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BN ceramics
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Fabrication and mechanical properties of laminated HfC–SiC/BN ceramics Liuyi Xiang, Laifei Cheng ∗ , Yi Hou, Fuyuan Wang, Liangjun Li, Litong Zhang Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, PR China Received 18 December 2013; received in revised form 29 March 2014; accepted 10 April 2014
Abstract Laminated HfC–SiC/BN ceramics were successfully fabricated by tape casting and hot pressing. Fully dense HfC–SiC ultra-high temperature ceramics with homogeneous structure were obtained. The introduction of the weak BN layer resulted in a slight decrease of the flexural strength but significantly improved the fracture toughness compared with monolithic HfC–SiC ceramics. The fracture toughness of laminated HfC–SiC/BN ceramics in the parallel direction peaked at 8.06 ± 0.46 MPa m1/2 , which increased by 115% than that of monolithic HfC–SiC ceramics. The composites showed non-catastrophic fracture behaviors in both parallel and perpendicular directions. It indicates that laminated structure design is a promising approach to obtain full density HfC–SiC ceramics with high fracture toughness. © 2014 Elsevier Ltd. All rights reserved. Keywords: Laminated HfC–SiC ceramics; Hot pressing; Microstructure; Mechanical properties
1. Introduction Hafnium carbide (HfC), as one of the most promising ultrahigh temperature ceramics (UHTC), has attracted considerable attention due to its excellent properties of high melting point and hardness, high thermal and electrical conductivity and excellent chemical and physical stability at high temperatures.1–3 These properties make it a potential candidate material for ultrahigh temperature structural applications, such as atmospheric re-entry, hypersonic flight, and high temperature electrodes, nozzles and armor.4,5 However, despite a lot of excellent properties for different applications, the lack of reliability in mechanical properties because of the low fracture toughness has prevented its use in the high temperature structural applications to date.6–8 For the high temperature anti-ablation structural applications, full density HfC ceramics with high fracture toughness and good thermal shock resistance are expected. The most promising candidates for improving ceramic toughness have been continuous fiber ceramic composites, which have shown high toughness and good thermal shock resistance.9 However, the fiber-reinforced
∗
Corresponding author. Tel.: +86 29 88494622; fax: +86 29 88494620. E-mail addresses: [email protected] (L. Xiang), [email protected] (L. Cheng).
ultra-high temperature ceramic composites are not full density. Ceramics with laminated structure have been considered as one simple and effective approach that can improve toughness and achieve full density.10–12 Over the past two decades, a lot of researches of laminated structure ceramics have been carried out in several systems, such as laminated SiC, Si3 N4 and Al2 O3 ceramic.13–17 However, the research on laminated ultrahigh temperature ceramics (LUHTC) has been rarely reported, only a few papers have been devoted to the study of laminated ZrB2 -based ceramics.18–20 In the present study, laminated HfC–SiC ceramics with BN interface layers were fabricated by tape casting and hot pressing. The microstructure and mechanical properties of the laminated HfC–SiC ceramic were investigated in detail. 2. Experimental 2.1. Materials and processing Commercially available HfC powders (cubic, average particle size 0.8 m, 99.6% purity, oxygen content ∼0.2 wt%, Shanghai Chao Wei Nami Technology Co. Ltd., China), SiC powders (-phase, average particle size 0.5 m, 99.5% purity, Shanghai Chao Wei Nami Technology Co. Ltd., China), MoSi2 powders (Tetragonal, average particle size 2 m, 99.9% purity,
http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.021 0955-2219/© 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Xiang L, et al. Fabrication and mechanical properties of laminated HfC–SiC/BN ceramics. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.021
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Shanghai st-nano science and technology Co. Ltd., China), BN powders (hexagonal, average particle size 0.6 m, 99.9% purity, Shanghai Chao Wei Nami Technology Co. Ltd., China) were used as raw materials. A mixture of isopropanol and toluene with volume ratio 1/1 were used as solvent. Triethyl phosphate was used as dispersant. Polyvinyl butyral (PVB) was used as binder. Glycerol and dioctyl phthalate with volume ratio 1/1 were used as plasticizer. In this study, the designed composition of the HfC–SiC matrix layers was 65 vol.% HfC, 30 vol.% SiC and sinter additives of 5 vol.% MoSi2 . 20 vol.% SiC was incorporated into the BN layers for the purpose of improving the bonding strength between matrix layers. The HfC–SiC sheets and BN sheets were first prepared separately using tape casting. The powder mixtures with solvent and dispersant were first ball milled for 24 h with ZrO2 balls as milling media, followed by adding binder and plasticizer. Then the slurries were ball milled for another 24 h. The volume ratios of solvent, dispersant, binder and plasticizer to powder mixtures in the HfC–SiC and BN slurries were 17:1:1:1:5 and 42:1:1:1:5, respectively. After homogenization, the slurries were degassed under vacuum to remove air bubbles. Tape casting was performed on small laboratory tape casting equipment (LY-150-1, Beijing Orient Sun-Tec Co. Ltd., Beijing, China). After drying at ambient temperature, the thicknesses of the HfC–SiC and BN green sheets were ∼330 m and 30 m, respectively. The dried green tapes were then cut into squares with dimensions of 40 mm × 40 mm. The HfC–SiC sheets and BN sheets were alternately stacked until the final thicknesses of as-prepared ceramics attained 3 mm for flexural strength test and 4 mm for fracture toughness test, respectively. For comparison, the monolithic HfC–SiC samples were fabricated using the same composition and processing of the laminated HfC–SiC samples, but without the BN layers. The stacked bodies were pressed by cold isostatic pressing at room temperature, and were then put into graphite dies. Binder removal was carried out under an argon atmosphere at 600 ◦ C for 2 h. Then the samples were hot pressed at 1950 ◦ C for 1 h under a specific pressure of 30 MPa in Ar atmosphere.
