Materials Science Forum Vols. 638-642 (2010) pp 979-984 Online available since 2010/Jan/12 at www.scientific.net © (2010) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.638-642.979
MICROSTRUCTURE AND SINTERING BEHAVIOR OF MULLITE-ZIRCONIA COMPOSITES F. Sahnoune1, a, N. Saheb2, b, M. Chegaar3, c, P. Goeuriot4, d 1
Laboratoire de Physique et Chimie des Matériaux, Université de M’sila, 28000, Algeria
2
Department of Mechanical Engineering, King Fahd University of Petroleum & Minerals, Dhahran, 31261, Kingdom of Saudi Arabia 3
Department of Physics, Ferhat Abbas University, 19000, Setif, Algeria
4
Department of Special Ceramics, ENSM, 158, cours Fauriel 42023 Saint-Etienne cedex 2, France
a
[email protected],
b
[email protected],
c
[email protected],
d
[email protected]
Keywords: Kaolin, Mullite, Reaction Sintering, Composites.
Abstract. In the present work, the structure and sintering behaviour of mullite-zirconia composites were investigated. The composites were prepared by reaction sintering of Algerian kaolin, α-Al2O3, and stabilized zirconia (3Y-TZP). The raw materials were wet ball milled in a planetary ball mill followed by attrition. The green samples shaped using a uniaxial press were sintered between 1100°C and 1600°C for 2 hours. The density was measured by the water immersion method. Phases present and change of the average crystallite size of the mullite phase as a function of sintering temperature and ZrO2 content were characterized through X-ray diffraction. Mulite grains had whiskers' shape; however, the increase of ZrO2 content changed the morphology of grains to near spherical shape. The microstructure of the samples revealed uniform distribution of ZrO2 particles; also, aluminium was uniformly distributed on all grains exception on zirconia grains. At least a relative density of 95 % was achieved for all samples at a relatively lower sintering temperature of 1500°C. In composites containing up to 16 wt. % ZrO2, the zirconia phase retained its tetragonal structure and the transformed part did not exceed 3 %. However, with the addition of 32 wt. % ZrO2 around 66 % of the tetragonal structure changed to monoclinic structure. Introduction Mullite is recognized as one of the most promising engineering materials for applications at elevated temperatures because of its high melting point, low thermal expansion, good chemical stability, excellent creep resistance and high strength at high temperatures [1, 2]. However, applications of mullite were limited because of its poor properties at ambient temperature. One way to improve the properties of mullite is to reinforce it by addition of particles, fibers or whiskers to produce composites with superior properties [1, 3-8]. Also, a well-known process to improve fracture toughness [1-2, 9-12] is phase transformation, under applied pressure, from tetragonal to monoclinic in zirconia particles. This stress-induced phase transformation is accompanied by a volume expansion (~4%) and shear deformation (~6%) [2, 13]. Although mullite-zirconia composites were synthesized through different methods [14] such as sintering of mixed mullite and ZrO2 powders, reaction sintering of mixtures of ZrO2 and mullite precursors, reaction sintering of zircon (ZrSiO4) and Al2O3, and crystallization of rapidly quenched melts in the ZrO2– Al2O3–SiO2 system with subsequent sintering; the reaction sintering method remained the most commonly used because of its low cost. In previous work we synthesised mullite through reaction sintering of Algerian kaolin and Al2O3 [15] and investigated the kinetics of mullite formation from Algerian kaolin [16]. Also, in another work, one of the authors investigated the effect of MgO addition and sintering parameters on mullite formation through reaction sintering Algerian kaolin and alumina [17]. The objectives of the present work is to synthesis mullite–zirconia composites through reaction sintering of Algerian kaolin, α- Al2O3, and ZrO2, and characterize the structure of the composites and investigate their sintering behaviour. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 41.98.221.232-13/11/10,18:27:59)
