Part A: applied scienc and manufacturing ELSEVIER mposites: Part A 31(2000)1015-1020 Short communication Oxidation behavior of three-dimensional Sic/Sic composites in air and combustion environment Laifei Cheng, Yongdong Xu, Litong Zhang, Xiaowei Yin State Key Laboratory of solidification Processing. Northwestern Polytechnical University, Xi'an Shaanxi 710072, People's Republic of China Received 28 May 1998; received in revised form 3 September 1999, accepted I March 2000 Abstract A three-dimensional Hi-Nicalon/SiC composite was prepared, the mechanical properties and weight changes before and after oxidation in air and combustion environment were investigated and compared in the present paper. The combustion gas with a high flow rate and a larger In c than in air, and the pyc interlayer was oxidized less. Weight loss of the composite in air was larger than that in combustion. The flexural strength did not decrease remarkably, in both air and combustion Oxidation took place along and weakened the two interfaces of the interlayer with the fibers and matrix besides removal of the interlayer, the fracture displacement was greatly increased after oxidation in combustion compared with those before and after oxidation in air. It was shown that the composite had an excellent oxidation resistance in a short period of exposure. This result was of very important significance in the development of a self-adaptable interlayer. C 2000 Elsevier Science ltd. All rights reserved Keyword: SiC/SIC composites 1. Introduction in various environments including air and combustion The oxidation behavior of Sic based fibers, including Nicalon Silicon carbide fiber reinforced silicon carbide compo- and Hi-Nicalon, has been investigated in air. A few of sites (SiC/SiC) are one of the best promising structural studies have been carried out on the oxidation of Sic/sic materials for high temperature applications [1-3 composites [5, 6], but the oxidation behavior Hi-Nicalon/ based fibers, such as Nicalon, have a better resistance to SiC composites, especially three-dimensional and in oxidation. However, the tensile strength of the fibers combustion has not been reported up to now. Because the decreases dramatically with a significant weight loss as fibers and matrix have a better resistance to oxidation, the the temperature is raised beyond 1200C. The large amount oxidation behavior of the Hi-Nicalon/SiC composite, espe- of oxygen(10-14%)is responsible for the degradation of cially the mechanical behavior, is very sensitive to the the fibers at high temperature. It is necessary to reduce the oxidation of the interphase. In the present paper, a three- oxygen content of the fibers to improve the thermal stability dimensional Hi-Nicalon/SiC composite was prepared, the of SiC/SiC. Recently, Hi-Nicalon with a low oxygen content mechanical properties and weight changes before and after (0.5% or less)was developed by Japan Nippon Carbon. oxidation in air and combustion environment were invest With the oxygen content being decreased, silicon carbide gated and compared fiber reinforced silicon carbide composites(Hi-Nicalon/ iC)are expected to be used at above 1400.C, and are being paid more and more attention [4] 2. Experimental procedure The oxidation behavior of ceramic matrix composites CMC)is infuenced cooperatively by the oxidation of the 2. 1. Fabrication of the composite fiber, matrix and interphase, and related to the oxidation onditions. a number of studies have been conducted on Hi-Nicalon silicon carbide fiber from Japan Ni the oxidation of Sic prepared by chemical vapor deposition Carbon was employed. The fiber preform was prepared by the three-dimensional braid method The cross-section 4 Corresponding author. Tel. 86-29-849127, fax: 86-29-8491000 dimension of the preform was 5 X 6 mm. The volume frac E-mailaddress:cmcres(@nwpu.edu.cn(CAifei tion of fibers was controlled in the range from 40 to 45% 1359-835X/00/S- see front matter e 2000 Elsevier Science Ltd. All rights reserved Pl:S1359-835X(00)00045-2