2.2. Characterization and testing The apparent density of the as-synthesized samples was measured by using Archimedes’ method using distilled water as the immersing medium as described in ASTM C20. The flexural strength was measured in three-point bending on size of 3.0 mm × 4.0 mm × 36.0 mm bars with a span of 30 mm and a crosshead speed of 0.5 mm/min at room temperature. The fracture toughness was evaluated by a single-edge notched beam (SENB) test with a span of 16 mm and a cross-head speed of 0.05 mm/min using 2 mm × 4 mm × 22 mm (width × thickness × length, respectively) bars on the same jig used for the flexural strength at room temperature. The notch was cut by electrical discharge machining and then finished with a razor blade and diamond paste. The notch length was about 2 mm, which was measured accurately by Scanning Electron Microscopy (SEM, S-4700, Hitachi, Japan). The
Fig. 1. Schematic showing the direction of testing (a) parallel to the stacking direction (nominated as ‘parallel direction’) and (b) perpendicular to the stacking direction (nominated as ‘perpendicular direction’).
fracture toughness was calculated according to the following equation21 : KIC =
a P S f B W 3/2 W
(1)
In Eq. (1), f(a/W) is a function of the notch length and thickness of specimen, as shown in Eq. (2). f
a a 1/2 a 3/2 a 5/2 = 2.9 − 4.6 + 21.8 W W W W a 7/2 a 9/2 − 37.6 + 38.7 W W
(2)
where P is the applied load, a is the notch length, B is the width of the specimen, W is the thickness of the specimen, and S is the supporting span. The mechanical properties of the laminated ceramic show a significant difference depending on the direction of testing, i.e. in the directions parallel and perpendicular to the stacking direction. Therefore, the mechanical properties were measured with two opposite orientations, as shown in Fig. 1. The flexural strength and fracture toughness were calculated by averaging five individual measurements. Microstructure analyses of the samples were conducted in a scanning electron microscope (SEM, S-4700, Hitachi, Japan) with an energy-dispersive spectroscopy (EDS) system.
Please cite this article in press as: Xiang L, et al. Fabrication and mechanical properties of laminated HfC–SiC/BN ceramics. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.021
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Fig. 2. SEM images of the polished cross-section of the laminated HfC–SiC/BN ceramic: (a and b) low magnification; (c and d) higher magnification of the HfC–SiC and BN layers.
Fig. 3. SEM images of fracture surface of the laminated HfC–SiC ceramics: (a and b) low magnification; (c and d) higher magnification of the HfC–SiC and BN layers.
Please cite this article in press as: Xiang L, et al. Fabrication and mechanical properties of laminated HfC–SiC/BN ceramics. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.021
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Fig. 4. SEM images of crack propagating paths of the laminated and monolithic HfC–SiC ceramics after the SENB test: (a–c) parallel direction of laminated ceramics ((a) propagating of a major crack with a zigzag path; (b) interlocking of tooth-like; and (c) delamination crack and crack bifurcation); (d) perpendicular direction of laminated ceramics; and (e) monolithic ceramic.
