Laboratoire des Composites Thermo Structuraux

A simple method was used to assemble single-walled carbon nanotubes into indefinitely long ribbons and fibers. The processing consists of dispersing the nanotubes in surfactant solutions, recondensing the nanotubes in the flow of a polymer solution to form a nanotube mesh, and then collating this mesh to a nanotube fiber. Flow-induced alignment may lead to a preferential orientation of the nanotubes in the mesh that has the form of a ribbon. Unlike classical carbon fibers, the nanotube fibers can be strongly bent without breaking. Their obtained elastic modulus is 10 times higher than the modulus of high-quality bucky paper.

C/SiC and SiC/SiC composites are tough ceramics when the fiber–matrix bonding is properly optimized, usually through a thin layer of an interfacial material referred to as the interphase. These composites can be fabricated by a variety of techniques that are briefly described and compared. The design of the interphase, matrix, and coating at the nanometer scale, in order to promote microcrack deflection and to enhance the oxidation resistance is discussed. Selected properties of the composites are presented and discussed. Examples of application in engines, heat shields, braking systems, and high‐temperature nuclear reactors are shown to illustrate the potential of these materials and the key points that still require research and development.

The oxidation of unidirectional SiC/C/SiC model composites has been investigated through thermogravimetric analysis, optical/electron microscopy, and electrical measurements. The influence of temperature and carbon interphase thickness on the oxidation of the composites is discussed. The oxidation involves three phenomena: (i) reaction of oxygen with the carbon interphase resulting in pores around the fibers, (ii) diffusion of oxygen and carbon oxides along the pores, and (iii) reaction of oxygen with the pore walls leading to the growth of silica layers on both the fibers and matrix. In composites with a thin carbon interphase (e.g., 0.1 μm) treated at T > 1000°C the pores are rapidly scaled by silica. Under such conditions, the oxidation damages are limited to the vicinity of the external surface and the materials exhibit a self‐healing character. Conversely, long exposures (300 h) at 900°C give rise to the formation of microcracks in the matrix related to mechanical stresses arising from the in situ SiC/SiO 2 conversion, fly, the self‐healing character is not observed in composites with a thick interphase (e.g., 1 μm) since carbon is totally consumed before silica can seal the pores.

A BN interphase has been deposited, by isothermal/isobaric chemical vapor infiltration (ICVI) from BF 3 ─NH 3 , within a preform made from ex‐polycarbosilane (ex‐PCS) fibers, at about 1000°C. In a second step, the BN‐treated preform was densified with SiC deposited from CH 3 SiCl 3 ─H 2 at about the same temperature. From a thermodynamic standpoint, ex‐PCS fibers could be regarded as unreactive vs the BF 3 ─NH 3 gas phase assuming they are coated with a thin layer of carbon or/and silica. The as‐deposited interphase consists of turbostratic BN (N/B < 1) containing oxygen. The SiC infiltration acts as an annealing treatment: (i) the BN interphase becomes almost stoichiometric and free of oxygen; (ii) the fibers undergo a decomposition process yielding a SiO 2 /C layer at the BN/fiber interface. The weaker link in the interfacial sequence seems to be the BN/SiO 2 interface. Deflection of microcracks arising from the failure of the matrix takes place at (or nearby) that particular interface.

A model, based on a simple axisymmettical fiber/interphase/ matrix assembly, is derived to depict the oxidation behavior of ID‐SiC/C/SiC composites within the temperature range 900–1300°C and for 10 < P O2 < 100 kPa. It takes into account (i) the changes versus time of the geometry of the annular pore resulting from the consumption by oxidation of the carbon interphase, (ii) the may transfers by diffusion along the pore of the reactant and products, and (iii) the chemical reactions with oxygen of both the pore walls (yielding silica) and the pore bottom (consisting of carbon). The model gives the gaseous species concentration and silica thickness profiles along the pore, the length of carbon consumed by oxidation, and the relative weight change. The model depicts in a satisfactory manner the features of the TGA curves recorded on actual composites and it is in excellent agreement with the measurements of the carbon interphase lengths consumed by oxidation. It shows that the oxidation resistance of ID‐SiC/C/SiC composites is better at high temperatures ( T 1100°C) and for thin carbon interphases ( e 0.1 μm). Under such conditions, the materials exhibit a self‐healing behavior.

