《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_C-SiC-12

MATERIALS CIENCE EIEERIG ELSEVIER Materials Science and Engineering A 435-436(2006)412-417 www.elsevier.com/locate/msea Tension-tension fatigue damage characteristics of a 3D SiC/SiC composite in H20-O2-Ar environment at 1300C Shoujun Wu", Laifei Cheng, Jun Zhang, Litong Zhang, Xingang Luan, Hui Mei, Peng Fang National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi'an Shaanxi 710072, People's Republic of China Received 27 May 2006: received in revised form 5 July 2006: accepted 14 July 2006 Abstract Tension-tension fatigue behaviors of a 3D woven SiC fiber(Hi-Nicalon)reinforced CVI SiC-matrix composite were measured in a H20-02-Ar environment at 1300C. The results indicated that the SiC/SiC had excellent fatigue resistance in H20-O2-Ar atmosphere Fiber/matrix interface debonding, sliding and fibers pull out in tension-tension fatigue tests were more obvious than those in monotonic tension tests. The cross-section surface of the monotonic tension specimens was coarse in whole, while that of the fatigue specimens were composed of ringed striations and smooth area. During tension-tension fatigue test, the coexisting damage characteristics were:(i) cracking of SiC matrix, disengaging of Sic matrix and grains growth; (ii) partial oxidation or burn out of pyrolytic carbon(PyC) interlayer and severe oxidation of fibers; (iii) fibers torn and fracture C 2006 Elsevier B. V. All rights reserved. Keywords: SiC/SiC composite: Tension-tension fatigue: Oxidation; Water vapor; Damage 1. Introduction vated temperature is important to their applications in harsh environments Silicon carbide fiber reinforced Sic-matrix composites fab- The fatigue behaviors of Sic/SiC composites have been by chemical vapor infiltration( CVI process(SiC/SiC) researched [7-13]. It should be noted that: firstly, the reinforce- en intensively researched as one of the most promising ments involved are mainly 2D woven Sic cloth which contain structural materials due to their high toughness, good a relative higher content of oxygen(14.4 or 15 wt%0)[7-9]and resistance to thermal shock, good mechanical properties at high the research interesting were focused on damage characteri- temperature, especially improved flaw tolerance and noncatas- zation by the effective dynamic modulus of elasticity and the trophic mode of failure [1-4]. As an example, SiC/SiC can be corresponding damping coefficient [8, 9]. Secondly, some of the used as combustor liners and turbine vanes for propulsion power involved composites contained boron [8, 10, 11] which had dis- generation tinct effects on the oxidation behavior [14], especially in wet In many cases, high-loading frequency and temperature capa- oxidizing environment. Thirdly, few researches were conducted bilities are required when SiC/SiC was used as some compo- in wet environment [11], and the experimental temperature was nents in reusable launch vehicles and/or space applications. much lower than the application environment. Hi-Nicalon fiber One of such applications is its use in a CMc bladed disk for is oxygen-free fibers consisting of a mixture of Sic-nanocrystals rocket engine turbopumps [5]. Moreover, in combustion pro-(5 nm in mean size)and free carbon [C/Si(at )ratio=1. 39] ess, substantial amounts of water vapor are produced from It is well known that the thermo-mechanical properties of fiber burning hydrocarbon fuels in air. Calculations showed that reinforced composites was greatly influenced by the architecture 10% of the combustion gas is water vapor under equilibrium of fiber preforms, especially chemical composition, microstruc conditions [6]. Therefore, the knowledge on fatigue behaviors ture of fibers and its strength as well as the service environ of this kind of materials in the moisture environment at ele- ments [1, 15-17. Thus, the fatigue behavior of the Hi-Nicalon fiber reinforced CVI SiC-matrix composites may be different form the reported ones. However, up to now, the research on Corresponding author. Tel. +8629 8848 6068 828: fax: +8629 88494620. the fatigue damage characteristics of Sic/SiC, especially the E-lmailaddress:shoujun_wu@163.com(Swu). research results of Hi-Nicalon fiber reinforced cvi SiC-matrix )6 Elsevier B v. All rights reserved

