COMPOSITES SCIENCE AND TECHNOLOGY ELSEVIER Composites Science and Technology 59(1999)813-819 Thermal stability of the low-oxygen-content silicon carbide fiber Hi-Nicalon TM Michio takeda Jun-ichi sakamoto. Yoshikazu imai. Hiroshi ichikawa Research Laboratory, Nippon Carbon Company Ltd, 1-1-1 Shin-ttrashina-cho, Kanagawa-ku, Yokohama 221-0031, Japan Received 8 September 1997; received in revised form 3 August 1998; accepted 7 January 1999 Abstract The low-oxygen SiC fiber, Hi-Nicalon M, was prepared by the pyrolysis of polycarbosilane fibers cured with electron-beam irradiation. This SiC fiber is continuous, in multi-filament form, and consists of Si-1. 39C-0010 by atomic ratio. Hi-Nicalon fiber has a high tensile strength and an elastic modulus of 2.8 and 270 GPa, respectively. This Sic fiber retains high strength and modulus even after exposure for 10 h at 1873 K in argon. It exhibits outstanding thermal stability as compared to other commercially available polymer-derived ceramic fibers. C 1999 Elsevier Science Ltd. All rights reserved Keywords: SiC; Ceramic fiber: Hi-NicalonTM: Thermal stability 1. ntroduction highly crystalline SiC fibers such as SylramicTM of Dow Corning[14], sintered SiC fiber of Ube Industries [15]. High-performance ceramic materials are needed in and Hi-NicalonTM Type S of Nippon Carbon [16] have advanced high-temperature technologies. In particular, been developed. Although these new Sic fibers were ceramic-matrix composites are the most promising mate- also reported to have high heat-resistance, these are still rials in these applications. The performance of a ceramic- of limited distribution or unavailable matrix composite(CMC) is highly dependent upon the In this work, the structural and mechanical chang properties of the reinforcement. A reinforcing fiber must of Hi-NicalonTM after thermal exposure tests have have thermal stability and sufficient mechanical properties been investigated by comparison with other (some even at elevated temperature. SiC fibers produced by commercially-available) ceramic fibers polymer pyrolysis have high tensile strength, high tensile modulus, and good thermal stability [l]. One highly com- mercialized polymer-derived SiC fiber, Nicalon TM, has 2. Experimental procedure been widely applied in many high-temperature structural materials[2, 3]. However, it is well known that the prop- 2.1. Fabrication of SiC fibers, Nicalon M and Hi- erties of SiC fibers containing oxygen are degraded as a Nicalon M result of carbothermal reduction reaction at high tem perature [4-6]. At elevated temperatures, decreases in the Fig. I shows the fabrication process for SiC fibers, tensile strength and creep resistance of SiC fibers are Nicalon M and Hi-NicalonM. Preceramic polymer observed [7-10]. As a reinforcement for CMCs, these polycarbosilane(PCS), was synthesized from dimethyl- characteristics will often allow only limited use dichlorosilane via polydimethylsilane. SiC fibers were It was reported that a SiC fiber with reduced oxygen prepared by the spinning of PCs, followed by curing content and improved thermal stability is obtained by and pyrolysis. PCS fibers with diameters of about 20 um using an irradiation-curing method [11-13]. This low- were obtained by the melt-spinning method. Two curing oxygen SiC fiber, Hi-Nicalon M, has now been success- processes for making fibers infusible, following pyr fully industrialized. More recently, stoichiometric, olysis, were available. One was by thermal oxidation curing at 473 K in air and the other was by irradiation 4604 rresponding author: Tel :+81-45.459.4692: fax: +81-45 curing with an electron beam Nicalon TM was obtained by the former process and Hi-Nicalon M by the latter 0266-353899/S-see atter c 1999 Elsevier Science Ltd. All rights reserved. PlI:S0266-3538(99)00012-3
