MATERIAL B ELSEVIE Materials Science and Engineering B 130(2006)215-219 elsevier. com/locate/mseb Comparison of oxidation behaviors of 3D C/Py c/Sic and sic/Pyc/sic composites in an O2-Ar atmosphere Shoujun Wu", Laifei Cheng, Litong Zhang, Yongdong Xu, Qing Zhang National Key Laboratory of Thermostricture Composite Materials, Northwestern Polytechnical University. Xi'an Shaanxi 710072, People's Republic of China Received 11 December 2005; received in revised form 15 March 2006; accepted 16 March 2006 Abstract Oxidation behaviors of three-dimensional C/PyC/SiC and SiC/PyC/Sic prepared by CvI processing were investigated in an O2-Ar atmosphere at 600C, 900C and 1200C, respectively, by using thermogravimetric analysis. After machining, both composites should be protected by CVD SiC coating, which was demonstrated effectively in improving the oxidation resistance of both composites. The oxidation behavior o SiC/PyC/SiC was different from that of C/Pyc/SiC. The oxidation kinetics of C/PyC/Sic was controlled by the rate of the reaction between carbon and oxygen at 600C and by the oxygen diffusion through the coating microcracks at 900C. The oxidation kinetics of SiC/PyC/Sic at both 600C and 900C were assumed to be controlled by the oxygen diffusion through channels of coating and matrix defects and looped pipelines instead of PyC interphase. At 1200C, the oxidation was controlled by oxygen diffusion through the SiO2 scale, which took place mainly on the surfaces of 2006 Elsevier B. v. All rights Keywords: C/PyC/SiC composite: SiC/PyC/SiC composite; Oxidation; TGA; SiC coating 1. ntroduction als surfaces. Up to now, the research on the com the oxidation kinetics of SiC/PyC/SiC and C/Py C/SiC is till cn As promising high-temperature structural materials, sili- limited n carbide fiber-reinforced silicon carbide c Sites with In the present paper, the oxidation behaviors of machined pyrolytic carbon interphase(SiC/PyC/SiC) and carbon fiber- 3D SiC/PyC/SiC and 3D C/PyC/SiC without coating and with einforced silicon carbide composites with pyrolytic carbon a CVD SiC coating were investigated, respectively, at 600C, interphase(C/PyC/SiC) exhibit excellent mechanical proper- 900C and 1200C in an O2-Ar environment, by analyzing the ties and thermal shock resistances [1-3. Oxidation behaviors weight and microstructural variations of C/Py C/SiC have been researched [4-8], showing that its oxi- dation kinetics can be divided into three domains(4, 6]: at low 2. Experimental procedure temperatures(T1100 C), microcrack is seac phases; at to prepare fiber preforms with a volume fraction of SiC fiber in and the oxidation is controlled by oxygen diffusion through step three dimensional(4-step 3D) braiding method in Nanjing Sio scale and the oxidation mainly took place on materi- Institute of Glass Fiber, P.R. China. In order to fabricate both composites, low pressure chemical vapor infiltration (LPCVD) process was employed to deposit pyrolytic carbon(PyC) Corresponding author. Tel: +86 29 8848 6068 828: fax: +86 298849 4 phase and the silicon carbide matrix. PyC was deposited on the E-imailaddress:shoujun_wu(@163.com(SWu) fiber by decompositions of C3H6 at 870C for I h at reduced 0921-5107/S-see front matter doi: 10. 1016/j.mse
Materials Science and Engineering B 130 (2006) 215–219 Comparison of oxidation behaviors of 3D C/PyC/SiC and SiC/PyC/SiC composites in an O2-Ar atmosphere Shoujun Wu ∗, Laifei Cheng, Litong Zhang, Yongdong Xu, Qing Zhang National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi’an Shaanxi 710072, People’s Republic of China Received 11 December 2005; received in revised form 15 March 2006; accepted 16 March 2006 Abstract Oxidation behaviors of three-dimensional woven C/PyC/SiC and SiC/PyC/SiC prepared by CVI processing were investigated in an O2-Ar atmosphere at 600 ◦C, 900 ◦C and 1200 ◦C, respectively, by using thermogravimetric analysis. After machining, both composites should be protected by CVD SiC coating, which was demonstrated effectively in improving the oxidation resistance of both composites. The oxidation behavior of SiC/PyC/SiC was different from that of C/PyC/SiC. The oxidation kinetics of C/PyC/SiC was controlled by the rate of the reaction between carbon and oxygen