Materials Science and Engineering A 435–436 (2006) 412–417 Tension–tension fatigue damage characteristics of a 3D SiC/SiC composite in H2O–O2–Ar environment at 1300 ◦C Shoujun Wu ∗, Laifei Cheng, Jun Zhang, Litong Zhang, Xingang Luan, Hui Mei, Peng Fang National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi’an Shaanxi 710072, People’s Republic of China Received 27 May 2006; received in revised form 5 July 2006; accepted 14 July 2006 Abstract Tension–tension fatigue behaviors of a 3D woven SiC fiber (Hi-Nicalon) reinforced CVI SiC–matrix composite were measured in a H2O–O2–Ar environment at 1300 ◦C. The results indicated that the SiC/SiC had excellent fatigue resistance in H2O–O2–Ar atmosphere. Fiber/matrix interface debonding, sliding and fibers pull out in tension–tension fatigue tests were more obvious than those in monotonic tension tests. The cross-section surface of the monotonic tension specimens was coarse in whole, while that of the fatigue specimens were composed of ringed striations and smooth area. During tension–tension fatigue test, the coexisting damage characteristics were: (i) cracking of SiC matrix, disengaging of SiC matrix and grains growth; (ii) partial oxidation or burn out of pyrolytic carbon (PyC) interlayer and severe oxidation of fibers; (iii) fibers torn and fracture. © 2006 Elsevier B.V. All rights reserved. Keywords: SiC/SiC composite; Tension–tension fatigue; Oxidation; Water vapor; Damage 1. Introduction Silicon carbide fiber reinforced SiC–matrix composites fab￾ricated by chemical vapor infiltration (CVI) process (SiC/SiC) have been intensively researched as one of the most promising thermal structural materials due to their high toughness, good resistance to thermal shock, good mechanical properties at high temperature, especially improved flaw tolerance and noncatas￾trophic mode of failure [1–4]. As an example, SiC/SiC can be used as combustor liners and turbine vanes for propulsion power generation. In many cases, high-loading frequency and temperature capa￾bilities are required when SiC/SiC was used as some compo￾nents in reusable launch vehicles and/or space applications. One of such applications is its use in a CMC bladed disk for rocket engine turbopumps [5]. Moreover, in combustion pro￾cess, substantial amounts of water vapor are produced from burning hydrocarbon fuels in air. Calculations showed that 5–10% of the combustion gas is water vapor under equilibrium conditions [6]. Therefore, the knowledge on fatigue behaviors of this kind of materials in the moisture environment at ele- ∗ Corresponding author. Tel.: +86 29 8848 6068 828; fax: +86 29 8849 4620. E-mail address: shoujun wu@163.com (S. Wu). vated temperature is important to their applications in harsh environments. The fatigue behaviors of SiC/SiC composites have been researched [7–13]. It should be noted that: firstly, the reinforce￾ments involved are mainly 2D woven SiC cloth which contain a relative higher content of oxygen (14.4 or 15 wt.%) [7–9] and the research interesting were focused on damage characteri￾zation by the effective dynamic modulus of elasticity and the corresponding damping coefficient [8,9]. Secondly, some of the involved composites contained boron [8,10,11] which had dis￾tinct effects on the oxidation behavior [14], especially in wet oxidizing environment. Thirdly, few researches were conducted in wet environment [11], and the experimental temperature was much lower than the application environment. Hi-Nicalon fiber is oxygen-free fibers consisting of a mixture of SiC-nanocrystals (≈5 nm in mean size) and free carbon [C/Si (at.) ratio = 1.39]. It is well known that the thermo-mechanical properties of fiber reinforced composites was greatly influenced by the architecture of fiber preforms, especially chemical composition, microstruc￾ture of fibers and its strength as well as the service environ￾ments [1,15–17]. Thus, the fatigue behavior of the Hi-Nicalon fiber reinforced CVI SiC–matrix composites may be different form the reported ones. However, up to now, the research on the fatigue damage characteristics of SiC/SiC, especially the research results of Hi-Nicalon fiber reinforced CVI SiC–matrix 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.07.041

S Wu et al. Materials Science and Engineering A 435-436(2006)412-417 posite fabricated by four-step three-dimensional (4-step 3D) long, 3 mm wide and 3 mm thick) were kept in the hot zone ater-containing environment have not been found and oxidizing atmospheres. Deionized water was used to engen- The present investigation presents the tension-tension fatigue der water vapor. The partial pressure of water vapor, oxy damage behaviors of a 3D PyC-interphase SiC/SiC composite gen and argon was 15, 8 and 78 kPa, respectively. The flux (reinforced by Hi-Nicalon SiC fibers)with a CVD SiC coating of gases was accurately controlled by a mass flow controller in a H2O-O2-Ar environments at 1300C. Much of analysis (5850 i series from BROOKS, in Japan) with a precision of and discussion will then focus on the fatigue characteristics of 0.1 SCCM. The tests were run under a maximum applied stress the composite of 120 MPa, at a frequency of I Hz