- Email: [email protected]
Pressureless sintering of sol-gel derived alumina–zirconia composites
Materials Science and Engineering A256 (1998) 265 – 270
Pressureless sintering of sol-gel derived alumina–zirconia composites Doni Jayaseelan a,*, Tadahiro Nishikawa b, Hideo Awaji b, F.D. Gnanam a a
b
Ceramic Di6ision, Department of Chemical Engineering, Anna Uni6ersity, Chennai-600 025, India Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagaya-466, Japan Received 30 January 1998; received in revised form 21 April 1998
Abstract Alumina–zirconia (pure zirconia, 12 mol % ceria stabilized zirconia and 3 mol% yttria stabilized zirconia) composites containing 5, 10, 15, 20 and 25 vol.% zirconia were prepared by sol-gel technique. The procedure involved the following steps: preparation of stable (hydrous) alumina and zirconia sols, mixing of sols in proper ratio, to obtain the final precursor with the desired composition and finally stabilizing the mixed sols. Thermal analysis was carried out for the dried precursor. The sol-gel derived precursors were calcined at different temperatures and their crystallization behavior was studied using X-ray diffraction (XRD) technique. Samples calcined at 950°C sintered well than those calcined at other temperatures. All the samples were sintered at 1530°C for 3 h by pressureless sintering, which yield upto 99.8% of the theoretical density of the composite. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Alumina – zirconia composites; Calcination; Pressureless sintering; Sol-gel
1. Introduction Zirconia-toughened alumina (ZTA) has received significant interest in recent years. The room temperature mechanical properties of alumina ceramics are improved significantly by the incorporation of well-dispersed tetragonal zirconia (t) polycrystals, where they transform into the monoclinic phase (m) under loading [1 –3]. Fracture toughness and strength of alumina can thus be increased remarkably by utilizing the martensitic (t m) transformation of zirconia particles dispersed in the alumina matrix [1 – 4]. The retention of t-ZrO2 has to be considered carefully until the composite is used for service. It has been recognized that the magnitude of toughening is complexly dependent on the microstructure of alumina – zirconia composite (i.e. volume fraction, size, shape, location and size distribution of ZrO2) [4,5]. Homogeneous, as well as fine dispersions of ZrO2 particles in the alumina matrix can be obtained by chemical mixing of the constituents in the solution or in the sol state. Hence the sol-gel * Corresponding author. E-mail: [email protected]
processing was used in the present work to prepare alumina/zirconia composite powder. Sol-gel processing arose as a new technique for the fabrication of high quality ceramic based composite. For complex oxides, it achieves ultra-homogeneous mixing of the several components on a molecular scale, low sintering temperature and hence fine-grained microstructure. Thus liquid precursor technology offers processing advantages and gives flexibility in tailoring the composite chemistry to obtain the desired properties [6–10]. Moreover, the processing conditions, composition, retention of t-phase of zirconia and the calcination temperature strongly influence the morphology of the powder and sintering behavior [10–14]. The arguments and observations considered to cause such modifications are changeable with the phase composition, crystallinity, crystallite size and pore-morphology, specific surface area, residual weight loss and subsequent shrinkage. Several methods of preparing alumina/zirconia powders have been reported elsewhere [6–14]. However, most of the work to date on ZTA has involved the use of high-purity alumina, and either hot pressing or sintering at very high temperature utilized in many cases to achieve a dense, fine-grained alumina–
0921-5093/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S0921-5093(98)00801-6
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zirconia composite. Therefore, the aim of this paper is to optimize the parameters of the sol-gel process technique in order to prepare highly sinterable alumina/zirconia composites using pressureless sintering. To perform this low temperature sintering, we had to study the effect of various experimental conditions, such as repeated alcohol washing, calcination temperature, stabilizing agents and compositions on the texture and the sintering behavior of the composites.
