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SiC composites fabricated through precursor infiltration and pyrolysis
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CERAMICS INTERNATIONAL
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Mechanical and microwave dielectric properties of KD-I SiCf/SiC composites fabricated through precursor infiltration and pyrolysis Hao Tiana, Hai-tao Liua,b, Hai-feng Chenga,n a
Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, College of Aerospace Science and Engineering, National University of Defense Technology, Changsha 410073, PR China b Science and Technology on Scramjet Laboratory, College of Aerospace Science and Engineering, National University of Defense Technology, Changsha 410073, PR China Received 1 October 2013; received in revised form 25 January 2014; accepted 26 January 2014
Abstract SiC fiber-reinforced SiC matrix (SiCf/SiC) composites, which employ a new type of KD-I SiC fibers (provided by the National University of Defense Technology, China) with in situ pyrocarbon (PyC) coating on the surface of the fibers as reinforcements, are fabricated through precursor infiltration and pyrolysis (PIP). The characteristics of the fiber surface are evaluated by scanning electron microscopy, X-ray photoelectron spectroscopy, Auger electron spectroscopy, and transmission electron microscopy. The mechanical and dielectric properties of KD-I SiCf/SiC composites are reported, and the effects of in situ PyC coating on the material properties are investigated. Results show that the KD-I SiC fiber has a special “skin (PyC phase)-core (Si–C–O phase)” structure. The composites possess excellent mechanical properties because of the in situ PyC coating on the surface of the fibers. The flexural strength and toughness are 268.8 MPa and 12.9 MPa m1/2, respectively. However, the dielectric constants are also remarkable, which is disadvantageous to microwave absorbing applications. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: C. Mechanical properties; C. Dielectric properties; SiCf/SiC composites; Precursor infiltration and pyrolysis
1. Introduction The demand for microwave absorbing materials, which are functional materials widely used in civil and military domains, have been increasing continuously at present because of the progressive uses of microwaves. These materials should ideally feature low density, favorable mechanical properties, and appropriate dielectric properties for strong microwave absorption over a wide frequency range. Moreover, these materials should serve at high temperature environments, particularly for applications in stealth technology for aircrafts such as supersonic cruise missiles and the battle plan of high Mach numbers [1–4]. SiC fiber-reinforced SiC matrix (SiCf/SiC) composites possess superior properties, such as excellent high temperature mechanical properties, good fracture and corrosion resistance, n
Corresponding author. Tel.: þ86 731 84573169; fax: þ 86 731 84576578. E-mail address: [email protected] (H.-f. Cheng).
and thermodynamic stability [5,6]. Given these excellent characteristics, SiCf/SiC composites are regarded as one of the most promising materials for structural applications at elevated temperatures. The excellent semiconductivity and relatively stable dielectric properties at elevated temperatures of these materials are also demonstrated in numerous practical and potential applications of high-temperature microwave absorption [7–9]. SiC fiber usually requires an appropriate coating on its surface to achieve promising reinforcement for composites because the interphase formed from the coating allows crack deflection, fiber pullout, and fiber/matrix (F/M) debonding, all of which provide excellent mechanical properties to SiCf/ SiC composites [10,11]. Different kinds of coatings have been suggested recently, such as carbon (C), boron nitride (BN), oxide and multilayer coatings ((X Y)n) [12–16]; however, carbon has been recognized as one of the most commonly used and effective coating materials. Aside from the favorable compatibility at the fiber/matrix interface, carbon layer
0272-8842/$ - see front matter & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2014.01.113 Please cite this article as: H. Tian, et al., Mechanical and microwave dielectric properties of KD-I SiCf/SiC composites fabricated through precursor infiltration and pyrolysis, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.01.113
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provides an adjustable electrical resistivity to SiC fibers, which remarkably affects the dielectric properties of SiCf/SiC composites [17]. The fabrication process is also one of the key factors that determine material properties. Concerning the manufacturing processes for the densification of fiber preforms, the most developed ones are chemical vapor infiltration (CVI) [18] and polymer infiltration and pyrolysis (PIP) [19,20]. Both techniques are based on the common principle of filling the porosity inside the fiber preforms with a SiC matrix resulting from decomposition of gaseous precursors (CVI) or pre-ceramic polymer precursors (PIP). General properties and issues of SiCf/SiC composites currently in vogue are summarized in Table 1. The mechanical behavior is highly dependent on composite microstructure, fiber types, interphases and manufacturing methods. In the present study, a new type of SiC fibers, namely, KD-I SiC fibers with pyrocarbon (PyC) coating on its surface, is employed as reinforcement for SiCf/SiC composites. The PyC layer is obtained in situ during the SiC fiber fabrication process. The mechanical and dielectric properties of KD-I SiCf/SiC composites fabricated through PIP are reported. The effects of the in situ PyC layer on material properties are also investigated.
2. Experimental procedure 2.1. Preparing KD-I SiCf/SiC composites KD-I SiC fibers, which were provided by the National University of Defense Technology, were produced following the route presented by Yajima [31], which involves three steps: (1) spinning the polycarbosilane (PCS) precursor in the molten state, (2) curing treatment to make the polymer fibers infusible, (3) and pyrolysis of the cured filaments in an inert atmosphere under different pyrolysis modes. KD-I SiC fibers
were fabricated through the one-step pyrolysis mode. The length of the temperature constant area in the pyrolysis furnace was 600 mm. The rolling speed was 60 mm/s. Nitrogen gas flew in from both sides of the corundum tube. The N2 flux was 2 L/min. The pyrolysis temperature was 1200 1C. The detailed preparation procedure could be found in Refs. [32–34]. The fibers had a mean diameter of 15 μm with 600 filaments in a bundle. The basic chemical composition was made of 34.7 at% silicon, 41.8 at% carbon, and 23.5 at% oxygen [35]. The typical characteristics of KD-I SiC fiber and other commercial SiC fibers are listed in Table 2. The reinforcements used to prepare 2D SiCf/SiC composites were plain-weave KD-I SiC fiber cloths. PCS, the precursor of the SiC matrix with a relative molecular mass of 1300 and a softening point of 210 1C, was synthesized in our laboratory. The 2D KD-I SiCf/SiC composites were fabricated through PIP. The detailed process could be found in Ref. [10].
