- Email: [email protected]
carbon composites
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5 9 ( 2 0 1 3 ) 1 7 6 –1 8 3
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Preparation of in situ grown silicon carbide nanofibers radially onto carbon fibers and their effects on the microstructure and flexural properties of carbon/carbon composites Xue-feng Lu a b
a,b,* ,
Peng Xiao
b,*
Key Lab. for Eco-textiles, Ministry of Education, Jiangnan University, Wuxi, Wuxi214122, PR China State Key Lab. for Powder Metallurgy, Central South University, Changsha 410083, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Silicon carbide nanofibers (SiCNFs) used as the second reinforcements of carbon/carbon
Received 3 September 2012
composites were grown radially on the carbon fiber surface. The microstructure of SiCNFs
Accepted 6 March 2013
and their effects on the microstructure and flexural properties of C/C composites were
Available online 13 March 2013
investigated. Results show that there are many defects such as twin crystals and stacking faults in SiCNFs which were grown by catalytic chemical vapor deposition. During the same process, the skin region of carbon fiber has changed. Several SiC layers are formed and the arrangement of the graphite layers around SiC layers is more orderly. In the next chemical vapor infiltration, due to the induction of SiCNFs, the middle textural pyrocarbon were formed firstly and then is the high textural pyrocarbon. The existence of SiCNFs also contributes to the three-phase interface between pyrocarbon, SiCNFs and carbon fibers, resulting in a good bond between carbon fiber and matrix. Those structural changes lead the better flexural properties of SiCNF–C/C composites compared with C/C composites. 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Carbon fiber reinforced carbon (C/C) composites have been successfully used in the fields of aviation, aerospace, nuclear power, medicine and so on, because of their excellent properties such as low density, high specific strength and modulus, high rupture toughness, good thermal shock resistance, high heat of ablation, chemical inertness [1,2]. The excellent properties of C/C composites are not only related to the performances of carbon fibers and matrix, but also dependent on the fiber/matrix interface. A good fiber/matrix interface is beneficial to load transfer, while a weak interface may impair the integrity of composites [3,4]. Therefore, many researches about improving the fiber/matrix interface were made. Recently, carbon nanotubes/nanofibers (CNTs/CNFs) used to modify the fiber/matrix has attracted a worldwide interest [5–11]. Many studies have reported that using CNTs/CNFs to
modify C/C composites can change the fiber/matrix interface structure [5], the pyrocarbon (PyC) structure [6], thermal conductivity [7] and mechanical properties of C/C composites [8– 11]. Our recent work [12] indicates that the existence of CNFs have influence on the interfacial structures of carbon fiber/ CNF, carbon fiber/PyC and CNF/PyC. So we wonder whether different nanofibers maybe have the same influence to the structure and properties of C/C composites. Silicon carbide nanofibers (SiCNFs) have the similar density, structure and chemical properties with CNTs/CNFs, which can also be used as the second reinforcements of C/C composites [13,14]. To date, there are many studies concerning the preparation and structure of SiCNFs, but little has been reported about the C/C composites modified by SiCNFs. However, when silicon carbide particles were dispersed in the matrix [15], the friction coefficient of C/C composites increased [16]. Silicon carbide was also used to displace some carbon to form the
* Corresponding authors: Fax: +86 731 88830131 (P. Xiao). E-mail addresses: [email protected] (X.-f. Lu), [email protected] (P. Xiao). 0008-6223/$ - see front matter 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.03.007
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dual matrixes which have good performance [17–19]. Introduced SiC whiskers into the SiC fiber preform by CVI method can result in a composite with a higher density and a uniform densification [20]. Therefore, we look forward to better properties of C/C composites modified by SiCNFs. In addition, although SiCNFs have similar structure with CNTs/CNFs, as the base surface of deposition, the surface structure of nanofibers has an important influence on the PyC deposition. In a word, it is necessary to study the effect of SiCNFs to structure of C/C composites. In our initial work, we have succeeded in preparing SiCNFs on the surface of carbon fibers by catalytic chemical vapor deposition (CCVD) [21,22] and found that there are two structure of SiCNFs grown by CCVD, layer-by-layer structure and scaly structure. In present study, layer-by-layer structured SiCNFs were grown radially on the carbon fiber surface to produce hybrid preform, and then the preform with SiCNFs was densified by chemical vapor deposition (CVI) to obtain SiCNF reinforced carbon/carbon (SiCNF–C/C) composites. The structure of in situ grown SiCNFs and their effects on the microstructure and flexural properties of C/C composites were investigated.
2.
Experimental
2.1.
