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eutectic microstructure of Al2O3–ZrO2 (Y2O3) ceramics prepared by spark plasma sintering
Materials Letters 175 (2016) 212–214
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Transitional/eutectic microstructure of Al2O3–ZrO2 (Y2O3) ceramics prepared by spark plasma sintering Xiaojian Xia a, Xiaoqiang Li a,n, Minai Zhang a, Donghai Zheng a,b a National Engineering Research Center of Near-net-shape Forming Technology for Metallic Materials, South China University of Technology, Guangzhou 510640, China b School of Engineering Technology, Beijing Normal University, Zhuhai 519087, China
art ic l e i nf o
a b s t r a c t
Article history: Received 28 January 2016 Received in revised form 18 March 2016 Accepted 3 April 2016 Available online 4 April 2016
Fully densified Al2O3–ZrO2 (Y2O3) eutectic ceramics without any binders were fabricated by a novel and efficient way, namely spark plasma sintering. The morphology of the specimen sintered at 1600 °C shows a typical lamellar eutectic microstructure, while the one sintered at 1500 °C manifests a transitional microstructure between sintering and casting. The latter specimen also exhibits high mechanical properties: the Vickers hardness and fracture toughness of it are 17.57 0.3 GPa and 7.47 0.4 MPa m1/2, respectively. & 2016 Elsevier B.V. All rights reserved.
Keywords: Ceramics Spark plasma sintering Eutectic Transitional microstructure Mechanical properties
1. Introduction Al2O3–ZrO2 (Y2O3) eutectic ceramics have a great advantage as widely utilized composite materials, because of their distinguishing properties: high strength combined with high toughness at high temperatures as well as excellent thermal stability and chemical resistance [1–2]. Directional solidification is the prevailing method to manufacture Al2O3–ZrO2 (Y2O3) eutectic with high performance, such as the Bridgman [3], micro-pulling down techniques [4], floating molten zone [5], and Laser Floating Zone (LFZ) [6–7]. However, these fabrication methods are inefficient and energy-consuming. Niu et al. [1] recently put forward an efficient and flexible method to prepare Al2O3–ZrO2 (Y2O3) eutectics, namely laser engineered net shaping (LENS). However, the mechanical properties of the prepared ceramic were slightly low: Vickers hardness and fracture toughness are 17.15 GPa and 4.79 MPa m1/2, respectively. It is necessary to find some efficient ways to prepare high performance Al2O3–ZrO2 (Y2O3) eutectic ceramics. Spark plasma sintering (SPS), a rapid and efficient sintering method, has attracted significant attention with a reported decrease of sintering temperature and shortening of soaking time in comparison to conventional sintering [8–9]. Due to its special sintering mechanism [10], SPS technique has yielded many high performance n
Corresponding author. E-mail address: [email protected] (X. Li).
http://dx.doi.org/10.1016/j.matlet.2016.04.031 0167-577X/& 2016 Elsevier B.V. All rights reserved.
materials. In this study, the fabrication of Al2O3–ZrO2 (Y2O3) ceramics by SPS with a typical eutectic microstructure or transitional microstructure is presented.
2. Material and methods The starting powders were α-Al2O3 ( 0.1 mm, purity 499.99%) and 3Y–ZrO2 (3 mol% Y2O3-stabilized, 0.08 mm, purity 4 99.9%). The Al2O3–ZrO2 (Y2O3) powders were mixed with the eutectic ratio of 58 wt% Al2O3 and 42 wt% ZrO2. The powder mixtures were wet mixed on a planetary ball mill (QM-3SP2, Nanjing NanDa Instrument Plant, China) in ethanol for 30 h using cemented carbide milling balls (ball-to-powder weight ratio was 10:1) and cemented carbide vials (250 mL). Then the milling process paused every 30 min for staying 18 min, subsequently restarted reversely at a speed of 200 r/min, and finally stopped after 60 cycles. In each time, 10 g of the finally obtained Al2O3–ZrO2 (Y2O3) powders were poured into a cylindrical graphite die with an inner diameter of 20 mm and an outer diameter of 50 mm. Then sintering was conducted on a Dr. Sinter Model SPS-825 system (Sumitomo Coal Mining Co. Ltd., Japan) in vacuum ( r6 Pa) at 1600/1500 °C for 5 min, with a heating rate of 100 °C/min and an applied pressure of 30 MPa. The specimens sintered at 1600/1500 °C are named AZ16/AZ15 for short. An infrared pyrometer (Z 570 °C) was focused at the bottom of a central core hole with a diameter of 2 mm in the die wall and about 7.5 mm away from the inner wall to
X. Xia et al. / Materials Letters 175 (2016) 212–214
213
Fig. 1. Densification curves for specimens (a) sintered at 1600 °C and (b) sintered at 1500 °C. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.).