at 600 ◦C and by the oxygen diffusion through the coating microcracks at 900 ◦C. The oxidation kinetics of SiC/PyC/SiC at both 600 ◦C and 900 ◦C were assumed to be controlled by the oxygen diffusion through channels of coating and matrix defects and looped pipelines instead of PyC interphase. At 1200 ◦C, the oxidation was controlled by oxygen diffusion through the SiO2 scale, which took place mainly on the surfaces of both composites. © 2006 Elsevier B.V. All rights reserved. Keywords: C/PyC/SiC composite; SiC/PyC/SiC composite; Oxidation; TGA; SiC coating 1. Introduction As promising high-temperature structural materials, silicon carbide fiber-reinforced silicon carbide composites with pyrolytic carbon interphase (SiC/PyC/SiC) and carbon fiberreinforced silicon carbide composites with pyrolytic carbon interphase (C/PyC/SiC) exhibit excellent mechanical properties and thermal shock resistances [1–3]. Oxidation behaviors of C/PyC/SiC have been researched [4–8], showing that its oxidation kinetics can be divided into three domains [4,6]: at low temperatures (T 1100 ◦C), microcrack is sealed by silica and the oxidation is controlled by oxygen diffusion through SiO2 scale and the oxidation mainly took place on materi- ∗ Corresponding author. Tel.: +86 29 8848 6068 828; fax: +86 29 8849 4620. E-mail address: shoujun wu@163.com (S. Wu). als surfaces. Up to now, the research on the comparison of the oxidation kinetics of SiC/PyC/SiC and C/PyC/SiC is till limited. In the present paper, the oxidation behaviors of machined 3D SiC/PyC/SiC and 3D C/PyC/SiC without coating and with a CVD SiC coating were investigated, respectively, at 600 ◦C, 900 ◦C and 1200 ◦C in an O2-Ar environment, by analyzing the weight and microstructural variations. 2. Experimental procedure 2.1. Sample preparation Hi-NicalonTM silicon carbide fiber from Japan Nippon Carbon and T-300 carbon fiber from Japan Toray were employed to prepare fiber preforms with a volume fraction of SiC fiber in the range of 40–45% and a braiding angle of 20◦ using fourstep three dimensional (4-step 3D) braiding method in Nanjing Institute of Glass Fiber, P.R. China. In order to fabricate both composites, low pressure chemical vapor infiltration (LPCVI) process was employed to deposit pyrolytic carbon (PyC) interphase and the silicon carbide matrix. PyC was deposited on the fiber by decompositions of C3H6 at 870 ◦C for 1 h at reduced 0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.03.012
216 S Wu et al. / Materials Science and Engineering B 130(2006)215-219 品a 0100200300400500600700800900 0100200300400500600700800900 Fig. 1. TGA curves of the composites without coating after oxidation for 15h: (a)3D C/PyC/SiC composite without coating: ( b)3D SiC/PyC/SiC without coatin pressure of 5 kPa in a CVI reactor, arriving to a thickness 3. Results and discussion of 0.2 um. Sic matrix was prepared at 1100C for 120 h at reduced pressure of 5 kPa by using methyltrichlorosilane(MTS Fig. 1 shows the TGA results of the uncoated 3D C/PyC/SiC CH3SiCl3 )with a molar ratio of 10 between H2 and MTS, which and SiC/Pyc/SiC after oxidation for 900 min. The uncoated was carried by bubbling hydrogen in gas phase and argon as C/Py C/Sic showed a remarkable weight loss in concave man- the dilute gas to slow down the chemical reaction rate during ner at 900C and 1200C, but a nearly linear curve at 600oC deposition. The as-received composites were machined and pol- The uncoated SiC/PyC/SiC showed a rapid weight loss in a lin- ished into 2.5 mm x 40mm x 30 mm samples. The as-received ear manner in the beginning at 600C, then the rate of weight yC/SiC has a density of 2.7 g/cmand a porosity of 7.3% loss decreased. The TG curve of uncoated SiC/Py C/SiC at and C/PyC/SiC has a density of 2. 