Availableonlineatwww.sciencedirect.com COMPOSITES ScienceDirect CIENCE AND TECHNOLOGY ELSEVIER Science and Technology 68(2008)602-607 w.elsevier. com/locate/compscitech Oxidation behavior of 2D C/Sic composite modified by SiBa particles in inter-bundle pores onei changqing Tong, Laifei Cheng" Xiaowei Yin, Litong Zhang, Yongdong Xu National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical Unirersity, Xian, Shaanxi 710072, PR China Received 21 May 2007: received in revised form 3 September 2007: accepted 18 October 2007 Available online 25 October 2007 Abstract In order to understand the oxidation resistance of a 2D C/Sic composite protected with self-healing filler in the inter-bundle pore SiBa particles as the filler were infiltrated into the inter- bundle pores of 2D C/Sic composite by slurry infiltration process. The SiBa particles were combined with Sic during the chemical vapor deposition(CVD) Sic coating process. Isothermal oxidation tests of the as-received modified composite were carried out in air at temperatures ranging from 500 to 1000C. SEM and EDS results showed that all the open inter- bundle pores of the C/SiC composite can be filled with SiB, particles. The SiBa filler can hinder the inwards diffusion of oxygen in air, and protect the carbon fibres and carbon interphase from oxidation. As a result, the modified composite lost weight slowly, and showed no obvious decrease in flexural strength at the temperatures of 500-900C for an oxidation time 10 h C 2007 Elsevier Ltd. All rights reserved Keywords: A. Carbon fibres; A. Ceramic-matrix composites; C. Crack; D. Scanning electron microscopy, E. Chemical vapor deposition 1. Introduction the oxidation resistance of C/SiC composites for long-term Carbon fibre reinforced silicon carbide composites(C/ Boron-bearing species are efficient at relatively low tem- Sic) exhibit attractive properties for thermostructural peratures(500-1000C) to improve the oxidation resis- applications, including low density, high strength and tance of C/SiC composites. They can form fluid oxide non-brittle mechanical behavior [1]. However, an as-fabri- phases(B2O3 or B-M-o ternary phase) during oxidation cated C/SiC composite has two important microstructural to fill cracks, slowing down the in-depth diffusion of oxy features: (a)large residual pore located among bundles and gen[8, 9]. So far, two kinds of approaches have been devel (b)matrix/coating microcracks resulting from coefficient of oped to protect the fibre and interface. The first approach thermal expansion(CTE) mismatch. The Sic matrix and is oxidation protection coating [10, 11]. Although the fluid lting are unable to protect the fibre from oxidation at phases formed during oxidation can fill coating cracks temperatures below the deposition temperature [2-5]. but the inter-bundle pores can still act as the path of According to previous researches on the oxidation behav- inwards diffusion of oxygen especially when the coating or of C/SiC, the specimens exposed to air lost maximum cannot be healed during oxidation [12]. The second weight at the temperatures ranging from 600 to 800C approach is oxidation protection matrix using multilayered [6, 7]. Therefore, various sealants are necessary to increase self-healing matrix or boron-based particles introduced in the Sic-matrix [5, 13, 14]. Compared with multilayered self-healing matrix fabricated by CVI method, boron-based E-mail adres chenglfanwpuedu cn(L Cheng). 620 particles introduced by slurry shorter processing period and lower cost. a simple way 0266-3538/- see front matter e 2007 Elsevier Ltd. All rights reserved do: 10.1016/j. compscitech 2007. 10.016
Oxidation behavior of 2D C/SiC composite modified by SiB4 particles in inter-bundle pores Changqing Tong, Laifei Cheng *, Xiaowei Yin, Litong Zhang, Yongdong Xu National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, PR China Received 21 May 2007; received in revised form 3 September 2007; accepted 18 October 2007 Available online 25 October 2007 Abstract In order to understand the oxidation resistance of a 2D C/SiC composite protected with self-healing filler in the inter-bundle pores, SiB4 particles as the filler were infiltrated into the inter-bundle pores of 2D C/SiC composite by slurry infiltration process. The SiB4 particles were combined with SiC during the chemical vapor deposition (CVD) SiC coating process. Isothermal oxidation tests of the as-received modified composite were carried out in air at temperatures ranging from 500 to 1000 C. SEM and EDS results showed that all the open inter-bundle pores of the C/SiC composite can be filled with SiB4 particles. The SiB4 filler can hinder the inwards diffusion of oxygen in air, and protect the carbon fibres and carbon interphase from oxidation. As a result, the modified composite lost weight slowly, and showed no obvious decrease in flexural strength at the temperatures of 500–900 C for an oxidation time of 10 h. 2007 Elsevier Ltd. All rights reserved. Keywords: A. Carbon fibres; A. Ceramic–matrix composites; C. Crack; D. Scanning electron microscopy; E. Chemical vapor deposition 1. Introduction Carbon fibre reinforced silicon carbide composites (C/ SiC) exhibit attractive properties for thermostructural applications, including low density, high strength and non-brittle mechanical behavior [1]. However, an as-fabricated C/SiC composite has two important microstructural features: (a) large residual pore located among bundles and (b) matrix/coating microcracks resulting from coefficient of thermal expansion (CTE) mismatch. The SiC matrix and coating are unable to protect the fibre from oxidation at temperatures below the deposition temperature [2–5]. According to previous researches on the oxidation behavior of C/SiC, the specimens exposed to air lost maximum weight at the temperatures ranging from 600 to 800 C [6,7]. Therefore, various sealants are necessary to increase the oxidation resistance of C/SiC composites for long-term use. Boron-bearing species are efficient at relatively low temperatures (500–1000 C) to improve the oxidation resistance of C/SiC composites. They can form fluid oxide phases (B2O3 or B–M–O ternary phase) during oxidation to fill cracks, slowing down the in-depth diffusion of oxygen [8,9]. So far, two kinds of approaches have been developed to protect the fibre and interface. The first approach is oxidation protection coating [10,11]. Although the fluid phases formed during oxidation can fill coating cracks, but the inter-bundle pores can still act as the path of inwards diffusion of oxygen especially when the coating cannot be healed during oxidation [12]. The second approach is oxidation protection matrix using multilayered self-healing matrix or boron-based particles introduced in the SiC-matrix [5,13,14]. Compared with multilayered self-healing matrix fabricated by CVI method, boron-based particles introduced by slurry infiltration method has shorter processing period and lower cost. A simple way 0266-3538/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2007.10.016 * Corresponding author. Tel.: +86 29 8848 6068; fax: +86 29 8849 4620. E-mail address: chenglf@nwpu.edu.cn (L. Cheng). www.elsevier.com/locate/compscitech Available online at www.sciencedirect.com Composites Science and Technology 68 (2008) 602–607 COMPOSITES SCIENCE AND TECHNOLOGY
C. Tong et al. Composites Science and Technology 68(2008)602-607 to introduce boron-based particles is to impregnate a fibre 2. 2. Oxidation tests preform with slurry consisting of a suspension of boron- based particles [8]. Up to now, there is still little knowledge The samples were exposed in dry air at specific temper on the oxidation behavior of 2D C/SiC composite pro- atures(between 500C and 1000 C)for 2, 5 and 10 h, tected with self-healing filler in the inter-bundle pores respectively. The oxidation weight change(Am/mo= 100 In the present paper, in order to understand the oxida- (m- mo)/mo, where mo is the sample mass at t=0 and m tion behavior of 2D C/SiC composite with self-healing par- that at t)was measured with an analytical balance(sensitiv ticles, SiB4 particles were infiltrated into the inter-bundle ity: 0.01 mg, Mettler AG 105 ). Three to six specimens were pores of 2D C/Sic composite by slurry infiltration. The measured in each case for statistical significance. xidation behavior of the as-fabricated composite in air at temperatures ranging from 500 to 1000C was 2.3. Characteristics of 2D C/SiC-SiB, composite investigated The flexural strength of the specimens before and after 2. Experiment procedure oxidation was measured by a three-point bending method which was carried out on an Instron 1195 machine at room 2. 1. Fabrication of the specimens temperature. The span dimension was 20 mm and the load ing rate was 0.5 mm/min Cross-sections of the C/SiC-SiB4 A two dimensional C/SiC composite was prepared by specimens were cut, then polished using diamond paste and low-pressure chemical vapor infiltration(LPCVI) process. cleaned in anhydrous ethanol. The sections and the sur- Preforms were prepared stacking plain weave carbon cloths faces of the specimens were observed on a scanning elec- (T-300)in a perforated graphite holder which was tightened tron microscopy(SEM, $2700). Energy dispersive X-ray to obtain the desired fibre volume fraction(generally 40%). spectrum(EDS) and micro-area X-ray diffractometry PyC was deposited on the fibre using C3H precursor at (XI <RD, Rigaku-D/MAX-2550) were recorded to identify 870C for I h at a reduced pressure of 5 kPa, yielding a element species and phase composition of the filler, thickness of 0.2 um. Methyltrichlorosilane(MTS, CH3- respectively. The XRD analytical conditions were as SiCl3) was used for the deposition of the Sic matrix. MTs follows: target, Cu; filter, Ni: voltage, 40kV; current apor was carried by bubbling hydrogen. The conditions 250 mA; diameter of the collimator, 30 um. for deposition of SiC matrix were as follows: the deposition temperature was 1000C, pressure was 5 kPa, time was 3. Results and discussion 240 h, and the molar ratio of H, to MTS was 10. Argon was employed as the dilute gas to slow down the chemical 3. 