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SiC composites fabricated by an optimized PIP process with a new precursor and a thermal molding method
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 6525–6532 www.elsevier.com/locate/ceramint
High-performance 3D SiC/PyC/SiC composites fabricated by an optimized PIP process with a new precursor and a thermal molding method Zheng Luon, Xingui Zhou, Jinshan Yu, Fei Wang Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, National University of Defense Technology, Changsha 410073, China Received 16 September 2013; received in revised form 12 November 2013; accepted 19 November 2013 Available online 28 November 2013
Abstract To improve the efficiency of the polymer impregnation and pyrolysis (PIP) process and the mechanical properties for SiC/SiC composites, 3-dimensional (3D) SiC/SiC were fabricated by a PIP process with a new precursor polymer and the thermal molding method. Liquid polyvinylcarbosilane (LPVCS) with active Si–H and –CHQCH2 groups was adopted as the SiC matrix precursor. The SiC/SiC composites with superior mechanical properties were efficiently fabricated. The fiber volume of the SiC/SiC was 50.4%. The bulk density and porosity of the SiC/SiC composites were 2.16 g cm 3 and 15.4% respectively. The flexural strength and fracture toughness of the SiC/SiC composites were 637.5 MPa and 29.8 MPa m1/2 respectively. The influences of LPVCS and molding pressure on the performances of the SiC/SiC composites were discussed in-depth. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: SiC/SiC composites; Optimized PIP process; New precursor polymer; Molding method; Mechanical properties
1. Introduction SiC/SiC composites exhibit excellent properties such as high-temperature strength, fracture toughness, chemical inertness, and low density [1–4]. Currently, the SiC/SiC composites are known to be one of the most promising candidates for aircraft engine hot-section components and fusion reactors [5–8]. SiC/SiC composites can be fabricated by chemical vapor infiltration (CVI), hot pressing (HP) and liquid silicon infiltration (LSI) [9–11]. The CVI process is difficult to operate and requires complex equipment. The HP process limits the components to simple plate shapes and the consequent machining is of high costs. The LSI process can develop thicker products and dense SiC matrix, but the residual silicon in the composite leads to the material strength degradation at elevated temperatures [12]. A hybrid process of SiC slurry impregnation (SI) and HP, nano-infiltrated transient eutecticphase (NITE) for example, has an advantage for the fabrication of dense SiC/SiC composite. The high temperature and n
Corresponding author. Tel.: þ86 731 84576397; fax: þ 86 731 84573165. E-mail address: [email protected] (Z. Luo).
pressure for the SiC fibers are the main challenge for the NITE process [13]. Among potential fabrication processes of the SiC/SiC composites, the PIP process is one of the main preparation processes of the SiC/SiC composites and has been widely employed because of its advantages in impregnation efficiency among fibers, microstructure control, large-scale components fabrication with complicated shapes and costs [14–15]. Main challenge of the PIP process has been made to reduce pores and cracks which are formed due to gas evolution and volumetric shrinkage of a pre-ceramic polymer during pyrolysis [16–19]. Effective consolidation to yield minimum amount of pores and cracks are very important technical issue for making a high performance SiC/SiC composites [20]. In order to control the pores and cracks, the utilization of high ceramic yield precursors and improved processes are expected to be much beneficial. In this work, the process development of high-performances 3D SiC/SiC composites was described. As the precursor, LPVCS was applied because of its advantages of sufficient stability at ambient temperature, low viscosity and high ceramic yield. To fabricate a composite of high density, main efforts were paid for optimizing molding pressure conditions.
