SiC composites

NEW 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.

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 difference 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 carbon/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

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

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 braiding 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-

Fig.1 Schematic of the structural cell for a 3D-C/SiC composite perform

Received date: 8 April 2008; Revised date: 2 April 2009 *Corresponding author. E-mail: [email protected] 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

HOU Jun-tao et al. / New Carbon Materials, 2009, 24(2): 173–177

stage II. The time of stage I becomes shorter and the DE value in stage I becomes smaller with exposure temperature. The DE value decreases with exposure temperature. Stage I may be a result of a direct oxidation of carbon fibers and interface carbon layers by the oxygen in air while stage II corresponds to a diffuse controlled oxidation of the inner part of the composites.

Fig.2

DE vs. time after the thermo-exposure at 600 °C, 900 °C and 1 300 °C in air

determining the damage degree of a 3D-C/SiC composite. The damage value may be above 1 if a relative change of electric resistance is used to evaluate the damage of a 3D-C/SiC composite. The damage evaluated by a relative change of elastic modulus is more reasonable and accurate. The 3D-C/SiC specimens were exposed at 600, 900, and 1 300 °C in air for different times (0, 0.5, 1, 1.5, 2, 2.5, 3.5, 5, 6.5, 8.5, 10, 11.5, 13.5, and 5 h). The elastic modulus was investigated using a three-point bending method under a small load. The test was carried out on a specimen at the same loading direction, the same supporting point, and the same loading point to obtain accurate data. The bending span was 40 mm and the loading rate was 0.3 mm/min. The specimens exposed at 1 300 °C for 15 h in air were bend to cause fracture at 1 300 °C in air. In addition, the fractured surface morphologies of the 3D-C/SiC specimens after exposure at 600, 900, and 1300 °C for 6 h were observed under a scanning electron microscope (SEM).

3 3.1

Results and discussion Damage evaluated by an elastic modulus change

The damage DE of a 3D-C/SiC composite after a thermo-exposure can be determined by elastic modulus as follows.

DE = 1 −

E′ E0

（1）

where, E ′ is the elastic modulus after thermo-exposure and E0 is the initial elastic modulus. Fig.2 shows the damage curves of DE versus time after the thermo-exposure at 600, 900, and 1300 °C in air for the 3D-C/SiC composites. It can be found that the sample exposed at 600 °C for 15 h has the largest DE of about 80%. The samples exposed at 900 and 1 300 °C have the DE value of about 38% and 16%, respectively. It should be noted that the damage curves can be divided into a sharp increasing stage (stage I ) and a steady increasing stage (stage II); stage I precedes

The coefficients of thermal expansion (CTE) of matrix and carbon fibers in the 3D-C/SiC composites are different. The longitudinal CTE of T300 carbon fibers is about 1×10-7/°C whereas that of the CVI SiC layers is 4.8×10-6/°C [8]. As the manufacturing temperature of the 3D-C/SiC composites is about 900-1 000 °C, the inner stress induced by the CTE difference between the matrix and carbon fibers leads to the formation of matrix micro-cracks in the composites when the composites are cooled down to room temperature. Similar observation has been found by Lamouroux[9] and Cheng[10]. The carbon fibers and the pyrolytic carbon interface layer can be oxidized above 400 °C. At a temperature between 400 and 600 °C, the SiC oxidation barrier coating layers cannot be oxidized by oxygen. Therefore, the matrix micro-cracks are the path of entrance for oxygen into the composites as well as the path of exit for oxide gases out of the composites. The matrix micro-cracks begin to be clogged as a result of thermal expansion of the matrix at 900 °C, which makes the diffusion of oxygen into the composites difficult. Therefore, the oxidation reactions take place mainly at the gaps in the fine fiber bundles and the damage is smaller at 900 °C than at 600 °C under the same thermal exposure times. The matrix micro-cracks are completely clogged at 1 300 °C and the SiC matrix begins to be oxidized to generate SiO2, which has fluidity similar to glass at this temperature and can seal the clogged matrix cracks. Thus, the oxidation damage at 1 300 °C is the lowest. 3.2

