Composites Science and Technology 69(2009)663-669 Contents lists available at ScienceDirect Soong Composites Science and Technology ELSEVIER journalhomepagewww.elsevier.com/locate/compscitech Creep of Nextelmm720/alumina-mullite ceramic composite at 1200C in air, argon, and steam M B. Ruggles-Wrenn*, C L. Genelin Air Force institute of Technology, wright-Patterson Air Force Base, AFT/ENY, 2950 Hobson Way, WPAFB, OH 45433-7765, USA ARTICLE IN F O ABSTRACT Article history: The tensile creep behavior of an Received 28 October 2008 Received in revised form 11 December 2008 1200C in laboratory air, in stean matrix reinforced with laminated, woven mullitealumina(Nextel 720)fibers, has no interface between Available online 14 January 2009 he 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 1200C. The elastic modulus was 74.5 GPa and e ultimate tensile strength was 153 MPa Tensile creep behavior was examined for creep stresses in the A Ceramic-matrix composites(CMCs) 70-140 MPa range. Primary and secondary creep regimes were observed in all tests. Creep run-out(set to 100 h)was achieved in laboratory air for creep stress levels <91 MPa. The presence of either steam or argon accelerated creep rates and reduced creep lifetimes. Composite microstructure, as well as damage B High-temperature properties and failure mechanisms were investigated D Fractography Published by Elsevier Ltd. 1 Introduction and non-catastrophic mode of failure. It is widely accepted that in order to avoid brittle fracture behavior in CMCs and improve the advances in power generation systems for aircraft engines, damage tolerance, a weak fiber/matrix interface is needed, which land-based turbines, rockets, and, most recently, hypersonic mis- serves to deflect matrix cracks and to allow subsequent fiber pull siles and flight vehicles have raised the demand for structural out [10-12]. It has been demonstrated that similar crack-deflecting materials that have superior long-term mechanical properties behavior can also be achieved by means of a finely distributed Ind retained properties under high temperature, high pressure rosity in the matrix instead of a separate interface between ma nd varying environmental factors, such as moisture [1]. Typic x and fibers [13]. This microstructural design philosophy implic components include combustors, nozzles and thermal insulation. itly accepts the strong fiber/matrix interface. The concept has been Ceramic-matrix composites(CMCs), capable of maintaining excel- successfully demonstrated for oxide-oxide composites lent strength and fracture toughness at high temperatures a [4,7, 9, 14, 15]. Resulting oxide/ oxide Cmcs exhibit damage toler prime candidate materials for such applications. Additionally, low- ance combined with inherent oxidation resistance. An extensive er densities of CMCs and their higher use temperatures, together review of the mechanisms and mechanical properties of porous with a reduced need for cooling air, allow for improved high-tem- matrix CMCs is given in [1, 16]. perature performance when compared to conventional nickel Porous-matrix oxide/oxide CMCs exhibit several behavior based superalloys 2]. Advanced reusable space launch vehicles trends that are distinctly different from those exhibited by tradi- will likely incorporate fiber-reinforced CMCs in critical propulsion tional CMCs with a fiber-matrix interface. Most Sic-fiber-contain- components [3. Because these applications require exposure to ing CMCs exhibit longer life under static loading and shorter life oxidizing environments, the thermodynamic stability and oxida- under cyclic loading [17. For these materials, fatigue is signifi tion resistance of CMCs are vital issues. The need for environmen- cantly more damaging than creep. Zawada et al. [18 examined tally stable composites motivated the development of CMCs based the high-temperature mechanical behavior of a porous matrix on environmentally stable oxide constituents [4-9). Nextel 610/aluminosilicate composite Results revealed excellent The main advantage of CMCs over monolithic ceramics is their fatigue performance at 1000C, the material exhibited high fatigue superior toughness, tolerance to the presence of cracks and defects, limit, long fatigue life and near 100% strength retention. Con ersely, creep lives were short, indicating low creep resistance Because creep was shown to be much more damaging than cy author.Tel:+19372553636:fax:+19376567053 lic loading to oxide-oxide CMCs with porous matrix[18-20]. hisx temperature creep resistance remains among the key issues the E-mail address: marina. ruggles-wrenneafit edu(MB. Ruggles-Wrenn) must be addressed before using these materials in advanced aero-