radiation, and the morphology of microstructure was observed by high-resolution scanning electron microscopy (SEM, Nova Nano 430, FEI, USA).
3. Results and discussion
Fig. 2. XRD patterns of as-milled powders and as-sintered specimens (1600/ 1500 °C).
obtain a realistic sample temperature. The density was measured based on the Archimedes principle and the theoretical density was calculated with the law of mixture to volume fractions of the constituents. The hardness was measured by a Vickers hardness tester (430SVA, Wilson Wolpert Co. Ltd., China) with a load of 98 N. The fracture toughness was obtained according to the formula given by Niihara et al. [11]. Additively, the elastic modulus was calculated with the Reuss rule of mixture [12]. Phase identification was conducted by an X-ray diffractometer (XRD, D8 Advance, Bruker Co., Germany) using Cu Kα
Fig. 1 shows the shrinkage displacement, shrinkage rate, pressure, and temperature varying with time for the AZ16/AZ15 specimens with a five-minute dwell time. It exhibits that the densification of the Al2O3–ZrO2 (Y2O3) composite starts at about 1130 °C and ends at around 1440 °C (see Fig. 1(a)). It is noteworthy that there is an excrescent growth for both shrinkage displacement and shrinkage rate when the sintering temperature rises above 1500 °C (see Fig. 1(a)). Meanwhile, the sintering pressure (circled in red) drops dramatically. These phenomena indicate that eutectic melting occurs in the alumina-zirconia system. As a result, there are no bulks but slags left since the entire specimen has melt and exuded outside of the graphite die. Thus to achieve intact bulks, some mixed Al2O3–ZrO2 (Y2O3) composite powders are sintered at 1500 °C which allows a complete densification with some liquid occurring but no melting entirely (see the red-circled section in Fig. 1(b)). The measured densities of AZ16/AZ15 are 4.77 7 0.1/4.74 70.1 g/cm3, respectively, both of which are slightly higher than the theoretical value of 4.66 g/cm3. This confusing fact could be caused by the two following reasons: (1) WC is inevitably mixed into the Al2O3–ZrO2 (Y2O3) powders which are milled with the cemented carbide vials and balls; (2) alumina can be dissolved in the zirconia [13], which will enhance the density value. Fig. 2 presents the XRD patterns of as-milled Al2O3–ZrO2 (Y2O3) powders before and after sintering process. It is clear that α-Al2O3 and t-ZrO2 are the primary and stable phases in this research.
Fig. 3. (a) Microstructure of the polished slags sintered at 1600 °C; (b) and (c), microstructure of bulk specimens sintered at 1500 °C.
