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Effects of SiC platelet and ZrSi2 additive on sintering and mechanical properties of ZrB2-based ceramics by hot-pressing
Materials and Design 34 (2012) 293–297
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Effects of SiC platelet and ZrSi2 additive on sintering and mechanical properties of ZrB2-based ceramics by hot-pressing Mingfu Wang a,b, Chang-An Wang b,⇑, Xinghong Zhang a a b
Center for Composite Materials, Harbin Institute of Technology, Harbin 150001, PR China State Key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China
a r t i c l e
i n f o
Article history: Received 27 April 2011 Accepted 9 August 2011 Available online 16 August 2011
a b s t r a c t Fully dense Zirconium Diboride (ZrB2)-based ceramics with Zirconium Silicide (ZrSi2) and Silicon Carbide platelet (SiCpl) additive were fabricated by hot-press sintering in flowing argon atmosphere. Their mechanical properties and microstructure were examined. The contents of ZrSi2 and SiCpl were optimized. The defined addition of ZrSi2 was beneficial for densification and improving the mechanical properties of the ZrB2-based ceramics. The addition of SiCpl obviously improved the fracture toughness of the ZrB2-based ceramics. The range of fracture toughness of ZrB2–SiCpl–ZrSi2 composite was 6.7–9.0 MPa m1/2. The flexural strength of the composites varied a little and the maximum was 597 MPa when the content of ZrSi2 and SiCpl was both 5 vol.%. XRD results showed that a small amount of Zirconium Carbide (ZrC) formed during sintering. The microstructure of the composites was fine, compact and homogeneous. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Refractory transition metal borides, such as Zirconium Diboride (ZrB2), have been referred to as ultra high temperature ceramics (UHTCs) due to their extremely high melting point (>3050 °C). ZrB2 ceramics also have high thermal and electrical conductivities, chemical inertness against molten metals, and great thermal shock resistance [1,2]. However, there are two main bottleneck problems that limit the development of ZrB2 ceramics. On one hand, ZrB2 has poor intrinsic sinterability due to its strong covalent bonds and low bulk and grain boundary diffusivity. The densification of ZrB2 powder generally requires very high temperatures (>2100 °C) [3] and external pressure. On the other hand, ZrB2 ceramics have low mechanical properties and poor high-temperature oxidation/ablation resistance. So far as we know, most researches on ZrB2-based ceramics are focused on these two aspects. In order to lower sintering temperature of ZrB2 ceramics, some sintering additives, e.g. nitrides: Silicon nitride (Si3N4), Aluminum nitride (AlN); silicides: Molybdenum disilicide (MoSi2), ZrSi2 and carbides: SiC, Boron Carbide (B4C), have been added to enhance sintering of ZrB2-based ceramics [4–10]. Besides some pressureassistant processes (e.g. hot-pressing [11,12] and spark plasma sintering [13]), recently, the ZrB2-based ceramics with MoSi2 or ZrSi2 additive were sintered by pressureless at a temperature below 1850 °C [14,15]. In their studies, MoSi2 and ZrSi2 are proved as good sintering additives for ZrB2-based ceramics. However, in their studies, a large amount of MoSi2 or ZrSi2 up to 40 vol.% was usually ⇑ Corresponding author. Tel./fax: +86 10 62785488. E-mail address: [email protected] (C.-A. Wang). 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.08.016
used for good densification of ZrB2. Excess MoSi2 or ZrSi2 addition destroyed the mechanical properties of the ZrB2-based ceramics, especially high temperature mechanical properties. In order to improve the mechanical properties and high temperature oxidation/ablation resistance of ZrB2 ceramics, many researches have been done through incorporation of some Si-rich substances (e.g. SiC whiskers or particles) as anti-oxidizing inhibitors into ZrB2 matrix [16–20]. A number of studies have shown that the introduction of SiC has beneficial effects not only on sinterability, but also on mechanical properties and resistance to oxidation [3,21,22]. The SiC reinforcements such as whiskers [23], particles [24], or fiber [25] were used in the previous researches, but using SiC platelets (SiCpl) as reinforcements for ZrB2 ceramics was rarely reported. In the previous research, SiC platelets were successfully used as reinforcements for some ceramics including Alumina (Al2O3) [26,27] and Si3N4 [28], and obvious reinforcing effects were obtained. Therefore, it is expected that incorporating SiC platelets into ZrB2 ceramics would improve the mechanical properties and oxidation resistance of the ZrB2 ceramics. In the present study, ZrSi2 and SiCpl were added together to fabricate ZrB2-based composites by hot-press sintering. The purpose is to reduce the addition of ZrSi2 and optimize the addition of SiCpl. Densification behavior, microstructure and mechanical properties were investigated and discussed. 2. Experiment procedure Commercially available ZrB2 powder (Grade B, Sanxing Corporation, Henan) was used in this study, which has a purity of >97% and a mean particle size of 2 lm. The SiC platelet (Dongfang Corporation,
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M. Wang et al. / Materials and Design 34 (2012) 293–297
Table 1 The designed compositions of the ZrB2-based ceramics. Sample
ZZ0 ZZ3 ZZ5 ZZ10 ZZS5 ZZS10 ZZS15
Composition (vol.%)
Sintering temperature (°C)
ZrB2
ZrSi2
SiCpl
100 97 95 90 90 85 80
0 3 5 10 5 5 5
0 0 0 0 5 10 15
1800 1800 1800 1800 1700, 1800, 1900 1800 1800
Fig. 2. Fractograph of Sample ZZ10 in which the elliptical marked area shows the collective glassy phase.
