ournal 1. Am Ceram Soc, s1 [9) 2269-77(1998) Creep and Fatigue Behavior in an Enhanced Sic/SiC Composite at High Temperature Shijie Zhu, tt Mineo Mizuno, ".T Yasuo Nagano, Jianwu Cao, .f Yutaka Kagawa, ".f and Hiroshi Kava Japan Fine Ceramics Center, Nagoya 456, Japan, Institute of Industrial Sciences, University of Tokyo, Tokyo 106, apan, and Petroleum Energy Center, Tokyo 106, Japan The monotonic tension, creep, and fatigue behavior of an enhanced SiC/SiC composite will become glass and flow to enhanced SiC/SiC composite was investigated at a tempera- seal the cracks at high temperatures. As a result, the glass phase ture of 1300C in air and argon. The improved creep and prevents further oxidation and heals the cracks, which may fatigue resistances were determined and compared to those improve the creep and fatigue resistance of the composite at of the standard SiC/SiC composite. The effects of additives high temperature glass-forming, boron-based particulates) in the matrix on The cyclic-fatigue behavior of CMCs at high temperatures is the creep and environmental resistance of the enhanced not well understood. Elements such as environmental factors, SiC/SiC composite were discussed. Crack propagation in creep of constituents, thermally induced stresses at interfaces, the matrix of the enhanced SiC/SiC composite was different and interfacial sliding resistance can cause the reduction of from that in the standard SiC/SiC composite. The filling of fatigue life at high temperatures. 8, 18-20 If the temperature is he glassy phases in the cracks prohibited the diffusion of lower than that for the onset of creep of the fibers and the oxygen from the environment. As a result, creep and fa- matrix, the decrease of sliding resistance due to the relaxation tigue properties in the enhanced siC/SiC composite in air at of the residual stress is attributed to the dominant fatigue high temperatures was improved. dation of the fibers(such as creep), the subject becomes complicated and is a priority for further re- search. 2 Fatigue and creep damage mechanisms can operate N THE recent decade, creep and fatigue of continuous-fiber simultaneously under high-temperature cyclic loading. Fatigu reinforced ceramic-matrix composites(CMCs) have been hot loading at high temperatures results in creep-fatigue interac- topics, -20 because these properties are very important for the tion, which causes a reduction in the number of cycles to fail application of CMCs. To obtain high fracture toughness and ure. Creep of the fiber and degradation of the interfacial sliding hermal shock resistance, CMCs are designed with weak inter- resistance have been considered to be the reasons for decreased faces between the fibers and the matrix; e. g, the interface in a fatigue resistance at high temperatures in standard SiC/SiC SiC/SiC composite is coated with carbon or BN. The weak con interface can cause cracks to deflect along the interfaces which Creep and fatigue tests at 1300@C were both conducted in air omits intact fibers to bridge matrix-crack faces. However, at the same maximum stresses, to compare the time-dependent although the use of weak interfaces can increase fracture re- deterioration with cyclic-dependent damage. The creep and fa- sistance, it is not compatible with creep and fatigue resistance tigue data of the enhanced SiC/SiC composite were compare at high temperature, which demands strong interfaces that re- with those of the standard SiC/SiC composite, to investigate sist the nucleation and growth of cavities 2, 3 the effects of the additives in the matrix on the mechanical The carbon layer in a SiC/SiC composite leads to low oxi- roperties. The creep and fatigue crack-propagation paths and dation resistance at high temperatures. 21-4Glass-forming, bo- mechanisms were studied. To understand the interaction of ron-based particulates that react with oxygen to produce a seal environmental creep, creep tests in pure argon were also con- ant glass that inhibits oxidation of the carbon layer can be ducted at the same temperature added to the matrix. This technology has been applied to SiC/SiC composites. SiC/SiC composites modified in this way are called enhanced SiC/SiC composites. 5 Il. Materials and Experimental Procedures Because the creep resistance of SiC fibers(NicalonTM, Nip- The composites used in this investigation were processed by pon Carbon, Tokyo, Japan)is lower than that of the SiC matrix, using chemical vapor infiltration(CVI) of Sic into plane matrix cracking is the dominant damage mechanism. 