Materials Science and Engineering A 419 (2006) 162–167

Microstructure of C/C composites prepared by chemical vapor infiltration method with vaporized kerosene as a precursor Jiping Wang ∗ , Junmin Qian, Zhihao Jin, Guanjun Qiao State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, PR China Received in revised form 9 December 2005; accepted 9 December 2005

Abstract The microstructures of two types of C/C composites prepared from different carbon felts by a rapid densification method, thermal gradient chemical vapor infiltration with vaporized kerosene as a precursor, at 1080–1120 ◦ C for 6 h were characterized by polarized light microscopy (PLM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman micro-spectrometry techniques. The experimental results show that the fibers in the two composites are both surrounded by ring-shaped pyrocarbons with rough laminar texture, but the thickness, the surface morphology of the pyrocarbons and the graphitizability of the composites depend much on the configurations of carbon felts. The C/C composite fabricated from a higher porosity carbon felt possesses larger thickness and rougher surface of pyrocarbon, and has a lower graphitizability after heat treatment at 2300 ◦ C for 2 h. © 2006 Elsevier B.V. All rights reserved. Keywords: Carbon/carbon composite; Microstructure; Pyrolytic carbon; Raman micro-spectrometry

1. Introduction The application of carbon/carbon (C/C) composites, mainly in aerospace industry as protection materials for atmosphere reentry and disc brakes of aircraft, requires a basic knowledge of their structural characteristics. The characteristics, including macroporosity, pyrocarbon texture, fiber orientations, microcrack and interfaces at different levels, etc. control the mechanical, thermal and frictional properties of C/C composites [1]. Therefore, during the past decades most attentions were paid on the microstructural investigations to obtain C/C composites with desired properties for many applications [2–6]. Since chemical vapor infiltration (CVI) is commonly utilized for C/C composites preparation [7], a correlation between the microstructures and the fabrication parameters has been intensively studied [8–10] and comprehensively reviewed by Oberlin [12] and Delhaes [11]. In general, the microstructures depend on the reinforcement configuration, the fabrication routes and processing conditions, as well as the used precursor and the surface property of carbon fiber [1,11,13–14], but a clear explanation for these relationships is still unavailable. On the other

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hand, new CVI techniques and the using of complex precursor have been developed to speed up fabrication process. Naturally, microstructural information of the fabricated C/C composites is also continuously investigated to give instructive and significant feedback on the fabrication process. Recently, we designed a new method, which combines the advantages of regular film boiling CVI and classical thermal gradient CVI, to realize a rapid deposition of C/C composites [15]. In this method, two (upper and lower) heat-sources are utilized to vaporize liquid precursor and to deposit pyrocarbons on preforms, respectively, in a cold wall reactor where the liquid precursor and the preform are separated. The reasons for rapid deposition have been attributed to the using of mixture of hydrocarbons as a precursor, the thermal gradient existing across the preform, and the short convection and diffusion paths of hydrocarbon from the liquid precursor to the depositing zone. Although principle analyses of this rapid densification method has been preformed by our previous work, further investigation is necessary and significative to get a better knowledge about the influences of the fabrication route and felt configurations on the microstructures of the composites. For this purpose, two types of C/C composites were prepared from different carbon felts in the present paper. The microstructures of the composites were studied by polarized light microscopy (PLM),

J. Wang et al. / Materials Science and Engineering A 419 (2006) 162–167 Table 1 The parameter of the carbon felt used as perform

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Table 2 The density, average density increase rate and porosity of the C/C composites

Type of carbon felt

Single fiber diameter (␮m)

Felt thickness (mm)

Bulk density (g/cm3 )

Porosity (%)

Type of C/C

Density (g/cm3 )

Density increase rate (g/(cm3 h))

Porosity (%)

A B

∼10 ∼6

10 10

∼0.15 ∼0.52

∼90 ∼70

A B

1.68 1.71

0.255 0.196

19 13

scanning electron microscope (SEM), X-ray diffraction (XRD) and Raman micro-spectrometry techniques. 2. Experimental procedure

densification, the upper surface of the preform is at a higher temperature (>1000 ◦ C) than that of the lower surface (∼170 ◦ C as measured in our experimental conditions). Therefore, an average temperature gradient (>80 ◦ C/mm) is formed in the thickness direction of the preform.

