M.B. Ruggles-Wrenn, CL Genelin/ Composites Science and Technology 69(2009)663-669 space applications. This study aims to evaluate the creep behavior from a high-pressure cylinder creating an inert gas environmen f Nextel720/ alumina-mullite(n720/AM), an oxide -oxide Cmc around the test section of the specimen. For elevated temperature with a porous matrix Creep tests were conducted at 1200C in testing, thermocouples were bonded to the specimen using alu- air, argon and steam environments for stress mina cement(Zircar) to calibrate the furnace on a periodic basis. 73 to 136 MPa Resulting creep performance The furnace controllers(using non-contacting thermocouples e on the use of these materials in high-tempe posed to the ambient environment near the test specimen)were ite microstructure. as well a adjusted to determine the settings needed to achieve the desired mechanisms are discussed temperature of the test specimen. The determined settings were then used in actual tests. The power settings for testing in steam 2. Material and experimental arrangements (argon) were determined by placing the specimen instrumented with thermocouples in steam(argon) environment and repeating aterial studied was Nextelm720/Jalumina-mullite(N720/ the furnace calibration procedure. Fracture surfaces of failed spec- an oxide-oxide ceramic composite composed of Nextelm720 imens were examined using SEM(FEI Quanta 200 HV)as well as an and a porous matrix, which consists of mullite and alumina optical microscope(Zeiss Discovery V12). The SEM specimens were particles in a sol-gel derived alumina. There is approximately carbon coated 12.5%(by volume)of mullite in the matrix composition. The com- All tests were performed at 1200C. Dog bone shaped speci posite, manufactured by COl Ceramics(San Diego, CA), was sup- mens of 152 mm total length with a 10-mm-wide gage section plied in a form of a 3.2 mm thick plate, comprised of 12 0 /90 were used in all tests. All specimens used in this study were cut woven layers, with a density of 2.63 g/cm and a fiber volume from a single plate. In all tests, a specimen was heated to test tem of approximately 40.4% Composite porosity was x26.8%. The lam- perature in 25 min, and held at temperature for additional 15 min nate was fabricated following the procedure described elsewhere prior to testing. In air, tensile tests were performed in stroke con 21]. No coating was applied to the fibers. The damage tolerance of trol with a constant displacement rate of 0.05 mm/s. In steam, the N720/ AM composite is enabled by a porous matrix. The overall monotonic tension tests were performed in load control with the microstructure of the CMC is presented in Fig. 1, which shows 0o constant rates of 0.0025 and 25 MPa/s. Creep-rupture tests were and 90o fiber tows as well as numerous matrix cracks In the case conducted in load control in accordance with the procedure in of the as-processed material, most are shrinkage cracks formed ASTM standard C 1337 in laboratory air, steam and argon. In all during processing rather than matrix cracks generated during creep tests the specimens were loaded to the creep stress level at oading the stress rate of 15 MPa/s Creep run-out was defined as 100 h A servocontrolled MTS mechanical testing machine equipped at a given creep stress. In each test, stress-strain data were re- with hydraulic water-cooled wedge grips, a compact two-zone corded during the loading to the creep stress level and the actual resistance-heated furnace. and two rature controllers was creep period. Thus both total strain and creep strain could be cal- used in all tests. an MTS TestStar ll digital controller was employed culated and examined. To determine the retained tensile strength for input signal generation and data acquisition. Strain measure- and modulus, specimens that achieved run-out were subjected to nent was accomplished with an MTS high-temperature air-cooled tensile test to failure at 1200C. It is worthy of note that in all tests niaxial extensometer of 125-mm gage length. Tests in steam reported below, the failure occurred within the gage section of the environment employed an alumina susceptor(tube with end caps). extensometer. In some cases one specimen was tested per test con- which fits inside the furnace. The specimen gage section is located dition. The authors recognize that this is a limited set of data. How inside the susceptor, with the ends of the specimen passing ever, extreme care was taken in generating the data. Selective through slots in the susceptor. Steam is introduced into the suscep duplicate tests have demonstrated the data to be very repeatable tor(through a feeding tubein a continuous stream with a slightly This exploratory effort serves to identify the behavioral trends positive pressure, expelling the dry air and creating a near 100% and to determine whether a more rigorous investigation should steam environment inside the susceptor. Tests in argon environ- be undertaken ment also employed an alumina susceptor. In this case ultra higl purity argon gas(99.999% pure) was supplied to the susceptor 3. Results and discussion 3. 1. Monotonic tension Tensile stress-strain behavior at 1200C in air was nearly lin to failure. Material exhibited typical fiber-dominated composite behavior. The average ultimate tensile strength (UTS)was 153 MPa, elastic modulus. 74.5 GPa, and failure strain. 0.34%. The tensile properties and stress-strain behavior of N720 /AM are sim- ilar to those exhibited by the n720/alumina (n720 / A)composite at Prior work [22, 23]revealed that at 1200C the monotonic ten- sile behavior and tensile properties of the n720 /A composite were strongly influenced by the loading rate. In the present study the ef- fects of loading rate on tensile behavior and tensile properties of the n720 AM composite were examined at 1200C in steam, in tests conducted at constant loading rates of 0.0025 and 25 MPa/ s (Fig. 2). The tensile stress-strain curves obtained at 25 MPa/s are ;802 nearly linear to failure. The average values of the UTS, elastic mod- ulus and failure strain were 150 MPa, 60.1 GPa, and 0.52% Fig. 1. Typical microstructure of the N720/AM ceramic composite. Micrograph tively. While the UTS values are consistent with the data courtesy of A. Szweda, COl Ceramics Inc. in tensile tests conducted in displacement control with thespace applications. This study aims to evaluate the creep behavior of NextelTM720/alumina–mullite (N720/AM), an oxide–oxide CMC with a porous matrix. Creep tests were conducted at 1200 C in air, argon and steam environments for stress levels ranging from 73 to 136 MPa. Resulting creep performance imposes limitations on the use of these materials in high-temperature applications. The composite microstructure, as well as damage and failure mechanisms are discussed. 