SiC composites fabricated by polymer infiltration and pyrolysis

NEW CARBON MATERIALS Volume 22, Issue 4, December 2007 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2007, 22(4): 327–331.

RESEARCH PAPER

Correlation of PyC/SiC interphase to the mechanical properties of 3D HTA C/SiC composites fabricated by polymer infiltration and pyrolysis ZHU Yun-zhou1,2, HUANG Zheng-ren1,*, DONG Shao-ming1, YUAN ming1,2, JIANG Dong-liang1 1

Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China;

2

Graduate School of Chinese Academy of Sciences, Beijing 100049, China

Abstract: 3D braided carbon fiber preforms were used to fabricate C/SiC composites using polymer infiltration and pyrolysis (PIP). Before the PIP, the preforms were coated by isothermal chemical vapor infiltration with methane to produce pyrocarbon (PyC) and then with hexamethyldisilazane to form SiC and produce the PyC/SiC interphase. The correlation of the PyC/SiC interphase to the microstructure and the mechanical properties of the fabricated composites was investigated. The flexural properties of the composites were measured using the three-point-bend test, and the fracture surfaces observed by SEM. The bend strengths were 247 MPa and 46 MPa, with and without the PyC/SiC interphase respectively. Long fiber pullout dominated the fracture surface for the composite with the PyC/SiC interphase, while for the one without the interphase, almost no fiber pullout was observed. Key Words: C/SiC composites; Interphase; Mechanical properties; PIP process

1

Introduction

Continuous fiber reinforced ceramic matrix composites (CFCCs) including C/C, C/SiC, and SiC/SiC have been widely recognized as the most promising high-temperature structural materials for gas turbine, brake disks, and first wall in fusion reactor etc, owing to their high toughness, low density, thermal and chemical stability, radiation tolerance, and so on[1–4]. When compared to C/C composites, C/SiC composites exhibit high mechanical properties and oxidation resistance[5] and are potential substitutes for C/C composites. These composites can be prepared by chemical vapor infiltration (CVI), reaction bonding (RB), and hot pressing (HP) [6, 7]. In recent years, with the development of high efficient preceramic polymer, the polymer infiltration and pyrolysis (PIP) method has aroused interest for an in-situ fabrication of such composites. Using this method, the composites with complex shape can also be easily fabricated at a relatively lower temperature. However, a large amount of active organic species will be released during the pyrolysis of the preceramic polymer. The strength of the reinforcing carbon fibers is sensitive to the pyrolysis. To prevent the adhering of carbon fibers to the stiff matrix, a layer of PyC was first deposited on the fiber surface. Subsequently, a layer of SiC was deposited around the PyC to prevent further chemical damage to the PyC and the carbon fibers in pyrolysis.

The objective of this study is to correlate the PyC/SiC interphase to the microstructure and the mechanical properties of 3D C/SiC composites fabricated by PIP, using HTA (Toho Tenax Co., Japan) carbon fibers as reinforcement.

2

Experimental

2.1

Sample preparation

3D braided C fiber preforms used as reinforcement for C/SiC composite fabrication in this article were braided by a four-step processing, which were supplied by Nanjing Glassfiber Research and Design Institute (Nanjing, China). The fiber volume fraction in the preforms was about 40%. The reinforcing carbon fibers were HTA carbon fibers. The typical parameters of the HTA carbon fibers are listed in Table 1. Table 1 Properties of HTA C fiber Diameter

Density

Filaments

Tensile strength

Elastic modulus

D/µm

ρ/g•cm–3

yarn/n

σ/MPa

E/GPa

7

1.76

3000

3920

235

Prior to the slurry infiltration process, the preforms were deposited with pyrocarbon (PyC) according to the following equation: CH4(g) → C(s) + 2H2(g)↑ (1) Methane (CH4) was used as carbon precursor under a pressure of 10 kPa with Ar as dilute gas by isothermal chemical vapor

