H. Mei et al Scripta Materialia 54(2006)163-168 under the constant load damaged the interface between the ible damage. Therefore, elongation of the surviving speci- fibers and the matrix, resulted in matrix cracking and bro- mens in the next monotonic tension seems to be limited ken fibers, and all these damages were recorded in the Thermal cycling damage to the modulus is more severe tested composites. The moduli calculated from the two lin- than to the strength of the C/SiC composites. This should ear portions of the curve were 48.86 GPa and 26.35 GPa, be partially ascribed to the brittleness of the Sic ceramic respectively, and the value is reduced by a factor of 0.5. matrix and its sensitivities to crack propagation The initial portion of the curve indicates a linear elastic behavior up to the proportional limit of 60 MPa and the 3. 4. Microstructural observations modulus is slightly lower than the virgin value of 50 GP given in Table 1. The stresses (less than 60 MPa)are not After the monotonic tensile test, coating surfaces, matrix high enough to produce extensive matrix cracking leading and fracture sections were observed on a SEM microscope reduction of modulus. However, after the stress of As shown in Fig. 6, cyclic unloading and reloading under 60 MPa, the slope of the curve decreases rapidly due to cyclic temperatures can result in matrix cracking the accumulated damage in the composite. The decrease (Fig. 6(a)) transversely, and wave-shaped coating cracks in modulus could be ascribed to matrix cracking, fiber frac- at regular spacing(about 278 um in Fig. 6(b). Fig. 7(a) ture and interfacial debonding during the thermal cycling shows that cracks appear in the Sic coating normally be- tests. Finally, the ultimate tensile strength is 131.4 MPa fore testing due to the mismatch of substrate and coating and the value is still 82% of the initial strength. The failure A typical superficial oxidation can also be found beneath strain approximates to 0.58% and is dramatically lower the coatings along the cracks in Fig. 7(b) Oxidation re- than the maximum strain during the thermal cycling tests gions were found along the opening cracks perpendicular (about 1.6%). The constant loading and thermal cycling to the substrate surface because these cracks were enlarged in testing led to increasing strain by forming the nonrevers- during the thermal cycling test with a constant load trix crack Fig. 6. Typical SEM micrograph showing(a) transverse cracks in the matrix and(b) wave-shaped coating cracks at relatively regular spacing after 50 thermal cycles. Matrix cracks marked by arrows in(a) are partially closed once unloaded. Oxidati Fig. 7. Typical cross section micrograph of the C/SiC composites(a) before and (b)after 50 thermal cycles under a constant load of 60 MPa in wet oxygeunder the constant load damaged the interface between the fibers and the matrix, resulted in matrix cracking and bro￾ken fibers, and all these damages were recorded in the tested composites. The moduli calculated from the two lin￾ear portions of the curve were 48.86 GPa and 26.35 GPa, respectively, and the value is reduced by a factor of 0.5. The initial portion of the curve indicates a linear elastic behavior up to the proportional limit of 60 MPa and the modulus is slightly lower than the virgin value of 50 GPa given in Table 1. The stresses (less than 60 MPa) are not high enough to produce extensive matrix cracking leading to reduction of modulus. However, after the stress of 60 MPa, the slope of the curve decreases rapidly due to the accumulated damage in the composite. The decrease in modulus could be ascribed to matrix cracking, fiber frac￾ture and interfacial debonding during the thermal cycling tests. Finally, the ultimate tensile strength is 131.4 MPa and the value is still 82% of the initial strength. The failure strain approximates to 0.58% and is dramatically lower than the maximum strain during the thermal cycling tests (about 1.6%). The constant loading and thermal cycling in testing led to increasing strain by forming the nonrevers￾ible damage. Therefore, elongation of the surviving speci￾mens in the next monotonic tension seems to be limited. Thermal cycling damage to the modulus is more severe than to the strength of the C/SiC composites. This should be partially ascribed to the brittleness of the SiC ceramic matrix and its sensitivities to crack propagation. 3.4. Microstructural observations After the monotonic tensile test, coating surfaces, matrix and fracture sections were observed on a SEM microscope. As shown in Fig. 6, cyclic unloading and reloading under cyclic temperatures can result in matrix cracking (Fig. 6(a)) transversely, and wave-shaped coating cracks at regular spacing (about 278 lm in Fig. 6(b)). Fig. 7(a) shows that cracks appear in the SiC coating normally be￾fore testing due to the mismatch of substrate and coating. A typical superficial oxidation can also be found beneath the coatings along the cracks in Fig. 7(b). Oxidation re￾gions were found along the opening cracks perpendicular to the substrate surface because these cracks were enlarged during the thermal cycling test with a constant load. Fig. 6. Typical SEM micrograph showing (a) transverse cracks in the matrix and (b) wave-shaped coating cracks at relatively regular spacing after 50 thermal cycles. Matrix cracks marked by arrows in (a) are partially closed once unloaded. Fig. 7. Typical cross section micrograph of the C/SiC composites (a) before and (b) after 50 thermal cycles under a constant load of 60 MPa in wet oxygen atmosphere. H. Mei et al. / Scripta Materialia 54 (2006) 163–168 167