2270 Journal of the American Ceramic Society-Zhu et al. Vol 8l. No 9 all th d were performed using a servo- To compare the creep and fatigue properties of the enhanced hydraulic testing system del mrS 810, MTS System Corp SiC/SiC composite with those of the standard SiC/SiC com- Eden Prairie, MN)at a temperature of 1300.C The monotonic posite, creep and fatigue tests of the standard SiC/SiC com- tensile tests were conducted in air, under a constant displace posite were conducted under the same conditions as those for ment rate of 0.5 mm/min. The specimens were allowed to soak the enhanced SiC/SiC composite in air. The creep and fatigue for -30 min at 1300%C before the tensile tests were started. The data of the standard SiC/SiC composite in argon were cited alignment between the upper and lower grips of the load unit from Zhu et al. 7 Mizuno et al. and Zhu et al. 10 for comparison was veri ified using the steel dummy specimen for verificatio After fracture, the specimens were examined by using optical that was supplied by MTS Corp. to allow a bending strain of microscopy and scanning electron microscopy (SEM) 5%. in accordance with ASTM Standard E 1012-89. Ana lytical and empirical analysis studies have concluded that, for lL. Results and discussion negligible effects on the estimates of the strength-distribution parameters(for example, the Weibull modulus and character- istic strength) of monolithic advanced ceramics, the allowable 1 Microstructures and Monotonic Tension percent bending, as defined in ASTM Practice E 1012, should Micrographs of the enhanced and standard SiC/SiC compos- ites in their original states are shown in Fig. 1. The differend tensile strength distributions of continuous-fiber-reinforced between them are that there are glassy phases in the matrix CMCs do not exist. ASTM Practice C 1275-94 has adopted the the enhanced SiC/SiC composite(the gray phases in the matrix recommendations for the tensile testing of monolithic advanced in Fig. I(c). The thickness of the carbon layer at the interfaces ceramics. Because CMCs have inelastic deformation, which of the enhanced SiC/SiC composite(0.5-0.6 um) is larger than can redistribute the stress state and sometimes lead to notch that of the standard SiC/SiC composite(0. 1-0. 2 um). More- insensitivity, a bending strain of 5% should not affect the over, there are more pores in the fiber bundles of the enhanced strength distribution SiC/SiC composite than in those of the standard SiC/SiC The fatigue tests were performed with a sinusoidal loading composite. frequency of 20 Hz in air. The stress ratio, which is defined The tensile stress-versus-strain relation of the enhanced and the ratio of minimum stress to maximum stress, was 0. 1 for the standard SiC/SiC composites at 1300%C is shown in Fig. 2. The testing temperature. Creep tests were conducted under a con curves of the enhanced SiC/SiC composite indicate linear elas- tant load in air and in an argon atmosphere. Creep strain was tic behavior up to the proportional limit of -70 MPa, and this measured directly from the gauge length of the specimen using a contact extensometer(Model MTS 632.53-F71,M MPa). The UTS of the enhanced SiC/SiC composite is almost System Corp )that had a measuring range of +2.5 mm over its the same as that of the standard SiC/SiC composite; however, gauge length of 25 mm. Periodically, partial unloading the strain at the UTS of the enhanced SiC/SiC composite is reloading was applied, to measure the modulus change durin much higher than that of the standard SiC/SiC composite. It is the creep tests. The specimens were allowed to soak for >30 evident that the addition of glass-forming particulates in the matrix increases the ductility of the composite. A possible rea A controlled-atmosphere furnace(Model MTS 659. Mrs son is the decrease of creep resistance of the matrix in the System Corp was used for the creep tests in the argon atmo- sphere. For the tests in argon, the chamber was first allowed to The Youngs modulus that is obtained from the linear ump down to <13.3 Pa(100 mtorr) and then the chamber was tion of the curve in the enhanced SiC/SiC composite is-89 backfilled with high-purity argon gas. These steps were re- GPa, which is lower than that at 1000C(127 GPa)and much ated three times to ensure a thorough purge. The argon gas lower than that of the standard SiC/SiC composite at 1300C was flowed through the chamber enough to equal five times the (200 GPa). The Young's modulus of the enhanced SiC/Sic chamber volume. The volume percentage of oxygen in the ity argon gas was <I ppm 1000C to 1300 C, whereas the Youngs modulus of the stan dard SiC/SiC composite decreases -23%. It was reported that the Youngs modulus of the ceramic-grade NicalonTM fibers decreased 21% as the temperature increased from 1000C to American Society for Testing and Materials, Philadelphia, PA 1300oC, which is similar to the change of the Youngs modu- 20um Fig 1. Microstructures of the standard and enhanced SiC/SiC composites(a)standard SiC/SiC composite, (b)enhanced SiC/SiC composite, and (c) glassy phases of the matrix of the enhanced SiC/SiC compositeAll the mechanical tests were performed using a servo￾hydraulic testing system (Model MTS 810, MTS System Corp., Eden Prairie, MN) at a temperature of 1300°C. The monotonic