SiC composites

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CERAMICS INTERNATIONAL

Ceramics International 40 (2014) 5191–5195 www.elsevier.com/locate/ceramint

Effect of in situ grown SiC nanowires on microstructure and mechanical properties of C/SiC composites Bingbing Peia,b, Yunzhou Zhua, Ming Yuana, Zhengren Huanga,n, Yinsheng Lia,b a

State Key Laboratory of High Performance Ceramics and Superfine microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China b University of the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China Received 21 September 2013; received in revised form 18 October 2013; accepted 18 October 2013 Available online 26 October 2013

Abstract Hexagonal-shaped SiC nanowires were in situ formed in C/SiC composites with ferrocene as catalyst in the densification process of polymer impregnation and pyrolysis. The effect of SiC nanowires on microstructure and properties of the composites were studied. The results show that the in situ formed SiC nanowires were hexagonal, mostly with diamer of about 250 nm, and grew by the vapor–liquid–solid (VLS) mechanism. The C/SiC composite with nanowires shows higher bulk density and flexural strength than the one with no SiC nanowires, and the high temperature flexural strength behavior of C/SiC composites with SiC nanowires was evaluated. Crown Copyright & 2013 Published by Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Silicon carbide nanowires; C/SiC composites; Polymer impregnation and pyrolysis; Polycarbosilane

1. Introduction Continuous carbon fiber reinforced silicon carbide matrix composites (C/SiC) are of promising candidates for applications such as aerospace industry, aircraft components due to the desirable properties of low density, non-brittle fracture, excellent thermal conductivity and high reliability at elevated temperatures [1–4]. Although the fracture tolerance of bulk SiC ceramic can be readily improved by the incorporation of carbon reinforcement fibers, such gains in fracture tolerance are basically from the fiber/matrix interfacial debonding and bridging/deflection of transverse matrix cracks by the fibers. However, the SiC matrix displays a brittle behavior similar to its bulk counterparts [5,6]. Therefore, challenges and interests maybe should follow on strengthening and toughening these week points. Recently, SiC nanowires have been widely studied as reinforcing elements in SiC coatings for its elongation of 200% before fracture and well matched coefficient of thermal expansion [7–10]. The SiC or SiC–Si coatings were toughened n

Corresponding author at: Chinese Academy of Sciences, Shanghai Institute of Ceramics, Shanghai 200050, China. Tel.: þ 86 21 52414901; fax: + 86 21 52413903. E-mail address: [email protected] (Z. Huang). E-mail address: [email protected] (B. Pei).

due to significant amounts of energy dissipated owing to the elastic extension of SiC nanowires [11,12]. The characteristics of fiber surface and fiber/matrix interface could be changed by the presence of SiC nanowires, and some properties, including interfacial bonding, energy absorption and thermal conductivity can be improved [13–15]. However, the related researches on the synthesis and properties of in situ forming SiC nanowires in SiC matrix of C/SiC composite are very few. In the present study, SiC nanowires were introduced into carbon fiber fabrics by pyrolysis of polycarbosilane with ferrocene (Fe(C5H5)2) as catalyst. The main interests were to investigate the effect of in situ grown SiC nanowires on density, open porosity, flexural strength and interface characters. More importantly, the high temperature flexural strength behavior of C/SiC composites with in situ grown SiC nanowires was evaluated. 2. Experimental procedures 2.1. Preform preparation The preforms were fabricated by alternatively stacked weftless plies and short-cut-fiber webs. Carbon fiber types of them were both PAN-based carbon fiber T700 (from Toray, Japan). Typical parameters of the carbon fiber are listed in

0272-8842/$ - see front matter Crown Copyright & 2013 Published by Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.10.077

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Table 1. Two successive weftless plies were oriented at an angle of 901. The fiber volume fraction in the preform was controlled at about 26%.

undertaken to densify the samples. The pyrolysis step was conducted at 1300 1C in flowing argon atmosphere. 2.3. Physical and mechanical properties

2.2. Preform densification Polycarbosilane (PCS; National University of Defense and Technology, China) was chosen as the preceramic polymer for densification of the fibrous performs by PIP process. The ceramic yield is 60.5% by TG test. The preforms were first infiltrated by slurry containing different concentrations of catalysts. After drying, the samples were pyrolyzed up to 1500 1C to convert the polymer into ceramic matrix and in situ growth SiC nanowires in flowing argon atmosphere. One preform underwent slurry without catalyst for comparison. Subsequently, another five PIP cycles using PCS as precursor (without catalyst) were

