H. Mei, L. Cheng / Materials Letters 59(2005)3246-3251 Fig. 3. Typical micrographs showing(a) two types of major fiber failure pattem:(i) physical fracture under thermal cycling and(in) chemical recession in oxidizing atmosphere. (b) Magnified view of the fiber oxidation breaking. fibers were susceptible to oxidation through the opening 3. 2. Thermal cycling damage to 2D C/SiC composites is cracks. It is apparent from the magnified view of the fiber limited and there exists a critical cycling number oxidation breaking(Fig. 3b)that the wet oxygen atmosphere made the fibers thinner and thinner with increasing time till The thermal cycling can result in a physical damage of C final breakage. In addition, the wet oxygen is corrosive to SiC composites, while the oxidizing atmospheres are CVD-SiC matrix covering the carbon fibers so that it can considered to be responsible for a chemical degradation penetrate into dense silica layers, leading to a superficial (i. e, oxidation). Therefore, thermal cycling damage in the oxidation. There is a general agreement within the literature wet oxygen atmosphere, actually, can be understood as a that oxygen and water vapor enhances the oxidation rate of coupled effect of above two factors: a physical damage Sic in the passive regime according to the following caused by thermal cycling and a chemical degradation eactions caused by oxidizing atmospheres. Meanwhile, the former can also provide the latter with the oxidation channels by SiC(s)+3/202(g)SiO2()+ CO(g) (4) opening the matrix cracks and then the latter become a dominant factor of the mechanical degradation in compo- SiC(s)+ 3H2O(g)SiO2(s)+ 3H2(g)+ CO() (5) sites. Correlation between relative residual strengths(i.e, a ratio of the residual strength to the virgin strength) of Water vapor plays a strong influence on the crystal- composites and thermal cycling number in the wet oxygen is lization of amorphous silica which could accelerate Sic shown in Fig. 4. It indicates that physical damage is very oxidation by promoting devitrification. Crystallized silica rapid compared to chemical recession, leading to a cannot act as an oxygen barrier by sealing the cracks remarkable decrease in mechanical properties at the early under cyclic temperature. Besides, the dense silica layer cycles. However, when the cycling number was more than a can be damaged by water. Water is dissolved as molecules critical value, the crack density was saturated, and the in the amorphous silica layer and then reacts with the further decrease in the strength of the composites mainly silicon-oxygen lattice to form Si-OH bonds [10]. The depended on oxidation, not thermal cycling. At this time formation of hydroxylic groups into the silica network the composites hardly had response to the destructive due to the presence of water vapor in an oxygen steam energy of thermal cycling because macro-cracks of matrix produces a less dense silica film which allows for faster diffusion of oxidizing species and/or re 104 [11]. Consequently, the role playing by Sic as an xidation barrier seems to be limited although its 10 oxidation is noticeable in H2O. 2D C/SiC composites under the thermal cycling are very sensitive to wet oxygen he effects of thermal cycling in the wet oxygen atmosphere on the mechanical strength can result from the following two causes: (i) the fracture of fibers under the thermal stress and/or the interface damage due to thermal mismatch, and (ii) the oxidation of the fibers along the 60 matrix cracks in the corrosive gas. The decrease in modulus Thermal cycling number of composites can be ascribed to matrix cracking, fiber Fig. 4. Correlation between relative residual strengths of composites and fracture and interface debonding thermal cycles in the wet oxygen atmosphere.fibers were susceptible to oxidation through the opening cracks. It is apparent from the magnified view of the fiber oxidation breaking (Fig. 3b) that the wet oxygen atmosphere made the fibers thinner and thinner with increasing time till final breakage. In addition, the wet oxygen is corrosive to CVD-SiC matrix covering the carbon fibers so that it can penetrate into dense silica layers, leading to a superficial oxidation. There is a general agreement within the literature that oxygen and water vapor enhances the oxidation rate of SiC in the passive regime according to the following reactions: SiCðsÞ þ 3=2O2ðgÞ YSiO2ðsÞ þ COðgÞ ð4Þ SiCðsÞ þ 3H2OðgÞ YSiO2ðsÞ þ 3H2ðgÞ þ COðgÞ ð5Þ Water vapor plays a strong influence on the crystal￾lization of amorphous silica which could accelerate SiC oxidation by promoting devitrification. Crystallized silica cannot act as an oxygen barrier by sealing the cracks under cyclic temperature. Besides, the dense silica layer can be damaged by water. Water is dissolved as molecules in the amorphous silica layer and then reacts with the silicon –oxygen lattice to form Si –OH bonds [10]. The formation of hydroxylic groups into the silica network due to the presence of water vapor in an oxygen steam produces a less dense silica film which allows for faster diffusion of oxidizing species and/or reaction products [11]. Consequently, the role playing by SiC as an oxidation barrier seems to be limited although its oxidation is noticeable in H2O. 2D C/SiC composites under the thermal cycling are very sensitive to wet oxygen atmosphere. The effects of thermal cycling in the wet oxygen atmosphere on the mechanical strength can result from the following two causes: (i) the fracture of fibers under the thermal stress and/or the interface damage due to thermal mismatch, and (ii) the oxidation of the fibers along the matrix cracks in the corrosive gas. The decrease in modulus of composites can be ascribed to matrix cracking, fiber fracture and interface debonding. 3.2. Thermal cycling damage to 2D C/SiC composites is limited and there exists a critical cycling number The thermal cycling can result in a physical damage of C/ SiC composites, while the oxidizing atmospheres are considered to be responsible for a chemical degradation (i.e., oxidation). Therefore, thermal cycling damage in the wet oxygen atmosphere, actually, can be understood as a coupled effect of above two factors: a physical damage caused by thermal cycling and a chemical degradation caused by oxidizing atmospheres. Meanwhile, the former can also provide the latter with the oxidation channels by opening the matrix cracks and then the latter become a dominant factor of the mechanical degradation in compo￾sites. Correlation between relative residual strengths (i.e., a ratio of the residual strength to the virgin strength) of composites and thermal cycling number in the wet oxygen is shown in Fig. 4. It indicates that physical damage is very rapid compared to chemical recession, leading to a remarkable decrease in mechanical properties at the early cycles. However, when the cycling number was more than a critical value, the crack density was saturated, and the further decrease in the strength of the composites mainly depended on oxidation, not thermal cycling. At this time, the composites hardly had response to the destructive energy of thermal cycling because macro-cracks of matrix Fig. 4. Correlation between relative residual strengths of composites and thermal cycles in the wet oxygen atmosphere. 100 µm 30 µm Coating Cracking a b Cracking Fig. 3. Typical micrographs showing (a) two types of major fiber failure pattern: (i) physical fracture under thermal cycling and (ii) chemical recession in oxidizing atmosphere. (b) Magnified view of the fiber oxidation breaking. H. Mei, L. Cheng / Materials Letters 59 (2005) 3246 – 3251 3249