Mechanical, electrical and thermal properties of ZrC

Journal of the European Ceramic Society 38 (2018) 3759–3766

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Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc

Original Article

Mechanical, electrical and thermal properties of ZrC-ZrB2-SiC ternary eutectic composites prepared by arc melting ⁎

T

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Rong Tua, Bing Xiaoa, Song Zhanga, Zhao Denga, Qizhong Lia,b, , Meijun Yanga,c, , Takashi Gotoa,d, Lianmeng Zhanga, Hitoshi Ohmorie a

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, China Hubei Key Laboratory of Advanced Technology for Automotive Components, Wuhan University of Technology, Wuhan, China c Center for Materials Research and Analysis, Wuhan University of Technology, Wuhan, China d Institute for Materials Research, Tohoku University, Sendai, 980-8577, Japan e Institute of Physical and Chemical Research, 2-1 Hirosawa, Wako, Saitama, 351-0198, Japan b

A R T I C LE I N FO

A B S T R A C T

Keywords: Arc-melting ZrC-ZrB2-SiC composites Hardness Electrical conductivity Thermal conductivity

ZrC-ZrB2-SiC composites were prepared by arc-melting in Ar atmosphere using ZrC, ZrB2 and SiC as starting materials. The ternary eutectic composition of 20ZrC-30ZrB2-50SiC (mol%) was first identified. SiC about 7 μm in length and 500 nm in diameter, ZrC about 4 μm in length and 1 μm in diameter, in rod-like microstructure, were uniformly dispersed in ZrB2 matrix of eutectic composite. The eutectic temperature of ZrC-ZrB2-SiC composite was approximately 2550 K. The Vickers Hardness and fracture toughness of eutectic composite was 23 GPa and 6.2 MPa m1/2, respectively. The electrical conductivity decreased from 7.2 × 107 to 1.75 × 106 S m−1 with the temperature increasing from 287 to 800 K. The thermal conductivity decreased from 85 to 61 W K−1 m−1 with increasing temperature from 287 to 973 K.

1. Introduction ZrC-ZrB2-SiC composites have been widely studied due to its high melting point, good wear resistance, high hardness and high fracture toughness as ultra-high temperature ceramic (UHTCs) [1,2]. Thus, ZrCZrB2-SiC composites have been demonstrated as a variety of hightemperature thermomechanical structural applications, such as furnace elements, plasma-arc electrodes, reusable launch vehicles, rocket engines and thermal protection structures for leading-edge parts on hypersonic re-entry space vehicles [2–4]. So far, this kind of ceramic composite has been mainly prepared by solid state sintering at high temperatures. Hot pressing (HP) and reactive hot pressing (RHP) are common ways to prepare ZrC-ZrB2-SiC composite. The composites prepared by RHP usually have lower Vickers Hardness because of the inhomogeneous microstructure compared with HP [2,5,6]. Despite ZrC-ZrB2-SiC composites has been prepared by both of these two ways, high temperature, high pressure and relatively long time is inevitable because of the strong covalent bond of ZrC, ZrB2 and SiC together with the small diffusion constants and high melting temperatures [7]. Spark plasma sintering (SPS) is another effective way to prepare ZrC-ZrB2-SiC composites at a lower temperature and in a shorter time compared with HP and RHP [8–10]. However, Valentine

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[11] et al. found that an amount of ZrO2 surveyed in ZrC-ZrB2-SiC composites because the SPS process could not provide enough time for assisting the carburization of zirconia. Thus, it is difficult to get pure ZrC-ZrB2-SiC composites through the SPS method. On the other hand, our research group has prepared several carbideboride composites using melting-solidification methods, by which the fully dense composites were obtained without pressure within 1 min for the systems of B4C-TiB2-SiC [12], TiC-TiB2-SiC [13], B4C-HfB2-SiC [14], ZrB2-SiC [15] and so on. These composites showed high performance at the eutectic compositions because of their self-assembly regular microstructure. In this study, the ZrC-ZrB2-SiC composites were prepared by arc melting, then the eutectic composition and temperature was determined. The effect of the composition on their microstructure, hardness, fracture toughness, electrical and thermal conductivity were investigated. 2. Experimental procedure ZrC, ZrB2 (Japan New Metals Co., Ltd., 99.5% in purity, 1–5 μm in diameter) and β−SiC (Ibiden Co., Ltd., 99% in purity, 0.5 μm in diameter) powders were weighed and mixed in agate mortar by adding a small amount of ethanol. The mixture of powders was then isostatically

