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Microstructural pathways for the densification of slip cast alumina
Materials Science and Engineering A352 (2003) 287 /293 www.elsevier.com/locate/msea
Microstructural pathways for the densification of slip cast alumina Ian Nettleship *, Richard McAfee Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, PA, USA Received 29 January 2002; received in revised form 21 November 2002
Abstract The effect of slip casting condition and hence particle packing on densification and microstructural evolution was evaluated during isothermal sintering of a commercial alumina powder at 1350 8C. While there was significant effect of flocculation on solid volume fraction at short sintering times there appeared to be no inherent barrier to the eventual attainment of the same limiting solid volume fraction. Microstructural pathways based on average microstructural parameters and microstructural populations revealed no effect of casting condition on grain-scale pathways at this temperature. It is thought that microstructural pathways at higher length scales based on grain clusters will reveal the effect of slip casting independent of grain coarsening behavior. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Microstructure; Densification; Alumina; Casting; Flocculation
1. Introduction The effects of forming on particle packing and the consequent effects on sintering have been of paramount importance in the powder processing of ceramics. It is therefore not surprising that it has been constantly debated over more than 40 years. The common approach to experimental studies has been the examination of the effect of green density on sintering. The green density can be altered using pressure in powder pressing or dispersion conditions in slip casting. The resulting interpretations have often been contradictory and lacking in quantitative microstructural information. Bruch [1] studied the effect of green density on intermediate and final stage sintering and concluded that there is a green density below which densification is inhibited and that even above this critical value the densification rate was affected by green density. Given that coarsening would increase the diffusion distance between the sources and sinks it is quite natural to interpret the effect of green density on densification in terms of coarsening and grain growth behavior. [2,3] Grain
* Corresponding author. Tel.: /1-412-624-9720; fax: /1-412-6248069 E-mail address: [email protected] (I. Nettleship).
growth or coarsening would obviously reduce the densification rate. If low green density resulted in coarsening behavior of the porous material it would compromise the ability of the system to reach high density. More recent studies using slip casting of fine powders reinforced this concept. [4] Highly ordered compacts from monosized silica resulted in high density translucent compacts at 1000 8C. In contrast, flocculation lowered the green density from 76 to 59% and lead to compacts that remained highly porous after the same sintering condition. Similar trends were found for fine alumina powders. Yeh and Sacks [5] cast samples at pH 4 and pH 9 then fired them at 1340 8C. The green density was 64% for pH 4 and 51% for pH 9. The pH 4 sample showed an enhanced densification rate and reached a solid volume fraction 0.99 after 72 h while the pH 9 sample reached only 0.90. To reach higher solid volume fractions (0.98) in a timely fashion the pH 9 sample was fired at 1450 8C, which resulted in a larger average grain size. Unfortunately the samples from the two different casting conditions were not sintered to high solid volume fraction at the same temperature and it is therefore difficult to separate the effects of sintering temperature and casting condition on the microstructural pathway. Occhionero and Halloran also looked at the effect of green density on grain growth and densification [6] and concluded that increasing the green
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density delayed grain growth during densification and thereby affected the densification rate. In studies where the effect of particle packing on densification and microstructural development was studied in terms of the effect of green density it is important to remember that different forming methods produce different particle packing behaviors. In this respect Zheng and Reed made a distinction between varying the green density by pressing granulated powders and varying the green density by changing the dispersion condition during slip casting. [7] They suggested that pressed granulated powder resulted in bimodal pore size distribution as measured by mercury intrusion porosimetry with the larger mode being the intergranular pores, which diminish as the pressure is increased. In cast structures the pore population was unimodal with a wider distribution at larger sizes due to the lowering of green density by flocculation. To describe pore structure evolution in sintering the authors defined micropores as pores smaller than half the particle size and macropores as intergranule pores larger than ten times the particle size. In pressed samples the proportion of macropores increased with sintered density because intragranular pores were preferentially removed. In cast samples all the pores were considered to be micropores. Hence, they could be fired to nearly the same sintered solid volume fraction even though the green densities can be very different. The general concept of the effect of green density on the densification and coarsening of alumina was further explored by Cameron and Raj who [8] compared the densification behavior of colloidally prepared compacts with a green density of 64% to dry pressed samples with a green density of 56%. The colloidally prepared sample showed better sintering kinetics. The authors suggest that densification behavior from earlier studies on dry pressed samples may be due to agglomeration, which lowers density and supports grain growth at solid volume fractions less than 0.9. Cameron and Raj also studied the effect of casting condition on the green density and the evolution of the average grain size and grain size distribution during densification of alumina. [9] This showed that when sintered at 1550 8C the sample cast in the dispersed condition sintered rapidly. In contrast the flocculated sample had a lower green density and sintered to a solid volume fraction of 0.9 over the same time frame. Notably the grain growth in the flocculated sample started at a much lower relative density than for the dispersed condition. They further showed that deformation of the flocculated green sample in uniaxial compression increased the green density and resulted in densification and grain growth behavior similar to that of the dispersed case. It can therefore be concluded that the microstructural pathway of alumina is affected by particle packing behavior
through its effect on grain growth when fired at 1550 8C. The effect of green density on densification has been studied for other systems. [10] Rahaman et al have shown that densification rate of pressed ZnO below a solid volume fraction of 0.8 decreased significantly as the green density increased. Above a solid volume fraction of 0.8 the densification rate was insensitive to green density. They also stated that there is no intrinsic barrier to achieving high sintered density even from compacts with very low green density. However, it must be pointed out that the experiments were conducted at constant heating rate and therefore the measurements of densification rates as a function of density were not conducted at the same temperature. The lack of an intrinsic barrier to densification of low green density materials has been reinforced by the recent work of Chen and Chen [11] who were able to sinter fine powders of CeO2 and Y2O3 to high density and a fine grain size at temperatures well below those previously used for these systems. This was achieved even with extremely low green densities. They explained this by the ability of a fine-grained porous network to evolve towards a common normalized pore size trajectory along which the materials progress and achieve high density. Presumably the fine powders and lower sintering temperature avoid coarsening of the grain structure, which might otherwise intervene in the densification process. The purpose of this report is to describe a series of experiments on a commercial alumina powder that were designed to carefully determine the effect of casting condition on isothermal densification kinetics and grainscale microstructural pathways at sintering temperatures low enough to avoid rapid grain growth. It was therefore possible to compare the behaviors for solid volume fractions above a solid volume fraction of 0.9.