2. Experimental details The parameters of the present study were stabilizing agents, volume fraction of zirconia (composition) and calcination temperature. During present investigation, three series of alumina/zirconia composites (additivefree zirconia, 12 mol% ceria-stabilized zirconia and 3 mol% yttria-stabilized zirconia) with different compositions such as 5, 10, 15, 20 and 25 vol.% of zirconia, were prepared. Fig. 1 shows the typical flow chart of the processing of alumina/zirconia composites. The procedure involved the following steps: preparation of stable (hydrous) boehmite and zirconyl oxalate (zirconyl oxalate, zirconyl cerium oxalate, zirconyl yttrium oxalate) sols, mixing of sols in proper ratio, to obtain
Fig. 2. DTA curve of as-dried alumina-15 vol.% Ce – zirconia precursor.
the final precursor with the desired composition and finally stabilizing the mixed sols. Aluminium isopropoxide [15] and zirconyl oxychloride [16] were used as the starting materials for preparing the respective sols. Water soluble salts of metal nitrates such as cerium nitrate and yttrium nitrate were used for stabilizing the t-phase of zirconia. The boehmite sol and zirconyl oxalate sol were mixed together in an appropriate ratio to have a desired composition and stirred vigorously for homogeneous mixing at room temperature for few hours. The pH of the composite sol was maintained in a slightly acidic region. After stabilizing the mixed sol, it became gel. It was repeatedly washed with alcohol and was oven dried at 110°C for few hours. The amorphous precursors were calcined at different temperatures 300, 700, 950 and 1250°C respectively. The as-dried and the calcined precursors were characterized by particle size analyzer, BET surface area analyzer and X-ray diffractometer. The powder of different compositions calcined at various temperatures were wet ball milled in alcohol, dried and sieved. The sieved powders were pressed into compacts, using an uniaxial press at a pressure of 240 MPa. PVA was added as the binder in the milling stage, for all the samples. The pellets were oven dried at 110°C for 24 h. The dried compacts were heated to 600°C and kept for 3 h to burn out the organic binder and then sintered to a maximum temperature of 1530°C. Bulk density of the sintered samples was measured by Archimede’s method. Microstructure of the sintered specimens were analyzed using scanning electron microscope (SEM).
3. Results and discussions
Fig. 1. Flow chart of sol-gel processing for alumina–zirconia composites.
The DTA of the as-dried powder is shown in Fig. 2. The endothermic peak at 100°C is due to the evaporation of absorbed water. The endothermic peak at 350°C
D. Jayaseelan et al. / Materials Science and Engineering A256 (1998) 265–270
corresponds to the transformation of pseudo boehmite into d-alumina. The exothermic peak at 450°C is attributed to the crystallization of t-zirconia. The small exothermic peak at 1230°C corresponds to the completion of a-alumina formation, i.e. conversion of remaining u-alumina to a-alumina. The calcined powders were characterized for their crystallinity using XRD. The first crystallite phases after calcination which appeared at low temperature were tetragonal phase of zirconia and d-phase of alumina (Fig. 3). The t-zirconia peaks were sharper than the d-alumina peaks, indicating the formation of well crystallized t-zirconia. Above 600°C, d-alumina was prominent but transformed into u, a-alumina at 950°C. However, after calcination at 1250°C, u-alumina transformed completely to a-alumina and hence the phases present were only t-zirconia and a-alumina. The transformation to a-alumina occurs at a higher temperature than usual (1100°C) in the presence of zirconia [8], which is evident from an exothermic peak at 1230°C in Fig. 2 (DTA). It is also clear from the X-ray patterns (Fig. 4), that zirconia grains retard the crystallization behavior of alumina. When the zirconia content is low, the intensity of alumina peaks is high and it decreases with increase in the zirconia content. On the other hand, the constraints imposed by the surrounding alumina grains influence the zirconia transformation. Alumina matrix with high elastic modulus and thermal
Fig. 3. XRD patterns of alumina-15 vol.% Ce–zirconia calcined at (a) 600°C; (b) 950°C; (c) 1250°C; and (d) 1400°C.
267
Fig. 4. XRD pattern of alumina-15 vol.% Ce – zirconia composites calcined at 1250°C for 3 h.
expansion coefficient restricts the expansion of zirconia occurring during t m transformation and reduces the transformation temperature. Zirconia present in the composite is therefore retained in the tetragonal form because of its fine particle size and the constraints imposed by the alumina grains. The t-phase of zirconia was retained in all the samples upto 1400°C. Synthesized powder of the composites is characterized and shown in Table 1. The specific surface area was determined on an automatic volumetric sorption analyzer, using N2 gas as an absorbate, by the BET method. An increase in the zirconia content or an increase in calcination temperature, respectively lead to a change in the microstructure. The particle size increases with increasing zirconia content and/or calcination temperature. The specific surface area for the powder at higher calcination temperature drastically decreases due to the formation of hard agglomerates and hence entrapment of pores during grain growth. At lower temperatures, as the hydrated powder is very fine, they agglomerate together. These agglomerates are broken into smaller particles, when calcined at 950°C and above this temperature they tend to become aggregate, accompanied by grain growth which is evident from the increase in grain size in Table 1. The mean particle diameter (d50) of the powder calcined at 950°C has relatively large specific surface area (around 85 m2 g − 1) which is desirable for the low temperature sintering. The in-
300 950 1250
300 950 1250
300 950 1250
5
15
25
g, d, u, a-phases of alumina, t-phase of zirconia.
