiC high-entropy ceramic matrix composites were reported for the first time. • The pyrolysis and the solid-solution mechanisms of (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C precursor were systematically studied. • The as-fabricated C f /(Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C-SiC composites present excellent mechanical and anti-ablation properties. In this work, C f /(Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C-SiC high-entropy ceramic matrix composites were reported for the first time. Based on the systematic study of the pyrolysis and solid-solution mechanisms of (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C precursor by Fourier transform infrared spectroscopy, TG-MS and XRD, C f /(Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C-SiC with uniform phase and element distribution were successfully fabricated by precursor infiltration and pyrolysis. The as-fabricated composites have a density and open porosity of 2.40 g/cm3 and 13.32 vol% respectively, with outstanding bending strength (322 MPa) and fracture toughness (8.24 MPa m1/2). The C f /(Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C-SiC composites also present excellent ablation resistant property at a heat flux density of 5 MW/m2, with linear and mass recession rates of 2.89 μm/s and 2.60 mg/s respectively. The excellent combinations of mechanical and ablation resistant properties make the C f /(Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C-SiC composites a new generation of reliable ultra-high temperature materials. [ABSTRACT FROM AUTHOR]

In this work, 3D-C f /HfC-SiC-based composites were fabricated and optimized via reactive melt infiltration (RMI) of Si into porous C f /HfC-C preforms prepared by a sol-gel processing. The physical and chemical processes involved during the fabrication were identified and analyzed in details. It is revealed that fibers and interphase of the composites can be eroded during carbothermal reduction process, which can be further aggravated during RMI, with the formation of Hf-containing substance on the fibers surface. The fibers and interphase degradation is mainly induced by the reactions between HfO 2 and C/SiC interphase layers at elevated temperatures. Accordingly, a two-step carbothermal reduction treatment was proposed for the optimization of the fabrication procedure. As a result, less fiber/interphase erosion and improved mechanical properties are achieved in the composites, with the bending strength increased by ∼49 % (from 214.1 ± 15.7 MPa to 319.0 ± 26.0 MPa). [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]

In this study, short carbon fibre (C sf) reinforced ZrB 2 -SiC composites were fabricated via direct ink writing followed by precursor infiltration and pyrolysis. The ablation behaviour of the composites was investigated at heat fluxes of 4–6 MW/m2 under an air plasma flame, which revealed excellent ablation resistance with negative linear ablation rates of − 0.1 to − 0.5 µm/s. The results showed that ZrO 2 -SiO 2 oxide layer was formed on the ablation surface after ablation, which provided effective protection to the internal materials. With an increase in heat flux, ZrO 2 precipitated from the SiO 2 melt first in the form of nano-tetragonal ZrO 2 (t -ZrO 2), owing to the vaporisation of SiO 2. During cooling, the nano t -ZrO 2 particles were retained, and some of them reacted with SiO 2 to form ZrSiO 4 , thus improving the thermal stability of the SiO 2 -ZrO 2 oxide layer. In contrast, t -ZrO 2 agglomerated and grew owing to capillary force and finally transformed into monoclinic ZrO 2 (m -ZrO 2). Moreover, a higher heat flux led to the size of precipitation ZrO 2 increases, meanwhile increasing the content of t -ZrO 2 and ZrSiO 4 in the oxide layer. • Short carbon fibers reinforced C sf /ZrB 2 -SiC composites were fabricated by direct ink writing, followed by precursor impregnation and pyrolysis. • During cooling, the nano t -ZrO 2 particles were retained, and some of them reacted with SiO 2 to form ZrSiO 4 , thus improving the thermal stability of the SiO 2 -ZrO 2 oxide layer. • The capillary force leads to the nano crystals gradually agglomerate and form dendritic structure. • From 4 to 6 MW/m2, the size of the dendritic structure ZrO 2 increased. [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]

