COMPOSITES SCIENCE AND TECHNOLOGY ELSEVIER Composites Science and Technology 61(2001)977-980 www.elsevier.com/locate/compscitech Oxidation behavior of 3D C/sic composites in two oxidizing environments Xiaowei Yin", Laifei Cheng, Litong Zhang, Yongdong Xu, Jianzhang Li State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, PR China Received 16 July 1999: received in revised form 13 June 2000: accepted 15 August 2000 Abstract The oxidative durability of a Sic-sealed 3D C/SiC composites was investigated at 1250 C for exposure duration of up to 9 h in the high-velocity flame of a burner rig. The testing results showed that the mass loss of the composites was related to the pores and the microcracks in the material, and the occurrence of a high-porosity zone in Sic matrix played a key role in understanding the t of environment on mass loss of the composites. Different oxidation behaviors of the SiC-sealed 3D C/Sic composites in ombustion environment and in dry air were also compared. The mechanical properties of the C/sic composites exposed in com- bustion gas were shown to decrease less apparently than in dry air, which mainly resulted from the low partial pressure of oxygen in combustion atmosphere. C 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon fibers; A. Silicon-carbide-matrix composites: B Oxidation; E. Chemical vapor infiltration; Combustion environments 1. Introduction 2. Experimental procedure Carbon/silicon carbide fibrous composites have been The composites used in the present study consisted of designed and developed for high-temperature structural SiC matrix reinforced three direction woven fiber arch applications such as engines and reentry thermal pro- tecture of carbon fibers(Hr T300, Toray). The fiber tection for spacecraft [1, 2]. The SiC matrix, used also in preform was first consolidated by some pyrocarbon (i.e the present study as an external sealing coating, theore- the interphase) formed from the in-pore cracking of tically protects the carbon fibers and their coating from C3 Hs, whose mean thickness was about 0.1 um, and oxidation. In fact, the high value of the fiber/matrix then infiltrated by Sic matrix formed in situ from thermal expansion coefficient mismatch in the C-fiber/ CH3 SiCl3 /H2, at about 1000 C, both treatments being Sic-matrix system already induced microcracks in both used according to the low-pressure chemical vapo the matrix and the seal-coating upon cooling from the infiltration process (LCvI)(P=0.01 MPa). These pro- processing temperature. Besides, restrained by the pre- cessing steps resulted in a material having a density paration process, a large amount of pores existed in the close to 2 g/cm, a fiber content of approximately 40 CVI SiC matrix. Both of the microcracks and the pores vol % and a residual porosity in the range 15-20% can render the highly reactive carbon constituents more Machining of the samples used in this study left uncoated accessible to oxygen in oxidizing environments [3, 4. By fibers on both sides of the specimens. Consequently, microstructural analysis, the reason for the mass loss of once cut at their final dimensions and prior to testing, ne 3D C/Sic composites exposed in the flame of a these samples were coated with a CVD-processed SiC burner rig was investigated in the present paper. The layer of 25 um in mean thickness different oxidation behaviors of 3D C/SiC(CvI)in two Two oxidizing environments were investigated: a static different oxidizing environments were also studied system (box furnace)and an open dynamic system (atmospheric-pressure burner rig). The burner rig facility 4 Corresponding author. Tel: +86-29-8491427; fax: +86-29. as designed with a high-velocity flame, in which the 8491000. partial pressures of O2 and H2O in the oxidation pro- ducts of jet fuel were 8180 and 13 500 Pa, respectively 0266-3538/01/ S.see front matter C 2001 Elsevier Science Ltd. All rights reserved. PII:S0266-3538(00)00190
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x. Yin et al. Composites Science and Technology 61(2001)977-980 The masses of the samples before and after oxidation 2.3x 10-6/k, corresponding to pure SiC, B is parabolic were determined by analyzing scale(e=0. 1 mg). Flexural rate constant of Sic and is bigger than 181 nm /min in strength was measured using the three-point-bending combustion environments at 1250C [6,7]. At room method. The span dimension was 20 mm, and the loading temperature, the maximum width of the microcracks rate was 0.5 mm. min. The tests were conducted on an was about 0.4 um. It is reasonable to assume that the Instron-1195 device at room temperature. The fracture width of the microcracks can be reduced to less than 0.2 surface was observed with a scanning electron micro- um when temperature is raised to 1250C [3]. Therefore, scope(SEM), ( JEOL 840) it can be known from Eq. (1) that tc is less than 2.9 h. The relative mass loss of carbon fibers however still increased apparently during the succeeding oxidation 3. Results and discussion process, which indicated that the occurrence of micro- cracks was not the major reason for the oxidation of 3.I. Mass loss as a function of time for C/Sic composite carbon phase in combustion gas In fact, restrained by the braiding method of fiber preform and CVI process, there exist a lot of residual As shown in Fig. 1, the relative mass loss of C/Sic pores in the interior of 3D C/SiC composites, as shown exposed in burner rig increases with the duration of in Fig 3. Parts of these pores are thought to be inter time, which indicates that gaseous products are pro- connected. The oxidizing gases may diffuse along these duced during oxidation and the amount increases with pores inwards the interior of the composites. The Sic the duration of time. Moreover, the mass loss of the matrix is thinner(about I um in thickness )in the areas samples in the first three hours of oxidation increases where there are a lot