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B4C composites infiltrated with molten silicon
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 6793–6798 www.elsevier.com/locate/ceramint
The preparation and properties of SiCw/B4C composites infiltrated with molten silicon Jieli Wang, Wensong Linn, Ziwang Jiang, Lihui Duan, Guoliang Yang College of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620, China Received 6 November 2013; accepted 2 December 2013 Available online 11 December 2013
Abstract Silicon carbide whisker (SiCw) toughened B4C composites have been prepared by pressureless infiltration of B4C–SiCw–C preforms with molten silicon under vacuum at 1500 1C. The effect of SiCw addition on bulk density, hardness, bending strength, fracture toughness and microstructure of SiCw/B4C composites is discussed. It is revealed that the addition of SiCw improves the fracture toughness of B4C ceramic, but reduces its bending strength at the same time. The maximum fracture toughness for SiCw/B4C composite with 24 wt% SiCw addition is 4.88 MPa m1/2, which is about 9% higher than that of the one without SiCw, but at the same time, the bending strength reduces to the minimum value 243 MPa, reduced by 25%. XRD analysis shows that the phase composition of reaction bonded SiCw/B4C composites is B4C, SiC, Si, and B12 (C, Si, B)3, with no residual C. And the main toughening mechanism of SiCw is whisker pulling up. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: B. Whiskers; C. Toughness and toughening; B4C; Reaction bonding
1. Introduction The outstanding properties of B4C, such as low specific weight, high hardness, good wear resistance, good mechanical properties, high melting point, adequate resistance to chemical agents and high neutron absorption cross-section, make it a valuable potential material for a variety of applications [1–5]. However, the realization of this potential is hindered by two prime issues: one is the very high temperature required for its sintering and the other is the low fracture toughness [4,6–9]. The high sintering temperature not only leads to rapid grain coarsening, but also generates equipmentrelated problems [10,11]. Its low fracture toughness has seriously hampered its practical application. Thus, the work of reducing the high sintering temperature and improving its fracture toughness is of great significance. The “reaction bonding” process has been proved to be a useful approach to fabricate fully dense, boron carbide-based composites at a low temperature (1450–1600 1C) [12–14]. n
Correspondence to: College of Materials Engineering, Shanghai University of Engineering Science, 333 Longteng Road, Shanghai 201620, China. Tel.: þ 86 21 67791198. E-mail address: [email protected] (W. Lin).
Using this method, reaction-bonded boron carbide (RBBC) can be prepared by pressureless infiltration of preforms, made of boron carbide with or without the addition of free carbon, with molten silicon in a graphite furnace under vacuum [15–18]. The methods of toughening boron carbide ceramic include particle toughening, whisker toughening, fiber toughening and so on [19–21]. Among all the methods mentioned above, many works focus on the whisker toughening, resulting from its good toughening effect and the simple preparation process. In the present paper, SiC whisker is considered to be an ideal toughening phase due to its high strength, high elastic modulus, good chemical stability, good resistance to high temperature and other characteristics [22–24]. In this work, SiCw was introduced into B4C ceramic to improve its fracture toughness, and dense SiCw/B4C composites with good fracture toughness were prepared by reaction bonding at 1500 1C. 2. Experimental Silicon carbide whiskers (Guangzhou Jiechuang Trade Co., Ltd., China), boron carbide powders (two kinds: F100 and F500, Mudanjiang Jingangzuan Boron Carbide Co., Ltd., China) and carbon black powders were used as major raw
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.2013.12.003
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Table 1 The properties of B4C powders. Type
Purity (%)
Btotal (%)
Ctotal (%)
d50 (μm)
F100 F500
96–98 94–95
77–80 74–78
17–21 17–21
125 12.871.0
materials. The properties of B4C powders are shown in Table 1, and the morphologies of raw materials are illustrated in Fig. 1. Firstly, the original boron carbide powders were soaked with HCl to remove B2O3 and the silicon carbide whiskers were soaked with HF to remove SiO2 in the raw material. Then the boron carbide powders and the silicon carbide whiskers were washed with distilled water to neutral condition. After that, they were dried in a vacuum oven. Secondly, the boron carbide powders and carbon black powders combined with paraffin wax solution (20 wt%) and dispersant (tetramethyl ammonium hydroxide) were ball-milled in distilled water for 24 h in an ND8-4L planetary mill. Then, the SiC whiskers, which were pre-dispersed in distilled water for 45 min by ultrasonication with tetramethyl ammonium hydroxide (TMAH, 25 wt%, Jiangsu, China) as dispersant, were mixed with the milled powders for 6 h. After milling, the slurries were dried at 60 1C in a vacuum oven to obtain the mixed powders. Then, the mixed powders were sieved through 60 meshes and were pressed into green compacts with dimensions 50 mm 25 mm 6 mm at 100 MPa for 30 s. After heat treatment at 900 1C for 5 h, the green compacts were transformed into the B4C–SiCw–C preforms. Finally, the preforms were reaction sintered by liquid Si infiltration (LSI) in a resistance heated graphite furnace at 1500 1C for 30 min in vacuum. The detailed processing procedure is shown in Fig. 2. Bulk density of SiCw/B4C composites and porosity of B4C–SiCw–C preforms were measured by the Archimedes method. Crystal structure of the SiCw/B4C composites was identified by X-ray diffraction (X0 Pert PRO, Holland PANalytical Company). Hardness of the SiCw/B4C composites was determined by a microhardness tester (HXD-1000TMSC/LCD, China). Microstructure of the SiCw/B4C composites was observed by scanning electron microscopy (SEM, S-3400N, Japan Hitachi Company). The tested bars for the flexural strength test had dimensions 3 mm 4 mm 36 mm and the flexural strength test was carried out with a crosshead speed of 0.5 mm/min and a span of 30 mm. The single edge notched beam (SENB) test was adopted to measure fracture toughness. The dimensions of the tested bars for the SENB test were 2 mm 4 mm 36 mm with a notch of 2 mm in depth and 0.2 mm in width. And the SENB tests were carried out with a crosshead speed of 0.05 mm/min and a span of 20 mm. Fig. 1. SEM micrographs of raw materials. (a) F100; (b) F500 (c) SiCw.
