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Consolidation of B4C-TaB2 eutectic composites by spark plasma sintering
Journal of Asian Ceramic Societies 3 (2015) 369–372
H O STED BY
Contents lists available at ScienceDirect
Journal of Asian Ceramic Societies journal homepage: www.elsevier.com/locate/jascer
Letter Consolidation of B4 C-TaB2 eutectic composites by spark plasma sintering
a r t i c l e
i n f o
Keywords: Spark plasma sintering Boron carbide Eutectic Tantalum diboride
a b s t r a c t The in situ synthesis/consolidation of B4 C-TaB2 eutectic composites by spark plasma sintering (SPS) is reported. The microstructure–property relations were determined for composites with the B4 C-TaB2 eutectic composition as functions of TaB2 content, and TaB2 -TaB2 interlamellar spacing. A clear maximum in fracture toughness was identified (∼4.5 MPa m1/2 ) for eutectic composites with interlamellar spacing between 0.9 and 1.1 m. The composites with the hypereutectic composition of 40 mol.% TaB2 obtained by SPS exhibited lower Vickers hardness (25–26 GPa) but higher indentation fracture toughness (up to 4.9 MPa m1/2 ) than eutectic composites with 30–35 mol.% of TaB2 . © 2015 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved.
1. Introduction Transition-metal diborides embedded in a boron carbide matrix exhibiting good mechanical properties are considered as attractive materials for high-temperature applications [1–8]. Among such systems, B4 C-MeV B2 eutectic composites have not been studied as extensively as boron carbide eutectic composites of other transition-metal diborides such as B4 C-TiB2 and B4 C-ZrB2 [1–5]. In particular, B4 C-TaB2 remains largely unexplored among the B4 C-MeIV−V B2 composites owing to its high eutectic temperature (∼2640 K), which is only slightly lower than that for the HfB2 eutectic with boron carbide (∼2650 K) [8]. Single-phase tantalum diboride (TaB2 ) possesses high hardness (24 GPa), chemical stability, a high melting point and good electrical and thermal conductivity. Furthermore, TaB2 is known for its high elastic modulus of 551 ± 8 GPa, as reported by Zhang et al. [9] for a 98% dense sample. Moreover, an earlier study by Claussen [10] suggested an elastic modulus (E) of 670 GPa for a fully dense TaB2 ceramic according to an extrapolation of data on porous ceramic tiles using E = 67,000 (1–2.5P), where P is the porosity of ceramic specimen. Furthermore, while studying single crystals of NbB2 and TaB2 , Nakano et al. [11] found that these metal diborides have good high-temperature hardness. Owing to these distinctive properties, TaB2 -based ceramics have excellent potential for use as cutting tools and as high-temperature materials. Recent studies of Demirskyi et al. [12,13] on B4 C-NbB2 ceramics showed that eutectic composites can be prepared in situ by spark plasma sintering (SPS). This method provides a new opportunity for studying non-oxide eutectic composites using the pressure during SPS as a controlling parameter. Namely, the pressure in SPS can be used to control the eutectic microstructure as well as to Peer review under responsibility of The Ceramic Society of Japan and the Korean Ceramic Society.
slightly decrease the eutectic temperature. Therefore, this allows the in situ synthesis of eutectic composites with melting points exceeding 2000 ◦ C during high-temperature SPS consolidation. The present communication extends a previous investigation on the B4 C-NbB2 system [12,13] and focuses on the B4 CTaB2 system, exploring the consolidation of eutectic composites near the eutectic point (Fig. 1). In addition, evaluation of the microstructure–property relations for B4 C-TaB2 ceramic eutectic composites prepared in situ by SPS is a main goal of the present communication.