Fig. 1. Experiment device for toughness test.
Ningxia) has a reported purity of >99% with 5 lm thickness and 20 lm edge length. The ZrSi2 powder (Jiayi Corporation, Shanghai) has a purity of >99% and a mean particle size of 2 lm. The designed compositions of ZrB2-based ceramics in the present work are shown in Table 1. The powder mixtures were ball milled by planetary mill for 4 h using agate balls and alcohol as milling media in nylon vessels, subsequently dried in a rotary evaporator and sieved with 60-mesh screen. The blended powder was put into a £ 50 mm graphite die with BN coating on the surface of the die wall. Hot-press sintering was conducted at designed temperatures in flowing argon atmosphere with 25 MPa pressure. The heating rate was 30 °C/min before 1200 °C and then 10 °C/min to the final temperature. 15 MPa pressure was carried on at 1500 °C and then gradually increased to 25 MPa at the final sintering temperature. The holding time at the final sintering temperature and pressure was 1 h. The sintered billets were cut by electrospark wire-electrode cutting. Specimen of 3 mm 4 mm 36 mm were polished for flexural strength test and 4 mm 6 mm 30 mm for toughness test. Bending strength was measured by a three-point bending test with a span of 30 mm and a crosshead speed of 0.5 mm/min. Fracture toughness was measured by the single-edged notch beam (SENB) method with a span of 24 mm and a crosshead speed of 0.05 mm/min, the width of the notch being <0.25 mm. Specific experiment device was shown in Fig. 1. The calculation formula was shown as Eq. (1).
K IC
3FL pffiffiffi c ¼Y 2 2bh
when the test was three point bending and L/h = 4.The density of the sintered bodies was measured by the Archimedes method, while the theoretical density was estimated with the rule of mixture. The experimental data were taken as the average of five measurements. The phase composition was determined by X-ray diffraction (Shimadzu XRD-7000, Japan). The microstructure was observed by scanning electron microscopy (Shimadzu SSX-550 super scan, Japan). 3. Results and discussion 3.1. Effect of ZrSi2 addition on hot-press sintered ZrB2 ceramics Table 2 shows the relative density and mechanical properties of ZrB2 ceramics sintered at 1800 °C with different ZrSi2 addition. It could be seen that the relative density increased with increase of the addition of ZrSi2. The ceramic was nearly full densification when 10 vol.% ZrSi2 was added. The addition of ZrSi2 was beneficial for improving the ceramic density. The main reason of high density was liquid phase ZrSi2 formation during sintering because the melting point of ZrSi2 is 1790 °C. The liquid phase promoted the densification significantly. The mechanical properties were also improved with the increase of ZrSi2 content from 0 to 5 vol.%. With further increasing in ZrSi2 content, the flexural strength and fracture toughness decrease remarkably. It was partially attributed to excessive formation of glassy phase which get together and would weaken combination among grains. Fig. 2 is the fractograph of Sample ZZ10 in which collective glassy phase could be observed clearly (the elliptical marked area in the figure). Through the above analysis it was obtained that 5 vol.% ZrSi2 addition was optimum for hot-press sintering. 3.2. Effect of sintering temperature on ZrB2–SiCpl–ZrSi2 ceramic composites
ð1Þ
2 3 in which Y is shape factor, and Y ¼ A0 þ A1 hc þ A2 hc þ A3 hc þ c 4 A4 h . The values of A are +1.93, 3.07, +14.53, 25.11, +25.80
The ZrB2-based ceramic composites with 5 vol.% ZrSi2 and 5 vol.% SiCpl additions were sintered at different temperatures (1700 °C, 1800 °C and 1900 °C) to confirm a suitable sintering point
Table 2 Mechanical properties of ZrB2 ceramics with different ZrSi2 addition. Sample
Amount of ZrSi2 addition (vol.%)
Relative density (%)
Fracture toughness (MPa m1/2)
Flexural strength (MPa)
ZZ0 ZZ3 ZZ5 ZZ10
0 3 5 10
92.7 98.2 99.0 99.6
5.81 ± 0.22 6.43 ± 0.17 6.72 ± 0.08 2.43 ± 0.16
514 ± 33 624 ± 19 580 ± 23 163 ± 40
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M. Wang et al. / Materials and Design 34 (2012) 293–297 Table 3 Mechanical properties of ZrB2–SiCpl–ZrSi2 composite ceramics sintered at different temperature. Sample
Initial composition (vol.%)
Sintering temperature (°C)
Relative density (%)
Fracture toughness (MPa m1/2)
Flexural strength (MPa)
