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J. P. Viricelle et al. Composites Science and Technology 61(2001)607-614 △m/m(%) T(C) Heat-treated e(m.2m01 SiC 1200 B C -0.05 800°C "- Carbon 1000 4., 200400600800 Fig 8. X-ray diffraction l500° C in helium pattem of a matrix I SiBC sample heated Fig 9. Thermal behaviour of the composite during calcination in wet atmosphere He-O2(20%)-CO2(5%H-HyO(2.3%), with a heating rate 4.1. Oxidation in wet atmosphere/He-O2(20% )-C0 (5%-H2012.3% spreading over the surface of the sample. The result is a 4.1.1. Increasing temperature strong weight gain in range(ii)600-1050oC. The varia The thermogravimetric curve obtained during the tions of the rate at 670 and 800C are not accurately calcination of a sample heated in a wet atmosphere with explained. They may correspond to a change in the a linear rate of 2 C/min until 1200oC and maintained at kinetics of B.C oxidation(670C)and to the beginning this temperature for 10 h is reported in Fig 9. It can be of the oxidation of the other constituents(SiBC, Sic)at seen that the relative weight change is small: -0.3%< 800C which modify the viscosity of the oxide scale Am/m, +0.05. However, the variations of the rate of Effectively, for individual constituents, at 670 C, matrix weight change (loss or gain and change of the slope) are 1 (SiBC) and Sic oxidation is negligible. The decrease important. Four temperature ranges can be considered of the weight gain rate at 670@C may be a consequence of the decrease of the reactive area due to the formation 20-600C, which corresponds to a weight loss of the continuous boron oxide layer which seals the 2. 600-1050oC, where a strong weight gain occurs porosity. In the third range (iii)1050-1200oC, the with various rates corresponding to three domains weight loss is a result of B2O3 consumption, either by (600-670,670-800,800-1050°C), volatilisation or reaction with water vapour, and of the 050-1200oC, with a weight loss which goes on change of the viscosity of the borosilicate [15]. Conse 4. the isotherm at 1200 C, where a stabilisation quently, the oxide layer is no more protective and the luring the first hour of the isothermal treatment. weight loss corresponds to B2O3 removal, but also to the weight change is observed for nearly 3 h before the carbon interphase oxidation. The partial stabilisa a final and continuous loss tion occurring during the beginning of the isotherm at 1200C is explained by a more important contribution The interpretation of this complex behaviour may be of the oxidation of matrices I and 3 which forms a explained by considering the behaviour of each con- borosilicate and silica stituent which was studied previously [18-20]. As the composite is porous and the atmosphere in the thermo- 4.1.2. Isothermal treatments balance saturated with water vapour at the beginning of Isothermal experiments have been performed at var- the experiment, the weight loss occurring in the range ious temperatures in the range 600-1200oC. The relative 20-400C is due to its desorption and evaporation. weight changes measured by thermogravimetry during Then, between 400 and 600oC, the continuous weight isothermal dwells of 20 h are reported in Fig. 10. The loss is the result of the combustion of the carbon inter- specific area of each sample and its changes after the phase, which is directly accessible by oxygen through isothermal treatment have been measured by adsorption the lateral faces of the sample. Above 600 C, the sudden of krypton at 77 K and calculated by the B ET change of behaviour (loss/ gain)is attributed to the method. The results are reported in Table 1. Because of contribution of the oxidation of boron carbide(matrix the presence of many constituents and of the complex 2)[15]. It occurs with a weight gain and the formation microstructure, it is very difficult to model the phenom- of a liquid boron oxide protects the interphase by ena quantitatively. We can only perform a description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