1. Composition and microstructure of the modified reaction rate of deposition. The as-received C/SiC compos- composite ites were machined and polished into specimens with a dimension of 3.0 x 4. x 30.0 mm. The apparent porosity, Fig. la shows that all the open inter-bundle pores of measured by the archimedes method, was 18-21% the C/SiC composite can be infiltrated with the SiB4 par A commercially available SiB4 powder, supplied by ticles. By EDS element analyses, the main elements of MaTeck GmbH, Germany, was employed for slurry infil- matrix among inter-bundle pores were Si, B and C ration. The purity and average particles size of SiB4 pow-( Fig. Ic). XRD results indicated that SiB4 and Sic were der were 99.5% and I um, respectively. The slurry was the main phase(Fig. 2), suggesting that the matrix was prepared by dispersing the SiBa powder(50 wt %)in deion- composed of SiB4 and SiC. Further, SiBa particles were ized H2O and adjusting PH to provide large electrostatic combined with Sic during the Cvd Sic coating process, repulsive forces between the particles. The slurry was ball as shown in Fig. Ib. However, few SiB4 particles were milled for 12 h in a polyethylene bottle with AlO3 balls infiltrated into intra-bundle micropores. The SiC matrix as the milling media was firstly deposited on the surface of carbon fibre, then q The specimens were divided into two groups. A Sic to the surface of the fibre bundles during the cvi pro- pating consisting of two layers was deposited by CVd cess. With the increase of the composite density, a contin on specimens of the first group. Those of the second group uous SiC matrix with a thickness of 20 um was formed were infiltrated with the SiB4 slurry under vacuum for I h. on the fibre bundles, which prevented the deposition of After drying at 80C, excess slurry was removed from the Sic in the intra-bundle pores. Therefore, SiB4 particles surface of the preforms, on which a two-layer SiC coatings cannot fill the intra-bundle pores during slurry infiltration were deposited with CVD process. The deposition condi- process tions of CVD Sic coating were the same as that of the CVI SiC matrix except for the deposition time. The depo- 3. 2. Composition and microstructure of the modified sition time of single-layer SiC coating was 80 h. composite after oxidation The as-fabricated C/SiC-SiBA composite attained a den sity of 2. 1 g/cm with the apparent porosity of 12%. The For the composite with SiB4 filler, the main reactions in content of SiB, in the composite was 3.9 wt% air from 500 to 1000C may take place as follows
to introduce boron-based particles is to impregnate a fibre preform with slurry consisting of a suspension of boronbased particles [8]. Up to now, there is still little knowledge on the oxidation behavior of 2D C/SiC composite protected with self-healing filler in the inter-bundle pores. In the present paper, in order to understand the oxidation behavior of 2D C/SiC composite with self-healing particles, SiB4 particles were infiltrated into the inter-bundle pores of 2D C/SiC composite by slurry infiltration. The oxidation behavior of the as-fabricated composite in air at temperatures ranging from 500 to 1000 C was investigated. 2. Experiment procedure 2.1. Fabrication of the specimens A two dimensional C/SiC composite was prepared by low-pressure chemical vapor infiltration (LPCVI) process. Preforms were prepared stacking plain weave carbon cloths (T-300) in a perforated graphite holder which was tightened to obtain the desired fibre volume fraction (generally 40%). PyC was deposited on the fibre using C3H6 precursor at 870 C for 1 h at a reduced pressure of 5 kPa, yielding a thickness of 0.2 lm. Methyltrichlorosilane (MTS, CH3- SiCl3) was used for the deposition of the SiC matrix. MTS vapor was carried by bubbling hydrogen. The conditions for deposition of SiC matrix were as follows: the deposition temperature was 1000 C, pressure was 5 kPa, time was 240 h, and the molar ratio of H2 to MTS was 10. Argon was employed as the dilute gas to slow down the chemical reaction rate of deposition. The as-received C/SiC composites were machined and polished into specimens with a dimension of 3.0 · 4.0 · 30.0 mm3 . The apparent porosity, measured by the Archimedes method, was 18–21%. A commercially available SiB4 powder, supplied by MaTecK GmbH, Germany, was employed for slurry infiltration. The purity and average particles size of SiB4 powder were 99.5% and 1 lm, respectively. The slurry was prepared by dispersing the SiB4 powder (50 wt %) in deionized H2O and adjusting PH to provide large electrostatic repulsive forces between the particles. The slurry was ball milled for 12 h in a polyethylene bottle with Al2O3 balls as the milling media. The specimens were divided into two groups. A SiC coating consisting of two layers was deposited by CVD on specimens of the first group. Those of the second group were infiltrated with the SiB4 slurry under vacuum for 1 h. After drying at 80 C, excess slurry was removed from the surface of the preforms, on which a two-layer SiC coatings were deposited with CVD process. The deposition conditions of CVD SiC coating were the same as that of the CVI SiC matrix except for the deposition time. The deposition time of single-layer SiC coating was 80 h. The as-fabricated C/SiC–SiB4 composite attained a density of 2.1 g/cm3 with the apparent porosity of 12%. The content of SiB4 in the composite was 3.9 wt%. 