MATERAL CHARACEERLATION ELSEVIER Materials Characterization 57(2006)6-11 Heat treatment effects on creep behavior of polycrystalline Sic fibers JJSha,, J.S. Park, T Hinoki, A Kohyama Graduate School of Energy Science, Kyoto University Gokasho, Uji, Kyoto 611-0011, Japan Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-001L, Japan Received 31 August 2005: received in revised form 26 November 2005: accepted 28 November 2005 Polycrystalline SiC fibers are being considered as potential reinforcement for ceramic matrix composites(CMCs). For these fibers with fine grain size, basic issues arise conceming thermo-mechanical properties and microstructural instability during fabrication and application of CMCs. To examine these issues, three commercially available Sic fibers were heat treated at elevated temperatures for I h in an Ar atmosphere. The creep resistance of Sic fibers was evaluated by the bend stress relaxation(BSr) method, and it was found that the creep resistance could be improved by heat treatment. Combining the results of the Bsr tests with the results of X-ray diffraction examinations indicated that the creep resistance of Sic fibers is mainly related to the B-s grain size and the composition at or adjacent to the grain boundaries. Also, the apparent activation energy of creep for both hi- Nicalon TM and Hi-Nicalon M type S fiber increased with increasing heat treatment temperature. In the case of Tyranno M-SA fiber, the apparent activation energy of creep did not show an obvious dependence on the heat treatment temperature. C 2005 Elsevier Inc. All rights reserved. Keywords: Heat treatment; SiC fibers; Creep; Apparent activation energy 1. Introduction tion. These fibers experience a pyrolysis/sintering process during fabrication and their microstructure and Ceramic matrix composites(CMCs) have been mechanical properties depend on the thermal history. On proposed as potential structural materials for advanced the other hand, the CMCs may be fabricated above the energy systems and propulsion systems [1-3]. Accept- fiber's processing temperature [6, 71, in which case the able performance of high temperature CMCs depends performance of the fibers could be changed by this latter upon judicious selection of ceramic fiber reinforcement thermal exposure. However, the effect of the thermal with the proper chemical, physical and mechanical history on the fibers properties in the CMCs requires further investigation to identify the factors which affect Recently developed SiC-based fibers with high the high temperature properties of these fibers. The crystallite structure and near stoichiometric composition creep resistance of SiC fibers is one of the most critical [4, 5], are promising reinforcement for CMCs fabrica- properties. However, there are significant difficulties in the experimental measurement of tensile creep of advanced small diameter(about 7-14 um) SiC fibers Corresponding author. Present address: Intemational Innovation Center, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan. Tel: +81 In the present study, in order to evaluate the creep 757535194;fax:+81757534841 resistance of these advanced Sic fibers and also to E-mail address: shajianjun(@ iic. kyoto-uLac jp(J. Sha) clarify the creep mechanism to support continuing 044-5803/S-see front matter o 2005 Elsevier Inc. All rights reserved. j. matcha. 2005. 11.019
Heat treatment effects on creep behavior of polycrystalline SiC fibers J.J. Sha a,⁎, J.S. Park b , T. Hinoki b , A. Kohyama b a Graduate School of Energy Science, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan b Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Received 31 August 2005; received in revised form 26 November 2005; accepted 28 November 2005 Abstract Polycrystalline SiC fibers are being considered as potential reinforcement for ceramic matrix composites (CMCs). For these fibers with fine grain size, basic issues arise concerning thermo-mechanical properties and microstructural instability during fabrication and application of CMCs. To examine these issues, three commercially available SiC fibers were heat treated at elevated temperatures for 1 h in an Ar atmosphere. The creep resistance of SiC fibers was evaluated by the bend stress relaxation (BSR) method, and it was found that the creep resistance could be improved by heat treatment. Combining the results of the BSR tests with the results of X-ray diffraction examinations indicated that the creep resistance of SiC fibers is mainly related to the β-SiC grain size and the composition at or adjacent to the grain boundaries. Also, the apparent activation energy of creep for both HiNicalon™ and Hi-Nicalon™ type S fiber increased with increasing heat treatment temperature. In the case of