Aging effects on the characteristics and sintering behavior of coprecipitated Al2O3ZrO2 powders

Ceramics International 211(1994) 379-384

Aging Effects on the Characteristics and Sintering Behavior of Coprecipitated AI203-ZrOa Powders Chih-Cheng Chen, Fu-Su Yen & Chi-Yuen Huang Department of Mineral and Petroleum Engineering, National Cheng Kung University, Tainan, 70101, Taiwan, R.O.C. (Received 9 December 1993; accepted 4 January 1994)

Abstract: Effects of gel aging on the characteristics of zirconia-toughened alumina (ZTA) composites were investigated. The composite powders were prepared by coprecipitation processes using a mixed solution of aluminum nitrate and zirconium oxynitrate. The coprecipitates were aged in an aqueous solution of pH 9 at room temperature. The occurrence of either intra- or intergranular ZrO2 particles in the AlzO3 matrix depended on the aging condition, which also affected the content of tetragonal ZrO2 (t-ZrOz). Aging treatment decreased the amount of intragranular ZrOz, as well as the agglomerate strength of calcined powders, both of which seemed to improve the sintered densities of ZTA ceramic bodies. Through suitable aging processes, sintered ZTA specimens with relative density >99% and intergranular ZrO2 particles were fabricated.

1 INTRODUCTION

synthesis, lz sol-gel processing, 4,13 15 and coprecipitared process =6 were used. Most of these fine powders exhibit relatively poor sinterable properties, due to an inadequate control of the agglomeration. ~7 Therefore, deagglomeration o f these powders was investigated to improve sinterability. These methods include the usage o f organic solvents as washing liquids,~7 19 grinding,ZO-ZZ spray-drying, 23 and freeze-drying, z4 Generally, hot pressing techniques were followed to m a k e the ceramic bodies. Furthermore, the ZrOz grain size o f Z T A ceramics using ultrafine powders were generally smaller than the suitable size o f phase transformation toughening, therefore, high ternperature annealing was employed to help the ZrO2 grains grow larger z5-26. Two kinds of ZrO2 particles exist in Z T A ceramics and can be roughly cataloged as: (1) faceted intergranular ZrO2 particles located between AI20 3 grains and (2) spherical intragranular ZrO 2 particles, which occur within AIzO 3 grains. Depending on the fabrication methods, Z r O 2 particles in the A1203 matrix can be either intraor intergranular. The intragranular ZrOz particles

An alumina-zirconia composite can be a structural ceramic with excellent mechanical properties, Several studies have been carried out to investigate this composite material. 1-3 Toughening o f zirconia-toughened alumina (ZTA) has been attributed to stress-induced transformation or microcrack nucleation. 3 Maintaining zirconia in tetragonal phase (t-ZrOz) is essential for stressinduced transformation toughening, whereas a uniform zirconia particle distribution is important for optimizing microcrack nucleation-induced toughening. 3 The dispersed ZrO2 particles must be smaller than a critical size to be retained in tetragonal phase. 4 This requires certain special techniques, especially when a uniform distribution o f ZrO2 particles in AIzO3 matrix is also desired, To meet these requirements, several processing approaches have been investigated. Methods including attrition milling, 3 CVD synthesis, 5 spray-ICP, 6 colloidal processing, 7 hydrothermal reaction, 8 evaporative decomposition, 9 hydrolysis coating, 1° polymer/powder flocculation, 11 plasma 379

Ceramics International 0272-8842/94/$7.00 © 1994 ElsevierScienceLimited, England and Techna S.r.l. Printed in Great Britain

Chih-Cheng Chen, Fu-Su Yen, Chi-Yuen Huang

380 often occur if the powder is prepared by wet chemical processes, such as sol-gel techniques.4 The size of the intergranular particles is generally larger than that of the intragranular particles, because of the faster coarsening kinetics associated with the grain boundaries, and has a well-defined critical size of t---)m phase transformation. 27 Intragranular particles are more resistant to t---)m phase transformation 4 and also weaken the mechanical properties of ZTA ceramics28. In this paper, A12Oa-ZrO2 powders were prepared by coprecipitation methods. Deagglomeration was investigated by using gel aging and alcohol washing. Emphasis was placed on the effect of aging on the characteristics of the gel powders, The occurrence of intra- or intergranular ZrOz particles of the ZTA ceramics was also examined. 2 EXPERIMENTAL