2.2. Oxidation tests The samples were exposed in dry air at specific temperatures (between 500 C and 1000 C) for 2, 5 and 10 h, respectively. The oxidation weight change (Dm/m0 = 100 (m m0)/m0, where m0 is the sample mass at t = 0 and m that at t) was measured with an analytical balance (sensitivity: 0.01 mg, Mettler AG 105). Three to six specimens were measured in each case for statistical significance. 2.3. Characteristics of 2D C/SiC–SiB4 composite The flexural strength of the specimens before and after oxidation was measured by a three-point bending method, which was carried out on an Instron 1195 machine at room temperature. The span dimension was 20 mm and the loading rate was 0.5 mm/min. Cross-sections of the C/SiC–SiB4 specimens were cut, then polished using diamond paste and cleaned in anhydrous ethanol. The sections and the surfaces of the specimens were observed on a scanning electron microscopy (SEM, S2700). Energy dispersive X-ray spectrum (EDS) and micro-area X-ray diffractometry (XRD, Rigaku-D/MAX-2550) were recorded to identify element species and phase composition of the filler, respectively. The XRD analytical conditions were as follows: target, Cu; filter, Ni; voltage, 40 kV; current, 250 mA; diameter of the collimator, 30 lm. 3. Results and discussion 3.1. Composition and microstructure of the modified composite Fig. 1a shows that all the open inter-bundle pores of the C/SiC composite can be infiltrated with the SiB4 particles. By EDS element analyses, the main elements of matrix among inter-bundle pores were Si, B and C (Fig. 1c). XRD results indicated that SiB4 and SiC were the main phase (Fig. 2), suggesting that the matrix was composed of SiB4 and SiC. Further, SiB4 particles were combined with SiC during the CVD SiC coating process, as shown in Fig. 1b. However, few SiB4 particles were infiltrated into intra-bundle micropores. The SiC matrix was firstly deposited on the surface of carbon fibre, then to the surface of the fibre bundles during the CVI process. With the increase of the composite density, a continuous SiC matrix with a thickness of 20 lm was formed on the fibre bundles, which prevented the deposition of SiC in the intra-bundle pores. Therefore, SiB4 particles cannot fill the intra-bundle pores during slurry infiltration process. 3.2. Composition and microstructure of the modified composite after oxidation For the composite with SiB4 filler, the main reactions in air from 500 to 1000 C may take place as follows: C. Tong et al. / Composites Science and Technology 68 (2008) 602–607 603
C. Tong et al. Composites Science and Techne 2 theta ( Fig. 2. Micro-area X-ray diffractometry of the SiB Sic matrix filled the inter-bundle pore in Fig. Ib SiBa-SiC matrix Figs. 3 and 4 show polished cross-section morpholog f the C/SiC-SiB4 composite after oxidation at 700 and 1000C for 10 h, respectively. At 700C, the matrix micro- SiC matrix cracks in the interior of the composite cannot be sealed, but they were covered by the dense SiB4-SiC matrix, as shown in Fig. 3a. Furthermore, a glassy material could not be apparently observed in the SiB4-SiC matrix due to the low reaction rate of SiB4 and Oz at low temperatures. with 5028 ￠ 50um a SiBa-SiC matrix c Crack SiC matrix 00八L b Fig. 1. SEM photographs of the as-received C/SiC-SiB, showing (a) lished cross-section morphology, (b) high magnification view of Fig. and (c)energy dispersive spectrum of the SiBy-SiC matrix filled the inter- bundle pore in Fig. 1b. Crack 2C(s)+O2(g)→2CO(g) H SiB4(s)+402(g)-2B2O31)+SiO2() B2O3(1)→B2O3(g) (3) 2SiC(s)+302(g)- 2SiO2(s)+ 2Co(g) According to the scanning electron micrograph apparent change was observed after the composite was oxi- dized at 500C. The morphologies of the oxidized at the Fig 3. SEM photographs of the C/SiC-SiB4 composite after oxidized at temperature of 600-800C and 900-1000C were similar (a)700 C for 10 h and(b)1000C for 10 h, showing the SiBg-Sic matrix with each other, respectively in the inter-bundle pore
2CðsÞ þ O2ðgÞ ! 2COðgÞ ð1Þ SiB4ðsÞ þ 4O2ðgÞ ! 2B2O3ðlÞ þ SiO2ðlÞ ð2Þ B2O3ðlÞ ! B2O3ðgÞ ð3Þ 2SiCðsÞ þ 3O2ðgÞ ! 2SiO2ðsÞ þ 2COðgÞ ð4Þ According to the scanning electron micrographs, no apparent change was observed after the composite was oxidized at 500 C. The morphologies of the oxidized at the temperature of 600–800 C and 900–1000 C were similar with each other, respectively. Figs. 3 and 4 show polished cross-section morphologies of the C/SiC–SiB4 composite after oxidation at 700 and 1000 C for 10 h, respectively. At 700 C, the matrix microcracks in the interior of the composite cannot be sealed, but they were covered by the dense SiB4–SiC matrix, as shown in Fig. 3a. Furthermore, a glassy material could not be apparently observed in the SiB4–SiC matrix due to the low reaction rate of SiB4 and O2 at low temperatures. With Fig. 1. SEM photographs of the as-received C/SiC–SiB4, showing (a) polished cross-section morphology, (b) high magnification view of Fig. 1a and (c) energy dispersive spectrum of the SiB4–SiC matrix filled the interbundle pore in Fig. 1b. Fig. 2. Micro-area X-ray diffractometry of the SiB4–SiC matrix filled the inter-bundle pore in Fig. 1b. Fig. 3. SEM photographs of the C/SiC–SiB4 composite after oxidized at (a) 700 C for 10 h and (b) 1000 C for 10 h, showing the SiB4–SiC matrix in the inter-bundle pore. 604 C. Tong et al. / Composites Science and Technology 68 (2008) 602–607