2. Experimental Procedures Two large 50-g samples of the same powder (Premalox, Alcoa, Pittsburgh, PA) were cast into plaster molds for this study. The first was dispersed in deionized water with an ammonium polyacrylate (Darvan C, Vanderbilt Co.) while the second was flocculated by mixing the powder in deionized water maintained at pH 9 with ammonium hydroxide. Both samples were cast with 20 wt.% solids. The particle size distribution of each slip was measured using centrifugal sedimentation after dilution into the correct absorption range. (Horiba CAPA 300). The median size for the dispersed powder was 0.3 mm while the median size of the flocculated powder was above 1 mm. The final shape of the compacts after drying made it difficult to estimate the green density. However, the green density of the
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dispersed sample was approximately 60% of theoretical density and the flocculated sample had a significantly larger volume. The effect of casting condition on density could be more accurately measured after prefiring at 1350 8C for 0.1 h and cutting the samples into sections. The fired solid volume fraction did not vary more than 2% for samples derived from the same cast. The solid volume fraction was measured using the Archimedes method in which the samples were evacuated before they were immersed in water. Sections of the prefired dispersed and flocculated casts were then heat treated for various times at 1350 8C ranging from 0.1 to 60 h. After firing the solid volume fraction was measured again and the samples were further sectioned and polished prior to thermal etching at 1325 8C for 18 min. The etched sections were examined in the scanning electron microscope (Philips FEG-XL 30). The etching treatment did not affect the solid volume fraction, which was checked again after etching. The volume fraction of pores was also evaluated using the area fraction of pores measured from at least six processed SEM images of the polished and etched sections without grain boundary reconstruction. The 95% confidence interval was calculated using the variance between the images. The images were also processed with grain boundary reconstruction that was manually completed. Then the number of grain boundary and pore boundary intercepts was evaluated on parallel test lines that were applied to the images. The surface area densities of solid/solid surface, SSS V (grain boundaries) and solid /vapor interface, SSV (pore V boundaries) were then calculated and the 95% confidence interval was calculated from the variance between the images. Finally the average grain intercept length and the average pore intercept length were calculated. The cross-sectional areas of the grains were also measured and the equivalent circular diameter calculated. At least 1300 grains were measured for each sintering condition. The results were unfolded into three dimensions using a Schwartz /Saltykov technique corrected for non-spherical grains. Information entropy was used to optimize the choice of class intervals and therefore the representation of the data. This avoids the need to smooth the unfolded data by fitting it to a chosen distribution function.
3. Results The effect of slip casting condition on densification can be clearly seen in Fig. 1. At the shortest sintering time of 0.1 h there was a significantly higher solid volume fraction of 0.82 for the dispersed case compared with the flocculated sample that had a solid volume fraction of 0.76. As the sintering time increased the difference between the results for the two casting
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Fig. 1. Solid volume fraction against time at temperature for flocculated and dispersed alumina at a sintering temperature of 1350 8C.
conditions decreased. Both sets of data fitted the common empirical logarithmic relationship up to a limiting solid volume. The dispersed sample departed from the fit at 10 h and did not densify further while the flocculated departed at 15 h. The Archimedes measurements for both cases, dispersed and flocculated, were always within the 95% confidence interval of the stereological results. Therefore, the image analysis results were considered to be representative at least for the volume fraction of solid. The measurement of the surface area density of solid/ solid contacts SSS V showed no significant difference between the two samples sintered to the same relative density. This quantity was relatively constant through the sintering process with values close to 3 mm 1 for both the dispersed case and the flocculated case below a solid volume fraction of 0.93. It decreased slightly above this solid volume fraction due to grain growth. Similar behavior has been observed in previous studies of alumina ceramics. [12,13] The relationship between the surface area density of pore boundaries SSV V and solid volume fraction is shown in Fig. 2. The linear relationship has also been observed in other studies on alumina [8,12,13] Consequently, the mean pore intercept remains unchanged through most of the sintering process. In the present study the mean pore intercept for both the dispersed and the flocculated samples were 0.2mm. For the microstructural pathways based on the surface area densities of boundaries as a function of solid volume fraction there is little effect of the two casting conditions on microstructural evolution.