2.2. Analytical methods Scanning electron microscopy (SEM) investigation was performed on a Hitachi FEG S4800 SEM (Hitachi Ltd., Japan) to analyze the morphology of SiC fibers and to observe the fracture surface of the composites. The specimens for transmission electron microscopy (TEM) observation were prepared by following the preparation procedure described by Appiah et al. [38]. JEOL JEM-2010 (JEOL Ltd., Japan) and Philips CM 200 FEG (Philips, the Netherlands) equipped with a Gatan imaging filter (GIF) system were used to characterize the cross section of the SiC fibers and the interfacial microstructure between fibers/matrices. The surface compositions of the SiC fibers were investigated by X-ray photoelectron spectroscopy (XPS) analysis using a VG ESCALAB MK II, with Al Kα radiation and calibrated against Au 4f7/2 and Cu 2p3/2 lines. The atom concentrations of carbon, silicon, and oxygen along the radial direction were measured by Auger electron
Table 1 Applications of carbon interphase in 2D-SiCf/SiC composites. Fiber type
Preparation process
Thickness of interphase (μm)
Mechanical properties (MPa)
References
Nicalon
CVI CVI PIP PIP
0.1 0.13 0.04 0.04
145 420 350 136
(TS) (FS) (FS) (TS)
[21] [22] [23] [24]
Hi-Nicalon
CVI PIP PIP
0.15 0.04 0.06
268 230 210 190
(TS) (FS) (TS) (FS)
[25] [23] [26]
Tyranno SA
CVI CVI
0.02 0.08 0.1 0.16
(FS) (TS) (FS) (FS) (TS)
[27] [28]
CVI CVI
380 272 356 606 230
[29] [30]
TS: Tensile strength; FS: Flexural strength. Please cite this article as: H. Tian, et al., Mechanical and microwave dielectric properties of KD-I SiCf/SiC composites fabricated through precursor infiltration and pyrolysis, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.01.113
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Table 2 Typical characteristics of KD-I SiC fiber and other commercial SiC fibers. Trade name
Diameter (μm)
Density (g cm 3)
Tensile strength (GPa)
Tensile modulus (GPa)
C/Si (atom)
KD-I Nicalona Hi-Nicalona Tyranno SAa
15 14 14 7.5
2.45 2.55 2.74 3.0
1.8–2.2 3.0 2.8 2.8
150–200 220 270 380
1.20 1.32 1.41 1.08
a
From references [36,37].
Fig. 1. SEM micrographs of KD-I SiC fibers.
spectroscopy (AES) using a nanoscanning auger system (PHI700, ULVAC-PHI). The density and porosity of KD-I SiCf/SiC composites were measured by the laws of Archimedes. The three-point bending test of the composites was performed at ambient temperature. The sample geometry was approximately 60l 4w 3t mm3. The support span was 50 mm. The crosshead speed was 0.5 mm/min and corresponded to a strain rate of 3 10 6 s 1. The complex permittivity of the composites was measured at a frequency between 8.2 GHz and 18 GHz using the waveguide method with a network analyzer Agilent 8720ET (Agilent Technologies, Inc., USA). 3. Results and discussion 3.1. Characteristics of the KD-I SiC fiber Fig. 1 shows the surface morphology of KD-I fibers. Each fiber notably exhibits a smooth filament surface and has an average diameter of approximately 15 μm. No obvious defect is found on the fiber surface. A proper surface morphology for the composite of the fibers is generally necessary to minimize stress concentration and provide a correct crack deflection behavior. The survey XPS spectrum recorded from the surface of KDI fibers is shown in Fig. 2. The signature peaks of C1s and O1s orbitals are found at 285 eV and 532 eV, respectively [39]. The C1s peak is attributed to the C–C bond. The appearance of an oxygen peak can be attributed to the impurity introduced by the sample plate. No Si line is observed in the overall XPS spectrum. Therefore, carbon is the main constituent of the surface layer of SiC fibers.
Fig. 3 shows the AES analysis results of the atom concentrations of carbon, silicon, and oxygen along the radial direction of KD-I fiber. The result of the AES analysis reveals that a carbon-rich layer approximately 15–20 nm thick is found in the circular outer portion of SiC fibers. A high-resolution TEM (HRTEM) analysis of SiC fibers has been adopted to further confirm the surface layer compositions of KD-I SiC fibers. The HRTEM cross-section image is shown in Fig. 4. A typical turbostratic stripe approximately 15–20 nm thick (in the white pane) is found on the surface of SiC fibers, which obviously contrasts with the inner area. According to our previous studies and numerous other references [40–42], the turbostratic structure is confirmed as a representative feature of the PyC phase, which is in agreement with the results of the XPS and AES analyses. The XPS, AES, and HRTEM analyses reveal that an in situ PyC layer approximately 15–20 nm thick is found on the surface of KD-I SiC fibers. The PyC surface characteristics of the fibers will undoubtedly affect the mechanical and electrical properties of SiCf/SiC composites. 3.2. Mechanical properties KD-I SiCf/SiC composites The properties of the fabricated KD-I SiCf/SiC composites are listed in Table 3. The flexural strength and toughness of the composites are 268.8 MPa and 12.9 MPa m1/2, respectively, thus exhibiting excellent mechanical properties. Fig. 5 shows the typical load–displacement curve of KD-I SiCf/SiC composites. These composites exhibit an obvious toughened fracture behavior. When the load reaches the maximum, the decrease is gradual, not sharp. The composites fail noncatastrophically, which differs from those of brittle
Please cite this article as: H. Tian, et al., Mechanical and microwave dielectric properties of KD-I SiCf/SiC composites fabricated through precursor infiltration and pyrolysis, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.01.113
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Fig. 2. XPS spectrum of KD-I SiC fibers.
