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Effect of HfC and SiC on microstructure and mechanical properties of HfB2-based ceramics
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CERAMICS INTERNATIONAL
Ceramics International 42 (2016) 7861–7867 www.elsevier.com/locate/ceramint
Short communication
Effect of HfC and SiC on microstructure and mechanical properties of HfB2-based ceramics Ye Yuana,b, Ji-Xuan Liua,n, Guo-Jun Zhanga,n a
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Shanghai 200050, China b University of the Chinese Academy of Sciences, Beijing 100049, China Received 19 December 2015; received in revised form 30 December 2015; accepted 7 January 2016 Available online 15 January 2016
Abstract SiC and HfC were used to tailor microstructures and improve properties of hot pressed HfB2-based ceramics. SiC and HfC particle, as the second phase, could hinder grain growth of HfB2. Compared with HfC, SiC grains grew slower, which was more effective to hinder grain growth of HfB2. The ternary ceramics HfB2–20vol%HfC–20vol%SiC had the finest microstructure, indicating the effect of SiC and HfC on grain growth. Several SiC clusters were found on related fracture surfaces. Large SiC clusters can cause high levels of residual stress and stress concentration, which are harmful to mechanical properties of boride-SiC ceramics. The predominant fracture mode of HfB2–20vol%SiC was transgranular, however, in HfB2–10vol%HfC–10vol%SiC and HfB2–20vol%HfC–20vol%SiC ceramics, the dominating fracture mode shift from transgranular to intergranular. With the incorporation of HfC, the fracture toughness of HH10S10 and HH20S20 increased to 4.34 MPa m1/2 and 5.05 MPa m1/2, which were 17.3% and 26.7% higher than HS20 (3.72 MPa m1/2), respectively. The flexural strengths of HfB2–HfC–SiC ceramics were improved to 750 MPa, which were about 1.3 times higher than that of HfB2–20SiC. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Hot pressing; B. Composites; C. Mechanical properties; E. Structural applications
1. Introduction HfB2-based ceramics have attracted lots of attentions during recent years [1–4]. Owing to a unique combination of excellent properties, including high melting point, high hardness, high electrical and thermal conductivity, and good corrosion resistance, HfB2-based ceramics are considered to be one important member of the ultra-high temperature ceramics (UHTCs) family, which has optimistic prospect to be used as suitable candidates for high temperature structural applications, such as sharp noses and leading edges for re-entry and hypersonic vehicles, cutting tools and molten metal crucibles [4]. As a member of the ultra-high temperature ceramics (UHTCs) family, HfC has a higher melting temperature, but lower thermal conductivity than HfB2 [1]. The addition of MC (M ¼ Hf, Zr) to MB2-SiC to form a ternary composite of n
Corresponding authors. E-mail addresses: [email protected] (J.-X. Liu), [email protected] (G.-J. Zhang). http://dx.doi.org/10.1016/j.ceramint.2016.01.067 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
MB2–MC–SiC can adjust the microstructure and properties of MB2–SiC composite. NASA reported that MB2–MC–SiC ceramics have superior resistance to ablation under an arc-jet environment [5], which is beneficial for the application of UHTCs. Recent years, a variety of research has been carried out on the ZrB2–SiC–ZrC system, including the densification behavior, microstructures evolution, as well as the fabrication processing and properties. Medri et al. [6] studied the densification and microstructures of ZrB2–SiC–ZrC ceramics fabricated by hot pressing and spark plasma sintering. Wu et al. [7] studied the in situ synthesis of ZrB2–SiC–ZrC by reactive hot pressing using Zr metal with B4C and silicon as starting materials. Guo et al. [8] mainly studied the properties of spark plasma sintered ZrB2–SiC–ZrC ceramics, including mechanical, thermal, and electrical properties. Liu et al. [9] investaged the roles of SiC and ZrC additions and their synergistic effects on the microstructures and properties of ZrB2-based ceramics. But studies on HfB2–HfC–SiC system
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are very limited. Monteverde [10] studied the in situ synthesis of HfB2–SiC–HfC by reactive hot pressing using Hf metal with B4C and silicon as starting materials, obtaining nearly full dense ceramics at 1900 1C. Ni et al. [11] studied the densification and microstructures of the hot pressed HfB2– SiC and HfB2–SiC–HfC ceramics. And analysis indicated that the incorporation of HfC promoted the densification and fined the microstructure of HfB2–SiC ceramics. In the multiphase ceramics, the second phase plays an important role on the microstructures. Researchers have reported that the secondphase particles could inhibit the grain growth of the matrix during sintering in several ceramic systems, such as HfB2–SiC [12], ZrB2–MoSi2 [13] and Al2O3–ZrO2 [14]. However, few papers reported the different roles of SiC and HfC additions and their effects on the microstructure and mechanical properties of the HfB2-based ceramics. In the present paper, HfB2-based ceramics with different amounts of SiC and HfC were densified by hot pressing sintering. The influence of each phase on microstructures and mechanical properties was investigated. Grain sizes of each phase were characterized and discussed.
