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SiC composites by a one-step Si infiltration reaction bonding
Materials Characterization 155 (2019) 109799
Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Joining of the Cf/SiC composites by a one-step Si infiltration reaction bonding
T
Xishi Wua,b, Bingbing Peia, Yunzhou Zhua, , Zhengren Huanga, ⁎
⁎
a
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, No. 588, Heshuo Road, Jiading District, Shanghai, China b University of Chinese Academy of Sciences, Beijing 100049, China
ARTICLE INFO
ABSTRACT
Keywords: Cf/SiC Reaction bonding Joint One-step Si infiltration
An improved joining technique by reaction bonded technology was studied. The final densification and joining of the reaction-sintered composites were simultaneously completed in the reaction bonding process by one-step Si infiltration process. This method achieves joints with strong interfacial bonding and high flexural strength. The joint obtained by the one-step silicon infiltration reaction has a more uniform microstructure, the joint flexural strength is increased to 203 MPa, and retention rate of the flexural strength is 96%, which is comparable to the flexural strength of the Cf/SiC substrate. The microstructure and interfacial evolution mechanism of the interlayer were discussed. The results show that a transition layer of 2–3 μm transition layer formed between the interlayer and the Cf/SiC substrate, which is composed of SiC crystal grains of about 0.5–1 μm. The formation of the transition layer is due to the carbon concentration difference at the interface between interlayer and substrate, resulting in the diffusion of carbon during the SieC reaction.
1. Introduction Carbon fiber reinforced silicon carbide matrix composites (Cf/SiC) have received considerable attention for their excellent high-temperature performances, excellent corrosion resistance, high specific rigidity, high thermal conductivity and exhibits low density [1,2]. Thus, Cf/SiC composites are possible materials in many areas, such as high-temperature structural components, aerospace, national defense, energy, and so on [3,4]. However, it is almost impossible to fabricate Cf/SiC composites components by forging, extruding or other plastic forming processes due to its extreme hardness and brittle nature. The rapid development of aerospace, national defense, energy, and other fields has put forward urgent requirements for Cf/SiC components with the complex shape or large size. Therefore, the most popular approach for fabricating Cf/SiC components is to join small or simple composites pieces together to form the desired structures. At present, a wide range of technologies has been developed for joining Cf/SiC composites, such as metallic brazebased joining [5,6], MAX phase joining [7,8], glass-ceramic bonding [9], diffusion [10], polymer-derived SiC joining [11] and SieC reaction joining [12–17]. The method of SieC reaction joining is to apply the reaction-sintered silicon carbide ceramic process to the joining of silicon carbide ⁎
ceramic and its composite material. Compared with other joining methods, the method of reaction joining has the advantages of high joint strength, good matching of the interlayer and the substrate, high application temperature and controllable structure of the interlayer [12]. In previous work [12–14], a variety of SiC ceramics and ceramic matrix composites were joined using this method, and the mechanical properties of the joints were evaluated. M Singh [15] studied the effect of the base material and joining process parameters on the high-temperature mechanical properties of the joint formed by the SieC reaction bonding. By measuring the electrical properties of the joint interface, Li, S.B [16] found no abrupt change in electrical resistivity around the joint area, which indicates that there is no steep gradient in the microstructure and properties of the SieC reaction joint interface. Luo, Z.H [17] used SiC/C tapes with different composition and thickness to join a pressureless sintered silicon carbide ceramic by SieC reaction bonding and obtained a sample having high bending strength by controlling the composition and thickness of the interlayer. However, the machining of the SiC ceramics and ceramic matrix composites before joining is quite difficult, and the process of silicon infiltration is repeated. Besshi, T [18] found that the joining of green bodies before sintering is useful in obtaining complexly shaped Al2O3 ceramic parts and effective for reducing manufacturing costs. In this paper, an improved
Corresponding authors. E-mail addresses: [email protected] (Y. Zhu), [email protected] (Z. Huang).
https://doi.org/10.1016/j.matchar.2019.109799 Received 10 April 2019; Received in revised form 19 June 2019; Accepted 24 June 2019 Available online 26 June 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.
Materials Characterization 155 (2019) 109799
X. Wu, et al.
Fig. 1. The microstructure of the Cf/C preform specimens.
joining technique by reaction bonded technology was studied. The Cf/C preform was joined before the silicon infiltration process, and the joining process was applied at the stage of the siliconization. The final densification and joining of the reaction-sintered composites were simultaneously completed in the reaction bonding process. The most important advantage of this technology is that the joining of the reaction-sintered composite by a one-step Si infiltration process, avoiding the repeated silicon infiltration process. The microstructure and mechanical properties of the joint were studied. The interfacial evolution mechanism of the joint area was also discussed.
characterized using scanning electron microscopy (SEM, S-4800, Hitachi) equipped with an energy dispersive spectroscopy (EDS) system. In order to study the morphology of the interlayer in more detail, the joint is etched with mixed acid of HF-HNO3 for 4 h to remove the residual silicon. The reactions are as follows:
3Si + 4HNO3
SiO2 + 4HF
3SiO2 + 4NO + 2H2 O
SiF4 + 2H2 O
(1) (2)
The high-resolution image electron diffraction was observed by transmission electron microscopy (TEM, JEM-2100F, JEOL Ltd., Japan).