Fig. 3. Atom concentrations (obtained by AES analysis) along the radical direction of KD-I SiC fibers.
Fig. 4. HRTEM micrograph of the carbon layer on the surface of KD-I SiC fibers.
monolithic ceramics. The significantly large extended area beyond the dotted line (Fig. 5) indicates that a great amount of fracture energy is consumed in the composites during the fracture process [13]. The in situ PyC coating on the fiber
surface contributes to enhancing the toughening mechanisms during composite fracture. The microstructure of the composites is investigated by SEM and TEM analyses to understand high strength in KD-I SiCf/SiC composites. Fig. 6 shows the fracture surfaces of KD-I SiCf/SiC composites after mechanical testing. The joint stress among the layers of 2D composites prepared by PIP is stress-large, and a layered fracture does not occur after testing (Fig. 6a). The fracture surface also exhibits an evident fiber pullout with pullout lengths exceeding 10 μm (Fig. 6b). The surface of the pulled-out fibers is smooth and free of any matrix, which shows that the chemical corrosion of SiC fibers during PIP is weak, and that debonding does not occur within the matrix. The interfacial debonding phenomenon is obvious between the fiber and the matrix (Fig. 6b). Fig. 7 presents the TEM cross-section images of KD-I SiCf/ SiC composite. The turbostratic PyC layer, which is formed by in situ PyC surface coating of KD-I SiC fibers (Fig. 7a and b), is approximately 25 nm thick. The interface between the fiber and the PyC interphase and between the PyC interphase and the matrix can be defined clearly (two red lines in Fig. 7b), that is, no obvious phenomenon of elemental interdiffusion or chemical reaction occurs within the PyC interphase. Therefore, the PyC interphase is a diffusion barrier for protecting SiC fibers from chemical damage during PIP. The highly aligned basal planes of the PyC also appear to be nearly parallel to the interfaces (two red lines). The approximately perfect orientation of the PyC interphase is generally very favorable for mechanical property improvement of SiCf/SiC composites. In this orientation, the load can be more easily transferred, and F/M debonding easily occurs because of the low van der Waals force between the basal planes of the PyC [38]. The preceding analysis shows that in situ PyC coating on the surface of SiC fibers prevents fiber damage and provides an appropriate interfacial bonding strength for SiCf/SiC composites, which provides KD-I SiCf/SiC composites with adequate fracture toughness. An F/M debonding region has been observed to investigate the microstructural transformations of the PyC interphase during F/M debonding, as shown in Fig. 7c. A highmagnification TEM image is presented in Fig. 7d. In the F/M debonding region, bridging and delamination are found within the PyC interphase. In the delamination region, the rupture boundaries of the PyC interphase are not even; thus, the cracks can be deflected and can propagate within the different basal planes of the PyC. In the bridging region, the cracks were also bridged by several basal planes of the PyC. However, the orientations of these basal planes are perpendicular to the interfaces. The orientation transformations may be induced by the stress arising from the cracks. The same phenomenon was observed by Boitier et al. in SiC fiberreinforced SiBC matrix composites [43]. The orientation transformations of the basal planes of the PyC can also consume energy, thus increasing the fracture work of SiCf/ SiC composites. In conclusion, the in situ PyC coating on the surface of the fiber for KD-I SiCf/SiC composites weakens interfacial
Please cite this article as: H. Tian, et al., Mechanical and microwave dielectric properties of KD-I SiCf/SiC composites fabricated through precursor infiltration and pyrolysis, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.01.113
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Table 3 Properties of the KD-I SiCf/SiC composites. Fiber volume fraction (%)
Density (g cm 3)
Porosity (%)
Flexural strength (MPa)
Fracture toughness (MPa m1/2)
45
2.04
18.4
268.8728
12.972
The imaginary part of the permittivity caused by electrical conductivity (ε″c ) can be described as follows: 00
εc ¼
s : ωε0
ð2Þ 00
Fig. 5. Load/displacement curve of KD-I SiCf/SiC composites.
bonding and effectively improves the accommodation between the fiber and the matrix. Most matrix cracks deflect when meeting the in situ PyC coating during the fracture process of the composites because of the proper combination of the fiber and the matrix. Therefore, the in situ PyC coating of the fibers can endow SiCf/SiC composites with excellent mechanical properties.