2. Experimental procedure 2.1. Powder processing and sintering The raw materials were HfB2 powders (purity4 99%, O: 0.15%, C: 0.04%, mean particle size is 1 μm), HfC powders (purity498%, free C: 0.47%, O: 0.72%, mean particle size is 225 nm), and commercial ɑ-SiC powders (purity498.5%, O: 1.04%, mean particle size is 0.45 μm, Changle Xinyuan Carborundum Micropowder Co. Ltd., Changle, China). HfB2 powders were synthesized from HfO2 and B4C [15], and HfC powders were synthesized from HfO2 and carbon black [16]. In this work, we prepared four HfB2-based composites including HfB220vol%SiC, HfB2–10vol%HfC–10vol%SiC, HfB2–20vol%HfC and HfB2–20vol%HfC–20vol%SiC, marked as HS20, HH10S10, HH20 and HH20S20. The starting powders were mixed in ethanol by rolling ball mill at speed of 30 rpm for 24 h, using Si3N4 balls as medium. Subsequently, the slurries were dried in rotary evaporator at 70 1C. The dried powders were sieved through 200 mesh screen. All composites were subsequently hot pressed in a graphite die with a rectangle cavity of 30 37 mm at 2000 1C for 1 h under a pressure of 30 MPa in argon. Before applying pressure, the powder compacts in the die were heated up to 1650 1C with a ramp rate of 10 1C/min and a dwelling period of 0.5 h in a vacuum environment of 20 Pa pressure, for the purpose of purging oxide impurities. After the hold at 1650 1C, the furnace atmosphere was changed to flowing argon purity 99.99%) at pressure of 105 Pa. The powder compact was continually heated to 2000 1C at 10 1C/min under uniaxial pressure of 30 MPa. After dwelling at 2000 1C for 1 h, the applied pressure was removed.
2.2. Sample characterization After removing the outer layer by grinding, the bulk densities of the as-sintered specimens were measured by the Archimedes method using deionized water as the liquid medium. Flexural strengths were measured by a four point bending test using test bars of size of 3 4 36 mm. One tensile surface of each specimen was polished and four edges were chamfered. All of the bending strength measurement was carried out using an Instron load frame at a cross-head displacement speed of 0.2 mm/min, in which the inner span was 10 mm and outer span was 30 mm. The data of average strength and standard deviation for each material were obtained based on the determination of five samples. Vickers hardness (HV) and fracture toughness (KIC) were determined using Vickers indentation (2100B Hardness Tester, Instron, Norwood, MA, USA) under load of 5 kg and dwell time of 10 s. All values of HV and KIC were the average of five measurements. The fracture toughness was calculated by the following equation [12,17]: 3 C1 þ C2 2 K IC ¼ P π ð tan βÞ 1 ð1Þ 4 Where P is the indentation load (N), C1 and C2 the measured diagonal crack lengths (m), and β an angle constant (681). Microstructures and chemical composition were analyzed using scanning electron microscopy (TM3000, Hitachi, Japan) along with energy-dispersive spectroscopy (EDS, Oxford INCA energy).
3. Results and discussion 3.1. Microstructures The relative densities of all HfB2-based ceramics are shown in Table 1. As is known to all, SiC is beneficial for the densification behavior of HfB2-based ceramics [1]. From the relative densities of samples, we could conclude that the existence of HfC can also accelerate the densification of HfB2-based ceramics. The average grain sizes of each phase were determined by the image software analysis from the SEM micrographs (shown in Table 1). HfB2 grain size in HH20 (2.97 μm) was larger than the one in HS20 (2.29 μm), which indicated that the grain growth of HfB2 was controlled by the grain size of the other phases (1.89 μm for HfC in HH20 and 1.33 μm for SiC in HS20). Based on the principle of Zener [18], the limited grain size of major phase (D) can be defined as d ð2Þ Dp f Where d and f represent the average diameter and the volume fraction of the added second phase particles. Compared with the grain size of HfB2 in HH10S10, the one in HH20S20 is smaller (2.57 μm compared with 2.01 μm). When the HfB2 content was fixed at 80 vol%, the HfB2 grain size increased
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Table 1 Summary of density and grain sizes of each phase in HfB2-based ceramics. Specimen Relative density (%)
Grain size for HfB2 (μm)
Grain size for SiC (μm)
Grain size for HfC (μm)
HS20 HH20 HH10S10 HH20S20
2.297 0.56 2.977 1.01 2.577 0.81 2.017 0.68
1.337 0.45 – 1.227 0.41 1.217 0.43
– 1.8970.62 1.5370.41 1.5170.43
98.8 98.7 98.8 99.8
with the decreasing SiC content. In HS20, HH10S10 and HH20, the average grain sizes of HfB2 are 2.29 μm, 2.57 μm and 2.97 μm. Average grain sizes of HfB2-based ceramics are shown in Fig. 1. Comparing the average grain sizes of different samples, we could find that HfB2–20HfC–20SiC had the finest grain size. Similarly, SiC particle could also prevent the grain growth of HfC, which led to finer grain size of HfC in HH20S20 (1.51 μm) than that in HH20 (1.89 μm). On the other hand, SiC grain size decreased from 1.33 μm in HS20 to 1.21 μm in HH20S20, which demonstrated the inhibiting effect of HfC on the grain growth of SiC, however the effect was not obvious. Microstructures of the sintered HfB2-based ceramics are shown in Fig. 2. As inspected by SEM，all of the ceramic bulks did not reveal any apparent residual porosity, which congruently agreed with their final relative density. In HfB2based ceramics, the SiC and HfC particulates were homogeneously distributed within the HfB2 skeleton. Although the polished surfaces of HfB2-based ceramics showed uniform microstructure, several SiC clusters were found on related fracture surfaces (Fig. 3). CTE values are 6.3 10 6 K 1 for HfB2, 4.7 10 6 K 1 for SiC, and 6.6 10 6 K 1 for HfC. Large SiC clusters can cause high levels of residual stress and stress concentration, and be harmful to mechanical properties of boride-SiC ceramics. In addition, in HH10S10 and HH20S20 ceramics, the SiC and HfC particulates were predominantly located at the multiple HfB2 grain junctions, which were believed to inhibit the grain growth of HfB2 via grain boundary pinning. 3.2. Mechanical properties The Young's modulus, Vickers’ hardness, fracture toughness, and bending strength of HfB2-based ceramics are summarized in Table 2. According P to a rule of mixture [19], namely the arithmetic average QiEi, where Ei and Qi are the Young's modulus and volume fractions of phase constituting the composite, the calculated Young's modulus values of HfB2-based ceramics are 513 GPa, 516 GPa, 514.8 GPa and 499.6 GPa for HS20, HH20, HS10H10 and HS20H20, respectively. In the case of linear properties, such as Young's modulus, it is well known that the arithmetic average generally overestimates the property. Obviously the tested values are below the calculated values. For Young's modulus calculation, the following data of Ei was used: 530 GPa [20] for HfB2, 460 GPa [21] for HfC, and 448 GPa [22] for SiC.
