Availableonlineatwww.sciencedirect.com COMPOSITES °" ScienceDirect CIENCE AND TECHNOLOGY ELSEVIER Composites Science and Technology 68(2008)1588-1595 w.elsevier. com/locate/compscitech Creep behavior of Nextel TMz 720/alumina ceramic composite with ±45° fiber orientation at1200°C☆ M B. Ruggles-Wrenn , G.T. Siegert,SS.Baek Department of Aeronautics and Astronautics, Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio 45433-7765, US.A Received 18 January 2007; received in revised form 2 May 2007: accepted 19 July 2007 Available online 1 august 2007 Abstract The tensile creep behavior of an oxide-oxide continuous fiber ceramic composite(CFCC)with +45 fiber orientation was investigated at 1200. in labor) fibers, has no interface between the fiber and matrix, and relies on the porous-matrix for flaw tolerance.The ten- in steam and in argon. The composite consists of a porous alumina matrix reinforced with laminated, woven mull- sile stress-strain behavior was investigated and the tensile properties measured at 1200C. The elastic modulus was 46 GPa and the ultimate tensile strength (UTS)was 55 MPa. Tensile creep behavior was examined for creep stresses in the 15-45 MPa range. Primary and secondary creep regimes were observed in all tests. Creep run- out(set to 100 h) was achieved in all test environments for creep stress levels 35 MPa, creep performance was best in laboratory air and worst in argon. The presence of either steam or argon accelerated creep rates and reduced creep life. Composite microstructure, as well as damage and failure mechanisms were investigated Keywords: A Ceramic-matrix composites(CMCs); A Oxides: B Creep; B. High-temperature properties; D. Fractography 1. Introduction Concurrent efforts in optimization of the CMCs and in Advances in aerospace propulsion technologies have design of the combustion chamber are expected to accelerate raised the demand for structural materials that have supe- the insertion of the CMCs into aerospace turbine rior long-term mechanical properties and retained proper- applications, such as combustor walls [3-5]. Because these ties under high-temperature, high pressure, and varying applications require exposure to oxidizing environments, environmental factors, such as moisture [l]. Ceramic-matrix the thermodynamic stability and oxidation resistance of omposites (CMCs), capable of maintaining excellent CMCs are vital issues. The need for environmentally stable strength and fracture toughness at high-temperatures are composites motivated the development of CMCs based on prime candidate materials for such aerospace applications. environmentally stable oxide constituents[6-11] Additionally, the lower densities of CMCs and their higher The main advantage of CMCs over monolithic ceramics use temperatures, together with a reduced need for cooling is their superior toughness, tolerance to the presence of air, allow for improved high-temperature performance when cracks and defects, and non-catastrophic mode of failure It is widely accepted that in order to avoid brittle fracture * The views expressed are those of the authors and do not reflect the behavior in CMCs and improve the damage tolerance, a official pol ion of the United States Air Force, Department of weak fiber /matrix interface is needed, which serves to Defense or the US government Corresponding author. Tel: +l 937 255 3636x4641: fax: +1 937 656 deflect matrix cracks and to allow subsequent fiber pullout [12-14]. It has been demonstrated that similar crack-deflect arina-ruggles-wrenn(@afit. edu(MB. Ruggles- ing behavior can also be achieved by means of a finely distributed porosity in the matrix instead of a separate 0266-3538S.see front matter Published by Elsevier Ltd. doi: 10.1016/j. compscitech. 2007.07.012
Creep behavior of NextelTM720/alumina ceramic composite with ±45 fiber orientation at 1200 C q M.B. Ruggles-Wrenn a,*, G.T. Siegert a , S.S. Baek b a Department of Aeronautics and Astronautics, Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio 45433-7765, USA b Agency for Defense Development, Daejeon, Republic of Korea Received 18 January 2007; received in revised form 2 May 2007; accepted 19 July 2007 Available online 1 August 2007 Abstract The tensile creep behavior of an oxide–oxide continuous fiber ceramic composite (CFCC) with ±45 fiber orientation was investigated at 1200 C in laboratory air, in steam and in argon. The composite consists of a porous alumina matrix reinforced with laminated, woven mullite/alumina (NextelTM720) fibers, has no interface between the fiber and matrix, and relies on the porous-matrix for flaw tolerance. The tensile stress–strain behavior was investigated and the tensile properties measured at 1200 C. The elastic modulus was 46 GPa and the ultimate tensile strength (UTS) was 55 MPa. Tensile creep behavior was examined for creep stresses in the 15–45 MPa range. Primary and secondary creep regimes were observed in all tests. Creep run-out (set to 100 h) was achieved in all test environments for creep stress levels 635 MPa. At creep stresses >35 MPa, creep performance was best in laboratory air and worst in argon. The presence of either steam or argon accelerated creep rates and reduced creep life. Composite microstructure, as well as damage and failure mechanisms were investigated. Published by Elsevier Ltd. Keywords: A. Ceramic–matrix composites (CMCs); A. Oxides; B. Creep; B. High-temperature properties; D. Fractography 1. Introduction Advances in aerospace propulsion technologies have raised the demand for structural materials that have superior long-term