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ZrB2-ZrSi2-SiC composites prepared by reactive spark plasma sintering
Int. Journal of Refractory Metals and Hard Materials 60 (2016) 104–107
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Short communication
ZrB2-ZrSi2-SiC composites prepared by reactive spark plasma sintering Gang Shao a, Xiaotong Zhao a, Hailong Wang a,⁎, Jianbao Chen a, Rui Zhang a,b, Bingbing Fan a, Hongxia Lu a, Hongliang Xu a, Deliang Chen a a b
School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, China Zhengzhou University of Aeronautics, Zhengzhou 450015, China
a r t i c l e
i n f o
Article history: Received 19 April 2016 Received in revised form 3 June 2016 Accepted 15 July 2016 Available online 17 July 2016 Keywords: Zirconium diboride Reactive spark plasma sintering Mechanical properties
a b s t r a c t ZrSi2 and SiC are good candidates to improve both sinterability and mechanical properties of ZrB2 ceramics, which were synthesized simultaneously by an in-situ reaction of ZrC and Si additives during the sintering processing in this work. The ZrB2 ceramic composites with different amount of ZrSi2 and SiC were fabricated by reactive spark plasma sintering (RSPS) method. X-ray diffraction, scanning microscopy and Archimedes's method are used to characterize the phase, microstructure and density of the composites. Meanwhile, fracture toughness and flexural strength of the obtained composites were investigated too. It's found that a fully dense composite can be achieved at 1500 °C by SPS. Both fracture toughness and flexural strength of ZrB2 ceramics increased with increasing the concentration of ZrSi2 and SiC additives and reached a maximum of 7.33 ± 0.24 MPa·m1/2 and 471 ± 15 MPa, respectively, with the ZrSi2 + SiC content of 30 wt%. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Due to the high melting temperature (N3000 °C), excellent strength at elevated temperatures, high thermal conductivity and outstanding thermal shock resistance, zirconium diboride (ZrB2) is considered as one of the most promising ceramics for ultra-high temperature applications in the fields of aerospace, refractory materials [1]. However, the drawback of ZrB2 ceramics is obvious too, poor sinterability, which is caused by the intrinsic features of ZrB 2 ceramics of high melting point, strong covalent bonds and low selfdiffusion coefficient [2,3]. Generally, there are three approaches to improve the sinterability and densification of ZrB2 ceramics at low temperature and pressure: The first one is using sintering additives to form a liquid phase which can improve the flowability and diffusivity of the composites and lower sintering temperatures. The second one is to increase the surface energy of ZrB2 powder by removing the surface oxide layer which can improve the reaction ability. The last one is to induce more defects by mechanical activation method [4]. Recently, adding sintering additives is a promising way to densification of ZrB2 ceramics, which is mostly adopted by researchers. For example, the addition of metals (e.g., Ni, Mo) and ceramics (e.g., SiC, MoSi2 , ZrSi2, Si3 N4 , AlN, ZrN) can significantly promote the densification and induce microstructure through liquid-phase sintering with much lower sintering temperatures than those of undoped compositions [5–9]. The mechanisms of these two cases are different. Metallic additives assist in densification by liquid phase sintering and ceramic additives (such as SiC) can react with the oxide layer of ZrB2 and expose fresh surfaces of
http://dx.doi.org/10.1016/j.ijrmhm.2016.07.011 0263-4368/© 2016 Elsevier Ltd. All rights reserved.
ZrB 2 increasing the surface energy [4]. Nowdays, spark plasma sintering is one of the most advanced processing techniques developed for densifying ceramic materials [10,11]. Spark plasma sintering possesses unique features of direct heating by current passing through powder compact and generating high-energy plasmas. These plasmas can induce special phenomena of particle surface activation, evaporation and local melting, which the conventional hotpress sintering cannot [12]. Therefore, spark plasma sintering is intensively used for sintering ZrB2 ceramics because of the aforementioned unique characters and fast heating/cooling rate, high applicable pressure [13–16]. Recently, the mixture of ZrSi2 and SiC has been used as an efficient sintering additive for densification of ZrB2 ceramics because it can improve both the sinterability and mechanical properties of the ZrB2 ceramics [17–19]. However, to the best knowledge of authors, there are no public reports on synthesis of ZrSi2 and SiC additives simultaneously from an in-situ reaction of ZrC and Si powders during sintering of ZrB2 ceramic composites. Meanwhile, ZrSi2 and SiC composites can be synthesized mechanically activated ZrC and Si powders with a mole ratio of 1:3, reported by In-Yong Ko and coworkers [20], which indicated that the formation of ZrSi 2 and SiC from the reactants of ZrC and Si can take place under a certain condition. In this work, 10–40 wt.% (ZrSi2 + SiC) additives were achieved by in-situ reaction of ZrC and Si powders with a mole ratio of 1:3 during spark plasma sintering of ZrB2-based ceramics. The densification behavior, microstructure and mechanical properties of the sintered ZrB2 ceramic composites were investigated and discussed.