Creep of NextelTM720/alumina–mullite ceramic composite at 1200 C in air, argon, and steam q M.B. Ruggles-Wrenn *, C.L. Genelin Air Force Institute of Technology, Wright-Patterson Air Force Base, AFIT/ENY, 2950 Hobson Way, WPAFB, OH 45433-7765, USA article info Article history: Received 28 October 2008 Received in revised form 11 December 2008 Accepted 8 January 2009 Available online 14 January 2009 Keywords: A. Ceramic–matrix composites (CMCs) A. Oxides B. Creep B. High-temperature properties D. Fractography abstract The tensile creep behavior of an oxide–oxide continuous fiber ceramic composite was investigated at 1200 C in laboratory air, in steam and in argon. The composite consists of a porous alumina–mullite 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 74.5 GPa and the ultimate tensile strength was 153 MPa. Tensile creep behavior was examined for creep stresses in the 70–140 MPa range. Primary and secondary creep regimes were observed in all tests. Creep run-out (set to 100 h) was achieved in laboratory air for creep stress levels 691 MPa. The presence of either steam or argon accelerated creep rates and reduced creep lifetimes. Composite microstructure, as well as damage and failure mechanisms were investigated. Published by Elsevier Ltd. 1. Introduction Advances in power generation systems for aircraft engines, land-based turbines, rockets, and, most recently, hypersonic missiles and flight vehicles 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]. Typical components include combustors, nozzles and thermal insulation. Ceramic–matrix composites (CMCs), capable of maintaining excellent strength and fracture toughness at high temperatures are prime candidate materials for such applications. Additionally, 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 nickelbased superalloys [2]. Advanced reusable space launch vehicles will likely incorporate fiber-reinforced CMCs in critical propulsion components [3]. 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 [4–9]. 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 [10–12]. 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 interface between matrix and fibers [13]. This microstructural design philosophy implicitly accepts the strong fiber/matrix interface. The concept has been successfully demonstrated for oxide–oxide composites [4,7,9,14,15]. 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 [1,16]. Porous-matrix oxide/oxide CMCs exhibit several behavior trends that are distinctly different from those exhibited by traditional CMCs with a fiber–matrix interface. Most SiC-fiber-containing CMCs exhibit longer life under static loading and shorter life under cyclic loading [17]. For these materials, fatigue is signifi- cantly more damaging than creep. Zawada et al. [18] examined the high-temperature mechanical behavior of a porous matrix NextelTM610/aluminosilicate composite. Results revealed excellent fatigue performance at 1000 C, the material exhibited high fatigue limit, long fatigue life and near 100% strength retention. Conversely, creep lives were short, indicating low creep resistance and limiting the use of that CMC to temperatures below 1000 C. Because creep was shown to be much more damaging than cyclic loading to oxide–oxide CMCs with porous matrix [18–20], hightemperature creep resistance remains among the key issues that must be addressed before using these materials in advanced aero- 0266-3538/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.compscitech.2009.01.002 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 3636; fax: +1 937 656 7053. E-mail address: marina.ruggles-wrenn@afit.edu (M.B. Ruggles-Wrenn). Composites Science and Technology 69 (2009) 663–669 Contents lists available at ScienceDirect Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
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 the
space 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 composite, manufactured by COI Ceramics (San Diego, CA), was supplied 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 laminate 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 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. Tests in argon environment 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 alumina cement (Zircar) to calibrate the furnace on a periodic basis. The furnace controllers (using non-contacting 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. 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 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. Dog bone shaped specimens 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 temperature in 25 min, and held at temperature for additional 15 min prior to testing. In air, tensile tests were performed in stroke control 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 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. 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 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 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 similar 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 tensile behavior and tensile properties of the N720/A composite were strongly influenced by the loading rate. In the present study the effects 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 modulus and failure strain were 150 MPa, 60.1 GPa, and 0.52%, respectively. 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
