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Microstructure and mechanical properties of laminated ZrB2–SiC ceramics with ZrO2 interface layers
Int. Journal of Refractory Metals and Hard Materials 30 (2012) 173–176
Contents lists available at SciVerse ScienceDirect
Int. Journal of Refractory Metals and Hard Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / I J R M H M
Microstructure and mechanical properties of laminated ZrB2–SiC ceramics with ZrO2 interface layers Chuncheng Wei ⁎, Xinghong Zhang, Ping Hu, Wenbo Han, Shuang Li National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin 150001, PR China
a r t i c l e
i n f o
Article history: Received 9 April 2011 Accepted 9 August 2011 Keywords: Laminated ZrB2 ZrO2 Fracture toughness
a b s t r a c t Laminated ZrB2–SiC ceramics with ZrO2 interface layers were successfully prepared by tape casting, laminating and hot pressing. The flexural strength and fracture toughness are 561 ± 20 MPa and 14.4 ± 0.3 MPa m1/2 for parallel direction, and 432 ± 18 MPa and 5.8 ± 0.3 MPa m 1/2 for perpendicular direction. The fracture toughness for parallel direction is improved significantly compared to monolithic ZrB2–SiC ceramics. The toughening mechanism was attributed to the deflection and branch of the crack and the new microcracks, which would increase the propagation path and fracture work. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction ZrB2 ceramics have been widely used in ultrahigh temperature structure and functional materials although their low thermal shock resistance and fracture toughness restricts the applications under severe circumstances such as the nose cone of the ultrasonic aircraft, the thermal part of the scramjet, etc. [1–7]. Accordingly, the low toughness needs to be improved in order to increase the resistance to thermal shock and the reliability and safety [8–10]. There are two conventional ways of strengthening–toughening of ceramics; one is to eliminate or reduce the raw crack, and the other is to incorporate the reinforcement phase (long fibers, whiskers, phase transformation reinforcements and particle reinforcements). For example, the toughness of ZrB2 ceramics reinforced by ZrO2 phase transformation is up to 6.6 MPa m 1/2[11–13], 6.2 by SiC whisker [14,15], 6.5 by nanosized SiC [16], and 6.1 by graphite [17,18] respectively. Nevertheless, the toughness does not improve significantly according to these data, and many researches have been done to find a new reinforcement way. Inspired by the microstructure of natural shells, the laminated composites design and fabrication call for the temperature withstanding and soft materials into the ceramics in recent years. The concept of laminated composite for improved performance of brittle materials is well established in several systems including laminated Si3N4 ceramics [19,20] and laminated SiC ceramics [21,22]. At present, laminated ZrB2 ultrahigh temperature ceramics reinforced by ZrO2 interface layers have been rarely investigated. In
⁎ Corresponding author. Tel./fax: + 86 451 86402382. E-mail address: [email protected] (C. Wei). 0263-4368/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2011.08.003
the present work, the ZrB2–SiC and ZrO2 green tapes were fabricated with alcohol-based tape casting, respectively. The green samples were stacked and then hot pressed. Finally, the laminated ZrB2–SiC ceramics were obtained successfully. The microstructure and mechanical properties of the laminated ZrB2–SiC ceramics were also investigated in detail. 2. Experimental procedure Commercially available ZrB2 powder (mean particle size 2 μm; Northwest Institute for Non-ferrous Metal Research, China), SiC (mean particle size 0.5 μm; Weifang Kaihua Micro-powder Co., Ltd., China) and ZrO2 (3Y) (mean particle size 50 nm; Zibo Guangtong Chemical Co., Ltd., China) were used as raw powders. The designed compositions of the laminated ZrB2–SiC ceramics with ZrO2 interface layers were 80 vol.% ZrB2, 10 vol.% SiC and 10 vol.% ZrO2. The powder mixtures of ZrB2 plus 20 vol.