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2. Experimental procedures Commercially available ZrB2 (purity ~ 97.0%, particle size b10 μm, Gongyi Sanxing Ceramics Materials Co. Ltd., Gongyi, China), ZrC (purity ~ 95.0%, particle size ~ 3 μm, Alfa Aesar, MA, USA) and Si (purity ~ 99.0%, size b5 μm, Alfa Aesar, MA, USA) were used as raw materials in this work. ZrSi2 and SiC were synthesized by the in-situ reaction of ZrC and Si powder according to the stoichiometric composition (ZrC:Si = 1:3 by molar ratio). Four compositions were designed in this work, illustrated as: ZBZSS10: ZrB2 + 10 wt.% (ZrSi2 + SiC), ZBZSS20: ZrB2 + 20 wt.% (ZrSi2 + SiC), ZBZSS30: ZrB2 + 30 wt.% (ZrSi2 + SiC), ZBZSS40: ZrB2 + 40 wt.% (ZrSi2 + SiC). ZrB2, ZrC and Si power were ball-milled for 4 h in a polyethylene tank using ZrO2 balls and ethanol as the grinding media. Then the slurry was filtered and dried by vacuum drying chamber. After that, the power mixtures were loaded into a graphite die (inner diameter of 30 mm) lined with graphite foil and densified using SPS (Dr. Sinter 2020, Sumitomo Coal Mining Co., Tokyo, Japan) in vacuum (~ 6 Pa) at 1500 °C for 10 min under an uniaxial pressure of 40 MPa (heating rate: 100 °C/min). A 12 ms-on and 2 ms-off pulse sequence were used. The heating process was controlled using a monochromatic optical pyrometer which was focused on the hole of side wall of graphite die. The shrinkages of specimens were detected by measuring the movement of lower electrode (resolution 0.01 mm) during the sintering and the shrinkage curves were recorded by a connected computer. After sintering, the bulk densities of sintered samples were measured using Archimedes principle. The theoretical densities of the sintered specimens were calculated by the rule of mixture. Flexural strength was measured by a three-point bending test (test bars 3 mm × 2 mm × 25 mm) with a span of 10 mm and a crosshead speed of 0.5 mm/min. Fracture toughness was tested by the single-edged notch beam (SENB) method (test bars 2 mm × 4 mm × 30 mm) with a span of 20 mm and a crosshead speed of 0.05 mm/min, the width of the notch being b 0.25 mm. Vickers hardness (HV) was carried out by Vickers indentation (HXD-1000TMC, China) with a load of 98 N for 15 s applied on the polished sections. The microstructure was observed by a scanning electron microscope (SEM, QUANTA-200, FEI, Netherlands). The phases of specimens were determined by X-ray diffraction analysis (XRD, XD-3, China) using Cu Kα radiation. 3. Results and discussion 3.1. Phase determination Fig. 1 is the XRD patterns of the sintered specimens. It was observed that the primary phase of ZrB2 was detected in all these four designed compositions, and a secondary phase of ZrSi2 and SiC were detected as well. The intensity of the phase peaks of ZrSi2 and SiC increases with increasing the amount of ZrC and Si, and no detectable ZrC and Si phases. These results indicated that ZrSi2 and SiC were formed by the in-situ reaction of ZrC and Si during the SPS sintering. At the same time, the composition design of ZrC:Si = 1:3 by molar ratio was proper and the in-situ reaction was completed adequately.
