Contour maps of mechanical properties in ternary ZrB2SiCZrC ceramic system

Scripta Materialia xxx (2015) xxx–xxx

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Contour maps of mechanical properties in ternary ZrB2ASiCAZrC ceramic system Hu-Lin Liu a,b, Ji-Xuan Liu a, Hai-Tao Liu a, Guo-Jun Zhang a,⇑ a b

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Shanghai 200050, China University of the Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e

i n f o

Article history: Received 11 May 2015 Revised 3 June 2015 Accepted 3 June 2015 Available online xxxx Keywords: Zirconium diboride (ZrB2) Composites Mechanical properties Microstructure Contour maps

a b s t r a c t Monolithic ZrB2, ZrB2ASiC, ZrB2AZrC and ZrB2ASiCAZrC were prepared by hot pressing in ternary ZrB2ASiCAZrC system. They exhibited different microstructural features. Grain sizes of each phase were smaller in the ternary composites. Contour maps of mechanical properties were also plotted in ZrB2ASiCAZrC ternary diagram based on experiments and software fitting. They established the relationships between chemical composition and properties in a visual way. Young’s Modulus and Vickers hardness had linear relationships with composition, while the evolution of the strength and toughness was complicated. Ó 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Zirconium diboride (ZrB2) based ultra-high-temperature ceramics (UHTCs) are the most potential candidates for high temperature structural applications, such as thermal protection systems and sharp-leading edge components in hypersonic vehicles. Silicon carbide (SiC) and/or zirconium carbide (ZrC) were normally introduced to the ZrB2 matrix [1,2]. These binary or ternary composites exhibit improved densification behavior and material properties, compared with the monolithic ZrB2 ceramics. The incorporated SiC phase could inhibit the grain growth of ZrB2 and improve oxidation resistance of the composites [3–5]. ZrB2 AZrC shows high eutectic temperature, high hardness and strong wear resistance [6,7]. ZrB2 ASiCAZrC ceramics performed superior ablation resistance in the conditions of air jets, high temperatures and oxygen atmosphere [8]. Therefore, ternary ZrB2ASiCAZrC system has attracted attentions as promising materials for ultra-high-temperature applications [9,10]. In this ternary system, the monolithic ZrB2, ZrB2ASiC and ZrB2AZrC binary composites and ZrB2ASiCAZrC ternary composites exhibit different properties and microstructures, which depend on the chemical composition [11]. Therefore, the relationships between chemical composition and properties should be established and expressed in a visual method. Contour maps, which contain several contour through points of equal values, have been widely used in geography to describe characteristics of ⇑ Corresponding author.

valleys and hills [12]. Mestral et al. [13] prepared 16 samples and plotted contour maps in TiB2ATiCASiC system to investigate the evolution of mechanical properties over the complete ternary diagram. In the present work, we will combine experiments and software fitting to plot contour maps of mechanical properties in ZrB2ASiCAZrC system. In order to guarantee the accuracy of the maps, we select nine samples and the content of the major phase ZrB2 is not less than 60 vol%. The raw materials were home synthesized ZrB2 powder [14] (D50 = 1.05 lm, 98% purity), a-SiC powder (D50 = 0.45 lm, 98.5% purity, Changle Xinyuan Carborundum Co. Ltd., Shandong, China) and home synthesized ZrC powder [15] (D50 = 0.85 lm, 99% purity). The chemical composition of the nine samples were presented in Table 1 and Fig. 1. ZrB2 containing 5 vol% B4C was deemed as the monolithic ceramic. B4C (D50 = 1.5 lm, Jingangzuan Boron Carbide Co. Ltd., Mudanjiang, China) was used as sintering aid to obtain dense monolithic ZrB2. Powder mixtures were ball-milled in alcohol for 24 h in polyethylene jars using Si3N4 milling medium balls, and dried by rotary evaporation. Then the mixed powder compacts were hot pressed at 1900 °C and 30 MPa for 1 h. The furnace was heated under vacuum below 1600 °C. Above this temperature, the atmosphere was switched to flowing argon gas. Bulk densities of the sintered samples were measured by the Archimedes method. Microstructures were observed on the acid etched surfaces, using scanning electron microscopy (SEM, TM3000, Hitachi, Japan). Flexural strengths were measured by