0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.11.105
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The relationships between flexure properties and microstructure of the SiC/SiC composites were discussed in-depth. 2. Materials and experimental procedure In the fabrication of composites, KD-I, which is a continuous SiC fiber provided by National University of Defense Technology (NUDT), was employed as the reinforcement. General characteristics of the KD-I are listed in Table 1 [21]. All samples were braided into 3D performs by three dimension and four-steps braiding technique. The fiber volume fraction and the braiding angle of the composites were 36.3% and 24.81 respectively. The PyC coating was prepared by chemical vapor deposition (CVD) method on the SiC fibers as the interface layer (Fig. 1). LPVCS was used as the matrix precursor, which was provided by NUDT. The preparation process comprises the following steps: (1) the treated perform was dipped into LPVCS' slurry by a vacuum infiltration method for 24 h; (2) a thermal molding method was applied after the first impregnation at 1 1C/min up to 300 1C; (3) the performance was pyrolyzed at 20 1C/min up to 1100 1C in an inert argon atmosphere; and (4) the route of impregnation and pyrolysis was repeated 9 times till weight increase was less than 1%. The densities and open porosities of the SiC/SiC composites were measured by Archimedes' method using kerosene oil as the immersion medium. The bending strength and modulus of the composites were characterized by three-point bending test following the general guidelines of the Chinese Standard GBT 6569-2006. The dimensions of a test sample were 4 4 65 mm3, the span length and the cross head speed were 50 mm and 0.5 mm/min, respectively. The fracture toughness was tested by a simple edge notch beam (SENB) method following GBT 23806-2009, with the specimen size of 4 8 65 mm3, the notch length of 4 mm, the crosshead speed of 0.05 mm/min Table 1 General characteristics of KD-I SiC fiber. Trademark
KD-I
Diameter/μm Density/g cm 3 Tensile strength/GPa Elastic modulus/GPa
13.3 2.45 1.8 170–180
and the support span of 40 mm. Five specimens were tested to estimate the mean for the mechanical tests. The microstructure of the fracture surfaces was characterized by a JSM-5600LV scanning electron microscope (SEM). 3. Results and discussion 3.1. Preform characterization The performances of the fiber reinforced composites are primarily determined by the perform architecture. Threedimension and four-steps braiding performs are composed by the alternating motion of the warp bundles and weft bundles in X and Y directions, following by a compacting motion in the Z direction (Fig. 2). Fiber bundles run parallel to the four body diagonals of the cell and all the fiber bundles take a nominally straight path until they change the direction [22–23] (Fig. 3). The 3D braided performs have the advantages of throughthickness reinforcement, low delamination tendency, high damage tolerance and near-net-shape manufacturing [24], but cause a large amount of inter-bundle pores forming inside the braided performs. 3.2. Utilization of LPVCS as matrix PCS has been widely used as a SiC precursor. It is a solid at room temperature and it was obtained from NUDT. The basic characteristics of LPVCS and PCS are listed in Table 2. LPVCS demonstrates a superior rheological characteristics and ceramic yield to PCS. The molecular structures of LPVCS and PCS are shown in Fig. 4. There are amount of active Si–H and –CHQCH2 groups in LPVCS and theses active groups can initiate cross-linking reaction when curing at 300 1C [25]. When heated to 1100 1C, the cured products transform to compact SiC ceramics with the shrinkage of the volume [26]. The macrophotographs of the precursor polymers and their cured and pyrolyzed products were shown in Fig. 5. The pyrolysis of PCS caused higher volume expansion and more pores than LPVCSs. This indicated that the curing and pyrolysis properties of LPVCS are advantages for reducing pores in PIP process. The IR spectrum of the LPVCS and cured products of LPVCS and PCS were shown in Fig. 6. LPVCS showed more numbers of absorption peaks than PCS in the bonds of 3055 cm 1 (stretching
Fig. 1. Microstructure of the KD-I SiC fibers: (a) before the CVD method and (b) after the CVD method.
Z. Luo et al. / Ceramics International 40 (2014) 6525–6532
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Fig. 2. Three-dimension and four-steps braiding technique.
Fig. 3. Architecture of 3D performs: (a) unit cell and (b) 3D fabric.
Table 2 Basic characteristics of LPVCS and PCS. LPVCS
PCS
State at RT Molar mass
Liquid 357
Solid 1742
Density/g cm 3 As-received After pyrolysis Impregnation Viscosity/mPa s Ceramic yield/%
1.04 2.42 Without solvent 20 59.5
1.15 2.34 Solved in Xylene 25 31.5
vibration of QCH2), 3016 cm 1 (stretching vibration of C–H), 1597 cm 1 (stretching vibration of CHQCH2), 1080 cm 1 (Si– O stretching vibration of Si–O), 933 cm 1 (out-of-plane
distortion vibration of SiCHQCH2) and 799 cm 1 (distortion vibration of Si(CH3)2). In addition, the absorption peaks of LPVCS in the bonds of 1350 cm 1 (distortion vibration of Si– CH3) and 1250 cm 1 (symmetric distortion vibration of Si–CH3) were stronger than the PCSs. The decrease of the absorption peaks in the bonds of 2100 cm 1 (stretching vibration of Si–H), 3055 cm 1, 3016 cm 1 and 1597 cm 1 could be observed at the cured products of LPVCS. This indicated that the addition reaction of Si–H bonds and CHQCH2 bonds was performed and bridge linked structure was formed by Si–CH2–CH2– Si bonds. XRD diffraction patterns for LPVCS- and PCS-pyrolyzed products were shown in Fig. 7. PCS and LPVCS-pyrolyzed products were almost crystallization. A small peak was observed at 26.51 for PCS and LPVCS-pyrolyzed products, which indicated the existence of free carbon.
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H2C CH3 Si H 3C
C H
CH
2
CH3 Si
H 3C
O
Si
CH3
O O Si
Si
O
CH
CH3
HC
CH3 H C 2
2
H3C
CH
Si
Si
H3C CH
CH2
H3C
H3C
H
Si CH2 Si
CH2
CH3
CH 2
CH3
CH3
H
Si CH2
Si
CH3
CH3
CH2
2
Si H3C
Fig. 4. The molecular structure of the precursor: (a) LPVCS and (b) PCS.
