[en] Microstructure of tungsten thin films deposited by RF-sputtering is studied as a function of their thicknesses. These films have been deposited on (100) oriented single crystal Si substrate and Si substrate covered by a 100-nm-thick Ti layer. The crystalline structure is studied by X-ray Diffraction (XRD) and Grazing Incidence XRD (GIXRD). The surface and the cross-section morphology are observed by high resolution Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). XRD patterns exhibit peaks corresponding to pure W phase. GIXRD analysis shows that the more the thickness increases, the more the film is oriented along the [110] direction. AFM observations show that films exhibit a particular morphology constituted of 'piles of platelets' oriented perpendicularly to the wafer surface. These 'piles of platelets' are in plane randomly oriented and are sometime observed upon all the thicknesses of the layer. This particular morphology is observed whatever the substrate is, and is explained by thin layer growth theories

[en] In this paper, the structure of nanometer tungsten thin films has been correlated to their surface morphology. Films have been deposited by RF-sputtering at a working pressure of 0.5 Pa and with a power density of 1.18 W/cm². Two phases with different morphology have been observed: W₃O with a nanograins structure is present in the first step of the tungsten growth; and, when the thickness is increased, a pure tungsten Wolfram phase (W) with a lamellar structure appears. We demonstrate that W₃O is related to a pollution of the target surface between two growth runs. We succeed to suppress this phase and to obtain pure tungsten Wolfram nanolayer, in order to realize (W/WO₃)_n multilayer

[en] Ceramic matrix composites have been manufactured at temperatures below 1100 C by a multistep process which consists in (i) a polymer impregnation of Si-C-O fibre preform and pyrolysis (PIP) in order to consolidate the preform; (ii) an impregnation with a slurry of submicron TiSi_2 powder, (iii) a heat treatment under nitrogen (N_2) in order to reduce the porosity thanks to the volume expansion associated with the nitridation of the active filler; and (iv) a final densification by PIP. Various liquid polymeric precursors were investigated to perform this final densification. The morphology, the chemical composition and the mechanical properties of the composites were characterized. Based on these characterizations, a pre-ceramic polymer precursor was chosen in order to produce a ceramic matrix composite. (authors)

[en] Ceramic materials containing oxygen and more precisely those containing barium aluminosilicate (BaAl₂Si₂O₈ - BAS) present interesting physical properties that make them useful for very high temperature applications. These materials are very sensitive to thermal shocks and must be reinforced by a fibrous structure containing also oxygen in order to improve their thermo-mechanical behaviour without altering their properties. The article describes the process to make them, it is based on a calefaction technique. A fibrous preform is immersed in a liquid precursor then the preform is heated which leads to the decomposition of the precursor and the deposition of matter inside the preform. The thermal gradient is so important between the surface of the preform and the boiling precursor that it leads to a densification of the material from the core of the preform to its periphery. The liquid precursor that has been used is a mix of 3 alkoxides: TEOS, ATSB, and BIP. The result is a ceramic with a tubular structure made of barium aluminosilicate reinforced by oxide fibers. (A.C.)

[en] Fabrication of multidirectional continuous carbon and silicon carbide fiber reinforced ceramic matrix composites (CMC) by a new short time hybrid process was studied. This process is based, first, on the deposition of fiber interphase and coating by chemical vapor infiltration, next, on the introduction of silicon nitride powders into the fibrous preform by slurry impregnation and, finally, on the densification of the composite by liquid phase spark plasma sintering (LP-SPS). The homogeneous introduction of the ceramic charges into the multidirectional fiber pre-forms was realized by slurry impregnation from highly concentrated and well-dispersed aqueous colloid suspensions. The chemical degradation of the carbon fibers during the fabrication was prevented by adapting the sintering pressure cycle. The composites manufactured are dense. Microstructural analyses were conducted to explain the mechanical properties achieved. One main important result of this study is that LP-SPS can be used in some hybrid processes to densify fiber reinforced CMC. (authors)

[en] The present work is part of the Fourth Generation Reactor Program, which describes the methodology and results for joining SiC substrates by metallic silicides with SiC powder reinforcements. The severest temperatures in service are in the range of 1000 C but short-time incursions at 1600 or 2000 C have to be anticipated. One of the key issues is the joining of SiC_f/SiC_m composites to seal the combustible cladding. We describe the results for joining SiC substrates in liquid state using TiSi_2. Joint integrity and joint strength can be improved by adding small SiC particles to the silicides powders. The assemblies are obtained in an inductive furnace. Cross-sections of the assembly, wettability tests, thermo-mechanical properties, and four-point bending tests are presented. (authors)