2. Materials and methods Commercially available TaB2 (dav = 1.0–1.5 m, <0.5 wt.% C, <0.5 wt.% N, <0.7 wt.% O, Lot # T301160, Japan New Metals Co. Ltd., Japan) and B4 C (dav = 1.5–5.0 m, B2 O3 <0.75 wt.%, Cfree <2 wt.%, Sinopharm Chemical Reagent Co. Ltd., Singapore) powders were used as starting materials. Powder mixtures of B4 C and 30, 33, 35 and 40 mol.% TaB2 were prepared by wet mixing in alcohol followed by drying at about 100 ◦ C. The resultant powder was screened through a 60-mesh screen. The SPS experiments were performed using the ‘Dr. Sinter’ apparatus (Sumitomo, Japan). Initially, a pressure of 20 MPa was applied to ensure sufficient electric contact between the powder tablet and the graphite die. After which pressure was then increased to 60 MPa and the temperature was increased to 800 ◦ C. A dwell time of 1 min at 800 ◦ C was used to focus a side pyrometer on the outer die wall surface. Then we increased the temperature at a heating rate of 100 ◦ C min−1 up to a sintering temperature of 1800 ◦ C with a dwell time of 1 min. This was followed by rapid heating (200 ◦ C min−1 ) to a temperature of 2350 ◦ C. A preset dwell time of 1 min was used at 2350 ◦ C to ensure homogeneity of specimens’ structure. This was followed by a cooling to a 1800 ◦ C at a rate of 150 ◦ C min−1 and a
http://dx.doi.org/10.1016/j.jascer.2015.08.001 2187-0764 © 2015 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved.
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Letter / Journal of Asian Ceramic Societies 3 (2015) 369–372
T, °C L 2800 TaB2+L Te = 2370±30 °C
B4C+L 2400
abc
B4C+TaB2
2000 B4C
d
20
40 60 TaB2,mol% →
80
TaB2
Fig. 1. Equilibrium phase diagram of B4 C-TaB2 system [8], where (a)–(d) correspond to the structures of ceramic composites (Fig. 2) with 30, 33, 35 and 40 mol.% TaB2 after SPS at 2350 ◦ C, respectively.
dwell time of 1 min at this temperature, and then cooling to room temperature was continued at a rate of 150 ◦ C min−1 . Importantly, to avoid the collapse of the sample due to liquid eutectic formation, the pressure was manually decreased from 60 to 10 MPa between 2150 and 2200 ◦ C. This pressure (10 MPa) was preserved until the end of the SPS consolidation. SPS was performed in an argon gas medium with a flow rate of 2 l min−1 . After SPS consolidation, specimens were first ground with SiC paper to form a flat surface, which was followed by grinding with diamond disks to a 0.5 m finish. Microstructural observations and analyses were carried out on the polished samples using SU 8000 cold-emission FE-SEM Hitachi and JEOL 6500-F scanning electron microscopes (SEMs), equipped with energy-dispersive spectroscopy (EDS) detectors. Hardness was determined by an MMT-7 Vickers hardness tester (Matsuzawa MMT-7; Matsuzawa SEIKI Co., Ltd., Tokyo, Japan), using loads from 100 to 1000 gf (i.e. 0.98–9.8 N) with dwell time of 15 s following the standard procedure (ASTM C 1327-15). The fracture toughness was calculated using the half-length of the crack (c) formed around the corners of indentations at loads (L) of 9.8 and 19.6 N using the KIC = 0.073 (L/c3/2 ) [14,15]. The hardness and fracture toughness for 20 points were averaged. 3. Results and discussion The phase diagram of B4 C-TaB2 is presented in Fig. 1, where the melting point for the eutectic composition with 31–33 mol.% of TaB2 is 2370 ± 30 ◦ C [8]. The data obtained in the present investigation are presented as symbols and letters, which correspond to the microstructures presented in Fig. 2. The results show that SPS at temperatures above 2300 ◦ C resulted in the formation of a eutectic structure. The maximum shrinkage rate [12,13] in case of reducing the pressure from 60 to 10 MPa at 2200 ◦ C was found to occur at 2290–2310 ◦ C. Compared with the value melting point of the B4 CTaB2 eutectic reported by Ordan’yan et al. [8], a decrease in the melting point of 50–80 ◦ C is apparent. This can be attributed [12] to
Fig. 3. Effect of microstructure parameters on (a, b) hardness and (c) fracture toughness of eutectic composites prepared by in situ SPS consolidation. Symbols for each composition have the same meaning as in Fig. 1.
Fig. 2. Typical structure of B4 C-TaB2 eutectic composites with (a) 30, (b) 33, (c) 35 and (d) 40 mol.% TaB2 after SPS at 2350 ◦ C (5000×). The insets present typical eutectic structures at magnification of 1000×. The scale bar is 5 m.