7 This woven 0/90 SiC-fiber preforms(made by DuPont Lanxide condition is undesirable for creep and environmental resistance Composites, Wilmington, DE). Before infiltration, the pre of the composites, because the increase in matrix stress via forms were coated with carbon by using chemical vapor del stress redistribution causes the development of periodic matrix sition( CVD), to decrease the interface bonding between the cracking. The glass-forming ulates in the matrix of the fibers and the matrix, thereby increasing the toughness. The size of the as-received composite panels was 200 mm x 200 mm with a thickness of 3.2 mm; the composite contained 40 vol% SiC fibers and had a porosity of 9.7%. The diameter of R. Naslain--contributing editor the SiC (NicalonTM) fiber was-12 um, and each bundle con- sisted of 500 fibers The tensile specimens were machined from the panels usil diamond cutting tools. The shape and dimensions of the sp mens for the monotonic tension, creep, and cyclic-fatigue tests inducted by the pet ole tm Entrgy tenter (pic as taphan development programs have been described by Mizuno et a1.&The surfaces of the ecimens were polished by an 800-grit grinding wheel before Lnstitute of Industrial Sciences, University of Tokyo testing; thus, the specimens were unprotected by a final CVI PEtroleum Energy Center run after machining
Creep and Fatigue Behavior in an Enhanced SiC/SiC Composite at High Temperature Shijie Zhu,†,‡ Mineo Mizuno,*,† Yasuo Nagano,*,† Jianwu Cao,*,† Yutaka Kagawa,*,‡ and Hiroshi Kaya§ Japan Fine Ceramics Center, Nagoya 456, Japan, Institute of Industrial Sciences, University of Tokyo, Tokyo 106, Japan, and Petroleum Energy Center, Tokyo 106, Japan The monotonic tension, creep, and fatigue behavior of an enhanced SiC/SiC composite was investigated at a temperature of 1300°C in air and argon. The improved creep and fatigue resistances were determined and compared to those of the standard SiC/SiC composite. The effects of additives (glass-forming, boron-based particulates) in the matrix on the creep and environmental resistance of the enhanced SiC/SiC composite were discussed. Crack propagation in the matrix of the enhanced SiC/SiC composite was different from that in the standard SiC/SiC composite. The filling of the glassy phases in the cracks prohibited the diffusion of oxygen from the environment. As a result, creep and fatigue properties in the enhanced SiC/SiC composite in air at high temperatures was improved. I. Introduction I N THE recent decade, creep and fatigue of continuous-fiberreinforced ceramic-matrix composites (CMCs) have been hot topics,1–20 because these properties are very important for the application of CMCs. To obtain high fracture toughness and thermal shock resistance, CMCs are designed with weak interfaces between the fibers and the matrix; e.g., the interface in a SiC/SiC composite is coated with carbon or BN. The weak interface can cause cracks to deflect along the interfaces, which permits intact fibers to bridge matrix-crack faces.1 However, although the use of weak interfaces can increase fracture resistance, it is not compatible with creep and fatigue resistance at high temperature, which demands strong interfaces that resist the nucleation and growth of cavities.2,3 The carbon layer in a SiC/SiC composite leads to low oxidation resistance at high temperatures.21–24 Glass-forming, boron-based particulates that react with oxygen to produce a sealant glass that inhibits oxidation of the carbon layer can be added to the matrix.6 This technology has been applied to SiC/SiC composites. SiC/SiC composites modified in this way are called enhanced SiC/SiC composites.5 Because the creep resistance of SiC fibers (Nicalon™, Nippon Carbon, Tokyo, Japan) is lower than that of the SiC matrix, matrix cracking is the dominant damage mechanism.7 This condition is undesirable for creep and environmental resistance of the composites, because the increase in matrix stress via stress redistribution causes the development of periodic matrix cracking.3 The glass-forming particulates in the matrix of the enhanced SiC/SiC composite will become glass and flow to seal the cracks at high temperatures. As a result, the glass phase prevents further oxidation and heals the cracks, which may improve the creep and fatigue resistance of the composite at high temperature. The cyclic-fatigue behavior of CMCs at high temperatures is not well understood. Elements such as environmental factors, creep of constituents, thermally induced stresses at interfaces, and interfacial sliding resistance can cause the reduction of fatigue life at high temperatures.8,18–20 If the temperature is lower than that for the onset of creep of the fibers and the matrix, the decrease of sliding resistance due to the relaxation