and with a stress ratio of R=0.5(R=Omin/o max), with a sinusoidal wave form. In order to 2. Materials and experimental procedure gain knowledge of the tension-tension fatigue life of the SiC/SiC in H2O-02-Ar environment, the tests were carried out until the 2.1. Sample preparation specimen fully failed. Microstructures of the samples before and after tests were SiC fiber(Hi-Nicalon, Nippon Carbon Co., Japan) pre- examined using a scanning electron microscope(SEM, $4700 form was prepared using four-step three-dimensional (4-step 3D)braiding method. Low pressure chemical vapor infiltra-3.Results tion(LPCvd) process was employed to deposit pyrolytic carbon (PyC) as an interphase and silicon carbide as a matrix. The vol- 3. 1. Monotonic tensile behavior of the SiC/SiC composite ume fraction of SiC fiber was about 40% and the braiding angle was about 200. PyC layer was deposited on the fiber by decom- The average ultimate tensile strength(UTS)of the SiC/SiC positions of C3 Hs at 870C for I h at reduced pressure of 5 kPa composites at room temperature measured from the monotonic in a CVI reactor, arriving to a thickness of 0. 2 um Methyl- tension test was 456+40MPa trichlorosilane(MTS, CH3 SiCl3) was used for the deposition of Fig. 2 shows the typical load-extension curve of the SiC/SiC SiC, carried by bubbling hydrogen in gas phase and argon measured under monotonic tension the dilute gas to slow down the chemical reaction rate during load-extension curve indicated initially a linear elastic behay deposition SiC matrix was prepared at 1100"C for 120h(30h ior up to the proportional limit of 1000N(corresponding to per times) at reduced pressure of 5 kPa, and the molar ratio of H2 about 130MPa), which was about 29% of the ultimate ten- to MTS was 10. The as-received SiC/SiC was cut into dog-bone sile strength. Then the curve showed a continuously decreasing emples(showed in Fig. 1)along the fibers longitudinal direc- slope and associated non-linear displacements. After that, the n. Density and open porosity of the samples were 2.7gcm-3 curve showed a linear portion till the fracture of the composite and 7.3%, respectively, measured by Archimedes method. A It should be noted that the load-extension curve for all spec CVD SiC coating was deposited on the samples for 20h to seal imens showed a slight steep drop in the final portion at the the open ends of the fiber load range from 2500 to 2800N (326-365 MPa), then the curve quickly restored. This monotonic tensile curve of the 4-step 3D 2.2. Test details SiC/SiC in this study is different from those observed in 2D Sic/SiC and those in 3D SiC/SiC with an orthogonal architecture The monotonic tensile tests were performed at room temper- [11,18) ature to determine the tensile properties of the material, namely, It has been supposed that the linear proportion in the ultimate tensile stress and to compare the damage character- load-extension curve represents the portion before any appre- istics. The load increased at a constant rate of 0.001 mms-up ciable amount of matrix crack, developed in the composite [ll] to fracture of the specimens. The displacement, load and strain The non-linear in curve or the transition was due to the sub were monitored Tension-tension fatigue tests of the SiC/SiC composite were conducted in a H20-02-Ar environment at 1300C. Tension-tension fatigue tests were conducted with integrated system including a resistance heating furnace and a servo- hydraulic machine(Model INSTRoN 8801 from INSTRON Ltd, England). Only the gauge parts of the specimens(40 mm 4-R8.5 Fig 1. A schematic showing of the dog-bone shape SiC/SiC specimen(the unit Fig. 2. Typical load-extension curves of the 3D SiC/SiC composite in monotonic

S. Wu et al. / Materials Science and Engineering A 435–436 (2006) 412–417 413 composite fabricated by four-step three-dimensional (4-step 3D) in water-containing environment have not been found. The present investigation presents the tension–tension fatigue damage behaviors of a 3D PyC-interphase SiC/SiC composite (reinforced by Hi-Nicalon SiC fibers) with a CVD SiC coating in a H2O–O2–Ar environments at 1300 ◦C. Much of analysis and discussion will then focus on the fatigue characteristics of the composite. 2. Materials and experimental procedure 2.1. Sample preparation SiC fiber (Hi-Nicalon, Nippon Carbon Co., Japan) pre￾form was prepared using four-step three-dimensional (4-step 3D) braiding method. Low pressure chemical vapor infiltra￾tion (LPCVI) process was employed to deposit pyrolytic carbon (PyC) as an interphase and silicon carbide as a matrix. The vol￾ume fraction of SiC fiber was about 40% and the braiding angle was about 20◦. PyC layer was deposited on the fiber by decom￾positions of C3H6 at 870 ◦C for 1 h at reduced pressure of 5 kPa in a CVI reactor, arriving to a thickness of 0.2