Fig. 5. Optical photographs of the precursor polymers and their cured and pyrolyzed products: (a) PCS; (b) pyrolyzed products of PCS; (c) LPVCS; (d) cured products of LPVCS; and (e) pyrolyzed products of LPVCS.
LPVCS 3016
1350
3055 2900
36.5° 26.5°
1597
60.1°
PCS
71.9°
933
2100
Intensity
Absorbance
2958
1250
Cured products of LPVCS
799 1080 2900 2958
LPVCS
2100
1250
PCS
1030
36.5°
26.5°
820
2900 2958 2100
1250
1030 820
4000
3600
3200
2800
2400
2000
1600
1200
800
400
Wavenumbers/cm-1 Fig. 6. IR spectrum of the LPVCS, cured products of LPVCS and PCS.
10
20
30
40
50
60
70
80
2θ/deg Fig. 7. XRD patterns of LPVCS- and PCS-pyrolyzed products.
Z. Luo et al. / Ceramics International 40 (2014) 6525–6532
SiC/SiC composites exhibited outstanding performance with the thermal molding process (1–9 MPa). The porosity was reduced and the fiber volume fraction was increased respectively. The density of the SiC/SiC composites increased first
2.2
2.0
1.8
Density/g·cm-3
The weight increase-cycle curves of SiC/SiC composites were shown in Fig. 8. The preparation cycles the SiC/SiC composites by LPVCS which were shorter than those of PCS obviously indicating the higher densification efficiency of LPVCS. The properties of SiC/SiC composites were listed in Table 3. The SiC/SiC composites by LPVCS exhibited higher density and lower porosity. The enhancement of densification attributed to the superior rheological characteristics, high ceramic yield and volume shrinkage during the pyrolysis process of LPVCS. The SiC/SiC composites by LPVCS exhibited excellent mechanical properties for high density. The flexural strength and fracture toughness were 401 MPa and 18.7 MPa m1/2 respectively.
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3.3. High-strength improvement by the molding method
0MPa 1MPa 3MPa 5MPa 7MPa 9MPa
1.6
1.4
1.2
As thermosetting of LPVCS from liquid to solid occurred at 300 1C, a thermal molding method was applied after the first impregnation for further enhancing the density of the composites. Density-cycle curves of the specimens under different molding pressures were shown in Fig. 9. The density of specimens with the molding method (1–9 MPa) exhibited higher density than the composites without the molding method (0 MPa) during the whole PIP process. The performance parameters of the SiC/SiC composites under different molding pressure were listed in Table 4. The
35
LPVCS PCS
1.0
0.8 0
1
2
3
4
5
6
7
8
9
10
Cycle/N Fig. 9. Density-cycle curves of specimens during the PIP process.
Table 4 Performance parameters of the SiC/SiC composites under different molding pressure. Pressure/ Fiber volume Density/ Porosity/ Flexure strength/ MPa fraction/% g cm 3 % MPa
Fracture toughness/ MPa m1/2
0 1 3 5 7 9
18.7 29.1 29.8 29.7 27.5 26.8
Weight increase (%)
30 25 20
41.2 48.2 50.4 50.5 48.7 48.2
2.11 2.14 2.16 2.16 2.15 2.14
21.8 16.6 15.4 16.6 17.8 18.2
401.1 619.4 637.5 629.2 591.2 557.9
15 10 700
5
(c)
(d)
(a) 0 MPa (b) 1 MPa (c) 3 MPa (d) 5 MPa (e) 7 MPa (f) 9 MPa
600
0 0
2
4
6
8
10
12
14
16
500
(b)
Fig. 8. weight increase-cycle curve of SiC/SiC by different precursors.
Table 3 Properties of the SiC/SiC composites.
Stress/MPa
Cycles (N) (a)
400
(f)
300
200
(e)
Polymer Density/ g cm 3
Open porosity/ %
Flexure strength/ MPa
Elastic modulus/ GPa
Fracture toughness/ MPa m1/2
6.7 13.1
401.1 348.0
62.7 44.3
18.7 13.8
100
0 0.00
LPVCS 2.11 PCS 2.01
0.01
0.02
0.03
0.04
0.05
0.06
Strain
Fig. 10. Stress–strain curves of SiC/SiC composites.