Corresponding authors at: State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, China. E-mail addresses: [email protected] (Q. Li), [email protected] (M. Yang).

https://doi.org/10.1016/j.jeurceramsoc.2018.04.028 Received 9 February 2018; Received in revised form 8 April 2018; Accepted 13 April 2018 Available online 16 April 2018 0955-2219/ © 2018 Elsevier Ltd. All rights reserved.


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Table 1 Composition in molar ratio of ZrC-ZrB2-SiC composites prepared by arc melting. Composition (mol%) ZrC 10 10 10 20 20 20 30 30 30

ZrB2 20 30 40 20 30 40 20 30 40

SiC 70 60 50 60 50 40 50 40 30

pressed into pellets 15 mm in diameter and 5 mm in thickness at 30 MPa. The pellets were melted by arc melting and solidified on a water-cooled copper base in an Ar atmosphere. The composition in molar ratio of the ZrC-ZrB2-SiC composites prepared by arc melting is shown in Table 1. The melting temperature of ZrC-ZrB2-SiC ternary composite was determined by monitoring the upper punch position of the powder compacts utilizing a spark plasma sintering (SPS) system. At the melting point, the position of the upper punch abruptly changed when the melts were extruded from the die. The actual temperature (real melting point, Tm) in the mold is different from the measure temperature (T), so several standard materials were used to calibrate the melting point, i.e., Cu, Ti and Al2O3 powders under the same conditions, as reported in the previous study [16]. The crystal phases of the specimens were examined on the cross section perpendicular to the growth direction by X-ray powder diffraction (XRD; Geigerflex, Rigaku Co., Japan). The microstructure and composition were investigated by scanning electron microscopy (FESEM, FEI Quanata FEG250, USA.) and electron probe microanalysis (EPMA). The phases and orientation relationships were determined by electron backscattered diffraction (EBSD, NordlysNano, Oxford, UK). The hardness of ZrC-ZrB2-SiC composites was measured by a Vickers micro-hardness tester (HM-221, Mitutoyo Co., Japan) at the load of 29.4 N for 10 s. The fracture toughness (KIC) was evaluated by an indentation method at the load of 29.4 N and calculated by Eq. (1) [17]: KIC = 0.0719(P/C3/2)

Fig. 1. XRD patterns on the cross section perpendicular to the growth direction of ZrC-ZrB2-SiC composites at (a) 10ZrC-30ZrB2-60SiC, (b) 20ZrC-30ZrB250SiC, (c) 30ZrC-40ZrB2-30SiC (mol%).

