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 consolidation 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 microstructure-property relations for the B4CNbB2 eutectic composites 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. and above often resulted in the collapse of the sample, which The hardness and fracture toughness were averaged for 20 points. suggests rapid liquid formation. Compared with the relationship between Tsh and values of the 3. Results and discussion melting point of the NbB2B4C eutectic reported by Ordan’yan 3.1 Effect of SPS parameters on eutectic forma- et al,12) a decrease in the melting point of 50200°C is apparent, tion which can be fully attributed to (i) the measurement of the temper- The phase diagram of NbB2B4C is presented in Fig. 1, where ature on the side of the graphite die rather than the specimen and the melting point for the eutectic composition is 2250 « 30°C.12) (ii) the difference between the local temperature and the com- The data obtained in the present investigation are presented as posite temperature.19),20) This temperature difference is mainly open circles in Fig. 1. The results show that SPS at temperatures determined by Td and the pressure, and may be related to the above 2200°C resulted in the formation of a eutectic structure. phenomena observed by Grasso et al. during the SPS of WC.23) Typical microstructures of the hypoeutectic, eutectic and hypereutectic composites are presented in Figs. 2(a)2(f ), respectively. 3.2 Structure and properties of eutectic compo- In all cases, the temperature at which composite was processed sites was 2250°C, and the pressure was decreased from 60 to 5 MPa In general values of identical were obtained for all com- at 2000°C. posites with the eutectic composition (Table 1). However, during

34 Journal of the Ceramic Society of Japan 123 [1] 33-37 2015 JCS-Japan

Fig. 3. Effect of pressure on the microstructure of ceramic composites with 15, 35, and 60 mol. % of NbB2 after SPS at 2200°C. White grains of NbB2 with size of higher than 6 ¯m can be clearly identiﬁed in case of 60 mol.% NbB2 composites and pressure of 10 and 5 MPa. (©1000, BSE). The scale bar is 10 ¯m.

Fig. 4. Effect of the pressure on consolidation of eutectic ceramic composites during SPS. (a) and (b) are typical SPS proﬁles of B4C-35 mol.% NbB2 ceramic composite consolidated at 2200°C. The dashed line represents the temperature of 2000°C at which pressure is decreased from 60 to 5 and 10 MPa, while the dotted line corresponds to the temperature with the maximum rate of shrinkage. (c) shows the shrinkage and shrinkage rate vs temperature in case of 60 to 5 MPa pressure decrease at 2000°C. (d) illustrates relationship between temperature of maximum shrinkage rate during consolidation by SPS (Tsh) and temperature of decrease in pressure from 60 to 5, 10 or 20 MPa (Td). The use of a pressure of 20 MPa at temperatures of 2100°C and above resulted in the collapse of samples.

35 JCS-Japan Demirskyi et al.: Fabrication, microstructure and properties of in situ synthesized B4C–NbB2 eutectic composites by spark plasma sintering

Table 1. Microstructural characteristics and properties of B4CNbB2 ceramics NbB content, Diameter of eutectic Hv0.1, Hv1.0, K , 2 , ¯m IC mol.% inclusions, d, ¯m GPa GPa MPa m1/2 1.2 « 0.15 1.2 « 0.1 48.5 « 1.1(B C) 27.5 « 0.7 4.8 « 0.4 15 4 38.5 « 1.1 26.3 « 1.3(B4C) 3.3 « 0.2(B4C) 30 1.1 « 0.15 1.1 « 0.1 33.2 « 1.2 26.8 « 0.4 5.0 « 0.3 35 1.2 « 0.17 1.1 « 0.1 32.3 « 1.1 26.3 « 1.1 5.2 « 0.6 40 1.1 « 0.12 1.2 « 0.2 34.3 « 0.7 26.6 « 0.6 5.3 « 0.3 60 0.8 « 0.15 0.7 « 0.15 35.3 « 1.5 27.2 « 0.7 7.2 « 0.4 0.9 « 0.17 1.0 « 0.15 35.3 « 1.5 28.3 « 0.4 5.8 « 0.5 27.3 « 0.7(NbB2) 22.1 « 0.45(NbB2) 3.4 « 0.4(NbB2) 0.9 « 0.1 1.3 « 0.3 37.3 « 1.1 29.5 « 0.3 5.6 « 0.2 80 1.0 « 0.2 1.2 « 0.3 33.3 « 1.1 25.9 « 1.3 4.2 « 0.4 3537 [12] ®®28.531.0 ®® 3537 [13] ® 12 ³4146* ®® Notes: Values for B4CNbB2 eutectic are reported if not speciﬁed otherwise. *load of 150 g was used.

Fig. 5. Effect of microstructure parameters on (a)(c) hardness and (d) fracture toughness of eutectic composites. (c) and (d) represent data on eutectic composites with ﬁxed rod inclusion size (d).

consolidation at a constant sintering temperature (Tsps in Fig. 4), those reported.12),13) A further increase in the load (1 kg, Hv1.0) a decrease in Td, changed the value of d and . In the hyper- resulted in a slight decrease in hardness to 2630 GPa. Compo- eutectic composites the size of NbB2 grains varied from 6 to 12 sites with the eutectic composition and with identical NbB2 ¯m (with some aggregates of 3035 ¯m), and a mean value of NbB2 spacing distance ( = 1.0 « 0.1 ¯m) showed almost iden- 8.3 « 2.5 ¯m was evaluated for hypereutectic composites formed tical hardness in the case of rod inclusion (d) size range of 0.7 with different value of Td and pressure. Signiﬁcantly, the in situ 1.5 ¯m. While at ﬁxed d (0.9 « 0.15 ¯m) hardness had a clear synthesis of the eutectic by SPS (Fig. 3) enabled the production maximum at of 1 ¯m. of a singlestep ceramic composite with a complex ‘composite Previous reports on laser-processed B4CTiB2 eutectic com- within a composite’ structure11) with the possibility of controlling posites24) showed that the hardness value for eutectic grains is the microstructure (i.e. grain size, eutectic colonies’ size, and extremely stable with indentation load from 5 to 10 N, results of NbB2NbB2 spacing distance) via the SPS parameters and the present study show that mean value of hardness slight composition. decrease. Further increase in load (up to 50 and 100 N) caused According to Table 1 and Fig. 5, composites with the eutectic spallation around indents in 80% of indents taken which did not composition exhibited microhardnesses (3038 GPa) similar to allow evaluating hardness.

36 Journal of the Ceramic Society of Japan 123 [1] 33-37 2015 JCS-Japan

It was shown that pressure during SPS allows controlling the size of eutectic colonies and in case of hypereutectic composites lead to NbB2 grain size decrease, thus conﬁrming that spark- plasma sintering is a ﬂexible method for tailoring of composites mechanical properties.

Acknowledgments This study was conducted with the ﬁnancial support from a JSPS Postdoctoral Fellowship provided by Japan Society for the Promotion of Science. Discussions with Dr. O. Vasylkiv are highly appreciated.

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