2. Experimental procedures
3. Results and discussion
The materials used for joining in this paper include Cf/SiC composites and Cf/C perform, all of which are prepared in our laboratory. The microstructure of the Cf/C preform is shown in Fig. 1. The density and the 3-point flexural strength of the Cf/SiC composites and Cf/C preform in this study are 2.77 ± 0.03 g/cm3, 210 ± 22 MPa, and 0.94 g/cm3, 95 ± 10 MPa, respectively. The precursor slurry, containing organic resin, solvent, dispersant, catalyst, and SiC powder, was used for joining. The powders used as inert filler were a commercially available α-SiC powder with an average particle size of 0.4 μm. The resoltype phenol-formaldehyde resin (Industrial level) was used as the carbon precursor. The ethyl alcohol (AR, Average molecular weight: 46.07) was used as a solvent. The dispersant was polyvinylpyrrolidone (K-30, AR, Average molecular weight: 58000), and the catalyst was hexamethylene-tetramine (AR, MW: 140.19), respectively. The PF, ethyl alcohol, polyvinylpyrrolidone, hexamethylene-tetramine, and α-SiC powder were mixed by ball milling for 6 h at room temperature. The Cf/C preform with a dimension of 6 mm × 40 mm × 20 mm was used for joining, and the joining area was 6 mm × 40 mm. The Cf/C preforms were first cleaned ultrasonically in ethyl alcohol for 20 min and then dried. The preparation of joining for Cf/SiC composites is schematically shown in Fig. 2. The joining samples were assembled in Cf/C/slurry/Cf/C to form a sandwich structure wherein the slurry was pasted between two Cf/C preform. Moreover, the assembled joining samples were loaded into the graphite die and were clamped by graphite nuts. The mated specimens were pyrolyzed at 800–1200 °C, respectively. Then, the mated specimens were reaction bonded with free Si at 1600 °C in a vacuum. The resulting joining sample is identified as RB-OS. As a comparison, the Cf/SiC composite was joined, and the resulting joining sample was identified as RB-TS. The connection process of RB-TS is the same as that of RB-OS. All the joined specimens were cut into rectangular bars with dimensions of 3 mm × 4 mm × 36 mm. The 3-point flexural strength of the joined specimens was tested on an Instron 5566 testing system (Instron-5566). The cross-head speed was 0.5 mm/min. The microstructure (polished cross-sections) of the joined specimens were
3.1. Microstructure of the joints Fig. 3 presents the cross-section microstructure of the RB-OS joint and RB-TS joint. Microstructural studies and phase analysis reveal that both of the joints are well-bonded without cracks and voids, and the interlayer is basically ‘fully dense’. This one-step silicon infiltration reaction bonding method achieves an effective joining of the Cf/SiC composite. The interlayer thicknesses of the RB-OS (Fig. 3b) and RB-TS (Fig. 3a) joints are approximately 55 μm and 12 μm, respectively. The detail of the interlayer is shown in Fig. 3c and d. EDS point analysis was performed for the points shown in Fig. 3(c) and (d), where the corresponding elemental results are listed in Table 1. The white particles (Fig. 3c, d(1)), the light gray phase (Fig. 3c, d (3)) and the dark gray zone (Fig. 3c, d (2)) in the interlayer are original α-SiC particles, newly formed SiC and residual Si phases, respectively. The chemical reaction between silicon and carbon results in the formation of β-SiC particles that precipitate on the original α-SiC grains and interconnect them. The pores between silicon carbide particles are filled with silicon [19]. The reliability of a joining sample depends on the weakest part of the joint, that is, the free silicon accumulation area [20]. There are many free silicon accumulation regions in the interlayer of the RB-TS joint. The microstructure of the RB-OS joint is more uniform than that of the RBTS joint, and the visible interface between the joint and the Cf/SiC composite is continuous. The results of the cross-sectional microstructure indicate that the uniform microstructure of the RB-OS joint is related to the surface of the Cf/C preform. As shown in Fig. 1, the surface of the Cf/C preform consists of a number of pores. During the joining of the Cf/C preform using the precursor slurry, the precursor slurry can infiltrate into the pores due to the capillarity and outer pressure. That will increases the stability of the joining process [21]. The outer pressure comes from the clamping pressure of the graphite mold and graphite nuts. The EDS areal mapping results of RB-TS joint and RB-OS joint are shown in Fig. 4. The interlayer of RB-TS joint (Fig. 4a, b, c) is composed of the elements Si, and C. The interlayer contains a higher content of silicon than the substrate, and there is ‘Silicon Island’ in the interlayer, 2
Materials Characterization 155 (2019) 109799
X. Wu, et al.
Fig. 2. The preparation process of RB-OS joint.
which influences the mechanical performance of joining. Compared with the RB-TS joint, the RB-OS joint (Fig. 4d, e, f) has a more uniform distribution of elements. From the Fig. 4, the C and Si elements are homogenously distributed in the interlayer (Fig. 4e, f). Also, the distribution of elements in the interlayer is not significantly different from that of the substrate, and there is no obvious interface and no aggregation of free Si. It indicates that the interface between the interlayer and the substrate materials were well matched throughout the whole joining processes.