Letter / Journal of Asian Ceramic Societies 3 (2015) 369–372
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Fig. 4. Crack propagation in B4 C-40 mol.% TaB2 ceramic composite after indentation with a load of 19.6 N. Crack narrowing was observed after the fracture of a TaB2 eutectic inclusion. (20,000×). The scale bar is 2 m. Insets show (a) eutectic fiber pullout and (b) microscopic crack deflection (50,000×, scale bar 1 m). Table 1 Microstructural characteristics and properties of B4 C-TaB2 ceramics. TaB2 content, mol.%
Interlamellar spacing, , m
Diameter of eutectic inclusions, d, m
Hv0.1a , GPa
Hv1.0b , GPa
KIC , MPa m1/2
30 33 35 40 40 35 NbB2 [12] TaB2 [8] B4 C [24]
1.1 ± 0.9 ± 1.5 ± 1.1 ± 1.1 ± 1.2 ± – –
1.4 ± 0.13 1.4 ± 0.12 1.4 ± 0.2 1.5–2.2 1.6 ± 0.3 1.1 ± 0.1 – –
34.5 ± 34.3 ± 33.5 ± 35.7 ± 36.5 ± 32.3 ± – –
26.3 ± 0.6 26.6 ± 0.5 27.7 ± 0.5 26.8 ± 0.3 25.9 ± 1.1 26.3 ± 1.1 25.6c 31.31 ± 0.79
4.4 ± 0.7 4.0 ± 0.2 3.1 ± 0.4 4.2 ± 0.2 4.66 ± 0.3 5.2 ± 0.6 4.5c 2.84 ± 0.11
a b c
0.12 0.15 0.2 0.12 0.14 0.17
0.7 1.1 1.1 0.7 1.1 1.1
Load of 100 gf (0.98 N) was used. Load of 1000 gf (9.8 N) was used. Load of 500 gf (4.9 N) was used.
(i) the measurement of the temperature on the side of the graphite die rather than that of the specimen and (ii) the difference between the local temperature and the composite temperature. Such temperature difference is affected by pressure during SPS, as observed by Grasso et al. [16,17] during the consolidation of WC. Typical microstructures of the eutectic structures of the composites with hypoeutectic, eutectic and hypereutectic composition are presented in Fig. 2(a)–(d). Since all samples were consolidated under the same conditions, it is clear that with increasing TaB2 content, the size of the diboride eutectic rods in eutectic colonies increases and in some cases exceeds 2 m. All specimens prepared during this study had almost full density. Some minor porosity was visible during examination of polished specimens by optical microscopy – it was introduced during polishing of composites, i.e. due to removal of TaB2 inclusions/grains. Anyway, we believe that application of the moderate pressure during cooling of the eutectic composite ensured enclosure of pores associated with melting of the ceramic composite. According to Fig. 3, at low indentation loads, the composites with the eutectic composition exhibited hardnesses (32–37 GPa) similar to those previously reported [8] (Fig. 3(a)). A further increase in the load (1 kgf, 9.8 N, Hv1.0) resulted in a slight decrease in hardness to 26–28 GPa, followed by a further decrease to 24–25 GPa at a 19.6 N load. This trend was previously reported by White and Dickey [18] and is typical for a brittle eutectic ceramic of B4 C-TiB2 . A decrease in interlamellar spacing () was found to unexpectedly decrease the hardness (Hv1.0) of the hypereutectic composite (40 mol.% TaB2 ) (Fig. 4). We suggest that such behavior can be
expected when the hardness of the eutectic composite is governed by the metal diboride hardness, where the size of the rod inclusions (d) is as crucial as the TaB2 -TaB2 interlamellar spacing () [13]. Therefore, a shift in maximum hardness for the B4 C-TaB2 eutectic composite with increasing was observed. To confirm this hypothesis, additional studies over a wide range of d and are required. The differences between the observed hardness values with that reported in the literature (∼24 GPa) can be explained by the diffusion of carbon into the ceramic specimen during SPS [19,20]. Namely, carbon diffusion in TaB2 , as suggested by Ordan’yan [5] (e.g. by means of TaC dissolving into TaB2 lattice), is limited to a few ˚ percent, but it causes decrease in the diboride lattice (rB = 0.91 A, ˚ resulting in the formation of tantalum carboborides rC = 0.77 A) [20]. The hardness and fracture toughness of such solid solutions are expected to be higher mainly due to the lattice distortion [21–23]. The slight increase in fracture toughness for the 40 mol.