mechanical properties and retained properties under high-temperature, high pressure, and varying environmental factors, such as moisture [1]. Ceramic–matrix composites (CMCs), capable of maintaining excellent strength and fracture toughness at high-temperatures are prime candidate materials for such aerospace applications. Additionally, the lower densities of CMCs and their higher use temperatures, together with a reduced need for cooling air, allow for improved high-temperature performance when compared to conventional nickel-based superalloys [2]. Concurrent efforts in optimization of the CMCs and in design of the combustion chamber are expected to accelerate the insertion of the CMCs into aerospace turbine engine applications, such as combustor walls [3–5]. Because these applications require exposure to oxidizing environments, the thermodynamic stability and oxidation resistance of CMCs are vital issues. The need for environmentally stable composites motivated the development of CMCs based on environmentally stable oxide constituents [6–11]. 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 widely accepted that in order to avoid brittle fracture behavior in CMCs and improve the damage tolerance, a weak fiber/matrix interface is needed, which serves to deflect matrix cracks and to allow subsequent fiber pullout [12–14]. It has been demonstrated that similar crack-deflecting behavior can also be achieved by means of a finely distributed porosity in the matrix instead of a separate 0266-3538/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.compscitech.2007.07.012 q The views expressed are those of the authors and do not reflect the official policy or position of the United States Air Force, Department of Defense or the US Government. * Corresponding author. Tel.: +1 937 255 3636x4641; fax: +1 937 656 7053. E-mail address: marina.ruggles-wrenn@afit.edu (M.B. RugglesWrenn). www.elsevier.com/locate/compscitech Available online at www.sciencedirect.com Composites Science and Technology 68 (2008) 1588–1595 COMPOSITES SCIENCE AND TECHNOLOGY
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 microstructural design philosophy implicitly accepts the strong fiber/ matrix interface. The concept has been successfully demonstrated 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 porousmatrix 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 components 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 NextelTM720 fibers. Several previous studies examined hightemperature 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 composite (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 coating was applied to the fibers. The damage tolerance of the N720/A composite is enabled by a porous-matrix. Representative micrograph of the untested material is presented in Fig. 1a, 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. Porous nature of the matrix is seen in Fig. 1b. 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 measurement 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 susceptor (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. An alumina susceptor was also used in tests conducted in argon environment. In this case high purity argon was introduced into the susceptor creating 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 thermocouples exposed 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. 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
M B. Ruggles-Wrenn et al / Composites Science and Technology 68(2008)1588-1595 the furnace for testing in argon, the specimen instrumented with two S-type thermocouples was placed in argon envi- 0°90° ronment. In the 18 mm specimen gage section, the e max1- UTS =192 MPa mum deviation from the nominal test temperature was ±3° C in steam and±2 In argon. The temperature con- troller set points determined for testing in steam were pproximately 100C higher than those determined for ±45° testing in air. The set points determined for testing in argon UTS=55 MPa were approximately 115c above those determined fo Fracture surfaces of failed specimens were examined T=12 using SEM(FEI Quanta 200 HV)as well as an optical microscope(Zeiss Discovery V12). The SEM specimens Strain (%) were carbon coated All tests were performed at 1200C. In all tests, a spec- 10C sile stress-strain curves for N720/A ceramic composite at imen was heated to test temperature in 25 min, and held at temperature for additional 15 min prior to testing. All test specimens used in this study were cut from a single plate to ifications shown in Fig. 2. Tensile tests were performed typical fiber-dominated composite behavior. The average in stroke control with a constant displacement rate of ultimate tensile strength (UTS)was 190 MPa, elastic modu 0.05 mm/s in laboratory air. Creep-rupture tests were con- lus, 76 GPa, and failure strain, 0.38%. These results agree ducted in load control in accordance with the procedure in well with the data reported earlier [20, 23]. In the case of ASTM standard C 1337 in laboratory air, steam and the #45 orientation, the nonlinear stress-strain behavior argon. In all creep tests the specimens were loaded to the sets in at fairly low stresses(15 MPa). As the stress creep stress level at the stress rate of 15 MPa/s. Creep approaches 50 MPa, appreciable inelastic strains develop run-out was defined as 100 h at a given creep stress. In each rapidly. The specimen achieves a strain of 0. 