Thermal stability of the low-oxygen-content silicon carbide ®ber, Hi-NicalonTM Michio Takeda*, Jun-ichi Sakamoto, Yoshikazu Imai, Hiroshi Ichikawa Research Laboratory, Nippon Carbon Company Ltd, 1-1-1 Shin-urashima-cho, Kanagawa-ku, Yokohama 221-0031, Japan Received 8 September 1997; received in revised form 3 August 1998; accepted 7 January 1999 Abstract The low-oxygen SiC ®ber, Hi-NicalonTM, was prepared by the pyrolysis of polycarbosilane ®bers cured with electron-beam irradiation. This SiC ®ber is continuous, in multi-®lament form, and consists of Si-1.39C-0.010 by atomic ratio. Hi-NicalonTM ®ber has a high tensile strength and an elastic modulus of 2.8 and 270 GPa, respectively. This SiC ®ber retains high strength and modulus even after exposure for 10 h at 1873 K in argon. It exhibits outstanding thermal stability as compared to other commercially available polymer-derived ceramic ®bers. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: SiC; Ceramic ®ber; Hi-NicalonTM; Thermal stability 1. Introduction High-performance ceramic materials are needed in advanced high-temperature technologies. In particular, ceramic-matrix composites are the most promising materials in these applications. The performance of a ceramicmatrix composite (CMC) is highly dependent upon the properties of the reinforcement. A reinforcing ®ber must have thermal stability and sucient mechanical properties even at elevated temperature. SiC ®bers produced by polymer pyrolysis have high tensile strength, high tensile modulus, and good thermal stability [1]. One highly commercialized polymer-derived SiC ®ber, NicalonTM, has been widely applied in many high-temperature structural materials [2,3]. However, it is well known that the properties of SiC ®bers containing oxygen are degraded as a result of carbothermal reduction reaction at high temperature [4±6]. At elevated temperatures, decreases in the tensile strength and creep resistance of SiC ®bers are observed [7±10]. As a reinforcement for CMCs, these characteristics will often allow only limited use. It was reported that a SiC ®ber with reduced oxygen content and improved thermal stability is obtained by using an irradiation-curing method [11±13]. This lowoxygen SiC ®ber, Hi-NicalonTM, has now been successfully industrialized. More recently, stoichiometric, highly crystalline SiC ®bers such as SylramicTM of Dow Corning [14], sintered SiC ®ber of Ube Industries [15], and Hi-NicalonTM Type S of Nippon Carbon [16] have been developed. Although these new SiC ®bers were also reported to have high heat-resistance, these are still of limited distribution or unavailable. In this work, the structural and mechanical changes of Hi-NicalonTM after thermal exposure tests have been investigated by comparison with other (some commercially-available) ceramic ®bers. 2. Experimental procedure 2.1. Fabrication of SiC ®bers, NicalonTM and HiNicalonTM Fig. 1 shows the fabrication process for SiC ®bers, NicalonTM and Hi-NicalonTM. Preceramic polymer, polycarbosilane (PCS), was synthesized from dimethyldichlorosilane via polydimethylsilane. SiC ®bers were prepared by the spinning of PCS, followed by curing, and pyrolysis. PCS ®bers with diameters of about 20 mm were obtained by the melt-spinning method. Two curing processes for making ®bers infusible, following pyrolysis, were available. One was by thermal oxidationcuring at 473 K in air and the other was by irradiationcuring with an electron beam. NicalonTM was obtained by the former process and Hi-NicalonTM by the latter. Composites Science and Technology 59 (1999) 813±819 0266-3538/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(99)00012-3 * Corresponding author: Tel.: +81-45-459-4692; fax: +81-45-459- 4604
M. Takeda et al./ Composites Science and Technology 59(1999)813-819 2.2. Thermal exposure test Dimethyldichlorosilane Cl-Si-C Polymer-derived ceramic fibers were heat-treated at Dechlorination high temperature in order to evaluate their thermal sta bility. Fibers tested included Hi-NicalonTM and Nica Dimethylpolysilane Carbon, Tyranno(Lox-M)fiber by Ube Industries and Rearrangement HPZ fiber by Dow Corning. Typical properties, includ ing chemical compositions, of these fibers are listed in Table 1 [17, 18]. The thermal exposure tests range from Polycarbosilane 1573 to 1873 K for 10 h in an argon atmosphere of 1 atm. Thermal exposure tests were carried out in argon Spinning n and air on Nicalon and Hi-Nicalon TM. These fibers Green Fiber were exposed in humid air (dew point is ca293 K)at 1273to1773Kfor10h. After the tests. the fibers were examined for w Electron Beam hange and mechanical properties at room temperature. Irradiation Oxidation The surfaces of the fibers and their crystal structures were analyzed by means of scanning electron micro- Cured Fiber scopy(SEM: model JSM-53 10, JEOL) and X-ray dif- fractometry (XRD: model Geigerflex, Rigaku Denki Pyrolysis Pyrolysis Co, Ltd ) respectively Si-C Fiber Si-C-O Fiber 3. Results and discussion (Hi-NICALON)(NICALON) 3. 