Search

The synthesis of Ti3SiC2/SiC composites was studied by testing 3TiC+2Si or 3TiC+2.2Si + xAl (x = 0 or 0.2) as sintering mixtures. In order to reveal the mechanism of reactive sintering of Ti3SiC2/SiC composites, the mixtures were sintered either by pressureless sintering or by sintering under pressure between 1200 °C and 1500 °C during 15min. The results showed that the nucleation of SiC grains occurs first at the surface of the TiC grains, limiting therefore the progression of the conversion at 1200 °C and 1300 °C. Silicon defects are the main limitation of the synthesis of Ti3SiC2/SiC composites because of volatilization and molten silicon losses. The addition of aluminum promotes the conversion by reacting with oxygen and resulting in the formation of Al2O3 at the grain boundaries. The optimum experimental parameters for the synthesis of Ti3SiC2/SiC composites were determined to be a TiC/Si/Al molar ratio of 3/2.2/0.2 maintained at 1500 °C for 15min.

In multicellular eukaryotic organisms, the initiation of DNA replication occurs asynchronously throughout S-phase according to a regulated replication timing program. Here, using Xenopus egg extracts, we showed that Yap (Yes-associated protein 1), a downstream effector of the Hippo signalling pathway, is required for the control of DNA replication dynamics. We found that Yap is recruited to chromatin at the start of DNA replication and identified Rif1, a major regulator of the DNA replication timing program, as a novel Yap binding protein. Furthermore, we show that either Yap or Rif1 depletion accelerates DNA replication dynamics by increasing the number of activated replication origins. In Xenopus embryos, using a Trim-Away approach during cleavage stages devoid of transcription, we found that either Yap or Rif1 depletion triggers an acceleration of cell divisions, suggesting a shorter S-phase by alterations of the replication program. Finally, our data show that Rif1 knockdown leads to defects in the partitioning of early versus late replication foci in retinal stem cells, as we previously showed for Yap. Altogether, our findings unveil a non-transcriptional role for Yap in regulating replication dynamics. We propose that Yap and Rif1 function as brakes to control the DNA replication program in early embryos and post-embryonic stem cells.

In multicellular eukaryotic organisms, the initiation of DNA replication occurs asynchronously throughout S-phase according to a regulated replication timing program. Here, using

International audience; Refractory materials hold great promise to develop functional multilayer coating for extreme environments and temperature applications but require high temperature and complex synthesis to overcome their strong atomic bonding and form a multilayer structure. Here, a spontaneous reaction producing sophisticated multilayer refractory carbide coatings on carbon fiber (CF) is reported. This approach utilizes a relatively low-temperature (950 °C) molten-salt process for forming refractory carbides. The reaction of titanium (Ti), chromium (Cr), and CF yields a complex, high-quality multilayer carbide coating composed of 1) Cr carbide (Cr 3 C 2 ), 2) Ti carbide, and 3) Cr 3 C 2 layers. The layered sequence arises from a difference in metal dissolutions, reactions, and diffusion rates in the salt media. The multilayer-coated CFs act as a permeable oxidation barrier with no crystalline degradation of the CFs after extreme temperature (1,200 °C) and environment (oxyacetylene flame) exposure. The synthesis of high-quality multilayer refractory coating in a fast, efficient, easy, and clean manner may answer the need for industrial applications that develop cheap and reliable extreme environment protection barriers.

International audience; To characterize the oxidation/corrosion behaviour of boron nitride (BN) at high temperature and to determine kinetics, thermo-gravimetric analyses were performed on BN flat coatings under dry (80% N 2 , 20% O 2) and wet conditions (10% H 2 O, 70% N 2 , 20% O 2), at temperatures ranging from 700 to 900 °C. The surface morphology was analysed by scanning electron microscopy. The aim of the study was to understand the complex behaviour of BN at high temperature under wet air. A model was then proposed to explain the recorded mass variations, by taking into account a competition between absorption and volatilization phenomena. Finally, an estimation of the amount of adsorbed species was extracted from these data.