2.1. Material preparation 2.2. Characterization Two types of carbon felts were used as preforms. Type A carbon felt obtained from Lanzhou carbon fiber plant, China, is made of short-cut carbon fibers in random directions. Type B carbon felt is a needle punched felt made of layered non-woven cloth in which carbon fiber is made in Jilin carbon plant, China. The parameters of the two felts are summarized in Table 1, where the porosities are calculated based on 1.76 g/cm3 of fiber density. A liquid hydrocarbon mixture, kerosene (molecular formula: C9 Hn −C16 Hm ) with a boiling temperature range of 180–260 ◦ C, was chosen as a precursor. The experimental device for preparing C/C composite is schematically shown in Fig. 1. During the processing, two susceptors are inductively heated by the inductor around the reactor. The lower susceptor is used to boil and vaporize the liquid kerosene which provides gaseous hydrocarbon for deposition. The upper susceptor is used to heat the surface of the preform. A thermocouple is located at the upper surface of the preform to measure the densification temperature. The densification of the preform was performed at 1080–1120 ◦ C for 6 h. During the

The densities and open porosities of the prepared C/C composites were measured by Archimedes principle for which distilled water was used; the average density increase rate was calculated. These results are given in Table 2. Microscopy observations on the polished surfaces and fracture surfaces of the C/C composites were carried out by polarized light microscope (PLM, Reichert, MeF3) and scanning electron microscope (SEM, Hitachi, S-2700) operated at 25 kV and 20 mA, respectively. Some composite materials were heat treated at 2300 ◦ C for 2 h under argon atmosphere on the purpose of investigating their graphitization degree. The crystalline structures of these composites were examined via X-ray diffraction (XRD, D/MAX-RA X-ray diffractometer) between 10 and 80◦ (2θ) to determine dspacing and crystallite size Lc . According to Bragg’s law, the d-spacing of the carbon (0 0 2) plane was determined using the following equation: d=

λ 2 sin θ

(1)

where λ is the wavelength of Cu K␣ radiation (0.1541 nm) and θ the diffraction angle in radians. Crystallite size Lc was obtained from the Scherrer equation: Lc =

0.9λ B cosθ

(2)

where B is the half maximum intensity in radians of the (0 0 2) peak. The polished surfaces of the heat-treated C/C composites were analyzed using Raman micro-spectroscopy (Jasco NRS-3100). A He–Ne laser was used with a fixed wavelength of 632.8 nm and the delivering power is around 1 mW on ∼2 ␮m2 . 3. Results and discussion 3.1. Morphology and texture of the C/C composites

Fig. 1. Sketch of experimental device for preparing C/C composites.

The polarized light micrographs of the C/C composites are shown in Fig. 2. At a low magnification observation of type A composite (Fig. 2a), it is clearly observed that the cross

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Fig. 2. Polarized light micrographs of type A composite (a and b) and type B composite (c and d).

sections and longitudinal sections of fibers are surrounded by concentric pyrocarbon deposits. The fibers are in random directions, the shape of macropores is irregular, and their distribution is uneven, which are caused by the irregular space existing between fibers in the carbon felt. Generally, the thickness around single fiber is about 20 ␮m (Fig. 2b), and the texture of pyrocarbon is rough laminar according to the classification method proposed by Pierson and Lieberman [1,16–18], showing numerous irregular extinction crosses (Fig. 2b). Circumferential cracks are found in the matrix, revealing that pyrocarbon consists of several layers and the adhesion strength between layers is weak. The fibers in type B composite are in a certain direction, and the space in interfiber (Fig. 2d) is much smaller than that in interbundle (Fig. 2c). Therefore, the thicknesses of pyrocarbon around the fibers are different, and are in the range of 1 to 8 ␮m. It can also be recognized that the texture is rough laminar and no cracks appear in the pyrocarbon matrix. The SEM images of type A composite and type B composite are given in Figs. 3 and 4, respectively. The relation between a single fiber and its surrounding pyrocarbon of type A composite shown in Fig. 3(a) suggests that the adhesion between them is strong because of the rough surface of the initial fiber. It is also observed that the pyrocarbon surface is much rough and consists of many convex spheres which looks like some onions formation (Fig. 3b). In type B composite, the pyrocarbon surface is smooth (Fig. 4a) and its thickness in the interfiber (Fig. 4b) is much smaller than that of in the interbundle (Fig. 4c).

Fig. 3. SEM micrographs of type A composite: fracture surface of a single fiber surrounded by pyrocarbon (a), and convex sphere shape of pyrocarbon surface (b).

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centration and ratio of surface area to pore volume and especially the type of the reacting molecules or free radicals [11–13]. 3.2. Growth feature of the pyrocarbon Although it is very difficult to know precisely the chemical reactions from the liquid kerosene to the solid deposition, the growth pattern of pyrocarbons can be studied by observing their morphology at different scales [4,6,12]. A fracture surface of pyrocarbon in type A composite (Fig. 5a) reveals that the pyrocarbon consists of several layers and each layer contains numerous sublayers (sublayer thickness <1 ␮m). This result turns out that the pyrocarbon grows in pattern of laminated pyrocarbons, and finally resulting in multiplayer structure in microscale. PLM micrograph (Fig. 5b) shows the pyrocarbon of type A composite consists of cone-like structures in both cross and longitudinal sections of fibers. Schematic models are proposed in Fig. 6 to illustrate these cone structures. It is considered that one cone structure either in the cross sections or in the longitudinal sections consists of many laminated pyrocarbons that have a same direction parallel to the fiber surface. With the increasing deposition time, new cones grow continuously in their favored directions, and finally form pyrocarbon with certain thicknesses.