% TaB2 confirms this observation. Alternatively, the decrease in the hardness value of the B4 C-TaB2 hypereutectic composites can be explained by the fact that the hardness of B4 C-TaB2 is still governed by the rule of mixtures, so that slightly higher diboride content results in decrease in hardness. Fig. 4 shows typical crack propagation observed after Vickers indentation for the 40 mol.% TaB2 eutectic composite after applying a 19.6 N load. The crack propagation path was both intergranular and intragranular. Eutectic inclusion removal leads to a decrease in the crack width since additional energy was expended during the fracture of TaB2 rods. Crack deflection in the case of an elongated eutectic inclusion was usually observed (see inset b). In parts of the materials, bridges were observed across the crack wake in form of
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Letter / Journal of Asian Ceramic Societies 3 (2015) 369–372
granules and fibrils (see insets a and b). These results suggest that the interface bonding strength between TaB2 and B4 C was higher than that usually observed for sintered composites [13]. In general, the fracture toughness of the eutectic composites was similar to the values reported for monolithic ceramics of TaB2 [9] and B4 C (2–4 MPa m1/2 ) [24,25] (Table 1). Therefore, further increase in the toughness of B4 C-TaB2 may be achieved by either the formation of a MeIV TaB2 solid solution or by modification of the boron carbide matrix, which are suggested as the next steps in ongoing research. In summary, B4 C-TaB2 ceramic composites were produced by high-temperature spark plasma sintering (2350 ◦ C) via the in situ formation of eutectic grains during the consolidation process. Uniform eutectic composites with a rod-like structure and wide interlamellar spacing () were prepared. The Vickers indentation hardness of B4 C-TaB2 eutectics formed by SPS was as high as 28 GPa at an indentation load of 9.8 N, and typical hardness–load behavior for a brittle ceramic was observed. An increase in maximum hardness was observed when was increased from 0.9 m to 1.3 m for the composites with 30–35 and 40 mol.% TaB2 which may be attributed to the fact that hardness of the metal diboride controls the hardness of hypereutectic composites. In spite of the increased hardness as compared with monolithic TaB2 , the B4 C-TaB2 eutectics are measured to have similar indentation fracture toughness between those of TaB2 and B4 C, although all the measured eutectic composites had a toughness of between 3 and 5 MPa m1/2 . Acknowledgement
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
D. Demirskyi and Y. Sakka, J. Am. Ceram. Soc., 97, 2376–2378 (2014). D. Demirskyi and Y. Sakka, J. Ceram. Soc. Jpn., 123, 33–37 (2015). B.R. Lawn and E.R. Fuller, J. Mater. Sci., 10, 2016–2024 (1975). K. Tanaka, J. Mater. Sci., 22, 1501–1508 (1987). S. Grasso, Y. Sakka, G. Maizza and C. Hu, J. Am. Ceram. Soc., 92, 2418–2421 (2009). G. Maizza, S. Grasso, Y. Sakka, T. Noda and O. Ohashi, Sci. Technol. Adv. Mater., 8, 644–654 (2007). R.M. White and E.C. Dickey, J. Eur. Ceram. Soc., 34, 2043–2050 (2014). I. Solodkyi, S.S. Xie, T. Zhao, H. Borodianska, Y. Sakka and O. Vasylkiv, J. Ceram. Soc. Jpn., 121, 950–955 (2013). D. Demirskyi and Y. Sakka, J. Eur. Ceram. Soc., 35, 405–410 (2015). C. Mroz, Am. Ceram. Soc. Bull., 73, 78–81 (1994). M. Moryiama, H. Aoki and Y. Kobayashi, J. Ceram. Soc. Jpn., 106, 1196–1200 (1998). D. Demirskyi, Y. Sakka and O. Vasylkiv, Ceram. Int., 41, 10828–10834 (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.022. S. Grasso, C. Hu, O. Vasylkiv, T.S. Suzuki, S. Guo, T. Nishimura and Y. Sakka, Scr. Mater., 64, 256–259 (2011). P. Badica, H. Borodianska, S. Xie, T. Zhao, D. Demirskyi, P. Li, A.I.Y. Tok, Y. Sakka and O. Vasylkiv, Ceram. Int., 40, 3053–3061 (2014).