3. Results and discussion 3.1. Microstructures SEM images of the polished cross-section of the laminated HfC–SiC/BN ceramic are shown in Fig. 2. It can be seen that the laminated structure can be observed clearly, in which the gray layers were HfC–SiC and the dark layers were BN. The BN layer was straight and its thickness was very uniform. The thicknesses of the HfC–SiC and BN layers were about 160 m and 10 m, respectively. According to the EDS analysis, the gray regions represent HfC matrix and the black regions represent SiC particles, as shown in Fig. 2(c). The SiC particles were homogeneously dispersed in the HfC matrix. No obvious pores were observed in the HfC–SiC layers, which was in agreement with that the relative density of the as-sintered ceramic peaked at 99.6%. In Fig. 2(d), obvious pits were observed in the BN layers, which indicated that the bonding strength of the BN layers
was weak, resulting in significant desquamation of BN particle during the polishing process. The above results indicated that the laminated HfC–SiC ceramics with full density and homogeneous structure can be obtained by tape casting and hot pressing. Fig. 3 shows the SEM images of fracture surface of the laminated HfC–SiC ceramics after the fracture toughness testing. As shown in Fig. 3(a), a typical step-wise fracture can be observed, which is caused by the weak interface deviating the initial straight crack path. Moreover, as can be seen clearly in Fig. 3(c), the fracture modes of the HfC–SiC matrix layer are coaction by intergranular and transgranular fracture. The change of the fracture mode also played an important role in improving the fracture toughness. During loading, the pull out of HfC and SiC particles could occur, which enlarged the crack propagation path and consumed lots of energy to inhibit the growth of crack. In addition, the interfacial de-bonding (Fig. 3(d)) of the HfC–SiC and BN layers occurred during the fracture process was observed, which absorbed more fracture energy.
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Table 1 Mechanical properties of the laminated and monolithic HfC–SiC ceramics. Samples
Load orientation
Flexural strength (MPa)
Fracture toughness (MPa m1/2 )
Laminated HfC–SiC ceramMonolithic HfC–SiC ceramics ics
Parallel Perpendicular
365 ± 36 400 ± 42
8.06 ± 0.46 5.61 ± 0.29
–
430 ± 26
3.73 ± 0.33
3.2. Mechanical properties Fig. 4 shows crack propagating paths of the laminated HfC–SiC/BN ceramic (Fig. 4(a)–(d)) and monolithic HfC–SiC ceramic (Fig. 4(e)) after the SENB test. Compared with the monolithic HfC–SiC ceramic, the laminated HfC–SiC/BN ceramic exhibited a zigzag crack propagating paths. The crack of the laminated HfC–SiC/BN ceramic in parallel direction propagated vertically first and then deflected into the soft BN layer when the soft-rigid interface was encountered. Then the crack propagated horizontally within the soft BN layer. As the load further increased, the crack turned back and propagated vertically again into the HfC–SiC layer. The crack propagated alternately within the HfC–SiC layer and the weak BN layer until the specimen finally fractured. Besides the deflection of crack from BN layers, there are the interlocking of tooth-like, debond layers (Fig. 4(b)), delamination crack and crack bifurcation (Fig. 4(c)) toughening mechanisms in the laminated HfC–SiC ceramic. The repetitious crack deflection and bifurcation markedly increased the crack propagating path and absorbed more fracture energy, which is favorable to improving the fracture toughness of the laminated HfC–SiC ceramic. The flexural strength and fracture toughness of the laminated and monolithic HfC–SiC ceramics are listed in Table 1. The flexural strength of the laminated HfC–SiC ceramics is 365 ± 36 MPa in parallel direction, and 400 ± 42 MPa in perpendicular direction, respectively, which was lower than that of the monolithic HfC–SiC ceramics (430 ± 26 MPa). The decrease in flexural strength is related to the introduction of the BN interlayer due to the weaker bonding of the HfC–SiC layer and the weak BN layer and the lower mechanical property of the weak BN layers as well as the flaws in the weak BN layers.19 Therefore, the total strength of laminated composites with an interfacial separating layer with a fixed composition was determined by the ceramic matrix layers. The stronger the matrix layer, the higher the strength of the laminated composites. The fracture toughness of the laminated HfC–SiC ceramics is 8.06 ± 0.46 MPa m1/2 in the parallel direction, and 5.61 ± 0.29 MPa m1/2 in the perpendicular direction, respectively. Compared with the monolithic HfC–SiC ceramics (3.73 ± 0.33 MPa m1/2 ), the fracture toughness of the laminated HfC–SiC ceramics in the parallel direction was increased by 115%. The remarkable improvement of the fracture toughness in the parallel direction is attributed to multiple toughening mechanisms including crack deflection, crack bifurcation and delamination crack. The difference of the strength and fracture toughness of laminated ceramics in different testing directions can be attributed to the different crack propagation paths during loading.14,19,22 To be specific, as to parallel direction, the
crack can deflect into the weak BN layer, and propagate horizontally within BN layer (Fig. 4(a)–(c)). However, for perpendicular direction (Fig. 4(d)), it is difficult for the crack to attain the horizontal propagation. This difference can lead to a variation of fracture mode. Therefore, the strength and fracture toughness of laminated ceramics in different testing directions showed an obvious anisotropy. In order to further investigate the fracture behavior and toughening mechanism of the laminated HfC–SiC ceramics, typical load–displacement curves of the monolithic and laminated HfC–SiC ceramics with parallel and perpendicular directions during three-point bending test are presented in Fig. 5. It can be seen that the curve of the monolithic HfC–SiC ceramics (Fig. 5(b)) has only one peak point, which illustrates that the fracture is a one-off event and the development of the crack is rapid. However, the curves of the laminated HfC–SiC/BN ceramics in
Fig. 5. The typical load–displacement curves of the (a) laminated and (b) monolithic HfC–SiC ceramics.