0.07
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and then decreased with the increase of the pressure. The main reason for decrease of the density at high molding pressure (5– 9 MPa) was the internal stress concentration and volume rebound. The similar trends were shown for the flexure strength and fracture toughness. That was probably related to the fiber damage by the high molding pressure. Typical stress– strain curves of specimens were shown in Fig. 10. The appropriate molding pressure was about 3 MPa. The flexure strength and fracture toughness were 637.5 MPa and 29.8 MPa m1/2 respectively. The fracture behavior of the SiC/SiC composites was shown in Fig. 11. Fiber pullouts on the fracture surface in Fig. 11(f) were shorter than the other specimens. It indicated that high molding pressure led to the strong interface bond of composites. Large quantities of long pulled-out fibers and matrix cracks could be seen on the fracture surfaces in Fig. 11(c). This was typical of crack arresting, deflecting and branching behaviors which led to the pseudo-ductile fracture mode of the composites. Morphologies of the fibers and interfaces of the composites under different molding pressure were shown in Fig. 12. The rupture and peeling of the PyC interface layer
were obvious in Fig. 12(a)–(e), illustrating the arrest, deflection and branching of the cracks. Fig. 12(f) exhibited strong interface bond between the fiber and the matrix.
4. Conclusions To fabricate high-performance SiC/SiC composites by the PIP process, the 3D braiding method, LPVCS and the thermal molding method were preformed. LPVCS could be compound very well into SiC perform owing to its low viscosity and excellent wettability. LPVCS as a thermosetting compound from liquid to solid at 300 1C providing the opportunity for the thermal molding method. Efficient densification could be achieved under the thermal molding method, but too high molding pressure produced the internal stress concentration, causing strong interface bond. The appropriate molding pressure was 3 MPa. The density of the SiC/SiC composites was 2.16 g cm 3. The flexural strength and fracture toughness of the SiC/SiC composites were 637.5 MPa and 29.8 MPa m1/2 respectively.
Fig. 11. Fracture surface of SiC/SiC composites under different molding pressure. (a) 0 MPa, (b) 1 MPa, (c) 3 MPa, (d) 5 MPa, (e) 7 MPa and (f) 9 MPa.
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Fig. 12. Morphology of the fibers and interfaces of the SiC/SiC composites under different molding pressure: (a) 0 MPa; (b) 1 MPa; (c) 3 MPa; (d) 5 MPa; (e) 7 MPa; and (f) 9 MPa.
Acknowledgments The authors acknowledge the support of the National Natural Science Foundation of China (Grant no. 50172228), the aid program for Innovative Group of National University of Defense Technology and the aid program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province. References [1] H.J. Yu, X.G. Zhou, et al., Mechanical properties of 3D KD-1 SiCf/SiC composites with engineered fibre-matrix interfaces, Compos. Sci. Technol. 71 (5) (2011) 699–704. [2] H.L. Wang, X.G. Zhou, et al., Microstructure, mechanical properties and reaction mechanism of KD-1 SiCf/SiC composites fabricated by chemical vapor infiltration and vapor silicon infiltration, Mater. Sci. Eng. A—struct. Mater. 528 (6) (2011) 2441–2445. [3] S. Zhao, X.G. Zhou, et al., Effect of heat treatment on microstructure and mechanical properties of PIP-SiC/SiC composites, Mater. Sci. Eng. A 599 (2013) 808–811.
[4] H.J. Yu, X.G. Zhou, et al., Mechanical behavior of SiCf/SiC composites with alternating PyC/SiC multilayer interphases, Mater. Des. 44 (2013) 320–324. [5] D. Brewer, HSR/EPM combustor materials development program, Mater. Sci. Eng. A A261 (1999) 284–291. [6] G.N. Morscher, G. Ojard, R. Miller, Y. Gowayerd, U. Santhosh, J. Ahmad, et al., Tensile creep and fatigue of Sylramic-iBN melt-infiltrated SiC matrix composites: retained properties, damage development, and failure mechanisms, Compos. Sci. Technol. 68 (2008) 3305–3313. [7] Y. Katoh, L.L. Snead, C.H. Henager, et al., Current status and critical issues for development of SiC composites for fusion applications, J. Nucl. Mater. 367–370 (2007) 659–671. [8] T. Nozawa, T. Hinoki, A. Hasegawa, et al., Recent advances and issues in development of silicon carbide composites for fusion applications, J. Nucl. Mater. 386–388 (2009) 622–627. [9] T. Taguchi, N. Igawa, R. Yamada, S. Jitsukawa, Effect of thick SiC interphase layers on microstructure, mechanical and thermal properties of reaction-bonded SiC/SiC composites, J. Phys. Chem. Solids 66 (2005) 576–580. [10] Y.D. Xu, L.F. Cheng, L.T. Zhang, X.W. Yin, H.F. Yin, High performance 3D textile Hi-Nicalon SiC/SiC composites by chemical vapor infiltration, Ceram. Int. 27 (2001) 565–570. [11] Z.S. Rak, A process for Cf/SiC composites using liquid polymer infiltration, J. Am. Ceram. Soc. 84 (10) (2001) 2235–2239.
