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SiC composites by introducing CNTs into the PyC interface
Materials Science & Engineering A 637 (2015) 123–129
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
The improvement in the mechanical and thermal properties of SiC/SiC composites by introducing CNTs into the PyC interface Wei Feng a, Litong Zhang a, Yongsheng Liu a,n, Xiaoqiang Li a, Laifei Cheng a, Shanlin Zhou b, Hui Bai a a
Science and Technology on Thermostructure Composite Materials Laboratory, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, China State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 10 March 2015 Accepted 4 April 2015 Available online 16 April 2015
Carbon nanotubes (CNTs) were introduced into the interface of SiC/SiC composites via electrophoretic deposition to improve their mechanical and thermal properties. SiC/SiC composites with pyrocarbon (PyC) and CNTs–PyC interfaces were denoted as SiC/SiC–P and SiC/SiC–CP, respectively. The results show that the flexural strength, fracture energy, interfacial shear strength and thermal conductivity increased upon the introduction of CNTs (SiC/SiC–CP) and were 1.174, 1.257, more than 2, and 2.158 times those of SiC/SiC–P, respectively. It was demonstrated that meshed CNTs on SiC fibres can strengthen the interfacial bonding between the fibre and matrix and improve the thermal conductivity of the interface. The meshed CNTs had positive effects on the mechanical and thermal properties of the SiC/SiC composites. & 2015 Elsevier B.V. All rights reserved.
Keywords: SiC/SiC composites CNTs Mechanical property Interface Thermal conductivity
1. Introduction The increasing interest in SiC/SiC composites for fusion and advanced fission energy applications comes from their excellent irradiation performance, including their very good high temperature fracture and creep, outstanding oxidation resistance, inherently low induced radioactivation and excellent irradiation performance [1–5]. In nuclear reactors, SiC/SiC composites face very severe conditions that require adequate mechanical properties and thermal conductivity, which have thus far not been achieved. These properties are highly dependant on their three constituents: the fibres, matrix and interface. In SiC/SiC composites, the interface plays a key role in their behaviour, such as preventing early failure of fibres, transferring load and heat between the fibres and matrix, and releasing part of the residual thermal stresses [6–10]. The most commonly used interfacial material in SiC/SiC composites for fusion/fission reactors is pyrocarbon (PyC) [11–13]. To improve the overall performance of SiC/ SiC composites, particularly to increase their mechanical strength and thermal conductivity, introducing carbon nanotubes (CNTs) into the PyC interface is highly promising. CNTs are characterized not only by the ability to increase the toughness of materials [14–18] but also by a relatively high thermal stability in non-oxidizing atmospheres. Extremely high thermal conductivities are theoretically predicted for CNTs due to the large *
Corresponding author. Tel.: þ86 29 8849 6068 823; fax: þ 86 29 8849 4620. E-mail addresses: [email protected], [email protected] (Y. Liu).
http://dx.doi.org/10.1016/j.msea.2015.04.006 0921-5093/& 2015 Elsevier B.V. All rights reserved.
phonon mean free path in the strong carbon sp2 bond network of the CNT walls ( 3000 W/(m K)) [19]. Many researchers have focused on CNTs, and CNTs-reinforced materials and CNTs/SiC [20], CNTs/Al2O3 [21,22] and CNTs/SiO2 [23] composites have been fabricated. However, few reports focused on the introduction of CNTs into SiC/SiC composites. In König’s work [24], CNTs were deposited onto SiC fibres as an interface in SiC/SiC composites for the first time, but there was no report on the relative mechanical or thermal properties. Although CNTs have many excellent properties and have been introduced into many ceramics, the effect of introducing CNTs into the PyC interface on the performance of SiC/SiC composites is still unclear. In this work, CNTs were introduced into the interface of SiC/SiC composites by electrophoretic deposition. The microstructure, three-point bending, interfacial shear strength and thermal conductivity of the SiC/SiC composites with a CNTs–PyC interface were characterized. These properties were compared to those of traditional SiC/SiC composites with a PyC interface.
2. Experimental 2.1. Preparation of CNT suspension Multi-wall carbon nanotubes, purity 4 95 wt%, 8–15 nm in diameter and 50 μm in length, were purchased from Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Sciences, Chengdu, China. A total of 0.05 g CNTs was dispersed in deionized water (1 L), and Triton X-100 was used as a dispersant.
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Tetramethyl-ammonium (TMAH) was used as an anionic surfactant and pH modifier. To form a CNT suspension, the pH level was adjusted to 11 to obtain a negatively charged zeta potential [29]. The suspension was homogenized using a strong probe-type ultrasound device (BILON-1500, China) for 30 min.