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both parallel and perpendicular directions (Fig. 5(a)) have several peak points, which illustrates that the fracture is no longer a rapid process but non-catastrophic behavior. This shows that the laminated ceramic composites exhibit a different fracture behavior from that of the monolithic ceramic. It indicates that such laminated structure design is a promising method to obtain high fracture toughness and reliability of HfC–SiC ceramics for the high temperature structural applications. 4. Conclusions Laminated HfC–SiC/BN ceramics were successfully fabricated by tape casting, laminating and hot pressing. The relative density of the as-sintered ceramics peaked at 99.6%. The composites showed non-catastrophic fracture behaviors in both parallel and perpendicular directions. The flexural strength and fracture toughness of the laminated HfC–SiC ceramics were 365 ± 36 MPa and 8.06 ± 0.46 MPa m1/2 for parallel direction, and 400 ± 42 MPa and 5.61 ± 0.29 MPa m1/2 for perpendicular direction, respectively. The flexural strength of laminated HfC–SiC ceramics slightly decreased compared with that of monolithic HfC–SiC ceramics, due to the introduction of the weak BN interlayer. However, the fracture toughness of parallel direction was significantly improved. The remarkable improvement of fracture toughness was attributed to multiple toughening mechanisms including crack deflection, crack bifurcation and delamination crack. It was found that such laminated structure design was a promising method to obtain full density and high fracture toughness HfC–SiC ceramics. The optimization of the thickness of each layer and the bonding strength control of the weak interface are also needed to investigate deeply, which can further improve the mechanical properties of the laminated HfC–SiC/BN ceramics. Acknowledgements This work was financially supported by the Natural Science Foundation of China (51272210, 51032006 and 90916030) and the 111 Project (B08040). The authors are grateful for the help of Professor Xiaowei Yin in the revision. References 1. Sciti D, Silvestroni L, Bellosi A. High-density pressureless-sintered HfCbased composites. J Am Ceram Soc 2006;89:2668–70. 2. Sun SK, Zhang GJ, Wu WW, Liu JX, Suzuki T, Sakka Y. Reactive spark plasma sintering of ZrC and HfC ceramics with fine microstructures. Scripta Mater 2013;69:139–42.
3. Sciti D, Guicciardi S, Nygren M. Densification and mechanical behavior of HfC and HfB2 fabricated by spark plasma sintering. J Am Ceram Soc 2008;91:1433–40. 4. Opeka MM, Talmy IG, Wuchina EJ, Zaykoski JA, Causey SJ. Mechanical, thermal, and oxidation properties of refractory hafnium and zirconium compounds. J Eur Ceram Soc 1999;19:2405–14. 5. Silvestroni L, Sciti D, Kling J, Lauterbach S, Kleebe HJ. Sintering mechanisms of zirconium and hafnium carbides doped with MoSi2 . J Am Ceram Soc 2009;92:1574–9. 6. Liu JX, Huang X, Zhang GJ. Pressureless sintering of hafnium carbide–silicon carbide ceramics. J Am Ceram Soc 2013;96: 1751–6. 7. Savino R, Fumo MDS, Silvestroni L, Sciti D. Arc-jet testing on HfB2 and HfC-based ultra-high temperature ceramic materials. J Eur Ceram Soc 2008;28:1899–907. 8. Silvestroni L, Bellosi A, Melandri C, Sciti D, Liu JX, Zhang GJ. Microstructure and properties of HfC and TaC-based ceramics obtained by ultrafine powder. J Eur Ceram Soc 2011;31:619–27. 9. Tomaszewski H, Weglarz H, Wajler A, Boniecki M, Kalinski D. Multilayer ceramic composites with high failure resistance. J Eur Ceram Soc 2007;27:1373–7. 10. Clegg WJ, Kendall K, Alford NM, Button TW, Birchall JD. A simple way to make tough ceramics. Nature 1990;347:455–7. 11. Ma J, Wang H, Weng L, Tan GEB. Effect of porous interlayers on crack deflection in ceramic laminates. J Eur Ceram Soc 2004;24: 825–31. 12. Vandeperre LJ, Kristoffersson A, Carlström E, Clegg WJ. Thermal shock of layered ceramic structures with crack-deflecting interfaces. J Am Ceram Soc 2001;84:104–10. 13. Zhang J, Huang R, Gu H, Jiang D, Lin Q, Huang Z. High toughness in laminated SiC ceramics from aqueous tape casting. Scripta Mater 2005;52:381–5. 