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Fig. 2. The solid /vapor surface area per unit volume is proportional to the pore volume (1 /VSV) for both the flocculated and dispersed S alumina. SSV V extrapolates to 0 at full density (VV /1) when all pore volume is removed.
Measurements were also taken for the grain size distribution. These results were taken from the same micrographs and unfolded into three dimensions using information entropy to choose the interval boundaries. The resulting size distributions were then fitted to three different distribution functions including: log-normal, Weibull and gamma. The three distribution functions gave sufficiently good fits to the data. It was therefore decided to fit the data with the log /normal distribution function since this gave better fits in most cases and is consistent with previous studies on fully dense ceramics. [14] Fig. 3 shows the median grain size for the dispersed and the flocculated condition. The median grain size from the population for the dispersed casting condition rises from 0.4 to 0.7 mm over the course of the experiments. Finally the coefficient of variation is plotted against solid volume fraction in Fig. 4. In this data there appears to be a change in the trend in the data above a solid volume fraction of 0.9. Both casting conditions gave a coefficient of variation of 0.25 below a solid volume fraction of 0.9 but then the values almost double at the longer sintering times. Again there is little difference between the characteristics of the grain populations for the samples cast under the different conditions and compared at the same solid volume fraction.
4. Discussion The results of this study show that casting of the flocculated slip slows the kinetics of densification
Fig. 3. Median grain size against solid volume fraction for flocculated and dispersed alumina sintered at 1350 8C.
Fig. 4. Grain size distribution coefficient of variation against solid volume fraction for flocculated and dispersed alumina sintered at 1350 8C.
consistent with the results of previous studies. [5,8,9] Although these previous studies were done with different alumina powders sintered at other temperatures, it is illuminating to compare the trends in densification behavior in the context of microstructural evolution. At short sintering times, in this study, the solid volume fraction of the flocculated materials was 0.06 lower than the dispersed cast. However, the flocculated and dispersed samples reached similar solid volume fractions at longer sintering times. This contrasts with previous
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studies [5,8,9] in which the dispersed sample maintained an appreciable advantage in solid volume fraction over the flocculated sample at all sintering times. Solid volume fractions above 0.9 were not achieved for the flocculated sample at the sintering times used in the previous studies. [5,8,9] The other striking difference between this study and that of Cameron and Raj involved the grain growth behavior. [8,9] In this study the median grain size only increased from 0.4 to 0.7 mm where as the grain size increased from just under 1 to almost 4 mm in the study of Cameron and Raj after firing at 1550 8C. The flocculated sample also coarsened significantly at lower densities in the latter study. In contrast, there was very little difference in the microstructural pathways for the flocculated and dispersed samples in the present study. This difference in the pathway and the consequent effects on densification may be explained by the fact that the sintering temperature used in the present study was 200 8C lower than that used by Cameron and Raj. It is thought that the enhanced grain growth behavior at 1550 8C effectively increases the diffusion distances required for densification. Therefore, the differences in sintering kinetics between the dispersed and flocculated samples reflects the differences in the grain growth behavior for samples fired at 1550 8C. A difference in sintering temperature could not explain a comparison of the results of this study with that of Yeh and Sacks [5] who fired their dispersed and flocculated alumina at 1340 8C, 10 8C below the sintering temperature used in this study. They were unable to reach high solid volume fraction with the flocculated sample for the sintering times used. When the flocculated samples were fired at 1450 8C they did reach high density but the microstructure was coarser. It is not known if the microstructure of the flocculated sample fired at 1340 8C coarsened in a way that would prevent the materials from reaching high density. However, their results were further examined by fitting the common logarithmic relationship to the densification data for the flocculated sample. The fit gave a Pearson’s correlation coefficient of 0.98 and showed no departure from this relationship at longer sintering times. Extrapolation of the results predicted that the flocculated sample would have to be sintered at 1340 8C for 200 h to reach a solid volume fraction of 0.95. This is well beyond the sintering times used in their study but suggests that high density could be reached if grain growth does not alter the densification kinetics. This would be consistent with the conclusions of Zheng and Reed [7,15] who have suggested that pores below a size corresponding to a critical ratio of pore size to mean particle size do not inhibit the eventual attainment of high density. Cameron and Raj also found that the grain size distribution was markedly wider for the flocculated
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condition when fired at 1550 8C. In the present study, this was not the case. The longer sintering time did result in a doubling of the coefficient of variation, mainly at solid volume fractions above 0.9. However there was no significant difference between the evolution of the grain size distribution for the flocculated and dispersed samples in this study. It is therefore suggested that the lower sintering temperature used in this study minimized grain growth and resulted in similar microstructural pathways for both grain populations. Unfortunately Cameron and Raj did not report the evolution of the average pore intercept. In the present study this quantity remained constant for both the dispersed and the flocculated materials as the density increased. This type of relationship is commonly observed for ceramics and powder metals. [12,13,16] It can therefore be concluded that preferential removal of small diameter pore channels did not occur to the extent that it affected