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Anisotropic grain growth in TiO2-doped alumina
MATERIALS SCIENCE & ENGINEERING ELSEVIER
Materials Science and Engineering A195 (1995) 169-178
A
Anisotropic grain growth in TiO2-doped alumina Debra S. Horn, Gary L. Messing Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802, USA
Abstract Grain growth in TiO2-doped alumina was studied in a high density, ultrafine matrix (0.4 ~m. Normal grain growth, anisotropic grain growth and abnormal grain growth were observed. With 0.15-0.4 wt.% TiO2, samples initially undergo normal grain growth until a crystal microstructure is attained and anisotropic grain growth in nucleated. Large anisotropic, platelet-shaped grains grow rapidly by a step growth process until impingement of the large grains essentially stops further growth. The volume fraction of anisotropic grains ranged from 20 to 100 vol.% suggesting that physical propertis dependent on grain shape and volume fraction can be tailored. Critical requirements are proposed for the in situ growth of anisotropic grains. Keywords: Anisotropy; Grain growth; Alumina; Doping
1. Introduction
There has been significant progress over the past 20 years in understanding how sintering and grain growth processes influence the attainment of dense ceramics with equiaxed and fine-grain microstructures (see Ref. [1] for example). Such microstructures, however, have limited high-temperature creep resistance and low fracture toughness, and thus are unsuitable for some mechanical applications. As a result, ceramic matrix composites with particle, fiber and whisker reinforcements have been developed. Although whiskerreinforced composites appear to have some of the best properties, the health hazards associated with handling whiskers have significantly curtailed industrial acceptance and implementation. Continuous-fiber-reinforced composites have been used in only limited applications because of the relatively high cost of manufacture. As a result of these shortcomings, research has been directed to the fabrication processes in which reinforcements can be produced in situ. Anisotropic grain growth is one class of in situ reaction that is relatively unexplored but appears to offer significant opportunity for the development of materials with self-reinforcing microstructures. Faber and Evans [2] predicted that disc- and rod-shaped grains increase fracture toughness in dense ceramics if the volume fraction of anisotropic grains is more than 0921-5093/95/$9.50 © 1995 - Elsevier Science S.A. All rights reserved SSDI 0921-5093(94)06516-0
10 vol.% for rod-shaped grains and more than 20 vol.% for disc-shaped grains. Silicon nitride (Si3N4) is exemplary of how the mechanical properties can be improved by optimizing anisotropic grain growth of acicular fl-Si3N 4 grains during the a-to-fl-Si3N 4 phase transformation [3]. Today, silicon nitrides with fracture toughnesses of 10-20 MPa m 1/2 are commercially produced. Also, the presence of platelet-shaped grains has resulted in very low clamping voltages in ZnO [4]. The development of fracture-resistant materials with anisotropic grains has been the result of extensive experimentation, but surprisingly there is little fundamental understanding about what parameters and processes control anisotropic grain growth in ceramics. Alumina is an excellent material in which to examine the development of anisotropic grains because of the extensive literature describing its grain growth, as well as the tendency of alumina to develop anisotropic tabular grains. To date, MgO doping has been used to purposely avoid development of this type of grain structure in relatively high purity alumina. Anisotropic grain growth in alumina, which is sometimes incorrectly referred to as abnormal grain growth, is usually associated with the presence of a liquid phase. In a recent study of sintered boehmite-drived aA1203, we observed extensive development of anisotropic grains [5]. This type of microstructure was traced to the presence of TiO2 in the commercial boehmite
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(7-A10(OH) sol. Titania TiO2) is present in Zieglerprocess-derived boehmite because a titanium compound is used as a catalyst in the process. Titania has been studied primarily as a sintering aid for alumina [6-9]. The influence of TiO 2 on anisotropic grain growth in alumina has not been examined in any detail. In contrast to the results in this paper, Hamano et al. did not observe anisotropic or exaggerated grain growth for TiO 2 concentrations up to 2 wt.% [9]. Song and Coble [10,11] correlated the development of anisotropic grains with the presence of specific concentrations of silica and either CaO, Na20 , SrO or BaO. They observed 'elongated' grains when alumina was codoped with CaO and TiO 2. Their results confirm earlier reports of enhanced grain growth when alumina is doped with TiO2 [6,7]. In this paper we report a series of grain growth experiments designed to determine what factors control anisotropic grain growth in TiO2-doped alumina and to determine what conditions lead to an anisotropic grain structure. These studies are unique relative to previous grain growth studies in that grain growth has been examined in the absence of porosity and starts with a high purity, ultrafine grain matrix of about 0.4/am.
2. Experimentalprocedure Aluminum sec-butoxide (ASB)(AI(OC4H)3 from Morton Thiokol, Inc., Alfa Products, Danvers, MA) was used to synthesize boehmite. The ASB was hydrolyzed by rapidly adding it to vigorously stirring water which had been heated to 90 °C in a clean Pyrex distilling flask [12]. The water-to-alkoxide ratio was about 100:1. After 30 min of stirring, the boehmite dispersion was peptized with nitric acid (HNO3) at an acidto-alkoxide ratio of 1:5. To prepare TiO2-doped boehmite, titanium isopropoxide was added to butanol and mixed with the ASB solution prior to hydrolysis. The same procedures used for the boehmite sol preparation were then used to prepare the doped boehmite sol. Disperison of the boehmite was evident when the opaque white sol became clear within seconds of the acid addition. After 8 days, the sol was removed from the flask and stored in a clean polyethylene bottle at room temperature. Impurity analysis of the boehmite sol was determined by atomic emission spectroscopy (Spectraspan 3; Spectrometrics, Beckman, CA). The boehmite sol was seeded with about 0.1 /am diameter a-A1203 particles which had been obtained by centrifuging a commercial alumina powder (A16SG, Alcoa Inc, Pittsburgh, PA). Ultrasonically dispersed seed particles were stirred into the boehmite sol
at a concentration of 5 x 1013 seeds per cubic centimeter of transformed y-Al203 (about 1.5 wt.% dry weight basis (dwb)) and the seeded sol was stirred for 1 day. Samples for the grain growth study were formed by casting the boehmite sols into a polyethylene beaker. The air-dried cast samples were about 100 p m thick. The dried samples were heated in a tube furnace from 400 to 1250 °C at 10 °C rain-~ and then sintered at 1250 °C for 1 h. To reduce contamination, the samples were placed in a platinum-foil-covered platinum crucible and heated in a high purity alumina tube during sintering and for the grain growth experiments. Isothermal grain growth studies were performed from 1300 °Cto 1550 °Cfor 1-16 h. For a comparative study of the effect of TiO2 on anisotropic grain growth in alumina powder compacts, a high purity, fine particle size a-Al203 (AKP-50; Sumitomo Chemical Co. Ltd., Tokyo) was doped with TiO 2 by adding titanium isopropoxide. The powders were uniaxially pressed at 245 MPa in a 1.2 cm diameter die. Scanning electron micrographs revealed the primary alumina particles to be on the order of 0.2/am. Powder compacts and thick boehmite gel samples were sintered simultaneously by heating in a box furnace at 10 °C min 1 from 400 to 1600 °C. For the comparative grain growth study, the samples were heated at 1600 °C in covered, high purity alumina crucibles for 1-16 h. The average grain sizes were determined from either 7 5 0 0 x or 10 0 0 0 x magnification scanning electron micrographs. The mean intercept length of a transverse line across 500 grains was measured for each data point with a digital image analyzer. The average intercept length was multiplied by 1.5 to obtain the average grain diameter of equiaxed grains. The mean transverse length is equal to twice the platelet grain thickness [13]. The average radius of an anisotropic grain was determined from the ratio of the number these grains per unit length NL and the number of anisotropic grains per unit area N~. The volume fraction of anisotropic grains is equal to the sum of the intercept lengths divided by the total transverse line length. The average aspect ratio was taken as the ratio of the average radius to average thickness. The microstructures of as-heated surfaces and sectioned samples were compared to determine whether surfaces are representative of grains in the sample interior. From the average grain diameter measurements it was concluded that the sample surfaces .were representative of the bulk microstructures. Therefore, because of the ease with which they could be prepared, sample surfaces, rather than cut and polished sections, were used for microstructural analysis.