1. Characterization of the low-oxygen SiC fiber, Hi- Fig1.Fabrication process of PCS-derived SiC fibers, Hi-Nicalon TM Nicalon/M and Nicalon Hi-Nicalonm fiber is a continuous multi-filament fiber bundle with 500 filaments of 14 um diameter Cured PCS fibers were converted to SiC fiber by pyrolysis Though Hi-Nicalon'M is fabricated from the same pre in an inert atmosphere. The diameter of the Sic fiber cursor as Nicalon M, it has a lower oxygen content of reduced to about 14 um. Elemental analysis and tensile 0.5 wt%, a higher elastic modulus of 280 GPa, and a properties of the SiC fibers were examined. Oxygen con- higher density of 2.74 g/cm. Fig. 2 shows the Auger tent was measured with a TC-436(LECO)analyzer. Depth electron spectroscopy(AES) depth profiles for Nica profiles of SiC fibers were measured by Auger electron lonM and Hi-Nicalon M. At the fiber surface Hi-Nica- spectroscopy(AES: model PHI-670, Perkin-Elmer). Ten- lonM has more carbon and much less oxygen, as sile strength and tensile modulus were measured by a sin- compared with Nicalon M fiber. These characteristics gle-filament method(JIS R 7601) with a gage length of 25 would cause differences in the interface behavior mm. (model UTM-2, Orientec Co, Ltd) CMCs properties of polymer-derived ceramic fibers NicalonTM NL-200 HPZ (Nippon Carbon (Nippon Carbon) (Ube Industries) Dow Corning) Fiber diameter (um) 10-12 Tensile strength(GPa) Tensile modulus(GPa) Density (g/cm) 2.7 2.55 Chemical composition(wt%) 11.7
Cured PCS ®bers were converted to SiC ®ber by pyrolysis in an inert atmosphere. The diameter of the SiC ®ber reduced to about 14 mm. Elemental analysis and tensile properties of the SiC ®bers were examined. Oxygen content was measured with a TC-436 (LECO) analyzer. Depth pro®les of SiC ®bers were measured by Auger electron spectroscopy (AES: model PHI-670, Perkin±Elmer). Tensile strength and tensile modulus were measured by a single-®lament method (JIS R 7601) with a gage length of 25 mm, (model UTM-2, Orientec Co., Ltd.). 2.2. Thermal exposure test Polymer-derived ceramic ®bers were heat-treated at high temperature in order to evaluate their thermal stability. Fibers tested included Hi-NicalonTM and NicalonTM (NL-202) SiC ®bers produced by Nippon Carbon, Tyranno (Lox-M) ®ber by Ube Industries and HPZ ®ber by Dow Corning. Typical properties, including chemical compositions, of these ®bers are listed in Table 1 [17,18]. The thermal exposure tests range from 1573 to 1873 K for 10 h in an argon atmosphere of 1 atm. Thermal exposure tests were carried out in argon and air on Nicalon and Hi-NicalonTM. These ®bers were exposed in humid air (dew point is ca293 K) at 1273 to 1773 K for 10 h. After the tests, the ®bers were examined for weight change and mechanical properties at room temperature. The surfaces of the ®bers and their crystal structures were analyzed by means of scanning electron microscopy (SEM: model JSM-53 10, JEOL) and X-ray diffractometry (XRD: model Geiger¯ex, Rigaku Denki Co., Ltd.), respectively. 3. Results and discussion 3.1. Characterization of the low-oxygen SiC ®ber, HiNicalonTM Hi-NicalonTM ®ber is a continuous, multi-®lament ®ber bundle with 500 ®laments of 14 mm diameter. Though Hi-NicalonTM is fabricated from the same precursor as NicalonTM, it has a lower oxygen content of 0.5 wt%, a higher elastic modulus of 280 GPa, and a higher density of 2.74 g/cm3 . Fig. 2 shows the Auger electron spectroscopy (AES) depth pro®les for NicalonTM and Hi-NicalonTM. At the ®ber surface Hi-NicalonTM has more carbon and much less oxygen, as compared with NicalonTM ®ber. These characteristics would cause dierences in the interface behavior in CMCs. Fig. 1. Fabrication process of PCS-derived SiC ®bers, Hi-NicalonTM and NicalonTM. Table 1 Typical properties of polymer-derived ceramic ®bers Property Hi-NicalonTM (Nippon Carbon) NicalonTM NL-200 (Nippon Carbon) TyrannoTM Lox-M (Ube Industries) HPZ (Dow Corning) Fiber diameter (mm) 14 14 11 10±12 Tensile strength (GPa) 2.8 3.0 3.3 2.8 Tensile modulus (GPa) 270 220 187 180 Density (g/cm3 ) 2.74 2.55 2.48 2.4 Chemical composition (wt%) Si 62.4 56.6 55.4 59 C 37.1 31.7 32.4 10 O 0.5 11.7 10.2 4 Ti 2.0 N 28 Cl 0.5 814 M. Takeda et al. / Composites Science and Technology 59 (1999) 813±819