1 g/emand a porosity of 900C was similar to the one of uncoated C/PyC/SiC except 5.0%0, respectively, measured by Archimedes method. CVd that the platform occurred much later. At 1200C, uncoated SiC coating was deposited on the machined samples to seal SiC/PyC/SiC firstly showed a rapid weight loss, then kept a con- the open ends of the fibers. Two layers of CVD SiC coating stant weight loss in a short duration, and finally gained weight were deposited on the SiC/Pyc/Sic and three layers of CVd in a parabolic manner. Sic coated on C/PyC/SiC. The process parameters of CVD Sic Fig. 2a shows the TGA results of the coated C/PyC/Sic coating were the same as the ones of Sic matrix except for the and SiC/PyC/Sic after oxidation for 900 min. Weight losses deposition duration of 20h, corresponding to one layer of coat- of the coated composites were much smaller than those of the uncoated ones, which indicated that CVD SiC coating was effec- tive in improving the oxidation resistances of both composites 2. 2. Oxidation tests The coated C/Pyc/Sic composite showed a nearly linear TG curve at 600oC. It should be noted that the weight loss of Oxidations of the composites with and without SiC coating the coated C/Pyc/SiC at 600C was much smaller than that were conducted in a mixture gas of Ar and O2 with a vol- at 900C and 1200C, respectively, within the initial duration ume ratio of 78/22 for 900 min in a thermogravimetric analyzer of 650 min SiC/Pyc/Sic composite lost weight at 600C and (TGA)with a sensitivity of +.000l mg(METTLER TOLEDO 900C, respectively, while gained weight at 1200C. Weight STAR) loss of C/PyCSiC was much larger than that of SiC/PyC/SiC After oxidation at 1200C for 900 min, weight gain of the 2.3. Sample characterization uncoated SiC/Py C/SiC was much larger than that of the coated one. Known from Fig. 1(b), SiC/PyC/Sic gained weight in a The microstructures of the samples before and after oxidation near-parabolic manner at 1200C, while lost weight at 600C were examined by using a scanning electron microscope(SEM, and 900C. S4700) The CTE of the composites and CVD Sic ceramic were All the above results indicated that oxidation behavior of 3D tested in argon by using DIL 402C, NETZSCH company, Selb, C/PyC/SiC was different from that of SiC/PyC/Sic due to the different composition and microstructure. Oxidation of carbon -0.05 3D SiC/S 0015 DC/iC 0 0100200300400500600700800 (a)TGA curves of coated 3D SiC/PyC/SiC and C/PyC/SiC after oxidation for 15 h; (b) the magnified view of the TGA curves of SiC/PyC/SiC in the initial
216 S. Wu et al. / Materials Science and Engineering B 130 (2006) 215–219 Fig. 1. TGA curves of the composites without coating after oxidation for 15 h: (a) 3D C/PyC/SiC composite without coating; (b) 3D SiC/PyC/SiC without coating. pressure of 5 kPa in a CVI reactor, arriving to a thickness of 0.2m. SiC matrix was prepared at 1100 ◦C for 120 h at reduced pressure of 5 kPa by using methyltrichlorosilane (MTS, CH3SiCl3) with a molar ratio of 10 between H2 and MTS, which was carried by bubbling hydrogen in gas phase and argon as the dilute gas to slow down the chemical reaction rate during deposition. The as-received composites were machined and polished into 2.5 mm × 4.0 mm × 30 mm samples. The as-received SiC/PyC/SiC has a density of 2.7 g/cm3 and a porosity of 7.3%, and C/PyC/SiC has a density of 2.1 g/cm3 and a porosity of 15.0%, respectively, measured by Archimedes method. CVD SiC coating was deposited on the machined samples to seal the open ends of the fibers. Two layers of CVD SiC coating were deposited on the SiC/PyC/SiC and three layers of CVD SiC coated on C/PyC/SiC. The process parameters of CVD SiC coating were the same as the ones of SiC matrix except for the deposition duration of 20 h, corresponding to one layer of coating. 2.2. Oxidation tests Oxidations of the composites with and without SiC coating were conducted in a mixture gas of Ar and O2 with a volume ratio of 78/22 for 900 min in a thermogravimetric analyzer (TGA) with a sensitivity of ±0.0001 mg (METTLER TOLEDO STARe). 2.3. Sample characterization The microstructures of the samples before and after oxidation were examined by using a scanning electron microscope (SEM, S4700). The CTE of the composites and CVD SiC ceramic were tested in argon by using DIL 402C, NETZSCH company, Selb, Company. 