27% at the max- test, stress-strain data were recorded during the loading to imum load. After the uts is reached, appreciable inelastic he creep stress level and the actual creep period. Thus both strains develop at near constant stress. These observations total strain and creep strain could be calculated and exam- are consistent with the results reported for the porous- ined. To determine the retained tensile strength and modu- matrix ceramic composites in the 45orientation [24, 25]. lus, specimens that achieved run-out were subjected to The elastic modulus(46 GPa)and UTS (55 MPa) obtained tensile test to failure at 1200C. In some cases one speci- for the +45 orientation are considerably lower than the men was tested per test condition. The authors recognize corresponding values for the 0/90 specimens. It is worthy that this is a limited set of data. However, extreme care of note that in all tension tests, as well as in all other tests was taken in generating the data. Selective duplicate tests reported herein, the failure occurred within the gage section have demonstrated the data to be very repeatable. This of the extensometer exploratory effort serves to identify the behavioral trends ind to determine whether a more rigorous investigation 3. 2. Creep-rupture should be undertaken Results of the creep-rupture tests for N720/A composite 3. Results and discussion with +45 fiber orientation are summarized in Table 1 where creep strain accumulation and rupture time are 3. Monotonic tension shown for each creep stress level and test environment Tensile stress-strain behavior at 1200C is typified inin Fig. 4. Creep curves produced in all tests at 15 and Fig 3. The stress-strain curves obtained for the 0/900 fiber 35 MPa exhibit primary and secondary creep regimes, but orientation are nearly linear to failure. Material exhibits no tertiary creep Transition from primary to secondary creep occurs late in creep life, primary creep persists during 叫+90 the first 40-50 h of the creep test. Note that creep run-out 8.0 of 100 h was achieved in all tests at 15 and 35 MPa, regard R=50 less of test environment While the test environment appears to have little influence on the appearance of the creep curves obtained at 15 and 35 MPa, it has a noticeable effect on the strain accumulated during 100 h of creep. For a given creep stress, the largest creep strains were accumu- lated in argon, followed by those accumulated in steam and Fig. 2. Test specimen, dimensions in air. In contrast, all creep curves obtained at 45 MPa
the furnace for testing in argon, the specimen instrumented with two S-type thermocouples was placed in argon environment. In the 18 mm specimen gage section, the maximum deviation from the nominal test temperature was ±3 C in steam and ±2 C in argon. The temperature controller set points determined for testing in steam were approximately 100 C higher than those determined for testing in air. The set points determined for testing in argon were approximately 115 C above those determined for testing in air. Fracture surfaces of failed specimens 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. In all tests, a specimen was heated to test temperature in 25 min, and held at temperature for additional 15 min prior to testing. All test specimens used in this study were cut from a single plate to specifications shown in Fig. 2. Tensile tests were performed in stroke control with a constant displacement rate of 0.05 mm/s in laboratory air. 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 recorded during the loading to the creep stress level and the actual creep period. Thus both total strain and creep strain could be calculated 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. In some cases one specimen was tested per test condition. The authors recognize that this is a limited set of data. However, 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 is typified in Fig. 3. The stress–strain curves obtained for the 0/90 fiber orientation are nearly linear to failure. Material exhibits typical fiber-dominated composite behavior. The average ultimate tensile strength (UTS) was 190 MPa, elastic modulus, 76 GPa, and failure strain, 0.38%. These results agree well with the data reported earlier [20,23]. In the case of the ±45 orientation, the nonlinear stress–strain behavior sets in at fairly low stresses (15 MPa). As the stress approaches 50 MPa, appreciable inelastic strains develop rapidly. The specimen achieves a strain of 0.27% at the maximum load. After the UTS is reached, appreciable inelastic strains develop at near constant stress. These observations are consistent with the results reported for the porousmatrix ceramic composites in the ±45 orientation [24,25]. The elastic modulus (46 GPa) and UTS (55 MPa) obtained for the ±45 orientation are considerably lower than the corresponding values for the 0/90 specimens. It is worthy of note that in all tension tests, as well as in all other tests reported herein, the failure occurred within the gage section of the extensometer. 3.2. Creep-rupture Results of the creep-rupture tests for N720/A composite with ±45 fiber orientation are summarized in Table 1, where creep strain accumulation and rupture time are shown for each creep stress level and test environment. Creep curves obtained in air, steam and argon are shown in Fig. 4. Creep curves produced in all tests at 15 and 35 MPa exhibit primary and secondary creep regimes, but no tertiary creep. Transition from primary to secondary creep occurs late in creep life, primary creep persists during the first 40–50 h of the creep test. Note that creep run-out of 100 h was achieved in all tests at 15 and 35 MPa, regardless of test environment. While the test environment appears to have little influence on the appearance of the creep curves obtained at 15 and 35 MPa, it has a noticeable effect on the strain accumulated during 100 h of creep. For a given creep stress, the largest creep strains were accumulated in argon, followed by those accumulated in steam and in air. In contrast, all creep curves obtained at 45 MPa R=50 50.0 76.0 8.0 9.0 5.0 Fig. 2. Test specimen, dimensions in mm. 0 50 100 150 200 0.0 0.1 0.2 0.3 0.4 Strain (%) Stress (MPa) T = 1200 ºC 0º/90º UTS = 192 MPa ±45º UTS = 55 MPa 0.5 Fig. 3. Tensile stress–strain curves for N720/A ceramic composite at 1200 C. 1590 M.B. Ruggles-Wrenn et al. / Composites Science and Technology 68 (2008) 1588–1595