of the residual stress is attributed to the dominant fatigue mechanism.8,19 When the temperature is high enough to produce strength degradation of the fibers (such as creep), the subject becomes complicated and is a priority for further research.12 Fatigue and creep damage mechanisms can operate simultaneously under high-temperature cyclic loading. Fatigue loading at high temperatures results in creep–fatigue interaction, which causes a reduction in the number of cycles to failure. Creep of the fiber and degradation of the interfacial sliding resistance have been considered to be the reasons for decreased fatigue resistance at high temperatures in standard SiC/SiC composites.8,19 Creep and fatigue tests at 1300°C were both conducted in air at the same maximum stresses, to compare the time-dependent deterioration with cyclic-dependent damage. The creep and fatigue data of the enhanced SiC/SiC composite were compared with those of the standard SiC/SiC composite, to investigate the effects of the additives in the matrix on the mechanical properties. The creep and fatigue crack-propagation paths and mechanisms were studied. To understand the interaction of environmental creep, creep tests in pure argon were also conducted at the same temperature. II. Materials and Experimental Procedures The composites used in this investigation were processed by using chemical vapor infiltration (CVI) of SiC into planewoven 0°/90° SiC-fiber preforms (made by DuPont Lanxide Composites, Wilmington, DE). Before infiltration, the preforms were coated with carbon by using chemical vapor deposition (CVD), to decrease the interface bonding between the fibers and the matrix, thereby increasing the toughness. The size of the as-received composite panels was 200 mm × 200 mm with a thickness of 3.2 mm; the composite contained 40 vol% SiC fibers and had a porosity of 9.7%. The diameter of the SiC (Nicalon™) fiber was ∼12 mm, and each bundle consisted of 500 fibers. The tensile specimens were machined from the panels using diamond cutting tools. The shape and dimensions of the specimens for the monotonic tension, creep, and cyclic-fatigue tests have been described by Mizuno et al.8 The surfaces of the specimens were polished by an 800-grit grinding wheel before testing; thus, the specimens were unprotected by a final CVI run after machining. R. Naslain—contributing editor Manuscript No. 191494. Received August 26, 1997; approved December 16, 1997. This work is a part of the automotive ceramic gas turbine development programs conducted by the Petroleum Energy Center (PEC) in Japan. *Member, American Ceramic Society. † Japan Fine Ceramics Center. ‡ Institute of Industrial Sciences, University of Tokyo. § Petroleum Energy Center. J. Am. Ceram. Soc., 81 [9] 2269–77 (1998) Journal 2269
2270 Journal of the American Ceramic Society-Zhu et al. Vol 8l. No 9 all th d were performed using a servo- To compare the creep and fatigue properties of the enhanced hydraulic testing system del mrS 810, MTS System Corp SiC/SiC composite with those of the standard SiC/SiC com- Eden Prairie, MN)at a temperature of 1300.C The monotonic posite, creep and fatigue tests of the standard SiC/SiC com- tensile tests were conducted in air, under a constant displace posite were conducted under the same conditions as those for ment rate of 0.5 mm/min. The specimens were allowed to soak the enhanced SiC/SiC composite in air. The creep and fatigue for -30 min at 1300%C before the tensile tests were started. The data of the standard SiC/SiC composite in argon were cited alignment between the upper and lower grips of the load unit from Zhu et al. 7 Mizuno et al. and Zhu et al. 10 for comparison was veri ified using the steel dummy specimen for verificatio After fracture, the specimens were examined by using optical that was supplied by MTS Corp. to allow a bending strain of microscopy and scanning electron microscopy (SEM) 5%. in accordance with ASTM Standard E 1012-89. Ana lytical and empirical analysis studies have concluded that, for lL. Results and discussion negligible effects on the estimates of the strength-distribution parameters(for example, the Weibull modulus and character- istic strength) of monolithic advanced ceramics, the allowable 1 Microstructures and Monotonic Tension percent bending, as defined in ASTM Practice E 1012, should Micrographs of the enhanced and standard SiC/SiC compos- ites in their original states are shown in Fig. 1. The differend tensile strength distributions of continuous-fiber-reinforced between them are that there are glassy phases in the matrix CMCs do not exist. ASTM Practice C 1275-94 has adopted the the enhanced SiC/SiC composite(the gray phases in the matrix recommendations for the tensile testing of monolithic advanced in Fig. I(c). The thickness of the carbon layer at the interfaces ceramics. Because CMCs have inelastic deformation, which of the enhanced SiC/SiC composite(0.5-0.6 um) is larger than can redistribute the stress state and sometimes lead to notch that of the standard SiC/SiC composite(0. 