CARBON PERGAMON Carbon38(2000)2103-2108 Oxidation behavior of three dimensional c/ Sic composites in air and combustion gas environments Laifei Cheng", Yongdong Xu, Litong Zhang, Xiaowei Yin State Key Laboratory of Solidification Processing, Northestern Polytechnical University, Xi an Shaanxi 710072, Peoples Republic of Received 29 June 1999; accepted 28 February 2000 Abstract A three dimensional C/SiC composite was prepared and flexural strengths during combustion atmosphere and weight changes in air were investigated. When oxidized in air, the C/Sic composite gained weight above the cracking temperature, and lost weight below the cracking temperature. The weight loss reached its maxumum value at about 700C. When oxidized during combustion atmosphere, the composite always lost weight due to the large temperature gradient along the specimen. The strengths were lowest at the area close to the nozzle wall where the flame temperature was about 700C. There were four oxidation zones along the specimens. There was an unoxidized zone (I)at the surporting end. Close to this was the cracking-oxidation zone (D). At the high-temperature end was the coating-oxidation zone (IV). Between the coating- oxidation and cracking- oxidation zones was the transition zone (Ill). Uniform, non-uniform and superficial oxidation regimes were observed which were considered to be responsible for the weight changes in air and strength changes during combustion atmosphere. 2000 Elsevier Science Ltd All rights reserved Keywords: A Carbon composites; B Oxidation; Combustion, 1. Introduction property changes of a C/S posite during oxidation, pecially the mechanical pre are very sensitive to Carbon fiber reinforced silicon carbide composites(C/ the oxidation of the carbo and the pyrocarbon SiC) are one of the most promising structural materials for interphase [4]. Sic prepared by chemical vapor deposition high temperature applications, and have been recently has been discovered to show different oxidation behaviors studied and applied as ceramic matrix composites [1, 2]. in various environments, including in air and in combus- C/SiC composites have a high thermal stability and are tion atmosphere [5-7. Some investigations have been usually considered useful up to 1650C, but they have a conducted on the oxidation behavior of C/Sic in air at low durability except in inert atmospheres. Because there lower temperatures [3, 8], but the oxidation behavior of is a large thermal expansion mismatch, the SiC matrix is C/SiC composites in combustion atmosphere has not been unable to protect the fibers from oxidation at temperatures eported, specially for woven composites. In the present below the deposition temperature. Although the deposition paper, a three-dimensional C/SiC composite was prepared of a pyrocarbon interphase by Cvi before depositing the and strength changes and weight changes after oxidation in Sic matrix can change the fracture behavior of the both air and combustion environment were investigated composites from brittle to toughened, and decrease the and compared cracking temperature by weakening the interfacial bond- ing, it can not prevent the matrix from forming cracks, especially in the case of woven composites [3] Carbon is known to react with oxygen above 400C, 2. Experimental procedure 2. 1. Fabrication of the composi *Corresponding author. Tel. +86-29-849-127, fax: +86-29- 49-1000. T-300 carbon fiber from Japan Toray was employed E-mailaddress:cmcres(@nwpu.edu.cn(L.Cheng) The fiber preforms were prepared by three-dimensional 0008-6223/00/S-see front matter 2000 Elsevier Science Ltd. All rights reserved PII:S0008-6223(00)00068-3
PERGAMON Carbon 38 (2000) 2103–2108 Oxidation behavior of three dimensional C/SiC composites in air and combustion gas environments 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 29 June 1999; accepted 28 February 2000 Abstract A three dimensional C/SiC composite was prepared and flexural strengths during combustion atmosphere and weight changes in air were investigated. When oxidized in air, the C/SiC composite gained weight above the cracking temperature, and lost weight below the cracking temperature. The weight loss reached its maxumum value at about 7008C. When oxidized during combustion atmosphere, the composite always lost weight due to the large temperature gradient along the specimen. The strengths were lowest at the area close to the nozzle wall where the flame temperature was about 7008C. There were four oxidation zones along the specimens. There was an unoxidized zone (I) at the surporting end. Close to this was the cracking-oxidation zone (II). At the high-temperature end was the coating-oxidation zone (IV). Between the coatingoxidation and cracking-oxidation zones was the transition zone (III). Uniform, non-uniform and superficial oxidation regimes were observed which were considered to be responsible for the weight changes in air and strength changes during combustion atmosphere. 