The as-prepared composites were cut and ground into 3 mm  4 mm  36 mm rectangles for open porosity, bulk density and three-point-bend testing. The Archimedes method was used to measure the density and open porosity of samples. The flexural strength by three-point-bend testing was conducted on the Instron 5566 (Canton, MA) universal testing machine, with the a cross-head speed of 0.5 mm/min and a span of 30 mm. Young's modulus was calculated from the data recorded during three-point-bend testing. The high temperature flexural strength of C/SiC composites with SiC nanowires was evaluated on the Instron 5500R (Canton, MA) universal testing

Table 1 Properties of T700 carbon fiber. Type

Diameter (μm)

Density (g/cm3)

Filaments/yarn

Tensile strength (MPa)

Elastic strength (GPa)

T700

6

1.80

1200

4900

296

Fig. 1. (a) The surface morphology of the carbon fiber fabric with nanowires, and (b) partially enlarged morphology of nanowires with the embedded EDS pattern.

Fig. 2. (a) and (b) EBSD of the as-received SiC nanowires.

Table 2 EBSD results of as-received SiC nanowire. Name

Crystal systerm

Laue group

a (Å)

b (Å)

c (Å)

α(1)

β(1)

γ(1)

SiC

Trigonal

7

3.07

3.07

52.87

90.00

90.00

120.00

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machine, with the same parameters as Instron 5566, and heating to 1100 1C at 6 1C/min and maintaining for 5 min in air. The as-prepared SiC nanowires were characterized by scanning transmission electron microscope (STEM, FEI Magellan 400, USA) with the analysis of electron back scattered diffraction (EBSD).The fracture surface was investigated by field emission scanning electron microscope (FESEM, JEOL 6700F, Tokyo, Japan) and the elemental analysis was conducted by energy dispersive spectroscopy (EDS).

3. Results and discussion 3.1. Morphology of SiC nanowires Fig. 1(a) shows the surface microstructure of the carbon fiber fabric with SiC nanowires. It can be seen that the grown nanowires are so dense that the carbon fiber fabric under the nanowires cannot be observed. The nanowires are randomly oriented with length of dozens of micrometers. The diameters of the nanowires range from 200 to 500 nm, with a uniform hexagonal-shaped structure (Fig. 1(b)). Only Si and C are detected by EDS analysis of the center of the nanowire, indicating a pure Si–C chemistry in nanowires (embedded in Fig. 1(b)). From electron back scattered diffraction (Fig. 2(a) and (b)) and Table 2 of the single SiC nanowire, the SiC nanowire is a single crystalline structure, and belongs to the trigonal system referred to α-SiC. The mechanism for formation of hexagonal-shaped SiC nanowires is not yet clear. However, the o 1 1 1 4 growth direction of cubic β-SiC

Fig. 3. The densities and open porosities of C/SiC composites as a function of the catalyst concentration.

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structure could make the staking sequences of atom layers forming a triangle of section in unit cell, and the hexagonal section can be formed by (1 1 1) planes in adjacent six-unit cell. The hexagonal shape of the synthesized nanowires maybe result from the o 1 1 1 4 growth direction of β-SiC crystal to some extent [16,17]. Therefore, we deduce that the hexagonalshaped SiC nanowire is a kind of core–shell structure with αSiC crystal shell and β-SiC crystalline core. During the whole experiment, iron that could pyrolyze from ferrocene was introduced as catalyst into the reaction system. However, no metallic droplets were observed at the tips of nanowires in electron microscopy, which was possibly due to the evaporation of iron under high temperature, indicating that the SiC nanowires grew through vapor–liquid–solids (VLS) mechanism [14,18]. It could be surmisable that the iron catalyst liquid droplets were formed under 1000 1C, then the SiO and CO gas, which were caused by the process of pyrolyzation of PCS, dissolved into droplets to form hexagonal-shaped SiC nanocrystal [8,14,19]. SiO þ CO-SiC þ CO2

(1)

3.2. Physical and mechanical properties Effect of catalyst concentration on density and porosity derivation of the final composites is shown in Fig. 3. The bulk

Fig. 4. Flexural stress–displacement curves of the composites with and without SiC nanowires.