E-mail address: [email protected] (G.-J. Zhang). http://dx.doi.org/10.1016/j.scriptamat.2015.06.005 1359-6462/Ó 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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Table 1 Chemical composition of the selected samples in ZrB2ASiCAZrC system. Codes

Chemical composition (vol%)

ZB ZS20 ZS10Z10 ZZ20 ZS20Z10 ZS10Z20 ZS40 ZZ40 ZS20Z20

ZrB2

SiC

ZrC

B4C

95 80 80 80 70 70 60 60 60

0 20 10 0 20 10 40 0 20

0 0 10 20 10 20 0 40 20

5 0 0 0 0 0 0 0 0

Fig. 1. Positions of the selected composition in ZrB2ASiCAZrC ternary diagram.

the four point bending test using the test bars with dimensions of 3  4  36 mm. The tensile faces were polished to 0.5 lm using diamond paste and the four edges were chamfered. The inner and outer spans were 10 mm and 30 mm, respectively, and cross-head speed was 0.5 mm/min. Hardness and fracture toughness were measured by Vickers indentation method, using a load of 5 kg for 10 s on the polished surface. The fracture toughness was calculated according to the following equation [16]:

K IC ¼ 0:026

! 1 1 E2 P2 a

ð1Þ

3

C2

where P is the indentation load (49.6 N), a is the half length of the indent, C is the half length of the crack, and E is the elastic modulus of ceramics. Young’s Modulus (E) was measured by an impulse excitation technique, according to the standard of GB/T 22315-2008

(China). All the reported values of the mechanical properties were the average of at least five measurements. The grain sizes were obtained on the SEM images using image analysis software (Image-Pro Plus, Version 7.0, Media Cybernetics, USA). The contour map of each property was plotted by Origin Pro software (Version 8.1, OriginLab Corp., USA). Densities of the selected samples are listed in Table 2. The relative densities were all above 99%. Typical microstructures of the sintered ceramics are compared in Fig. 2. In the composites (Fig. 2(b)–(f)), the gray, black and white phases are ZrB2, SiC and ZrC, respectively. And SiC and ZrC particles were well distributed in the ZrB2 matrix. However, the larger black grains and smaller black spots in monolithic ZrB2 (Fig. 2(a)) are B4C and pores, respectively. The total amount of the black phases was about 5.6%, according to their area fraction in Fig. 2(a) measured by Image-Pro software. Subtracting the B4C content (5%), the porosity of monolithic ZrB2 was about 0.6%. It was consistent with the result from the Archimedes measurement. Furthermore, monolithic, binary and ternary ceramics exhibited different microstructural features. The introduction of ZrC seemed to promote elongation of ZrB2 grains, as highlighted by the arrows in Fig. 2(c). In addition, SiC or ZrC clusters containing several SiC or ZrC grains existed in the composites, which were believed to be harmful to mechanical properties. The clusters tended to be isolated when SiC or ZrC content was below 20 vol%, while SiC and ZrC particles formed interconnected network in ZS40 and ZZ40 (Fig. 2(e) and (f)). The grain sizes of each phase varied with chemical composition, as listed in Table 2. The grain size of ZrB2 decreased dramatically from the monolithic ceramic (8 lm) to the composites added with SiC and/or ZrC (<3 lm). The grain size of the second phase (SiC or ZrC) was smaller in the ternary composite than in the binary composite. In addition, the sizes of SiC and ZrC clusters were also influenced by their content. The maximum sizes of SiC or ZrC clusters decreased from 9 lm to 6 lm when SiC or ZrC volumes decreased from 20% to 10%. However, the sizes of SiC and ZrC clusters cannot be measured in ZS40 and ZZ40, due to the interconnected structure of SiC and ZrC. These different microstructure features of ZrB2-based ceramics would influence their mechanical properties. Fig. 3 depicts the contour maps of mechanical properties in ZrB2ASiCAZrC ternary diagram. The relevant data of the selected samples are also listed in Table 3. Different properties have different change trends with chemical composition. Linear models can describe the evolution of Young’s Modulus and Vickers hardness with the volume fractions of SiC and ZrC, as shown in Fig. 3(a) and (b). The equations are expressed as follows:

E ¼ 531:37  58:81V 1  119:81V 2

ð2Þ

Hv ¼ 16:5 þ 11:86V 1 þ 0:14V 2

ð3Þ

where E is Young’s Modulus, Hv is Vickers hardness, V1 and V2 are volume fractions of SiC and ZrC. It indicated that elastic modulus