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MaterialsScience and EngineeringA195 (1995) 169-178
3. Results
All grain growth samples were more than 99% dense and had an initial grain size of about 0.4/~m. Fig. 1 shows the three types of microstructures obtained in this study. The equiaxed microstructure in Fig. l(a) is typical of samples that undergo normal grain growth.
171
Fig. l(b) is characteristic of samples that undergo anisotropic grain growth and will be discussed more extensively below. Fig. l(c) is characteristic of abnormal grain growth. Fig. 2 summarizes the range of times, temperatures and T i O 2 concentrations where these types of grain growth were observed. Fig. 3 shows the grain growth kinetics for undoped and TiO2-doped samples at 1300 °C. The grain size increases slightly with increasing T i O 2 c o n t e n t . After 16 h at 1300 °C the grain size of the 0.4 wt.% sample is only about 0.78/tm and that of the undoped sample is 0.6/~m. In comparison, the average grain size of the 0.6 wt.% T i O 2 sample, which undergoes abnormal grain growth, is 2.4/~m after 1 h and 13 /~m after 16 h at 1300 °C. As seen in Fig. 2, alumina undergoes normal grain growth at 0.4 wt.% or less TiO2 for relatively low temperatures, i.e. 1350 °C or less and for short times. The grain growth kinetics for samples in the normal grain growth regime fit the complete parabolic grain growth law (i.e. including the initial grain size). Fig. 4 shows the grain growth kinetics for the 0.6 wt.% TiO2-doped samples. At 1300 °C these samples increased to an approximately 12/~m grain size after 16 h in contrast with a grain size of only 0.78/~m in the 0.4 wt.% T i O 2 sample. Increasing the grain growth temperature results in the growth of very large grains of irregular shape. Clearly, anisotropic grain growth does not occur under these conditions. The grain boundary mobilities for samples with parabolic growth kinetics were calculated from G 2 - G 0 2 = 2),gbMt where ~gb is the grain boundary energy and is assumed to be constant at 0.3 J m -2 for this calculation. The mobilities of the 0.15 wt.% T i O zdoped alumina are 1.1x 10 -17 m 4 j - 1 s -1 and 1.3X10-17 m 4 j - 1 S-1 at 1300 °C and 1350 °C, respectively. At 1300 °C the mobility of the 0.4 wt.% TiO2-doped alumina was 1.2 x 1 0 -17 m 4 j - i s - 1 . The undoped alumina has mobilities of 7.8 x 10-18 m 4 j - l S -1 and 9.8x 10 -18 m 4 j -1 s -I at 1300 °C and 1350 °C, respectively. These results clearly demonstrate that TiO2 increases the grain boundary mobility in alumina. These data are discussed more fully elsewhere [14]. 3.1. Anisotropic grain growth
Fig. 1. Microstructures observed in TiO2-doped alumina and indicative of (A) normal grain growth, (B) anisotropic grain growth, and (C) abnormal grain growth.
Grains are observed to undergo anisotropic grain growth after a period of normal grain growth with the 'incubation' time decreasing with increasing temperature. The microstructure of the 0.15 wt.% sample at the onset of anisotropic grain growth consists of equiaxed grains of about 0.8 ktm average diameter. The onset of anisotropic grain growth appears to be established by this characteristic microstructure and not necessarily by the time or temperature conditions. The 0.4 wt.% sample undergoes normal grain growth to an average
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D.S. Horn, G.L. Messing
Temperature (°C) Titania Concentration
Time
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1350
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0.4 w t %
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Fig. 2. Map of the conditions resulting in normal, anisotropic and abnormal grain growth in TiO2-doped alumina.
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size of about 1.0 # m prior to the onset of anisotropic grain growth. In all cases, the intercept lengths range from approximately 0.5 to 1.5/~m which suggests that the onset or 'nucleation' of anisotropic grain growth is a function of matrix grain size and grain size distribution. The volume fraction of anisotropic grains is plotted in Fig. 5 as a function of time. Interestingly, the volume fraction of anisotropic grains remains constant at about 25 vol.% for 0.15 wt.% TiO2 even after 16 h at 1400 °C. Increasing the temperature by 50 °C or increasing the TiO 2 concentration to 0.4 wt.% increases the volume fraction of anisotropic grains to about 60 vol.% after 16 h (Fig. 6). When the temperature and time are further increased the microstructure consists of only anisotropic grains. The surface perpendicular to the thickness direction was identified as the basal plane. This observation is consistent with previous reports of basal plane facets in alumina sintered in the presence of a liquid grain boundary phase [15-17]. The growth kinetics for the radial and thickness dimensions of the anisotropic grains are plotted in Fig. 7 and Fig. 8 for the 0.15 wt.% TiO 2 and 0.4 wt.% TiO2, respectively. Anisotropic grain growth is observed to initiate at a lower temperature and after a shorter incubation period with the 0.4 wt.% TiO 2. Radial growth kinetics are seen tobe significantly faster than the rate of grain thickening. Once anisotropic grain growth begins, the grain growth exponent changes from 2 to 3 (see Tables 1 and 2). Initially, the growth kinetics increase rapidly and then
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MaterialsScience and Engineering A195 (1995) 169-178 Time (hours)
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Fig. 7. Kinetics for radial growth (top) and thickening (bottom) of anisotropic grains in 0.15 w t . % T i O 2 - d o p e d alumina.
Fig. 6. Anisotropic grain structure of 0. lwt.%TiO 2-doped alumina after heating at 1500 °C for 2 h.
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slow significantly with the growth exponent reaching very high values which do not correspond to any existing grain growth models. The reduced kinetics after long periods is a result of impingement of the anisotropic grains and the significant loss of driving force for grain growth processes. Because the grains are so large and the basal surfaces straight, there is little driving force for grain thickening and the aspect ratio remains relatively constant at about 10 over a range of processing temperatures and times. Recall, however, that the volume fraction of anisotropic grains changes with temperature. The boundary mobility for radial growth and thickening can be calculated from M=K(nTgbGan-2) -1 where K is the growth constant, n is the grain growth exponent, and Ga is the average grain size [18]. A more correct analysis would include the grain boundary energies of the specific growth planes, but unfortunately this information is not known. The mobilities are tabulated in Tables 1 and 2 as a function of temperature. At low temperatures the mobilities for normal grain growth are similar to values reported by Hand-
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Fig. 8. Kinetics for radial growth (top) and thickening (bottom) of anisotropic grains in 0.4wt.%TiO2-doped alumina.