Preparation of samples
Carbon fiber warp unidirectional fabric shown in Fig. 1a (260 g/m2, Yixing Tianniao Co. Ltd., China) was used as preform materials. Before using, the carbon fiber fabric with the area of 10 · 10 mm2 was soaked in acetone for 24 h to remove the sizing agent and other adsorbents on the fiber surface, followed by repeated washing in deionizer water. Carbon fiber used here was polyacrylonitrile (PAN) based carbon fiber (T700, 12 K). Nickel particles used as catalysts were obtained on the fiber surface by electroplating, using 10 wt% nickel sulfate as electrolyte. Electrolytic nickel plate was used as the anode, with a unidirectional fabric as the cathode. The electronic current intensity was 10 A. The electroplating time is 10 min. After Ni particles growth, the carbon unidirectional preform was prepared by stacking the carbon unidirectional fabrics in the same direction using self-made graphite clamp (shown in Fig. 1b). SiCNFs were then grown in situ on the fiber surface of the unidirectional carbon preform by CCVD. The CCVD process
(a)
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(b)
was performed at the temperature of 1000 C under the pressure of 500 Pa. Methyltrichorosilane (MTS) was used as the source of silicon carbide, with hydrogen as carrier. The volume ratio of hydrogen to MTS was 60. Argon was fed at a flow rate of 200 sccm as dilute gas. The total flow rate was 400 sccm. Finally, the preform with SiCNFs was densified to obtain SiCNF–C/C composites by CVI. The CVI process was performed at the temperature of 1000 C under the pressure of 10 kPa, with propylene as carbon source and nitrogen as carrier gas. The flow ratio of propylene to nitrogen was 1/2. For comparison, C/C composites with the same fiber content were prepared during the same CVI process.
2.2.
Analytical methods
The bulk density and open porosity of the two composites were measured by Archimedes’ method. The accuracy of the electronic balance was 0.1 mg. Scanning electron microscopy (SEM, JEOL TSM-6360LA), transmission electron microscopy (TEM, JEOL 2010F), polarized light micrograph (PLM, MeF3A) and Raman spectroscopy were employed for the structural characterization. Unpolarized visible Raman spectra were excited with the 633 nm line of a He–Ne laser under nitrogen protection, and the spot size is 1 lm. To investigate the influence of SiCNFs on the graphitization degree of the composites, signals were collected from the skin region of fiber, fiber/PyC interface and PyC, as shown in Fig. 2.
2.3.
Test of mechanical properties
Specimens for mechanical testing were prepared from the two different composites, each measuring 55 · 10 · 4 mm3. The cutting directions were divided into the carbon fiber axial direction and the carbon fiber radial direction (Fig. 3). Six specimens were machined from each one of composites and tested subsequently. Flexural testing using the threepoint bend configuration was performed in a universal testing machine (CSS-44100) equipped with a flexural test fixture. The span-to-depth ratio was 25. The crosshead speed was equivalent to a strain rate of 0.2 mm/min at the compressive face. All presented data was the average value of the five valid data of the specimens for each processing condition.
Carbon unidirectional fabric
Carbon fiber Carbon fiber
Polypropylene fiber Fig. 1 – image of carbon unidirectional fabric (a) and carbon unidirectional preform (b).
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123 PyC
fiber
Fig. 2 – Schematic of Raman spectrum analysis.
Fig. 3 – Sketch map of flexing intension test (a) the radial direction (?); (b) the axial direction (k).
Fig. 5 shows the morphology of SiCNFs on the surface of carbon fibers. Disorderly arranged SiCNFs are grown radially on the surface of every individual fiber. Most SiCNFs are very long, and even some SiCNFs between the two adjacent carbon fibers are in cross-contact with each other. The diameter of SiCNFs is about 20–150 nm and the aspect ratio is about 50. There are few Ni catalysts located on the top of SiCNFs, rather than on the surface of carbon fibers. The Ni catalysts on the nanofiber top are much smaller than that on the fiber surface after electroplating as shown in Fig. 4. Therefore, we can deduce that during CCVD process the Ni dendrites split off from the globular-cactus shaped Ni particles. SiC molecules decomposed from MTS then deposit on these dendrites, consequently resulting in the SiCNFs grown along the dendrites. High resolution TEM (HRTEM) images and the typical diffraction pattern of SiCNFs are shown in Fig. 6. It is obvious that atomic layers are stacked in the direction vertical to the nanofiber axis. The interplanar spacing observed by HRTEM is 0.25 nm, corresponding to the interplanar spacing of the (1 1 1) crystal plane in the cubic b-SiC. It can be also observed that there are many stacking faults, which can be used to infer that SiCNF has a quasi-periodic twin crystal structure. The twins are periodically arranged along the nanofiber axial direction (h1 1 1i direction). Each twin crystal plane consists of 3–10 atomic layers, and the adjacent twin crystal planes form the angle of 141. The diffraction fringes shown in Fig. 6b are vertical to {1 1 1} stacking fault, which further confirms the existence of twin crystals and stacking faults in the SiCNF grown by CCVD.
3.2.
3.
Results and discussion
3.1.
The preparation of SiCNFs
Fig. 4 shows the morphology of the carbon fiber after electroplating Ni. It can be seen that small Ni particles are distributed uniformly on the fiber surface. Based on the TEM analysis (Fig. 4b), we found that the globular-cactus shaped Ni particles with the diameter of about 0.05–0.5 lm are composed of many small dendrites which can be fractured at high temperature.