Fig. 1. XRD patents of the sintered ZrB2-ZrSi2-SiC ceramic composites by SPS at 1500 °C.
temperature of densification and this effect enhanced with the ZrSi2 and SiC formed. In particular, the onset temperature of densification was about 80 °C lower for ZBZSS40 than that for ZBZSS10. After the densification started, rapid shrinkage was produced, accompanying with a further increasing temperature, and the densification rate has an order of ZBZS40 N ZBZS30 N ZBZS20 N ZBZS10. During the rapid shrinkage, the shrinkage beings about at 1126 °C. This indicated that the ZrSi2 and SiC additions not only lowered the onset temperature of densification but also accelerated the shrinkage rate during SPS sintering. Meanwhile, during the isothermal heating, ZBZSS30 and ZBZSS40 had a plateau in the density versus time curves and the shrinkage stopped immediately after heating to isothermal temperature. However, the shrinkage process progressed continuously for ZBZSS10 andZBZSS20, and no plateau was observed. Fig. 3 shows the effect of ZrSi2 and SiC contents on the density and hardness of ZrB2-ZrSi2-SiC ceramics. It found that the density of ZrB2ZrSi2-SiC composites increased with increasing amount of ZrC and Si. The relative density of the ZBZSS10 and ZBZSS20 composites was 72.6% and 95.5% at 1500 °C, respectively. And the relative density of the ZBZSS30 and the ZBZSS40 composites reached 100% at 1500 °C. This revealed that the densification behavior of ZrB2 was improved by increasing the amount of ZrC and Si, and the fully dense ZrB2-ZrSi2-SiC composites were obtained finally. This is attributed to the formation of ZrSi2 and SiC by the in-situ reaction which can adhere the ZrB2 particles together during SPS sintering.
3.2. Densification behavior The typical shrinkage curves of ZrB2-ZrSi2-SiC composites with different compositions sintered at 1500 °C by spark plasma sintering are presented In Fig. 2. It can be seen that the displacement curves of ZrB2-ZrSi2-SiC composites were pretty similar. And the initial temperature of densification declined with increasing the concentration of ZrCSi additives: 978 °C for ZBZSS10, 945 °C for ZBZSS20, 924 °C for ZBZSS30, and 897 °C for ZBZSS40. This indicated that the formation of ZrSi2 and SiC by an in-situ reaction during sintering which reduced the onset
Fig. 2. Shrinkage of the ZrB2-ZrSi2-SiC ceramic composites during spark plasma sintering.
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Fig. 3. Relative density and Vickers hardness of ZrB2-ZrSi2-SiC composites.
3.3. Mechanical properties The flexural strength and fracture toughness of ZrB2-ZrSi2-SiC composites at room temperature are shown in Fig. 4. The room-temperature flexure strength of the ZrB2-ZrSi2-SiC composites was improved by increasing ZrSi2 and SiC contents from 168 MPa (ZBZSS10) to 471 MPa (ZBZSS30). The main reason of low flexure strength (168 MPa) for the ZBZSS10 composites is the low density of 72.6% which was caused by lack of ZrSi2 and SiC. However, when the content of ZrSi2 and SiC over 30 wt%, the flexure strength of the composites reduced a little bit. The appearance of this phenomenon could be caused by the excess of ZrSi2 and SiC concentration which might induce unwanted aggregation and relative ununiform microstructure. Therefore, the flexure strength of the composites reached a maximum of 471 MPa with a proper ZrSi2 and SiC concentration of 30 wt% formed during sintering. The change tendency of fracture toughness of the composites was similar to that of flexure strength which increased firstly and then decreased with increasing the ZrSi2 and SiC concentration and accompanied with a maximum of 7.33 MPa·m1/2 at 30 wt.% of ZrSi2 and SiC. The high fracture toughness of ZBZSS30 sample is mostly attributed to the high density and fine microstructure. The suitable amount of ZrSi2 and SiC could enhance the interfacial strength and restrict to the crack growth which
Fig. 4. Flexural strength and fracture toughness of ZrB2-ZrSi2-SiC composites.
Fig. 5. SEM images of the fracture surface of the sintered ZrB2-ZrSi2-SiC composites.
G. Shao et al. / Int. Journal of Refractory Metals and Hard Materials 60 (2016) 104–107
improved both flexural strength and fracture toughness remarkably. The hardness of composites was shown in Fig. 3 which possessed the similar trend with those of flexural strength and fracture toughness. The highest hardness was 18.10 ± 0.51 GPa with ZBZSS30 sample (30 wt.% of ZrSi2 and SiC) which is much higher than that of the ZBZSS10 sample with the value of 9.83 ± 0.57 GPa. The hardness of ZBZSS40 sample only slightly decreased compared with that of ZBZSS30 one. The significant improvement of hardness of the composites is due to the greatly raised of relative density from 72.6% of ZBZSS10 to 100% of ZBZSS30 which is benefited from the formation of ZrSi2 and SiC.