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X. Xia et al. / Materials Letters 175 (2016) 212–214
Table 1 Fabrication conditions and physical properties of the Al2O3–ZrO2 (Y2O3) composite ceramics. Specimens
Fabrication temperature (°C)
Relative density (%)
Lamellar interspacing (μm)
Hardness (GPa)
Fracture toughness (MPa m1/2)
AZ16 AZ15 Niu et al. [1]
1600 1500 Z 1860
100 99.4 7 0.2 –
0.7 70.1 – 0.1
15.6 70.5 17.5 7 0.3 17.15
3.8 7 0.6 7.4 7 0.4 4.79
Besides, WC phase appears in the as-milled powders and the assintered specimens (AZ16/AZ15). The content of tungsten are determined by potassium thiocynate photometric method (GB/T 14352.1-1993, China National Standards), and the reckoned WC content is about 3.0 wt%. Considering WC as part of the specimens, the recalculated theoretical density is 4.75 g/cm3. Thus the relative density (RD) of AZ16 is 100.4 70.2% indicating the formation of solid solution. Given AZ16 has been completely densified, the addition of solid solution to RD is 0.4%. Considering this, the RD of bulk specimen (1500 °C) has actually reached 99.4%, which means the Al2O3–ZrO2 (Y2O3) ceramic sintered at 1500 °C has been fully densified. Fig. 3 shows the representative BSE morphology of specimens sintered at 1600/1500 °C. According to EDS results, the slag (AZ16) consists of bright columnar t-ZrO2 phase and dark Al2O3 phase (see Fig. 3(a)). As expected, it shows a columnar colony microstructure formed by facet alumina grains growing along [0001] which contains an ordered distribution of t-ZrO2. This microstructure is similar to that of the Al2O3–ZrO2 (Y2O3) eutectic specimen fabricated by LENS [1] and LFZ [6–7]. However, the eutectic lamellar interspacing (λ) is much coarser than the counterpart reported by Niu et al., which leads to poorer mechanical properties (see Table 1). Fig. 3(c) shows the microstructure of the bulk specimen (AZ15) which displays that the continuous net-shaped microstructure has completely taken place of lamellar microstructure. In the microstructure of AZ15, the white phase distributed like a continuous net is t-ZrO2, while the black matrix phase is α-Al2O3. This microstructure looks like the counterpart fabricated via pressureless sintering (1450 °C) by Lorenzo-Martin et al. [14], but more regular, continuous and interconnected. When the sintering temperature is above 1500 °C, much faster grain boundary migration and grain boundary diffusion lay the foundation of rapidly yielding liquid eutectic ceramics. On the other hand, when the sintering temperature is around 1500 °C, it does not lead to melting entirely but yield some liquid. This liquid can considerably accelerate the diffusion which is responsible for this highly regular net-shaped microstructure. Evidently, it is regarded as a transitional microstructure between sintering and casting. In the inset of Fig. 3(b), the highly bright dotted phase is identified as WC according to EDS result. These WC particles are dispersed homogeneously, which as a consequence may partly strengthen the Al2O3–ZrO2 (Y2O3) ceramic. Comparing to the hardness and indentation fracture toughness of the specimens listed in Table 1, the bulk (AZ15) exhibits excellent mechanical properties: 17.57 0.3 GPa and 7.4 70.4 MPa m1/2, respectively. Such high fracture toughness results from the transitional microstructure: continuous and fine net-shaped t-ZrO2, where the cracks have to go through. Thus the cracks need consuming more energy for a transformation of intergranular to transgranular fracture or deflecting frequently, and the crack propagation is suspended (see Fig. 3(c)).
4. Conclusions In summary, the Al2O3–ZrO2 (Y2O3) eutectic ceramic can be fabricated via SPS. When sintered at 1600 °C, the morphology of the specimen presents typical lamellar eutectic microstructure. When sintered at 1500 °C, bulk specimens are obtained with a 99.4% relative density. In this case, the morphology of the bulk specimen possesses transitional microstructure comprised of continuous-net-shaped t-ZrO2 and isolated α-Al2O3. Consequently an Al2O3–ZrO2 (Y2O3) ceramic with superior mechanical properties is obtained: the hardness and fracture toughness are 17.5 70.3 GPa and 7.470.4 MPa m1/2, respectively. However, further detailed investigations are needed to understand why this composite ceramic presents so high mechanical properties and determine whether the WC introduced by milling is beneficial to the improved properties.
Acknowledgements This work was financed by the National Nature Science Foundation of China (No. 51474108 and No. 51174095), the Fundamental Research Funds for Central Universities (No. D2153590), the Open Foundation of National Engineering Research Center of Near-NetShape Forming for Metallic Materials (No. 2015004) and the Nature Science Foundation of Guangdong Province, China (No. 2014A030313234).