into thin slices and carefully polished to 250 μm in thickness. Then, the two specimens were attached to a graphite sheet with a hole 3 mm in diameter in the centre to enable the fibres to be pushed-out. Ten fibres perpendicular to the thickness were selected for testing in each specimen. The interfacial shear strength (IFSS) can be determined by the following equation:
2.2. Preparation of SiC/SiC composites with CNTs–PyC and PyC interfaces
τ=
Thin CNTs interfaces were individually deposited on 10 layers of SiC cloth (0.25K, plain weave, made in China), which was pretreated with sodium dioctyl-sulfosuccinate (SDODD) solution to improve wettability [24], by electrophoretic deposition (EPD) at 10–15 V for 15–20 min. The basic information of the SiC fibre is shown in Table 1. Electrodes made of graphite were vertically immersed in the aqueous suspension in a cell, and the SiC fabric was placed in front of the electrode during the EPD. After depositing the CNTs, the 10 layers were stacked with a fabric layer orientation of 0°/90°. Another SiC fabric was prepared using 10 layers of SiC cloth without the CNT interface in the same way. Then, a PyC layer was deposited on these two SiC fabrics using a C3H6 precursor at 870 °C and a reduced pressure of 5 kPa via low pressure chemical vapour deposition (LPCVI). The SiC matrix was then densely deposited in the two preforms by CVI at a reduced pressure of 2 kPa and a temperature of approximately 1050 °C using methyltrichlorosilane (MTS, CH3SiCl3) with a H2:MTS molar ratio of 10. This process was achieved by bubbling hydrogen gas through the MTS. Argon was used as a diluent to slow the chemical reaction rate during the deposition. Afterwards, the two SiC/SiC composites with PyC (SiC/SiC–P) and CNTs–PyC (SiC/SiC–CP) interfaces were each machined and polished into 3 samples with dimensions of φ12.7 mm 3 mm and 5 samples with dimensions of 40 mm 5 mm 3 mm. 2.3. Characterization
Table 1 The basic parameters of the SiC fibre. C/SiC
Diameter (μm)
Filament strength (GPa)
1.124
12.43
3.33
Table 2 Properties of SiC/SiC–CP and SiC/SiC–P. Sample
Density (g/ cm3)
SiC/SiC–CP 2.62 7 0.03 SiC/SiC–P 2.60 7 0.03
(1)
where p is the debonding load, a is the radius of the fibre and h is the thickness of the specimen. A Laser Flash Apparatus LFA 427, made by NETZSCH Company, was employed for measurement of the thermal conductivity from room temperature to 1000 °C. The thermal conductivity of the SiC/ SiC composites was calculated from the following equations:
K = αρCp
(2)
α = 1.38L2/π 2t1/2
(3)
where α is the thermal diffusion, ρ is the density, Cp is the specific heat, L is the specimen thickness and t1/2 is the time required to reach half of the total temperature rise on the rear surface of the specimen. Cp and α were measured directly through the laser flash method, and the thermal conductivity was calculated via Eq. (2). A transmission electron microscope (G-20, FEI-Tecnai, USA) operating at an accelerating voltage of 200 kV was used for the transmission electron microscopy (TEM) analysis. The micro-morphologies were observed by a scanning electronic microscope (SEM, JEOL6700F, Tokyo, Japan).
3. Results and discussion 3.1. Microstructure
The flexural strength of the composites was measured using a three-point bending test (SANS CMT 4304, Sans Testing Machine, Shen Zhen, China) with a span of 30 mm at a loading rate of 0.5 mm/min. The force–displacement curves were recorded by a computer. To determine the influence of the deposited CNTs on the interfacial bonding strength between the fibre and matrix, a single fibre push-out test was performed. A Nano Indentor TI 950 Triboindenter (Hysitron), located in the State Key Laboratory for the Mechanical Behaviour of Materials (in Xi'an Jiaotong University), was used to measure the fibre/matrix interfacial shear strength of the SiC/SiC composites with different interfaces at room temperature. The indenter was conical with a 60° tape angle, and the curvature radius at the tip was 5 μm. The two composites were cut
SiC fibre
p 2πah
Bending strength Fracture energy (MPa) (kJ/m2)
IFSS (MPa)
505 7 23 430 7 17
455 26.6 7 4.1
90.4 73.4 71.9 72.8
The two composites have almost the same density; the introduction of CNTs into the interface has no obvious effect on the density of the SiC/SiC composites (Table 2). The microstructure of the SiC fibre is shown in Fig. 1a; the surface of the SiC fibre is quite smooth. After the deposition of the CNTs, the SiC fibres are covered by a thin layer of CNTs, and the surface of the fibres is roughened (Fig. 1b). The CNTs introduced by the EPD form a reticulate structure on the surface of the SiC fibres (Fig. 1c), and the CNTs are well dispersed. Fig. 2 shows the interface morphologies of SiC/SiC–P (Fig. 2a) and SiC/SiC–CP (Fig. 2b). The CNT layer is very thin, so the thicknesses of the PyC and CNTs–PyC interfaces are both approximately 100 nm. However, the PyC interface is uneven, and the boundary is coarse. When a thin layer of CNTs deposits on the SiC fibre before the PyC, the morphology of the PyC interface becomes quite even, and the boundary is smooth. This might be due to the CNTs' large surface area, which makes the deposition of the PyC easier. The surface condition and structure of the fibre have important effects on the structure and bonding state of the CVI–PyC interface [25]. To analyse the structural difference between PyC deposited on fibre and on CNTs, TEM is employed. Fig. 3 shows the TEM images of PyC deposited on CNTs and SiC fibres. The CNTs are surrounded by the PyC deposited on them (Fig. 3a). Fig. 3b and c shows high resolution TEM images of the PyC deposited on SiC fibre and on CNTs. The PyC deposited on the SiC fibres presents a low nano-textural order without an obvious orientation and a low texture. The PyC deposited on the CNTs shows a morphology of short graphene fringes partially oriented in the same direction (marked by rectangles) and longer, highly curved graphene fringes (marked by circles), which confirms that the PyC deposited on CNTs has a higher texture than that
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Fig. 1. Surface morphology of SiC fibre: (a) without CNTs; (b) with CNTs deposited on the fibre.