Furthermore, the relative intensity of ZrC (200) and ZrB2 (101) in the ZrC-ZrB2-SiC composite were significantly higher than that in raw powders, indicating that ZrC (200) and ZrB2 (101) preferred orientation along growth direction. Fig. 2 depicts the typical backscattering electron microstructure of ZrC-ZrB2-SiC composites on the cross section perpendicular to the growth direction, where the black phase is SiC, the white phase ZrC and the gray phase ZrB2. The rod-like SiC and ZrC-ZrB2-SiC ternary eutectic (TE) textures are observed at the composition of 10ZrC-20ZrB2-70SiC (mol%) as shown in Fig. 2(a), implying a SiC rich composition comparing to TE. In Fig. 2(b), the microstructure of 20ZrC-40ZrB2-40SiC (mol%) composite contains ZrC-ZrB2 binary eutectic (BE) and ZrC-ZrB2SiC ternary eutectic (TE), suggesting the content of ZrC and ZrB2 is higher than ternary eutectic composite. Fig. 2(c) shows TE and the long striped composite containing irregular shaped ZrC and SiC in 30ZrC20ZrB2-50SiC (mol%) composite, which means the content of ZrC and SiC was higher than TE. The microstructure shows the eutectic textures surrounded by spherical ZrC grains and ZrB2 grains which have a tendency to be more idiomorphic in the composite of 30ZrC-30ZrB2-40SiC (mol%) in Fig. 2(d), suggesting the content of ZrC and ZrB2 was higher than that of the TE composition. Fig. 3 depicts the microstructure of 20ZrC-30ZrB2-50SiC (mol%) ternary eutectic composite for the cross section (Fig. 3(a) and (b)) perpendicular to the growth direction and parallel to the growth direction (Fig. 3(c) and (d)). The eutectic texture contained the rod-like black SiC phase (about 7 μm in length and 500 nm in diameter) and white ZrC phase (about 4 μm in length and 1 μm in diameter), which are uniformly dispersed in the gray ZrB2 matrix. The microstructure of ZrC-ZrB2-SiC eutectic composite might have a specific growing direction, i.e., ZrC (200) and ZrB2 (101) preferred orientation as showed in Fig. 1. EPMA analysis indicated that the eutectic composition was (18.6 ± 3)ZrC- (35.2 ± 2)ZrB2-(46.2 ± 3)SiC (mol%). Fig. 4 shows the phase diagram of ZrC-ZrB2-SiC and TiC-TiB2-SiC ternary system. [18,19]. The microstructure of ZrC-ZrB2-SiC can be roughly categorized into TE + BE2, TE + SiC, TE + BE3, TE + ZrB2, TE + ZrC + SiC, TE + BE2+ZrB2, where TE represents the ZrC-ZrB2SiC ternary eutectic, BE2 for ZrB2-SiC binary eutectic and BE3 for ZrCZrB2 binary eutectic. The big red solid circle corresponds to the ternary eutectic point of ZrC-ZrB2-SiC system and the big blue one to the TiCTiB2-SiC system. The binary point of TiB2-SiC [20] and ZrB2-SiC [15]

(1)

Where, P is the indentation load (N) and C is the half of average cracks length (m). The hardness and fracture toughness values were an average of 10 points. Electrical conductivity (σ) was measured by a four-probed method (ZEM-3, Ulvac Riko, Japan) using rectangular specimens (3 × 3 × 10 mm). Thermal conductivity (κ) was calculated from the Eq. (2) κ = λCPd

(2)

Where, λ is thermal diffusivity, CP specific heat of each composite and d density. The thermal diffusivity was tested by a laser flash method using the Netzsch LFA457 system for disk-shaped specimens (6 mm in diameter and 2 mm in thickness). Electrical and thermal conductivity measurements were conducted from room temperature to 973 K in a vacuum.

3. Results and discussion Fig. 1 shows the typical XRD patterns of the cross section perpendicular to the growth direction at the composition of (a) 10ZrC-30ZrB260SiC, (b) 20ZrC-30ZrB2-50SiC, (c) 30ZrC-40ZrB2-30SiC (mol%). Only the initial phases of ZrC, ZrB2 and SiC, without other phases, were identified, implying no chemical reactions among them and consisting of a ternary system with the initial compounds as the end members. 3760


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Fig. 2. SEM images on the cross section perpendicular to growth direction of ZrC-ZrB2-SiC composites with compositions of (a) 10ZrC-20ZrB2-70SiC, (b) 20ZrC40ZrB2-40SiC, (c) 30ZrC-20ZrB2-50SiC, (d) 30ZrC-30ZrB2-40SiC (mol%) composites.