Table 1 EDS results of the RB-OS joint and RB-TS joint. Position
Fig. 3(c) Fig. 3(d)
3.2. Mechanical property and failure model The flexural strengths of the RB-OS joint and RB-TS joint at room temperature are shown in Fig. 5. The flexural strength of the RB-TS joint and RB-OS joint is 175 ± 13 MPa and 203 ± 24 MPa, respectively. The retention rate of the flexural strength of the RB-TS joint and RB-OS joint is 83 and 96%, respectively. The strength retention rate refers to the ratio of the flexural strength of the joint to the flexural
Composition (at. %)
1 2 3 1 2 3
Si
C
54.46 94.05 52.18 50.89 95.89 50.62
44.54 5.95 47.82 49.11 4.11 49.38
Possible phase
SiC Residual Si SiC SiC Residual Si SiC
strength of the Cf/SiC substrate. The flexural strength of the RB-OS joint is comparable to the flexural strength of the Cf/SiC substrate (210 ± 22 MPa). The flexural strength of RB-OS joint is higher than that of RB-TS. That is because that the RB-OS joint has a more uniform microstructure and no aggregation of free Si. The micrograph of the
Fig. 3. The secondary electron image of the RB-TS and RB-OS joint (a) (c) RB-TS (b) (d) RB-OS. 3
Materials Characterization 155 (2019) 109799
X. Wu, et al.
Fig. 4. The backscattered electron images and elemental analysis results of RB-TS joint (a, b, c) and RB-OS joint (d, e, f).
Flexural Strength
MPa
250 200
203±24
210±21
RB-OS
Substrate
destroyed. It means that the reliability of joined specimens depended on Cf/SiC substrate materials, not the interlayer. The microstructure of the RB-TS and RB-OS joints after acid corrosion by a mixture of HF-HNO3 are shown in Fig. 7. It can be noted that the HF-HNO3 acid mixture etches residual Si of the joined specimens. The pores that appear on the surface of the joined specimens originated from the removal of residual Si. Compared with RB-TS joints, fewer pores can be seen on the polished surface under the same etching condition for the interlayer of the RB-OS joint. At the joint interfaces, it mainly contains SiC/SiC and Si/SiC interfaces, as well as a small amount of Si/Si interface. There is a difference between the thermal expansion coefficients of silicon and silicon carbide. When the joined samples are cooled from a high temperature, axial residual stresses are generated near the SiC-Si interface of the joint [22]. That is detrimental to the mechanical properties of the joined samples. Therefore, more SiC-SiC bonding regions can achieve better mechanical properties. It can be seen in Fig. 7a, there is a clear joint interface in the RB-TS joint, and there are fewer SiC-SiC bonding regions and more SiC-Si bonding regions on the interface. However, there is no obvious joint interface in the RB-OS joint (Fig. 7b), and there are more SiC-SiC bonding regions and fewer SiC-Si bonding regions on the interface. That means that a one-step infiltration reaction sintering method can achieve a more stable joint.
175±13
150 100 50 0 RB-TS
Fig. 5. The flexural strength of RB-TS, RB-OS joints and Cf/SiC substrate.
joined bars after mechanical testing is shown in Fig. 6. It can be seen from Fig. 6a, after mechanical testing, the fracture of the RB-TS joined specimens were always found to occur through the interlayer. It means that the strength of the RB-TS joined specimens was determined by that of the interlayer. It should be noted that in Fig. 6b, the fracture was always found in the substrate. However, the joint region does not be
Fig. 6. The optical images of the joined bars after mechanical testing. 4
Materials Characterization 155 (2019) 109799
X. Wu, et al.
Fig. 7. The microstructure of joined specimens after HF-HNO3 acid mixture corrosion. (a) RB-TS (b) RB-OS.
3.3. Joining process and mechanism
and in the substrate. Furthermore, the TEM micrograph in Fig. 8d shows that there was a layer of newly formed SiC phase, which grew on the surface of the original SiC particles (Fig. 8d). It can be known from the electron diffraction image (Fig. 8d) that there are α-SiC phase and β-SiC phase in regions A and B, respectively. It reveals that the boundary was clear. According to the solution-precipitation mechanism [23,24], the
The TEM and HRTEM micrographs at the joining interface of the RB-OS joint are shown in Fig. 8. From the Fig. 8a, b, c, it can be seen that there was a 2–3 μm transition layer between the interlayer and the substrate, which is composed of SiC crystal grains of about 0.5–1 μm. Its grain size was in between the Size of SiC grain formed in the interlayer
Fig. 8. TEM image and the corresponding HRTEM of Cf/SiC-Cf/SiC RB-OS joint. 5
Materials Characterization 155 (2019) 109799
X. Wu, et al.
Fig. 9. RB-OS joint mechanism schematic.
SieC reaction process is as follows: At first, the reaction begins when the molten silicon is sufficiently flowing, and carbon transports within the liquid silicon to form CeSi pairs. Then, the CeSi atom pairs will become supersaturated in the liquid silicon and form new SiC layer on the surface of the SiC substrate. However, there are some differences in the SieC reaction process for the RB-OS joining process. When the CeSi atom pair is formed, there is a carbon concentration difference at the interface between interlayer and substrate (Fig. 9a), i.e., the carbon concentration in the interlayer was smaller than in the substrate. At this time, carbon would diffuse from the high concentration zone to the low concentration zone, as shown in Fig. 9b. As the diffusion rate of carbon was high-speed, a supersaturated solution of carbon was formed at the α-SiC-Si interface. From the thermodynamically most favorable site, carbon precipitates at the interface between α-SiC and liquid Si to form β-SiC, so that β-SiC particles grow and the total volume of β-SiC phase increases (see in Fig. 9c). That also explains why there was a transition layer between the interlayer and the substrate.