Fig. 2. The morphologies of the two interfaces: (a) PyC; (b) CNTs–PyC.
deposited on SiC fibres. According to Chen Jie's research, CNTs can act as nuclei of small hydrocarbon molecules in deposition because of the π–π conjugated electronic structure of CNTs, and small hydrocarbon molecules can deposit, grow, coalesce and align in the direction vertical to the perpendicular surface of the CNTs [26]. 3.2. Mechanical properties Fig. 4 shows typical bending strength–displacement curves derived from the three-point bending test for SiC/SiC–P and SiC/ SiC–CP. These two load–displacement curves display several common features: (1) an initial linear region, which reflects the elastic response of these two composites; (2) a nonlinear region
until the maximum load, mainly caused by matrix cracking, interfacial debonding and fibre sliding; and (3) a quick reduction in the load after reaching its maximum, perhaps related to the failure of a significant fraction of the SiC fibres. SiC/SiC–CP has a higher maximum load than SiC/SiC–P. The average flexural strengths of SiC/SiC–CP and SiC/SiC–P are 505 723 MPa and 430 717 MPa (Table 2), respectively. The fracture energies are obtained from the area below the load–displacement curves until the maximum load [27]. The fracture energies of SiC/SiC–CP and SiC/SiC–P are 90.4 73.4 kJ/m2 and 71.972.8 kJ/m2 (Table 2), respectively. These results indicate that SiC/SiC–CP has a higher strength and absorbs more energy before fracture.
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Fig. 3. (a) TEM image of PyC deposited on CNTs; (b) HRTEM of PyC deposited on SiC fibre; (c) HRTEM of PyC deposited on CNTs.
Fig. 4. Typical bending strength–displacement curves for SiC/SiC composites with PyC interface and CNTs–PyC interface.
Typical SEM fracture surfaces of these two composites are shown in Fig. 5. Debonding and fibre pull-outs are observed on the fracture surfaces. The average fibre pull-out length of SiC/SiC–P is approximately 200 μm, while that of SiC/SiC–CP is less than 100 μm. The main functions of the interface in SiC/SiC composites are to transfer the load from the matrix to the fibres and to deflect the matrix cracks at the interface to obtain improved fracture tolerance [28]. A strong interfacial bonding between the fibre and
matrix always leads to a shorter pull-out length of the fibre. This phenomenon indicates that SiC/SiC–CP has a stronger fibre/matrix bonding than SiC/SiC–P. It seems that the interfacial bonding strength between the SiC fibres and SiC matrix is strengthened by CNTs. To verify the difference in the interfacial bonding strength between these two interfaces, a single fibre push-out test is carried out to quantitatively evaluate the actual interfacial shear strength. Fig. 6 shows SEM images of the pushed-in and pushedout fibre, as observed from the top (Fig. 6a) and bottom (Fig. 6b), respectively. Fig. 7 shows typical indentation load–displacement curves of SiC/SiC–CP and SiC/SiC–P. In Fig. 7a, when the load is held steady, debonding between the fibre and matrix occurs, and the fibre is pushed-out from the matrix. The fibre diameter is approximately 12.43 μm, and the interfacial shear strength can be calculated according to Eq. (1). The IFSS of PyC is approximately 26.67 4.1 MPa. No fibres are pushed-out in SiC/SiC–CP at the same test condition, and according to the load–displacement curve in Fig. 7b, the IFSS of the CNTs–PyC interface is greater than 55 MPa (Table 2). This is in accordance with the assumptions and the results revealed by the three-point bending tests. The meshed CNTs introduced by EPD on the surface of the SiC fibres before the deposition of PyC act as a reinforced phase that can strengthen the
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Fig. 5. SEM micrographs of the fracture surface of two composites after the three-point bending test: (a) SiC/SiC–P; (b) SiC/SiC–CP.
Fig. 6. SEM of the fibre after a single fibre push-out test: (a) fibre push-in as observed from the top surface; (b) fibre push-out as observed from the bottom surface.