M B. Ruggles-Wrenn et al/ Composites Science and Technology 68(2008)1588-1595 Table l during the first third of the creep life At the intermediate ry of creep-rupture results for the N720/A ceramic composite with stress of 40 MPa, only primary and secondary creep are +450fiber orientation at 1200 C in laboratory air, steam and argon observed in air and in steam, but all three creep regimes are seen In argon. Environment Creep stress Creep strain Time to rupture (%) Minimum creep rate was reached in all tests. Creep rate as a function of applied stress is presented in Fig. 5, where 13.3 results for N720/A composite with 00/90 fiber orientation l1.9 75,569 from prior work [20] are included for comparison. It is seen 19 that in air the secondary creep rate of the +45 orientation can be as high as 10 times that of the 0%/90%orientation.In 17.8 steam, the +45 creep rate can be as high as 10 times the 2,615 Steam 0.6 0/90 rate. This result is hardly surprising, considering Argon 360000 that the creep-rupture of the 0 /90 orientation is likely Argon 550555055500 dominated by creep-rupture of the Nextel720 fibers It Argon is recognized that Nextel720 fiber has the best creep per Argon Argon 3.55 17 formance of any commercially available polycrystalline Argon oxide fiber. The superior high-temperature creep perfor mance of the Nextel720 fibers results from the high con- tent of mullite, which has a much better creep resistance than alumina [26]. Conversely, the creep-rupture of the +45 orientation is largely dominated by an exceptionally weak porous alumina matrix. For both fiber orientations, T=1200° the minimum creep rates increase with increasing applied stress. In the case of the 0/90 orientation, the secondary creep rate increases by two orders of magnitude as the creep stress increases from 80 to 154 MPa. For a given creep stress, creep rate in steam is approximately an order of magnitude higher than that in air. In the case of the +45 fiber orientation, for stresses <40 MPa creep rate is relatively unaffected by environment. Creep rates obtained in all tests at 15 and 35 MPa are <10s. As the creep stress increases to 45 MPa, the creep rate in air increases by a3 orders of magnitude. The creep rate obtained at 100000200000300000400000500000 Time(s) 45 MPa in steam remains close to that obtained in air while in argon the creep rate increases even more dramat ically. The creep rate in argon is at least one order of mag nitude higher than the rates obtained in air and in steam at Pa, Argon 45 MPa 40 MPa, Argon 1E+00 1.E-01 0e90°,Ar ▲0°0°, Steam 1E22 △ 1.E04 100 Time(s) 9 1.E-0 Fig 4. Creep curves for N720/A composite with #45 fiber orientation at 1E07 1200C in air, steam and argon: (a) time scale chosen to shor 1E08 T=1200 rains accumulated at 15-35 MPa and (b) time scale reduced to show the reep curves obtained at 45 MPa. 1E-09 how primary, secondary and tertiary creep. Transition from primary to secon creep occurs almost immedi- Fig. 5. Minimum creep rate as a function of applied stress for N720/A ceramic composite at 1200C in laboratory air, steam and argon. Data for ately, and secondary transitions to tertiary creep 0/90 fiber orientation from Ruggles-Wrenn et al [20] are also shown
show primary, secondary and tertiary creep. Transition from primary to secondary creep occurs almost immediately, and secondary creep transitions to tertiary creep during the first third of the creep life. At the intermediate stress of 40 MPa, only primary and secondary creep are observed in air and in steam, but all three creep regimes are seen in argon. Minimum creep rate was reached in all tests. Creep rate as a function of applied stress is presented in Fig. 5, where results for N720/A composite with 0/90 fiber orientation from prior work [20] are included for comparison. It is seen that in air the secondary creep rate of the ±45 orientation can be as high as 106 times that of the 0/90 orientation. In steam, the ±45 creep rate can be as high as 105 times the 0/90 rate. This result is hardly surprising, considering that the creep-rupture of the 0/90 orientation is likely dominated by creep-rupture of the NextelTM720 fibers. It is recognized that NextelTM720 fiber has the best creep performance of any commercially available polycrystalline oxide fiber. The superior high-temperature creep performance of the NextelTM720 fibers results from the high content of mullite, which has a much better creep resistance than alumina [26]. Conversely, the creep-rupture of the ±45 orientation is largely dominated by an exceptionally weak porous alumina matrix. For both fiber orientations, the minimum creep rates increase with increasing applied stress. In the case of the 0/90 orientation, the secondary creep rate increases by two orders of magnitude as the creep stress increases from 80 to 154 MPa. For a given creep stress, creep rate in steam