3.4. Microstructure analysis SEM images on fracture surfaces of ZrB2-ZrSi2-SiC ceramics are shown in Fig. 5. Homogeneous microstructures were observed for all ZrB2 ceramic composites with well dispersed ZrSi2 and SiC particles. Meanwhile, a decreasing of grain size and changing of fracture modes were found with increasing the amount of ZrSi2 and SiC. The ZBZSS10 composite exhibits a flat fracture surface, which shows a transgranular fracture mode for most areas, while the fracture surfaces of the ZBZSS20, ZBZSS30 and ZBZSS40 composites are much rougher than that of the ZBZSS10, and show a concave–convex appearance. In addition, an interesting phenomenon can be observed from the fracture surfaces of ZBZSS20, ZBZSS30 and ZBZSS40 composites: there are many pits on the fracture surfaces and grain boundaries. The existence of these pits could be owed to the grains pull-out and fracturing of grain boundaries. These above-mentioned effects indicated that the existence of ZrSi2 and SiC in ZrB2 composites caused a mixed transgranular and intergranular fracture mode and lead to the increased strength and toughness of the composites.
4. Conclusions ZrB2 ceramics have been densified by spark plasma sintering at 1500 °C using ZrC and Si as sintering additives. ZrSi2 and SiC were formed by an in-situ reaction during SPS sintering process which played an important role in promoting densification and improving the sinterability of ZrB2 ceramics. The density of the ZrB2-ZrSi2-SiC composites increases with increasing amount of ZrSi2 and SiC, and the fully dense ZrB2 ceramics were obtained at a content of ZrSi2-SiC over 30 wt.%. Both fracture toughness and flexural strength of the composites were increased with increasing the content of ZrSi2 and SiC. The maximum flexural strength and fracture toughness were 471 ± 15 MPa and 7.33 ± 0.24 MPa·m1/2, respectively for the ZBZSS30 sample. And the highest hardness of the composites was as high as 18.10 ± 0.51 GPa. The formation of ZrSi2 and SiC in the composites caused a mixed transgranular and intergranular fracture mode which attributed to the excellent mechanical properties.
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Acknowledgements The authors appreciate the financial support from the National Natural Science Foundation of China (Grant No. 51402264), China Postdoctoral Science Foundation (Grant No. 2014M561996) and Excellent Young Faculty Research Foundation of Zhengzhou University (Grant No. 1421320049). References [1] S.Q. Guo, Densification of ZrB2-based composites and their mechanical and physical properties: a review, J. Eur. Ceram. Soc. 29 (2009) 995–1011. [2] N.P. Vafa, M.S. Asl, M.J. Zamharir, Reactive hot pressing of ZrB2-based composites with changes in ZrO2/SiC ratio and sintering conditions. Part I: densification behavior, Ceram. Int. 41 (2015) 8388–8396. [3] W.M. Guo, S.X. Gu, Y. You, Densification and thermal stability of hot-pressed Si3N4ZrB2 ceramics, J. Am. Ceram. Soc. 98 (2015) 3651–3654. [4] J.K. Sonber, A.k. Suri, Synthesis and consolidation of zirconium diboride: review, Adv. Appl. Ceram. 110 (2011) 321–334. [5] M. Khoeini, A. Nemati, M. Zakeri, Comprehensive study on the effect of SiC and carbon additives on the pressureless sintering and microstructural and mechanical characteristics of new ultra-high temperature ZrB2 ceramics, Ceram. Int. 41 (2015) 11456–11463. [6] R.Z. Wang, W.G. Li, Effects of microstructures and flaw evolution on the fracture strength of ZrB2-MoSi2 composites under high temperatures, J. Alloys Compd. 644 (2015) 582–588. [7] Z. Wang, P. Zhou, Z.J. Wu, Effect of surface oxidation on thermal shock resistance of ZrB2-SiC-ZrC ceramic at temperature difference from 800 to 1900 °C, Corros. Sci. 98 (2015) 233–239. [8] S.Q. Guo, T. Nishimura, Y. Kagawa, Low-temperature hot pressing of ZrB2-based ceramics with ZrSi2 additives, Int. J. Appl. Ceram. Technol. 