International audience; In Part A of this study, infiltrations experiments of porous SiC samples by hexadecane with pore-size distributions comprising small and large pores were realized. Two successive stages were identified during the filling of these samples corresponding to the infiltration of the two types of pores. The experimental data were successfully treated with a new analytical function. In Part B, it was found that this function can also be applied to the analysis of the mass gain during molten silicon infiltration at 1500°C. Prior to silicon infiltration, it was found that the operating temperature induces a shift of the pore size distributions towards larger values. A dissolution-recrystallisation mechanism can also occur during the infiltration of silicon. During the first stage, liquid silicon fills rapidly larger pores than hexadecane. The kinetics are significantly larger with liquid silicon. Consequently, the durations for the complete filling are very short with molten silicon.

International audience; The lack of robust interphases between carbon and most metals prevent the exploration of the full scope potential of carbon-based metal matrix composites. Here, we demonstrated a scalable and straightforward way to produce strong interphase between copper (Cu) and carbon fibers (CFs) by designing a tailored titanium oxide-carbide coating (TiOy-TiCx) on CFs in a molten salt process. The oxide-carbide composition in the graded layer strongly depends on the coating temperature (800–950 °C). A coating with a high TiOy content obtained at a low coating temperature (800 °C) contributes to better molten-Cu wetting and strong adhesion energy between CFs and Cu during a subsequent exposure at 1200 °C. The Cu wetting angle for the TiOy-TiCx-CF sample obtained at 800 °C was ∼80° ± 5° with a Cu surface coverage of ∼50% versus ∼115° and ∼10% for the TiCx-CF sample made at 950 °C. The kinetic analysis of the coating process step by step suggests a growth rate limited by the mass-transfer through the coated layer. This method provides a novel approach to improve the thermal conductivity of Cu/C composite for thermal management applications.

International audience; The characterization of the porosity of ceramic materials is of prime importance for numerous applications. The work is being presented in two parts: A and B. In this part (Part A), several porous SiC samples exhibiting different sizes and porosity spectra were characterized by capillary infiltration of hexadecane at room temperature. The model materials contain a bimodal pore population generated by cracks and porogens. The monitoring of the weight increase in time for each sample evidence the occurrence of two successive filling stages with specific kinetics, except for the reference samples which contain only one kind of pore. The first kinetic is found clearly larger than the second. A new analytical function was used to identify the transition times and the kinetics of each stage. The various values of the kinetics revealed the occurrence of different infiltration mechanisms. The same model correlates silicon infiltration observations as well (Part B).

International audience; A SiC/SiC-Si composite can be synthesized by molten silicon infiltration in a SiC preform impregnated with a slurry of SiC powder. The kinetics of molten silicon infiltration in such a porous material are unknown, and the experimental measurement is complex. The calculation of the rise kinetics for a liquid in a porous material is possible by applying the Washburn modified equation with an accurate description of the pores. The monitoring of the mass and front height variations during hexadecane infiltration in a submicronic SiC packed powder and in a preform filled with a SiC powder was used to characterize the porosity. In the case of a fibrous preform, the residual porosity is composed of inter-grain pores and cracks formed during the drying of the slurry. The results of this study suggest that the liquid progresses first in the inter-grain porosity of the powder. The rise kinetics of molten silicon into a preform were then evaluated by calculation based on the pore description obtained from hexadecane capillary infiltration experiments.