C/C-ZrC and C/SiC-ZrC composites were fabricated by a joint process of slurry impregnation and chemical vapor infiltration, in which ZrC matrix was obtained by slurry impregnation process, while C or SiC matrix was introduced by chemical vapor infiltration process. The as fabricated C/C-ZrC and C/SiC-ZrC composites have densities of 1.67 g cm −3 and 1.91 g cm −3 respectively. Tensile strength is 89.4±8.4 MPa and 182.2±14.0 MPa respectively for the as prepared C/C-ZrC and C/SiC-ZrC. Ablation behavior of the C/C-ZrC and C/SiC-ZrC composites under air plasma was studied and compared in detail. Due to different oxidation resistance and heat transfer capacity of the matrix, these two ZrC based composites showed various ablation behavior. The linear erosion rate is 48 µm s −1 and 39 µm s −1 respectively for C/C-ZrC and C/SiC-ZrC composites. [ABSTRACT FROM AUTHOR]

Reaction bonded B 4 C-SiC composites were prepared by infiltrating silicon melt into porous B 4 C-SiC green preforms at 1500 °C in vacuum. The porous green preform was obtained from a mixture of polycarbosilane (PCS) and particle size graded B 4 C after pre-sintering at 1600 °C. For the first time, PCS was used to adjust the phase composition and microstructure of the reaction bonded boron carbide composites. It is indicated that the addition of PCS and its content has a significant influence on the microstructure as well as the mechanical properties of the subsequent reaction bonded B 4 C-SiC composites. For the B 4 C-SiC composite with 5 wt% PCS added, a flexural strength of 319±12 MPa, and an elastic modulus of 402±18 GPa can be achieved, which is 23% and 15% higher than those of the composite without PCS addition, respectively. While, with the higher content of PCS addition, the mechanical properties of the composites are decreased drastically due to the large amount of residual Si agglomeration in the composites. The reaction mechanisms as well as their microstructure evolution processes correlated with the mechanical properties of the reaction bonded B 4 C-SiC composites are further discussed in our work. [ABSTRACT FROM AUTHOR]

For the first time, air plasma ablation behavior of C f /(Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C–SiC high-entropy ceramic matrix composites was studied systematically under a heat flux of 5 MW/m2, which provided a quasi-real hypersonic service environment at a temperature up to 2430 °C. The C f /(Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C–SiC composites present excellent ablation-resistant performance with a linear recession rate of ∼2.89 μm/s and mass recession rate of ∼2.60 mg/s, which can be attributed to the dense and stable oxides layer formed on the sample surface. (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C is oxidized to high-entropy oxide (TiZrHfNbTa)O x at the ablation center and highly viscous SiO 2 melt with uniformly dispersed (TiZrHfNbTa)O x microspheres is formed. While at the edge of the ablation center, precipitation occurs during cooling and the oxides layer turns to a plate-like (Hf 0.5 Zr 0.5 O 2)' skeleton surrounded by (TiNbTaO 7-y)' nanocrystals and continuous SiO 2 melt. In contrast, oxidation of (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C to produce (Hf 0.5 Zr 0.5 O 2)' and (TiNbTaO 7-y)' is dominant at the ablation transition area and outer area with lower temperature. The multiphase oxides formed during ablation provide a stable and highly self-healing protective layer for the internal materials at ultra-high temperatures. [ABSTRACT FROM AUTHOR]

• Internal C f /SiBCN composites endure severe thermal damage and complex reactions during ablation. • Thermal damage of SiBCN matrix results from crystallization and degradation of amorphous SiBCN. • Ring-like SiC is formed covering the fibers, providing further protection to the carbon fibers. Ablation behavior and thermal damage mechanisms of C f /SiBCN composites were investigated in plasma ablation flame under a heat flux of 4.02 MW/m2, simulating a hypersonic service environment at ∼2200 °C. Oxide layer formed on the composites surface effectively protects internal C f /SiBCN composites from further ablation. However, the internal C f /SiBCN composites endure severe thermal damage and complex reactions, leading to decomposition of SiBCN matrix and formation of α/β-SiC, β-Si 3 N 4 grains, amorphous C layer and ring-like SiC layer. Based on detailed microstructure and phase composition analysis, thermal damage and microstructure evolution mechanisms of C f /SiBCN composites during plasma ablation are clarified. [ABSTRACT FROM AUTHOR]

