B. Ruggles-Wrenn et al. Composites: Part A 37(2006)2029-2040 demonstrated in various turbine components [2]. Many of of intermittent moisture exposure on the high-temperature these demonstration components have exhibited acceler- fatigue durability of five different CMCs, including N720/ ated degradation after only a few hours in service environ- A Fatigue testing of N720 /A was performed at 1200C, ment. It is now widely recognized that the thermodynamic fatigue stress levels were <120 MPa. Cyclic loading and stability and oxidation resistance of CMCs have become moisture exposure were applied alternately Zawada et al. Important issues. observed no degradation in fatigue performance or Sintered structural ceramics are known to exhibit degra- retained strength with intermittent moisture exposure dation in high-temperature environments. Non-oxide fiber/ In the present study, fatigue and creep-rupture testing of non-oxide matrix composites generally show poor oxida- N720/A specimens was conducted both in laboratory air tion resistance [3, 4]. The degradation involves oxidation and in 100% steam environment at high temperatures of fibers and fiber coatings, and is accelerated by the pres-(1200 and 1330C). Applied stress levels used in both ence of moisture [5-7]. Numerous studies addressed oxida- creep-rupture and fatigue tests(<170 MPa) were consider- tion of Sic in moist environments [8-13]. Opila and Hann ably higher than those employed in previous studies. As is [9] and Pila [10, 11] reported that the presence of water seen in detail, effects of steam environment on fatigue and vapor increased the rate of Sio growth on SiC at high especially on creep performance cannot be neglected temperature, which led to accelerated rates of Sic reces- sion. Degradation of BN fiber coatings in moist environ- 2. Experimental procedure ments has also been a subject of extensive research [14-20]. Non-oxide fiber/oxide matrix composites or oxide 2.1. Material fiber/non-oxide matrix composites do not exhibit high oxidation resistance either. For these materials. the high The composite studied was a commercially available permeability constant for the diffusion of oxygen results material(N720/A, COI Ceramics, San Diego, CA)consist- in rapid oxygen permeation through the oxide matrix ing of NextelM720 fibers in a porous alumina matrix, sup- [21]. These considerations motivated the development of plied in a form of 2.8 mm thick plates. The plates consisted environmentally stable or: Composites(CFCCs) based on of 12 0/900 woven layers, with a density of 2.78g/cm The main advantage of CMCs over monolithic ceramics fiber coating. The fiber fabric was infiltrated with the is their superior toughness, tolerance to the presence of matrix in a sol-gel process. After drying with a"vacuum cracks and defects, and non-catastrophic mode of failure. bag"technique under low pressure and low temperature, It is now well recognized that CFCCs can be designed to the composite was pressureless sintered [49]. Matrix poros- exhibit non-brittle fracture behavior and improved damage ity was w24%. Such porosity level renders the matrix suffi- tolerance by introducing a weak fiber/matrix interface, ciently weak and gives the composite excellent damage which serves to deflect matrix cracks and to allow subse- tolerance during loading. Representative micrographs of quent fiber pull-out [31-33]. It has recently been demon- the untested as-received material are shown in Fig. I strated that similar crack-deflecting behavior can also be Fig. l(a) shows 0 and 90 fiber tows as well as numerous achieved by means of a finely distributed porosity in the matrix cracks. In the case of untested material, most are matrix instead of a separate interface between matrix and shrinkage cracks formed during processing rather than fibers [34]. This microstructural design philosophy implic- matrix cracks generated during loading. Porous nature of itly accepts the formation of strong interfaces. It builds the matrix is seen in Fig. on the experience with porous interlayers as crack deflec tion paths [35, 36] and extends the concept to utilize a por- 2. 2. Mechanical testing ous matrix as a surrogate. The concept has been successfully demonstrated for oxide-oxide composites A servocontrolled MTS mechanical testing machine [22, 26, 30, 37-41]. Resulting oxide/oxide CFCCs exhibit equipped with hydraulic water-cooled collet grips, a com damage tolerance combined with inherent oxidation resis- pact two-zone resistance-heated furnace, and two tempera tance. An extensive review of the mechanisms and mechat ture controllers was used in all tests. An mrs Teststar ical properties of porous-matrix CMCs is given in [42]. digital controller was employed for input signal generation The objective of this study is to investigate effects of and data acquisition. Strain measurement was accom- steam environment on high-temperature mechanical plished with an MTS high-temperature air-cooled uniaxial behavior and durability of an oxide-oxide CFCC, consist- extensometer. For elevated temperature testing, thermo- ing of a porous alumina matrix reinforced with the couples were bonded to the specimens to calibrate the fur- Nextel720 fibers. Several previous studies examined nace on a periodic basis. The furnace controller(using a high-temperature mechanical behavior of this material non-contacting thermocouple exposed to the ambient envi [2, 43], and [44]. Unlike its counterpart reinforced with ronment near the test specimen) was adjusted to determine Sic fibers [45-48], the CFCC exhibited steady-state creep. the power setting needed to achieve the desired tempera- Creep rates were 10-8-10-7s-l, similar to those expected ture of the test specimen. Thus determined power setting from fibers alone. Zawada et al. [2] investigated the effect was then used in actual tests. The power setting for