m. Methyl￾trichlorosilane (MTS, CH3SiCl3) was used for the deposition of SiC, carried by bubbling hydrogen in gas phase and argon as the dilute gas to slow down the chemical reaction rate during deposition. SiC matrix was prepared at 1100 ◦C for 120 h (30 h per times) at reduced pressure of 5 kPa, and the molar ratio of H2 to MTS was 10. The as-received SiC/SiC was cut into dog-bone samples (showed in Fig. 1) along the fibers longitudinal direc￾tion. Density and open porosity of the samples were 2.7 g cm−3 and 7.3%, respectively, measured by Archimedes method. A CVD SiC coating was deposited on the samples for 20 h to seal the open ends of the fibers. 2.2. Test details The monotonic tensile tests were performed at room temper￾ature to determine the tensile properties of the material, namely, the ultimate tensile stress and to compare the damage character￾istics. The load increased at a constant rate of 0.001 mm s−1 up to fracture of the specimens. The displacement, load and strain were monitored. Tension–tension fatigue tests of the SiC/SiC composite were conducted in a H2O–O2–Ar environment at 1300 ◦C. Tension–tension fatigue tests were conducted with integrated system including a resistance heating furnace and a servo￾hydraulic machine (Model INSTRON 8801 from INSTRON Ltd., England). Only the gauge parts of the specimens (40 mm Fig. 1. A schematic showing of the dog-bone shape SiC/SiC specimen (the unit was mm). long, 3 mm wide and 3 mm thick) were kept in the hot zone and oxidizing atmospheres. Deionized water was used to engen￾der water vapor. The partial pressure of water vapor, oxy￾gen and argon was 15, 8 and 78 kPa, respectively. The flux of gases was accurately controlled by a mass flow controller (5850 i series from BROOKS, in Japan) with a precision of 0.1 SCCM. The tests were run under a maximum applied stress of 120 MPa, at a frequency of 1 Hz and with a stress ratio of R = 0.5 (R = σmin/σmax), with a sinusoidal wave form. In order to gain knowledge of the tension–tension fatigue life of the SiC/SiC in H2O–O2–Ar environment, the tests were carried out until the specimen fully failed. Microstructures of the samples before and after tests were examined using a scanning electron microscope (SEM, S4700). 3. Results 3.1. Monotonic tensile behavior of the SiC/SiC composite The average ultimate tensile strength (UTS) of the SiC/SiC composites at room temperature measured from the monotonic tension test was 456 ± 40 MPa. Fig. 2 shows the typical load–extension curve of the SiC/SiC measured under monotonic tension at room temperature. The load–extension curve indicated initially a linear elastic behav￾ior up to the proportional limit of 1000 N (corresponding to about 130 MPa), which was about 29% of the ultimate ten￾sile strength. Then the curve showed a continuously decreasing slope and associated non-linear displacements. After that, the curve showed a linear portion till the fracture of the composite. It should be noted that the load–extension curve for all spec￾imens showed a slight steep drop in the final portion at the load range from 2500 to 2800 N (326–365 MPa), then the curve quickly restored. This monotonic tensile curve of the 4-step 3D SiC/SiC in this study is different from those observed in 2D SiC/SiC and those in 3D SiC/SiC with an orthogonal architecture [11,18]. It has been supposed that the linear proportion in load–extension curve represents the portion before any appre￾ciable amount of matrix crack, developed in the composite [11]. The non-linear in curve or the transition was due to the sub￾Fig. 2. Typical load–extension curves of the 3D SiC/SiC composite in monotonic tension at room temperature

S. Wu et al./ Materials Science and Engineering A 435-436(2006)412-17 Fig. 3. Cross-section morphologies of the 3D SiC/SiC composite after:(a)monotonic tension test;(b) tension-tension fatigue test. Fig. 4.(a) Fiber/matrix interface debonding:(b)fibers sliding and pull out in the SiC/SiC composite after tension-tension fatigue sequent matrix/fiber interface debonding and sliding of fibers. pull out in tension-tension fatigue tested specimens was more The final linear portion was mainly owing to the damage regime and longer than that in monotonic tested specimens. Moreover, [19]. At this study, due to the fibers preform was prepared by the fibers in tension-tension fatigue tested specimens were much 4-step 3D method, the braiding angle may be reduced when the looser, while those in monotonic tested specimens kept bunch matrix damaged under the tension load. Consequently, the slight iness. These results indicate that fiber/matrix interface debond teep drop in the final