is approximately an order of magnitude higher than that in air. In the case of the ±45 fiber orientation, for stresses <40 MPa creep rate is relatively unaffected by environment. Creep rates obtained in all tests at 15 and 35 MPa are 6 105 s 1 . As the creep stress increases to 45 MPa, the creep rate in air increases by 3 orders of magnitude. The creep rate obtained at 45 MPa in steam remains close to that obtained in air, while in argon the creep rate increases even more dramatically. The creep rate in argon is at least one order of magnitude higher than the rates obtained in air and in steam at 45 MPa. 0 5 10 15 20 25 0 100000 200000 300000 400000 500000 Time (s) Strain (%) 15 MPa, Steam 35 MPa, Steam T = 1200 ºC 15 MPa, Air 15 MPa, Argon 35 MPa, Argon 40 MPa, Air 35 MPa, Air 0.0 1.0 2.0 3.0 4.0 5.0 0 50 100 150 200 Time (s) Strain (%) 45 MPa, Argon T = 1200 ºC 40 MPa, Air 40 MPa, Steam 45 MPa, Steam 45 MPa, Air 40 MPa, Argon Fig. 4. Creep curves for N720/A composite with ±45 fiber orientation at 1200 C in air, steam and argon: (a) time scale chosen to show creep strains accumulated at 15–35 MPa and (b) time scale reduced to show the creep curves obtained at 45 MPa. 1.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 10 100 1000 Creep Stress (MPa) Creep Strain Rate (s-1) 0º/90º, Air 0º/90º, Steam ±45º, Air ±45º, Steam ±45º, Argon T = 1200 ºC Fig. 5. Minimum creep rate as a function of applied stress for N720/A ceramic composite at 1200 C in laboratory air, steam and argon. Data for 0/90 fiber orientation from Ruggles-Wrenn et al [20] are also shown. Table 1 Summary of creep-rupture results for the N720/A ceramic composite with ±45 fiber orientation at 1200 C in laboratory air, steam and argon environments Environment Creep stress (MPa) Creep strain (%) Time to rupture (s) Air 15 3.38 360,000* Air 35 13.3 360,000* Air 40 11.9 75,569 Air 45 1.48 119 Steam 15 4.80 360,000* Steam 35 17.8 360,000* Steam 40 4.92 2,615 Steam 45 0.65 59 Argon 15 7.05 360000* Argon 35 20.5 360000* Argon 40 3.58 64 Argon 40 3.36 49 Argon 45 3.55 17 Argon 45 4.12 22 * Run-out. M.B. Ruggles-Wrenn et al. / Composites Science and Technology 68 (2008) 1588–1595 1591
M B. Ruggles-Wrenn et al / Composites Science and Technology 68(2008)1588-1595 Stress-rupture behavior is summarized in Fig. 6, where Table 2 results for N720/A composite with 0/90 fiber orientation Retained properties of the N720/A specimens with*45fiber orientation from prior work [20] are also included. As expected, creep subjected to prior creep at 1200C in laboratory air, steam and argon life decreases with increasing applied stress for both fiber orientations. In the case of the 0/90 orientation, the pres Environment Creep etained Retained Strain at ence of steam dramatically reduced creep lifetimes. The ength modulu failure(%) (MPa) GPa) reduction in creep life due to steam was at least 90% for applied stress levels >100 MPa, and 82% for the applied Air 0.15 stress of 80 MPa. Because the creep performance of the 0%/Steam 63 90 orientation is dominated by the fibers, fiber degrada- Steam tion is a likely source of the composite degradation. Recent Argon 73.0 68.8 studies [27, 28] suggest that the loss of mullite from the fiber may be the mechanism behind the degraded creep perfor mance in steam. Alternatively, poor creep resistance in steam may be due to a stress-corrosion mechanism. In this case,crack growth in the fiber could be caused by a chem- 100 ical interaction of water molecules with mechanically 100 h at 15 MPa strained Si-o bonds at the crack tip with the rate of chem- Steam ical reaction increasing exponentially with applied stress 75 As-Processed 29-34]. In the case of the +45 orientation, environment has little effect on the creep lifetimes (up to 100 h) for applied stresses 40 MPa, creep lifetimes can be reduced by as much as an order of magni 100 h at 15 MPa tude in the presence of steam. An even greater reduction in creep life is seen in the presence of argon. Further experi- ments would be required to understand the cause of such T=1200 drastic degradation of the creep performance in argon Retained strength and modulus of the specimens that Strain (%) achieved a run-out are summarized in Table 2. Tensile stress-strain curves obtained for the specimens subjected prior creep are presented in Fig. 7 together with the te 100h at 35 MPa sile stress-strain curve for the as-processed material. while 75 the specimens subjected to prior creep in all environments 100h at 35 MPa As-Processed in Steam exhibit increased tensile strength and stifness, their capac- y for inelastic straining appears to be considerably reduced. The pre-crept specimens produced higher propor tional limits and much lower failure strains than the as-pro- cessed material. Since the tensile properties of the T=1200°c 0.1 0.2 Strain T=1200°c ■0°/90 ▲0°90°, Steam Effects of prior creep at 1200C in laboratory air, steam and argon 口±45°,Air nsile stress-strain behavior of N720/A with +45 fiber orientation △±45° Stean creep stress:(a)15 MPa and (b)35 MPa. a150 ◇±45°, Argon composite with the +45 fiber orientation are largely dom- inated by the matrix, results indicate that matrix strength ening(most likely due to additional sintering) may be 50fUTSxs taking place during the 100 h of creep 3.3. Composite microstructure E+001E+011.E+021.E+031.E+041.E+051.E+061E+07 (s) Fracture surfaces of the N720/A specimens with +45 Fig. 6. Creep stress vs time to rupture for n720/A ceramic composite at fiber orientation tested in creep at 45 MPa are presented 1200C in laboratory air, steam and argon. Data for 0/900 fiber in Figs. 8 and 9. It is seen that the fracture occurred along orientation from Ruggles-Wrenn et al. [20] the plane at 45 to the loading direction. The failure