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon composites; B. Oxidation; Combustion, 1. Introduction property changes of a C/SiC composite during oxidation, specially the mechanical properties, are very sensitive to Carbon fiber reinforced silicon carbide composites (C/ the oxidation of the carbon fibers and the pyrocarbon SiC) are one of the most promising structural materials for interphase [4]. SiC prepared by chemical vapor deposition high temperature applications, and have been recently has been discovered to show different oxidation behaviors studied and applied as ceramic matrix composites [1,2]. in various environments, including in air and in combusC/SiC composites have a high thermal stability and are tion atmosphere [5–7]. Some investigations have been usually considered useful up to 16508C, but they have a conducted on the oxidation behavior of C/SiC in air at low durability except in inert atmospheres. Because there lower temperatures [3,8], but the oxidation behavior of is a large thermal expansion mismatch, the SiC matrix is C/SiC composites in combustion atmosphere has not been unable to protect the fibers from oxidation at temperatures reported, specially for woven composites. In the present below the deposition temperature. Although the deposition paper, a three-dimensional C/SiC composite was prepared of a pyrocarbon interphase by CVI before depositing the and strength changes and weight changes after oxidation in SiC matrix can change the fracture behavior of the both air and combustion environment were investigated composites from brittle to toughened, and decrease the and compared. cracking temperature by weakening the interfacial bonding, it can not prevent the matrix from forming cracks, especially in the case of woven composites [3]. 2. Experimental procedure Carbon is known to react with oxygen above 4008C, 2.1. Fabrication of the composite *Corresponding author. Tel.: 186-29-849-127; fax: 186-29- 849-1000. T-300E carbon fiber from Japan Toray was employed. E-mail address: cmcres@nwpu.edu.cn (L. Cheng) The fiber preforms were prepared by three-dimensional 0008-6223/00/$ – see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S0008-6223(00)00068-3
2104 L. Cheng et al. Carbon 38(2000)2103-2108 53.52% 46.48% Remainder 3.279 40.25% tunnel which had a nozzle of 170 mm in diameter. Fig. 1 is 1. holder 2. norzle 3 chamber 4 oil pipe schematic of the high-temperature wind tunnel. The speci- 5. entrance 6. spray 7 igniter 8. pecimens mens were held on the nozzle by fixing one end with bolts ( Fig. 2). Parameters of the oxidation tests were given in Fig. 1. Schematic of high temperature wind tunnel. Table 1 Composition of the combustion gas was listed in Table 2. The composition was measured by a gas-analyzer and then tested and verified by calculattion. The error braid method. The dimension of the preforms was 4 between the results measured and those calculated was less mmX5 mmx 150 mm. The volume fraction of fibers was than 5%. The composition varied little from the wall to the controlled in the range from 40% to 45% forms centor of the chamber because the vere deposited with a pyrolysis carbon(Pyc)and Sic by rapidly in the combustion gas which had a very high low-pressure chemical vapor infiltration (LPCVi). The Reynolds number. Oxidation tests in dry air at 1250C PyC interficial layer was deposited for I h at 870oC and 5 were conducted in a furnace heated with SiC rods, and the kPa. The conditions for deposition of the SiC matrix were short specimens with a length of 40 mm were employed as follow: deposition temperature was 1100C, pressure After cut from the prepared composite, the specimens were was 5 kPa, time was 20 h, flow of H, was about 350 ml deposited with Sic for 5 h to seal the open ends of the in,and the molar ratio of H, and mts (Methyltrich- fibers lorosilane) was 10. Density of the composite varied from 1.93 g/cm'to 2.01 