linear portion is supposed to come from ing, sliding and fibers pull out in tension-tension fatigue tests the reduction of this braiding angle were more obvious than those in monotonic tension tests, as shown in Fig 4 3.2. Tension-tension fatigue damage characteristics Fig 5 shows the comparison of cross-section morphologie of fibers in the Sic/Sic composite after tension-tension fatigue Tension-tension fatigue of the 3D SiC/SiC composite in the test and after monotonic tension test, respectively. It shows that H20-02-Ar environments at 1300C lasted up to 105 h and the whole cross-section surface of the monotonic tension tested 12 min indicating the composite has excellent fatigue capacity. specimens was coarse. However, the cross-section surface of However, the damage characteristics should be clarified, because the tension-tension fatigue tested specimens can be obviously it is important to design, evaluate and maintain the components divided into two different areas: one area was composed of made of this SiC/SiC composite in engines application. In this ringed striations which took up about half of the whole section, part, damage characteristics after the tension-tension fatigue while the other area was smooth. tests were reported Fig. 6 shows the micromorphologies of Sic matrix in the Fig 3 shows the cross-section of the specimens after mono- SiC/SiC after tension-tension fatigue test. It can be seen that tonic tension test and tension-tension fatigue tests. The fibers three changes occurred in the matrix. Firstly, the matrix cracked Fig. 5. Cross-section morphologies of the fibers in 3D SiC/SiC composite after(a)monotonic tension test; (b)tension-tension fatigue test

414 S. Wu et al. / Materials Science and Engineering A 435–436 (2006) 412–417 Fig. 3. Cross-section morphologies of the 3D SiC/SiC composite after: (a) monotonic tension test; (b) tension–tension fatigue test. Fig. 4. (a) Fiber/matrix interface debonding; (b) fibers sliding and pull out in the SiC/SiC composite after tension–tension fatigue. sequent matrix/fiber interface debonding and sliding of fibers. The final linear portion was mainly owing to the damage regime [19]. At this study, due to the fibers preform was prepared by 4-step 3D method, the braiding angle may be reduced when the matrix damaged under the tension load. Consequently, the slight steep drop in the final linear portion is supposed to come from the reduction of this braiding angle. 3.2. Tension–tension fatigue damage characteristics Tension–tension fatigue of the 3D SiC/SiC composite in the H2O–O2–Ar environments at 1300 ◦C lasted up to 105 h and 12 min indicating the composite has excellent fatigue capacity. However, the damage characteristics should be clarified, because it is important to design, evaluate and maintain the components made of this SiC/SiC composite in engines application. In this part, damage characteristics after the tension–tension fatigue tests were reported. Fig. 3 shows the cross-section of the specimens after mono￾tonic tension test and tension–tension fatigue tests. The fibers pull out in tension–tension fatigue tested specimens was more and longer than that in monotonic tested specimens. Moreover, the fibers in tension–tension fatigue tested specimens were much looser, while those in monotonic tested specimens kept bunch￾iness. These results indicate that fiber/matrix interface debond￾ing, sliding and fibers pull out in tension–tension fatigue tests were more obvious than those in monotonic tension tests, as shown in Fig. 4. Fig. 5 shows the comparison of cross-section morphologies of fibers in the SiC/SiC composite after tension–tension fatigue test and after monotonic tension test, respectively. It shows that the whole cross-section surface of the monotonic tension tested specimens was coarse. However, the cross-section surface of the tension–tension fatigue tested specimens can be obviously divided into two different areas: one area was composed of ringed striations which took up about half of the whole section, while the other area was smooth. Fig. 6 shows the micromorphologies of SiC matrix in the SiC/SiC after tension–tension fatigue test. It can be seen that three changes occurred in the matrix. Firstly, the matrix cracked Fig. 5. Cross-section morphologies of the fibers in 3D SiC/SiC composite after: (a) monotonic tension test; (b) tension–tension fatigue test

