Composites Science and Technology 68(2008)2260-2266 Contents lists available at Science Direct Composites Science and Technology ELSEVIER journalhomepagewww.elsevier.com/locate/compscitech Creep behavior in interlaminar shear of NextelmM720/ alumina ceramic composite at elevated temperature in air and in steam" M.B. Ruggles-Wrenn, P D. Laffey Department of Aeronautics and Astronautics, Air Force institute of Technology, wright-Patterson Air Force Base, OH 45433-7765, United States ARTICLE INFO A BSTRACT The creep behavior in interlaminar shear of an oxide-oxide ceramic matrix composite(CMc) eceived 14 March 2008 ated at 1200C in laboratory air and in steam using double-notch shear test specimens. The ceived in revised form 4 April 2008 Accepted 9 April 2008 consists of a porous alumina matrix reinforced with laminated, woven mullite/alumina(Ne Available online 15 April 2008 fibers, has no interface between the fiber and matrix, and relies on the porous matrix for fla The interlaminar shear properties were measured. The creep behavior was examined for interlamir near stresses in the 4-6.5 MPa range. Primary and secondary creep regimes were observed in all tests A Ceramic-matrix composites(CMCs onducted in air. In steam, the composite exhibited primary, secondary and tertiary creep In air, creep run-out defined as 100 h at creep stress was achieved in all tests. In the presence of steam, creep perfor- mance deteriorated rapidly and run-out was achieved only at 4 MPa (50% of the interlaminar shear B High-temperature properties strength at 1200C). The retained properties of all specimens that achieved run-out were characterized. D Fractography Composite microstructure, as well as damage and failure mechanisms were investigated. Matrix degra- dation appears to be the cause of reduced creep lifetimes in steam Published by Elsevier Ltd. 1 Introduction development of CMCs based on environmentally stable oxide con- stituents 6-11 Advances in power generation systems for aircraft engines, The main advantage of CMCs over monolithic ceramics is their land-based turbines, rockets, and, most recently, hypersonic mis- superior toughness, tolerance to the presence of cracks and de- siles and flight vehicles have raised the demand for structural fects, and non-catastrophic mode of failure. It is widely accepted materials that have superior long-term mechanical properties that in order to avoid brittle fracture behavior in CMCs and im- and retained properties under high temperature, high pressure, prove the damage tolerance, a weak fiber/matrix interface and varying environmental factors, such as moisture [1. Typical needed which serves to deflect matrix cracks and to allow subse- omponents include combustors, nozzles and thermal insulati quent fiber pullout [12-14]. It has been demonstrated that similar Ceramic-matrix composites(CMCs), capable of maintaining excel- crack-deflecting behavior can also be achieved by means of a fi lent strength and fracture toughness at high temperatures are nely distributed porosity in the matrix instead of a separate inter- prime candidate materials for such applications. Additionally, low- face between matrix and fibers [15. This microstructural design with a reduced need for cooling air, allow for improved high-ten The concept has been successfully demonstrated for oxide -oxide perature performance when compared to conventional nickel- composites [6,9, 11, 16, 17]. Resulting oxide/oxide CMCs exhibit based superalloys [2]. Concurrent efforts in optimization of the damage tolerance combined with inherent oxidation resistance. CMCs and in design of the combustion chamber are expected to An extensive review of the mechanisms and mechanical proper accelerate the insertion of the Cmcs into aerospace turbine engine ties of porous-matrix CMCs is given in [18, 19 tions require exposure to oxidizing environments, the thermody- with monolithic ceramics, two-dimensional laminated CMCpared pplications, such as combustor walls 3-5]. Because these applica While CMCs exhibit improved damage tolerance cor namic stability and oxidation resistance of CMCs are vital issues. more susceptible to failure in the matrix-rich interlaminar regions. The need for environmentally stable composites motivated the The interlaminar failure or delamination may ultimately lead to loss of stiffness and accelerate structural failure [20]. A number of studies assessed the behavior of CMCs in shear [20-23]. choi expressed are those of the authors and do not reflect the official tion of the United States Air Force, Department of Defense or the Us et al.[24-27]assessed the high-temperature life limiting behavior in interlaminar shear of several non-oxide CMCs, including three ding author.Tel:+19372553636x4641;fax:+19376567053 Sic fiber-reinforced CMCs and one carbon -fiber-reinforced cmc .ruggles-wrenn@afit.edu(MB. Ruggles-Wrenn) Choi and co-workers determined the interlaminar shear strength ront matter Published by Elsevier Ltd