g/cm. Taking account of the SiC 2.3. Measurements of the compos coating formed on the composite surface in the CVi process, the density was nominal For the long specimens, flexural strengths before and 2.2. Oxidation tests after oxidation in combustion atmosphere were measured by three-point bending method with a span of 20 mm Oxidation tests in combustion atmosphere with the flame Along the every specimen, a strength distribution with the temperature being 1250C were conducted by direct use distance from the high-temperature end to the low-tem- perature was obtained. For the short specimens, weigl changes after oxidation in air were measured bolts 3. Results and discusion time(Fig. 3), it could be clearly seen that the C/SiC composite always lost weight by a parabolic law when oxidized in combustion atmosphere with the flame tem- ature be oxidized in air at 1250%C. It was shown that the carbon Fig. 2. Method of holding specimens in combustion atmosphere. fibers and the Pyc interphase in the composite were not Table I Parameters of the oxidation tests in combustion Flow Relative change 0991Kg/s 200m/s 1209-1250°C 1.1×103Pa 0.986kg/cm
2104 L. Cheng et al. / Carbon 38 (2000) 2103 –2108 Table 2 Composition of the combustion gas Air Pure combustion 53.52% 46.48% O Remainder CO H O 2 22 13.27% 40.25% 33.21% 13.27% the long composite specimens in a high-temperature wind tunnel which had a nozzle of 170 mm in diameter. Fig. 1 is schamatic of the high-temperature wind tunnel. The specimens were held on the nozzle by fixing one end with bolts (Fig. 2). Parameters of the oxidation tests were given in Table 1. Composition of the combustion gas was listed in Fig. 1. Schematic of high temperature wind tunnel. Table 2. The composition was measured by a gas-analyzer, and then tested and verified by calculattion. The error braid method. The dimension of the preforms was 4 between the results measured and those calculated was less mm35 mm3150 mm. The volume fraction of fibers was than 5%. The composition varied little from the wall to the controlled in the range from 40% to 45%. The preforms centor of the chamber because the gases diffused very were deposited with a pyrolysis carbon (PyC) and SiC by rapidly in the combustion gas which had a very high low-pressure chemical vapor infiltration (LPCVI). The Reynolds number. Oxidation tests in dry air at 12508C PyC interficial layer was deposited for 1 h at 8708C and 5 were conducted in a furnace heated with SiC rods, and the kPa. The conditions for deposition of the SiC matrix were short specimens with a length of 40 mm were employed. as follow: deposotion temperature was 11008C, pressure After cut from the prepared composite, the specimens were was 5 kPa, time was 20 h, flow of H was about 350 ml deposited with SiC for 5 h to seal the open ends of the 2 21 min , and the molar ratio of H and MTS (Methyltrich- fibers. 2 lorosilane) was 10. Density of the composite varied from 3 3 1.93 g/cm to 2.01 g/cm . Taking account of the SiC 2.3. Measurements of the composite coating formed on the composite surface in the CVI process, the density was nominal. For the long specimens, flexural strengths before and after oxidation in combustion atmosphere were measured 2.2. Oxidation tests by three-point bending method with a span of 20 mm. Along the every specimen, a strength distribution with the Oxidation tests in combustion atmosphere with the flame distance from the high-temperature end to the low-tem- temperature being 12508C were conducted by direct use of perature was obtained. For the short specimens, weight changes after oxidation in air were measured. 3. Results and discusion From the relations of weight changes with oxidation time (Fig. 3), it could be clearly seen that the C/SiC composite always lost weight by a parabolic law when oxidized in combustion atmosphere with the flame temperature being 12508C, but always gained weight when oxidized in air at 12508C. It was shown that the carbon Fig. 2. Method of holding specimens in combustion atmosphere. fibers and the PyC interphase in the composite were not Table 1 Parameters of the oxidation tests in combustion Flow Temperature Pressure Discharge Rate Distribution Relative change Entrance Exit 5 3 0.991 Kg/s 200 m/s 1209–12508C ,5% 1.1310 Pa 0.986 kg/cm