the average pore intercept for either casting condition. This is inconsistent with the idea that the dominant change in the pore phase during densification is the preferential removal of small intra-aggregate pores in a hierarchical microstructure consisting of porous aggregates and large inter-aggregate pores. [17] The results of this study are in agreement with those of Rahaman et al on ZnO [10] and Chen and Chen on CeO2 and Y2O3 [11] in that low green density does not create an inherent obstacle to the achievement of high solid volume fraction independent of the grain growth behavior. It also implies that flocculated materials could be fired to high density if coarsening of the grain structure can be suppressed at solid volume fractions below 0.9. This underlines the importance of determining the microstructural pathway when studying the effect of processing variables on densification. Conclusions regarding the effect of green density on densification behavior should not be made in the absence of quantitative microstructural information. One important question remains to be considered. This involves the sensitivity of the microstructural changes at the scale of individual grains and pores to changes in powder processing variables. All of the grainscale microstructure pathways measured in this study are insensitive to the state of dispersion in casting. It is possible that the microstructures of the dispersed sample and the flocculated sample have a common microstructural pathway. Before this can be simply stated, it is necessary to examine the microstructure pathway at other scales for which differences might be anticipated. In the case of this study, it would be reasonable to suggest that a change in processing that has such a profound effect on the densification kinetics would also affect the evolution of the microstructure. From our qualitative understanding of the effect of flocculation on particle packing it would be logical to suggest that the state of dispersion affects the agglomeration of the
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particles and manifestations of this difference may therefore be observable on some higher microstructural scale based on clusters of particles or grains. One would suspect that flocculation could result in particle clusters that are larger and more porous. If this is the case the effect on particle packing should be observable at the ‘cluster scale’ independent of the effect of temperature on grain growth and densification. The common focus of quantitative microstructural studies on the scale of grains and pores may obscure higher scale effects simply due to the size of the field of view required for accurate measurements. However, observation of the microstructure at higher scale appears to show some differences between the microstructures achieved in this study. Fig. 5 shows low magnification micrographs of the dispersed and flocculated condition fired to the same solid volume fraction of 0.85. It is not difficult to see that the microstructures contain dense regions in both samples and these regions appear to be larger in the flocculated sample. The relatively dense clusters appear to be surrounded by lower density regions containing pores
of much the same size. This last point would explain why the state of dispersion does not affect the average pore intercept. The origin of clusters is believed to be the local packing of the particles during the forming operation and differential sintering during initial and intermediate stage densification. In this study some association of particles has occurred even in the dispersed condition, although at a smaller scale. These preliminary observations have yet to be quantified. Further understanding of the effect of particle packing awaits the development of measurement techniques tailored to the observation of clustering in a major phase. Recently developed cluster characterization methods are focused on minor phases in composite microstructures [18,19] and may not be appropriate to high volume fractions when feature spacings become small. Nonetheless a direct approach to quantifying the effect of powder processing variables on local particle packing and sintering may well be achieved by determination of microstructural pathways at higher ‘hidden scales’ based on particle clusters.
5. Conclusions Alumina has been fired to a solid volume fraction of 0.96 at 1350 8C when slip cast in the dispersed (10 h) and the flocculated (15 h) state. Although there was an appreciable difference in the kinetics of densification there was little effect on the microstructure pathway at the scale of grains and pores. It may be concluded that there is no inherent obstacle to densification of flocculated casts of alumina at 1350 8C independent of the grain growth behavior. Therefore low temperature sintering of flocculated alumina will eventually give high density although the sintering times may be very long. It can also be concluded that microstructural pathways based on average microstructural parameters were insensitive to the effects of the casting condition in the absence of pronounced grain growth. Microstructural pathways based on the populations of grains also showed little sensitivity. It is thought that microstructural pathways for grain clusters will be more sensitive to casting condition.
Acknowledgements The authors would like to acknowledge financial support from the National Science Foundation under the grant CMS 9700062.
Fig. 5. Top, Dispersed alumina sintered at 1350 8C for 0.6 h. Bottom, flocculated alumina sintered for 1.1 h. Both have a solid volume fraction of approximately 0.85.
References [1] C.A. Bruch, Am. Ceram. Soc. Bull. 41 (1962) 799.
I. Nettleship, R. McAfee / Materials Science and Engineering A352 (2003) 287 /293 [2] T.K. Gupta, J. Am. Ceram. Soc. 55 (1972) 276. [3] C.G. Greskovich, K.W. Lay, J. Am. Ceram. Soc. 55 (1972) 142. [4] M.D. Sacks, T.Y. Tseng, J. Am. Ceram. Soc. 67 (1984) 532. [5] T.S. Yeh, M.D. Sacks, Effect of green microstructure on sintering of alumina, in: C.A. Handwerker, J.E. Blendell, W.A. Kaysser (Eds.), Ceramic Transactions, vol. 7, The American Ceramic Society, Ohio, 1990, pp. 309 /331. [6] M.A. Occhionero, J.W. Halloran, The Influence of Green density on Sintering, in: G.C. Kuczynski, A.E. Miller, G.A. Gordon (Eds.), Materials Science Research, vol. 16, Plenum Press, New York, 1984, pp. 89 /102. [7] J. Zheng, J.S. Reed, Am. Ceram. Soc. Bull. 71 (1992) 1410. [8] C.P. Cameron, R. Raj, J. Am. Ceram. Soc. 71 (1988) 1031. [9] C.P. Cameron, R. Raj, J. Am. Ceram. Soc. 73 (1990) 2032.