Aqueous solutions of Al(NO3)3.9H20 (Ishizu Pharmaceutical Co. Ltd., Japan) and ZrO(NO3)2.2H20 (Ishizu Pharmaceutical Co. Ltd., Japan) were used as the starting materials. The precipitates were obtained by adding NHaOH solution into the mixed aqueous solution to a pH value of 9 under the conditions of 25°C and continuous stirring. The A1203-ZrO 2 powders were prepared at a ratio of 85 A1203/15 ZrO 2 vol%. The gels were then aged at room temperature in the same containers for 0.5, 6, 12, 24 and 40 h, separately (sample A, A6, A12, A24, A40). After aging they were washed twice with distilled water and ethyl alcohol (C2H5OH) (Seoul Chemical Industry Co. Ltd., Korea), and were filtered and dried at 80°C for 48 h. All sampies were calcined at 900°C for 1 h and the calcined powders were ground to less than 200 mesh size. Green compacts were prepared by a cylindrical die, 1-2 cm in diameter, under a 500 MPa pressure, and were sintered at 1550°C (heating rate 10°C/min)for4 h. The bulk density of the sintered bodies was determined by Archimedes' method, and the relative densities were calculated with respect to the theoretical densities of A120 3= 3.987 g/cm3, t-ZrO2=6-100 g/cm3 and m-ZrO2=5.560 g/cm 3. Mineral phase analysis was performed by XRD (Rigaku, D/MAX liB), using CuK,, radiation. The ratio of tetragonal to monoclinic ZrO 2 was determined by XRD using the integrated peak intensities of tetragonal (111) and the monoclinic (111) and (11 ~).29 The strength of agglomerates was determined by Pj (joining pressure or break point pressure). 17 The value of Pj is a measure for determining the compressive strength of agglomerates. The microstructure was observed using scanning

electron microscopy (SEM) (Joel JSM-35F) on polished and thermally etched surfaces of the samples. Average grain size was measured by the linear intercept method 3° from about 200 grains for each sample. 3 RESULTS

AND

DISCUSSION

3.1. Phase analysis To understand the sequence of phase development affected by the duration of aging of coprecipitated gels, a series of XRD data of the aged gels was taken (Table 1). The XRD pattern of the dried gels without aging showed the presence of pseudoboehmite (4AIO(OH).H20) and amorphous ZrO2. After the gels were aged in pH 9 solution at room temperature and dried, pseudoboehmite gradually transformed to bayerite (AI(OH)3) of high (OH) content, which was similar to the situation reported previously--that pseudoboehmite transforms to bayerite under alkali environment.3~ After calcining at 900°C for 1 h, the XRD patterns showed that pseudoboehmite and bayerite were transformed into 8-A1203 and 0-A1203, and amorphous ZrO2 was transformed into t-ZrO2 (Table 1). If the calcination temperature increased, phase transformation of 8-A1203 ---) 0-A1203 ---) t~A1203 took place. 31 It was found that either 8A1203or 0-A1203 in green compacts transformed to ot-Al203 when heated at 1550°C for 4 h. It has been pointed out that the 0 ~ a-A1203 phase transformation followed the nucleation and growth process. 32 During the grain growing process, a considerable amount of fine pores that existed in the alumina matrix were redistributed. Some pores were observed to be trapped within the fast growing a-A1203 grains. 32 ZrO2 particles in A1203 matrix could behave like the pores. During the fast growth of a-A1203 grains from finegrained matrix, the ZrO2 particles could be trapped within the ot-A1203 grains. This analogy can be explained by the microstructures of ZTA sintered bodies (Fig. 1)in this study. Table 1. Phase changes st varied aging conditions Aging time (hours)

Dried gel (80°C, 24h)

Calcined

Sintered

powders

bodies

0.5 6 12 24

P, A P ,(B), A P, B, A P, B, A P, B, A

(900oc, lh) (,~), (8), t (~), (0), t & 0, t (8), 0, t (8), 0, t

(1550°C,4h) a, t, (m) a , t , (m) a, t , m a, t, m a, (t), m

40

P: Pseudoboehmite

B: Bayerite A: A m o r p h o u s

~ : ~ A I20 3

( ) : Weak

t:t-ZrO 2

m:m-ZrO 2

0 : 0-AI20 a

a : (~-AI20 a

381

Sintering behavior of A1203-Zr02 powders 600 0 Gel powder

"-.500' \ "-'400 a~

~, 300

i

~ ]

200

~

tO0

t

I i • ~(E I " ~ t • ~', ~ tI~ t '% " "*p , *" ' ~ j [ ~ a ~ ~, , : ~ *' " ~ . ~ m ,~ .),~, ~ ~ . [ "~k '~ ~" a'?j '~li:lll~r~ I1~ Fig. 1. Scanning electron micrograph ofA1203-15 vol% ZrO2 ceramics, taken with secondary electron image, so that light areas correspond to ZrO2 particles. Five samples were prepared with varied aging time: (a) sample A, 0.5 h. (b) sample A6, 6 h. (c) sample A12, 12 h. (d) sample A24, 24 h (e) sample A40, 40 h. (bar -- 1 /zm).