tensile tests were conducted in air, under a constant displace￾ment rate of 0.5 mm/min. The specimens were allowed to soak for ∼30 min at 1300°C before the tensile tests were started. The alignment between the upper and lower grips of the load unit was verified using the steel dummy specimen for verification that was supplied by MTS Corp. to allow a bending strain of <5%, in accordance with ASTM¶ Standard E 1012-89. Ana￾lytical and empirical analysis studies have concluded that, for negligible effects on the estimates of the strength-distribution parameters (for example, the Weibull modulus and character￾istic strength) of monolithic advanced ceramics, the allowable percent bending, as defined in ASTM Practice E 1012, should not be >5%. Similar studies of the effect of bending on the tensile strength distributions of continuous-fiber-reinforced CMCs do not exist. ASTM Practice C 1275-94 has adopted the recommendations for the tensile testing of monolithic advanced ceramics. Because CMCs have inelastic deformation, which can redistribute the stress state and sometimes lead to notch insensitivity, a bending strain of 5% should not affect the strength distribution. The fatigue tests were performed with a sinusoidal loading frequency of 20 Hz in air. The stress ratio, which is defined as the ratio of minimum stress to maximum stress, was 0.1 for the testing temperature. Creep tests were conducted under a con￾stant load in air and in an argon atmosphere. Creep strain was measured directly from the gauge length of the specimen by using a contact extensometer (Model MTS 632.53-F71, MTS System Corp.) that had a measuring range of ±2.5 mm over its gauge length of 25 mm. Periodically, partial unloading– reloading was applied, to measure the modulus change during the creep tests. The specimens were allowed to soak for >30 min at 1300°C before creep or cyclic-fatigue tests were started. A controlled-atmosphere furnace (Model MTS 659, MTS System Corp.) was used for the creep tests in the argon atmo￾sphere. For the tests in argon, the chamber was first allowed to pump down to <13.3 Pa (100 mtorr) and then the chamber was backfilled with high-purity argon gas. These steps were re￾peated three times to ensure a thorough purge. The argon gas was flowed through the chamber enough to equal five times the chamber volume. The volume percentage of oxygen in the high-purity argon gas was <1 ppm. To compare the creep and fatigue properties of the enhanced SiC/SiC composite with those of the standard SiC/SiC com￾posite, creep and fatigue tests of the standard SiC/SiC com￾posite were conducted under the same conditions as those for the enhanced SiC/SiC composite in air. The creep and fatigue data of the standard SiC/SiC composite in argon were cited from Zhu et al.7 Mizuno et al.8 and Zhu et al.10 for comparison. After fracture, the specimens were examined by using optical microscopy and scanning electron microscopy (SEM). III. Results and Discussion (1) Microstructures and Monotonic Tension Micrographs of the enhanced and standard SiC/SiC compos￾ites in their original states are shown in Fig. 1. The differences between them are that there are glassy phases in the matrix of the enhanced SiC/SiC composite (the gray phases in the matrix in Fig. 1(c)). The thickness of the carbon layer at the interfaces of the enhanced SiC/SiC composite (0.5–0.6 mm) is larger than that of the standard SiC/SiC composite (0.1–0.2 mm). More￾over, there are more pores in the fiber bundles of the enhanced SiC/SiC composite than in those of the standard SiC/SiC composite. The tensile stress-versus-strain relation of the enhanced and standard SiC/SiC composites at 1300°C is shown in Fig. 2. The curves of the enhanced SiC/SiC composite indicate linear elas￾tic behavior up to the proportional limit of ∼70 MPa, and this stress is ∼30% of the ultimate tensile strength (UTS) (225 MPa). The UTS of the enhanced SiC/SiC composite is almost the same as that of the standard SiC/SiC composite; however, the strain at the UTS of the enhanced SiC/SiC composite is much higher than that of the standard SiC/SiC composite. It is evident that the addition of glass-forming particulates in the matrix increases the ductility of the composite. A possible rea￾son is the decrease of creep resistance of the matrix in the enhanced SiC/SiC composite. The Young’s modulus that is obtained from the linear por￾tion of the curve in the enhanced SiC/SiC composite is ∼89 GPa, which is lower than that at 1000°C (127 GPa) and much lower than that of the standard SiC/SiC composite at 1300°C (200 GPa). The Young’s modulus of the enhanced SiC/SiC composite decreases 30% as the temperature increases from 1000°C to 1300°C, whereas the Young’s modulus of the stan￾dard SiC/SiC composite decreases ∼23%. It was reported that the Young’s modulus of the ceramic-grade Nicalon™ fibers decreased 21% as the temperature increased from 1000°C to 1300°C,25 which is similar to the change of the Young’s modu- ¶ American Society for Testing and Materials, Philadelphia, PA. Fig. 1. Microstructures of the standard and enhanced SiC/SiC composites ((a) standard SiC/SiC composite, (b) enhanced SiC/SiC composite, and (c) glassy phases of the matrix of the enhanced SiC/SiC composite). 2270 Journal of the American Ceramic Society—Zhu et al. Vol. 81, No. 9