Key Engineering Materials Vols. 317-318 (2006) pp. 159-162 online at http://www.scientific.net © (2006) Trans Tech Publications, Switzerland

Rapid Fabrication of C/C-SiC Composites Ji-Ping Wanga, Zhi-Hao Jinb and Guan-Jun Qiaoc State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, 710049, China a

b

c

[email protected], [email protected], [email protected]

Keywords: C/C Composites. Chemical Liquid-Vaporized Infiltration. Reactive Melt Infiltration. C/C-SiC Composites.

Abstract. C/C-SiC composites, namely carbon fiber reinforced silicon carbide and pyrocarbon matrices, were fabricated in two steps in this study. Firstly, C/C composites were prepared by a rapid economical densification process of chemical liquid-vaporized infiltration. PAN based felt and 2-Dimensional carbon fibers were chosen as preform, respectively. A liquid hydrocarbon, kerosene, was used as a precursor. The C/C composites were processed in a temperature range of 900-1100ºC for 150 minutes. Subsequently, C/C-SiC composites were fabricated from the C/C composites and silicon powder by reactive melt infiltration method. Densities, open porosities of the C/C and the C/C-SiC composites were investigated. Structural properties of the C/C-SiC composites were studied by optical microscopy. X-ray diffraction was used to identify the element and the crystal phase of the composites. It was shown that the density of C/C composite reached to 1.72 g/cm3 based on the 2D carbon fibers by CLVI method. Microstructure observation of the C/C composite revealed that the pyrocarbon is layer concentric around the fibers. It was found that during the RMI processing β-SiC was formed through the reaction only between liquid silicon and pyrocarbon, while carbon fiber was not damaged. Free silicon remains in the C/C-SiC composites because of insufficient reaction with the pyrocarbon. Introduction C/C-SiC composites, which have the combined advantages of fiber reinforcement and ceramic matrix, are widely used as structural materials in military, space, and aircraft industry areas [1-3]. Their high mass-specific characteristics and smooth frictional behavior, as well as high strength and thermal shock resistance maintained up to extreme temperature are important properties for applications. The fabrication of C/C-SiC materials includes two steps: preparation of C/C composites and subsequently fabrication of C/C-SiC composites. As for the preparation of C/C composites, two primary routes are 1) impregnation of the preform by an organic liquid precursor, followed by pyrolysis treatments; and 2) Chemical Vapor Infiltration (CVI). However, several cycles are needed to obtain sufficiently high density and thus a long densification time is necessary with these methods [4]. Fortunately another C/C composite process has been developed, named as chemical liquid-vaporized infiltration (CLVI) [4, 5]. Profiting from the high thermal-gradient existing in the preform and the closer diffusion approach of the precursor during the processing, two outstanding characteristic was reported with CLVI method: 1) densification rate is about 100 times faster compared to that of isothermal, isobaric CVI; 2) fabrication is completed in one cycle infiltration run without removing the preform during the procedure [4]. For the fabrication of C/C-SiC composites, the reactive melt infiltration (RMI) [6] method has advantages over conventional sintering, such as a near-net-shaped product and full densification at relatively low temperature. During the RMI procedure, silicon melt infiltrates into a porous matrix of C/C composites with the driving force of capillarity, and the reaction between silicon and carbon occurs instantaneously. The fabrication can be completed within minutes to hours depending on the preform microstructure.

