18 Journal of the American Ceramic Sociery-Zhu et al. Vol. 82. No the creep and fatigue behavior of a Hi-NicalonTM fiber-rein- A controlled-atmosphere furnace(Model MTS 659, MTS forced SiC composite System Corp. was used. For the tests in argon, the chamber Creep and fatigue tests of the Hi-Nicalon TM/SiC composite was first allowed to pump down to <13.3 Pa(100 mtorr), and were both performed in air at the same maximum stresses, to then the chamber was backfilled with high-purity argon gas ompare time-dependent deterioration with cyclic-dependent The steps were repeated three times to ensure a thorough purge amage Creep tests in pure argon also were conducted at the Enough argon gas was flowed through the chamber to equal ame temperature to understand the effect of environment five times the chamber volume. The volume percentage of the creep. The mechanical behavior of the Hi-Nicalon TM/Sic oxygen in the high-purity argon gas was <l ppm. After frac- omposite was compared with the enhanced SiC/SiC and ture, the specimens were examined by using both optical mi- tandard SiC/SiC composites to investigate the effects of im- croscopy and scanning electron microscopy(SEM) proved fibers and the matrix on the creep and fatigue resistance IlL. Results Il. Materials and Experimental Procedure () Microstructures and Monotonic Tension The composites used in this investigation were processed via The satin-woven structure and matrix microstructure of the Hi-Nicalon TM/SiC composite in the original state are shown the chemical vapor infiltration(CVI)of SiC into satin-woven Fig. 1. The width of the fiber bundles in the satin-woven struc- 0%/90 Hi-Nicalon TM fiber preforms(made by Du Pont Lanxide le ture is -1.5 mm(Fig. I(a). There are glassy phases in the Composites, Wilmington, DE). Before the infiltration, the chanced Sic matrix(dark phases in the matrix shown in Fi orms were coated with carbon via chemical vapor deposition I(b)) VD), to decrease the interface bonding between the fibers Stress-versus-strain curves of the Hi-Nicalon TM/SiC. en- and the matrix, thereby Increasing to The compos hanced SiC/SiC, and standard SiC/SiC composites at a tem- tes, which were processed as 200 mm x 200 mm panels with perature of 1300C are shown in Fig. 2. The enhanced SiC/SiC a thickness of 3.2 mm, contained 40 vol% SiC fibers and 9. 7% composite consists of normal Nicalon TM fibers and the en- hanced SiC matrix. which is the same as that in the Hi- insisted of 500 fibers NicalonTM/SiC composite. The curve for the Hi-Nicalon TM/SiC The tensile specimens were machined from th amond cutting tools. The shape and dimensions of the speci- mens for the monotonic-tension, creep, and cyclic-fatigue tests have been described by Zhu and co-workers. 7, 8, 0, II The sur- aces of the specimens were machined by using an 800 grit ding wheel before testing. The specimens were not pro- tected by a final Cvi run after machining All the mechanical tests were performed with a servo- e Eden Prairie, MN)at a temperature of 1300.C. The monotonic tensile tests were conducted in air under a constant stress rate o The alignment between the upper and lower grips of the load f 50 MI unit was verified using a steel dummy specimen for verifica- tion.Analytical and empirical analysis studies concluded that or negligible effects on the estimates of the strength distribu- tion parameters(for example, the Weibull modulus and the characteristic strength) of monolithic advanced ceramics, the allowable bending percentage, as defined in ASTM Practice E 12, should not be >5% Similar studies of the effect of bend- reinforced CMCs do not exist. ASTM Practice C 1275-94(a) mation, which can redistribute the stress state and sometimes lead to notch insensitivity, a bending strain of 5% should no affect the strength distribution The fatigue tests were performed with a sinusoidal de quency of 20 Hz in air. The stress ratio (r), which was defined as the ratio of minimum stress to maximum stress, was 0.1 for fatigue tests. Creep tests were conducted under constant load in both air and argon. Creep strain was measured directly from the gauge length of the specimen by using a contact xtensometer(Model 632.53-F71, MTS System Corp ) which has a measuring range of +2.5 mm over its gauge length of 25 mm. Repeated unloading-reloading, with a rate of 50 MPa/s, was ed to measure the modulus change during the creep tests. The specimens were allowed to soak for >30 min at a temperature of 1300C before starting monotonic-tension, 4um creep, or cyclic-fatigue tests Fig. 1.