Tyranno™-SA fiber, the apparent activation energy of creep did not show an obvious dependence on the heat treatment temperature. © 2005 Elsevier Inc. All rights reserved. Keywords: Heat treatment; SiC fibers; Creep; Apparent activation energy 1. Introduction Ceramic matrix composites (CMCs) have been proposed as potential structural materials for advanced energy systems and propulsion systems [1–3]. Acceptable performance of high temperature CMCs depends upon judicious selection of ceramic fiber reinforcement with the proper chemical, physical and mechanical properties. Recently developed SiC-based fibers with high crystallite structure and near stoichiometric composition [4,5], are promising reinforcement for CMCs fabrication. These fibers experience a pyrolysis/sintering process during fabrication and their microstructure and mechanical properties depend on the thermal history. On the other hand, the CMCs may be fabricated above the fiber's processing temperature [6,7], in which case the performance of the fibers could be changed by this latter thermal exposure. However, the effect of the thermal history on the fiber's properties in the CMCs requires further investigation to identify the factors which affect the high temperature properties of these fibers. The creep resistance of SiC fibers is one of the most critical properties. However, there are significant difficulties in the experimental measurement of tensile creep of advanced small diameter (about 7–14 μm) SiC fibers. In the present study, in order to evaluate the creep resistance of these advanced SiC fibers, and also to clarify the creep mechanism to support continuing Materials Characterization 57 (2006) 6–11 ⁎ Corresponding author. Present address: International Innovation Center, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan. Tel.: +81 75 753 5194; fax: +81 75 753 4841. E-mail address: shajianjun@iic.kyoto-u.ac.jp (J.J. Sha). 1044-5803/$ - see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2005.11.019
J. Sha et al. / Materials Characterization 57(2006)6-1l Table 1 long-term BSR tests, the fibers were tied into loops Nominal properties of SiC fibers provided by manufacturers a constant radius and subjected to a specific Sic C/ Oxygen Strength Modulus Density Diameter temperature treatment in air. The BSr parameter, m, was fiber Si (wt % (GPa) ( GPa) (g/cm) (H m) taken to be the average for 3-5 fibers for each condition HNL1.390.528 270 HNLs1050.2 3. Results and discussion TySA1.08<0.5 3.1. Effect of initial applied strain on the stress optimization of the properties, the bend stress relaxation relaxation parameter: m method(BSR)[8] was used. The creep behavior was studied before and after heat treatment. In addition, the Fig. I shows the I h bsr test at 1300C in air for apparent activation energy of creep was calculated by investigating the effect of initial applied strain on the applying a cross-cut method to the results of the long- stress relaxation parameter. A minor influence of initial term bSR tests applied strain on m was observed for all three fibers Also, it was evident that the applied stress for near 2. Experimental procedure stoichiometric SiC fibers (i.e. HNLS and TySA)relaxed much less than that of the HNL fiber. The applied stress 2. Materials and heat treatment on the hnL fiber was nearly fully relaxed in this test condition. Also, the negligible strain depe endence The fibers examined in this study are Hi-Nicalon observed in these tests supported grain boundary (HND)[4], Hi-Nicalon TM Type-S(HNLS)[4 and sliding, accommodated by diffusion, as the principal Tyranno TM SA (TySA)[5]. The first two fiber types creep mechanism during the stress relaxation of these are from Nippon Carbon Co. Japan, and the last one is Sic fibers [8, 9] from Ube Industry Co. Japan. Table 1 lists the fibers properties as provided by the manufacturers, and it can 3.2. BSR test be seen that HNLS and TySA fibers have near stoichiometric composition. The fibers were heat treated Fig. 2(a)-(c)show the stress relaxation parameter in Ar under a pressure of 10 Pa, and held for 1 h at (m)of as-received and heat treated fibers, as a function various temperatures in the range 1400 to 1900C. After of temperature and time. The duration of the stress heat treatment. th e crista alline size of B-SiC was relaxation tests was 1, 10, 25 and 50 h. The stress estimated from the half-value width of the (111) peak relaxation follows an S-shaped curve and it is quite by employing the Scherrer's formula obvious that both the time and temperature required for L=K认/(Dcos) full relaxation increased with increasing heat treatment temperature(HTT). For longer time BSR tests, the S where K is a constant(taken as 0.9); the CuKo ength (i.e, 1=0. 