The oxidation behavior of a 2D woven C/SiC composite partly protected with a SiC seal coating and heat‐treated (stabilized) at 1600°C in inert gas has been investigated through an experimental approach based on thermogravimetric analyses and optical/electron microscopy. Results of the tests, performed under flowing oxygen, have shown that the oxidation behavior of the composite material in terms of oxidation kinetics and morphological evolutions is related to the presence of thermal microcracks in the seal coating as well as in the matrix. Three different temperature domains exist. At low temperatures (<800°C), the mechanisms of reaction between carbon and oxygen control the oxidation kinetics and are associated with a uniform degradation of the carbon reinforcement. At intermediate temperatures, (between 800° and 1100°C), the oxidation kinetics are controlled by the gas‐phase diffusion through a network of microcracks in the SiC coating, resulting in a nonuniform degradation of the carbon phases. At high temperatures (>1100°C), such diffusion mechanisms are limited by sealing of the microcracks by silica; therefore, the degradation of the composite remains superficial. The study of the oxidation behavior of (i) the heat‐treated composite in a lower oxygen content environment (dry air) and (ii) the as‐processed (unstabilized) composite in dry oxygen confirms the different mechanisms proposed to explain the oxidation behavior of the composite material.

The fibre-matrix (FM) interfacial zone plays a key role in the mechanical behaviour of Si-C(O)/SiC inverse composites fabricated by chemical vapour infiltration (CVI) from ex-polycarbosilane Si-C(O) fibres precoated with pyrocarbon or boron nitride. It consists not only of the C (or BN) main interphase, but also of very thin secondary interphases (i.e. carbon and silica) resulting from the decomposition of the metastable Si-C(O) fibres thought to occur during the fabrication of the fibres and/or the composites. The FM interfacial zone may play two complementary roles: (i) it provides low-energy microcrack propagation paths parallel to the fibre axis, and (ii) it may act as a compliant buffer for the relaxation of the residual thermal compression stresses on the interface. The FM bond strength is low or moderate when a quasi-continuous thin layer of anisotropic carbon is present between the fibre and the main interphase and when the fibre surface remains smooth. The mechanical behaviour in tension is non-brittle with a wide non-linear stress-strain domain when the FM bonding is low or moderate, and tends to become brittle when the bonding is too strong. Finally, the thickness (and presumably the microtexture) of the carbon main interphase plays an important role in the oxidation resistance of the material, a self-healing behaviour being observed at high temperatures with a thin interphase. Replacing carbon by BN and adding an external coating of SiC (or Si3N4) to the composite improves its oxidation resistance.

Pyrocarbon (PyC), the common interphase for SiC/SiC, is not stable under severe environmental conditions. It could be replaced by boron nitride more resistant to oxidation but poorly compatible with nuclear applications. Other materials, such as ternary carbides seem promising but their use in SiC/SiC has not been demonstrated. The most efficient way to improve the behavior of PyC interphase in severe environments is to replace part of PyC by a material displaying a better compatibility, such as SiC itself. Issues related to the design and behavior of layered interphases are reviewed with a view to demonstrate their interest in high‐temperature nuclear reactors.

Relations between fracture toughness and fiber/matrix interphases were examined on various SiC/SiC composites made by chemical vapor infiltration (CVI) and reinforced with woven fiber bundles. Strong and weak fiber/matrix bondings were obtained using multilayered interphases consisting of various combinations of carbon and SiC layers of different thickness and using fibers which had been previously treated. Fracture toughness was estimated using the J ‐ integral and using strain energy release rate computed with a model taking into account the presence of a process zone of matrix microcracks. Both approaches evidenced similar trends. It appeared that higher toughness was obtained with those composites possessing strong interphases and subject to dense matrix microcracking.

