NEW CARBON MATERIALS Availableonlineatwww.sciencedirect.com Volume 24 Issue 2 June 2009 Online English edition of the Chinese language journal Sciencedirect ite this article as: New carbon materials 2009. 24(2): 173-177 RESEARCH The influence of high temperature exposure to air on the damage to 3D-C/Sic composites HOU Jun-tao, QIAO Sheng- ru*, ZHANG Cheng-yu, ZHANG Yue-bingt Uitra-High-Temperature Structural Composite Laboratory, Northwestem Polytechnical University, Xi an 710072, China; National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, China Abstract: 3D-C/SiC composites, exposed in air at 600, 900, and 1 300C for 0 to 15 h, were investigated by three point bend tests at oom temperature, SEM, and energy dispersive spectroscopy. The results show that the damage curves, expressed as a relative change of elastic modulus, of the composites for a 15 h exposure, could be divided into a sharply increasing stage(stage I)and a steady increasing stage(stage II). Stage I may be caused by a direct oxidation of the carbon fibers and interface carbon layers by the oxygen in air, and stage Il may be caused by a diffuse controlled oxidation of the inner part of the composites. The matrix micro-cracks, induced by a dif- ference of coefficients of thermal expansion between matrix and carbon fibers in the cooling process after composite preparation act as oxygen diffuse paths and are where the oxidation takes place. The fact that the damage decreases with temperature for the same exposure time may be caused by the crack shrinking at high temperature, which decreases the oxidizable surface area and inhibits the diffusion of oxygen into the composites. Key Words: 3D-C/SiC, Thermo-exposure; Damage; Flexural behaviors 1 Introductlon ume fraction of carbon fibers of 40-45%, a porosity of about 17% volume ratio, a density of about 2.0 g/cm3, a pyrolytic Ceramic matrix composites(CMCs), including ca carbon layer thickness of about 200 nm, and an SiC oxIdation bon/silicon carbide(C/SiC)composites, are promising for use barrier coating layer of about 50 um. The specimens were as high temperature structural materials. This kind of materials machined to have a dimension of 50 mmx5 mmx35mm for have a high strength to density ratio and a high temperature the test performance over conventional superalloys in order that little or no cooling is required-31 2.2 Test procedure During the services of C/SiC composites, mechanical After vacuum fatigue, the residual strength of a damage can be caused by an applied stress, which decreases 3D-C/Sic composite does not decrease; however, it increase their modulus- On the other hand, chemical damage may slightly. Therefore, residual strength is not suitable for take place in the high temperature environment, also causing a decrease of their modulus. Up to now, there are few damage evolution after a thermo-exposure at high temperature In this study, the flexural strength and damage evolution of a three-dimensional C/SiC composite (3D-C/SiC) afte thermo-exposure are investigated preliminarily. 2 Materlals and procedure 2.1 Materials The preforms for the 3D-C/SiC composites were woven by a three-dimensional braiding technique, in which T300 carbon fiber bundles were used as weaving yarns and a braid ng angle was 22 as shown in Fig. 1. Pyrolytic carbon layer and SiC matrix were deposited by chemical vapor infiltration Fig 1 Schematic of the structural cell for a 3D-C/SiC composite (CVI)at 900-1 000C. The 3D-C/SiC composite had a vol orm Copyrighto2009, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DO:Io.lol61872-5805(0860046-3NEW CARBON MATERIALS Volume 24, Issue 2, June 2009 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2009, 24(2):173–177. Received date: 8 April 2008; Revised date: 2 April 2009 *Corresponding author. E-mail: blao@nwpu.edu.cn Copyright©2009, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-5805(08)60046-3 RESEARCH PAPER The influence of high temperature exposure to air on the damage to 3D-C/SiC composites HOU Jun-tao1 , QIAO Sheng-ru1 *, ZHANG Cheng-yu1 , ZHANG Yue-bing2 1 Ultra-High-Temperature Structural Composite Laboratory, Northwestern Polytechnical University, Xi’an 710072,China; 2 National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, China Abstract: 3D-C/SiC composites, exposed in air at 600, 900, and 1 300 °C for 0 to 15 h, were investigated by three point bend tests at room temperature, SEM, and energy dispersive spectroscopy. The results show that the damage curves, expressed as a relative change of elastic modulus, of the composites for a 15 h exposure, could be divided into a sharply increasing stage (stage I) and a steady increasing stage (stage II). Stage I may be caused by a direct oxidation of the carbon fibers and interface carbon layers by the oxygen in air, and stage II may be caused by a diffuse controlled oxidation of the inner part of the composites. The matrix micro-cracks, induced by a dif￾ference of coefficients of thermal expansion between matrix and carbon fibers in the cooling process after composite preparation act as oxygen diffuse paths and are where the oxidation takes place. The fact that the damage decreases with temperature for the same exposure time may be caused by the crack shrinking at high temperature, which decreases the oxidizable surface area and inhibits the diffusion of oxygen into the composites. Key Words: 3D-C/SiC; Thermo-exposure; Damage; Flexural behaviors 1 Introduction Ceramic matrix composites (CMCs), including car￾bon/silicon carbide (C/SiC) composites, are promising for use as high temperature structural materials. This kind of materials have a high strength to density ratio and a high temperature performance over conventional superalloys in order that little or no cooling is required[1-3]. During the services of C/SiC composites, mechanical damage can be caused by an applied stress, which decreases their modulus[4-7]. On the other hand, chemical damage may take place in the high temperature environment, also causing a decrease of their modulus. Up to now, there are few reports on damage evolution after a thermo-exposure at high temperature. In this study, the flexural strength and damage evolution of a three-dimensional C/SiC composite (3D-C/SiC) after thermo-exposure are investigated preliminarily. 2 Materials and procedure 2.1 Materials The preforms for the 3D-C/SiC composites were woven by a three-dimensional braiding technique, in which T300 carbon fiber bundles were used as weaving yarns and a braid￾ing angle was 22º as shown in Fig.1. Pyrolytic carbon layer and SiC matrix were deposited by chemical vapor infiltration (CVI) at 900-1 000 °C. The 3D-C/SiC composite had a vol￾ume fraction of carbon fibers of 40-45%, a porosity of about 17 % volume ratio， a density of about 2.0 g/cm3, a pyrolytic carbon layer thickness of about 200 nm, and an SiC oxidation barrier coating layer of about 50 µm. The specimens were machined to have a dimension of 50 mm×5 mm×3.5mm for the test. 2.2 Test procedure After vacuum fatigue ， the residual strength of a 3D-C/SiC composite does not decrease; however, it increases slightly. Therefore, residual strength is not suitable for Fig.1 Schematic of the structural cell for a 3D-C/SiC composite perform