Fig. 7. Typical indentation load vs. displacement curves of SiC/SiC composites with (a) PyC interface; (b) CNTs–PyC interface.
bonding between the fibre and the matrix, so SiC/SiC–CP has a stronger interface than SiC/SiC–P. The interface is a critical part of a composite because load transfers from the matrix to the fibre and vice versa must occur through the interface. A weak interface (PyC) debonds easily, and load transfers through the debonded interface are poor. The mechanical properties of the SiC/SiC–P composites are low. Upon adding CNTs into the PyC interface, the meshed CNTs on the SiC fibres can strengthen the bonding between the fibre and matrix, and SiC/SiC–CP has a stronger interface than SiC/SiC–P. In this stronger interface, cracks that appear in the matrix can be deflected into short, branched multiple cracks [29, 30]. Short
debonding segments, as well as improved load transfer, allow further cracking of the matrix via the scale effect [31, 32], leading to a higher density of matrix cracks, and the sliding friction of the strong interface and the multiple cracking of the matrix then increase the energy absorption. SiC/SiC–CP therefore has a higher flexural strength and fracture energy than SiC/SiC–P. 3.3. Thermal conductivity The thermal conductivity measured in this experiment is the traverse thermal conductivity, along the thickness of the samples. Based on the measured thermal diffusivity and specific data, the
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thermal conductivities are slightly reduced due to phonon–phonon interactions. In the test of thermal conductivity, the direction of the heat flux is perpendicular to the fibre and interface. The schematic of heat transfer in the thickness of the SiC/SiC composites is shown in Fig. 9a, and a simplified form using a serial structural model, analogous to current transfer in a series circuit, is shown in Fig. 9b. The effective traverse thermal conductivity can be calculated by Eq. (4):
v v v 1 = m + i + f Keff Km Ki Kf
Fig. 8. Thermal conductivity of SiC/SiC composites with different interfaces.
(4)
where Keff is the effective traverse thermal conductivity, and Km, Ki and Kf are the thermal conductivities of the matrix, interface and fibre, respectively. vm, vi, and vf are the volume fractions of the matrix, interface and fibre, respectively. The two composites are quite similar except for the interface. The CNTs–PyC interface can be seen as CNTs dispersed into the continuous PyC (Fig. 10). The thermal conductivity of CNTs–PyC can be calculated by Eq. (5):
( )(2 1 − v (1 − )(
1 + 2vc 1 − Ki = Kp
c
Kp Kc
Kp Kc
Kp
Kp
Kc
Kc
+1
+1
)
) (5)
where Ki, Kp and Kc are the thermal conductivities of the interface, PyC and CNTs, respectively, and vc is the CNTs volume fraction. The thermal conductivity of the CNTs ( 3000 W/(m K)) is much higher than that of PyC, so Eq. (5) can be simplified as follows:
Ki = Kp
Fig. 9. Schematic diagram of heat transfer along the thickness of SiC/SiC composites.
1 + 2vc 1 − vc
(6)
Upon adding CNTs into the PyC interface, Ki increases, so the interfacial thermal conductivity of SiC/SiC–CP is higher than that of SiC/SiC–P. Additionally, the texture of PyC is higher than that of pure CVI–PyC, so the thermal conductivity of PyC deposited on CNTs is higher than that deposited on fibres [33]. In general, the introduction of CNTs into the PyC interface increases the thermal conductivity of the interface (Ki). Considering the quite thin CNTs layer, the change in the interfacial volume fraction (vi) in these two composites can be neglected. Then, with the increase of Ki, Keff increases. Hence, the traverse thermal conductivity of SiC/SiC–CP is higher than that of SiC/SiC–P.
4. Conclusions The effects of CNTs on the microstructure, mechanical and thermal properties of SiC/SiC composites are investigated. The improved mechanical and thermal properties of SiC/SiC composites have been demonstrated by introducing CNTs into the PyC interface. The conclusions can be summarized as follows: Fig. 10. Serial structural model of heat transfer along the thickness of SiC/SiC composites.
thermal conductivity of SiC/SiC composites with different interfaces can be obtained (shown in Fig. 8). Without CNTs in the interface, the thermal conductivity of SiC/ SiC–P is 8.2 W/(m K) at room temperature. Upon adding CNTs into the interface, the thermal conductivity of SiC/SiC–CP increases to 17.7 W/(m K), which is 2.16 times higher than that of SiC/SiC–P at room temperature. With the increase of the test temperature, the
(1) Compared with a pure PyC interface, the uniformity and texture of the PyC interface improves greatly by introducing CNTs, resulting in the short graphene fringes partially orienting in the same direction and longer, highly curved graphene fringes. (2) The meshed CNTs introduced on the surface of the SiC fibres before the deposition of the PyC interface can strengthen the bonding between the fibre and matrix. The IFSS between the SiC fibres and SiC matrix is improved by more than 2 times, from 26.674.1 MPa to above 55 MPa. A stronger interface has a positive effect on the mechanical properties of SiC/SiC composites. (3) Due to not only the extremely high thermal conductivity of CNTs but also the improved thermal conductivity of the more highly
W. Feng et al. / Materials Science & Engineering A 637 (2015) 123–129
textured PyC obtained by the introduction of CNTs into the PyC interface, the traverse thermal conductivity of the SiC/SiC composites increases by 2.16 times. The heat transfer in the thickness of the SiC/SiC composites can be simplified using a serial structural model. Introducing CNTs into the interface improves the thermal conductivity of the interface, and thereby, the overall thermal conductivity of the SiC/SiC composite is improved.
Acknowledgements The authors acknowledge the support of the Chinese National Foundation for Natural Sciences Under Contracts (Nos. 51332004, 51472201), the Fundamental Research Funds for the Central Universities (No. 3102014KYJD011), the Fund of the State Key Laboratory of Solidification Processing in NWPU (No. SKLSP201401), the Research Fund of Northwestern Polytechnical University (20120204), and the Major National Scientific Instrument and Equipment Development Project (2011YQ12007504).