Fractured surface morphologies

Fig.3 shows the SEM images and energy dispersive spectra of the specimens after thermo-exposure at different temperatures for 6 h. After thermo-exposure for 6 h, the gaps appear obvious between carbon fibers and matrix. At 1 300 °C, the color of matrix around the fibers seems to be brighter than that of the matrix elsewhere. It can be found in the energy spectra that the silica contents vary slightly and the carbon contents drop clearly with temperature. However, the oxygen contents increase sharply only at 1 300 °C. The appearance of the gaps between fibers and matrix indicates that the oxidation of carbon interface layer and parts of fibers occurs. The oxidation activation energy of the carbon fibers is above 30 kcal /mol and that of the carbon interface layer is 26 kcal/mol[11-12]. In addition, the bonding of SiC matrix and pyrolytic carbon interface layer is weaker than that of carbon fibers and pyrolytic carbon interface layer. As a result, the matrix micro-cracks induced by residual stress exist mainly in the area between the SiC matrix and the pyrolytic carbon interface layer. Thus, the oxidation of the 3D-C/SiC

DU Gui-xiang et al. / New Carbon Materials, 2009, 24(1): 331–338

Fig.3 SEM morphology and energy spectrum after thermo-exposure at (a) 600 °C, ( b) 900 °C, and (c) 1 300 °C for 6h

composites begins first on the interface between carbon fibers and pyrolytic carbon and then expands into the pyrolytic carbon layer instead of carbon fibers. The carbon contents decrease with temperature as a result of the oxidation as revealed by the energy dispersive spectra. The high oxygen content at 1 300 °C demonstrates that SiO2 is formed, which can seal the matrix micro-cracks and prevent oxygen from diffusing into the composites. 3.3

Flexural strength and fracture strain

Table 1 lists the change of mechanical strength of sample after exposure at 1 300 °C for 15h in air. It can be found that the fracture strain of the 3D-C/SiC composite exposed at 1 300 °C for 15 h in air is only about 1/40 of the initial value and the flexural strength drops to about 44% of the initial value. The degradation of these two properties is far larger than that of the elastic modulus. The volume fractions of carbon fibers and pyrolytic carbon interface layer decrease after thermo-exposure, which indicates that the effective area to load burden is decreased and the porosity is increased. As described in the references[13-14], the increase of porosity can lead to a decrease of the elastic modulus and the flexural strength in brittle materials.

Table 1

The change of mechanical strength of the sample after exposure at 1 300 °C for 15 h in air

Percentage retained

3.4

Elastic

Flexural

Fracture

modulus

strength

strain

72%

44%

2.5%

Life prediction

The critical value of DE is about 0.29 for 3D-C/SiC composites[4], above which the composite damage is obvious and the bearing capacity decreases sharply. In engineering, the relative change of elastic modulus is generally below 10%, i.e., the value of DE is below 0.1. The damage curve was fitted by a cubic equation for each temperature using the multiple regression method. The fitted equation is shown as follows. DE= -0.00004t3 - 0.0003 t2 + 0.0633t ;

(2)

900 °C,

DE= 0.0002 t3 - 0.0049 t2+ 0.0498t ;

(3)

1 300 °C,

DE= 0.0001 t3 - 0.0025 t2 + 0.0237t .

(4)

600 °C,

The life times corresponding to the DE value of 0.1 and 0.29 were evaluated by the fitted equations at each temperature and are listed in Table 2.

HOU Jun-tao et al. / New Carbon Materials, 2009, 24(2): 173–177

Table 2

Life time of the 3D-C/SiC composites exposed at 600 °C, 900 °C, and 1 300 °C in air at DE of 0.1 and 0.29

Damage time

4

600 °C

900 °C

1 300 °C

DE=0.1

1.6 h

3.3 h

10.2 h

DE=0.29

4.8 h

11.3 h

>15 h

Conclusions

The damage curves of the 3D- C/SiC composites exposed at different temperatures in air can be divided into a sharp increasing stage (stage I) and a steady increasing stage (stage II); stage I precedes stage II. The damage evaluated by the elastic modulus decreases with temperature under the same exposure temperature. The fracture strain and flexural strength decrease to 2.5% and 44% of the initial value after thermo-exposure at 1 300 °C for 15 h, whose degradations are considerably more obvious than that of the elastic modulus.

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