Fig. 3. SEM photos of 20ZrC-30ZrB2-50SiC (mol%) eutectic composite for the cross section perpendicular to the growth direction (a, b) and parallel to the growth direction (c, d). 3761


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Fig. 6. Composition dependence of Vickers micro-hardness of ZrC-ZrB2-SiC composites.

40ZrB2-60SiC (mol%) composite and 43ZrC-57ZrB2 (mol%) composite have been reported as 2570 K [15] and 2933 ± 40 K [25], respectively, which is higher than the ternary eutectic temperature in this study. Fig. 6 shows the Vickers hardness (HV) of ZrC-ZrB2-SiC composites. The Vickers hardness of monolithic ZrC, ZrB2 and SiC is 20–22 [26], 21–23 [27] and 28 GPa [27], respectively. It indicates that with the increase of SiC content may result in a slight increase in hardness. In this study, as predicted, with the increasement of SiC, the HV slightly increased except for the 20ZrC-30ZrB2-50SiC (mol%) ternary eutectic composite. The 30ZrC-20ZrB2-50SiC (mol%) composite and 20ZrC30ZrB2-50SiC (mol%) ternary eutectic composite had the highest and second highest hardness value of 23.3 and 23.0 GPa, respectively, which might have been attributed by the large part of uniform and fine eutectic microstructure as shown in Figs. 2(c) and 3. It appears that the HV depends on the microstructure rather than the composition. Fig. 7 shows the composition dependence of fracture toughness (KIC) of ZrC-ZrB2-SiC composites calculated from Eq. (1). The fracture toughness of monolithic ZrC (4 MPa m1/2 [28]), ZrB2 (3.5–4.2 MPa m1/2 [27]) and SiC (3–4 MPa m1/2) is very close. The KIC may be influenced significantly by the microstructure [29]. The 30ZrC-40ZrB2-30SiC (mol %) composite and 20ZrC-30ZrB2-50SiC (mol%) ternary eutectic composite has the highest and second highest KIC of 6.7 and 6.2 MPa m1/2, much higher than the single-phase fracture toughness values. This may attribute to the uniform and fine structure of eutectic microstructure. Fig. 8 shows the morphology of indentation and cracks around the indentation of ZrC-ZrB2-SiC composites. The propagation of cracks at the eutectic composite is mostly transgranular in a zig-zag route (Fig. 8(a, b)), which implies the strong bonding between the interface.

Fig. 4. Phase diagram of ZrC-ZrB2-SiC and TiC-TiB2-SiC [18] ternary systems.

was very close and on the extension line of MeC and the ternary eutectic points, indicating the molar ratio of MeB2 to SiC in ternary eutectic composites is almost the same as that in binary eutectic composites. In another word, MeC may have a relatively stronger effect on the ternary eutectic composition than MeB2. The binary points of ZrC-ZrB2 [21] and TiC-TiB2 [22] system are also obviously different. Although ZrC and TiC has similar crystal structure, their lattice constant is quite different, i.e., 0.4693 nm for ZrC and 0.4329 nm for TiC, while the difference between ZrB2 (a = 0.3169, c = 0.3530 nm) and TiB2 (a = 0.3029, c = 0.3228 nm) is small, which might have been resulted in the significantly different binary and ternary eutectic composition involving MeC. Fig. 5 shows the relationship of calibrated temperature and the upper punch position (L) of the ZrC, ZrB2 and SiC mixture powder in the eutectic composition that was measured by SPS system. As temperature rising, L increased firstly and then kept constant, finally decreased dramatically at 2550 K. Thus, the eutectic temperature of ZrC-ZrB2-SiC might have been 2550 K, which is significantly lower than the melting point of ZrB2 (about 3500 K [4]), ZrC (3420 K [23]) and SiC (2818 K [24], decomposition temperature). The binary eutectic temperature of

Fig. 5. Temperature dependence of upper punch position of ZrC-ZrB2-SiC powder compacts in the eutectic composition.