ZZS5–1700 ZZS5–1800 ZZS5–1900
ZrB2 + 5ZrSi2 + 5SiCpl ZrB2 + 5ZrSi2 + 5SiCpl ZrB2 + 5ZrSi2 + 5SiCpl
1700 1800 1900
98.4 98.7 98.7
7.89 ± 0.20 8.98 ± 0.30 4.90 ± 0.10
616 ± 47 597 ± 23 205 ± 19
Table 4 Mechanical properties of ZrB2–SiCpl–ZrSi2 composite ceramics with different SiC platelet content. Sample
Initial composition (vol.%)
Sintering temperature (°C)
R.D (%)
Fracture toughness (MPa m1/2)
Flexural strength (MPa)
Vicke’s hardness (GPa)
ZZS0 ZZS5 ZZS10 ZZS15
ZrB2 + 5ZrSi2 ZrB2 + 5ZrSi2 + 5SiCpl ZrB2 + 5ZrSi2 + 10SiCpl ZrB2 + 5ZrSi2 + 15SiCpl
1800 1800 1800 1800
99.0 98.7 98.3 98.2
6.72 ± 0.08 8.98 ± 0.30 8.22 ± 0.04 8.31 ± 0.21
580 ± 23 597 ± 23 584 ± 24 490 ± 17
16.9 ± 0.1 17.2 ± 0.5 16.9 ± 0.3 17.8 ± 0.4
Table 4 shows the mechanical properties of the ceramic composites with SiCpl and ZrSi2 additions. The relative density of the specimens was all above 98.2%. It indicated that the ceramic composites had achieved excellent densification. Meanwhile the relative density decreased with the SiCpl content increasing. It was probably due to that the large size SiC platelets with twodimensional structure hinder densification process of the ceramic composites. The increasing SiCpl content induced the difficulty in densification of the ceramic composites. The hardness showed little variance with the changing SiCpl content. The flexural strength of the ceramic composite was around 600 MPa and decreased to 490 MPa when SiCpl content was 15 vol.%. Excessive SiCpl addition decreased the relative density of the ceramic composites, and subsequently decreased the flexural strength. For Sample ZZS5, the fracture toughness achieved a maximum of 8.98 MPa m1/2 and
meanwhile the flexural strength was 597 MPa. The toughness was 32% higher than the previous work of our group [13]. The microstructure of Sample ZZS5 was observed by SEM using a fracture surface which is shown in Fig. 3. The SiC platelets were distributed homogeneously in the whole matrix. The grains were fine, compact and homogeneous. The average grain size was 3–4 lm which indicated that there was no abnormal grain growth during the sintering process. Fig. 4 shows the X-ray diffraction patterns of the ZrB2–SiCpl–ZrSi2 composites ceramics. It was observed that the main phase was ZrB2 and a small quantity of ZrC was formed when both ZrSi2 and SiCpl were added. The EDX analysis also demonstrated the existence of ZrC in the ceramic composites. It could be explained in the term of chemical bonding energy. C needs less energy to form compound with Zr compared with Si which was in the same main group. ZrC is more stable and formed more easily than ZrSi2 under the same condition. During the hot-press sintering at the temperature of 1800 °C and the pressure of 25 MPa, Si atom in ZrSi2 which was near SiC platelet was replaced by C and then ZrC formed. The replaced Si atoms moved to the interface of grains and formed amorphous phase Si. There was no Si founded in the composite and the probably reason was its volatilization at so high temperature with so long time (1800 °C and 1 h). Further investigation was carried out on the microstructure to explain the high toughness of ZZS5. Fig. 5 shows the fracture surface of specimen. The SiC platelets were distributed homogeneously in the ZrB2 matrix. It can be seen that both pulled-out SiC platelets and holes formed after SiC platelet pulled out during
Fig. 3. Fractograph of Sample ZZS5.
Fig. 4. X-ray diffraction patterns of the ZrB2–SiCpl–ZrSi2 ceramic composites.
for this material system. The relative density, flexural strength and fracture toughness were tested and shown in Table 3. It could be seen that the sintering temperature of 1800 °C or higher is suitable for the ZrB2 + 5ZrSi2 + 5SiCp ceramic composites. Furthermore, the mechanical properties of ZZS5-1800 were better than that of ZZS51900. The decrease of mechanical properties of ZZS5-1900 may the result of over burning. Then 1800 °C was selected as the sintering temperature from the three temperatures. It can be seen that ZrSi2 could bring down the hot press sintering temperature and enhance the densification compared with reported work [29,30]. 3.3. Effect of SiCpl content on ZrB2–SiCpl–ZrSi2 ceramic composites
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Fig. 5. A typical SEM photograph of cross section of the ZrB2 ceramic composites.