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[12] Yunzhou Zhu, Zhengren Huang, Shaoming Dong, et al., Manufacturing 2D carbon-fiber-reinforced SiC matrix composites by slurry infiltration and PIP process, Ceram. Int. 34 (2008) 1201–1205. [13] Ji Yeon Park, Myung Hoon Jeong, Kim Weon-Ju, Characterization of slurry infiltrated SiCf/SiC prepared by electrophoretic deposition, J. Nucl. Mater. 442 (2013) 5390–5393. [14] J.A. Cornie, Y.M. Chiang, D.R. Uhlmann, et al., Processing of metal and ceramic matrix composites, Ceram. Bull. 65 (2) (1986) 293–303. [15] R. Jones, A. Szweda, D. Petrak, Polymer derived ceramic matrix composites, Compos.: Part A 30 (1990) 569–575. [16] J.R. Walker, B.E., P.F. Rice, et al., Preparation and properties of monolithic and composite ceramics produced by polymer pyrolysis, Am. Ceram. Soc. Bull. 62 (8) (1983) 916–923. [17] H. Yoshida, N. Miyata, M. Sagawa, S. Ishikawa, K. Naito, N. Enomoto, C. Yamagishi, Preparation of unidirectionally reinforced carbon-SiC composite by repeated infiltration of polycarbosilane, J. Ceram. Soc. Jpn. 100 (4) (1992) 454–458. [18] D.W. Shin, H. Tanaka., Low-temperature processing of ceramic woven fabric/ceramic matrix composites, J. Am. Ceram. Soc. 77 (1) (1994) 97–104. [19] T. Tanaka, N. Tamari, I. Kondoh, M. Iwasa, Fabrication and mechanical properties of 3-dimensional Tyranno fiber reinforced SiC composites by
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 6525–6532 www.elsevier.com/locate/ceramint
High-performance 3D SiC/PyC/SiC composites fabricated by an optimized PIP process with a new precursor and a thermal molding method Zheng Luon, Xingui Zhou, Jinshan Yu, Fei Wang Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, National University of Defense Technology, Changsha 410073, China Received 16 September 2013; received in revised form 12 November 2013; accepted 19 November 2013 Available online 28 November 2013
Abstract To improve the efficiency of the polymer impregnation and pyrolysis (PIP) process and the mechanical properties for SiC/SiC composites, 3-dimensional (3D) SiC/SiC were fabricated by a PIP process with a new precursor polymer and the thermal molding method. Liquid polyvinylcarbosilane (LPVCS) with active Si–H and –CHQCH2 groups was adopted as the SiC matrix precursor. The SiC/SiC composites with superior mechanical properties were efficiently fabricated. The fiber volume of the SiC/SiC was 50.4%. The bulk density and porosity of the SiC/SiC composites were 2.16 g cm 3 and 15.4% respectively. The flexural strength and fracture toughness of the SiC/SiC composites were 637.5 MPa and 29.8 MPa m1/2 respectively. The influences of LPVCS and molding pressure on the performances of the SiC/SiC composites were discussed in-depth. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: SiC/SiC composites; Optimized PIP process; New precursor polymer; Molding method; Mechanical properties
1. Introduction SiC/SiC composites exhibit excellent properties such as high-temperature strength, fracture toughness, chemical inertness, and low density [1–4]. Currently, the SiC/SiC composites are known to be one of the most promising candidates for aircraft engine hot-section components and fusion reactors [5–8]. SiC/SiC composites can be fabricated by chemical vapor infiltration (CVI), hot pressing (HP) and liquid silicon infiltration (LSI) [9–11]. The CVI process is difficult to operate and requires complex equipment. The HP process limits the components to simple plate shapes and the consequent machining is of high costs. The LSI process can develop thicker products and dense SiC matrix, but the residual silicon in the composite leads to the material strength degradation at elevated temperatures [12]. A hybrid process of SiC slurry impregnation (SI) and HP, nano-infiltrated transient eutecticphase (NITE) for example, has an advantage for the fabrication of dense SiC/SiC composite. The high temperature and n
Corresponding author. Tel.: þ86 731 84576397; fax: þ 86 731 84573165. E-mail address: [email protected] (Z. Luo).
pressure for the SiC fibers are the main challenge for the NITE process [13]. Among potential fabrication processes of the SiC/SiC composites, the PIP process is one of the main preparation processes of the SiC/SiC composites and has been widely employed because of its advantages in impregnation efficiency among fibers, microstructure control, large-scale components fabrication with complicated shapes and costs [14–15]. Main challenge of the PIP process has been made to reduce pores and cracks which are formed due to gas evolution and volumetric shrinkage of a pre-ceramic polymer during pyrolysis [16–19]. Effective consolidation to yield minimum amount of pores and cracks are very important technical issue for making a high performance SiC/SiC composites [20]. In order to control the pores and cracks, the utilization of high ceramic yield precursors and improved processes are expected to be much beneficial. In this work, the process development of high-performances 3D SiC/SiC composites was described. As the precursor, LPVCS was applied because of its advantages of sufficient stability at ambient temperature, low viscosity and high ceramic yield. To fabricate a composite of high density, main efforts were paid for optimizing molding pressure conditions.
