Influence de l'interface fibre/matrice sur le comportement mécanique en température des composites C/C

The aim of this work is to understand the influence of the interfacial fibre/matrix bonding (F/M) on the mechanical behaviour of 3D carbon/carbon composites. For that purpose, three pyrocarbon interphases were deposited with different nature and thickness (100-300nm) by means of chemical vapour infiltration (I-CVI) inside carbon fibre preforms. After the deposition of the interphase, the same conventional matrix of pyrocarbon was infiltrated at industrial stage for the different materials. The so-obtained carbon/interphase/carbon composites were characterized from a structuraland thermo-mechanical point of view. The anisotropy of the different interphases measured by the optical extinction angle (Ae) method, varied from 13 to 22°. These variations were confirmed by the measurement of the orientation angle by means of electron diffraction (60°>OA>40°). From room temperature to 1000°C, the influence of the F/M interface on tensile mechanical behaviour is very low compared to those of the inter-bundle and interlaminar interfaces as well as the 3D architecture. In this range, as the test temperature becomes closer and closer to the processing one, the Young’s modulus increases. This is related to the decrease of the residual stresses and the cancelllation of the F/M debonding. On the contrary, at high temperature (between 1800 and 2000°C), the fibre becomes viscoplastic and the behaviour of the material becomes speed-dependent. In these conditions, the compliant role of the interphase is crucial to fit the fibre and matrix elongation mismatching under stress. The mechanical behaviour and performances vary depending on the interphase anisotropy: an interphase (Ae=16°) with a less ordered microstructure than that of matrix (Ae=22°), allows to fit the strong deformation of the fibre with respect to that of the matrix. The early failure of the fibre is thus delayed thanks to a visco-plastic behaviour of the interphase strongly time-dependant. When the interphase had a too poor anisotropy, then the strength of the F/M interface increases and the mechanical behaviour of the composite becomes brittle.L’objectif de cette thèse est de comprendre l’influence de la liaison fibre/matrice (F/M) sur le comportement mécanique de composites carbone/carbone 3D. Pour cela des interphases de nature et d’épaisseur (100-300nm) différentes ont été déposées par infiltration chimique en phase vapeur (CVI) au sein de préformes constituées de fibres de carbone. A la suite du dépôt de ces différentes interphases, une matrice de pyrocarbone conventionnelle a été déposée en four industriel de façon équivalente pour tous les matériaux. Les composites carbone/interphase/carbone ainsi réalisés ont été caractérisés aux plans physico-chimique et thermomécanique. L’anisotropie des interphases a été mesurée par la technique optique de l’angle d’extinction (13°<Ae<22°) et confirmée par diffraction électronique en mesurant l’angle d’ouverture (60°<OA