Investigation of diffusion barrier coatings on SiC monofilaments for use in titanium-based composites

International audienceThe development of titanium-based composites has been hindered largely because of interfacial diffusion. When SiC is in contact with titanium at elevated temperatures, metal silicide (TisSi3) and carbide (TIC) are formed. The aim of our work is to reduce the fibre-matrix interaction at high temperatures by interposing compounds between the metal and SiC reinforcement. Procedures are described for the formation of thin layers on silicon carbide fibres (Sigma) by sputter deposition. Sputtering offers good control over density and stress state, thickness and composition. Past work has shown that sputtered Y203 deposited on SiC, SiC-Si or metal can reduce the degree of ceramic-metal reaction. Use of the Gibbs free energy of formation of TiO2 provides a simple basis for the assessment of this material for use in a titanium matrix. Since TiSi 2 and silicon are also present in the Ti-Si diagram, they could act as diffusion barriers. Coatings of TiSi 2, Y20 3 and ZrO~ have been produced by sputter deposition. The stoichiometry of the coatings has been revealed by Rutherford backscattering spectrometry. Coated fibres have been used to prepare titanium-based composites by hot pressing. Annealing between 900 and 1100 °C has been performed. Scanning microscopy, Auger electron spectroscopy and transmission electron microscopy have been used to investigate the inter-facial products

A methodological improvement of dynamic fracture toughness evaluations using an instrumented Charpy impact tester

L'objectif de l'étude est d'essayer une méthode de détermination de la ténacité dynamique basée sur la mesure du déplacement d'un point de l'éprouvette. Il est aussi possible d'éviter le traitement par une analyse dynamique de l'effort généralement enregistré au niveau de l'impacteur pour obtenir le chargement effectif de l'éprouvette au cours du temps lors d'un essai de type CHARPY. Un capteur de déplacement sans contact est utilisé afin de mesurer la flèche au voisinage de l'entaille de l'éprouvette au cours du choc. De la valeur de la flèche à l'amorçage de la rupture peut être déduit le facteur critique d'intensité de contraintes à l'aide d'une analyse quasistatique. L'utilisation de cette méthode pour caractériser la ténacité dynamique du PMMA donne une valeur comparable aux résultats trouvés dans la littérature.The study aims at the investigation of a method based on the displacement measurement of a specimen point for the determination of dynamic toughness. Thus, it is possible to avoid the treatment of the force generally recorded through the impactor by a dynamic analysis in order to obtain the true load undergone by the specimen during a CHARPY impact test. Deflection of the impacted specimen is measured with the help of a non contacting displacement transducer. Then, a static calibration is used to derive the fracture toughness from the deflection of the beam at fracture initiation. Characterizing the dynamic fracture toughness of PMMA with this experimental procedure gives rise to a value consistent with results reported in the literature

Powder route processing of carbon fibres reinforced titanium matrix composites

Titanium matrix composites reinforced by continuous carbon fibres were prepared, using powder metallurgy and for part, techniques derived from ceramic and polymer matrix composites processing methods. Major difficulties encountered during fibres/matrix coupling and composites consolidation, were overcome through specific studies related to preparation of fine titanium powder, impregnation of carbon fibre yarns by a slurry containing matrix powder in a viscous transfer fluid made of polymeric binder in solvent, thermal elimination of binder, as well as mechanisms of consolidation and densification of titanium matrix under pressure and moderately high temperature