3.2 Intra- and intergranular ZrO 2 The polished and thermally etched surface of the specimens sintered at 1550°C for 4 h (Fig. 1) showed that, ZrO 2 particles were present as either intergranules or intragranules in the A1203 matrix when the coprecipitated gels were aged for different lengths of time (Fig. 1). Sample A (0.5 h aging) had a limited amount of intergranular ZrO2, and most Zr02 particles were intragranular (Fig. l a). As the time of aging became longer, the number of intragranular ZrO 2 particles decreased, and when the aging time was longer than 12 h, ZrO 2 particles were almost all intergranular (Figs l c-e). This result can be explained by the surface area of calcined powders of varying aging time (Fig. 2). With increasing aging time the surface area of calcined powders became less. At the same time, the surface energy of calcined powders decreased, which caused the a-Al203 grains to grow slowly. As a result, ZrO2 particles could not be easily trapped within a-A120 3 grains, and ZrO2

,01 0

i

l £0

t

I 20

l

I 30

i

I 40

Aging Time (hours) Fig. 2. Plots of surface area vs aging time for gel powders and calcined powders. particles were almost intergranular for aging times above 12 h. Figures l a and l b also showed that samples A and A6, prepared with shorter aging time, had both intergranular and intragranular ZrO 2 particles, and the former were larger in size. Previous study 27 has revealed that the intergranular particles were generally larger than the intragranules, because of the faster coarsening kinetics associated with the grain boundaries. The intragranular ZrO 2 particles coarsen at a slower rate for a high temperature sintering process, because bulk diffusion is so slow in this system. 27 This previous study could explain why the intragranular ZrOz particles were smaller than intergranules, and why the intragranular ZrO 2 particles were almost smaller than 0-2/zm in this study (Figs la and lb).

3.3 The critical size of t-~m-ZrO 2 phase transformation Figure 3 shows that, with increasing aging time, t-ZrO2 content of the sintered bodies gradually decreased and a 2-stage declining phenomenon was shown. In the first stage, where the aging time was less than 6 h, the t-ZrO2 contents present in the ~00

~

s~,,o~a boa,~ c~55o.c,4h,)

~ao o~ 80 ~ 40 L zo °b

' 1'o ' z'0 ' a'0 ' 4'0 Aging Time (hours) Fig. 3. t-ZrO 2 contents of A1203-15 vol% ZrO2 ceramics for varied aging time.

382

Chih-Cheng Chen, Fu-Su Yen, Chi-Yuen Huang

loo ,., a0

It is clear that aging treatment caused Z r O 2 grains to grow (Fig. 4). When the size of ZrO 2 grains was larger than the critical size of ~0.4/zm (Fig. 5), they transformed into m-ZrO2, which decreased t-ZrO2 content (Fig. 3).

~o

3.4. Sintering behavior

S,,,teTe~ bo~*s (1550"C. 4hT)

~, 4 0

otq i 20 *~ 0.4 o.'e o.'a t.b Average ZrOe Grain Size (jam) Fig. 4. Average ZrO2 grain sizes in A1203-15 rot% ZrO2 ceramics with varyingaging time. I

o

I

I

o.z

I

I

I

I

samples were more than 80%. In the stage of aging time above 12 h, the t-ZrO 2 content of the sintered bodies became less than 50% (Fig. 3). This result can be explained by the average Z r O 2 grain size of ZTA sintered bodies (Figs 4 and 5). Figure 4 shows that, with increasing aging time, the average ZrO2 grain size of ZTA sintered bodies gradually increased. It is apparent that some ZrO2 grains grew larger than the critical size of t--)m-ZrO2 phase transformation under longer aging time. To determine the critical size above which tZrO2 content decreased rapidly, the average ZrO2 grain sizes were measured by the linear intercept method 3° from a series of micrographs of samples at various aging time (Fig. 5). Figure 5 shows that the critical size of t---~m-ZrO2 phase transformation for pure Z r O 2 is ---0.4 /xm. Above this size, the t-ZrO2 content drastically decreased• This size is similar to the value reported by Messing ~5which was derived from sol-gel processing, but below the values of 0.5-0.8/xm reported by others. 1'4,33-34 This result may be due to differences in grain size measurement techniques or processing methods,