3. Results and discussion Fig. 1 shows the TGA results of the uncoated 3D C/PyC/SiC and SiC/PyC/SiC after oxidation for 900 min. The uncoated C/PyC/SiC showed a remarkable weight loss in concave manner at 900 ◦C and 1200 ◦C, but a nearly linear curve at 600 ◦C. The uncoated SiC/PyC/SiC showed a rapid weight loss in a linear manner in the beginning at 600 ◦C, then the rate of weight loss decreased. The TG curve of uncoated SiC/PyC/SiC at 900 ◦C was similar to the one of uncoated C/PyC/SiC except that the platform occurred much later. At 1200 ◦C, uncoated SiC/PyC/SiC firstly showed a rapid weight loss, then kept a constant weight loss in a short duration, and finally gained weight in a parabolic manner. Fig. 2a shows the TGA results of the coated C/PyC/SiC and SiC/PyC/SiC after oxidation for 900 min. Weight losses of the coated composites were much smaller than those of the uncoated ones, which indicated that CVD SiC coating was effective in improving the oxidation resistances of both composites. The coated C/PyC/SiC composite showed a nearly linear TG curve at 600 ◦C. It should be noted that the weight loss of the coated C/PyC/SiC at 600 ◦C was much smaller than that at 900 ◦C and 1200 ◦C, respectively, within the initial duration of 650 min. SiC/PyC/SiC composite lost weight at 600 ◦C and 900 ◦C, respectively, while gained weight at 1200 ◦C. Weight loss of C/PyC/SiC was much larger than that of SiC/PyC/SiC. After oxidation at 1200 ◦C for 900 min, weight gain of the uncoated SiC/PyC/SiC was much larger than that of the coated one. Known from Fig. 1(b), SiC/PyC/SiC gained weight in a near-parabolic manner at 1200 ◦C, while lost weight at 600 ◦C and 900 ◦C. All the above results indicated that oxidation behavior of 3D C/PyC/SiC was different from that of SiC/PyC/SiC due to the different composition and microstructure. Oxidation of carbon Fig. 2. (a) TGA curves of coated 3D SiC/PyC/SiC and C/PyC/SiC after oxidation for 15 h; (b) the magnified view of the TGA curves of SiC/PyC/SiC in the initial 400 min
S. Wu et al. Materials Science and Engineering B 130(2006)215-219 Matrix micro cracks Fig 3. Morphologies (a)3D C/PyC/SiC; (b)3D SiC/PyC/Sie 3D SiC/SiC atrix micro crack CVD SiC 3D C/SiC 0060080010001200 Fig. 5. CTE of C/PyC/SiC, SiC/PyC/SiC, and CVD SiC as a function of tem- Fig 4. A fine matrix microcrack in SiC/PyC/SiC. Thus, no tensile stress was formed in the matrix during cooling process and there were few thermal cracks forming in the fiber and Pyc interphase in C/PyC/SiC and PyC interphase matrix, as shown in Fig 3(b). Only very fine microcracks could in SiC/Py C/SiC led to the weight loss, while oxidation of be observed in the matrix of SiC/Py C/SiC(Fig 4). SiC matrix, fiber and coating resulted in weight gain. Weight Fig. 5 shows CTE of 3D C/PyC/SiC, SiC/PyC/SiC and CVD change of both composites during oxidation was directly SiC as a function of temperature raging from 25C to 1250C related to the defects existing in the coating and the matrix. CTE of SiC/PyC/SiC was nearly the same as that of CVD SiC, The defects are mainly the microcracks and micropores in the especially at high temperature. CTE of C/Pyc/Sic was much ating and the matrix resulted from the mismatch of thermal smaller than that of CVD Sic over the full temperature range expansion coefficient (CTE) between the composite and the Because of the larger CTE mismatch between the Sic coating coating, and between the fiber and the matrix, respectively. In and the composite, microcracks were produced in the coating of C/PyC/SiC, the T300 carbon fiber was an anisotropic material, C/PyC/SiC (Fig. 6(a). On the contrary, no microcracks could characterized by two CTEs: CTE of-0 1 to-1.1 x 10-boC- observed in the coating of SiC/PyC/SiC, as shown in Fig. 6(b) in the longitudinal direction and a CTE of 7.0 x 10-0o0 Some pores existing between the fiber bundles were observed in the radial direction [9, 10). The CtE of SiC matrix was which were not fully sealed by the two layers of coating, as about 4.6x 10-6oC-I[11]. Hence, the matrix encountered shown in Fig. 7. Below the coating preparation temperature, tensile stress during cooling process and matrix microcracks these defects will act as channels for oxygen diffusion inward, formed perpendicular to the direction of fiber axis, as shown resulting in oxidation of carbon phase. The fraction of carbon in Fig. 3(a). In SiC/PyC/SiC composite, CTE of Hi-Nicalon phase in the coated C/PyC/SiC was much higher than that in the SiC fiber was in the range of 3. 1 to 3. 5 x 10-6oC-I[2, 12]. coated SiC/PyC/Sic in the present study. Therefore, the coated Microcrack Fig. 6. Surface morphologies of CVD SiC coated on:(a)3D C/PyC/SiC;(b)3D SiC/PyC/SiC