crack extension on the fracture surface. It indicates that more fracture energy would be released through SiC platelet pulling out during the fracture process. Therefore, SiC platelet toughening is a key toughening mechanism which is beneficial for improving the toughness of the ZrB2 ceramic composites. Fig. 6 shows the fracture surface photographs of ZrB2 ceramic without and with ZrSi2 addition. It could be seen from Fig. 6a that the fracture surface was nearly flat, therefore, transgranular
fracture is a dominating fracture mode. In addition, some holes also could be found on the fracture surface which shows the poor densification of pure ZrB2 material without ZrSi2 as sintering additive. Fig. 6b shows the cross section of the ZrB2 ceramics with ZiSi2 addition. We can obviously see that the fracture surface was very rugged which clearly indicates crack deflection toughening mechanism. It means that cracks pass more paths during the crack propagation which naturally consumes more energy and subsequently improves the toughness of the ceramic composites. Meanwhile the fracture mode turned to mixture mode of intergranular fracture and transgranular fracture from single transgranular fracture. In addition, it could be also seen that the microstructure of the ZrB2/ZrSi2 composite ceramic was more compact compared with Fig. 6a. It indicates that ZrSi2 addition promotes the sintering densification of ZrB2 ceramic. The density data in Table 2 also confirmed this result. The work of Yang et al. [31] and Li et al. [32] on ZrB2-based material system also indicates that crack deflection could improve the toughness of ceramic composites. In summary, the main toughening mechanisms in the ZrB2/ SiCpl/ZrSi2 ceramic composites can be considered as SiC platelet toughening and crack deflection. This is why the fracture toughness of the ceramic composites was improved and could achieved as high as nearly 9 MPa m1/2. 4. Conclusions Near full dense ZrB2 matrix composites with SiC platelet and ZrSi2 additive were fabricated by hot-press sintering. The content of ZrSi2 and SiCpl was optimized. A small quantity of ZrC was formed during sintering when SiCpl and ZrSi2 both added. The grains of the ZrB2–SiCpl–ZrSi2 ceramic composites were fine, compact and homogeneous. The mechanical properties of the ceramic composites were improved. The flexural strength was around 600 MPa and the maximum fracture toughness was as high as 8.98 MPa m1/2. The main toughening mechanisms in the ZrB2/ SiCpl/ZrSi2 ceramic composites were SiC platelet toughening and crack deflection. Acknowledgment This work was supported by the 973 program from Ministry of Science and Technology of China (Grant No. 5133102-4). References
Fig. 6. Fracture surface photographs of the ZrB2 ceramic composites without (a) and with ZrSi2 addition (b).
[1] Morz C. Zirconium diboride. Am Ceram Soc Bull 1994;6:141–2. [2] Fahrenholtz WG, Hilmas GE. Refractory diborides of zirconium and hafnium. J Am Ceram Soc 2007;5:1347–64. [3] Pastor H. Metallic borides: preparation of solid bodies—sintering methods and properties of solid bodies. In: Matkovich VI, editor. Boron and Refractory Borides. New York: Springer-Verlag; 1977. p. 454–93. [4] Monteverde F, Bellosi A. Effect of the addition of silicon nitride on sintering behavior and microstructure of zirconium diboride. Scripta Mater 2002;3:223–8. [5] Weng L, Zhang XH, Han JC, Han WB. The effect of Si3N4 on microstructure, mechanical properties and oxidation resistance of HfB2-based composite. J Compos Mater 2009;2:113–23. [6] Monteverde F, Bellosi A. Beneficial effects of AlN as sintering aid on microstructure and mechanical properties of hot-pressed ZrB2. Adv Eng Mater 2003;5:509–12. [7] Guo SQ, Nishimura Toshiyuki, Mizuguchi Takashi, Kagawa Yutaka. Mechanical properties of hot-pressed ZrB2–MoSi2–SiC composites. J Euro Ceram Soc 2008;28:1891–8. [8] Guo SQ, Kagawa Yutaka, Nishimura Toshiyuki. Mechanical behavior of twostep hot-pressed ZrB2-basedcomposites with ZrSi2. J Euro Ceram Soc 2009;29:787–94. [9] Meng SH, Chen HB, Hu JH, Wang ZW. Radiative properties characterization of ZrB2–SiC-based ultrahigh temperature ceramic at high temperature. Mater Des 2011;32:377–81. [10] Zhu SM, Fahrenholtz WG, Hilmas GE, Zhang SC. Microwave sintering of a ZrB2– B4C particulate ceramic composite. Composites Part A 2008;39:449–53.
M. Wang et al. / Materials and Design 34 (2012) 293–297 [11] Sun X, Han WB, Liu Q, Hu P, Hong CQ. ZrB2-ceramic toughened by refractory metal Nb prepared by hot-pressing. Mater Des 2010;31:4427–31. [12] Monteverde F, Bellosi A. Development and characterization of metal-diboridebased composites toughened with ultra-fine SiC particulates. Solid State Sci 2005;7:622–30. [13] Wang HL, Wang CA, Yao XF, Fang DN. Processing and mechanical properties of zirconium diboride-based ceramics prepared by spark plasma sintering. J Am Ceram Soc 2007;7:1992–7. [14] Sciti D, Guicciardi S, Bellosi A. Properties of a pressureless-sintered ZrB2–MoSi2 ceramic composite. J Am Ceram Soc 2006;7:2320–2. [15] Guo SQ, Kagawa Yutaka, Nishimura Toshiyuki, Tanaka Hidehiko. Pressureless sintering and physical properties of ZrB2-based composites with ZrSi2 additive. Scripta Mater 2008;58:579–82. [16] Monteverde F. The thermal stability in air of hot-pressed diboride matrix composites for uses at ultra-high temperatures. Corros Sci 2005;8:2020–33. [17] Opila E, Levine S, Lorincz J. Oxidation of ZrB2- and HfB2-based ultra-high temperature ceramics: effect of Ta additions. J Mater Sci 2004;19:5969–77. [18] Brach M, Sciti D, Balbo A, Bellosi A. Short-term oxidation of a ternary composite in the system AlN–SiC–ZrB2. J Euro Ceram Soc 2005;10:1771–80. [19] Levine SR, Opila EJ, Halbig MC, Kiser JD, Singh M, Salem JA. Evaluation of ultrahigh temperature ceramics for aeropropulsion use. J Euro Ceram Soc 2002; 14-15:2757–67. [20] Li JP, Meng SH, Han JC, Zhang XH. Study on explosive compaction process of ZrB2–SiCw ultra high temperature ceramics. Rare Met Mate Eng 2006(Suppl. 2):177–80. [21] Chamberlain AL, Fahrenholtz WG, Hilmas GE. High-strength zirconiuim diboride-based ceramics. J Am Ceram Soc 2004;6:1170–2.