Figure 6 shows that relative densities of ZTA sintered specimens increased with aging time, and the aging treatment decreased agglomerate strength of calcined powders. The relative densities of sintered samples were above 98% when the aging time was longer than 12 h. Sample A24 had the highest relative density of 99.5%. When the aging time increased, the agglomerate strength decreased from 60 MPa to ~30 MPa. The agglomerates of all samples could be fractured by the forming pressure (500 MPa), therefore, the green densities of all samples were almost the same (Fig. 6). The agglomerate effects on the sintered densities of all samples seemed negligible under this condition. However, the aging treatment changed the surface area (Fig. 2), and the driving force during sintering was different. Consequently, the sintered densities were very different under various aging times• This result can be explained by the microstructure of sample A (aging time 0.5 h) (Fig. la), which shows some pores being trapped within the A1203 grains. The reason is that the calcined powders of sample A were very active with a high surface area (Fig. 2) and the fast growth of a - A I 2 0 3 grains trapped the pores within the grains• The intragranular pores were difficult to remove by sintering, which caused sample A to have a low relative density. On the other hand, aging treatment could prevent the fast growth of a-A1203 grains, the intragranular pores of sintered bodies almost disappeared when the aging time was above 12 h, and the relative densities became larger.

-190

N 0.8

~

~

z~,4gg~,,,,,-~, s~-.,,#uL • c~e~ ~

~ 80 •

80

El

"tO

.,-4

;

/

° ,o

oo . 40

~

o

.

2

--'--7, ...... ~: 4o ~.4.----= =

'$

~

300

0

I

I

t0

I

I

20

t

I

30

I

I

40

Aging Time (hours) Fig. 5. Relative t-ZrO2 contents retained as a function of

zirconia grain size in 15 vol% ZTA.

'

x'o

'

fo

'

3'0

~ '

"n

zo

Io 4'0

~: ,-o

o

Aging Time ~lzours) Fig. 6. The effects of the aging time on the agglomerate strength, green densities and sintered densities of 15 vol%

ZTA.

383

Sintering behavior o f AleO3-ZrO2 powders

z.0

well as the agglomerate strength of calcined

,

powders, both of which seem to increase the

"-" ~ .~ l.s o~ •,~ t.0 ~'

sintered densities of ZTA ceramics. Through a suitable aging process, the Z T A sintered specimen containing intergranular ZrO 2 particles and with relative density >99% could be achieved under the condition of 900°C/lh calcination and 1550°C/4h sintering. It is clear that coprecipitation methods via aging treatments offer significant advantages for the processing of Z T A ceramics. A major process advantage for ZTA production is that one can engineer the ZrO2 grain size and t/m ratios of ZTA ceramics rather than resort to high cost hot pressing and annealing treatments.

s~,,t,,ed

0.5

bo~

(,sso'c. 4hr)

¢~ ,~ °o

'

t'0 ' z'0 ' a'0 ' 4'0 Aging Time (hours) Fig. 7. Average A1203 grain sizes in AlzO3-15 vol% ZrO2 ceramics with varying aging t i m e . The average A120 3 grain size of sample A was 1.87 /zm when the aging time was 0.5 h (Fig. 7). When the aging time increased, the average Al203 grain sizes gradually decreased until aging time was above 12 h. The average AI20 3 grain sizes were about 1.2 /xm. With respect to the aging time, the tendency of the average A1203 grain sizes (Fig. 7) was similar to the tendency of surface area (Fig. 2). When the calcined powders had a high surface area, the t~-A1203 grains grew quickly, because the high surface area increased the sintered driving force during sintering. For example, the calcined powders of sample A were active powders with high surface area, which caused the a-AlaO 3 grains to grow rapidly. Consequently, the AI20 3 grain size of sample A was larger than other samples. This result supported the interpretation of the occurrence of intragranular ZrO 2 particles and pores, which were trapped within the fast growth of t~-Al20 3 grains. 4 CONCLUSIONS Aging of a coprecipitated gel had a significant effect on the characteristics of Z T A composites. The Z T A coprecipitated powders had a high surface area, which caused the ot-Al20 3 grains to grow rapidly. The rapid growth of cz-Al203 grains trapped pores and ZrO 2 particles. Aging treatment decreased the surface area of Z T A coprecipitated powders, and diminished the intragrangular ZrO2 particles. When the aging time was above 12 h, Z T A composites could prevent intragranular ZrO2 particles, which often resulted from a wet chemical process. The ZrO2 grain size and t-ZrO 2 content could be controlled by aging time. The critical size of t--->m-ZrO2 phase transformation was about 0-4 /xm in this study. Aging treatment can also decrease the a m o u n t of intragranular pores, as