Stress-rupture behavior is summarized in Fig. 6, where results for N720/A composite with 0/90 fiber orientation from prior work [20] are also included. As expected, creep life decreases with increasing applied stress for both fiber orientations. In the case of the 0/90 orientation, the presence of steam dramatically reduced creep lifetimes. The reduction in creep life due to steam was at least 90% for applied stress levels P100 MPa, and 82% for the applied stress of 80 MPa. Because the creep performance of the 0/ 90 orientation is dominated by the fibers, fiber degradation is a likely source of the composite degradation. Recent studies [27,28] suggest that the loss of mullite from the fiber may be the mechanism behind the degraded creep performance in steam. Alternatively, poor creep resistance in steam may be due to a stress-corrosion mechanism. In this case, crack growth in the fiber could be caused by a chemical interaction of water molecules with mechanically strained Si–O bonds at the crack tip with the rate of chemical reaction increasing exponentially with applied stress [29–34]. In the case of the ±45 orientation, environment has little effect on the creep lifetimes (up to 100 h) for applied stresses 635 MPa. For stresses P40 MPa, creep lifetimes can be reduced by as much as an order of magnitude in the presence of steam. An even greater reduction in creep life is seen in the presence of argon. Further experiments would be required to understand the cause of such drastic degradation of the creep performance in argon. Retained strength and modulus of the specimens that achieved a run-out are summarized in Table 2. Tensile stress–strain curves obtained for the specimens subjected to prior creep are presented in Fig. 7 together with the tensile stress–strain curve for the as-processed material. While the specimens subjected to prior creep in all environments exhibit increased tensile strength and stiffness, their capacity for inelastic straining appears to be considerably reduced. The pre-crept specimens produced higher proportional limits and much lower failure strains than the as-processed material. Since the tensile properties of the composite with the ±45 fiber orientation are largely dominated by the matrix, results indicate that matrix strengthening (most likely due to additional sintering) may be taking place during the 100 h of creep. 3.3. Composite microstructure Fracture surfaces of the N720/A specimens with ±45 fiber orientation tested in creep at 45 MPa are presented in Figs. 8 and 9. It is seen that the fracture occurred along the plane at 45 to the loading direction. The failure 0 50 100 150 200 250 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 Time (s) Stress (MPa) 0º/90º, Air 0º/90º, Steam ±45º, Air ±45º, Steam ±45º, Argon UTS0/90 UTS±45 T = 1200 ºC Fig. 6. Creep stress vs time to rupture for N720/A ceramic composite at 1200 C in laboratory air, steam and argon. Data for 0/90 fiber orientation from Ruggles-Wrenn et al. [20]. Table 2 Retained properties of the N720/A specimens with ±45 fiber orientation subjected to prior creep at 1200 C in laboratory air, steam and argon environments Environment Creep stress (MPa) Retained strength (MPa) Retained modulus (GPa) Strain at failure (%) Air 15 61.1 64.5 0.15 Air 35 58.3 48.4 0.14 Steam 15 67.0 63.4 0.17 Steam 35 53.4 58.3 0.07 Argon 15 73.0 68.8 0.14 Argon 35 49.4 55.3 0.09 25 50 75 100 Strain (%) Strain (%) Stress (MPa) 100 h at 15 MPa in Steam 100 h at 15 MPa in Air 100 h at 15 MPa in Argon As-Processed T = 1200 ºC 0 0 25 50 75 100 0 0.1 0.2 0.3 0 0.1 0.2 0.3 Stress (MPa) As-Processed T = 1200 ºC 100 h at 35 MPa in Air 100 h at 35 MPa in Steam 100 h at 35 MPa in Argon Fig. 7. Effects of prior creep at 1200 C in laboratory air, steam and argon on tensile stress–strain behavior of N720/A with ±45 fiber orientation. Prior creep stress: (a) 15 MPa and (b) 35 MPa. 1592 M.B. Ruggles-Wrenn et al. / Composites Science and Technology 68 (2008) 1588–1595
M.B. Ruggles-Wrenn et al. Composites Science and Technology 68(2008)1588-1595 1593 s5 mm 5 mm 5 mm Fig 8. Fracture surfaces obtained in creep tests conducted at 45 MPa at 1200C in:(a)air,(b) steam and (c) argon 50 um 100 um 100um Fig 9. SEM micrographs of the fracture surfaces obtained in creep tests conducted at 45 MPa at 1200C in:(a) air, (b)steam and (c)argon. occurs primarily through the matrix, with only minimal typical for dense-matrix CMCs. As seen in the SEM micro- fiber fracture. a diffuse localized deformation band is seen graphs(Fig. 11), the amount of matrix material remaining in all specimens(Fig 8). Following localization, substantial bonded to the fibers is greater than that in the specimens additional straining occurs within the localized band prior subjected to creep at 45 MPa. The difference is particularly to specimen failure Deformation within the localized band pronounced in the case of specimens tested in steam is accommodated by the extensive fragmentation of the Recent studies [35-37] investigated effects of thermal matrix and rotation of the fiber tows towards the loading aging on the physical and mechanical properties of compos- direction. Damage within this band is highly dilatational, ites consisting of Nextel720 fibers and a porous-matrix of as evidenced by the pronounced through-thickness swell- mullite and alumina. For a composite with a pure alumina ing, which is attributed to the