Effect of SiCNFs on the structure of C/C composites
The polished transverse section of the C/C and SiCNF–C/C composites viewed by polarized-light microscopy is shown in Fig. 7. For C/C composites, PyC around carbon fibers is in the shape of circular shell and has some homocentric annular cracks, which is the typical smooth laminar (SL) PyC. The fiber/PyC interface is ring shaped and loose with cracks. For SiCNF–C/C composites (Fig. 7b), PyC around carbon fibers is many tiny particles with a strong optical reflectivity. These tiny particles are PyC with SiCNFs being wrapped inside. At the edge of pore, although SiCNFs cannot be discerned, the
Fig. 4 – Morphology of carbon fiber after electroplating (a) and Ni particles (b).
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Fig. 5 – SEM images of in situ grown SiCNFs on the surface of fibers. (a) Low magnification; (b) High magnification.
(a)
(b)
11 1 11 1
200
200
1 11
1 11
Fig. 6 – TEM image (a) and the diffraction pattern (b) of the SiCNF.
growth of PyC around SiCNFs is coarse conical. These characteristics belong to the rough laminar (RL) PyC which is more graphitizable. The fiber/PyC interface is tight and jagged. Therefore, during the same CVI process, the in situ grown SiCNFs are beneficial to form the higher textured PyC and the better fiber/PyC interface. In addition, there are some pink areas shown in Fig. 7b, which are mostly composed by agglomerated SiCNFs. Internal pores between agglomerated SiCNFs are too small. Even if having filled the pores utterly, the PyC around the nanofibers is too thin to discern the anisotropy under PLM.
(a)
Based on the samples without high temperature treatment, Raman spectra were introduced to study the integrity of graphite structure. Fig. 8 shows the results of two composites at different locations (shown in Fig. 2). There are two main bands in Raman spectra of carbon materials. One is at about 1580 cm1 (G band) corresponding to graphitic in-plane vibration with E2g symmetry and characterizing for the integrality of SP2 hybrid orbital structure. The other is at 1330 cm1 corresponding to the defect lattice vibration. The reciprocal of D and G band intensity ratio, 1/R, is proportional to graphitization degree [23–26]. Table 1 shows the 1/R values of two composites at different locations. It can be seen that 1/R value of SiCNF-C/C composites is much higher than that of C/C composites at corresponding locations. This is a further confirmation that the existence of SiCNFs leads to the formation of the higher textured PyC. It also can be seen that the 1/R value at location 1 of SiCNF–C/C composites is higher than that of C/C composites, which indicates a much better graphitic structure in the skin region of carbon fiber. Thus it is a reasonable postulate that the skin region structure of carbon fiber has changed during the CCVD process. The SEM photographs of the fracture surfaces of C/C and SiCNF–C/C composites are shown in Fig. 9. For C/C composites, the fracture surface is smooth and the interface between carbon fiber and PyC is clear. PyC around carbon fiber is thick and exhibits clear annular cracks. While for SiCNF–C/C composites shown in Fig. 9b, the fracture surface is rough and no
(b)
CF PyC
CF
PyC SiCNFs+PyC
SiCNF
Fig. 7 – PLM micrographs of C/C (a) and SiCNF–C/C (b) composites.
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D
(a)
600
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500
500
3
200
2
100
1
0 600
800
cps
cps
300
1000
1200
1400
D G
400
G
400
(b)
1600 -1
300
3
200
2
100
1
0 600
1800
800
1000
1200
1400
-1
1600
1800
Wavenumber/cm
Wavenumber/cm
Fig. 8 – Raman spectra of C/C (a) and SiCNF-C/C (b) composites at different locations.
obvious fiber/PyC interface can be observed. There are some tiny particles on the surface of carbon fiber, and next are protuberant globular drops. Both these tiny particles and drops are SiCNFs covered with PyC. The pores between SiCNFs close to the carbon fiber surface are very small, resulting in the formation of these tiny particles. Far from the carbon fiber surface, PyC is deposited preferentially around the SiCNFs and shaped into protuberant globular drops. For further research the microstructure of SiCNF–C/C composites, TEM are introduced and the images are shown in Fig. 10. It can be seen from Fig. 10a that SiCNFs are wrapped in PyC and the fiber/PyC interface is obvious. Higher magnification image (Fig. 10b) of the fiber/PyC interface shows that the disordered arranged SiCNFs make the fiber/PyC interface more complicated. PyC are deposited preferentially on the
Table 1 – 1/R value of corresponding locations from Raman spectra. Sample
Location
C/C SiCNF –C/C
1
2
3
0.457 0.578
0.486 0.891
0.444 0.772
(a)
surface of SiCNFs, and the three-phase interface among PyC, SiCNFs and fibers are formed. Fig. 10c is the HRTEM image of region 1 in Fig. 10b. It can be seen that the interface between PyC and SiCNFs is unobvious, and the angle of two adjacent graphite layers close to SiCNFs is about 141o, which is in good agreement with the angle of two adjacent twin crystal planes in SiCNFs (seen in Fig. 6). Graphite layers far from the SiCNFs are parallel to the axial direction of SiCNFs. Fig. 10d is the HRTEM image of region 2 in Fig. 10b. Several SiC layers exist on the skin region of carbon fiber and graphite layers around the SiC layers are more orderly, which agreed with the result of Raman spectra (Fig. 8).