International audience; This work is the study of the reactivity during the capillary infiltration of liquid silicon or Si-Ti molten alloys in SiC or SiC+TiC compacts to form TiSi2/SiC composites. The main aim was to identify the thermodynamic and kinetic limitations of this process. Preliminary thermodynamic analyses of the equilibria in the Ti-Si-C system were performed to select the compositions of the liquid and the operating temperatures. Three cases were chosen: 1) infiltration of molten TiSi2 in pure SiC compacts at 1550°C, 2) reactive infiltration of pure liquid silicon in SiC+TiC compacts at 1450°C, and 3) reactive infiltration of the molten eutectic Ti0.16Si0.84 alloy in SiC+TiC compacts at 1380°C. The compacts were prepared from mixtures of micronic SiC (α or β polytypes) and TiC powders. The compositions of the powder mixtures were calculated to fill totally the porosity of the compacts of about 50% or with an excess of TiC. The heat treatments of the pellets at 1380, 1450 and 1550°C were performed with a holding duration of 1 h in high vacuum. Experimental results evidenced that the interactions between the liquid and the powders are complex. The obtained materials differ more or less from the expected composites that are generally not dense and contain variable quantities of free silicon. It is found that these experimental results can be explained by advanced thermodynamic calculations. This work proves that the activity gradients play a determining role during the infiltration process by initiating the dissolution and the diffusion of atoms in the liquids.

International audience; Liquid silicon Infiltration (LSI) is a fast and economical process to manufacture SiC-based ceramics. For a better understanding of reactive melt infiltration of liquid silicon, the wetting and infiltration of porous graphite by molten silicon were investigated at 1450, 1500 and 1550°C for duration comprised between 10 s and 1 h. Infiltrations tests were performed in an argon atmosphere with an inductively heated furnace operating with heating and cooling rates of 300°C.min-1. The formation and growth of SiC grains were investigated at the outer surface and within graphitic carbon substrates with 11% porosity and a narrow pore size distribution centered at 2 µm. The length of the infiltrated zone and the SiC crystals growth were determined from scanning electron microscopy. Rapid spreading and infiltration of molten silicon are observed from the first 60 seconds. The growth rate of the interfacial SiC layer obeys a fourth-power law with an activation energy of 26030 kJ.mol-1. Pore filling by SiC is limited by volume diffusion with an activation energy equal to 32040 kJ.mol-1 .

International audience; This paper relates an unconventional technique based on the supercritical fluid technology for manufacturing dense ceramic matrix composites, which is known as the Supercritical Fluid Chemical Deposition process (SFCD). By using a conventional precursor in a solvent above its critical coordinates, it is possible to faster the deposition rate of a SiC-based matrix on fibrous preforms in comparison to the Chemical Vapor Deposition technique for which a low pressure (1–50 kPa) is recommended for a similar range of temperatures 800–1000 °C (mixture N2: tetramethylsilane 99.2:0.8 vol.%). The microstructure of the coating or matrix is characterized by optical microscopy, Spectroscopy Electron Microscopy and Electron Dispersive X-ray spectroscopy. The influence of the main experimental parameters (temperature, pressure, residence time) is studied on the chemical composition of the ceramic deposits. The opportunity to tune the C:SiC molar ratio opens the door towards multilayered interfaces exhibiting multifunctional properties.

International audience; SiCpowder/Simatrix composites represent a new class of microstructurally toughened materials. The interactions between molten silicon and submicronic SiC powder have been considered since it could originate some limitations on the final properties of the material. Experiments putting in interaction a SiC powder and molten Si were performed while heating up to final values ranging between 1450 and 1600°C for duration up to 8 hours. The volume ratio of SiC and silicon was equal to one and SiC particles were freely dispersed within the liquid. X-ray diffraction analyses demonstrated that the apparent crystallites size increase of SiC powder followed a ripening law corresponding to a limitation either by volume diffusion or by dissolution into the liquid. Depending on the relevant mechanism, the activation energy of the crystallites' growth has been found equal to 35750 kJ.mol-1 or 44157 kJ.mol-1. An agglomeration-coarsening process of SiC particles was also identified which promoted a quick formation of larger particles.