S Wu et al. Materials Science and Engineering A 435-436(2006)412-417 Fig. 6.(a)Cracking of SiC matrix;(b and c) disengaging of SiC matrix and grains growth in the 3D SiC/SiC composite after tension-tension fatigue perpendicularly to longitudinal direction of fiber bundles as deposited Sic-nanogranules [20] grew under the cooperation shown in Fig. 6(a). Secondly, the SiC matrix zigzag delam- of stress inducement [21, 22] and long time exposure at high ination at interface of two depositions, as shown in Fig. 6(b temperature and c). Thirdly, the Sic grains evidently grow up, as shown in The cracking of matrix resulted in the diffusion of oxidizing Fig. 6(c) gas, namely oxygen and water vapor, into the PyC interlayer, which led to oxidation of PyC and SiC phases at 1300Cin two 4. Discussion possible manners: (1)the opening width of microcracks was much narrow and the amount of oxygen was few for burning out During monotonic loading, the fibers were first undamaged, of PyC before the specimens fractured. In this case, oxidation then the volume fraction of broken fibers also increased lead- consumption of Pyc interlayer was predominant and the oxi ng to the sliding areas at the fiber/matrix interface. Hence, the dation of the Sic phase was rather less as shown in Fig. 8(a), failure behavior of ceramic-matrix composites depends mainly which was favorable to subsequent oxidation and fibers pull on two microstructural parameters: (i) the individual failure out.(2) The microcracks were relatively wider than the for- stress distribution of the fibers and (ii) the characteristics of the mer, the amount of oxygen was too much for the burn out of iber/matrix interface. In fiber reinforced CVI matrix compos- PyC before specimens fractured. In this case, the Sic phase was ite, the fiber bundles were tightly constrained by CVI matrix, severely oxidized, as shown in Fig. 8(b-e), so that the fibers acting as a unitary unit under load. As shown in Fig. 7, the and matrix adhered to each other by the formed Sio2 as shown PyC interphase was closely bonded to both CVI SiC matrix in Fig. 8(b). Thus, the sliding and pull out of fibers will be and Hi-Nicalon fiber. Therefore, it was difficult for the Hi- hampered by the adhesion of Sio2. As a result, due to adhe Nicalon SiC fiber to debond and to be pulled out from the sion between fiber and matrix by the formed SiOz, some of the silicon carbide matrix, which resulted in the fracture of fibers fibers will be unilaterally torn during pull out under tensile load, was mainly in bundles. Though the stress was much lower for as shown in Fig. 8(f. It is obviously that all of these different matrix cracking or the composite fracture during tension-tension types of damage characteristics were coexistent. On the other fatigue load, it resulted in the braiding architecture of the hand, the formed SiO2 may crack during the change of tension composite changed, i.e. braiding angle reduction due to the load. repeat loading. The braiding angle reduction led to microcracks As shown in Fig 9, when the specimen was under tensile propagation and reduction of constrains between matrix and load, the tensile stress in fibers flexural outboard was higher fibers, which result in the cracking of the SiC matrix at the than that in inboard because the fibers were tortuous. Under interface of two depositions. Thus, the 3D SiC/SiC compos- tension-tension fatigue, when the load is higher than average ite after tension-tension fatigue tests showed a more obvious value, microcracks on the fibers outboard surface will extend pull out than that in monotonic tension tests. Furthermore, the in radial direction. When the load was below average level, the stress in the whole fiber might be released and there were no microcracks propagation occurred. Consequently, the microc racks propagation is off and on when the specimens are loaded Matrix Interphase layer fluctuating tensile load which led to the fiber shows ringed stri- ations. When the microcracks propagated to where is the center of fibers, it is supposed that the fibers will suddenly brittle frac- tured due to stress concentration and the effective stress was much higher than tensile strength. Thus, the cross-section sur- face of fibers showed a smooth area as a result of brittle fracture of fibers Fiber Furthermore it can be concluded that these damage char- acteristics of 4-step 3D SiC/SiC composite in tension-tension fatigue was directly related to the braiding architecture of fiber Fig. 7. Typical micromorphology of fiberlinterphase/matrix in the as-received preform, which determined the characteristics of stress state 3D SiC/SiC composite matrix changes and fibers damage and pull out