testingdemonstrated in various turbine components [2]. Many of these demonstration components have exhibited acceler￾ated degradation after only a few hours in service environ￾ment. It is now widely recognized that the thermodynamic stability and oxidation resistance of CMCs have become important issues. Sintered structural ceramics are known to exhibit degra￾dation in high-temperature environments. Non-oxide fiber/ non-oxide matrix composites generally show poor oxida￾tion resistance [3,4]. The degradation involves oxidation of fibers and fiber coatings, and is accelerated by the pres￾ence of moisture [5–7]. Numerous studies addressed oxida￾tion of SiC in moist environments [8–13]. Opila and Hann [9] and Opila [10,11] reported that the presence of water vapor increased the rate of SiO2 growth on SiC at high temperature, which led to accelerated rates of SiC reces￾sion. Degradation of BN fiber coatings in moist environ￾ments has also been a subject of extensive research [14–20]. Non-oxide fiber/oxide matrix composites or oxide fiber/non-oxide matrix composites do not exhibit high oxidation resistance either. For these materials, the high permeability constant for the diffusion of oxygen results in rapid oxygen permeation through the oxide matrix [21]. These considerations motivated the development of continuous fiber ceramic composites (CFCCs) based on environmentally stable oxide constituents [22–30]. The main advantage of CMCs over monolithic ceramics is their superior toughness, tolerance to the presence of cracks and defects, and non-catastrophic mode of failure. It is now well recognized that CFCCs can be designed to exhibit non-brittle fracture behavior and improved damage tolerance by introducing a weak fiber/matrix interface, which serves to deflect matrix cracks and to allow subse￾quent fiber pull-out [31–33]. It has recently been demon￾strated that similar crack-deflecting behavior can also be achieved by means of a finely distributed porosity in the matrix instead of a separate interface between matrix and fibers [34]. This microstructural design philosophy implic￾itly accepts the formation of strong interfaces. It builds on the experience with porous interlayers as crack deflec￾tion paths [35,36] and extends the concept to utilize a por￾ous matrix as a surrogate. The concept has been successfully demonstrated for oxide–oxide composites [22,26,30,37–41]. Resulting oxide/oxide CFCCs exhibit damage tolerance combined with inherent oxidation resis￾tance. An extensive review of the mechanisms and mechan￾ical properties of porous-matrix CMCs is given in [42]. The objective of this study is to investigate effects of steam environment on high-temperature mechanical behavior and durability of an oxide–oxide CFCC, consist￾ing of a porous alumina matrix reinforced with the NextelTM720 fibers. Several previous studies examined high-temperature mechanical behavior of this material [2,43], and [44]. Unlike its counterpart reinforced with SiC fibers [45–48], the CFCC exhibited steady-state creep. Creep rates were 108 –107 s 1 , similar to those expected from fibers alone. Zawada et al. [2] investigated the effect of intermittent moisture exposure on the high-temperature fatigue durability of five different CMCs, including N720/ A. Fatigue testing of N720/A was performed at 1200 C, fatigue stress levels were 6120 MPa. Cyclic loading and moisture exposure were applied alternately. Zawada et al. observed no degradation in fatigue performance or retained strength with intermittent moisture exposure. In the present study, fatigue and creep-rupture testing of N720/A specimens was conducted both in laboratory air and in 100% steam environment at high temperatures (1200 and 1330 C). Applied stress levels used in both creep-rupture and fatigue tests (6170 MPa) were consider￾ably higher than those employed in previous studies. As is seen in detail, effects of steam environment on fatigue and especially on creep performance cannot be neglected. 2. Experimental procedure 2.1. Material The composite studied was a commercially available material (N720/A, COI Ceramics, San Diego, CA) consist￾ing of NextelTM720 fibers in a porous alumina matrix, sup￾plied in a form of 2.8 mm thick plates. The plates consisted of 12 0/90 woven layers, with a density of 2.78 g/cm3 and a fiber volume of approximately 44%. There was no fiber coating. The fiber fabric was infiltrated with the matrix in a sol–gel process. After drying with a ‘‘vacuum bag’’ technique under low pressure and low temperature, the composite was pressureless sintered [49]. Matrix poros￾ity was 24%. Such porosity level renders the matrix suffi- ciently weak and gives the composite excellent damage tolerance during loading. Representative micrographs of the untested as-received material are shown in Fig. 1. Fig. 1(a) shows 0 and 90 fiber tows as well as numerous matrix cracks. In the case of untested material, most are shrinkage cracks formed during processing rather than matrix cracks generated during loading. Porous nature of the matrix is seen in Fig. 1(b). 2.2. Mechanical testing A servocontrolled MTS mechanical testing machine equipped with hydraulic water-cooled collet grips, a com￾pact two-zone resistance-heated furnace, and two tempera￾ture controllers was used in all tests. An MTS TestStar 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. For elevated temperature testing, thermo￾couples were bonded to the specimens to calibrate the fur￾nace on a periodic basis. The furnace controller (using a non-contacting thermocouple exposed to the ambient envi￾ronment near the test specimen) was adjusted to determine the power setting needed to achieve the desired tempera￾ture of the test specimen. Thus determined power setting was then used in actual tests. The power setting for testing 2030 M.B. Ruggles-Wrenn et al. / Composites: Part A 37 (2006) 2029–2040