1-0. 2 um). More- insensitivity, a bending strain of 5% should not affect the over, there are more pores in the fiber bundles of the enhanced strength distribution SiC/SiC composite than in those of the standard SiC/SiC The fatigue tests were performed with a sinusoidal loading composite. frequency of 20 Hz in air. The stress ratio, which is defined The tensile stress-versus-strain relation of the enhanced and the ratio of minimum stress to maximum stress, was 0. 1 for the standard SiC/SiC composites at 1300%C is shown in Fig. 2. The testing temperature. Creep tests were conducted under a con curves of the enhanced SiC/SiC composite indicate linear elas- tant load in air and in an argon atmosphere. Creep strain was tic behavior up to the proportional limit of -70 MPa, and this measured directly from the gauge length of the specimen using a contact extensometer(Model MTS 632.53-F71,M MPa). The UTS of the enhanced SiC/SiC composite is almost System Corp )that had a measuring range of +2.5 mm over its the same as that of the standard SiC/SiC composite; however, gauge length of 25 mm. Periodically, partial unloading the strain at the UTS of the enhanced SiC/SiC composite is reloading was applied, to measure the modulus change durin much higher than that of the standard SiC/SiC composite. It is the creep tests. The specimens were allowed to soak for >30 evident that the addition of glass-forming particulates in the matrix increases the ductility of the composite. A possible rea A controlled-atmosphere furnace(Model MTS 659. Mrs son is the decrease of creep resistance of the matrix in the System Corp was used for the creep tests in the argon atmo- sphere. For the tests in argon, the chamber was first allowed to The Youngs modulus that is obtained from the linear ump down to <13.3 Pa(100 mtorr) and then the chamber was tion of the curve in the enhanced SiC/SiC composite is-89 backfilled with high-purity argon gas. These steps were re- GPa, which is lower than that at 1000C(127 GPa)and much ated three times to ensure a thorough purge. The argon gas lower than that of the standard SiC/SiC composite at 1300C was flowed through the chamber enough to equal five times the (200 GPa). The Young's modulus of the enhanced SiC/Sic chamber volume. The volume percentage of oxygen in the ity argon gas was <I ppm 1000C to 1300 C, whereas the Youngs modulus of the stan dard SiC/SiC composite decreases -23%. It was reported that the Youngs modulus of the ceramic-grade NicalonTM fibers decreased 21% as the temperature increased from 1000C to American Society for Testing and Materials, Philadelphia, PA 1300oC, which is similar to the change of the Youngs modu- 20um Fig 1. Microstructures of the standard and enhanced SiC/SiC composites(a)standard SiC/SiC composite, (b)enhanced SiC/SiC composite, and (c) glassy phases of the matrix of the enhanced SiC/SiC composite
All the mechanical tests were performed using a servohydraulic testing system (Model MTS 810, MTS System Corp., Eden Prairie, MN) at a temperature of 1300°C. The monotonic tensile tests were conducted in air, under a constant displacement rate of 0.5 mm/min. The specimens were allowed to soak for ∼30 min at 1300°C before the tensile tests were started. The alignment between the upper and lower grips of the load unit was verified using the steel dummy specimen for verification that was supplied by MTS Corp. to allow a bending strain of <5%, in accordance with ASTM¶ Standard E 1012-89. Analytical and empirical analysis studies have concluded that, for negligible effects on the estimates of the strength-distribution parameters (for example, the Weibull modulus and characteristic strength) of monolithic advanced ceramics, the allowable percent bending, as defined in ASTM Practice E 1012, should not be >5%. Similar studies of the effect of bending on the tensile strength distributions of continuous-fiber-reinforced CMCs do not exist. ASTM Practice C 1275-94 has adopted the recommendations for the tensile testing of monolithic advanced ceramics. Because CMCs have inelastic deformation, which can redistribute the stress state and sometimes lead to notch insensitivity, a bending strain of 5% should not affect the strength distribution. The fatigue tests were performed with a sinusoidal loading frequency