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Materials and Experimental Procedures Raw kaolin (DD3, from Guelma, Algeria) was used in this investigation, its chemical composition as determined by XRF is reported elsewhere [16, 17]. α-Al2O3 powder (MARTOXTD MDS-6) and zirconia containing 3 mol% Y2O3 (3Y-TZP, CRIGERAM) were added to kaolin to obtain 100/00, 92/08, 84/16, 76/24 and 68/32 mullite/ZrO2 composites (by wt. %), they are named KA00Z, KA08Z, KA16Z, KA24Z and KA32Z respectively. The powders were charged into cylindrical zirconia vials (250 ml in volume) together with 15 zirconia balls (10mm in diameter). Water was added to the mixture at a ratio of 2:1. Ammonium polymethacylate (1 wt%) was used as dispersant. Ammoniac was added to adjust the pH of the suspension at approximately 10.5. The ball-milling experiments were performed in a high-energy planetary ball mill (Fritsch P6) and were carried out at room temperature at a rotation speed of 250 rpm. The mixture was ball milled for 5 hours followed by attrition for 1 hour using ZrO2 balls (diameter of 1.2 mm) at a speed of 1250 rpm. After attrition, the slurry was dried at 110°C and subsequently granulated by sieving. A cold uniaxial press was used to shape bars (25x5x2 mm) and discs of 8 mm diameter and compact them at 75 MPa. The green samples were sintered between 1100°C and 1600°C for 2 hours, a heating rate of 10°C /min was used. The microstructure of sintered samples was characterized using a JEOL scanning electron microscope (SEM) model JSM 5600 equipped with energy dispersive spectrometer (EDS). The density of samples was measured by the water immersion method using a KERN densimeter. Phases present and their transformations as well as the change of the average crystallite size of the mullite phase as a function of sintering temperature and ZrO2 content were characterized using a Bruker x-ray diffractometer model D8 (Cu Kα radiation and a Ni-filter) operated at 40 kV, 40 mA with a scanning speed of 0.3°/min and a step of 0.05. Results and Discussion Figure 1 (left) shows the microstructure of samples KA00Z, KA08Z, KA16Z, KA24Z and KA32Z sintered at 1600°C for 2 hours. These samples were polished then heat treated 15 minutes at a temperature lower than the sintering temperature by 150°C, after that they were coated with gold to study the microstructure at the surface. The microstructure of fractured and gold coated samples containing 0, 8, 24 and 32 wt.% of ZrO2 is shown in Fig.1 (right). Figure 2(a) and (b) show the microstructure of samples KA08Z and KA32Z, respectively, sintered at 1600°C for 2 hours. These samples were polished then heat-treated 15 minutes at a temperature lower than the sintering temperature by 150°C, after that they were coated with carbon to study the distribution of phases in the samples. As can be seen from Fig.1 and Fig. 2 mulite grains have whiskers' shape; however, the increase of ZrO2 content changed the morphology of grains to near spherical shape. The distribution of Zr in the KA08Z and KA32Z samples is shown in Fig. 2(c) and (d) respectively. It is clear that ZrO2 particles are uniformly distributed in the samples. The distribution of Al in the KA08Z and KA32Z samples is shown in Fig. 2(e) and (f) respectively. Aluminium is distributed uniformly on all grains exception on zirconia grains this implies the end of interaction between aluminium and SiO2 to form mullite. The energy spectrum of KA08Z and KA32Z samples is presented in Fig.2 (g) and (h). The EDS spectrum shows the existence of only aluminium, silicon and zirconium in addition to oxygen. More information on the formation of mullite through reaction sintering of Algerian Kaolin and Al2O3 as well as phase transformations that take place during sintering is reported elsewhere [15]. In samples containing ZrO2, ZrSiO4 starts to form at 1250°C through the reaction of ZrO2 and SiO2 and will again dissolve to its main ingredients at 1350°C. The monoclinic ZrO2 starts to form at 1400°C and its fraction increases with the increase of temperature up to 1600°C. Analysis of qualitative and quantitative XRD data showed that in composites containing up to 16 wt. % ZrO2, the zirconia phase retained its tetragonal structure and the transformed part did not exceed 3 %. However, with the addition of 32 wt. % ZrO2 around 66 % of the tetragonal structure changed to monoclinic structure. Also, the increase of ZrO2 content led to the decrease of the average crystallite size of the mullite phase from 140 nm to 90 nm.
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AK00Z
AK08Z
AH24Z
AK32Z Figure 1: SEM micrographs of samples sintered 2h at 1600°C, polished and thermally etched (left) and fractured (right).
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ghfkg h
55
KA08Z
ZrO2
Al2O3
KA32Z
ZrO2
Al2O3
Figure 2: SEM micrographs of KA08Z (a) and KA32Z (b) samples sintered 2h at 1600°C, distribution of Zr (c, d) and Al (e, f) and EDS septum (g, h).