M.B. Ruggles-Wrenn, CL. Genelin/Composites Science and Technology 69(2009)663-669 Following the procedure in 30,31, experimental data obtained in steam were plotted as log(UTS)vS log(applied stress rate). De- 25 MPa/s spite the limited number of tensile tests conducted at different 150 stress rates in steam, the fit of data to Eq. 4) was good with a coef- ficient of correlation in regression >0.980. This implies that the de- 0.0025MPa/s layed failure of N720 AM in steam at 1200C can be described by the empirical power law in Eq(1). Hence the parameters n=18 and D= 128. 13 were determined by a linear from the slope and intercept, respectively The crack growth exponent n represents a measure of suscepti- bility of the material to subcritical crack growth. Typically for brit- T=1200°c, Steam tle materials, the susceptibility is considered high for n50. Thus at1200° 0.00 250.500.751001251.50 in steam the n720 AM composite exhibits significant susceptibility Strain(%) to subcritical crack growth. Similar results showing relatively high Fig. 2. Tensile stress-strain curves for N720/AM ceramic composite obtained in susceptibility to delayed failure at 1200C with n=12 were re- ests conducted with loading rates of 0.0025 and 25 MPa/s at 1200"C in steam. ported for N720/A composite [23]. In addition, Milz et al. [32] Effect of loading rate on stress-strain behavior and ultimate tensile strength is and Goering et al. 33] found that the N720 fibers exhibited consid erable susceptibility to delayed failure at temperatures >1000C, with the values of n ranging from 9 to 18. Choi et al. [24, 25] found 0.05 mm/s in air, modulus values are nearly 20% lower and failure that several non-oxide Cmcs exhibited significant susceptil stains are 53% higher than the corresponding values obtained in temperatures >1100C, with the n values ranging from 6 to 20 ir. Note that for N720/AM at 1200C, the displacement rate of 0.05 mm/s is equivalent to the stress rate of 25 MPa/s provi 3. 2. Creep-rupture elastic material behavior is assumed A change in loading rate by four orders or magnitude has a sig- Results of the creep-rupture tests for N720 /AM composite ar nificant effect on tensile stress-strain response and tensile proper presented in Table 1 and in Fig 3. Creep curves produced in all ties at 1200C in steam. As seen in Fig. 2, the tensile stress-strain tests conducted in air exhibit primary and secondary creep re- curves produced at 0.0025 MPa/s are strongly nonlinear. The gimes, but no tertiary creep Transition from primary to secondary 0.0025 MPa s stress-strain curves depart from linearity at a low creep occurs early in creep life, primary creep persists during the stress of 20 MPa. As the stress continues to increase, appreciable first w10 h of the creep test. Creep strain decreases as the applied elastic strains develop The failure strains ranging from 0.97 to stress increases from 73 to 136 MPa. Creep strains accumulate 26% are nearly two to four times those obtained at 25 MPa/s In at stresses 91 MPa exhibited by this composite at 1200C in steam is similar to that where only primary and secondary creep regimes are observed. exhibited by monolithic ceramics at elevated temperature In contrast, creep curves obtained at 73 MPa in argon and in steam [24, 25).In the case of monolithic ceramics, the time(rate)-depen- show primary, secondary and tertiary creep In argon, transition dence of strength(also known as delayed failure)has been shown from secondary to tertiary creep occurs after w70 h of creep life. to proceed by environmentally-assisted subcritical crack growth In steam, the secondary creep transitions to tertiary creep after 6-28]. The subcritical crack growth rate can be described by only x10 h. Moreover, the test environment has a significant effect the empirical power law [29-31 on creep strains accumulated at stresses <91 MPa. At 73 MPa, the creep strain produced in argon is nearly 10 times and the creep (1) strain produced in steam, four times that accumulated in air At 91 MPa, the presence of argon increases creep strain by a factor where a is the crack size, t is time, K is the mode I stress intensity factor, Kic is the critical stress intensity factor re toug ess)under mode I loading, and a and n are th parameters In the case of loading at constant st he frac Table ture strength af can be derived as a function of applied stress rate Summary of creep-rupture results for the N720/AM ceramic composite at 1200.C in o as follows [29-31: laboratory air, argon, and steam environments Cr= D(a)(n+1) (2) Test environment Creep stress(MPa) Creep strain(%) Time to rupture(h Here d is a crack growth parameter, associated with inert N720/alumina-mullite strength a, n and crack geometry, given by 04 2(n+1)o where Y is a geometry fac lated to flaw shape and its orienta tion with respect to the of applied stress. By taking loga- ste rithms of both sides Eq be expressed Steam Steam 0.11 log a t log d