Abstract BN films deposited from a BF 3 NH 3 precursor, under chemical vapour infiltration conditions, on plane sintered α‐SiC substrates were analysed by XPS. The films are non‐stoichiometric with an N/B atomic ratio of <1. They also contain significant amounts of oxygen atoms, homogeneously distributed in the film and thought to replace partly the nitrogen atoms in the turbostratic hexagonal network. As a result, ternary BN x O y species are formed locally. Near the BN/SiC interface, the oxygen concentration increases owing to the occurrence of ternary SiN x O y species, thought to be the result of an oxinitriding reaction on the substrate surface with the gas phase containing residual oxygen, at the very beginning of the BN deposition process.

A model for the oxidation kinetics of SiC‐coated 2D C/SiC composites is developed on the basis of mechanisms derived from TGA data (between 700° and 1500°C in dry oxygen or air, P = 100 kPa). Carbon/oxygen reaction, gas phase diffusion through microcracks present in the external SiC coating, and silica growth are the main phenomena taken into account in the modeling. A morphological characterization of the microcracks network based on a compression test and SEM observations has been developed. The differential equations of the model are solved according to an iterative procedure. The mass variations of the composite during an oxidation test, as derived from the model, are in good agreement with the experimental data. The model is then used to predict the oxidation rate variations when (i) the oxygen partial pressure is decreased, and (ii) the state of damage of the external SiC coating is increased by mechanical loading.

A microcomposite test procedure for evaluating the constituent properties of CMCs produced by chemical vapor infiltration is described. The analysis and experimental results demonstrate that the interface sliding resistance t o can be obtained from the unload/reload hysteresis, after one (or more) matrix crack has been induced. The nonlinear strain can also be used to provide an independent determination of t o , as well as give values for the misfit strain and the interface debond energy. Results obtained on SiC/C/SiC and SiC/BN/SiC materials are evaluated.

The tensile behavior of CVI SiC/SiC composites with Hi‐Nicalon type‐S (Hi‐NicalonS) or Tyranno‐SA3 (SA3) fibers was investigated using minicomposite test specimens. Minicomposites contain a single tow. The mechanical behavior was correlated with microstructural features including tow failure strength and interface characteristics. The Hi‐NicalonS fiber‐reinforced minicomposites exhibited a conventional damage‐tolerant response, comparable to that observed on composites reinforced by untreated Nicalon or Hi‐Nicalon fibers and possessing weak fiber/matrix interfaces. The SA3 fiber‐reinforced minicomposites exhibited larger interfacial shear stresses and erratic behavior depending on the fiber PyC coating thickness. Differences in the mechanical behavior were related to differences in the fiber surface roughness.

The present study is focused on the application of a ceramic tubular high temperature heat exchanger with engineered cellular architectures. Thermal design and optimization to maximise the radiative heat transfer has been investigated both experimentally and computationally. Numerical models were designed involving various arrangements of cells and their different sizes (while the total heat transfer area remains constant). They were 3D-printed by Stereolithography (SLA) and subsequently sintered. Heat transfer tests were performed both with a high temperature pressure drop test and by CFD simulations on 2D and 3D models. The computational results agree with the experimental data. We found that radial heat transfer in a tube increases by 160% to 280%, if a ceramic lattice is inserted, in respect of an empty tube. Moreover, the arrangement of cells and their size significantly influences the radiative heat transfer showing (for a given array) its top performances above 773 K. Geometries with large cells outside and small cells inside in the radial direction allow radiation to penetrate better through the core of the porous body. With this engineered ceramic lattices it is possible to reduce the tube length by one third to obtain more compact heat exchangers than an empty tubular solution.

The interfacial properties of SiC/SiC composites with interphases that consist of (C‐SiC) sequences deposited on the fibers have been determined by single‐fiber push‐out tests. The matrix has been reinforced with either as‐received or treated Nicalon fibers. The measured interfacial properties are correlated with the fiber‐coatingbond strength and the number of interlayers. For the composites reinforced with as‐received (weakly bonded) fibers, interfacial characteristics are extracted from the nonlinear portion of the stress‐displacement curve by fitting Hsueh's push‐out model. The interfacial characteristics are controlled by the carbon layer adjacent to the fiber. The resistance to interface crack growth and fiber sliding increases as the number of (C‐SiC) sequences increases. For the composites reinforced with treated (strongly bonded) fibers, the push‐out curves exhibit an uncommon upward curvature, which reflects different modes of interphase cracking and a contribution of fiber roughness.