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Materials Science and Engineering A195 (1995) 169-178
Table 1 Grain growth exponents and mobilities for 0.15 wt.% TiO 2doped alumina Temperature
(oc)
1300 1350 1400 1450 1500 1550
Thickness parameters
Radial parameters
Growth exponent
Mobility (m4j - 1s- 1)
Growth exponent
Mobility (m4 j - 1S-
2 2 2 3 8 14
1.1 × 10 -17 1.3×10 -17 2.7× 10 -17 6.7×10 -17 2.6x 10 -16 1.5×10 -16
2 2 3 10 24 33
1.1 x 10 -17 1.3×10 -17 6.3×10 -16 2.1×10 -14 8.7×10 -12 6.3×10 -12
1)
Table 2 Grain growth exponents and mobilities for 0.4 wt.% TiOE-doped alumina Temperature
Thickness parameters
Radial parameters
(of)
Growth exponent
Mobility (m4j - l s 1)
Growth exponent
1300 1350 1400 1450 1500 1550
2 2 2 11 8 12
1.2×10 -17 3.6x 10 -16 6.7x 10 -a6 3.8 x 10 16 2.0 x 10 -16 2.0 x 10 -16
2 3 3 5 6 8
Mobility (m4 j - 1s- 1) 1.2x 10 -17 4.0×10 -14 4.3x 10 -14 2.7 x 10 -13 3.0 x 10 -13 3.2x 10 -13
werker et al. [19]. T h e b o u n d a r y mobility for radial growth is almost two orders of magnitude greater than for thickening. With increasing temperature the mobility significantly decreases as a result of the increased grain size. T h e mobilities in the thickening direction, however, are much less than in the radial growth direction because of the flatness of these boundaries. This result agrees qualitatively with R6del and Glaeser's [20] study of the growth of sapphire single crystals into a polycrystalline alumina matrix. T h e y observed that prismatic {1120} surfaces grew further (i.e. faster) into the polycrystalline matrix than basal {0001} surfaces. To observe how anisotropic grains begin to grow, a 0.4 wt.% T i O 2 sample was heated intermittently in a furnace at 1500 °C and the same grains were observed in a scanning electron microscope after each thermal treatment. A few large grains of about 2 ~ m were first observed after 2.5 min. Fig. 9 shows a sequence of micrographs of the same sample heated for 5 - 9 min. In Fig. 9(A) anisotropic grains were observed to form in a matrix of significantly smaller equiaxed grains after 5 min at 1500 °C. After 9 min (Fig. 9(C)) there has been extensive growth of only a few grains. T h e large grain a
Fig. 9. Development of anisotropic grains in 0.4wt.%TiO2doped alumina at 1500 °C after (A) 5 min, (B) 7 min and (C) 9 min.
has developed into an anisotropic grain and consumed a number of finer grains. In fact it has consumed several anisotropic grains that were several times larger than the average matrix grain size. Grain a increased from 5.5 to 8 ktm in length and from 1.5 to 2 / ~ m in width between 5 and 9 min. However, the b o u n d a r y border-
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Materials Science and Engineering A195 (1995) 169-178
ing grain b on the right has not changed during this period because it is straight and has too little driving force for movement relative to more curved boundaries. After 9 min no new anisotropic grains develop. The only notable change between 7 and 9 min is that the matrix grains doubled in size. The radial and thickening growth kinetics of anisotropic grains in the 0.15 wt.% T i O 2 sample are plotted in Fig. 10. Some of the anisotropic grains undergo very rapid growth, or growth 'spurts', over a short period of time. These kinetics are different from Figs. 3, 6 and 7 in that the data were obtained for the s a m e grains at each time. As a result these data are more sensitive to the initial growth of anisotropic grains. It is evident that the average size measurements are not as sensitive to the details of the growth process. The rapid growth seen at the beginning of anisotropic grain growth has been discussed in the literature and is more accurately termed stepped grain growth [21,22]. The above results suggest that the ultrafine grain size of the alumina is responsible for the growth of anisotropic grains. To determine whether these results could be duplicated with a coarser alumina, a high purity commercial alumina was doped with T i O 2 and sintered to full density. The initial grains were equiaxed and had an average grain size of about 0.8/~m. Samples containing 0.15 and 0.4 wt.% TiO2 were heated at 1500 °C for up to 70 h. The samples underwent normal grain growth and showed no evidence of anisotropic grain growth. These results are similar to those of Hamano et al. [9] who only reported equiaxed grains in T i O zdoped alumina samples heated for 2 h at 1700 °C.
4. Discussion
Grain growth in tx-AI203 has been the subject of active research for many years. Until recently, most aluminas contained high impurity concentrations, were
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175
not fully dense and had greater than 1/zm grain diameter. Consequently, the impurities combined with silica and formed liquid phases which have been shown to result in anisotropic grain growth. In most grain growth studies, the determination of grain growth parameters such as the grain growth exponent, mobility and kinetics has been compromised by liquid phases and/ or porosity and their effect on grain boundary motion. While much has been learned from these earlier studies, only the more recent literature in which the growth of single-crystal facets into a dense, polycrystalline matrix [19,20] and when impurities were carefully controlled and measured [19,23,27] are relevant to this study. The alkoxide-derived alumina contained 80 ppm MgO, 20 ppm SiO2 and 50 ppm CaO. Sodium, iron and other cations were not detected. The commercial alumina powder used for the grain growth study contained 10 ppm MgO, 20 ppm SiO2, 2 ppm CaO and similarly small concentrations of TiO 2, Fe203, ZnO and CuO. Sodium was not detected. The impurity analyses are critical for ascertaining whether a liquid grain boundary phase can form during sintering and subsequent grain growth. For example, some researchers [ 10,19,23] suggest that growth of anisotropic grains is not possible without a liquid grain boundary phase. Silica, sodium oxide and calcia are the most commonly observed components of grain boundary films. In the above case, no sodium was detected in either the boehmite or commercial alumina. Some grain growth experiments were repeated by using boehrnite derived from a higher purity aluminum isopropoxide. The MgO and CaO concentrations were the same but the SiO2 content was reduced to 5 ppm. There were no differences noted in the grain growth studies with this higher purity boehmite. Note that the SiO2 content of the higher purity boehmite sample is lower than that of the commercial high purity alumina used in this study. Anisotropic grain growth in the higher purity sample but not in the commercial alumina samples suggests that S i O 2 is not critical for anisotropic grain growth in TiOz-doped alumina. In addition to the above analysis, a series of transmission electron microscopy (TEM) and electron microprobe experiments were performed on the grain growth samples to determine whether a grain boundary phase was responsible for anisotropic grain growth. Energy-dispersive spectroscopy (EDS) during scanning electron microscopy (SEM) and TEM, and wavelength-dispersive spectroscopy in a microprobe, detected only aluminum and titanium. In an effort to enrich the impurity level on the grain boundaries like Handwerker et al. [19], samples were annealed for 24 h at 1400 °C. No second phase formed nor were impurities detected by EDS.
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Grain boundaries were also observed by T E M under diffuse dark field conditions. No grain boundaries with an obvious amorphous film were observed. It should be noted that the search and characterization of nanometer-scale boundary films is better done by high resolution electron microscope, but during the period of this study one was not available. A few TiO2 precipitates of about 100 nm diameter were observed on grain boundaries and at triple points, but in some samples no TiO 2 precipitates were observed. The number of precipitate particles was too low to affect boundary motion. Also, the precipitates did not appear to be associated with any specific boundaries. In contrast with Hamano et al. [9], no AI2TiO 5 precipitates were observed. As shown above, TiO2 enhances diffusion in alumina which agrees with Hamano et al. [9]. At 1500 °C boundary mobilities of anisotropic grains in 0.15 wt.% TiO2-doped alumina are comparable to boundary mobilities in pure alumina at 1650°C. However, as seen in Fig. 11, the scale of the micro-
structures is approximately the same but the TiO 2doped sample has distinctly anisotropic grains whereas the undoped sample consists of more irregular-shaped grains. Apparently, other factors besides enhanced diffusion cause anisotropic grain growth in TiO2doped alumina. The above analyses are inconclusive about the location of the TiO 2 and its specific role during anisotropic grain growth. The solubility of TiO 2 in alumina has been reported to be 0.27 wt.% above 1150°C [9]. However, it has been reported that Ti 4+ has a strong tendency to segregate to the grain boundary with a segregation coefficient of about 120-600 in 0.06 wt.% TiOz-doped alumina at 1300 °C [25]. Based on their sintering studies, Bagley et al. reported that TiO2 solubility in A120 3 was grain size dependent with a greater TiO 2 solubility in finer particles [8]. Interestingly, they also reported a sintering mechanism change between 0.5 and 1.0/zm which is the same grain size range at which anisotropic grain growth was observed to initiate. Kroger presented a model to explain the grain size dependence of grain boundary segregation of TiOz [26]. Li and Kingery detected significant TiO 2 segregation in doped alumina [27].