0.05 mm/s in air, modulus values are nearly 20% lower and failure stains are 53% higher than the corresponding values obtained in air. Note that for N720/AM at 1200 C, the displacement rate of 0.05 mm/s is equivalent to the stress rate of 25 MPa/s provided elastic material behavior is assumed. A change in loading rate by four orders or magnitude has a significant effect on tensile stress–strain response and tensile properties at 1200 C in steam. As seen in Fig. 2, the tensile stress–strain curves produced at 0.0025 MPa/s are strongly nonlinear. The 0.0025 MPa/s stress–strain curves depart from linearity at a low stress of 20 MPa. As the stress continues to increase, appreciable inelastic strains develop. The failure strains ranging from 0.97 to 1.26% are nearly two to four times those obtained at 25 MPa/s. In contrast, the average UTS value of 94 MPa is 39% lower than the average strength value obtained at 25 MPa/s. The strong dependence of tensile strength on loading rate exhibited by this composite at 1200 C in steam is similar to that exhibited by monolithic ceramics at elevated temperatures [24,25]. In the case of monolithic ceramics, the time (rate)-dependence of strength (also known as delayed failure) has been shown to proceed by environmentally-assisted subcritical crack growth [26–28]. The subcritical crack growth rate can be described by the empirical power law [29–31]: da dt ¼ A KI KIC n ð1Þ where a is the crack size, t is time, KI is the mode I stress intensity factor, KIC is the critical stress intensity factor (or fracture toughness) under mode I loading, and A and n are the slow crack growth parameters. In the case of loading at constant stress rate, the fracture strength rf can be derived as a function of applied stress rate r_ as follows [29–31]: rf ¼ Dðr_ Þ 1=ðnþ1Þ ð2Þ Here D is a crack growth parameter, associated with inert strength ri, n and crack geometry, given by: D ¼ 2ðn þ 1ÞK2 IC rn2 i AY2 ðn 2Þ " #1=ðnþ1Þ ð3Þ where Y is a geometry factor related to flaw shape and its orientation with respect to the direction of applied stress. By taking logarithms of both sides Eq. (2) can be expressed as: logrf ¼ 1 n þ 1 logr_ þ logD ð4Þ Following the procedure in [30,31], experimental data obtained in steam were plotted as log (UTS) vs. log (applied stress rate). Despite the limited number of tensile tests conducted at different stress rates in steam, the fit of data to Eq. (4) was good with a coef- ficient of correlation in regression >0.980. This implies that the delayed failure of N720/AM in steam at 1200 C can be described by the empirical power law in Eq. (1). Hence the parameters n = 18 and D = 128.13 were determined by a linear regression analysis from the slope and intercept, respectively. The crack growth exponent n represents a measure of susceptibility of the material to subcritical crack growth. Typically for brittle materials, the susceptibility is considered high for n 6 20, intermediate for 30 6 n 6 50, and low for n > 50. Thus at 1200 C in steam the N720/AM composite exhibits significant susceptibility to subcritical crack growth. Similar results showing relatively high susceptibility to delayed failure at 1200 C with n = 12 were reported for N720/A composite [23]. In addition, Milz et al. [32] and Goering et al. [33] found that the N720 fibers exhibited considerable susceptibility to delayed failure at temperatures P1000 C, with the values of n ranging from 9 to 18. Choi et al. [24,25] found that several non-oxide CMCs exhibited significant susceptibility at temperatures P1100 C, with the n values ranging from 6 to 20. 3.2. Creep–rupture Results of the creep–rupture tests for N720/AM composite are presented in Table 1 and in Fig. 3. Creep curves produced in all tests conducted in air exhibit primary and secondary creep regimes, but no tertiary creep. Transition from primary to secondary creep occurs early in creep life, primary creep persists during the first 10 h of the creep test. Creep strain decreases as the applied stress increases from 73 to 136 MPa. Creep strains accumulated at stresses 6114 MPa considerably exceed the failure strain obtained in the tension test. The test environment appears to have little influence on the appearance of the creep curves obtained at stresses P91 MPa, where only primary and secondary creep regimes are observed. In contrast, creep curves obtained at 73 MPa in argon and in steam show primary, secondary and tertiary creep. In argon, transition from secondary to tertiary creep occurs after 70 h of creep life. In steam, the secondary creep transitions to tertiary creep after only 10 h. Moreover, the test environment has a significant effect on creep strains accumulated at stresses 691 MPa. At 73 MPa, the creep strain produced in argon is nearly 10 times and the creep strain produced in steam, four times that accumulated in air. At 91 MPa, the presence of argon increases creep strain by a factor Fig. 2. Tensile stress–strain curves for N720/AM ceramic composite obtained in tests conducted with loading rates of 0.0025 and 25 MPa/s at 1200 C in steam. Effect of loading rate on stress–strain behavior and ultimate tensile strength is evident. Table 1 Summary of creep–rupture results for the N720/AM ceramic composite at 1200 C in laboratory air, argon, and steam environments. Test environment Creep stress (MPa) Creep strain (%) Time to rupture (h) N720/alumina–mullite Air 73 0.60 >100a Air 91 0.59 >100a Air 114 0.47 22.3 Air 136 0.28 0.59 Argon 73 5.86 92.8 Argon 91 2.99 18.8 Argon 114 0.66 0.45 Argon 136 0.62 0.07 Steam 73 2.49 37.0 Steam 91 1.57 4.18 Steam 114 0.48 0.38 Steam 136 0.11 0.01 a Run-out. M.B. Ruggles-Wrenn, C.L. Genelin / Composites Science and Technology 69 (2009) 663–669 665