154056 nm);D, the half-value of the B-Sic (111) peak; and 8, the Bragg angle 5°for-siC(11l). 2.2. Measurement of BsR creep resistance The creep resistance of Sic fibers was assessed by 04 the BSR method [8], and could be expressed as the stress relaxation parameter. The stress relaxation parameter was defined as m=1-Ro/Ra, where Ro and R. are the initial radius and residual radius of the fiber loop Fibers are considered more creep resistant as the m value increases from 0 to 1. In the present BSr tests, the 00.1020.304050.60.70.80.9 Initial applied strain( effect of initial applied strain on the stress relaxation parameter,m,was investigated at 1300C in air for I h, Fig. 1. Bend stress relaxation parameter, m, of three fibers vs the initial by tying the fibers into loops with different radius. For applied strain tested at 1300.C in air for 1 h
optimization of the properties, the bend stress relaxation method (BSR) [8] was used. The creep behavior was studied before and after heat treatment. In addition, the apparent activation energy of creep was calculated by applying a cross-cut method to the results of the longterm BSR tests. 2. Experimental procedure 2.1. Materials and heat treatment The fibers examined in this study are Hi-Nicalon™ (HNL) [4], Hi-Nicalon™ Type-S (HNLS) [4] and Tyranno™ SA (TySA) [5]. The first two fiber types are from Nippon Carbon Co. Japan, and the last one is from Ube Industry Co. Japan. Table 1 lists the fiber's properties as provided by the manufacturers, and it can be seen that HNLS and TySA fibers have near stoichiometric composition. The fibers were heat treated in Ar under a pressure of 105 Pa, and held for 1 h at various temperatures in the range 1400 to 1900 °C. After heat treatment, the crystalline size of β-SiC was estimated from the half-value width of the (111) peak by employing the Scherrer's formula: L ¼ Kd k=ðDd coshÞ ð1Þ where K is a constant (taken as 0.9); λ, the CuKα wavelength (i.e., λ= 0.154056 nm); D, the half-value width of the β-SiC (111) peak; and θ, the Bragg angle (θ= 17.5° for β-SiC (111)). 2.2. Measurement of BSR creep resistance The creep resistance of SiC fibers was assessed by the BSR method [8], and could be expressed as the stress relaxation parameter. The stress relaxation parameter was defined as m= 1−R0 /Ra, where R0 and Ra are the initial radius and residual radius of the fiber loop. Fibers are considered more creep resistant as the m value increases from 0 to 1. In the present BSR tests, the effect of initial applied strain on the stress relaxation parameter, m, was investigated at 1300 °C in air for 1 h, by tying the fibers into loops with different radius. For long-term BSR tests, the fibers were tied into loops with a constant radius and subjected to a specific time/ temperature treatment in air. The BSR parameter, m, was taken to be the average for 3–5 fibers for each condition. 3. Results and discussion 3.1. Effect of initial applied strain on the stress relaxation parameter, m Fig. 1 shows the 1 h BSR test at 1300 °C in air for investigating the effect of initial applied strain on the stress relaxation parameter. A minor influence of initial applied strain on m was observed for all three fibers. Also, it was evident that the applied stress for nearstoichiometric SiC fibers (i.e. HNLS and TySA) relaxed much less than that of the HNL fiber. The applied stress on the HNL fiber was nearly fully relaxed in this test condition. Also, the negligible strain dependence of m observed in these tests supported grain boundary sliding, accommodated by diffusion, as the principal creep mechanism during the stress relaxation of these SiC fibers [8,9]. 3.2. BSR test Fig. 2 (a)–(c) show the stress relaxation parameter (m) of as-received and heat treated fibers, as a function of temperature and time. The duration of the stress relaxation tests was 1, 10, 25 and 50 h. The stress relaxation follows an S-shaped curve and it is quite obvious that both the time and temperature required for full relaxation increased with increasing heat treatment temperature (HTT). For longer time BSR tests, the STable 1 Nominal properties of SiC fibers provided by manufacturers SiC fiber C/ Si Oxygen (wt.%) Strength (GPa) Modulus (GPa) Density (g/cm3 ) Diameter (μ m) HNL 1.39 0.5 2.8 270 2.74 14 HNLS 1.05 0.2 2.6 420 3.1 12 TySA 1.08 b0.5 2.6 400 3.0 7 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 HNL TySA HNLS Initial applied strain (%) Stress relaxation parameter, m Fig. 1. Bend stress relaxation parameter, m, of three fibers vs. the initial applied strain tested at 1300 °C in air for 1 h. J.J. Sha et al. / Materials Characterization 57 (2006) 6–11 7