C. Tong et al. Composites Science and Technology 68(2008)602-607 Oxidized fibres E Crack 150um 2 Glass Glass 100 Fig. 4. SEM photographs of the C/SiC-SiBa composite after oxidized at Fig. 5. The surface morphology of the C/SiC-SiB4 composite after l000° C for I0h, (a)carbon fibres were oxidized due to the in the glas oxidized at 1000.C for 10 h, showing(a) glass in the microcrack; (b) pores inward diffusion of through mircocracks and(b) carbon fibres were protected by the increase of oxidation temperature, B2O3. xSio2 glass volatilization by the reactions(1)and(3), and the weight nay be formed [15]. At 1000C, micro-pores of the SiB 4- gain attributed to the SiB4 and Sic oxidation in the com iC matrix were occupied with fluid B203.xSiOz, which posite by chemical reactions(2)and (, ine ON y, 200 1o led to the healing of microcrack in the Sic matrix, as shown Fig. 6 shows the effect of oxidation ti in Fig. 3b. However, the oxidation near the external surface tion behavior of the 2D C/SiC-SiB4 and 2D C/SiC of the samples was not uniform Fig 4a shows that part of posites with a two-layer CVD SiC coating from carbon fibres in some regions near the SiC coating, without 1000C. At 500C, the relative weight changes of C/ SiBa filler, were oxidized due to the inwards diffusion of SiC-SiB4 were negligible due to the oxidation rate of the oxygen through the coating microcracks. As a contrast, in composite was relatively slow. At temperatures ranging the similar regions with SiB4 filler, the formed borosilicate from 600 to 1000oC, the weight loss increased in the initial glass hindered the inwards diffusion of oxygen, which pro- 5 h, indicating that oxygen diffused into the C/SiC-SiB4 tected carbon fibres from oxidation(Fig 4b). Fig 5a shows composite, resulting in the oxidation of the carbon phase the glass filled in a coating microcrack, which implied that Obviously, the increase of temperature led to the accelera the microcracks of coating were not fully closed at tion of the oxidation rate. Although the formation of B,O3 1000C, leading to the outwards flowing of glass. Further- and SiO could lead to mass gains theoretically, the weight more, pores could be observed in the glass flowing out loss of the C/SiC-SiB4 composite was still large due to the hrough the microcracks of coating(Fig. 5b ), which con- fact that carbon consumption with a mass loss was pre- firmed the volatilization of B2O3. Previous reports [15, 16] dominant. After oxidation for 5 h, the oxidation rate of also mentioned that the volatilization of B2O3 at 1000C. the samples decreased due to the formation of borosilicate glass phase caused by the solution of Sio2 with B2O3 and 3.3. Oxidation behavior of the modified composite then self-protection film formed, which subsequently hin dered the diffusion of oxygen into the intra-bundle pores The weight variation of C/SiC-SiB4 materials is due to of the composite. At 600C, the C/SiC-SiB4 composite weight loss from the carbon oxidation and the B 2O3 showed a tendency of weight gaining during the oxidation
the increase of oxidation temperature, B2O3 Æ xSiO2 glass may be formed [15]. At 1000 C, micro-pores of the SiB4– SiC matrix were occupied with fluid B2O3 Æ xSiO2, which led to the healing of microcrack in the SiC matrix, as shown in Fig. 3b. However, the oxidation near the external surface of the samples was not uniform. Fig. 4a shows that part of carbon fibres in some regions near the SiC coating, without SiB4 filler, were oxidized due to the inwards diffusion of oxygen through the coating microcracks. As a contrast, in the similar regions with SiB4 filler, the formed borosilicate glass hindered the inwards diffusion of oxygen, which protected carbon fibres from oxidation (Fig. 4b). Fig. 5a shows the glass filled in a coating microcrack, which implied that the microcracks of coating were not fully closed at 1000 C, leading to the outwards flowing of glass. Furthermore, pores could be observed in the glass flowing out through the microcracks of coating (Fig. 5b), which con- firmed the volatilization of B2O3. Previous reports [15,16] also mentioned that the volatilization of B2O3 at 1000 C. 3.3. Oxidation behavior of the modified composite The weight variation of C/SiC–SiB4 materials is due to the weight loss from the carbon oxidation and the B2O3 volatilization by the reactions (1) and (3), and the weight gain attributed to the SiB4 and SiC oxidation in the composite by chemical reactions (2) and (4). Fig. 6 shows the effect of oxidation time on the oxidation behavior of the 2D C/SiC–SiB4 and 2D C/SiC composites with a two-layer CVD SiC coating from 500 to 1000 C. At 500 C, the relative weight changes of C/ SiC–SiB4 were negligible due to the oxidation rate of the composite was relatively slow. At temperatures ranging from 600 to 1000 C, the weight loss increased in the initial 5 h, indicating that oxygen diffused into the C/SiC–SiB4 composite, resulting in