(a) Satin- woven structure of the Hi-Nicalon TM fiber bundles he steel dummy specimen plied by MTS Sy with astm stathe creep and fatigue behavior of a Hi-Nicalon™ fiber-rein￾forced SiC composite. Creep and fatigue tests of the Hi-Nicalon™/SiC composite were both performed in air at the same maximum stresses, to compare time-dependent deterioration with cyclic-dependent damage. Creep tests in pure argon also were conducted at the same temperature to understand the effect of environment on the creep. The mechanical behavior of the Hi-Nicalon™/SiC composite was compared with the enhanced SiC/SiC11 and standard SiC/SiC composites10 to investigate the effects of im￾proved fibers and the matrix on the creep and fatigue resistance at high temperature. II. Materials and Experimental Procedures The composites used in this investigation were processed via the chemical vapor infiltration (CVI) of SiC into satin-woven 0°/90° Hi-Nicalon™ fiber preforms (made by Du Pont Lanxide Composites, Wilmington, DE). Before the infiltration, the pre￾forms were coated with carbon via chemical vapor deposition (CVD), to decrease the interface bonding between the fibers and the matrix, thereby increasing the toughness. The compos￾ites, which were processed as 200 mm × 200 mm panels with a thickness of 3.2 mm, contained 40 vol% SiC fibers and 9.7% porosity. The diameter of a fiber was ∼12 mm, and each bundle consisted of 500 fibers. The tensile specimens were machined from the panels using diamond cutting tools. The shape and dimensions of the speci￾mens for the monotonic-tension, creep, and cyclic-fatigue tests have been described by Zhu and co-workers.7,8,10,11 The sur￾faces of the specimens were machined by using an 800 grit grinding wheel before testing. The specimens were not pro￾tected by a final CVI run after machining. All the mechanical tests were performed with 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 stress rate of 50 MPa/s. The alignment between the upper and lower grips of the load unit was verified using a steel dummy specimen for verifica￾tion.¶ Analytical and empirical analysis studies concluded that, for negligible effects on the estimates of the strength distribu￾tion parameters (for example, the Weibull modulus and the characteristic strength) of monolithic advanced ceramics, the allowable bending percentage, as defined in ASTM Practice E 1012, should not be >5%. Similar studies of the effect of bend￾ing on the tensile strength distributions of continuous-fiber￾reinforced CMCs do not exist. ASTM Practice C 1275-94 adopted the recommendations for the tensile testing of mono￾lithic advanced ceramics. Because CMCs have inelastic defor￾mation, 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 (r), which was defined as the ratio of minimum stress to maximum stress, was 0.1 for fatigue tests. Creep tests were conducted under constant load in both air and argon. Creep strain was measured directly from the gauge length of the specimen by using a contact extensometer (Model 632.53-F71, MTS System Corp.), which has a measuring range of ±2.5 mm over its gauge length of 25 mm. Repeated unloading–reloading, with a rate of 50 MPa/s, was applied to measure the modulus change during the creep tests. The specimens were allowed to soak for >30 min at a temperature of 1300°C before starting monotonic-tension, creep, or cyclic-fatigue tests. A controlled-atmosphere furnace (Model MTS 659, MTS System Corp.) was used. 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. The steps were repeated three times to ensure a thorough purge. Enough argon gas was flowed through the chamber to equal five times the chamber volume. The volume percentage of oxygen in the high-purity argon gas was <1 ppm. After frac￾ture, the specimens were examined by using both optical mi￾croscopy and scanning electron microscopy (SEM). III. Results (1) Microstructures and Monotonic Tension The satin-woven structure and matrix microstructure of the Hi-Nicalon™/SiC composite in the original state are shown in Fig. 1. The width of the fiber bundles in the satin-woven struc￾ture is ∼1.5 mm (Fig. 1(a)). There are glassy phases in the enhanced SiC matrix (dark phases in the matrix shown in Fig. 1(b)). Stress-versus-strain curves of the Hi-Nicalon™/SiC, en￾hanced SiC/SiC, and standard SiC/SiC composites at a tem￾perature of 1300°C are shown in Fig. 2. The enhanced SiC/SiC composite consists of normal Nicalon™ fibers and the en￾hanced SiC matrix, which is the same as that in the Hi￾Nicalon™/SiC composite. The curve for the Hi-Nicalon™/SiC ¶ The steel dummy specimen was supplied by MTS System Corp. It was designed to allow the bending strain to be <5%, in accordance with ASTM Standard E 1012-89. Fig. 1. (a) Satin-woven structure of the Hi-Nicalon™ fiber bundles in the Hi-Nicalon™/SiC composite; (b) glassy phases (dark blocks) in the matrix of the Hi-Nicalon™/SiC composite. 118 Journal of the American Ceramic Society—Zhu et al. Vol. 82, No. 1