comminution of the matrix matrix, a porosity reduction of 6% was observed after a during deformation [25, 18]. Note that the SEM micro- 10 min exposure at 1200C [36, 37]. For a composite with a graphs(Fig. 9)reveal only small amounts of matrix parti- mullite/alumina matrix, strengthening of the matrix and cles bonded to the fiber surfaces. The test environment the fiber-matrix interfaces was observed following aging at appears to have little effect on the failure mechanism 1200C [35]. Additional sintering of the matrix during the and/or fracture surface appearance aging treatments was considered to be associated predomi- By contrast, the fracture surfaces obtained for specimens nantly with Al2O3. The short duration of the creep tests at subjected to prior creep at 35 MPa(Figs. 10 and ll)reveal 45 MPa does not allow for any substantial strengthening that the failure mechanism in this case includes extensive of the matrix to occur. However, it is likely that additional fiber fracture. The fracture surface of the specimen pre- sintering of the matrix occurred during the 100 h creep tests crept in air(Fig. 10a)shows areas of fiber failure, while at 35 MPa. The resultant strengthening of the matrix is man- retaining some of the V-shape fracture typically seen in ifested in the retained properties of the composite. Results in the porous-matrix composites with +45 orientation. Spec- Table 2 show that after 100 h of prior creep at 35 MPa, the imens subjected to prior creep in steam and in argon frac- modulus and the tensile strength increase, and the inelastic tured along the planes nearly orthogonal to the loading straining capability of the composite decreases. The direction. Fracture surfaces in Fig. 10b and c suggest that strengthening is also manifested in the change in the failure specimens pre-crept in steam and in argon failed cata- mechanism. The failure of the composite subjected to prior strophically at the maximum load, with the majority of creep at 35 MPa in steam and in argon is dominated by fiber the fibers breaking in the process. Such failure process is fracture, while the as-processed material fails predominantly
occurs primarily through the matrix, with only minimal fiber fracture. A diffuse localized deformation band is seen in all specimens (Fig. 8). Following localization, substantial additional straining occurs within the localized band prior to specimen failure. Deformation within the localized band is accommodated by the extensive fragmentation of the matrix and rotation of the fiber tows towards the loading direction. Damage within this band is highly dilatational, as evidenced by the pronounced through-thickness swelling, which is attributed to the comminution of the matrix during deformation [25,18]. Note that the SEM micrographs (Fig. 9) reveal only small amounts of matrix particles bonded to the fiber surfaces. The test environment appears to have little effect on the failure mechanism and/or fracture surface appearance. By contrast, the fracture surfaces obtained for specimens subjected to prior creep at 35 MPa (Figs. 10 and 11) reveal that the failure mechanism in this case includes extensive fiber fracture. The fracture surface of the specimen precrept in air (Fig. 10a) shows areas of fiber failure, while retaining some of the V-shape fracture typically seen in the porous-matrix composites with ±45 orientation. Specimens subjected to prior creep in steam and in argon fractured along the planes nearly orthogonal to the loading direction. Fracture surfaces in Fig. 10b and c suggest that specimens pre-crept in steam and in argon failed catastrophically at the maximum load, with the majority of the fibers breaking in the process. Such failure process is typical for dense-matrix CMCs. As seen in the SEM micrographs (Fig. 11), the amount of matrix material remaining bonded to the fibers is greater than that in the specimens subjected to creep at 45 MPa. The difference is particularly pronounced in the case of specimens tested in steam. Recent studies [35–37] investigated effects of thermal aging on the physical and mechanical properties of composites consisting of NextelTM720 fibers and a porous-matrix of mullite and alumina. For a composite with a pure alumina matrix, a porosity reduction of 6% was observed after a 10 min exposure at 1200 C [36,37]. For a composite with a mullite/alumina matrix, strengthening of the matrix and the fiber-matrix interfaces was observed following aging at 1200 C [35]. Additional sintering of the matrix during the aging treatments was considered to be associated predominantly with Al2O3. The short duration of the creep tests at 45 MPa does not allow for any substantial strengthening of the matrix to occur. However, it is likely that additional sintering of the matrix occurred during the 100 h creep tests at 35 MPa. The resultant strengthening of the matrix is manifested in the retained properties of the composite. Results in Table 2 show that after 100 h of prior creep at 35 MPa, the modulus and the tensile strength increase, and the inelastic straining capability of the composite decreases. The strengthening is also manifested in the change in the failure mechanism. The failure of the composite subjected to prior creep at 35 MPa in steam and in argon is dominated by fiber fracture, while the as-processed material fails predominantly Fig. 8. Fracture surfaces obtained in creep tests conducted at 45 MPa at 1200 C in: (a) air, (b) steam and (c) argon. Fig. 9. SEM micrographs of the fracture surfaces obtained in creep tests conducted at 45 MPa at 1200 C in: (a) air, (b) steam and (c) argon. M.B. Ruggles-Wrenn et al. / Composites Science and Technology 68 (2008) 1588–1595 1593