the oxidation of the carbon phase. Obviously, the increase of temperature led to the acceleration of the oxidation rate. Although the formation of B2O3 and SiO2 could lead to mass gains theoretically, the weight loss of the C/SiC–SiB4 composite was still large due to the fact that carbon consumption with a mass loss was predominant. After oxidation for 5 h, the oxidation rate of the samples decreased due to the formation of borosilicate glass phase caused by the solution of SiO2 with B2O3 and then self-protection film formed, which subsequently hindered the diffusion of oxygen into the intra-bundle pores of the composite. At 600 C, the C/SiC–SiB4 composite showed a tendency of weight gaining during the oxidation Fig. 4. SEM photographs of the C/SiC–SiB4 composite after oxidized at 1000 C for 10 h, showing (a) carbon fibres were oxidized due to the inward diffusion of oxygen through mircocracks and (b) carbon fibres were protected by glass sealant from oxidation. Fig. 5. The surface morphology of the C/SiC–SiB4 composite after oxidized at 1000 C for 10 h, showing (a) glass in the microcrack; (b) pores in the glass. C. Tong et al. / Composites Science and Technology 68 (2008) 602–607 605
C. Tong et al. Composites Science and Technology 68(2008)602-607 2h f环 一一10h C/SiC-SiB C/SiC-SiB4 1000 0 Temperature (C) Fig. 6. Relationship of weight change and temperature in the 2D C/Sic- Fig. 7. Flexural strength change of the 2D C/SiC-SiB4 and 2D C/Sic SiB, and 2D C/Sic osites with a two-layer SiC coating after composites with a two-layer Sic coating after oxidized at different oxidizing for different periods of time. of 10 h. This is in good agreement with the Matsushita et al's research results [15], indicating the oxidation of investigation, the oxidation process of C/SiC-SiB4 may SiBa powder started at 500C and the maximum value of be summarized as follows: the weight gain at 600C when it oxidized at 200- 1100C for 10 h in air. Above 900C, the weight loss (1) inwards diffusion of molecular oxygen and outwards increased with increasing of oxidation time due to the diffusion of the gaseous products through the micr SiB4 filler can not hinder the inwards diffusion of oxygen cracks in the coating and matrix; ugh the coating microcracks into the region close to (2) the chemical reaction of carbon with O2 leading to ting. As a result, the carbon interphase and fibre would the weight loss, while the reactions between SiB be oxidized continuously. However, the carbon reinforce- and Sic with O, leading to the weight gain ment was uniformly degraded for C/SiC at temperatures (3) the decrease of the oxidation rate of the samples after below 800C [17], and the weight loss reached its maxi- the formation of borosilicate glass phase in the mum value at 700oC after oxidizing for 10 h. From 700 regions in which there exists SiB4 filler, subsequently to 1000C, the weight loss of C/Sic rapidly decreased hindering the inwards diffusion of oxygen into com- because the matrix cracks and the coating cracks decreased posite; however, further inwards diffusion of oxygen rapidly with an increasing temperature. The oxidation through coating microcracks into the regions close behavior is similar to that of the 3D C/SiC [4] to the coating in which there exists no SiBa filler still Fig. 7 shows the relationship between residual strength resulting in the oxidation of carbon phase and temperature of the C/SiC-SiB4 and 2D C/SiC with wo-layer CVD SiC coating composites oxidized in air at 4. Conclusions different temperatures for 10 h. It could be found that the flexural strength of C/SiC-SiB4 was about 342 MPa, the SiBa particles can be infiltrated into the open inter-bun- value was smaller than that of C/Sic before oxidizing. dle pores of C/SiC composite by slurry infiltration process. After oxidation at temperatures from 500 to 900C, the As filler in C/SiC composite, SiB, can hinder the inward strength of the C/SiC-SiB4 has a little change. However, diffusion of oxygen, and the oxidation of carbon fibres the strength of C/SiC rapidly decreased and reached its took place mainly in the region close to the SiC coating minimum value at 700C, which indicated that SiB, filler As a result, the flexural strengths of the modified compos- can hinder the inwards diffusion of oxygen in air, and pro- ites did not decrease obviously after oxidation at the tem- tect the carbon fibres and carbon interphase from oxida- peratures of 500-900C for 10 h tion. At 1000C, a noticeable decrease in flexural In order to further improve the oxidation resistance of strength of the C/SiC-SiB4 composite was observed. The C/SiC composite, it will be modified by not only self-heal- reason for the decrease is thought to be a high oxidation ing filler, but also self-healing coating rate of the carbon in the early stage of the oxidation Another possible reason is that the partial deterioration Acknowledgments of the self-healing film close to the coating, which resulted in the further oxidation of carbon near the surface The authors acknowledge financial suppor All the above results indicated that oxidation behavior Natural Science Foundation of China( Contract No of C/SiC-SiB4 was different from that of C/sic due to 90405015), National Young Elitists Foundation(Contract the existence of SiB4 particles On the basis of the above No. 50245208)