Dmytro Demirskyi a,b,∗ National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan b Temasek Laboratories at NTU, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore, Singapore
a
Yoshio Sakka National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Oleg Vasylkiv a,b National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan b Temasek Laboratories at NTU, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore, Singapore
a
This study was conducted with financial support from a JSPS Postdoctoral Fellowship by The Japan Society for the Promotion of Science. References [1] I. Bogomol, H. Borodianska, T. Zhao, T. Nishimura, Y. Sakka, P. Loboda and O. Vasylkiv, Scr. Mater., 71, 17–20 (2014). [2] I. Bogomol, T. Nishimura, O. Vasylkiv, Y. Sakka and P. Loboda, J. Alloys Compd., 485, 677–681 (2009). [3] I. Bogomol, S. Grasso, T. Nishimura, Y. Sakka, P. Loboda and O. Vasylkiv, Ceram. Int., 38, 3947–3953 (2012). [4] R.M. White and E.C. Dickey, J. Am. Ceram. Soc., 94, 4032–4039 (2011). [5] Yu.G. Tkachenko, S.S. Ordan’yan, D.Z. Yurchenko, V.K. Yulyugin, G.A. Bovkun and V.I. Unrod, Inorg. Mater. (USSR), 15, 704–708 (1979). [6] V. Paderno, Y. Paderno, V. Filippov and A. Liashchenko, J. Solid State Chem., 177, 523–528 (2004). [7] S.S. Ordan’yan, Refractories, 34, 268–271 (1993). [8] S.S. Ordan’yan, A.I. Dmitriev, K.T. Bizhev and E.K. Stepanenko, Sov. Powder Metall. Met. Ceram., 26, 834–836 (1987). [9] X. Zhang, G.E. Hilmas and W.G. Fahrenholtz, Mater. Lett., 62, 4251–4253 (2008). [10] N. Claussen, Mater. Sci. Eng., 4, 245–246 (1969). [11] K. Nakano, K. Nakamura, T. Okubo and T. Sugimura, J. Less-Common Met., 84, 79–85 (1982).
∗ Corresponding
author. Present address: Temasek Laboratories at NTU, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore, Singapore. Tel.: +65 93415641. E-mail addresses: [email protected], [email protected] (D. Demirskyi). 3 June 2015 29 July 2015 3 August 2015 Available online 24 August 2015
H O STED BY
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Journal of Asian Ceramic Societies journal homepage: www.elsevier.com/locate/jascer
Letter Consolidation of B4 C-TaB2 eutectic composites by spark plasma sintering
a r t i c l e
i n f o
Keywords: Spark plasma sintering Boron carbide Eutectic Tantalum diboride
a b s t r a c t The in situ synthesis/consolidation of B4 C-TaB2 eutectic composites by spark plasma sintering (SPS) is reported. The microstructure–property relations were determined for composites with the B4 C-TaB2 eutectic composition as functions of TaB2 content, and TaB2 -TaB2 interlamellar spacing. A clear maximum in fracture toughness was identified (∼4.5 MPa m1/2 ) for eutectic composites with interlamellar spacing between 0.9 and 1.1 m. The composites with the hypereutectic composition of 40 mol.% TaB2 obtained by SPS exhibited lower Vickers hardness (25–26 GPa) but higher indentation fracture toughness (up to 4.9 MPa m1/2 ) than eutectic composites with 30–35 mol.% of TaB2 . © 2015 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved.
1. Introduction Transition-metal diborides embedded in a boron carbide matrix exhibiting good mechanical properties are considered as attractive materials for high-temperature applications [1–8]. Among such systems, B4 C-MeV B2 eutectic composites have not been studied as extensively as boron carbide eutectic composites of other transition-metal diborides such as B4 C-TiB2 and B4 C-ZrB2 [1–5]. In particular, B4 C-TaB2 remains largely unexplored among the B4 C-MeIV−V B2 composites owing to its high eutectic temperature (∼2640 K), which is only slightly lower than that for the HfB2 eutectic with boron carbide (∼2650 K) [8]. Single-phase tantalum diboride (TaB2 ) possesses high hardness (24 GPa), chemical stability, a high melting point and good electrical and thermal conductivity. Furthermore, TaB2 is known for its high elastic modulus of 551 ± 8 GPa, as reported by Zhang et al. [9] for a 98% dense sample. Moreover, an earlier study by Claussen [10] suggested an elastic modulus (E) of 670 GPa for a fully dense TaB2 ceramic according to an extrapolation of data on porous ceramic tiles using E = 67,000 (1–2.5P), where P is the porosity of ceramic specimen. Furthermore, while studying single crystals of NbB2 and TaB2 , Nakano et al. [11] found that these metal diborides have good high-temperature hardness. Owing to these distinctive properties, TaB2 -based ceramics have excellent potential for use as cutting tools and as high-temperature materials. Recent studies of Demirskyi et al. [12,13] on B4 C-NbB2 ceramics showed that eutectic composites can be prepared in situ by spark plasma sintering (SPS). This method provides a new opportunity for studying non-oxide eutectic composites using the pressure during SPS as a controlling parameter. Namely, the pressure in SPS can be used to control the eutectic microstructure as well as to Peer review under responsibility of The Ceramic Society of Japan and the Korean Ceramic Society.