[3] S. Schmidt, S. Beyer, H. Knabe, H. Immich, R. Meistring, A. Gessler, Acta Astronaut. 55 (3–9) (2004) 409–420. [4] T. Ishikawa, S. Kajii, K. Matsanaga, T. Hogani, Y. Kohtoku, T. Nagasawa, Atough thermally conductive silicon carbide composite with high strength up to 1600°C in air, Science 282 (1998) 1295–1297. [5] Q. Zhang, Jie Zhang, Chunfeng Liu, Effect of brazing parameters on microstructure and mechanical properties of C-f/SiC and Nb-1Zr joints brazed with Ti-Co-Nb filler alloy[J], J. Eur. Ceram. Soc. 37 (3) (2017) 931–937. [6] H.P. Xiong, Chen Bo, Yu Pan, Joining of C-f/SiC composite with a Cu-Au-Pd-V brazing filler and interfacial reactions, J. Eur. Ceram. Soc. 34 (6) (2014) 1481–1486. [7] P. Tatarko, et al., High temperature properties of the monolithic CVD beta-SiC materials joined with a pre-sintered MAX phase Ti3SiC2 interlayer via solid-state diffusion bonding, J. Eur. Ceram. Soc. 37 (4) (2017) 1205–1216. [8] H. Yang, et al., Thickness-dependent phase evolution and bonding strength of SiC ceramics joints with active Ti interlayer, J. Eur. Ceram. Soc. 37 (4) (2017) 1233–1241. [9] C. Isola, M. Salvo, M. Ferraris, M.A. Montorsi, J. Eur. Ceram. Soc. 18 (1998) 1017–1024. [10] S.J. Li, J.J. Zhang, X.B. Liang, H.P. Duan, Z. Yan, Int. J. Mod. Phys. B 17 (2003) 1777–1781. [11] B.F. Zhou, K.Q. Feng, H.L. Zhou, Joining of SiC ceramic by using the liquid polyvinylphenylsiloxane, Adv. Appl. Ceram. 117 (4) (2018) 212–216. [12] B.H. Rabin, Joining of silicon-carbide silicon-carbide composites and dense siliconcarbide using combustion reactions in the titanium-carbon-nickel system, J. Am. Ceram. Soc. 75 (1) (1992) 131–135. [13] M. Singh, Ceramic joining technology, Adv. Mater. Process. 154 (4) (1998) 89–90. [14] M. Singh, Microstructure and mechanical properties of reaction-formed joints in reaction-bonded silicon carbide ceramics, J. Mater. Sci. 33 (24) (1998) 5781–5787. [15] J.M. Fernandez, et al., Interfacial and thermomechanical characterization of reaction formed joints in silicon carbide-based materials, J. Eur. Ceram. Soc. 20 (14–15) (2000) 2641–2648. [16] S.B. Li, et al., Reaction forming of joints in silicon carbide ceramic materials, Mater. Sci. Eng. A (483–84) (2008) 747–750. [17] Z.H. Luo, et al., Development of SiC-SiC joint by reaction bonding method using SiC/C tapes as the interlayer, J. Eur. Ceram. Soc. 32 (14) (2012) 3819–3824. [18] T. Besshi, et al., Joining of ceramic green bodies and sintering characteristics, J. Mater. Process. Technol. 132 (1–3) (2003) 269–273. [19] Yumin Zhang, Zhiyong Yuan, Yufeng Zhou, Gelcasting of silicon carbide ceramics using phenolic resin and furfuryl alcohol as the gel former, Ceram. Int. 40 (2014) 7873–7878. [20] Y. Katoh, et al., Radiation-tolerant joining technologies for silicon carbide ceramics and composites, J. Nucl. Mater. 448 (1–3) (2014) 497–511. [21] Y.L. Huo, et al., The Effects of Different Slurry on the Joining Characteristics of SiC Green Body. High-Performance Ceramics Vii, Pts 1 and 2, vols. 512-515, (2012), pp. 411–414. [22] H.P. Kirchner, J.C. Conway Jr., A.E. Segall, J. Am. Ceram. Soc. 70 (1987) 104. [23] R. Pampuch, J. Bialoskorski, E. Walasek, Mechanism of reactions in the Si+Cf system and the self-propagating high-temperature synthesis of silicon carbide, Ceram. Int. 13 (1987) 63–68. [24] J.N. Ness, T.F. Page, Microstructural evolution in reaction-bonded silicon carbide, J. Mater. Sci. 21 (1986) 1377.
4. Conclusions It has been demonstrated that the one-step silicon infiltration reaction bonding can achieve a joint with strong interfacial bonding and high flexural strength. A transition layer of 2–3 μm transition layer formed between the interlayer and the Cf/SiC substrate, which is composed of SiC crystal grains of about 0.5–1 μm. The flexural strength of RB-OS joints reached 203 ± 24 MPa, which is comparable to the flexural strength of the Cf/SiC substrate. The more homogenous microstructure of interlayer and the formation of the transition layer were the main reasons for the high flexural strength. Acknowledgments This work was supported by Natural Science Foundation of Shanghai (No. 16ZR1440900). References [1] B. Yang, X.G. Zhou, J.S. Yu, The properties of Cf/SiC composites prepared from different precursors, Ceram. Int. 41 (2015) 4207–4213. [2] K. Jian, Z.H. Chen, Q.S. Ma, Effects of pyrolysis processes on the microstructures and mechanical properties of Cf/SiC composites using polycarbosilane, Mater. Sci. Eng. A 390 (2005) 154–158.