Eq. (2) shows that εc is mostly determined by electrical conductivity. Therefore, analyzing the electrical conductivity of KD-I SiC fibers is necessary. Given the special microstructure of KD-I SiC fibers (Section 3.1), we propose a “skin (PyC phase)-core (Si–C–O phase)” model to simulate the electrical properties of KD-I fibers. The microstructure model of KD-I SiC fibers is shown in Fig. 9. In this model, the Si–C–O phase acts as the high electrical resistance phase, and the PyC coating acts as the low electrical resistance phase. The electrical resistance of SiC fibers can be derived using Eq. (3) according to Ohm's law: 1=Rf ¼ 1=RPyC þ 1=RSi C O ;
ð3Þ
where Rf , RPyC , and RSi C O are the electrical resistance of SiC fibers, PyC layer, and Si–C–O phase, respectively. Given that the electrical conductivity of PyC is significantly higher than that of the Si–C–O phase [32,34], then Eq. (4) can be attained as
3.3. Dielectric properties of KD-I SiCf/SiC composites
Rf ¼ ρf l=ðπr 2 Þ RPyC ¼ ρPyC l=ðπr 2 πðr tÞ2 Þ;
Fig. 8 reveals the ε0 and ε″ of KD-I SiCf/SiC composites within the frequency range of 8–18 GHz. The permittivity of SiCf/SiC composites is remarkable. The ε0 of SiCf/SiC composites ranges from 40 to 15, whereas ε″ ranges from 50 to 5, which are larger than those of composites without PyC coating, as reported in Refs.[7,14]. An appreciable frequency dispersion effect is also observed, that is, ε0 and ε″ of SiCf/SiC composites both decrease as the microwave frequency increases. The imaginary part of the permittivity (ε″) of the composites can be calculated based on the Debye equation:
where ρf and ρPyC are the electrical resistivities of SiC fibers and PyC layers, respectively; r is the radius of SiC fibers; and t is the thickness of the PyC coating. Given r⪢t, then Eq. (4) can be simplified as
ε″ ¼
ðεs ε1 Þωτ=ð1þ ðωτÞ2 Þ þ s ; ε0 ω
ð1Þ
where εs is the static permittivity, ε1 is the permittivity at a high frequency limit, ω is the angular frequency, τ is the relaxation time, and s is the electrical conductivity of the composites. Eq. (1) shows that ε″ is determined by the relaxation time and electrical conductivity of the composites. For most dielectric materials, conductivity losses are the most important factors that affect the permittivity [44,45], and thus, should first be considered.
ρf ¼ ρPyC r=2t:
ð4Þ
ð5Þ
Eq. (5), shows that the effective electrical resistivity of KD-I fibers is proportional to that of the PyC layers and inversely proportional to the thickness of the PyC layers. The electrical resistivity of PyC derived from PCS is approximately 10 2 Ω cm [46,47]. The analysis in Section 3.1 reveals that when the thickness of the PyC layers is approximately 15–20 nm, the effective electrical resistivity of KD-I fibers is approximately 100–101 Ω cm based on Eq. (5), which is significantly higher than the electrical resistivity value of approximatelty 10 14 Ω cm of SiC fibers without PyC coating. Therefore, the ε00 of KD-I SiCf/SiC composites is remarkable at the X-band because of in situ PyC coating on the surface of the fiber. The real part of the permittivity is also determined by the electrical conductivity [46]; thus, the real part of the permittivity of KD-I SiCf/SiC composites with in situ PyC coating is also signifificantly large.
Please cite this article as: H. Tian, et al., Mechanical and microwave dielectric properties of KD-I SiCf/SiC composites fabricated through precursor infiltration and pyrolysis, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.01.113
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Fig. 6. SEM micrographs of the fracture surface of KD-I SiCf/SiC composites.
Fig. 7. TEM cross-section micrographs of KD-I SiCf/SiC composites. (For interpretation of the references to color in this figure the reader is referred to the web version of this article.)
However, the reflectivity for a given microwave absorbing material generally follows the following equation [48]: ffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffi tan hð2πjf dpffiffiffiffiffiffiffiffiffiffiffiffiffi ε0 jε″=cÞ ε0 jε″ pffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffi ; ffi R ¼ 20 Log tan hð2πjf d ε0 jε″=cÞ þ ε0 jε″ where R is the reflectivity, f is the frequency, d is the thickness of the material, j is the imaginary unit, and c is the vacuum
speed of light. Determining that a large permittivity leads to strong reflection and weak absorption of microwave for KD-I SiCf/SiC composites is easy when using a computer-aided software. On all accounts, KD-I SiCf/SiC composites prepared through PIP possess excellent mechanical properties because of the in situ PyC coating on the surface of the fibers.
Please cite this article as: H. Tian, et al., Mechanical and microwave dielectric properties of KD-I SiCf/SiC composites fabricated through precursor infiltration and pyrolysis, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.01.113
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microwave. Accordingly, the PyC coating on the surface of KD-I SiC fibers should first be removed before SiCf/SiC composites are used for microwave-absorbing applications.
Acknowledgments The authors appreciate the financial support of the National Natural Science Foundation of China (51202291), Aid Program for Innovative Group of National University of Defense Technology, and Aid program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province. Fig. 8. Dielectric constant of KD-I SiCf/SiC composites.
References
Pyrocarbon layer Si-C-O interphase
Fig. 9. Microstructure model of KD-I SiC fibers.