Creep behavior in interlaminar shear of NextelTM720/alumina ceramic composite at elevated temperature in air and in steamq M.B. Ruggles-Wrenn *, P.D. Laffey Department of Aeronautics and Astronautics, Air Force Institute of Technology, Wright-Patterson Air Force Base, OH 45433-7765, United States article info Article history: Received 14 March 2008 Received in revised form 4 April 2008 Accepted 9 April 2008 Available online 15 April 2008 Keywords: A. Ceramic–matrix composites (CMCs) A. Oxides B. Creep B. High-temperature properties D. Fractography abstract The creep behavior in interlaminar shear of an oxide–oxide ceramic matrix composite (CMC) was evaluated at 1200 C in laboratory air and in steam using double-notch shear test specimens. 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 interlaminar shear properties were measured. The creep behavior was examined for interlaminar shear stresses in the 4–6.5 MPa range. Primary and secondary creep regimes were observed in all tests conducted in air. In steam, the composite exhibited primary, secondary and tertiary creep. In air, creep run-out defined as 100 h at creep stress was achieved in all tests. In the presence of steam, creep performance deteriorated rapidly and run-out was achieved only at 4 MPa (50% of the interlaminar shear strength at 1200 C). The retained properties of all specimens that achieved run-out were characterized. Composite microstructure, as well as damage and failure mechanisms were investigated. Matrix degradation appears to be the cause of reduced creep lifetimes in steam. 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]. 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 fi- nely distributed porosity in the matrix instead of a separate 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 porous-matrix CMCs is given in [18,19]. While CMCs exhibit improved damage tolerance compared with monolithic ceramics, two-dimensional laminated CMCs are more susceptible to failure in the matrix-rich interlaminar regions. The interlaminar failure or delamination may ultimately lead to loss of stiffness and accelerate structural failure [20]. A number of studies assessed the behavior of CMCs in shear [20–23]. Choi et al. [24–27] assessed the high-temperature life limiting behavior in interlaminar shear of several non-oxide CMCs, including three SiC fiber-reinforced CMCs and one carbon-fiber-reinforced CMC. Choi and co-workers determined the interlaminar shear strength 0266-3538/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.compscitech.2008.04.009 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. Ruggles-Wrenn). Composites Science and Technology 68 (2008) 2260–2266 Contents lists available at ScienceDirect Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
MB. Ruggles-Wrenn, P D. Laffey Composites Science and Technology 68(2008)2260-2266 as a function of test rate using double-notch shear specimens and consisting of a porous alumina matrix reinforced with Nextel M720 emonstrated that the interlaminar shear strength (ILSS)degraded fibers. There is no fiber coating. The composite was supplied in a with decreasing test rate. A phenomenological, power-law based form of 5.2-mm thick plates comprised of 24 00/90 woven layers crack growth model was proposed to account for the degradation with a density of 2.83 g/cm, a fiber volume of 46.6%, and of the IlSS of the composite at elevated temperatures. High-tem- matrix porosity of w%. The fiber fabric was infiltrated with the perature stress-rupture tests in interlaminar shear were employed matrix in a sol-gel process. the laminate was dried with a"vac- to validate the proposed model. These studies focused on the non- uum bag"technique under low pressure and low temperature, oxide CMcs with matrix-dense interlaminar regions, where the and then pressureless sintered [32]. Representative micrograph interlaminar failure is controlled by the fiber-matrix interface In of the untested material is presented in Fig. la, which shows 0o the case of the porous-matrix oxide-oxide CMCs, interlaminar and 90 fiber tows as well as numerous matrix cracks In the case shear failure is controlled by the exceptionally weak porous of the as-processed material, most are shrinkage cracks formed matrix. The objective of this effort is to evaluate the interlaminar during processing. Porous nature of the matrix is seen in Fig. 1b shear strength and to investigate the creep behavior in interlami The double-notch shear(DNS)test specimens measuring ar shear of an oxide-oxide Cmc consisting of a porous alumina 150 mm x 20 mm were cut from a single N720 A panel. The thick- matrix reinforced with the Nextel 720 fibers. Several previous ness of the specimens was the same as the nominal thickness of studies examined the in-plane mechanical behavior of this com- the composite panel, i.e. N5.2 mm. The notches of 0.5 mm width posite [28-31 at elevated temperature. This study investigates were extended to the middle of each test specimen within creep behavior of the Nextel 720/alumina(N720/A)composite in +0.05 mm so that shear failure occurred on the plane between interlaminar shear at 1200C in air and in steam environments. the notch tips. The distance between the notches was 13 mm. The composite microstructure, as well as damage and failure Schematic of the dNS specimen and notch details are shown in mechanisms are discussed Fig. 2a and b, respectively. Dimensions of the DNS specimens used in this effort were different from those recommended in the astm 2. Material and experimental arrangements Standard C1425. The 13-mm distance between the notches was chosen specifically to enable the measurement of compressive The material studied was Nextel M720/alumina(N720JA), an strain between the notch tips with an MTS high-temperature xide-oxide CMC(manufactured by COl Ceramics, San Diego, CA) extensometer of 125-mm gage length. The overall specimen 三200um Fig. 1. As-processed material: (a)overview and (b) porous nature of the matrix is evident. Load 20 150 13 5mm Load Fig. 2. Double-notch shear specimen: (a) configuration and dimensions and (b) notch details