3.3. The mechanism of the structural change of C/C composites modified by SiCNFs According to the above analysis, the existence of SiCNFs leads to the structural changes of the skin region of carbon fiber and the formation of PyC. These structural changes of C/C composites modified by in situ grown SiCNFs may respectively occur in two processes, which need to be further confirmed. One is the formation of SiC layers and more orderly stacking of graphite layers in the skin region of carbon fiber during the CVD process. The other is the formation of the higher textured PyC during the CVI process.
(b)
PyC CF
CF SiCNFs+PyC
Fig. 9 – SEM images of C/C (a) and SiCNF-C/C (b) composites.
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(a)
(b)
2
CF
CF
SiCNFs+PyC
1
(d)
(c) PyC
SiC
SiCNF
CF PyC
141o
Fig. 10 – TEM images of SiCNF-C/C composite. (a) Low magnigication; (b) high magnification; (c) enlargement of the area 1 in Fig. 9b and (d) enlargement of the area 2 in Fig. 9b.
In the CVD process, MTS are broken down to yield a small amount of silicon atoms and SiC. Silicon then reacts with graphite layers in the skin region of carbon fiber to form silicon carbide layers. At the same time, the SiC adsorbed on the Ni particles penetrates into the skin region of carbon fiber by the diffusion in the Ni particles. Consequently, several SiC layers exist in the skin region of carbon fiber and graphite layers around them are orderly stacking. In the CVI process, SiCNFs with special structure can influence and induce the deposition of PyC. There are two mechanisms of PyC deposition [27]. One is surface active spots controlled mode in which carbon atoms on the solid surface increase by the way of chemical adsorption, surface migration and dehydrogenation of hydrocarbon gas. The other is surface condensation controlled mode in which carbon atoms increase by two ways. One way is physical adsorption, molecular rearrangement and dehydrogenation on the solid surface after the polymerization of aromatic compounds. The other way is molecular rearrangement and dehydrogenation after the polymerization of molecular which adsorbed on the solid surface. The influence of deposition surface is dependent on
the ratio of gas phase reaction rate to surface reaction rate [28]. When PyC deposited on the surface of SiCNFs (shown in Fig. 11), due to surface defects of SiCNFs which can be the active spots, carbon atoms are adsorbed and fill the defects on the SiCNF surface, and then the angle of graphite layers takes place. With the deposition of carbon atoms, the induction of defects decreases and transmission effect of
Graphite layers
SiCNF Axial direction
Fig. 11 – Schematic of deposition of PyC induced by SiCNF.
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Table 2 – Flexural property of the two composites. Sample
Density/g cm3
SiCNF–C/C C/C
1.67 1.67
Open porosity 9.9 7.4
surface structure also decreases. Carbon atoms are deposited on the PyC covered on the SiCNFs. Then the deposition of PyC is mainly controlled by condensation and nucleation. The structure of PyC is dependent on the resident time of the hydrocarbon gas. Due to the many small pores between SiCNFs, the resident time of the hydrocarbon gas increases. Consequently, the carbon atoms deposited on the nanofiber surface form the graphite layers which are parallel to the axial direction of SiCNFs. When the graphite layers further increase, the carbon atoms start to stack freely, which results in the formation of the low textured PyC.
3.4. Effect of in situ grown SiCNFs on the mechanical properties of C/C composites The flexural properties of the two composites are shown in Table 2. It is obvious that the flexural properties of SiCNF–C/ C composites increase both in the axial and radial direction, compared with the C/C composites. It also can be seen that the effect of SiCNFs on the flexural properties in the axial direction is more obvious than in the radial direction. For unidirectional composites, the strength of composites in the radial direction depends on the fiber strength, while in the axial direction,the strength of those composites depends on the matrix and on the bonding between fibers and matrix. The existence of SiCNFs leads to the matrix structural change and better fiber/matrix interface. Therefore, the strength of SiCNF–C/C composites in the axial direction is increased by 180%, and the modulus increased by 60%, while in the radial direction, the strength and modulus are only increased by 16% and 22%.
4.
Conclusions
SiCNFs growing on the surface of carbon fibers have many defects, such as micro-twins and stacking faults. Due to the defects, PyC is preferentially deposited on the surface of SiCNFs, and the adjacent graphite layers of PyC form the same angle as the adjacent twin crystal planes of SiCNFs. Then SiCNFs induce the formation of the high textured PyC. The existence of SiCNFs also contributes to form the three-phase interface among PyC, SiCNFs and carbon fibers, resulting in a good bond between fiber and matrix. In addition, several SiC layers in the skin region of carbon fiber lead to the better orderly arrangement of graphite layers around them. Due to those structural changes, the strength of SiCNF-C/C composites in the axial direction is increased by 180% and the modulus increased by 60%, compared with C/C composites. While in the radial direction, the strength and modulus are only increased by 16% and 22%, respectively.
r?/MPa
E?/GPa
rk/MPa
Ek/GPa
207 179
41 33
25 9
11 7
Acknowledgements This project was supported by the State Key Development Program for Basic Research in China (Grant No. 2011CB605804). The authors express appreciation to Professor L. L. He and PhD. student Y. Q. Lu from the Institute of Metal Research, Chinese Academy of Sciences, for denoting time to do the TEM characterization.