8 (2011) 1425–1435. [9] L. Rangaraj, C. Divakar, V. Jayaram, Fabrication and mechanisms of densification of ZrB2-based ultra high temperature ceramics by reactive hot pressing, J. Eur. Ceram. Soc. 30 (2010) 129–138. [10] M. Tokida, Trends in advanced spark plasma sintering system and technology, J. Soc. Powder Technol. Jpn. 30 (1993) 790–804. [11] M. Nygren, Z. Shen, On the preparation of bio-, nano- and structural ceramics and composites by spark plasma sintering, Solid State Sci. 5 (2003) 125–131. [12] J. Lin, Y. Huang, H. Zhang, Y. Yang, Y. Wu, Spark plasma sintering of ZrO2 fiber toughened ZrB2-based ultra-high temperature ceramics, Ceram. Int. 41 (2015) 10336–10340. [13] C. Ang, A. Seeber, T. Williams, Y.B. Cheng, SPS densification and microstructure of ZrB2 composites derived from sol–gel ZrC coating, J. Eur. Ceram. Soc. 34 (2014) 2875–2883. [14] H.L. Wang, B.B. Fan, L. Feng, The fabrication and mechanical properties of SiC/ZrB2 laminated ceramic composite prepared by spark plasma sintering, Ceram. Int. 38 (2012) 5015–5022. [15] L. Zhang, D.A. Pejakovic, J. Marschall, M. Gasch, Thermal and electrical transport properties of spark plasma sintered HfB2 and ZrB2 ceramics, J. Am. Ceram. Soc. 94 (2011) 2562–2570. [16] L.S. Walker, W.R. Pinc, E.L. Corral, Powder processing effects on the rapid lowtemperature densification of ZrB2-SiC ultra-high temperature ceramic composites using spark plasma sintering, J. Am. Ceram. Soc. 95 (2012) 194–203. [17] M.F. Wang, C.A. Wang, X.H. Zhang, Effects of SiC platelet and ZrSi2 additive on sintering and mechanical properties of ZrB2-based ceramics by hot-pressing, Mater. Des. 34 (2012) 293–297. [18] J.J. Sha, J. Li, S.H. Wang, Toughening effect of short carbon fibers in the ZrB2-ZrSi2 ceramic composites, Mater. Design. 75 (2015) 160–165. [19] O.N. Grigoriev, B.A. Galanov, V.A. Kotenko, Oxidation of ZrB2–SiC–ZrSi2 ceramics in oxygen, J. Eur. Ceram. Soc. 30 (2010) 2397–2405. [20] I.Y. Ko, J.H. Park, J.K. Yoon, ZrSi2-SiC composite obtained from mechanically activated ZrC + 3Si powders by pulsed current activated combustion synthesis, Ceram. Int. 36 (2010) 817–820.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Short communication
ZrB2-ZrSi2-SiC composites prepared by reactive spark plasma sintering Gang Shao a, Xiaotong Zhao a, Hailong Wang a,⁎, Jianbao Chen a, Rui Zhang a,b, Bingbing Fan a, Hongxia Lu a, Hongliang Xu a, Deliang Chen a a b
School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, China Zhengzhou University of Aeronautics, Zhengzhou 450015, China
a r t i c l e
i n f o
Article history: Received 19 April 2016 Received in revised form 3 June 2016 Accepted 15 July 2016 Available online 17 July 2016 Keywords: Zirconium diboride Reactive spark plasma sintering Mechanical properties
a b s t r a c t ZrSi2 and SiC are good candidates to improve both sinterability and mechanical properties of ZrB2 ceramics, which were synthesized simultaneously by an in-situ reaction of ZrC and Si additives during the sintering processing in this work. The ZrB2 ceramic composites with different amount of ZrSi2 and SiC were fabricated by reactive spark plasma sintering (RSPS) method. X-ray diffraction, scanning microscopy and Archimedes's method are used to characterize the phase, microstructure and density of the composites. Meanwhile, fracture toughness and flexural strength of the obtained composites were investigated too. It's found that a fully dense composite can be achieved at 1500 °C by SPS. Both fracture toughness and flexural strength of ZrB2 ceramics increased with increasing the concentration of ZrSi2 and SiC additives and reached a maximum of 7.33 ± 0.24 MPa·m1/2 and 471 ± 15 MPa, respectively, with the ZrSi2 + SiC content of 30 wt%. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Due to the high melting temperature (N3000 °C), excellent strength at elevated temperatures, high thermal conductivity and outstanding thermal shock resistance, zirconium diboride (ZrB2) is considered as one of the most promising ceramics for ultra-high temperature applications in the fields of aerospace, refractory materials [1]. However, the drawback of ZrB2 ceramics is obvious too, poor sinterability, which is caused by the intrinsic features of ZrB 2 ceramics of high melting point, strong covalent bonds and low selfdiffusion coefficient [2,3]. Generally, there are three approaches to improve the sinterability and densification of ZrB2 ceramics at low temperature and pressure: The first one is using sintering