International audience; Infiltration of molten metal into ceramic performs without external pressure is a promising fabrication method for metal-matrix composites. Thus, this research is focused on the kinetic of capillary infiltration in the processing of Si/SiC composites. The calculation of the infiltration kinetics for a liquid in a porous material is possible by applying the modified-Washburn equation which involves the effective diameter of the pores. This equation implies a K value depending on the wetting angle, the density, the viscosity and the surface tension of the liquid at a given temperature. In this study, the main aim was to measure the K Si value of molten silicon infiltration into porous SiC at 1500°C. On this way, consolidated β-SiC powder compacts were prepared for infiltration experiments. The effects of the SiC particles presintering on the pore size were examined as they can affect molten silicon infiltration. So, the monitoring of the mass increase during hexadecane rise was used to determine the effective diameter of the pores. It was evidenced that an increase of 15.5% of the initial effective diameter occurs. Consequently, a corresponding correction has to be applied to the initial effective diameter of the silicon-infiltrated compacts. Infiltrations of pure molten silicon into SiC compacts were successfully carried out at 1500°C under high vacuum (5.10-3 mbar) and the average value of K Si was found equal to (5.8±0.4)×10-6 cm 5 .s.g-2. The effect of molten silicon rise on the porosity was also considered.

International audience; Metal matrix composites are currently being investigated for thermal management applications. In the case of a copper/carbon (Cu/C) composite system, a particular issue is the lack of affinity between the Cu matrix and the C reinforcements. Titanium-alloyed Cu (Cu-Ti) powders were introduced in a Cu/C powder mixture and sintered under load at a temperature at which the Cu-Ti powders became liquid, while the rest of the Cu and C remained solid. Fully dense materials were obtained (porosity of less than 5%). The creation of regular and homogeneous interphases was confirmed. All Ti reacted with the carbon, hence purifying the Cu matrix. Thermal conductivities were enhanced as compared with the Cu/C composites without interphase. The chemical analyses are in agreement with thermodynamic simulations carried out to predict the phase transformation during the sintering process.

This paper describes a new method to prepare a matrix for Ceramic Matrix Composites (CMC) at low temperatures from a solid/gas chemical reaction. Contrary to previous works, a TiSi 2 powder is fully nitrided at 1100 °C leading to the formation of Si 3 N 4 and TiN phases while avoiding the presence of free silicon. This new result can be obtained by the addition of a chemical element promoting a full chemical conversion. In the present case, thermochemical computations led to select nickel (Ni) as this chemical element.

The evolution of TiC reinforcement during the high-temperature consolidation step of a particulate-reinforced Ti matrix composite has been studied. A four-step scenario has been highlighted starting with the dissolution of the smallest particles to reach C saturation of the Ti matrix, followed by a change in the TiC stoichiometry from the initial TiC0.96 composition to the equilibrium composition (TiC0.57). This change in composition induces an increase in both the total mass fraction of reinforcement and the particle diameter. The diameter increase promotes contact between individual particles in the most reinforced domains and initiates an aggregation phenomenon that is responsible for the observed high growth rate of particles for heat treatment times shorter than 1 h. Finally Ostwald ripening is responsible for the growth of particles for longer heat treatment times.

Ternary iron silicide aluminide γ-Al3FeSi crystals were grown from two Al—Fe—Si melts quenched in cold water. The crystal structures were determined from single-crystal X-ray data: trigonal symmetry, space group R-3 (n°148), unit cell parameters a = 10.2223(2) Å, c = 19.6791(4) Å (V = 1781 Å3) for the Si-poorer crystal and a = 10.1987(2) Å, c = 19.5320(3) Å (V = 1759 Å3) for the Si-richer one. The structure of γ-Al3FeSi may be described in terms of Al-cubes connected together by Al—Al pairs. The structure contains also Al cuboctaedra with one Fe—Al mixed atom in the center. The average chemical formula obtained from the refinements is Al3FeSi. This phase shows a partial disorder on the aluminium network because of the substitution of aluminum atoms by silicon. This substitution mechanism is at the origin of the large homogeneity range of this phase.