Calcination temperature (°C)
ZrO2 (12 Ce) Vol.%
284.5 95 3.44
271.9 89 2.68
256.9 84.5 1.34
3.9 1.8 2.8 6.3 3.6 4 7.2 3.6 4
94 95 90.3 95 93 90.8
91.3 90.8 90.9
91.2 90.3 91.4
90.7 90.4 90.8
Mean particle size (d50) (mm)
96 93 90.4
Specific surface area (m2 g−1)
Table 1 Powder characterization of alumina-12 Ce–zirconia precursors after calcination at different temperatures
4.6 13.9 383
5.1 15 518
5 17 1094
90.7 92.1 915
90.4 92.1 918
90.8 91.2 912
Grain size (nm)
1.88 3.91
2.14 4.27
3.6
Zirconia
4.2 4.8
5.4
6.7
Alumina
Crystallite size (nm)
g d, u, a, t a, t
g d, u, a, t a, t
g d, u, a, t a, t
Identified phases
268 D. Jayaseelan et al. / Materials Science and Engineering A256 (1998) 265–270
D. Jayaseelan et al. / Materials Science and Engineering A256 (1998) 265–270
crease in surface area of the powder is mainly due to repeated alcohol washing, wet milling and other processing conditions. Change of bulk density after sintering with calcination temperature is shown in Fig. 5. High bulk density was obtained for the composite powder calcined at 950°C, compared to calcined powder at lower or higher temperature irrespective of the type of zirconia. Due to the presence of higher amount of hydrates, weight loss was observed after sintering for powder calcined at low temperatures. Since, some energy is utilized for this evolution of hydrates (endothermic process) during sintering of the compacts, only a less amount of energy is available for sintering. When escaping, the hydrates will generate the residual pores. Thus the pores trapped within a-alumina grains during the final stage of conventional sintering are extremely difficult to remove. These two factors are responsible for the low bulk density for the powder calcined at lower temperatures. The decrease in bulk density above 1000°C is most likely caused by the grain growth accompanying the phase transformation of alumina. The microstructural development with ua-alumina phase transformation is obviously the most important sintering step for the densification of transition alumina containing composites. The concept of simultaneous sintering and phase transformation may provide a better opportunity for low temperature sin-
Fig. 5. Bulk density of three kinds of alumina-15 vol.% zirconia versus calcination temperature.
269
Fig. 6. Bulk density of three series of alumina – zirconia composites pressureless sintered at 1530°C for 3 h.
tering of alumina/zirconia composite than the more conventional approach on the basis of alumina. The optimal zirconia particle size, surface area and distribution obtained by calcination would benefit stacking and sintering alumina/zirconia powder compact. The shorter diffusion length and higher surface curvature of the sub-micron sized powder helped to lower the sintering temperature and minimize coarsening. Therefore, the optimal calcination condition for densification of alumina/zirconia composite is 950°C for 3 h. Kurita and Hori [17] also reported an optimal calcination between 800 and 1000°C. Those ZTA specimens were prepared by the CVD method had a higher relative density. Fig. 6 shows the sintered density of the samples of the three series of the composites calcined at 950°C and sintered at 1530°C for 3 h, with respect to zirconia content. The sintered compacts yield density up to 99.8% of the theoretical density. It is observed from the Fig. 6, that the relative density of the sintered composite containing unstabilized zirconia decreases with respect to zirconia content, unlike Ce and Y-stabilized zirconia containing composites. When the amount of t-phase is calculated from the XRD, it is observed that nearly 90–95% of the t-phase is retained for the Ce and Y-stabilized zirconia for the entire composition, whereas for the unstablilized zirconia, the t-phase decreases with increase in the zirconia content. This t–m phase transformation of zirconia causes
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4. Conclusion Highly sinterable alumina/zirconia composites were prepared by sol-gel technique. Calcination at 950°C for 3 h is found to be the optimal calcination temperature for the best densification of alumina/zirconia composites. All the samples could be sintered at 1530°C for 3 h by pressureless sintering to more than 98% of the theoretical density. By optimizing the processing condition and calcination temperature, the sol-mixed sample was sintered well at relatively low temperature compare to the conventional and other processing methods. These specimens are expected to have better mechanical properties according to their microstrucutre.