Short communication Oxidation behavior of three-dimensional SiC/SiC composites in air and combustion environment Laifei Cheng*, Yongdong Xu, Litong Zhang, Xiaowei Yin State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an Shaanxi 710072, People’s Republic of China Received 28 May 1998; received in revised form 3 September 1999; accepted 1 March 2000 Abstract A three-dimensional Hi-Nicalon/SiC composite was prepared, the mechanical properties and weight changes before and after oxidation in air and combustion environment were investigated and compared in the present paper. The combustion gas with a high flow rate and a larger amount of H2O accelerated the silica formation on the fibers and matrix, the oxidation channel could be sealed much earlier in combustion than in air, and the PyC interlayer was oxidized less. Weight loss of the composite in air was larger than that in combustion. The flexural strength did not decrease remarkably, in both air and combustion. Oxidation took place along and weakened the two interfaces of the interlayer with the fibers and matrix besides removal of the interlayer, the fracture displacement was greatly increased after oxidation in combustion compared with those before and after oxidation in air. It was shown that the composite had an excellent oxidation resistance in a short period of exposure. This result was of very important significance in the development of a self-adaptable interlayer. q 2000 Elsevier Science Ltd. All rights reserved. Keyword: SiC/SiC composites 1. Introduction Silicon carbide fiber reinforced silicon carbide composites (SiC/SiC) are one of the best promising structural materials for high temperature applications [1–3]. SiC based fibers, such as Nicalon, have a better resistance to oxidation. However, the tensile strength of the fibers decreases dramatically with a significant weight loss as the temperature is raised beyond 12008C. The large amount of oxygen (10–14%) is responsible for the degradation of the fibers at high temperature. It is necessary to reduce the oxygen content of the fibers to improve the thermal stability of SiC/SiC. Recently, Hi-Nicalon with a low oxygen content (0.5% or less) was developed by Japan Nippon Carbon. With the oxygen content being decreased, silicon carbide fiber reinforced silicon carbide composites (Hi-Nicalon/ SiC) are expected to be used at above 14008C, and are being paid more and more attention [4]. The oxidation behavior of ceramic matrix composites (CMC) is influenced cooperatively by the oxidation of the fiber, matrix and interphase, and related to the oxidation conditions. A number of studies have been conducted on the oxidation of SiC prepared by chemical vapor deposition in various environments, including air and combustion. The oxidation behavior of SiC based fibers, including Nicalon and Hi-Nicalon, has been investigated in air. A few of studies have been carried out on the oxidation of SiC/SiC composites [5,6], but the oxidation behavior Hi-Nicalon/ SiC composites, especially three-dimensional and in combustion has not been reported up to now. Because the fibers and matrix have a better resistance to oxidation, the oxidation behavior of the Hi-Nicalon/SiC composite, especially the mechanical behavior, is very sensitive to the oxidation of the interphase. In the present paper, a threedimensional Hi-Nicalon/SiC composite was prepared, the mechanical properties and weight changes before and after oxidation in air and combustion environment were investigated and compared. 2. Experimental procedure 2.1. Fabrication of the composite Hi-Nicalone silicon carbide fiber from Japan Nippon Carbon was employed. The fiber preform was prepared by the three-dimensional braid method. The cross-section dimension of the preform was 5 × 6 mm: The volume fraction of fibers was controlled in the range from 40 to 45%. Composites: Part A 31 (2000) 1015–1020 1359-835X/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S1359-835X(00)00045-2 www.elsevier.com/locate/compositesa * Corresponding author. Tel.: 1 86-29-849127; fax: 1 86-29-8491000. E-mail address: cmcres@nwpu.edu.cn (C. Laifei)
L Cheng et al. /Composites: Part A 31(2000)1015-1020 combustion at 1250C were conducted in a high temperature wind tunnel which had a nozzle of 170 mm in diameter. The maximum temperature at the flame center was 1350.C Fig I is the schematic of the high temperature wind tunnel. A special holder for specimens was designed and fabricated In order to raise the efficiency of the oxidation tests, the holder was star shaped and five arms in it could be used Fig. 2 is the schematic of the holder with specimens. The I holder 2. norzle 3. chamber 4 oil pipe specimens were kept apart from each other by blocks of silicon nitride ceramic. The parameters of the oxidation tests are given in Table 1. The composition of the combus- Fig 1 Schematic of high temperature wind tunnel tion gas has been listed in Table 2 The preforms were deposited with a pyrolysis carbon(PyC) and Sic by low pressure chemical vapor deposition 3. Results and discussion (LPCVD). The hot zone of the Cvi furnace is 60 mm X 200 mm. The interfacial layer of PyC was deposited for 1 h The density and open porosity of the SiC/SiC compo- at 870C and 5 kPa with a C3H flow of 200 ml min-1 The sIte are given in Table 3. It could be clearly seen