Ablation behavior of 3D-C f /SiBCN composites with (PyC/SiC) 3 multi-layers as interphase was investigated in plasma ablation flame at a heat flux of 4.02 MW m−2. During ablation, amorphous SiBCN matrix is firstly oxidized and forms a borosilicate glass, in which B 2 O 3 is volatilized as the ablation going on. At the ablation center with higher temperature, SiC and Si 3 N 4 grains are precipitated from the amorphous SiBCN matrix. And then, these SiC and Si 3 N 4 grains are oxidized, forming a certain viscous silica rich protective coating on the sample surface. Moreover, it is revealed that the SiC layers in (PyC/SiC) 3 multi-layers interphase can protect the carbon fibers from ablation damage effectively. Compared with the C f / (PyC) SiBCN composites, much improved ablation resistance is achieved for the C f / (PyC/SiC)3 SiBCN composites. The linear and mass recession rates of the C f / (PyC/SiC)3 SiBCN composites are as low as 1.7 ± 0.4 μm s−1 and 0.0427 ± 0.0053 mg mm−2s−1, which are ∼43 % and ∼21 % lower than those of C f / (PyC) SiBCN. [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]

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]

Three-dimensional C f /SiC-ZrC-ZrB 2 composites were fabricated by a joint process of sol-gel and reactive melt infiltration. Ablation test of the composites was performed under air plasma. The composites showed negative recession rates at 1800–2000 °C, while positive recession rates above 2000 °C. Results revealed that dense SiO 2 -ZrO 2 -B 2 O 3 layer formed on the sample surface prevented the ablation process extending inward the samples. At higher temperature, active oxidation of SiC and severe evaporation of oxide phase significantly impaired the integrity of the oxide layer and inevitably deteriorated the resistance of the oxide layer to the air plasma. [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]

In the present study, a novel liquid polycarbosilane (LPCS) with a ceramic yield as high as 83% was applied to develop 3D needle-punched C f /SiC composites via polymer impregnation and pyrolysis process (PIP). The cross-link and ceramization processes of LPCS were studied in detail by FT-IR and TG-DSC; a compact ceramic was obtained when LPCS was firstly cured at 120 °C before pyrolysis. It was found that the LPCS-C f /SiC composites possessed a higher density (2.13 g/cm 3 ) than that of the PCS-C f /SiC composites even though the PIP cycle for densification was obviously reduced, which means a higher densification efficiency. Logically, the LPCS-C f /SiC composites exhibited superior mechanical properties. The shorter length and rougher surfaces of pulled-out fibers indicated the LPCS-C f /SiC composites to possess a stronger bonding between matrix and PyC interphase compared with the PCS-C f /SiC composites. [ABSTRACT FROM AUTHOR]

Three-dimensional C f /SiC-ZrC-ZrB 2 composites were fabricated by reactive melt infiltration of ZrSi 2 alloys into porous C f /B 4 C-C preforms prepared through a sol-gel process. The influence of pore structure of the C f /B 4 C-C preforms on microstructure and mechanical performances of the C f /SiC-ZrC-ZrB 2 composites was studied. It was found that the microstructure and mechanical performances of the composites showed a strong dependence on the pore structure of the preforms. Residual ZrSi 2 content in the composites was significantly reduced with the decreases in the porosity and pore size of the C f /B 4 C-C preforms. At the same time, fiber erosion in the composites was also gradually mitigated, which led to a significant improvement in the fracture behaviors of the composite, as failure mode of the composites changed from brittle to pseudo-plastic. When the porosity of the preforms decreased from 53.7% to 31.3%, the flexural strength of the composites increased from 53 MPa to 184 MPa. [ABSTRACT FROM AUTHOR]

• The difference of ablation behavior for ZrC- and ZrB 2 -based coatings was firstly investigated and reported. • ZrB 2 -based coating showed better ablation resistance including lower surface temperature and superior oxidation resistance. • The ablation mechanism of ZrC- and ZrB 2 -based coatings was also analyzed based on thermodynamics predictions. In the present work, five kinds of UHTCs coatings (ZrC, ZrC-SiC, ZrB 2 , ZrB 2 -SiC, ZrB 2 -ZrC-SiC) were fabricated. The difference of ablation behavior between ZrB 2 - and ZrC-based coatings was evaluated and focused on. The microstructure evolution of resulted oxide layer were characterized in detail. The results showed that the ablation resistance of ZrB 2 -based coating was better than that of ZrC-based coating under the same environment, exhibiting lower surface temperature, thinner thickness of both the total oxide layer and the SiC-depleted layer. The possible reasons for the phenomena were supposed based on microstructure characters and thermodynamic analysis. [ABSTRACT FROM AUTHOR]