From literature data presently available, the decomposition temperature and the nature of the decomposition reaction of the ternary compound {alpha}-AlFeSi (also designated as {alpha}{sub H} or {tau}{sub 5}) are not clearly identified. Moreover, some uncertainties remain concerning its crystal structure. The crystallographic structure and thermochemical behaviour of the ternary compound {alpha}-AlFeSi were meticulously studied. The crystal structure of {alpha}-AlFeSi was examined at room temperature from X-ray single crystal intensity data. It presents hexagonal symmetry, space group P6{sub 3}/mmc with unit cell parameters (293 K) a=12.345(2) A and c=26.210(3) A (V=3459 A{sup 3}). The average chemical formula obtained from refinement is Al{sub 7.1}Fe{sub 2}Si. From isothermal reaction-diffusion experiments and Differential Thermal Analysis, the title compound decomposes peritectically upon heating into {theta}-Fe{sub 4}Al{sub 13}(Si), {gamma}-Al{sub 3}FeSi and a ternary Al-rich liquid. Under atmospheric pressure, the temperature of this reversible transformation has been determined to be 772{+-}12 {sup o}C. -- Graphical abstract: Partial representation of the crystal structure of the {alpha}-Al{sub 7.1}Fe{sub 2}Si compound. Display Omitted Highlights: The main findings of our work are: {yields} a detailed X-rays crystal structure determination of the ternary compound {alpha}-AlFeSi. {yields} The precision of the silicon atoms positions in the crystal structure. {yields} A precised determination ofmore » the decomposition temperature of this compound.« less

In this study, we examined the effect induced by the addition of a TiC interlayer on the stability of the Mo/SiC system at high temperature. Indeed, Mo/SiC couple is unstable at high temperature with formation of Mo2C and Mo5Si3Cx phases. In order to limit the degradation of Mo mechanical properties, a TiC film was inserted between Mo and SiC. Samples used in this work were prepared on metallic wires substrates, SiC and TiC being deposited by CVD. The protection given by TiC layer was considered in the 1473–1673 K temperature range and for TiC thicknesses up to about 60 μm. From our results, TiC is not effective enough to mitigate C and Si atoms diffusion. Nevertheless, a notable reduction of the reaction extent is obtained at any temperatures. The so-observed effect depends on the TiC thickness and the temperature. In actual fact, TiC efficiency increases when temperature and/or TiC layer thickness increases without reaching a complete protection.

Owing to the development of high temperature applications using SiC, this study examines the effect of TiN coatings against Si and C atoms diffusion in presence of two refractory metals (W or Mo). Indeed, SiC reacts rapidly with these metals at elevated temperatures generating unacceptable reaction zones and a mechanical embrittlement. Samples used during this work were prepared on wire and planar metallic substrates, SiC and TiN being deposited by CVD. The effect given by TiN interlayers with thicknesses between 10 and 20 μm was considered at 1773 K. From our results, TiN can be considered as an effective diffusion barrier in presence of W, whereas it ensures no real protection in the case of Mo-containing samples.

Solid-state reactions between SiC films and pure Mo or Re metal substrates on thermal annealing between 1373 and 1873 K for various durations have been investigated. The behaviour of Mo–47.5Re/SiC couple was also examined to establish the effect of the mix on stability. SiC coatings were deposited on metallic wires by hot filament rapid chemical vapour deposition from a gas mixture of tetramethylsilane (TMS) and hydrogen at 1373 K under normal pressure. The interface zones were characterized using optical microscopy, X-ray diffraction, and electron probe microanalysis. All analyses reveal that SiC reacts with substrates. The formation of a rhenium silicide was established on pure Re whereas a carbide and a silicon carbide containing molybdenum were formed on pure Mo. The diffusion kinetics were established from the thicknesses of reaction zones as a function of annealing durations. The outstanding fact of this work was the higher degree of reactivity observed on Mo–47.5Re alloy in comparison with pure Mo or Re as if there is a synergic effect. The apparent activation energy determined on pure Mo, pure Re and Mo–47.5Re substrates are equal to 64, 99 and 74 kcal mol −1 , respectively.