References [1] F.Y. Niu, D.J. Wu, G.Y. Ma, J.T. Wang, M.H. Guo, B. Zhang, Scr. Mater. 95 (2015) 39–41. [2] A. Laidoune, K. Lebbou, D. Bahloul, M. Smadi, M. Zereg, Opt. Mater. 32 (2010) 731–734. [3] Y. Waku, N. Nakagawa, T. Wakamoto, H. Ohtsubo, K. Shimizu, Y. Kohtoku, J. Mater. Sci. 33 (1998) 1217–1225. [4] J.H. Lee, A. Yoshikawa, H. Kaiden, K. Lebbou, T. Fukuda, D.H. Yoon, Y. Waku, J. Cryst. Growth 231 (2001) 179–185. [5] N.R. Harlan, R.I. Merino, J.I. Pena, A. Larrea, V.M. Orera, C. Gonzalez, P. Poza, J. Llorca, J. Am. Ceram. Soc. 85 (2002) 2025–2032. [6] I. de Francisco, R.I. Merino, V.M. Orera, A. Larrea, J.I. Pena, J. Eur. Ceram. Soc. 25 (2005) 1341–1350. [7] J. Ramirez-Rico, J. Martinez-Fernandez, J.I. Pena, D. Singh, J. Routbort, Mater. Sci. Eng. A 541 (2012) 61–66. [8] Z.J. Shen, H. Peng, J. Liu, M. Nygren, J. Eur. Ceram. Soc. 24 (2004) 3447–3452. [9] D. Galusek, J. Sedlacek, J. Chovanec, M. Michalkova, Ceram. Int. 41 (2015) 9692–9700. [10] Z.J. Shen, M. Johnsson, Z. Zhao, M. Nygren, J. Am. Ceram. Soc. 85 (2002) 1921–1927. [11] K. Niihara, R. Morena, D.P.H. Hasselman, J. Mater. Sci. Lett. 1 (1982) 13–16. [12] C.L. Hsieh, W.H. Tuan, Mater. Sci. Eng. A 393 (2005) 133–139. [13] L. Gao, Q. Liu, J.S. Hong, H. Miyamoto, S.D. De la Torre, A. Kakitsuji, K. Liddell, D. P. Thompson, J. Mater. Sci. 33 (1998) 1399–1403. [14] C. Lorenzo-Martin, D. Singh, J. Routbort, G. Chen, Mater. Sci. Eng. A 517 (2009) 78–84.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Transitional/eutectic microstructure of Al2O3–ZrO2 (Y2O3) ceramics prepared by spark plasma sintering Xiaojian Xia a, Xiaoqiang Li a,n, Minai Zhang a, Donghai Zheng a,b a National Engineering Research Center of Near-net-shape Forming Technology for Metallic Materials, South China University of Technology, Guangzhou 510640, China b School of Engineering Technology, Beijing Normal University, Zhuhai 519087, China
art ic l e i nf o
a b s t r a c t
Article history: Received 28 January 2016 Received in revised form 18 March 2016 Accepted 3 April 2016 Available online 4 April 2016
Fully densified Al2O3–ZrO2 (Y2O3) eutectic ceramics without any binders were fabricated by a novel and efficient way, namely spark plasma sintering. The morphology of the specimen sintered at 1600 °C shows a typical lamellar eutectic microstructure, while the one sintered at 1500 °C manifests a transitional microstructure between sintering and casting. The latter specimen also exhibits high mechanical properties: the Vickers hardness and fracture toughness of it are 17.57 0.3 GPa and 7.47 0.4 MPa m1/2, respectively. & 2016 Elsevier B.V. All rights reserved.
Keywords: Ceramics Spark plasma sintering Eutectic Transitional microstructure Mechanical properties
1. Introduction Al2O3–ZrO2 (Y2O3) eutectic ceramics have a great advantage as widely utilized composite materials, because of their distinguishing properties: high strength combined with high toughness at high temperatures as well as excellent thermal stability and chemical resistance [1–2]. Directional solidification is the prevailing method to manufacture Al2O3–ZrO2 (Y2O3) eutectic with high performance, such as the Bridgman [3], micro-pulling down techniques [4], floating molten zone [5], and Laser Floating Zone (LFZ) [6–7]. However, these fabrication methods are inefficient and energy-consuming. Niu et al. [1] recently put forward an efficient and flexible method to prepare Al2O3–ZrO2 (Y2O3) eutectics, namely laser engineered net shaping (LENS). However, the mechanical properties of the prepared ceramic were slightly low: Vickers hardness and fracture toughness are 17.15 GPa and 4.79 MPa m1/2, respectively. It is necessary to find some efficient ways to prepare high performance Al2O3–ZrO2 (Y2O3) eutectic ceramics. Spark plasma sintering (SPS), a rapid and efficient sintering method, has attracted significant attention with a reported decrease of sintering temperature and shortening of soaking time in comparison to conventional sintering [8–9]. Due to its special sintering mechanism [10], SPS technique has yielded many high performance n
Corresponding author. E-mail address: [email protected] (X. Li).