Fig. 1. Average Grain Size of HfB2-bsded Ceramics.
The Vickers’ hardness Hv5 of the HS20 was 21.07 GPa, which is consistent with the reported values (19–21 GPa) for HfB2-20vol% SiC by Marschall et al. [23] and Gasch et al. [24] The hardness of HH20 was 20.17 GPa, which is lower than that of HS20. In my analysis, there should be two factors determining the hardness of HH20. Firstly, the intrinsic hardness of HfC (27 GPa) and SiC (26 GPa) is very close. Then, the grain sizes of HH20 are much larger than those in HS20. Generally, the finer grain size can increase the frequency with which dislocations encounter grain boundaries, thus heightening the amount of stress required for deformation. At a given indent load, less deformation zone and higher hardness could be expected in grain-refined material. Therefore, HS20 had a higher hardness compared with HH20. Similarly, grain-refined HH10S10 had a higher hardness than HH20. In regard to HH20S20, the microstructure played an important role in the value of hardness. Compared with the others, HH20S20 had the finest microstructure and highest hardness. The fracture toughness of HS20 ceramic was 3.72 MPa m1/2, which was close to the reported value (3.95 MPa m1/2) for hot pressed HfB2-20vol% SiC by Ni et al [11]. With the incorporation of HfC, the fracture toughness of HH10S10 and HH20S20 increased to 4.34 MPa m1/2 and 5.05 MPa m1/2, which were 17.3% and 26.7% higher than HS20. To elucidate the toughening mechanisms, the propagating paths of some Vickersindentation-induced cracks on polished surfaces of HfB2-based ceramics were observed by SEM (Fig. 4). As the predominant fracture mode of HS20 was transgranular (Fig. 5), crack deflection appeared neglectable in the crack propagation path. However, in HH10S10 and HH20S20 ceramics, the dominating fracture mode shift from transgranular to intergranular. In general, most of the impurity would locate at the grain boundaries, which led to a weak interface. In this case, the propagating crack would deflect and split more easily along the grain boundaries. As shown in Fig. 4, the indentation crack paths in HH10S10 and HH20S20 appeared more tortuous than those in HS20 ceramics. More crack deflection and crack
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Fig. 2. SEM images of the polished (a)–(d) and etched (e)–(h) surfaces of HfB2-based ceramics.
bridging in the HH10S10 and HH20S20 ceramics increased energy dissipation during crack propagation, resulting in higher measured toughness values. The four-point flexural strength of HS20, HH10S10 and HH20S20 were 585 MPa, 800 MPa and 758 MPa respectively. With the incorporation of HfC, the bending strength increased significantly. The flexural strength of ceramics is inversely proportional to critical flaw present in the microstructure, as
described by Griffith equation [25]: s¼
K IC pffiffiffi Y a
ð3Þ
Where s is the flexural strength, KIC is the fracture toughness, Y is geometric constant, and a is the critical flaw size in the material. Y can be 1.28 for boride-SiC composite ceramic [26]. Based on Eq. (3), the calculated critical flaw sizes in HS20,
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Fig. 3. SEM images of SiC clusters on related fracture surfaces of HfB2-based ceramics.
Table 2 Room temperature mechanical properties of HfB2-based ceramics. Specimens
E (GPa)
Hv5 (GPa)
KΙC (MPa m1/2)
sfour (MPa)
HS20 HH20 HH10S10 HH20S20
508 497 511 482
21.0770.19 20.1770.24 20.6470.21 21.4670.39
3.727 0.14 3.367 0.12 4.347 0.12 5.057 0.23
5857 103 7047 76 8007 146 7587 108
Fig. 4. SEM images of indentation crack propagation of HfB2-based ceramics.
HH10S10 and HH20S20 are 24 μm, 18 μm and 27 μm, respectively, which are similar to the size of SiC clusters (see Fig. 3). In previous work, SiC inclusions causing large residual stress and stress concentration had been identified as the critical flaw in ZrB2–SiC ceramics. Thus, it is reasonable to conclude that SiC clusters are also the strength-limiting flaws in HfB2-based ceramics. The CET values of HfB2 and HfC are
very close, so the residual stress in HH20 is lower than the one in HS20. Therefore the flexural strength of HH20 is higher than HS20. According to the weakest link theory, fracture always starts at the weakest point. In other words, the fracture in particlereinforced ceramics was controlled by tensile zones rather than compressive zones [27]. For the ceramic containing SiC, the
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Fig. 5. SEM images of the fractured surfaces of HfB2-based ceramics.