L. Cheng et al. Carbon 38(2000)2103-2108 2105 202468 Ain combustion 6 10 Fig. 3. Relations of weight changes with oxidation time in air and in combustion atmosphere at 1250C oxidized in air, but were oxidized in combustion atmos- the weight loss reached its maximum value at about 700C There was al ways some area where it was below the A SiC coating which was about 100 um in thickness cracking temprature due to the large temperature gradient Fig. 4)was formed on the composite surface in the last along the specimens when they were oxidized in combus- stage of the CVI process. Thickness of the coating for tion atmosphere. As a result, the composite always lost sealing of the fiber ends was about 50 um. The coating weight when oxidized in combustion atmosphere could prevent the fibers and the interphase from oxidation Fig. 6 was the strength changes along the specimens above the craking temperature. According to the ex after oxidation in combustion atmosphere. Fig. 6a was perimental results of oxidation in air at different tempera- typical of the strength changes for all the three specimens ures, a curve of weight change with temperature wa The strengths firstly decreased little at the area close to the obtained(Fig. 5). The cracking temperature of the coating flame center, and then decreased step by step at the middle was 1050.C and was little lower than the deposition area. After reached their lowest values around the nozzle temperature which was 1100C. Oxidation was only con- wall, they increased. There was a strength gradiend along ducted on the coating surface above the cracking tempera- the specimen before oxidation( Fig. 6e). The strength was ture, oxidation behavior of the composite depended lower at the end which faced to the reactive gas flow in the that of CVD Sic which always gained weight when CVI process. Oxidation of the fibers caused by H2O could oxidized in air. Below the cracking temperature, oxidation be responsible for the strength gradient befroe oxidation. of the fibers and Pyc interfaces lead to weight loss because Some of the H,o was from the H2, and the other was the cracks became the oxidation channel, and furthermore absorbed in the fumace and on the tube walls of the gas system. Obviously, the strength gradient had an effect the strength distribution along the specimens after oxid tion in combustion atmosphere. When the gas-facing end of the specimen was in the fame, the strength distribution oxidation time lh 400600800100012001400 Fig. 4. SEM micrograph of the CVD Sic coating on a C/Sic Fig. 5. Weight change with oxidation temperature in air
L. Cheng et al. / Carbon 38 (2000) 2103 –2108 2105 Fig. 3. Relations of weight changes with oxidation time in air and in combustion atmosphere at 12508C. oxidized in air, but were oxidized in combustion atmos- the weight loss reached its maximum value at about 7008C. phere. There was always some area where it was below the A SiC coating which was about 100 mm in thickness cracking temprature due to the large temperature gradient (Fig. 4) was formed on the composite surface in the last along the specimens when they were oxidized in combusstage of the CVI process. Thickness of the coating for tion atmosphere. As a result, the composite always lost sealing of the fiber ends was about 50 mm. The coating weight when oxidized in combustion atmosphere. could prevent the fibers and the interphase from oxidation Fig. 6 was the strength changes along the specimens above the craking temperature. According to the ex- after oxidation in combustion atmosphere. Fig. 6a was perimental results of oxidation in air at different tempera- typical of the strength changes for all the three specimens. tures, a curve of weight change with temperature was The strengths firstly decreased little at the area close to the obtained (Fig. 5). The cracking temperature of the coating flame center, and then decreased step by step at the middle was 10508C and was little lower than the deposition area. After reached their lowest values around the nozzle temperature which was 11008C. Oxidation was only con- wall, they increased. There was a strength gradiend along ducted on the coating surface above the cracking tempera- the specimen before oxidation (Fig. 6e). The strength was ture, oxidation behavior of the composite depended upon lower at the end which faced to the reactive gas flow in the that of CVD SiC which always gained weight when CVI process. Oxidation of the fibers caused by H O could 2 oxidized in air. Below the cracking temperature, oxidation be responsible for the strength gradient befroe oxidation. of the fibers and PyC interfaces lead to weight loss because Some of the H O was from the H , and the other was 2 2 the cracks became the oxidation channel, and furthermore absorbed in the furnace and on the tube walls of the gas system. Obviously, the strength gradient had an effect on the strength distribution along the specimens after oxidation in combustion atmosphere. When the gas-facing end of the specimen was in the flame, the strength distribution Fig. 4. SEM micrograph of the CVD SiC coating on a C/SiC composite. Fig. 5. Weight change with oxidation temperature in air
