HfB2-CNTs composites with enhanced mechanical properties prepared by spark plasma sintering

Author’s Accepted Manuscript HfB2-CNTs composites with enhanced mechanical properties prepared by spark plasma sintering Hua Jin, Songhe Meng, Weihua Xie, Chenghai Xu, Jiahong Niu www.elsevier.com/locate/ceri

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S0272-8842(16)31990-3 http://dx.doi.org/10.1016/j.ceramint.2016.10.200 CERI14079

To appear in: Ceramics International Received date: 10 October 2016 Revised date: 29 October 2016 Accepted date: 30 October 2016 Cite this article as: Hua Jin, Songhe Meng, Weihua Xie, Chenghai Xu and Jiahong Niu, HfB2-CNTs composites with enhanced mechanical properties prepared by spark plasma sintering, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.10.200 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

HfB2-CNTs composites with enhanced mechanical properties prepared by spark plasma sintering Hua Jin, Songhe Meng, Weihua Xie*, Chenghai Xu, Jiahong Niu National Key laboratory of Science and Technology on advanced composites in Special Environments, Harbin Institute of Technology, Harbin 150001, P.R. China *

Corresponding

author:

Weihua

Xie,

Tel/Fax:

+86-451-86412259.

[email protected] Abstract HfB2-x vol.%CNTs (x=0, 5, 10, and 15) composites are prepared by spark plasma sintering. The influence of CNTs content and sintering temperature on densification, microstructure and mechanical properties is studied. Compared with pure HfB2 ceramic, the sinterability of HfB2-CNTs composites is remarkably improved by the addition of CNTs. Appropriate addition of CNTs (10 vol.%) and sintering temperature (1800 °C) can achieve the highest mechanical properties: the hardness, flexural strength and fracture toughness are measured to be 21.8±0.5 GPa, 894±60 MPa, and 7.8±0.2 MPa·m1/2, respectively. This is contributed to the optimal combination of the relative density, grain size and the dispersion of CNTs. The crack deflection, CNTs debonding and pull-out are observed and supposed to exhaust more fracture energy during the fracture process.

Keywords: B. Composites; C. Mechanical properties; E. Structural applications

1 Introduction Due to the unique properties of Hafnium diboride (HfB2), such as high melting temperature (3380°C), good mechanical properties, excellent thermal performance and

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electrical characteristics, it has been considered as a valuable candidate material for high-temperature structural parts, refractory linings, cutting tools, electrodes and microelectronics [1-3]. However, the low intrinsic sinterability and brittleness of HfB2 are still the obstacle to limit the wider application of HfB2-based ceramics [4, 5]. Researches indicated there are two approaches to enhance the densification, including reduction of starting particle size and the use of sintering aids (such as SiC, B4C, TiSi2, MoSi2, and so on) [6-9]. In this paper, high purity fine HfB2 powder has been used as the starting powder. Carbon nanotubes (CNTs) has been selected here to enhance the HfB2 ceramics, which has been proved to be an effective reinforcement in composites since its first discovery [10-12]. Spark plasma sintering has emerged as a nonconventional powder consolidation method with higher heating rates, lower sintering temperatures and shorter dwelling times by using a pulsed DC current to active and improve sintering kinetics [13, 14]. The densification, microstructures and mechanical properties of spark plasma sintered HfB2-CNTs composites are presented and discussed. 2 Experimental procedures Commercially available HfB2 powder (1-2 μm, >99.9%, Found Star Science and Technology, Beijing, China) and multi-walled CNTs (Mean diameter and length are 40-60 nm and 5-15 μm, respectively, >99.9%, Shenzhen Nanotech Port Co. Ltd., China) were used as raw material. The powders were weighed in proportion to the stoichiometric ratio to yield HfB2-x vol.%CNTs (x=0, 5, 10, and 15). The multi-walled CNTs were first dispersed in ethanol for 1 h by ultrasonic vibration, and then ball-mixed with HfB2 powders for 6 h in a polyethylene bottle using silicon carbide balls and ethanol as the grinding media. After mixing, the slurry was dried at 80 °C for 24 h in vacuum evaporator and then sieved through a 100-mesh sieve. The resulting powder