3. Results and discussion 3.1. Bulk density results Variation in SiCw addition with bulk density of SiCw/B4C composites is shown in Fig. 3. And variation in SiCw addition
with relative density and porosity of B4C–SiCw–C preforms is shown in Fig. 4. With the increase of SiCw addition, the bulk density increases firstly and then decreases slowly (see Fig. 3). The increase of SiCw addition reduces the stacking density of the mixed powders, and thus leads to high porosity and low
J. Wang et al. / Ceramics International 40 (2014) 6793–6798 B4C powders (F100/F500=2)
Carbon black
TMAH
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Paraffin wax solution
Ball milling B4C-C suspension
SiC whisker
Ball milling B4C-C-SiCw suspension Drying Mixed powders Sieving + Pressing Green compacts Pyrolysising B4C-SiCw-C preforms
Fig. 4. Variation in SiCw addition with relative density and porosity of B4C– SiCw–C preforms.
Reaction sintering SiCw/B4C composites
Fig. 2. Fabrication process of SiCw/B4C composites.
of SiCw/B4C composites is primarily influenced by the increase of SiCw addition. During the reaction sintering process, the liquid silicon infiltrates and fills up all the excessive pores in the B4C– SiCw–C preform under the action of capillary pressure. As a result, when the addition of SiCw increases from 18 wt% to 24 wt %, the porosity of B4C–SiCw–C preform increases sharply (Fig. 4), which leads to a high amount of silicon, and thus results in a decrease in the bulk density finally. 3.2. Phase composition and structure analysis Fig. 5 exhibits the XRD patterns for SiCw/B4C composites. The original ceramic with no SiCw and the composite with 24 wt% SiCw are shown, respectively, in Fig. 5(a) and (b). It can be found that there are B4C, SiC, Si, and B12 (C, Si, B)3 phases existing in the samples. And no residual carbon is found in the diffraction pattern. This indicates that carbon has reacted with molten silicon completely during the sintering period. The B12 (C, Si, B)3 phase is a new solid solution phase, which is formed by the reaction of boron carbide and the liquid silicon at high temperature [13,15,25].
Fig. 3. Variation in SiCw addition with bulk density of SiCw/B4C composites.
relative density of the B4C–SiCw–C preform (see Fig. 4). Because the SiCw/B4C composites consist of SiC, B4C, B12 (C, Si, B)3 and Si phases, the bulk density is decided by the amount of the above four phases and their densities. The increase of SiCw addition has two effects on bulk density. On one hand, the increase of SiCw can improve the bulk density to high density (B4C: 2.52 g/cm3, Si: 2.4 g/cm3, and SiC: 3.2 g/ cm3). On the other hand, the increase of SiCw can lead to a high amount of silicon, resulting in a decrease in bulk density. With the increase of SiCw addition from 0 wt% to 18 wt%, the porosity of B4C–SiCw–C preform increases slowly (Fig. 4), which is due to not so many whiskers. When the addition of SiCw is 18 wt%, the bulk density reaches the maximum (2.632 g/cm3) (Fig. 3). It is concluded that the bulk density
3.3. Hardness results Variation in SiCw addition with hardness of SiCw/B4C composites is depicted in Fig. 6. We can see that with the increase of SiCw addition, the hardness of SiCw/B4C composites goes down. In the first stage, when the addition of SiCw reaches 6 wt%, the trend of the variation is downward. Then in the second stage, the hardness of SiCw/B4C composites decreases dramatically. The hardness of SiCw/B4C composites reduces from 3179 kg/mm2 (the original ceramic without SiCw addition) to 2811 kg/mm2 (SiCw addition of 24 wt%). Because the hardness of SiCw/B4C composites is directly related to the addition and distribution of SiC, B4C and Si phases, the hardness order of the above three phases is B4C4SiC4Si. These results reveal that with the increased addition of SiCw, the proportion of B4C in the SiCw/B4C
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Fig. 5. X-ray diffraction pattern of SiCw/B4C composites.