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[22] Guo WM, Zhang GJ, Zou J, Kan YM, Wang PL. Effect of Yb2O3 addition on hotpressed ZrB2–SiC ceramics. Adv Eng Mater 2008;8:759–62. [23] Zhu T, Xu L, Zhang XH, Han WB, Hu P, Weng L. Densification, microstructure and mechanical properties of ZrB2–SiCw ceramic composites. J Euro Ceram Soc 2009;13:2893–901. [24] Peng F, Speyer RF. Oxidation resistance of fully dense ZrB2 with SiC, TaB2, and TaSi2 additives. J Am Ceram Soc 2008;5:1489–94. [25] Tang SF, Deng JY, Wang SJ, Liu WC. Fabrication and characterization of an ultra-high-temperature carbon fiber-reinforced ZrB2–SiC matrix composite. J Am Ceram Soc 2007;10:3320–2. [26] Kaya C, Kaya F. Processing, toughness improvement and microstructural analysis of SiC platelet-reinforced Al2O3/Y-TZP nano-ceramic matrix composites. Mater Sci Eng 1998;A247:75–80. [27] Kaya C, Kaya F. On the toughening mechanisms of SiC platelet-reinforced Al2O3/Y-TZP nano-ceramic matrix composites. Ceram Int 1999;25:359–66. [28] Choi BJ, Koh YH. Mechanical properties of Si3N4–SiC three-layer composite materials. J Am Ceram Soc 1998;81:2725–8. [29] Rezaie A, Fahrenholtz WG, Hilmas GE. Effect of hot pressing time and temperature on the microstructure and mechanical properties of ZrB2–SiC. J Mater Sci 2007;42:2735–44. [30] Hu P, Wang Z. Flexural strength and fracture behavior of ZrB2–SiC ultra-high temperature ceramic composites at 1800 °C. J Euro Ceram Soc 2010;30:1021–6. [31] Yang FY, Zhang XH, Han JC, Du SY. Processing and mechanical properties of short carbon fibers toughened zirconium diboride-based ceramics. Mater Des 2008;29:1817–20. [32] Li G, Han WB, Wang BL. Effect of BN grain size on microstructure and mechanical properties of the ZrB2–SiC–BN composites. Mater Des 2011;32:401–5.
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Effects of SiC platelet and ZrSi2 additive on sintering and mechanical properties of ZrB2-based ceramics by hot-pressing Mingfu Wang a,b, Chang-An Wang b,⇑, Xinghong Zhang a a b
Center for Composite Materials, Harbin Institute of Technology, Harbin 150001, PR China State Key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China
a r t i c l e
i n f o
Article history: Received 27 April 2011 Accepted 9 August 2011 Available online 16 August 2011
a b s t r a c t Fully dense Zirconium Diboride (ZrB2)-based ceramics with Zirconium Silicide (ZrSi2) and Silicon Carbide platelet (SiCpl) additive were fabricated by hot-press sintering in flowing argon atmosphere. Their mechanical properties and microstructure were examined. The contents of ZrSi2 and SiCpl were optimized. The defined addition of ZrSi2 was beneficial for densification and improving the mechanical properties of the ZrB2-based ceramics. The addition of SiCpl obviously improved the fracture toughness of the ZrB2-based ceramics. The range of fracture toughness of ZrB2–SiCpl–ZrSi2 composite was 6.7–9.0 MPa m1/2. The flexural strength of the composites varied a little and the maximum was 597 MPa when the content of ZrSi2 and SiCpl was both 5 vol.%. XRD results showed that a small amount of Zirconium Carbide (ZrC) formed during sintering. The microstructure of the composites was fine, compact and homogeneous. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Refractory transition metal borides, such as Zirconium Diboride (ZrB2), have been referred to as ultra high temperature ceramics (UHTCs) due to their extremely high melting point (>3050 °C). ZrB2 ceramics also have high thermal and electrical conductivities, chemical inertness against molten metals, and great thermal shock resistance [1,2]. However, there are two main bottleneck problems that limit the development of ZrB2 ceramics. On one hand, ZrB2 has poor intrinsic sinterability due to its strong covalent bonds and low bulk and grain boundary diffusivity. The densification of ZrB2 powder generally requires very high temperatures (>2100 °C) [3] and external pressure. On the other hand, ZrB2 ceramics have low mechanical properties and poor high-temperature oxidation/ablation resistance. So far as we know, most researches on ZrB2-based ceramics are focused on these two aspects. In order to lower sintering temperature of ZrB2 ceramics, some sintering additives, e.g. nitrides: Silicon nitride (Si3N4), Aluminum nitride (AlN); silicides: Molybdenum disilicide (MoSi2), ZrSi2 and carbides: SiC, Boron Carbide (B4C), have been added to enhance sintering of ZrB2-based ceramics [4–10]. Besides some pressureassistant processes (e.g. hot-pressing [11,12] and spark plasma sintering [13]), recently, the ZrB2-based ceramics with MoSi2 or ZrSi2 additive were sintered by pressureless at a temperature below 1850 °C [14,15]. In their studies, MoSi2 and ZrSi2 are proved as good sintering additives for ZrB2-based ceramics. However, in their studies, a large amount of MoSi2 or ZrSi2 up to 40 vol.% was usually ⇑ Corresponding author. Tel./fax: +86 10 62785488. E-mail address: [email protected] (C.-A. Wang). 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.08.016
