42 J Vicens et al. Aerospace Science and Technology 7(2003)135-146 M Sio F 0.1m Fig. 8. Silica layer formed at the fibre/matrix interface in a sample annealed at 1223 K for 50 h in silica(0.3 um) at the fibre/matrix interface can be also ob- According to these authors, the silica activity in the glass- served by TEM after the creep test in the oxidized part of the ceramic matrix is suggested to play a primary role to sample(Fig. 8) control the chemical reaction which creates the carbon laver The formation of the carbon layer by reaction(1)is rate- limited by the diffusion of silica from the fibre to the 4. Models for layers formation on glass-ceramic matrix through the silica activity gradient. This gradient is composites determined by the basicity of the matrix glass. As pointed out by Le Strat and Le Strat et al. [25, 26], the Cooper- A first detailed investigation of layer formation at the Chyung model is based upon the hypotheses that Sic fibre/matrix interface has been performed by Cooper Chyung [12] in SiC/LAS and SiC/CAS composites. TEM SIC, C and Sioz, implying that the activities of these phases studies have revealed a carbon-rich reaction layer in all are equal to unity. Several studies have subsequently shown that Sic Nicalon fibres contain a significant amount of the generation of silica and carbon was thermodynamically silica oxycarbide and a very small amount of sioz [211 favored by the following chemical reaction is not thermodynamically stable [24]. The microstructural SiC(s)+ O2(g)- SiOz(s)+ C(s) observation of a silica-rich region between the matrix and142 J. Vicens et al. / Aerospace Science and Technology 7 (2003) 135–146 Fig. 8. Silica layer formed at the fibre/matrix interface in a sample annealed at 1223 K for 50 h in air. silica (∼0.3 µm) at the fibre/matrix interface can be also ob￾served by TEM after the creep test in the oxidized part of the sample (Fig. 8). 4. Models for layers formation on glass–ceramic composites A first detailed investigation of layer formation at the fibre/matrix interface has been performed by Cooper et Chyung [12] in SiC/LAS and SiC/CAS composites. TEM studies have revealed a carbon-rich reaction layer in all composites. According to thermochemical considerations, the generation of silica and carbon was thermodynamically favored by the following chemical reaction: SiC(s) + O2(g) → SiO2(S) + C(s) (1) According to these authors, the silica activity in the glass– ceramic matrix is suggested to play a primary role to control the chemical reaction which creates the carbon layer. The formation of the carbon layer by reaction (1) is rate￾limited by the diffusion of silica from the fibre to the matrix through the silica activity gradient. This gradient is determined by the basicity of the matrix glass. As pointed out by Le Strat and Le Strat et al. [25,26], the Cooper– Chyung model is based upon the hypotheses that SiC Nicalon fibres are thermodynamically stable and contain SiC, C and SiO2, implying that the activities of these phases are equal to unity. Several studies have subsequently shown that SiC Nicalon fibres contain a significant amount of silica oxycarbide and a very small amount of SiO2 [21]. Other studies have demonstrated that the SiC Nicalon fibre is not thermodynamically stable [24]. The microstructural observation of a silica-rich region between the matrix and