of 20 Hz in air. The stress ratio, which is defined as the ratio of minimum stress to maximum stress, was 0.1 for the testing temperature. Creep tests were conducted under a constant load in air and in an argon atmosphere. Creep strain was measured directly from the gauge length of the specimen by using a contact extensometer (Model MTS 632.53-F71, MTS System Corp.) that had a measuring range of ±2.5 mm over its gauge length of 25 mm. Periodically, partial unloading– reloading was applied, to measure the modulus change during the creep tests. The specimens were allowed to soak for >30 min at 1300°C before creep or cyclic-fatigue tests were started. A controlled-atmosphere furnace (Model MTS 659, MTS System Corp.) was used for the creep tests in the argon atmosphere. For the tests in argon, the chamber was first allowed to pump down to <13.3 Pa (100 mtorr) and then the chamber was backfilled with high-purity argon gas. These steps were repeated three times to ensure a thorough purge. The argon gas was flowed through the chamber enough to equal five times the chamber volume. The volume percentage of oxygen in the high-purity argon gas was <1 ppm. To compare the creep and fatigue properties of the enhanced SiC/SiC composite with those of the standard SiC/SiC composite, creep and fatigue tests of the standard SiC/SiC composite were conducted under the same conditions as those for the enhanced SiC/SiC composite in air. The creep and fatigue data of the standard SiC/SiC composite in argon were cited from Zhu et al.7 Mizuno et al.8 and Zhu et al.10 for comparison. After fracture, the specimens were examined by using optical microscopy and scanning electron microscopy (SEM). III. Results and Discussion (1) Microstructures and Monotonic Tension Micrographs of the enhanced and standard SiC/SiC composites in their original states are shown in Fig. 1. The differences between them are that there are glassy phases in the matrix of the enhanced SiC/SiC composite (the gray phases in the matrix in Fig. 1(c)). The thickness of the carbon layer at the interfaces of the enhanced SiC/SiC composite (0.5–0.6 mm) is larger than that of the standard SiC/SiC composite (0.1–0.2 mm). Moreover, there are more pores in the fiber bundles of the enhanced SiC/SiC composite than in those of the standard SiC/SiC composite. The tensile stress-versus-strain relation of the enhanced and standard SiC/SiC composites at 1300°C is shown in Fig. 2. The curves of the enhanced SiC/SiC composite indicate linear elastic behavior up to the proportional limit of ∼70 MPa, and this stress is ∼30% of the ultimate tensile strength (UTS) (225 MPa). The UTS of the enhanced SiC/SiC composite is almost the same as that of the standard SiC/SiC composite; however, the strain at the UTS of the enhanced SiC/SiC composite is much higher than that of the standard SiC/SiC composite. It is evident that the addition of glass-forming particulates in the matrix increases the ductility of the composite. A possible reason is the decrease of creep resistance of the matrix in the enhanced SiC/SiC composite. The Young’s modulus that is obtained from the linear portion of the curve in the enhanced SiC/SiC composite is ∼89 GPa, which is lower than that at 1000°C (127 GPa) and much lower than that of the standard SiC/SiC composite at 1300°C (200 GPa). The Young’s modulus of the enhanced SiC/SiC composite decreases 30% as the temperature increases from 1000°C to 1300°C, whereas the Young’s modulus of the standard SiC/SiC composite decreases ∼23%. It was reported that the Young’s modulus of the ceramic-grade Nicalon™ fibers decreased 21% as the temperature increased from 1000°C to 1300°C,25 which is similar to the change of the Young’s modu- ¶ American Society for Testing and Materials, Philadelphia, PA. Fig. 1. Microstructures of the standard and enhanced SiC/SiC composites ((a) standard SiC/SiC composite, (b) enhanced SiC/SiC composite, and (c) glassy phases of the matrix of the enhanced SiC/SiC composite). 2270 Journal of the American Ceramic Society—Zhu et al. Vol. 81, No. 9