of 10 h. This is in good agreement with the Matsushita et al’s research results [15], indicating the oxidation of SiB4 powder started at 500 C and the maximum value of the weight gain at 600 C when it oxidized at 200– 1100 C for 10 h in air. Above 900 C, the weight loss increased with increasing of oxidation time due to the SiB4 filler can not hinder the inwards diffusion of oxygen through the coating microcracks into the region close to coating. As a result, the carbon interphase and fibre would be oxidized continuously. However, the carbon reinforcement was uniformly degraded for C/SiC at temperatures below 800 C [17], and the weight loss reached its maximum value at 700 C after oxidizing for 10 h. From 700 to 1000 C, the weight loss of C/SiC rapidly decreased because the matrix cracks and the coating cracks decreased rapidly with an increasing temperature. The oxidation behavior is similar to that of the 3D C/SiC [4]. Fig. 7 shows the relationship between residual strength and temperature of the C/SiC–SiB4 and 2D C/SiC with a two-layer CVD SiC coating composites oxidized in air at different temperatures for 10 h. It could be found that the flexural strength of C/SiC–SiB4 was about 342 MPa, the value was smaller than that of C/SiC before oxidizing. After oxidation at temperatures from 500 to 900 C, the strength of the C/SiC–SiB4 has a little change. However, the strength of C/SiC rapidly decreased and reached its minimum value at 700 C, which indicated that SiB4 filler can hinder the inwards diffusion of oxygen in air, and protect the carbon fibres and carbon interphase from oxidation. At 1000 C, a noticeable decrease in flexural strength of the C/SiC–SiB4 composite was observed. The reason for the decrease is thought to be a high oxidation rate of the carbon in the early stage of the oxidation. Another possible reason is that the partial deterioration of the self-healing film close to the coating, which resulted in the further oxidation of carbon near the surface. All the above results indicated that oxidation behavior of C/SiC–SiB4 was different from that of C/SiC due to the existence of SiB4 particles. On the basis of the above investigation, the oxidation process of C/SiC–SiB4 may be summarized as follows: (1) inwards diffusion of molecular oxygen and outwards diffusion of the gaseous products through the microcracks in the coating and matrix; (2) the chemical reaction of carbon with O2 leading to the weight loss, while the reactions between SiB4 and SiC with O2 leading to the weight gain; (3) the decrease of the oxidation rate of the samples after the formation of borosilicate glass phase in the regions in which there exists SiB4 filler, subsequently hindering the inwards diffusion of oxygen into composite; however, further inwards diffusion of oxygen through coating microcracks into the regions close to the coating in which there exists no SiB4 filler still resulting in the oxidation of carbon phase. 4. Conclusions SiB4 particles can be infiltrated into the open inter-bundle pores of C/SiC composite by slurry infiltration process. As filler in C/SiC composite, SiB4 can hinder the inwards diffusion of oxygen, and the oxidation of carbon fibres took place mainly in the region close to the SiC coating. As a result, the flexural strengths of the modified composites did not decrease obviously after oxidation at the temperatures of 500–900 C for 10 h. In order to further improve the oxidation resistance of C/SiC composite, it will be modified by not only self-healing filler, but also self-healing coating. Acknowledgments The authors acknowledge the financial support of Natural Science Foundation of China (Contract No. 90405015), National Young Elitists Foundation (Contract No. 50245208). -8 -6 -4 -2 0 2 400 600 800 1000 Temperature (ºC) Weight chang (%) C/SiC-SiB4 C/SiC 2h 5h 10h 10h Fig. 6. Relationship of weight change and temperature in the 2D C/SiC– SiB4 and 2D C/SiC composites with a two-layer SiC coating after oxidizing for different periods of time. 0 100 200 300 400 500 0 200 400 600 800 1000 Temperature (ºC) Flexural strength (MPa) C/SiC-SiB4 C/SiC Fig. 7. Flexural strength change of the 2D C/SiC–SiB4 and 2D C/SiC composites with a two-layer SiC coating after oxidized at different temperatures for 10 h. 606 C. Tong et al. / Composites Science and Technology 68 (2008) 602–607