M. Takeda et al. Composites Science and Technology 59(1999)813-819 3. 2. Thermal stability of polymer-derived ceramic fibers were about 20%. The silicon carbonitride fiber. HPZ significantly exposure testing is shown in Fig 3. After exposure at thermal exposure test 1873 K, the weight loss of Nicalon M and TyrannoTM Fig 4 shows the tensile strengths of the fibers mea- sured at room temperature after thermal exposure etching rate 121nm/ min(SiO TyrannoM and HPZ fibers lose their strength almost completely after exposure at 1673 K. while NicalonTM lost its strength after exposure at 1773 K. However, Hi NicalonTM retains a good strength of 2 GPa after HI-NICALON exposure at 1773K, and 1. 4 GPa after exposure at 1873 K for 10 h in argon. Fig. 5 shows the elastic moduli of the fibers. Hi- Nicalon exhibits a high elastic modulus O Nicalon NL200 △ Tyranno Lox M PUTTERING TIME(Min) (b) 0000 As15001600170018001900 SPUTTERING TIME (Min) Temperature(K) Fig. 2. AES depth profiles of Hi- Nicalon"M and NicalonM Fig. 4. Tensile strength of ceramic fibers after the exposure for 10 h il 。8 0 00 150 o Nicalon NL20 ○ Nical △ Tyranno Lox M Tyranno LOx M As15001600170018001900 As15001600170018001900 received Temperature (K) Temperature Fig 3. Weight change of ceramic fibers after the exposure for 10 h in Fig. 5. Tensile modulus of ceramic fibers after the exposure for 10 h in argon. argon
3.2. Thermal stability of polymer-derived ceramic ®bers The weight change of the Hi-NicalonTM, NicalonTM, TyrannoTM, and HPZ ceramic ®bers after thermal exposure testing is shown in Fig. 3. After exposure at 1873 K, the weight loss of NicalonTM and TyrannoTM were about 20%. The silicon carbonitride ®ber, HPZ, signi®cantly decreased to about 50% of its starting weight. On the other hand, the low-oxygen SiC ®ber, Hi-NicalonTM, shows little change (<1%) during the thermal exposure test. Fig. 4 shows the tensile strengths of the ®bers measured at room temperature after thermal exposure. TyrannoTM and HPZ ®bers lose their strength almost completely after exposure at 1673 K, while NicalonTM lost its strength after exposure at 1773 K. However, HiNicalonTM retains a good strength of 2 GPa after exposure at 1773 K, and 1.4 GPa after exposure at 1873 K for 10 h in argon. Fig. 5 shows the elastic moduli of the ®bers. Hi-NicalonTM exhibits a high elastic modulus Fig. 3. Weight change of ceramic ®bers after the exposure for 10 h in argon. Fig. 2. AES depth pro®les of Hi-NicalonTM and NicalonTM. Fig. 4. Tensile strength of ceramic ®bers after the exposure for 10 h in argon. Fig. 5. Tensile modulus of ceramic ®bers after the exposure for 10 h in argon. M. Takeda et al. / Composites Science and Technology 59 (1999) 813±819 815
816 M. Takeda et al./ Composites Science and Technology 59(1999)813-819 and retains it even after exposure at 1873 K. Fig. 6 retains a high strength of 1. 4 GPa and modulus of shows seM photographs of the fibers after thermal GPa even after 1873 K exposure exposure testing. NicalonTM forms small particles and The silicon oxycarbide-based fibers, NicalonM and changes to a rough fiber surface after the exposure at Tyranno TM, mainly decompose according to the following 1873 K. TyrannoM and HPZ fibers exhibit obvious reaction at elevated temperature smooth filament surface, showing no change in appea: 9 structural damage. However, Hi-Nicalon'M has SixC1O2→xSiC+zCO+( ance Fig. 7 shows SEM photographs of Nicalon and Hi-Nicalon'M after exposure at 2273 K for I h in argon. The weight loss of about 20% in Nicalon and Tyr Hi-NicalonTM has a smooth fiber surface, but, by con- anno M fibers will be explained by co gas evolution, trast, Nicalon M has changed into clusters of large Sic and some SiO evaporation would occur simultaneously crystals. The NicalonTM, TyrannoTM, and HPZ fibers In the case of Si-N-C-O fiber, the HPZ shows a more decreased substantially in their weight and strength complicated phenomenon. The weight loss of HPZ after the thermal exposure test, whereas, Hi-Nicalon M would be caused not only by Co or Sio evaporation As received 1873K 1ohours In argon Hi-Nicalon Nicalon (NL202) 10 um Tyranno LOX-M) HPZ 10 Fig. 6. SEM photographs of as received ceramic fibers and those after the exposure for 10 h in argon at 1873 K