http://dx.doi.org/10.1016/j.matlet.2016.04.031 0167-577X/& 2016 Elsevier B.V. All rights reserved.
materials. In this study, the fabrication of Al2O3–ZrO2 (Y2O3) ceramics by SPS with a typical eutectic microstructure or transitional microstructure is presented.
2. Material and methods The starting powders were α-Al2O3 ( 0.1 mm, purity 499.99%) and 3Y–ZrO2 (3 mol% Y2O3-stabilized, 0.08 mm, purity 4 99.9%). The Al2O3–ZrO2 (Y2O3) powders were mixed with the eutectic ratio of 58 wt% Al2O3 and 42 wt% ZrO2. The powder mixtures were wet mixed on a planetary ball mill (QM-3SP2, Nanjing NanDa Instrument Plant, China) in ethanol for 30 h using cemented carbide milling balls (ball-to-powder weight ratio was 10:1) and cemented carbide vials (250 mL). Then the milling process paused every 30 min for staying 18 min, subsequently restarted reversely at a speed of 200 r/min, and finally stopped after 60 cycles. In each time, 10 g of the finally obtained Al2O3–ZrO2 (Y2O3) powders were poured into a cylindrical graphite die with an inner diameter of 20 mm and an outer diameter of 50 mm. Then sintering was conducted on a Dr. Sinter Model SPS-825 system (Sumitomo Coal Mining Co. Ltd., Japan) in vacuum ( r6 Pa) at 1600/1500 °C for 5 min, with a heating rate of 100 °C/min and an applied pressure of 30 MPa. The specimens sintered at 1600/1500 °C are named AZ16/AZ15 for short. An infrared pyrometer (Z 570 °C) was focused at the bottom of a central core hole with a diameter of 2 mm in the die wall and about 7.5 mm away from the inner wall to
X. Xia et al. / Materials Letters 175 (2016) 212–214
213
Fig. 1. Densification curves for specimens (a) sintered at 1600 °C and (b) sintered at 1500 °C. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.).
radiation, and the morphology of microstructure was observed by high-resolution scanning electron microscopy (SEM, Nova Nano 430, FEI, USA).
3. Results and discussion
Fig. 2. XRD patterns of as-milled powders and as-sintered specimens (1600/ 1500 °C).
obtain a realistic sample temperature. The density was measured based on the Archimedes principle and the theoretical density was calculated with the law of mixture to volume fractions of the constituents. The hardness was measured by a Vickers hardness tester (430SVA, Wilson Wolpert Co. Ltd., China) with a load of 98 N. The fracture toughness was obtained according to the formula given by Niihara et al. [11]. Additively, the elastic modulus was calculated with the Reuss rule of mixture [12]. Phase identification was conducted by an X-ray diffractometer (XRD, D8 Advance, Bruker Co., Germany) using Cu Kα
Fig. 1 shows the shrinkage displacement, shrinkage rate, pressure, and temperature varying with time for the AZ16/AZ15 specimens with a five-minute dwell time. It exhibits that the densification of the Al2O3–ZrO2 (Y2O3) composite starts at about 1130 °C and ends at around 1440 °C (see Fig. 1(a)). It is noteworthy that there is an excrescent growth for both shrinkage displacement and shrinkage rate when the sintering temperature rises above 1500 °C (see Fig. 1(a)). Meanwhile, the sintering pressure (circled in red) drops dramatically. These phenomena indicate that eutectic melting occurs in the alumina-zirconia system. As a result, there are no bulks but slags left since the entire specimen has melt and exuded outside of the graphite die. Thus to achieve intact bulks, some mixed Al2O3–ZrO2 (Y2O3) composite powders are sintered at 1500 °C which allows a complete densification with some liquid occurring but no melting entirely (see the red-circled section in Fig. 1(b)). The measured densities of AZ16/AZ15 are 4.77 7 0.1/4.74 70.1 g/cm3, respectively, both of which are slightly higher than the theoretical value of 4.66 g/cm3. This confusing fact could be caused by the two following reasons: (1) WC is inevitably mixed into the Al2O3–ZrO2 (Y2O3) powders which are milled with the cemented carbide vials and balls; (2) alumina can be dissolved in the zirconia [13], which will enhance the density value. Fig. 2 presents the XRD patterns of as-milled Al2O3–ZrO2 (Y2O3) powders before and after sintering process. It is clear that α-Al2O3 and t-ZrO2 are the primary and stable phases in this research.