6
Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Joining of the Cf/SiC composites by a one-step Si infiltration reaction bonding
T
Xishi Wua,b, Bingbing Peia, Yunzhou Zhua, , Zhengren Huanga, ⁎
⁎
a
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, No. 588, Heshuo Road, Jiading District, Shanghai, China b University of Chinese Academy of Sciences, Beijing 100049, China
ARTICLE INFO
ABSTRACT
Keywords: Cf/SiC Reaction bonding Joint One-step Si infiltration
An improved joining technique by reaction bonded technology was studied. The final densification and joining of the reaction-sintered composites were simultaneously completed in the reaction bonding process by one-step Si infiltration process. This method achieves joints with strong interfacial bonding and high flexural strength. The joint obtained by the one-step silicon infiltration reaction has a more uniform microstructure, the joint flexural strength is increased to 203 MPa, and retention rate of the flexural strength is 96%, which is comparable to the flexural strength of the Cf/SiC substrate. The microstructure and interfacial evolution mechanism of the interlayer were discussed. The results show that a transition layer of 2–3 μm transition layer formed between the interlayer and the Cf/SiC substrate, which is composed of SiC crystal grains of about 0.5–1 μm. The formation of the transition layer is due to the carbon concentration difference at the interface between interlayer and substrate, resulting in the diffusion of carbon during the SieC reaction.
1. Introduction Carbon fiber reinforced silicon carbide matrix composites (Cf/SiC) have received considerable attention for their excellent high-temperature performances, excellent corrosion resistance, high specific rigidity, high thermal conductivity and exhibits low density [1,2]. Thus, Cf/SiC composites are possible materials in many areas, such as high-temperature structural components, aerospace, national defense, energy, and so on [3,4]. However, it is almost impossible to fabricate Cf/SiC composites components by forging, extruding or other plastic forming processes due to its extreme hardness and brittle nature. The rapid development of aerospace, national defense, energy, and other fields has put forward urgent requirements for Cf/SiC components with the complex shape or large size. Therefore, the most popular approach for fabricating Cf/SiC components is to join small or simple composites pieces together to form the desired structures. At present, a wide range of technologies has been developed for joining Cf/SiC composites, such as metallic brazebased joining [5,6], MAX phase joining [7,8], glass-ceramic bonding [9], diffusion [10], polymer-derived SiC joining [11] and SieC reaction joining [12–17]. The method of SieC reaction joining is to apply the reaction-sintered silicon carbide ceramic process to the joining of silicon carbide ⁎
ceramic and its composite material. Compared with other joining methods, the method of reaction joining has the advantages of high joint strength, good matching of the interlayer and the substrate, high application temperature and controllable structure of the interlayer [12]. In previous work [12–14], a variety of SiC ceramics and ceramic matrix composites were joined using this method, and the mechanical properties of the joints were evaluated. M Singh [15] studied the effect of the base material and joining process parameters on the high-temperature mechanical properties of the joint formed by the SieC reaction bonding. By measuring the electrical properties of the joint interface, Li, S.B [16] found no abrupt change in electrical resistivity around the joint area, which indicates that there is no steep gradient in the microstructure and properties of the SieC reaction joint interface. Luo, Z.H [17] used SiC/C tapes with different composition and thickness to join a pressureless sintered silicon carbide ceramic by SieC reaction bonding and obtained a sample having high bending strength by controlling the composition and thickness of the interlayer. However, the machining of the SiC ceramics and ceramic matrix composites before joining is quite difficult, and the process of silicon infiltration is repeated. Besshi, T [18] found that the joining of green bodies before sintering is useful in obtaining complexly shaped Al2O3 ceramic parts and effective for reducing manufacturing costs. In this paper, an improved
Corresponding authors. E-mail addresses: [email protected] (Y. Zhu), [email protected] (Z. Huang).
https://doi.org/10.1016/j.matchar.2019.109799 Received 10 April 2019; Received in revised form 19 June 2019; Accepted 24 June 2019 Available online 26 June 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.
Materials Characterization 155 (2019) 109799
X. Wu, et al.
Fig. 1. The microstructure of the Cf/C preform specimens.
joining technique by reaction bonded technology was studied. The Cf/C preform was joined before the silicon infiltration process, and the joining process was applied at the stage of the siliconization. The final densification and joining of the reaction-sintered composites were simultaneously completed in the reaction bonding process. The most important advantage of this technology is that the joining of the reaction-sintered composite by a one-step Si infiltration process, avoiding the repeated silicon infiltration process. The microstructure and mechanical properties of the joint were studied. The interfacial evolution mechanism of the joint area was also discussed.
characterized using scanning electron microscopy (SEM, S-4800, Hitachi) equipped with an energy dispersive spectroscopy (EDS) system. In order to study the morphology of the interlayer in more detail, the joint is etched with mixed acid of HF-HNO3 for 4 h to remove the residual silicon. The reactions are as follows:
3Si + 4HNO3
SiO2 + 4HF
3SiO2 + 4NO + 2H2 O
SiF4 + 2H2 O
(1) (2)
The high-resolution image electron diffraction was observed by transmission electron microscopy (TEM, JEM-2100F, JEOL Ltd., Japan).