14. Windsheimer H, Travitzky N, Hofenauer A, Greil P. Laminated object manufacturing of preceramic-paper-derived Si–SiC composites. Adv Mater 2007;19:4515–9. 15. She J, Inoue T, Ueno K. Multilayer Al2 O3 /SiC ceramics with improved mechanical behavior. J Eur Ceram Soc 2000;20:1771–5. 16. Ohji T, Shigegaki Y, Miyajima T. Fracture resistance behavior of multilayered silicon nitride. J Am Ceram Soc 1997;80:991–4. 17. Mawdsley JR, Kovar D, Halloran JW. Fracture behavior of alumina/monazite multilayer laminates. J Am Ceram Soc 2000;83: 802–8. 18. Zhou P, Hu P, Zhang X, Han W. Laminated ZrB2 –SiC ceramic with improved strength and toughness. Scripta Mater 2011;64:276–9. 19. Wei C, Zhang X, Hu P, Han W, Tian G. The fabrication and mechanical properties of bionic laminated ZrB2 -SiC/BN ceramic prepared by tape casting and hot pressing. Scripta Mater 2011;65:791–4. 20. Lü Z, Jiang D, Zhang J, Lin Q, Huang Z. ZrB2 -SiC laminated ceramic composites. J Eur Ceram Soc 2012;32:1435–9. 21. Yin X, He S, Zhang L, Fan S, Cheng L, Tian G, et al. Fabrication and characterization of a carbon fibre reinforced carbon-silicon carbidetitanium silicon carbide hybrid matrix composite. Mater Sci Eng A 2010;527: 835–41. 22. Wei C, Zhang X, Hu P, Han W, Li S. Microstructure and mechanical properties of laminated ZrB2 -SiC ceramics with ZrO2 interface layers. Int J Refract Met Hard Mater 2012;30:173–6.
Please cite this article in press as: Xiang L, et al. Fabrication and mechanical properties of laminated HfC–SiC/BN ceramics. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.021
ARTICLE IN PRESS Available online at www.sciencedirect.com
ScienceDirect Journal of the European Ceramic Society xxx (2014) xxx–xxx
Fabrication and mechanical properties of laminated HfC–SiC/BN ceramics Liuyi Xiang, Laifei Cheng ∗ , Yi Hou, Fuyuan Wang, Liangjun Li, Litong Zhang Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, PR China Received 18 December 2013; received in revised form 29 March 2014; accepted 10 April 2014
Abstract Laminated HfC–SiC/BN ceramics were successfully fabricated by tape casting and hot pressing. Fully dense HfC–SiC ultra-high temperature ceramics with homogeneous structure were obtained. The introduction of the weak BN layer resulted in a slight decrease of the flexural strength but significantly improved the fracture toughness compared with monolithic HfC–SiC ceramics. The fracture toughness of laminated HfC–SiC/BN ceramics in the parallel direction peaked at 8.06 ± 0.46 MPa m1/2 , which increased by 115% than that of monolithic HfC–SiC ceramics. The composites showed non-catastrophic fracture behaviors in both parallel and perpendicular directions. It indicates that laminated structure design is a promising approach to obtain full density HfC–SiC ceramics with high fracture toughness. © 2014 Elsevier Ltd. All rights reserved. Keywords: Laminated HfC–SiC ceramics; Hot pressing; Microstructure; Mechanical properties
1. Introduction Hafnium carbide (HfC), as one of the most promising ultrahigh temperature ceramics (UHTC), has attracted considerable attention due to its excellent properties of high melting point and hardness, high thermal and electrical conductivity and excellent chemical and physical stability at high temperatures.1–3 These properties make it a potential candidate material for ultrahigh temperature structural applications, such as atmospheric re-entry, hypersonic flight, and high temperature electrodes, nozzles and armor.4,5 However, despite a lot of excellent properties for different applications, the lack of reliability in mechanical properties because of the low fracture toughness has prevented its use in the high temperature structural applications to date.6–8 For the high temperature anti-ablation structural applications, full density HfC ceramics with high fracture toughness and good thermal shock resistance are expected. The most promising candidates for improving ceramic toughness have been continuous fiber ceramic composites, which have shown high toughness and good thermal shock resistance.9 However, the fiber-reinforced
∗
Corresponding author. Tel.: +86 29 88494622; fax: +86 29 88494620. E-mail addresses: [email protected] (L. Xiang), [email protected] (L. Cheng).