as a function of test rate using double-notch shear specimens and demonstrated that the interlaminar shear strength (ILSS) degraded with decreasing test rate. A phenomenological, power-law based crack growth model was proposed to account for the degradation of the ILSS of the composite at elevated temperatures. High-temperature stress-rupture tests in interlaminar shear were employed to validate the proposed model. These studies focused on the nonoxide CMCS with matrix-dense interlaminar regions, where the interlaminar failure is controlled by the fiber-matrix interface. In the case of the porous-matrix oxide–oxide CMCs, interlaminar shear failure is controlled by the exceptionally weak porous matrix. The objective of this effort is to evaluate the interlaminar shear strength and to investigate the creep behavior in interlaminar shear of an oxide–oxide CMC consisting of a porous alumina matrix reinforced with the NextelTM720 fibers. Several previous studies examined the in-plane mechanical behavior of this composite [28–31] at elevated temperature. This study investigates creep behavior of the NextelTM720/alumina (N720/A) composite in interlaminar shear at 1200 C in air and in steam environments. 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), an oxide–oxide CMC (manufactured by COI Ceramics, San Diego, CA) consisting of a porous alumina matrix reinforced with NextelTM720 fibers. There is no fiber coating. The composite was supplied in a form of 5.2-mm thick plates comprised of 24 0/90 woven layers, with a density of 2.83 g/cm3 , a fiber volume of 46.6%, and matrix porosity of 22%. 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, and then pressureless sintered [32]. 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. Porous nature of the matrix is seen in Fig. 1b. The double-notch shear (DNS) test specimens measuring 150 mm 20 mm were cut from a single N720/A panel. The thickness of the specimens was the same as the nominal thickness of the composite panel, i.e. 5.2 mm. The notches of 0.5 mm width were extended to the middle of each test specimen within ±0.05 mm so that shear failure occurred on the plane between the notch tips. The distance between the notches was 13 mm. Schematic of the DNS specimen and notch details are shown in Fig. 2a and b, respectively. Dimensions of the DNS specimens used in this effort were different from those recommended in the ASTM Standard C1425. The 13-mm distance between the notches was chosen specifically to enable the measurement of compressive strain between the notch tips with an MTS high-temperature extensometer of 12.5-mm gage length. The overall specimen Fig. 1. As-processed material: (a) overview and (b) porous nature of the matrix is evident. Fig. 2. Double-notch shear specimen: (a) configuration and dimensions and (b) notch details. M.B. Ruggles-Wrenn, P.D. Laffey / Composites Science and Technology 68 (2008) 2260–2266 2261
M.B. Ruggles-Wrenn, P D. Laffey / Composites Science and Technology 68(2008)2260-2266 length of 150 mm ensures that the local stress fields at the notch tips are not influenced by the external loading at the specimen ends A servocontrolled MTS mechanical testing machine equipped with hydraulic water-cooled collet grips, a compact two-zone Shear plane esistance-heated furnace and two temperature controllers was used in all tests. An MTS TestStar ll digital controller was employed input signal generation and data acquisition For elevated tem- erature testing, thermocouples were bonded to the specimens sing alumina cement(Zircar) to calibrate the furnace on a peri- dic basis. Tests in steam environment employed an alumina sus- ptor( 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 con- tinuous stream with a slightly positive pressure, expelling the dry air and creating a near 100% steam environment inside the suscep- tor Strain measurement was accomplished with an MTS high-te perature air-cooled uniaxial extensometer of 125-mm gage length. For strain measurement, the extensometer rods were placed as close to the notch tips of the specimen as possible. fracture sur- aces of failed specimens were examined using an SEM(FEI Quanta 200 HV) as well as an optical microscope(Zeiss Discovery V12). Fig 3. Micrograph of a typical specimen after interlaminar shear failure(side view). The SEM specimens were carbon coated All tests were conducted at 1200C. In all tests, a specimen was heated to the test temperature at a rate of 1C/s, and held at tem- perature for additional 30 min prior to testing. To investigate the nterlaminar shear properties and stress-rupture behavior, the T=1200°c DNS specimens were loaded in compression along the specime axis, as shown in Fig 2a. Monotonic tests were performed in dis- lacement control at the displacement rate of 0.05 mm/s in labora- f 5 MPa/s Creep run- out was defined as 100 h at a given creep Steam stress. In each test, stress and 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 interlaminar strength, speci- ens that achieved creep run-out were subjected to monotonic tests to failure at 1200 oC. 0.00 0.10 0.15 The shear stress values reported here represent the average Compressive Strain (%) stress,t, between the notches along the prospective shear plane calculated a 12g C iteiamnd in stear stress vs compressive strain for N/Z0/A Composite at similar ILSS values(8 MPa)in short-beam shear tests conducted where Pa is the applied force, Wis the specimen width and Ln is the at room temperature on the N720/ mullite-alumina specimens sub- istance between the notches. Although the shear stress distribu- jected to prior heat treatment at 1200.C The DNS specimen tested tion between the notches is not uniform, the average stresses pro- in steam was aged for 24 h at 1200C in steam prior to testing vided by Eq. (1)are useful when comparing interlaminar shear Prior aging in steam had negligible effect on ILSS of N720/A com- jected to identical mechanical tests in different environments. It is age strength value of 8.44 MPa. However, exposure vorthy of note that in all tests reported below, the DNS specimens noticeably influenced the stress-strain behavior of the composite failed in shear mode along the shear plane. Fig. 3 shows a typical While the stress-strain curve obtained in air departs from linearity shear failure of a DNS specimen. at the shear stress of approximately 4 MPa, the stress-strain curve obtained in steam becomes markedly nonlinear at a much lower 3. Results and discussion shear stress of 1.5 MPa. as the shear stress continues to increase in the test conducted in steam, appreciable inelastic strain devel- 3. 