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mixtures were placed into a graphite die with an inner diameter of 30 mm and then spark plasma sintered at 1700 °C-1850 °C, with a heating rate of 150 °C·min−1 for 8 min under a uniaxial load of 30 MPa in vacuum. Bulk density of the sintered samples was measured using the Archimedes method with deionized water as the immersing medium. The relative density was calculated by dividing the bulk density by the theoretical density. Phases were identified by the conventional X-ray diffraction (XRD; PANalytical X'Pert PRO, Holland, CuKα=1.5418 Å). The microstructural observations of the samples were carried out by scanning electron microscopy (SEM, ZEISS EVO18, Germany). The grain sizes were determined from SEM images of the fracture surface using an image analysis software package and estimated by measuring at least 100 grains. Hardness was measured by Vickers’ indentation with a 50 N load applied for 10 s on polished sections. Bending strength was measured by a three-point bending test, using a 12 mm span and a crosshead speed of 0.5 mm·min−1, according to the Chinese Standard GB/T6569-2006 [15]. Test samples were machined into bars of 2 mm × 3 mm × 18 mm (width × height × length, respectively), and polished with diamond slurries down to a 1μm finish. The edges of all the specimens were chamfered to minimize the effect of stress concentration due to machining flaws. Fracture toughness (KIC) was evaluated by a single-edge notched beam test with a 16 mm span and a crosshead speed of 0.05 mm·min−1, on the same jig used for the flexural strength, according to the Chinese Standard GB/T23806-2009 [16]. The test bars, 2 mm × 4 mm × 22 mm (width × height × length, respectively), were notched with a 0.1 mm thick diamond saw and the notch length was about 0.50 of the bar height. A minimum number of six specimens were tested for each experimental condition. 3 Results and discussions

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The variation of the relative density as a function of CNTs content and sintering temperature has been listed in Table 1. Generally, the relative density is increased with the increasing sintering temperature, and also increased with the increasing CNTs content until 10 vol.% but decreased when the CNTs content is up to 15 vol.%. The relative density of specimens is supposed to be dependent on the dispersion of CNTs. Similarly, the mechanical properties of HfB2-x vol.%CNTs composites are increased with the increasing sintering temperature, and the optimal hardness (21.8±0.5 GPa), flexural strength (894±60 MPa) and fracture toughness (7.8±0.2 MPa·m1/2) are achieved in composite with 10 vol.% CNTs sintered at 1800 °C, which are much higher than that of monolithic HfB2 sintered at the same condition as shown in Table 1. XRD patterns of HfB2-x vol.%CNTs composites sintered at different temperatures are shown in Fig.1. Characteristic peaks of HfB2 (JCPDS: 12-0234) are identified for all samples. Increasing the CNTs content from 5 vol.% to 15 vol.% results in heightening the peaks of carbon phase (JCPDS: 26-1080). The main components HfB2 and CNTs are apart, no other phases can be found. Meanwhile, the phase compositions of the samples sintered at different temperatures are similar, consisting of a major phase HfB2 and trace CNTs. It is indicated that there is no obvious reaction between HfB2 and CNTs. SEM micrographs of the fracture surface of the typical HfB2-CNTs composites sintered at different temperatures are illustrated in Fig.2-Fig.4. Compared with the pure HfB2, the grain size of HfB2 in composites is decreased with the increasing CNTs content but increased with the increasing sintering temperature. CNTs clusters are located at the grain boundary in the composites which are believed to inhibit the HfB2 grain growth via grain boundary pinning, little or no pores can be detected here which is consistent with the relative density measurements. At a constant sintering temperature as seen in Fig.3(a,b) and Fig.4(a,b), the CNTs clusters with porous are found to be

