Materials Letters 82 (2012) 36–38

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Materials Letters

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Preparation of C/SiC composites by pulse chemical liquid–vapor deposition process

Min Mei a, Xinbo He a, Xuanhui Qu a, Haifeng Hu b,⁎, Yudi Zhang b, Si'an Chen b

a School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, Chinab National Key Laboratory of Science and Technology on Advanced Ceramic Fibers and Composites, College of Aerospace and Materials Engineering, National University of Defense Technology,Changsha 410073, China

⁎ Corresponding author. Tel./fax: +86 73184576269.E-mail address: hfhu_nudt@nudt.edu.cn (H. Hu).

0167-577X/$ – see front matter © 2012 Elsevier B.V. Aldoi:10.1016/j.matlet.2012.05.030

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 March 2012Accepted 8 May 2012Available online 17 May 2012

Keywords:Carbon/silicon carbideCeramic compositesPulse chemical liquid–vapor depositionMicrostructure

C/SiC composites were prepared by pulse chemical liquid–vapor deposition (pulse CLVD) process in 3 hwith liq-uid polycarbosilane as precursor. The microstructure and properties of the C/SiC composites were studied. Theresults show that the apparent density and open porosity of the C/SiC composites are 1.81 g⋅cm−3 and 8.0%, re-spectively. The matrix consists of β-SiC, α-SiC and pyrolytic carbon, with the grain size of β-SiC of ~42 nm. Andthe carbon fiber adheres to the matrix with a strong interface, therefore the C/SiC composites exhibit a brittlefracture behavior with flexural strength of 138.0 MPa, flexural modulus of 31.0 GPa.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

C/SiC composites are widely used in aeronautic and astronautics fortheir low density, high strength and excellent anti-oxidation propertiesat high temperatures [1,2]. Themajormethods for preparing C/SiC com-posites are chemical vapor infiltration (CVI), precursor infiltration andpyrolysis (PIP), and reactive melt infiltration (RMI) process. However,both of CVI and PIP processes need a long processing period to obtaindesired density [3,4], thus increasing the cost and restricting their fur-ther applications, while RMIprocesswith short preparation period, usu-ally leads to relative low mechanical properties [5].

Chemical liquid–vapor deposition (CLVD) process [6], which wasfirst used to prepare C/C composites, combining the advantages ofhigh concentration infiltration from PIP process and continuous infil-tration (deposition) from CVI process, had a densification rate about100 times higher than that of conventional CVI process. However,the biggest problem in CLVD process was how to realize homoge-neous deposition throughout the articles, e.g., for a cylindrical article,usually a gradient deposition would occur when the articles were rel-atively thick or the deposition temperature was above 1200 °C [7,8].Unfortunately, because of the characteristics of CLVD process, the de-position temperature should be kept above 1200 °C to obtain high de-position efficiency. To improve the temperature gradient existed inthe preform, Delhaes [9] introduced a method by wrapping twolayers of a PTFE membrane around the preform with a decrease of500 °C of the thermal gradient. A step-by-step heating process [10]was also used to improve the homogeneity of the composites,

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obtaining a uniform density of 1.60~1.74 g⋅cm−3. In addition, a sand-wiched heat-source with a porous adiabatic material outside adoptedto reduce the thermal loss had been reported [11].

The above reports all contained a continuous heating process. Inthis paper, however, a pulse heating step was adopted to improvethe infiltration, and accordingly the microstructures and mechanicalproperties of the C/SiC composites were studied.

2. Experimental procedure

2.1. Raw materials

Plain weave PAN-based carbon fiber cloth (Toray T-300, 3 k,200 g⋅m−2) was used as the reinforcement. A liquid polycarbosilanewith a boiling temperature of 120~200 °C was chosen as the deposi-tion precursor.

The preform, which had a thickness of 10 mm and fiber volumefraction of 42%, was prepared by wrapping carbon fiber cloth arounda graphite cylinder with diameter of 30 mm, then stitching along thecircumferential direction to fix it.

2.2. Preparation of samples

The preform and graphite cylinder were set in an inductive coilwhich was powered by an inductive heating device (30~90 KHz,5~50 KVA), and the sample together with the coil were immersedin the liquid polycarbosilane.

The sample was firstly heated to 500 °C in 5 min, then to 1600 °Cby pulse heating in 2.5 h, followed by a holding step of 0.5 h at1600 °C. In the pulse heating cycle, the heating time and time intervalwere 5 min and 1 min, respectively.

Table 1Properties of C/SiC composites.

Density(g⋅cm−3)

Porosity(%)

Flexural strength(MPa)

Flexural modulus(GPa)

PIP⁎ 1.92 5 330 65CLVD⁎⁎ 1.75 25 100±10 27±2Pulse CLVD 1.81 8.0 138.0±1.0 31.0±2.0

⁎ Data from Ref. [14].⁎⁎ Data from Ref. [15].

37M. Mei et al. / Materials Letters 82 (2012) 36–38

2.3. Measurements

The apparent density and open porosity of the C/SiC compositeswere measured by Archimedes' method with kerosene as medium.The density of SiC matrix was also measured by Archimedes' methodwith 100-mesh sieved SiC matrix powder in a quantitative volumebottle, using water as medium.