Table 3 Flexural properties of C/SiC composites without and with SiC nanowires.

Composite without SiC nanowires Composite with SiC nanowiresa Composite with SiC nanowiresa a

Catalyst concentration is 0.1 mol/L. Under 1100 1C in air.

b

Density (g/cm3)

Porosity (%)

Flexural strength (MPa)

Elastic modulus (GPa)

1.2270.01 1.6270.02 1.6270.02

38.97 0.3 23.57 0.2 23.57 0.2

8775 167 714 77.576b

7.270.6 11.57 0.3 –

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density almost increases with the increase of catalyst content, while the open porosity had an opposite tendency. As the catalyst concentration increased from 0.0125 mol/L to 0.1 mol/L, the density of C/SiC composite with SiC nanowires increases from 1.34 g/cm3 to 1.62 g/cm3, and the corresponding open porosity decreases from 36.6% to 23.5%, respectively. Compared to the C/SiC composite without SiC nanowires, its density and porosity are 1.22 g/cm3 and 38.9%, respectively even after 6 cycles. Further enlargement of catalyst concentration, it is unlikely to obviously decrease its porosity, which might be attributed to the

Fig. 5. Flexural stress–displacement curves of the composites with SiC nanowires under 1100 1C for 5 min in air.

closed pores forming during the first time infiltration. For the convenience of comparison, physical and mechanical testing data of the composites with and without SiC nanowires are summarized in Table 3. It is seen that the two composites exhibited quite different physical and bending properties.

3.3. Fracture behavior Fig. 4 shows the stress–displacement curves of these two composites. The curves can be divided into three regions: an initial linear step followed by a large nonlinear step and finally a quasi-linear step. The typical fracture behaviors of the C/SiC composites with and without SiC nanowires subjected to flexural stress are shown in Fig. 4. Moreover, Fig. 5 shows that the C/SiC composites with SiC nanowires could obtain 77.5 MPa under 1100 1C for 5 min in air. As can be seen, the two composites both show a typical non-brittle failure behavior. However, bending characteristics such as initial slope, failure stress are quite different. The C/SiC composites with SiC nanowires exhibit better flexural properties than the ones without SiC nanowires. The result is consistent with the data listed in Table 3. Fig. 6 shows the typical fracture surfaces of the fabricated composites after bending test. As shown in Fig. 6(a), the fracture surface is very planar and few short pullout fibers are observed for composite without SiC nanowires. A number of long pullout fibers are observed in Fig. 6(b), whereas the debonding and pullout of nanowires are rarely observed due to a strong bonding between SiC nanowire and SiC matrix. Some small cracks are observed in SiC matrix without in situ forming

Fig. 6. SEM micrographs of the fracture surface of C/SiC composites. (a) without SiC nanowires, (b) with SiC nanowires, (c) and (d) partially enlarged morphology of SiC matrix.

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SiC nanowires, otherwise than the dense matrix incorporation of SiC nanowires. Moreover, the nanowire bridging, microcrack deflection and SiC nanowire network are observed in Fig. 6(c), as well as the small cracks on the wall of pullout carbon fiber that could be due to the SiC nanowires debonding (Fig. 6(d)), indicating that the elastic deformation of the nanowires may result in a substantial contribution to the increased strength and toughness of the matrix [20]. Additionally, high density causes the matrix to transfer loading effectively, and then improves the flexural strength of composites.

[4]

[5]

[6]

[7]

4. Conclusions [8]

Hexagonal-shaped SiC nanowires were formed in carbon fiber preform by impregnating PCS precursor with ferrocene catalyst, followed by the high-temperature pyrolysis. The incorporation of SiC nanowires not only increased the final density, but also improved the flexural property, achieved maximum levels of which are a maximum stress of 167 MPa with a low density of 1.62 g/cm3. Compared to the composite without SiC nanowires, a 92% increase in flexural property was achieved. The flexural strength of composites with SiC nanowires decreases to the level of 77.5 MPa under 1100 1C in air. The present results clearly show the possibility of increasing flexural stress by in situ SiC nanowires in carbon fiber preform. However, further studies on interphase design of the reinforcing fiber and in situ SiC nanowire are quite necessary for property modification of the composites. Acknowledgments This work has been supported by the National Natural Science Foundation of China under Grant no. 51102256 and Institute Innovation Foundation under Grant no. Y12ZC2120G.

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

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