L. Cheng et al. Carbon 38(2000)2103-2108 400 300 200 1 一与杀 200 YO--C before oxidation 0 150 Distance from flame center(mm Fig. 6. Flexural strength changes along the specimens after oxidation in combustion atmosphere would be like that shown in Fig. 6c. Conversely, the unoxidized zone was the cracking-oxidation zone (D) rength distribution would be like that shown in Fig. 6b. which was in the temperature range from 400C to 1050C The strength distribution was related to the oxidation oxidation of the fibers taking place by oxygen diffusion behavior of the composite. There were four oxidation through the cracks. At the high-temperature end was the cones along the specimens when they were oxidized in coating-oxidation zone (Iv), oxidation taking place only combustion atmosphere(Fig. 7). The unoxidized zone (I) on the coating surface due to no coating cracks Between was at the supporting end, with no oxidation taking place the coating-oxidation and the cracking-oxidation zones was because the temperature was below 400C. Close to the the transition (n, oxidation of the fibers taking 2 600 6080 Distance from the flame center(mm Fig. 7. Effect of temperature gradient on flexural strength of a C/SiC composite
2106 L. Cheng et al. / Carbon 38 (2000) 2103 –2108 Fig. 6. Flexural strength changes along the specimens after oxidation in combustion atmosphere. would be like that shown in Fig. 6c. Conversely, the unoxidized zone was the cracking-oxidation zone (II) strength distribution would be like that shown in Fig. 6b. which was in the temperature range from 4008C to 10508C, The strength distribution was related to the oxidation oxidation of the fibers taking place by oxygen diffusion behavior of the composite. There were four oxidation through the cracks. At the high-temperature end was the zones along the specimens when they were oxidized in coating-oxidation zone (IV), oxidation taking place only combustion atmosphere (Fig. 7). The unoxidized zone (I) on the coating surface due to no coating cracks. Between was at the supporting end, with no oxidation taking place the coating-oxidation and the cracking-oxidation zones was because the temperature was below 4008C. Close to the the transition zone (III), oxidation of the fibers taking Fig. 7. Effect of temperature gradient on flexural strength of a C/SiC composite
L. Cheng et al. Carbon 38(2000)2103-2108 place by oxyge on through the matrix although it of the reactant gases took place, and the strengths de- was above the temperature. After oxidized out, creased. For the area close to the nozzle wall where the carbon fibers e channels of oxygen diffusion temperature was below 800%C, uniform oxidation which The zone Ill was formed because the zone Il expand was controlled by reaction took place, and the strength tawards the zone IV. Consequently, the zone Ill increased decreased greatly. Fig. 8 showed the three oxidation with increasing oxidation time. After oxidation of 9 h in regimes. In the area where uniform oxidation took place, combustion atmosphere, the zone Ill was about 40 mm in only Sic matrix were remainded(Fig. &c). In the area width. The strengths of the specimens were lowest at the where non-uniform oxidation took place, gaps were pro- area close to the nozzle wall where the flame temperature duced at the fiber-matrix interfaces(Fig. &b). In the area was 700"C less or more, which was basically identical with where superficial oxidation took place, fibers close to the the experimental results of oxidation in air surface were damaged and those inside were not oxidized An oxidation model presented by Lamouroux 3] could Fig. &a) be used to explain the oxidation mechanism of the C/SiC For high-temperature structural applications, C/SiC osite in combustion atmosphere. For the area close to components would be always used in an oxidizing and the temperature was 50C, gradient