that they changed little. Fig. 3 depicts the microstructure of conditions for deposition of SiC matrix were as follows: the the SiC/SiC composite. It was not difficult to make out deposition temperature was 1100C, pressure was 5 kPa, time was 20 h flow of H, was about. and the molar ratio that the deposition of the Sic matrix around the fiber of H2 and methyltrichlorosilane(MTS)was 10. The thick- filaments was homogeneous and the thickness of the ness of the interfacial layer was calculated to be about SiC layer was 5 um less or more. This could be 0.1 um by the weight gain after deposition of PyC. The concluded more from the preform of Hi-Nicalon" than weight gain after deposition of the SiC matrix was about from that of t-300 100%. The SiC coating were prepared on the specimens for Table 4 contains the weight changes and mechanical 5 h to seal the open ends of the fibers after cutting from the properties of the Hi-Nicalon/SiC composite before and after oxidation in air and combustion for 3 h. It could be prepared composite found that the composite lost little in weight after oxidation 2. 2. Measurements of the composite in both air and combustion, the flexural strength did not have a remarkable decrease. no matter which environment Specimens with a length of 40 mm were cut from the the composite was oxidized in, and the fracture displace- composite. The flexural strength and weight change of the ment was greatly increased after oxidation in combustion specimens before and after oxidation were first measured compared with those before and after oxidation in air. and then the fracture work and fracture displacement were Consequently, the fracture work was also increased greatly luctuation of the weight changes was considered to be curves. The flexural strength was measured by the three- caused by the holding and the surface oxidation of the speci- point bending method with a span of 30 mm loss. Because the specimens had not been machined, differ 2.3. Oxidation tests ent specimens had different surface roughness and then different weight gain after oxidation. These results reveal Oxidation tests in dry air were conducted in a furnace that the properties of the composite changed little after heated with SiC rods at 1250.C. Oxidation tests in oxidation in air, and increased obviously after oxidation in water out 4, belts 5. press blocks 6. space blocks Fig. 2. Schematic of substrate holder: (a)the back;(b)the side; (c)the front
The preforms were deposited with a pyrolysis carbon (PyC) and SiC by low pressure chemical vapor deposition (LPCVI). The hot zone of the CVI furnace is B60 mm × 200 mm: The interfacial layer of PyC was deposited for 1 h at 8708C and 5 kPa with a C3H6 flow of 200 ml min21 . The conditions for deposition of SiC matrix were as follows: the deposition temperature was 11008C, pressure was 5 kPa, time was 20 h, flow of H2 was about, and the molar ratio of H2 and methyltrchlorosilane (MTS) was 10. The thickness of the interfacial layer was calculated to be about 0.1 mm by the weight gain after deposition of PyC. The weight gain after deposition of the SiC matrix was about 100%. The SiC coating were prepared on the specimens for 5 h to seal the open ends of the fibers after cutting from the prepared composite. 2.2. Measurements of the composite Specimens with a length of 40 mm were cut from the composite. The flexural strength and weight change of the specimens before and after oxidation were first measured, and then the fracture work and fracture displacement were determined by the steady stage of the load–displacement curves. The flexural strength was measured by the threepoint bending method with a span of 30 mm. 2.3. Oxidation tests Oxidation tests in dry air were conducted in a furnace heated with SiC rods at 12508C. Oxidation tests in combustion at 12508C were conducted in a high temperature wind tunnel which had a nozzle of 170 mm in diameter. The maximum temperature at the flame center was 13508C. Fig. 1 is the schematic of the high temperature wind tunnel. A special holder for specimens was designed and fabricated. In order to raise the efficiency of the oxidation tests, the holder was star shaped and five arms in it could be used. Fig. 2 is the schematic of the holder with specimens. The specimens were kept apart from each other by blocks of silicon nitride ceramic. The parameters of the oxidation tests are given in Table 1. The composition of the combustion gas has been listed in Table 2. 