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[10] M.N. Rahaman, L.C. De Jonghe, M.Y. Chu, J. Am. Ceram. Soc. 74 (1991) 514. [11] P.-L. Chen, I.-W. Chen, J. Am. Ceram. Soc. 79 (1996) 3129. [12] M.D. Lehigh, I. Nettleship, A Stereological examination of porous alumina microstructures during densification, in: R.M. German, G.L. Messing, R.G. Cornell (Eds.), Sintering Technology, Marcel Dekker, New York, 1996, pp. 61 /68. [13] N.J. Shaw, R.J. Brook, J. Am. Ceram. Soc. 69 (1986) 107. [14] S.K. Kurtz, F.M.A. Carpay, J. Appl. Phys. 51 (1980) 5745. [15] J. Zheng, J.S. Reed, J. Am. Ceram. Soc. 72 (1989) 810. [16] E.H. Aigeltinger, H.E. Exner, Metall. Trans. 8A (1977) 421. [17] F.F. Lange, J. Am. Ceram. Soc. 67 (1984) 83. [18] J.E. Spowart, B. Maruyama, D.B. Miracle, Mater. Sci. Eng. A307 (2001) 51. [19] J.M. Missiaen, J.M. Chaix, J. Micro. 175 (1994) 195.
Microstructural pathways for the densification of slip cast alumina Ian Nettleship *, Richard McAfee Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, PA, USA Received 29 January 2002; received in revised form 21 November 2002
Abstract The effect of slip casting condition and hence particle packing on densification and microstructural evolution was evaluated during isothermal sintering of a commercial alumina powder at 1350 8C. While there was significant effect of flocculation on solid volume fraction at short sintering times there appeared to be no inherent barrier to the eventual attainment of the same limiting solid volume fraction. Microstructural pathways based on average microstructural parameters and microstructural populations revealed no effect of casting condition on grain-scale pathways at this temperature. It is thought that microstructural pathways at higher length scales based on grain clusters will reveal the effect of slip casting independent of grain coarsening behavior. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Microstructure; Densification; Alumina; Casting; Flocculation
1. Introduction The effects of forming on particle packing and the consequent effects on sintering have been of paramount importance in the powder processing of ceramics. It is therefore not surprising that it has been constantly debated over more than 40 years. The common approach to experimental studies has been the examination of the effect of green density on sintering. The green density can be altered using pressure in powder pressing or dispersion conditions in slip casting. The resulting interpretations have often been contradictory and lacking in quantitative microstructural information. Bruch [1] studied the effect of green density on intermediate and final stage sintering and concluded that there is a green density below which densification is inhibited and that even above this critical value the densification rate was affected by green density. Given that coarsening would increase the diffusion distance between the sources and sinks it is quite natural to interpret the effect of green density on densification in terms of coarsening and grain growth behavior. [2,3] Grain
* Corresponding author. Tel.: /1-412-624-9720; fax: /1-412-6248069 E-mail address: [email protected] (I. Nettleship).
growth or coarsening would obviously reduce the densification rate. If low green density resulted in coarsening behavior of the porous material it would compromise the ability of the system to reach high density. More recent studies using slip casting of fine powders reinforced this concept. [4] Highly ordered compacts from monosized silica resulted in high density translucent compacts at 1000 8C. In contrast, flocculation lowered the green density from 76 to 59% and lead to compacts that remained highly porous after the same sintering condition. Similar trends were found for fine alumina powders. Yeh and Sacks [5] cast samples at pH 4 and pH 9 then fired them at 1340 8C. The green density was 64% for pH 4 and 51% for pH 9. The pH 4 sample showed an enhanced densification rate and reached a solid volume fraction 0.99 after 72 h while the pH 9 sample reached only 0.90. To reach higher solid volume fractions (0.98) in a timely fashion the pH 9 sample was fired at 1450 8C, which resulted in a larger average grain size. Unfortunately the samples from the two different casting conditions were not sintered to high solid volume fraction at the same temperature and it is therefore difficult to separate the effects of sintering temperature and casting condition on the microstructural pathway. Occhionero and Halloran also looked at the effect of green density on grain growth and densification [6] and concluded that increasing the green
0921-5093/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5093(02)00899-7
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density delayed grain growth during densification and thereby affected the densification rate. In studies where the effect of particle packing on densification and microstructural development was studied in terms of the effect of green density it is important to remember that different forming methods produce different particle packing behaviors. In this respect Zheng and Reed made a distinction between varying the green density by pressing granulated powders and varying the green density by changing the dispersion condition during slip casting. [7] They suggested that pressed granulated powder resulted in bimodal pore size distribution as measured by mercury intrusion porosimetry with the larger mode being the intergranular pores, which diminish as the pressure is increased. In cast structures the pore population was unimodal with a wider distribution at larger sizes due to the lowering of green density by flocculation. To describe pore structure evolution in sintering the authors defined micropores as pores smaller than half the particle size and macropores as intergranule pores larger than ten times the particle size. In pressed samples the proportion of macropores increased with sintered density because intragranular pores were preferentially removed. In cast samples all the pores were considered to be micropores. Hence, they could be fired to nearly the same sintered solid volume fraction even though the green densities can be very different. The general concept of the effect of green density on the densification and coarsening of alumina was further explored by Cameron and Raj who [8] compared