However, the dielectric constants are also remarkable, which is disadvantageous to microwave-absorbing applications because of the potential to result in lower input impedance and a mismatch in the impedance between the composites and free space. These two mutually contradictory aspects will limit the application of the fibers in microwave-absorbing materials. In the future, we will first remove the PyC coating and then introduce a low dielectric constant BN coating on the surface of KD-I SiC fibers for practical microwave-absorbing applications. 4. Conclusions (1) KD-I SiC fibers have a special “skin (PyC phase)-core (Si– C–O phase)” structure. A PyC layer approximately15– 20 nm thick is found on the surface of fibers, which remarkably affects the mechanical and dielectrical properties of KD-I SiCf/SiC composites. (2) Excellent mechanical properties of 268.8 MPa in flexural strength and 12.9 MPa m1/2 in fracture toughness are obtained in KD-I SiCf/SiC composites prepared through PIP. This result is primarily attributed to the in situ PyC coating on the surface of the fibers that can weaken interfacial bonding and effectively improve accommodation between the fiber and the matrix. (3) The permittivity of KD-I SiCf/SiC composites is remarkable at the X-band because of the in situ PyC coating on the surface of the fibers. However, a large permittivity leads to strong reflection and weak absorption of
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Please cite this article as: H. Tian, et al., Mechanical and microwave dielectric properties of KD-I SiCf/SiC composites fabricated through precursor infiltration and pyrolysis, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.01.113
CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Mechanical and microwave dielectric properties of KD-I SiCf/SiC composites fabricated through precursor infiltration and pyrolysis Hao Tiana, Hai-tao Liua,b, Hai-feng Chenga,n a
Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, College of Aerospace Science and Engineering, National University of Defense Technology, Changsha 410073, PR China b Science and Technology on Scramjet Laboratory, College of Aerospace Science and Engineering, National University of Defense Technology, Changsha 410073, PR China Received 1 October 2013; received in revised form 25 January 2014; accepted 26 January 2014
Abstract SiC fiber-reinforced SiC matrix (SiCf/SiC) composites, which employ a new type of KD-I SiC fibers (provided by the National University of Defense Technology, China) with in situ pyrocarbon (PyC) coating on the surface of the fibers as reinforcements, are fabricated through precursor infiltration and pyrolysis (PIP). The characteristics of the fiber surface are evaluated by scanning electron microscopy, X-ray photoelectron spectroscopy, Auger electron spectroscopy, and transmission electron microscopy. The mechanical and dielectric properties of KD-I SiCf/SiC composites are reported, and the effects of in situ PyC coating on the material properties are investigated. Results show that the KD-I SiC fiber has a special “skin (PyC phase)-core (Si–C–O phase)” structure. The composites possess excellent mechanical properties because of the in situ PyC coating on the surface of the fibers. The flexural strength and toughness are 268.8 MPa and 12.9 MPa m1/2, respectively. However, the dielectric constants are also remarkable, which is disadvantageous to microwave absorbing applications. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: C. Mechanical properties; C. Dielectric properties; SiCf/SiC composites; Precursor infiltration and pyrolysis
1. Introduction The demand for microwave absorbing materials, which are functional materials widely used in civil and military domains, have been increasing continuously at present because of the progressive uses of microwaves. These materials should ideally feature low density, favorable mechanical properties, and appropriate dielectric properties for strong microwave absorption over a wide frequency range. Moreover, these materials should serve at high temperature environments, particularly for applications in stealth technology for aircrafts such as supersonic cruise missiles and the battle plan of high Mach numbers [1–4]. SiC fiber-reinforced SiC matrix (SiCf/SiC) composites possess superior properties, such as excellent high temperature mechanical properties, good fracture and corrosion resistance, n
Corresponding author. Tel.: þ86 731 84573169; fax: þ 86 731 84576578. E-mail address: [email protected] (H.-f. Cheng).
and thermodynamic stability [5,6]. Given these excellent characteristics, SiCf/SiC composites are regarded as one of the most promising materials for structural applications at elevated temperatures. The excellent semiconductivity and relatively stable dielectric properties at elevated temperatures of these materials are also demonstrated in numerous practical and potential applications of high-temperature microwave absorption [7–9]. SiC fiber usually requires an appropriate coating on its surface to achieve promising reinforcement for composites because the interphase formed from the coating allows crack deflection, fiber pullout, and fiber/matrix (F/M) debonding, all of which provide excellent mechanical properties to SiCf/ SiC composites [10,11]. Different kinds of coatings have been suggested recently, such as carbon (C), boron nitride (BN), oxide and multilayer coatings ((X Y)n) [12–16]; however, carbon has been recognized as one of the most commonly used and effective coating materials. Aside from the favorable compatibility at the fiber/matrix interface, carbon layer
0272-8842/$ - see front matter & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2014.01.113 Please cite this article as: H. Tian, et al., Mechanical and microwave dielectric properties of KD-I SiCf/SiC composites fabricated through precursor infiltration and pyrolysis, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.01.113
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provides an adjustable electrical resistivity to SiC fibers, which remarkably affects the dielectric properties of SiCf/SiC composites [17]. The fabrication process is also one of the key factors that determine material properties. Concerning the manufacturing processes for the densification of fiber preforms, the most developed ones are chemical vapor infiltration (CVI) [18] and polymer infiltration and pyrolysis (PIP) [19,20]. Both techniques are based on the common principle of filling the porosity inside the fiber preforms with a SiC matrix resulting from decomposition of gaseous precursors (CVI) or pre-ceramic polymer precursors (PIP). General properties and issues of SiCf/SiC composites currently in vogue are summarized in Table 1. The mechanical behavior is highly dependent on composite microstructure, fiber types, interphases and manufacturing methods. In the present study, a new type of SiC fibers, namely, KD-I SiC fibers with pyrocarbon (PyC) coating on its surface, is employed as reinforcement for SiCf/SiC composites. The PyC layer is obtained in situ during the SiC fiber fabrication process. The mechanical and dielectric properties of KD-I SiCf/SiC composites fabricated through PIP are reported. The effects of the in situ PyC layer on material properties are also investigated.