Table 2 Densities and characteristic grain sizes of the selected samples. Samples

ZB ZS20 ZS10Z10 ZZ20 ZS20Z10 ZS10Z20 ZS40 ZZ40 ZS20Z20

Density (g/cm3)

5.94 5.52 5.88 6.22 5.59 5.94 4.95 6.30 5.63

Relative density (%)

100.3 100.2 100.2 100.1 100.2 100.2 100.2 99.2 99.8

Grain size (lm) Average ZrB2

Max ZrB2

Average SiC

Max SiC cluster

Average ZrC

Max ZrC cluster

8.24 ± 2.42 2.47 ± 0.80 2.74 ± 1.19 3.11 ± 1.35 2.24 ± 0.69 2.38 ± 0.68 2.36 ± 0.65 2.34 ± 0.61 2.25 ± 0.69

13.84 4.82 5.77 6.67 4.46 4.63 4.22 4.17 4.07

– 1.29 ± 0.45 1.16 ± 0.44 – 1.15 ± 0.34 1.09 ± 0.29 1.41 ± 0.36 – 1.15 ± 0.36

– 10.75 6.56 – 9.18 6.86 – – 9.03

– – 1.57 ± 0.69 1.93 ± 0.70 1.61 ± 0.63 1.66 ± 0.48 – 1.97 ± 0.71 1.57 ± 0.53

– – 5.81 8.16 5.94 8.74 – – 8.83

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Fig. 2. Typical microstructures on the etched surfaces of the selected ceramics: (a) ZB, (b) ZS20, (c) ZZ20, (d) ZS20Z20, (e) ZS40, and (f) ZZ40, respectively.

Fig. 3. Contour maps of mechanical properties in the ternary diagram: (a) elastic modulus, (b) hardness, (c) flexural strength and (d) toughness, respectively.

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Table 3 Mechanical properties of the selected samples at room temperature. Samples

E (GPa)

Hv (GPa)

KIC (MPam1/2)

r (MPa)

Calculated flaw size (lm)

ZB ZS20 ZS10Z10 ZZ20 ZS20Z10 ZS10Z20 ZS40 ZZ40 ZS20Z20

535 ± 1 521 ± 1 509 ± 1 504 ± 1 507 ± 1 500 ± 1 510 ± 1 487 ± 1 494 ± 1

16.2 ± 0.3 19.3 ± 0.5 17.7 ± 0.2 16.3 ± 0.1 18.6 ± 0.3 18.0 ± 0.2 21.2 ± 0.2 16.8 ± 0.1 18.4 ± 0.3

2.84 ± 0.12 3.89 ± 0.26 4.60 ± 0.23 4.48 ± 0.19 4.81 ± 0.33 4.44 ± 0.17 3.97 ± 0.22 3.44 ± 0.14 4.84 ± 0.28

449 ± 43 562 ± 65 851 ± 85 794 ± 49 734 ± 107 755 ± 57 731 ± 87 633 ± 63 785 ± 72

10.22 ± 2.14 12.20 ± 3.26 7.45 ± 1.66 8.12 ± 1.22 10.95 ± 3.53 8.81 ± 1.49 7.51 ± 1.97 7.56 ± 1.63 9.70 ± 2.10

and hardness of near fully dense ceramics were dominated by chemical composition, and they have little relationships with microstructures. However, the relationships of flexural strength and toughness with chemical composition were too complicated to be expressed by simple equations. Based on the contour maps (Fig. 3(c) and (d)), the composites located in the region around ZS10Z10 and ZZ20 have relative higher strength (above 800 MPa), while relative higher toughness is obtained in the region around ZS20Z10 and ZS20Z20 (above 4.64 MPa m1/2). It indicated that both chemical composition and microstructures controlled the strength and toughness. Fig. 4(a) represents typical indention crack paths of ZrB2-based ceramics. The cracks propagated through ZrB2 and SiC grains directly, while they went around ZrC grains resulting in crack deflection. Furthermore, the existence of SiC grains could pin crack tips and was also helpful to improving toughness. Consequently, ZrB2ASiCAZrC had larger toughness than monolith ZrB2 and binary composites. The different crack paths were attributed to the different thermal residual stress state induced by thermal expansion mismatch of each phase (CTE of ZrB2, SiC and ZrC [11]: 6.5  106 K1, 4.7  106 K1 and 7.1  106 K1, respectively).