1. Interlaminar shear strength ops. Failure strain measured in steam is some 40% greater than that obtained in The monotonic stress-strain response is presented in Fig. 4 Three specimens were tested in air and three specimens were 3. 2. Creep-rupture tested in steam. In air, the average interlaminar shear strength (ILSS)was 8.25 MPa. Such relatively low value of the ILSS(com Results of the creep-rupture tests are summarized pared to the ilSS values for the dense-matrix CMCs)can be attrib- where rupture time and creep strain accumulation are for uted to the high matrix porosity. Note that Levi et al. [15 obtained each test environment and applied shear stress level
length of 150 mm ensures that the local stress fields at the notch tips are not influenced by the external loading at the specimen ends. A servocontrolled MTS mechanical testing machine equipped with hydraulic water-cooled collet 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. For elevated temperature testing, thermocouples were bonded to the specimens using alumina cement (Zircar) to calibrate the furnace on a periodic basis. 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. Strain measurement was accomplished with an MTS high-temperature air-cooled uniaxial extensometer of 12.5-mm gage length. For strain measurement, the extensometer rods were placed as close to the notch tips of the specimen as possible. Fracture surfaces of failed specimens were examined using an SEM (FEI Quanta 200 HV) as well as an optical microscope (Zeiss Discovery V12). The SEM specimens were carbon coated. All tests were conducted at 1200 C. In all tests, a specimen was heated to the test temperature at a rate of 1 C/s, and held at temperature for additional 30 min prior to testing. To investigate the interlaminar shear properties and stress-rupture behavior, the DNS specimens were loaded in compression along the specimen axis, as shown in Fig. 2a. Monotonic tests were performed in displacement control at the displacement rate of 0.05 mm/s in laboratory air and in steam environment. In creep-rupture tests specimens were loaded to the creep stress level at the stress rate of 5 MPa/s. Creep run-out was defined as 100 h at a given creep stress. In each test, stress and 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 interlaminar strength, specimens that achieved creep run-out were subjected to monotonic tests to failure at 1200 C. The shear stress values reported here represent the average stress, s, between the notches along the prospective shear plane calculated as s ¼ Pa WLn ð1Þ where Pa is the applied force, W is the specimen width and Ln is the distance between the notches. Although the shear stress distribution between the notches is not uniform, the average stresses provided by Eq. (1) are useful when comparing interlaminar shear strength values and stress-rupture responses of specimens subjected to identical mechanical tests in different environments. It is worthy of note that in all tests reported below, the DNS specimens failed in shear mode along the shear plane. Fig. 3 shows a typical shear failure of a DNS specimen. 3. Results and discussion 3.1. Interlaminar shear strength The monotonic stress–strain response is presented in Fig. 4. Three specimens were tested in air and three specimens were tested in steam. In air, the average interlaminar shear strength (ILSS) was 8.25 MPa. Such relatively low value of the ILSS (compared to the ILSS values for the dense-matrix CMCs) can be attributed to the high matrix porosity. Note that Levi et al. [15] obtained similar ILSS values (8 MPa) in short-beam shear tests conducted at room temperature on the N720/mullite-alumina specimens subjected to prior heat treatment at 1200 C. The DNS specimen tested in steam was aged for 24 h at 1200 C in steam prior to testing. Prior aging in steam had negligible effect on ILSS of N720/A composite. The specimens aged at 1200 C in steam produced the average strength value of 8.44 MPa. However, exposure to steam noticeably influenced the stress–strain behavior of the composite. While the stress–strain curve obtained in air departs from linearity at the shear stress of approximately 4 MPa, the stress–strain curve obtained in steam becomes markedly nonlinear at a much lower shear stress of 1.5 MPa. As the shear stress continues to increase in the test conducted in steam, appreciable inelastic strain develops. Failure strain measured in steam is some 40% greater than that obtained in air. 3.2. Creep-rupture Results of the creep-rupture tests are summarized in Table 1, where rupture time and creep strain accumulation are shown for each test environment and applied shear stress level. Creep strain Fig. 3. Micrograph of a typical specimen after interlaminar shear failure (side view). 0 2 4 6 8 10 12 0.00 0.05 0.10 0.15 0.20 Shear Stress (MPa) Compressive Strain (%) T = 1200 ºC Air Steam Fig. 4. Interlaminar shear stress vs compressive strain for N720/A composite at 1200 C in air and in steam. 2262 M.B. Ruggles-Wrenn, P.D. Laffey / Composites Science and Technology 68 (2008) 2260–2266