matrix was under tensile stresses and the tensile stresses decreased with the distance from SiC grains. Therefore, the scale of the weakest zones was similar to the size of the largest SiC clusters for the HfB2-based ceramics with SiC. 4. Conclusions In the present work, both SiC and HfC were used as the reinforced particles to tailor the microstructures and improve the mechanical properties of hot pressed HfB2-based ceramics. (1) SiC and HfC particles, as the second phase, could hinder HfB2 grain growth. But the effect of SiC addition was greater than HfC addition. When SiC and HfC were introduced into the HfB2 matrix simultaneously, a synergistic effect of inhibiting grain growth was achieved. HfB2– 20HfC–20SiC had the finest microstructure. (2) In HfB2-based ceramics, the SiC and HfC particulates were homogeneously distributed within the HfB2 skeleton. However several SiC clusters were found on related fracture surfaces. Large SiC clusters can cause high levels of residual stress and stress concentration, and be harmful to mechanical properties of boride-SiC ceramics. Based on Griffith equation, the calculated critical flaw sizes in HS20, HH10S10 and HH20S20 are similar to the size of SiC clusters. (3) Most of the impurity would locate at the grain boundaries, which led to a weak interface. So the propagating crack would deflect and split more easily along the grain boundaries. The indentation crack paths in HH10S10 and HH20S20 appeared more tortuous than those in HS20
ceramics. Therefore the fracture toughness of HH20S20 improved to 5.05 MPa m1/2. Acknowledgments The authors express their gratitude to financial supports from the National Natural Science Foundation of China (No. 51272266), the Science and Technology Commission of Shanghai (No. 15ZR1445200), the State Key Laboratory of High Performance Ceramics and Superfine Microstructure of Shanghai Institute of Ceramics are gratefully acknowledged. References [1] W.G. Fahrenholtz, G.E. Hilmas, I.G. Talmy, J.A. Zaykoski, Refractory Diborides of Zirconium and Hafnium, J. Am. Ceram. Soc. 90 (2007) 1347–1364. [2] F. Monteverde, Hot pressing of hafnium diboride aided by different sinter additives, J. Mater. Sci. 43 (2007) 1002–1007. [3] E. Zapata-Solvas, D.D. Jayaseelan, H.T. Lin, P. Brown, W.E. Lee, Mechanical properties of ZrB2- and HfB2-based ultra-high temperature ceramics fabricated by spark plasma sintering, J. Eur. Ceram. Soc. 33 (2013) 1373–1386. [4] T.I.G. Opeka M M, E.J. Wuchina, Mechanical, thermal, and oxidation properties of refractory hafnium and zirconium compounds, J. Eur. Ceram. Soc. 19 (1999) 2405–2414. [5] K.M. White BJ, U.S. Patent No. 5,750,450,. ZrB2–ZrC–SiC Ablation resistant zirconium and hafnium ceramics,1998. [6] V. Medri, F. Monteverde, A. Balbo, A. Bellosi, Comparison of ZrB2– ZrC–SiC composites fabricated by spark plasma sintering and hotpressing, Adv. Eng. Mater. 7 (2005) 159–163. [7] W.W. Wu, G.J. Zhang, Y.M. Kan, P.L. Wang, Reactive hot pressing of ZrB2–SiC–ZrC ultra high-temperature ceramics at 1800 1C, J. Am. Ceram. Soc. 89 (2006) 2967–2969.
Y. Yuan et al. / Ceramics International 42 (2016) 7861–7867 [8] S.-Q. Guo, Y. Kagawa, T. Nishimura, D. Chung, J.-M. Yang, Mechanical and physical behavior of spark plasma sintered ZrC–ZrB2–SiC composites, J. Eur. Ceram. Soc. 28 (2008) 1279–1285. [9] H.L. Liu, G.J. Zhang, J.-X. Liu, H. Wu, Synergetic roles of ZrC and SiC in ternary ZrB2–SiC–ZrC ceramics, J. Eur. Ceram. Soc. 35 (2015) 4389–4397. [10] F. Monteverde, Progress in the fabrication of ultra-high-temperature ceramics: “in situ” synthesis, microstructure and properties of a reactive hot-pressed HfB2–SiC composite, Compos. Sci. Technol. 65 (2005) 1869–1879. [11] D.W. Ni, J.X. Liu, G.J. Zhang, Microstructure refinement and mechanical properties improvement of HfB2-SiC composites with the incorporation of HfC, J. Eur. Ceram. Soc. 32 (2012) 2557–2563. [12] D.W. Ni, G.J. Zhang, Y.M. Kan, P.L. Wang, Hot pressed HfB2 and HfB2–20 vol%SiC ceramics based on HfB2 powder synthesized by borothermal reduction of HfO2, Int. J. Appl. Ceram. Technol. 7 (2010) 830–836. [13] W.M. Guo, Z.G. Yang, G.J. Zhang, Microstructural evolution of ZrB2– MoSi2 composites during heat treatment, Ceram. Int. 37 (2011) 2931–2935. [14] F. Lange, M.M. Hirlinger, Hindrance of grain growth in Al2O3 by ZrO2 inclusions, J. Am. Ceram. Soc. 67 (1984) 164–168. [15] D.W. Ni, G.J. Zhang, Y.M. Kan, P.L. Wang, Synthesis of monodispersed fine hafnium diboride powders using carbo/borothermal reduction of hafnium dioxide, J. Am. Ceram. Soc. 91 (2008) 2709–2712. [16] J.X. Liu, Y.M. Kan, G.J. Zhang, Synthesis of ultra-fine Hafnium Carbide powder and its pressureless sintering, J. Am. Ceram. Soc. 93 (2010) 980–986. [17] A G Evans, E.A. Charles, Fracture toughness determinations by indentation, J. Am. Ceram. Soc. 59 (1976).