Fig. 3. (a) Microstructure of the polished slags sintered at 1600 °C; (b) and (c), microstructure of bulk specimens sintered at 1500 °C.
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X. Xia et al. / Materials Letters 175 (2016) 212–214
Table 1 Fabrication conditions and physical properties of the Al2O3–ZrO2 (Y2O3) composite ceramics. Specimens
Fabrication temperature (°C)
Relative density (%)
Lamellar interspacing (μm)
Hardness (GPa)
Fracture toughness (MPa m1/2)
AZ16 AZ15 Niu et al. [1]
1600 1500 Z 1860
100 99.4 7 0.2 –
0.7 70.1 – 0.1
15.6 70.5 17.5 7 0.3 17.15
3.8 7 0.6 7.4 7 0.4 4.79
Besides, WC phase appears in the as-milled powders and the assintered specimens (AZ16/AZ15). The content of tungsten are determined by potassium thiocynate photometric method (GB/T 14352.1-1993, China National Standards), and the reckoned WC content is about 3.0 wt%. Considering WC as part of the specimens, the recalculated theoretical density is 4.75 g/cm3. Thus the relative density (RD) of AZ16 is 100.4 70.2% indicating the formation of solid solution. Given AZ16 has been completely densified, the addition of solid solution to RD is 0.4%. Considering this, the RD of bulk specimen (1500 °C) has actually reached 99.4%, which means the Al2O3–ZrO2 (Y2O3) ceramic sintered at 1500 °C has been fully densified. Fig. 3 shows the representative BSE morphology of specimens sintered at 1600/1500 °C. According to EDS results, the slag (AZ16) consists of bright columnar t-ZrO2 phase and dark Al2O3 phase (see Fig. 3(a)). As expected, it shows a columnar colony microstructure formed by facet alumina grains growing along [0001] which contains an ordered distribution of t-ZrO2. This microstructure is similar to that of the Al2O3–ZrO2 (Y2O3) eutectic specimen fabricated by LENS [1] and LFZ [6–7]. However, the eutectic lamellar interspacing (λ) is much coarser than the counterpart reported by Niu et al., which leads to poorer mechanical properties (see Table 1). Fig. 3(c) shows the microstructure of the bulk specimen (AZ15) which displays that the continuous net-shaped microstructure has completely taken place of lamellar microstructure. In the microstructure of AZ15, the white phase distributed like a continuous net is t-ZrO2, while the black matrix phase is α-Al2O3. This microstructure looks like the counterpart fabricated via pressureless sintering (1450 °C) by Lorenzo-Martin et al. [14], but more regular, continuous and interconnected. When the sintering temperature is above 1500 °C, much faster grain boundary migration and grain boundary diffusion lay the foundation of rapidly yielding liquid eutectic ceramics. On the other hand, when the sintering temperature is around 1500 °C, it does not lead to melting entirely but yield some liquid. This liquid can considerably accelerate the diffusion which is responsible for this highly regular net-shaped microstructure. Evidently, it is regarded as a transitional microstructure between sintering and casting. In the inset of Fig. 3(b), the highly bright dotted phase is identified as WC according to EDS result. These WC particles are dispersed homogeneously, which as a consequence may partly strengthen the Al2O3–ZrO2 (Y2O3) ceramic. Comparing to the hardness and indentation fracture toughness of the specimens listed in Table 1, the bulk (AZ15) exhibits excellent mechanical properties: 17.57 0.3 GPa and 7.4 70.4 MPa m1/2, respectively. Such high fracture toughness results from the transitional microstructure: continuous and fine net-shaped t-ZrO2, where the cracks have to go through. Thus the cracks need consuming more energy for a transformation of intergranular to transgranular fracture or deflecting frequently, and the crack propagation is suspended (see Fig. 3(c)).