M.B. Ruggles-Wrenn, CL. Genelin/Composites Science and Technology 69(2009)663-669 200H O N720/AN MPa, Argon 73 MPa, Argon 91 MPa. Air 73 MPa Air E031E-021.E-011E+001E+011E+021E+03 Time(h) 08 Fig. 5. Creep stress vs time to rupture for N720/AM and N720 A CMCs at 1200" in T=1200° team. Data for N720/A from Ruggles-Wrenn et al. [34 All data are 114 MPa, Argon 136 MPa, Argon adjusted for V,=0.40. 14 MPa Steam is nearly two orders of magnitude higher than that in air. In steam, the n20/ AM creep rates are slightly lower than those observed for N720/A, especially for applied stress levels 95% for applied stress levels of 5, while the presence of steam increases creep strain by nearly a >91 MPa, and 63% for the applied stress of 73 MPa In steam, creep actor of 3 run-out was not achieved In argon, specimen tested at 73 MPa sur Minimum creep rate was reached in all tests. Creep rate as a vived 92.8 h, almost achieving creep run-out of 100h.However,at function of applied stress is presented in Fig. 4, where results for stresses >91 MPa, the presence of argon degraded creep lifetimes N720/A from prior work [34] are included for comparison. In air, by at least 80%. It is notable that in air and in steam the creep life- 73 MPa to 136 MPa In ai, thie the creep stress incretoxim mom times of N720) AM were similar to those of N720/A.Conversely, ar- secondary creep rate of N720/AM the two composites. The presence of had a beneficial effect is approximately an order of magnitude lower than that of N720/ on creep performance of N720/A, increasing the creep lifetimes A for a given creep stress. The N720/AM creep rates increase dra- at least twofold matically in steam. For a given creep stress, the creep rate in steam Retained strength and modulus of the specimens that achieved creep run-out in air are summarized in Table 2. Tensile stress- strain curves obtained for the specimens subjected to prior creep 1.0E-04 are presented in Fig. 6 together with the tensile stress-strain curve M,Ar■N720A.Ai for the as-processed material. Prior creep appears to have a bene- 10E05△N720/ AM, Argon ficial effect on tensile strength. The strength of the N720 /AM spec- imens subjected to 100 h of prior creep in air was x10% higher than the uts of the untested material. Nevertheless a reduction g10E06 in modulus was observed. Modulus loss due to prior creep a 73 MPa was 6%, and modulus loss due to creep at 91 MPa was 9%. Results of the present study reveal that at 1200C the presence 西1.0E07 of steam dramatically reduces creep lifetimes of N720/AM. Because the creep performance of the composite with 0/90 orientation is 1.0E08 dominated by the fibers, fiber degradation is a likely source of the T=1200°c omposite degradation. It is possible that environmentally-assisted subcritical crack growth in the n720 fibers is the mechanism be- 405060708090100 00 hind reduced creep resistance of N720 /AM composite at 1200C in steam. In this case, subcritical(slow crack growth in the fiber Max Stress(MPa) is caused by a chemical interaction of water molecules with Fig 4. Minimum creep mechanically strained Si-o bonds at the crack ramic composites at 1200.C in air, argon and steam. Data for N720/A from of chemical reaction increasing exponentially with applied stress Ruggles-Wrenn et al. [34]. All data are adjusted for Vr=0. 40. 27,35-42
of 5, while the presence of steam increases creep strain by nearly a factor of 3. Minimum creep rate was reached in all tests. Creep rate as a function of applied stress is presented in Fig. 4, where results for N720/A from prior work [34] are included for comparison. In air, the minimum creep rate of N720/AM increases by approximately two orders of magnitude as the creep stress increases from 73 MPa to 136 MPa. In air, the secondary creep rate of N720/AM is approximately an order of magnitude lower than that of N720/ A for a given creep stress. The N720/AM creep rates increase dramatically in steam. For a given creep stress, the creep rate in steam is nearly two orders of magnitude higher than that in air. In steam, the N720/AM creep rates are slightly lower than those observed for N720/A, especially for applied stress levels 6114 MPa. It is noteworthy that the presence of argon also accelerates the creep rates of N720/AM. The N720/AM creep rates obtained in argon are close to those produced in steam for a given applied stress. In contrast, the presence of argon did not increase the N720/A creep rates. In argon, creep rates of N720/A were somewhat lower than those obtained in air. Stress–rupture behavior is summarized in Fig. 5, where results for N720/A from prior work [34] are also included. As expected, creep life decreases with increasing applied stress. In air, the creep run-out stress for N720/AM was 91 MPa. The presence of steam drastically reduced the creep lifetimes of N720/AM. The reduction in creep life due to steam was P95% for applied stress levels P91 MPa, and 63% for the applied stress of 73 MPa. In steam, creep run-out was not achieved. In argon, specimen tested at 73 MPa survived 92.8 h, almost achieving creep run-out of 100 h. However, at stresses P91 MPa, the presence of argon degraded creep lifetimes by at least 80%. It is notable that in air and in steam the creep lifetimes of