3. Results and discussion The density and open porosity of the SiC/SiC composite are given in Table 3. It could be clearly seen that they changed little. Fig. 3 depicts the microstructure of the SiC/SiC composite. It was not difficult to make out that the deposition of the SiC matrix around the fiber filaments was homogeneous and the thickness of the SiC layer was 5 mm less or more. This could be concluded more from the preform of Hi-Nicalone than from that of T-300e. Table 4 contains the weight changes and mechanical properties of the Hi-Nicalon/SiC composite before and after oxidation in air and combustion for 3 h. It could be found that the composite lost little in weight after oxidation in both air and combustion, the flexural strength did not have a remarkable decrease, no matter which environment the composite was oxidized in, and the fracture displacement was greatly increased after oxidation in combustion compared with those before and after oxidation in air. Consequently, the fracture work was also increased greatly. The fluctuation of the weight changes was considered to be caused by the holding and the surface oxidation of the specimens. The holding of the specimens led to different weight loss. Because the specimens had not been machined, different specimens had different surface roughness and then different weight gain after oxidation. These results reveal that the properties of the composite changed little after oxidation in air, and increased obviously after oxidation in 1016 L. Cheng et al. / Composites: Part A 31 (2000) 1015–1020 Fig. 1. Schematic of high temperature wind tunnel. Fig. 2. Schematic of substrate holder: (a) the back; (b) the side; (c) the front
1017 he oxidation tests Temperature Pressure 0.991kg/s 200m/s 1209-1250°C 5% 1.1×105Pa 0.986kg/cm3 forming non-bridge SIOH groups. This decreased the silica Composition of the combustion gas viscosity and increased the diffusion of H2o in the silica Air(53.52%) Pure combustion(46.48%) [10, 11. For the oxidation rate of CVD SiC, there was O, Remainde controlled by the reaction rate, and the later was controlled 40.25% 33.21% 13.27% by the diffusion rate of oxygen in the silica. The lineal stage was about I min short and was negligible compared with the parabolic stage when oxidized in air. The fast diffusion of combustion. This was exciting and very important for H]0 in the silica changed the controlling step of the oxida- applications of this composite in advanced engines. If tion rate from diffusion to reaction when oxidized in only favorable interlayers were prepared, could CMC exhi- combustion and the lineal stage was prolonged for about bit a tough fracture behavior? Most often the pyrocarbon 1 h [12, 13]. The silica formation in combustion was much (Py C)interlayer was applied, and the CVD BN interlayer faster and had a lower viscosity than that formed in air, the had been used in place of PyC recently for improving the oxidation channel could be sealed much earlier, and the Pyc oxidation resistance [7]. Unfortunately, PyC had very poor interlayer should be oxidized less( Fig. 4). The weight gain oxidation resistance and bn was not sufficiently better to be produced by silica formation in combustion was larger than viable at above 1300.C. Up to now there are no demon- that in air, and the weight loss caused by the interlayer strated oxidation resistant interface approaches [8]. For C/ oxidation was smaller. Consequently, the weight loss of SiC composites, it is not difficult to understand the effect of the specimens in air should be larger than that in combus- interfacial oxidation on properties. For Nicalon/SiC compo- tion, similar to that confirmed by the test result. At the same sites, it was considered that the degradation of mechanical time, this result showed that influence of the erosion caused properties after short periods of exposure was due to the by the combustion gas with a high flow rate of 200/s on the oxidation and removal of the PyC interlayer [9]. For the weight loss could be neglected at the test temperature Hi-Nicalon/SiC composite investigated in the present Compared with the still air, the combustion gas decreased paper, the PyC interlayer not only provided a sufficiently the boundary layer thickness on the specimens, promoted weak interface, but also could further weaken the interfacial the inward diffusion of oxidizing gases and the outward binding after a relatively longer period of exposure in the xidizing atmosphere. This result was of great significance in developing a self-adaptable interlayer, although proper- ties of the Hi-Nicalon/SiC composite after much longer oxidation are still to be studied A silica film would be formed on the cvd sic matrix or the hi-Nicalon fibers after oxidation in both air and combus- tion. Because the oxidation of Sic produced a weight gain and a volume expansion, the oxidation of the Pyc interlayer should be responsible for the weight loss of the specimens The oxidation channel of the Pyc interlayer could be considered to start from the intersection of bundles. which was connected with the open interbundle porosity. Because there was a large amount of HO in the combustion gas (Table 2), the molecule net of the silica was broken by Fig 3. Microstructures of the SiC/SiC composite as prepare Density and open porosity SiC/SiC the composite Number 4 5 6 2.47 2.48 2.49 2.47 Open porosity (%