5. Anisotropic grain growth
Fig. 11. Comparison of microstructures having the same grain boundary mobility: (A) undoped alumina at 1650 °C for 16 h; (B) 0.4wt.%TiO2-doped alumina at 1500 °C for 12 h.
It is evident that anisotropic grain growth in TiO 2doped alumina depends on a number of different factors. For example, the presence of a silica-based liquid does not appear to be essential for anisotropic grain growth. However, CaO may be important as it was present in significant concentrations in the boehmite-derived samples but not in the commercial samples. Bae and Baik [24] recently showed that the critical CaO concentration for exaggerated grain growth in high purity alumina was only 30 ppm when no silica was present and the sample was heated at 1900 °C. While the role of impurities is still unclear, it is evident that anisotropic grains develop in only a narrow range of TiO 2 concentrations. The initial microstructure appears to be a critical factor for anisotropic grain growth. Contrary to earlier reports, anisotropic grain growth is not necessarily associated with the removal of the last few per cent of porosity or the attainment of full density [6,10]. For example, rapid growth of large anisotropic grains was not observed until after an incubation period. Instead, nucleation of anisotropic grains was observed to occur once a small population of grains had reached a critical size while at the same time the remaining grains did not significantly coarsen. The importance of a critical TiO2 concentration for anisotropic grain growth has been clearly demon-
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Materials Science and Engineering A195 (1995) 169-178
strated. The undoped samples do not display anisotropic grain growth at any of the times and temperatures investigated. Above 0.6 wt.% TiO2, grains grow in an abnormal manner. Between these two limits, anisotropic grains were observed. Again, this result demonstrates that enhanced diffusion alone is not responsible for grain growth since rapid diffusion leads to abnormal growth in the 0.6 wt.% TiO2 sample. Anisotropic grain growth in a polycrystalline matrix is analogous to the growth of a single crystal. Similar to the growth of single crystals, the growth of shaped grains in a polycrystalline matrix requires slower diffusion and a sufficient driving force for material transport. In the above system, slower growth at low temperatures limits the number of grains that grow to the critical size for anisotropic growth, and also limits matrix coarsening, thus preserving the driving force for the growth of anisotropic grains. T i O 2 appears to segregate to the grain boundary where it may act to poison growth on specific surfaces (e.g. basal growth), adsorb on specific surfaces to change the relative magnitude of the surface energies, or change the local diffusional growth processes. It is evident from these studies that a balance of the above processes is required to tailor the microstructure in a desired manner. As initially stated in the Introduction, this work was motivated by an interest in improving the fracture behavior of alumina. Fig. 12 shows the fracture surface of a TiO2-doped sample heated to 1400 °C for 1 h. It is obvious that the crack path is significantly altered relative to equiaxed, fine-grained alumina. As determined by the indentation method, the fracture toughness of this sample is 5.2 MPa m 1/2 vs. a fracture toughness of 3.2 MPa m l/2 for an ultrafine, equiaxed alumina. This preliminary result suggests that aniso-
177
tropic grain growth may be an effective manner for obtaining tougher ceramics.
6. Summarizing remarks Three types of grain growth are observed in T i O 2alumina. Alumina undergoes normal grain growth at low TiO 2 concentrations which changes to anisotropic grain growth after an incubation period. During the incubation period, a few grains grow to a critical size and then grow rapidly into the fine-grainsize matrix. Subsequent impingement of the large anisotropic grains and the loss of grain boundary area essentially stop further growth. At high TiO2 concentrations only abnormal grains form. Obviously, grain growth is a result of a variety of factors. In the past, many studies focused on one or two factors that were perceived as significant to grain growth in alumina. Unfortunately, the samples were porous or contained unwanted impurities. This study has demonstrated that a silica-based liquid is not required for anisotropic grain growth. However, the possible importance of calcia is intriguing and will be further examined. While T i O 2 enhances the rate of diffusion, it is evident that it is not sufficient to cause anisotropic grain growth. We can conclude from this study that some of the requirements for anisotropic grain growth in polycrystalline materials include (1) a fully dense matrix to avoid boundary-pore interactions, (2) an ultrafine matrix to provide sufficient driving force for anisotropic growth, (3) transport regulation to initiate anisotropic grain growth but limit matrix coarsening, (4) dopants to modify the growth process, and (5) preferably a material with non-isometric crystal structure. As shown in a computer simulation of anisotropic grain growth, a material with a large difference in interfacial energy as a function of crystallographic direction also facilitates anisotropic grain growth [28]. Clearly, more research is needed to establish the relative importance of these factors and what other oxide ceramics satisfy these requirements. Nevertheless, it is evident that the materials are now of suitable quality to permit a more exacting investigation of these phenomena and to tailor materials with desirable microstructures. doped
Acknowledgments
Fig. 12. Fracture surface of TiO2-doped alumina after 1 h at 1400 °C.
The authors gratefully acknowledge the support of the US Office of Naval Research under Grant N00014-94-1-0007.
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References [1] C.A. Handwerker, J.A. Blendell and W.A. Kaysser (eds.), Sintering of Ceramics, American Ceramic Society, Westerville, OH, 1990. [2] K.T. Faber and A.G. Evans, Acta Metall., 26 (1988) 469. [3] F.E Lange, Bull. Am. Ceram. Soc., 62(1983) 1369. [4] L.T. Bowen and E Avella, J. Appl. Phys., 54 (1983) 2764. [5] D.S. Horn, MS Thesis, Pennsylvania State University, University Park, PA, 1988. [6] H. Calhoon and C. Christenson, J. Am. Ceram. Soc., 39 (1956) 337. [7] I.B. Cutler, C. Bradshaw, C. Christenson and E. Hyatt, J. Am. Ceram. Soc., 40(1957) 134. [8] R. Bagley, I.B. Cutler and D.L. Johnson, J. Am. Ceram. Soc., 53(1970) 136. [9] K. Hamano, C. Hwang, Z. Nagagawa and Y. Ohya, Yogyo KyokaiShi, 94(1986) 505. [10] H. Song and R.L. Coble, J. Am. Ceram. Soc., 73 (1990) 2077. [11] H. Song and R.L. Coble, J. Am. Ceram. Soc., 73 (1990) 2086. [12] B. Yoldas, Bull. Am. Ceram. Soc., 54(1975) 289. [13] R.T. DeHoff and EN. Rhines (eds.), Quantitative Microscopy, McGraw-Hill, New York, 1968. [14] D.S. Horn and G.L. Messing, J. Am. Ceram. Soc., submitted for publication.