S. Wu et al. / Materials Science and Engineering B 130 (2006) 215–219 217 Fig. 3. Morphologies of SiC matrix in: (a) 3D C/PyC/SiC; (b) 3D SiC/PyC/SiC. Fig. 4. A fine matrix microcrack in SiC/PyC/SiC. fiber and PyC interphase in C/PyC/SiC and PyC interphase in SiC/PyC/SiC led to the weight loss, while oxidation of SiC matrix, fiber and coating resulted in weight gain. Weight change of both composites during oxidation was directly related to the defects existing in the coating and the matrix. The defects are mainly the microcracks and micropores in the coating and the matrix resulted from the mismatch of thermal expansion coefficient (CTE) between the composite and the coating, and between the fiber and the matrix, respectively. In C/PyC/SiC, the T300 carbon fiber was an anisotropic material, characterized by two CTEs: CTE of −0.1 to −1.1 × 10−6 ◦C−1 in the longitudinal direction and a CTE of 7.0 × 10−6 ◦C−1 in the radial direction [9,10]. The CTE of SiC matrix was about 4.6 × 10−6 ◦C−1 [11]. Hence, the matrix encountered tensile stress during cooling process and matrix microcracks formed perpendicular to the direction of fiber axis, as shown in Fig. 3(a). In SiC/PyC/SiC composite, CTE of Hi-Nicalon SiC fiber was in the range of 3.1 to 3.5 × 10−6 ◦C−1 [2,12]. Fig. 5. CTE of C/PyC/SiC, SiC/PyC/SiC, and CVD SiC as a function of temperature. Thus, no tensile stress was formed in the matrix during cooling process and there were few thermal cracks forming in the matrix, as shown in Fig. 3(b). Only very fine microcracks could be observed in the matrix of SiC/PyC/SiC (Fig. 4). Fig. 5 shows CTE of 3D C/PyC/SiC, SiC/PyC/SiC and CVD SiC as a function of temperature raging from 25 ◦C to 1250 ◦C. CTE of SiC/PyC/SiC was nearly the same as that of CVD SiC, especially at high temperature. CTE of C/PyC/SiC was much smaller than that of CVD SiC over the full temperature range. Because of the larger CTE mismatch between the SiC coating and the composite, microcracks were produced in the coating of C/PyC/SiC (Fig. 6(a)). On the contrary, no microcracks could be observed in the coating of SiC/PyC/SiC, as shown in Fig. 6(b). Some pores existing between the fiber bundles were observed, which were not fully sealed by the two layers of coating, as shown in Fig. 7. Below the coating preparation temperature, these defects will act as channels for oxygen diffusion inward, resulting in oxidation of carbon phase. The fraction of carbon phase in the coated C/PyC/SiC was much higher than that in the coated SiC/PyC/SiC in the present study. Therefore, the coated Fig. 6. Surface morphologies of CVD SiC coated on: (a) 3D C/PyC/SiC; (b) 3D SiC/PyC/SiC
S Wu et al. / Materials Science and Engineering B 130(2006)215-219 Interphase layer Fiber Fig. 7. Unsealed pore in CVD SiC coating. Fig 9. Typical microstructure of the SiC/PyC/SiC composite 3D C/PyC/SiC had a much higher weight loss than that of the weight loss of the composite showed a slight decrease with the coated 3D SiC/PyC/SiC. increase of time and this tendency became more obvious at high At 600 C, the microcracks in coated C/PyC/SiC was so wide temperature that oxygen diffused inwards rapidly, leading to the oxidation Typical microstructure of 3D SiC/PyC/SiC was showed in of carbon phases. The oxidation kinetics was controlled by the Fig. 9. It can be seen that between the fiber and the matrix a rate of the reaction between carbon and oxygen, resulting in layered PyC interphase with a thickness of 200 nm could be a uniform degradation of carbon reinforcement, as shown in observed and gaps were scarcely observed at the interfaces both Fig. 8(a). The microcracks was still wide at 900C, and the oxi- between the fiber and the interphase and between the matrix dation was controlled by gaseous diffusion through the coating and the interphase. Consequently, as the oxidation proceeded, microcracks, leading to a nonuniform consumption of carbon the PyC was consumed and looped pipelines would be formed phases, as shown in Fig. &(b). Though the microcracks became along the fibers which resulted in the oxidation progressing narrower as temperature increased, the reaction rate between