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[18] (a) G.S. Rohrer, Introduction to grains, phases, and interfaces – an interpretation of microstructure, Trans. AIME 175 (1948) 15–51; (b) C.S. Smith, Metall. Mater. Trans. A 41 (2010) 1063–1100. [19] S. Torquato, Modelling of physical properties of composites materials, Int. J. Solids. Struct. 37 (2000) 411–422. [20] R. Kotelnikov, S. Bashlykov, Z. Galiakbarov, A. Kashtanov, Handbook of High Melting Point Materials, Metallgiya Press, Moscow, 1969. [21] H.L. Brown, Elastic properties of some polycrystalline transition-metal monocarbides, J. Chem. Phys. 45 (1966) 547. [22] J.F. Lynch, Engineering Property Data on Selected Ceramics. Volume II. Carbides, in, DTIC Document, 1979. [23] J. Marschall, H. Manning, D.C. Erlich, Microhardness and high-velocity impact resistance of HfB2/SiC and ZrB2/SiC composites, J. Mater. Sci. 39 (2004) 5959–5968. [24] M. Gasch, E. Irby, D. Ellerby, Processing, properties and arc jet oxidation of hafnium diboride/silicon carbide ultra high temperature ceramics, J. Mater. Sci. Technol. 39 (2004) 5925–5937. [25] A. Rezaie, W.G. Fahrenholtz, G.E. Hilmas, Effect of hot pressing time and temperature on the microstructure and mechanical properties of ZrB2–SiC, J. Mater. Sci. 42 (2007) 2735–2744. [26] J.X. Liu, G.J. Zhang, F.F. Xu, W.-W. Wu, H.T. Liu, Y. Sakka, T. Nishimura, T.S. Suzuki, D.W. Ni, J. Zou, Densification, microstructure evolution and mechanical properties of WC doped HfB2–SiC ceramics, J. Eur. Ceram. Soc. 35 (2015) 2707–2714. [27] Y.W. Bao, C.C. Liu, J.L. Huang, Effects of residual stresses on strength and toughness of particle-reinforced TiN/Si3N4 composite: Theoretical investigation and FEM simulation, Mater. Sci. Eng. A 434 (2006) 250–258.
CERAMICS INTERNATIONAL
Ceramics International 42 (2016) 7861–7867 www.elsevier.com/locate/ceramint
Short communication
Effect of HfC and SiC on microstructure and mechanical properties of HfB2-based ceramics Ye Yuana,b, Ji-Xuan Liua,n, Guo-Jun Zhanga,n a
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Shanghai 200050, China b University of the Chinese Academy of Sciences, Beijing 100049, China Received 19 December 2015; received in revised form 30 December 2015; accepted 7 January 2016 Available online 15 January 2016
Abstract SiC and HfC were used to tailor microstructures and improve properties of hot pressed HfB2-based ceramics. SiC and HfC particle, as the second phase, could hinder grain growth of HfB2. Compared with HfC, SiC grains grew slower, which was more effective to hinder grain growth of HfB2. The ternary ceramics HfB2–20vol%HfC–20vol%SiC had the finest microstructure, indicating the effect of SiC and HfC on grain growth. Several SiC clusters were found on related fracture surfaces. Large SiC clusters can cause high levels of residual stress and stress concentration, which are harmful to mechanical properties of boride-SiC ceramics. The predominant fracture mode of HfB2–20vol%SiC was transgranular, however, in HfB2–10vol%HfC–10vol%SiC and HfB2–20vol%HfC–20vol%SiC ceramics, the dominating fracture mode shift from transgranular to intergranular. With the incorporation of HfC, the fracture toughness of HH10S10 and HH20S20 increased to 4.34 MPa m1/2 and 5.05 MPa m1/2, which were 17.3% and 26.7% higher than HS20 (3.72 MPa m1/2), respectively. The flexural strengths of HfB2–HfC–SiC ceramics were improved to 750 MPa, which were about 1.3 times higher than that of HfB2–20SiC. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Hot pressing; B. Composites; C. Mechanical properties; E. Structural applications
1. Introduction HfB2-based ceramics have attracted lots of attentions during recent years [1–4]. Owing to a unique combination of excellent properties, including high melting point, high hardness, high electrical and thermal conductivity, and good corrosion resistance, HfB2-based ceramics are considered to be one important member of the ultra-high temperature ceramics (UHTCs) family, which has optimistic prospect to be used as suitable candidates for high temperature structural applications, such as sharp noses and leading edges for re-entry and hypersonic vehicles, cutting tools and molten metal crucibles [4]. As a member of the ultra-high temperature ceramics (UHTCs) family, HfC has a higher melting temperature, but lower thermal conductivity than HfB2 [1]. The addition of MC (M ¼ Hf, Zr) to MB2-SiC to form a ternary composite of n
Corresponding authors. E-mail addresses: [email protected] (J.-X. Liu), [email protected] (G.-J. Zhang). http://dx.doi.org/10.1016/j.ceramint.2016.01.067 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
