Silicon carbide (SiC) is a high-performance ceramic renowned for its excellent strength, thermal stability, and corrosion resistance, making it highly critical for advanced applications. Yet, achieving reliable joints remains challenging, especially given the need for localized heating rather than bulk heating of an entire SiC component. Laser-assisted joining has emerged as a promising alternative, offering the advantages of localized heating, rapid processing without the need for pressure, and precise energy control that significantly minimizes impact on adjacent materials. This study examines the feasibility of using two different infrared diode lasers for pressure-less, localized joining of SiC/SiC tubes to SiC/SiC end-plugs. The results are compared with those obtained using conventional furnaces. A silica-alumina-yttria-based glass is utilized as the joining material. The morphology, microstructure, and mechanical strength of the joints are analyzed, with strength evaluated through push tests designed to detach the end-plug from the tube.

While presenting similar properties, the Hi-Nicalon Type S (HNS) and Tyranno SA3 (TSA3) SiC fibers exhibit different mechanical behaviors when used as reinforcement in SiC/SiC composites. Indeed, the HNS-reinforced composites exhibit a pseudoductile mechanical behavior whereas the TSA3-based composites show low ductility. Even though the differences in their grain size and surface roughness could explain a part of this phenomenon, the chemical composition and microstructure of the fibers outermost surface play a key role. The recent availability of the new Tyranno SA4 (TSA4) SiC fiber allowed the processing of composites showing the expected pseudoductile mechanical behavior in ceramic matrix composites, even without an interphase. Therefore, this result shows that the TSA4 surface should be different from its predecessors. In order to characterize the surface, X-ray photoelectron spectroscopy (XPS), auger electron spectroscopy (AES), and transmission electron microscopy (TEM) were performed on the HNS, TSA3, and TSA4 fibers. The presence of an organized boron nitride layer of dozens of nanometers in thickness on the TSA4 fiber surface was evidenced. This layer already acts as an interphase material, guaranteeing cracks deflection, and is responsible for the pseudoductile behavior of composites made of this new fiber, reducing the interfacial shear stress at the fiber/matrix interface.

To ensure the leak tightness of SiC/SiC composites cladding, niobium and tantalum have been retained as liner/coating materials for their high melting point, ductility and weldability; however, their chemical compatibility at high temperatures towards SiC remains to be assessed. In the literature, large discrepancies in the composition of the reaction zone and the kinetics were noticed between some metallic liners and SiC. In this work, diffusion couple experiments between Nb and Ta with SiC and SiC/SiC were conducted at high temperatures (1050–1500 °C) to determine the diffusion paths and the reaction kinetics in order to estimate the lifetime of such coatings in nominal conditions. A detailed analysis of the interaction area was conducted as a function of temperature by a combination of experimental characterizations and thermodynamic calculations. No significant difference in the sandwich cladding materials was observed. The interfacial reactivity was found to be strongly higher than expected from literature data. C and Si were evidenced as the main diffusing species in the Nb/SiC and Ta/SiC systems. From the reaction layer thickness extrapolation in gas-cooled fast reactor operating conditions, niobium but especially tantalum have been approved as liner material in hybrid CMC/metal cladding materials from a chemical compatibility point of view.

Silicon carbide fiber-reinforced silicon carbide matrix (SiC/SiC) composites are candidate materials for cladding of light water reactor (LWR) fuels. Loss of fission product gas retention due to the formation of microcrack networks is considered a potential failure mechanism for SiC/SiC-cladded fuels. In this study, a variety of SiC/SiC composite tubes were irradiated with and without an LWR-relevant radial heat flux in the High Flux Isotope Reactor, followed by detailed characterization with X-ray computed tomography (XCT). This first set of XCT data for neutron-irradiated samples confirmed that the internal stresses arising from a combination of temperature gradients and irradiation-induced swelling act as the primary driver for cracking. While the observed cracking patterns varied depending on the tube architectures, the sharp edges of relatively large pores were found to be the common stress concentrator. These findings are useful to help improve the design and manufacturing of SiC/SiC fuel claddings for reduced failure probability.

