J.P. Viricelle et al/ Composites Science and Technology 61(2001)607-614 Summary of the oxidation behaviour of the composite in a wet atmosphere He-O2(20%)CO2 (5%H-H2O023%) Temperature (C) Carbon interphase Matrix 2 B4C Matrices 1-3 SiBC-Sic Global weight change Explanations Am<0 quasi-linear and No protection, calcinate 650-700 △n<0 △m≌0weak interphase by B,O3 Partial protection of 100-1200 Anso prolate aloctin B o, tim △m≥0 rotection of the interphase by silica and borosilicate from 900C due to borosilicate formation, but it has no in dry oxygen, whereas it was only observed at 650@ in action concerning the interphase protection in this case. the presence of water vapour Table 2 summarises the results observed in the wet At 700 and 800oC, the phenomena are the same as in atmosphere He-O2(20%)-CO2 (5%)-H2O(2.3%) wet oxygen. The mass variation is a balance between the weight loss(carbon calcination) and the weight gain 4.2. Oxidation in dry atmosphere/He-O2(20%6) (B4C oxidation), and at 800 C, the porosity is sealed by CO2(5% the oxide film(△S<0) The main difference between the two atmospheres Experiments similar to those previously described concerns the 900oC experiment. In wet oxygen, the have been performed in a dry atmosphere. In this con- weight gain was not stabilised after 20 h and the poros dition, the weight changes remain very small and the ity was not closed. In the dry atmosphere, the behaviour inal absolute variations after 20 h are lower than 0.08% at 900C is the same as at 800C: the interphase is (e0.5 mg) for all temperatures in the range 600-1200 C protected. At 900 C, the oxidation rate of matrix 2 is ( Table 3). These values have been obtained by weighing quite higher in dry than in wet conditions and B2O3 the samples before and after treatment. The corre- volatilisation is negligible without water vapour. Con- sponding kinetic curves cannot be exploited because of sequently, the quantity of borosilicate glass produced is their bad reliability. The variations of the specific area more important and explains why the composite is pro- have been determined and the comparison with the tected in these conditions values obtained in a wet atmosphere(Table 3)indicates At higher temperatures(>1000oC). the main protec the same trend except at 900C. tive effect is the oxidation of the fibers and of the sic At 600C, the weight loss remains very small but the matrix and the global behaviour is nearly the same specific area increases and clearly indicates that the dry and in wet oxygen interphase is oxidised. The difference concerning the weight change with experiments performed in wet atmosphere is explained by the oxidation rate of matrix 5. Conclusion 2 at 600oC, which is considerably higher in dry oxygen [15]. Consequently, boron oxide formation is more The oxidation behaviour in a O2/H,O mixture or in important and its protective effect is significant at 600oc dry oxygen of a 2.5D(SiC)/C/(SiBC)m composite very complicated on account of the multi-layered struc Table 3 ture of the matrix. For the studied samples cut from Variations of weight and specific area of samples oxidised 20 h in dry plates, the oxygen access via the bulk of the composite is not limited and oxygen can react directly with the car- Dry atmosphere Wet atmosphere bon interphase of the cross-sectioned yarns. Conse quently, up to 600C, the role of the external layers Temperature(C) mo(%)△s(m2g-)△s(m2g- BC(1, 2), SiC(2, 3, 4, 5)and SibC(1, 2)is negligible for the 0.03 +0.210 interphase protection. Their weak oxidation will only +0.0 result in an additional weight gain as a result of silica +0.06 and borosilicate formation. The global behaviour is +0.06 +0.270 mainly explained by the oxidation of boron carbide in the range 650-900oC and by the oxidation of Nicalon fibers and silicon carbide Sic(1)at higher temperatures ),,
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