the densification behavior of colloidally prepared compacts with a green density of 64% to dry pressed samples with a green density of 56%. The colloidally prepared sample showed better sintering kinetics. The authors suggest that densification behavior from earlier studies on dry pressed samples may be due to agglomeration, which lowers density and supports grain growth at solid volume fractions less than 0.9. Cameron and Raj also studied the effect of casting condition on the green density and the evolution of the average grain size and grain size distribution during densification of alumina. [9] This showed that when sintered at 1550 8C the sample cast in the dispersed condition sintered rapidly. In contrast the flocculated sample had a lower green density and sintered to a solid volume fraction of 0.9 over the same time frame. Notably the grain growth in the flocculated sample started at a much lower relative density than for the dispersed condition. They further showed that deformation of the flocculated green sample in uniaxial compression increased the green density and resulted in densification and grain growth behavior similar to that of the dispersed case. It can therefore be concluded that the microstructural pathway of alumina is affected by particle packing behavior
through its effect on grain growth when fired at 1550 8C. The effect of green density on densification has been studied for other systems. [10] Rahaman et al have shown that densification rate of pressed ZnO below a solid volume fraction of 0.8 decreased significantly as the green density increased. Above a solid volume fraction of 0.8 the densification rate was insensitive to green density. They also stated that there is no intrinsic barrier to achieving high sintered density even from compacts with very low green density. However, it must be pointed out that the experiments were conducted at constant heating rate and therefore the measurements of densification rates as a function of density were not conducted at the same temperature. The lack of an intrinsic barrier to densification of low green density materials has been reinforced by the recent work of Chen and Chen [11] who were able to sinter fine powders of CeO2 and Y2O3 to high density and a fine grain size at temperatures well below those previously used for these systems. This was achieved even with extremely low green densities. They explained this by the ability of a fine-grained porous network to evolve towards a common normalized pore size trajectory along which the materials progress and achieve high density. Presumably the fine powders and lower sintering temperature avoid coarsening of the grain structure, which might otherwise intervene in the densification process. The purpose of this report is to describe a series of experiments on a commercial alumina powder that were designed to carefully determine the effect of casting condition on isothermal densification kinetics and grainscale microstructural pathways at sintering temperatures low enough to avoid rapid grain growth. It was therefore possible to compare the behaviors for solid volume fractions above a solid volume fraction of 0.9.
2. Experimental Procedures Two large 50-g samples of the same powder (Premalox, Alcoa, Pittsburgh, PA) were cast into plaster molds for this study. The first was dispersed in deionized water with an ammonium polyacrylate (Darvan C, Vanderbilt Co.) while the second was flocculated by mixing the powder in deionized water maintained at pH 9 with ammonium hydroxide. Both samples were cast with 20 wt.% solids. The particle size distribution of each slip was measured using centrifugal sedimentation after dilution into the correct absorption range. (Horiba CAPA 300). The median size for the dispersed powder was 0.3 mm while the median size of the flocculated powder was above 1 mm. The final shape of the compacts after drying made it difficult to estimate the green density. However, the green density of the
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dispersed sample was approximately 60% of theoretical density and the flocculated sample had a significantly larger volume. The effect of casting condition on density could be more accurately measured after prefiring at 1350 8C for 0.1 h and cutting the samples into sections. The fired solid volume fraction did not vary more than 2% for samples derived from the same cast. The solid volume fraction was measured using the Archimedes method in which the samples were evacuated before they were immersed in water. Sections of the prefired dispersed and flocculated casts were then heat treated for various times at 1350 8C ranging from 0.1 to 60 h. After firing the solid volume fraction was measured again and the samples were further sectioned and polished prior to thermal etching at 1325 8C for 18 min. The etched sections were examined in the scanning electron microscope (Philips FEG-XL 30). The etching treatment did not affect the solid volume fraction, which was checked again after etching. The volume fraction of pores was also evaluated using the area fraction of pores measured from at least six processed SEM images of the polished and etched sections without grain boundary reconstruction. The 95% confidence interval was calculated using the variance between the images. The images were also processed with grain boundary reconstruction that was manually completed. Then the number of grain boundary and pore boundary intercepts was evaluated on parallel test lines that were applied to the images. The surface area densities of solid/solid surface, SSS V (grain boundaries) and solid /vapor interface, SSV (pore V boundaries) were then calculated and the 95% confidence interval was calculated from the variance between the images. Finally the average grain intercept length and the average pore intercept length were calculated. The cross-sectional areas of the grains were also measured and the equivalent circular diameter calculated. At least 1300 grains were measured for each sintering condition. The results were unfolded into three dimensions using a Schwartz /Saltykov technique corrected for non-spherical grains. Information entropy was used to optimize the choice of class intervals and therefore the representation of the data. This avoids the need to smooth the unfolded data by fitting it to a chosen distribution function.