used for good densification of ZrB2. Excess MoSi2 or ZrSi2 addition destroyed the mechanical properties of the ZrB2-based ceramics, especially high temperature mechanical properties. In order to improve the mechanical properties and high temperature oxidation/ablation resistance of ZrB2 ceramics, many researches have been done through incorporation of some Si-rich substances (e.g. SiC whiskers or particles) as anti-oxidizing inhibitors into ZrB2 matrix [16–20]. A number of studies have shown that the introduction of SiC has beneficial effects not only on sinterability, but also on mechanical properties and resistance to oxidation [3,21,22]. The SiC reinforcements such as whiskers [23], particles [24], or fiber [25] were used in the previous researches, but using SiC platelets (SiCpl) as reinforcements for ZrB2 ceramics was rarely reported. In the previous research, SiC platelets were successfully used as reinforcements for some ceramics including Alumina (Al2O3) [26,27] and Si3N4 [28], and obvious reinforcing effects were obtained. Therefore, it is expected that incorporating SiC platelets into ZrB2 ceramics would improve the mechanical properties and oxidation resistance of the ZrB2 ceramics. In the present study, ZrSi2 and SiCpl were added together to fabricate ZrB2-based composites by hot-press sintering. The purpose is to reduce the addition of ZrSi2 and optimize the addition of SiCpl. Densification behavior, microstructure and mechanical properties were investigated and discussed. 2. Experiment procedure Commercially available ZrB2 powder (Grade B, Sanxing Corporation, Henan) was used in this study, which has a purity of >97% and a mean particle size of 2 lm. The SiC platelet (Dongfang Corporation,
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M. Wang et al. / Materials and Design 34 (2012) 293–297
Table 1 The designed compositions of the ZrB2-based ceramics. Sample
ZZ0 ZZ3 ZZ5 ZZ10 ZZS5 ZZS10 ZZS15
Composition (vol.%)
Sintering temperature (°C)
ZrB2
ZrSi2
SiCpl
100 97 95 90 90 85 80
0 3 5 10 5 5 5
0 0 0 0 5 10 15
1800 1800 1800 1800 1700, 1800, 1900 1800 1800
Fig. 2. Fractograph of Sample ZZ10 in which the elliptical marked area shows the collective glassy phase.
Fig. 1. Experiment device for toughness test.
Ningxia) has a reported purity of >99% with 5 lm thickness and 20 lm edge length. The ZrSi2 powder (Jiayi Corporation, Shanghai) has a purity of >99% and a mean particle size of 2 lm. The designed compositions of ZrB2-based ceramics in the present work are shown in Table 1. The powder mixtures were ball milled by planetary mill for 4 h using agate balls and alcohol as milling media in nylon vessels, subsequently dried in a rotary evaporator and sieved with 60-mesh screen. The blended powder was put into a £ 50 mm graphite die with BN coating on the surface of the die wall. Hot-press sintering was conducted at designed temperatures in flowing argon atmosphere with 25 MPa pressure. The heating rate was 30 °C/min before 1200 °C and then 10 °C/min to the final temperature. 15 MPa pressure was carried on at 1500 °C and then gradually increased to 25 MPa at the final sintering temperature. The holding time at the final sintering temperature and pressure was 1 h. The sintered billets were cut by electrospark wire-electrode cutting. Specimen of 3 mm 4 mm 36 mm were polished for flexural strength test and 4 mm 6 mm 30 mm for toughness test. Bending strength was measured by a three-point bending test with a span of 30 mm and a crosshead speed of 0.5 mm/min. Fracture toughness was measured by the single-edged notch beam (SENB) method with a span of 24 mm and a crosshead speed of 0.05 mm/min, the width of the notch being <0.25 mm. Specific experiment device was shown in Fig. 1. The calculation formula was shown as Eq. (1).