along these looped pipelines. Because the dimension of these oxygen and carbon phases increased following the Arrhenius looped pipelines in the radial direction of fiber was too small for law. Thus, the weight loss of the coated C/Pyc/SiC at 600C oxygen molecule to diffuse through freely, the oxidation was was much smaller than that at 900C and 1200C, respectively, assumed to be controlled by oxygen diffusion. The TG curve within the initial 650 min and weight loss decreased as tem- of coated 3D SiC/PyC/SiC at 600C showed a concave curve perature increased after oxidation for 900 min. For uncoated manner similar to the one of coated 3D C/Py C/SiC at 900oC C/PyC/SiC, the carbon phases (i.e. carbon fiber and PyC)were The corresponding microstructure resulted from the gaseou fully exposed to oxygen, thus the composite showed a rapid diffusion-control mechanism was referred to the nonuniform weight loss. As depicted in the experimental, volume fraction of oxidation of PyC interphase, as shown in Fig. 10. Though the fiber was about 40-45%, thus the maximum weight loss corre- width of oxygen diffusion channels formed by the defects sponding to the fiber burn-out was about 46% for C/PyC/Sic SiC coating and matrix decreased with the increase of tempera and 1.8% for SiC/PyC/SiC. When the fiber and the matrix ture, the oxidation reaction rate obeyed the Arrhenius equation expanded to their original lengths at the oxidation tempera- Moreover, at 900C, the oxidation rate of Sic in air was ver ture as high as the material processing temperature, residual slow [13]. Hence, the coated 3D SiC/PyC/SiC showed a higher thermal stresses were relieved and the pre-existing microcracks weight loss than the one at 600C ecame narrower. Moreover, formation of silica scale due to At 1200C, the width of matrix microcracks became very oxidation of the silicon carbide can fill the microcracks, act- narrow due to the thermal expansion, and the oxidation of Sic ing as a sealant at high temperatures. The higher the tempera- became significant, controlled by oxygen diffusion through Sio ture was, the higher the growth rate of oxide scale was. Thus, scale. Furthermore, the volume expansion of Sio2 may seal the Fig 8. Cross-section morphologies of the coated 3D C/Pyc/SiC composite after oxidation for 900 min at:(a)600 C:(b)900oC
218 S. Wu et al. / Materials Science and Engineering B 130 (2006) 215–219 Fig. 7. Unsealed pore in CVD SiC coating. 3D C/PyC/SiC had a much higher weight loss than that of the coated 3D SiC/PyC/SiC. At 600 ◦C, the microcracks in coated C/PyC/SiC was so wide that oxygen diffused inwards rapidly, leading to the oxidation of carbon phases. The oxidation kinetics was controlled by the rate of the reaction between carbon and oxygen, resulting in a uniform degradation of carbon reinforcement, as shown in Fig. 8(a). The microcracks was still wide at 900 ◦C, and the oxidation was controlled by gaseous diffusion through the coating microcracks, leading to a nonuniform consumption of carbon phases, as shown in Fig. 8(b). Though the microcracks became narrower as temperature increased, the reaction rate between oxygen and carbon phases increased following the Arrhenius law. Thus, the weight loss of the coated C/PyC/SiC at 600 ◦C was much smaller than that at 900 ◦C and 1200 ◦C, respectively, within the initial 650 min and weight loss decreased as temperature increased after oxidation for 900 min. For uncoated C/PyC/SiC, the carbon phases (i.e. carbon fiber and PyC) were fully exposed to oxygen, thus the composite showed a rapid weight loss. As depicted in the experimental, volume fraction of fiber was about 40–45%, thus the maximum weight loss corresponding to the fiber burn-out was about 46% for C/PyC/SiC and 1.8% for SiC/PyC/SiC. When the fiber and the matrix expanded to their original lengths at the oxidation temperature as high as the material processing temperature, residual thermal stresses were relieved and the pre-existing microcracks became narrower. Moreover, formation of silica scale due to oxidation of the silicon carbide can fill the microcracks, acting as a sealant at high temperatures. The higher the temperature was, the higher the growth rate of oxide scale was. Thus, Fig. 9. Typical