September 1998 Creep and Fatigue Behavior in an Enhanced SiC/Sic Composite at High Temperature 2271 300 250 n200 150 1300ˇc 100 Enhanced, Air 50 Standard. ar 0.002 0.003 0.005 0.006 Strai Fig. 2. Monotonic tensile stress versus strain of the standard SiC/SiC composite in argon and the enhanced SiC/SiC composite in air, at 1300oC (the displacement rate was 0.5 mm/min). s modulus of the and moreso the matrix, contribute to the reduction of the modu- enhanced SiC matrix is-70 GPa at 1300oC. Because the cree lus of the enhanced SiC/SiC composite with temperature. Un- rate of the ceramic-grade Nicalon M fibers at 1300oC is suffi- ently hanced SiC matrix. The Young's modulus of the matrix can be within the time frame of their tests, as shown in Fig 3 calculated if the mixture law of modulus is assumed for the composite, which is (2) Creep and fatigue E=E4+Em(1--) Plots of the tensile creep strain versus time in the enhanced SiC/SiC composite at different maximum applied stresses in air where Ee, Es and Em are the Youngs moduli of the at 1300C are shown in Fig. 3. Only the transient creep stage fiber, and the matrix, respectively; Ve is the equivale exists at stresses >90 MPa, which is similar to the creep of fraction of fibers, and Ip is the volume fraction of or Nicalon TM fibers at 1300oC.28, 29 A long transient creep stage he standard SiC/SiC composite at room temperature, E and very short tertiary creep stage appear at 90 MPa. The 260 GPa. Er= 190 GPa 25 and Vn 10%. When v= 20% steady-state or minimum strain rates of cyclic creep are lower Em 320 GPa, which is similar to the measured Young's than those of static creep(Figs. 3 and 4 ). The steady-state of modulus of the CVI SiC matrix(300 GPa2). For the enhanced minimum strain rates of cyclic creep are calculated using the 0.007 0.014 0.006 1300C. 150 MPa. Air 0.005 0.01 0.004 0.008 Creep 0.003 E0.006 口 Fatigue 0.002 0,004 0.001 a Fatigue 0.002 03006009001200 5x1041×10 Time (9 Fig 3. Tensile creep strain versus time of the enhanced SiC/SiC composite under constant load(creep) and cyclic loading(fatigue)in air at 1300C loads of (a)150 and(b)90 MPa)
lus of the standard SiC/SiC composite. Therefore, the fibers, and moreso the matrix, contribute to the reduction of the modulus of the enhanced SiC/SiC composite with temperature. Unfortunately, there are no Young’s modulus data for the enhanced SiC matrix. The Young’s modulus of the matrix can be calculated if the mixture law of modulus is assumed for the composite, which is Ec 4 Ef Vf e + Em(1 − Vf e − Vp) (1) where Ec, Ef , and Em are the Young’s moduli of the composite, fiber, and the matrix, respectively; Vf e is the equivalent volume fraction of fibers, and Vp is the volume fraction of pores. For the standard SiC/SiC composite at room temperature, Ec 4 260 GPa,8 Ef 4 190 GPa,25 and Vp 4 10%. When Vf e 4 20%, Em ≈ 320 GPa, which is similar to the measured Young’s modulus of the CVI SiC matrix (300 GPa26). For the enhanced SiC/SiC composite, the calculated Young’s modulus of the enhanced SiC matrix is ∼70 GPa at 1300°C. Because the creep rate of the ceramic-grade Nicalon™ fibers at 1300°C is sufficiently high,25,27–29 the Young’s modulus is time dependent, within the time frame of their tests, as shown in Fig. 3. (2) Creep and Fatigue Plots of the tensile creep strain versus time in the enhanced SiC/SiC composite at different maximum applied stresses in air at 1300°C are shown in Fig. 3. Only the transient creep stage exists at stresses >90 MPa, which is similar to the creep of Nicalon™ fibers at 1300°C.28,29 A long transient creep stage and very short tertiary creep stage appear at 90 MPa. The steady-state or minimum strain rates of cyclic creep are lower than those of static creep (Figs. 3 and 4). The steady-state of minimum strain rates of cyclic creep are calculated using the Fig. 3. Tensile creep strain versus time of the enhanced SiC/SiC composite under constant load (creep) and cyclic loading (fatigue) in air at 1300°C (loads of (a) 150 and (b) 90 MPa). Fig. 2. Monotonic tensile stress versus strain of the standard SiC/SiC composite in argon and the enhanced SiC/SiC composite in air, at 1300°C (the displacement rate was 0.5 mm/min). September 1998 Creep and Fatigue Behavior in an Enhanced SiC/SiC Composite at High Temperature 2271
2272 Journal of the American Ceramic Society-Zhu et al. Vol 81. No 9 eep,n=8 1300°c,Air 如d 后 6 1300“c Standard. air Maximum stress, MP Stress, MPa Fig. 4. Minimum creep strain rate versus stress of the enhanced ig. 6. Minimum creep strain rate versus stress of the standard and SiC/SiC composite under constant load (creep) and cyclic loading enhanced SiC/SiC composites under constant load in air and in argon (fatigue)in air at 1300C atl300°C naximum strain, i.e, without considering the strain of the un- loading cycle one hand, is much improved. On the other hand, this condition We assume that creep strain rates(e)of the composites can is partly because the creep rates of Sic fibers in argon are be described by the power law igher than those in air. 27 Figure 6 also shows that the creep rates of the enhanced SiC/SiC composite in argon are higher than those of the stan- dard SiC/SiC composite, because of the low creep resistance of he enhanced SiC matrix. However, in air, the