Effect of hydrothermal corrosion on the fracture strength of SiC layer in tristructural-isotropic fuel particles

The hydrothermal corrosion behavior of SiC layer in tristructural-isotropic (TRISO) fuel particles and its effect on the fracture strength were investigated. The corrosion test was performed using the static autoclave at 400°C/10.3 MPa. The SiC layer exhibited a thickness loss and the corrosion rate followed a linear law. During corrosion, carbon was formed on the SiC surface due to the loss of Si. The corrosion was found preferentially occurred at the grain boundary of SiC, leading to the grain detachment and pit formation. The rate determining step of the corrosion was SiO₂ formation rather than SiO₂ dissolution in the hydrothermal environment. The fracture strength of SiC shell after corrosion was evaluated using the crush test. It showed a slight decrease with an increase in corrosion time, due to the thickness reduction in SiC layer. The results of this study demonstrated that the SiC in TRISO particles has good corrosion resistance in the hydrothermal environment.     

Carbon Interlayer Between CVD SiC and SiO2 in High‐Temperature Passive Oxidation

The oxidation behavior of high‐purity silicon carbide (SiC) prepared by chemical vapor deposition was investigated by thermogravimetry, transmission electron microscopy, and Raman spectroscopy in the temperature range 1534–1902 K in pure O₂. The carbon layer formed at the SiC/SiO₂ interface upon oxidation above 1784 K. Raman peaks corresponding to D‐ and G‐bands could be identified from the carbon layer. Bubbles were observed in the SiO₂ scale after the oxidation at 1873 K. This could be attributed to the accumulation of CO gas at the SiC/SiO₂ interface, resulting in the formation of the carbon layer and bubbles. These suggest that the oxidation rate of SiC is limited by the outward diffusion of CO in the SiO₂ scale in this temperature range.     

Oxidation resistance and microstructure evolution of ZrB2–SiC–La2O3/SiC dual-layer coating on siliconized graphite at 1800 °C under low air pressures

The oxidation behaviors of a ZrB₂–SiC–La₂O₃/SiC dual-layer coating on siliconized graphite at 1800 °C under low air pressures (50, 5 and 0.5 kPa) were investigated. The results showed that with the decrease of air pressure, the oxidation kinetics of the coated samples changed from parabolic weight gain to linear weight loss. A protective oxide scale consisted of ZrO₂ and SiO₂ with La dispersed was formed on the coating surface after oxidation in 50 kPa air. The oxide scale formed in 5 kPa air was full of bubbles. Only porous ZrO₂ layer was left on the coating surface after oxidation in 0.5 kPa air. At 1800 °C, the active oxidation of SiC occurred and gaseous SiO formed at the coating/oxide interface. The surface volatilization of SiO became severe with the decrease of air pressure, resulting in the presence of non-protective oxide scale.     

Interaction of Embedded Tri-Isotropic Fuel Particles with Melt of Alkaline Borosilicate Glass

A silicon carbide (SiC) layer is an outer-coated layer of spent tri-isotropic (TRISO) fuel particles and it is known to be a pressure vessel for retaining fission products, and preventing contamination in the primary circuit of a nuclear reactor. The goal of this article is to elucidate the chemical bonding and an interface formation of an alkaline borosilicate glass (ABG) with the coating layer of TRISO fuel particles. Particular emphasis is placed on the analysis of the intermediate chemical phase at the interface SiC/glass as a function of the material impurity and thickness of the SiC layer. The findings provide valuable information regarding the restriction parameters of immobilisation TRISO particles in glass. The interaction between the glass and SiC caused a total destruction of a thin SiC layer (10 μm), a random partial interaction to a thick SiC layer (40 μm) and formation of bubbles (CO₂, CO) to an inner pyrolitic carbon (IPYC). The Raman spectroscopy analysis revealed that the interaction of ABG with the SiC layer occurred at a point, where a low excess of carbon was co-deposited during chemical vapour deposition process. The interaction resulted in a formation of a mono-crystal SiC, dispersed in vitreous silica as a crystalline inclusion.     