and retains it even after exposure at 1873 K. Fig. 6 shows SEM photographs of the ®bers after thermal exposure testing. NicalonTM forms small particles and changes to a rough ®ber surface after the exposure at 1873 K. TyrannoTM and HPZ ®bers exhibit obvious structural damage. However, Hi-NicalonTM has a smooth ®lament surface, showing no change in appearance. Fig. 7 shows SEM photographs of NicalonTM and Hi-NicalonTM after exposure at 2273 K for 1 h in argon. Hi-NicalonTM has a smooth ®ber surface, but, by contrast, NicalonTM has changed into clusters of large SiC crystals. The NicalonTM, TyrannoTM, and HPZ ®bers decreased substantially in their weight and strength after the thermal exposure test, whereas, Hi-NicalonTM retains a high strength of 1.4 GPa and modulus of 250 GPa even after 1873 K exposure. The silicon oxycarbide-based ®bers, NicalonTM and TyrannoTM, mainly decompose according to the following reaction at elevated temperature: SixCyOz ! xSiC zCO y ÿ x ÿ zC The weight loss of about 20% in NicalonTM and TyrannoTM ®bers will be explained by CO gas evolution, and some SiO evaporation would occur simultaneously. In the case of Si±N±C±O ®ber, the HPZ shows a more complicated phenomenon. The weight loss of HPZ would be caused not only by CO or SiO evaporation, Fig. 6. SEM photographs of as received ceramic ®bers and those after the exposure for 10 h in argon at 1873 K. 816 M. Takeda et al. / Composites Science and Technology 59 (1999) 813±819
M. Takeda et al./ Composites Science and Technology 59(1999)813-819 10 um x2,k81 20 um Fig. 7. SEM photographs of(a) Hi-Nicalon M and(b) Nicalon M after the exposure for 10 h in argon at 2273K but also by N2 elimination, which is known in silicon nitride ceramics Si3N4→3Si+2N2 1673 K 10 hours These fibers evolved CO, Sio, and N2 gas at high tem perature because of their decomposition, resulting in weight loss. Oxygen and nitrogen in silicon non-oxide 台A别 ceramics are unstable at elevated temperature in an inert 1573 K 10 hours atmosphere. Hi-Nicalon'm exhibited outstanding ther- mal stability as compared to the other polymer-derived As Recelved ceramic fibers The changes in physical and chemical properties after thermal exposure were examined. Fig. 8 shows XRD 405060708090 patterns for NicalonM and Hi-Nicalon TM fibers before nd after thermal exposure. Obvious B-Sic crystal growth and a disordered stacking structure [19are Fig 9. X-ray difiraction patterns of HPZ fiber before and after ther. observed in both fibers after exposure at 1873 K for 10 h mal exposure tests. in argon. The B-Sic crystallite size of Hi-NicalonTM after the treatment is smaller than that of Nicalon TM.In the case of Hi-Nicalon TM. excess carbon in the fiber 30L△ Tyranno Lox M 口HPz Hi-Nicalon c 20 As15001600170018001900 3040506070 Temperature(K) Fig. 8. X-ray diffraction patterns of Hi-NicalonM and Nicalon Fig. 10. B-SiC crystalline size of ceramic fibers after the exposure for before and after thermal exposure test. 10 h in argon
but also by N2 elimination, which is known in silicon nitride ceramics. Si3N4 ! 3Si 2N2 These ®bers evolved CO, SiO, and N2 gas at high temperature because of their decomposition, resulting in weight loss. Oxygen and nitrogen in silicon non-oxide ceramics are unstable at elevated temperature in an inert atmosphere. Hi-NicalonTM exhibited outstanding thermal stability as compared to the other polymer-derived ceramic ®bers. The changes in physical and chemical properties after thermal exposure were examined. Fig. 8 shows XRD patterns for NicalonTM and Hi-NicalonTM ®bers before and after thermal exposure. Obvious -SiC crystal growth and a disordered stacking structure [19] are observed in both ®bers after exposure at 1873 K for 10 h in argon. The -SiC crystallite size of Hi-NicalonTM after the treatment is smaller than that of NicalonTM. In the case of Hi-NicalonTM, excess carbon in the ®ber Fig. 8. X-ray diraction patterns of Hi-NicalonTM and NicalonTM before and after thermal exposure test. Fig. 9. X-ray diraction patterns of HPZ ®ber before and after thermal exposure tests. Fig. 7. SEM photographs of (a) Hi-NicalonTM and (b) NicalonTM after the exposure for 10 h in argon at 2273 K. Fig. 10. -SiC crystalline size of ceramic ®bers after the exposure for 10 h in argon. M. Takeda et al. / Composites Science and Technology 59 (1999) 813±819 817