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
The improvement in the mechanical and thermal properties of SiC/SiC composites by introducing CNTs into the PyC interface Wei Feng a, Litong Zhang a, Yongsheng Liu a,n, Xiaoqiang Li a, Laifei Cheng a, Shanlin Zhou b, Hui Bai a a
Science and Technology on Thermostructure Composite Materials Laboratory, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, China State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 10 March 2015 Accepted 4 April 2015 Available online 16 April 2015
Carbon nanotubes (CNTs) were introduced into the interface of SiC/SiC composites via electrophoretic deposition to improve their mechanical and thermal properties. SiC/SiC composites with pyrocarbon (PyC) and CNTs–PyC interfaces were denoted as SiC/SiC–P and SiC/SiC–CP, respectively. The results show that the flexural strength, fracture energy, interfacial shear strength and thermal conductivity increased upon the introduction of CNTs (SiC/SiC–CP) and were 1.174, 1.257, more than 2, and 2.158 times those of SiC/SiC–P, respectively. It was demonstrated that meshed CNTs on SiC fibres can strengthen the interfacial bonding between the fibre and matrix and improve the thermal conductivity of the interface. The meshed CNTs had positive effects on the mechanical and thermal properties of the SiC/SiC composites. & 2015 Elsevier B.V. All rights reserved.
Keywords: SiC/SiC composites CNTs Mechanical property Interface Thermal conductivity
1. Introduction The increasing interest in SiC/SiC composites for fusion and advanced fission energy applications comes from their excellent irradiation performance, including their very good high temperature fracture and creep, outstanding oxidation resistance, inherently low induced radioactivation and excellent irradiation performance [1–5]. In nuclear reactors, SiC/SiC composites face very severe conditions that require adequate mechanical properties and thermal conductivity, which have thus far not been achieved. These properties are highly dependant on their three constituents: the fibres, matrix and interface. In SiC/SiC composites, the interface plays a key role in their behaviour, such as preventing early failure of fibres, transferring load and heat between the fibres and matrix, and releasing part of the residual thermal stresses [6–10]. The most commonly used interfacial material in SiC/SiC composites for fusion/fission reactors is pyrocarbon (PyC) [11–13]. To improve the overall performance of SiC/ SiC composites, particularly to increase their mechanical strength and thermal conductivity, introducing carbon nanotubes (CNTs) into the PyC interface is highly promising. CNTs are characterized not only by the ability to increase the toughness of materials [14–18] but also by a relatively high thermal stability in non-oxidizing atmospheres. Extremely high thermal conductivities are theoretically predicted for CNTs due to the large *
Corresponding author. Tel.: þ86 29 8849 6068 823; fax: þ 86 29 8849 4620. E-mail addresses: [email protected], [email protected] (Y. Liu).
http://dx.doi.org/10.1016/j.msea.2015.04.006 0921-5093/& 2015 Elsevier B.V. All rights reserved.
phonon mean free path in the strong carbon sp2 bond network of the CNT walls ( 3000 W/(m K)) [19]. Many researchers have focused on CNTs, and CNTs-reinforced materials and CNTs/SiC [20], CNTs/Al2O3 [21,22] and CNTs/SiO2 [23] composites have been fabricated. However, few reports focused on the introduction of CNTs into SiC/SiC composites. In König’s work [24], CNTs were deposited onto SiC fibres as an interface in SiC/SiC composites for the first time, but there was no report on the relative mechanical or thermal properties. Although CNTs have many excellent properties and have been introduced into many ceramics, the effect of introducing CNTs into the PyC interface on the performance of SiC/SiC composites is still unclear. In this work, CNTs were introduced into the interface of SiC/SiC composites by electrophoretic deposition. The microstructure, three-point bending, interfacial shear strength and thermal conductivity of the SiC/SiC composites with a CNTs–PyC interface were characterized. These properties were compared to those of traditional SiC/SiC composites with a PyC interface.
2. Experimental 2.1. Preparation of CNT suspension Multi-wall carbon nanotubes, purity 4 95 wt%, 8–15 nm in diameter and 50 μm in length, were purchased from Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Sciences, Chengdu, China. A total of 0.05 g CNTs was dispersed in deionized water (1 L), and Triton X-100 was used as a dispersant.
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Tetramethyl-ammonium (TMAH) was used as an anionic surfactant and pH modifier. To form a CNT suspension, the pH level was adjusted to 11 to obtain a negatively charged zeta potential [29]. The suspension was homogenized using a strong probe-type ultrasound device (BILON-1500, China) for 30 min.