combustion. This was exciting and very important for applications of this composite in advanced engines. If only favorable interlayers were prepared, could CMC exhibit a tough fracture behavior? Most often the pyrocarbon (PyC) interlayer was applied, and the CVD BN interlayer had been used in place of PyC recently for improving the oxidation resistance [7]. Unfortunately, PyC had very poor oxidation resistance and BN was not sufficiently better to be viable at above 13008C. Up to now there are no demonstrated oxidation resistant interface approaches [8]. For C/ SiC composites, it is not difficult to understand the effect of interfacial oxidation on properties. For Nicalon/SiC composites, it was considered that the degradation of mechanical properties after short periods of exposure was due to the oxidation and removal of the PyC interlayer [9]. For the Hi-Nicalon/SiC composite investigated in the present paper, the PyC interlayer not only provided a sufficiently weak interface, but also could further weaken the interfacial binding after a relatively longer period of exposure in the oxidizing atmosphere. This result was of great significance in developing a self-adaptable interlayer, although properties of the Hi-Nicalon/SiC composite after much longer oxidation are still to be studied. A silica film would be formed on the CVD SiC matrix or the Hi-Nicalon fibers after oxidation in both air and combustion. Because the oxidation of SiC produced a weight gain and a volume expansion, the oxidation of the PyC interlayer should be responsible for the weight loss of the specimens. The oxidation channel of the PyC interlayer could be considered to start from the intersection of bundles, which was connected with the open interbundle porosity. Because there was a large amount of H2O in the combustion gas (Table 2), the molecule net of the silica was broken by forming non-bridge SiOH groups. This decreased the silica viscosity and increased the diffusion of H2O in the silica [10,11]. For the oxidation rate of CVD SiC, there was a lineal stage and a parabolic stage. The former was controlled by the reaction rate, and the later was controlled by the diffusion rate of oxygen in the silica. The lineal stage was about 1 min short and was negligible compared with the parabolic stage when oxidized in air. The fast diffusion of H2O in the silica changed the controlling step of the oxidation rate from diffusion to reaction when oxidized in combustion and the lineal stage was prolonged for about 1 h [12,13]. The silica formation in combustion was much faster and had a lower viscosity than that formed in air, the oxidation channel could be sealed much earlier, and the PyC interlayer should be oxidized less (Fig. 4). The weight gain produced by silica formation in combustion was larger than that in air, and the weight loss caused by the interlayer oxidation was smaller. Consequently, the weight loss of the specimens in air should be larger than that in combustion, similar to that confirmed by the test result. At the same time, this result showed that influence of the erosion caused by the combustion gas with a high flow rate of 200/s on the weight loss could be neglected at the test temperature. Compared with the still air, the combustion gas decreased the boundary layer thickness on the specimens, promoted the inward diffusion of oxidizing gases and the outward L. Cheng et al. / Composites: Part A 31 (2000) 1015–1020 1017 Table 1 Parameters of the oxidation tests Flow Temperature Pressure Discharge Rate Distribution Relative change Entrance Exit 0.991 kg/s 200 m/s 1209–12508C , 5% 1:1 × 105 Pa 0.986 kg/cm3 Table 2 Composition of the combustion gas Air (53.52%) Pure combustion (46.48%) O2 Remainder CO2 H2O 13.27% 40.25% 33.21% 13.27% Table 3 Density and open porosity SiC/SiC the composite Number 1 2 3 4 5 6 Average Density (g/cm3 ) 2.46 2.46 2.46 2.47 2.48 2.49 2.47 Open porosity (%) 13 13 14 15 15 15 14 Fig. 3. Microstructures of the SiC/SiC composite as prepared