1594 M B. Ruggles-Wrenn et al / Composites Science and Technology 68(2008)1588-1595 (a) (b)(c) 5/mm 5 mm mm Fig 10. Fracture surfaces obtained in tensile tests conducted on specimens subjected to 100 h of prior creep at 35 MPa at 1200C in: (a)air. (b) steam and a 10oum厂100m 100m Fig. Il. SEM micrographs of the fracture surfaces obtained in tensile tests conducted on specimens subjected to 100 h of prior creep at 35 MPa at 1200C in:(a) air, (b) steam and (c)argon. through matrix damage and interply delamination. The are observed. For a given applied stress creep strain accu- specimen pre-crept in air appears to have failed through a mulation is highest in argon, followed by that in steam combination of fiber fracture and matrix failure. It is possi- and in air ble that the sintering of the matrix is accelerated in the pres- Creep strain rates range from 4.2 x 10-6to 9.4 x 10-3s-1 ence of steam. It is also somewhat surprising that in air, from 58x 10 to 1. 4 x 10-s in steam, and from strengthening of the matrix also occurs in argon environ- 4.9x10 to 1.6x 10S in argon For creep stress levels ment, where no oxygen is present. 40 MPa, creep departs from linearity at a low stress of 15 MPa. Once lifetimes were drastically reduced in the presence of steam the UTS= 55 MPa and the corresponding strain of 0.27% and especially in the presence of argon. are reached the softening commences. Considerable inelas- For all test environments investigated in tests of less than tic strains develop at stresses 50 MPa 100 h duration, the failure occurs primarily through matrix The creep-rupture behavior of the N720/A composite damage and interplay delamination, with minimal fiber frac with +45 fiber orientation was characterized for stress lev- ture. For test durations >100 h, the failure mechanism els ranging from 15 to 45 MPa at 1200C in air, steam and dominated by fiber fracture. It is possible that the matrix argon environments. For the stress levels <35 MPa the undergoes additional sintering during the long-term tests material exhibits primary and secondary creep regimes. Additional sintering and consequently strengthening of the At 45 MPa, primary, secondary and tertiary creep regimes matrix may be behind the change in failure mechanisms
through matrix damage and interply delamination. The specimen pre-crept in air appears to have failed through a combination of fiber fracture and matrix failure. It is possible that the sintering of the matrix is accelerated in the presence of steam. It is also somewhat surprising that strengthening of the matrix also occurs in argon environment, where no oxygen is present. 4. Concluding remarks The tensile stress–strain behavior of the N720/A composite with ±45 fiber orientation was investigated and the tensile properties measured at 1200 C. The elastic modulus was 46 GPa and the UTS was 55 MPa. These properties are significantly lower than the corresponding values for the 0/90 fiber orientation. The stress–strain behavior departs from linearity at a low stress of 15 MPa. Once the UTS = 55 MPa and the corresponding strain of 0.27% are reached, the softening commences. Considerable inelastic strains develop at stresses 50 MPa. The creep-rupture behavior of the N720/A composite with ±45 fiber orientation was characterized for stress levels ranging from 15 to 45 MPa at 1200 C in air, steam and argon environments. For the stress levels 635 MPa the material exhibits primary and secondary creep regimes. At 45 MPa, primary, secondary and tertiary creep regimes are observed. For a given applied stress creep strain accumulation is highest in argon, followed by that in steam and in air. Creep strain rates range from 4.2 · 106 to 9.4 · 103 s 1 in air, from 5.8 · 106 to 1.4 · 102 s 1 in steam, and from 4.9 · 106 to 1.6 · 101 s 1 in argon. For creep stress levels 635 MPa environment has little effect on rate, all creep rates are less than 105 s 1 . At 45 MPa, creep rates in all environments increase by at least three orders of magnitude. At 45 MPa, creep rate in argon is about one order of magnitude higher than the rates produced in air and in steam. Creep run-out of 100 h was achieved at applied stress levels 635 MPa in all environments. The run-out specimens exhibited an increase in stiffness and in strength. Prior creep significantly diminished composite’s capability for inelastic straining. For applied stresses P40 MPa, creep lifetimes were drastically reduced in the presence of steam and especially in the presence of argon. For all test environments investigated in tests of less than 100 h duration, the failure occurs primarily through matrix damage and interplay delamination, with minimal fiber fracture. For test durations P100 h, the failure mechanism is dominated by fiber fracture. It is possible that the matrix undergoes additional sintering during the long-term tests. Additional sintering and consequently strengthening of the matrix may be behind the change in failure mechanisms. Fig. 10. Fracture surfaces obtained in tensile tests conducted on specimens subjected to 100 h of prior creep at 35 MPa at 1200 C in: (a) air, (b) steam and (c) argon. Fig. 11. SEM micrographs of the fracture surfaces obtained in tensile tests conducted on specimens subjected to 100 h of prior creep at 35 MPa at 1200 C in: (a) air, (b) steam and (c) argon. 1594 M.B. Ruggles-Wrenn et al. / Composites Science and Technology 68 (2008) 1588–1595
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