M.B. Ruggles-Wrenn et al. Composites Science and Technology 68(2008)1588-1595 interface between matrix and fibers [15]. This microstruc- low temperature, then pressureless sintered [22]. No coat ural design philosophy implicitly accepts the strong fiber/ ing was applied to the fibers. The damage tolerance of matrix interface. The concept has been successfully demon- the N720/A composite is enabled by a porous-matrix. Rep- strated for oxide-oxide composites [6, 9, 11, 16, 17]. Resulting resentative micrograph of the untested material is pre- oxide/oxide CMCs exhibit damage tolerance combined sented in Fig. la, which shows 0 and 90 fiber tows with inherent oxidation resistance. An extensive review of well as numerous matrix cracks In the case of the as-pro- the mechanisms and mechanical properties of porous- cessed material, most are shrinkage cracks formed during matrix CMCs is given in [18, 19 processing rather than matrix cracks generated during In many potential applications oxide-oxide CMCs will loading Porous nature of the matrix is seen in Fig. Ib be subject to multiaxial states of stress. The woven CMC A servocontrolled MTS mechanical testing machine materials developed for use in aerospace engine compo- equipped with hydraulic water-cooled wedge grips, a com- nents are typically made from 0/900 fiber architectures. pact two-zone resistance-heated furnace, and two tempera- However, the highest loads in structural components are ture controllers was used in all tests. An MTS TestStar Il not always applied in the direction of the reinforcing fibers. digital controller was employed for input signal generation As a result, the components could experience stresses and data acquisition. Strain measurement was accom- approaching the off-axis tensile and creep strengths. The plished with an MTS high-temperature air-cooled uniaxial objective of this effort is to investigate the off-axis tensile extensometer of 12.5 mm gage length. Tests in steam envi and creep behaviors of an oxide-oxide CMC consisting ronment employed an alumina susceptor(tube with end of a porous alumina matrix reinforced with the Nex- caps), which fits inside the furnace. The specimen gage sec tel720 fibers. Several previous studies examined high- tion is located inside the susceptor, with the ends of the temperature mechanical behavior of this composite in the specimen passing through slots in the susceptor. Steam is 0/90 fiber orientation [2, 20, 21]. This study investigates introduced into the susceptor(through a feeding tube) tensile and creep behavior of the NextelM720/alumina a continuous stream with a slightly positive pressure, expel- (N720/A) composite in the +45 orientation at 1200c ling the dry air and creating a near 100% steam environ- in air, steam and argon environments. Creep tests were ment inside the susceptor. An alumina susceptor was also onducted at stress levels ranging from 15 to 45 MPa. used in tests conducted in argon environment. In this case Results reveal that test environment has a noticeable effect high purity argon was introduced into the susceptor creat- on creep life. The composite microstructure, as well as ing an inert gas environment around the test section of the damage and failure mechanisms are discussed. specimen. For elevated-temperature testing, two S-type thermocouples were bonded to the specimen using alumina 2. Material and experimental arrangements cement(Zircar) to calibrate the furnace on a periodic basis The furnace controllers(using non-contacting S-type ther- The material studied was NextelM720/Alumina(N720/ mocouples exposed to the ambient environment near the A), a commercially available oxide-oxide ceramic compos- test specimen) were adjusted to determine the setting te(COI Ceramics, San Diego, CA), consisting of a porous needed to achieve the desired temperature of the test spec- alumina matrix reinforced with Nextel720 fibers. The imen. The determined settings were then used in actual composite was supplied in a form of a 2. 8 mm thick plate, tests. Within the 18-mm gage section of the test specimen, comprised of 12 0/90 woven layers, with a density of the maximum deviation from the nominal test temperature 2.77 g/cm'and a fiber volume of approximately 45%. was +l C. The power settings for testing in steam were Matrix porosity was m24%. The fiber fabric was infiltrated determined by placing the specimen instrumented with with the matrix in a sol-gel process. The laminate was dried two S-type thermocouples in steam environment and with a"vacuum bag"technique under low pressure and repeating the furnace calibration procedure. To calibrate (a) 200um 05m Fig. I. As-received material: (a)overview, (b) porous nature of the matrix is