K IC
3FL pffiffiffi c ¼Y 2 2bh
when the test was three point bending and L/h = 4.The density of the sintered bodies was measured by the Archimedes method, while the theoretical density was estimated with the rule of mixture. The experimental data were taken as the average of five measurements. The phase composition was determined by X-ray diffraction (Shimadzu XRD-7000, Japan). The microstructure was observed by scanning electron microscopy (Shimadzu SSX-550 super scan, Japan). 3. Results and discussion 3.1. Effect of ZrSi2 addition on hot-press sintered ZrB2 ceramics Table 2 shows the relative density and mechanical properties of ZrB2 ceramics sintered at 1800 °C with different ZrSi2 addition. It could be seen that the relative density increased with increase of the addition of ZrSi2. The ceramic was nearly full densification when 10 vol.% ZrSi2 was added. The addition of ZrSi2 was beneficial for improving the ceramic density. The main reason of high density was liquid phase ZrSi2 formation during sintering because the melting point of ZrSi2 is 1790 °C. The liquid phase promoted the densification significantly. The mechanical properties were also improved with the increase of ZrSi2 content from 0 to 5 vol.%. With further increasing in ZrSi2 content, the flexural strength and fracture toughness decrease remarkably. It was partially attributed to excessive formation of glassy phase which get together and would weaken combination among grains. Fig. 2 is the fractograph of Sample ZZ10 in which collective glassy phase could be observed clearly (the elliptical marked area in the figure). Through the above analysis it was obtained that 5 vol.% ZrSi2 addition was optimum for hot-press sintering. 3.2. Effect of sintering temperature on ZrB2–SiCpl–ZrSi2 ceramic composites
ð1Þ
2 3 in which Y is shape factor, and Y ¼ A0 þ A1 hc þ A2 hc þ A3 hc þ c 4 A4 h . The values of A are +1.93, 3.07, +14.53, 25.11, +25.80
The ZrB2-based ceramic composites with 5 vol.% ZrSi2 and 5 vol.% SiCpl additions were sintered at different temperatures (1700 °C, 1800 °C and 1900 °C) to confirm a suitable sintering point
Table 2 Mechanical properties of ZrB2 ceramics with different ZrSi2 addition. Sample
Amount of ZrSi2 addition (vol.%)
Relative density (%)
Fracture toughness (MPa m1/2)
Flexural strength (MPa)
ZZ0 ZZ3 ZZ5 ZZ10
0 3 5 10
92.7 98.2 99.0 99.6
5.81 ± 0.22 6.43 ± 0.17 6.72 ± 0.08 2.43 ± 0.16
514 ± 33 624 ± 19 580 ± 23 163 ± 40
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M. Wang et al. / Materials and Design 34 (2012) 293–297 Table 3 Mechanical properties of ZrB2–SiCpl–ZrSi2 composite ceramics sintered at different temperature. Sample
Initial composition (vol.%)
Sintering temperature (°C)
Relative density (%)
Fracture toughness (MPa m1/2)
Flexural strength (MPa)
ZZS5–1700 ZZS5–1800 ZZS5–1900
ZrB2 + 5ZrSi2 + 5SiCpl ZrB2 + 5ZrSi2 + 5SiCpl ZrB2 + 5ZrSi2 + 5SiCpl
1700 1800 1900
98.4 98.7 98.7
7.89 ± 0.20 8.98 ± 0.30 4.90 ± 0.10
616 ± 47 597 ± 23 205 ± 19
Table 4 Mechanical properties of ZrB2–SiCpl–ZrSi2 composite ceramics with different SiC platelet content. Sample
Initial composition (vol.%)
Sintering temperature (°C)
R.D (%)
Fracture toughness (MPa m1/2)
Flexural strength (MPa)
Vicke’s hardness (GPa)
ZZS0 ZZS5 ZZS10 ZZS15
ZrB2 + 5ZrSi2 ZrB2 + 5ZrSi2 + 5SiCpl ZrB2 + 5ZrSi2 + 10SiCpl ZrB2 + 5ZrSi2 + 15SiCpl
1800 1800 1800 1800
99.0 98.7 98.3 98.2
6.72 ± 0.08 8.98 ± 0.30 8.22 ± 0.04 8.31 ± 0.21
580 ± 23 597 ± 23 584 ± 24 490 ± 17
16.9 ± 0.1 17.2 ± 0.5 16.9 ± 0.3 17.8 ± 0.4
Table 4 shows the mechanical properties of the ceramic composites with SiCpl and ZrSi2 additions. The relative density of the specimens was all above 98.2%. It indicated that the ceramic composites had achieved excellent densification. Meanwhile the relative density decreased with the SiCpl content increasing. It was probably due to that the large size SiC platelets with twodimensional structure hinder densification process of the ceramic composites. The increasing SiCpl content induced the difficulty in densification of the ceramic composites. The hardness showed little variance with the changing SiCpl content. The flexural strength of the ceramic composite was around 600 MPa and decreased to 490 MPa when SiCpl content was 15 vol.%. Excessive SiCpl addition decreased the relative density of the ceramic composites, and subsequently decreased the flexural strength. For Sample ZZS5, the fracture toughness achieved a maximum of 8.98 MPa m1/2 and
