Papers by Author: Dominique Rouby

Abstract: Non-oxide composites are interesting materials for long term applications at high temperature under oxidizing atmosphere. To improve their lifetime, self-healing [Si-B-C] matrix has been developed recently. On this new generation of composite, fatigue has been studied at high temperature (up to 1200°C) in air. For the SiCf/[Si-B-C] composite, lifetime at 600°C is higher than 1000 h for a tension/compression cyclic loading of -50/+170 MPa. Nevertheless, the mechanical behaviour during the cyclic fatigue test is different at 600°C from that at 1200°C. Contrarily to 1200°C, at 600°C no specific evolution of the classical parameters used to characterize the mechanical behaviour during fatigue allows us to estimate the approach to fracture,. This indicates that the fatigue phenomena involved at 600°C are different from those involved at 1200°C. At 600°C, lifetime is mainly controlled by slow-crack-growth in the fibres in presence of air, and at 1200°C lifetime is controlled by fibre creep and oxidation.

Abstract: Unidirectional SiC/SiC minicomposites were produced by infiltration of a Hi-Nicalon fibre tow by using pulsed chemical vapour deposition (P-CVD). The sequence of precursor gazes inlet steps determines the structure of the Pyrocarbon/TiC layered interphase deposited first on the filaments. The SiC matrix is then deposited with a given thickness. The different interphase structures will be described. The interface behaviour was characterized by indentation method. The load is applied on the fibre-end at a polished cross-section of the composite, and the displacement of that fibre-end is measured. The push-in test on thick specimens shows that the fibre debonds at a given fibre stress and then slides under a certain frictional shear stress. Both parameters increase when the TiC layers become discontinuous, like aligned TiC clusters in a Pyrocarbon matrix. The load-displacement behaviour is in very good agreement with theory when the interfacial shear stress is kept constant, non depending on relative slip displacement. The push-through test on thin composite slices gives directly access to sliding resistance because the fibre is less compressed. The load displacement curves obtained will be described and analysed. For example, sliding velocity jumps revealed a slight increase in sliding resistance with velocity. During push-back, a seating drop was recorded as the fibre returns in its initial position, illustrating a certain effect of roughness.

Abstract: Non-oxide ceramic-matrix composites (CMCs) are subjected to be used in aeronautic applications which require very long duration (up to 100 000h) of materials at high temperatures and under air. Recently a self-healing [Si-B-C] matrix has been developped to enhance strongly the lifetime of CMCs under air. The aim of this work is to study the mechanical behaviour of a SiCf/[Si-B-C] composite with a self-healing matrix under static fatigue, and to determine its lifetime. During the mechanical tests, acoustic emission is detected in order to characterize the damage of the composite in addition to the measurement of the longitudinal deformation of the composite. The analysis of acoustic emission follows a non-supervised procedure of classification. Each event of acoustic emission is described by a set of several parameters, and the total activity can be divided in four classes. The assignment of each class to a damage mechanism is required to follow the spread of damage during fatigue and to determine the mechanisms controlling the failure of the composite.