2. Experimental procedure 2.1. Preparing KD-I SiCf/SiC composites KD-I SiC fibers, which were provided by the National University of Defense Technology, were produced following the route presented by Yajima [31], which involves three steps: (1) spinning the polycarbosilane (PCS) precursor in the molten state, (2) curing treatment to make the polymer fibers infusible, (3) and pyrolysis of the cured filaments in an inert atmosphere under different pyrolysis modes. KD-I SiC fibers
were fabricated through the one-step pyrolysis mode. The length of the temperature constant area in the pyrolysis furnace was 600 mm. The rolling speed was 60 mm/s. Nitrogen gas flew in from both sides of the corundum tube. The N2 flux was 2 L/min. The pyrolysis temperature was 1200 1C. The detailed preparation procedure could be found in Refs. [32–34]. The fibers had a mean diameter of 15 μm with 600 filaments in a bundle. The basic chemical composition was made of 34.7 at% silicon, 41.8 at% carbon, and 23.5 at% oxygen [35]. The typical characteristics of KD-I SiC fiber and other commercial SiC fibers are listed in Table 2. The reinforcements used to prepare 2D SiCf/SiC composites were plain-weave KD-I SiC fiber cloths. PCS, the precursor of the SiC matrix with a relative molecular mass of 1300 and a softening point of 210 1C, was synthesized in our laboratory. The 2D KD-I SiCf/SiC composites were fabricated through PIP. The detailed process could be found in Ref. [10].
2.2. Analytical methods Scanning electron microscopy (SEM) investigation was performed on a Hitachi FEG S4800 SEM (Hitachi Ltd., Japan) to analyze the morphology of SiC fibers and to observe the fracture surface of the composites. The specimens for transmission electron microscopy (TEM) observation were prepared by following the preparation procedure described by Appiah et al. [38]. JEOL JEM-2010 (JEOL Ltd., Japan) and Philips CM 200 FEG (Philips, the Netherlands) equipped with a Gatan imaging filter (GIF) system were used to characterize the cross section of the SiC fibers and the interfacial microstructure between fibers/matrices. The surface compositions of the SiC fibers were investigated by X-ray photoelectron spectroscopy (XPS) analysis using a VG ESCALAB MK II, with Al Kα radiation and calibrated against Au 4f7/2 and Cu 2p3/2 lines. The atom concentrations of carbon, silicon, and oxygen along the radial direction were measured by Auger electron
Table 1 Applications of carbon interphase in 2D-SiCf/SiC composites. Fiber type
Preparation process
Thickness of interphase (μm)
Mechanical properties (MPa)
References
Nicalon
CVI CVI PIP PIP
0.1 0.13 0.04 0.04
145 420 350 136
(TS) (FS) (FS) (TS)
[21] [22] [23] [24]
Hi-Nicalon
CVI PIP PIP
0.15 0.04 0.06
268 230 210 190
(TS) (FS) (TS) (FS)
[25] [23] [26]
Tyranno SA
CVI CVI
0.02 0.08 0.1 0.16
(FS) (TS) (FS) (FS) (TS)
[27] [28]
CVI CVI
380 272 356 606 230
[29] [30]
TS: Tensile strength; FS: Flexural strength. Please cite this article as: H. Tian, et al., Mechanical and microwave dielectric properties of KD-I SiCf/SiC composites fabricated through precursor infiltration and pyrolysis, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.01.113
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Table 2 Typical characteristics of KD-I SiC fiber and other commercial SiC fibers. Trade name
Diameter (μm)
Density (g cm 3)
Tensile strength (GPa)
Tensile modulus (GPa)
C/Si (atom)
KD-I Nicalona Hi-Nicalona Tyranno SAa
15 14 14 7.5
2.45 2.55 2.74 3.0
1.8–2.2 3.0 2.8 2.8
150–200 220 270 380
1.20 1.32 1.41 1.08
a
From references [36,37].
Fig. 1. SEM micrographs of KD-I SiC fibers.
spectroscopy (AES) using a nanoscanning auger system (PHI700, ULVAC-PHI). The density and porosity of KD-I SiCf/SiC composites were measured by the laws of Archimedes. The three-point bending test of the composites was performed at ambient temperature. The sample geometry was approximately 60l 4w 3t mm3. The support span was 50 mm. The crosshead speed was 0.5 mm/min and corresponded to a strain rate of 3 10 6 s 1. The complex permittivity of the composites was measured at a frequency between 8.2 GHz and 18 GHz using the waveguide method with a network analyzer Agilent 8720ET (Agilent Technologies, Inc., USA). 3. Results and discussion 3.1. Characteristics of the KD-I SiC fiber Fig. 1 shows the surface morphology of KD-I fibers. Each fiber notably exhibits a smooth filament surface and has an average diameter of approximately 15 μm. No obvious defect is found on the fiber surface. A proper surface morphology for the composite of the fibers is generally necessary to minimize stress concentration and provide a correct crack deflection behavior. The survey XPS spectrum recorded from the surface of KDI fibers is shown in Fig. 2. The signature peaks of C1s and O1s orbitals are found at 285 eV and 532 eV, respectively [39]. The C1s peak is attributed to the C–C bond. The appearance of an oxygen peak can be attributed to the impurity introduced by the sample plate. No Si line is observed in the overall XPS spectrum. Therefore, carbon is the main constituent of the surface layer of SiC fibers.
Fig. 3 shows the AES analysis results of the atom concentrations of carbon, silicon, and oxygen along the radial direction of KD-I fiber. The result of the AES analysis reveals that a carbon-rich layer approximately 15–20 nm thick is found in the circular outer portion of SiC fibers. A high-resolution TEM (HRTEM) analysis of SiC fibers has been adopted to further confirm the surface layer compositions of KD-I SiC fibers. The HRTEM cross-section image is shown in Fig. 4. A typical turbostratic stripe approximately 15–20 nm thick (in the white pane) is found on the surface of SiC fibers, which obviously contrasts with the inner area. According to our previous studies and numerous other references [40–42], the turbostratic structure is confirmed as a representative feature of the PyC phase, which is in agreement with the results of the XPS and AES analyses. The XPS, AES, and HRTEM analyses reveal that an in situ PyC layer approximately 15–20 nm thick is found on the surface of KD-I SiC fibers. The PyC surface characteristics of the fibers will undoubtedly affect the mechanical and electrical properties of SiCf/SiC composites. 3.2. Mechanical properties KD-I SiCf/SiC composites The properties of the fabricated KD-I SiCf/SiC composites are listed in Table 3. The flexural strength and toughness of the composites are 268.8 MPa and 12.9 MPa m1/2, respectively, thus exhibiting excellent mechanical properties. Fig. 5 shows the typical load–displacement curve of KD-I SiCf/SiC composites. These composites exhibit an obvious toughened fracture behavior. When the load reaches the maximum, the decrease is gradual, not sharp. The composites fail noncatastrophically, which differs from those of brittle
Please cite this article as: H. Tian, et al., Mechanical and microwave dielectric properties of KD-I SiCf/SiC composites fabricated through precursor infiltration and pyrolysis, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.01.113
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Fig. 2. XPS spectrum of KD-I SiC fibers.