[15] W. Kaysser, M. Sprissler, C.A. Handwerker and J.E. Blendell, J. Am. Ceram. Soc., 70(1987) 339. [16] Y. Simpson and C.B. Carter, J. Am. Ceram. Soc., 73 (1990) 2391. [17] D. Susnitsky and C.B. Carter, J. Am. Ceram. Soc., 73(1990) 2485. [18] M. Yan, R. Cannon and H. Bowen in R. Fulrath and J.A. Pask (eds.), Ceramic Microstructures '76, Westview Press, Boulder, CO, 1976. [19] C.A. Handwerker, P.A. Morris and R.L. Coble, J. Am. Ceram. Soc., 72(1989) 130. [20] J. R6del and A.M. Glaeser, J. Am. Ceram. Soc., 73 (1990) 3292. [21] G. Abbruzzese, K. L/icke and H. Eichelkraut, Trans. Iron Steel lnst. Jpn., (1988) 818. [22] H. Eichelkraut, G. Abbruzzese and K. L/icke, Acta Metall., 36 (1988) 55. [23] S. Bennison and M.P. Harmer, J. Am. Ceram. Soc., 68 (1985)C22. [24] S. Bae and S.Baik, J. Am. Ceram. Soc., 76(1993) 1065. [25] R. Langensiepen, PhD Dissertation, Pennsylvania State University, University Park, PA, 1986. [26] EA. Kroger, J. Am. Ceram. Soc., 67(1984) 390. [27] C.W. Li and W.D. Kingery, in W.D. Kingery (ed.), Structure and Properties of MgO and Polycrystalline A12 03, American Ceramic Society, Westerville, OH, 1984. [28] W. Yang, L.Q. Chen and G.L. Messing, Mater. Sci. Eng., A195(1995).
Materials Science and Engineering A195 (1995) 169-178
A
Anisotropic grain growth in TiO2-doped alumina Debra S. Horn, Gary L. Messing Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802, USA
Abstract Grain growth in TiO2-doped alumina was studied in a high density, ultrafine matrix (0.4 ~m. Normal grain growth, anisotropic grain growth and abnormal grain growth were observed. With 0.15-0.4 wt.% TiO2, samples initially undergo normal grain growth until a crystal microstructure is attained and anisotropic grain growth in nucleated. Large anisotropic, platelet-shaped grains grow rapidly by a step growth process until impingement of the large grains essentially stops further growth. The volume fraction of anisotropic grains ranged from 20 to 100 vol.% suggesting that physical propertis dependent on grain shape and volume fraction can be tailored. Critical requirements are proposed for the in situ growth of anisotropic grains. Keywords: Anisotropy; Grain growth; Alumina; Doping
1. Introduction
There has been significant progress over the past 20 years in understanding how sintering and grain growth processes influence the attainment of dense ceramics with equiaxed and fine-grain microstructures (see Ref. [1] for example). Such microstructures, however, have limited high-temperature creep resistance and low fracture toughness, and thus are unsuitable for some mechanical applications. As a result, ceramic matrix composites with particle, fiber and whisker reinforcements have been developed. Although whiskerreinforced composites appear to have some of the best properties, the health hazards associated with handling whiskers have significantly curtailed industrial acceptance and implementation. Continuous-fiber-reinforced composites have been used in only limited applications because of the relatively high cost of manufacture. As a result of these shortcomings, research has been directed to the fabrication processes in which reinforcements can be produced in situ. Anisotropic grain growth is one class of in situ reaction that is relatively unexplored but appears to offer significant opportunity for the development of materials with self-reinforcing microstructures. Faber and Evans [2] predicted that disc- and rod-shaped grains increase fracture toughness in dense ceramics if the volume fraction of anisotropic grains is more than 0921-5093/95/$9.50 © 1995 - Elsevier Science S.A. All rights reserved SSDI 0921-5093(94)06516-0
10 vol.% for rod-shaped grains and more than 20 vol.% for disc-shaped grains. Silicon nitride (Si3N4) is exemplary of how the mechanical properties can be improved by optimizing anisotropic grain growth of acicular fl-Si3N 4 grains during the a-to-fl-Si3N 4 phase transformation [3]. Today, silicon nitrides with fracture toughnesses of 10-20 MPa m 1/2 are commercially produced. Also, the presence of platelet-shaped grains has resulted in very low clamping voltages in ZnO [4]. The development of fracture-resistant materials with anisotropic grains has been the result of extensive experimentation, but surprisingly there is little fundamental understanding about what parameters and processes control anisotropic grain growth in ceramics. Alumina is an excellent material in which to examine the development of anisotropic grains because of the extensive literature describing its grain growth, as well as the tendency of alumina to develop anisotropic tabular grains. To date, MgO doping has been used to purposely avoid development of this type of grain structure in relatively high purity alumina. Anisotropic grain growth in alumina, which is sometimes incorrectly referred to as abnormal grain growth, is usually associated with the presence of a liquid phase. In a recent study of sintered boehmite-drived aA1203, we observed extensive development of anisotropic grains [5]. This type of microstructure was traced to the presence of TiO2 in the commercial boehmite
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(7-A10(OH) sol. Titania TiO2) is present in Zieglerprocess-derived boehmite because a titanium compound is used as a catalyst in the process. Titania has been studied primarily as a sintering aid for alumina [6-9]. The influence of TiO 2 on anisotropic grain growth in alumina has not been examined in any detail. In contrast to the results in this paper, Hamano et al. did not observe anisotropic or exaggerated grain growth for TiO 2 concentrations up to 2 wt.% [9]. Song and Coble [10,11] correlated the development of anisotropic grains with the presence of specific concentrations of silica and either CaO, Na20 , SrO or BaO. They observed 'elongated' grains when alumina was codoped with CaO and TiO 2. Their results confirm earlier reports of enhanced grain growth when alumina is doped with TiO2 [6,7]. In this paper we report a series of grain growth experiments designed to determine what factors control anisotropic grain growth in TiO2-doped alumina and to determine what conditions lead to an anisotropic grain structure. These studies are unique relative to previous grain growth studies in that grain growth has been examined in the absence of porosity and starts with a high purity, ultrafine grain matrix of about 0.4/am.