C. Tong et aL. Composites Science and Technology 68(2008)602-607 References 9 Naslain R, Guette A, Rebillat F, Pailler R, Langlais F, Bourrat X Boron-bearing species in ceramic matrix com [I Besmann TM, Sheldon BW, Lowden RA. Vapor phase fabricatio erospace applications. J Solid State Chem 2004: 177(2): 449-56 and properties of continuous filament ceramic composites. Science [o] Goujard S, Vandenbulcke L, Tawil H Oxidation behat 1991;253(6):11049 [2] Lamouroux F, Bourrat X, Nasalain R. Structure/oxidation behavi chemical-vapour-deposition polylayers coatings. J Mater Sci relationship in the carbonaceous of 2D-C/PyC/SiC composites. [11] Cheng LF. Xu YD, Zhang LT, Gao R. Effect of glass sealing on the 1994:29(23):6212-20 3]Xu YD, Zhang LT, Cheng LF, Yan DT. Microstructure and oxidation behavior of three dimensional C/SiC composites in air. mechanical properties of three-dimensional carbon/ silicon carbide arbon2001:39(8):1127-33 composites fabricated by chemical vapor infiltration. Carbon [12] Wu SJ, Cheng LF, Zhang LT, Xu YD, Zhang Q. Comparison of 1998:36(7-8):1051-6. xidation behaviors of 3D C/PyC/SiC and SiC/PyC/SiC composites [4 Cheng LF, Xu YD, Zhang LT, Yin Xw. Oxidation behavior from in an OrAr atmosphere. Mater Sci Eng B 2006: 130(1-3): 215-9 oom temperature to 1500C of 3D-C/SiC composites with different [13] Viricelle JP, Goursa P, Bahloul-Hourlie D Oxidation behaviour of a coatings. J Am Ceram Soc 2002: 85(4): 989-91 multi-layer ceramic-matrix composite(SiC)/C/(SiBC)m Compos Sci 5]Laumouroux F, Bertrand S, Pailler R, Naslain R, Cataldi M Technol2001:61:607-14 [14]Quemard L, Rebillat F, Guette A, Tawil H, Louchet-Pouillerie C. tes. Compos Sci Technol 1999; 59(7): 1073-85 f-healing mechanisms of a Sic fiber reinforced [6Xu YD, Cheng LF, Zhang LT, Ying HF, Zhou wC. Oxidation eramic matrix composite in high pressure steam environments. J Eur behavior and mechanical properties of C/SiC composites with Si- Ceram soc2007;27(4):2085-94 MoSiz oxidation protection coating. J Mater Sci 1999 34(24): [5]Matsushita J1, Yutaka S. Oxidation resistance of silicon tetra boride powder. J Ceram Soc Jpn 1997: 105(10): 922-4 [7] Cheng LF, Xu YD, Zhang LT, Yin XW. Effect of carbon interlayer [6] Golowko EL, Makarenko GN, VoitovichRF, Fedorus VBOxidation C/SiC composites with a coating from room f boron silicide and materials based on it. Powder metall Met Ceram temperature to 1500C. Mater Sci Eng A 2001: 300(1-2): 219-25. 1993:32(11-12):917-20. [8]Naslain R. Design, preparation and properties of non-oxide CMCs [17] Lamouroux F, Camus G, Thebault J. Kinetics and mechanisms of for application in engines and nuclear reactors: an overview. Compos xidation of 2D woven C/SiC composites: I Experimental approach. Sci Technol2004:642):155-70. J Am Ceram Soc 1994: 77(8): 2049-57
References [1] Besmann TM, Sheldon BW, Lowden RA. Vapor phase fabrication and properties of continuous filament ceramic composites. Science 1991;253(6):1104–9. [2] Lamouroux F, Bourrat X, Nasalain R. Structure/oxidation behavior relationship in the carbonaceous of 2D-C/PyC/SiC composites. Carbon 1993;31(8):1273–88. [3] Xu YD, Zhang LT, Cheng LF, Yan DT. Microstructure and mechanical properties of three-dimensional carbon/silicon carbide composites fabricated by chemical vapor infiltration. Carbon 1998;36(7–8):1051–6. [4] Cheng LF, Xu YD, Zhang LT, Yin XW. Oxidation behavior from room temperature to 1500 C of 3D-C/SiC composites with different coatings. J Am Ceram Soc 2002;85(4):989–91. [5] Laumouroux F, Bertrand S, Pailler R, Naslain R, Cataldi M. Oxidation-resistant carbon-fiber-reinforced ceramic–matrix composites. Compos Sci Technol 1999;59(7):1073–85. [6] Xu YD, Cheng LF, Zhang LT, Ying HF, Zhou WC. Oxidation behavior and mechanical properties of C/SiC composites with Si– MoSi2 oxidation protection coating. J Mater Sci 1999;34(24): 6009–14. [7] Cheng LF, Xu YD, Zhang LT, Yin XW. Effect of carbon interlayer on oxidation behavior of C/SiC composites with a coating from room temperature to 1500 C. Mater Sci Eng A 2001;300(1–2):219–25. [8] Naslain R. Design, preparation and properties of non-oxide CMCs for application in engines and nuclear reactors: an overview. Compos Sci Technol 2004;64(2):155–70. [9] Naslain R, Guette A, Rebillat F, Pailler R, Langlais F, Bourrat X. Boron-bearing species in ceramic matrix composites for long-term aerospace applications. J Solid State Chem 2004;177(2):449–56. [10] Goujard S, Vandenbulcke L, Tawil H. Oxidation behaviour of twoand three-dimensional C/SiC thermostructural materials protected by chemical-vapour-deposition polylayers coatings. J Mater Sci 1994;29(23):6212–20. [11] Cheng LF, Xu YD, Zhang LT, Gao R. Effect of glass sealing on the oxidation behavior of three dimensional C/SiC composites in air. Carbon 2001;39(8):1127–33. [12] Wu SJ, Cheng LF, Zhang LT, Xu YD, Zhang Q. Comparison of oxidation behaviors of 3D C/PyC/SiC and SiC/PyC/SiC composites in an O2–Ar atmosphere. Mater Sci Eng B 2006;130(1–3):215–9. [13] Viricelle JP, Goursa P, Bahloul-Hourlie D. Oxidation behaviour of a multi-layer ceramic–matrix composite (SiC)f/C/(SiBC)m. Compos Sci Technol 2001;61:607–14. [14] Quemard L, Rebillat F, Guette A, Tawil H, Louchet-Pouillerie C. Self-healing mechanisms of a SiC fiber reinforced multi-layered ceramic matrix composite in high pressure steam environments. J Eur Ceram Soc 2007;27(4):2085–94. [15] Matsushita JI, Yutaka S. Oxidation resistance of silicon tetra boride powder. J Ceram Soc Jpn 1997;105(10):922–4. [16] Golovko E´I, Makarenko GN, Voitovich RF, Fedorus VB. Oxidation of boron silicide and materials based on it. Powder Metall Met Ceram 1993;32(11–12):917–20. [17] Lamouroux F, Camus G, Thebault J. Kinetics and mechanisms of oxidation of 2D woven C/SiC composites: I, Experimental approach. J Am Ceram Soc 1994;77(8):2049–57. C. Tong et al. / Composites Science and Technology 68 (2008) 602–607 607