of pores, which render the highly more apparently than in the ensuing 6 h reactive carbon constituents more accessible to oxygen Owing to the fiber/matrix thermal expansion coeffi- in oxidizing environments. As shown in Fig. 3(a) and cient mismatch. there exist some microcracks in the (b), carbon fibers were oxidized locally and even con- Sic composites(Fig. 2). Microcracks allowed oxygen to sumed completely in the areas where the density of diffuse in depth and to react with the carbonaceous pores was higher. In the first 3 h, the microcracks and phases. The microcracks are very narrow at 1250@C and pores took effect on the oxidation of C/Sic together the width of microcracks decreases with the increase of which, however, was affected only by the pores there oxidation time, due to the thickening of SiO2 film after. Therefore, the oxidation rate of the C/Sic com- formed on the surface of microcrack wall, which leads posites in the last six hours was lower than in the first 3 to the reduction of the oxidation rate of carbon fibers h The time necessary for the sealing of microcracks(tc) 3. 2. Different oxidation behaviors of C/Sic composites can be expressed as the following equation, deduced in combustion gas and in air from Ref [6] As shown in Table 1, the mass loss of C/SiC in com- 1「H0 tc (1) bustion environments is lower than in dry air.The mechanical properties of C/SiC decrease noticeably after oxidized in combustion gas. After oxidized in dry where h is the width of microcracks the thermal expansion coefficient of the matrix (0) is about 15 Time(b)6 841229KUX2,989188 Fig 1. Relative mass variations as a function of time for the C/sic composites in the combustion environment. Fig. 2. Microcracks on the surface of C/Sic composites
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x. Yin et al. Composites Science and Technology 61(2001)977-980 x2, 880 Fig 3. SEM fractographs of a burner rig C/SiC sample, showing:(a)the carbon fibers oxidizing and(b) the carbon fibers fully depleted. sI, however, the mechanical properties decrease drasti- oxygen in oxidizing atmosphere takes a main effect on cally. The main result to note in Fig. 4(a) is the very the oxidation of carbon. The decrease of oxygen partial fibrous nature of the failure. Fig. 4(b)illustrates the flat pressure will lead to the reducing of oxidation rate of and brittle fractures and the degradation of C fibers is in carbon fibers, described by the following expression [3] a large area of the sample. The above observation agrees with the indication in Table i that the oxidation k(po,)=Ipo,"k/(latm) rate of carbon fibers in combustion environments is lower than in dry air where k is the linear oxidation rate of carbon fibers, Po As we know, the reaction energy of H2O with C is is the oxygen partial pressure in gaseous environment, much higher than that of O2 with C, that is to say, the and q is the apparent reaction order with respect to oxygen(q >0). Since the oxygen partial pressure in the combustion atmosphere was much lower than in dry air, results of C/Sic composites before and after oxidation at the degradation of carbon fibers in the former was not or 3 h as serious as in the latter Relatio On the other hand, oxygen diffuses rapidly in the Flexural variations(%) strength pores of the composites exposed in the combustion (MPa)(KJm2, atmosphere, and the global reactivity of carbon phases is lowered by the decrease of oxygen partial pressure. Therefore, a more homogeneous degradation of carbon Burner rig(O2, H:O) -8.73 203.5 Box furnace(dry air) -9.92 1350 phases may take place in combustion atmosphere which is consistent with the content in Fig 4(a). 15kv 500um CSCK Fig 4. SEM fractographs of C/Sic samples oxidized at 1250C for 3h in:(a) combustion gas and(b)dry air, showing different fiber pullout in dif- ferent oxidizing atmospheres
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x. Yin et al. Composites Science and Technology 61(2001)977-980 4. Conclusions Scientific Foundation of the Peoples Republic of China Studies on the oxidation behaviors of 3D C/SiC composit ed in combustion gas and dry air References respectively have shown the following results [ Berton B, Bacos MP, Demange D, Lahaye J. High temperature behavior of the hot structure materials of Hermes Space Shuttle. 1. Weight loss of the 3D C/SiC composite is related In: Naslain R, Lamalle J, Zulian JL, editors. Composite materials to the pores and microcracks in the CVI Sic matrix and d the pores are proved to play a key role in understanding the effect of environment on the 2 Cavalier JC, Lacombe A, Rouges JM. Ceramic matrix compo oxidation behavior of the composites Massiah A editors. Developments in the science and technology 2. Mechanical properties of the C/SiC samples oxi- of composite materials. London: Eisevier, 1989.p 99-110 dized in combustion gas decrease less apparently 3 Lamouroux F, Camus G. Kinetics and mechanisms of oxidation han in dry air, which mainly result from the low of 2D woven C/SiC composites: I, experimental approach. J Am partial pressure of oxygen in combustion atmo- CeramSoc I994;77(8):204957 (4 Lamouroux F, Bourrat x, Sevely J, Naslain R. Structure/ oxida- sphere tion behavior relations in the carboneous constituents of 2Dc (T300)/P, C/SiC(CVI) composites. Carbon 1993: 31(8): 1273-88 Nathan S Jacobson. corrosion of silicon-based ceramics in com- Acknowledgements bustion environments. J Am Ceram Soc 1993: 76(1): 3-28. [6 Huger M, Fargeot D, Gault C. Ultrasonic characterization of This work was supported by National Natural Scien xidation mechanisms in nicalon/C/SiC composites. J Am Ceram Soc1994:77(10):255460. ific Foundation of the People's Republic of China [7 Pila EJ. Oxidation kinetics of chemically vapor-deposited sili- under the Contract No. 59772030 and Nation Defense con carbide in wet oxygen. J Am Ceram Soc 1994: 77(3): 730-6
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