% SiC were ball-mixed for 24 h in a polyethylene bottle using ZrO2 balls and ethanol as the grinding media, followed by adding polyvinyl butyral resin (PVB) and polyethylene glycol-6000 (PEG-6000) as adhesive and plasticizer. Then the slurries were ball milled for another 12 h. The mass ratio of ethanol, PVB and PEG-6000 to ZrB2–SiC mixtures in slurry was 15:1:1:10. The raw ZrB2–SiC sheets and ZrO2 sheets were formed by tape casting. The ZrB2–SiC sheets and ZrO2 sheets were alternately stacked until the desired compositions were achieved. The stacked ZrB2–SiC/ZrO2 body in a graphite mold was heated to 650 °C at a heating rate of 2 °C/min and held for 30 min in a vacuum furnace and then the stacked ZrB2–SiC/ZrO2 body was hot-pressed at 1850 °C under a uniaxial load of 20 MPa for 60 min. The microstructural features of the laminated ZrB2–SiC ceramics were observed by scanning electron microscopy (SEM, FEI Sirion, Holland) with simultaneous chemical analysis by energy dispersive spectroscopy (EDS, EDAX Inc.). The
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phase composition was determined by X-ray diffraction (XRD, Rigaku, Dmax-rb, CuKa= 1.5418 Å). The specimens for mechanical tests were prepared by cutting and machining the final products to the size of 3 mm × 4 mm × 36 mm and 2 mm × 4 mm × 22 mm. The flexural strength was tested in three point bending using a 30-mm span and a crosshead speed of 0.5 mm/min. Fracture toughness was analyzed by a single-edge notched beam (SENB) test with a 16-mm span, a cross-head speed of 0.05 mm/min, a 2 mm notch depth and a smaller than 0.2 mm opening width. As the laminated ZrB2–SiC ceramics showed a preferred orientation, the mechanical properties of laminated ZrB2–SiC ceramics should depend on the testing direction. Therefore, two batches of specimens were machined with opposite orientations, that is, parallel and perpendicular to the laminate direction, respectively. A minimum number of 5 specimens were tested for each experimental condition.
The microstructure of the laminated ZrB2 ceramics fracture is shown in Fig. 3. From Fig. 3a, the fracture of the ZrB2 layers is plain, but some obvious deflection appears in the ZrO2 layers, which makes the fracture become ladder. It indicates that the neighboring interfacial layer possesses much residual stress. From Fig. 3c and d, the microcracks emerge in the ZrO2 layer. The stress of the crack tip is relaxed by absorbing much energy by the microcracks, so the fracture toughness is improved significantly. Because of the different coefficients of thermal expansion between the ZrB2–SiC layers and the ZrO2 layers, the interface of adjacent layers results in residual stress. The thermal stresses can be estimated using Hsueh's formula [23].
σ0 =
αm −αg ΔT ð1 + vm Þ
ð1 + vg ÞEm
3. Results and discussion 3.1. Microstructure Fig. 1 shows the XRD patters of the surface and the side in the laminated ZrB2 ceramics. There exists only ZrB2 and SiC on the surface, as shown in Fig. 1a. It indicates that hard laminates were exposed on the surface and the soft ones have been dropped when the specimen is machined. However, there appears obvious tetragonal ZrO2 on the sides (Fig. 1b), which shows that ZrO2 (3Y) has not transformed into cubic ZrO2 but sintered to tetragonal one at 1850 °C. It is beneficial to phase transformation reinforcement, that is to say tetragonal ZrO2 transforms into monoclinic ZrO2[11–13]. Fig. 2 shows the SEM images of the cross section for parallel specimens. From Fig. 2a, it is clearly seen that the ZrB2–SiC layers and ZrO2 layers arrange alternately, the interface is straight and clear, and the thicknesses are uniform too. The ZrB2–SiC layers are dark and thick, whereas the ZrO2 layers are light and thin separating the ZrB2– SiC layers. The thickness of the ZrB2–SiC layers is 280 μm and the thickness of the ZrO2 layers is about 30 μm. The volume fraction of ZrO2 is 9.6%, which indicates that alcohol-based tape casting could control the composition of laminated ZrB2–SiC ceramics precisely. From Fig. 2b, it can be seen that the interfacial bonding properties are good. The analyzed results from the enlarged picture of Fig. 2c with EDS shows that the black dot is SiC, and the gray one is ZrB2. SiC is distributed among the ZrB2–SiC layers homogeneously. Fig. 2d is the enlarged picture of the ZrO2 layer, which is compact and no pores can be seen. SiC particle is pressed into the ZrO2 layer. Whether or not ZrB2 particle is pressed into the ZrO2 layer could not be observed directly because of the similar colors. The ZrB2 layer and ZrO2 layer immerse each other and form gradient layer, so the interfacial bonding properties are better.