L Cheng et al./Composites: Part A 31(2000)1015-1020 Fig. 4. Schematic representation of SiC/C/SiC oxidation mechanisms. Fig. 5. The fracture surfaces of the Hi-Nicalon/Sic after oxidation diffusion of products, and accelerated the silica formation paper, a Hi-Nicalon/SiC composite with a larger thickness on the fibers and matrix [14 of the Pyc interlayer was prepared. Fig. 6 shows the SEM In the oxidation process, on the one hand the removal of micrograph of the interlayer. After oxidation for I h in air the PyC interlayer weakened the interfacial binding, on the (Fig. 7a), the Pyc close to the section was oxidized out, and other hand, the fibers and matrix were bonded together by some fibers were bonded with the matrix by the silica the silica formed so that the interfacial binding was strength- formed. After oxidation for 7h (Fig. 7b), more fibers were ened. Because the oxidation channel was sealed much bonded with the matrix earlier, the interface of the fibers and matrix bonded by silica in combustion was much longer than that in air. and hen the fracture displacement of the composite in combus- tion was much larger than that in air. The fact that the fracture displacement in air was little larger and that in com bustion was much larger than that before oxidation showed that the weakened interface by oxidation in both air and combustion was much longer than that bonded The reasonable explanation was that oxidation took place along and weakened the two interfaces of the interlayer with the fibers and matrix, besides removal of the interlayer. This could be also confirmed by the fact that pull out of the fibers on fracture surfaces of specimens oxidized in combustion was much longer than that in air(Fig. 5) For improving the oxidation resistance of the SiC/Sic 2KX10,8881mlD22 opposites, the thickness of the PyC interlayer should not be larger than 0.2 um[5]. In order to demonstrate the oxida- Fig. 6 SEM micrograph of the interlayer in the SiC/SiC composite tion behavior of the PyC interfacial layer presented in this
diffusion of products, and accelerated the silica formation on the fibers and matrix [14]. In the oxidation process, on the one hand the removal of the PyC interlayer weakened the interfacial binding; on the other hand, the fibers and matrix were bonded together by the silica formed so that the interfacial binding was strengthened. Because the oxidation channel was sealed much earlier, the interface of the fibers and matrix bonded by silica in combustion was much longer than that in air, and then the fracture displacement of the composite in combustion was much larger than that in air. The fact that the fracture displacement in air was little larger and that in combustion was much larger than that before oxidation showed that the weakened interface by oxidation in both air and combustion was much longer than that bonded. The reasonable explanation was that oxidation took place along and weakened the two interfaces of the interlayer with the fibers and matrix, besides removal of the interlayer. This could be also confirmed by the fact that pull out of the fibers on fracture surfaces of specimens oxidized in combustion was much longer than that in air (Fig. 5). For improving the oxidation resistance of the SiC/SiC composites, the thickness of the PyC interlayer should not be larger than 0.2 mm [5]. In order to demonstrate the oxidation behavior of the PyC interfacial layer presented in this paper, a Hi-Nicalon/SiC composite with a larger thickness of the PyC interlayer was prepared. Fig. 6 shows the SEM micrograph of the interlayer. After oxidation for 1 h in air (Fig. 7a), the PyC close to the section was oxidized out, and some fibers were bonded with the matrix by the silica formed. After oxidation for 7 h (Fig. 7b), more fibers were bonded with the matrix. 1018 L. Cheng et al. / Composites: Part A 31 (2000) 1015–1020 Fig. 5. The fracture surfaces of the Hi-Nicalon/SiC after oxidation. Fig. 4. Schematic representation of SiC/C/SiC oxidation mechanisms. Fig. 6. SEM micrograph of the interlayer in the SiC/SiC composite as prepared
L Cheng et al/Composites: Part A 31(2000)1015-1020 1019 Table 4 Weight change and properties of Hi-Nicalon SiC/SiC composite before and after oxidation in air and combustion for 3 h loss, properties Condition 2 Average -0.12 0.44 0.29 0.60 Flexural strength(MPa) In combustion 65 Fracture work(kJ As prepared In combustion Fracture displacement(mm 0.55 in In combustion The thicker the PyC interlayer, the larger the length of the ity, then it had a higher strength. Because the PyC inter- interlayer consumed by oxidation in both air and combi layer was well deposited, the composite had a larger tion. For Nicalon/SiC composites this meant that the fibers fracture displacement and fracture work. would be oxidized more strongly and lose strength more 2. The flexural strength