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5 9 ( 2 0 1 3 ) 1 7 6 –1 8 3
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Preparation of in situ grown silicon carbide nanofibers radially onto carbon fibers and their effects on the microstructure and flexural properties of carbon/carbon composites Xue-feng Lu a b
a,b,* ,
Peng Xiao
b,*
Key Lab. for Eco-textiles, Ministry of Education, Jiangnan University, Wuxi, Wuxi214122, PR China State Key Lab. for Powder Metallurgy, Central South University, Changsha 410083, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Silicon carbide nanofibers (SiCNFs) used as the second reinforcements of carbon/carbon
Received 3 September 2012
composites were grown radially on the carbon fiber surface. The microstructure of SiCNFs
Accepted 6 March 2013
and their effects on the microstructure and flexural properties of C/C composites were
Available online 13 March 2013
investigated. Results show that there are many defects such as twin crystals and stacking faults in SiCNFs which were grown by catalytic chemical vapor deposition. During the same process, the skin region of carbon fiber has changed. Several SiC layers are formed and the arrangement of the graphite layers around SiC layers is more orderly. In the next chemical vapor infiltration, due to the induction of SiCNFs, the middle textural pyrocarbon were formed firstly and then is the high textural pyrocarbon. The existence of SiCNFs also contributes to the three-phase interface between pyrocarbon, SiCNFs and carbon fibers, resulting in a good bond between carbon fiber and matrix. Those structural changes lead the better flexural properties of SiCNF–C/C composites compared with C/C composites. 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Carbon fiber reinforced carbon (C/C) composites have been successfully used in the fields of aviation, aerospace, nuclear power, medicine and so on, because of their excellent properties such as low density, high specific strength and modulus, high rupture toughness, good thermal shock resistance, high heat of ablation, chemical inertness [1,2]. The excellent properties of C/C composites are not only related to the performances of carbon fibers and matrix, but also dependent on the fiber/matrix interface. A good fiber/matrix interface is beneficial to load transfer, while a weak interface may impair the integrity of composites [3,4]. Therefore, many researches about improving the fiber/matrix interface were made. Recently, carbon nanotubes/nanofibers (CNTs/CNFs) used to modify the fiber/matrix has attracted a worldwide interest [5–11]. Many studies have reported that using CNTs/CNFs to
modify C/C composites can change the fiber/matrix interface structure [5], the pyrocarbon (PyC) structure [6], thermal conductivity [7] and mechanical properties of C/C composites [8– 11]. Our recent work [12] indicates that the existence of CNFs have influence on the interfacial structures of carbon fiber/ CNF, carbon fiber/PyC and CNF/PyC. So we wonder whether different nanofibers maybe have the same influence to the structure and properties of C/C composites. Silicon carbide nanofibers (SiCNFs) have the similar density, structure and chemical properties with CNTs/CNFs, which can also be used as the second reinforcements of C/C composites [13,14]. To date, there are many studies concerning the preparation and structure of SiCNFs, but little has been reported about the C/C composites modified by SiCNFs. However, when silicon carbide particles were dispersed in the matrix [15], the friction coefficient of C/C composites increased [16]. Silicon carbide was also used to displace some carbon to form the
* Corresponding authors: Fax: +86 731 88830131 (P. Xiao). E-mail addresses: [email protected] (X.-f. Lu), [email protected] (P. Xiao). 0008-6223/$ - see front matter 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.03.007
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dual matrixes which have good performance [17–19]. Introduced SiC whiskers into the SiC fiber preform by CVI method can result in a composite with a higher density and a uniform densification [20]. Therefore, we look forward to better properties of C/C composites modified by SiCNFs. In addition, although SiCNFs have similar structure with CNTs/CNFs, as the base surface of deposition, the surface structure of nanofibers has an important influence on the PyC deposition. In a word, it is necessary to study the effect of SiCNFs to structure of C/C composites. In our initial work, we have succeeded in preparing SiCNFs on the surface of carbon fibers by catalytic chemical vapor deposition (CCVD) [21,22] and found that there are two structure of SiCNFs grown by CCVD, layer-by-layer structure and scaly structure. In present study, layer-by-layer structured SiCNFs were grown radially on the carbon fiber surface to produce hybrid preform, and then the preform with SiCNFs was densified by chemical vapor deposition (CVI) to obtain SiCNF reinforced carbon/carbon (SiCNF–C/C) composites. The structure of in situ grown SiCNFs and their effects on the microstructure and flexural properties of C/C composites were investigated.
2.
Experimental
2.1.