The flexural strength was obtained by using the three-point bend-ing test (WDW-100, China) on 5 specimens of 3×4×35 mm with aspan of 30 mm and a crosshead speed of 0.5 mm⋅min−1. The load di-rection was perpendicular to the plane of carbon cloth. The crystallinephase and grain size of the matrix were determined by X-ray diffrac-tion (Rigaku D/max-RB12, Japan). The microstructures and elementalanalysis of the C/SiC composites were characterized by scanning elec-tron microscopy (JSM-6360, Japan) with an energy-dispersive spec-troscopy (EDS) system.

Fig. 2. Fracture surfaces of the C/SiC composites prepared by pulse CLVD.

3. Results and discussion

3.1. Density

Table 1 listed the densities from different processes. It was obviousthat simple PIP process, CVLD, and pulse CLVD gave densities of 1.95,1.75, and 1.81 g⋅cm−3, respectively.

Comparing to the composites prepared by CVI or PIP [12,13], thelower density and porosity of the composites prepared by pulse CLVDare due to the low density of the SiC matrix, which is 2.1 g⋅cm−3. Theincrease of the density may be attributed to the pulse heating process.Once the heating is interrupted, the by-products concentrated in the in-terior of the preform can diffuse out of the preform easily due to the de-crease of temperature, at the same time, more precursor liquid candiffuse into the inner part of the preform, when heating is resumed,the liquid will vaporize and form deposits at the pores between thefiber bundles, so the interior of the preform could be well densified,thereby attaining a much more dense and homogeneous article.

Fig. 1. Stress–displacement curves of the sample prepared by CLVD and pulse CLVD.

3.2. Mechanical properties

Table 1 lists the mechanical properties of the C/SiC compositesprepared by CLVD [15] and pulse CLVD process, respectively. It isclear that the mechanical properties of the C/SiC composites preparedby pulse CLVD were all higher than that of the C/SiC composites pre-pared by CLVD process.

Fig. 1 shows the typical stress–displacement curve of the C/SiCcomposites prepared by CLVD and pulse CLVD, which can reveal thefracture behavior of the C/SiC composites. To the sample preparedby CLVD process, which is denoted as Cs, the stress–displacementcurve shows low stress with a long and flat curve, indicating thatthe SiC matrix cannot transfer the load effectively due to the low den-sity and the large pores between bundles and carbon cloth layers, ac-cordingly leading to a low stress fracture of the composites. As tosample prepared by pulse CLVD, which is denoted as PCs, the stress

Fig. 3. XRD patterns of the deposited SiC matrix.

Fig. 4. EDS analysis of the C/SiC composites.

38 M. Mei et al. / Materials Letters 82 (2012) 36–38

first increases linearly with the increase of displacement, afterreaching the maximum value, the stress decreases sharply, indicatinga brittle fracture behavior. And this is further supported by SEM im-ages in Fig. 2.

3.3. Microstructure and phase characterization

The density and interfacial structure are the important factors todetermine the mechanic properties of the C/SiC composites [16].Fig. 2 is the fracture surfaces of the C/SiC composites prepared bypulse CLVD. As shown in Fig. 2(a), the low magnification SEM imageof the sample reveals that the C/SiC composite shows a dense micro-structure with few pores between fiber bundles or layers. As shown inFig. 2(b), only short fiber pull-outs are observed, and the interface be-tween the fiber and SiC matrix is not very clear, which indicates thatthe interfacial bonding between carbon fiber and matrix is strong[17]. This characteristic may be ascribed to the Si atoms diffusedinto the carbon fibers at the temperature above 1600 °C [4]. There-fore, the C/SiC composite exhibits low fracture strength and a brittlefracture behavior.

Fig. 3 shows the XRD patterns of the deposited SiC matrix. TheXRD analysis has clearly identified that the matrix consisted of β-SiC polytype (3 C), α-SiC polytype and carbon. The peaks that belongto β-SiC are much higher and sharper than that of α-SiC and carbon,indicating that β-SiC is the major component of the matrix with ahigh degree of crystallinity. Debye-Scherrer analysis of the highest in-tensity (111) peak at full width at half maximum intensity (FWHM)shows grain size of ~42 nm, which means that the deposits are strongand dense [18].

Fig. 4 shows the EDS analysis of the C/SiC composites. The EDSanalysis clearly shows that the chemical compositions of SiC matrixare Si and C, and Si/C atomic ratio is close to 1. Compared with theSiC matrix from PCS pyrolysis by PIP process [16] which has a10 wt% carbon rich in the matrix, the SiC matrix by CLVD process ismore pure and highly consistent with that from CVI process withMTS reagents.

4. Conclusions

The C/SiC composites were rapidly prepared by an improved pulseCLVD process in 3 h, with a density of 1.81 g⋅cm−3 and an averageflexural strength of 138.0 MPa. In pulse CLVD process, the precursorin the inner part of the preform exhausted by heating can bereplenished at the interval time, thus the inter bundle and interlayer pores can be well densified and a homogeneous structure isobtained.

Acknowledgements

The authors are grateful to the 863 Project of China for providingthe financial support under Grant No. 2006AA03Z558.

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