MB2–MC–SiC can adjust the microstructure and properties of MB2–SiC composite. NASA reported that MB2–MC–SiC ceramics have superior resistance to ablation under an arc-jet environment [5], which is beneficial for the application of UHTCs. Recent years, a variety of research has been carried out on the ZrB2–SiC–ZrC system, including the densification behavior, microstructures evolution, as well as the fabrication processing and properties. Medri et al. [6] studied the densification and microstructures of ZrB2–SiC–ZrC ceramics fabricated by hot pressing and spark plasma sintering. Wu et al. [7] studied the in situ synthesis of ZrB2–SiC–ZrC by reactive hot pressing using Zr metal with B4C and silicon as starting materials. Guo et al. [8] mainly studied the properties of spark plasma sintered ZrB2–SiC–ZrC ceramics, including mechanical, thermal, and electrical properties. Liu et al. [9] investaged the roles of SiC and ZrC additions and their synergistic effects on the microstructures and properties of ZrB2-based ceramics. But studies on HfB2–HfC–SiC system
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are very limited. Monteverde [10] studied the in situ synthesis of HfB2–SiC–HfC by reactive hot pressing using Hf metal with B4C and silicon as starting materials, obtaining nearly full dense ceramics at 1900 1C. Ni et al. [11] studied the densification and microstructures of the hot pressed HfB2– SiC and HfB2–SiC–HfC ceramics. And analysis indicated that the incorporation of HfC promoted the densification and fined the microstructure of HfB2–SiC ceramics. In the multiphase ceramics, the second phase plays an important role on the microstructures. Researchers have reported that the secondphase particles could inhibit the grain growth of the matrix during sintering in several ceramic systems, such as HfB2–SiC [12], ZrB2–MoSi2 [13] and Al2O3–ZrO2 [14]. However, few papers reported the different roles of SiC and HfC additions and their effects on the microstructure and mechanical properties of the HfB2-based ceramics. In the present paper, HfB2-based ceramics with different amounts of SiC and HfC were densified by hot pressing sintering. The influence of each phase on microstructures and mechanical properties was investigated. Grain sizes of each phase were characterized and discussed.
2. Experimental procedure 2.1. Powder processing and sintering The raw materials were HfB2 powders (purity4 99%, O: 0.15%, C: 0.04%, mean particle size is 1 μm), HfC powders (purity498%, free C: 0.47%, O: 0.72%, mean particle size is 225 nm), and commercial ɑ-SiC powders (purity498.5%, O: 1.04%, mean particle size is 0.45 μm, Changle Xinyuan Carborundum Micropowder Co. Ltd., Changle, China). HfB2 powders were synthesized from HfO2 and B4C [15], and HfC powders were synthesized from HfO2 and carbon black [16]. In this work, we prepared four HfB2-based composites including HfB220vol%SiC, HfB2–10vol%HfC–10vol%SiC, HfB2–20vol%HfC and HfB2–20vol%HfC–20vol%SiC, marked as HS20, HH10S10, HH20 and HH20S20. The starting powders were mixed in ethanol by rolling ball mill at speed of 30 rpm for 24 h, using Si3N4 balls as medium. Subsequently, the slurries were dried in rotary evaporator at 70 1C. The dried powders were sieved through 200 mesh screen. All composites were subsequently hot pressed in a graphite die with a rectangle cavity of 30 37 mm at 2000 1C for 1 h under a pressure of 30 MPa in argon. Before applying pressure, the powder compacts in the die were heated up to 1650 1C with a ramp rate of 10 1C/min and a dwelling period of 0.5 h in a vacuum environment of 20 Pa pressure, for the purpose of purging oxide impurities. After the hold at 1650 1C, the furnace atmosphere was changed to flowing argon purity 99.99%) at pressure of 105 Pa. The powder compact was continually heated to 2000 1C at 10 1C/min under uniaxial pressure of 30 MPa. After dwelling at 2000 1C for 1 h, the applied pressure was removed.
2.2. Sample characterization After removing the outer layer by grinding, the bulk densities of the as-sintered specimens were measured by the Archimedes method using deionized water as the liquid medium. Flexural strengths were measured by a four point bending test using test bars of size of 3 4 36 mm. One tensile surface of each specimen was polished and four edges were chamfered. All of the bending strength measurement was carried out using an Instron load frame at a cross-head displacement speed of 0.2 mm/min, in which the inner span was 10 mm and outer span was 30 mm. The data of average strength and standard deviation for each material were obtained based on the determination of five samples. Vickers hardness (HV) and fracture toughness (KIC) were determined using Vickers indentation (2100B Hardness Tester, Instron, Norwood, MA, USA) under load of 5 kg and dwell time of 10 s. All values of HV and KIC were the average of five measurements. The fracture toughness was calculated by the following equation [12,17]: 3 C1 þ C2 2 K IC ¼ P π ð tan βÞ 1 ð1Þ 4 Where P is the indentation load (N), C1 and C2 the measured diagonal crack lengths (m), and β an angle constant (681). Microstructures and chemical composition were analyzed using scanning electron microscopy (TM3000, Hitachi, Japan) along with energy-dispersive spectroscopy (EDS, Oxford INCA energy).