Preparation of samples
Carbon fiber warp unidirectional fabric shown in Fig. 1a (260 g/m2, Yixing Tianniao Co. Ltd., China) was used as preform materials. Before using, the carbon fiber fabric with the area of 10 · 10 mm2 was soaked in acetone for 24 h to remove the sizing agent and other adsorbents on the fiber surface, followed by repeated washing in deionizer water. Carbon fiber used here was polyacrylonitrile (PAN) based carbon fiber (T700, 12 K). Nickel particles used as catalysts were obtained on the fiber surface by electroplating, using 10 wt% nickel sulfate as electrolyte. Electrolytic nickel plate was used as the anode, with a unidirectional fabric as the cathode. The electronic current intensity was 10 A. The electroplating time is 10 min. After Ni particles growth, the carbon unidirectional preform was prepared by stacking the carbon unidirectional fabrics in the same direction using self-made graphite clamp (shown in Fig. 1b). SiCNFs were then grown in situ on the fiber surface of the unidirectional carbon preform by CCVD. The CCVD process
(a)
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(b)
was performed at the temperature of 1000 C under the pressure of 500 Pa. Methyltrichorosilane (MTS) was used as the source of silicon carbide, with hydrogen as carrier. The volume ratio of hydrogen to MTS was 60. Argon was fed at a flow rate of 200 sccm as dilute gas. The total flow rate was 400 sccm. Finally, the preform with SiCNFs was densified to obtain SiCNF–C/C composites by CVI. The CVI process was performed at the temperature of 1000 C under the pressure of 10 kPa, with propylene as carbon source and nitrogen as carrier gas. The flow ratio of propylene to nitrogen was 1/2. For comparison, C/C composites with the same fiber content were prepared during the same CVI process.
2.2.
Analytical methods
The bulk density and open porosity of the two composites were measured by Archimedes’ method. The accuracy of the electronic balance was 0.1 mg. Scanning electron microscopy (SEM, JEOL TSM-6360LA), transmission electron microscopy (TEM, JEOL 2010F), polarized light micrograph (PLM, MeF3A) and Raman spectroscopy were employed for the structural characterization. Unpolarized visible Raman spectra were excited with the 633 nm line of a He–Ne laser under nitrogen protection, and the spot size is 1 lm. To investigate the influence of SiCNFs on the graphitization degree of the composites, signals were collected from the skin region of fiber, fiber/PyC interface and PyC, as shown in Fig. 2.
2.3.
Test of mechanical properties
Specimens for mechanical testing were prepared from the two different composites, each measuring 55 · 10 · 4 mm3. The cutting directions were divided into the carbon fiber axial direction and the carbon fiber radial direction (Fig. 3). Six specimens were machined from each one of composites and tested subsequently. Flexural testing using the threepoint bend configuration was performed in a universal testing machine (CSS-44100) equipped with a flexural test fixture. The span-to-depth ratio was 25. The crosshead speed was equivalent to a strain rate of 0.2 mm/min at the compressive face. All presented data was the average value of the five valid data of the specimens for each processing condition.
Carbon unidirectional fabric
Carbon fiber Carbon fiber
Polypropylene fiber Fig. 1 – image of carbon unidirectional fabric (a) and carbon unidirectional preform (b).
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123 PyC
fiber
Fig. 2 – Schematic of Raman spectrum analysis.
Fig. 3 – Sketch map of flexing intension test (a) the radial direction (?); (b) the axial direction (k).
Fig. 5 shows the morphology of SiCNFs on the surface of carbon fibers. Disorderly arranged SiCNFs are grown radially on the surface of every individual fiber. Most SiCNFs are very long, and even some SiCNFs between the two adjacent carbon fibers are in cross-contact with each other. The diameter of SiCNFs is about 20–150 nm and the aspect ratio is about 50. There are few Ni catalysts located on the top of SiCNFs, rather than on the surface of carbon fibers. The Ni catalysts on the nanofiber top are much smaller than that on the fiber surface after electroplating as shown in Fig. 4. Therefore, we can deduce that during CCVD process the Ni dendrites split off from the globular-cactus shaped Ni particles. SiC molecules decomposed from MTS then deposit on these dendrites, consequently resulting in the SiCNFs grown along the dendrites. High resolution TEM (HRTEM) images and the typical diffraction pattern of SiCNFs are shown in Fig. 6. It is obvious that atomic layers are stacked in the direction vertical to the nanofiber axis. The interplanar spacing observed by HRTEM is 0.25 nm, corresponding to the interplanar spacing of the (1 1 1) crystal plane in the cubic b-SiC. It can be also observed that there are many stacking faults, which can be used to infer that SiCNF has a quasi-periodic twin crystal structure. The twins are periodically arranged along the nanofiber axial direction (h1 1 1i direction). Each twin crystal plane consists of 3–10 atomic layers, and the adjacent twin crystal planes form the angle of 141. The diffraction fringes shown in Fig. 6b are vertical to {1 1 1} stacking fault, which further confirms the existence of twin crystals and stacking faults in the SiCNF grown by CCVD.
3.2.
3.
Results and discussion
3.1.
The preparation of SiCNFs
Fig. 4 shows the morphology of the carbon fiber after electroplating Ni. It can be seen that small Ni particles are distributed uniformly on the fiber surface. Based on the TEM analysis (Fig. 4b), we found that the globular-cactus shaped Ni particles with the diameter of about 0.05–0.5 lm are composed of many small dendrites which can be fractured at high temperature.