N720/AM were similar to those of N720/A. Conversely, argon environment had opposing effects on the creep performance of the two composites. The presence of argon had a beneficial effect on creep performance of N720/A, increasing the creep lifetimes at least twofold. Retained strength and modulus of the specimens that achieved creep run-out in air are summarized in Table 2. Tensile stress– strain curves obtained for the specimens subjected to prior creep are presented in Fig. 6 together with the tensile stress–strain curve for the as-processed material. Prior creep appears to have a bene- ficial effect on tensile strength. The strength of the N720/AM specimens subjected to 100 h of prior creep in air was 10% higher than the UTS of the untested material. Nevertheless, a reduction in modulus was observed. Modulus loss due to prior creep at 73 MPa was 6%, and modulus loss due to creep at 91 MPa was 9%. Results of the present study reveal that at 1200 C the presence of steam dramatically reduces creep lifetimes of N720/AM. Because the creep performance of the composite with 0/90 orientation is dominated by the fibers, fiber degradation is a likely source of the composite degradation. It is possible that environmentally-assisted subcritical crack growth in the N720 fibers is the mechanism behind reduced creep resistance of N720/AM composite at 1200 C in steam. In this case, subcritical (slow) crack growth in the fiber is 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 [27,35–42]. 0 1 2 3 4 5 0 10 20 30 40 50 Strain (%) Time (h) 73 MPa, Air T = 1200 ºC 91 MPa, Steam 73 MPa, Argon 91 MPa, Argon 91 MPa, Air 73 MPa, Steam 0.0 0.2 0.4 0.6 0.8 0.0 0.2 0.4 0.6 0.8 Strain (%) Time (h) 114 MPa, Air 136 MPa, Air 114 MPa, Argon 114 MPa, Steam T = 1200 ºC 136 MPa, Argon 136 MPa Steam a b Fig. 3. Creep strain vs. time curves for N720/alumina–mullite composite at 1200 C in air, steam and argon: (a) at 73 and 91 MPa and (b) at 114 and 136 MPa. 1.0E-09 1.0E-08 1.0E-07 1.0E-06 1.0E-05 1.0E-04 40 Strain Rate (s-1) Max Stress (MPa) N720/AM, Air N720/A, Air N720/AM, Steam N720/A, Steam N720/AM, Argon N720/A, Argon T = 1200 ºC 50 60 70 80 90 100 200 Fig. 4. Minimum creep rate as a function of applied stress for N720/AM and N720/A ceramic composites at 1200 C in air, argon and steam. Data for N720/A from Ruggles-Wrenn et al. [34]. All data are adjusted for Vf = 0.40. 0 50 100 150 200 250 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 Stress (MPa) Time (h) N720/AM, Air N720/A, Air N720/AM, Argon N720/A, Argon N720/AM, Steam N720/A, Steam N720/AM, Steam-Prediction N720/A, Steam-Prediction T = 1200 ºC Fig. 5. Creep stress vs. time to rupture for N720/AM and N720/A CMCs at 1200 C in air, argon and steam. Data for N720/A from Ruggles-Wrenn et al. [34]. All data are adjusted for Vf = 0.40. 666 M.B. Ruggles-Wrenn, C.L. Genelin / Composites Science and Technology 69 (2009) 663–669
M.B. Ruggles-Wrenn CL Genelin/Composites Science and Technology 69(2009)663-669 2 ined properties of the N720/AM specimens subjected to prior creep at 1200C in air. Creep stress(MPa) Retained strength(MPa) Strength retention(%) Strain at failure (% 109 0.34 tr 100 h at 91 MPa Air where t and o are time to failure and applied creep stress, Fig 5 shows the predicted creep lifetimes at 1200C in steam N720 AM composite(solid line) together with the prediction for the N720 /A composite(dashed line)from prior work [23]. Good agreement between the prediction and the experimental results indicates that the environmentally-assisted slow crack growth is T=1200°c indeed the governing failure mechanism for both CMCs at 1200C in steam. These findings point to the environmentally-as- 0 sisted subcritical crack growth in N720 fibers as the mechanism Strain(%) behind the degraded creep performance of N720/ AM at 1200C Fig. 6. Effects of prior creep at 1200 C in laboratory air on tensile stress-strain in steam. However, further experiments would be required to ehavior of N720/AM understand the mechanism responsible for the considerable degr dation of the creep performance in argon ow)crack growth as the dominant time-dependent failure 3.3. Composite microstructure mechanism, time to failure under constant stress(creep lifetime) can be predicted from constant stress-rate test data by using the Optical micrographs of the fracture surfaces obtained in the 73 linear elastic crack growth model [24, 25]. Using the empirical and 136 MPa creep tests are shown in Fig. 7. The N720/AM speci- power-law crack-velocity formulation in Eq (1)the time to failure nens tested at 73 MPa( Fig. 7a-c)show a somewhat greater degree under constant stress can be obtained in the form 43 of uncorrelated fiber fracture than those tested at 136 MPa( Fig 7d-f). Furthermore, specimens tested at 73 MPa have noticeably 2Kco-o (5) have little effect on the fracture surface topography. For a given creep stress, fracture surfaces obtained in steam or argon are sim- terms of the crack growth parameters obtained from constant A previous study [23 revealed that for N720/A composite tress-rate data as tested at 1200 C in air and in steam the fracture surface appear A () c 10 mm 0 10 mm 10 mm 10 mm 0 mm 10 mm Fig. 7. Optical micrographs of the fracture surfaces of specimens tested in creep at 1200": (a)at 73 MPa in air, (b )at 73 MPa in argon, (c)at 73 MPa in steam, (d)at 136 MPa in air, (e)at 136 MPa in argon, and(f)at 136 MPa in steam