into thin slices and carefully polished to 250 μm in thickness. Then, the two specimens were attached to a graphite sheet with a hole 3 mm in diameter in the centre to enable the fibres to be pushed-out. Ten fibres perpendicular to the thickness were selected for testing in each specimen. The interfacial shear strength (IFSS) can be determined by the following equation:
2.2. Preparation of SiC/SiC composites with CNTs–PyC and PyC interfaces
τ=
Thin CNTs interfaces were individually deposited on 10 layers of SiC cloth (0.25K, plain weave, made in China), which was pretreated with sodium dioctyl-sulfosuccinate (SDODD) solution to improve wettability [24], by electrophoretic deposition (EPD) at 10–15 V for 15–20 min. The basic information of the SiC fibre is shown in Table 1. Electrodes made of graphite were vertically immersed in the aqueous suspension in a cell, and the SiC fabric was placed in front of the electrode during the EPD. After depositing the CNTs, the 10 layers were stacked with a fabric layer orientation of 0°/90°. Another SiC fabric was prepared using 10 layers of SiC cloth without the CNT interface in the same way. Then, a PyC layer was deposited on these two SiC fabrics using a C3H6 precursor at 870 °C and a reduced pressure of 5 kPa via low pressure chemical vapour deposition (LPCVI). The SiC matrix was then densely deposited in the two preforms by CVI at a reduced pressure of 2 kPa and a temperature of approximately 1050 °C using methyltrichlorosilane (MTS, CH3SiCl3) with a H2:MTS molar ratio of 10. This process was achieved by bubbling hydrogen gas through the MTS. Argon was used as a diluent to slow the chemical reaction rate during the deposition. Afterwards, the two SiC/SiC composites with PyC (SiC/SiC–P) and CNTs–PyC (SiC/SiC–CP) interfaces were each machined and polished into 3 samples with dimensions of φ12.7 mm 3 mm and 5 samples with dimensions of 40 mm 5 mm 3 mm. 2.3. Characterization
Table 1 The basic parameters of the SiC fibre. C/SiC
Diameter (μm)
Filament strength (GPa)
1.124
12.43
3.33
Table 2 Properties of SiC/SiC–CP and SiC/SiC–P. Sample
Density (g/ cm3)
SiC/SiC–CP 2.62 7 0.03 SiC/SiC–P 2.60 7 0.03
(1)
where p is the debonding load, a is the radius of the fibre and h is the thickness of the specimen. A Laser Flash Apparatus LFA 427, made by NETZSCH Company, was employed for measurement of the thermal conductivity from room temperature to 1000 °C. The thermal conductivity of the SiC/ SiC composites was calculated from the following equations:
K = αρCp
(2)
α = 1.38L2/π 2t1/2
(3)
where α is the thermal diffusion, ρ is the density, Cp is the specific heat, L is the specimen thickness and t1/2 is the time required to reach half of the total temperature rise on the rear surface of the specimen. Cp and α were measured directly through the laser flash method, and the thermal conductivity was calculated via Eq. (2). A transmission electron microscope (G-20, FEI-Tecnai, USA) operating at an accelerating voltage of 200 kV was used for the transmission electron microscopy (TEM) analysis. The micro-morphologies were observed by a scanning electronic microscope (SEM, JEOL6700F, Tokyo, Japan).
3. Results and discussion 3.1. Microstructure
The flexural strength of the composites was measured using a three-point bending test (SANS CMT 4304, Sans Testing Machine, Shen Zhen, China) with a span of 30 mm at a loading rate of 0.5 mm/min. The force–displacement curves were recorded by a computer. To determine the influence of the deposited CNTs on the interfacial bonding strength between the fibre and matrix, a single fibre push-out test was performed. A Nano Indentor TI 950 Triboindenter (Hysitron), located in the State Key Laboratory for the Mechanical Behaviour of Materials (in Xi'an Jiaotong University), was used to measure the fibre/matrix interfacial shear strength of the SiC/SiC composites with different interfaces at room temperature. The indenter was conical with a 60° tape angle, and the curvature radius at the tip was 5 μm. The two composites were cut
SiC fibre
p 2πah
Bending strength Fracture energy (MPa) (kJ/m2)
IFSS (MPa)
505 7 23 430 7 17
455 26.6 7 4.1
90.4 73.4 71.9 72.8
The two composites have almost the same density; the introduction of CNTs into the interface has no obvious effect on the density of the SiC/SiC composites (Table 2). The microstructure of the SiC fibre is shown in Fig. 1a; the surface of the SiC fibre is quite smooth. After the deposition of the CNTs, the SiC fibres are covered by a thin layer of CNTs, and the surface of the fibres is roughened (Fig. 1b). The CNTs introduced by the EPD form a reticulate structure on the surface of the SiC fibres (Fig. 1c), and the CNTs are well dispersed. Fig. 2 shows the interface morphologies of SiC/SiC–P (Fig. 2a) and SiC/SiC–CP (Fig. 2b). The CNT layer is very thin, so the thicknesses of the PyC and CNTs–PyC interfaces are both approximately 100 nm. However, the PyC interface is uneven, and the boundary is coarse. When a thin layer of CNTs deposits on the SiC fibre before the PyC, the morphology of the PyC interface becomes quite even, and the boundary is smooth. This might be due to the CNTs' large surface area, which makes the deposition of the PyC easier. The surface condition and structure of the fibre have important effects on the structure and bonding state of the CVI–PyC interface [25]. To analyse the structural difference between PyC deposited on fibre and on CNTs, TEM is employed. Fig. 3 shows the TEM images of PyC deposited on CNTs and SiC fibres. The CNTs are surrounded by the PyC deposited on them (Fig. 3a). Fig. 3b and c shows high resolution TEM images of the PyC deposited on SiC fibre and on CNTs. The PyC deposited on the SiC fibres presents a low nano-textural order without an obvious orientation and a low texture. The PyC deposited on the CNTs shows a morphology of short graphene fringes partially oriented in the same direction (marked by rectangles) and longer, highly curved graphene fringes (marked by circles), which confirms that the PyC deposited on CNTs has a higher texture than that
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Fig. 1. Surface morphology of SiC fibre: (a) without CNTs; (b) with CNTs deposited on the fibre.