Effect of SiCNFs on the structure of C/C composites
The polished transverse section of the C/C and SiCNF–C/C composites viewed by polarized-light microscopy is shown in Fig. 7. For C/C composites, PyC around carbon fibers is in the shape of circular shell and has some homocentric annular cracks, which is the typical smooth laminar (SL) PyC. The fiber/PyC interface is ring shaped and loose with cracks. For SiCNF–C/C composites (Fig. 7b), PyC around carbon fibers is many tiny particles with a strong optical reflectivity. These tiny particles are PyC with SiCNFs being wrapped inside. At the edge of pore, although SiCNFs cannot be discerned, the
Fig. 4 – Morphology of carbon fiber after electroplating (a) and Ni particles (b).
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Fig. 5 – SEM images of in situ grown SiCNFs on the surface of fibers. (a) Low magnification; (b) High magnification.
(a)
(b)
11 1 11 1
200
200
1 11
1 11
Fig. 6 – TEM image (a) and the diffraction pattern (b) of the SiCNF.
growth of PyC around SiCNFs is coarse conical. These characteristics belong to the rough laminar (RL) PyC which is more graphitizable. The fiber/PyC interface is tight and jagged. Therefore, during the same CVI process, the in situ grown SiCNFs are beneficial to form the higher textured PyC and the better fiber/PyC interface. In addition, there are some pink areas shown in Fig. 7b, which are mostly composed by agglomerated SiCNFs. Internal pores between agglomerated SiCNFs are too small. Even if having filled the pores utterly, the PyC around the nanofibers is too thin to discern the anisotropy under PLM.
(a)
Based on the samples without high temperature treatment, Raman spectra were introduced to study the integrity of graphite structure. Fig. 8 shows the results of two composites at different locations (shown in Fig. 2). There are two main bands in Raman spectra of carbon materials. One is at about 1580 cm1 (G band) corresponding to graphitic in-plane vibration with E2g symmetry and characterizing for the integrality of SP2 hybrid orbital structure. The other is at 1330 cm1 corresponding to the defect lattice vibration. The reciprocal of D and G band intensity ratio, 1/R, is proportional to graphitization degree [23–26]. Table 1 shows the 1/R values of two composites at different locations. It can be seen that 1/R value of SiCNF-C/C composites is much higher than that of C/C composites at corresponding locations. This is a further confirmation that the existence of SiCNFs leads to the formation of the higher textured PyC. It also can be seen that the 1/R value at location 1 of SiCNF–C/C composites is higher than that of C/C composites, which indicates a much better graphitic structure in the skin region of carbon fiber. Thus it is a reasonable postulate that the skin region structure of carbon fiber has changed during the CCVD process. The SEM photographs of the fracture surfaces of C/C and SiCNF–C/C composites are shown in Fig. 9. For C/C composites, the fracture surface is smooth and the interface between carbon fiber and PyC is clear. PyC around carbon fiber is thick and exhibits clear annular cracks. While for SiCNF–C/C composites shown in Fig. 9b, the fracture surface is rough and no
(b)
CF PyC
CF
PyC SiCNFs+PyC
SiCNF
Fig. 7 – PLM micrographs of C/C (a) and SiCNF–C/C (b) composites.
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D
(a)
600
5 9 ( 2 0 1 3 ) 1 7 6 –1 8 3
500
500
3
200
2
100
1
0 600
800
cps
cps
300
1000
1200
1400
D G
400
G
400
(b)
1600 -1
300
3
200
2
100
1
0 600
1800
800
1000
1200
1400
-1
1600
1800
Wavenumber/cm
Wavenumber/cm
Fig. 8 – Raman spectra of C/C (a) and SiCNF-C/C (b) composites at different locations.
obvious fiber/PyC interface can be observed. There are some tiny particles on the surface of carbon fiber, and next are protuberant globular drops. Both these tiny particles and drops are SiCNFs covered with PyC. The pores between SiCNFs close to the carbon fiber surface are very small, resulting in the formation of these tiny particles. Far from the carbon fiber surface, PyC is deposited preferentially around the SiCNFs and shaped into protuberant globular drops. For further research the microstructure of SiCNF–C/C composites, TEM are introduced and the images are shown in Fig. 10. It can be seen from Fig. 10a that SiCNFs are wrapped in PyC and the fiber/PyC interface is obvious. Higher magnification image (Fig. 10b) of the fiber/PyC interface shows that the disordered arranged SiCNFs make the fiber/PyC interface more complicated. PyC are deposited preferentially on the
Table 1 – 1/R value of corresponding locations from Raman spectra. Sample
Location
C/C SiCNF –C/C
1
2
3
0.457 0.578
0.486 0.891
0.444 0.772
(a)
surface of SiCNFs, and the three-phase interface among PyC, SiCNFs and fibers are formed. Fig. 10c is the HRTEM image of region 1 in Fig. 10b. It can be seen that the interface between PyC and SiCNFs is unobvious, and the angle of two adjacent graphite layers close to SiCNFs is about 141o, which is in good agreement with the angle of two adjacent twin crystal planes in SiCNFs (seen in Fig. 6). Graphite layers far from the SiCNFs are parallel to the axial direction of SiCNFs. Fig. 10d is the HRTEM image of region 2 in Fig. 10b. Several SiC layers exist on the skin region of carbon fiber and graphite layers around the SiC layers are more orderly, which agreed with the result of Raman spectra (Fig. 8).