+
ð1−2vg Þ
ð1Þ
Ep
Where σ0 is the thermal residual stress, α is the thermal expansion coefficient, v is the Poisson's ratio, E is Young's modulus, and ΔTis the temperature change (negative for cooling). The subscripts m and g refer to the matrix (ZrB2 or SiC) and ZrO2, respectively. For the laminated ZrB2-based ceramic composites, EZrB2= 490 GPa, ESiC= 550 GPa, E−6 /K, ZrO2=200 GPa,νZrB2= 0.17, νSiC= 0.14, vZrO2= 0.3,αZrB2= 6.7 × 10 αSiC= 4.7 × 10− 6/K, and αZrO2 =11× 10− 6/K [11,18]. The value of ΔT is calculated from hot-pressing temperature to room temperature, namely, 1850 °C− 25 °C = 1825 °C. According to formula, the maximum thermal residual stresses at ZrB2–ZrO2 and SiC–ZrO2 interfaces were evaluated to be about 2.07 GPa and 3.24 GPa, respectively. The existence of such a large stress may weaken the interphase boundaries, leading to a fracture along such boundaries [18]. Fig. 4a, b and c are the microstructures of the cracks propagating paths for parallel specimens after the fracture toughness testing. From Fig. 4a, it is found that the main crack deflects along the ZrO2 interface and forms the step-like cracks microstructure at last, which greatly extends effective crack length and absorbs more fracture energy. Besides, the main cracks, branches and microcracks occurring in the ZrO2 interface further absorb the fracture energy, which also improves fracture toughening. While the fracture in perpendicular direction is brittle fracture, the path of crack is shorter and plainer, which can be seen in Fig. 4d. 3.2. Mechanical properties The mechanical properties of the laminated ZrB2 ceramics are listed in Table 1. The fracture toughness is 14.4 ± 0.3 MPa m 1/2 for parallel direction, and 5.8 ± 0.3 MPa m 1/2 for perpendicular direction, respectively. The fracture toughness for parallel direction was
Fig. 1. XRD spectra obtained from the surface and the side of the laminated ZrB2–SiC ceramics.
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175
Fig. 2. SEM images of a polished cross-section of the laminated ZrB2–SiC ceramics.
approximately improved by 220% than that of ZrB2–SiC composites (4.0–4.5 MPa m 1/2) [18], 122% than that of ZrB2–nano-SiC composites (6.3–6.5) [16], 132% than that of ZrB2–SiCw composites (5.8–6.2 MPa m 1/2) [14,15], and 118% than that of ZrB2–SiC–ZrO2 composites (5.6– 6.6 MPa m 1/2) [11–13]. The improvement in fracture toughness lies in the deflecting, and branching cracks prolonged the propagation path when meeting the ZrO2 interface layers. The ZrO2 phase transformation reinforcement is another reason. The flexural strength of the laminated ZrB2 ceramics is 561 ± 20 MPa for parallel direction, and 432 ± 18 MPa in perpendicular direction, respectively. The difference is due to the brittleness fracture for
perpendicular direction and the successive fracture with crack damage tolerance for parallel direction [21]. These phenomena indicate that fracture manners have effect on the flexural strength of the ceramics. Accordingly, the ZrO2 interface layers play an important role in improving the fracture toughness of the laminated ZrB2–SiC ceramics. 4. Conclusions The laminated ZrB2–SiC ceramics with ZrO2 interface layers are successfully prepared by tape casting, laminating and hot pressing. The flexural strength and fracture toughness are 561 ± 20 MPa and
Fig. 3. SEM images of fracture surface of the laminated ZrB2–SiC ceramics.
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Fig. 4. SEM images of parallel (a, b and c) and perpendicular (d) specimens after SENB test.