meanwhile the flexural strength was 597 MPa. The toughness was 32% higher than the previous work of our group [13]. The microstructure of Sample ZZS5 was observed by SEM using a fracture surface which is shown in Fig. 3. The SiC platelets were distributed homogeneously in the whole matrix. The grains were fine, compact and homogeneous. The average grain size was 3–4 lm which indicated that there was no abnormal grain growth during the sintering process. Fig. 4 shows the X-ray diffraction patterns of the ZrB2–SiCpl–ZrSi2 composites ceramics. It was observed that the main phase was ZrB2 and a small quantity of ZrC was formed when both ZrSi2 and SiCpl were added. The EDX analysis also demonstrated the existence of ZrC in the ceramic composites. It could be explained in the term of chemical bonding energy. C needs less energy to form compound with Zr compared with Si which was in the same main group. ZrC is more stable and formed more easily than ZrSi2 under the same condition. During the hot-press sintering at the temperature of 1800 °C and the pressure of 25 MPa, Si atom in ZrSi2 which was near SiC platelet was replaced by C and then ZrC formed. The replaced Si atoms moved to the interface of grains and formed amorphous phase Si. There was no Si founded in the composite and the probably reason was its volatilization at so high temperature with so long time (1800 °C and 1 h). Further investigation was carried out on the microstructure to explain the high toughness of ZZS5. Fig. 5 shows the fracture surface of specimen. The SiC platelets were distributed homogeneously in the ZrB2 matrix. It can be seen that both pulled-out SiC platelets and holes formed after SiC platelet pulled out during
Fig. 3. Fractograph of Sample ZZS5.
Fig. 4. X-ray diffraction patterns of the ZrB2–SiCpl–ZrSi2 ceramic composites.
for this material system. The relative density, flexural strength and fracture toughness were tested and shown in Table 3. It could be seen that the sintering temperature of 1800 °C or higher is suitable for the ZrB2 + 5ZrSi2 + 5SiCp ceramic composites. Furthermore, the mechanical properties of ZZS5-1800 were better than that of ZZS51900. The decrease of mechanical properties of ZZS5-1900 may the result of over burning. Then 1800 °C was selected as the sintering temperature from the three temperatures. It can be seen that ZrSi2 could bring down the hot press sintering temperature and enhance the densification compared with reported work [29,30]. 3.3. Effect of SiCpl content on ZrB2–SiCpl–ZrSi2 ceramic composites
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Fig. 5. A typical SEM photograph of cross section of the ZrB2 ceramic composites.
crack extension on the fracture surface. It indicates that more fracture energy would be released through SiC platelet pulling out during the fracture process. Therefore, SiC platelet toughening is a key toughening mechanism which is beneficial for improving the toughness of the ZrB2 ceramic composites. Fig. 6 shows the fracture surface photographs of ZrB2 ceramic without and with ZrSi2 addition. It could be seen from Fig. 6a that the fracture surface was nearly flat, therefore, transgranular
fracture is a dominating fracture mode. In addition, some holes also could be found on the fracture surface which shows the poor densification of pure ZrB2 material without ZrSi2 as sintering additive. Fig. 6b shows the cross section of the ZrB2 ceramics with ZiSi2 addition. We can obviously see that the fracture surface was very rugged which clearly indicates crack deflection toughening mechanism. It means that cracks pass more paths during the crack propagation which naturally consumes more energy and subsequently improves the toughness of the ceramic composites. Meanwhile the fracture mode turned to mixture mode of intergranular fracture and transgranular fracture from single transgranular fracture. In addition, it could be also seen that the microstructure of the ZrB2/ZrSi2 composite ceramic was more compact compared with Fig. 6a. It indicates that ZrSi2 addition promotes the sintering densification of ZrB2 ceramic. The density data in Table 2 also confirmed this result. The work of Yang et al. [31] and Li et al. [32] on ZrB2-based material system also indicates that crack deflection could improve the toughness of ceramic composites. In summary, the main toughening mechanisms in the ZrB2/ SiCpl/ZrSi2 ceramic composites can be considered as SiC platelet toughening and crack deflection. This is why the fracture toughness of the ceramic composites was improved and could achieved as high as nearly 9 MPa m1/2. 4. Conclusions Near full dense ZrB2 matrix composites with SiC platelet and ZrSi2 additive were fabricated by hot-press sintering. The content of ZrSi2 and SiCpl was optimized. A small quantity of ZrC was formed during sintering when SiCpl and ZrSi2 both added. The grains of the ZrB2–SiCpl–ZrSi2 ceramic composites were fine, compact and homogeneous. The mechanical properties of the ceramic composites were improved. The flexural strength was around 600 MPa and the maximum fracture toughness was as high as 8.98 MPa m1/2. The main toughening mechanisms in the ZrB2/ SiCpl/ZrSi2 ceramic composites were SiC platelet toughening and crack deflection. Acknowledgment This work was supported by the 973 program from Ministry of Science and Technology of China (Grant No. 5133102-4). References
Fig. 6. Fracture surface photographs of the ZrB2 ceramic composites without (a) and with ZrSi2 addition (b).
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