ultra-high temperature ceramic composites are not full density. Ceramics with laminated structure have been considered as one simple and effective approach that can improve toughness and achieve full density.10–12 Over the past two decades, a lot of researches of laminated structure ceramics have been carried out in several systems, such as laminated SiC, Si3 N4 and Al2 O3 ceramic.13–17 However, the research on laminated ultrahigh temperature ceramics (LUHTC) has been rarely reported, only a few papers have been devoted to the study of laminated ZrB2 -based ceramics.18–20 In the present study, laminated HfC–SiC ceramics with BN interface layers were fabricated by tape casting and hot pressing. The microstructure and mechanical properties of the laminated HfC–SiC ceramic were investigated in detail. 2. Experimental 2.1. Materials and processing Commercially available HfC powders (cubic, average particle size 0.8 m, 99.6% purity, oxygen content ∼0.2 wt%, Shanghai Chao Wei Nami Technology Co. Ltd., China), SiC powders (-phase, average particle size 0.5 m, 99.5% purity, Shanghai Chao Wei Nami Technology Co. Ltd., China), MoSi2 powders (Tetragonal, average particle size 2 m, 99.9% purity,
http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.021 0955-2219/© 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Xiang L, et al. Fabrication and mechanical properties of laminated HfC–SiC/BN ceramics. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.021
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Shanghai st-nano science and technology Co. Ltd., China), BN powders (hexagonal, average particle size 0.6 m, 99.9% purity, Shanghai Chao Wei Nami Technology Co. Ltd., China) were used as raw materials. A mixture of isopropanol and toluene with volume ratio 1/1 were used as solvent. Triethyl phosphate was used as dispersant. Polyvinyl butyral (PVB) was used as binder. Glycerol and dioctyl phthalate with volume ratio 1/1 were used as plasticizer. In this study, the designed composition of the HfC–SiC matrix layers was 65 vol.% HfC, 30 vol.% SiC and sinter additives of 5 vol.% MoSi2 . 20 vol.% SiC was incorporated into the BN layers for the purpose of improving the bonding strength between matrix layers. The HfC–SiC sheets and BN sheets were first prepared separately using tape casting. The powder mixtures with solvent and dispersant were first ball milled for 24 h with ZrO2 balls as milling media, followed by adding binder and plasticizer. Then the slurries were ball milled for another 24 h. The volume ratios of solvent, dispersant, binder and plasticizer to powder mixtures in the HfC–SiC and BN slurries were 17:1:1:1:5 and 42:1:1:1:5, respectively. After homogenization, the slurries were degassed under vacuum to remove air bubbles. Tape casting was performed on small laboratory tape casting equipment (LY-150-1, Beijing Orient Sun-Tec Co. Ltd., Beijing, China). After drying at ambient temperature, the thicknesses of the HfC–SiC and BN green sheets were ∼330 m and 30 m, respectively. The dried green tapes were then cut into squares with dimensions of 40 mm × 40 mm. The HfC–SiC sheets and BN sheets were alternately stacked until the final thicknesses of as-prepared ceramics attained 3 mm for flexural strength test and 4 mm for fracture toughness test, respectively. For comparison, the monolithic HfC–SiC samples were fabricated using the same composition and processing of the laminated HfC–SiC samples, but without the BN layers. The stacked bodies were pressed by cold isostatic pressing at room temperature, and were then put into graphite dies. Binder removal was carried out under an argon atmosphere at 600 ◦ C for 2 h. Then the samples were hot pressed at 1950 ◦ C for 1 h under a specific pressure of 30 MPa in Ar atmosphere.
2.2. Characterization and testing The apparent density of the as-synthesized samples was measured by using Archimedes’ method using distilled water as the immersing medium as described in ASTM C20. The flexural strength was measured in three-point bending on size of 3.0 mm × 4.0 mm × 36.0 mm bars with a span of 30 mm and a crosshead speed of 0.5 mm/min at room temperature. The fracture toughness was evaluated by a single-edge notched beam (SENB) test with a span of 16 mm and a cross-head speed of 0.05 mm/min using 2 mm × 4 mm × 22 mm (width × thickness × length, respectively) bars on the same jig used for the flexural strength at room temperature. The notch was cut by electrical discharge machining and then finished with a razor blade and diamond paste. The notch length was about 2 mm, which was measured accurately by Scanning Electron Microscopy (SEM, S-4700, Hitachi, Japan). The
Fig. 1. Schematic showing the direction of testing (a) parallel to the stacking direction (nominated as ‘parallel direction’) and (b) perpendicular to the stacking direction (nominated as ‘perpendicular direction’).
fracture toughness was calculated according to the following equation21 : KIC =
a P S f B W 3/2 W
(1)
In Eq. (1), f(a/W) is a function of the notch length and thickness of specimen, as shown in Eq. (2). f
a a 1/2 a 3/2 a 5/2 = 2.9 − 4.6 + 21.8 W W W W a 7/2 a 9/2 − 37.6 + 38.7 W W
(2)
where P is the applied load, a is the notch length, B is the width of the specimen, W is the thickness of the specimen, and S is the supporting span. The mechanical properties of the laminated ceramic show a significant difference depending on the direction of testing, i.e. in the directions parallel and perpendicular to the stacking direction. Therefore, the mechanical properties were measured with two opposite orientations, as shown in Fig. 1. The flexural strength and fracture toughness were calculated by averaging five individual measurements. Microstructure analyses of the samples were conducted in a scanning electron microscope (SEM, S-4700, Hitachi, Japan) with an energy-dispersive spectroscopy (EDS) system.
