Materials Letters 60 (2006) 1269 – 1272 www.elsevier.com/locate/matlet

Improvement of film boiling chemical vapor infiltration process for fabrication of large size C/C composite Ji-ping Wang ⁎, Jun-min Qian, Guan-jun Qiao, Zhi-hao Jin State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, People's Republic of China Received 21 June 2005; accepted 5 November 2005 Available online 28 November 2005

Abstract An improved film boiling chemical vapor infiltration process was developed to fabricate a large size C/C composite with homogeneous density and microstructure. The C/C composite was prepared by processing a disc-shaped carbon felt preform, whose upper and lower sides were fixed and heated simultaneously by two flat surfaces of two heat sources, with kerosene as a precursor at 1050 °C for 3 h at an atmospheric pressure. The in-situ temperature distribution along the radial direction of the preform upper surface was analyzed to get better information and control of the process. Experimental results show that the average density of the composite of Φ 110 × 10 mm3 size is about 1.72 g/cm3 and its maximal difference along radial direction is 0.05 g/cm3. Polarized light microscopy (PLM) and scanning electron microscopy (SEM) reveal that the carbon fibers of the composite are surrounded by ring-shaped pyrocarbons with a thickness of ∼ 20 μm, and that pyrocarbons are delaminated to 4–6 layers. A schematic model is proposed to analyze the process by dividing the reactor into different regions associated with specific functions. © 2005 Elsevier B.V. All rights reserved. Keywords: Carbon/carbon composite; Chemical vapor infiltration; Rapid densification; Microstructure

1. Introduction Carbon/carbon (C/C) composites are widely applied in many fields for their low density, excellent thermal and mechanical properties with smooth frictional behavior and good biocompatibility [1]. Currently, the main method for fabricating C/C composites in industry is the isothermal chemical vapor infiltration (CVI) technique. However, it has a major intrinsic drawback, namely, a long processing period is inevitable to obtain desired density [2,3]. Fortunately, another method called as film boiling chemical vapor infiltration (FBCVI) or chemical liquid-vaporized infiltration (CLVI) [4–7] has been developed to increase the deposition efficiency. It appears very attractive to prepare C/C composite in a short processing time with a high carbon yield which is about one order of magnitude larger than by classic isothermal CVI. It is known that the FBCVI method involves a strong thermal gradient inside cold wall reactor. A mobile densification front is created in a porous preform which is directly immersed into a ⁎ Corresponding author. Tel.: +86 29 82667942; fax: +86 29 82665443. E-mail address: [email protected] (J. Wang). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.11.012

liquid hydrocarbon precursor. The principle, the experimental device, and the influences of some basic parameters (temperature, pressure, precursors, etc.) have already been well studied [3,5,6]. Further investigations are carried out experimentally or theoretically to reveal the complex chemical reactions leading to the pyrocarbon matrix in a confined place and the role of the heat and mass transfers inside porous preform [8–11]. Nevertheless, these studies were mainly carried out in laboratory reactors. The prepared C/C composites are usually of thin-walled tubular shape with small dimensions (the wall thickness is below 35 mm). Moreover, spatial density gradients exist in the composites, where the density at the interior regions near the heat source is the highest and that at the outer surfaces near the liquid precursor is the lowest [11,12]. Therefore, further improvement of this process is necessary for preparing large bulk C/C composite of more regular shape with homogeneous density distribution and uniform microstructure. For this purpose, a double heat source design is firstly developed in the present work. A large size C/C composite disc was fabricated by this improved FBCVI method. To get better information and control of the whole process, the in-situ temperature distribution in the preform was recorded and

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heating rate was controlled by adjusting the power of an inductive generator. N2 flowed through the reactor for safety consideration. With the increasing deposition time, the precursor was consumed and resupplied from the precursor inlet to the reactor. 2.2. Characterization

Fig. 1. Sketch of experimental device for preparing C/C composite disc.