ACKNOWLEDGEMENT The a'uthors would like to express their thanks to the National Science Council of the Republic of China for supporting this project (NSC 82-0405E006-061) and Prof. H. S. Liu for his valuable discussion.

REFERENCES 1. CLAUSSEN, N., Fracture toughness of A1203 with an unstabilized ZrO2 dispersed phase. J. Am. Ceram. Soc., 59(1-2) (1976) 451. 2. CLAUSSEN, N., STEEB, J. & PABST, R. F., Effect on induced microcrack on the fracture toughness of ceramics. Bull. Am Ceram. Soc., 56(6) (1972) 559-62. 3. CLAUSSEN, N. & RI]HLE, M., Design of transformation-toughened ceramics. In Advances in Ceramics, Vol. 3, Science and Technology of Zirconia, ed. A. H. Heuer & L.W. Hobbs. The American Ceramic Society, Columbus, OH, 1981, pp. 137-63. 4. HEUER, A. H., CLAUSSEN, N., KRIVEN, W. M. & ROHLE, M., Stability of tetragonal ZrO2 particles in ceramic matrices. J. Am. Ceram. Soc., 65(12) (1982) 643-50. 5. HORI, S., YOSHIMURA, M. & SOMIYA, S., A1203-ZrO2 ceramics prepared from CVD powders. In Advances in Ceramics, Vol. 12, ed. N. Claussen, A. H. Heuer & M. Rtihle. The American Ceramic Society, Columbus, OH, 1984, pp. 794-805. 6. KAGAWA, M., KIKUCHI, M., SYONO, Y. & NAGAE, T., Stability of ultrafine tetragonal ZrO2 coprecipiated with AI203 by the spray-ICP technique. J. Am. Ceram. Soc.,66(11)(1983)751-54.

7. AKSAY, I. A., LANGE, F. F. & DAVIS, B. I., Uniformity of AI203-ZrO2 composites by colloidal filtration. J. Am. Ceram. Soc., 66(10) (1983) C190-C92. 8. YOSHIMURA, M., KIKUGAWA, S. & SOMIYA, S., Alumina-zirconia fine powders prepared by hydrothermal oxidation. Yogyo-Kyokai-Shi, 91(4)(1983)43-48. 9. SPROSON, D. W., & MESSING, G. L., Preparation of alumina-zirconia powders by evaporative decomposition of solutions. J. Am. Ceram. Soc., 67(5) (1984) C92-C93. 10. FEGLEY, B., JR., WHITE, P. & BOWEN, H. K., Preparation of zirconia-alumina powders by zirconium alkoxide hydrolysis. J. Am. Ceram. Soc., 68(2) (1985) c60-c62. 11. MOFFATT, W. C., WHITE, P. & BOWEN, H. K., Production of alumina-zirconia composite ceramics by nonsolvent flocculation of polymer-containing powder