Table 1 Mechanical properties of the laminated ZrB2–SiC ceramics. Orientation
Flexural strength (MPa)
Fracture toughness (MPa . m1/2)
Parallel Perpendicular
561 ± 20 432 ± 18
14.4 ± 0.3 5.8 ± 0.3
14.4 ± 0.3 MPa m 1/2 in parallel direction, and 432 ± 18 MPa and 5.8 ± 0.3 MPa m 1/2 in perpendicular direction. The difference lies in the brittleness fracture in perpendicular direction and the successive fracture with crack damage tolerance in parallel direction. The fracture toughness in parallel direction improves significantly. The improvement is ascribed to that the deflecting and branching cracks prolonged the propagation path when meeting the ZrO2 interface layers. The ZrO2 phase transformation reinforcement contributes to another reason. Acknowledgements This work was supported by the National Science Foundation (50972029) of China and project (HIT.KLOF.2009026) supported by the Key Laboratory Opening Funding of National Key Laboratory of Advanced Composites in Special Environments. References [1] Monteverde F, Bellosi A. Development and characterization of metal-diboridebased composites toughened with ultra-fine SiC particulates. Solid State Sci 2005;7:622–30. [2] Yan Y, Huang Z, Dong S, Jiang D. Pressureless sintering of high-density ZrB2–SiC ceramic composites. J Am Ceram Soc 2006;89:3589–92. [3] Zhu S, Fahrenholtz WG, Hilmas GE. Influence of silicon carbide particle size on the microstructure and mechanical properties of zirconium diboride-silicon carbide ceramics. J Eur Ceram Soc 2007;27:2077–83. [4] Hwang SS, Vailiev AL, Padture NP. Improved processing and oxidation-resistance of ZrB2 ultra-high temperature ceramics containing SiC nanodispersoids. Mater Sci Eng A 2007;464:216–22.
[5] Han J, Hu P, Zhang X, Meng S. Oxidation behavior of zirconium diboride-silicon carbide at 1800 °C. Scr Mater 2007;57:825–8. [6] Monteverde F. Beneficial effects of an ultra-fine α-SiC incorporation on the sinterability and mechanical properties of ZrB2. Appl Phys A 2006;82:329–37. [7] Ni D, Zhang G, Kan Y, Sakka Y. Highly textured ZrB2-based ultrahigh temperature ceramics via strong magnetic field alignment. Scr Mater 2009;60:615–8. [8] Chamberlain AL, Fahrenholtz WG, Hilmas GE, Ellerby DT. High strength zirconium diboride-based ceramics. J Am Ceram Soc 2004;87:1170–2. [9] Zhang SC, Hilmas GE, Fahrenholtz WG. Pressureless sintering of ZrB2–SiC ceramics. J Am Ceram Soc 2008;91:26–32. [10] Guo SQ. Densification of ZrB2-based composites and their mechanical and physical properties: a review. J Eur Ceram Soc 2009;29:995–1011. [11] Zhang XH, Li WJ, Hong CQ, Han WB, Han JC. Microstructure and mechanical properties of hot pressed ZrB2–SiCp–ZrO2 composites. Mater Lett 2008;62:2404–6. [12] Bakshi SD, Basu B, Mishra SK. Microstructure and mechanical properties of sinterHIPed ZrO2–ZrB2 composites. Composites: Part A 2006;37:2128–35. [13] Li WJ, Zhang XH, Hong CQ, Han WB, Han JC. Preparation, microstructure and mechanical properties of ZrB2–ZrO2 ceramics. J Eur Ceram Soc 2009;29:779–86. [14] Zhang XH, Xu L, Du SY, Liu CY, Han JC, Han WB. Spark plasma sintering and hot pressing of ZrB2–SiCW ultra-high temperature ceramics. J Alloys Compd 2008;466: 241–5. [15] Zhang P, Hu P, Zhang XH, Han JC, Meng SH. Processing and characterization of ZrB2–SiCW ultra-high temperature ceramics. J Alloys Compd 2009;472:358–62. [16] Liu Q, Han WB, Hu P. Microstructure and mechanical properties of ZrB2–SiC nanocomposite ceramic. Scr Mater 2009;61:690–2. [17] Zhang XH, Wang Z, Sun X, Han WB, Hong CQ. Effect of graphite flake on the mechanical properties of hot pressed ZrB2–SiC ceramics. Mater Lett 2008;62: 4360–6. [18] Wang Z, Wang S, Zhang XH, Hu P, Han WB, Hong CQ. Effect of graphite flake on microstructure as well as mechanical properties and thermal shock resistance of ZrB2–SiC matrix ultrahigh temperature ceramics. J Alloys Compd 2009;484:390–4. [19] Krstic Z, Krstic VD. Fracture toughness of concentric Si3N4-based laminated structures. J Eur Ceram Soc 2009;29:1825–9. [20] Krstic Z, Krstic VD. Young's modulus, density and phase composition of pressureless sintered self-sealed Si3N4/BN laminated structures. J Eur Ceram Soc 2008;28:723–30. [21] Li DY, Qiao GJ, Jin ZH. R-curve behavior of laminated SiC/BN ceramics. Ceram Int 2004;30:213–7. [22] Zhang JX, Huang R, Gu H, Jiang DL, Lin QL, Huang ZR. High toughness in laminated SiC ceramics from aqueous tape casting. Scr Mater 2005;52:381–5. [23] Hsueh CH, Becher PF, Angelini P. Effects of interfacial films on thermal stresses in whisker-reinforced ceramics. J Am Ceram Soc 1988;71:929–33.