3.3. The mechanism of the structural change of C/C composites modified by SiCNFs According to the above analysis, the existence of SiCNFs leads to the structural changes of the skin region of carbon fiber and the formation of PyC. These structural changes of C/C composites modified by in situ grown SiCNFs may respectively occur in two processes, which need to be further confirmed. One is the formation of SiC layers and more orderly stacking of graphite layers in the skin region of carbon fiber during the CVD process. The other is the formation of the higher textured PyC during the CVI process.
(b)
PyC CF
CF SiCNFs+PyC
Fig. 9 – SEM images of C/C (a) and SiCNF-C/C (b) composites.
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(a)
(b)
2
CF
CF
SiCNFs+PyC
1
(d)
(c) PyC
SiC
SiCNF
CF PyC
141o
Fig. 10 – TEM images of SiCNF-C/C composite. (a) Low magnigication; (b) high magnification; (c) enlargement of the area 1 in Fig. 9b and (d) enlargement of the area 2 in Fig. 9b.
In the CVD process, MTS are broken down to yield a small amount of silicon atoms and SiC. Silicon then reacts with graphite layers in the skin region of carbon fiber to form silicon carbide layers. At the same time, the SiC adsorbed on the Ni particles penetrates into the skin region of carbon fiber by the diffusion in the Ni particles. Consequently, several SiC layers exist in the skin region of carbon fiber and graphite layers around them are orderly stacking. In the CVI process, SiCNFs with special structure can influence and induce the deposition of PyC. There are two mechanisms of PyC deposition [27]. One is surface active spots controlled mode in which carbon atoms on the solid surface increase by the way of chemical adsorption, surface migration and dehydrogenation of hydrocarbon gas. The other is surface condensation controlled mode in which carbon atoms increase by two ways. One way is physical adsorption, molecular rearrangement and dehydrogenation on the solid surface after the polymerization of aromatic compounds. The other way is molecular rearrangement and dehydrogenation after the polymerization of molecular which adsorbed on the solid surface. The influence of deposition surface is dependent on
the ratio of gas phase reaction rate to surface reaction rate [28]. When PyC deposited on the surface of SiCNFs (shown in Fig. 11), due to surface defects of SiCNFs which can be the active spots, carbon atoms are adsorbed and fill the defects on the SiCNF surface, and then the angle of graphite layers takes place. With the deposition of carbon atoms, the induction of defects decreases and transmission effect of
Graphite layers
SiCNF Axial direction
Fig. 11 – Schematic of deposition of PyC induced by SiCNF.
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Table 2 – Flexural property of the two composites. Sample
Density/g cm3
SiCNF–C/C C/C
1.67 1.67
Open porosity 9.9 7.4
surface structure also decreases. Carbon atoms are deposited on the PyC covered on the SiCNFs. Then the deposition of PyC is mainly controlled by condensation and nucleation. The structure of PyC is dependent on the resident time of the hydrocarbon gas. Due to the many small pores between SiCNFs, the resident time of the hydrocarbon gas increases. Consequently, the carbon atoms deposited on the nanofiber surface form the graphite layers which are parallel to the axial direction of SiCNFs. When the graphite layers further increase, the carbon atoms start to stack freely, which results in the formation of the low textured PyC.
3.4. Effect of in situ grown SiCNFs on the mechanical properties of C/C composites The flexural properties of the two composites are shown in Table 2. It is obvious that the flexural properties of SiCNF–C/ C composites increase both in the axial and radial direction, compared with the C/C composites. It also can be seen that the effect of SiCNFs on the flexural properties in the axial direction is more obvious than in the radial direction. For unidirectional composites, the strength of composites in the radial direction depends on the fiber strength, while in the axial direction,the strength of those composites depends on the matrix and on the bonding between fibers and matrix. The existence of SiCNFs leads to the matrix structural change and better fiber/matrix interface. Therefore, the strength of SiCNF–C/C composites in the axial direction is increased by 180%, and the modulus increased by 60%, while in the radial direction, the strength and modulus are only increased by 16% and 22%.
4.
Conclusions
SiCNFs growing on the surface of carbon fibers have many defects, such as micro-twins and stacking faults. Due to the defects, PyC is preferentially deposited on the surface of SiCNFs, and the adjacent graphite layers of PyC form the same angle as the adjacent twin crystal planes of SiCNFs. Then SiCNFs induce the formation of the high textured PyC. The existence of SiCNFs also contributes to form the three-phase interface among PyC, SiCNFs and carbon fibers, resulting in a good bond between fiber and matrix. In addition, several SiC layers in the skin region of carbon fiber lead to the better orderly arrangement of graphite layers around them. Due to those structural changes, the strength of SiCNF-C/C composites in the axial direction is increased by 180% and the modulus increased by 60%, compared with C/C composites. While in the radial direction, the strength and modulus are only increased by 16% and 22%, respectively.
r?/MPa
E?/GPa
rk/MPa
Ek/GPa
207 179
41 33
25 9
11 7
Acknowledgements This project was supported by the State Key Development Program for Basic Research in China (Grant No. 2011CB605804). The authors express appreciation to Professor L. L. He and PhD. student Y. Q. Lu from the Institute of Metal Research, Chinese Academy of Sciences, for denoting time to do the TEM characterization.
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