temperature environment. Besides the working superficial oxidation which was controlled by diffusion temperature, the temperature distribution on a special C/ through the silica film on the SiC coating took place, thus SiC component should be taken into consideration, and the strengths decreased little. For the area where the different methods should be employed for the oxidation temperature was between 800oC and 1050C, non-uniform protection of the composite in different temperature oxidation which was controlled by reaction and transport ranges. Therefore, giving consideration to both High-tem- U 3,8891 Fig. 8. Fracture surface of the C/SiC composite after oxidation in combustion atmosphere (a): superficial oxidation, (b )non-uniform oxidation,(c): uniform oxidation
L. Cheng et al. / Carbon 38 (2000) 2103 –2108 2107 place by oxygen diffusion through the matrix although it of the reactant gases took place, and the strengths dewas above the cracking temperature. After oxidized out, creased. For the area close to the nozzle wall where the carbon fibers became the channels of oxygen diffusion. temperature was below 8008C, uniform oxidation which The zone III was formed because the zone II expanded was controlled by reaction took place, and the strengths tawards the zone IV. Consequently, the zone III increased decreased greatly. Fig. 8 showed the three oxidation with increasing oxidation time. After oxidation of 9 h in regimes. In the area where uniform oxidation took place, combustion atmosphere, the zone III was about 40 mm in only SiC matrix were remainded (Fig. 8c). In the area width. The strengths of the specimens were lowest at the where non-uniform oxidation took place, gaps were proarea close to the nozzle wall where the flame temperature duced at the fiber-matrix interfaces (Fig. 8b). In the area was 7008C less or more, which was basically identical with where superficial oxidation took place, fibers close to the the experimental results of oxidation in air. surface were damaged and those inside were not oxidized An oxidation model presented by Lamouroux [3] could (Fig. 8a). be used to explain the oxidation mechanism of the C/SiC For high-temperature structural applications, C/SiC composite in combustion atmosphere. For the area close to components would be always used in an oxidizing and the flame center where the temperature was above 10508C, gradient temperature environment. Besides the working superficial oxidation which was controlled by diffusion temperature, the temperature distribution on a special C/ through the silica film on the SiC coating took place, thus SiC component should be taken into consideration, and the strengths decreased little. For the area where the different methods should be employed for the oxidation temperature was between 8008C and 10508C, non-uniform protection of the composite in different temperature oxidation which was controlled by reaction and transport ranges. Therefore, giving consideration to both High-temFig. 8. Fracture surface of the C/SiC composite after oxidation in combustion atmosphere (a): superficial oxidation, (b:) non-uniform oxidation, (c): uniform oxidation
L. Cheng et al. Carbon 38(2000)2103-2108 perature range and low-temperature range was very im- were observed which were considered to be responsible portant and difficult for the oxidation protection of C/SiC for the weight changes in air and strength changes in combustion atmosphere 4. References 1. When oxidized in air, the C/Sic composite gained weight because oxidation was only conducted on the [1] Capoto AG, Lackey WJ. Fabrication of fiber reinforced coating surface above the cracking temperature, and lost ceramic matrix composites by chemical vapor infiltration. Ceram Eng Sci Porc 1984; 5(7-8): 654-67 weight because the cracks became oxidation channels 22] Besmann TM, Sheldon Bw, Lowden RA. Vapor phase elow the cracking temperature. The weight loss fabrication and properties of continuous filament ceramic reached its maxumum value at about 700C mposites Science 1991: 253(6): 1 104-9 2. When oxidized in combustion atmosphere, the compo- [3] Lamouroux F, Bourrat X, Naslain R.Structure/oxidation site always lost weight because there was always some ehavior relationship in the carbonaceous constituents of area where it was below the cracking temperature due 2D-C/PyC/SiC composites. Carbon 1993, 31(8): 1273-88 to the large temperature gradient along the specimens 14 