Contents lists available at SciVerse ScienceDirect
Int. Journal of Refractory Metals and Hard Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / I J R M H M
Microstructure and mechanical properties of laminated ZrB2–SiC ceramics with ZrO2 interface layers Chuncheng Wei ⁎, Xinghong Zhang, Ping Hu, Wenbo Han, Shuang Li National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin 150001, PR China
a r t i c l e
i n f o
Article history: Received 9 April 2011 Accepted 9 August 2011 Keywords: Laminated ZrB2 ZrO2 Fracture toughness
a b s t r a c t Laminated ZrB2–SiC ceramics with ZrO2 interface layers were successfully prepared by tape casting, laminating and hot pressing. The flexural strength and fracture toughness are 561 ± 20 MPa and 14.4 ± 0.3 MPa m1/2 for parallel direction, and 432 ± 18 MPa and 5.8 ± 0.3 MPa m 1/2 for perpendicular direction. The fracture toughness for parallel direction is improved significantly compared to monolithic ZrB2–SiC ceramics. The toughening mechanism was attributed to the deflection and branch of the crack and the new microcracks, which would increase the propagation path and fracture work. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction ZrB2 ceramics have been widely used in ultrahigh temperature structure and functional materials although their low thermal shock resistance and fracture toughness restricts the applications under severe circumstances such as the nose cone of the ultrasonic aircraft, the thermal part of the scramjet, etc. [1–7]. Accordingly, the low toughness needs to be improved in order to increase the resistance to thermal shock and the reliability and safety [8–10]. There are two conventional ways of strengthening–toughening of ceramics; one is to eliminate or reduce the raw crack, and the other is to incorporate the reinforcement phase (long fibers, whiskers, phase transformation reinforcements and particle reinforcements). For example, the toughness of ZrB2 ceramics reinforced by ZrO2 phase transformation is up to 6.6 MPa m 1/2[11–13], 6.2 by SiC whisker [14,15], 6.5 by nanosized SiC [16], and 6.1 by graphite [17,18] respectively. Nevertheless, the toughness does not improve significantly according to these data, and many researches have been done to find a new reinforcement way. Inspired by the microstructure of natural shells, the laminated composites design and fabrication call for the temperature withstanding and soft materials into the ceramics in recent years. The concept of laminated composite for improved performance of brittle materials is well established in several systems including laminated Si3N4 ceramics [19,20] and laminated SiC ceramics [21,22]. At present, laminated ZrB2 ultrahigh temperature ceramics reinforced by ZrO2 interface layers have been rarely investigated. In
⁎ Corresponding author. Tel./fax: + 86 451 86402382. E-mail address: [email protected] (C. Wei). 0263-4368/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2011.08.003
the present work, the ZrB2–SiC and ZrO2 green tapes were fabricated with alcohol-based tape casting, respectively. The green samples were stacked and then hot pressed. Finally, the laminated ZrB2–SiC ceramics were obtained successfully. The microstructure and mechanical properties of the laminated ZrB2–SiC ceramics were also investigated in detail. 2. Experimental procedure Commercially available ZrB2 powder (mean particle size 2 μm; Northwest Institute for Non-ferrous Metal Research, China), SiC (mean particle size 0.5 μm; Weifang Kaihua Micro-powder Co., Ltd., China) and ZrO2 (3Y) (mean particle size 50 nm; Zibo Guangtong Chemical Co., Ltd., China) were used as raw powders. The designed compositions of the laminated ZrB2–SiC ceramics with ZrO2 interface layers were 80 vol.% ZrB2, 10 vol.% SiC and 10 vol.% ZrO2. The powder mixtures of ZrB2 plus 20 vol.% SiC were ball-mixed for 24 h in a polyethylene bottle using ZrO2 balls and ethanol as the grinding media, followed by adding polyvinyl butyral resin (PVB) and polyethylene glycol-6000 (PEG-6000) as adhesive and plasticizer. Then the slurries were ball milled for another 12 h. The mass ratio of ethanol, PVB and PEG-6000 to ZrB2–SiC mixtures in slurry was 15:1:1:10. The raw ZrB2–SiC sheets and ZrO2 sheets were formed by tape casting. The ZrB2–SiC sheets and ZrO2 sheets were alternately stacked until the desired compositions were achieved. The stacked ZrB2–SiC/ZrO2 body in a graphite mold was heated to 650 °C at a heating rate of 2 °C/min and held for 30 min in a vacuum furnace and then the stacked ZrB2–SiC/ZrO2 body was hot-pressed at 1850 °C under a uniaxial load of 20 MPa for 60 min. The microstructural features of the laminated ZrB2–SiC ceramics were observed by scanning electron microscopy (SEM, FEI Sirion, Holland) with simultaneous chemical analysis by energy dispersive spectroscopy (EDS, EDAX Inc.). The