2. Experimentalprocedure Aluminum sec-butoxide (ASB)(AI(OC4H)3 from Morton Thiokol, Inc., Alfa Products, Danvers, MA) was used to synthesize boehmite. The ASB was hydrolyzed by rapidly adding it to vigorously stirring water which had been heated to 90 °C in a clean Pyrex distilling flask [12]. The water-to-alkoxide ratio was about 100:1. After 30 min of stirring, the boehmite dispersion was peptized with nitric acid (HNO3) at an acidto-alkoxide ratio of 1:5. To prepare TiO2-doped boehmite, titanium isopropoxide was added to butanol and mixed with the ASB solution prior to hydrolysis. The same procedures used for the boehmite sol preparation were then used to prepare the doped boehmite sol. Disperison of the boehmite was evident when the opaque white sol became clear within seconds of the acid addition. After 8 days, the sol was removed from the flask and stored in a clean polyethylene bottle at room temperature. Impurity analysis of the boehmite sol was determined by atomic emission spectroscopy (Spectraspan 3; Spectrometrics, Beckman, CA). The boehmite sol was seeded with about 0.1 /am diameter a-A1203 particles which had been obtained by centrifuging a commercial alumina powder (A16SG, Alcoa Inc, Pittsburgh, PA). Ultrasonically dispersed seed particles were stirred into the boehmite sol
at a concentration of 5 x 1013 seeds per cubic centimeter of transformed y-Al203 (about 1.5 wt.% dry weight basis (dwb)) and the seeded sol was stirred for 1 day. Samples for the grain growth study were formed by casting the boehmite sols into a polyethylene beaker. The air-dried cast samples were about 100 p m thick. The dried samples were heated in a tube furnace from 400 to 1250 °C at 10 °C rain-~ and then sintered at 1250 °C for 1 h. To reduce contamination, the samples were placed in a platinum-foil-covered platinum crucible and heated in a high purity alumina tube during sintering and for the grain growth experiments. Isothermal grain growth studies were performed from 1300 °Cto 1550 °Cfor 1-16 h. For a comparative study of the effect of TiO2 on anisotropic grain growth in alumina powder compacts, a high purity, fine particle size a-Al203 (AKP-50; Sumitomo Chemical Co. Ltd., Tokyo) was doped with TiO 2 by adding titanium isopropoxide. The powders were uniaxially pressed at 245 MPa in a 1.2 cm diameter die. Scanning electron micrographs revealed the primary alumina particles to be on the order of 0.2/am. Powder compacts and thick boehmite gel samples were sintered simultaneously by heating in a box furnace at 10 °C min 1 from 400 to 1600 °C. For the comparative grain growth study, the samples were heated at 1600 °C in covered, high purity alumina crucibles for 1-16 h. The average grain sizes were determined from either 7 5 0 0 x or 10 0 0 0 x magnification scanning electron micrographs. The mean intercept length of a transverse line across 500 grains was measured for each data point with a digital image analyzer. The average intercept length was multiplied by 1.5 to obtain the average grain diameter of equiaxed grains. The mean transverse length is equal to twice the platelet grain thickness [13]. The average radius of an anisotropic grain was determined from the ratio of the number these grains per unit length NL and the number of anisotropic grains per unit area N~. The volume fraction of anisotropic grains is equal to the sum of the intercept lengths divided by the total transverse line length. The average aspect ratio was taken as the ratio of the average radius to average thickness. The microstructures of as-heated surfaces and sectioned samples were compared to determine whether surfaces are representative of grains in the sample interior. From the average grain diameter measurements it was concluded that the sample surfaces .were representative of the bulk microstructures. Therefore, because of the ease with which they could be prepared, sample surfaces, rather than cut and polished sections, were used for microstructural analysis.
D.S. Horn, G.L. Messing /
MaterialsScience and EngineeringA195 (1995) 169-178
3. Results
All grain growth samples were more than 99% dense and had an initial grain size of about 0.4/~m. Fig. 1 shows the three types of microstructures obtained in this study. The equiaxed microstructure in Fig. l(a) is typical of samples that undergo normal grain growth.
171
Fig. l(b) is characteristic of samples that undergo anisotropic grain growth and will be discussed more extensively below. Fig. l(c) is characteristic of abnormal grain growth. Fig. 2 summarizes the range of times, temperatures and T i O 2 concentrations where these types of grain growth were observed. Fig. 3 shows the grain growth kinetics for undoped and TiO2-doped samples at 1300 °C. The grain size increases slightly with increasing T i O 2 c o n t e n t . After 16 h at 1300 °C the grain size of the 0.4 wt.% sample is only about 0.78/tm and that of the undoped sample is 0.6/~m. In comparison, the average grain size of the 0.6 wt.% T i O 2 sample, which undergoes abnormal grain growth, is 2.4/~m after 1 h and 13 /~m after 16 h at 1300 °C. As seen in Fig. 2, alumina undergoes normal grain growth at 0.4 wt.% or less TiO2 for relatively low temperatures, i.e. 1350 °C or less and for short times. The grain growth kinetics for samples in the normal grain growth regime fit the complete parabolic grain growth law (i.e. including the initial grain size). Fig. 4 shows the grain growth kinetics for the 0.6 wt.% TiO2-doped samples. At 1300 °C these samples increased to an approximately 12/~m grain size after 16 h in contrast with a grain size of only 0.78/~m in the 0.4 wt.% T i O 2 sample. Increasing the grain growth temperature results in the growth of very large grains of irregular shape. Clearly, anisotropic grain growth does not occur under these conditions. The grain boundary mobilities for samples with parabolic growth kinetics were calculated from G 2 - G 0 2 = 2),gbMt where ~gb is the grain boundary energy and is assumed to be constant at 0.3 J m -2 for this calculation. The mobilities of the 0.15 wt.% T i O zdoped alumina are 1.1x 10 -17 m 4 j - 1 s -1 and 1.3X10-17 m 4 j - 1 S-1 at 1300 °C and 1350 °C, respectively. At 1300 °C the mobility of the 0.4 wt.% TiO2-doped alumina was 1.2 x 1 0 -17 m 4 j - i s - 1 . The undoped alumina has mobilities of 7.8 x 10-18 m 4 j - l S -1 and 9.8x 10 -18 m 4 j -1 s -I at 1300 °C and 1350 °C, respectively. These results clearly demonstrate that TiO2 increases the grain boundary mobility in alumina. These data are discussed more fully elsewhere [14]. 3.1. Anisotropic grain growth
Fig. 1. Microstructures observed in TiO2-doped alumina and indicative of (A) normal grain growth, (B) anisotropic grain growth, and (C) abnormal grain growth.
Grains are observed to undergo anisotropic grain growth after a period of normal grain growth with the 'incubation' time decreasing with increasing temperature. The microstructure of the 0.15 wt.% sample at the onset of anisotropic grain growth consists of equiaxed grains of about 0.8 ktm average diameter. The onset of anisotropic grain growth appears to be established by this characteristic microstructure and not necessarily by the time or temperature conditions. The 0.4 wt.% sample undergoes normal grain growth to an average
172
/ Materials Science and Engineering A195 (1995) 169-178
D.S. Horn, G.L. Messing
Temperature (°C) Titania Concentration
Time
1300
1350
1400
1450
1500
1550
(h)
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0.4 w t %
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Fig. 2. Map of the conditions resulting in normal, anisotropic and abnormal grain growth in TiO2-doped alumina.
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size of about 1.0 # m prior to the onset of anisotropic grain growth. In all cases, the intercept lengths range from approximately 0.5 to 1.5/~m which suggests that the onset or 'nucleation' of anisotropic grain growth is a function of matrix grain size and grain size distribution. The volume fraction of anisotropic grains is plotted in Fig. 5 as a function of time. Interestingly, the volume fraction of anisotropic grains remains constant at about 25 vol.% for 0.15 wt.% TiO2 even after 16 h at 1400 °C. Increasing the temperature by 50 °C or increasing the TiO 2 concentration to 0.4 wt.% increases the volume fraction of anisotropic grains to about 60 vol.% after 16 h (Fig. 6). When the temperature and time are further increased the microstructure consists of only anisotropic grains. The surface perpendicular to the thickness direction was identified as the basal plane. This observation is consistent with previous reports of basal plane facets in alumina sintered in the presence of a liquid grain boundary phase [15-17]. The growth kinetics for the radial and thickness dimensions of the anisotropic grains are plotted in Fig. 7 and Fig. 8 for the 0.15 wt.% TiO 2 and 0.4 wt.% TiO2, respectively. Anisotropic grain growth is observed to initiate at a lower temperature and after a shorter incubation period with the 0.4 wt.% TiO 2. Radial growth kinetics are seen tobe significantly faster than the rate of grain thickening. Once anisotropic grain growth begins, the grain growth exponent changes from 2 to 3 (see Tables 1 and 2). Initially, the growth kinetics increase rapidly and then
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MaterialsScience and Engineering A195 (1995) 169-178 Time (hours)
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Fig. 7. Kinetics for radial growth (top) and thickening (bottom) of anisotropic grains in 0.15 w t . % T i O 2 - d o p e d alumina.
