H Mei et al. /Materials Science and Engineering A 460-461(2007)306-313 p su ide surfac 150kv120mmx35 Fig. 1. SEM micrographs showing fiber architectures of the as-fabricated C/SiC composites, (a) 2D and(b)braided 3D preform structures at about 1000oC. The volume fractions of fibers for the 3d 23. Measurements and observations braided architecture and 2d architecture were about 40 and 37%0, respectively. The dog-bone shaped test samples were cut Mechanical strengths of the composite specimens before from the fabricated composite plates and further coated with Sic and after thermal cycles were measured using an Instron y I-CVI under the same conditions(thickness a 50 um). Fiber tester(Model 1196, Instron Ltd, High Wycombe, England) at architectures of the as-received 2D and braided 3D composite room temperature. Fractured sections and coating surfaces were mples are shown in Fig. la and b The virgin properties of the observed with a scanning electron microscope(SEM, HITACHI s-received composite samples are listed in Table 1 S-4700). 2.2. Thermal cycling test 3. Results and discussion Thermal cycling experiments were conducted with an 3.1. Monotonic tensile behaviors integrated system (see the details in Fig. 2 of Ref. [13)), including an induction heating furnace and a servo-hydraulic The static tensile monotonic stress-strain behaviors of the tester(Instron 8801 d ). 2D and braided 3D C/SiC composite materials were measured The dimensions of the dog-bone shaped specimens were to rupture on the Instron tester with a loading rate of 0.001 mm/s 185 mm x 3 mm x 3 mm as illustrated in the middle of the fur- at room temperature. Typical monotonic tensile stress-strain nace. As shown later in Fig 3, thermal cycling was carried out curves, obtained from one example of several 2D and braided between 900 and 1200C over a period of 120s(temperature 3D C/SiC composite samples, are shown in Fig. 2. The com- gradient ATN300C) Only the middle parts of the specimens posites behave as a typical damageable material, exhibiting an (40 mm long, 3 mm wide, and 3 mm thick) were kept in the hot extensive non-linear stress-strain domain up to rupture because zone and wet oxygen atmosphere: 7.90 vol %o O2/14.85 vol %o of the presence of the processing-induced microcracks and of H,O/77.25 vol. Ar Loading mode was tension-tension fatigue the damage accumulation nature of materials during testing (sine wave, frequency: 1 Hz, stress: 60+20 MPa, and stress ratio The linear deformation of the braided 3D C/SiC composite is R=0.5). Strains were assessed directly from gauge length of clearly limited up to about 50 MPa(referred to as"proportional specimen by a contact Instron extensometer with a gauge length limit"or first-matrix cracking stress oM), after which the behav of 10mm. Coefficient of thermal expansion(CTEof the com- iors become non-linear. Moreover, the slope of the tensile curve posite was assessed by a dilatometer(Model DIL 402C, Netszch continuously decreases as the stress increases. However, for the Ltd, Selb, Germany) 2D architecture composite, the non-linearity starts almost from Table Properties of the as- 3D C/SiC composites Materials Density (GPa) Strength(MPa) Failure strain(%6) Porosity(%) CTE (x10-6oC-I 900°C1000°C1100°C1200°C 9123 52.45 0.71 4.2785452654.6125 364153.77664.32934.3719 fficient of thermal expansion.H. Mei et al. / Materials Science and Engineering A 460–461 (2007) 306–313 307 Fig. 1. SEM micrographs showing fiber architectures of the as-fabricated C/SiC composites, (a) 2D and (b) braided 3D preform structures. at about 1000 ◦C. The volume fractions of fibers for the 3D braided architecture and 2D architecture were about 40 and 37%, respectively. The dog-bone shaped test samples were cut from the fabricated composite plates and further coated with SiC by I-CVI under the same conditions (thickness ≈ 50m). Fiber architectures of the as-received 2D and braided 3D composite samples are shown in Fig. 1a and b. The virgin properties of the as-received composite samples are listed in Table 1. 2.2. Thermal cycling test Thermal cycling experiments were conducted with an integrated system (see the details in Fig. 2 of Ref. [13]), including an induction heating furnace and a servo-hydraulic tester (Instron 8801, Instron Ltd., High Wycombe, England). The dimensions of the dog-bone shaped specimens were 185 mm × 3 mm × 3 mm as illustrated in the middle of the fur￾nace. As shown later in Fig. 3, thermal cycling was carried out between 900 and 1200 ◦C over a period of 120 s (temperature gradient T ≈ 300 ◦C). Only the middle parts of the specimens (40 mm long, 3 mm wide, and 3 mm thick) were kept in the hot zone and wet oxygen atmosphere: 7.90 vol.% O2/14.85 vol.% H2O/77.25 vol.% Ar. Loading mode was tension–tension fatigue (sine wave, frequency: 1 Hz, stress: 60 ± 20 MPa, and stress ratio R = 0.5). Strains were assessed directly from gauge length of specimen by a contact Instron extensometer with a gauge length of 10 mm. Coefficient of thermal expansion (CTE) of the com￾posite was assessed by a dilatometer (Model DIL 402 C, Netszch Ltd., Selb, Germany). 2.3. Measurements and observations Mechanical strengths of the composite specimens before and after thermal cycles were measured using an Instron tester (Model 1196, Instron Ltd., High Wycombe, England) at room temperature. Fractured sections and coating surfaces were observed with a scanning electron microscope (SEM, HITACHI S-4700). 3. Results and discussion 3.1. Monotonic tensile behaviors The static tensile monotonic stress–strain behaviors of the 2D and braided 3D C/SiC composite materials were measured to rupture on the Instron tester with a loading rate of 0.001 mm/s at room temperature. Typical monotonic tensile stress–strain curves, obtained from one example of several 2D and braided 3D C/SiC composite samples, are shown in Fig. 2. The com￾posites behave as a typical damageable material, exhibiting an extensive non-linear stress–strain domain up to rupture because of the presence of the processing-induced microcracks and of the damage accumulation nature of materials during testing. The linear deformation of the braided 3D C/SiC composite is clearly limited up to about 50 MPa (referred to as “proportional limit” or first-matrix cracking stress σM), after which the behav￾iors become non-linear. Moreover, the slope of the tensile curve continuously decreases as the stress increases. However, for the 2D architecture composite, the non-linearity starts almost from Table 1 Properties of the as-received 2D and braided 3D C/SiC composites Materials Density (g/cm3) Modulus (GPa) Strength (MPa) Failure strain (%) Porosity (%) CTE (×10−6 ◦C−1) 900 ◦C 1000 ◦C 1100 ◦C 1200 ◦C 2D C/SiC 2.20 91.23 252.45 0.71 10.66 4.2785 4.5265 4.6125 4.1855 3D C/SiC 2.16 142.85 413.76 0.92 13.74 3.6415 3.7766 4.3293 4.3719 CTE, Coefficient of thermal expansion.