J. P. Viricelle et al/ Composites Science and Technology 61(2001)607-614 and a qualitative explanation of the behaviour during and the protection becomes more effective. Microscopic each isothermal treatment observations of samples oxidised at 800 C show that all external surfaces are covered by a glassy oxide layer 4.1.2.1. 600-800C range. The kinetic curves(Fig. 10) (Fig. 11). Hence, the interphase is protected(no further confirm the important change of behaviour above carbon combustion) and the substantial weight gain 600C, described previously (Fig. 9). During the iso- corresponds mainly to B4 C oxidation, limited by oxygen therm at 600C, a continuous and quasi-linear weight diffusion though the oxide layer [15] loss is observed. The linearity confirms that the main These results are confirmed by the variation of the phenomenon is the carbon interphase combustion, and specific area(Table 1). An increase of the area is linked thus that there is no protection by boron carbide oxi- to the creation of porosity and thus to carbon con- dation at this temperature. Effectively, the linearity is sumption, whereas a decrease is the result of porosity characteristic of a chemical regime where the reactive closure mainly explained by oxide formation. The var interface(carbon/oxygen) is constant and corresponds iation measured at 600C(AS=So *8)clearly points out to the area of the annular pores created by carbon con- a significant consumption of carbon. This variation sumption [4]. At a slightly higher temperature 650@C, decreases for increasing temperatures(650, 700oC)but the weight change becomes very small. It is negligible remains large (AS=S *2 at 700C). This information during the first 8 h and then a slow weight loss(com- supports that the small weight change measured by pared to the rate at 600oC)is observed. As explained thermogravimetry at these temperatures corresponds to previously, two phenomena occur: carbon combustion a balance between the loss of carbon and the formation (Am<0)and boron carbide oxidation(Am>0). The of boron oxide. At 800C, the change in area is very measured variation is the result of the balance between small and negative, which confirms that boron oxide has the two reactions. With increasing temperatures up to sealed all the porosity created by the interphase degra- 800C, kinetics of boron carbide oxidation increases dation as it can be seen in Fig. 11 In the range 600-800oC, the composite behaviour is thus mainly explained by the reactions of two con stituents: the interphase and matrix 2(B.C). The oxi- dation of matrix 1 (SiBC)is very limited and negligible 900°C for matrix 3 (SiC) and Sic fibers for experiments of 20 h 600C Appears to be a critical temperature 1000c 4.1.2.2. 800-1200C range. The maximum weight gain is obtained at 800C. For increasing temperatures(900, 1000, 1200oC), the final weight gain decreases but the 650°C shape of the curves is quite different for each tempera ture(Fig. 10) At 800C, the main phenomenon is the oxidation of BC according to a diffusion limited regime in agree- ment with the results of matrix 2 oxidation [15]. The ⊥1L 020040060080010001200 tion of atmosphere He-O2(20%CO2(5%H-H0(2.3%) Table l Initial specific area(So) of uncoated samples and variations(AS)after treatment in a wet atmosphere for 20 h Temperature(°C (m2g-1) 0.025 0.01l 0.00 +0.270 Smear=0.017 Fig. Il. Optical micrograph (x90) of a sample oxidised at 800C without surface preparation)  F        0       3 ""2 ""
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