Fig. 6. Variation in SiCw addition with hardness of SiCw/B4C composites.
composites subsides, which leads to the measured value of hardness slumps obviously. 3.4. Toughening mechanism studies It is essential to study the toughening mechanism of SiCw for its applications, and it is directly related to the fracture morphologies of SiCw/B4C composites. The effect of SiCw addition on the fracture morphologies of SiCw/B4C composites is shown in Fig. 7(a)–(e). As shown in Fig. 7(a), the fracture is extremely uneven when there is no SiCw, and there are many toughening nest and metal torn edges, indicating intergranular fracture. From Fig. 7(b), when the addition of SiCw reaches 6 wt %, the amount of toughening nest and metal torn edges becomes less than that of the case with no SiCw (Fig. 7(a)). It is mainly because the addition of SiCw is small (6 wt%) and part of SiCw dissolves during the reaction bonding process. The results indicate that the toughening effect is not obvious, and the fracture still suggests the characteristics of intergranular fracture. When the SiCw addition is 12 wt%, there is not only a small
amount of metal torn edges and whisker toughening nest in the composites, but also some “holes” exist, which result from the pulling out of the whisker (Fig. 7(c)). We conclude that SiCw has a certain toughening effect. A silicon carbide whisker with the length of about 1.2 mm and the diameter of about 0.3 μm can be found in Fig. 7(d). It is reasoned that it is left after pulling out from the B4C matrix under the action of loads. The pulling out of SiCw from the B4C matrix is an energy-consuming process, which can prevent the expansion of the crack and improve the fracture toughness of B4C ceramic. When the addition of SiCw is 24 wt%, there are much SiCw existing in it, as shown in Fig. 7 (e). It can be found that some whiskers are still in the burl profile, and some whiskers are pulled out from the matrix. This indicates that the bonding between SiCw and matrix weakens, which is beneficial to the fracture toughness of the composites. To further clarify the toughening mechanism of SiCw/B4C composites, the flexural strength test and the SENB test were performed. Variations of the bending strength and fracture toughness are shown in Fig. 8 with SiCw addition in the range from 0 to 24 wt%. The bending strength of SiCw/B4C composites decreases with the increase of SiCw addition gradually. When the addition of SiCw is 24 wt%, the bending strength reduces to the minimum value 243 MPa, which is about 25% lower than that of the case without SiCw. We can see that the addition of SiCw does not have an effect on enhancing the bending strength, but decreases it. Basically, SiCw is considered to be an ideal enhancement phase for its excellent mechanical properties such as high elastic modulus, high bending strength and so on. But the toughening effect of whisker not only depends on the performance of whisker and matrix itself such as strength, elastic modulus and thermal expansion coefficient, but also depends on a good matching strength between whisker and matrix. For the above results of the bending strength, we think that there are following reasons. (1) During the process of molten Si infiltration, part of SiCw is dissolved into SiC particles. As mentioned above, only a small amount of SiCw is found in Fig. 7(d) and (e). It is speculated that part of SiCw has dissolved and turned into SiC particles during the reaction bonding process. (2) A large internal stress formed due to the mismatch of thermal expansion coefficient of SiCw and B4C, which resulted in the generation of the microcrack within the materials and made the strength decrease. (3) With the increase of SiCw addition, the defects such as the non-uniformity distribution of SiCw and the porosity of B4C–SiCw–C preforms increased. At the same time, the bonding between B4C and SiCw weakened, which decreased the interface bonding strength. While carrying, the weak interface bonding strength was not conducive to the transfer of load to SiCw. So, it led to the decrease in the bending strength. The fracture toughness of SiCw/B4C composites increases with SiCw addition in the range from 0 to 24 wt% (Fig. 8). When the addition of SiCw is 24 wt%, the fracture toughness of SiCw/B4C composites reaches the maximum, 4.88 MPa m1/ 2 , which is about 9% higher than that of the case without SiCw. It is concluded that the toughening mechanism of SiCw mainly includes crack bridging, crack deflection and the pulling out
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Fig. 7. SEM images of SiCw/B4C composites with different SiCw additions: (a) 0 wt%; (b) 6 wt%; (c) 12 wt%; (d) 18 wt%; and (e) 24 wt%.
effect. The phenomenon of SiCw pulling out can be seen obviously in Fig. 7(d). When SiCw is pulled out from the B4C matrix, part of energy of the external load can be consumed due to the interface friction, which improves the fracture toughness of SiCw/B4C composites. These results reveal that the toughening mechanism can be thought as the pulling out effect. 4. Conclusions
Fig. 8. Variation in SiCw addition with bending strength and fracture toughness of SiCw/B4C composites.