Effect of La2O3 on the oxidation resistance of SiC ceramic at 1973 K: Experimental and theoretical study

The effect of La₂O₃ on the oxidation resistance of SiC ceramic at 1973 K was investigated using isothermal oxidation test and first-principles calculations. The SiC ceramic with La₂O₃ shows a better oxidation resistance compared with that without La₂O₃ due to the in situ formed La₂Si₂O₇ in SiO₂ glass layer after oxidation at 1973 K. First-principles calculations based on density functional theory were applied to analyze the solution behaviors of La atom in the surface of SiO₂ and La₂Si₂O₇. The solution energy of La atom in SiO₂ (0 1 1) is −19.05 eV, which is far less than −4.19 eV in La₂Si₂O₇ (2 0 1) with a La vacancy, thus resulting in that La atom in La₂Si₂O₇ (2 0 1) diffuses into SiO₂ (0 1 1). The SiO₂ lattice with an interstitial La atom is more stable than that with a substitutional La atom and the interstitial La breaks the nearest Si–O bond to form La–O and La–Si bonds, which is beneficial to improving the high-temperature stability of SiO₂. Experimental and theoretical results indicate that the formation of refractory La₂Si₂O₇ phase enhances the stability of SiO₂ glass layer, so as to protect SiC ceramic from further oxidation at 1973 K.     

Effect of temperature gradient on the erosion behavior of SiC coating for carbon/carbon composites in a combustion environment

SiC coating with a thickness of 50–70 µm was prepared on the surface of C/C composites by in-situ reaction method. The SiC coated C/C composites were then tested in a wind tunnel where a temperature gradient from 200 to 1600 °C could be obtained to investigate their erosion behavior. The results of wind tunnel test indicated that the service life of C/C composites was prolonged from 0.5 to 44 h after applying the SiC coating. After the wind tunnel test, three typical oxidation morphologies, including glassy SiO₂ layer, porous SiO₂ layer and clusters of honeycomb-like SiO₂ grains, were found on the SiC coated C/C composites. With the decrease of oxidation temperature, the amount of glassy SiO₂ declined and the thermal stress increased, which induced the cracking followed by the degradation of the SiC coating.     

Interfacial microstructure and mechanical properties of SiC joints achieved by reactive air brazing using Ag-V2O5 filler

SiC ceramics are successfully brazed via reactive air brazing using Ag-V₂O₅ fillers. The wettability of SiC ceramics by Ag-V₂O₅ fillers is investigated. Interfacial microstructure of SiC joints is analyzed by scanning electron microscopy and transmission electron microscopy with energy dispersive spectroscopy. Effect of the brazing filler composition on the microstructure and mechanical properties of SiC joints is studied in detail. The V₂O₅ from the brazing fillers is found to react intensively with SiC, and the SiO₂ reaction layer with the thickness of ?7 μm is formed on the SiC surface which ensures a good wetting of the brazing filler on SiC ceramics. The brazing seam is composed of Ag and VO₂ with small amount of remaining V₂O₅. The maximum shear strength (?58 MPa) is achieved when using the optimized brazing process (Ag-8V₂O₅, 1050 ℃/30 min, the loading pressure is ?20 kPa and the cooling rate is 2 ℃/min).     

Low-temperature joining of SiC ceramics with Y2O3-Al2O3 interlayer by SiO2-based liquid phase extrusion strategy

In order to reduce the joining temperature of SiC ceramics by glass-ceramic joining, some oxides were usually introduced into to Y₂O₃–Al₂O₃ for reducing the eutectic temperature. However, the joints might have poor high-temperature resistance due to the low melting point of the joining layer. In the present work, based on novel SiO₂-based liquid phase extrusion strategy, joining of SiC ceramics with Y₂O₃–Al₂O₃ interlayer was carried out by using Y₂O₃–Al₂O₃–SiO₂ as the filler through spark plasma sintering (SPS). The SiO₂-free interlayer of Y₂O₃–Al₂O₃ was used for comparison. It was found that SiC joints using Y₂O₃–Al₂O₃ could be only joined at a high temperature of 1800 °C, and the thickness of the interlayer was about 20 μm. The shear strength of the joint obtained at 1800 °C was 89.62 ± 4.67 MPa and the failure located in the SiC matrix. By contrast, reliable joining of SiC ceramics could be finished at as low as 1550 °C by extrusion of SiO₂-containing liquid phase when using Y₂O₃–Al₂O₃–SiO₂ as the interlayer, alongside the interlayer thickness of only several microns. The joint strengths after joining at 1550 °C was 84.90 ± 3.48 MPa and the failure located in matrix position. The joining mechanism was discussed by combining the detailed microstructure analysis and phase diagram.     