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Fig. 2. SEM images of the polished cross-section of the laminated HfC–SiC/BN ceramic: (a and b) low magnification; (c and d) higher magnification of the HfC–SiC and BN layers.
Fig. 3. SEM images of fracture surface of the laminated HfC–SiC ceramics: (a and b) low magnification; (c and d) higher magnification of the HfC–SiC and BN layers.
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Fig. 4. SEM images of crack propagating paths of the laminated and monolithic HfC–SiC ceramics after the SENB test: (a–c) parallel direction of laminated ceramics ((a) propagating of a major crack with a zigzag path; (b) interlocking of tooth-like; and (c) delamination crack and crack bifurcation); (d) perpendicular direction of laminated ceramics; and (e) monolithic ceramic.
3. Results and discussion 3.1. Microstructures SEM images of the polished cross-section of the laminated HfC–SiC/BN ceramic are shown in Fig. 2. It can be seen that the laminated structure can be observed clearly, in which the gray layers were HfC–SiC and the dark layers were BN. The BN layer was straight and its thickness was very uniform. The thicknesses of the HfC–SiC and BN layers were about 160 m and 10 m, respectively. According to the EDS analysis, the gray regions represent HfC matrix and the black regions represent SiC particles, as shown in Fig. 2(c). The SiC particles were homogeneously dispersed in the HfC matrix. No obvious pores were observed in the HfC–SiC layers, which was in agreement with that the relative density of the as-sintered ceramic peaked at 99.6%. In Fig. 2(d), obvious pits were observed in the BN layers, which indicated that the bonding strength of the BN layers
was weak, resulting in significant desquamation of BN particle during the polishing process. The above results indicated that the laminated HfC–SiC ceramics with full density and homogeneous structure can be obtained by tape casting and hot pressing. Fig. 3 shows the SEM images of fracture surface of the laminated HfC–SiC ceramics after the fracture toughness testing. As shown in Fig. 3(a), a typical step-wise fracture can be observed, which is caused by the weak interface deviating the initial straight crack path. Moreover, as can be seen clearly in Fig. 3(c), the fracture modes of the HfC–SiC matrix layer are coaction by intergranular and transgranular fracture. The change of the fracture mode also played an important role in improving the fracture toughness. During loading, the pull out of HfC and SiC particles could occur, which enlarged the crack propagation path and consumed lots of energy to inhibit the growth of crack. In addition, the interfacial de-bonding (Fig. 3(d)) of the HfC–SiC and BN layers occurred during the fracture process was observed, which absorbed more fracture energy.
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Table 1 Mechanical properties of the laminated and monolithic HfC–SiC ceramics. Samples
Load orientation
Flexural strength (MPa)
Fracture toughness (MPa m1/2 )
Laminated HfC–SiC ceramMonolithic HfC–SiC ceramics ics
Parallel Perpendicular
365 ± 36 400 ± 42
8.06 ± 0.46 5.61 ± 0.29
–
430 ± 26
3.73 ± 0.33
3.2. Mechanical properties Fig. 4 shows crack propagating paths of the laminated HfC–SiC/BN ceramic (Fig. 4(a)–(d)) and monolithic HfC–SiC ceramic (Fig. 4(e)) after the SENB test. Compared with the monolithic HfC–SiC ceramic, the laminated HfC–SiC/BN ceramic exhibited a zigzag crack propagating paths. The crack of the laminated HfC–SiC/BN ceramic in parallel direction propagated vertically first and then deflected into the soft BN layer when the soft-rigid interface was encountered. Then the crack propagated horizontally within the soft BN layer. As the load further increased, the crack turned back and propagated vertically again into the HfC–SiC layer. The crack propagated alternately within the HfC–SiC layer and the weak BN layer until the specimen finally fractured. Besides the deflection of crack from BN layers, there are the interlocking of tooth-like, debond layers (Fig. 4(b)), delamination crack and crack bifurcation (Fig. 4(c)) toughening mechanisms in the laminated HfC–SiC ceramic. The repetitious crack deflection and bifurcation markedly increased the crack propagating path and absorbed more fracture energy, which is favorable to improving the fracture toughness of the laminated HfC–SiC ceramic. The flexural strength and fracture toughness of the laminated and monolithic HfC–SiC ceramics are listed in Table 1. The flexural strength of the laminated HfC–SiC ceramics is 365 ± 36 MPa in parallel direction, and 400 ± 42 MPa in perpendicular direction, respectively, which was lower than that of the monolithic HfC–SiC ceramics (430 ± 26 MPa). The decrease in flexural strength is related to the introduction of the BN interlayer due to the weaker bonding of the