In the framework of SiC/SiC composite development for nuclear applications, the influence of pyrocarbon interphase texture and thickness on the mechanical behavior both on as-processed materials and on irradiated materials is a major concern. Thus, the PyC interphase influence has to be clearly addressed to define its optimal chemical vapor infiltration processing parameters. For this purpose, specific 2.5D SiC/SiC composites reinforced with Hi-Nicalon S fibers and with two kinds of PyC texture and thickness were produced. Transmission electronic microscopy allowed PyC thickness and microstructure/texture characterizations, whereas push-out and tensile tests were employed as experimental mechanical procedures. The original result is that PyC nature directly influences the interfacial shear stress and failure mode of the weakest interface, regardless of the PyC thickness within the studied range. Adhesive failure or cohesive failure are highlighted depending on the PyC CVI deposition mechanisms. Similar post-irradiation characterizations will be required to assess the role of irradiation on the PyC microstructure/texture evolution and mechanical behavior of these materials.

Cladding thermal conductivity is an important physical property in assessing the performance of silicon carbide (SiC)-cladded fuels for nuclear reactors. However, there is a significant lack of reliable data, particularly for irradiated materials, because the geometry complicates the measurement. This study investigates the thermal diffusivity of coupons with a curvature, machined from SiC fiber–reinforced SiC matrix composite tubes, with and without neutron irradiation under light water reactor–relevant temperature and dose conditions. The tested materials included full composite and duplex SiC composite tubes. The measurements were conducted using a modern flash diffusivity apparatus. The analyzed area on the specimen during diffusivity testing was reduced for improved measurement accuracy due to sample curvature. Post-irradiation measurements showed that the effects of neutron irradiation on thermal conductivity (e.g. thermal defect resistivity) are different between SiC composite plates versus tubes. The difference was explained by higher matrix density of the tube than the plate. This study provides reliable thermal properties of prototypic SiC composite tubes useful for fuel performance modeling of SiC-based cladding.

Ceramic matrix composites reinforced by external polymerization of acrylonitrile (ex-PAN) carbon fibers with pyrocarbon (PyC) interphase are attractive materials for thermomechanical applications. Nevertheless, C/SiC composites suffer from a low damage tolerance. A 1600 degrees C thermal pretreatment of the carbon fibers led to an improvement of the mechanical properties of the composites. Even if this heat treatment was seen to modify the fiber microstructure and texture, the changes were not sufficient to explain the observed improvements. The consequences of the thermal treatment on carbon fibers were studied by high resolution transmission electron microscopy and physicochemical analyses. The fiber/matrix debonding was also apprehended by analyzing the interfacial regions of C/SiC composites. The key role of the fiber surface structure on the fiber/matrix (F/M) coupling was highlighted. The microstructural reorganization of the heat-treated fibers surfaces induces a high F/M bonding strength and leads to better damage tolerance for the C/SiC composites. (c) 2021TheAuthors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)

The development and availability of a new 3rd generation SiC fiber, the Tyranno SA4 (SA4), are promising for the processing of higher neutron and/or corrosion resistant SiC/SiC composites. Despite its promising properties, especially the higher crystallinity and thermal conductivity than the Hi-Nicalon S fiber, the previous Tyranno SA3 (SA3) reinforcement leads to low damage tolerant SiC/SiC composites, restraining its use as a reinforcement. This is the consequence of very high interfacial shear stress, whatever the pyrocarbon interphase thickness. In this work, tubular samples where produced with both reinforcements and with two different pyrocarbon interphase thicknesses for tensile mechanical characterizations to access the potential benefit of the new SA4 fibers. The tensile mechanical properties of SA4 composites are highly enhanced compared to SA3 composites. The low damage tolerance drawback of SA3 composites is solved with higher failure strain for SA4-based composites. Tensile mechanical tests also highlight an unusual influence of pyrocarbon interphase thickness on the composites tensile modulus and proportional limit stress. The thinner interphase (approximate to 60 nm) is the most interesting for repeatable mechanical properties and induces high proportional limit stress. Unloading - reloading cycles during tensile mechanical tests also highlight the benefit of this new fiber compared to Hi-Nicalon S. This work demonstrates that the substitution of SA3 by the new SA4 SiC fiber reinforcement in the processing of SiC/SiC composites is a great opportunity for the ceramic matrix composites development and especially for nuclear applications. (C) 2021 Elsevier B.V. All rights reserved.