analyzed. The density, porosity and microstructures of the prepared composite are characterized by Archimedes principle, polarized light microscopy and scanning electron microscopy techniques. Finally, a schematic model is proposed to study the densification process. 2. Experimental procedure 2.1. Material preparation 2.1.1. Preform and precursor A PAN-based carbon felt (thickness: 10 mm, bulk density: ∼ 0.20 g/cm3, fiber diameter: Φ 9–13 μm) was used as a preform in this study. A liquid hydrocarbon mixture, kerosene (molecular formula: C10Hn–C16Hm) with a boiling temperature range of 180–230 °C, was chosen as a precursor, which was proven to be a feasible and efficient precursor [12]. 2.1.2. Experimental set-up The experimental device is schematically shown in Fig. 1. In a cylindrical quartz glass reactor, two graphite cylinders (H1 and H2) with diameter of Φ110 mm were placed at the same axis as the reactor and inductively heated by an inductor coil. The preform cut in disc shape (Φ110 mm) was fixed between the lower surface of H1 and the upper surface of H2 and heated by both of them. The residual surfaces of the two graphite cylinders were wrapped with a thermal insulator. Both the cylinders and the preform were immersed into the liquid precursor. Three thermocouples (T1, T2 and T3) were located at the interface of H1 and the preform. Their distances to the axis are 0 mm, 25 mm and 50 mm, respectively. Thus the inner, middle and outer temperatures of the upper surface of the preform can be simultaneously measured by them during the process. 2.1.3. Processing The deposition of the preform was performed at 1050 °C (measured by T1 thermocouple) for 3 h at an atmospheric pressure by the improved FBCVI. During the process, the

The dimensions of the as-prepared C/C composites are Φ 110 × 10 mm3. In order to study the homogeneity of density and microstructure, three specimens labeled as S1, S2 and S3 were cut off from the composite along the radial direction, which was located at the distance of 0 mm, 25 mm and 50 mm to the axis (corresponding to the T1, T2 and T3 location), respectively. The densities and open porosities of the three specimens were measured using Archimedes principle. Microscopy observations on the polished surfaces and the fracture surfaces of the specimens were carried out by polarized light microscope (PLM, Reichert, MeF3) and scanning electron microscope (SEM, Hitachi, S-2700) operated at 20 kV and 20 mA, respectively. 3. Results and discussion 3.1. Temperature distribution in the preform The key point of this process is the control of temperature distribution in the preform, either in axial or in radial direction [5,11]. Therefore, we designed a double heat source. This creates two extensive thermal gradients in the axial direction of the preform. Both sides of the preform are heated simultaneously by H1 and H2. The temperatures of the upper and lower surfaces of the preform are the highest. The deposition firstly occurred on these two surfaces and then the formed densification zones moved successively to the lower temperature zone. Thus, a rapid single cycle densification of the preform can be achieved due to the double high temperature gradients along the axis. In the radial direction of the preform, a uniform temperature of the heat surface is required to obtain uniform thickness and microstructure of the composite. Under the experimental conditions of the present paper, the temperature distribution in the radial direction of the upper surface of the preform is shown in Fig. 2, which was recorded

Fig. 2. Temperatures vs. time of three thermocouples recorded during the process.

J. Wang et al. / Materials Letters 60 (2006) 1269–1272 Table 1 Densities and open porosities of S1, S2 and S3 specimens Specimen

S1

S2

S3

Density (g/cm3) Porosity (%)

1.70 12.9

1.75 10.4

1.72 11.7

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and complex [11]. In this study, the temperatures mainly depend on the height of H1, the thickness of the thermal insulator, and the inductive heat frequency. Thus, the desired temperature distribution along the upper surface of the preform can be obtained by adjusting such parameters. 3.2. Density and open porosity of the composite

simultaneously by three thermocouples during the process. It can be seen that a very high heating rate was obtained using inductive heating, and the temperatures exceeded 900 °C in 10 min. During the deposition process, when T1 temperature was kept at 1050 °C, T2 temperature was about 30 °C higher and T3 temperature was about 40 °C lower than T1 temperature. An expected even temperature distribution along the upper surface of the preform was obtained. The temperature distribution in radial direction of the preform was the balance of the inductive heating and thermal exchanges between the heaters and the surrounding environment in the reactor. Theoretical calculations of the thermal exchange in the preform are very difficult

Fig. 3. PLM micrographs of polished sections of S1 (a), S2 (b) and S3 (c) specimens.