The addition of SiCw has a significant effect on the mechanical properties of B4C ceramic, which improves the fracture toughness of B4C ceramic while reducing its bending strength. The main toughening mechanism of the SiCw/B4C composites is the pulling out of SiCw from the B4C matrix. Transformation of SiC whisker to SiC particle, formation of microcracks within the material and weakening of interface
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bonding strength are the main reasons for the decline in the bending strength. Acknowledgments This work was financially supported by Innovation Program of Shanghai Municipal Education Commission (No. 13ZZ133) and Shanghai University of Engineering Science Innovative Research Project for Graduate Students (No. 13KY0507). References [1] F. Thevenot, Boron carbide—a comprehensive review, J. Eur. Ceram. Soc. 6 (1990) 205–225. [2] A. Simona, T. CsákÓ, C. Jeynes, T. Szörényi, High lateral resolution 2D mapping of the B/C ratio in a boron carbide film formed by femtosecond pulsed laser deposition, Nucl. Instrum. Methods Phys. Res. B 249 (2006) 454–457. [3] A. Schwetz, L.S. Sigl, L. Pfau, Mechanical properties of injection molded B4C–C ceramics, J Solid State Chem. 133 (1997) 68–76. [4] L. Levin, N. Frage, M.P. Dariel, A novel approach for the preparation of B4C-based cermets, Int. J. Refractory Metals & Hard Mater. 18 (2–3) (2000) 131–135. [5] D. Mallick, T.K. Kayal, J. Ghosh, O.P. Chakrabarti, S. Biswas, H. S. Maiti, Development of multi-phase B–Si–C ceramic composite by reaction sintering, Ceram. Int. 35 (2009) 1667–1669. [6] N. Frage, L. Levin, M.P. Dariel, The effect of the sintering atmosphere on the densification of B4C ceramics, J. Solid State Chem. 177 (2004) 410–414. [7] F. Thevenot, A review on boron carbide, Key Eng. Mater. 56-57 (1991) 59–88. [8] G.I. Kalandadze, S.O. Shalamberidze, A.B. Peikrishvili, Sintering of boron and boron carbide, J. Solid State Chem. 154 (2000) 194–198. [9] H. Lee, R.F. Speyer, Pressureless sintering of boron carbide, J. Am. Ceram. Soc. 89 (2003) 1468–1473. [10] S.L. Dole, S. Prochazka, R.H. Doremus, Microstructural coarsening during sintering of boron carbide, J. Am. Ceram. Soc. 72 (1989) 958–966. [11] S. Prochazka, S.L. Dole, C.A. Hejna, Abnormal grain growth and microcracking in boron carbide, J. Am. Ceram. Soc. 68 (1985) 235–236.
[12] M.K. Aghajanian, B.N. Morgan, J.R. Singh, J. Mears, R.A. Wolffe, A new family of reaction bonded ceramics for armor applications, Ceram. Trans. 134 (2002) 527–539. [13] S. Hayun, H. Dilman, M.P. Dariel, N. Frage, The effect of aluminum on the microstructure and phase composition of boron carbide infiltrated with silicon, Mater. Chem. Phys. 118 (2009) 490–495. [14] S. Hayun, H. Dilman, M. Dariel, N. Frage, S. Dub, The effect of carbon source on the microstructure and the mechanical properties of reaction bonded boron carbide, Adv. Sintering Sci. Technol.: Ceram. Trans. 209 (2010) 29–39. [15] S. Hayun, A. Weizmann, M.P. Dariel, N. Frage, Microstructural evolution during the infiltration of boron carbide with molten silicon, J. Eur. Ceram. Soc. 30 (2010) 1007–1014. [16] S. Hayun, N. Frage, M.P. Dariel, The morphology of ceramic phases in BxC–SiC–Si infiltrated composites, J. Solid State Chem. 179 (2006) 2875–2879. [17] S. Hayun, N. Frage, M.P. Dariel, E. Zaretsky, Y. Ashuah, Dynamic response of B4C–SiC ceramic composites, in: Medvedovski (Ed.), Ceramic Armor and Armor Systems II, John Wiley & Sons, Inc., Hoboken, NJ, 2006, pp. 147–156. [18] S. Hayun, D. Rittel, N. Frage, M.P. Dariel, Static and dynamic mechanical properties of infiltrated B4C–Si composites, Mater. Sci. Eng. A 487 (2008) 405–409. [19] S. Hiroshi, K. Atsushi, I. Kiyoshi, N. Akira, A probabilistic approach to the toughening mechanism in short-fiber-reinforced ceramic–matrix composites, Compos. Sci. Technol. 61 (2001) 281–288. [20] G.L. Zhao, C.Z. Huang, H.L. Liu, B. Zou, H.T. Zhu, J. Wang, Preparation of in-situ growth TaC whiskers toughening Al2O3ceramic matrix composite, Int. J. Refract. Met. Hard Mater. 36 (2013) 122–125. [21] D. Sciti, et al., Combined effect of SiC chopped fibers and SiC whiskers on the toughening of ZrB2, Ceram. Int. (2013). [22] S. Li, Y.M. Zhang, J.C. Han, Y.F. Zhou, Fabrication and characterization of SiC whisker reinforced reaction bonded SiC composite, Ceram. Int. 39 (2013) 449–455. [23] Y.M. Zhang, S. Li, J.C. Han, Y.F. Zhou, Fabrication and characterization of random chopped fiber reinforced reaction bonded silicon carbide composite, Ceram. Int. 38 (2012) 1261–1266. [24] S. Li, Y.M. Zhang, J.C. Han, Y.F. Zhou, Fabrication and characterization of SiC whisker reinforced reaction bonded SiC composite, Ceram. Int. 39 (2013) 449–455. [25] F.M. Shi, X.W. Yin, X.M. Fan, L.F. Cheng, L.T. Zhang, A new route to fabricate SiB4 modified C/SiC composites, J. Eur. Ceram. Soc. 30 (2010) 1955–1962.
CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 6793–6798 www.elsevier.com/locate/ceramint
The preparation and properties of SiCw/B4C composites infiltrated with molten silicon Jieli Wang, Wensong Linn, Ziwang Jiang, Lihui Duan, Guoliang Yang College of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620, China Received 6 November 2013; accepted 2 December 2013 Available online 11 December 2013
Abstract Silicon carbide whisker (SiCw) toughened B4C composites have been prepared by pressureless infiltration of B4C–SiCw–C preforms with molten silicon under vacuum at 1500 1C. The effect of SiCw addition on bulk density, hardness, bending strength, fracture toughness and microstructure of SiCw/B4C composites is discussed. It is revealed that the addition of SiCw improves the fracture toughness of B4C ceramic, but reduces its bending strength at the same time. The maximum fracture toughness for SiCw/B4C composite with 24 wt% SiCw addition is 4.88 MPa m1/2, which is about 9% higher than that of the one without SiCw, but at the same time, the bending strength reduces to the minimum value 243 MPa, reduced by 25%. XRD analysis shows that the phase composition of reaction bonded SiCw/B4C composites is B4C, SiC, Si, and B12 (C, Si, B)3, with no residual C. And the main toughening mechanism of SiCw is whisker pulling up. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: B. Whiskers; C. Toughness and toughening; B4C; Reaction bonding
1. Introduction The outstanding properties of B4C, such as low specific weight, high hardness, good wear resistance, good mechanical properties, high melting point, adequate resistance to chemical agents and high neutron absorption cross-section, make it a valuable potential material for a variety of applications [1–5]. However, the realization of this potential is hindered by two prime issues: one is the very high temperature required for its sintering and the other is the low fracture toughness [4,6–9]. The high sintering temperature not only leads to rapid grain coarsening, but also generates equipmentrelated problems [10,11]. Its low fracture toughness has seriously hampered its practical application. Thus, the work of reducing the high sintering temperature and improving its fracture toughness is of great significance. The “reaction bonding” process has been proved to be a useful approach to fabricate fully dense, boron carbide-based composites at a low temperature (1450–1600 1C) [12–14]. n
Correspondence to: College of Materials Engineering, Shanghai University of Engineering Science, 333 Longteng Road, Shanghai 201620, China. Tel.: þ 86 21 67791198. E-mail address: [email protected] (W. Lin).
Using this method, reaction-bonded boron carbide (RBBC) can be prepared by pressureless infiltration of preforms, made of boron carbide with or without the addition of free carbon, with molten silicon in a graphite furnace under vacuum [15–18]. The methods of toughening boron carbide ceramic include particle toughening, whisker toughening, fiber toughening and so on [19–21]. Among all the methods mentioned above, many works focus on the whisker toughening, resulting from its good toughening effect and the simple preparation process. In the present paper, SiC whisker is considered to be an ideal toughening phase due to its high strength, high elastic modulus, good chemical stability, good resistance to high temperature and other characteristics [22–24]. In this work, SiCw was introduced into B4C ceramic to improve its fracture toughness, and dense SiCw/B4C composites with good fracture toughness were prepared by reaction bonding at 1500 1C. 2. Experimental Silicon carbide whiskers (Guangzhou Jiechuang Trade Co., Ltd., China), boron carbide powders (two kinds: F100 and F500, Mudanjiang Jingangzuan Boron Carbide Co., Ltd., China) and carbon black powders were used as major raw
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.2013.12.003
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Table 1 The properties of B4C powders. Type
Purity (%)
Btotal (%)
Ctotal (%)
d50 (μm)
F100 F500
96–98 94–95
77–80 74–78
17–21 17–21
125 12.871.0
materials. The properties of B4C powders are shown in Table 1, and the morphologies of raw materials are illustrated in Fig. 1. Firstly, the original boron carbide powders were soaked with HCl to remove B2O3 and the silicon carbide whiskers were soaked with HF to remove SiO2 in the raw material. Then the boron carbide powders and the silicon carbide whiskers were washed with distilled water to neutral condition. After that, they were dried in a vacuum oven. Secondly, the boron carbide powders and carbon black powders combined with paraffin wax solution (20 wt%) and dispersant (tetramethyl ammonium hydroxide) were ball-milled in distilled water for 24 h in an ND8-4L planetary mill. Then, the SiC whiskers, which were pre-dispersed in distilled water for 45 min by ultrasonication with tetramethyl ammonium hydroxide (TMAH, 25 wt%, Jiangsu, China) as dispersant, were mixed with the milled powders for 6 h. After milling, the slurries were dried at 60 1C in a vacuum oven to obtain the mixed powders. Then, the mixed powders were sieved through 60 meshes and were pressed into green compacts with dimensions 50 mm 25 mm 6 mm at 100 MPa for 30 s. After heat treatment at 900 1C for 5 h, the green compacts were transformed into the B4C–SiCw–C preforms. Finally, the preforms were reaction sintered by liquid Si infiltration (LSI) in a resistance heated graphite furnace at 1500 1C for 30 min in vacuum. The detailed processing procedure is shown in Fig. 2. Bulk density of SiCw/B4C composites and porosity of B4C–SiCw–C preforms were measured by the Archimedes method. Crystal structure of the SiCw/B4C composites was identified by X-ray diffraction (X0 Pert PRO, Holland PANalytical Company). Hardness of the SiCw/B4C composites was determined by a microhardness tester (HXD-1000TMSC/LCD, China). Microstructure of the SiCw/B4C composites was observed by scanning electron microscopy (SEM, S-3400N, Japan Hitachi Company). The tested bars for the flexural strength test had dimensions 3 mm 4 mm 36 mm and the flexural strength test was carried out with a crosshead speed of 0.5 mm/min and a span of 30 mm. The single edge notched beam (SENB) test was adopted to measure fracture toughness. The dimensions of the tested bars for the SENB test were 2 mm 4 mm 36 mm with a notch of 2 mm in depth and 0.2 mm in width. And the SENB tests were carried out with a crosshead speed of 0.05 mm/min and a span of 20 mm. Fig. 1. SEM micrographs of raw materials. (a) F100; (b) F500 (c) SiCw.