J. Sha et al. / Materials Characterization 57(2006)6-1/ est temperature [c] Test temperature [c] E 1 500140013001200 09▲ 三0.8 三08 0.7 0.71 1600C HT-1h 0.7 0.5 0.5 0.5 04 耍04 0.2 (b) (c) 70.80.9 Reciprocal temperature [10oc-11 Reciprocal temperature[10oC] Reciprocal temperature [10oc- Fig. 2. Temperature and time dependence of m of SiC fibers after heat treatment at elevated temperatures:(a)HNL fiber; (b)HNLS fiber; (c) TysA fiber,(HT: heat treatment). shaped curves are shifted to the right, and the shapes of In the present work, the O values were found(from these curves are slightly different at the low and high m=0.3, 0.5, 0.7, which represent the high temperature, moderate temperature and low temperature regions)to Heat treatment was seen to improve the creep be 563, 598 and 445 kJ/mol for the as-received HNL resistance of SiC fibers In Fig. 2(a), improved creep fibers; 707, 692, and 500 kJ/mol for as-received HNLS resistance was observed for the hnL fibers heat treated at fibers; 707, 774 and 525 kJ/mol for as-received TySA 1400C. The applied stress in as-received HNL fiber was nearly fully relaxed at 1300C after 1 h; however, for the Table 2 HNL fiber heat treated at 1600C, full stress relaxation Apparent activation energies for SiC-based fibers with different required 50 h at 1400C. Fig. 2(b)and(c)show the m ratios for HNLS and TySA fibers after heat treatment at Fiber condition BSR parameter In(h) t2(h) o different temperatures. The creep resistance was im- (kJ/mol) proved for both fibers after heat treatment at 1600C If we take m=0.5 as an arbitrary value for which we can compare test results, two trends are evident. First, he relaxation temperature for m=0.5 increased with an HNL1400°HTT0.7 652 increase of the HTT. Second, at the level of m=0.5 the 1 h BSR tests show a higher relaxation temperature HNL=1600°HTT0.7 than that of longer time BSR test, as would be expected 3.3. Apparent ad HNLS-as received 0.7 Since stress relaxation is a thermally activated HNLs--16000 process for ceramic materials, the apparent activation energy of creep, 0, can then be determined from the A(1/n) spacing between the curves at a constant m TySA-as received 0 value(cross-cut method) in the plots of stress relaxation parameter vs reciprocal temperature as indicated in Fig. TySA-1600C HTT 0.7 2(a)-(c). From these figures, the o for a given m value, can be calculated from the relationship TysA-1780°CHr0.7 111111111111111 50602 50774 (2)1ysA-1900c0H07 TT where R is the gas constant(8.314 J/mol K) Means no data are available for this m value
shaped curves are shifted to the right, and the shapes of these curves are slightly different at the low and high temperature regions. Heat treatment was seen to improve the creep resistance of SiC fibers. In Fig. 2 (a), improved creep resistance was observed for the HNL fibers heat treated at 1400 °C. The applied stress in as-received HNL fiber was nearly fully relaxed at 1300 °C after 1 h; however, for the HNL fiber heat treated at 1600 °C, full stress relaxation required 50 h at 1400 °C. Fig. 2 (b) and (c) show the m ratios for HNLS and TySA fibers after heat treatment at different temperatures. The creep resistance was improved for both fibers after heat treatment at 1600 °C. If we take m= 0.5 as an arbitrary value for which we can compare test results, two trends are evident. First, the relaxation temperature for m= 0.5 increased with an increase of the HTT. Second, at the level of m= 0.5, the 1 h BSR tests show a higher relaxation temperature than that of longer time BSR test, as would be expected. 3.3. Apparent activation energy of creep Since stress relaxation is a thermally activated process for ceramic materials, the apparent activation energy of creep, Q, can then be determined from the Δ(1/T) spacing between the curves at a constant m value (cross-cut method) in the plots of stress relaxation parameter vs. reciprocal temperature as indicated in Fig. 2 (a)–(c). From these figures, the Q for a given m value, can be calculated from the relationship: Q ¼ Rd ln t2 t1 1 T1 − 1 T2 ð2Þ where R is the gas constant (8.314 J/mol K). In the present work, the Q values were found (from m= 0.3, 0.5, 0.7, which represent the high temperature, moderate temperature and low temperature regions) to be 563, 598 and 445 kJ/mol for the as-received HNL fibers; 707, 692, and 500 kJ/mol for as-received HNLS fibers; 707, 774 and 525 kJ/mol for as-received TySA 0.5 0.6 0.7 0.8 0.9 1 0.5 0.6 0.7 0.8 0.9 1 0.5 0.6 0.7 0.8 0.9 1 As received-1h As received-10h 1400C HT-1h 1400C HT-25h 1600C HT-1h 1600C HT-50h 1500 1200 1000 Test temperature [°C] Reciprocal temperature [10-3 °C-1] Stress relaxation parameter, m (1/T) (a) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 As received-1h As-received-50h 1600C HT-1h 1600C HT-50h Test temperature [°C] Reciprocal temperature [10-3 °C-1] Stress relaxation parameter, m (b) As received-1h As received-50h 1600C HT-1h 1600C HT-50h 1780C HT-1h 1780C HT-50h 1900C HT-1h 1900C HT-50h Test temperature [°C] Reciprocal temperature [10-3 °C-1] Stress relaxation parameter, m (c) 1400 1300 1500 1200 1000 1400 1300 1500 1200 1000 1400 1300 Fig. 2. Temperature and time dependence of m of SiC fibers after heat treatment at elevated temperatures: (a) HNL fiber; (b) HNLS fiber; (c) TySA fiber, (HT: heat treatment). Table 2 Apparent activation energies for SiC-based fibers with different conditions Fiber condition BSR parameter (m) t1 (h) t2 (h) Q (kJ/mol) HNL—as received 0.7 1 10 445 0.5 1 10 598 0.3 1 10 563 HNL—1400 °C HTT 0.7 1 25 525 0.5 1 25 622 0.3 1 25 652 HNL—1600 °C HTT 0.7 1 50 x 0.5 1 50 929 0.3 1 50 775 HNLS—as received 0.7 1 50 500 0.5 1 50 692 0.3 1 50 707 HNLS—1600 °C HTT 0.7 1 50 664 0.5 1 50 929 0.3 1 50 756 TySA—as received 0.7 1 50 525 0.5 1 50 774 0.3 1 50 707 TySA—1600 °C HTT 0.7 1 50 561 0.5 1 50 692 0.3 1 50 581 TySA—1780 °C HTT 0.7 1 50 x 0.5 1 50 602 0.3 1 50 774 TySA—1900 °C HTT 0.7 1 50 614 0.5 1 50 551 0.3 1 50 x x: Means no data are available for this m value. 8 J.J. Sha et al. / Materials Characterization 57 (2006) 6–11
J. Sha et al. / Materials Characterization 57(2006)6-1l (a) (11 (c)(11 (311) 160°cHT1=(200 222) 1400C HTT 1400°cHTT 1600°cHT As-received As-received As-received wwW WAANW/W Fig 3. XRD patterns for fibers heat treated at different temperatures:(a) HNL fibers;(b) HNLS fibers; (c) TySA fibers fibers, as listed in Table 2. From these results, we can see(20=35.7, d-0 251 nm),(220)(20=60.0, d=0. 154 the g values increase with an increase of the test nm)and(311)(20=720o, d=0.131 nm) planes. After temperature. The same change in apparent activation heat treatment at temperatures > 1600C, two other energy was also found in earlier studies. The large peaks, which are indexed as the(200) and(222)planes activation energies for the high temperature regions- of the B-SiC phase, become obvious i.e. low values of m-could be related to the concurrent Using Scherrer's formula, the apparent crystallite microstructure change during the BSr tests [9] size of B-SiC, Dsic, was calculated from the half-value The apparent activation energies for creep for heat width of the(111) peak reated fibers were also calculated and compared with Fig. 4 show the correlation between 1 h stress those of as-received fibers (Table 2). The o value for relaxation temperatures for m=0.5 and the crystallite HNL and hnls fibers increased with an increase in the heat treatment temperature. However, no obvious HTT -HNL fiber dependence of the apparent activation energy for TySA fibers was observed (Table 2) -TysA fiber The apparent activation energies obtained in our work are in acceptable agreement with those for carbon 9 and silicon self-diffusion in SiC [10, 11], which suggests 9 1400- ■-+」2 that the thermally activated diffusion mechanism plays an important role on the creep resistance of si materials The apparent activation energy is a crucial parameter 1200 -·---■ in the evaluation of servic times of sic materials [2] 3. 4. XRD characterization nitia|12001400160 18002000 Heat treatment temperature (c) Fig 3(a)-(c)show typical X-ray diffraction(XRD) pattens of Sic fibers after 1 h heat treatments in Ar. The Fig. 4. Correlation between I h relaxation XRD pattens of the as-received Sic-based fibers show m=0.5 and crystallite size of B-sic for fibers heat treated at elevated hree main peaks which were assigned to the (lll) temperatures in Ar for 1 h
fibers, as listed in Table 2. From these results, we can see the Q values increase with an increase of the test temperature. The same change in apparent activation energy was also found in earlier studies. The large activation energies for the high temperature regions— i.e., low values of m—could be related to the concurrent microstructure change during the BSR tests [9]. The apparent activation energies for creep for heat treated fibers were also calculated and compared with those of as-received fibers (Table 2). The Q value for HNL and HNLS fibers increased with an increase in the heat treatment temperature. However, no obvious HTT dependence of the apparent activation energy for TySA fibers was observed (Table 2). The apparent activation energies obtained in our work are in acceptable agreement with those for carbon and silicon self-diffusion in SiC [10,11], which suggests that the thermally activated diffusion mechanism plays an important role on the creep resistance of SiC materials. The apparent activation energy is a crucial parameter in the evaluation of service lifetimes of SiC materials [12]. 