0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.11.105
6526
Z. Luo et al. / Ceramics International 40 (2014) 6525–6532
The relationships between flexure properties and microstructure of the SiC/SiC composites were discussed in-depth. 2. Materials and experimental procedure In the fabrication of composites, KD-I, which is a continuous SiC fiber provided by National University of Defense Technology (NUDT), was employed as the reinforcement. General characteristics of the KD-I are listed in Table 1 [21]. All samples were braided into 3D performs by three dimension and four-steps braiding technique. The fiber volume fraction and the braiding angle of the composites were 36.3% and 24.81 respectively. The PyC coating was prepared by chemical vapor deposition (CVD) method on the SiC fibers as the interface layer (Fig. 1). LPVCS was used as the matrix precursor, which was provided by NUDT. The preparation process comprises the following steps: (1) the treated perform was dipped into LPVCS' slurry by a vacuum infiltration method for 24 h; (2) a thermal molding method was applied after the first impregnation at 1 1C/min up to 300 1C; (3) the performance was pyrolyzed at 20 1C/min up to 1100 1C in an inert argon atmosphere; and (4) the route of impregnation and pyrolysis was repeated 9 times till weight increase was less than 1%. The densities and open porosities of the SiC/SiC composites were measured by Archimedes' method using kerosene oil as the immersion medium. The bending strength and modulus of the composites were characterized by three-point bending test following the general guidelines of the Chinese Standard GBT 6569-2006. The dimensions of a test sample were 4 4 65 mm3, the span length and the cross head speed were 50 mm and 0.5 mm/min, respectively. The fracture toughness was tested by a simple edge notch beam (SENB) method following GBT 23806-2009, with the specimen size of 4 8 65 mm3, the notch length of 4 mm, the crosshead speed of 0.05 mm/min Table 1 General characteristics of KD-I SiC fiber. Trademark
KD-I
Diameter/μm Density/g cm 3 Tensile strength/GPa Elastic modulus/GPa
13.3 2.45 1.8 170–180
and the support span of 40 mm. Five specimens were tested to estimate the mean for the mechanical tests. The microstructure of the fracture surfaces was characterized by a JSM-5600LV scanning electron microscope (SEM). 3. Results and discussion 3.1. Preform characterization The performances of the fiber reinforced composites are primarily determined by the perform architecture. Threedimension and four-steps braiding performs are composed by the alternating motion of the warp bundles and weft bundles in X and Y directions, following by a compacting motion in the Z direction (Fig. 2). Fiber bundles run parallel to the four body diagonals of the cell and all the fiber bundles take a nominally straight path until they change the direction [22–23] (Fig. 3). The 3D braided performs have the advantages of throughthickness reinforcement, low delamination tendency, high damage tolerance and near-net-shape manufacturing [24], but cause a large amount of inter-bundle pores forming inside the braided performs. 3.2. Utilization of LPVCS as matrix PCS has been widely used as a SiC precursor. It is a solid at room temperature and it was obtained from NUDT. The basic characteristics of LPVCS and PCS are listed in Table 2. LPVCS demonstrates a superior rheological characteristics and ceramic yield to PCS. The molecular structures of LPVCS and PCS are shown in Fig. 4. There are amount of active Si–H and –CHQCH2 groups in LPVCS and theses active groups can initiate cross-linking reaction when curing at 300 1C [25]. When heated to 1100 1C, the cured products transform to compact SiC ceramics with the shrinkage of the volume [26]. The macrophotographs of the precursor polymers and their cured and pyrolyzed products were shown in Fig. 5. The pyrolysis of PCS caused higher volume expansion and more pores than LPVCSs. This indicated that the curing and pyrolysis properties of LPVCS are advantages for reducing pores in PIP process. The IR spectrum of the LPVCS and cured products of LPVCS and PCS were shown in Fig. 6. LPVCS showed more numbers of absorption peaks than PCS in the bonds of 3055 cm 1 (stretching
Fig. 1. Microstructure of the KD-I SiC fibers: (a) before the CVD method and (b) after the CVD method.
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Fig. 2. Three-dimension and four-steps braiding technique.
Fig. 3. Architecture of 3D performs: (a) unit cell and (b) 3D fabric.