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Therefore, if CLVI method was utilized together with the RMI method, we can expect a rapid fabrication of C/C-SiC materials. This can decrease the procedure period and reduce the processing costs. Consequently, in the present study, we attempt to use CLVI combined with the RMI method to get a rapid densification process for C/C-SiC composites. The density, open porosity, microstructure and crystalline phases of fabricated C/C-SiC composites were studied respectively. Experimental procedures Materials. Two types of carbon preform were chosen in this study in order to investigate the influence of different architecture of the fibers on the microstructure of the C/C-SiC composites. One is a PAN-based felt (CF) in which the diameter of the fibers is ~13µm. Another is PAN-based 2-Dimensional carbon fiber (C2D) (two-bundle fiber stacked with 45o direction) with a fiber diameter of 6µm. A liquid hydrocarbon, kerosene, was chosen as a precursor to prepare the C/C composites. Silicon powder was used in the RMI processing for fabrication of the C/C-SiC composites. Composites process. Fig. 1 is the processing scheme of the C/C-SiC composites. Firstly the preform (CF or C2D) was Felt or 2D Carbon fiber fixed around the graphite susceptor and placed in the reactor Drying filled with kerosene. Temperature range of the CLVI o o processing is from 900 C to 1100 C. After keeping at this 110ºC, 2~4h range for 150 minutes, the CF/C (C/C composites made from CLVI CF) and the C2D/C (C/C composites made from C2D) were N2, 900~1100ºC, 150min densified, respectively. Subsequently, the CF/C and C2D/C were cut and placed in C/C a graphite crucible covered with certain silicon powder. During the RMI method, the sample was heated to 1550ºC Reactive melt infiltration and held at this temperature for 30 minutes in a vacuum of 10-1Pa, 1550ºC, 30min 10-1Pa. The CF/C-SiC (the C/C-SiC composites made from CF/C) and C2D/C-SiC (the C/C-SiC composites made from C/C-SiC C2D/C) were obtained, respectively. Characterization The density and open porosity of the Fig. 1. Processing scheme of fabricaC/C and the C/C-SiC composites were measured. ting C/C-SiC composites. Microscopy observation on the polished C/C and C/C-SiC specimens was carried out by reflection optical microscopy (OM). X-ray diffraction (XRD) measurement was utilized to determine the crystalline phases of the C/C-SiC composites. Results and discussion Property of the prepared composites Table 1 gives the density and open porosity of the C/C and the C/C-SiC composites. The C2D/C and CF/C have densities of 1.62 g/cm3 and 1.72 g/cm3 respectively. This result indicates that CLVI is a promising method with a very high densification rate, compared with CVI. The rapid densification is attributed to the high thermal-gradient existing within the preform and the closer diffusion approach of precursor in which the preform is immersed during the whole procedure. On the other hand, it also proves that the kerosene is an effective precursor for formation of pyrocarbon. During the CLVI processing, kerosene firstly vaporized and then decomposed into a great number of compounds. However, the mechanism of chemical reactions occurring from the starting product (liquid kerosene) to the ending deposition (solid pyrocarbon) is still not revealed clearly because it is difficult to precisely predict the temperature of the reaction part and analyze the in-situ gaseous or other by-products.

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The densities of CF/C-SiC and C2D/C-SiC are both close to 2.0g/cm3. After RMI procedure, the pores in the C/C composites are primarily filled with formed SiC and some free Si. But the quantitative content of SiC, Si and C phase is not easy to measure. Table 1 Density and porosity of the composites

Density [g/cm3] Porosity [%]

CF/C

C2D/C

1.62 24.3

1.72 9.7

CF/C-SiC C2D/C-SiC 1.98 1.2

2.06 0.9

■ ■: β-SiC XRD analysis of the C/C-SiC composites The ▼: Si XRD patterns of CF/C-SiC (a) and C2D/C-SiC (b) are ▲: C shown in Fig. 2. It can be seen that β-SiC is observed ■ ▲ ■ ▼ in both patterns, indicating that the formation of (b) β-SiC has taken place. The fact that the Si phase is also observed reveals that residual free Si still exists. (a) Comparatively, the residual free carbon in C2D/C-SiC is higher than that in CF/C-SiC. This can be attributed to the higher fiber volume content and higher density 10 20 30 40 50 60 70 Diffraction angle, 2θ (degrees) of C2D/C. Therefore, by adjusting the fiber volume and density of the C/C composites, we can control Fig. 2. XRD patterns of (a) CF/C-SiC and (b) different contents of C, SiC, and Si in the C/C-SiC C2D/C-SiC. composites. Microstructure characteristics of the C/C composites Microscopy observation on the C/C composites is shown in Fig. 3. The C/C composites under crossed polarizer have optically anisotropic domains. It can be observed that the fibers in both CF and C2D are surrounded concentrically by a ring pyrocarbon with different thickness. The pyrocarbon deposited on the surface of CF is generally close to 30 µm thick, while that deposited on the C2D fiber is about 10 µm thick. It also can be seen that pores distributed in both C/C composites. The pores volume of CF/C is higher than that of C2D/C. Small cracks in the pyrocarbon are formed in CF/C (see Fig. 3, a). This can be attributed to the thermal coefficient mismatch between the random textured fibers and the oriented deposit during the cooling stage. The surface of the pyrocarbon is rough both in CF/C and C2D/C. (a)