Glime WH, Cawley JD. Oxidation of carbon fibers and films The strengths of the specimens were lowest at the area in ceramic matrix composites: A weak link process. Carbon close to the nozzle wall where the flame temperature 1995;33(8:1053-60 was about700°C [] Narushima T, Goto T, Iguchi Y, Hirai T. High-temperature 3. There were four oxidation zones along the specimens oxidation of chemically vapor-deposited silicon carbide in when they were oxidized in combustion atmosphere wet oxygen at 1823 to 1923 K. J Am Ceram Soc The unoxidized zone ()was at the supporting end 1990,73(12:1580-4. where the temperature was below 400C. Close to the 6 Narushima T, Goto T, Iguchi Y, Hirai T. High-temperature unoxidized zone was the cracking- oxidation zone(n) active oxidation of chemically vapor-deposited silicon car- where the temperature was in the range from 400C to bide in CO-CO,. J Am Ceram Soc 1990, 73(12): 2521-4. 1050C. At the high-temperature end was the coating- [7 Jacobson NS. Corrosion of silicon-based ceramics in com- bustion environments. J Am Ceram Soc 1990, 76(10): 3-28 oxidation zone(Iv) where the temperature was 1250%C Between the coating-oxidation and the cracking-oxida- 18 Jouin JM, Lamouroux F, Naslain R. Oxidation kinetics tion zones was the transition zone (Ill) where it was modeling of a 2D woven carbon fiber silicon carbide composite. In: Naslain R, editor, High temperature above the cracking temperature matrix composites, Bordeaux: Woodhead Publication 4. Uniform, non-uniform and superficial oxidation regimes
2108 L. Cheng et al. / Carbon 38 (2000) 2103 –2108 perature range and low-temperature range was very im- were observed which were considered to be responsible portant and difficult for the oxidation protection of C/SiC for the weight changes in air and strength changes in composites. combustion atmosphere. 4. Conclusions References 1. When oxidized in air, the C/SiC composite gained [1] Capoto AG, Lackey WJ. Fabrication of fiber reinforced weight because oxidation was only conducted on the ceramic matrix composites by chemical vapor infiltration. coating surface above the cracking temperature, and lost Ceram Eng Sci Porc 1984;5(7–8):654–67. weight because the cracks became oxidation channels [2] Besmann TM, Sheldon BW, Lowden RA. Vapor phase below the cracking temperature. The weight loss fabrication and properties of continuous filament ceramic reached its maxumum value at about 7008C. composites. Science 1991;253(6):1104–9. 2. When oxidized in combustion atmosphere, the compo- [3] Lamouroux F, Bourrat X, Naslain R. Structure/oxidation site always lost weight because there was always some behavior relationship in the carbonaceous constituents of area where it was below the cracking temperature due 2D-C/PyC/SiC composites. Carbon 1993;31(8):1273–88. to the large temperature gradient along the specimens. [4] Glime WH, Cawley JD. Oxidation of carbon fibers and films The strengths of the specimens were lowest at the area in ceramic matrix composites: A weak link process. Carbon 1995;33(8):1053–60. close to the nozzle wall where the flame temperature was about 7008C. [5] Narushima T, Goto T, Iguchi Y, Hirai T. High-temperature oxidation of chemically vapor-deposited silicon carbide in 3. There were four oxidation zones along the specimens wet oxygen at 1823 to 1923 K. J Am Ceram Soc when they were oxidized in combustion atmosphere. 1990;73(12):1580–4. The unoxidized zone (I) was at the supporting end [6] Narushima T, Goto T, Iguchi Y, Hirai T. High-temperature where the temperature was below 4008C. Close to the active oxidation of chemically vapor-deposited silicon car- unoxidized zone was the cracking-oxidation zone (II) bide in CO–CO . J Am Ceram Soc 1990;73(12):2521–4. 2 where the temperature was in the range from 4008C to [7] Jacobson NS. Corrosion of silicon-based ceramics in com- 10508C. At the high-temperature end was the coating- bustion environments. J Am Ceram Soc 1990;76(10):3–28. oxidation zone (IV) where the temperature was 12508C. [8] Jouin JM, Lamouroux F, Naslain R. Oxidation kinetics Between the coating-oxidation and the cracking-oxida- modeling of a 2D woven carbon fiber silicon carbide matrix tion zones was the transition zone (III) where it was composite. In: Naslain R, editor, High temperature ceramic above the cracking temperature. matrix composites, Bordeaux: Woodhead Publications, 1993, 4. Uniform, non-uniform and superficial oxidation regimes pp. 707–14