HfC–SiC layer and the weak BN layer and the lower mechanical property of the weak BN layers as well as the flaws in the weak BN layers.19 Therefore, the total strength of laminated composites with an interfacial separating layer with a fixed composition was determined by the ceramic matrix layers. The stronger the matrix layer, the higher the strength of the laminated composites. The fracture toughness of the laminated HfC–SiC ceramics is 8.06 ± 0.46 MPa m1/2 in the parallel direction, and 5.61 ± 0.29 MPa m1/2 in the perpendicular direction, respectively. Compared with the monolithic HfC–SiC ceramics (3.73 ± 0.33 MPa m1/2 ), the fracture toughness of the laminated HfC–SiC ceramics in the parallel direction was increased by 115%. The remarkable improvement of the fracture toughness in the parallel direction is attributed to multiple toughening mechanisms including crack deflection, crack bifurcation and delamination crack. The difference of the strength and fracture toughness of laminated ceramics in different testing directions can be attributed to the different crack propagation paths during loading.14,19,22 To be specific, as to parallel direction, the
crack can deflect into the weak BN layer, and propagate horizontally within BN layer (Fig. 4(a)–(c)). However, for perpendicular direction (Fig. 4(d)), it is difficult for the crack to attain the horizontal propagation. This difference can lead to a variation of fracture mode. Therefore, the strength and fracture toughness of laminated ceramics in different testing directions showed an obvious anisotropy. In order to further investigate the fracture behavior and toughening mechanism of the laminated HfC–SiC ceramics, typical load–displacement curves of the monolithic and laminated HfC–SiC ceramics with parallel and perpendicular directions during three-point bending test are presented in Fig. 5. It can be seen that the curve of the monolithic HfC–SiC ceramics (Fig. 5(b)) has only one peak point, which illustrates that the fracture is a one-off event and the development of the crack is rapid. However, the curves of the laminated HfC–SiC/BN ceramics in
Fig. 5. The typical load–displacement curves of the (a) laminated and (b) monolithic HfC–SiC ceramics.
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both parallel and perpendicular directions (Fig. 5(a)) have several peak points, which illustrates that the fracture is no longer a rapid process but non-catastrophic behavior. This shows that the laminated ceramic composites exhibit a different fracture behavior from that of the monolithic ceramic. It indicates that such laminated structure design is a promising method to obtain high fracture toughness and reliability of HfC–SiC ceramics for the high temperature structural applications. 4. Conclusions Laminated HfC–SiC/BN ceramics were successfully fabricated by tape casting, laminating and hot pressing. The relative density of the as-sintered ceramics peaked at 99.6%. The composites showed non-catastrophic fracture behaviors in both parallel and perpendicular directions. The flexural strength and fracture toughness of the laminated HfC–SiC ceramics were 365 ± 36 MPa and 8.06 ± 0.46 MPa m1/2 for parallel direction, and 400 ± 42 MPa and 5.61 ± 0.29 MPa m1/2 for perpendicular direction, respectively. The flexural strength of laminated HfC–SiC ceramics slightly decreased compared with that of monolithic HfC–SiC ceramics, due to the introduction of the weak BN interlayer. However, the fracture toughness of parallel direction was significantly improved. The remarkable improvement of fracture toughness was attributed to multiple toughening mechanisms including crack deflection, crack bifurcation and delamination crack. It was found that such laminated structure design was a promising method to obtain full density and high fracture toughness HfC–SiC ceramics. The optimization of the thickness of each layer and the bonding strength control of the weak interface are also needed to investigate deeply, which can further improve the mechanical properties of the laminated HfC–SiC/BN ceramics. Acknowledgements This work was financially supported by the Natural Science Foundation of China (51272210, 51032006 and 90916030) and the 111 Project (B08040). The authors are grateful for the help of Professor Xiaowei Yin in the revision. References 1. Sciti D, Silvestroni L, Bellosi A. High-density pressureless-sintered HfCbased composites. J Am Ceram Soc 2006;89:2668–70. 2. Sun SK, Zhang GJ, Wu WW, Liu JX, Suzuki T, Sakka Y. Reactive spark plasma sintering of ZrC and HfC ceramics with fine microstructures. Scripta Mater 2013;69:139–42.
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Please cite this article in press as: Xiang L, et al. Fabrication and mechanical properties of laminated HfC–SiC/BN ceramics. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.021