Wet-oxygen corrosion resistance and mechanism of bi-layer Mullite/SiC coating for Cf/SiC composites

A bi-layer oxidation-resistant coating consisting of a mullite outer coating, and a SiC inner coating on the surface of C_f/SiC composites was prepared by the chemical vapour deposition and an air spray sol-gel process, and its corrosion behavior was evaluated in a wet-oxygen coupling environment. Results show that the formation of SiO₂ glass layer and its reaction with mullite particles to form aluminosilicate glass layer, leading to an increase in the density of the mullite outer coating, so that the weight loss of bi-layer Mullite/SiC coating coated C/SiC sample was only 1.11 × 10^?3 g·cm^?2 in the first 100 h of oxidation. However, the weight loss of the coated sample reached 26.82 × 10^?3 g·cm^?2 after 200 h of oxidation due to a part of the mullite outer coating was detached. The SiO₂ glass phase reacted with water vapour to generate gaseous Si(OH)_x, which created distinct holes on the surface of the SiO₂ glass layer or inside the molten aluminosilicate glass layer. Eventually, the mullite outer coating was blistered and detached from the surface of the sample due to the combination and growth of holes.     

Degradation mechanism of near stoichiometric SiC fibers after air and Ar–H2O–O2 corrosion at 1000–1500°C

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–O₂ 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.     

Autonomous crack healing ability of SiC dispersed Yb2Si2O7 by oxidations in air and water vapor

Yb₂Si₂O₇ is a popular environmental barrier coating; however, it decomposes into Yb₂SiO₅ in high-temperature steam environments. The thermal mismatch between Yb₂Si₂O₇ and Yb₂SiO₅ leads to the cracking and failure of the disilicate coating via oxidation. Dispersing SiC nanofillers into the Yb₂Si₂O₇ matrix is suggested to maintain the Yb₂Si₂O₇ matrix and promote crack self-healing. This study is aimed at clarifying the effect of water vapor on the self-healing ability of such composites. X-ray diffraction analysis and scanning electron microscopy were used to monitor the surface composition and the crack formation, respectively, in 10 vol% SiC-dispersed Yb₂Si₂O₇ composites. Annealing at temperatures higher than 750 °C in air or in a water vapor rich atmosphere led to strength recovery and the self-healing of indentation-induced surface cracks owing to volume expansion during the oxidation of SiC. The self-healing effect was influenced by the oxidation time and temperature. Rapid diffusion of H₂O as an oxidizer into the SiO₂ layer promoted self-healing in a water vapor rich atmosphere. However, accelerated oxidation at temperatures higher than 1150 °C formed bubbles on the surface. Fabricating composites with a small amount of Yb₂SiO₅ will be a solution to these problems.     

Intergranular film thickness of self-reinforced silicon carbide ceramics

The intergranular film of self-reinforced SiC ceramics prepared by hot pressing and further annealing with SiO₂–Y₂O₃ and SiO₂–Al₂O₃ as sintering additives was observed by high-resolution transmission electron microscopy. The film thickness of SiC ceramics with SiO₂–Y₂O₃ was 1.2 nm whereas that of ceramics with SiO₂–Al₂O₃ was 0.8 nm. Based on the refined continuum model, an explanation on the variation of thickness with sintering additives is given. It seems that the behavior of intergranular glassy film of SiC ceramics is akin to that of Si₃N₄ ceramics.     

Oxidation behaviour of nano-sized SiC particulate reinforced Alon composites

Silicon carbide (SiC)-aluminium oxynitride (Alon) ceramic composites exhibited improved mechanical properties, but the high temperature oxidation behaviour was unknown. The aim of this investigation was to identify oxidation characteristics and kinetics of 8 wt% SiC-Alon composites over a temperature range between 700 °C and 1200 °C in air. The Alon matrix and SiC particles near the surface were oxidized to form Al₂O₃ and SiO₂, respectively. The starting oxidation temperature of Alon was observed to be about 1000 °C. While the addition of nano-sized SiC particles resulted in a reduced starting oxidation temperature due to the large cumulative surface area and high total surface energy, the oxidation resistance at higher temperatures of 1100 °C and 1200 °C was remarkably enhanced. The oxidation kinetics changed from a linear weight gain for pure Alon into a logarithmic weight gain for the composites due to the formation of a dense protective oxidation layer arising from the presence of SiO₂.     