microstructure of the SiC/PyC/SiC composite. weight loss of the composite showed a slight decrease with the increase of time and this tendency became more obvious at high temperature. Typical microstructure of 3D SiC/PyC/SiC was showed in Fig. 9. It can be seen that between the fiber and the matrix a layered PyC interphase with a thickness of 200 nm could be observed and gaps were scarcely observed at the interfaces both between the fiber and the interphase and between the matrix and the interphase. Consequently, as the oxidation proceeded, the PyC was consumed and looped pipelines would be formed along the fibers which resulted in the oxidation progressing along these looped pipelines. Because the dimension of these looped pipelines in the radial direction of fiber was too small for oxygen molecule to diffuse through freely, the oxidation was assumed to be controlled by oxygen diffusion. The TG curve of coated 3D SiC/PyC/SiC at 600 ◦C showed a concave curve manner similar to the one of coated 3D C/PyC/SiC at 900 ◦C. The corresponding microstructure resulted from the gaseousdiffusion-control mechanism was referred to the nonuniform oxidation of PyC interphase, as shown in Fig. 10. Though the width of oxygen diffusion channels formed by the defects in SiC coating and matrix decreased with the increase of temperature, the oxidation reaction rate obeyed the Arrhenius equation. Moreover, at 900 ◦C, the oxidation rate of SiC in air was very slow [13]. Hence, the coated 3D SiC/PyC/SiC showed a higher weight loss than the one at 600 ◦C. At 1200 ◦C, the width of matrix microcracks became very narrow due to the thermal expansion, and the oxidation of SiC became significant, controlled by oxygen diffusion through SiO2 scale. Furthermore, the volume expansion of SiO2 may seal the Fig. 8. Cross-section morphologies of the coated 3D C/PyC/SiC composite after oxidation for 900 min at: (a)600 ◦C; (b)900 ◦C
S. Wu et al. Materials Science and Engineering B 130(2006)215-219 Fig 10. Cross section morphologies of coated 3D SiC/PyC/SiC composite after oxidation for 900 min at: (a)600 C; ( b)900C. coating defects In the beginning, the oxidation of Sic resulted 3. Compared to C/PyC/SiC composite, the better oxidation in the formation of protective SiO2 scale and weight gain in a resistance of the SiC fiber and the smaller Cte mismatch parabolic manner. The oxidation mainly took place on the coat- between the SiC fiber and the matrix and between the com- ing surface. For uncoated 3D SiC/PyC/SiC, the oxidation took posite and the coating lead to a better oxidation resistance of place along the looped pipelines around the fiber due to the con- SiC/PyC/SiC composite umption of PyC. Sio would be formed not only on the outer surface of the composites but also along the fibers. Under the Acknowledgments cooperation of the weight loss due to PyC consumption and the weight gains due to Sioz formation, a dynamic weight changes The authors acknowledge the support of the Chinese National equilibrium would occur. Then, weight gain due to the Sio2 Foundation for Natural Sciences under Contract No. 90405015 formation would overcome weight loss due to the Pyc con- and the NSFC Distinguished Young Scholar under Contract No sumption, thus the composite firstly showed a rapid weight loss 50425208 200 at 1200C, then the weight loss kept constant, finally, weight loss decreased and became weight gain in a parabolic manner. references Weight gain of the uncoated SiC/PyC/SiC was higher than that of the coated one 凹 R. Naslain, Compos. Sci. Technol. 64(200417663 4. Conclusions [3] P. Fenici, A.J. Frias Rebelo, R.H. Jones, A. Kohyama, L.L. Snead, J Nuc. Mater.258-263(1998)215-225 [4 F. Lamouroux. G. Camus, J. Am. Ceram. Soc. 77(8)(1994)2049- 1. CVD SiC coating can effectively improve the oxidation resis- tance of machined 3D C/PyC/SiC and 3D SiC/PyC/SiC in the [5]S. Goujard, L Vandenbulcke, I Mater. Sci. 29(1994)6212-6220 O2-Ar atmosphere [6 x.w. Yin, L.F. Cheng, L T. Zhang, Y.D. Xu, X.G. luan, Mater. Sci. 2. The oxidation kinetics of coated C/Pyc/Sic was controlled Technol.1702001)727-730. by the rate of the reaction between carbon and oxygen [7x. w. Yin, LE Cheng, LT Zhang, Y.D. Xu, JZ LI, Compos. Sci. at 600C and by oxygen