Received date: 2006-10-28; Revised date: 2007-12-03 Foundation item: National 973 Programme *Corresponding author. Tel:+86-21-52414323, E-mail: [email protected] Copyright © 2007, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

ZHU Yun-zhou et al. / New Carbon Materials, 2007, 22(4): 327–331

infiltration (ICVI). More details about the CVI apparatus can be found in literature[8]. The flow rates of CH4 and Ar were 20 mL/min and 100 mL/min, respectively. The thickness of the PyC coating was controlled at about 200 nm. A new precursor hexamethyldisilazane (HMDS) was selected for the deposition of the SiC phase around PyC with H2 as both dilute and carrying gas according to equation 2. The deposited thickness was estimated to be 1–2 µm. NH[Si(CH3)3]2(l) → SiC(s)+C(s)+H2(g)↑+NH3(g)↑ (2)

Subsequently, the infiltration and pyrolysis were repeated five times using PCS as the precursor without filler. 2.2

The density of each sample was measured using the Archimedes method. The flexural strength was obtained by the three-point-bend test on the Instron 5566 universal testing machine, with a sample dimension of 2.5 mm × 4 mm × 36 mm, a cross-head speed of 0.5 mm/min, and a span of 24 mm. The polished cross sections and the fracture surfaces after the bending test were imaged by scanning electron microscopy (SEM).

3

Fig. 1

TG curve for PCS pyrolysis

The preceramic polymer for SiC matrix in the present experiment was polycarbosilane (PCS) (from National University of Defense and Technology, China). The ceramic yield was obtained as mass fraction 60.5% by the TG test, as plotted in Fig.1. As the filler material for the matrix, SiC powders (Germany) with an average grain size of 0.5 µm were utilized. The preforms after deposition by PyC and SiC were first dipped into a slurry containing mass fraction 40% SiC powder, and mass fraction 10% PCS in vacuum. After drying, the samples were pyrolyzed in nitrogen atmosphere at a high-temperature (1200°C) to convert the polymer into ceramic matrix.

Characterization

Results and discussion

The properties of the two kinds of composites are listed in Table 2. It can be easily seen that the flexural strength was significantly improved by depositing the PyC/SiC interphase on the fibers. The room-temperature bending strength for the PyC/SiC coated composite reached 247 MPa. However, for the composite with bare reinforcing fiber, the flexural strength was at a quite low level of only 46 MPa even though no significant difference in the bulk density and porosity were observed. Considerably increased failure displacement was observed for the coated composite. As reported in literatures [9, 10], the Si element in the polymer-derived matrix diffuses into the carbon fiber during the pyrolysis at high temperature, which leads to strong fiber/matrix bonding and a decline in the fiber strength. When a PyC/SiC interphase was deposited, on one hand, the loose PyC weakened the interfacial bonding and prevented the adhesion of stiff SiC to the fiber directly, and on the other hand, the deposited SiC layer acting as a barrier avoided the diffusion of Si from the matrix to the fiber. As a result, a significantly high strength of the fiber was retained and the flexural strength for the composite was considerably improved.

Table 2 Properties of the as-fabricated C/SiC composites Density ρ/g•cm-3

Porosity q/%

Uncoated

1.59

PyC/SiC coated

1.66

Flexual strength σ/MPa

Failure displacement d/mm

21.1

46 ± 4

0.07 ± 0.02

16.0

247 ± 21

0.17 ± 0.02

Fig.2 shows the cross-sectional micrographs of the fabricated C/SiC composites. As seen in Fig.2a, some isolated small pores can be observed in the intra-bundle areas even after several infiltration-pyrolysis cycles, which was a commonly observable phenomenon in the PIP-derived samples. These dispersed residual pores were ascribed to the shrinkage of the infiltrated PCS on pyrolysis and the difficulty for achieving effective polymer infiltration after the matrix was formed in the first cycle slurry infiltration. Generally, during PIP, the size and the number of residual pores remaining in the inter- and intra-bundle areas will gradually decrease when the PIP cycles proceed and then hinder further polymer infil-

tration. When the residual pores were small enough, the viscous PCS solution could not be effectively infiltrated into the consolidated body. At this time, the process should be stopped. In Fig. 2b, a reduced number of pores was observed, which is consistent with the porosity data listed in Table 2 for these two composites. At the same time, homogeneously deposited PyC/SiC interphase was observed at the fiber boundary. The thickness of the SiC layer was estimated to be about 2 µm and the PyC interphase could not be distinguished clearly from the carbon fibers for the same element composition.