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increased with increasing amount of CNTs, and the HfB2 particles are homogeneously surrounded by these clusters. However, more porous rope-like structure of CNTs clusters is observed by further CNT addition (≥15 vol.%), resulting in the slight reduction of relative density as listed in Table 1. Based on the previous investigations [17], the hardness of ceramics is weakened by the introduction of weak second phases, such as carbon/graphite, h-BN, and pores. Accordingly, the hardness of HfB2-CNTs composites is increased with the increasing densification due to the reduction of pores. As known, the flexural strength of the structural materials is determined by the relative density and the grain size [18, 19]. σ = σ0 + kd−1/2

(1)

σ = σ0 exp(−kα)

(2)

where σ is the strength of materials, σ0 is the strength of materials without any defect, k is material constants, d is grain size, and α is residual porosity. Hence, the strength of the HfB2-CNTs composites is improved with increasing relative density and decreasing grain size in comparison with the pure HfB2 ceramics. Meanwhile, the measured fracture toughness of HfB2-CNTs is notably higher than the measured results for the pure HfB2. As shown in Fig.2, the fracture mode of pure HfB2 is mainly transgranular. However, the mixed fracture mode, including transgranular and intergranular, is found in the HfB2-CNTs composites (Fig.3 and Fig.4). As depicted in the high magnifications, a perfect interface between CNTs and the HfB2 matrix is observed, confirmed that there is no obvious reaction between CNTs and HfB2, which is consistent with the XRD analysis. Significant pits and nanotube roots are observed on the fracture surface of the HfB2-CNTs composites due to the CNTs debonding and pull-out. Besides, the rough fracture surface of the composites and rugged crack propagation path reveal the occurrence of crack deflection. It is known that when relatively brittle matrix is

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toughened by reinforcement with obvious aspect ratio like CNTs, CNTs debonding can be detected around the weak CNTs/matrix interface. Hence, the intact CNTs can be found when the crack propagates around them, and the length and area of opening cracks are amplified, which would induce crack deflection to decrease the stress intensity around the crack tip. Furthermore, when the crack finally propagates, more fracture energy can be exhausted from frictional sliding in CNTs pullout. However, the fracture toughness of HfB2-CNTs is not always increased with the increasing CNTs content and measured to be decreased at higher amount of CNTs additions (≥15vol.%) due to the formation of porous rope-like structure of CNTs, and the cracks could propagate easily as encountered these CNTs clusters in the composites. 4. Conclusions The effect of CNTs content on microstructures and mechanical properties of spark plasma sintered HfB2-CNTs is investigated. The hardness, flexural strength and fracture toughness of the HfB2-CNTs composites are determined by the relative density, grain size and the dispersion of CNTs. The highest hardness (21.8±0.5 GPa), flexural strength (894±60 MPa) and fracture toughness (7.8±0.2 MPa·m1/2) are achieved in composite with 10 vol.% CNTs sintered at 1800 °C, which are much higher than that of pure HfB2. The significant crack deflection, CNTs debonding and pull-out are observed, which can exhaust more fracture energy during the fracture process. This investigation points out a promising method for improving the mechanical properties of HfB2-based ceramics. Acknowledgements This work was supported by the National Natural Science Foundation of China (11502058), the Heilongjiang Postdoctoral Science Foundation Funded Project (LBH-Z15071) and Aerospace Innovation Fund. References

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10. J. Lin, Y. Yang, H. Zhang, Z. Wu, Y. Huang. Effect of sintering temperature on the mechanical properties and microstructure of carbon nanotubes toughened TiB2 ceramics densified by spark plasma sintering, Mater. Lett. 166 (2016) 280-283. 11. J. Lin, Y. Huang, H. Zhang, Y. Yang, N. Li. Microstructure and mechanical properties of spark plasma sintered ZrB2-SiC-MWCNT composites, Ceram. Int. 41 (2015) 15261-15265. 12. B. Xu, Ch. Hong, S. Zhou, J. Han, X. Zhang. High-temperature erosion resistance of ZrB2-based ceramic coating for lightweight carbon/carbon composites under simulated atmospheric re-entry conditions by high frequency plasma wind tunnel test, Ceram. Int. 42 (2016) 9511-9518. 13. A.J. Mackie, G.D. Hatton, H.G.C. Hamilton, J.S. Dean, R. Goodall. Carbon uptake and distribution in Spark Plasma Sintering (SPS) processed Sm(Co,Fe,Cu,Zr)z, Mater. Lett. 171 (2016) 14-17. 14. B. Basu, T. Venkateswaran. Microstructure and properties of spark plasma-sintered ZrO2-ZrB2 nanoceramic composites, J. Am. Ceram. Soc. 89 [8] (2006) 2405-2412. 15. GB/T6569-2006.