creep rates in the where A is a constant, n the stress exponent fo 2 the enhanced SiC/SiC composite are substantially lower than those activation energy for creep, R the gas constar in the standard SiC/SiC composite olute temperature, The stress exponent for cycl 10, All the stress exponents(n) for the creep of the enhanced and which is slightly higher than that for static andard SiC/SiC composites in air and argon are much higher (Fg.4). han the stress exponent(n= 1-2.5) for the creep of Nicalon Although there is an obvious difference in creep rates be- fibers.27-29 However, in a severe-matrix-cracking AL,O,/SiC a given maximum stress are almost the same(Fig. 5). If a consistent with that of the Al2 O, fiber ip of the composite is In Nicalon TM-fiber- shorter time at the maximum stress under fatigue is considered, reinforced glass-ceramics, n for creep of the composite is also a longer time to rupture under fatigue should be expected, the same as that of NicalonTM fibers. The creep rate is con- ompared to that for creep. The same time to rupture under trolled by the creep of fibers in either severe- matrix-cracking fatigue and creep means that the real behavior of fatigue is Al2O,/SiC composites or glass-ceramic-matrix SiC/calcium more complicated, which will be discussed in the last section. aluminosilicate(SiC/CAS) composites, because of their weak Figure 6 shows the creep rates of the enhanced and standard matrix. In SiC/SiC composites, the Sic SiC/SiC composites in air and argon. For the standard SiC/Sic resistance. Therefore, the creep-rate-co of the composite, the creep rates in air are much higher than those in enhanced SiC/SiC composite should be argon, because of the oxidation effects on creep. Conversely, of the fibers constrained by the matrix, an creep of free for the enhanced SiC/SiC composite, which implies that the Figure 7 shows that the time to rupture in air is longer than oxidation resistance of the enhanced SiC/SiC composite that in argon at a given stress in the enhanced SiC/SiC com- 1300°c Cree ague Strength Standard. Ai 50 10210310410 102 TIme to Rupture (s) TIme to Rupture, s Fig. 5. under constant in air at I300°C
maximum strain, i.e., without considering the strain of the unloading cycle. We assume that creep strain rates (e . ) of the composites can be described by the power law e˙ = Asn expS− Q RTD (2) where A is a constant, n the stress exponent for creep, Q the activation energy for creep, R the gas constant, and T the absolute temperature. The stress exponent for cyclic creep is 10, which is slightly higher than that for static creep (n 4 8) (Fig. 4). Although there is an obvious difference in creep rates between creep and fatigue (Fig. 4), their time-to-rupture values at a given maximum stress are almost the same (Fig. 5). If a shorter time at the maximum stress under fatigue is considered, a longer time to rupture under fatigue should be expected, compared to that for creep. The same time to rupture under fatigue and creep means that the real behavior of fatigue is more complicated, which will be discussed in the last section. Figure 6 shows the creep rates of the enhanced and standard SiC/SiC composites in air and argon. For the standard SiC/SiC composite, the creep rates in air are much higher than those in argon, because of the oxidation effects on creep. Conversely, the creep rates in argon are evidently higher than those in air for the enhanced SiC/SiC composite, which implies that the oxidation resistance of the enhanced SiC/SiC composite, on one hand, is much improved. On the other hand, this condition is partly because the creep rates of SiC fibers in argon are higher than those in air.27 Figure 6 also shows that the creep rates of the enhanced SiC/SiC composite in argon are higher than those of the standard SiC/SiC composite, because of the low creep resistance of the enhanced SiC matrix. However, in air, the creep rates in the enhanced SiC/SiC composite are substantially lower than those in the standard SiC/SiC composite. All the stress exponents (n) for the creep of the enhanced and standard SiC/SiC composites in air and argon are much higher than the stress exponent (n 4 1–2.5) for the creep of Nicalon™ fibers.27–29 However, in a severe-matrix-cracking Al2O3/SiC composite, the stress exponent n for creep of the composite is consistent with that of the Al2O3 fiber.11 In Nicalon™-fiberreinforced glass-ceramics, n for creep of the composite is also the same as that of Nicalon™ fibers. The creep rate is controlled by the creep of fibers in either severe-matrix-cracking Al2O3/SiC composites or glass-ceramic-matrix SiC/calcium aluminosilicate (SiC/CAS) composites, because of their weak matrix. In SiC/SiC composites, the SiC matrix has high creep resistance. Therefore, the creep-rate-controlling process of the enhanced SiC/SiC composite should be considered to be creep of the fibers constrained by the matrix, rather than creep of free fibers. Figure 7 shows that the time to rupture in air is longer than that in argon at a given stress in the enhanced SiC/SiC comFig. 