Fig. 4. SEM micrographs of type B composite: smooth pyrocarbon surface and pullout fibers (a), small thickness of pyrocarbon in interfiber (b) and the large thickness of pyrocarbon in interbundle (c).

Both PLM and SEM results reveal that the configuration of carbon felt has a great influence on the thickness and surface morphology of the pyrocarbon. Type A carbon felt with higher initial porosity (∼90%) allows rapid convection and diffusion of the vaporized kerosene from the liquid surface to the hot deposition zone. The growth of the pyrocarbon around single fiber is not limited by the neighbor fibers because of the great space existing between them. As a result, the pyrocarbon thicknesses in type A composite are larger than that in type B composite; this consists with the data shown in Table 2 that a higher density increase rate is achieved for type A composite. The pyrocarbons surface morphology and texture of the composites are controlled by the local conditions during the deposition, such as gas con-

Fig. 5. Pyrocarbon growth pattern of type A composite: (a) layer and sublayer structure and (b) cone-shaped pyrocarbon structure in cross and longitudinal sections of fibers.

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Fig. 6. Model of cone-shaped pyrocarbon structure in the cross section view (a) and the longitudinal section view (b) of fiber.

Table 3 Values of d0 0 2 and Lc of the C/C composites after heat treatment Type of composite

d0 0 2 -spacing (nm)

Crystallite size Lc (nm)

A B

0.3393 0.3382

26.5 28.8

3.3. Graphitization of the C/C composites The d-spacing of the (0 0 2) peaks and the crystallite sizes of two types of C/C composites after heat treatment are summarized in Table 3. Compared with the results in previous literature [1,6], it can be recognized that the two composites have a strong graphitizability. Moreover, a slight difference between the two composites is also detected: the lower d-spacing value and larger crystal size Lc indicate that type B composite is a more graphitelike material. Raman spectra of two types of composites after heat treatment are given in Fig. 7. The explored sites in the polished surface of the composites are illustrated in Fig. 7(a). It can be seen that the typical D, G and D bands are detected in the Raman spectra of the fibers and the pyrocarbons (Fig. 7b and c), where the intrinsic graphite peak (G band) is at 1580 cm−1 , the additional bands (D and D ) of disordered carbons are at around 1340–1350 cm−1 and 1610 cm−1 , respectively [19,20]. The locations of the corresponding peaks are fixed, but their intensities change at the different explored sites. The intensity ratios R = ID /IG+D of different Raman spectra lines shown in Fig. 7(b) and (c) have been determined and summarized in Table 4, the integration intervals are 1200–1480 cm−1 for ID and 1480–1700 cm−1 for IG+D . It was recognized [19,21] that R is inversely proportional to the microcrystalline in-plane

Table 4 Intensity ratios R values of the carbon fiber (Rfiber ) and the pyrocarbon (Rpyrocarbon ) Raman spectra Type of composite

Rfiber

Rpyrocarbon

A

1.31

Inner 0.57

B

1.28

Middle 0.68 0.42

Outer 0.61

Fig. 7. Raman spectra of the carbon fibers and the pyrocarbons after heat treatment: (a) analysis locations of type A (left) and type B (right) composites, where F1, F2 represent fibers, PC1–PC3 represent inner, middle and outer pyrocarbons of type A composite, PC4 represents middle pyrocarbon of type B composite; (b) the Raman spectra of carbon fibers; (c) the Raman spectra of pyrocarbons.

size (La ) and the ability of graphitization. The fiber in the two composites is poorly graphitizable, and the pyrocarbon in type B composite has a higher graphitizability than that in type A composite, which confirms the XRD results. Moreover, the pyrocarbons in type A composite at different locations show various microcrystalline structures, and this result cannot be achieved by PLM or XRD techniques.

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4. Conclusions The C/C composites based on different carbon felt were prepared by thermal gradient CVI at 1080–1120 ◦ C for 6 h using vaporized kerosene as a precursor at atmosphere pressures. The following results are obtained via PLM, SEM, XRD and Raman micro-spectrometry techniques. The fibers in the two composites are surrounded by laminar pyrocarbons in the form of rough laminar texture. Configuration of the carbon felt has a great effect on the thickness and the surface morphology of the pyrocarbon. Type A composite based on the carbon felt with higher initial porosity has larger pyrocarbon thicknesses, rougher pyrocarbon surface, and lower graphitizability than that of type B composite. Raman spectrometry results reveal that fiber is poorly graphitizable, and in type A composite the pyrocarbon at different locations has different microcrystalline structures. Acknowledgements This study was supported by the National Natural Science Foundation of China (No. 50272051). References [1] E. Fitzer, L.M. Manocha, Carbon Reinforcements and Carbon/Carbon Composites, Springer–Verlag, New York, 1998.

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