did not have a remarkable decrease, greatly. It was considered that the thickness of the PyC no matter which environment the composite was interlayer should be less than 0 I um in order to oxidized in. It was shown that oxidation had little improve the oxidation resistance of the interlayer [6] effect on strength of the fibers after short periods of For the Hi-Nicalon/SiC composite, the interlayer could exposure be thicker because the fibers retained strength much 3. Compared with air, the larger amount of H2O in combus more than Nicalon fibers after oxidation. This could tion changed the controlling step of the oxidation rate of be favorable for keeping a weak interfacial binding SiC from diffusion to reaction when oxidized in combus- after longer periods of exposure in both air and combus- tion. and the silica formation was faster. The combustion tion, especially to prevent the fibers and matrix from gas with a high flow rate decreased the boundary layer being bonded by silica and producing brittle behavio thickness on the Hi-Nicalon/SiC composite, promoted after oxidation in air the inward diffusion of oxidizing gases and the outward diffusion of products, and accelerated the silica formation on the fibers and matrix. Consequently, the oxidation 4. Conclusions channel could be sealed much earlier, and the Pyc inter layer was oxidized les 1. When the Hi-Nicalon/SiC composite prepared, had a 4. Because oxidation took place along and weakened the higher density, a lower porosity and a better homogene- two interfaces of the interlayer with the fibers and matrix 081 387728K818,8881NmWD23 Fig. 7. SEM micrograph of the interlayer in the SiC/SiC composite after oxidation
The thicker the PyC interlayer, the larger the length of the interlayer consumed by oxidation in both air and combustion. For Nicalon/SiC composites this meant that the fibers would be oxidized more strongly and lose strength more greatly. It was considered that the thickness of the PyC interlayer should be less than 0.1 mm in order to improve the oxidation resistance of the interlayer [6]. For the Hi-Nicalon/SiC composite, the interlayer could be thicker because the fibers retained strength much more than Nicalon fibers after oxidation. This could be favorable for keeping a weak interfacial binding after longer periods of exposure in both air and combustion, especially to prevent the fibers and matrix from being bonded by silica and producing brittle behavior after oxidation in air. 4. Conclusions 1. When the Hi-Nicalon/SiC composite prepared, had a higher density, a lower porosity and a better homogeneity, then it had a higher strength. Because the PyC interlayer was well deposited, the composite had a larger fracture displacement and fracture work. 2. The flexural strength did not have a remarkable decrease, no matter which environment the composite was oxidized in. It was shown that oxidation had little effect on strength of the fibers after short periods of exposure. 3. Compared with air, the larger amount of H2O in combustion changed the controlling step of the oxidation rate of SiC from diffusion to reaction when oxidized in combustion, and the silica formation was faster. The combustion gas with a high flow rate decreased the boundary layer thickness on the Hi-Nicalon/SiC composite, promoted the inward diffusion of oxidizing gases and the outward diffusion of products, and accelerated the silica formation on the fibers and matrix. Consequently, the oxidation channel could be sealed much earlier, and the PyC interlayer was oxidized less. 4. Because oxidation took place along and weakened the two interfaces of the interlayer with the fibers and matrix L. Cheng et al. / Composites: Part A 31 (2000) 1015–1020 1019 Table 4 Weight change and properties of Hi-Nicalon SiC/SiC composite before and after oxidation in air and combustion for 3 h Weight loss, properties Condition 1 2 3 Average Weight change (%) As prepared 0000 In air 20.12 20.30 20.90 20.44 In combustion 20.27 20.29 20.60 20.39 Flexural strength (MPa) As prepared 583 596 590 In air 504 579 605 563 In combustion 542 543 656 581 Fracture work (kJ/m2 ) As prepared 27 20 23 In air 23 29 20 24 In combustion 17 60 60 46 Fracture displacement (mm) As prepared 0.55 0.61 0.58 in air 0.60 0.65 0.74 0.66 In combustion 0.58 1.03 1.37 0.99 Fig. 7. SEM micrograph of the interlayer in the SiC/SiC composite after oxidation in air