slightly decrease the eutectic temperature. Therefore, this allows the in situ synthesis of eutectic composites with melting points exceeding 2000 ◦ C during high-temperature SPS consolidation. The present communication extends a previous investigation on the B4 C-NbB2 system [12,13] and focuses on the B4 CTaB2 system, exploring the consolidation of eutectic composites near the eutectic point (Fig. 1). In addition, evaluation of the microstructure–property relations for B4 C-TaB2 ceramic eutectic composites prepared in situ by SPS is a main goal of the present communication.
2. Materials and methods Commercially available TaB2 (dav = 1.0–1.5 m, <0.5 wt.% C, <0.5 wt.% N, <0.7 wt.% O, Lot # T301160, Japan New Metals Co. Ltd., Japan) and B4 C (dav = 1.5–5.0 m, B2 O3 <0.75 wt.%, Cfree <2 wt.%, Sinopharm Chemical Reagent Co. Ltd., Singapore) powders were used as starting materials. Powder mixtures of B4 C and 30, 33, 35 and 40 mol.% TaB2 were prepared by wet mixing in alcohol followed by drying at about 100 ◦ C. The resultant powder was screened through a 60-mesh screen. The SPS experiments were performed using the ‘Dr. Sinter’ apparatus (Sumitomo, Japan). Initially, a pressure of 20 MPa was applied to ensure sufficient electric contact between the powder tablet and the graphite die. After which pressure was then increased to 60 MPa and the temperature was increased to 800 ◦ C. A dwell time of 1 min at 800 ◦ C was used to focus a side pyrometer on the outer die wall surface. Then we increased the temperature at a heating rate of 100 ◦ C min−1 up to a sintering temperature of 1800 ◦ C with a dwell time of 1 min. This was followed by rapid heating (200 ◦ C min−1 ) to a temperature of 2350 ◦ C. A preset dwell time of 1 min was used at 2350 ◦ C to ensure homogeneity of specimens’ structure. This was followed by a cooling to a 1800 ◦ C at a rate of 150 ◦ C min−1 and a
http://dx.doi.org/10.1016/j.jascer.2015.08.001 2187-0764 © 2015 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved.
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Letter / Journal of Asian Ceramic Societies 3 (2015) 369–372
T, °C L 2800 TaB2+L Te = 2370±30 °C
B4C+L 2400
abc
B4C+TaB2
2000 B4C
d
20
40 60 TaB2,mol% →
80
TaB2
Fig. 1. Equilibrium phase diagram of B4 C-TaB2 system [8], where (a)–(d) correspond to the structures of ceramic composites (Fig. 2) with 30, 33, 35 and 40 mol.% TaB2 after SPS at 2350 ◦ C, respectively.