Fig. 3. Atom concentrations (obtained by AES analysis) along the radical direction of KD-I SiC fibers.
Fig. 4. HRTEM micrograph of the carbon layer on the surface of KD-I SiC fibers.
monolithic ceramics. The significantly large extended area beyond the dotted line (Fig. 5) indicates that a great amount of fracture energy is consumed in the composites during the fracture process [13]. The in situ PyC coating on the fiber
surface contributes to enhancing the toughening mechanisms during composite fracture. The microstructure of the composites is investigated by SEM and TEM analyses to understand high strength in KD-I SiCf/SiC composites. Fig. 6 shows the fracture surfaces of KD-I SiCf/SiC composites after mechanical testing. The joint stress among the layers of 2D composites prepared by PIP is stress-large, and a layered fracture does not occur after testing (Fig. 6a). The fracture surface also exhibits an evident fiber pullout with pullout lengths exceeding 10 μm (Fig. 6b). The surface of the pulled-out fibers is smooth and free of any matrix, which shows that the chemical corrosion of SiC fibers during PIP is weak, and that debonding does not occur within the matrix. The interfacial debonding phenomenon is obvious between the fiber and the matrix (Fig. 6b). Fig. 7 presents the TEM cross-section images of KD-I SiCf/ SiC composite. The turbostratic PyC layer, which is formed by in situ PyC surface coating of KD-I SiC fibers (Fig. 7a and b), is approximately 25 nm thick. The interface between the fiber and the PyC interphase and between the PyC interphase and the matrix can be defined clearly (two red lines in Fig. 7b), that is, no obvious phenomenon of elemental interdiffusion or chemical reaction occurs within the PyC interphase. Therefore, the PyC interphase is a diffusion barrier for protecting SiC fibers from chemical damage during PIP. The highly aligned basal planes of the PyC also appear to be nearly parallel to the interfaces (two red lines). The approximately perfect orientation of the PyC interphase is generally very favorable for mechanical property improvement of SiCf/SiC composites. In this orientation, the load can be more easily transferred, and F/M debonding easily occurs because of the low van der Waals force between the basal planes of the PyC [38]. The preceding analysis shows that in situ PyC coating on the surface of SiC fibers prevents fiber damage and provides an appropriate interfacial bonding strength for SiCf/SiC composites, which provides KD-I SiCf/SiC composites with adequate fracture toughness. An F/M debonding region has been observed to investigate the microstructural transformations of the PyC interphase during F/M debonding, as shown in Fig. 7c. A highmagnification TEM image is presented in Fig. 7d. In the F/M debonding region, bridging and delamination are found within the PyC interphase. In the delamination region, the rupture boundaries of the PyC interphase are not even; thus, the cracks can be deflected and can propagate within the different basal planes of the PyC. In the bridging region, the cracks were also bridged by several basal planes of the PyC. However, the orientations of these basal planes are perpendicular to the interfaces. The orientation transformations may be induced by the stress arising from the cracks. The same phenomenon was observed by Boitier et al. in SiC fiberreinforced SiBC matrix composites [43]. The orientation transformations of the basal planes of the PyC can also consume energy, thus increasing the fracture work of SiCf/ SiC composites. In conclusion, the in situ PyC coating on the surface of the fiber for KD-I SiCf/SiC composites weakens interfacial
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Table 3 Properties of the KD-I SiCf/SiC composites. Fiber volume fraction (%)
Density (g cm 3)
Porosity (%)
Flexural strength (MPa)
Fracture toughness (MPa m1/2)
45
2.04
18.4
268.8728
12.972
The imaginary part of the permittivity caused by electrical conductivity (ε″c ) can be described as follows: 00
εc ¼
s : ωε0
ð2Þ 00
Fig. 5. Load/displacement curve of KD-I SiCf/SiC composites.
bonding and effectively improves the accommodation between the fiber and the matrix. Most matrix cracks deflect when meeting the in situ PyC coating during the fracture process of the composites because of the proper combination of the fiber and the matrix. Therefore, the in situ PyC coating of the fibers can endow SiCf/SiC composites with excellent mechanical properties.