According to Selsing’s model of a single spherical particle imbedded in an infinite isotropic elastic matrix [17], there would be a micro internal stress P presented on the spherical particle:

P¼

2ðap  am ÞDTEm ð1 þ mm Þ þ 2bð1  2mp Þ

ð4Þ

where a is the coefficient of thermal expansion (CTE), m is the Poisson’s ratio, the subscripts m and p represent the matrix and the particle. DT is temperature change during cooling process. b = Em/Ep. Radial (rr) and tangential (rt) stresses in the ZrB2 matrix are expressed as

(



rr ¼ P Rr 3  rt ¼  12 P Rr 3

ð5Þ

where r is the particle radius, R refers to the distance from the center of the particle. Because the thermal expansion coefficient of ZrC is larger than ZrB2, residual stress field in ZrB2 matrix around the ZrC particle is tensile stress in radial direction and compressive stress in tangential direction. Therefore, the propagated cracks near ZrC particles are parallel to tangential direction and vertical to radial direction, i.e., crack deflection occurred in ZrB2 matrix around ZrC particles [18,19]. On the contrary, the cracks near SiC particles are parallel to radial direction and were easier to propagate across SiC particles than ZrC particles, due to the smaller CTE of SiC than ZrB2. In addition, the crack paths were similar in ZS20 and ZS40, and cracks propagated across ZrB2 and SiC grains. However, the crack path was more tortuous in ZZ20 than ZZ40. The cracks trended to go across ZrC grains rather than around ZrC grains when the ZrC content increased to 40 vol%. It would be attributed to the interconnected network of ZrC and the changed thermal stress field in the composite. As a result, ZS20 and ZS40 had similar toughness (3.9 MPa m1/2), while the toughness was smaller in ZZ40 (3.44 MPa m1/2) than in ZZ20 (4.48 MPa m1/2). The correlation between microstructures and flexural strengths can be analyzed by Griffith equation [20]:

Fig. 4. Correlation of microstructures with the toughness and strengths: (a) typical crack paths of different ceramics, (b) the contour map of the calculated flaw sizes, and (c) the contour map of relevant grain sizes controlling ceramic strengths, respectively.

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K Y a

r ¼ pICffiffiffi

ð6Þ

where r is the flexural strength, KIC is the toughness, Y is a geometric constant, and a is the critical flaw size in the ceramic. Based on the SEM observations of the polished surfaces, no macroscopic defects were identified as probable fracture origins. In this case, critical flaw sizes can be taken as characteristic grain sizes, and the geometric constant Y was set as 1.98 [21,22]. The calculated flaw sizes are also listed in Table 3 based on Eq. (6) using the measured strength and toughness. And the standard deviation arises from that of the strength and toughness. In general, the strength of the monolithic ZrB2 was controlled by ZrB2 grain sizes, whereas it was influenced by particle sizes (SiC or ZrC) in the composite [23,24]. According to the thermal stress fields in composites, ZrC grains were under tensile stresses in ZrB2AZrC, and the matrix was under compressive stress. Therefore, the largest ZrC clusters controlled the strengths of ZrB2AZrC. Nevertheless, ZrB2 matrix was under tensile stress in ZrB2ASiC, and the tensile stress decreased with the increasing distance from SiC grains. When ZrC was added to ZrB2ASiC, ZrC grains were also under tensile stresses. But the stresses were lower in ZrC grains than in ZrB2 matrix around SiC grains because of the smaller difference in CTE between ZrC and ZrB2. Therefore, the largest tensile stress existed in ZrB2 matrix around SiC clusters for the composite containing SiC. And the largest SiC clusters played a dominant role on the strengths of ZrB2ASiC-based ceramics. Fig. 4(c) was the contour map of the relevant grain sizes controlling the ceramic strength using data in Table 2. The change trends of relevant grain sizes in this map were consistent with the contour map of calculated flaw sizes (Fig. 4(b)). It indicated that the strengths of ZrB2-based ceramics were controlled by the microstructures, especially the grain sizes of second phases. In summary, monolithic, binary and ternary ceramics were hot pressed in ternary ZrB2ASiCAZrC system. They exhibited different microstructural features. Grain sizes of each phase were smaller in the ternary composites. Contour maps of mechanical properties were also plotted in ZrB2ASiCAZrC ternary diagram based on the experimental results and software fitting. They established the relationships between chemical composition and properties in a visual way. Young’s Modulus and Vickers hardness had linear relationships with the volume fractions of SiC and ZrC. However, the evolution of strength and toughness were complicated. The