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The bulk density of KA00Z, KA08Z, KA16Z, KA24Z and KA32Z samples sintered 2 h at different sintering temperatures is shown in Fig. 3; and the relative density (ratio between experimental and theoretical densities) of KA00Z, KA16Z, and KA32Z samples sintered 2h at different sintering temperatures is presented in Fig. 4. Theoretical values of the density of phases from literature [2, 18-20] presented in table 1 were used to calculate the theoretical density of samples. Phases present in the samples and their weight fractions were obtained from qualitative and quantitative XRD data. Table 1: The theoretical bulk density of phases Phases
Al2O3
t-ZrO2
m-ZrO2
ZrSiO4
SiO2Am
SiO2c
SiO2q
Mullite
ρ (gr/cm3)
3.98
6.80
5.85
4.6
2.20
2.32
2.26
3.16
It is clearly seen from Fig. 4 that the relative density of samples exceeded 95% for temperatures greater than or equal to 1500°C. The bulk and relative densities remained unchanged after 1500°C. This is an indication of the complete formation of the stable phases i.e. mullite and zirconia with its two phases. For sample containing 32 wt. % ZrO2, the relative density reached 95 % at only a temperature of 1400°C, and it exceeded 97 % at higher temperatures. The change of bulk density with sintering temperature can be divided in three stages. Initially the density increased with temperature in the first stage which ended at 1300°C for sample containing 0 and 8 wt. % ZrO2 and at 1350°C for sample containing 16, 24 and 32 wt. % ZrO2. This difference is due to the presence of different phases in the samples. In the second stage which ended at around 1450°C there was a decrease of the bulk density. However, the relative density did not decrease. This is due to phase transformations taking place in the samples. The third stage between 1450°C and 1600°C is characterized by the formation of stable closed pores and the elimination of open pores; in this stage the density remained almost unchanged. It is believed that the grain size (particle size) has an important effect on the densification of samples. The smaller the grain size the shorter the sintering time and the lower the sintering temperature for the formation of primary and secondary mullite. This is due to the fact that less energy is required for Al and Si to diffuse for a short distance to from the stable phases [15, 16]. 100
x=0 x=8 x=16 x=24 x=32
3.7 3.5 bulk density
AK00Z AK16Z AK32Z
95
Relative bulk density (%)
3.9
3.3 3.1 2.9
90
85
80
75
2.7 70
2.5 1150
1250
1350
1450
1550
temperature °C
Figure 3: Bulk density of samples
1650
1100
1200
1300
1400
1500
1600
0
T e m p e ra tu re ( C )
Figure 4: Relative density of samples
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Conclusion Mullite-zirconia composites were prepared by reaction sintering Algerian kaolin, α-Al2O3, and stabilized zirconia (3Y-TZP). Mulite grains had whiskers' shape; however, the increase of ZrO2 content changed the morphology of grains to near spherical shape. The microstructure of the samples revealed uniform distribution of ZrO2 particles; also, aluminium was uniformly distributed on all grains exception on zirconia grains which indicates the end of interaction between aluminium and SiO2 to form mullite. At least a relative density of 95 % was achieved for all samples at a relatively lower sintering temperature of 1500°C. The change of the bulk density with sintering temperature was characterized by three stages. The density increased with temperature in the first stage and decreased in the second stage, however, the relative density did not decrease. The third stage was characterized by the formation of stable closed pores and the elimination of open pores; in this stage the density remained almost unchanged. In composites containing up to 16 wt. % ZrO2, the zirconia phase retained its tetragonal structure and the transformed part did not exceed 3 %. However, with the addition of 32 wt. % ZrO2 around 66 % of the tetragonal structure changed to monoclinic structure. Also, the increase of ZrO2 content led to the decrease of the average crystallite size of the mullite phase from 140 nm to 90 nm. Acknowledgements: The authors are grateful to the technical support from Laboratoire de Physique et Chimie des Matériaux, Université de M’sila, Algeria, and the Department of Special Ceramics, ENSM, France. Also, the corresponding author would like to acknowledge the financial support from King Fahd University of Petroleum and Minerals, Kingdom of Saudi Arabia. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
X.H. Jin, L. Gao, Y.R. Chen and Q.M. Yuan: J. Eur. Ceram. Soc. Vol. 20 (2000), p. 2115. H.C. Park, T.Y. Yang, S.Y. Yoon and R. Stevens: Mate. Sci. A Vol. 405 (2005), p. 233. N. Claussen: J. Phys. Vol. 47 (1986), p. 693. Z. F. Yang, H. Y. Xu, J. Q. Tan, and Q. M. Yuan: J. Chin. Ceram. Soc. Vol. 17 (1989), p. 467. J. S. Hong . X. X, Huang. J. K, Guo and B. S, Li: J Chin. Ceram. Soc. Vol. 20 (1992), p. 410. P. F. Becher and T. W Tiegs: J. Am. Ceram. Soc. Vol. 70 (1987), p. 651. R. Ruh and K. S. Mazdiyashi: J. Am. Ceram. Soc. Vol. 71 (1988), p. 503. J. K. J. Guo. Chin. Ceram. Soc. Bull. Vol. 14 (1995), p. 18. J.S. Moya and M.I. Osendi: J. Mater. Sci. Vol 19 (1984), p. 2909. S. Prochazka, J.S. Wallace and N. Claussen: J. Am. Ceram. Soc. Vol. 66 (1983), p. 125. Q.M. Yuan, J.Q. Tan and Z.G. Jin: J. Am. Ceram. Soc. Vol. 69 (1986), p. 265. J.S. Moya and P. Miranzo, in: High Tech Ceramics, Edited by P. Vincenzini. Elsevier Science Publishers B.V, Amsterdam (1987). D.J. Green, R.H.J. Hannink and M.V. Swain: Transformation Toughening Ceramics, CRC Press Inc., FL, (1989). H. Schneider, K. Okada and J. Pask: Mullite and Mullite Ceramics, John Wiley & Sons, New York, (1994) F. Sahnoune, M. Chegaar, N. Saheb, P. Goeuriot and F. Valdivieso: App. Clay Sci. Vol. 38 (2008), p. 304. F. Sahnoune, M. Chegaar, N. Saheb, P. Gueuriot, and F. Valdivieso: Adv. App. Ceram. Vol. 107 (2008) p. 9. M. Heraiz, A. Merrouche and N. Saheb: Adv. App. Ceram. Vol. 105 (2006), p. 285. K. Das, S.K. Das, B. Mukherjee and G. Banerjee: Inter. Ceram. Vol. 47 (1998), p. 304. C.Y. Chen, G.S. Lan and W.H. Tuan: J. Eur. Ceram. Soc. Vol. 20 (2000), p. 2519. CRC materials science and engineering handbook, edited by James F. Shackelford and William Alexander, (1995).