MB. Ruggles-Wrenn, P D. Laffey Composites Science and Technology 68(2008)2260-2266 2263 1E-03 rupture tests in interlaminar shear for the N720/ A ceramic composite ratory air and in steam environment stress(MPa) Time to rupture (s i 1.E-04IASteam Laboratory air 0.14 360.000 正105 3600004 3.21 2.1 1E07 T=1200° ' s time curves obtained at 1200 C in air and in steam are shown in Fig 5 10 The creep curves obtained at 6.5 MPa in air exhibit primary and Creep Stress(MPa) secondary creep regimes. Transition from primary to secondary Fig. 6. Minimum creep rate as a function of applied stress for N720JA ceramic creep occurs fairly early in creep life. Secondary creep is likely to composite at 1200C in air and in steam. persist for the duration of the creep lifetime. In reep run-out of 100 h is achieved at the shear stress of 6.5 MPa (78.5% ILSS The strains accumulated during 100 h at 6.5 MPa are compara to those obtained in the monotonic test. Creep curve produced in T=1200°c steam at the shear stress of 4 MPa also exhibits only primary and ILSS. Air ondary creep regimes. In contrast, creep curves obtained in at 5 and 6.5 MPa show primary, secondary and tertiary immediately. Secondary creep persists for x70% of the creep life 8 before transitioning to tertiary creep. Creep strain accumulation first increases as the applied shear stress increases from 4 to 5 MPa, then decreases as the applied stress increases to 6.5 MPa. It is noteworthy that in steam, all accumulated creep strains are at least an order of magnitude higher than the failure strain ob- tained in the monotonic test In steam creep run-out was achieved nly at 4 MPa(50% ILSS). 1.E+03 1.E+04 1.E+05 1.E+06 Minimum creep rate was measured in all tests. Creep strain rate as a function of applied stress is shown in Fig. 6. In steam, the min- mum creep rate increases by a factor of 10 when applied stress in- fg 7. Interlaminar shear stress vs time to rupture for N720/A composite at creases from 4 to 5 MPa. At 6.5 MPa, creep rate in steam is at least wo orders of magnitude higher than that in air. Stress-rupture Retained interlaminar shear strength values of the specimens behavior is summarized in Fig. 7, where applied shear stress is that achieved a run -out at 6.5 MPa in air and at 4 MPa in steam plotted vs time to rupture at 1200C in air and in steam. In are given in Table 2. The stress-strain curves obtained for the creep life(up to 100 h)appears to be relatively independent of ap- N720/A specimens subjected to pri in interlaminar shear plied stress up to 78% ILSS All creep tests conducted at 6.5 MPa inin air and in steam are presented in Fig. &a and b, respectively air achieved a run-out. For applied shear stress >5 MPa the pres- The IlSS of the specimen pre-crept at 6. 5 MPa in air has increased ence of steam dramatically reduced creep lifetimes. At 6.5 MPa, by nearly 37% compared to the ILSS of the as-processed specimen the reduction in creep life due to steam was 94% as compared to Conversely prior creep in steam has degraded the interlaminar the run-out of 100 h shear strength of N720/A by about 27%. The specimen pre-crept at 4 MPa in steam retained less than 75% of its ILSS. Prior creep in either environment had little qualitative effect on stress-strain 5 MPa T=1200°c 3.3. Composite microstructure 6.5 MPa 方20 pression to failure at 1200C in air is shown in Fig 9. Delamination Table 2 Retained interlaminar shear properties of the n720/A specimens subjected to prior 4 MPa creep in interlaminar shear at 1200C Creep stress etained interlaminar shear strength ailure strain 400006000080000 100000 Laboratory air 6.5 0.22 Fig. 5. Creep strain vs time curves for N720JA laminar shear stresses in the 4-6.5 MPa range at 1200" C in air and in steam
vs time curves obtained at 1200 C in air and in steam are shown in Fig. 5. The creep curves obtained at 6.5 MPa in air exhibit primary and secondary creep regimes. Transition from primary to secondary creep occurs fairly early in creep life. Secondary creep is likely to persist for the duration of the creep lifetime. In air, creep run-out of 100 h is achieved at the shear stress of 6.5 MPa (78.5% ILSS). The strains accumulated during 100 h at 6.5 MPa are comparable to those obtained in the monotonic test. Creep curve produced in steam at the shear stress of 4 MPa also exhibits only primary and secondary creep regimes. In contrast, creep curves obtained in steam at 5 and 6.5 MPa show primary, secondary and tertiary creep. Transition from primary to secondary creep occurs almost immediately. Secondary creep persists for 70% of the creep life before transitioning to tertiary creep. Creep strain accumulation first increases as the applied shear stress increases from 4 to 5 MPa, then decreases as the applied stress increases to 6.5 MPa. It is noteworthy that in steam, all accumulated creep strains are at least an order of magnitude higher than the failure strain obtained in the monotonic test. In steam creep run-out was achieved only at 4 MPa (50% ILSS). Minimum creep rate was measured in all tests. Creep strain rate as a function of applied stress is shown in Fig. 6. In steam, the minimum creep rate increases by a factor of 10 when applied stress increases from 4 to 5 MPa. At 6.5 MPa, creep rate in steam is at least two orders of magnitude higher than that in air. Stress-rupture behavior is summarized in Fig. 7, where applied shear stress is plotted vs time to rupture at 1200 C in air and in steam. In air, creep life (up to 100 h) appears to be relatively independent of applied stress up to 78% ILSS. All creep tests conducted at 6.5 MPa in air achieved a run-out. For applied shear stress P5 MPa the presence of steam dramatically reduced creep lifetimes. At 6.5 MPa, the reduction in creep life due to steam was 94% as compared to the run-out of 100 h. Retained interlaminar shear strength values of the specimens that achieved a run-out at 6.5 MPa in air and at 4 MPa in steam are given in Table 2. The stress–strain curves obtained for the N720/A specimens subjected to prior creep in interlaminar shear in air and in steam are presented in Fig. 8a and b, respectively. The ILSS of the specimen pre-crept at 6.5 MPa in air has increased by nearly 37% compared to the ILSS of the as-processed specimen. Conversely, prior creep in steam has degraded the interlaminar shear strength of N720/A by about 27%. The specimen pre-crept at 4 MPa in steam retained less than 75% of its ILSS. Prior creep in either environment had little qualitative effect on stress–strain behavior. 