Acknowledgements The authors would like to thank Dr Wilfried Wunderlich for the useful suggestions and discussions in preparing the manuscript. One of the authors (DJ) would like to acknowledge the financial support of MHRD, DST and CSIR, India and AIEJ-Japan, during his period of research.
References
Fig. 7. Microstructure of alumina-25 vol.% Ce–zirconia composite sintered at 1530°C for 3 h; (a) polished sample; and (b) thermally etched at 1450°C for 30 min.
volume expansion leading to a decrease in the relative density of the composite. Fig. 7 shows the typical microstructure of alumina/25% Ce – zirconia sintered at 1530°C for 3 h. The average grain size of alumina is 2 mm and the composite has attained full densification. It is seen from the etched specimen that the zirconia grains are uniformly distributed in the alumina matrix. It is inferred that zirconia content influences the zirconia spatial distribution and hence at higher volume content of zirconia such as 25% ZrO2 (Fig. 7), the zirconia becomes less isolated and more continuous and interconnected.
.
[1] R.C. Garvie, R.H. Hannink, R.T. Pascoe, Nature Lond. 258 (1975) 703. [2] N. Claussen, J. Am. Ceram. Soc. 59 (1976) 49. [3] W.M. Kriven, J. Am. Ceram. Soc. 71 (1988) 1021. [4] J. Wang, R. Stevens, J. Mat. Sci. 24 (1988) 3421. [5] A.H. Heuer, N. Claussen, W.M. Kriven, M. Ruhle, J. Am. Ceram. Soc. 65 (1982) 642. [6] P.A. Badkar, J.E. Bailey, J. Mater. Sci. 11 (1976) 1794. [7] E.A. Pugar, P.E.D. Morgan, J. Am. Ceram. Soc. 69 (1986) 120. [8] M. Balasubramanian, S.K. Malhotra, C.V. Gokularathnam, Br. Ceram. Trans. 95 (1996) 263. [9] I.M. Low, R. McPherson, J. Mater. Sci. 24 (1989) 892. [10] J.C. Debsidkar, J. Mater. Sci. 22 (1987) 2237. [11] M.A.C.G. Van de Graaf, J.H.H. Ter Maat, A.J. Burggraaf, J. Mater. Sci. 20 (1985) 1407. [12] P. den Exter, L. Winnubst, T.H.P. Leuwerink, A.J. Burggraaf, J. Am. Ceram. Soc. 77 (1994) 2376. [13] C. Brodhag, J.P. Bach, F. Thevnot, M. Deletter, Mater. Sci. Eng. A109 (1989) 53. [14] C.S. Hwang, S.C. Tsaur, J. Mater. Sci. 27 (1992) 6791. [15] B.E. Yoldas, J. Appl. Chem. Biotech. 23 (1973) 803. [16] T. Settu, R. Gobinathan, Bull. Chem. Soc. Jpn. 67 (1994) 1999. [17] R. Kurita, S. Hori, US Patent 4 665 040 (1987).