S. Wu et al. / Materials Science and Engineering A 435–436 (2006) 412–417 415 Fig. 6. (a) Cracking of SiC matrix; (b and c) disengaging of SiC matrix and grains growth in the 3D SiC/SiC composite after tension–tension fatigue. perpendicularly to longitudinal direction of fiber bundles as shown in Fig. 6(a). Secondly, the SiC matrix zigzag delam￾ination at interface of two depositions, as shown in Fig. 6(b and c). Thirdly, the SiC grains evidently grow up, as shown in Fig. 6(c). 4. Discussion During monotonic loading, the fibers were first undamaged, then the volume fraction of broken fibers also increased lead￾ing to the sliding areas at the fiber/matrix interface. Hence, the failure behavior of ceramic–matrix composites depends mainly on two microstructural parameters: (i) the individual failure stress distribution of the fibers and (ii) the characteristics of the fiber/matrix interface. In fiber reinforced CVI matrix compos￾ite, the fiber bundles were tightly constrained by CVI matrix, acting as a unitary unit under load. As shown in Fig. 7, the PyC interphase was closely bonded to both CVI SiC matrix and Hi-Nicalon fiber. Therefore, it was difficult for the Hi￾Nicalon SiC fiber to debond and to be pulled out from the silicon carbide matrix, which resulted in the fracture of fibers was mainly in bundles. Though the stress was much lower for matrix cracking or the composite fracture during tension–tension fatigue load, it resulted in the braiding architecture of the composite changed, i.e. braiding angle reduction due to the repeat loading. The braiding angle reduction led to microcracks propagation and reduction of constrains between matrix and fibers, which result in the cracking of the SiC matrix at the interface of two depositions. Thus, the 3D SiC/SiC compos￾ite after tension–tension fatigue tests showed a more obvious pull out than that in monotonic tension tests. Furthermore, the Fig. 7. Typical micromorphology of fiber/interphase/matrix in the as-received 3D SiC/SiC composite. deposited SiC-nanogranules [20] grew under the cooperation of stress inducement [21,22] and long time exposure at high temperature. The cracking of matrix resulted in the diffusion of oxidizing gas, namely oxygen and water vapor, into the PyC interlayer, which led to oxidation of PyC and SiC phases at 1300 ◦C in two possible manners: (1) the opening width of microcracks was much narrow and the amount of oxygen was few for burning out of PyC before the specimens fractured. In this case, oxidation consumption of PyC interlayer was predominant and the oxi￾dation of the SiC phase was rather less as shown in Fig. 8(a), which was favorable to subsequent oxidation and fibers pull out. (2) The microcracks were relatively wider than the for￾mer, the amount of oxygen was too much for the burn out of PyC before specimens fractured. In this case, the SiC phase was severely oxidized, as shown in Fig. 8(b–e), so that the fibers and matrix adhered to each other by the formed SiO2 as shown in Fig. 8(b). Thus, the sliding and pull out of fibers will be hampered by the adhesion of SiO2. As a result, due to adhe￾sion between fiber and matrix by the formed SiO2, some of the fibers will be unilaterally torn during pull out under tensile load, as shown in Fig. 8(f). It is obviously that all of these different types of damage characteristics were coexistent. On the other hand, the formed SiO2 may crack during the change of tension load. As shown in Fig. 9, when the specimen was under tensile load, the tensile stress in fibers flexural outboard was higher than that in inboard because the fibers were tortuous. Under tension–tension fatigue, when the load is higher than average value, microcracks on the fibers’ outboard surface will extend in radial direction. When the load was below average level, the stress in the whole fiber might be released and there were no microcracks propagation occurred. Consequently, the microc￾racks propagation is off and on when the specimens are loaded fluctuating tensile load which led to the fiber shows ringed stri￾ations. When the microcracks propagated to where is the center of fibers, it is supposed that the fibers will suddenly brittle frac￾tured due to stress concentration and the effective stress was much higher than tensile strength. Thus, the cross-section sur￾face of fibers showed a smooth area as a result of brittle fracture of fibers. Furthermore, it can be concluded that these damage char￾acteristics of 4-step 3D SiC/SiC composite in tension–tension fatigue was directly related to the braiding architecture of fiber preform, which determined the characteristics of stress state, matrix changes and fibers damage and pull out

