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Fabrication, Microstructure and Properties of Insitu Synthesized B4C

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Fabrication, Microstructure and Properties of Insitu Synthesized B4C Journal of the Ceramic Society of Japan 123 [1] 33-37 2015 Paper Fabrication, microstructure and properties of in situ synthesized B4C–NbB2 eutectic composites by spark plasma sintering Dmytro DEMIRSKYI³,‡ and Yoshio SAKKA National Institute for Materials Science, 1–2–1 Sengen, Tsukuba, Ibaraki 305–0047, Japan Fabrication, microstructure and properties of in situ formation of strengthened eutectic composites, consisting of a matrix (B4C, and B4C­NbB2 composite) and also containing regularly distributed whiskers of NbB2 by spark plasma sintering (SPS) is reported. It has shown that pressure and the temperature at which pressure is decreased during SPS may be used as a means of controlling the eutectic formation and composites microstructure. The microstructure-property relations were determined for composites of NbB2­B4C eutectic composition as a function of NbB2 content, eutectic inclusion rod size and NbB2­NbB2 spacing distance. It was found that the composites with the eutectic composition of 30­35 mol.% NbB2 obtained by SPS exhibit a high Vickers hardness (26­27 GPa) and a maximum in indentation fracture toughness (³7 MPa·m1/2) in case of interlamelar spacing distance of 0.8 « 0.4 ¯m. ©2015 The Ceramic Society of Japan. All rights reserved. Key-words : Spark–plasma sintering, Eutectic, B4C, NbB2 [Received August 25, 2014; Accepted October 17, 2014] 1. Introduction Boride and carbide ceramics, because of their highly covalent bonding, are utilized in many applications requiring high hardness, fracture toughness and strength. Composites of these materials may offer an additional pathway for microstructural and property control via enhanced toughening and strengthening mechanisms at room and high-temperatures.1)­7) IV­V Quasi­binary alloys of B4C with Me B2 diborides are one of examples of such ceramic composite systems being widely investigated for structural applications.7)­11) Eutectic inclusions with a rodlike structure ensure an improved balance between the hardness, fracture toughness and strength of such bulk ceramic composites as compared with individual bulk ceramics.7)­14) Several methods for production of the eutectics have been proposed for the non-oxide ceramics which include laser-melting, directional solidification using zonal melting, arc-melting, micro- ­ 12) ­ wave heating and lately spark-plasma sintering (SPS). Fig. 1. Equilibrium phase diagram of B4C NbB2 system, where (a) (f ) correspond to the structures of ceramic composites with 15, 30, 35, 40, Furthermore recent research on eutectic (B C­TiB )­B C 4 2 4 60 and 80 mol.% NbB after SPS at 2200°C (in Fig. 2), respectively. composites underlines the possibility designing multihierarchal 2 eutectic composites using the directional solidification (DS) and spark­plasma sintering (SPS)15),16). A two­step method for the production of B4C­TiB2 eutectic composites consisted of DS The present study extended investigation on a B4C­NbB2 12),13) followed by milling, etching and finally consolidation by SPS. As system whose composition contains 35­37 mol.% NbB2, a result, a complex structure consisting of a ‘composite within a and explored the consolidation of composites in the hypoeutectic, composite’ provided a significant increase in room and high- eutectic and hypereutectic regions (Fig. 1). IV­V temperature strength. Among quasi­binary alloys of B4C with Me B2 diborides Furthermore, recent study conducted on NbB2­B4C compo- B4C­NbB2 system has the highest volume of metal diboride sites suggested that in case of SPS eutectic composites tend to content and one of the lowest melting points.3) In addition, form in situ,14) which implies that after the optimization desired niobium diboride was selected owing to its high hardness, high ceramic composite with ‘simple’ eutectic or more complicated, elastic modulus and high chemical stability under long­term i.e. ‘composite within a composite’ structure can be obtained. storage as a powder. In previous paper we succeeded in con- solidation of composites of the B4C­NbB2 system with eutectic ³ Corresponding author: D. Demirskyi; E-mail: demirskyi.dmytro@ inclusions and found that hardness of eutectic composites was nims.go.jp. higher than that of boron carbide matrix in case of hypereutectic ‡ 14) Present address: National Institute for Materials Science, 1­2­1 composition (60 mol.