Pressureless sintering of sol-gel derived alumina–zirconia composites Doni Jayaseelan a,*, Tadahiro Nishikawa b, Hideo Awaji b, F.D. Gnanam a a
b
Ceramic Di6ision, Department of Chemical Engineering, Anna Uni6ersity, Chennai-600 025, India Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagaya-466, Japan Received 30 January 1998; received in revised form 21 April 1998
Abstract Alumina–zirconia (pure zirconia, 12 mol % ceria stabilized zirconia and 3 mol% yttria stabilized zirconia) composites containing 5, 10, 15, 20 and 25 vol.% zirconia were prepared by sol-gel technique. The procedure involved the following steps: preparation of stable (hydrous) alumina and zirconia sols, mixing of sols in proper ratio, to obtain the final precursor with the desired composition and finally stabilizing the mixed sols. Thermal analysis was carried out for the dried precursor. The sol-gel derived precursors were calcined at different temperatures and their crystallization behavior was studied using X-ray diffraction (XRD) technique. Samples calcined at 950°C sintered well than those calcined at other temperatures. All the samples were sintered at 1530°C for 3 h by pressureless sintering, which yield upto 99.8% of the theoretical density of the composite. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Alumina – zirconia composites; Calcination; Pressureless sintering; Sol-gel
1. Introduction Zirconia-toughened alumina (ZTA) has received significant interest in recent years. The room temperature mechanical properties of alumina ceramics are improved significantly by the incorporation of well-dispersed tetragonal zirconia (t) polycrystals, where they transform into the monoclinic phase (m) under loading [1 –3]. Fracture toughness and strength of alumina can thus be increased remarkably by utilizing the martensitic (t m) transformation of zirconia particles dispersed in the alumina matrix [1 – 4]. The retention of t-ZrO2 has to be considered carefully until the composite is used for service. It has been recognized that the magnitude of toughening is complexly dependent on the microstructure of alumina – zirconia composite (i.e. volume fraction, size, shape, location and size distribution of ZrO2) [4,5]. Homogeneous, as well as fine dispersions of ZrO2 particles in the alumina matrix can be obtained by chemical mixing of the constituents in the solution or in the sol state. Hence the sol-gel * Corresponding author. E-mail: [email protected]
processing was used in the present work to prepare alumina/zirconia composite powder. Sol-gel processing arose as a new technique for the fabrication of high quality ceramic based composite. For complex oxides, it achieves ultra-homogeneous mixing of the several components on a molecular scale, low sintering temperature and hence fine-grained microstructure. Thus liquid precursor technology offers processing advantages and gives flexibility in tailoring the composite chemistry to obtain the desired properties [6–10]. Moreover, the processing conditions, composition, retention of t-phase of zirconia and the calcination temperature strongly influence the morphology of the powder and sintering behavior [10–14]. The arguments and observations considered to cause such modifications are changeable with the phase composition, crystallinity, crystallite size and pore-morphology, specific surface area, residual weight loss and subsequent shrinkage. Several methods of preparing alumina/zirconia powders have been reported elsewhere [6–14]. However, most of the work to date on ZTA has involved the use of high-purity alumina, and either hot pressing or sintering at very high temperature utilized in many cases to achieve a dense, fine-grained alumina–
0921-5093/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S0921-5093(98)00801-6
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zirconia composite. Therefore, the aim of this paper is to optimize the parameters of the sol-gel process technique in order to prepare highly sinterable alumina/zirconia composites using pressureless sintering. To perform this low temperature sintering, we had to study the effect of various experimental conditions, such as repeated alcohol washing, calcination temperature, stabilizing agents and compositions on the texture and the sintering behavior of the composites.
2. Experimental details The parameters of the present study were stabilizing agents, volume fraction of zirconia (composition) and calcination temperature. During present investigation, three series of alumina/zirconia composites (additivefree zirconia, 12 mol% ceria-stabilized zirconia and 3 mol% yttria-stabilized zirconia) with different compositions such as 5, 10, 15, 20 and 25 vol.% of zirconia, were prepared. Fig. 1 shows the typical flow chart of the processing of alumina/zirconia composites. The procedure involved the following steps: preparation of stable (hydrous) boehmite and zirconyl oxalate (zirconyl oxalate, zirconyl cerium oxalate, zirconyl yttrium oxalate) sols, mixing of sols in proper ratio, to obtain
Fig. 2. DTA curve of as-dried alumina-15 vol.% Ce – zirconia precursor.
the final precursor with the desired composition and finally stabilizing the mixed sols. Aluminium isopropoxide [15] and zirconyl oxychloride [16] were used as the starting materials for preparing the respective sols. Water soluble salts of metal nitrates such as cerium nitrate and yttrium nitrate were used for stabilizing the t-phase of zirconia. The boehmite sol and zirconyl oxalate sol were mixed together in an appropriate ratio to have a desired composition and stirred vigorously for homogeneous mixing at room temperature for few hours. The pH of the composite sol was maintained in a slightly acidic region. After stabilizing the mixed sol, it became gel. It was repeatedly washed with alcohol and was oven dried at 110°C for few hours. The amorphous precursors were calcined at different temperatures 300, 700, 950 and 1250°C respectively. The as-dried and the calcined precursors were characterized by particle size analyzer, BET surface area analyzer and X-ray diffractometer. The powder of different compositions calcined at various temperatures were wet ball milled in alcohol, dried and sieved. The sieved powders were pressed into compacts, using an uniaxial press at a pressure of 240 MPa. PVA was added as the binder in the milling stage, for all the samples. The pellets were oven dried at 110°C for 24 h. The dried compacts were heated to 600°C and kept for 3 h to burn out the organic binder and then sintered to a maximum temperature of 1530°C. Bulk density of the sintered samples was measured by Archimede’s method. Microstructure of the sintered specimens were analyzed using scanning electron microscope (SEM).