Effect of SiC whiskers on the anti-oxidation properties of Yb2Si2O7/mullite/SiC tri-layer environmental barrier coating for CMCs

The mullite and ytterbium disilicate (β-Yb₂Si₂O₇) powders as starting materials for the Yb₂Si₂O₇/mullite/SiC tri-layer coating are synthesized by a sol–gel method. The effect of SiC whiskers on the anti-oxidation properties of Yb₂Si₂O₇/mullite/SiC tri-layer coating for C/SiC composites in the air environment is deeply studied. Results show that the formation temperature and complete transition temperature of mullite were 800–1000 and 1300°C, respectively. Yb₂SiO₅, α-Yb₂Si₂O₇, and β-Yb₂Si₂O₇ were gradually formed between 800 and 1000°C, and Yb₂SiO₅ and α-Yb₂Si₂O₇ were completely transformed into β-Yb₂Si₂O₇ at a temperature above 1200°C. The weight loss of Yb₂Si₂O₇/(SiC_w–mullite)/SiC tri-layer coating coated specimens was 0.15 × 10⁻³ g cm⁻² after 200 h oxidation at 1400°C, which is lower than that of Yb₂Si₂O₇/mullite/SiC tri-layer coating (2.84 × 10⁻³ g cm⁻²). The SiC whiskers in mullite middle coating can not only alleviate the coefficient of thermal expansion difference between mullite middle coating and β-Yb₂Si₂O₇ outer coating, but also improve the self-healing performance of the mullite middle coating owing to the self-healing aluminosilicate glass phase formed by the reaction between SiO₂ (oxidation of SiC whiskers) and mullite particles.     

Oxidation protection of carbon/carbon composites with SiC/indialite coating for intermediate temperatures

In order to improve the oxidation resistance of carbon/carbon composites at intermediate temperatures, a novel double-layer SiC/indialite coating was prepared by a simple and low-cost method. The internal SiC transition layer was prepared by pack cementation and the external indialite glass–ceramic coating was produced by in situ crystallization of ternary MgO–Al₂O₃–SiO₂ glass. The microstructures and morphologies of coating were determined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). Oxidation resistance of the as-coated C/C composites was evaluated in ambient air at temperature from 800 °C to 1200 °C. Nearly neglectable mass loss was measured after 100 h isothermal oxidation test, indicating that SiC/indialite coating possesses excellent oxidation protection ability. The as-coated samples have a good thermal shock resistance and no obvious damage was found in the coating even after suffered more than 11 thermal cycles between test temperature and room temperature. The oxidation protection mechanism of this coating was also discussed.     

TEM Study of the High‐Temperature Oxidation Behavior of Hot‐Pressed ZrB2–SiC Composites

The oxidation behaviors of ZrB₂‐ 30 vol% SiC composites were investigated at 1500°C in air and under reducing conditions with oxygen partial pressures of 10⁴ and 10 ^{? 8} Pa, respectively. The oxidation of ZrB₂ and SiC were analyzed using transmission electron microscopy (TEM). Due to kinetic difference of oxidation behavior, the three layers (surface silica‐rich layer, oxide layer, and unreacted layer) were observed over a wide area of specimen in air, while the two layers (oxide layer, and unreacted layer) were observed over a narrow area in specimen under reducing condition. In oxide layer, the ZrB₂ was oxidized to ZrO₂ accompanied by division into small grains and the shape was also changed from faceted to round. This layer also consisted of amorphous SiO₂ with residual SiC and found dispersed in TEM. Based on TEM analysis of ZrB₂ – SiC composites tested under air and low oxygen partial pressure, the ZrB₂ begins to oxidize preferentially and the SiC remained without any changes at the interface between oxidized layer and unreacted layer.     

Surface oxidation behavior in air and O2-H2O-Ar atmospheres of continuous freestanding SiC films derived from polycarbosilane