additives to form a liquid phase which can improve the flowability and diffusivity of the composites and lower sintering temperatures. The second one is to increase the surface energy of ZrB2 powder by removing the surface oxide layer which can improve the reaction ability. The last one is to induce more defects by mechanical activation method [4]. Recently, adding sintering additives is a promising way to densification of ZrB2 ceramics, which is mostly adopted by researchers. For example, the addition of metals (e.g., Ni, Mo) and ceramics (e.g., SiC, MoSi2 , ZrSi2, Si3 N4 , AlN, ZrN) can significantly promote the densification and induce microstructure through liquid-phase sintering with much lower sintering temperatures than those of undoped compositions [5–9]. The mechanisms of these two cases are different. Metallic additives assist in densification by liquid phase sintering and ceramic additives (such as SiC) can react with the oxide layer of ZrB2 and expose fresh surfaces of
http://dx.doi.org/10.1016/j.ijrmhm.2016.07.011 0263-4368/© 2016 Elsevier Ltd. All rights reserved.
ZrB 2 increasing the surface energy [4]. Nowdays, spark plasma sintering is one of the most advanced processing techniques developed for densifying ceramic materials [10,11]. Spark plasma sintering possesses unique features of direct heating by current passing through powder compact and generating high-energy plasmas. These plasmas can induce special phenomena of particle surface activation, evaporation and local melting, which the conventional hotpress sintering cannot [12]. Therefore, spark plasma sintering is intensively used for sintering ZrB2 ceramics because of the aforementioned unique characters and fast heating/cooling rate, high applicable pressure [13–16]. Recently, the mixture of ZrSi2 and SiC has been used as an efficient sintering additive for densification of ZrB2 ceramics because it can improve both the sinterability and mechanical properties of the ZrB2 ceramics [17–19]. However, to the best knowledge of authors, there are no public reports on synthesis of ZrSi2 and SiC additives simultaneously from an in-situ reaction of ZrC and Si powders during sintering of ZrB2 ceramic composites. Meanwhile, ZrSi2 and SiC composites can be synthesized mechanically activated ZrC and Si powders with a mole ratio of 1:3, reported by In-Yong Ko and coworkers [20], which indicated that the formation of ZrSi 2 and SiC from the reactants of ZrC and Si can take place under a certain condition. In this work, 10–40 wt.% (ZrSi2 + SiC) additives were achieved by in-situ reaction of ZrC and Si powders with a mole ratio of 1:3 during spark plasma sintering of ZrB2-based ceramics. The densification behavior, microstructure and mechanical properties of the sintered ZrB2 ceramic composites were investigated and discussed.
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2. Experimental procedures Commercially available ZrB2 (purity ~ 97.0%, particle size b10 μm, Gongyi Sanxing Ceramics Materials Co. Ltd., Gongyi, China), ZrC (purity ~ 95.0%, particle size ~ 3 μm, Alfa Aesar, MA, USA) and Si (purity ~ 99.0%, size b5 μm, Alfa Aesar, MA, USA) were used as raw materials in this work. ZrSi2 and SiC were synthesized by the in-situ reaction of ZrC and Si powder according to the stoichiometric composition (ZrC:Si = 1:3 by molar ratio). Four compositions were designed in this work, illustrated as: ZBZSS10: ZrB2 + 10 wt.% (ZrSi2 + SiC), ZBZSS20: ZrB2 + 20 wt.% (ZrSi2 + SiC), ZBZSS30: ZrB2 + 30 wt.% (ZrSi2 + SiC), ZBZSS40: ZrB2 + 40 wt.% (ZrSi2 + SiC). ZrB2, ZrC and Si power were ball-milled for 4 h in a polyethylene tank using ZrO2 balls and ethanol as the grinding media. Then the slurry was filtered and dried by vacuum drying chamber. After that, the power mixtures were loaded into a graphite die (inner diameter of 30 mm) lined with graphite foil and densified using SPS (Dr. Sinter 2020, Sumitomo Coal Mining Co., Tokyo, Japan) in vacuum (~ 6 Pa) at 1500 °C for 10 min under an uniaxial pressure of 40 MPa (heating rate: 100 °C/min). A 12 ms-on and 2 ms-off pulse sequence were used. The heating process was controlled using a monochromatic optical pyrometer which was focused on