4. Minimum creep strain rate versus stress of the enhanced SiC/SiC composite under constant load (creep) and cyclic loading (fatigue) in air at 1300°C. Fig. 5. Maximum stress versus time to rupture of the enhanced SiC/ SiC composite under constant load (creep) and cyclic loading (fatigue) in air at 1300°C. Fig. 6. Minimum creep strain rate versus stress of the standard and enhanced SiC/SiC composites under constant load in air and in argon at 1300°C. Fig. 7. Stress versus time to rupture of the standard and enhanced SiC/SiC composites under constant load in air and in argon at 1300°C. 2272 Journal of the American Ceramic Society—Zhu et al. Vol. 81, No. 9
September 1998 Creep and Fatigue Behavior in an Enhanced SiC/SiC Composite at High Temperature 2273 000 posite, whereas the time to rupture in air is much shorter than 1300c that in argon at a given stress in the standard SiC/SiC compos- ite. The time to rupture of the enhanced SiC/SiC composite is much longer than that of the standard SiC/SiC composite in air Although the creep rate of the enhanced SiC/SiC composite in argon higher than that of the standard SiC/SiC composite in the time to rupture of the enhanced SiC/SiC composite is still longer than that of the standard SiC/SiC composite. This phenomenon demonstrates that the addition of a glassy phase in he matrix of the enhanced SiC/SiC composite increases creep C=- Standard, Air rates, but much improves the total creep time to rupture in 10 argon. The reason for this result may be understood by the 10102103104105106107 match of the creep resistance between the fibers and the matrix ated Cycles to Failure SiC(NicalonTM) fibers is lower than that of the SiC ma trix.7,30-34 Creep of fibers transfers the stress onto the matrix Fig.8. Maximum stress versus number of cycles to failure of the and causes matrix cracks. Matrix cracking reloads the fibers enhanced SiC/SiC composite in air and the standard SiC/SiC compos- As the matrix creep resistance decreases, creep relaxation of ite in air and in argon under cyclic loading at 1300C he matrix may decrease the matrix cracking and stress con- centration near large pores, at which creep cracks are often Plots of the cyclic-fatigue life versus the maximum stress of he enhanced SiC/SiC composite in air and the standard SiC/ M2 030um (a ▲ Pores 25um for 2 h
posite, whereas the time to rupture in air is much shorter than that in argon at a given stress in the standard SiC/SiC composite. The time to rupture of the enhanced SiC/SiC composite is much longer than that of the standard SiC/SiC composite in air. Although the creep rate of the enhanced SiC/SiC composite in argon is higher than that of the standard SiC/SiC composite in argon, the time to rupture of the enhanced SiC/SiC composite is still longer than that of the standard SiC/SiC composite. This phenomenon demonstrates that the addition of a glassy phase in the matrix of the enhanced SiC/SiC composite increases creep rates, but much improves the total creep time to rupture in argon. The reason for this result may be understood by the match of the creep resistance between the fibers and the matrix. As stated in the Introduction section, the creep resistance of SiC (Nicalon™) fibers is lower than that of the SiC matrix.7,30–34 Creep of fibers transfers the stress onto the matrix and causes matrix cracks. Matrix cracking reloads the fibers. As the matrix creep resistance decreases, creep relaxation of the matrix may decrease the matrix cracking and stress concentration near large pores, at which creep cracks are often initiated.7 Plots of the cyclic-fatigue life versus the maximum stress of the enhanced SiC/SiC composite in air and the standard SiC/ Fig. 8. Maximum stress versus number of cycles to failure of the enhanced SiC/SiC composite in air and the standard SiC/SiC composite in air and in argon under cyclic loading at 1300°C. Fig. 9. Micrographs depicting crack propagation in specimens of the enhanced SiC/SiC composite (a) fatigued in air at 1300°C and a load of 90 MPa for 2.8 × 106 cycles, (b) crept in argon at 1300°C and a load of 90 MPa for 2 h, and (c) crept in air at 1300°C and a load of 90 MPa for 2 h. September 1998 Creep and Fatigue Behavior in an Enhanced SiC/SiC Composite at High Temperature 2273