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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