the hole of side wall of graphite die. The shrinkages of specimens were detected by measuring the movement of lower electrode (resolution 0.01 mm) during the sintering and the shrinkage curves were recorded by a connected computer. After sintering, the bulk densities of sintered samples were measured using Archimedes principle. The theoretical densities of the sintered specimens were calculated by the rule of mixture. Flexural strength was measured by a three-point bending test (test bars 3 mm × 2 mm × 25 mm) with a span of 10 mm and a crosshead speed of 0.5 mm/min. Fracture toughness was tested by the single-edged notch beam (SENB) method (test bars 2 mm × 4 mm × 30 mm) with a span of 20 mm and a crosshead speed of 0.05 mm/min, the width of the notch being b 0.25 mm. Vickers hardness (HV) was carried out by Vickers indentation (HXD-1000TMC, China) with a load of 98 N for 15 s applied on the polished sections. The microstructure was observed by a scanning electron microscope (SEM, QUANTA-200, FEI, Netherlands). The phases of specimens were determined by X-ray diffraction analysis (XRD, XD-3, China) using Cu Kα radiation. 3. Results and discussion 3.1. Phase determination Fig. 1 is the XRD patterns of the sintered specimens. It was observed that the primary phase of ZrB2 was detected in all these four designed compositions, and a secondary phase of ZrSi2 and SiC were detected as well. The intensity of the phase peaks of ZrSi2 and SiC increases with increasing the amount of ZrC and Si, and no detectable ZrC and Si phases. These results indicated that ZrSi2 and SiC were formed by the in-situ reaction of ZrC and Si during the SPS sintering. At the same time, the composition design of ZrC:Si = 1:3 by molar ratio was proper and the in-situ reaction was completed adequately.
Fig. 1. XRD patents of the sintered ZrB2-ZrSi2-SiC ceramic composites by SPS at 1500 °C.
temperature of densification and this effect enhanced with the ZrSi2 and SiC formed. In particular, the onset temperature of densification was about 80 °C lower for ZBZSS40 than that for ZBZSS10. After the densification started, rapid shrinkage was produced, accompanying with a further increasing temperature, and the densification rate has an order of ZBZS40 N ZBZS30 N ZBZS20 N ZBZS10. During the rapid shrinkage, the shrinkage beings about at 1126 °C. This indicated that the ZrSi2 and SiC additions not only lowered the onset temperature of densification but also accelerated the shrinkage rate during SPS sintering. Meanwhile, during the isothermal heating, ZBZSS30 and ZBZSS40 had a plateau in the density versus time curves and the shrinkage stopped immediately after heating to isothermal temperature. However, the shrinkage process progressed continuously for ZBZSS10 andZBZSS20, and no plateau was observed. Fig. 3 shows the effect of ZrSi2 and SiC contents on the density and hardness of ZrB2-ZrSi2-SiC ceramics. It found that the density of ZrB2ZrSi2-SiC composites increased with increasing amount of ZrC and Si. The relative density of the ZBZSS10 and ZBZSS20 composites was 72.6% and 95.5% at 1500 °C, respectively. And the relative density of the ZBZSS30 and the ZBZSS40 composites reached 100% at 1500 °C. This revealed that the densification behavior of ZrB2 was improved by increasing the amount of ZrC and Si, and the fully dense ZrB2-ZrSi2-SiC composites were obtained finally. This is attributed to the formation of ZrSi2 and SiC by the in-situ reaction which can adhere the ZrB2 particles together during SPS sintering.
3.2. Densification behavior The typical shrinkage curves of ZrB2-ZrSi2-SiC composites with different compositions sintered at 1500 °C by spark plasma sintering are presented In Fig. 2. It can be seen that the displacement curves of ZrB2-ZrSi2-SiC composites were pretty similar. And the initial temperature of densification declined with increasing the concentration of ZrCSi additives: 978 °C for ZBZSS10, 945 °C for ZBZSS20, 924 °C for ZBZSS30, and 897 °C for ZBZSS40. This indicated that the formation of ZrSi2 and SiC by an in-situ reaction during sintering which reduced the onset
Fig. 2. Shrinkage of the ZrB2-ZrSi2-SiC ceramic composites during spark plasma sintering.