3.4. XRD characterization Fig. 3 (a)–(c) show typical X-ray diffraction (XRD) patterns of SiC fibers after 1 h heat treatments in Ar. The XRD patterns of the as-received SiC-based fibers show three main peaks which were assigned to the (111) (2θ= 35.7°; d= 0.251 nm), (220) (2θ= 60.0°; d= 0.154 nm) and (311) (2θ= 72.0°; d= 0.131 nm) planes. After heat treatment at temperatures ≥1600 °C, two other peaks, which are indexed as the (200) and (222) planes of the β-SiC phase, become obvious. Using Scherrer's formula, the apparent crystallite size of β-SiC, DSiC, was calculated from the half-value width of the (111) peak. Fig. 4 show the correlation between 1 h stress relaxation temperatures for m= 0.5 and the crystallite Fig. 3. XRD patterns for fibers heat treated at different temperatures: (a) HNL fibers; (b) HNLS fibers; (c) TySA fibers. 1000 1100 1200 1300 1400 1500 1600 1200 1400 1600 1800 2000 0 5 10 15 20 25 30 35 40 HNL fiber HNLS fiber TySA fiber Initial Crystallite size of SiC (111), DSiC/nm Heat treatment temperature (°C) Stress relaxation temperature for m = 0.5 (°C) Fig. 4. Correlation between 1 h stress relaxation temperature for m= 0.5 and crystallite size of β-SiC for fibers heat treated at elevated temperatures in Ar for 1 h. J.J. Sha et al. / Materials Characterization 57 (2006) 6–11 9
J. Sha et al. / Materials Characterization 57(2006)6-1/ size of B-SiC for fibers heat treated at elevated fibers [17], which contain small amounts of temperatures in Ar for 1 h. The improved creep boron. The creep rate decreased significantly by resistance was observed at temperatures where the post-fabrication thermal treatments that remove crystallite size started to increase except for TySA fiber. boron from the bulk of these fibers According to Fig. 4, the heat treatment effects could be characterized as follows: (i)Improved creep 4. Conclusions resistance and grain growth of the HNL fibers started at1400°C;(i) When the htt was<1600°C,the Polycrystalline Sic fibers were heat treated at crystallite size of B-SiC in the hNLS and TySA fibers elevated temperatures for I h in an Ar atmosphe remained unchanged, but for higher temperature heat The creep resistance of SiC fibers evaluated by BSr treatments a drastic coarsening in crystallite size method could be improved by heat treatment. The occurred for the HNLS fibers;(iii) The improved apparent activation energies for creep were calculated creep resistance of the TySa fibers was obtained prior from the results of the long-term BSR test by a cross cut to grain growth method using data for the stress relaxation parameter, m It is obvious that heat treatment has a significant plotted as a function of reciprocal temperature. The main ffect on the crystallite size of HNL and hnls fibers results can be summarized as follows: and results in the improved creep resistance. It is thought hat almost all creep mechanisms in ceramics obey (1) Combining the results of the creep tests with those form [131 indicated that sistance of Sic fibers was mainly related to the es exp(-O/RT) B-SiC grain size and, probably, the composition at and adjacent to the grain boundaries here A is a constant; o, applied stress;n,stress (2) The apparent creep activation energy for both Hi- ponent;d, grain size; m, exponent of inverse grain NicalonTM and Hi-Nicalon TM type s fibers size:o, apparent active energy; R, gas constant; and T, increased with an increase in the htt. In the absolute temperature. According to Eq(3), the fibers case of Tyranno TM-SA fibers, the apparent creep with larger grain sizes exhibit higher creep resistance. activation energy did not exhibit a significant Furthermore, the Bsr creep resistance is not only dependence on the hTt. dependent on the grain size, but also on other factors ()A negligible strain dependence of m was observed This is illustrated by the following facts in these tests. This supports the judgment that grain boundary sliding, accommodated by diffu- (1)For the 1600C heat treated HNL fiber, the creep ion, is the principal creep mechanism during the resistance was close to those of as-received near- stress relaxation of these sic fibers. This stoichiometric fibers despite the fact that the grain emphasizes the importance of the composition at sizes were much smaller than those of the latter and adjacent to the g fibers. The excess carbon distributed in the grain boundary of the HNL fiber might inhibit the References coalescence of B-SiC, which results in a stable grain boundary structure [14 [1] Naslain R. Design, preparation and properties of non-oxide (2) In the case of TySA fiber, the grain size of B-SiC CMCs for application in engines and nuclear reactors: an appears not to be very dependent on the HTT, and overview. Compos Sci Technol 2004: 64: 155-70. [2] Kohyama A. Advanced ceramic composite materials for