L Cheng et al./Composites: Part A 31(2000)1015-1020 besides removal of the interlayer, the fracture displace- [6] Filipuzzi L, Naslain R. Oxidation mechanisms and kinetics of ment and the fracture work were greatly increased after ID-SiC/C/SiC composite materials. I. An experimental approach. J oxidation in combustion compared with those before and Am Ceram Soc1994;77(2):459-66. after oxidation in air [7 Naslain. Fiber-matrix interphases and interfaces in ceramic matrix composites processed by CVl. Composites Interface 1993: 1(3): 253 [8]Kerans RJ. Control of fiber-matrix interface properties in ceramic composites. In: Naslain R, editor. High temperature ceramic matrix References composites, Bordeaux: Woodhead, 1993. P. 301-12 [9]Lowden RA, More KL, Schwarz OJ, Vaughn NL. Improved fiber [1 Capoto AG, Lackey WJ. Fabrication of fiber reinforced ceramic matrix interlayers for Nicalon/SiC composites. In: Naslain R, editor. natrix composites by chemical vapor infiltration. Ceram Engng Sci High temperature ceramic matrix composites, Bordeaux: Woodhead, Pro1984:5(7/8):654-67 1993.p.215-29 [2]Besmann TM, Sheldon BW, Lowden RA. Vapor phase fabrication [10] Irene EA, Ghez R. Silicon oxidation studies: the role of H2O. J nd properties of continuous filament ceramic composites. Science Electrochem Soc1977124(11):1757-61. 1991253(6):1104-9 [11 Jorgensen PJ, Wadsworth ME, Cutle IB Effect of water vapor 3] Naslain R. CVI composites. In: Wareen R, editor. Ceramic matrix oxidation of silicon carbide. J Am Ceram Soc 1961: 44(6): 258-6 composites, London: Chapman Hall, 1992. P. 199-243. [12] Narushima T, Goto T, Iguchi Y, Hirai T. High-temperature oxidation 4 Cholon G, Czerniak M, Pailler R, Bourrat X, Naslain R, Olry P, of chemically vapor-deposited silicon carbide in wet oxygen at 1823 Loison S. Silicon carbide fibers: pre to1923K. J Am Ceram Soc1990,73(12):1580-4 Sic based fibers with a low oxygen content. In: Naslain R, edito [13] Narushima T, Goto T, Yokoyama Y, Iguchi Y, Hirai T High temperature ceramic matrix composites, Bordeaux: Woodhead, temperature active oxidation of chemically vapor-deposited 1993.p.109-16 carbide in CO-CO atmosphere. J Am Ceram Soc 1990 5] Filipuzzi L, Gamus G, Naslain R. Oxidation mechanisms and kinetics 2521-4 f ID-SiC/C/SiC composite materials. Il Modeling. J Am Ceram Soc [14] Luthra KL. Oxidation of carbon/carbon composites-a theoretical analysis. Carbon 1988, 26(2): 217-24
besides removal of the interlayer, the fracture displacement and the fracture work were greatly increased after oxidation in combustion compared with those before and after oxidation in air. References [1] Capoto AG, Lackey WJ. Fabrication of fiber reinforced ceramic matrix composites by chemical vapor infiltration. Ceram Engng Sci Proc 1984;5(7/8):654–67. [2] Besmann TM, Sheldon BW, Lowden RA. Vapor phase fabrication and properties of continuous filament ceramic composites. Science 1991;253(6):1104–9. [3] Naslain R. CVI composites. In: Wareen R, editor. Ceramic matrix composites, London: Chapman & Hall, 1992. p. 199–243. [4] Chollon G, Czerniak M, Pailler R, Bourrat X, Naslain R, Olry P, Loison S. Silicon carbide fibers: preparation and characterization of SiC based fibers with a low oxygen content. In: Naslain R, editor. High temperature ceramic matrix composites, Bordeaux: Woodhead, 1993. p. 109–16. [5] Filipuzzi L, Gamus G, Naslain R. Oxidation mechanisms and kinetics of 1D-SiC/C/SiC composite materials. II. Modeling. J Am Ceram Soc 1994;77(2):467–80. [6] Filipuzzi L, Naslain R. Oxidation mechanisms and kinetics of 1D-SiC/C/SiC composite materials. I. An experimental approach. J Am Ceram Soc 1994;77(2):459–66. [7] Naslain. Fiber–matrix interphases and interfaces in ceramic matrix composites processed by CVI. Composites Interface 1993;1(3):253– 86. [8] Kerans RJ. Control of fiber–matrix interface properties in ceramic composites. In: Naslain R, editor. High temperature ceramic matrix composites, Bordeaux: Woodhead, 1993. p. 301–12. [9] Lowden RA, More KL, Schwarz OJ, Vaughn NL. Improved fiber– matrix interlayers for Nicalon/SiC composites. In: Naslain R, editor. High temperature ceramic matrix composites, Bordeaux: Woodhead, 1993. p. 215–29. [10] Irene EA, Ghez R. Silicon oxidation studies: the role of H2O. J Electrochem Soc 1977;124(11):1757–61. [11] Jorgensen PJ, Wadsworth ME, Cutle IB. Effect of water vapor on oxidation of silicon carbide. J Am Ceram Soc 1961;44(6):258–61. [12] Narushima T, Goto T, Iguchi Y, Hirai T. High-temperature oxidation of chemically vapor-deposited silicon carbide in wet oxygen at 1823 to 1923 K. J Am Ceram Soc 1990;73(12):1580–4. [13] Narushima T, Goto T, Yokoyama Y, Iguchi Y, Hirai T. Hightemperature active oxidation of chemically vapor-deposited silicon carbide in CO–CO2 atmosphere. J Am Ceram Soc 1990;76(10): 2521–4. [14] Luthra KL. Oxidation of carbon/carbon composites—a theoretical analysis. Carbon 1988;26(2):217–24. 1020 L. Cheng et al. / Composites: Part A 31 (2000) 1015–1020