Solid-state reactions between SiC films and W–xRe (x = 0, 5 and 25 at%) substrates on thermal annealing between 1673 K and 1873 K for various durations have been investigated. SiC coatings were deposited on metallic wires by hot filament chemical vapour deposition (HFCVD) from a gas mixture of tetramethylsilane (TMS) and hydrogen at 1373 K under normal pressure. The interface zones were characterized using scanning electron and optical microscopies, X-ray diffraction and electron microprobe microanalysis. All analyses reveal that SiC reacts with substrates. Various metal silicides and carbides were formed in layered reaction and the presence of these phases was confirmed by electron probe microanalysis. The effects of rhenium on the reactivity were established by the determination of growth kinetics deducted from the thicknesses of reaction zones as a function of annealing time. It has been found that an increase in the diffusion kinetics and activation energy with the quantity of rhenium in the tungsten wire.

The binary rare-earth metal silicides RESi1.7(RE = Dy, Ho) were prepared by tin flux synthesis between 800–1000 °C. Single crystals were obtained after electrochemical dissolving of molten ingots. Their crystal structures were determined from single-crystal X-ray data: orthorhombic symmetry, space groupImm2; unit cell parametersa= 19.144(1),b= 8.2562(3),c= 6.6571(3) Å anda= 19.063(1),b= 8.2120(2),c= 6.6313(2) Å for DySi1.7and HoSi1.7, respectively. They represent a new ordered structure type, which is a derivative of the defect AlB2type. The main structural feature is the occurrence of a vacancy ordering in the two-dimensional planar network of silicon atoms, leading to rows of edge-shared twelve-membered rings separated by distorted hexagonal rings. Structural comparison with the parent hexagonal silicide Yb3Si5(Th3Pd5type), the structure of which has been re-determined on single crystal, is also given.

International audience; Little information is available about the evolution of SiC in presence of molten silicon. In this context, two kinds of experiments relative to the interaction between SiC substrates and molten Si were performed between 1450 and 1600°C with: 1) dense α-SiC bars and 2) dense α-SiC pellets coated with a β-SiC layer deposited by CVD. The results obtained indicate that an important crystal growth occurs in any cases. Large and facetted SiC crystals have grown on the surface of the SiC substrates and on the whole surface of molten silicon. These crystals are formed by dissolution-growth mechanism of SiC substrates with kinetics depending on the temperature. Low thermal gradients within the samples and between the inner parts and/or the surface of the samples could noticeably promote the evolution of the system by generating a carbon flux. The interaction between molten silicon and SiC is of importance because it could have some effects on the physical and chemical properties of the materials. It is well known that large grains are deleterious for mechanical properties.

Ponge as Reader of Michaux : Deep Grounds for Controversy. Ponge, classicist, purifier of the baroque, rather despised Michaux, «orientalist» — the baroque as excess. But are Pong's manifestos ("La Table") not excessive in view of the anthropological nature of Michaux' explorations ("Grandes Épreuves de l'esprit")!, Roger Jérôme. Ponge, lecteur de Michaux : un différend sans merci. In: Littérature, n°115, 1999. Henri Michaux. pp. 70-86.

International audience; The manufacturing of silicon carbide reinforced ceramic matrix composites by a hybrid process is explored. Fibre preforms are infiltrated with TiSi 2 powders using the slurry method. Using TiSi 2 active filler leads to reduce the porosity by the subsequent formation of nitride phases after treatment under N 2 atmosphere at low temperatures (≤ 1100°C). Taking into account the influence of the specific surface area of the powder on the nitridation rate, it is shown that it is possible to produce nitrides TiN and Si 3 N 4 at 1100°C with an interesting volume expansion inside the composite. To complete the densification of the composite, a polymer impregnation and pyrolysis (PIP) process are performed with a liquid polymeric precursor. Characterizations of the composites show that mechanical properties are improved with the presence of the TiN and Si 3 N 4 phases, and the number of PIP cycles.