dwell time of 1 min at this temperature, and then cooling to room temperature was continued at a rate of 150 ◦ C min−1 . Importantly, to avoid the collapse of the sample due to liquid eutectic formation, the pressure was manually decreased from 60 to 10 MPa between 2150 and 2200 ◦ C. This pressure (10 MPa) was preserved until the end of the SPS consolidation. SPS was performed in an argon gas medium with a flow rate of 2 l min−1 . After SPS consolidation, specimens were first ground with SiC paper to form a flat surface, which was followed by grinding with diamond disks to a 0.5 m finish. Microstructural observations and analyses were carried out on the polished samples using SU 8000 cold-emission FE-SEM Hitachi and JEOL 6500-F scanning electron microscopes (SEMs), equipped with energy-dispersive spectroscopy (EDS) detectors. Hardness was determined by an MMT-7 Vickers hardness tester (Matsuzawa MMT-7; Matsuzawa SEIKI Co., Ltd., Tokyo, Japan), using loads from 100 to 1000 gf (i.e. 0.98–9.8 N) with dwell time of 15 s following the standard procedure (ASTM C 1327-15). The fracture toughness was calculated using the half-length of the crack (c) formed around the corners of indentations at loads (L) of 9.8 and 19.6 N using the KIC = 0.073 (L/c3/2 ) [14,15]. The hardness and fracture toughness for 20 points were averaged. 3. Results and discussion The phase diagram of B4 C-TaB2 is presented in Fig. 1, where the melting point for the eutectic composition with 31–33 mol.% of TaB2 is 2370 ± 30 ◦ C [8]. The data obtained in the present investigation are presented as symbols and letters, which correspond to the microstructures presented in Fig. 2. The results show that SPS at temperatures above 2300 ◦ C resulted in the formation of a eutectic structure. The maximum shrinkage rate [12,13] in case of reducing the pressure from 60 to 10 MPa at 2200 ◦ C was found to occur at 2290–2310 ◦ C. Compared with the value melting point of the B4 CTaB2 eutectic reported by Ordan’yan et al. [8], a decrease in the melting point of 50–80 ◦ C is apparent. This can be attributed [12] to
Fig. 3. Effect of microstructure parameters on (a, b) hardness and (c) fracture toughness of eutectic composites prepared by in situ SPS consolidation. Symbols for each composition have the same meaning as in Fig. 1.
Fig. 2. Typical structure of B4 C-TaB2 eutectic composites with (a) 30, (b) 33, (c) 35 and (d) 40 mol.% TaB2 after SPS at 2350 ◦ C (5000×). The insets present typical eutectic structures at magnification of 1000×. The scale bar is 5 m.
Letter / Journal of Asian Ceramic Societies 3 (2015) 369–372
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Fig. 4. Crack propagation in B4 C-40 mol.% TaB2 ceramic composite after indentation with a load of 19.6 N. Crack narrowing was observed after the fracture of a TaB2 eutectic inclusion. (20,000×). The scale bar is 2 m. Insets show (a) eutectic fiber pullout and (b) microscopic crack deflection (50,000×, scale bar 1 m). Table 1 Microstructural characteristics and properties of B4 C-TaB2 ceramics. TaB2 content, mol.%
Interlamellar spacing, , m
Diameter of eutectic inclusions, d, m
Hv0.1a , GPa
Hv1.0b , GPa
KIC , MPa m1/2
30 33 35 40 40 35 NbB2 [12] TaB2 [8] B4 C [24]
1.1 ± 0.9 ± 1.5 ± 1.1 ± 1.1 ± 1.2 ± – –
1.4 ± 0.13 1.4 ± 0.12 1.4 ± 0.2 1.5–2.2 1.6 ± 0.3 1.1 ± 0.1 – –
34.5 ± 34.3 ± 33.5 ± 35.7 ± 36.5 ± 32.3 ± – –
26.3 ± 0.6 26.6 ± 0.5 27.7 ± 0.5 26.8 ± 0.3 25.9 ± 1.1 26.3 ± 1.1 25.6c 31.31 ± 0.79
4.4 ± 0.7 4.0 ± 0.2 3.1 ± 0.4 4.2 ± 0.2 4.66 ± 0.3 5.2 ± 0.6 4.5c 2.84 ± 0.11
a b c
0.12 0.15 0.2 0.12 0.14 0.17
0.7 1.1 1.1 0.7 1.1 1.1
Load of 100 gf (0.98 N) was used. Load of 1000 gf (9.8 N) was used. Load of 500 gf (4.9 N) was used.