2. Experimental procedures
3. Results and discussion
The materials used for joining in this paper include Cf/SiC composites and Cf/C perform, all of which are prepared in our laboratory. The microstructure of the Cf/C preform is shown in Fig. 1. The density and the 3-point flexural strength of the Cf/SiC composites and Cf/C preform in this study are 2.77 ± 0.03 g/cm3, 210 ± 22 MPa, and 0.94 g/cm3, 95 ± 10 MPa, respectively. The precursor slurry, containing organic resin, solvent, dispersant, catalyst, and SiC powder, was used for joining. The powders used as inert filler were a commercially available α-SiC powder with an average particle size of 0.4 μm. The resoltype phenol-formaldehyde resin (Industrial level) was used as the carbon precursor. The ethyl alcohol (AR, Average molecular weight: 46.07) was used as a solvent. The dispersant was polyvinylpyrrolidone (K-30, AR, Average molecular weight: 58000), and the catalyst was hexamethylene-tetramine (AR, MW: 140.19), respectively. The PF, ethyl alcohol, polyvinylpyrrolidone, hexamethylene-tetramine, and α-SiC powder were mixed by ball milling for 6 h at room temperature. The Cf/C preform with a dimension of 6 mm × 40 mm × 20 mm was used for joining, and the joining area was 6 mm × 40 mm. The Cf/C preforms were first cleaned ultrasonically in ethyl alcohol for 20 min and then dried. The preparation of joining for Cf/SiC composites is schematically shown in Fig. 2. The joining samples were assembled in Cf/C/slurry/Cf/C to form a sandwich structure wherein the slurry was pasted between two Cf/C preform. Moreover, the assembled joining samples were loaded into the graphite die and were clamped by graphite nuts. The mated specimens were pyrolyzed at 800–1200 °C, respectively. Then, the mated specimens were reaction bonded with free Si at 1600 °C in a vacuum. The resulting joining sample is identified as RB-OS. As a comparison, the Cf/SiC composite was joined, and the resulting joining sample was identified as RB-TS. The connection process of RB-TS is the same as that of RB-OS. All the joined specimens were cut into rectangular bars with dimensions of 3 mm × 4 mm × 36 mm. The 3-point flexural strength of the joined specimens was tested on an Instron 5566 testing system (Instron-5566). The cross-head speed was 0.5 mm/min. The microstructure (polished cross-sections) of the joined specimens were
3.1. Microstructure of the joints Fig. 3 presents the cross-section microstructure of the RB-OS joint and RB-TS joint. Microstructural studies and phase analysis reveal that both of the joints are well-bonded without cracks and voids, and the interlayer is basically ‘fully dense’. This one-step silicon infiltration reaction bonding method achieves an effective joining of the Cf/SiC composite. The interlayer thicknesses of the RB-OS (Fig. 3b) and RB-TS (Fig. 3a) joints are approximately 55 μm and 12 μm, respectively. The detail of the interlayer is shown in Fig. 3c and d. EDS point analysis was performed for the points shown in Fig. 3(c) and (d), where the corresponding elemental results are listed in Table 1. The white particles (Fig. 3c, d(1)), the light gray phase (Fig. 3c, d (3)) and the dark gray zone (Fig. 3c, d (2)) in the interlayer are original α-SiC particles, newly formed SiC and residual Si phases, respectively. The chemical reaction between silicon and carbon results in the formation of β-SiC particles that precipitate on the original α-SiC grains and interconnect them. The pores between silicon carbide particles are filled with silicon [19]. The reliability of a joining sample depends on the weakest part of the joint, that is, the free silicon accumulation area [20]. There are many free silicon accumulation regions in the interlayer of the RB-TS joint. The microstructure of the RB-OS joint is more uniform than that of the RBTS joint, and the visible interface between the joint and the Cf/SiC composite is continuous. The results of the cross-sectional microstructure indicate that the uniform microstructure of the RB-OS joint is related to the surface of the Cf/C preform. As shown in Fig. 1, the surface of the Cf/C preform consists of a number of pores. During the joining of the Cf/C preform using the precursor slurry, the precursor slurry can infiltrate into the pores due to the capillarity and outer pressure. That will increases the stability of the joining process [21]. The outer pressure comes from the clamping pressure of the graphite mold and graphite nuts. The EDS areal mapping results of RB-TS joint and RB-OS joint are shown in Fig. 4. The interlayer of RB-TS joint (Fig. 4a, b, c) is composed of the elements Si, and C. The interlayer contains a higher content of silicon than the substrate, and there is ‘Silicon Island’ in the interlayer, 2
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Fig. 2. The preparation process of RB-OS joint.
which influences the mechanical performance of joining. Compared with the RB-TS joint, the RB-OS joint (Fig. 4d, e, f) has a more uniform distribution of elements. From the Fig. 4, the C and Si elements are homogenously distributed in the interlayer (Fig. 4e, f). Also, the distribution of elements in the interlayer is not significantly different from that of the substrate, and there is no obvious interface and no aggregation of free Si. It indicates that the interface between the interlayer and the substrate materials were well matched throughout the whole joining processes.
