C composites prepared by a RCLD method

International Journal of Minerals, Metallurgy and Materials Volume 16, Number 3, June 2009, Page 334

Materials

Microstructure and properties of SiC gradiently coated Cf/C composites prepared by a RCLD method Jun-hua Chen1), Guang-li Chen2), Hao-ran Geng2), and Yan Wang2) 1) School of Materials Science and Engineering, Shandong University, Jinan 250061, China 2) School of Materials Science and Engineering, University of Jinan, Jinan 250022, China (Received 2008-06-07)

Abstract: The SiC gradiently coated carbon fiber/carbon (Cf/C) composites were prepared by a two-step rapid chemical liquid deposition (RCLD) method. The microstructure and properties of the composites were investigated using X-ray diffraction, scanning electron microscopy together with energy dispersive X-ray analysis, bending tests, and oxidation tests. The experimental results show that the surface layer of the composites is composed of SiC, pyrocarbon, and carbon fibers. Their inner area consists of pyrocarbon and carbon fibers. The SiC content gradiently decreases with increasing distance from the outer surface to the center of the composites. Furthermore, the thickness of the SiC layer increases with increasing tetraethylorthosilicate content and deposition time. SiC coatings have no significant influence on the bending strength of the composites. However, the oxidation resistance of the composites increases with increasing thickness of the SiC layer. Key words: carbon fiber reinforced composites; chemical liquid deposition; bending strength; oxidation resistance

[This work was financially supported by the National Natural Science Foundation of China (No.50371047).]

1. Introduction Carbon materials are promising structural candidates for high temperature applications. They exhibit a superior thermo-mechanical behavior at temperatures above 2000ºCin the absence of oxygen. However, their applications have been restricted to an inert atmosphere because of the oxidation above 500ºC [1-3]. Therefore, the improvement of the oxidation resistance of carbon materials is a key issue for their applications at high temperature. Many studies have been performed to improve the oxidation resistance of carbon materials over past decades [4-9]. Antioxidation additives such as Al, Si, and SiC are introduced in carbon-containing refractory ceramics [5-6]. Carbon materials can also be coated with refractory ceramics such as SiC, Si3N4, or oxide glass layer [8-9]. In addition, carbon fiber reinforced SiC composites have also been developed [7]. SiC has excellent high-temperature oxidation resistance and chemical compatibility with carbon. SiC coatings are usually prepared by chemical vapor deposition (CVD) processes but tend Corresponding author: Hao-ran Geng, E-mail: [email protected] © 2009 University of Science and Technology Beijing. All rights reserved.

to peel off from the substrate due to high thermal stress and poor adhesion. Although the surface treatment of carbon fiber/carbon (Cf/C) composites is adopted, the degradation of mechanical properties and high fabrication cost limit their engineering applications [10-12]. In our former work, Cf/SiC composites have been produced by a rapid chemical liquid deposition (RCLD) method [13]. This method is easily controlled with high yield rate and low cost. The present work aimed to prepare SiC gradiently coated Cf/C composites by using this RCLD method. The microstructure and properties of the composites have also been investigated.

2. Experimental 2.1. Preparation of Cf/C composites Carbon fibers (T-300, Japan Toray) with a diameter of about 7 μm were used in this work. The surface of the carbon fibers was pretreated by an electrochemical method. 30 bundles of the fibers were coated using Also available online at www.sciencedirect.com

J.H. Chen et al., Microstructure and properties of SiC gradiently coated Cf/C composites prepared by a RCLD method

resin and then treated in a furnace at 300ºCfor 30 min. A rigid precursor was obtained owing to the solidification and hardening effect of the resin. The RCLD apparatus is schematically shown in Fig. 1. The rigid precursor was fixed on a heating rod and was dipped into kerosene. The precursor was deposited in the reaction furnace with a pressure of 0.1 MPa at 900ºCfor 5-6 h. The Cf/C composites with a porous surface were then obtained, and the bulk density of the composites was 1.63 g/cm3.

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deposition time) are listed in Table 1. The bulk density of the composites was 1.9 g/cm3. The microstructure of the composites was characterized using a scanning electron microscope (SEM, Hitachi S2500) together with an energy dispersive X-ray analyzer (EDX). The phases in the composites were identified by an X-ray diffractometer (XRD, Rigaku D/max-Rb).

2.2. Formation of SiC gradiently coated Cf/C composites The low density Cf/C composite sample was fixed on the heating rod and dipped into a mixture of kerosene and tetraethylorthosilicate (TEOS). The sample was heated in the furnace with a pressure of 0.1 MPa at 1200ºC for different durations. 6 SiC coated Cf/C composite samples (S1-S6) were prepared and the corresponding process parameters (TEOS contents and Table 1.

Process parameters and bending strength of the SiC gradiently coated Cf/C composites (S1-S6) and Cf/C composite Parameter

S1

S2

S3

S4

S5

S6

Cf/C composite

TEOS content / wt% Deposition time / h Bending strength / MPa

20 2 130.17

20 4 141.74

30 2 133.06

30 4 146.30

40 2 133.21

40 4 151.60

— — 118.10

2.3. Bending strength tests and oxidation tests The specimens with dimensions of 30 mm×5 mm×5 mm were cut from the full-sized samples. The bending strength of the composites was measured by the three-point bending test [2, 14] using a universal testing machine (Instron 5569). The flexural strength (Vf) was calculated using the following equation:

Vf

Fig. 1. Schematic representation of the RCLD apparatus: 1—control system; 2—smoke pipe; 3—thermocouple; 4—gathering system; 5—floodlight; 6—heating system; 7—protection system; 8—water tank; 9—cooling system; 10—precursor; 11—furnace.