diffusion through the coating Technol.61(7(2001)977-980 [8]RM. Sullivan, Carbon43(2)(2005)275-285 microcracksat900c.Theoxidationkineticsofcoated[9]http://www.torayusa.com/cfa/product.html. iC/PyC/SiC at both 600C and 900C were controlled by [10] Y.D. Xu, L.F. Cheng, L.T. Zhang. H.F. Ying, J. Mater. Sci. 34(1999) oxygen diffusion through the channels composed of defects 60096014 in the coating and matrix and looped pipelines instead of [Il w. Yang, A Kohyama, T Noda,Y.Katoh, THinoki,. Araki,J.Yu. PyC interphase. At 1200C, the oxidation was controlled by J.Nucl. Mater.307-311(2002)1088-1092 [12]H. Ichikawa, Ann. Chim. Sci. Mat. 25(2000)523-528 oxygen diffusion through the Sioz scale, which took place [13]CE Ramberg, G Cruciani, K.E. Spear, R.E. Tressler, C.F. Ramberg Jr. mainly on the sample surface of the both composites J.Am. Ceram.Soc.79(11)(1996)2897-291l
S. Wu et al. / Materials Science and Engineering B 130 (2006) 215–219 219 Fig. 10. Cross section morphologies of coated 3D SiC/PyC/SiC composite after oxidation for 900 min at: (a)600 ◦C; (b)900 ◦C. coating defects. In the beginning, the oxidation of SiC resulted in the formation of protective SiO2 scale and weight gain in a parabolic manner. The oxidation mainly took place on the coating surface. For uncoated 3D SiC/PyC/SiC, the oxidation took place along the looped pipelines around the fiber due to the consumption of PyC. SiO2 would be formed not only on the outer surface of the composites but also along the fibers. Under the cooperation of the weight loss due to PyC consumption and the weight gains due to SiO2 formation, a dynamic weight changes equilibrium would occur. Then, weight gain due to the SiO2 formation would overcome weight loss due to the PyC consumption, thus the composite firstly showed a rapid weight loss at 1200 ◦C, then the weight loss kept constant, finally, weight loss decreased and became weight gain in a parabolic manner. Weight gain of the uncoated SiC/PyC/SiC was higher than that of the coated one. 4. Conclusions 1. CVD SiC coating can effectively improve the oxidation resistance of machined 3D C/PyC/SiC and 3D SiC/PyC/SiC in the O2-Ar atmosphere. 2. The oxidation kinetics of coated C/PyC/SiC was controlled by the rate of the reaction between carbon and oxygen at 600 ◦C and by oxygen diffusion through the coating microcracks at 900 ◦C. The oxidation kinetics of coated SiC/PyC/SiC at both 600 ◦C and 900 ◦C were controlled by oxygen diffusion through the channels composed of defects in the coating and matrix and looped pipelines instead of PyC interphase. At 1200 ◦C, the oxidation was controlled by oxygen diffusion through the SiO2 scale, which took place mainly on the sample surface of the both composites. 3. Compared to C/PyC/SiC composite, the better oxidation resistance of the SiC fiber and the smaller CTE mismatch between the SiC fiber and the matrix, and between the composite and the coating lead to a better oxidation resistance of SiC/PyC/SiC composite. Acknowledgments The authors acknowledge the support of the Chinese National Foundation for Natural Sciences under Contract No. 90405015 and the NSFC Distinguished Young Scholar under Contract No. 50425208, 2004. References [1] B.N. Cox, F.W. Zok, Curr. Opin. Solid St. M. 1 (1996) 666–673. [2] R. Naslain, Compos. Sci. Technol. 64 (2004) 155–170. [3] P. Fenici, A.J. Frias Rebelo, R.H. Jones, A. Kohyama, L.L. Snead, J. Nucl. Mater. 258–263 (1998) 215–225. [4] F. Lamouroux, G. Camus, J. Am. Ceram. Soc. 77 (8) (1994) 2049– 2057. [5] S. Goujard, L. Vandenbulcke, J. Mater. Sci. 29 (1994) 6212–6220. [6] X.W. Yin, L.F. Cheng, L.T. Zhang, Y.D. Xu, X.G. luan, Mater. Sci. Technol. 17 (2001) 727–730. [7] X.W. Yin, L.F. Cheng, L.T. Zhang, Y.D. Xu, J.Z. Li, Compos. Sci. Technol. 61 (7) (2001) 977–980. [8] R.M. Sullivan, Carbon 43 (2) (2005) 275–285. [9] http://www.torayusa.com/cfa/product.html. [10] Y.D. Xu, L.F. Cheng, L.T. Zhang, H.F. Ying, J. Mater. Sci. 34 (1999) 6009–6014. [11] W. Yang, A. Kohyama, T. Noda, Y. Katoh, T. Hinoki, H. Araki, J. Yu, J. Nucl. Mater. 307–311 (2002) 1088–1092. [12] H. Ichikawa, Ann. Chim. Sci. Mat. 25 (2000) 523–528. [13] C.E. Ramberg, G. Cruciani, K.E. Spear, R.E. Tressler, C.F. Ramberg Jr., J. Am. Ceram. Soc. 79 (11) (1996) 2897–2911