3.3. Composite microstructure A typical fracture surface of the DNS specimen tested in compression to failure at 1200 C in air is shown in Fig. 9. Delamination Table 1 Results of creep-rupture tests in interlaminar shear for the N720/A ceramic composite at 1200 C in laboratory air and in steam environment Creep stress (MPa) Creep strain (%) Time to rupture (s) Laboratory air 6.5 0.14 360,000a 6.5 0.24 360,000a Steam 4.0 1.65 360,000a 5.0 3.21 29,040 6.5 2.13 20,400 a Run-out. 0.0 1.0 2.0 3.0 4.0 0 20000 40000 60000 80000 100000 Creep Strain (%) Time (s) T = 1200 ºC 6.5 MPa 5 MPa 4 MPa 6.5 MPa Air Steam Fig. 5. Creep strain vs time curves for N720/A composite obtained at applied interlaminar shear stresses in the 4–6.5 MPa range at 1200 C in air and in steam. 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1 10 Creep Strain Rate (s-1) Creep Stress (MPa) Air Steam T = 1200 ºC Fig. 6. Minimum creep rate as a function of applied stress for N720/A ceramic composite at 1200 C in air and in steam. 0 2 4 6 8 10 1.E+03 1.E+04 1.E+05 1.E+06 Shear Stress (MPa) Time (s) Air Steam T = 1200 ºC ILSS, Air Fig. 7. Interlaminar shear stress vs time to rupture for N720/A composite at 1200 C. Table 2 Retained interlaminar shear properties of the N720/A specimens subjected to prior creep in interlaminar shear at 1200 C Creep stress (MPa) Retained interlaminar shear strength (MPa) Failure strain (%) Laboratory air 6.5 11.2 0.22 Steam 4.0 6.13 0.19 M.B. Ruggles-Wrenn, P.D. Laffey / Composites Science and Technology 68 (2008) 2260–2266 2263
M.B. Ruggles-Wrenn, P D. Laffey / Composites Science and Technology 68(2008)2260-2266 a and clean, indicating that only a single fiber layer is associated with delamination, some rough areas exposing debris and fiber break age are also visible In the course of delamination the departing fi- a bers leave distinct troughs in the remaining matrix (see higher magnification views in Fig. 9b and c). Small amounts of the matrix material remain bonded to the fibers exposed during delamination y contrast, the fracture surface of the dNS specimen tested in compression following 100 h of prior creep at 6.5 MPa at 1200C case includes considerable fiber fracture. The rougher fracture su face shows increased damage in fiber tows, frequently exposing T=1200°c,Air multiple 0 /90 fiber layers. Only some limited areas show clean delamination of a single fiber layer from the matrix-rich regions 0.00 0.25 (Fig. 10a). n in Fig. 10c, the failure also involves extensive Compressive Strain (%) damage to the matrix. Recent studies [33, 34 demonstrated that for a composite con- b sisting of Nextel720 fibers in a porous alumina matrix, a porosity T=1200°c, Stean reduction of -6% was observed after a 10-min exposure at 1200C, which was caused by additional sintering of the matrix. It is likely 24 h at o MPa in steam that additional sintering of the matrix occurred during the 100 h creep test at 6.5 MPa. The resultant strengthening of the matrix is manifested in the retained ILSS of the composite Results in Table 2 show that the ILSS increased after prior creep at 6.5 MPa. The strengthening is also manifested in the change of the failure mech- anism. The failure of the pre-crept composite involves extensive fi ber fracture, while the as-processed material fails predominantly through matrix damage and interply delamination. 100 h at 4 MPa in steam The fracture surfaces obtained in steam(Figs. 11 and 12)are considerably more violent and rough than those obtained in air. The fracture surface produced in creep at 6.5 MPa(Fig. 11a and Compressive Strain(%) b)as well as the fracture surface obtained in compression test on a specimen subjected to 100 h of prior creep at 4 MPa(Fig. 12a ig. 8. Effects of prior creep in interlaminar shear on interlaminar shear stress- and b)reveal extensive fracture of fiber tows. As seen in Figs. 11c compressive strain behavior of N720/A ceramic composite at 1200C: (a)in air and and 12c, the amount of matrix material remaining bonded to the b)in steam. exposed fibers is greater than that in the specimens tested in air. In fact, it appears that during tests of over 100 h duration con- of the woven 0 /90 fiber layers from the matrix-rich regions a ducted in steam, the fiber tows become bonded together by the pears to be the primary mechanism of interlaminar shear failure. matrix material and then fail in coordinated fashion. It is possible While most of the fracture surface shown in Fig. 9a is fairly smooth that the sintering of the matrix is accelerated in the presence of 1. 0 mm Fig 9. Fracture surface of the DNS specimen tested in compression to failure at 1200C in air. Fig. 10. Fracture surface of the dNS specimen tested in compression to failure following 100 h at 6.5 MPa at 1200C in air