Table 2 Basic characteristics of LPVCS and PCS. LPVCS
PCS
State at RT Molar mass
Liquid 357
Solid 1742
Density/g cm 3 As-received After pyrolysis Impregnation Viscosity/mPa s Ceramic yield/%
1.04 2.42 Without solvent 20 59.5
1.15 2.34 Solved in Xylene 25 31.5
vibration of QCH2), 3016 cm 1 (stretching vibration of C–H), 1597 cm 1 (stretching vibration of CHQCH2), 1080 cm 1 (Si– O stretching vibration of Si–O), 933 cm 1 (out-of-plane
distortion vibration of SiCHQCH2) and 799 cm 1 (distortion vibration of Si(CH3)2). In addition, the absorption peaks of LPVCS in the bonds of 1350 cm 1 (distortion vibration of Si– CH3) and 1250 cm 1 (symmetric distortion vibration of Si–CH3) were stronger than the PCSs. The decrease of the absorption peaks in the bonds of 2100 cm 1 (stretching vibration of Si–H), 3055 cm 1, 3016 cm 1 and 1597 cm 1 could be observed at the cured products of LPVCS. This indicated that the addition reaction of Si–H bonds and CHQCH2 bonds was performed and bridge linked structure was formed by Si–CH2–CH2– Si bonds. XRD diffraction patterns for LPVCS- and PCS-pyrolyzed products were shown in Fig. 7. PCS and LPVCS-pyrolyzed products were almost crystallization. A small peak was observed at 26.51 for PCS and LPVCS-pyrolyzed products, which indicated the existence of free carbon.
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H2C CH3 Si H 3C
C H
CH
2
CH3 Si
H 3C
O
Si
CH3
O O Si
Si
O
CH
CH3
HC
CH3 H C 2
2
H3C
CH
Si
Si
H3C CH
CH2
H3C
H3C
H
Si CH2 Si
CH2
CH3
CH 2
CH3
CH3
H
Si CH2
Si
CH3
CH3
CH2
2
Si H3C
Fig. 4. The molecular structure of the precursor: (a) LPVCS and (b) PCS.
Fig. 5. Optical photographs of the precursor polymers and their cured and pyrolyzed products: (a) PCS; (b) pyrolyzed products of PCS; (c) LPVCS; (d) cured products of LPVCS; and (e) pyrolyzed products of LPVCS.
LPVCS 3016
1350
3055 2900
36.5° 26.5°
1597
60.1°
PCS
71.9°
933
2100
Intensity
Absorbance
2958
1250
Cured products of LPVCS
799 1080 2900 2958
LPVCS
2100
1250
PCS
1030
36.5°
26.5°
820
2900 2958 2100
1250
1030 820
4000
3600
3200
2800
2400
2000
1600
1200
800
400
Wavenumbers/cm-1 Fig. 6. IR spectrum of the LPVCS, cured products of LPVCS and PCS.
10
20
30
40
50
60
70
80
2θ/deg Fig. 7. XRD patterns of LPVCS- and PCS-pyrolyzed products.
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SiC/SiC composites exhibited outstanding performance with the thermal molding process (1–9 MPa). The porosity was reduced and the fiber volume fraction was increased respectively. The density of the SiC/SiC composites increased first
2.2
2.0
1.8
Density/g·cm-3
The weight increase-cycle curves of SiC/SiC composites were shown in Fig. 8. The preparation cycles the SiC/SiC composites by LPVCS which were shorter than those of PCS obviously indicating the higher densification efficiency of LPVCS. The properties of SiC/SiC composites were listed in Table 3. The SiC/SiC composites by LPVCS exhibited higher density and lower porosity. The enhancement of densification attributed to the superior rheological characteristics, high ceramic yield and volume shrinkage during the pyrolysis process of LPVCS. The SiC/SiC composites by LPVCS exhibited excellent mechanical properties for high density. The flexural strength and fracture toughness were 401 MPa and 18.7 MPa m1/2 respectively.
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3.3. High-strength improvement by the molding method
0MPa 1MPa 3MPa 5MPa 7MPa 9MPa
1.6
1.4
1.2
As thermosetting of LPVCS from liquid to solid occurred at 300 1C, a thermal molding method was applied after the first impregnation for further enhancing the density of the composites. Density-cycle curves of the specimens under different molding pressures were shown in Fig. 9. The density of specimens with the molding method (1–9 MPa) exhibited higher density than the composites without the molding method (0 MPa) during the whole PIP process. The performance parameters of the SiC/SiC composites under different molding pressure were listed in Table 4. The
35
LPVCS PCS
1.0
0.8 0
1
2
3
4
5
6
7
8
9
10
Cycle/N Fig. 9. Density-cycle curves of specimens during the PIP process.