Fig. 7. Composition dependence of fracture toughness of ZrC-ZrB2-SiC composites. 3762


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Fig. 8. SEM images of indentation and cracks around the indentation of ZrC-ZrB2-SiC composite at 20ZrC-30ZrB2-50SiC (a, b) and 20ZrC-40ZrB2-40SiC (c, d) (mol%).

The other composites’ cracks are a mix of transgranular and intergranular (Fig. 8(c, d)). Most deflection occurred around the SiC phase due to residual strain which resulted from the difference of elastic modulus and/or thermal expansion mismatches between different phases. Branching and bridging, which can consume excess energy of crack propagation, prevent the crack from expansion, as toughening mechanism are also found in the microstructure Fig. 8 (d). The small grain size in 20ZrC-30ZrB2-50SiC eutectic composite may account for the higher fracture toughness than the others. Fig. 9 summarizes the HV and KIC of ZrC-ZrB2-SiC ternary composites, ZrB2-SiC, ZrC-ZrB2 and ZrC-SiC binary composites (mol%) in literature and the present study. The mechanical properties can be mainly divided into two areas, the upper left of ZrC-ZrB2-SiC ternary composites and the lower right of binary composites including ZrB2-SiC, ZrCZrB2 and ZrC-SiC composites. In theory, since ZrC has lower Vickers hardness (20–22 GPa) [26] than ZrB2 and SiC while almost the same fracture toughness (4 MPa) [28], the addition of ZrC may decrease the hardness and keep the fracture toughness. In literature, ZrC-ZrB2-SiC sintered composites have the same level of KIC but lower HV than ZrB2SiC composites [15,30–38], which is coincident with theoretical prediction. In the present study, the KIC and HV of the arc-melted ZrC-ZrB2SiC ternary composites were higher than most of the sintered ZrC-ZrB2SiC composites [1,39–42]. Moreover, the ZrC-ZrB2-SiC ternary eutectic composite has lower HV but higher KIC compared with the arc-melted ZrB2-SiC binary composite [15]. Fig. 10 shows the temperature dependence of electrical conductivity (σ) of ZrC-ZrB2-SiC composites and the reference values of ZrC [15], ZrB2 [43] and SiC [12], together with ZrB2-SiC [15], B4C-HfB2-SiC [14] and TiC-TiB2-SiC [13] eutectic composites. As the temperature increases, the electrical conductivity of ZrC-ZrB2-SiC composite slightly decreased, showing a metallic conductor behavior. Compared with other components, the eutectic composite had the highest electrical conductivity (7.2 × 107 − 1.75 × 106 S m−1). The σ of ZrC-ZrB2-SiC

Fig. 9. Hardness and fracture toughness of ZrC, ZrB2 and SiC binary and ternary composites.

eutectic composite was slightly lower than that of 40ZrB2-60SiC eutectic composite (2.1 × 107 − 6.2 × 105 S m−1), but significantly higher than those of 70B4C-30HfB2 and 34TiC-22TiB2-44SiC eutectic composite because the ZrB2 and ZrC has higher σ than TiC, TiB2 and HfB2. Guo et al.[41] reported the σ of 20ZrC-30ZrB2-50SiC composite at room temperature was 1.2 × 106 S m−1, much lower than that of eutectic composite in this study, suggesting that the fine and uniform eutectic microstructure could significantly improve the σ . Good 3763


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Fig. 10. (a) Temperature dependence of electrical conductivity of ZrC-ZrB2-SiC composite. (b) Schematics diagram for measurement of the electrical conductivity.

Fig. 11. (a) Temperature dependence of thermal conductivity of ZrC-ZrB2-SiC composite. (b) Schematics diagram for measurement of the thermal conductivity.