Self-assembled plasmonic metamaterials are fabricated from silver nanoparticles covered with a silica shell. These metamaterials demonstrate topological darkness or selective suppression of reflection connected to global properties of the Fresnel coefficients. The optical properties of the studied structures are in good agreement with effective medium theory. The results suggest a practical way of achieving high phase sensitivity in plasmonic metamaterials.

The creep behavior of Hi‐Nicalon, Hi‐Nicalon S, and Tyranno SA3 fibers is investigated at temperatures up to 1700°C. Tensile tests were carried out on a high‐capability fiber testing apparatus in which the fiber is heated uniformly under vacuum. Analysis of initial microstructure and composition of fibers was performed using various techniques. All the fibers experienced a steady‐state creep. Primary creep was found to be more or less significant depending on fiber microstructure. Steady‐state creep was shown to result from grain‐boundary sliding. Activation energy and stress exponents were determined. Creep mechanisms are discussed on the basis of activation energy and stress exponent data. Finally, tertiary creep was observed at very high temperatures. Tertiary creep was related to volatilization of SiC. Results are discussed with respect to fiber microstructure.

The interfacial characteristics of SiC/C/SiC composites with different fiber‐coating bond strengths have been investigated using single‐fiber push‐out tests. Previous studies have shown that weak or strong bonds can be obtained by using as‐received or treated fibers, respectively, and that the stress‐strain behavior is improved with the treated fibers. This effect results from multiple branching of the cracks within the interphase. The model used to extract interfacial characteristics from nanoindentation and microindentation tests does not consider the presence of an interphase. However, the results highlight the significant effect of the interphase on the interfacial parameters, as well as the effect of roughness along the sliding surfaces. For the composite with treated fibers, the uncommon upward curvature of the push‐out curves is related to different modes of crack propagation in the interphase. Different techniques are required to analyze the interfacial properties, such as nanoindentation and microindentation with push‐out and push‐back tests.

SiC/SiC minicomposites that comprise different pyrocarbon/silicon carbide ((PyC/SiC) n ) multilayered interphases and a tow of SiC fibers (Hi‐Nicalon) have been prepared via pressure‐pulsed chemical vapor infiltration. Pyrocarbon and SiC were deposited from propane and a CH 3 SiCl 3 /H 2 mixture, respectively. The microstructure of the interphases has been investigated using transmission electron microscopy. The mechanical tensile behavior of the minicomposites at room temperature exhibits the classical features of tough composites, regardless of the characteristics of the (PyC/SiC) sequences. The interfacial shear stress has been determined from the width of hysteresis loops upon unloading/reloading and from the crack‐spacing distance at saturation. All the experimental data indicate that the strength of the fiber/interphase interfaces is rather weak (∼50 MPa).

Soil moisture is a key parameter in different environmental applications, suchas hydrology and natural risk assessment. In this paper, surface soil moisture mappingwas carried out over a basin in France using satellite synthetic aperture radar (SAR)images acquired in 2006 and 2007 by C-band (5.3 GHz) sensors. The comparisonbetween soil moisture estimated from SAR data and in situ measurements shows goodagreement, with a mapping accuracy better than 3%. This result shows that themonitoring of soil moisture from SAR images is possible in operational phase. Moreover,moistures simulated by the operational Météo-France ISBA soil-vegetation-atmospheretransfer model in the SIM-Safran-ISBA-Modcou chain were compared to radar moistureestimates to validate its pertinence. The difference between ISBA simulations and radarestimates fluctuates between 0.4 and 10% (RMSE). The comparison between ISBA andgravimetric measurements of the 12 March 2007 shows a RMSE of about 6%. Generally,these results are very encouraging. Results show also that the soil moisture estimatedfrom SAR images is not correlated with the textural units defined in the European Soil Geographical Database (SGDBE) at 1:1000000 scale. However, dependence was observed between texture maps and ISBA moisture. This dependence is induced by the use of the texture map as an input parameter in the ISBA model. Even if this parameter is very important for soil moisture estimations, radar results shown that the textural map scale at 1:1000000 is not appropriate to differentiate moistures zones.