(i) the measurement of the temperature on the side of the graphite die rather than that of the specimen and (ii) the difference between the local temperature and the composite temperature. Such temperature difference is affected by pressure during SPS, as observed by Grasso et al. [16,17] during the consolidation of WC. Typical microstructures of the eutectic structures of the composites with hypoeutectic, eutectic and hypereutectic composition are presented in Fig. 2(a)–(d). Since all samples were consolidated under the same conditions, it is clear that with increasing TaB2 content, the size of the diboride eutectic rods in eutectic colonies increases and in some cases exceeds 2 m. All specimens prepared during this study had almost full density. Some minor porosity was visible during examination of polished specimens by optical microscopy – it was introduced during polishing of composites, i.e. due to removal of TaB2 inclusions/grains. Anyway, we believe that application of the moderate pressure during cooling of the eutectic composite ensured enclosure of pores associated with melting of the ceramic composite. According to Fig. 3, at low indentation loads, the composites with the eutectic composition exhibited hardnesses (32–37 GPa) similar to those previously reported [8] (Fig. 3(a)). A further increase in the load (1 kgf, 9.8 N, Hv1.0) resulted in a slight decrease in hardness to 26–28 GPa, followed by a further decrease to 24–25 GPa at a 19.6 N load. This trend was previously reported by White and Dickey [18] and is typical for a brittle eutectic ceramic of B4 C-TiB2 . A decrease in interlamellar spacing () was found to unexpectedly decrease the hardness (Hv1.0) of the hypereutectic composite (40 mol.% TaB2 ) (Fig. 4). We suggest that such behavior can be
expected when the hardness of the eutectic composite is governed by the metal diboride hardness, where the size of the rod inclusions (d) is as crucial as the TaB2 -TaB2 interlamellar spacing () [13]. Therefore, a shift in maximum hardness for the B4 C-TaB2 eutectic composite with increasing was observed. To confirm this hypothesis, additional studies over a wide range of d and are required. The differences between the observed hardness values with that reported in the literature (∼24 GPa) can be explained by the diffusion of carbon into the ceramic specimen during SPS [19,20]. Namely, carbon diffusion in TaB2 , as suggested by Ordan’yan [5] (e.g. by means of TaC dissolving into TaB2 lattice), is limited to a few ˚ percent, but it causes decrease in the diboride lattice (rB = 0.91 A, ˚ resulting in the formation of tantalum carboborides rC = 0.77 A) [20]. The hardness and fracture toughness of such solid solutions are expected to be higher mainly due to the lattice distortion [21–23]. The slight increase in fracture toughness for the 40 mol.% TaB2 confirms this observation. Alternatively, the decrease in the hardness value of the B4 C-TaB2 hypereutectic composites can be explained by the fact that the hardness of B4 C-TaB2 is still governed by the rule of mixtures, so that slightly higher diboride content results in decrease in hardness. Fig. 4 shows typical crack propagation observed after Vickers indentation for the 40 mol.% TaB2 eutectic composite after applying a 19.6 N load. The crack propagation path was both intergranular and intragranular. Eutectic inclusion removal leads to a decrease in the crack width since additional energy was expended during the fracture of TaB2 rods. Crack deflection in the case of an elongated eutectic inclusion was usually observed (see inset b). In parts of the materials, bridges were observed across the crack wake in form of
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granules and fibrils (see insets a and b). These results suggest that the interface bonding strength between TaB2 and B4 C was higher than that usually observed for sintered composites [13]. In general, the fracture toughness of the eutectic composites was similar to the values reported for monolithic ceramics of TaB2 [9] and B4 C (2–4 MPa m1/2 ) [24,25] (Table 1). Therefore, further increase in the toughness of B4 C-TaB2 may be achieved by either the formation of a MeIV TaB2 solid solution or by modification of the boron carbide matrix, which are suggested as the next steps in ongoing research. In summary, B4 C-TaB2 ceramic composites were produced by high-temperature spark plasma sintering (2350 ◦ C) via the in situ formation of eutectic grains during the consolidation process. Uniform eutectic composites with a rod-like structure and wide interlamellar spacing () were prepared. The Vickers indentation hardness of B4 C-TaB2 eutectics formed by SPS was as high as 28 GPa at an indentation load of 9.8 N, and typical hardness–load behavior for a brittle ceramic was observed. An increase in maximum hardness was observed when was increased from 0.9 m to 1.3 m for the composites with 30–35 and 40 mol.% TaB2 which may be attributed to the fact that hardness of the metal diboride controls the hardness of hypereutectic composites. In spite of the increased hardness as compared with monolithic TaB2 , the B4 C-TaB2 eutectics are measured to have similar indentation fracture toughness between those of TaB2 and B4 C, although all the measured eutectic composites had a toughness of between 3 and 5 MPa m1/2 . Acknowledgement
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Dmytro Demirskyi a,b,∗ National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan b Temasek Laboratories at NTU, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore, Singapore
a
Yoshio Sakka National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Oleg Vasylkiv a,b National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan b Temasek Laboratories at NTU, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore, Singapore
a
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∗ Corresponding
author. Present address: Temasek Laboratories at NTU, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore, Singapore. Tel.: +65 93415641. E-mail addresses: [email protected], [email protected] (D. Demirskyi). 3 June 2015 29 July 2015 3 August 2015 Available online 24 August 2015