ZrC is much higher than that of SiC, the σ of the eutectic composite is mainly determined by the content of ZrB2 and ZrC following the EMA model. Fig. 11(a) shows the temperature dependence of thermal conductivity (κ) for the ZrC-ZrB2-SiC composites parallel to the growth direction and the reference values of ZrC [44], ZrB2 [15] and SiC [15]. Some eutectic composites prepared by arc melting, i.e., ZrB2-SiC [15], B4C-TiB2-SiC [12] and TiC-TiB2-SiC [18] are also included. The thermal conductivity of ZrC-ZrB2-SiC increased with decreasing ZrC content because of the lowest κ of ZrC among the three components. The κ of ZrC-ZrB2-SiC eutectic composite was 85-61 W K−1 m−1 in the temperature range of 298–973 K, higher than ZrB2-SiC, B4C-TiB2-SiC and TiC-TiB2-SiC eutectic composites. The thermal conductivity was also compared with those simulated by the parallel, series and EMA models as expressed in Eqs. (3)–(5), where σ should be substituted by thermal conductivity (κ) [18]. Fig. 11(b) shows the schematics diagram of the specimens microstructure and the measurement. The microstructure of the specimens is rod-like SiC and ZrC dispersing in ZrB2 mixture matrix, while the laser pulsation through the specimens parallels to the rod-like

electrical conductivity indicates the electrical discharge machining can be used for the ZrC-ZrB2-SiC composite [41]. The σ of eutectic composite of ZrC-ZrB2-SiC is simulated by three models: parallel, series and effective medium approximation models (EMA) express as Eqs. (3)–(5), respectively [18].

σcal (parallel) = VA σA + VB σB + VC σC

(3)

−1 σcal (series )

(4)

=

VA σA−1

+

VB σB−1

+

VC σC−1

σcal (EMA) − σC σcal (EMA) − σB σcal (EMA) − σA VC = 0 VB + VA + 2σcal (EMA) + σA 2σcal (EMA) + σB 2σcal (EMA) + σC

(5)

Where σ is electrical conductivity, V is a volume fraction, and subscripts (A, B and C) indicate the component of the composite. It is clear that the σ is closest to the calculated value of σEMA, which might have been related to the microstructure of the ZrC-ZrB2-SiC ternary eutectic composite. Fig. 10(b) shows the schematics diagram of the specimens microstructure and the measurement. The microstructure of specimen is rod-like SiC and ZrC dispersed in ZrB2 matrix, while the measurement current flows perpendicular to the rod-like SiC. Due to the σ of ZrB2 and 3764


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SiC. The κ of the eutectic composite follows the parallel model, owing to the rod-like eutectic structure parallel to the measurement direction.

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4. Conclusions [15]

ZrC-ZrB2-SiC is a ternary eutectic system with the eutectic composition of 20ZrC-30ZrB2-50SiC (mol%) and eutectic temperature of 2550 K. The microstructure of ZrC-ZrB2-SiC ternary eutectic is composed of the rod-like SiC phase and ZrC phase uniformly dispersed in the ZrB2 matrix. The ZrC-ZrB2-SiC eutectic composite had relatively higher microhardness and fracture toughness than the binary and ternary composites prepared by solid state sintering. The electrical conductivity of the ZrC-ZrB2-SiC composites was in the order of 105– 106 Sm−1, decreased with increasing temperature and showed the highest value at the eutectic composite. The thermal conductivity of ZrC-ZrB2-SiC composites increased with increasing ZrB2 and decreasing ZrC content, showing 60–85 W K−1 m−1 for the eutectic composite. The fine and regular microstructure of the eutectic composite may contribute to the excellent mechanical, electrical and thermal properties.

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Acknowledgement

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This work was supported by the National Natural Science Foundation of China (Nos. 51372188, and 51521001) and by the 111 Project (B13035). This research was also supported by the International Science & Technology Cooperation Program of China (2014DFA53090) and the Natural Science Foundation of Hubei Province, China (2016CFA006), and the National Key Research and Development Program of China (2017YFB0310400), and the Fundamental Research Funds for the Central Universities (WUT: 2017II43GX, 2017III032, 2017YB004), and Science Challenge Project (No.TZ2016001), and the State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (WUT, Grant No: 2017-KF-5).

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