3PL 2bh 2

(1)

where P is the loading pressure, L the span of the bending test, b the width, and h the thickness of the specimen. 5 measurements were carried out, and the average value was taken for each sample. The oxidation tests were performed to evaluate the oxidation resistance of the composites. The specimens (I8 mm×10 mm) were put into a furnace and held at 400, 500, 600, 700, 800, and 900ºC for 5, 10, and 20 min. The oxided specimens were weighed, and the mass losses were calculated.

3. Results and discussion Fig. 2(a) shows the cross-section microstructure of the SiC gradiently coated Cf/C composite (S6). The corresponding EDX area analysis of C and Si are shown in Figs. 2(b) and 2(c), respectively. It can be

seen from Fig. 2(b) that the inner area of the sample is C-rich and the outer layer is C-poor. In contrast, the inner area of the sample is Si-poor and the outer layer is Si-rich shown in Fig. 2(c). The XRD result shows that the sample comprises two phases: carbon and SiC, as shown in Fig. 3. The XRD result in combination with EDX analysis confirms that the composite is coated with a SiC layer. The SiC layer is about 1 mm in thickness. Moreover, there exists an obvious interface between the Si-rich and C-rich layers, as highlighted by an arrow in Fig. 2(a). The high magnification microstructure of the composite (S6) is shown in Fig. 4. The outer layer of the sample is composed of carbon fibers, pyrocarbon, and SiC as marked by three arrows in Fig. 4(a). Carbon fibers are coated by a thin pyrocarbon layer and SiC fills in the interstice of the bundles of carbon fibers. This can be verified by the EDX result as shown in Fig. 5. The inner region of the composite only consists of two phases: carbon fibers and pyrocarbon as shown in Fig. 4(b). Furthermore, the SEM observation shows that the content of SiC decreases and the number of carbon fibers increases with increasing distance from the outer surface to the center of the sample, indicating that the Cf/C composites are gradiently coated by SiC and good interface adhesion can be achieved, as clearly seen in Figs. 2 and 4.

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Fig. 3.

XRD pattern of the composite S6.

Fig. 4. SEM images showing the microstructure of the outer layer (a) and inner area (b) of the composite (S6). Fig. 2. SEM image showing the cross-section microstructure of the SiC gradiently coated Cf/C composite S6 (a) and corresponding EDX area analysis of C (b) and Si (c).

The examination of all other 5 samples (S1-S5) demonstrated that the phase constitution of all the 6 samples was identical and the microstructure was similar. In addition, the thickness of the SiC layer increased with increasing deposition time and TEOS contents. In this work, a two-step deposition method was used to prepare the SiC gradiently coated Cf/C composites. In the first step, the Cf/C composites with a porous surface were produced. In the second step, the TEOS entered the porous surface and assisted to form a SiC layer.

Fig. 5.

EDX point analysis of position 3 in Fig. 4(a).

As listed in Table 1, the bending strength of SiC gradiently coated Cf/C composites is almost the same as that of Cf/C composites without coatings. This indicates that SiC coatings have no obvious influence on the bending strength of Cf/C composites. In addition,

J.H. Chen et al., Microstructure and properties of SiC gradiently coated Cf/C composites prepared by a RCLD method

the process parameters including TEOS content and deposition time have no marked effect on the bending strength of SiC gradiently coated Cf/C composites. The mechanical properties of the composites are mainly dependent upon Cf/C composites prepared by the first step. Fig. 6 shows the temperature dependence of relative mass losses of SiC gradiently coated Cf/C composites. It can be seen that the relative mass losses are almost maintained at zero with increasing temperature up to 600ºC for all the samples. The relative mass losses markedly increase with further increasing temperature owing to the oxidation reaction of carbon in the composites. It should be noted that the S6 sample is stable up to 800ºC, indicating the excellent antioxidation property due to its thicker SiC layer.

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rate of mass losses decreases. This indicates that deposition time has a significant effect on the oxidation resistance of SiC gradiently coated Cf/C composites at higher temperatures. At the beginning of oxidation, pyrocarbon in the surface of the composites reacts with oxygen and the relative content of SiC increases. Therefore, oxidation would be blocked off by SiC [15].

4. Conclusion SiC gradiently coated Cf/C composites have been successfully prepared by a two-step RCLD method. Moreover, the thickness of the SiC layer increases with increasing TEOS content and deposition time. The surface layer of SiC gradiently coated Cf/C composite is composed of SiC, pyrocarbon, and carbon fibers. The inner area of the composites consists of pyrocarbon and carbon fibers. There is an obvious interface between the surface layer and the inner area. Furthermore, the SiC content gradiently decreases with increasing distance from the outer surface to the center of the composites. The SiC coatings have no significant influence on the bending strength of Cf/C composites. However, the oxidation resistance of SiC gradiently coated Cf/C composites increases with increasing thickness of the SiC layer.

References Fig. 6. Temperature dependence of the relative mass losses of SiC gradiently coated Cf/C composites (The holding time at each temperature is 20 min).

Fig. 7 shows the relation between relative mass losses and different oxidation time at 900ºC. The mass

Fig. 7. Correlation between relative mass losses and oxidation time at 900ºCfor SiC gradiently coated Cf/C composites.

losses first increase rapidly with time and then change a little with prolonging time for the samples S2, S4, and S6. For the samples S1, S3, and S5, the mass losses continuously increase with prolonging time, but the

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