(b)

15µm

15µm

Fig. 3. OM micrographs of (a) CF/C and (b) C2D/C . Microstructure characteristics of the C/C-SiC composites The OM micrographs of CF/C-SiC and C2D/C-SiC are given in Fig. 4. It reveals different contents and distribution of the carbon fiber (F), pyrocarbon (Cpy), SiC and free Si. Each phase has been denoted in the figure. It can be observed that a well-bonded silicon carbide layer was formed by reaction of the carbon substrate with molten silicon during the RMI processing. For the formed SiC layers, the interface between the SiC and pyrocarbon appears to be continuous, while the interface between the SiC and metal is discontinuous. The different interface appearance indicates that the SiC located at the pyrocarbon side of the interface

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layer was formed during the in situ reaction between liquid silicon and solid carbon; the discontinuous SiC surface contact with the silicon metal was produced by precipitation from a supersaturated Si-C solution during the cooling period [9]. Molten silicon expanded to fill all porous space in the preform and formed solid silicon phase as the temperature cooled, ultimately resulting in a fully dense matrix containing free Si and SiC (see Fig 4). It can also be found that the SiC layer is only formed through the reaction between the pyrocarbon and the melt silicon, whilst the carbon fiber is not damaged. This is a very important advanced characteristic of the procedure for the fabrication of C/C-SiC composites, in which the excellent mechanical properties of the carbon fibers are maintained. Consequently, C/C-SiC composites can have promising characteristics. (b)

(a)

Si SiC

SiC

Si

Cpy

Cpy F 15µm

F

15µm

Fig. 4. OM micrographs of (a) CF/C-SiC and (b) C2D/C-SiC. Conclusion C/C-SiC composites were fabricated with rapid processes. Firstly C/C composites were prepared by CLVI method in 150 minutes. CF/C and C2D/C attained a density of 1.62g/cm3 and 1.72g/cm3, respectively. Subsequently with the RMI processing in 30 minutes, the low porosity (~1%) C/C-SiC with density of about 2.0g/cm3 was obtained. During the RMI, β-SiC was formed through the reaction between Si and pyrocarbon; whilst the carbon fibers were not damaged. The excellent mechanical property of carbon fiber can be maintained. Acknowledgement The authors acknowledge the support of the National Natural Science Foundation of China (No.50272051). References [1] K. M. Prewo: Am. Ceram. Soc. Bull. Vol. 68 (2) (1989), p. 395. [2] W. Krenkel: Ceram. Eng. Sci. Proc. Vol. 24 (4) (2003), p. 583. [3] E. Fitzer, L. M. Manocha: Carbon reinforcements and carbon/carbon composites (Springer -Verlag, New York, 1998. p.295). [4] I. Golecki: Mater. Sci. Eng. R. Vol. 20 (1997), p. 37. [5] E. Bruneton, B. Natcy, A. Oberlin: Carbon. Vol. 35 (10) (1997), p. 1593. [6] Hillig. William B: Am. Ceram. Soc. Bull. Vol. 73 (4) (1994), p. 56. [7] David P. Stinton, A. J. Caputo, Richard A. Lowden: Am. Ceram. Soc. Bull. Vol. 65 (2) (1989), p. 347. [8] G. Rajesh, RB. Bhagat: Ceramic Eng Sci Proc. Vol. 17(3) (1996), p. 34. [9] Li. Jian-Guo, Hausner. Hans: J Am Ceram Soc. Vol. 79(4) (1996), p. 873.