Effect of plate-like alumina on the properties of alumina ceramics prepared by gel-casting

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Effect of Plate-like Alumina on the Properties of Alumina Ceramics Prepared by Gel-casting Meiqi Cao, Qingzhi Yan, Xianhui Li, Yingying Mi

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S0921-5093(13)01046-0 http://dx.doi.org/10.1016/j.msea.2013.09.061 MSA30314

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Materials Science & Engineering A

Received date: 24 June 2013 Revised date: 15 September 2013 Accepted date: 18 September 2013 Cite this article as: Meiqi Cao, Qingzhi Yan, Xianhui Li, Yingying Mi, Effect of Plate-like Alumina on the Properties of Alumina Ceramics Prepared by Gelcasting, Materials Science & Engineering A, http://dx.doi.org/10.1016/j. msea.2013.09.061 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of Plate-like Alumina on the Properties of Alumina Ceramics Prepared by Gel-casting Meiqi Cao᧨Qingzhi Yan*᧨Xianhui Li᧨Yingying Mi Institute of Special Ceramics and Powder Metallurgy, University of Science & Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing, China. *᧶corresponding author, Fax: +86-01062334951; Tel: +86-01062334951; E-mail: [email protected] Abstract: In this work, alumina ceramics were prepared via gel-casting by mixing plate-like and granular alumina powders. Plate-like alumina with a hexagonal or disk-shaped morphology decreased the viscosity of the suspension remarkably, which made the casting process much easier. As the plate-like alumina content varied from 10 wt%, 30 wt%, 50 wt%, 70 wt% to 100 wt%, the density and bending strength of the sintered ceramics increased firstly then decreased and reached their maximum at the content of 50 wt%. However, the microhardness and fracture toughness presented negligible dependence on plate-like alumina content. It is worth noting that all samples with plate-like alumina displayed lower density and bending strength but higher fracture toughness than the sample without plate-like alumina. Keywords᧶plate-like alumina; gel-casting; viscosity; fracture toughness

1. Introduction Excellent properties of alumina, such as chemical, thermal stability, high strength, high hardness, wear resistance, make it can be applied to various engineering field [1]. However, its lack of reliability and low fracture toughness limits its application [2].

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Gel-casting has been considered as a promising method to fabricate complex-shaped alumina component due to its advantages such as controllable casting, rapid forming cycle, minimal moulding defects, fabricating green bodies with high mechanical strength and stiffness, low shrinkage during drying and sintering processes, and ability to processing complex-shaped and near-net-shaped ceramic parts [3-5]. In gel-casting process, the suspension slurry with high solid loading and low viscosity is crucial for high-quality ceramics [6]. But the solid loading and viscosity are contradictory for gel-casting: high solid loading inevitably leads to high viscosity if granular alumina powders are used as raw materials [7]. In this work, we try to obtain high solid loading and low viscosity suspension slurry for gel-casting by using plate-like alumina as raw materials. Plate-like alumina is a single crystal of -Al2O3 with a hexagonal or disk-shaped morphology [8]. The radial dimension and thickness of plate-like alumina are in micron and nano-scales, respectively. Therefore, it has dual performances of both micron powder and nano powders, which are appropriate for surface activity and good adhesion. It can easily combine with an active group without agglomerate [9]. Plate-like alumina is supposed to improve the fluidity of the slurry. Meanwhile, plate-like grains has been reported to improve fracture toughness due to form crack bridging easily in ceramics [10-13]. We prepared alumina ceramics via gel-casting with plate-like alumina powder contents ranging from 0 to 100 wt% to investigate the effects of plate-like alumina on the microstructure and mechanical properties of alumina ceramics.

2. Experimental 2

2.1 Materials and processing Commercially available granular alumina (-Al2O3, d50=2.02m) and plate-like alumina (-Al2O3, diameter of 3m, thickness of 0.5m) powders were used as the starting materials. Six batches were mixed using the two powders and additives containing Y2O3 and MgO. The relative contents of plate-like alumina powder in these batches were 0, 10, 30, 50, 70 and 100 wt%. The essential reagents in gel-casting are commercially available, including monomers: acrylamide, C2H3CONH2 (AM) and N,N’-methylenebisacrylamide, (C2H3CONH)2CH2 (MBAM); initiator: ammonium bisulphate

(NH4)2S2O8

(APS);

catalyst:

N,N,N’,N’-tetramethy-lenediamine,

(CH3)2N(CH2)2N(CH3)2 (TEMED); dispersant: ammonium polyacrylate (PAA-NH4). All reagents were chemically pure. A premixed solution of monomers was prepared in deionized water with an appropriate amount of AM and MBAM. Six batches of ceramic powders with 2wt% dispersant were mixed in the premixed solution. The suspensions were mechanically stirred for at least 1 h and degassed for 10 min until no bubbles were released. The initiator and catalyst were then added. Afterwards, the slurry was cast into a mold. After gelation, the green body was demolded and dried at room temperature. Finally, the green compacts were sintered at 1800 °C in air. A flow chart of the process is shown in Fig. 1. 2.2 Testing methods The rheological properties of the slurries were determined using a rotational viscometer (NXS-11B, China) at a constant temperature of 25°C. The bulk densities of

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the samples were measured by Archimedes’ method. The sintered specimens with a dimension of 3 mm × 4 mm × 30 mm were ground and polished to test. The flexural strength was measured by three-point bending test with a span of 13.10 mm at a crosshead speed of 0.5 mm/min. The micro hardness was measured on the polished surfaces by using a diamond pyramid indenter at a load of 300 N and a loading duration of 10 s. Fracture toughness value (KIC) was measured by indentation test on Wilson-wolpert Tukon 2100B (INSTRON), and the load and loading time were 49N and 10 s, respectively. The morphology of dispersed powders and the microstructure of the fracture surface were observed with scanning electron microscope (LEO1450 SEM) after the samples were coated with carbon.

3. Results and discussions The SEM micrographs of plate-like alumina and granular alumina particles are shown in Fig. 2. The plate-like alumina powders have a hexagonal platelet shape, an approximate thickness of 0.5m, and size of 3 m. The granular alumina powders have a granular morphology, with an average particle size of 2.02 m. Six slurries with 50vol % solids loading were prepared with mixed granular and plate-like alumina powders as raw materials. The relative contents of plate-like alumina powder changed from 0, 10 wt%, 30 wt%, 50 wt%, 70 wt% to 100 wt%.Fig. 3 shows the effect of the plate-like alumina content on the viscosity of the suspensions. All the suspensions exhibit a shear-thinning behavior which is beneficial for the slurry casting. The viscosity of the suspension is largely influenced by the plate-like alumina. With the increase of plate-like alumina content, the viscosity of the suspensions rapidly decreases.

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As can be seen, at 20 s-1 shear rate, with the plate-like alumina content increase from 0 to 100wt%, the viscosity of the suspensions decrease from 1105 mPa·s to 224 mPa·s. The fluidity of the slurry is improved significantly with the addition of plate-like alumina. The mechanical properties of alumina ceramics with different plate-like alumina powder contents are shown in Table. 1. It is observed that the density of the sintered ceramic increases firstly, reaching the maximum value of 3.88 g/cm3 at 50 wt% plate-like alumina content, and then decreases to 3.28 g/cm3. This phenomenon can be attributed to the larger grain size of plate-like alumina with lower sintering activity [8]. The variation of the bending strength is in accordance with the density. As the plate-like alumina content varied from 10 wt% to 100 wt%, the bending strength of the sample increases firstly then decreases. When the content is 50 wt%, the bending strength reaches a maximum value of 264.16 MPa, which is still lower than that of the sample with pure granular alumina (310.83 MPa). Fig. 4 shows SEM images of the fracture surface of sintered materials with various plate-like alumina contents. As seen in figure 4, larger grains and pores increase with increasing plate-like alumina content, resulting in lower sintering density. We can conclude that elevated sintering temperature is necessary for larger plate-like alumina particle to be densified [14]. Given the temperature limitation of the furnace, the further sintering experiment is not yet conducted. However, the fracture toughness and microhardness of the alumina ceramics display negligible dependence on the plate-like alumina content. It is worth noting that

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all samples with plate-like alumina display lower density and bending strength but higher fracture toughness than the sample without plate-like alumina. Fig. 5 shows the indentation crack propagations on the polished surface of sintered ceramic prepared from starting powder with (a) 0 wt% and (b) 50 wt% plate-like alumina powder. Crack path in sample prepared with pure granular alumina powder is almost straight (Fig. 5a), whereas crack deflection and crack bridging are observed in sample prepared with 50 wt% plate-like alumina (Fig. 5b). The crack deflects and crack bridging can consume more impact energy, enhancing fracture toughness.

4. Conclusion The effects of plate-like alumina on the microstructure and mechanical properties of alumina ceramics were investigated. Plate-like alumina can improve the fluidity of the slurry, and the viscosity of suspensions rapidly decreased with increasing plate-like alumina content. The microstructure of the fracture surface of the sample prepared with granular alumina was relatively uniform. The grain size and porosity increased with increasing plate-like alumina powder content. As the plate-like alumina content varied from 10 wt%, 30 wt%, 50 wt%, 70 wt% to 100 wt%, the density and bending strength of the sintered ceramics increased firstly then decreased, reaching their maximum value of 3.88 g/cm3 and 264.16 MPa at the content of 50 wt%. Although the density and bending strength of the sintered ceramics decreased with the addition of plate-like alumina, the fracture toughness of the alumina ceramics increased by adding plate-like alumina. The maximum fracture toughness of alumina ceramics with 50 wt% plate-like alumina powder reached up to 3.58 Mpa.m1/2. The plate-like alumina particles formed

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crack deflection and crack bridging in the ceramics matrix, which enhanced the fracture toughness. We suppose that the bending strength and fracture toughness of alumina ceramics with plate-like particles can be further improved when they are sintered at elevated temperature.