3. Results and discussion 3.1. Bulk density results Variation in SiCw addition with bulk density of SiCw/B4C composites is shown in Fig. 3. And variation in SiCw addition
with relative density and porosity of B4C–SiCw–C preforms is shown in Fig. 4. With the increase of SiCw addition, the bulk density increases firstly and then decreases slowly (see Fig. 3). The increase of SiCw addition reduces the stacking density of the mixed powders, and thus leads to high porosity and low
J. Wang et al. / Ceramics International 40 (2014) 6793–6798 B4C powders (F100/F500=2)
Carbon black
TMAH
6795
Paraffin wax solution
Ball milling B4C-C suspension
SiC whisker
Ball milling B4C-C-SiCw suspension Drying Mixed powders Sieving + Pressing Green compacts Pyrolysising B4C-SiCw-C preforms
Fig. 4. Variation in SiCw addition with relative density and porosity of B4C– SiCw–C preforms.
Reaction sintering SiCw/B4C composites
Fig. 2. Fabrication process of SiCw/B4C composites.
of SiCw/B4C composites is primarily influenced by the increase of SiCw addition. During the reaction sintering process, the liquid silicon infiltrates and fills up all the excessive pores in the B4C– SiCw–C preform under the action of capillary pressure. As a result, when the addition of SiCw increases from 18 wt% to 24 wt %, the porosity of B4C–SiCw–C preform increases sharply (Fig. 4), which leads to a high amount of silicon, and thus results in a decrease in the bulk density finally. 3.2. Phase composition and structure analysis Fig. 5 exhibits the XRD patterns for SiCw/B4C composites. The original ceramic with no SiCw and the composite with 24 wt% SiCw are shown, respectively, in Fig. 5(a) and (b). It can be found that there are B4C, SiC, Si, and B12 (C, Si, B)3 phases existing in the samples. And no residual carbon is found in the diffraction pattern. This indicates that carbon has reacted with molten silicon completely during the sintering period. The B12 (C, Si, B)3 phase is a new solid solution phase, which is formed by the reaction of boron carbide and the liquid silicon at high temperature [13,15,25].
Fig. 3. Variation in SiCw addition with bulk density of SiCw/B4C composites.
relative density of the B4C–SiCw–C preform (see Fig. 4). Because the SiCw/B4C composites consist of SiC, B4C, B12 (C, Si, B)3 and Si phases, the bulk density is decided by the amount of the above four phases and their densities. The increase of SiCw addition has two effects on bulk density. On one hand, the increase of SiCw can improve the bulk density to high density (B4C: 2.52 g/cm3, Si: 2.4 g/cm3, and SiC: 3.2 g/ cm3). On the other hand, the increase of SiCw can lead to a high amount of silicon, resulting in a decrease in bulk density. With the increase of SiCw addition from 0 wt% to 18 wt%, the porosity of B4C–SiCw–C preform increases slowly (Fig. 4), which is due to not so many whiskers. When the addition of SiCw is 18 wt%, the bulk density reaches the maximum (2.632 g/cm3) (Fig. 3). It is concluded that the bulk density
3.3. Hardness results Variation in SiCw addition with hardness of SiCw/B4C composites is depicted in Fig. 6. We can see that with the increase of SiCw addition, the hardness of SiCw/B4C composites goes down. In the first stage, when the addition of SiCw reaches 6 wt%, the trend of the variation is downward. Then in the second stage, the hardness of SiCw/B4C composites decreases dramatically. The hardness of SiCw/B4C composites reduces from 3179 kg/mm2 (the original ceramic without SiCw addition) to 2811 kg/mm2 (SiCw addition of 24 wt%). Because the hardness of SiCw/B4C composites is directly related to the addition and distribution of SiC, B4C and Si phases, the hardness order of the above three phases is B4C4SiC4Si. These results reveal that with the increased addition of SiCw, the proportion of B4C in the SiCw/B4C
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Fig. 5. X-ray diffraction pattern of SiCw/B4C composites.
