Cette thèse s’inscrit dans une volonté d’adapter un procédé hydrothermal au traitement de la surface de fibres céramiques utilisées lors de la fabrication des composites à matrice céramique (CMCs). Le procédé conventionnel développé par la société Safran Ceramics se réalise en plusieurs étapes dont la principale consiste à dissoudre les phases oxydées de la surface des fibres Nicalon dans des bains d’acides. En conséquence, leurs propriétés chimiques de surface sont homogènes et un film de carbone microporeux est généré à la surface des fibres afin d’améliorer sa compatibilité chimique avec l’interphase de pyrocarbone qui y est déposée. Nous avons proposé de substituer ce procédé par un traitement par voie hydrothermale. En effet, l’eau possédant des propriétés physicochimiques ajustables en fonction des paramètres pression et température, il a été possible de modifier les propriétés de surface des fibres Nicalon d’une manière identique être productible au procédé conventionnel. L’efficacité et la compétitivité de ce traitement ont pu être démontrées par l’obtention de fibres avec des propriétés de surface optimales en une seule étape. Par la suite, l’étude du mécanisme réactionnel a révélé une attaque sélective des atomes de Si de la fibre selon des réactions d’hydrolyse. Puis, l’étude thermodynamique réalisée a mis en avant un régime à dominante cinétique. Finalement, les propriétés mécaniques des composites fabriqués à partir de tissus de fibres traités selon ce nouveau procédé ont été conformes aux objectifs. Ceci nous a donc permis de qualifier le traitement des fibres Nicalon par voie hydrothermale. / This thesis project was carried out in order to develop a hydrothermal processfor the surface treatment of ceramic fibres which are integrated into the fabrication of ceramicmatrix composites (CMCs). A conventional process was developed by Safran Ceramics tomodify the surface chemistry of the Nicalon fibres following 3 steps. The main step consistsin dissolving the oxidised phases at the fibre surface by the use of strong acids. As aconsequence, the chemical homogeneity of the surface is enhanced and a microporouscarbon film is generated helping its compatibilization with the pyrocarbon interphase that isdeposited in between the fibres and the matrix. It was suggested to substitute thisconventional process by a hydrothermal treatment. Indeed, as water displays tunablephysico-chemical properties regarding the temperature and pressure conditions, it waspossible to recover fibres demonstrating reproducible and similar characteristics. Theefficiency and competitivity of the hydrothermal treatment have been assessed throughoptimised surface properties obtained after one single step. Next, the mechanisminvestigation revealed a selective attack of the Si atoms contained in the fibre via hydrolysisreactions. Then, the thermodynamic study pointed out the fact that the process wasdominated by a kinetic regim. Finally, the mechanical caracterisation of the CMCs made ofhydrothermal treated fibres showed results which met all the requirements. These finalobservations allowed us to complete the qualification of the hydrothermal process to treat thesurface of Nicalon fibres.

The role of an interfacial barrier at the fiber-matrix interface in the heat conduction behavior of an uniaxial silicon carbide fiber-reinforced reaction-bonded silicon nitride and the effect of fiber orientation on the heat conduction characteristics of carbon fiber-reinforced borosilicate glass was investigated. In the study of the effect of an interfacial thermal barrier, a composite with fibers having a carbon-rich coating of about 3 J.l m was chosen as the reference material. The fiber-matrix interface was then modified by preferential oxidation of the carbon coating on the fibers, using fibers with no carbon coating and using hotisostatic-pressing (HIP) after nitridation. The formation of an interfacial gap at the interface due to thermal expansion mismatch between the fiber and the matrix in reference and HIP'd composites, and removal of carbon coating for oxidized composites, resulted in the dependence of thermal diffusivity/conductivity on the surrounding . atmosphere. This effect was attributed to gaseous heat transfer at the interface. However, no atmospheric effects were observed for composites with fibers without the carbon coating due to very strong bonding between the fiber and the matrix. HIP'ing increased the thermal diffusivity/conductivity of the composites due to densification of the matrix, crystallization of the fibers and increased physical contact at the interface. Removal of the interfacial carbon layer by preferential oxidation lowered the interfacial conductance considerably, due to decrease in the direct thermal contact between the fibers and the matrix. Interfacial contact conductance determined from measurements made in vacuum for reference and HIP'd composites increased rapidly with increasing temperature in accordance with interfacial gap closure. These observations indicate that the heat conduction behavior of all the composites investigated was strongly affected by the existence of an interfacial thermal barrier, for heat transfer transverse to the fiber direction. / Ph. D.

This diploma thesis will discuss possibilities of using inorganic fibers to improve the mechanical properties of the resulting physical body bricks. Assessment of the effect of fiber length, fiber thickness is in the order of micrometers. In this work we were used fibers which are commercially produced for the purpose of reinforcement, but also waste fibers, which mainly serves as an insulator.