3. Results The effect of slip casting condition on densification can be clearly seen in Fig. 1. At the shortest sintering time of 0.1 h there was a significantly higher solid volume fraction of 0.82 for the dispersed case compared with the flocculated sample that had a solid volume fraction of 0.76. As the sintering time increased the difference between the results for the two casting
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Fig. 1. Solid volume fraction against time at temperature for flocculated and dispersed alumina at a sintering temperature of 1350 8C.
conditions decreased. Both sets of data fitted the common empirical logarithmic relationship up to a limiting solid volume. The dispersed sample departed from the fit at 10 h and did not densify further while the flocculated departed at 15 h. The Archimedes measurements for both cases, dispersed and flocculated, were always within the 95% confidence interval of the stereological results. Therefore, the image analysis results were considered to be representative at least for the volume fraction of solid. The measurement of the surface area density of solid/ solid contacts SSS V showed no significant difference between the two samples sintered to the same relative density. This quantity was relatively constant through the sintering process with values close to 3 mm 1 for both the dispersed case and the flocculated case below a solid volume fraction of 0.93. It decreased slightly above this solid volume fraction due to grain growth. Similar behavior has been observed in previous studies of alumina ceramics. [12,13] The relationship between the surface area density of pore boundaries SSV V and solid volume fraction is shown in Fig. 2. The linear relationship has also been observed in other studies on alumina [8,12,13] Consequently, the mean pore intercept remains unchanged through most of the sintering process. In the present study the mean pore intercept for both the dispersed and the flocculated samples were 0.2mm. For the microstructural pathways based on the surface area densities of boundaries as a function of solid volume fraction there is little effect of the two casting conditions on microstructural evolution.
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Fig. 2. The solid /vapor surface area per unit volume is proportional to the pore volume (1 /VSV) for both the flocculated and dispersed S alumina. SSV V extrapolates to 0 at full density (VV /1) when all pore volume is removed.
Measurements were also taken for the grain size distribution. These results were taken from the same micrographs and unfolded into three dimensions using information entropy to choose the interval boundaries. The resulting size distributions were then fitted to three different distribution functions including: log-normal, Weibull and gamma. The three distribution functions gave sufficiently good fits to the data. It was therefore decided to fit the data with the log /normal distribution function since this gave better fits in most cases and is consistent with previous studies on fully dense ceramics. [14] Fig. 3 shows the median grain size for the dispersed and the flocculated condition. The median grain size from the population for the dispersed casting condition rises from 0.4 to 0.7 mm over the course of the experiments. Finally the coefficient of variation is plotted against solid volume fraction in Fig. 4. In this data there appears to be a change in the trend in the data above a solid volume fraction of 0.9. Both casting conditions gave a coefficient of variation of 0.25 below a solid volume fraction of 0.9 but then the values almost double at the longer sintering times. Again there is little difference between the characteristics of the grain populations for the samples cast under the different conditions and compared at the same solid volume fraction.
4. Discussion The results of this study show that casting of the flocculated slip slows the kinetics of densification
Fig. 3. Median grain size against solid volume fraction for flocculated and dispersed alumina sintered at 1350 8C.
Fig. 4. Grain size distribution coefficient of variation against solid volume fraction for flocculated and dispersed alumina sintered at 1350 8C.
consistent with the results of previous studies. [5,8,9] Although these previous studies were done with different alumina powders sintered at other temperatures, it is illuminating to compare the trends in densification behavior in the context of microstructural evolution. At short sintering times, in this study, the solid volume fraction of the flocculated materials was 0.06 lower than the dispersed cast. However, the flocculated and dispersed samples reached similar solid volume fractions at longer sintering times. This contrasts with previous
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studies [5,8,9] in which the dispersed sample maintained an appreciable advantage in solid volume fraction over the flocculated sample at all sintering times. Solid volume fractions above 0.9 were not achieved for the flocculated sample at the sintering times used in the previous studies. [5,8,9] The other striking difference between this study and that of Cameron and Raj involved the grain growth behavior. [8,9] In this study the median grain size only increased from 0.4 to 0.7 mm where as the grain size increased from just under 1 to almost 4 mm in the study of Cameron and Raj after firing at 1550 8C. The flocculated sample also coarsened significantly at lower densities in the latter study. In contrast, there was very little difference in the microstructural pathways for the flocculated and dispersed samples in the present study. This difference in the pathway and the consequent effects on densification may be explained by the fact that the sintering temperature used in the present study was 200 8C lower than that used by Cameron and Raj. It is thought that the enhanced grain growth behavior at 1550 8C effectively increases the diffusion distances required for densification. Therefore, the differences in sintering kinetics between the dispersed and flocculated samples reflects the differences in the grain growth behavior for samples fired at 1550 8C. A difference in sintering temperature could not explain a comparison of the results of this study with that of Yeh and Sacks [5] who fired their dispersed and flocculated alumina at 1340 8C, 10 8C below the sintering temperature used in this study. They were unable to reach high solid volume fraction with the flocculated sample for the sintering times used. When the flocculated samples were fired at 1450 8C they did reach high density but the microstructure was coarser. It is not known if the microstructure of the flocculated sample fired at 1340 8C coarsened in a way that would prevent the materials from reaching high density. However, their results were further examined by fitting the common logarithmic relationship to the densification data for the flocculated sample. The fit gave a Pearson’s correlation coefficient of 0.98 and showed no departure from this relationship at longer sintering times. Extrapolation of the results predicted that the flocculated sample would have to be sintered at 1340 8C for 200 h to reach a solid volume fraction of 0.95. This is well beyond the sintering times used in their study but suggests that high density could be reached if grain growth does not alter the densification kinetics. This would be consistent with the conclusions of Zheng and Reed [7,15] who have suggested that pores below a size corresponding to a critical ratio of pore size to mean particle size do not inhibit the eventual attainment of high density. Cameron and Raj also found that the grain size distribution was markedly wider for the flocculated