Eq. (2) shows that εc is mostly determined by electrical conductivity. Therefore, analyzing the electrical conductivity of KD-I SiC fibers is necessary. Given the special microstructure of KD-I SiC fibers (Section 3.1), we propose a “skin (PyC phase)-core (Si–C–O phase)” model to simulate the electrical properties of KD-I fibers. The microstructure model of KD-I SiC fibers is shown in Fig. 9. In this model, the Si–C–O phase acts as the high electrical resistance phase, and the PyC coating acts as the low electrical resistance phase. The electrical resistance of SiC fibers can be derived using Eq. (3) according to Ohm's law: 1=Rf ¼ 1=RPyC þ 1=RSi C O ;
ð3Þ
where Rf , RPyC , and RSi C O are the electrical resistance of SiC fibers, PyC layer, and Si–C–O phase, respectively. Given that the electrical conductivity of PyC is significantly higher than that of the Si–C–O phase [32,34], then Eq. (4) can be attained as
3.3. Dielectric properties of KD-I SiCf/SiC composites
Rf ¼ ρf l=ðπr 2 Þ RPyC ¼ ρPyC l=ðπr 2 πðr tÞ2 Þ;
Fig. 8 reveals the ε0 and ε″ of KD-I SiCf/SiC composites within the frequency range of 8–18 GHz. The permittivity of SiCf/SiC composites is remarkable. The ε0 of SiCf/SiC composites ranges from 40 to 15, whereas ε″ ranges from 50 to 5, which are larger than those of composites without PyC coating, as reported in Refs.[7,14]. An appreciable frequency dispersion effect is also observed, that is, ε0 and ε″ of SiCf/SiC composites both decrease as the microwave frequency increases. The imaginary part of the permittivity (ε″) of the composites can be calculated based on the Debye equation:
where ρf and ρPyC are the electrical resistivities of SiC fibers and PyC layers, respectively; r is the radius of SiC fibers; and t is the thickness of the PyC coating. Given r⪢t, then Eq. (4) can be simplified as
ε″ ¼
ðεs ε1 Þωτ=ð1þ ðωτÞ2 Þ þ s ; ε0 ω
ð1Þ
where εs is the static permittivity, ε1 is the permittivity at a high frequency limit, ω is the angular frequency, τ is the relaxation time, and s is the electrical conductivity of the composites. Eq. (1) shows that ε″ is determined by the relaxation time and electrical conductivity of the composites. For most dielectric materials, conductivity losses are the most important factors that affect the permittivity [44,45], and thus, should first be considered.
ρf ¼ ρPyC r=2t:
ð4Þ
ð5Þ
Eq. (5), shows that the effective electrical resistivity of KD-I fibers is proportional to that of the PyC layers and inversely proportional to the thickness of the PyC layers. The electrical resistivity of PyC derived from PCS is approximately 10 2 Ω cm [46,47]. The analysis in Section 3.1 reveals that when the thickness of the PyC layers is approximately 15–20 nm, the effective electrical resistivity of KD-I fibers is approximately 100–101 Ω cm based on Eq. (5), which is significantly higher than the electrical resistivity value of approximatelty 10 14 Ω cm of SiC fibers without PyC coating. Therefore, the ε00 of KD-I SiCf/SiC composites is remarkable at the X-band because of in situ PyC coating on the surface of the fiber. The real part of the permittivity is also determined by the electrical conductivity [46]; thus, the real part of the permittivity of KD-I SiCf/SiC composites with in situ PyC coating is also signifificantly large.
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Fig. 6. SEM micrographs of the fracture surface of KD-I SiCf/SiC composites.
Fig. 7. TEM cross-section micrographs of KD-I SiCf/SiC composites. (For interpretation of the references to color in this figure the reader is referred to the web version of this article.)
However, the reflectivity for a given microwave absorbing material generally follows the following equation [48]: ffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffi tan hð2πjf dpffiffiffiffiffiffiffiffiffiffiffiffiffi ε0 jε″=cÞ ε0 jε″ pffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffi ; ffi R ¼ 20 Log tan hð2πjf d ε0 jε″=cÞ þ ε0 jε″ where R is the reflectivity, f is the frequency, d is the thickness of the material, j is the imaginary unit, and c is the vacuum
speed of light. Determining that a large permittivity leads to strong reflection and weak absorption of microwave for KD-I SiCf/SiC composites is easy when using a computer-aided software. On all accounts, KD-I SiCf/SiC composites prepared through PIP possess excellent mechanical properties because of the in situ PyC coating on the surface of the fibers.
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microwave. Accordingly, the PyC coating on the surface of KD-I SiC fibers should first be removed before SiCf/SiC composites are used for microwave-absorbing applications.
Acknowledgments The authors appreciate the financial support of the National Natural Science Foundation of China (51202291), Aid Program for Innovative Group of National University of Defense Technology, and Aid program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province. Fig. 8. Dielectric constant of KD-I SiCf/SiC composites.
References
Pyrocarbon layer Si-C-O interphase
Fig. 9. Microstructure model of KD-I SiC fibers.
However, the dielectric constants are also remarkable, which is disadvantageous to microwave-absorbing applications because of the potential to result in lower input impedance and a mismatch in the impedance between the composites and free space. These two mutually contradictory aspects will limit the application of the fibers in microwave-absorbing materials. In the future, we will first remove the PyC coating and then introduce a low dielectric constant BN coating on the surface of KD-I SiC fibers for practical microwave-absorbing applications. 4. Conclusions (1) KD-I SiC fibers have a special “skin (PyC phase)-core (Si– C–O phase)” structure. A PyC layer approximately15– 20 nm thick is found on the surface of fibers, which remarkably affects the mechanical and dielectrical properties of KD-I SiCf/SiC composites. (2) Excellent mechanical properties of 268.8 MPa in flexural strength and 12.9 MPa m1/2 in fracture toughness are obtained in KD-I SiCf/SiC composites prepared through PIP. This result is primarily attributed to the in situ PyC coating on the surface of the fibers that can weaken interfacial bonding and effectively improve accommodation between the fiber and the matrix. (3) The permittivity of KD-I SiCf/SiC composites is remarkable at the X-band because of the in situ PyC coating on the surface of the fibers. However, a large permittivity leads to strong reflection and weak absorption of
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Please cite this article as: H. Tian, et al., Mechanical and microwave dielectric properties of KD-I SiCf/SiC composites fabricated through precursor infiltration and pyrolysis, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.01.113