SiC/SiC composites have aroused interest for various nuclear applications. For example, the use of SiC/SiC components in Sodium-cooled Fast Reactors in replacement of steel wrapper tubes should improve the safety. However, the chemical compatibility between the liquid sodium and SiC or SiC/SiC composites and its effect on their mechanical properties remain unknown. Corrosion tests were conducted on CVD SiC and both undamaged and pre-damaged SiC/SiC in liquid sodium at 823 K up to 20 0 0 h in two different sodium batches, a purified and an oxygen-rich. No strong corrosion of the CVD SiC and no degradation of the mechanical properties of SiC/SiC composites were observed in both conditions, even with mechanical pre-damaging prior to immersion. A strong increase in ultimate tensile strength was highlighted after exposure, only for the undamaged samples. This phenomenon is attributed to the healing by liquid sodium of the microcracks generated during the manufacturing process. (C) 2020 Published by Elsevier B.V.

SiC/SiC composites reinforced with 3rd generation SiC fibers (Hi-Nicalon S and Tyranno SA3 fibers) are promising candidates for thermomechanical applications in high technology industries. Both composites exhibited a pseudo-ductile mechanical behavior but the HNS/PyC/SiC composite reaches higher failure strains than TSA3/PyC/SiC ones. The mechanical behavior of SiC/SiC composites is linked to the way PyC is bonded to the fiber surface. Analyses have shown that these interactions and the Fiber/Matrix debonding behavior depend strongly on the nature of the carbon on the SiC fiber surface, which is different according to the SiC fiber. In order to understand the mechanism governing the chemical adhesion at the PyC/SiC fiber interface, the formation, the chemistry and the structure of the surface carbon layer were studied. Understanding the origin of this carbon has allowed elucidating the local interaction mechanisms of the studied SiC/SiC composites.

Cracks play an essential role in the degradation of the thermomechanical behavior of ceramic matrix composites. However, characterizing their complex 3D geometries within a complex microstructure is still a challenge. This paper presents a series of procedures, based on X-ray tomographic images, to evaluate the applied 3D strains, including their through-thickness gradients, and to detect and quantify the induced crack networks in ceramic matrix composites. Digital volume correlation and some dedicated image processing algorithms are employed. A novel method is proposed to estimate the opening, orientation and surface area of the detected cracks. The proposed procedures are applied to the images of a SiC/SiC composite tube that has been tested in situ under uniaxial tension with synchrotron X-ray computed tomography.

Crack initiation and propagation in three braided SiC/SiC composite tubes with different braiding angles are investigated by in situ tensile tests with synchrotron micro-computed tomography. Crack networks are precisely detected after an image subtraction procedure based on Digital Volume Correlation. FFT based simulations are performed on the full-resolution 3D images to assess elastic stress/strain fields. Quantitative measurements of the crack geometries are performed using a novel method based on grey levels. The results show that braiding angle has no obvious effect on the location of crack onsets (initiation always occurs at tow interfaces), whereas it significantly affects the paths of crack propagation. This work provides an explicit demonstration of the crack propagation scenarios with respect to the mesoscopic fibre architectures.

Damage initiation is an important issue to understand the mechanical behavior of ceramic matrix composites. In the present work, a braided SiC/SiC composite tube was studied by FFT simulation tightly linked with micro-computed tomography (µCT) observations performed during an in situ uniaxial tensile test, which provide both the real microstructure, with a good description of local microstructural geometries, and location of cracks at the onset of damage. The FFT method was proven applicable to tubular structures and efficient to complete the large-scale simulation on a full resolution µCT scan (∼6.7 billion voxels) within a short time. The edge effect due to the numerical periodic boundary conditions prescribed on the real and not rigorously periodic microstructure was quantified. The obtained stress field was compared to the cracks detected by the in situ µCT observations of the same composite tube. This one-to-one comparison showed that cracks preferentially initiated at tow interfaces, where sharp edges of macropores are mostly located and generate stress concentrations.