In this contribution, thermodynamic computational calculations firstly carried out on Ar-Si-C-O/Ar-Si-C-O-H database demonstrate that passive oxidation is main reaction of continuous freestanding SiC films in both air and 14%H₂O/8%O₂/78%Ar atmospheres. SiC films were subsequently annealed at 1300?°C, 1400?°C and 1500?°C for 1?h in air and O₂-H₂O-Ar atmospheres. Results suggest that modulus, hardness and resistivity decrease whereas crystallite size of β-SiC and α-cristobalite increase with elevated annealing temperature. In particular, hardness of wet oxidized samples is lower than that of air oxidized ones. Additionally, their oxidation kinetics models were also established and verified by annealing at 1200?°C in air and wet oxygen for different time from 1?h to 100?h. Oxidation of continuous freestanding SiC films is identified to follow parabolic oxidation kinetics, and water could effectively enhance the oxidation rates. It is revealed that SiO₂ layer can protect SiC films from further oxidation, and their thickness increases with prolonged annealing time. In this study, a dense and uniform SiO₂ layer with a thickness of 1.1–1.6?µm was produced for sacrificial and passivation layer based on suitable thermal oxidation process (annealing at 1000?°C for 5?h in O₂-H₂O-Ar environment). Interestingly, fast diffusion paths in this oxide layer could effectively accelerate oxidation process of SiC films. These obtained achievements would promote further applications of SiC films on microelectromechanical systems (MEMS) devices in harsh environments.     

High-temperature oxidation induced layering process for Si–C–B–N ceramic fibers with SiC nanograins

High-temperature oxidation resistance is important for Si–C–B–N ceramic fibers when reinforcing ceramic matrix composites with superior reliability and faulting tolerance. At present, few studies have investigated on the high-temperature oxidation behavior of Si–C–B–N fibers, limiting their further applications. In this work, we analyzed the high-temperature oxidation process of Si–C–B–N ceramic fibers with SiC nanograins (SiBCN-SiCn fibers) at 1000–1500 °C in air. SiBCN-SiCn fibers stated to be oxidized at 1000 °C, with the formation of thin oxide layer. After oxidizing at 1300 °C, obvious oxide layer that mainly consisted of amorphous SiO₂ could be detected. Further oxidizing at 1500 °C caused the thickness increment of oxide layer, which could inhibit the oxidation products (CO, N₂) to release away from the fibers. The remained CO and N₂ may react with SiC nanograins to form SiO₂ and graphite-like g-C₃N₄, causing the formation of additional transition layer. Our finding may support useful information for the applications of SiBCN-SiCn fibers under harsh environment.     

Silicon carbide fiber oxidation behavior in the presence of boron nitride

The oxidation behavior of Sylramic SiC fibers without a boron nitride surface layer was compared to Sylramic iBN SiC fibers with a boron nitride surface layer by conducting thermogravimetric analysis in dry O₂ at temperatures ranging from 800 to 1300°C for times up to 100 hours. Sylramic fibers followed the Deal and Grove oxidation kinetic model. A transient period of accelerated oxidation kinetics was observed with Sylramic iBN fibers. Raman spectroscopic analysis of oxidized fibers provided evidence for a borosilicate glass structure. The boron concentrations in the oxides, quantified by inductively coupled plasma‐optical emission spectrometry, were correlated with the weight change behavior, oxide thickness, and fiber recession of the oxidized fibers. Oxides formed from Sylramic iBN fibers were typically higher in boron concentration, which led to initial rapid oxidation rates that were 3‐10 times faster than observed for pure SiC. Slower oxidation rates followed as the oxide surface became increasingly enriched with SiO₂ due to boria volatilization, thus limiting boria effects on SiC fiber oxidation kinetics. The accelerated high‐temperature oxidation of SiC fibers due to the presence of BN are discussed in terms of the borosilicate glass structure and composition.     

Microstructure and corrosion behavior of in-situ grown Y3Si2C2 coated SiC fibers exposed to air and wet-oxygen at 1400 ℃

Y₃Si₂C₂ ternary ceramics were in-situ grown on the third-generation Chinese commercial SiC fiber (KD-SA SiC fiber) surface via molten salt method. Microstructures and oxidation/corrosion behavior of in-situ grown Y₃Si₂C₂ coated SiC fibers exposed to air and wet-oxygen at 1400 ℃ were investigated. Results indicated that the layered Y₃Si₂C₂ slices with thickness of approximately 15 nm can be successfully in-situ grown on SiC fibers. The product on the fibers surface after oxidation/corrosion at 1400 ℃ for 1 h in both ambient air and wet-oxygen are Y₂Si₂O₇ and SiO₂. Moreover, microstructural characterization indicates that the immigration and expansion of gaseous bubbles induced by oxidation product, mainly CO, result in microstructural differences of SiC fiber specimens, and finally oxidation mechanism based on the microstructural difference were proposed.