References [1] L.D. Hart (Ed.), Alumina Chemicals: Science and Technology Handbook, The American Ceramic Society Inc, Westerville, OH, 1990. [2] Chang-Gi Ha, Yeon-Gil Jung, et al. Mater. Sci. Eng. A. 337 (2002) 212-221. [3] A.C. Young, O.O. Omatete, M.A. Janney, J. Am. Ceram. Soc. 74(3) (1991) 612–618. [4] Mehrdad Kokabi, Ali Akbar Babaluo, Abolfazl Barati, J. Eur. Ceram. Soc. 26 (2006) 3083–3090. [5] H. Akhondi, E. Taheri-Nassaj, H. Sarpoolaky, A. Taavoni-Gilan, Ceram. Int. 35 (2009) 1033-1037. [6] R. Gilissen, J.P. Erauw, A. Smolders, E. Vanswijgenhoven, J. Luyten, Mater. Des. 21 (2000) 251-257. [7] Xiaolin Liu, Yong Huang, Jinlong Yang, Ceram. Int. 28(2002) 159-164. [8] E. Champion, S. Gautier, D. Bernache-assollant, J. Mater. Sci.-Mater. Med. 7 (1996) 125-130. [9] Yiquan Wu, Yufeng Zhang, Xiaoxian Huang, Jing-kun Guo, Ceram. Int. 27 (2001) 265-268. [10] Kim H. J. , Kim T. G. , Kim J. J. , Park S. S. , Hong S. S. , Lee G. D. b. , J. Phys.

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Chem. Solids. 69 (2008) 1521-1524. [11] Zhu Lihui, Liu Wei, et a1, Effect of plate-like alumina seeds on alumina ceramics. J. Mater. Sci. Eng. 26 (2008) 44-47. [12] Yu Jia-wei, Liao Qilong, J. Funct. Mater. 42 (2011) 1833-1835. [13] Manuel Belmonte, Jose S. Moya, Pilar Miranzo, J. Am. Ceram. Soc. 78(6) (1995) 1661-1667. [14] Mehdi Rahimian, Naser Ehsani, Nader Parvin, Hamid reza Baharvandi, J. Mater. Process. Technol. 209 (2009) 5387-5393.

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Figure Captions: Fig. 1 SEM micrograph of plate-like alumina (a) and granular alumina (b) particles Fig. 2 Flow chart of the preparation of alumina ceramic Fig. 3 Effect of plate-like alumina content on the viscosity of suspensions Fig. 4 SEM photographs of the sintered materials prepared from starting powder with (a) 0 wt%, (b) 10 wt%, (c) 30 wt%, (d) 50 wt%, (e) 70 wt% and (f) 100 wt% contents of plate-like alumina powder Fig. 5 SEM micrographs showing indentation crack propagations in sintered ceramics prepared from starting powder with (a) 0 wt% and (b) 50 wt% contents of plate-like alumina powder

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List of tables: Table.1 Experimental results

Plate-like Density

Bending strength

Micro hardness

Fracture toughness

g/cm3

MPa

GPa

MPa.m1/2

0

3.93

310.82

10.66

3.16

10%

3.60

196.80

12.97

3.57

30%

3.71

225.90

11.62

3.51

50%

3.88

264.16

10.09

3.58

70%

3.28

124.50

13.44

3.50

100%

3.33

160.49

12.24

3.36

alumina content

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*Figure(s)

Fig. 1 Flow chart of the preparation of alumina ceramic

*Figure(s)

Fig. 2 SEM micrograph of plate-like alumina (a) and granular alumina (b) particles

*Figure(s)

Fig. 3 Effect of plate-like alumina content on the viscosity of suspensions

Figure(s)

Fig. 4 SEM photographs of the sintered materials prepared from starting powder with (a) 0 wt%, (b) 10 wt%, (c) 30 wt%, (d) 50 wt%, (e) 70 wt% and (f) 100 wt% contents of plate-like alumina powder

*Figure(s)

Fig. 5 SEM micrographs showing indentation crack propagations in sintered ceramics prepared from starting powder with (a) 0 wt%, (b) 50 wt% contents of plate-like alumina powder