Fig. 2. The morphologies of the two interfaces: (a) PyC; (b) CNTs–PyC.
deposited on SiC fibres. According to Chen Jie's research, CNTs can act as nuclei of small hydrocarbon molecules in deposition because of the π–π conjugated electronic structure of CNTs, and small hydrocarbon molecules can deposit, grow, coalesce and align in the direction vertical to the perpendicular surface of the CNTs [26]. 3.2. Mechanical properties Fig. 4 shows typical bending strength–displacement curves derived from the three-point bending test for SiC/SiC–P and SiC/ SiC–CP. These two load–displacement curves display several common features: (1) an initial linear region, which reflects the elastic response of these two composites; (2) a nonlinear region
until the maximum load, mainly caused by matrix cracking, interfacial debonding and fibre sliding; and (3) a quick reduction in the load after reaching its maximum, perhaps related to the failure of a significant fraction of the SiC fibres. SiC/SiC–CP has a higher maximum load than SiC/SiC–P. The average flexural strengths of SiC/SiC–CP and SiC/SiC–P are 505 723 MPa and 430 717 MPa (Table 2), respectively. The fracture energies are obtained from the area below the load–displacement curves until the maximum load [27]. The fracture energies of SiC/SiC–CP and SiC/SiC–P are 90.4 73.4 kJ/m2 and 71.972.8 kJ/m2 (Table 2), respectively. These results indicate that SiC/SiC–CP has a higher strength and absorbs more energy before fracture.
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Fig. 3. (a) TEM image of PyC deposited on CNTs; (b) HRTEM of PyC deposited on SiC fibre; (c) HRTEM of PyC deposited on CNTs.
Fig. 4. Typical bending strength–displacement curves for SiC/SiC composites with PyC interface and CNTs–PyC interface.
Typical SEM fracture surfaces of these two composites are shown in Fig. 5. Debonding and fibre pull-outs are observed on the fracture surfaces. The average fibre pull-out length of SiC/SiC–P is approximately 200 μm, while that of SiC/SiC–CP is less than 100 μm. The main functions of the interface in SiC/SiC composites are to transfer the load from the matrix to the fibres and to deflect the matrix cracks at the interface to obtain improved fracture tolerance [28]. A strong interfacial bonding between the fibre and
matrix always leads to a shorter pull-out length of the fibre. This phenomenon indicates that SiC/SiC–CP has a stronger fibre/matrix bonding than SiC/SiC–P. It seems that the interfacial bonding strength between the SiC fibres and SiC matrix is strengthened by CNTs. To verify the difference in the interfacial bonding strength between these two interfaces, a single fibre push-out test is carried out to quantitatively evaluate the actual interfacial shear strength. Fig. 6 shows SEM images of the pushed-in and pushedout fibre, as observed from the top (Fig. 6a) and bottom (Fig. 6b), respectively. Fig. 7 shows typical indentation load–displacement curves of SiC/SiC–CP and SiC/SiC–P. In Fig. 7a, when the load is held steady, debonding between the fibre and matrix occurs, and the fibre is pushed-out from the matrix. The fibre diameter is approximately 12.43 μm, and the interfacial shear strength can be calculated according to Eq. (1). The IFSS of PyC is approximately 26.67 4.1 MPa. No fibres are pushed-out in SiC/SiC–CP at the same test condition, and according to the load–displacement curve in Fig. 7b, the IFSS of the CNTs–PyC interface is greater than 55 MPa (Table 2). This is in accordance with the assumptions and the results revealed by the three-point bending tests. The meshed CNTs introduced by EPD on the surface of the SiC fibres before the deposition of PyC act as a reinforced phase that can strengthen the
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Fig. 5. SEM micrographs of the fracture surface of two composites after the three-point bending test: (a) SiC/SiC–P; (b) SiC/SiC–CP.
Fig. 6. SEM of the fibre after a single fibre push-out test: (a) fibre push-in as observed from the top surface; (b) fibre push-out as observed from the bottom surface.
