J. P. Viricelle et al. Composites Science and Technology 61(2001)607-614 The oxidation of the material was recorded by ther gravimetry using a thermobalance (Setaram B60) Matrix 3 SiC(5) The weight changes were continuously monitored. A Matrix 1 SiBC( 2) platinum wire and an alumina crucible were used to hold the sample in the furnace. The experiments were Matrix 3 SiC(4) performed with a total flow rate of 1. 8 1/h either in a dry atmosphere consisting of oxygen (20 vol %) carbon Matrix 1 SiBC(1) dioxide (5 vol %)and helium or a wet atmosphere obtained by the bubbling of the previous one in a water Matrix 3 Sic(3 bath regulated at a constant temperature(20C). The Matrix 2 BC(2) resulting water vapour content was about 2.3 voL% Carbon dioxide was used to control the activity as it is Matrix 3 SiC(2) produced by the oxidation reaction. Moreover, this gas is present in the combustion atmosphere in real appli Matrix 2 BC(1) cations of composite materials. Increasing-temperature Matrix 3 Sic(1) experiments with a low heating rate of 2C/min until 1200 C have been conducted in order to identify the Carbon interphase various steps during oxidation. Isothermal kinetic curves were then obtained in the range 600-1200%C for a duration of 20 h. The temperature ramp(10C/min) Fig 1. Schematic illustration of the composite architecture was conducted under dry helium until the desired tem- perature was reached. The chosen atmosphere for the experiments was then introduced and this constituted SiC (5) the starting time of the isotherm ∈SiBC(2 SC(4) ∈SiBC(1) 3. Material SiC (3) ←B4C(2 The composite architecture is of crucial importance for the oxidation behaviour The material is a 2 5D network of Nicalon NL202 fibers coated with a pyr- carbon interphase of approximately 0.1 um thick ness. The multilayered matrix is then deposited by Fig. 2. Optical micrograph(x 180)of the composite, external layers chemical vapour infiltration or deposition depending on the layer. The sequence of the deposit is sum marised in Fig. 1. It can be seen that the matrix is composed of three different constituents: a phase containing silicon, boron and carbon noted SiBC and referred to as matrix 1, a phase containing boron and sc(5)一 carbon noted B. C, called matrix 2, and a third phase containing silicon and carbon noted SiC, matrix 3 The samples used in this study( 8 mm x8 mm x 4 SiC (4 mm)were obtained by machining larger plates perpen dicularly to the 2D fiber plane. In the four lateral cut faces( 8 mm x4 mm), the fibers and the interphase are thus unprotected by the matrix which was deposited before machining, as shown in Fig. 2. The sequence of the matrix layers inside the yarn network depends on SiC (2 the interyarn porosity which is progressively filled and losed during infiltration cycles. The layers SiC(1)+ Fig 3. Scanning electron micrograph in the bulk of the composite. B,C(1) have a total thickness of about I um. The sequence SiC(2)+(BC(2)fills the intrayarn porosity osition of the boron-carbide phase, matrix Figs 3 and 4). The specific surface area of the samples 2, B4C(2)and of the SiBC phase, matrix 1, SiBC(1)and is very low, about 0.02 m/g, indicating that there is SiBC( been determined by microanalysis. The no microporosity. The total macroporosity is about oxidation behaviour of samples Ith a composition 10% close to that of matrix 2 has been studied and presented-      /       0  0       +, - / 0 0  /       :   /        /             -    /     /   >/   (C         0  0  ', 1!    . 1       /         0         /    0          ',
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