fusion/ the improved creep resistance was obtained prior fission reactor systems. Ceramics 2004: 39: 838-42 [in Japanese]- to increase of crystallite size. This result indicates [] Ohnabe H, Masaki S, Onozuka M, Miyahara K, Sasa T. that the stability of the grain boundaries must be Potential application of ceramic matrix composites to aero- changed during the heat treatment. For TySA engine components. Compos, Part A Appl Sci Manuf 199930:489-96 fibers containing Al it has been observed that [4] Ichikawa H. Recent advances in Nicalon ceramic fibers including during heat treatment it is possible that Al is Hi-Nicalon Type S. Ann Chim Sci Mat 2000 52: 523-8. educed in the bulk of the fiber due to migration to [5] Ishikawa T, Kohtoka Y, Kumagawa K, Yamamura T, Nagasawa the fiber surface. this leads to a decrease in the sic T. High-strength alkali-resistant sintered Sic fiber stable self-diffusion coefficients and thereby a decrease 2200°C. Nature1998;391:773-4 in the creep rate [15]. Similar results have been 6] Dong S, Katoh Y, Kohyama A. Preparation of Sic/SiC composites by hot pressing, using Tyranno SA fiber as observed in Sylramic fibers [16] and UF-HM reinforcement. J Am Ceram Soc 2003: 86: 26-32
size of β-SiC for fibers heat treated at elevated temperatures in Ar for 1 h. The improved creep resistance was observed at temperatures where the crystallite size started to increase except for TySA fiber. According to Fig. 4, the heat treatment effects could be characterized as follows: (i) Improved creep resistance and grain growth of the HNL fibers started at 1400 °C; (ii) When the HTT was b1600 °C, the crystallite size of β-SiC in the HNLS and TySA fibers remained unchanged, but for higher temperature heat treatments a drastic coarsening in crystallite size occurred for the HNLS fibers; (iii) The improved creep resistance of the TySA fibers was obtained prior to grain growth. It is obvious that heat treatment has a significant effect on the crystallite size of HNL and HNLS fibers and results in the improved creep resistance. It is thought that almost all creep mechanisms in ceramics obey constitutive relations of the form [13]: e s ¼ Ad rn 1 d m expð−Q=RTÞ ð3Þ where A is a constant; σ, applied stress; n, stress exponent; d, grain size; m, exponent of inverse grain size; Q, apparent active energy; R, gas constant; and T, absolute temperature. According to Eq. (3), the fibers with larger grain sizes exhibit higher creep resistance. Furthermore, the BSR creep resistance is not only dependent on the grain size, but also on other factors. This is illustrated by the following facts: (1) For the 1600 °C heat treated HNL fiber, the creep resistance was close to those of as-received nearstoichiometric fibers despite the fact that the grain sizes were much smaller than those of the latter fibers. The excess carbon distributed in the grain boundary of the HNL fiber might inhibit the coalescence of β-SiC, which results in a stable grain boundary structure [14]. (2) In the case of TySA fiber, the grain size of β-SiC appears not to be very dependent on the HTT, and the improved creep resistance was obtained prior to increase of crystallite size. This result indicates that the stability of the grain boundaries must be changed during the heat treatment. For TySA fibers containing Al it has been observed that, during heat treatment it is possible that Al is reduced in the bulk of the fiber due to migration to the fiber surface; this leads to a decrease in the SiC self-diffusion coefficients and thereby a decrease in the creep rate [15]. Similar results have been observed in Sylramic fibers [16] and UF-HM fibers [17], which contain small amounts of boron. The creep rate decreased significantly by post-fabrication thermal treatments that remove boron from the bulk of these fibers. 4. Conclusions Polycrystalline SiC fibers were heat treated at elevated temperatures for 1 h in an Ar atmosphere. The creep resistance of SiC fibers evaluated by BSR method could be improved by heat treatment. The apparent activation energies for creep were calculated from the results of the long-term BSR test by a cross cut method using data for the stress relaxation parameter, m, plotted as a function of reciprocal temperature. The main results can be summarized as follows: (1) Combining the results of the creep tests with those from XRD examinations, indicated that the creep resistance of SiC fibers was mainly related to the β-SiC grain size and, probably, the composition at and adjacent to the grain boundaries. (2) The apparent creep activation energy for both HiNicalon™ and Hi-Nicalon™ type S fibers increased with an increase in the HTT. In the case of Tyranno™-SA fibers, the apparent creep activation energy did not exhibit