Table 4 Performance parameters of the SiC/SiC composites under different molding pressure. Pressure/ Fiber volume Density/ Porosity/ Flexure strength/ MPa fraction/% g cm 3 % MPa
Fracture toughness/ MPa m1/2
0 1 3 5 7 9
18.7 29.1 29.8 29.7 27.5 26.8
Weight increase (%)
30 25 20
41.2 48.2 50.4 50.5 48.7 48.2
2.11 2.14 2.16 2.16 2.15 2.14
21.8 16.6 15.4 16.6 17.8 18.2
401.1 619.4 637.5 629.2 591.2 557.9
15 10 700
5
(c)
(d)
(a) 0 MPa (b) 1 MPa (c) 3 MPa (d) 5 MPa (e) 7 MPa (f) 9 MPa
600
0 0
2
4
6
8
10
12
14
16
500
(b)
Fig. 8. weight increase-cycle curve of SiC/SiC by different precursors.
Table 3 Properties of the SiC/SiC composites.
Stress/MPa
Cycles (N) (a)
400
(f)
300
200
(e)
Polymer Density/ g cm 3
Open porosity/ %
Flexure strength/ MPa
Elastic modulus/ GPa
Fracture toughness/ MPa m1/2
6.7 13.1
401.1 348.0
62.7 44.3
18.7 13.8
100
0 0.00
LPVCS 2.11 PCS 2.01
0.01
0.02
0.03
0.04
0.05
0.06
Strain
Fig. 10. Stress–strain curves of SiC/SiC composites.
0.07
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and then decreased with the increase of the pressure. The main reason for decrease of the density at high molding pressure (5– 9 MPa) was the internal stress concentration and volume rebound. The similar trends were shown for the flexure strength and fracture toughness. That was probably related to the fiber damage by the high molding pressure. Typical stress– strain curves of specimens were shown in Fig. 10. The appropriate molding pressure was about 3 MPa. The flexure strength and fracture toughness were 637.5 MPa and 29.8 MPa m1/2 respectively. The fracture behavior of the SiC/SiC composites was shown in Fig. 11. Fiber pullouts on the fracture surface in Fig. 11(f) were shorter than the other specimens. It indicated that high molding pressure led to the strong interface bond of composites. Large quantities of long pulled-out fibers and matrix cracks could be seen on the fracture surfaces in Fig. 11(c). This was typical of crack arresting, deflecting and branching behaviors which led to the pseudo-ductile fracture mode of the composites. Morphologies of the fibers and interfaces of the composites under different molding pressure were shown in Fig. 12. The rupture and peeling of the PyC interface layer
were obvious in Fig. 12(a)–(e), illustrating the arrest, deflection and branching of the cracks. Fig. 12(f) exhibited strong interface bond between the fiber and the matrix.
4. Conclusions To fabricate high-performance SiC/SiC composites by the PIP process, the 3D braiding method, LPVCS and the thermal molding method were preformed. LPVCS could be compound very well into SiC perform owing to its low viscosity and excellent wettability. LPVCS as a thermosetting compound from liquid to solid at 300 1C providing the opportunity for the thermal molding method. Efficient densification could be achieved under the thermal molding method, but too high molding pressure produced the internal stress concentration, causing strong interface bond. The appropriate molding pressure was 3 MPa. The density of the SiC/SiC composites was 2.16 g cm 3. The flexural strength and fracture toughness of the SiC/SiC composites were 637.5 MPa and 29.8 MPa m1/2 respectively.
Fig. 11. Fracture surface of SiC/SiC composites under different molding pressure. (a) 0 MPa, (b) 1 MPa, (c) 3 MPa, (d) 5 MPa, (e) 7 MPa and (f) 9 MPa.
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Fig. 12. Morphology of the fibers and interfaces of the SiC/SiC composites under different molding pressure: (a) 0 MPa; (b) 1 MPa; (c) 3 MPa; (d) 5 MPa; (e) 7 MPa; and (f) 9 MPa.
Acknowledgments The authors acknowledge the support of the National Natural Science Foundation of China (Grant no. 50172228), the aid program for Innovative Group of National University of Defense Technology and the aid program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province. References [1] H.J. Yu, X.G. Zhou, et al., Mechanical properties of 3D KD-1 SiCf/SiC composites with engineered fibre-matrix interfaces, Compos. Sci. Technol. 71 (5) (2011) 699–704. [2] H.L. Wang, X.G. Zhou, et al., Microstructure, mechanical properties and reaction mechanism of KD-1 SiCf/SiC composites fabricated by chemical vapor infiltration and vapor silicon infiltration, Mater. Sci. Eng. A—struct. Mater. 528 (6) (2011) 2441–2445. [3] S. Zhao, X.G. Zhou, et al., Effect of heat treatment on microstructure and mechanical properties of PIP-SiC/SiC composites, Mater. Sci. Eng. A 599 (2013) 808–811.
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