M. Takeda et al. Composites Science and Technology 59(1999)813-819 Hi-Nicalon 3 15 0.5 0 140016001800 1200140016001800 Received Temperature( K) Received Temperature(K) Fig. 11. Tensile strength of SiC fibers after 10 h exposure at elevated Fig. 12. Oxide layer thickness of Sic fibers after 10 h exposure at ele- temperature in vated temperature in air(% H2O) would inhibit crystal growth. Nicalon'M contains less 1673 K exposure in argon for 10 h. Fig. 10 shows the excess carbon after thermal exposure because it decom- crystallite sizes of ceramic fibers, Hi-Nicalon, Nica- poses into near-stoichiometric composition. The Si-C lonM, TyrannoTM, and HPZ, after the thermal exposure O ceramic material prepared by oxidation-curing of test. The crystallite sizes of any fibers grow with increasing PCS loses carbon and oxygen as CO evolution by the temperature. The crystallite size of the oxidation-cured carbothermal reduction and changes into a nearly stoi- fibers, Nicalon M and Tyranno TM, significantly increases chiometric composition of SiC. This near-stoichiometric above 1673 K. On the other hand, the irradiation-cured SiC material tends to grow into larger crystals at a fiber, Hi-Nicalon TM has smaller crystal growth above higher temperature than a Sic containing excess carbon 1773 K compared to the oxidation-cured fiber, because of prepared by the irradiation-curing process Fig 9 shows the presence of excess carbon in the fiber. HPZ would KRD patterns for the HPZ fiber. The HPZ fiber shows decompose to form free silicon after thermal exposure. amorphous XRD characteristics in the as-received state Excess silicon in SiC accelerates crystal growth extensively and after exposure at 1573 K for 10 h, although exten The oxidation-cured SiC fibers, NicalonTM and Tyr sive crystallization of SiC and Si, N4 was observed after annoTM, decreased in their weight. The Si-C-N-O βsicB-S|c Hi-Nicalon 673 K 10 h treated 1673 K 1 h treated As Fabricated 10 20 30 40 Fig. 13. XRD patterns of SiC fibers before and after 1673 K exposure for 10 h in air(2% H2O)
would inhibit crystal growth. NicalonTM contains less excess carbon after thermal exposure because it decomposes into near-stoichiometric composition. The Si±C± O ceramic material prepared by oxidation-curing of PCS loses carbon and oxygen as CO evolution by the carbothermal reduction and changes into a nearly stoichiometric composition of SiC. This near-stoichiometric SiC material tends to grow into larger crystals at a higher temperature than a SiC containing excess carbon prepared by the irradiation-curing process. Fig. 9 shows XRD patterns for the HPZ ®ber. The HPZ ®ber shows amorphous XRD characteristics in the as-received state and after exposure at 1573 K for 10 h, although extensive crystallization of SiC and Si3N4 was observed after 1673 K exposure in argon for 10 h. Fig. 10 shows the crystallite sizes of ceramic ®bers, Hi-NicalonTM, NicalonTM, TyrannoTM, and HPZ, after the thermal exposure test. The crystallite sizes of any ®bers grow with increasing temperature. The crystallite size of the oxidation-cured ®bers, NicalonTM and TyrannoTM, signi®cantly increases above 1673 K. On the other hand, the irradiation-cured ®ber, Hi-NicalonTM has smaller crystal growth above 1773 K compared to the oxidation-cured ®ber, because of the presence of excess carbon in the ®ber. HPZ would decompose to form free silicon after thermal exposure. Excess silicon in SiC accelerates crystal growth extensively. The oxidation-cured SiC ®bers, NicalonTM and TyrannoTM, decreased in their weight. The Si±C±N±O Fig. 11. Tensile strength of SiC ®bers after 10 h exposure at elevated temperature in air (2% H2O). Fig. 12. Oxide layer thickness of SiC ®bers after 10 h exposure at elevated temperature in air (2% H2O). Fig. 13. XRD patterns of SiC ®bers before and after 1673 K exposure for 10 h in air (2% H2O). 818 M. Takeda et al. / Composites Science and Technology 59 (1999) 813±819