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Fig. 3. Relative density and Vickers hardness of ZrB2-ZrSi2-SiC composites.
3.3. Mechanical properties The flexural strength and fracture toughness of ZrB2-ZrSi2-SiC composites at room temperature are shown in Fig. 4. The room-temperature flexure strength of the ZrB2-ZrSi2-SiC composites was improved by increasing ZrSi2 and SiC contents from 168 MPa (ZBZSS10) to 471 MPa (ZBZSS30). The main reason of low flexure strength (168 MPa) for the ZBZSS10 composites is the low density of 72.6% which was caused by lack of ZrSi2 and SiC. However, when the content of ZrSi2 and SiC over 30 wt%, the flexure strength of the composites reduced a little bit. The appearance of this phenomenon could be caused by the excess of ZrSi2 and SiC concentration which might induce unwanted aggregation and relative ununiform microstructure. Therefore, the flexure strength of the composites reached a maximum of 471 MPa with a proper ZrSi2 and SiC concentration of 30 wt% formed during sintering. The change tendency of fracture toughness of the composites was similar to that of flexure strength which increased firstly and then decreased with increasing the ZrSi2 and SiC concentration and accompanied with a maximum of 7.33 MPa·m1/2 at 30 wt.% of ZrSi2 and SiC. The high fracture toughness of ZBZSS30 sample is mostly attributed to the high density and fine microstructure. The suitable amount of ZrSi2 and SiC could enhance the interfacial strength and restrict to the crack growth which
Fig. 4. Flexural strength and fracture toughness of ZrB2-ZrSi2-SiC composites.
Fig. 5. SEM images of the fracture surface of the sintered ZrB2-ZrSi2-SiC composites.
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improved both flexural strength and fracture toughness remarkably. The hardness of composites was shown in Fig. 3 which possessed the similar trend with those of flexural strength and fracture toughness. The highest hardness was 18.10 ± 0.51 GPa with ZBZSS30 sample (30 wt.% of ZrSi2 and SiC) which is much higher than that of the ZBZSS10 sample with the value of 9.83 ± 0.57 GPa. The hardness of ZBZSS40 sample only slightly decreased compared with that of ZBZSS30 one. The significant improvement of hardness of the composites is due to the greatly raised of relative density from 72.6% of ZBZSS10 to 100% of ZBZSS30 which is benefited from the formation of ZrSi2 and SiC.
3.4. Microstructure analysis SEM images on fracture surfaces of ZrB2-ZrSi2-SiC ceramics are shown in Fig. 5. Homogeneous microstructures were observed for all ZrB2 ceramic composites with well dispersed ZrSi2 and SiC particles. Meanwhile, a decreasing of grain size and changing of fracture modes were found with increasing the amount of ZrSi2 and SiC. The ZBZSS10 composite exhibits a flat fracture surface, which shows a transgranular fracture mode for most areas, while the fracture surfaces of the ZBZSS20, ZBZSS30 and ZBZSS40 composites are much rougher than that of the ZBZSS10, and show a concave–convex appearance. In addition, an interesting phenomenon can be observed from the fracture surfaces of ZBZSS20, ZBZSS30 and ZBZSS40 composites: there are many pits on the fracture surfaces and grain boundaries. The existence of these pits could be owed to the grains pull-out and fracturing of grain boundaries. These above-mentioned effects indicated that the existence of ZrSi2 and SiC in ZrB2 composites caused a mixed transgranular and intergranular fracture mode and lead to the increased strength and toughness of the composites.
4. Conclusions ZrB2 ceramics have been densified by spark plasma sintering at 1500 °C using ZrC and Si as sintering additives. ZrSi2 and SiC were formed by an in-situ reaction during SPS sintering process which played an important role in promoting densification and improving the sinterability of ZrB2 ceramics. The density of the ZrB2-ZrSi2-SiC composites increases with increasing amount of ZrSi2 and SiC, and the fully dense ZrB2 ceramics were obtained at a content of ZrSi2-SiC over 30 wt.%. Both fracture toughness and flexural strength of the composites were increased with increasing the content of ZrSi2 and SiC. The maximum flexural strength and fracture toughness were 471 ± 15 MPa and 7.33 ± 0.24 MPa·m1/2, respectively for the ZBZSS30 sample. And the highest hardness of the composites was as high as 18.10 ± 0.51 GPa. The formation of ZrSi2 and SiC in the composites caused a mixed transgranular and intergranular fracture mode which attributed to the excellent mechanical properties.
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