Table 1 EDS results of the RB-OS joint and RB-TS joint. Position
Fig. 3(c) Fig. 3(d)
3.2. Mechanical property and failure model The flexural strengths of the RB-OS joint and RB-TS joint at room temperature are shown in Fig. 5. The flexural strength of the RB-TS joint and RB-OS joint is 175 ± 13 MPa and 203 ± 24 MPa, respectively. The retention rate of the flexural strength of the RB-TS joint and RB-OS joint is 83 and 96%, respectively. The strength retention rate refers to the ratio of the flexural strength of the joint to the flexural
Composition (at. %)
1 2 3 1 2 3
Si
C
54.46 94.05 52.18 50.89 95.89 50.62
44.54 5.95 47.82 49.11 4.11 49.38
Possible phase
SiC Residual Si SiC SiC Residual Si SiC
strength of the Cf/SiC substrate. The flexural strength of the RB-OS joint is comparable to the flexural strength of the Cf/SiC substrate (210 ± 22 MPa). The flexural strength of RB-OS joint is higher than that of RB-TS. That is because that the RB-OS joint has a more uniform microstructure and no aggregation of free Si. The micrograph of the
Fig. 3. The secondary electron image of the RB-TS and RB-OS joint (a) (c) RB-TS (b) (d) RB-OS. 3
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Fig. 4. The backscattered electron images and elemental analysis results of RB-TS joint (a, b, c) and RB-OS joint (d, e, f).
Flexural Strength
MPa
250 200
203±24
210±21
RB-OS
Substrate
destroyed. It means that the reliability of joined specimens depended on Cf/SiC substrate materials, not the interlayer. The microstructure of the RB-TS and RB-OS joints after acid corrosion by a mixture of HF-HNO3 are shown in Fig. 7. It can be noted that the HF-HNO3 acid mixture etches residual Si of the joined specimens. The pores that appear on the surface of the joined specimens originated from the removal of residual Si. Compared with RB-TS joints, fewer pores can be seen on the polished surface under the same etching condition for the interlayer of the RB-OS joint. At the joint interfaces, it mainly contains SiC/SiC and Si/SiC interfaces, as well as a small amount of Si/Si interface. There is a difference between the thermal expansion coefficients of silicon and silicon carbide. When the joined samples are cooled from a high temperature, axial residual stresses are generated near the SiC-Si interface of the joint [22]. That is detrimental to the mechanical properties of the joined samples. Therefore, more SiC-SiC bonding regions can achieve better mechanical properties. It can be seen in Fig. 7a, there is a clear joint interface in the RB-TS joint, and there are fewer SiC-SiC bonding regions and more SiC-Si bonding regions on the interface. However, there is no obvious joint interface in the RB-OS joint (Fig. 7b), and there are more SiC-SiC bonding regions and fewer SiC-Si bonding regions on the interface. That means that a one-step infiltration reaction sintering method can achieve a more stable joint.
175±13
150 100 50 0 RB-TS
Fig. 5. The flexural strength of RB-TS, RB-OS joints and Cf/SiC substrate.
joined bars after mechanical testing is shown in Fig. 6. It can be seen from Fig. 6a, after mechanical testing, the fracture of the RB-TS joined specimens were always found to occur through the interlayer. It means that the strength of the RB-TS joined specimens was determined by that of the interlayer. It should be noted that in Fig. 6b, the fracture was always found in the substrate. However, the joint region does not be
Fig. 6. The optical images of the joined bars after mechanical testing. 4
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Fig. 7. The microstructure of joined specimens after HF-HNO3 acid mixture corrosion. (a) RB-TS (b) RB-OS.
3.3. Joining process and mechanism
and in the substrate. Furthermore, the TEM micrograph in Fig. 8d shows that there was a layer of newly formed SiC phase, which grew on the surface of the original SiC particles (Fig. 8d). It can be known from the electron diffraction image (Fig. 8d) that there are α-SiC phase and β-SiC phase in regions A and B, respectively. It reveals that the boundary was clear. According to the solution-precipitation mechanism [23,24], the
The TEM and HRTEM micrographs at the joining interface of the RB-OS joint are shown in Fig. 8. From the Fig. 8a, b, c, it can be seen that there was a 2–3 μm transition layer between the interlayer and the substrate, which is composed of SiC crystal grains of about 0.5–1 μm. Its grain size was in between the Size of SiC grain formed in the interlayer
Fig. 8. TEM image and the corresponding HRTEM of Cf/SiC-Cf/SiC RB-OS joint. 5
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Fig. 9. RB-OS joint mechanism schematic.
SieC reaction process is as follows: At first, the reaction begins when the molten silicon is sufficiently flowing, and carbon transports within the liquid silicon to form CeSi pairs. Then, the CeSi atom pairs will become supersaturated in the liquid silicon and form new SiC layer on the surface of the SiC substrate. However, there are some differences in the SieC reaction process for the RB-OS joining process. When the CeSi atom pair is formed, there is a carbon concentration difference at the interface between interlayer and substrate (Fig. 9a), i.e., the carbon concentration in the interlayer was smaller than in the substrate. At this time, carbon would diffuse from the high concentration zone to the low concentration zone, as shown in Fig. 9b. As the diffusion rate of carbon was high-speed, a supersaturated solution of carbon was formed at the α-SiC-Si interface. From the thermodynamically most favorable site, carbon precipitates at the interface between α-SiC and liquid Si to form β-SiC, so that β-SiC particles grow and the total volume of β-SiC phase increases (see in Fig. 9c). That also explains why there was a transition layer between the interlayer and the substrate.
