Examination of the interaction between liquid silicon and bulk silicon carbide

International audienceLittle 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

Capillary infiltration of hexadecane in packed SiC powder and in SiC/SiC preforms: Pore description and calculation of molten Si infiltration

International audienceA 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

Protection against oxidation, by CVD or SPS coatings of hafnium carbide and silicon carbide, on carbon/carbon composites

protection of carbon/carbon composites against oxidation at high temperatures. However HfC and most of metallic carbides present a non stoechiometric composition with carbon vacancies. As a consequence, the oxidation resistance is poor at low temperatures (500-1000°C). In order to overcome this main drawback the HfC can be associated with silicon carbide (SiC) presenting a better oxidation resistance at lower temperatures. Two coating routes have been studied; the first one is the Chemical Vapour Deposition which enables to obtain very thin coatings and the second one is the Spark Plasma Sintering technique which permits to get new microstructures of coatings. On first hand, this study describes the CVD conditions for the deposition of HfC from the metallic hafnium pellets to get hafnium chlorides followed by the reduction of the chlorides by H2 and the deposition of HfC with the methane as carbon precursor. This enables to get an alternated multilayer microstructure made of a first layer of SiC on top of which the first layer of HfC is deposited and so on to a ten alternated layer deposit [1]. Please click Additional Files below to see the full abstract

Microstructural evolution of SiC powder in molten silicon

International audienceSiCpowder/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 35750 kJ.mol-1 or 44157 kJ.mol-1. An agglomeration-coarsening process of SiC particles was also identified which promoted a quick formation of larger particles

Une nouvelle approche dans la compréhension de l'interaction SiC/Ti

La morphologie de la zone de réaction du système SiC/Ti a été analysée par microscopie électronique à balayage. La composition des différents constituants a été déterminée par spectroscopie d'électrons Auger et par microsonde électronique. Les analyses montrent que la zone de réaction est constituée des trois composés Ti5Si3Cx, TiCx et Ti3Si. Le diagramme de phases du système Ti - Si - C a été complété à 900°C. Le suivi de l'évolution morphologique de la zone de réaction interfaciale en fonction du temps de recuit de diffusion à 900°C, a permis de déterminer la loi de croissance des particules de TiCx et de préciser les mécanismes en jeu dans l'interaction SiC/Ti

Low temperature matrix synthesis by activated nitridation of a TiSi2 powder

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SiC/Si composites elaboration by capillary infiltration of molten silicon

International audienceInfiltration 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