THERMEC 2009 doi:10.4028/www.scientific.net/MSF.638-642 Microstructure and Sintering Behavior of Mullite-Zirconia Composites doi:10.4028/www.scientific.net/MSF.638-642.979 References [1] X.H. Jin, L. Gao, Y.R. Chen and Q.M. Yuan: J. Eur. Ceram. Soc. Vol. 20 (2000), p. 2115. doi:10.1016/S0955-2219(00)00108-4 [2] H.C. Park, T.Y. Yang, S.Y. Yoon and R. Stevens: Mate. Sci. A Vol. 405 (2005), p. 233. doi:10.1016/j.msea.2005.06.005 [3] N. Claussen: J. Phys. Vol. 47 (1986), p. 693. [4] Z. F. Yang, H. Y. Xu, J. Q. Tan, and Q. M. Yuan: J. Chin. Ceram. Soc. Vol. 17 (1989), p. 467. [5] J. S. Hong . X. X, Huang. J. K, Guo and B. S, Li: J Chin. Ceram. Soc. Vol. 20 (1992), p. 410. [6] P. F. Becher and T. W Tiegs: J. Am. Ceram. Soc. Vol. 70 (1987), p. 651. doi:10.1111/j.1151-2916.1987.tb05734.x [7] R. Ruh and K. S. Mazdiyashi: J. Am. Ceram. Soc. Vol. 71 (1988), p. 503. doi:10.1111/j.1151-2916.1988.tb05902.x [8] J. K. J. Guo. Chin. Ceram. Soc. Bull. Vol. 14 (1995), p. 18. [9] J.S. Moya and M.I. Osendi: J. Mater. Sci. Vol 19 (1984), p. 2909. doi:10.1007/BF01026966 [10] S. Prochazka, J.S. Wallace and N. Claussen: J. Am. Ceram. Soc. Vol. 66 (1983), p. 125. doi:10.1111/j.1151-2916.1983.tb11004.x [11] Q.M. Yuan, J.Q. Tan and Z.G. Jin: J. Am. Ceram. Soc. Vol. 69 (1986), p. 265. doi:10.1111/j.1151-2916.1986.tb07422.x [12] J.S. Moya and P. Miranzo, in: High Tech Ceramics, Edited by P. Vincenzini. Elsevier Science Publishers B.V, Amsterdam (1987). [13] D.J. Green, R.H.J. Hannink and M.V. Swain: Transformation Toughening Ceramics, CRC Press Inc., FL, (1989).
[14] H. Schneider, K. Okada and J. Pask: Mullite and Mullite Ceramics, John Wiley & Sons, New York, (1994) [15] F. Sahnoune, M. Chegaar, N. Saheb, P. Goeuriot and F. Valdivieso: App. Clay Sci. Vol. 38 (2008), p. 304. doi:10.1016/j.clay.2007.04.013 [16] F. Sahnoune, M. Chegaar, N. Saheb, P. Gueuriot, and F. Valdivieso: Adv. App. Ceram. Vol. 107 (2008) p. 9. doi:10.1179/174367607X228007 [17] M. Heraiz, A. Merrouche and N. Saheb: Adv. App. Ceram. Vol. 105 (2006), p. 285. doi:10.1179/174367606X146676 [18] K. Das, S.K. Das, B. Mukherjee and G. Banerjee: Inter. Ceram. Vol. 47 (1998), p. 304. [19] C.Y. Chen, G.S. Lan and W.H. Tuan: J. Eur. Ceram. Soc. Vol. 20 (2000), p. 2519. doi:10.1016/S0955-2219(00)00125-4 [20] CRC materials science and engineering handbook, edited by James F. Shackelford and William Alexander, (1995).