[3] S. Schmidt, S. Beyer, H. Knabe, H. Immich, R. Meistring, A. Gessler, Acta Astronaut. 55 (3–9) (2004) 409–420. [4] T. Ishikawa, S. Kajii, K. Matsanaga, T. Hogani, Y. Kohtoku, T. Nagasawa, Atough thermally conductive silicon carbide composite with high strength up to 1600°C in air, Science 282 (1998) 1295–1297. [5] Q. Zhang, Jie Zhang, Chunfeng Liu, Effect of brazing parameters on microstructure and mechanical properties of C-f/SiC and Nb-1Zr joints brazed with Ti-Co-Nb filler alloy[J], J. Eur. Ceram. Soc. 37 (3) (2017) 931–937. [6] H.P. Xiong, Chen Bo, Yu Pan, Joining of C-f/SiC composite with a Cu-Au-Pd-V brazing filler and interfacial reactions, J. Eur. Ceram. Soc. 34 (6) (2014) 1481–1486. [7] P. Tatarko, et al., High temperature properties of the monolithic CVD beta-SiC materials joined with a pre-sintered MAX phase Ti3SiC2 interlayer via solid-state diffusion bonding, J. Eur. Ceram. Soc. 37 (4) (2017) 1205–1216. [8] H. Yang, et al., Thickness-dependent phase evolution and bonding strength of SiC ceramics joints with active Ti interlayer, J. Eur. Ceram. Soc. 37 (4) (2017) 1233–1241. [9] C. Isola, M. Salvo, M. Ferraris, M.A. Montorsi, J. Eur. Ceram. Soc. 18 (1998) 1017–1024. [10] S.J. Li, J.J. Zhang, X.B. Liang, H.P. Duan, Z. Yan, Int. J. Mod. Phys. B 17 (2003) 1777–1781. [11] B.F. Zhou, K.Q. Feng, H.L. Zhou, Joining of SiC ceramic by using the liquid polyvinylphenylsiloxane, Adv. Appl. Ceram. 117 (4) (2018) 212–216. [12] B.H. Rabin, Joining of silicon-carbide silicon-carbide composites and dense siliconcarbide using combustion reactions in the titanium-carbon-nickel system, J. Am. Ceram. Soc. 75 (1) (1992) 131–135. [13] M. Singh, Ceramic joining technology, Adv. Mater. Process. 154 (4) (1998) 89–90. [14] M. Singh, Microstructure and mechanical properties of reaction-formed joints in reaction-bonded silicon carbide ceramics, J. Mater. Sci. 33 (24) (1998) 5781–5787. [15] J.M. Fernandez, et al., Interfacial and thermomechanical characterization of reaction formed joints in silicon carbide-based materials, J. Eur. Ceram. Soc. 20 (14–15) (2000) 2641–2648. [16] S.B. Li, et al., Reaction forming of joints in silicon carbide ceramic materials, Mater. Sci. Eng. A (483–84) (2008) 747–750. [17] Z.H. Luo, et al., Development of SiC-SiC joint by reaction bonding method using SiC/C tapes as the interlayer, J. Eur. Ceram. Soc. 32 (14) (2012) 3819–3824. [18] T. Besshi, et al., Joining of ceramic green bodies and sintering characteristics, J. Mater. Process. Technol. 132 (1–3) (2003) 269–273. [19] Yumin Zhang, Zhiyong Yuan, Yufeng Zhou, Gelcasting of silicon carbide ceramics using phenolic resin and furfuryl alcohol as the gel former, Ceram. Int. 40 (2014) 7873–7878. [20] Y. Katoh, et al., Radiation-tolerant joining technologies for silicon carbide ceramics and composites, J. Nucl. Mater. 448 (1–3) (2014) 497–511. [21] Y.L. Huo, et al., The Effects of Different Slurry on the Joining Characteristics of SiC Green Body. High-Performance Ceramics Vii, Pts 1 and 2, vols. 512-515, (2012), pp. 411–414. [22] H.P. Kirchner, J.C. Conway Jr., A.E. Segall, J. Am. Ceram. Soc. 70 (1987) 104. [23] R. Pampuch, J. Bialoskorski, E. Walasek, Mechanism of reactions in the Si+Cf system and the self-propagating high-temperature synthesis of silicon carbide, Ceram. Int. 13 (1987) 63–68. [24] J.N. Ness, T.F. Page, Microstructural evolution in reaction-bonded silicon carbide, J. Mater. Sci. 21 (1986) 1377.
4. Conclusions It has been demonstrated that the one-step silicon infiltration reaction bonding can achieve a joint with strong interfacial bonding and high flexural strength. A transition layer of 2–3 μm transition layer formed between the interlayer and the Cf/SiC substrate, which is composed of SiC crystal grains of about 0.5–1 μm. The flexural strength of RB-OS joints reached 203 ± 24 MPa, which is comparable to the flexural strength of the Cf/SiC substrate. The more homogenous microstructure of interlayer and the formation of the transition layer were the main reasons for the high flexural strength. Acknowledgments This work was supported by Natural Science Foundation of Shanghai (No. 16ZR1440900). References [1] B. Yang, X.G. Zhou, J.S. Yu, The properties of Cf/SiC composites prepared from different precursors, Ceram. Int. 41 (2015) 4207–4213. [2] K. Jian, Z.H. Chen, Q.S. Ma, Effects of pyrolysis processes on the microstructures and mechanical properties of Cf/SiC composites using polycarbosilane, Mater. Sci. Eng. A 390 (2005) 154–158.
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