Biological cells are able to sense the stiffness, geometry and topography of their environment and sensitively respond to it. For this purpose, they actively apply contractile forces to the extracellular space, which can be determined by traction force microscopy. Thereby cells are cultured on elastically deformable substrates and cellular traction patterns are quanti- tatively reconstructed from measured substrate deformations, by solving the inverse elastic problem. In this thesis we investigate the influence of environmental topography to cellular force generation and the distribution of intracellular tension. For this purpose, we reconstruct traction forces on wavy elastic substrates, using a novel technique based on finite element methods. In order to relate forces to single cell-matrix contacts and different structures of the cytoskeleton, we then introduce another novel variant of traction force microscopy, which introduces cell contraction modeling into the process of cellular traction reconstruction. This approach is robust against experimental noise and does not need regularisation. We apply this method to experimental data to demonstrate that different types of actin fibers in the cell statistically show different contractilities. We complete our investigation by simulation studies considering cell colonies and single cells as thermoelastically contracting continuum coupled to an elastic substrate. In particular we examined the effect of geometry on cellular behavior in collective cell migration and tissue invasion during tumor metastasis.

International audience; The aim the present work is to determine the reaction kinetics between a TiSi 2 powder with gaseous nitrogen. Isothermal nitridation of TiSi 2 powders with fine (1.4µm) and medium (4.5µm) particle size has been studied in pure nitrogen atmosphere ranging from 1000 to 1200°C for duration up to 50 hours. The isothermal nitridation kinetics of TiSi 2 powders were investigated by thermogravimetry. The nitridation rate strongly depends on the particle size and temperature. The smaller particle size exhibits the higher nitridation rate due to its larger surface area. The conversion process is complex with nucleation and growth of TiN at the surface of the grain and Si 3 N 4 inside the grain promoted by Kirkendall effect with an influence of the volume increase.; Le but de ce travail est de déterminer la cinétique de réaction d'une poudre de TiSi2 avec de l'azote gazeux. La nitruration isotherme de poudres de TiSi2 d'une granulométries fine (1.4μm) et moyenne (4.5μm) a été étudiée dans une atmosphère d'azote pur entre 1000 et 1200°C pendant une durée allant jusqu'à 50 heures. La cinétique isotherme de nitruration de poudres de TiSi2 ont été étudiés par thermogravimétrie. Le taux de nitruration dépend fortement de la taille des particules et de la température. Les particules les plus petites présentent le taux de nitruration le plus grand en raison de leur plus grande surface. Le processus de conversion est complexe et met en oeuvre la nucléation et la croissance de TiN à la surface des grains et de Si3N4 à l'intérieur du grain, ce comportement est favorisé l'effet Kirkendall avec une influence sur le gain de volume.

International audience; In Reactive Chemical Vapour Deposition (RCVD), the absence of one element of the deposited carbide in the initial gas phase involves the consumption/conversion of the solid substrate. In this way, the growth of a continuous carbide layer on the substrate requires solid-phase diffusion of the reagent. In this work, a parametric study of the RCVD of titanium carbide from pyrocarbon (PyC) and an H2-TiCl4 mixture has been carried out. Conversion ratio, PyC consumption and carbide layer growth kinetics have been determined at 1000°C. The influence of the H2/TiCl 4 dilution ratio has been also investigated. The apparent inter-diffusion coefficient of the carbon through the TiC deposited layer and the direct apparent reaction rate were determined from a comparison between simulations based on a Deal-Grove-type model and the experimental results. The study has been completed with FTIR spectrometry analyses of the gases.

International audience; Ti +15%TiCp composites were prepared by three different powder metallurgy techniques: free sintering, HIPing and direct powder extrusion. The microstructures and textures were characterized by EBSD while the Ti/TiC reaction, which takes place during heat treatment, was characterized using X-ray diffraction and Rietveld analysis. The mechanical properties were obtained by the mean of tensile tests. TiC particles effectively reinforce the strength of the Ti matrix by at least 40% indicating the Ti/TiC reaction did not deteriorate the mechanical properties. The correlation between the microstructural features and the mechanical properties is discussed and the processes are compared. Direct powder extrusion was shown to be the most suitable process to obtain ductile composites (epsilon = 4.6%).