ZHU Yun-zhou et al. / New Carbon Materials, 2007, 22(4): 327–331

Fig. 2 SEM micrographs for the two composites with (a) un-coated fiber and (b) coated fiber

Fig.3 shows the fracture surfaces of the two composites after the bending test. As seen in Fig.3a, the fracture surface of the un-coated composite was very plane and no pullout fibers were observed in un-coated composites, which indicated that the fibers were seriously damaged in the pyrolysis, causing a strength decrease. The SEM image of high magnification shown in Fig.3b better illustrates the smooth no-fiber-pullout fracture surface. With the increase of the load in the testing process, cracks evolved in the matrix first and propagated directly perpendicular to the fibers. However, for the decreased strength of the fibers, the cracks were not deflected along the fibers/matrix interface. Thus, fibers were sheared off by the crack, and a smooth fracture surface was generated. As

for the high retention of strength for the coated fibers, the cracks were deflected parallel to the fiber axis along the loosely formed interphase and allowed long fiber pullout[11]. As seen in Fig.3c, long pull-out fibers dominated the fracture surface, indicating high retention of the fiber strength and formation of weak interphase between the fibers and the matrix. A typical long pore left by the pull-out fibers is shown in Fig.3d. For the composite with PyC/SiC interphase, the loosely formed PyC interphase avoided strong bonding between the fibers and the PCS-derived matrix, demonstrating extensive fiber debonding from the matrix and long fiber pullout.

Fig. 3 Fracture micrographs of the two kinds of composites (a) and (b) for un-coated composite, (c) and (d) for PyC/SiC coated composite

ZHU Yun-zhou et al. / New Carbon Materials, 2007, 22(4): 327–331

In Fig.4, the stress-displacement curves of the two composites are plotted. With the incorporation of the PyC/SiC interphase, the fracture behavior was completely changed. The curve for the uncoated composite demonstrated a catastrophic and early failure mode, whereas for the PyC/SiC coated composite, the curve indicated a typical non-brittle fracture behavior for the fiber-toughened composite. Linear deformation, matrix cracking, fiber debonding from the matrix, and bundle failure are the main characteristics during the loading process. With increase of the load, the sample exhibited a linear deformation stage, and then a derivation was observed at a stress of about 220 MPa, indicating the occurrence of microcracking in the matrix. After reaching maximum load, the stress dropped gradually, demonstrating a pseudo-ductile fracture behavior for the fiber pullout, bridging, and sliding. The failure curves were consistent with the characteristics of the fracture surfaces observed by SEM.

Acknowledgements The authors are grateful to the 973 program for the financial support.

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Fig. 4 Flexural stress-displacement curves of the composites

4

Conclusions

3D C/SiC composites were fabricated by the polymer infiltration and pyrolysis process, using bare and PyC/SiC coated HTA C fibers. The mechanical properties and microstructures were investigated. Chemical erosion to the carbon fibers was avoided to some extent by PyC/SiC deposition. As a result, the bending strength of the composites was enhanced remarkably to 247 MPa and the composite exhibited a typical non-brittle fracture behavior when the PyC/SiC interphase was deposited on the fiber surface by the ICVI process. Further investigation will be performed on the optimization of the interphase thickness and enhancement of bulk density by incorporation of nano-scale SiC filler to obtain better mechanical properties.

layered interphases processed by temperature-pulsing chemical vapor infiltration[J]. Physics Status Solidi, 2006, 203(8): R58– R60.

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