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phase composition was determined by X-ray diffraction (XRD, Rigaku, Dmax-rb, CuKa= 1.5418 Å). The specimens for mechanical tests were prepared by cutting and machining the final products to the size of 3 mm × 4 mm × 36 mm and 2 mm × 4 mm × 22 mm. The flexural strength was tested in three point bending using a 30-mm span and a crosshead speed of 0.5 mm/min. Fracture toughness was analyzed by a single-edge notched beam (SENB) test with a 16-mm span, a cross-head speed of 0.05 mm/min, a 2 mm notch depth and a smaller than 0.2 mm opening width. As the laminated ZrB2–SiC ceramics showed a preferred orientation, the mechanical properties of laminated ZrB2–SiC ceramics should depend on the testing direction. Therefore, two batches of specimens were machined with opposite orientations, that is, parallel and perpendicular to the laminate direction, respectively. A minimum number of 5 specimens were tested for each experimental condition.
The microstructure of the laminated ZrB2 ceramics fracture is shown in Fig. 3. From Fig. 3a, the fracture of the ZrB2 layers is plain, but some obvious deflection appears in the ZrO2 layers, which makes the fracture become ladder. It indicates that the neighboring interfacial layer possesses much residual stress. From Fig. 3c and d, the microcracks emerge in the ZrO2 layer. The stress of the crack tip is relaxed by absorbing much energy by the microcracks, so the fracture toughness is improved significantly. Because of the different coefficients of thermal expansion between the ZrB2–SiC layers and the ZrO2 layers, the interface of adjacent layers results in residual stress. The thermal stresses can be estimated using Hsueh's formula [23].
σ0 =
αm −αg ΔT ð1 + vm Þ
ð1 + vg ÞEm
3. Results and discussion 3.1. Microstructure Fig. 1 shows the XRD patters of the surface and the side in the laminated ZrB2 ceramics. There exists only ZrB2 and SiC on the surface, as shown in Fig. 1a. It indicates that hard laminates were exposed on the surface and the soft ones have been dropped when the specimen is machined. However, there appears obvious tetragonal ZrO2 on the sides (Fig. 1b), which shows that ZrO2 (3Y) has not transformed into cubic ZrO2 but sintered to tetragonal one at 1850 °C. It is beneficial to phase transformation reinforcement, that is to say tetragonal ZrO2 transforms into monoclinic ZrO2[11–13]. Fig. 2 shows the SEM images of the cross section for parallel specimens. From Fig. 2a, it is clearly seen that the ZrB2–SiC layers and ZrO2 layers arrange alternately, the interface is straight and clear, and the thicknesses are uniform too. The ZrB2–SiC layers are dark and thick, whereas the ZrO2 layers are light and thin separating the ZrB2– SiC layers. The thickness of the ZrB2–SiC layers is 280 μm and the thickness of the ZrO2 layers is about 30 μm. The volume fraction of ZrO2 is 9.6%, which indicates that alcohol-based tape casting could control the composition of laminated ZrB2–SiC ceramics precisely. From Fig. 2b, it can be seen that the interfacial bonding properties are good. The analyzed results from the enlarged picture of Fig. 2c with EDS shows that the black dot is SiC, and the gray one is ZrB2. SiC is distributed among the ZrB2–SiC layers homogeneously. Fig. 2d is the enlarged picture of the ZrO2 layer, which is compact and no pores can be seen. SiC particle is pressed into the ZrO2 layer. Whether or not ZrB2 particle is pressed into the ZrO2 layer could not be observed directly because of the similar colors. The ZrB2 layer and ZrO2 layer immerse each other and form gradient layer, so the interfacial bonding properties are better.