2. Material and experimental arrangements The material studied was NextelTM720/alumina–mullite (N720/ AM), an oxide–oxide ceramic composite composed of NextelTM720 fibers and a porous matrix, which consists of mullite and alumina particles in a sol–gel derived alumina. There is approximately 12.5% (by volume) of mullite in the matrix composition. The com￾posite, manufactured by COI Ceramics (San Diego, CA), was sup￾plied in a form of a 3.2 mm thick plate, comprised of 12 0/90 woven layers, with a density of 2.63 g/cm3 and a fiber volume of approximately 40.4%. Composite porosity was 26.8%. The lam￾inate was fabricated following the procedure described elsewhere [21]. No coating was applied to the fibers. The damage tolerance of the N720/AM composite is enabled by a porous matrix. The overall microstructure of the CMC is presented in Fig. 1, which shows 0 and 90 fiber tows as well as numerous matrix cracks. In the case of the as-processed material, most are shrinkage cracks formed during processing rather than matrix cracks generated during loading. A servocontrolled MTS mechanical testing machine equipped with hydraulic water-cooled wedge grips, a compact two-zone resistance-heated furnace, and two temperature controllers was used in all tests. An MTS TestStar II digital controller was employed for input signal generation and data acquisition. Strain measure￾ment was accomplished with an MTS high-temperature air-cooled uniaxial extensometer of 12.5-mm gage length. Tests in steam environment employed an alumina susceptor (tube with end caps), which fits inside the furnace. The specimen gage section is located inside the susceptor, with the ends of the specimen passing through slots in the susceptor. Steam is introduced into the suscep￾tor (through a feeding tube) in a continuous stream with a slightly positive pressure, expelling the dry air and creating a near 100% steam environment inside the susceptor. Tests in argon environ￾ment also employed an alumina susceptor. In this case ultra high purity argon gas (99.999% pure) was supplied to the susceptor from a high-pressure cylinder creating an inert gas environment around the test section of the specimen. For elevated temperature testing, thermocouples were bonded to the specimen using alu￾mina cement (Zircar) to calibrate the furnace on a periodic basis. The furnace controllers (using non-contacting thermocouples ex￾posed to the ambient environment near the test specimen) were adjusted to determine the settings needed to achieve the desired temperature of the test specimen. The determined settings were then used in actual tests. The power settings for testing in steam (argon) were determined by placing the specimen instrumented with thermocouples in steam (argon) environment and repeating the furnace calibration procedure. Fracture surfaces of failed spec￾imens were examined using SEM (FEI Quanta 200 HV) as well as an optical microscope (Zeiss Discovery V12). The SEM specimens were carbon coated. All tests were performed at 1200 C. Dog bone shaped speci￾mens of 152 mm total length with a 10-mm-wide gage section were used in all tests. All specimens used in this study were cut from a single plate. In all tests, a specimen was heated to test tem￾perature in 25 min, and held at temperature for additional 15 min prior to testing. In air, tensile tests were performed in stroke con￾trol with a constant displacement rate of 0.05 mm/s. In steam, monotonic tension tests were performed in load control with the constant rates of 0.0025 and 25 MPa/s. Creep–rupture tests were conducted in load control in accordance with the procedure in ASTM standard C 1337 in laboratory air, steam and argon. In all creep tests the specimens were loaded to the creep stress level at the stress rate of 15 MPa/s. Creep run-out was defined as 100 h at a given creep stress. In each test, stress–strain data were re￾corded during the loading to the creep stress level and the actual creep period. Thus both total strain and creep strain could be cal￾culated and examined. To determine the retained tensile strength and modulus, specimens that achieved run-out were subjected to tensile test to failure at 1200 C. It is worthy of note that in all tests reported below, the failure occurred within the gage section of the extensometer. In some cases one specimen was tested per test con￾dition. The authors recognize that this is a limited set of data. How￾ever, extreme care was taken in generating the data. Selective duplicate tests have demonstrated the data to be very repeatable. This exploratory effort serves to identify the behavioral trends and to determine whether a more rigorous investigation should be undertaken. 3. Results and discussion 3.1. Monotonic tension Tensile stress–strain behavior at 1200 C in air was nearly linear to failure. Material exhibited typical fiber-dominated composite behavior. The average ultimate tensile strength (UTS) was 153 MPa, elastic modulus, 74.5 GPa, and failure strain, 0.34%. The tensile properties and stress–strain behavior of N720/AM are sim￾ilar to those exhibited by the N720/alumina (N720/A) composite at 1200 C [20]. Prior work [22,23] revealed that at 1200 C the monotonic ten￾sile behavior and tensile properties of the N720/A composite were strongly influenced by the loading rate. In the present study the ef￾fects of loading rate on tensile behavior and tensile properties of the N720/AM composite were examined at 1200 C in steam, in tests conducted at constant loading rates of 0.0025 and 25 MPa/s (Fig. 2). The tensile stress–strain curves obtained at 25 MPa/s are nearly linear to failure. The average values of the UTS, elastic mod￾ulus and failure strain were 150 MPa, 60.1 GPa, and 0.52%, respec￾tively. While the UTS values are consistent with the data obtained in tensile tests conducted in displacement control with the rate of Fig. 1. Typical microstructure of the N720/AM ceramic composite. Micrograph courtesy of A. Szweda, COI Ceramics Inc. 664 M.B. Ruggles-Wrenn, C.L. Genelin / Composites Science and Technology 69 (2009) 663–669