Fig. 6. Variation in SiCw addition with hardness of SiCw/B4C composites.
composites subsides, which leads to the measured value of hardness slumps obviously. 3.4. Toughening mechanism studies It is essential to study the toughening mechanism of SiCw for its applications, and it is directly related to the fracture morphologies of SiCw/B4C composites. The effect of SiCw addition on the fracture morphologies of SiCw/B4C composites is shown in Fig. 7(a)–(e). As shown in Fig. 7(a), the fracture is extremely uneven when there is no SiCw, and there are many toughening nest and metal torn edges, indicating intergranular fracture. From Fig. 7(b), when the addition of SiCw reaches 6 wt %, the amount of toughening nest and metal torn edges becomes less than that of the case with no SiCw (Fig. 7(a)). It is mainly because the addition of SiCw is small (6 wt%) and part of SiCw dissolves during the reaction bonding process. The results indicate that the toughening effect is not obvious, and the fracture still suggests the characteristics of intergranular fracture. When the SiCw addition is 12 wt%, there is not only a small
amount of metal torn edges and whisker toughening nest in the composites, but also some “holes” exist, which result from the pulling out of the whisker (Fig. 7(c)). We conclude that SiCw has a certain toughening effect. A silicon carbide whisker with the length of about 1.2 mm and the diameter of about 0.3 μm can be found in Fig. 7(d). It is reasoned that it is left after pulling out from the B4C matrix under the action of loads. The pulling out of SiCw from the B4C matrix is an energy-consuming process, which can prevent the expansion of the crack and improve the fracture toughness of B4C ceramic. When the addition of SiCw is 24 wt%, there are much SiCw existing in it, as shown in Fig. 7 (e). It can be found that some whiskers are still in the burl profile, and some whiskers are pulled out from the matrix. This indicates that the bonding between SiCw and matrix weakens, which is beneficial to the fracture toughness of the composites. To further clarify the toughening mechanism of SiCw/B4C composites, the flexural strength test and the SENB test were performed. Variations of the bending strength and fracture toughness are shown in Fig. 8 with SiCw addition in the range from 0 to 24 wt%. The bending strength of SiCw/B4C composites decreases with the increase of SiCw addition gradually. When the addition of SiCw is 24 wt%, the bending strength reduces to the minimum value 243 MPa, which is about 25% lower than that of the case without SiCw. We can see that the addition of SiCw does not have an effect on enhancing the bending strength, but decreases it. Basically, SiCw is considered to be an ideal enhancement phase for its excellent mechanical properties such as high elastic modulus, high bending strength and so on. But the toughening effect of whisker not only depends on the performance of whisker and matrix itself such as strength, elastic modulus and thermal expansion coefficient, but also depends on a good matching strength between whisker and matrix. For the above results of the bending strength, we think that there are following reasons. (1) During the process of molten Si infiltration, part of SiCw is dissolved into SiC particles. As mentioned above, only a small amount of SiCw is found in Fig. 7(d) and (e). It is speculated that part of SiCw has dissolved and turned into SiC particles during the reaction bonding process. (2) A large internal stress formed due to the mismatch of thermal expansion coefficient of SiCw and B4C, which resulted in the generation of the microcrack within the materials and made the strength decrease. (3) With the increase of SiCw addition, the defects such as the non-uniformity distribution of SiCw and the porosity of B4C–SiCw–C preforms increased. At the same time, the bonding between B4C and SiCw weakened, which decreased the interface bonding strength. While carrying, the weak interface bonding strength was not conducive to the transfer of load to SiCw. So, it led to the decrease in the bending strength. The fracture toughness of SiCw/B4C composites increases with SiCw addition in the range from 0 to 24 wt% (Fig. 8). When the addition of SiCw is 24 wt%, the fracture toughness of SiCw/B4C composites reaches the maximum, 4.88 MPa m1/ 2 , which is about 9% higher than that of the case without SiCw. It is concluded that the toughening mechanism of SiCw mainly includes crack bridging, crack deflection and the pulling out
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Fig. 7. SEM images of SiCw/B4C composites with different SiCw additions: (a) 0 wt%; (b) 6 wt%; (c) 12 wt%; (d) 18 wt%; and (e) 24 wt%.
effect. The phenomenon of SiCw pulling out can be seen obviously in Fig. 7(d). When SiCw is pulled out from the B4C matrix, part of energy of the external load can be consumed due to the interface friction, which improves the fracture toughness of SiCw/B4C composites. These results reveal that the toughening mechanism can be thought as the pulling out effect. 4. Conclusions
Fig. 8. Variation in SiCw addition with bending strength and fracture toughness of SiCw/B4C composites.