+
ð1−2vg Þ
ð1Þ
Ep
Where σ0 is the thermal residual stress, α is the thermal expansion coefficient, v is the Poisson's ratio, E is Young's modulus, and ΔTis the temperature change (negative for cooling). The subscripts m and g refer to the matrix (ZrB2 or SiC) and ZrO2, respectively. For the laminated ZrB2-based ceramic composites, EZrB2= 490 GPa, ESiC= 550 GPa, E−6 /K, ZrO2=200 GPa,νZrB2= 0.17, νSiC= 0.14, vZrO2= 0.3,αZrB2= 6.7 × 10 αSiC= 4.7 × 10− 6/K, and αZrO2 =11× 10− 6/K [11,18]. The value of ΔT is calculated from hot-pressing temperature to room temperature, namely, 1850 °C− 25 °C = 1825 °C. According to formula, the maximum thermal residual stresses at ZrB2–ZrO2 and SiC–ZrO2 interfaces were evaluated to be about 2.07 GPa and 3.24 GPa, respectively. The existence of such a large stress may weaken the interphase boundaries, leading to a fracture along such boundaries [18]. Fig. 4a, b and c are the microstructures of the cracks propagating paths for parallel specimens after the fracture toughness testing. From Fig. 4a, it is found that the main crack deflects along the ZrO2 interface and forms the step-like cracks microstructure at last, which greatly extends effective crack length and absorbs more fracture energy. Besides, the main cracks, branches and microcracks occurring in the ZrO2 interface further absorb the fracture energy, which also improves fracture toughening. While the fracture in perpendicular direction is brittle fracture, the path of crack is shorter and plainer, which can be seen in Fig. 4d. 3.2. Mechanical properties The mechanical properties of the laminated ZrB2 ceramics are listed in Table 1. The fracture toughness is 14.4 ± 0.3 MPa m 1/2 for parallel direction, and 5.8 ± 0.3 MPa m 1/2 for perpendicular direction, respectively. The fracture toughness for parallel direction was
Fig. 1. XRD spectra obtained from the surface and the side of the laminated ZrB2–SiC ceramics.
C. Wei et al. / Int. Journal of Refractory Metals and Hard Materials 30 (2012) 173–176
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Fig. 2. SEM images of a polished cross-section of the laminated ZrB2–SiC ceramics.
approximately improved by 220% than that of ZrB2–SiC composites (4.0–4.5 MPa m 1/2) [18], 122% than that of ZrB2–nano-SiC composites (6.3–6.5) [16], 132% than that of ZrB2–SiCw composites (5.8–6.2 MPa m 1/2) [14,15], and 118% than that of ZrB2–SiC–ZrO2 composites (5.6– 6.6 MPa m 1/2) [11–13]. The improvement in fracture toughness lies in the deflecting, and branching cracks prolonged the propagation path when meeting the ZrO2 interface layers. The ZrO2 phase transformation reinforcement is another reason. The flexural strength of the laminated ZrB2 ceramics is 561 ± 20 MPa for parallel direction, and 432 ± 18 MPa in perpendicular direction, respectively. The difference is due to the brittleness fracture for
perpendicular direction and the successive fracture with crack damage tolerance for parallel direction [21]. These phenomena indicate that fracture manners have effect on the flexural strength of the ceramics. Accordingly, the ZrO2 interface layers play an important role in improving the fracture toughness of the laminated ZrB2–SiC ceramics. 4. Conclusions The laminated ZrB2–SiC ceramics with ZrO2 interface layers are successfully prepared by tape casting, laminating and hot pressing. The flexural strength and fracture toughness are 561 ± 20 MPa and
Fig. 3. SEM images of fracture surface of the laminated ZrB2–SiC ceramics.
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Fig. 4. SEM images of parallel (a, b and c) and perpendicular (d) specimens after SENB test.
Table 1 Mechanical properties of the laminated ZrB2–SiC ceramics. Orientation
Flexural strength (MPa)
Fracture toughness (MPa . m1/2)
Parallel Perpendicular
561 ± 20 432 ± 18
14.4 ± 0.3 5.8 ± 0.3
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