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properties of TiB2 ceramics enhanced by SiC particles and carbon nanotubes, Ceram. Int. 42 (2016) 4627-4631. 18. Z. Zhang, X. Shen, F. Wang, S. Lee, L. Wang. Densification behavior and mechanical properties of the spark plasma sintered monolithic TiB2 ceramics, Mater. Sci. Eng. A 527 (2010) 5947-5951. 19. J. Lin, Y. Huang, H. Zhang, Y. Yang, Y. Wu. Spark plasma sintering of ZrO 2 fiber toughened ZrB2-based ultra-high temperature ceramics, Ceram. Int. 41 (2015) 10336-10340.

Fig.1 XRD patterns of HfB2-x vol.%CNTs composites sintered at different temperatures. Fig.2 SEM image of fracture surface of pure HfB2 sintered at 1850 °C. Fig.3 SEM images of fracture surface of HfB2 -5 vol.%CNTs sintered at 1800 °C. Fig.4 SEM images of fracture surface of HfB2 -10 vol.%CNTs sintered at different temperatures: (a,b) 1800 °C and (c,d) 1850 °C.

Table 1 Density and mechanical properties of HfB2-x vol.%CNTs composites. Sintering temperature (°C) 1700

1750

1800

Material Pure HfB2 HfB2-5vol.%CNTs HfB2-10vol.%CNTs HfB2-15vol.%CNTs Pure HfB2 HfB2-5vol.%CNTs HfB2-10vol.%CNTs HfB2-15vol.%CNTs Pure HfB2 HfB2-5vol.%CNTs HfB2-10vol.%CNTs

Relative Flexural Hardness density strength (GPa) (%) (MPa) 76.2 298±35 79.6 305±47 82.8 342±38 82.3 330±45 88.4 385±50 93.7 16.5±0.6 517±62 98.2 19.4±0.4 723±48 97.8 19.1±0.6 688±65 92.5 15.4±0.8 445±76 96.9 18.7±0.9 650±94 99.6 21.8±0.5 894±60 9

Fracture toughness (MPa·m1/2) 2.4±0.1 2.6±0.3 2.8±0.4 2.7±0.2 3.4±0.3 4.2±0.4 6.4±0.1 6.0±0.3 3.7±0.2 5.3±0.4 7.8±0.2

1850

HfB2-15vol.%CNTs Pure HfB2 HfB2-5vol.%CNTs HfB2-10vol.%CNTs HfB2-15vol.%CNTs

99.3 95.8 98.5 99.4 99.2

21.0±0.6 18.0±0.7 19.8±0.5 21.4±0.6 20.8±0.3

809±45 636±53 744±76 856±82 795±58

7.2±0.4 4.8±0.3 6.8±0.2 7.6±0.1 6.9±0.2

Fig.1 XRD patterns of HfB2-x vol.%CNTs composites sintered at different temperatures.

5μm Fig.2 SEM image of fracture surface of pure HfB2 sintered at 1850 °C.

10

(a)

(b)

5μm

500nm

Fig.3 SEM images of fracture surface of HfB2 -5 vol.%CNTs sintered at 1800 °C.

(b)

(a)

5μm (c)

500nm (d)

5μm

500nm

Fig.4 SEM images of fracture surface of HfB2 -10 vol.%CNTs sintered at different temperatures: (a,b) 1800 °C and (c,d) 1850 °C.

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