The densities and open porosities of the three specimens are shown in Table 1. The density increases up to 1.70 g/cm3 from the initial density ∼ 0.2 g/cm3 after 3 h of densification. This result reveals that a rapid densification of the preform is achieved by the improved FBCVI process. Comparing the values of S1, S2 and S3, we can see that densities along the radial direction of the composite are close and there is a small difference of 0.05 g/cm3 among them. This uniform density distribution in the composite is the result of controlling the radial temperature distribution of the preform. The S2 density is a little higher than S1 and S3 because its corresponding deposition temperature (T2) is higher, which leads to a higher densification rate. The open porosity of the composite in Table 1 is higher than 10%. We can expect an increase of the density with increasing of densification time.

Fig. 4. SEM micrographs of the fracture surface of S1 (a), S2 (b) and S3 (c) specimens.

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carbon source for the infiltration and densification can be transported to the preform and reaction zone rapidly. Based on this schematic model, it can be concluded that the improved FBCVI process has the following three advantages compared with the classical one: (1) a more rapid densification rate because of the existing double densification front; (2) the diameter of the disc is not limited by the densification thickness; and (3) the as-prepared disc can be easily removed from the graphite cylinder surface without destroying them.

4. Conclusions

Fig. 5. A schematic model of the improved FBCVI process.

3.3. Microstructure of the composite It is known that the qualities of the C/C composites, such as density and mechanical, electrical or thermal properties are determined by the microstructure of pyrocarbon [13]. Fig. 3 shows the PLM micrographs of the polished surfaces of S1, S2 and S3 specimens. It can be seen that the fibers are surrounded concentrically by a ring pyrocarbon which has optically anisotropic domains (dark cross appears systematically in the bright deposit). The thicknesses of pyrocarbons are all about 20 μm and the textures are smooth laminar according to the classification method proposed by Pierson and Lieberman [14–16]. Small cracks can also be seen in pyrocarbon matrix and at interface of fiber and pyrocarbon. This can be attributed to the thermal expanding coefficients mismatch between the fibers and the pyrocarbon, and the damage during preparing specimen. The SEM imaging of the fracture surfaces of three specimens are shown in Fig. 4. It shows that the surface of the pyrocarbon surrounding one fiber is rough and laid up with 5–6 layers. Comparisons of the PLM or SEM micrographs of S1, S2 and S3 reveal that there are no distinct differences, which indicates that the microstructures of pyrocarbon are similar.

An improved FBCVI method was developed and used to prepare a large size C/C composite disk of Φ 110 × 10 mm3 at 1050 °C for 3 h at an atmospheric pressure. The average density of the resulting composite is about 1.72 g/cm3, and the maximum difference of density along the radial direction is 0.05 g/cm3. The carbon fibers of the composite are surrounded by ring-shaped pyrocarbons with a thickness of ∼ 20 μm, and pyrocarbon is delaminated to 4–6 layers. A schematic model was proposed to analyze the process by dividing the reactor into different regions associated with specific functions. The double-heat source design allows a double densification front which can accelerate the densification theoretically. Moreover, the diameter of the C/C composite disc is not limited by the densification thickness. Therefore, this process will have a promising application for rapid preparation of large size C/C composite. Acknowledgement This study was supported by the National Natural Science Foundation of China (No. 50272051). References

3.4. Model of the improved FBCVI In order to get a better understanding of the densification process, a schematic model of the improved FBCVI is presented in Fig. 5. In this model, the differences of temperature along the radial direction and that in upper and lower surface of the preform are negligible. We divided the reactor into different regions and endowed different functions to each of the regions. 1. Densified preform: during the process, two high thermal gradients exist in the axial direction of the preform due to the use of two heat sources. The upper and lower surfaces of the preform contacted with the hot surface are firstly densified. The densification zones moved successively to lower temperature zone along the thermal gradient. 2. Densification zone: the chemical reactions occurred in these zones where steep thermal gradient exist. The decomposition of the precursors is complex, including many unit reactions and a large number of intermediate species [6,10,11]. 3. Undensified zone: in the undensified porous preform, the heat and mass exchange in a complex way. The precursor is fed from the outside and the reaction products such as hydrogen have to be exhausted. 4. Boiling precursor: the boiling precursor outside is considered as a mass reservoir for carbon precursor surrounding the preform. The

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