4. Conclusions In summary, the Al2O3–ZrO2 (Y2O3) eutectic ceramic can be fabricated via SPS. When sintered at 1600 °C, the morphology of the specimen presents typical lamellar eutectic microstructure. When sintered at 1500 °C, bulk specimens are obtained with a 99.4% relative density. In this case, the morphology of the bulk specimen possesses transitional microstructure comprised of continuous-net-shaped t-ZrO2 and isolated α-Al2O3. Consequently an Al2O3–ZrO2 (Y2O3) ceramic with superior mechanical properties is obtained: the hardness and fracture toughness are 17.5 70.3 GPa and 7.470.4 MPa m1/2, respectively. However, further detailed investigations are needed to understand why this composite ceramic presents so high mechanical properties and determine whether the WC introduced by milling is beneficial to the improved properties.
Acknowledgements This work was financed by the National Nature Science Foundation of China (No. 51474108 and No. 51174095), the Fundamental Research Funds for Central Universities (No. D2153590), the Open Foundation of National Engineering Research Center of Near-NetShape Forming for Metallic Materials (No. 2015004) and the Nature Science Foundation of Guangdong Province, China (No. 2014A030313234).
References [1] F.Y. Niu, D.J. Wu, G.Y. Ma, J.T. Wang, M.H. Guo, B. Zhang, Scr. Mater. 95 (2015) 39–41. [2] A. Laidoune, K. Lebbou, D. Bahloul, M. Smadi, M. Zereg, Opt. Mater. 32 (2010) 731–734. [3] Y. Waku, N. Nakagawa, T. Wakamoto, H. Ohtsubo, K. Shimizu, Y. Kohtoku, J. Mater. Sci. 33 (1998) 1217–1225. [4] J.H. Lee, A. Yoshikawa, H. Kaiden, K. Lebbou, T. Fukuda, D.H. Yoon, Y. Waku, J. Cryst. Growth 231 (2001) 179–185. [5] N.R. Harlan, R.I. Merino, J.I. Pena, A. Larrea, V.M. Orera, C. Gonzalez, P. Poza, J. Llorca, J. Am. Ceram. Soc. 85 (2002) 2025–2032. [6] I. de Francisco, R.I. Merino, V.M. Orera, A. Larrea, J.I. Pena, J. Eur. Ceram. Soc. 25 (2005) 1341–1350. [7] J. Ramirez-Rico, J. Martinez-Fernandez, J.I. Pena, D. Singh, J. Routbort, Mater. Sci. Eng. A 541 (2012) 61–66. [8] Z.J. Shen, H. Peng, J. Liu, M. Nygren, J. Eur. Ceram. Soc. 24 (2004) 3447–3452. [9] D. Galusek, J. Sedlacek, J. Chovanec, M. Michalkova, Ceram. Int. 41 (2015) 9692–9700. [10] Z.J. Shen, M. Johnsson, Z. Zhao, M. Nygren, J. Am. Ceram. Soc. 85 (2002) 1921–1927. [11] K. Niihara, R. Morena, D.P.H. Hasselman, J. Mater. Sci. Lett. 1 (1982) 13–16. [12] C.L. Hsieh, W.H. Tuan, Mater. Sci. Eng. A 393 (2005) 133–139. [13] L. Gao, Q. Liu, J.S. Hong, H. Miyamoto, S.D. De la Torre, A. Kakitsuji, K. Liddell, D. P. Thompson, J. Mater. Sci. 33 (1998) 1399–1403. [14] C. Lorenzo-Martin, D. Singh, J. Routbort, G. Chen, Mater. Sci. Eng. A 517 (2009) 78–84.