M. Takeda et al./ Composites Science and Technology 59(1999)813-819 ceramic fiber, HPZ, also decomposed with extra- References ordinary weight loss and crystallization. By contrast, Hi-NicalonTM, the SiC fiber with low oxygen contents [1] Yajima S, Hayashi J, Omori M, Okamura K. Development of prepared by irradiation-curing showed less change in SiC fibre with high tensile strength. Nature 1976: 261: 683-5 properties, showing much better thermal stability 2 Prewo KM. Brennan JJ, Layden GK. Fiber-reinforced glasses and glass-ceramics for high-performance applications. Am Ceram oc Bull986:65(2)305-14 33. Oxidation resistance 3 Strife JR, Brennan JJ, Prewo KM. Status of continuous fiber. reinforced ceramic processing techI The tensile strength of SiC fibers after exposure for 10 Ceram Eng Sci Proc 1990; 11(7-8): 8 h in humid air is shown in Fig. ll. All fibers tested 4 Clark TJ, Arons RM, Stamatoff JB, Rabe J. Thermal degrada- decreased their strength as compared with as-received on of Nicalon M SiC fibers. Ceram Eng Sci Proc 1985: 6(7- s,and showed lower strengths after exposure 5 Coustumer PL, Monthioux M, Oberlin A. Thermal degradatic higher temperatures. The strength of Hi-NicalonTM is mechanisms of a Nicalon M fiber as deduced from TEM obse roughly the same as that of Nicalon in the range of rations. Int Symp Carbon 1, Tsukuba, Japan 1990: 182-5 1273-1473K. much higher in 1573 and 1673 K. both [6 Delverdier O, Monthioux M, Oberlin A, Mocaer, D. Thermal evo- fibers became too weak to measure their strength after lutions of a cured (fibered) and uncured PCS-based ceramic micr 1773 K exposure for 10 h in humid air. The thickness of textural aspect. Int Symp Carbon 1, Tsukuba, Japan 1990. 190-3 Pysher DJ, Goretta KC, Hodder Jr. RS, Tressler RE Strengths the oxidation layers on the fibers after the test is shown of ceramic fibers at elevated temperatures. J Am Ceram Soc in Fig. 12. The oxide layer thickness on two kinds of the 1989;72(2):2848 fibers is almost the same. XRD patterns of as-fabricated 18] Simon G, Bunsell AR The creep of silicon carbide fibresJMat and oxidized fibers are shown in Fig. 13. Cristobalite formed in both SiC fibers after 1673 K exposure. Fur 9 Simon G, Bunsell AR. Mechanical and structural characteriza on of the Nicalon M silicon carbide fibre. J Mat Sci thermore, Sic crystal growth bserved in Nica 19849:3649-57 lonM by narrowing the XRD peak of B-SiCll1 [10] DiCarlo JA. Creep-related limitations of current ceramic fibers (0=35%)as shown in Fig. 13. Nicalon fiber is not only oxidized but also thermally degraded, as previously [1 Okamura K, Sato M Seguchi T Kawanishi S High-temperature strength improvement of SiC-O fiber by the reduction of oxygen atmosphere. Hi-NicalonTM fiber shows better oxidation content. Proceedings of the Ist Japanese International SAMPE resistance than Nicalon M in the range of 1573-1673K Symposium 1989: 929-34 on a strength basis [12] Takeda M, Imai Y, Ichikawa H, Ishikawa T, Seguchi T, Oka- mura K. Properties of the low oxygen content SiC fiber on high temperature heat treatment. Ceram Eng Sci Proc 1991; 12(7- 8):1007-18 4. Conclusions [13 Takeda M, Imai Y, Ichikawa H, Ishikawa T, Kasai N, Seguchi T, The properties of the low-oxygen SiC fiber, Hi-Nica fibers derived from polycarbosilane. Ceram Eng Sci Proc 1992:13(7-8):209-17 lonTM,prepared by the irradiation-curing method were (14) Lipowitz J, Rabe JA, Zangvil A, Xu Y Structure and properties investigated. This low-oxygen SiC fiber has excellent mechanical properties, which have a high tensile metric B-SiC composition. Ceram Eng Sci Proc I 17 strength of 2.8 GPa and a high tensile modulus of 270 GPa. Hi-NicalonTM retains high strength and modulus [5] Ishikawa T, Kohtoku y, kumagawa K, Yamamura T, Naga after exposure at 1873 K for 10 h in argon. It exhibits Lox-M. and HPz. as for oxidation in air. the oxide Ceram Eng Sci Proc 1996: 17(4):35-42. layer thickness of Nicalon TM and Hi-Nicalon M is [7 Lipowitz J. Polymer-derived ceramic fibers. Cer Bull almost the same. However. Hi-Nicalon TM fiber retained 1991:70(12):1888-94 higher strength than Nicalon in the range of 1573- [19] Honjo K, Shindo A Crystallinity of Sic coating on carbon fiber 1673K. Yogyo-kyokal-shi 1986: 94(1): 172-88
ceramic ®ber, HPZ, also decomposed with extraordinary weight loss and crystallization. By contrast, Hi-NicalonTM, the SiC ®ber with low oxygen contents prepared by irradiation-curing showed less change in properties, showing much better thermal stability. 