SiC/SiC composites reinforced with Hi-Nicalon S 3rd generation SiC fibers are promising candidates for high temperature aeronautical applications and fuel cladding in nuclear reactors. An original patented process was used to produce SiC/SiC tubular samples for dimensional and tensile mechanical characterizations. The process route is described in detail to further understanding of specific features. SiC/SiC samples show a low porosity and excellent geometrical tolerance. Highly reproducible tensile mechanical behavior is observed with good mechanical failure characteristics. Some specific features were highlighted during mechanical characterizations with loading/unloading sequences. For instance, an original mechanical behavior was observed before the onset of matrix multicracking (deviation from linearity). To our knowledge, it is the first time that this kind of behavior has been detected on SiC/SiC composites, due to the processing route leading to the microstructure of specific samples.

Silicon carbide ceramics are really appreciated in energy applications, especially when extreme environments are involved. Their reliability and thermomechanical performances beyond 1000°C are higher than that of superalloys, therefore they may significantly contribute to the reduction of Critical Raw Materials use, which is a crucial point for sustainability issues. The examples discussed in the present paper are taken from European projects, in the field of nuclear energy (IV gen fission nuclear reactors) and renewable energy production (Concentrated Solar Power) and demonstrate a common approach based on optimizing and assessing the ceramic materials, considering current international standards, but also prenormative research and accelerated ageing tests in expected working environments. In this way, together with a simultaneous process implementation for the production of components in the size and geometry required for the applications, high target TRL (5-7) can be demonstrated, creating the competencies, the methods and the results needed for subsequent industrialisation.

Matrix cracking in SiC/SiC composite tubes is characterised from in situ tensile tests performed under x-ray synchrotron tomography. Digital volume correlation is used to estimate semi-local strains and to detect and extract cracks from the 3D images. Two populations of cracks can be distinguished. Their spatial distribution within the tube thickness is studied and their evolution is quantitatively analysed, such as subvoxel opening.

SiC/SiC composite tubes are studied as materials for nuclear fuel cladding. A thorough understanding on the mechanisms of damage to this material requires both an experimental and a numerical study. In situ tensile tests were performed under X-ray tomography at the SOLEIL synchrotron. Post processing methods have been developed to analyze the microstructure, measure the deformation and characterize qualitatively and quantitatively the damage mechanisms inside the material. The tomographic images provide 3D descriptions on the microstructure, which are direct input data for the numerical simulation based on FFT. The use of real microstructures makes it possible to combine the simulation results directly with the experimental observations.

This paper presents the results of tests performed on selected cladding materials in a well-defined atmosphere that reproduces the environment expected in a gas-fast nuclear reactor. In particular, the feasibility of silicon carbide composites is investigated in helium with impurities by means of a thermogravimetric device. The temperatures of the tests are in the range of normal operation conditions (900-1000 °C) and accident conditions (up to 1500 °C) of a gas fast reactor. Post-test analyses are carried out with scanning electron microscope. This work reports the transition temperature between passive and active oxidation of silicon carbide in helium and impurities atmosphere. Kinetics constants are provided for the linear weight loss.

L’objectif de cette communication au symposium « calculs epoustouflants en mecanique des fluides, materiaux et structures » est de montrer, au travers d’un code massivement parallelise, AMITEX_FFTP, les possibilites offertes par les « methodes FFT » pour repousser les limites de la simulation numerique (taille et temps de calcul) en mecanique des materiaux heterogenes. Les simulations numeriques de grands agregats polycristallins, ou de microstructures directement obtenues par tomographie X, beneficient pleinement de ces methodes parallelisees.