% NbB2). The objective of this inves- Sengen, Tsukuba-city, Ibaraki 305­0047, phone +81-(0)29-859- tigation was (i) to explore spark-plasma sintering as a method for 2461. producing the eutectic composites with different microstructures ©2015 The Ceramic Society of Japan 33 DOI http://dx.doi.org/10.2109/jcersj2.123.33 JCS-Japan Demirskyi et al.: Fabrication, microstructure and properties of in situ synthesized B4C–NbB2 eutectic composites by spark plasma sintering and composition systematically and (ii) to evaluate the micro- structure-property relations for the B4C­NbB2 eutectic compo- sites in situ synthesized during consolidation in more details. 2. Materials and Methods Commercially available NbB2 (dav = 1.0­2.4 ¯m, <0.5 wt % C, <0.5 wt % N, <0.7 wt % O, Wako Pure Chemicals, Japan) and B4C(dav = 1.5­5.0 ¯m, B2O3 < 0.75 wt %,Cfree < 2wt%, Sinopharm Chemical Reagent Co. Ltd., Singapore) powders were used as starting materials. A powder mixture of B4C and 15, 30, 35, 40, 60 or 80 mol.% NbB2 was prepared via by wet-chemical 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’ Fig. 2. Structures of B4C NbB2 ceramic composites with (a) 15, (b) 30, (c) 35, (d) 40, (e) 60 and (f ) 80 mol.% of NbB after SPS at 2200°C. (Sumitomo, Japan). Initially, a pressure of 20 MPa was applied to 2 (©5000). ensure sufficient electric contact between the powder tablet and the graphite die, which was then increased to 60 MPa and the temperature was increased to 800°C. A dwell time of 1 min at Interestingly, we noticed the residual NbB2 grains were present 800°C was used to focus a side pyrometer on the outer die wall in the hypoeutectic and eutectic compositions, which suggests a surface. Then we increased a temperature at a heating rate of difference in the local (intergrain) temperature, which has been 100°C·min¹1 up to a sintering temperature of 1800°C with a dwell widely reported for SPS.19)­23) To investigate the effect of proc- time of 1 min. This was followed by rapid heating (200°C·min¹1) essing parameters such as the applied pressure, and the temper- to a temperature exceeding 2200°C. To avoid the collapse of ature of pressure decrease (Td), we selected values of Td of the sample due to liquid eutectic formation, the pressure was 2000­2150°C with a step of 50°C. Pressures of 5, 10 and 20 MPa manually decreased between 2000 and 2150°C.14) SPS was were used. performed in an argon gas medium with a flow rate of 2 l·min¹1. The effect of pressure and Td on microstructure of eutectic Sintered specimens were first ground with SiC paper to form a composites is presented in Fig. 3. We also noticed that by flat surface, which was followed by grinding with diamond disks decreasing the pressure to 10 MPa the number of unreacted NbB2 to a 0.5 ¯m finish. Microstructural observations and analyses grains in the case of the hypereutectic and eutectic compositions were carried out on the polished sections using a Jeol JSM-5400- may be decreased, as well as the NbB2 grain size. These grains F field emission scanning electron microscope (SEM) and a can be clearly as white phase in 60 mol.% NbB2 eutectic Hitachi TM3000 in the back-scattering mode (BSE), both of composites with a size ranging from 3 ¯m (20 MPa) to 6­12 ¯m which were equipped with an energy dispersive spectrometer. (10 and 20 MPa). It should be noted that in case of using the Hardness was determined by a MMT-7 Vickers hardness tester pressure of 5 MPa the size of the eutectic colonies may be (Buehler, Japan) with a load of 1000 g applied for a dwell time increased, although the average parameters of eutectic compo- of 15 s following the standard procedure (ASTM C 1327-03). sites such as rod inclusion size (d) and NbB2­NbB2 spacing The microhardness of individual phases of the composites was distance (­) remain unchanged and are affected mainly by other determined using a load of 100­500 g. SPS parameters such as sintering temperature and Td. The fracture toughness was evaluated by a hardness tester The typical SPS sintering profile during the processing of [Hardness Testing Machine (AVK-A), Akashi Co. Japan], under 35 mol.% NbB2 composite is presented in Fig. 4. It is clear that loads of 9.8 and 19.6 N from the half-length of a crack formed between 2000 and 2200°C a rapid increase in shrinkage takes around the indentations by using the following equation.17) place, which is attributed to liquid formation. This occurred after 1=2 3=2 the pressure was decreased at 2000°C from 60 to 5 MPa. The KIC ¼ 0:0016 ðEc=HÞ ðP=c Þð1Þ value of the Td parameter has a strong linear correlation with Here, Ec is the Young’s moduli of the NbC­TiB2 composites that the temperature of rapid shrinkage (Tsh) [Fig. 4(d)], allowing a were calculated from the Young’s moduli of NbB2 (550 GPa) and decrease of processing temperature (Tsps) if necessary. Interest- 18) B4C (460 GPa), assuming the rule of mixtures. H is Vickers ingly, a pressure of 20 MPa applied at temperatures of 2100°C hadness (GPa), P is load (N), c is half of the average crack length.
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