For many glass and ceramic materials that exhibit subcritical (slow) crack growth as the dominant time-dependent failure mechanism, time to failure under constant stress (creep lifetime) can be predicted from constant stress-rate test data by using the linear elastic crack growth model [24,25]. Using the empirical power-law crack-velocity formulation in Eq. (1) the time to failure under constant stress can be obtained in the form [43]: tf ¼ 2K2 ICrn2 i AY2 ðn 2Þ " #rn ð5Þ Combining Eqs. (5) and (3), creep lifetime can be expressed in terms of the crack growth parameters obtained from constant stress-rate data as: tf ¼ Dnþ1 ðn þ 1Þ " #rn ð6Þ where tf and r are time to failure and applied creep stress, respectively. Fig. 5 shows the predicted creep lifetimes at 1200 C in steam for N720/AM composite (solid line) together with the predictions for the N720/A composite (dashed line) from prior work [23]. Good agreement between the prediction and the experimental results indicates that the environmentally-assisted slow crack growth is indeed the governing failure mechanism for both CMCs at 1200 C in steam. These findings point to the environmentally-assisted subcritical crack growth in N720 fibers as the mechanism behind the degraded creep performance of N720/AM at 1200 C in steam. However, further experiments would be required to understand the mechanism responsible for the considerable degradation of the creep performance in argon. 3.3. Composite microstructure Optical micrographs of the fracture surfaces obtained in the 73 and 136 MPa creep tests are shown in Fig. 7. The N720/AM specimens tested at 73 MPa (Fig. 7a–c) show a somewhat greater degree of uncorrelated fiber fracture than those tested at 136 MPa (Fig. 7d–f). Furthermore, specimens tested at 73 MPa have noticeably longer damage zones. Notably the test environment appears to have little effect on the fracture surface topography. For a given creep stress, fracture surfaces obtained in steam or argon are similar to those obtained in air. A previous study [23] revealed that for N720/A composite tested at 1200 C in air and in steam, the fracture surface appearTable 2 Retained properties of the N720/AM specimens subjected to prior creep at 1200 C in air. Creep stress (MPa) Retained strength (MPa) Strength retention (%) Retained modulus (GPa) Modulus retention (%) Strain at failure (%) 73 169 110 70.1 94 0.35 91 167 109 67.5 91 0.34 0 50 100 150 200 250 0.0 0.1 0.2 0.3 0.4 0.5 Stress (MPa) Strain (%) T = 1200ºC As-Processed 100 h at 91 MPa, Air 100 h at 73 MPa, Air Fig. 6. Effects of prior creep at 1200 C in laboratory air on tensile stress–strain behavior of N720/AM. Fig. 7. Optical micrographs of the fracture surfaces of specimens tested in creep at 1200 C: (a) at 73 MPa in air, (b) at 73 MPa in argon, (c) at 73 MPa in steam, (d) at 136 MPa in air, (e) at 136 MPa in argon, and (f) at 136 MPa in steam. M.B. Ruggles-Wrenn, C.L. Genelin / Composites Science and Technology 69 (2009) 663–669 667
L.B. Ruggles-Wrenn, CL. Genelin/Composit 500 空(b 1.0mm c 2.0mm (a) 0 mm 500 f 10m Fig 8. SEM micrographs of the fracture surfaces of specimens tested in creep at 1200C: (a)at 73 MPa in air. ( b)at 73 MPa in argon, (c)at 73 MPa in steam, (d)at 136 MPa in air,(e)at 136 MPa in argon, and (f at 136 MPa in steam. ance could be correlated with the failure time, with a predomi- strain obtained in the tension test. Primary, secondary and tertiary nantly planar fracture surface corresponding to a short life and fi- creep regimes are observed in argon and in steam. Creep strains brous fracture indicating longer life. In the case of N720/A, the accumulated at stresses 91 MPa. in Fig. Sa achieved a 100-h creep run-out and failed in a subse- The n720 AM fracture surfaces obtained at 1200C are domi- quent tensile test, while the specimen in Fig. 8d failed after mere nated by regions of planar fracture. The near-planar fracture su 36 s of creep faces suggest the loss of matrix porosity and subsequent matrix densification due to additional sintering. The fracture surface 4. Concluding remarks appearance cannot be directly correlated with the creep lifetime The tensile stress-strain behavior of the N720/ AM composite Acknowledgement was investigated and the tensile properties measured at 1200C. The uts was 153 MPa. the elastic modulus was 74. 5 GPa and the The financial support of the Air Force Research Laboratory, Pro- failure strain was 0.34%. The stress-strain curve was nearly linear pulsion Directorate(Dr. R. Sikorski and Dr. J. Zelina)is highly The influence of loading rate was explored at 1200C in steam, in tests conducted with constant loading rates of 0.0025 and References 25 MPa/s. The tensile properties were stre influenced by the ding rate. As the loading rate decreases by four orders of magn [11 Zok F. Developments in oxide fiber composites. J Am Ceram Soc de, the uts drops by 39%. At 0.0025 MPa/ s, the tensile stress- [21 Zawada LP, Staehler J. Steel S. Consequence of intermittent exposure to strain behavior is distinctly nonlinear. Considerable inelastic strains develop as the stress exceeds 20 MPa. The creep-rupture behavior of the N720/ AM composite [3I Schmidt S, Beyer S, Knabe H, Immich H, Meistring R, Gessler A. Advanced rials for current and future propulsion characterized for stress levels ranging from 73 to 136 MPa at in air, argon and steam environments. In air N720/ AM 41 Szweda A Millard ML Harrison MG. Fiber-reinforced ceramic-matrix primary and secondary creep regimes. Creep strains accu- 5 Sim SM, Kerans R). Slurry infiltration and 3-D woven composites Ceram Eng at stresses <114 MPa considerably exceed the failure Sci Proc1992:13(9-10):632-41