The addition of SiCw has a significant effect on the mechanical properties of B4C ceramic, which improves the fracture toughness of B4C ceramic while reducing its bending strength. The main toughening mechanism of the SiCw/B4C composites is the pulling out of SiCw from the B4C matrix. Transformation of SiC whisker to SiC particle, formation of microcracks within the material and weakening of interface
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bonding strength are the main reasons for the decline in the bending strength. Acknowledgments This work was financially supported by Innovation Program of Shanghai Municipal Education Commission (No. 13ZZ133) and Shanghai University of Engineering Science Innovative Research Project for Graduate Students (No. 13KY0507). References [1] F. Thevenot, Boron carbide—a comprehensive review, J. Eur. Ceram. Soc. 6 (1990) 205–225. [2] A. Simona, T. CsákÓ, C. Jeynes, T. Szörényi, High lateral resolution 2D mapping of the B/C ratio in a boron carbide film formed by femtosecond pulsed laser deposition, Nucl. Instrum. Methods Phys. Res. B 249 (2006) 454–457. [3] A. Schwetz, L.S. Sigl, L. Pfau, Mechanical properties of injection molded B4C–C ceramics, J Solid State Chem. 133 (1997) 68–76. [4] L. Levin, N. Frage, M.P. Dariel, A novel approach for the preparation of B4C-based cermets, Int. J. Refractory Metals & Hard Mater. 18 (2–3) (2000) 131–135. [5] D. Mallick, T.K. Kayal, J. Ghosh, O.P. Chakrabarti, S. Biswas, H. S. Maiti, Development of multi-phase B–Si–C ceramic composite by reaction sintering, Ceram. Int. 35 (2009) 1667–1669. [6] N. Frage, L. Levin, M.P. Dariel, The effect of the sintering atmosphere on the densification of B4C ceramics, J. Solid State Chem. 177 (2004) 410–414. [7] F. Thevenot, A review on boron carbide, Key Eng. Mater. 56-57 (1991) 59–88. [8] G.I. Kalandadze, S.O. Shalamberidze, A.B. Peikrishvili, Sintering of boron and boron carbide, J. Solid State Chem. 154 (2000) 194–198. [9] H. Lee, R.F. Speyer, Pressureless sintering of boron carbide, J. Am. Ceram. Soc. 89 (2003) 1468–1473. [10] S.L. Dole, S. Prochazka, R.H. Doremus, Microstructural coarsening during sintering of boron carbide, J. Am. Ceram. Soc. 72 (1989) 958–966. [11] S. Prochazka, S.L. Dole, C.A. Hejna, Abnormal grain growth and microcracking in boron carbide, J. Am. Ceram. Soc. 68 (1985) 235–236.
[12] M.K. Aghajanian, B.N. Morgan, J.R. Singh, J. Mears, R.A. Wolffe, A new family of reaction bonded ceramics for armor applications, Ceram. Trans. 134 (2002) 527–539. [13] S. Hayun, H. Dilman, M.P. Dariel, N. Frage, The effect of aluminum on the microstructure and phase composition of boron carbide infiltrated with silicon, Mater. Chem. Phys. 118 (2009) 490–495. [14] S. Hayun, H. Dilman, M. Dariel, N. Frage, S. Dub, The effect of carbon source on the microstructure and the mechanical properties of reaction bonded boron carbide, Adv. Sintering Sci. Technol.: Ceram. Trans. 209 (2010) 29–39. [15] S. Hayun, A. Weizmann, M.P. Dariel, N. Frage, Microstructural evolution during the infiltration of boron carbide with molten silicon, J. Eur. Ceram. Soc. 30 (2010) 1007–1014. [16] S. Hayun, N. Frage, M.P. Dariel, The morphology of ceramic phases in BxC–SiC–Si infiltrated composites, J. Solid State Chem. 179 (2006) 2875–2879. [17] S. Hayun, N. Frage, M.P. Dariel, E. Zaretsky, Y. Ashuah, Dynamic response of B4C–SiC ceramic composites, in: Medvedovski (Ed.), Ceramic Armor and Armor Systems II, John Wiley & Sons, Inc., Hoboken, NJ, 2006, pp. 147–156. [18] S. Hayun, D. Rittel, N. Frage, M.P. Dariel, Static and dynamic mechanical properties of infiltrated B4C–Si composites, Mater. Sci. Eng. A 487 (2008) 405–409. [19] S. Hiroshi, K. Atsushi, I. Kiyoshi, N. Akira, A probabilistic approach to the toughening mechanism in short-fiber-reinforced ceramic–matrix composites, Compos. Sci. Technol. 61 (2001) 281–288. [20] G.L. Zhao, C.Z. Huang, H.L. Liu, B. Zou, H.T. Zhu, J. Wang, Preparation of in-situ growth TaC whiskers toughening Al2O3ceramic matrix composite, Int. J. Refract. Met. Hard Mater. 36 (2013) 122–125. [21] D. Sciti, et al., Combined effect of SiC chopped fibers and SiC whiskers on the toughening of ZrB2, Ceram. Int. (2013). [22] S. Li, Y.M. Zhang, J.C. Han, Y.F. Zhou, Fabrication and characterization of SiC whisker reinforced reaction bonded SiC composite, Ceram. Int. 39 (2013) 449–455. [23] Y.M. Zhang, S. Li, J.C. Han, Y.F. Zhou, Fabrication and characterization of random chopped fiber reinforced reaction bonded silicon carbide composite, Ceram. Int. 38 (2012) 1261–1266. [24] S. Li, Y.M. Zhang, J.C. Han, Y.F. Zhou, Fabrication and characterization of SiC whisker reinforced reaction bonded SiC composite, Ceram. Int. 39 (2013) 449–455. [25] F.M. Shi, X.W. Yin, X.M. Fan, L.F. Cheng, L.T. Zhang, A new route to fabricate SiB4 modified C/SiC composites, J. Eur. Ceram. Soc. 30 (2010) 1955–1962.