3.3. Oxidation resistance The tensile strength of SiC ®bers after exposure for 10 h in humid air is shown in Fig. 11. All ®bers tested decreased their strength as compared with as-received ®bers, and showed lower strengths after exposure at higher temperatures. The strength of Hi-NicalonTM is roughly the same as that of NicalonTM in the range of 1273±1473 K, much higher in 1573 and 1673 K. Both ®bers became too weak to measure their strength after 1773 K exposure for 10 h in humid air. The thickness of the oxidation layers on the ®bers after the test is shown in Fig. 12. The oxide layer thickness on two kinds of the ®bers is almost the same. XRD patterns of as-fabricated and oxidized ®bers are shown in Fig. 13. Cristobalite formed in both SiC ®bers after 1673 K exposure. Furthermore, SiC crystal growth was observed in NicalonTM by narrowing the XRD peak of -SiC111 ( 35) as shown in Fig. 13. NicalonTM ®ber is not only oxidized but also thermally degraded, as previously discussed, during thermal exposure tests in an argon atmosphere. Hi-NicalonTM ®ber shows better oxidation resistance than NicalonTM in the range of 1573±1673 K on a strength basis. 4. Conclusions The properties of the low-oxygen SiC ®ber, Hi-NicalonTM, prepared by the irradiation-curing method were investigated. This low-oxygen SiC ®ber has excellent mechanical properties, which have a high tensile strength of 2.8 GPa and a high tensile modulus of 270 GPa. Hi-NicalonTM retains high strength and modulus after exposure at 1873 K for 10 h in argon. It exhibits outstanding thermal stability compared with other polymer-derived ceramic ®bers, NicalonTM, TyrannoTM Lox-M, and HPZ. As for oxidation in air, the oxide layer thickness of NicalonTM and Hi-NicalonTM is almost the same. However, Hi-NicalonTM ®ber retained a higher strength than NicalonTM in the range of 1573± 1673 K. References [1] Yajima S, Hayashi J, Omori M, Okamura K. Development of a SiC ®bre with high tensile strength. Nature 1976;261:683±5. [2] Prewo KM, Brennan JJ, Layden GK. Fiber-reinforced glasses and glass-ceramics for high-performance applications. Am Ceram Soc Bull 1986;65(2):305±14. [3] Strife JR, Brennan JJ, Prewo KM. Status of continuous ®berreinforced ceramic matrix composite processing technology. Ceram Eng Sci Proc 1990;11(7±8):871±919. [4] Clark TJ, Arons RM, Stamato JB, Rabe J. Thermal degradation of NicalonTM SiC ®bers. Ceram Eng Sci Proc 1985;6(7± 8):576±88. [5] Coustumer PL, Monthioux M, Oberlin A. Thermal degradation mechanisms of a NicalonTM ®ber as deduced from TEM observations. Int Symp Carbon 1, Tsukuba, Japan 1990: 182±5. [6] Delverdier O, Monthioux M, Oberlin A, Mocaer, D. Thermal evolutions of a cured (®bered) and uncured PCS-based ceramic microtextural aspect. Int Symp Carbon 1, Tsukuba, Japan 1990: 190±3. [7] Pysher DJ, Goretta KC, Hodder Jr. RS, Tressler RE. Strengths of ceramic ®bers at elevated temperatures. J Am Ceram Soc 1989;72(2):284±8. [8] Simon G, Bunsell AR. The creep of silicon carbide ®bres. J Mat Sci Lett 1983;2:80±2. [9] Simon G, Bunsell AR. Mechanical and structural characterization of the NicalonTM silicon carbide ®bre. J Mat Sci 1984;19:3649±57. [10] DiCarlo JA. Creep-related limitations of current ceramic ®bers. Proceedings of the International Workshop Adv. Inorg. Fiber Technology, Melbourne, Australia, 1992. p. 67. [11] Okamura K, Sato M, Seguchi T, Kawanishi S. High-temperature strength improvement of Si±C±O ®ber by the reduction of oxygen content. Proceedings of the 1st Japanese International SAMPE Symposium 1989: 929±34. [12] Takeda M, Imai Y, Ichikawa H, Ishikawa T, Seguchi T, Okamura K. Properties of the low oxygen content SiC ®ber on high temperature heat treatment. Ceram Eng Sci Proc 1991;12(7± 8):1007±18. [13] Takeda M, Imai Y, Ichikawa H, Ishikawa T, Kasai N, Seguchi T, Okamura K. Thermal stability of the low oxygen silicon carbide ®bers derived from polycarbosilane. Ceram Eng Sci Proc 1992;13(7±8):209±17. [14] Lipowitz J, Rabe JA, Zangvil A, Xu Y. Structure and properties of SylramicTM silicon carbide ®berÐa polycrystalline, stoichiometric -SiC composition. Ceram Eng Sci Proc 1997;18(3):147± 57. [15] Ishikawa T, Kohtoku Y, Kumagawa K, Yamamura T, Nagasawa T. High-strength alkali-resistant sintered SiC ®bre stable to 2200C. Nature 1998;391:773±4. [16] Takeda M, Sakamoto J, Saeki A, Ichikawa H. Mechanical and structural analysis of silicon carbide ®ber Hi-NicalonTM Type S. Ceram Eng Sci Proc 1996;17(4):35±42. [17] Lipowitz J. Polymer-derived ceramic ®bers. Cer Bull 1991;70(12):1888±94. [18] TyrannoTM ®ber Technical Data Sheet, Ube Industries. [19] Honjo K, Shindo A. Crystallinity of SiC coating on carbon ®ber. Yogyo-kyokai-shi 1986;94(1):172±88. M. Takeda et al. / Composites Science and Technology 59 (1999) 813±819 819