of the woven 0/90 fiber layers from the matrix-rich regions appears to be the primary mechanism of interlaminar shear failure. While most of the fracture surface shown in Fig. 9a is fairly smooth and clean, indicating that only a single fiber layer is associated with delamination, some rough areas exposing debris and fiber breakage are also visible. In the course of delamination the departing fi- bers leave distinct troughs in the remaining matrix (see higher magnification views in Fig. 9b and c). Small amounts of the matrix material remain bonded to the fibers exposed during delamination. By contrast, the fracture surface of the DNS specimen tested in compression following 100 h of prior creep at 6.5 MPa at 1200 C in air (see Fig. 10a and b) reveals that the failure mechanism in this case includes considerable fiber fracture. The rougher fracture surface shows increased damage in fiber tows, frequently exposing multiple 0/90 fiber layers. Only some limited areas show clean delamination of a single fiber layer from the matrix-rich regions (Fig. 10a). As seen in Fig. 10c, the failure also involves extensive damage to the matrix. Recent studies [33,34] demonstrated that for a composite consisting of NextelTM720 fibers in a porous alumina matrix, a porosity reduction of 6% was observed after a 10-min exposure at 1200 C, which was caused by additional sintering of the matrix. It is likely that additional sintering of the matrix occurred during the 100 h creep test at 6.5 MPa. The resultant strengthening of the matrix is manifested in the retained ILSS of the composite. Results in Table 2 show that the ILSS increased after prior creep at 6.5 MPa. The strengthening is also manifested in the change of the failure mechanism. The failure of the pre-crept composite involves extensive fi- ber fracture, while the as-processed material fails predominantly through matrix damage and interply delamination. The fracture surfaces obtained in steam (Figs. 11 and 12) are considerably more violent and rough than those obtained in air. The fracture surface produced in creep at 6.5 MPa (Fig. 11a and b) as well as the fracture surface obtained in compression test on a specimen subjected to 100 h of prior creep at 4 MPa (Fig. 12a and b) reveal extensive fracture of fiber tows. As seen in Figs. 11c and 12c, the amount of matrix material remaining bonded to the exposed fibers is greater than that in the specimens tested in air. In fact, it appears that during tests of over 100 h duration conducted in steam, the fiber tows become bonded together by the matrix material and then fail in coordinated fashion. It is possible that the sintering of the matrix is accelerated in the presence of 0 2 4 6 8 10 12 0.00 0.05 0.10 0.15 0.20 0.25 Shear Stress (MPa) Compressive Strain (%) T = 1200 ºC, Air As-processed 100 h at 6.5 MPa in air 0 2 4 6 8 10 12 0.00 0.05 0.10 0.15 0.20 0.25 Shear Stress (MPa) Compressive Strain (%) T = 1200 ºC, Steam 24 h at 0 MPa in steam 100 h at 4 MPa in steam Fig. 8. Effects of prior creep in interlaminar shear on interlaminar shear stress– compressive strain behavior of N720/A ceramic composite at 1200 C: (a) in air and (b) in steam. Fig. 9. Fracture surface of the DNS specimen tested in compression to failure at 1200 C in air. Fig. 10. Fracture surface of the DNS specimen tested in compression to failure following 100 h at 6.5 MPa at 1200 C in air. 2264 M.B. Ruggles-Wrenn, P.D. Laffey / Composites Science and Technology 68 (2008) 2260–2266
MB. Ruggles-Wrenn, P D. Laffey Composites Science and Technology 68(2008)2260-2266 2265 b Fig. 11. Fracture surface of the dNS specimen tested in creep at 6.5 MPa at 1200C in steam. 10m um Fig. 12. Fracture surface of the DNS specimen tested in compression to failure following 100 h at 4.0 MPa at 1200" in steam. steam. However, while prior creep in air resulted in strengthening creep run-out was achieved only at 4 MPa. The run-out specimen of the matrix, retained properties in Table 2 suggest that prior retained only 75% of its ILSS For applied stress >5 MPa, the pres- creep in steam causes weakening of the matrix. The retained ilSs ence of steam drastically reduced creep lifetimes. The hydrother of the specimen pre-crept in steam is noticeably lower than the al weakening of the alumina matrix due to incorporation of ILSS of the as-processed composite. ydrogen defects during testing at 1200C in steam may be behind Recent studies [35,36 show that water attacks grain bound the degraded creep performance of the N720/A in steam. aries and degrades the strength of the polycrystalline alumina. In tests of short duration conducted in air, the failure occurs pri- Kronenberg et al. [36 found two types of hydrogen defects incor- marily through delamination of the woven 0/90 fiber layers from porated in alumina specimens subjected to heat treatment at tem- the matrix-rich regions, with minimal fiber fracture. generally om 850c to 1025C under 1500 MPa only one fiber layer is associated with delamination For test dura- hydrostatic pressure in the presence of water, followed by com- tions >100 h, the failure mechanism includes considerable fiber pression tests at temperatures in the 630-850C range The infra- fracture. It is possible that the matrix undergoes additional sinter red measurements revealed interstitial hydrogens in the bulk and ing during the long-term tests conducted in air. In tests conduct molecular water clusters near surfaces, grain boundaries and in steam, the failure mechanism is dominated by fiber fracture. cracks of the hydrothermally treated alumina specimens. Further more, the presence of hydrogen defects reduced the yield stress of Acknowledgements fine-grained alumina by a factor of six. It is possible that hydrogen defects that are introduced into the alumina matrix of the n720/A The financial support of Dr R. Sikorski and Dr J. Zelina, Propul- specimens during tests conducted at 1200C in steam are behind sion Directorate, Air Force Research Laboratory is highly the degradation of creep performance in steam. 