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composites located in the region around ZrB2A10SiCA10ZrC (number in vol%) and ZrB2A20ZrC had relative higher strength (above 800 MPa), while relative higher toughness was obtained in the region around ZrB2A20SiCA10ZrC and ZrB2A20SiCA20ZrC (above 4.64 MPa m1/2). It indicated that both microstructures and composition influenced the strength and toughness. These maps should be useful to the composition design in ZrB2ASiCAZrC ternary system. Acknowledgments The authors express their gratitude to financial supports from the National Natural Science Foundation of China (No. 51272266), and the State Key Laboratory of High Performance Ceramics and Superfine Microstructure of Shanghai Institute of Ceramics are gratefully acknowledged. References [1] V. Medri, F. Monteverde, A. Balbo, A. Bellosi, Adv. Eng. Mater. 7 (2005) 159. [2] W.G. Fahrenholtz, G.E. Hilmas, I.G. Talmy, J.A. Zaykoski, J. Am. Ceram. Soc. 90 (2007) 1347. [3] J.K. Sonber, A.K. Suri, Adv. Appl. Ceram. 110 (2011) 321. [4] P.A. Williams, R. Sakidja, J.H. Perepezko, P. Ritt, J. Eur. Ceram. Soc. 32 (2012) 3875. [5] J.C. Han, P. Hu, X.H. Zhang, S.H. Meng, Scr. Mater. 57 (2007) 825. [6] D.S. King, G.E. Hilmas, W.G. Fahrenholtz, J. Eur. Ceram. Soc. 34 (2014) 3549. [7] G.J. Zhang, M. Ando, J.F. Yang, T. Ohji, S. Kanzaki, J. Eur. Ceram. Soc. 24 (2004) 171. [8] J. Bull, M.J. White, L. Kaufman, United States Patent, 1998. [9] W.W. Wu, G.J. Zhang, Y.M. Kan, P.L. Wang, J. Am. Ceram. Soc. 91 (2008) 2501. [10] M. Jalaly, M. Tamizifar, M.S. Bafghi, F.J. Gotor, J. Alloy Compd. 581 (2013) 782. [11] S.Q. Guo, Y. Kagawa, T. Nishimura, D. Chung, J.M. Yang, J. Eur. Ceram. Soc. 28 (2008) 1279. [12] D.F. Merriam, P.H.A. Sneath, J. Geophys. Res. 71 (1966) 1105. [13] F. Demestral, F. Thevenot, J. Mater. Sci. 26 (1991) 5547. [14] D.W. Ni, G.J. Zhang, Y.M. Kan, P.L. Wang, J. Am. Ceram. Soc. 91 (2008) 2709. [15] X.G. Wang, J.X. Liu, Y.M. Kan, G.J. Zhang, J. Eur. Ceram. Soc. 32 (2012) 1795. [16] G.J. Zhang, Z.Y. Deng, N. Kondo, J.F. Yang, T. Ohji, J. Am. Ceram. Soc. 83 (2000) 2330. [17] J. Selsing, J. Am. Ceram. Soc. 44 (1961) 419. [18] G.C. Wei, P.F. Becher, J. Am. Ceram. Soc. 67 (1984) 571. [19] G.J. Zhang, X.M. Yue, Z.Z. Jin, J. Chin. Ceram. Soc. 23 (1995) 365. [20] S. Zhu, W.G. Fahrenholtz, G.E. Hilmas, Scr. Mater. 59 (2008) 123. [21] A. Rezaie, W.G. Fahrenholtz, G.E. Hilmas, J. Mater. Sci. 42 (2007) 2735. [22] J.B. Wachtman, W.R. Cannon, M.J. Matthewson, Mechanical Properties of Ceramics, John Wiley & Sons Inc., New York, 2009. [23] J. Watts, G. Hilmas, W.G. Fahrenholtz, J. Am. Ceram. Soc. 94 (2011) 4410. [24] H.B. Ma, H.L. Liu, J. Zhao, F.F. Xu, G.J. Zhang, J. Eur. Ceram. Soc. 35 (2015) 2699.

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