Fig. 7. Typical indentation load vs. displacement curves of SiC/SiC composites with (a) PyC interface; (b) CNTs–PyC interface.
bonding between the fibre and the matrix, so SiC/SiC–CP has a stronger interface than SiC/SiC–P. The interface is a critical part of a composite because load transfers from the matrix to the fibre and vice versa must occur through the interface. A weak interface (PyC) debonds easily, and load transfers through the debonded interface are poor. The mechanical properties of the SiC/SiC–P composites are low. Upon adding CNTs into the PyC interface, the meshed CNTs on the SiC fibres can strengthen the bonding between the fibre and matrix, and SiC/SiC–CP has a stronger interface than SiC/SiC–P. In this stronger interface, cracks that appear in the matrix can be deflected into short, branched multiple cracks [29, 30]. Short
debonding segments, as well as improved load transfer, allow further cracking of the matrix via the scale effect [31, 32], leading to a higher density of matrix cracks, and the sliding friction of the strong interface and the multiple cracking of the matrix then increase the energy absorption. SiC/SiC–CP therefore has a higher flexural strength and fracture energy than SiC/SiC–P. 3.3. Thermal conductivity The thermal conductivity measured in this experiment is the traverse thermal conductivity, along the thickness of the samples. Based on the measured thermal diffusivity and specific data, the
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thermal conductivities are slightly reduced due to phonon–phonon interactions. In the test of thermal conductivity, the direction of the heat flux is perpendicular to the fibre and interface. The schematic of heat transfer in the thickness of the SiC/SiC composites is shown in Fig. 9a, and a simplified form using a serial structural model, analogous to current transfer in a series circuit, is shown in Fig. 9b. The effective traverse thermal conductivity can be calculated by Eq. (4):
v v v 1 = m + i + f Keff Km Ki Kf
Fig. 8. Thermal conductivity of SiC/SiC composites with different interfaces.
(4)
where Keff is the effective traverse thermal conductivity, and Km, Ki and Kf are the thermal conductivities of the matrix, interface and fibre, respectively. vm, vi, and vf are the volume fractions of the matrix, interface and fibre, respectively. The two composites are quite similar except for the interface. The CNTs–PyC interface can be seen as CNTs dispersed into the continuous PyC (Fig. 10). The thermal conductivity of CNTs–PyC can be calculated by Eq. (5):
( )(2 1 − v (1 − )(
1 + 2vc 1 − Ki = Kp
c
Kp Kc
Kp Kc
Kp
Kp
Kc
Kc
+1
+1
)
) (5)
where Ki, Kp and Kc are the thermal conductivities of the interface, PyC and CNTs, respectively, and vc is the CNTs volume fraction. The thermal conductivity of the CNTs ( 3000 W/(m K)) is much higher than that of PyC, so Eq. (5) can be simplified as follows:
Ki = Kp
Fig. 9. Schematic diagram of heat transfer along the thickness of SiC/SiC composites.
1 + 2vc 1 − vc
(6)
Upon adding CNTs into the PyC interface, Ki increases, so the interfacial thermal conductivity of SiC/SiC–CP is higher than that of SiC/SiC–P. Additionally, the texture of PyC is higher than that of pure CVI–PyC, so the thermal conductivity of PyC deposited on CNTs is higher than that deposited on fibres [33]. In general, the introduction of CNTs into the PyC interface increases the thermal conductivity of the interface (Ki). Considering the quite thin CNTs layer, the change in the interfacial volume fraction (vi) in these two composites can be neglected. Then, with the increase of Ki, Keff increases. Hence, the traverse thermal conductivity of SiC/SiC–CP is higher than that of SiC/SiC–P.
4. Conclusions The effects of CNTs on the microstructure, mechanical and thermal properties of SiC/SiC composites are investigated. The improved mechanical and thermal properties of SiC/SiC composites have been demonstrated by introducing CNTs into the PyC interface. The conclusions can be summarized as follows: Fig. 10. Serial structural model of heat transfer along the thickness of SiC/SiC composites.
thermal conductivity of SiC/SiC composites with different interfaces can be obtained (shown in Fig. 8). Without CNTs in the interface, the thermal conductivity of SiC/ SiC–P is 8.2 W/(m K) at room temperature. Upon adding CNTs into the interface, the thermal conductivity of SiC/SiC–CP increases to 17.7 W/(m K), which is 2.16 times higher than that of SiC/SiC–P at room temperature. With the increase of the test temperature, the
(1) Compared with a pure PyC interface, the uniformity and texture of the PyC interface improves greatly by introducing CNTs, resulting in the short graphene fringes partially orienting in the same direction and longer, highly curved graphene fringes. (2) The meshed CNTs introduced on the surface of the SiC fibres before the deposition of the PyC interface can strengthen the bonding between the fibre and matrix. The IFSS between the SiC fibres and SiC matrix is improved by more than 2 times, from 26.674.1 MPa to above 55 MPa. A stronger interface has a positive effect on the mechanical properties of SiC/SiC composites. (3) Due to not only the extremely high thermal conductivity of CNTs but also the improved thermal conductivity of the more highly
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textured PyC obtained by the introduction of CNTs into the PyC interface, the traverse thermal conductivity of the SiC/SiC composites increases by 2.16 times. The heat transfer in the thickness of the SiC/SiC composites can be simplified using a serial structural model. Introducing CNTs into the interface improves the thermal conductivity of the interface, and thereby, the overall thermal conductivity of the SiC/SiC composite is improved.
Acknowledgements The authors acknowledge the support of the Chinese National Foundation for Natural Sciences Under Contracts (Nos. 51332004, 51472201), the Fundamental Research Funds for the Central Universities (No. 3102014KYJD011), the Fund of the State Key Laboratory of Solidification Processing in NWPU (No. SKLSP201401), the Research Fund of Northwestern Polytechnical University (20120204), and the Major National Scientific Instrument and Equipment Development Project (2011YQ12007504).
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