evident.interface between matrix and fibers [15]. This microstruc￾tural design philosophy implicitly accepts the strong fiber/ matrix interface. The concept has been successfully demon￾strated for oxide–oxide composites [6,9,11,16,17]. Resulting oxide/oxide CMCs exhibit damage tolerance combined with inherent oxidation resistance. An extensive review of the mechanisms and mechanical properties of porous￾matrix CMCs is given in [18,19]. In many potential applications oxide–oxide CMCs will be subject to multiaxial states of stress. The woven CMC materials developed for use in aerospace engine compo￾nents are typically made from 0/90 fiber architectures. However, the highest loads in structural components are not always applied in the direction of the reinforcing fibers. As a result, the components could experience stresses approaching the off-axis tensile and creep strengths. The objective of this effort is to investigate the off-axis tensile and creep behaviors of an oxide–oxide CMC consisting of a porous alumina matrix reinforced with the Nex￾telTM720 fibers. Several previous studies examined high￾temperature mechanical behavior of this composite in the 0/90 fiber orientation [2,20,21]. This study investigates tensile and creep behavior of the NextelTM720/alumina (N720/A) composite in the ±45 orientation at 1200 C in air, steam and argon environments. Creep tests were conducted at stress levels ranging from 15 to 45 MPa. Results reveal that test environment has a noticeable effect on creep life. The composite microstructure, as well as damage and failure mechanisms are discussed. 2. Material and experimental arrangements The material studied was NextelTM720/Alumina (N720/ A), a commercially available oxide–oxide ceramic compos￾ite (COI Ceramics, San Diego, CA), consisting of a porous alumina matrix reinforced with NextelTM720 fibers. The composite was supplied in a form of a 2.8 mm thick plate, comprised of 12 0/90 woven layers, with a density of 2.77 g/cm3 and a fiber volume of approximately 45%. Matrix porosity was 24%. The fiber fabric was infiltrated with the matrix in a sol–gel process. The laminate was dried with a ‘‘vacuum bag’’ technique under low pressure and low temperature, then pressureless sintered [22]. No coat￾ing was applied to the fibers. The damage tolerance of the N720/A composite is enabled by a porous-matrix. Rep￾resentative micrograph of the untested material is pre￾sented in Fig. 1a, which shows 0 and 90 fiber tows as well as numerous matrix cracks. In the case of the as-pro￾cessed material, most are shrinkage cracks formed during processing rather than matrix cracks generated during loading. Porous nature of the matrix is seen in Fig. 1b. A servocontrolled MTS mechanical testing machine equipped with hydraulic water-cooled wedge grips, a com￾pact two-zone resistance-heated furnace, and two tempera￾ture controllers was used in all tests. An MTS TestStar II digital controller was employed for input signal generation and data acquisition. Strain measurement was accom￾plished with an MTS high-temperature air-cooled uniaxial extensometer of 12.5 mm gage length. Tests in steam envi￾ronment employed an alumina susceptor (tube with end caps), which fits inside the furnace. The specimen gage sec￾tion is located inside the susceptor, with the ends of the specimen passing through slots in the susceptor. Steam is introduced into the susceptor (through a feeding tube) in a continuous stream with a slightly positive pressure, expel￾ling the dry air and creating a near 100% steam environ￾ment inside the susceptor. An alumina susceptor was also used in tests conducted in argon environment. In this case high purity argon was introduced into the susceptor creat￾ing an inert gas environment around the test section of the specimen. For elevated-temperature testing, two S-type thermocouples were bonded to the specimen using alumina cement (Zircar) to calibrate the furnace on a periodic basis. The furnace controllers (using non-contacting S-type ther￾mocouples exposed to the ambient environment near the test specimen) were adjusted to determine the settings needed to achieve the desired temperature of the test spec￾imen. The determined settings were then used in actual tests. Within the 18-mm gage section of the test specimen, the maximum deviation from the nominal test temperature was ±1 C. The power settings for testing in steam were determined by placing the specimen instrumented with two S-type thermocouples in steam environment and repeating the furnace calibration procedure. To calibrate Fig. 1. As-received material: (a) overview, (b) porous nature of the matrix is evident. M.B. Ruggles-Wrenn et al. / Composites Science and Technology 68 (2008) 1588–1595 1589