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condition when fired at 1550 8C. In the present study, this was not the case. The longer sintering time did result in a doubling of the coefficient of variation, mainly at solid volume fractions above 0.9. However there was no significant difference between the evolution of the grain size distribution for the flocculated and dispersed samples in this study. It is therefore suggested that the lower sintering temperature used in this study minimized grain growth and resulted in similar microstructural pathways for both grain populations. Unfortunately Cameron and Raj did not report the evolution of the average pore intercept. In the present study this quantity remained constant for both the dispersed and the flocculated materials as the density increased. This type of relationship is commonly observed for ceramics and powder metals. [12,13,16] It can therefore be concluded that preferential removal of small diameter pore channels did not occur to the extent that it affected the average pore intercept for either casting condition. This is inconsistent with the idea that the dominant change in the pore phase during densification is the preferential removal of small intra-aggregate pores in a hierarchical microstructure consisting of porous aggregates and large inter-aggregate pores. [17] The results of this study are in agreement with those of Rahaman et al on ZnO [10] and Chen and Chen on CeO2 and Y2O3 [11] in that low green density does not create an inherent obstacle to the achievement of high solid volume fraction independent of the grain growth behavior. It also implies that flocculated materials could be fired to high density if coarsening of the grain structure can be suppressed at solid volume fractions below 0.9. This underlines the importance of determining the microstructural pathway when studying the effect of processing variables on densification. Conclusions regarding the effect of green density on densification behavior should not be made in the absence of quantitative microstructural information. One important question remains to be considered. This involves the sensitivity of the microstructural changes at the scale of individual grains and pores to changes in powder processing variables. All of the grainscale microstructure pathways measured in this study are insensitive to the state of dispersion in casting. It is possible that the microstructures of the dispersed sample and the flocculated sample have a common microstructural pathway. Before this can be simply stated, it is necessary to examine the microstructure pathway at other scales for which differences might be anticipated. In the case of this study, it would be reasonable to suggest that a change in processing that has such a profound effect on the densification kinetics would also affect the evolution of the microstructure. From our qualitative understanding of the effect of flocculation on particle packing it would be logical to suggest that the state of dispersion affects the agglomeration of the
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particles and manifestations of this difference may therefore be observable on some higher microstructural scale based on clusters of particles or grains. One would suspect that flocculation could result in particle clusters that are larger and more porous. If this is the case the effect on particle packing should be observable at the ‘cluster scale’ independent of the effect of temperature on grain growth and densification. The common focus of quantitative microstructural studies on the scale of grains and pores may obscure higher scale effects simply due to the size of the field of view required for accurate measurements. However, observation of the microstructure at higher scale appears to show some differences between the microstructures achieved in this study. Fig. 5 shows low magnification micrographs of the dispersed and flocculated condition fired to the same solid volume fraction of 0.85. It is not difficult to see that the microstructures contain dense regions in both samples and these regions appear to be larger in the flocculated sample. The relatively dense clusters appear to be surrounded by lower density regions containing pores
of much the same size. This last point would explain why the state of dispersion does not affect the average pore intercept. The origin of clusters is believed to be the local packing of the particles during the forming operation and differential sintering during initial and intermediate stage densification. In this study some association of particles has occurred even in the dispersed condition, although at a smaller scale. These preliminary observations have yet to be quantified. Further understanding of the effect of particle packing awaits the development of measurement techniques tailored to the observation of clustering in a major phase. Recently developed cluster characterization methods are focused on minor phases in composite microstructures [18,19] and may not be appropriate to high volume fractions when feature spacings become small. Nonetheless a direct approach to quantifying the effect of powder processing variables on local particle packing and sintering may well be achieved by determination of microstructural pathways at higher ‘hidden scales’ based on particle clusters.
5. Conclusions Alumina has been fired to a solid volume fraction of 0.96 at 1350 8C when slip cast in the dispersed (10 h) and the flocculated (15 h) state. Although there was an appreciable difference in the kinetics of densification there was little effect on the microstructure pathway at the scale of grains and pores. It may be concluded that there is no inherent obstacle to densification of flocculated casts of alumina at 1350 8C independent of the grain growth behavior. Therefore low temperature sintering of flocculated alumina will eventually give high density although the sintering times may be very long. It can also be concluded that microstructural pathways based on average microstructural parameters were insensitive to the effects of the casting condition in the absence of pronounced grain growth. Microstructural pathways based on the populations of grains also showed little sensitivity. It is thought that microstructural pathways for grain clusters will be more sensitive to casting condition.
Acknowledgements The authors would like to acknowledge financial support from the National Science Foundation under the grant CMS 9700062.
Fig. 5. Top, Dispersed alumina sintered at 1350 8C for 0.6 h. Bottom, flocculated alumina sintered for 1.1 h. Both have a solid volume fraction of approximately 0.85.
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