384 dispersions. In Ceramic Powder Science II, ed. G . L . Messing, E. R. Fuller & H. Hausner, The American Ceramic Society, Columbus, OH, 1988, pp. 645-53. 12. KOH, S. C. H. K., AIK, K. & MCPHERSON, R., Sintering characteristics of A1203-ZrO2 plasma-synthesized powders. In Advances in Ceramics, 1Iol. 124A, Science and Technology of Zirconia III, ed. S. Srmiya, N. Yamamoto & H. Yanagida. The American Ceramic Society, Westerville, OH, 1986, pp. 293-300. 13. TSUKUMA, K., TAKAHATA, T. & SHIOMI, M., Strength and fracture toughness of Y-TZP, Ce-TZP, y-TZP/AI203, and Ce-TZP/A1203. ibid, 124B, (1988) 721-28. 14. BECHER, P. F., Transient thermal stress behavior in ZrO2-toughened A1203. J. Am. Ceram. Soc., 64(1) (1981) 37-39. 15. MESSING, G. L. & KUMAGAL, M., Low-temperature sintering of seeding sol-gel-derived, ZrO2-toughened A1203 composites, ibid, 72(1) (1989) 40~. 16. HASSELMAN, D. P. H. & SYED, R., The thermal diffusivity and conductivity of transformation-toughened solid solution of alumina and chromia. J. Mat. Sci., 20 (1985) 2549-56. 17. VAN DE GEAFF, M. A. C. G. & BURGGRAFF, A. J., Wet-chemical preparation of Zirconia powders: Their microstructure and behavior. In Design of Advanced Ceramics, VoL 12, Science and Technology of Zirconia II, ed. N. Claussen, M. R0hle & A. H. Heuer, The American Ceramic Society, Columbus, OH, 1983, pp. 744-65. 18. REIJNEN, P. & FIRATLI, A. A. C., Sintering and microstructure of multiphase ceramics. Interceram, 6(1984) 19-21. 19. PUGAR, E. A. & MORGAN, P. E. D., Coupled grain growth effects in A1203/10 vol% ZrO 2. J. Am. Ceram. Soc., 69(6)(1986)C120-C123. 20. NIESZ, D. E. & BNNETT, R. B.; Structure and properties of agglomerates. In Ceramic Processing Before Firing, ed. G. Y. Onoda, Jr. & L. L. Hench. Wiley, New York, 1978, pp. 61-73. 21. TREMPER, R. T. & GORDON, R. S., Agglomeration effects on the sintering of alumina powders prepared by autoclaving aluminum metal, ibid, 153-176. 22. JOHNSON, D. W., Jr., NITTI, D. J. & BERRIN, L., High purity reactive alumina powders: I I . particle size and agglomeration study. Bull. Aml Ceram. Soc., 67(2) (1984) 83-89.

Chih-Cheng Chen, Fu-Su Yen, ~ Chi-Yuen Huang 23. RESMUSSEN, M. D., AKING, M., MILIUS, D. & MCTAGGART, M. G., Effect of spray drying on the sintering of Y 2 0 3 . Bull. Am. Ceram. Soc., 64(2) (1985) 314-18. 24. XUE, L. A., RILEY, F. L. & BROOK, R. J., Tert-butyl alcohol a medium for freeze-drying: Application to barium titanate. Br. Ceram. Trans. J., 85(2)(1986)47-48. 25. PKADA, K., YOSHIZAWA, Y. I. & SAKUMA, T., Grain-size distribution in AI203-ZrO2 generated by hightemperature annealing. J. Am. Ceram. Soc.,74(11)(1991) 2820-23. 26. WlTEK, S. R. & BULTER, E. P., Zirconia particle coarsening and the effect of zirconia additions on the mechanical properties of certain commercial alumina, ibid, 69(7) (1982) 610-614. 27. KIBBEL, B. W. & HEUER, A. H., Ripening of interand intragranular ZrO2 particles in ZrO:-toughened AI203. InDesign of Advanced Ceramics, Vol. 12, Science and Technology of Zirconia II, ed. N. Claussen, M. ROhle & A. H. Heuer, The American Ceramic Society, Columbus, OH, 1983, pp. 415-24: 28. CHEN, C. C. & YEN, F. S., Mechanical properties of inter- and intragranular ZrO2 particles in ZrO2-toughened A1203 composites prepared by coprecipitated process. (Unpublished work). 29. GARVIE, R. C. & NICHOLSON, P. S., Phase analysis in zirconia system. J. Am. Ceram. Soc., 55(6) (1972) 303-305. 30. WURST, J. C. & NELSON, J. A., Linear intercept technique for measuring grain size in two-phase polycrystailine ceramics. J. Am. Ceram. Soc., 55(2) (1972) 109. 31. WEFERS, K. & BELL, G. M., Oxides and hydroxides of aluminum. Alcoa Research Lab., Technical Paper, No. 19, 1972. 32. BADKAR, P. A. & BAILEY, J. E., The mechanism of simultaneous sintering and phase transformation in alumina. J. Mat. Sci., 11(1976) 1794-1806. 33. LANGE, F. F. & GREEN, D. J., Effects of inclusion size on the retention of tetragonal ZrO2 : Theory and experiments. In Advances in Ceramics, Vol. 3, ed. A. H. Heuer & L. W. Hobbs. The American Ceramic Society, Columbus, OH, 1981, pp. 217-25. 34. GUPA, T. K., BECHTOLD, J. H., KUZNICHI, R. C., ADOFF, L. H. & ROSSING, B. R., Stabilization of tetragonal phase in polycrystalline zirconia. J. Mat. Sci., 12 (1977) 2421-26.