Search

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]

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]

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]

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]