a significant dependence on the HTT. (3) A negligible strain dependence of m was observed in these tests. This supports the judgment that grain boundary sliding, accommodated by diffusion, is the principal creep mechanism during the stress relaxation of these SiC fibers. This emphasizes the importance of the composition at and adjacent to the grain boundaries. References [1] Naslain R. Design, preparation and properties of non-oxide CMCs for application in engines and nuclear reactors: an overview. Compos Sci Technol 2004;64:155–70. [2] Kohyama A. Advanced ceramic composite materials for fusion/ fission reactor systems. Ceramics 2004;39:838–42 [in Japanese]. [3] Ohnabe H, Masaki S, Onozuka M, Miyahara K, Sasa T. Potential application of ceramic matrix composites to aeroengine components. Compos, Part A Appl Sci Manuf 1999;30:489–96. [4] Ichikawa H. Recent advances in Nicalon ceramic fibers including Hi-Nicalon Type S. Ann Chim Sci Mat 2000;52:523–8. [5] Ishikawa T, Kohtoka Y, Kumagawa K, Yamamura T, Nagasawa T. High-strength alkali-resistant sintered SiC fiber stable to 2200 °C. Nature 1998;391:773–4. [6] Dong S, Katoh Y, Kohyama A. Preparation of SiC/SiC composites by hot pressing, using Tyranno SA fiber as reinforcement. J Am Ceram Soc 2003;86:26–32. 10 J.J. Sha et al. / Materials Characterization 57 (2006) 6–11
J.. Sha et al. / Materials Characterization 57(2006)6-1l [7 Lee SP, Katoh Y, Park JS, Dong S, Kohyama A, Suyama S, et al. [13 Gallardo-Lopez A, Munoz A, Martinez-Femandez J, Dominguez Microstructural and mechanical characteristics of siC/SiC Rodriguez A. High-temperature compressive creep of liquid composites with modified RS process. J Nucl Mater 2001 phase sintered silicon carbide. Acta Mater 1999: 47: 2185-9 [14 Takeda M, Saeki A, Sakamoto J, Imai Y, Ichikawa H Properties [8] Morscher GN, Dicarlo JA. A simple test for thermomechanical of polycarbosilane-derived silicon carbide fibers with various C/ evaluation of ceramic fibers. J Am Ceram Soc 1992: 75: 136-40 Si compositions. Compos Sci Technol 1999: 59: 787-92. 9 Morscher GN, Lewinsohn CA, Bakis CE, Tressler RE. [15 Bunsell AR, Berger MH. Fine diameter ceramic fibers. J Eur Comparison of bend stress relaxation and tensile creep of CVD Ceram Soc2000;20:2249-60 SiC fibers. J Am Ceram Soc 1995: 78: 3244-52. [16 Dicarlo JA, Yun HM, Hurst JB. Fracture mechanisms for SiC [10 Hon MH, Davis RF. Self-diffusion of C in polycrystalline B- fibers and SiC/SiC composites under stress-rupture conditions at SiC. J Mater Sci 1979: 14: 2411-21 high temperatures. Appl Math Comput 2004: 152: 473-81 [11] Hong JD, Hon MH, Davis RF. Si-30 diffusion in alpha-SiC and [17] Sacks MD. Effects of composition and heat treatment conditions beta-SiC. Am Ceram Soc Bull 1979: 58: 348 on the tensile strength and creep resistance of Sic-based fibers. J [12]Zhu S, Mizuno M, Kagawa Y, Mutoh Y Monotonic tension, Eur Ceram Soc 1999: 19: 2305-15 fatigue and creep behavior of Sic-fiber-reinforced Sic-matrix composites: a review. Compos Sci Technol 1999: 59: 833-51
[7] Lee SP, Katoh Y, Park JS, Dong S, Kohyama A, Suyama S, et al. Microstructural and mechanical characteristics of SiC/SiC composites with modified RS process. J Nucl Mater 2001; 289:30–6. [8] Morscher GN, Dicarlo JA. A simple test for thermomechanical evaluation of ceramic fibers. J Am Ceram Soc 1992;75:136–40. [9] Morscher GN, Lewinsohn CA, Bakis CE, Tressler RE. Comparison of bend stress relaxation and tensile creep of CVD SiC fibers. J Am Ceram Soc 1995;78:3244–52. [10] Hon MH, Davis RF. Self-diffusion of 14C in polycrystalline β- SiC. J Mater Sci 1979;14:2411–21. [11] Hong JD, Hon MH, Davis RF. Si-30 diffusion in alpha-SiC and beta-SiC. Am Ceram Soc Bull 1979;58:348. [12] Zhu S, Mizuno M, Kagawa Y, Mutoh Y. Monotonic tension, fatigue and creep behavior of SiC-fiber-reinforced SiC-matrix composites: a review. Compos Sci Technol 1999;59:833–51. [13] Gallardo-Lopez A, Munoz A, Martinez-Fernandez J, DominguezRodriguez A. High-temperature compressive creep of liquid phase sintered silicon carbide. Acta Mater 1999;47:2185–95. [14] Takeda M, Saeki A, Sakamoto J, Imai Y, Ichikawa H. Properties of polycarbosilane-derived silicon carbide fibers with various C/ Si compositions. Compos Sci Technol 1999;59:787–92. [15] Bunsell AR, Berger MH. Fine diameter ceramic fibers. J Eur Ceram Soc 2000;20:2249–60. [16] Dicarlo JA, Yun HM, Hurst JB. Fracture mechanisms for SiC fibers and SiC/SiC composites under stress-rupture conditions at high temperatures. Appl Math Comput 2004;152:473–81. [17] Sacks MD. Effects of composition and heat treatment conditions on the tensile strength and creep resistance of SiC-based fibers. J Eur Ceram Soc 1999;19:2305–15. J.J. Sha et al. / Materials Characterization 57 (2006) 6–11 11