3. Results and discussions
Fig. 1. Flow chart of sol-gel processing for alumina–zirconia composites.
The DTA of the as-dried powder is shown in Fig. 2. The endothermic peak at 100°C is due to the evaporation of absorbed water. The endothermic peak at 350°C
D. Jayaseelan et al. / Materials Science and Engineering A256 (1998) 265–270
corresponds to the transformation of pseudo boehmite into d-alumina. The exothermic peak at 450°C is attributed to the crystallization of t-zirconia. The small exothermic peak at 1230°C corresponds to the completion of a-alumina formation, i.e. conversion of remaining u-alumina to a-alumina. The calcined powders were characterized for their crystallinity using XRD. The first crystallite phases after calcination which appeared at low temperature were tetragonal phase of zirconia and d-phase of alumina (Fig. 3). The t-zirconia peaks were sharper than the d-alumina peaks, indicating the formation of well crystallized t-zirconia. Above 600°C, d-alumina was prominent but transformed into u, a-alumina at 950°C. However, after calcination at 1250°C, u-alumina transformed completely to a-alumina and hence the phases present were only t-zirconia and a-alumina. The transformation to a-alumina occurs at a higher temperature than usual (1100°C) in the presence of zirconia [8], which is evident from an exothermic peak at 1230°C in Fig. 2 (DTA). It is also clear from the X-ray patterns (Fig. 4), that zirconia grains retard the crystallization behavior of alumina. When the zirconia content is low, the intensity of alumina peaks is high and it decreases with increase in the zirconia content. On the other hand, the constraints imposed by the surrounding alumina grains influence the zirconia transformation. Alumina matrix with high elastic modulus and thermal
Fig. 3. XRD patterns of alumina-15 vol.% Ce–zirconia calcined at (a) 600°C; (b) 950°C; (c) 1250°C; and (d) 1400°C.
267
Fig. 4. XRD pattern of alumina-15 vol.% Ce – zirconia composites calcined at 1250°C for 3 h.
expansion coefficient restricts the expansion of zirconia occurring during t m transformation and reduces the transformation temperature. Zirconia present in the composite is therefore retained in the tetragonal form because of its fine particle size and the constraints imposed by the alumina grains. The t-phase of zirconia was retained in all the samples upto 1400°C. Synthesized powder of the composites is characterized and shown in Table 1. The specific surface area was determined on an automatic volumetric sorption analyzer, using N2 gas as an absorbate, by the BET method. An increase in the zirconia content or an increase in calcination temperature, respectively lead to a change in the microstructure. The particle size increases with increasing zirconia content and/or calcination temperature. The specific surface area for the powder at higher calcination temperature drastically decreases due to the formation of hard agglomerates and hence entrapment of pores during grain growth. At lower temperatures, as the hydrated powder is very fine, they agglomerate together. These agglomerates are broken into smaller particles, when calcined at 950°C and above this temperature they tend to become aggregate, accompanied by grain growth which is evident from the increase in grain size in Table 1. The mean particle diameter (d50) of the powder calcined at 950°C has relatively large specific surface area (around 85 m2 g − 1) which is desirable for the low temperature sintering. The in-
300 950 1250
300 950 1250
300 950 1250
5
15
25
g, d, u, a-phases of alumina, t-phase of zirconia.