3. Results and discussion 3.1. Microstructures The relative densities of all HfB2-based ceramics are shown in Table 1. As is known to all, SiC is beneficial for the densification behavior of HfB2-based ceramics [1]. From the relative densities of samples, we could conclude that the existence of HfC can also accelerate the densification of HfB2-based ceramics. The average grain sizes of each phase were determined by the image software analysis from the SEM micrographs (shown in Table 1). HfB2 grain size in HH20 (2.97 μm) was larger than the one in HS20 (2.29 μm), which indicated that the grain growth of HfB2 was controlled by the grain size of the other phases (1.89 μm for HfC in HH20 and 1.33 μm for SiC in HS20). Based on the principle of Zener [18], the limited grain size of major phase (D) can be defined as d ð2Þ Dp f Where d and f represent the average diameter and the volume fraction of the added second phase particles. Compared with the grain size of HfB2 in HH10S10, the one in HH20S20 is smaller (2.57 μm compared with 2.01 μm). When the HfB2 content was fixed at 80 vol%, the HfB2 grain size increased
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Table 1 Summary of density and grain sizes of each phase in HfB2-based ceramics. Specimen Relative density (%)
Grain size for HfB2 (μm)
Grain size for SiC (μm)
Grain size for HfC (μm)
HS20 HH20 HH10S10 HH20S20
2.297 0.56 2.977 1.01 2.577 0.81 2.017 0.68
1.337 0.45 – 1.227 0.41 1.217 0.43
– 1.8970.62 1.5370.41 1.5170.43
98.8 98.7 98.8 99.8
with the decreasing SiC content. In HS20, HH10S10 and HH20, the average grain sizes of HfB2 are 2.29 μm, 2.57 μm and 2.97 μm. Average grain sizes of HfB2-based ceramics are shown in Fig. 1. Comparing the average grain sizes of different samples, we could find that HfB2–20HfC–20SiC had the finest grain size. Similarly, SiC particle could also prevent the grain growth of HfC, which led to finer grain size of HfC in HH20S20 (1.51 μm) than that in HH20 (1.89 μm). On the other hand, SiC grain size decreased from 1.33 μm in HS20 to 1.21 μm in HH20S20, which demonstrated the inhibiting effect of HfC on the grain growth of SiC, however the effect was not obvious. Microstructures of the sintered HfB2-based ceramics are shown in Fig. 2. As inspected by SEM，all of the ceramic bulks did not reveal any apparent residual porosity, which congruently agreed with their final relative density. In HfB2based ceramics, the SiC and HfC particulates were homogeneously distributed within the HfB2 skeleton. Although the polished surfaces of HfB2-based ceramics showed uniform microstructure, several SiC clusters were found on related fracture surfaces (Fig. 3). CTE values are 6.3 10 6 K 1 for HfB2, 4.7 10 6 K 1 for SiC, and 6.6 10 6 K 1 for HfC. Large SiC clusters can cause high levels of residual stress and stress concentration, and be harmful to mechanical properties of boride-SiC ceramics. In addition, in HH10S10 and HH20S20 ceramics, the SiC and HfC particulates were predominantly located at the multiple HfB2 grain junctions, which were believed to inhibit the grain growth of HfB2 via grain boundary pinning. 3.2. Mechanical properties The Young's modulus, Vickers’ hardness, fracture toughness, and bending strength of HfB2-based ceramics are summarized in Table 2. According P to a rule of mixture [19], namely the arithmetic average QiEi, where Ei and Qi are the Young's modulus and volume fractions of phase constituting the composite, the calculated Young's modulus values of HfB2-based ceramics are 513 GPa, 516 GPa, 514.8 GPa and 499.6 GPa for HS20, HH20, HS10H10 and HS20H20, respectively. In the case of linear properties, such as Young's modulus, it is well known that the arithmetic average generally overestimates the property. Obviously the tested values are below the calculated values. For Young's modulus calculation, the following data of Ei was used: 530 GPa [20] for HfB2, 460 GPa [21] for HfC, and 448 GPa [22] for SiC.
Fig. 1. Average Grain Size of HfB2-bsded Ceramics.
The Vickers’ hardness Hv5 of the HS20 was 21.07 GPa, which is consistent with the reported values (19–21 GPa) for HfB2-20vol% SiC by Marschall et al. [23] and Gasch et al. [24] The hardness of HH20 was 20.17 GPa, which is lower than that of HS20. In my analysis, there should be two factors determining the hardness of HH20. Firstly, the intrinsic hardness of HfC (27 GPa) and SiC (26 GPa) is very close. Then, the grain sizes of HH20 are much larger than those in HS20. Generally, the finer grain size can increase the frequency with which dislocations encounter grain boundaries, thus heightening the amount of stress required for deformation. At a given indent load, less deformation zone and higher hardness could be expected in grain-refined material. Therefore, HS20 had a higher hardness compared with HH20. Similarly, grain-refined HH10S10 had a higher hardness than HH20. In regard to HH20S20, the microstructure played an important role in the value of hardness. Compared with the others, HH20S20 had the finest microstructure and highest hardness. The fracture toughness of HS20 ceramic was 3.72 MPa m1/2, which was close to the reported value (3.95 MPa m1/2) for hot pressed HfB2-20vol% SiC by Ni et al [11]. With the incorporation of HfC, the fracture toughness of HH10S10 and HH20S20 increased to 4.34 MPa m1/2 and 5.05 MPa m1/2, which were 17.3% and 26.7% higher than HS20. To elucidate the toughening mechanisms, the propagating paths of some Vickersindentation-induced cracks on polished surfaces of HfB2-based ceramics were observed by SEM (Fig. 4). As the predominant fracture mode of HS20 was transgranular (Fig. 5), crack deflection appeared neglectable in the crack propagation path. However, in HH10S10 and HH20S20 ceramics, the dominating fracture mode shift from transgranular to intergranular. In general, most of the impurity would locate at the grain boundaries, which led to a weak interface. In this case, the propagating crack would deflect and split more easily along the grain boundaries. As shown in Fig. 4, the indentation crack paths in HH10S10 and HH20S20 appeared more tortuous than those in HS20 ceramics. More crack deflection and crack
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Fig. 2. SEM images of the polished (a)–(d) and etched (e)–(h) surfaces of HfB2-based ceramics.
bridging in the HH10S10 and HH20S20 ceramics increased energy dissipation during crack propagation, resulting in higher measured toughness values. The four-point flexural strength of HS20, HH10S10 and HH20S20 were 585 MPa, 800 MPa and 758 MPa respectively. With the incorporation of HfC, the bending strength increased significantly. The flexural strength of ceramics is inversely proportional to critical flaw present in the microstructure, as
described by Griffith equation [25]: s¼
K IC pffiffiffi Y a
ð3Þ
Where s is the flexural strength, KIC is the fracture toughness, Y is geometric constant, and a is the critical flaw size in the material. Y can be 1.28 for boride-SiC composite ceramic [26]. Based on Eq. (3), the calculated critical flaw sizes in HS20,
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Fig. 3. SEM images of SiC clusters on related fracture surfaces of HfB2-based ceramics.