4. Concluding remarks References The creep behavior of the N720/A ceramic composite in inter- I1 Ohnabe H, Masaki SOnozuka M, Miyahara K,Sasa TPotential application of laminar shear was assessed and the interlaminar shear properties were measured at 1200C in laboratory air and in steam using [21 Zawada LP Staehler J Steel S Consequence of intermittent exposure to strength(ILSS)was 8.25 MPa. Prior aging for 24 h at 1200C in (31 Parlier M, Ritti MH. State of the art and perspectives for oxide/oxide steam had no effect on ILSS. The N720/A composite exhibits pri- mary and secondary creep regimes in air. In steam, primary, sec. [4 Mattoni MA, Yang JY, Levi CG, Zok Fw-2: 211-21. Zawada LP. Effects of combustor rig ondary and tertiary creep regimes are observed. Creep strains us-matrix oxide composite. Int J Appl Ceram Technol 2(2):133-40 accumulated in steam are at least an order of magnitude larger [5] Parthasarathy TA, Zawada LP, John R, Cinibulk MK, Zelina J. Evaluation of than those produced in air. Creep strain rates were approximately 21×10-7s- I in ai, and ranged from34×10-6to7.×10-5s-1 Technol 2005:2(2):122-32 novel combustor wall application. Int JApp Ceram in steam. At 6.5 MPa, creep rate in steam is at least two orders of posite member and m No.5601674,1997 magnitude higher than the creep rate produced in air. [7 Sim SM, Kerans R]. Slurry infiltration and 3-D woven composites Ceram Eng Creep run-out was achieved in all tests conducted in air. The 18) Moore EH, Mah T, Keller KA 3D composite fabrication through matrix slurry un-out specimens exhibited substantial increase in ILSS In steam, pressure infiltration. Ceram Eng Sci Proc 1994: 15(4): 1 13-20
steam. However, while prior creep in air resulted in strengthening of the matrix, retained properties in Table 2 suggest that prior creep in steam causes weakening of the matrix. The retained ILSS of the specimen pre-crept in steam is noticeably lower than the ILSS of the as-processed composite. Recent studies [35,36] show that water attacks grain boundaries and degrades the strength of the polycrystalline alumina. Kronenberg et al. [36] found two types of hydrogen defects incorporated in alumina specimens subjected to heat treatment at temperatures ranging from 850 C to 1025 C under 1500 MPa hydrostatic pressure in the presence of water, followed by compression tests at temperatures in the 630–850 C range. The infrared measurements revealed interstitial hydrogens in the bulk and molecular water clusters near surfaces, grain boundaries and cracks of the hydrothermally treated alumina specimens. Furthermore, the presence of hydrogen defects reduced the yield stress of fine-grained alumina by a factor of six. It is possible that hydrogen defects that are introduced into the alumina matrix of the N720/A specimens during tests conducted at 1200 C in steam are behind the degradation of creep performance in steam. 4. Concluding remarks The creep behavior of the N720/A ceramic composite in interlaminar shear was assessed and the interlaminar shear properties were measured at 1200 C in laboratory air and in steam using double-notch shear test specimens. The interlaminar shear strength (ILSS) was 8.25 MPa. Prior aging for 24 h at 1200 C in steam had no effect on ILSS. The N720/A composite exhibits primary and secondary creep regimes in air. In steam, primary, secondary and tertiary creep regimes are observed. Creep strains accumulated in steam are at least an order of magnitude larger than those produced in air. Creep strain rates were approximately 2.1 107 s1 in air, and ranged from 3.4 106 to 7.0 105 s1 in steam. At 6.5 MPa, creep rate in steam is at least two orders of magnitude higher than the creep rate produced in air. Creep run-out was achieved in all tests conducted in air. The run-out specimens exhibited substantial increase in ILSS. In steam, creep run-out was achieved only at 4 MPa. The run-out specimen retained only 75% of its ILSS. For applied stress P5 MPa, the presence of steam drastically reduced creep lifetimes. The hydrothermal weakening of the alumina matrix due to incorporation of hydrogen defects during testing at 1200 C in steam may be behind the degraded creep performance of the N720/A in steam. In tests of short duration conducted in air, the failure occurs primarily through delamination of the woven 0/90 fiber layers from the matrix-rich regions, with minimal fiber fracture. Generally, only one fiber layer is associated with delamination. For test durations >100 h, the failure mechanism includes considerable fiber fracture. It is possible that the matrix undergoes additional sintering during the long-term tests conducted in air. In tests conducted in steam, the failure mechanism is dominated by fiber fracture. Acknowledgements The financial support of Dr. R. Sikorski and Dr. J. Zelina, Propulsion Directorate, Air Force Research Laboratory is highly appreciated. References [1] Ohnabe H, Masaki S, Onozuka M, Miyahara K, Sasa T. Potential application of ceramic matrix composites to aero-engine components. Composites: Part A 1999;30:489–96. [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] Parlier M, Ritti MH. State of the art and perspectives for oxide/oxide composites. Aerospace Sci Technol 2003;7:211–21. [4] Mattoni MA, Yang JY, Levi CG, Zok FW, Zawada LP. Effects of combustor rig exposure on a porous-matrix oxide composite. Int J Appl Ceram Technol 2005;2(2):133–40. [5] Parthasarathy TA, Zawada LP, John R, Cinibulk MK, Zelina J. Evaluation of oxide–oxide composites in a novel combustor wall application. Int J App Ceram Technol 2005;2(2):122–32. [6] Szweda A, Millard ML, Harrison MG. Fiber-reinforced ceramic–matrix composite member and method for making. US Patent No. 5 601 674, 1997. [7] Sim SM, Kerans RJ. Slurry infiltration and 3-D woven composites. Ceram Eng Sci Proc 1992;13(9–10):632–41. [8] Moore EH, Mah T, Keller KA. 3D composite fabrication through matrix slurry pressure infiltration. Ceram Eng Sci Proc 1994;15(4):113–20. Fig. 11. Fracture surface of the DNS specimen tested in creep at 6.5 MPa at 1200 C in steam. Fig. 12. Fracture surface of the DNS specimen tested in compression to failure following 100 h at 4.0 MPa at 1200 in steam. M.B. Ruggles-Wrenn, P.D. Laffey / Composites Science and Technology 68 (2008) 2260–2266 2265
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