M. Leparoux et al. in boron as compared with nitrogen, that cannot the volatility of boron oxide, it is not surprising to be evidenced by our analyses find such a phase in different parts of the compo- site, especially in the tows, inside cracks perpend cular or parallel to the fibers (Figs 6 and 7) 4 Discussion Sometimes it can even link the cracks walls. The EELS analysis along a decohesion at the fiber /BN The application of a compliant interphase such as interface confirms that the Si, O and B elements are carbon or boron nitride on treated Nicalon M fibers present(Section 3. 2. 2, Analysis of the cracks has been found to improve the mechanical beha- analysis of a crack inside the bn interphase vior of the SiC/SiC composites at room tempera- also an incrcasc in oxygen contcnt and the ture. The drawbacks of carbon are obvious in presence with Si-o bonds formation oxidizing atmosphere, and bn should exhibil a From EELS study, it appears that the bulk better oxidation resistance, especially at moderate boron nitride interphase can be modified at 600C temperature such as 600C. However the ICVI-BN in air. However the decrease of the o, C and N deposits always present a turbostratic structure. contents should be examined by further analyses, Even if the temperature of 600 C is low for the particularly at different distances from the far end onset of hexagonal-BN oxidation, poorer lattice of the cracks, i.e. different conditions of ambient organization might be affected at this temperature. air access. This modification could be attributed to The availability of boron oxide could then allow the relatively poor organization of the structure the formation of a glassy phase with various and the initial incorporation of C and o impurities amounts of boron. It is indeed well-known that Because its composition was not changed by the boron significantly decreases the viscosity of glas- ICVI processing of the silicon carbide matrix from ses. The melting temperature of B2O3 is about 480C23 and the eutectic temperature of the pseudo-binary SiO2-B2O3 is, for example, about 450 C24 When considering the flowing ability and link 1 um Fig. 6. Vitreous phase linking the trcated Nicalon m fiber and 10 um the bn interphase (a) deflection at the fiber/BN the BN/matrix and into the Fig. 5. Cracks deflection in a 2D-SiC/BN/SiC composite Fig. 7. Glassy phase in a crack propagating between two tested in tension at 600%C in air neighboring fibers720 M. Leparoux et al. in boron as compared with nitrogen, that cannot be evidenced by our analyses. 4 Discussion The application of a compliant interphase such as carbon or boron nitride on treated Nicalon@ fibers has been found to improve the mechanical beha￾vior of the Sic/Sic composites at room tempera￾ture. The drawbacks of carbon are obvious in oxidizing atmosphere, and BN should exhibit a better oxidation resistance, especially at moderate temperature such as 600°C. However the ICVI-BN deposits always present a turbostratic structure. Even if the temperature of 600°C is low for the onset of hexagonal-BN oxidation, poorer lattice organization might be affected at this temperature. The availability of boron oxide could then allow the formation of a glassy phase with various amounts of boron. It is indeed well-known that boron significantly decreases the viscosity of glas￾ses. The melting temperature of Bz03 is about 480”C23 and the eutectic temperature of the pseudo-binary Si02-B203 is, for example, about 450”C.24 When considering the flowing ability and Fig. 5. Cracks deflection in a 2D-SiC/BN/SiC composite tested in tension at 600°C in air. the volatility of boron oxide, it is not surprising to find such a phase in different parts of the compo￾site, especially in the tows, inside cracks perpendi￾cular or parallel to the fibers (Figs 6 and 7). Sometimes it can even link the cracks walls. The EELS analysis along a decohesion at the fiber/BN interface confirms that the Si, 0 and B elements are present (Section 3.2.2, Analysis of the cracks). The analysis of a crack inside the BN interphase reveals also an increase in oxygen content and the silicon presence with Si-0 bonds formation. From EELS study, it appears that the bulk boron nitride interphase can be modified at 600°C in air. However the decrease of the 0, C and N contents should be examined by further analyses, particularly at different distances from the far end of the cracks, i.e. different conditions of ambient air access. This modification could be attributed to the relatively poor organization of the structure and the initial incorporation of C and 0 impurities. Because its composition was not changed by the ICVI processing of the silicon carbide matrix from Fig. 6. Vitreous phase linking the treated Nicalon@ fiber and the BN interphase. Fig. 7. Glassy phase in a crack propagating between two neighboring fibers