Table 2 Room temperature mechanical properties of HfB2-based ceramics. Specimens
E (GPa)
Hv5 (GPa)
KΙC (MPa m1/2)
sfour (MPa)
HS20 HH20 HH10S10 HH20S20
508 497 511 482
21.0770.19 20.1770.24 20.6470.21 21.4670.39
3.727 0.14 3.367 0.12 4.347 0.12 5.057 0.23
5857 103 7047 76 8007 146 7587 108
Fig. 4. SEM images of indentation crack propagation of HfB2-based ceramics.
HH10S10 and HH20S20 are 24 μm, 18 μm and 27 μm, respectively, which are similar to the size of SiC clusters (see Fig. 3). In previous work, SiC inclusions causing large residual stress and stress concentration had been identified as the critical flaw in ZrB2–SiC ceramics. Thus, it is reasonable to conclude that SiC clusters are also the strength-limiting flaws in HfB2-based ceramics. The CET values of HfB2 and HfC are
very close, so the residual stress in HH20 is lower than the one in HS20. Therefore the flexural strength of HH20 is higher than HS20. According to the weakest link theory, fracture always starts at the weakest point. In other words, the fracture in particlereinforced ceramics was controlled by tensile zones rather than compressive zones [27]. For the ceramic containing SiC, the
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Fig. 5. SEM images of the fractured surfaces of HfB2-based ceramics.
matrix was under tensile stresses and the tensile stresses decreased with the distance from SiC grains. Therefore, the scale of the weakest zones was similar to the size of the largest SiC clusters for the HfB2-based ceramics with SiC. 4. Conclusions In the present work, both SiC and HfC were used as the reinforced particles to tailor the microstructures and improve the mechanical properties of hot pressed HfB2-based ceramics. (1) SiC and HfC particles, as the second phase, could hinder HfB2 grain growth. But the effect of SiC addition was greater than HfC addition. When SiC and HfC were introduced into the HfB2 matrix simultaneously, a synergistic effect of inhibiting grain growth was achieved. HfB2– 20HfC–20SiC had the finest microstructure. (2) In HfB2-based ceramics, the SiC and HfC particulates were homogeneously distributed within the HfB2 skeleton. However several SiC clusters were found on related fracture surfaces. Large SiC clusters can cause high levels of residual stress and stress concentration, and be harmful to mechanical properties of boride-SiC ceramics. Based on Griffith equation, the calculated critical flaw sizes in HS20, HH10S10 and HH20S20 are similar to the size of SiC clusters. (3) Most of the impurity would locate at the grain boundaries, which led to a weak interface. So the propagating crack would deflect and split more easily along the grain boundaries. The indentation crack paths in HH10S10 and HH20S20 appeared more tortuous than those in HS20
ceramics. Therefore the fracture toughness of HH20S20 improved to 5.05 MPa m1/2. Acknowledgments The authors express their gratitude to financial supports from the National Natural Science Foundation of China (No. 51272266), the Science and Technology Commission of Shanghai (No. 15ZR1445200), the State Key Laboratory of High Performance Ceramics and Superfine Microstructure of Shanghai Institute of Ceramics are gratefully acknowledged. References [1] W.G. Fahrenholtz, G.E. Hilmas, I.G. Talmy, J.A. Zaykoski, Refractory Diborides of Zirconium and Hafnium, J. Am. Ceram. Soc. 90 (2007) 1347–1364. [2] F. Monteverde, Hot pressing of hafnium diboride aided by different sinter additives, J. Mater. Sci. 43 (2007) 1002–1007. [3] E. Zapata-Solvas, D.D. Jayaseelan, H.T. Lin, P. Brown, W.E. Lee, Mechanical properties of ZrB2- and HfB2-based ultra-high temperature ceramics fabricated by spark plasma sintering, J. Eur. Ceram. Soc. 33 (2013) 1373–1386. [4] T.I.G. Opeka M M, E.J. Wuchina, Mechanical, thermal, and oxidation properties of refractory hafnium and zirconium compounds, J. Eur. Ceram. Soc. 19 (1999) 2405–2414. [5] K.M. White BJ, U.S. Patent No. 5,750,450,. ZrB2–ZrC–SiC Ablation resistant zirconium and hafnium ceramics,1998. [6] V. Medri, F. Monteverde, A. Balbo, A. Bellosi, Comparison of ZrB2– ZrC–SiC composites fabricated by spark plasma sintering and hotpressing, Adv. Eng. Mater. 7 (2005) 159–163. [7] W.W. Wu, G.J. Zhang, Y.M. Kan, P.L. Wang, Reactive hot pressing of ZrB2–SiC–ZrC ultra high-temperature ceramics at 1800 1C, J. Am. Ceram. Soc. 89 (2006) 2967–2969.
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