Fig. 6. Anisotropic grain structure of 0. lwt.%TiO 2-doped alumina after heating at 1500 °C for 2 h.
Time (minutes) I
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=L
slow significantly with the growth exponent reaching very high values which do not correspond to any existing grain growth models. The reduced kinetics after long periods is a result of impingement of the anisotropic grains and the significant loss of driving force for grain growth processes. Because the grains are so large and the basal surfaces straight, there is little driving force for grain thickening and the aspect ratio remains relatively constant at about 10 over a range of processing temperatures and times. Recall, however, that the volume fraction of anisotropic grains changes with temperature. The boundary mobility for radial growth and thickening can be calculated from M=K(nTgbGan-2) -1 where K is the growth constant, n is the grain growth exponent, and Ga is the average grain size [18]. A more correct analysis would include the grain boundary energies of the specific growth planes, but unfortunately this information is not known. The mobilities are tabulated in Tables 1 and 2 as a function of temperature. At low temperatures the mobilities for normal grain growth are similar to values reported by Hand-
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Fig. 8. Kinetics for radial growth (top) and thickening (bottom) of anisotropic grains in 0.4wt.%TiO2-doped alumina.
174
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Materials Science and Engineering A195 (1995) 169-178
Table 1 Grain growth exponents and mobilities for 0.15 wt.% TiO 2doped alumina Temperature
(oc)
1300 1350 1400 1450 1500 1550
Thickness parameters
Radial parameters
Growth exponent
Mobility (m4j - 1s- 1)
Growth exponent
Mobility (m4 j - 1S-
2 2 2 3 8 14
1.1 × 10 -17 1.3×10 -17 2.7× 10 -17 6.7×10 -17 2.6x 10 -16 1.5×10 -16
2 2 3 10 24 33
1.1 x 10 -17 1.3×10 -17 6.3×10 -16 2.1×10 -14 8.7×10 -12 6.3×10 -12
1)
Table 2 Grain growth exponents and mobilities for 0.4 wt.% TiOE-doped alumina Temperature
Thickness parameters
Radial parameters
(of)
Growth exponent
Mobility (m4j - l s 1)
Growth exponent
1300 1350 1400 1450 1500 1550
2 2 2 11 8 12
1.2×10 -17 3.6x 10 -16 6.7x 10 -a6 3.8 x 10 16 2.0 x 10 -16 2.0 x 10 -16
2 3 3 5 6 8
Mobility (m4 j - 1s- 1) 1.2x 10 -17 4.0×10 -14 4.3x 10 -14 2.7 x 10 -13 3.0 x 10 -13 3.2x 10 -13
werker et al. [19]. T h e b o u n d a r y mobility for radial growth is almost two orders of magnitude greater than for thickening. With increasing temperature the mobility significantly decreases as a result of the increased grain size. T h e mobilities in the thickening direction, however, are much less than in the radial growth direction because of the flatness of these boundaries. This result agrees qualitatively with R6del and Glaeser's [20] study of the growth of sapphire single crystals into a polycrystalline alumina matrix. T h e y observed that prismatic {1120} surfaces grew further (i.e. faster) into the polycrystalline matrix than basal {0001} surfaces. To observe how anisotropic grains begin to grow, a 0.4 wt.% T i O 2 sample was heated intermittently in a furnace at 1500 °C and the same grains were observed in a scanning electron microscope after each thermal treatment. A few large grains of about 2 ~ m were first observed after 2.5 min. Fig. 9 shows a sequence of micrographs of the same sample heated for 5 - 9 min. In Fig. 9(A) anisotropic grains were observed to form in a matrix of significantly smaller equiaxed grains after 5 min at 1500 °C. After 9 min (Fig. 9(C)) there has been extensive growth of only a few grains. T h e large grain a
Fig. 9. Development of anisotropic grains in 0.4wt.%TiO2doped alumina at 1500 °C after (A) 5 min, (B) 7 min and (C) 9 min.
has developed into an anisotropic grain and consumed a number of finer grains. In fact it has consumed several anisotropic grains that were several times larger than the average matrix grain size. Grain a increased from 5.5 to 8 ktm in length and from 1.5 to 2 / ~ m in width between 5 and 9 min. However, the b o u n d a r y border-
D.S. Horn, G.L. Messing
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Materials Science and Engineering A195 (1995) 169-178
ing grain b on the right has not changed during this period because it is straight and has too little driving force for movement relative to more curved boundaries. After 9 min no new anisotropic grains develop. The only notable change between 7 and 9 min is that the matrix grains doubled in size. The radial and thickening growth kinetics of anisotropic grains in the 0.15 wt.% T i O 2 sample are plotted in Fig. 10. Some of the anisotropic grains undergo very rapid growth, or growth 'spurts', over a short period of time. These kinetics are different from Figs. 3, 6 and 7 in that the data were obtained for the s a m e grains at each time. As a result these data are more sensitive to the initial growth of anisotropic grains. It is evident that the average size measurements are not as sensitive to the details of the growth process. The rapid growth seen at the beginning of anisotropic grain growth has been discussed in the literature and is more accurately termed stepped grain growth [21,22]. The above results suggest that the ultrafine grain size of the alumina is responsible for the growth of anisotropic grains. To determine whether these results could be duplicated with a coarser alumina, a high purity commercial alumina was doped with T i O 2 and sintered to full density. The initial grains were equiaxed and had an average grain size of about 0.8/~m. Samples containing 0.15 and 0.4 wt.% TiO2 were heated at 1500 °C for up to 70 h. The samples underwent normal grain growth and showed no evidence of anisotropic grain growth. These results are similar to those of Hamano et al. [9] who only reported equiaxed grains in T i O zdoped alumina samples heated for 2 h at 1700 °C.
4. Discussion
Grain growth in tx-AI203 has been the subject of active research for many years. Until recently, most aluminas contained high impurity concentrations, were
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Fig. 10. Radial and thickeninggrowth kinetics in 0.15wt.%TiO2doped alumina at 1500 °C.
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not fully dense and had greater than 1/zm grain diameter. Consequently, the impurities combined with silica and formed liquid phases which have been shown to result in anisotropic grain growth. In most grain growth studies, the determination of grain growth parameters such as the grain growth exponent, mobility and kinetics has been compromised by liquid phases and/ or porosity and their effect on grain boundary motion. While much has been learned from these earlier studies, only the more recent literature in which the growth of single-crystal facets into a dense, polycrystalline matrix [19,20] and when impurities were carefully controlled and measured [19,23,27] are relevant to this study. The alkoxide-derived alumina contained 80 ppm MgO, 20 ppm SiO2 and 50 ppm CaO. Sodium, iron and other cations were not detected. The commercial alumina powder used for the grain growth study contained 10 ppm MgO, 20 ppm SiO2, 2 ppm CaO and similarly small concentrations of TiO 2, Fe203, ZnO and CuO. Sodium was not detected. The impurity analyses are critical for ascertaining whether a liquid grain boundary phase can form during sintering and subsequent grain growth. For example, some researchers [ 10,19,23] suggest that growth of anisotropic grains is not possible without a liquid grain boundary phase. Silica, sodium oxide and calcia are the most commonly observed components of grain boundary films. In the above case, no sodium was detected in either the boehmite or commercial alumina. Some grain growth experiments were repeated by using boehrnite derived from a higher purity aluminum isopropoxide. The MgO and CaO concentrations were the same but the SiO2 content was reduced to 5 ppm. There were no differences noted in the grain growth studies with this higher purity boehmite. Note that the SiO2 content of the higher purity boehmite sample is lower than that of the commercial high purity alumina used in this study. Anisotropic grain growth in the higher purity sample but not in the commercial alumina samples suggests that S i O 2 is not critical for anisotropic grain growth in TiOz-doped alumina. In addition to the above analysis, a series of transmission electron microscopy (TEM) and electron microprobe experiments were performed on the grain growth samples to determine whether a grain boundary phase was responsible for anisotropic grain growth. Energy-dispersive spectroscopy (EDS) during scanning electron microscopy (SEM) and TEM, and wavelength-dispersive spectroscopy in a microprobe, detected only aluminum and titanium. In an effort to enrich the impurity level on the grain boundaries like Handwerker et al. [19], samples were annealed for 24 h at 1400 °C. No second phase formed nor were impurities detected by EDS.