ance could be correlated with the failure time, with a predominantly planar fracture surface corresponding to a short life and fi- brous fracture indicating longer life. In the case of N720/A, the near-planar fracture surfaces were attributed to matrix densification and subsequent loss of matrix porosity, which resulted in decreased damage tolerance. In contrast, the SEM micrographs of the N720/AM fracture surfaces obtained in creep tests at 1200 C in air, argon and steam (Fig. 8) show that for N720/AM, the fracture surface appearance cannot be directly correlated with the creep lifetime. All fracture surfaces in Fig. 8 are dominated by planar regions of coordinated fiber failure. Compare the fracture surface of the specimen tested at 73 MPa in air (Fig. 8a) and that of the specimen tested at 136 MPa in steam (Fig. 8d). The two fracture surfaces have essentially the same topography. Yet the specimen in Fig. 8a achieved a 100-h creep run-out and failed in a subsequent tensile test, while the specimen in Fig. 8d failed after mere 36 s of creep. 4. Concluding remarks The tensile stress–strain behavior of the N720/AM composite was investigated and the tensile properties measured at 1200 C. The UTS was 153 MPa, the elastic modulus was 74.5 GPa, and the failure strain was 0.34%. The stress–strain curve was nearly linear to failure. The influence of loading rate was explored at 1200 C in steam, in tests conducted with constant loading rates of 0.0025 and 25 MPa/s. The tensile properties were strongly influenced by the loading rate. As the loading rate decreases by four orders of magnitude, the UTS drops by 39%. At 0.0025 MPa/s, the tensile stress– strain behavior is distinctly nonlinear. Considerable inelastic strains develop as the stress exceeds 20 MPa. The creep–rupture behavior of the N720/AM composite was characterized for stress levels ranging from 73 to 136 MPa at 1200 C in air, argon and steam environments. In air N720/AM exhibits primary and secondary creep regimes. Creep strains accumulated at stresses 6114 MPa considerably exceed the failure strain obtained in the tension test. Primary, secondary and tertiary creep regimes are observed in argon and in steam. Creep strains accumulated at stresses 691 MPa in argon and in steam are significantly larger than those produced in air. Minimum creep rate was reached in all tests. Creep strain rates range from 9.2 109 to 8.7 107 s1 in air. The presence of steam or argon accelerates creep rates of N720/AM by nearly two orders of magnitude. Creep run-out of 100 h was achieved at applied stress levels 691 MPa in air. The run-out specimens exhibited an increase in strength, but stiffness loss of up to 9% was observed. The presence of steam or argon dramatically reduced creep lifetimes. The reduction in creep lifetime due to steam was 63% at 73 MPa and 98% at 136 MPa. The reduction in creep lifetimes due to argon was at least 80% at stresses P91 MPa. The N720/AM fracture surfaces obtained at 1200 C are dominated by regions of planar fracture. The near-planar fracture surfaces suggest the loss of matrix porosity and subsequent matrix densification due to additional sintering. The fracture surface appearance cannot be directly correlated with the creep lifetime. Acknowledgement The financial support of the Air Force Research Laboratory, Propulsion Directorate (Dr. R. Sikorski and Dr. J. Zelina) is highly appreciated. References [1] Zok F. Developments in oxide fiber composites. J Am Ceram Soc 2006;89(11):3309–24. [2] Zawada LP, Staehler J, Steel S. Consequence of intermittent exposure to moisture and salt fog on the high-temperature fatigue durability of several ceramic–matrix composites. J Am Ceram Soc 2003;86(8):1282–91. [3] Schmidt S, Beyer S, Knabe H, Immich H, Meistring R, Gessler A. Advanced ceramic matrix composite materials for current and future propulsion technology applications. Acta Astronaut 2004;55:409–20. [4] Szweda A, Millard ML, Harrison MG. Fiber-reinforced ceramic–matrix composite member and method for making. US Pat. No. 5 601674; 1997. [5] Sim SM, Kerans RJ. Slurry infiltration and 3-D woven composites. Ceram Eng Sci Proc 1992;13(9–10):632–41. Fig. 8. SEM micrographs of the fracture surfaces of specimens tested in creep at 1200 C: (a) at 73 MPa in air, (b) at 73 MPa in argon, (c) at 73 MPa in steam, (d) at 136 MPa in air, (e) at 136 MPa in argon, and (f) at 136 MPa in steam. 668 M.B. Ruggles-Wrenn, C.L. Genelin / Composites Science and Technology 69 (2009) 663–669
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