416 S. Wu et al./ Materials Science and Engineering A 435-436(2006)412-17 SiO, Fiber Matrix d SiO2 produced at the matrix crack m Fig. 8. Different types of oxidation and fibers damage in the SiC/SiC composite after tension-tension fatigue in wet oxidizing environments at 1300 C for 105 h and 12 min: (a) partial oxidation of PyC interlayer; (b)adhesion between fibers and matrix;(c) SiOz locally formed at the matrix cracks; (d)magnified view of (c); (e)unilateral oxidation damage on the fiber surface (f) fibers torn damage. Fiber 十+ Fig 9.(a) SEM image of the fibers preform fabricated by 4-step 3D braiding method; (b) schematic of stress state of the fibers and matrix under tensile load 5. Conclusions Fiber/matrix interface debonding, sliding and fibers pull out in tension-tension fatigue tests were more obvious than those A three-dimensional SiC/Sic composite was prepared by in monotonic tension tests. The fracture surface of the mono- chemical vapor infiltration process, and tension-tension fatigue tonic tension specimens was coarse while that of the fatigue test of the SiC/SiC composite was carried out in a H20-O2-Ar specimens shows two different areas, namely ringed striations environment at 1300 C. The results indicated that the SiC/Sic and smooth area. during tension-tension fatigue the coexisting has excellent fatigue capacity as the tests lasted up to 105 h and damage characteristics were: (i) cracking of Sic matrix, dis- 12 min engaging of SiC matrix and grains growth; (ii) PyC interlayer

416 S. Wu et al. / Materials Science and Engineering A 435–436 (2006) 412–417 Fig. 8. Different types of oxidation and fibers damage in the SiC/SiC composite after tension–tension fatigue in wet oxidizing environments at 1300 ◦C for 105 h and 12 min: (a) partial oxidation of PyC interlayer; (b) adhesion between fibers and matrix; (c) SiO2 locally formed at the matrix cracks; (d) magnified view of (c); (e) unilateral oxidation damage on the fiber surface; (f) fibers torn damage. Fig. 9. (a) SEM image of the fibers preform fabricated by 4-step 3D braiding method; (b) schematic of stress state of the fibers and matrix under tensile load. 5. Conclusions A three-dimensional SiC/SiC composite was prepared by chemical vapor infiltration process, and tension–tension fatigue test of the SiC/SiC composite was carried out in a H2O–O2–Ar environment at 1300 ◦C. The results indicated that the SiC/SiC has excellent fatigue capacity as the tests lasted up to 105 h and 12 min. Fiber/matrix interface debonding, sliding and fibers pull out in tension–tension fatigue tests were more obvious than those in monotonic tension tests. The fracture surface of the mono￾tonic tension specimens was coarse while that of the fatigue specimens shows two different areas, namely ringed striations and smooth area. During tension–tension fatigue, the coexisting damage characteristics were: (i) cracking of SiC matrix, dis￾engaging of SiC matrix and grains growth; (ii) PyC interlayer

S Wu et al. Materials Science and Engineering A 435-436(2006)412-417 partial oxidation or burn out and fibers severe oxidation due to [6] N.S. Jacobson, J. Am. Ceram Soc. 76(1993)3-28 diffusion of oxidizing gas through the matrix cracks; (iii)fibers [71 S. Pasquier, J. Lamon, R.Naslain, Compos. Part A Appl. 29(1998) torn and fracture 1157-1164 Tension-tension fatigue damage characteristics of fiber rein [8] M. Mizuno, SJ. Zhu, Y Kagawa, H. Kaya, J. Eur. Ceram Soc. 18(1998) forced composite were directly affected by braiding architecture [9]V Kostopoulos, Y.Z. Pappas, Y P Markopoulos, J. Eur. Ceram. Soc of the fibers preform (1999)207-215 [10] S Zhu, M. Mizuno, Y Kagawa, Y. Mutoh, Compos. Sci. Technol. 59(1999) Acknowledgements 83-851 [11 S Mall, Mater. Sci. Eng. A 412(2005)165-170. 2]P The authors acknowledge the support of the Chinese National [13] Y. Miyashita, K. Kanda, S. Zhu, Y.Mutoh, M. Mizuno, A.J.McEvily, Int Foundation for Natural Sciences under Contract No. 90405015 J. Fatigue24(2002)24l-248 the NSFC Distinguished Young Scholar under Contract No. [14J. Schlichting, High Temp. -High Press. 14(1982)717-724 50425208(2004), the program for Changjiang Scholars and 16lM Takeda sak moit imate ichikawa com)s Sci technol so (1999)813-819 [171 M. Takeda, A Urano, J. Sakamoto, Y Imai, J. Nucl. Mater. 258-263(1998) References 1594-1599 [18] P. Lipetzky, G.J. Dvorak, N.S. Stoloff, Mater. Sci. Eng. A216(1996)11-19 [1 R. Naslain, Compos. Sci. Technol. 64 (2004)155-170. [19]D. Beyerley, S.M. Spearing, F.w. Zok, A.G. Evans, J. Am. Ceram Soc. 75 [2] S Schmidt, S. Beyer, H. Knabe, et al., Acta Astronaut. 55(2004)409-420. (1992)2719-2725. [3]J. Kimmel, N. Miriyala, J. Price, K. More, et al., J. Eur. Ceram Soc. 22 [20] Y.D. Xu, L F. Cheng, L.T. Zhang, J. Mater. Process. Technol. 101(2000) (2002)2769-2775 47-51. [4] C.G. Papakonstantinou, P Balaguru, R E. Lyon, Compos. Part B Eng. 32 [21] A.J. Haslam, D Moldovan, V. Yamakov, D Wolf, S.R. Phillpot, H Gleiter, 2001)637-649 Acta mater:.5102003)2097-2112. [5] M.R. Effinger, G.G. Genge, J D. Kiser, Adv. Mater Process. 157(2000) [22] F.R. N Nabarro, Scripta Mater. 39(1998)1681-1683

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