Calcination temperature (°C)
ZrO2 (12 Ce) Vol.%
284.5 95 3.44
271.9 89 2.68
256.9 84.5 1.34
3.9 1.8 2.8 6.3 3.6 4 7.2 3.6 4
94 95 90.3 95 93 90.8
91.3 90.8 90.9
91.2 90.3 91.4
90.7 90.4 90.8
Mean particle size (d50) (mm)
96 93 90.4
Specific surface area (m2 g−1)
Table 1 Powder characterization of alumina-12 Ce–zirconia precursors after calcination at different temperatures
4.6 13.9 383
5.1 15 518
5 17 1094
90.7 92.1 915
90.4 92.1 918
90.8 91.2 912
Grain size (nm)
1.88 3.91
2.14 4.27
3.6
Zirconia
4.2 4.8
5.4
6.7
Alumina
Crystallite size (nm)
g d, u, a, t a, t
g d, u, a, t a, t
g d, u, a, t a, t
Identified phases
268 D. Jayaseelan et al. / Materials Science and Engineering A256 (1998) 265–270
D. Jayaseelan et al. / Materials Science and Engineering A256 (1998) 265–270
crease in surface area of the powder is mainly due to repeated alcohol washing, wet milling and other processing conditions. Change of bulk density after sintering with calcination temperature is shown in Fig. 5. High bulk density was obtained for the composite powder calcined at 950°C, compared to calcined powder at lower or higher temperature irrespective of the type of zirconia. Due to the presence of higher amount of hydrates, weight loss was observed after sintering for powder calcined at low temperatures. Since, some energy is utilized for this evolution of hydrates (endothermic process) during sintering of the compacts, only a less amount of energy is available for sintering. When escaping, the hydrates will generate the residual pores. Thus the pores trapped within a-alumina grains during the final stage of conventional sintering are extremely difficult to remove. These two factors are responsible for the low bulk density for the powder calcined at lower temperatures. The decrease in bulk density above 1000°C is most likely caused by the grain growth accompanying the phase transformation of alumina. The microstructural development with ua-alumina phase transformation is obviously the most important sintering step for the densification of transition alumina containing composites. The concept of simultaneous sintering and phase transformation may provide a better opportunity for low temperature sin-
Fig. 5. Bulk density of three kinds of alumina-15 vol.% zirconia versus calcination temperature.
269
Fig. 6. Bulk density of three series of alumina – zirconia composites pressureless sintered at 1530°C for 3 h.
tering of alumina/zirconia composite than the more conventional approach on the basis of alumina. The optimal zirconia particle size, surface area and distribution obtained by calcination would benefit stacking and sintering alumina/zirconia powder compact. The shorter diffusion length and higher surface curvature of the sub-micron sized powder helped to lower the sintering temperature and minimize coarsening. Therefore, the optimal calcination condition for densification of alumina/zirconia composite is 950°C for 3 h. Kurita and Hori [17] also reported an optimal calcination between 800 and 1000°C. Those ZTA specimens were prepared by the CVD method had a higher relative density. Fig. 6 shows the sintered density of the samples of the three series of the composites calcined at 950°C and sintered at 1530°C for 3 h, with respect to zirconia content. The sintered compacts yield density up to 99.8% of the theoretical density. It is observed from the Fig. 6, that the relative density of the sintered composite containing unstabilized zirconia decreases with respect to zirconia content, unlike Ce and Y-stabilized zirconia containing composites. When the amount of t-phase is calculated from the XRD, it is observed that nearly 90–95% of the t-phase is retained for the Ce and Y-stabilized zirconia for the entire composition, whereas for the unstablilized zirconia, the t-phase decreases with increase in the zirconia content. This t–m phase transformation of zirconia causes
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4. Conclusion Highly sinterable alumina/zirconia composites were prepared by sol-gel technique. Calcination at 950°C for 3 h is found to be the optimal calcination temperature for the best densification of alumina/zirconia composites. All the samples could be sintered at 1530°C for 3 h by pressureless sintering to more than 98% of the theoretical density. By optimizing the processing condition and calcination temperature, the sol-mixed sample was sintered well at relatively low temperature compare to the conventional and other processing methods. These specimens are expected to have better mechanical properties according to their microstrucutre.
Acknowledgements The authors would like to thank Dr Wilfried Wunderlich for the useful suggestions and discussions in preparing the manuscript. One of the authors (DJ) would like to acknowledge the financial support of MHRD, DST and CSIR, India and AIEJ-Japan, during his period of research.
References
Fig. 7. Microstructure of alumina-25 vol.% Ce–zirconia composite sintered at 1530°C for 3 h; (a) polished sample; and (b) thermally etched at 1450°C for 30 min.
volume expansion leading to a decrease in the relative density of the composite. Fig. 7 shows the typical microstructure of alumina/25% Ce – zirconia sintered at 1530°C for 3 h. The average grain size of alumina is 2 mm and the composite has attained full densification. It is seen from the etched specimen that the zirconia grains are uniformly distributed in the alumina matrix. It is inferred that zirconia content influences the zirconia spatial distribution and hence at higher volume content of zirconia such as 25% ZrO2 (Fig. 7), the zirconia becomes less isolated and more continuous and interconnected.
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