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Grain boundaries were also observed by T E M under diffuse dark field conditions. No grain boundaries with an obvious amorphous film were observed. It should be noted that the search and characterization of nanometer-scale boundary films is better done by high resolution electron microscope, but during the period of this study one was not available. A few TiO2 precipitates of about 100 nm diameter were observed on grain boundaries and at triple points, but in some samples no TiO 2 precipitates were observed. The number of precipitate particles was too low to affect boundary motion. Also, the precipitates did not appear to be associated with any specific boundaries. In contrast with Hamano et al. [9], no AI2TiO 5 precipitates were observed. As shown above, TiO2 enhances diffusion in alumina which agrees with Hamano et al. [9]. At 1500 °C boundary mobilities of anisotropic grains in 0.15 wt.% TiO2-doped alumina are comparable to boundary mobilities in pure alumina at 1650°C. However, as seen in Fig. 11, the scale of the micro-
structures is approximately the same but the TiO 2doped sample has distinctly anisotropic grains whereas the undoped sample consists of more irregular-shaped grains. Apparently, other factors besides enhanced diffusion cause anisotropic grain growth in TiO2doped alumina. The above analyses are inconclusive about the location of the TiO 2 and its specific role during anisotropic grain growth. The solubility of TiO 2 in alumina has been reported to be 0.27 wt.% above 1150°C [9]. However, it has been reported that Ti 4+ has a strong tendency to segregate to the grain boundary with a segregation coefficient of about 120-600 in 0.06 wt.% TiOz-doped alumina at 1300 °C [25]. Based on their sintering studies, Bagley et al. reported that TiO2 solubility in A120 3 was grain size dependent with a greater TiO 2 solubility in finer particles [8]. Interestingly, they also reported a sintering mechanism change between 0.5 and 1.0/zm which is the same grain size range at which anisotropic grain growth was observed to initiate. Kroger presented a model to explain the grain size dependence of grain boundary segregation of TiOz [26]. Li and Kingery detected significant TiO 2 segregation in doped alumina [27].
5. Anisotropic grain growth
Fig. 11. Comparison of microstructures having the same grain boundary mobility: (A) undoped alumina at 1650 °C for 16 h; (B) 0.4wt.%TiO2-doped alumina at 1500 °C for 12 h.
It is evident that anisotropic grain growth in TiO 2doped alumina depends on a number of different factors. For example, the presence of a silica-based liquid does not appear to be essential for anisotropic grain growth. However, CaO may be important as it was present in significant concentrations in the boehmite-derived samples but not in the commercial samples. Bae and Baik [24] recently showed that the critical CaO concentration for exaggerated grain growth in high purity alumina was only 30 ppm when no silica was present and the sample was heated at 1900 °C. While the role of impurities is still unclear, it is evident that anisotropic grains develop in only a narrow range of TiO 2 concentrations. The initial microstructure appears to be a critical factor for anisotropic grain growth. Contrary to earlier reports, anisotropic grain growth is not necessarily associated with the removal of the last few per cent of porosity or the attainment of full density [6,10]. For example, rapid growth of large anisotropic grains was not observed until after an incubation period. Instead, nucleation of anisotropic grains was observed to occur once a small population of grains had reached a critical size while at the same time the remaining grains did not significantly coarsen. The importance of a critical TiO2 concentration for anisotropic grain growth has been clearly demon-
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strated. The undoped samples do not display anisotropic grain growth at any of the times and temperatures investigated. Above 0.6 wt.% TiO2, grains grow in an abnormal manner. Between these two limits, anisotropic grains were observed. Again, this result demonstrates that enhanced diffusion alone is not responsible for grain growth since rapid diffusion leads to abnormal growth in the 0.6 wt.% TiO2 sample. Anisotropic grain growth in a polycrystalline matrix is analogous to the growth of a single crystal. Similar to the growth of single crystals, the growth of shaped grains in a polycrystalline matrix requires slower diffusion and a sufficient driving force for material transport. In the above system, slower growth at low temperatures limits the number of grains that grow to the critical size for anisotropic growth, and also limits matrix coarsening, thus preserving the driving force for the growth of anisotropic grains. T i O 2 appears to segregate to the grain boundary where it may act to poison growth on specific surfaces (e.g. basal growth), adsorb on specific surfaces to change the relative magnitude of the surface energies, or change the local diffusional growth processes. It is evident from these studies that a balance of the above processes is required to tailor the microstructure in a desired manner. As initially stated in the Introduction, this work was motivated by an interest in improving the fracture behavior of alumina. Fig. 12 shows the fracture surface of a TiO2-doped sample heated to 1400 °C for 1 h. It is obvious that the crack path is significantly altered relative to equiaxed, fine-grained alumina. As determined by the indentation method, the fracture toughness of this sample is 5.2 MPa m 1/2 vs. a fracture toughness of 3.2 MPa m l/2 for an ultrafine, equiaxed alumina. This preliminary result suggests that aniso-
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tropic grain growth may be an effective manner for obtaining tougher ceramics.
6. Summarizing remarks Three types of grain growth are observed in T i O 2alumina. Alumina undergoes normal grain growth at low TiO 2 concentrations which changes to anisotropic grain growth after an incubation period. During the incubation period, a few grains grow to a critical size and then grow rapidly into the fine-grainsize matrix. Subsequent impingement of the large anisotropic grains and the loss of grain boundary area essentially stop further growth. At high TiO2 concentrations only abnormal grains form. Obviously, grain growth is a result of a variety of factors. In the past, many studies focused on one or two factors that were perceived as significant to grain growth in alumina. Unfortunately, the samples were porous or contained unwanted impurities. This study has demonstrated that a silica-based liquid is not required for anisotropic grain growth. However, the possible importance of calcia is intriguing and will be further examined. While T i O 2 enhances the rate of diffusion, it is evident that it is not sufficient to cause anisotropic grain growth. We can conclude from this study that some of the requirements for anisotropic grain growth in polycrystalline materials include (1) a fully dense matrix to avoid boundary-pore interactions, (2) an ultrafine matrix to provide sufficient driving force for anisotropic growth, (3) transport regulation to initiate anisotropic grain growth but limit matrix coarsening, (4) dopants to modify the growth process, and (5) preferably a material with non-isometric crystal structure. As shown in a computer simulation of anisotropic grain growth, a material with a large difference in interfacial energy as a function of crystallographic direction also facilitates anisotropic grain growth [28]. Clearly, more research is needed to establish the relative importance of these factors and what other oxide ceramics satisfy these requirements. Nevertheless, it is evident that the materials are now of suitable quality to permit a more exacting investigation of these phenomena and to tailor materials with desirable microstructures. doped
Acknowledgments
Fig. 12. Fracture surface of TiO2-doped alumina after 1 h at 1400 °C.
The authors gratefully acknowledge the support of the US Office of Naval Research under Grant N00014-94-1-0007.
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