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PREPARATION OF DENSE TETRAGONAL ZIRCONIA CERAMICS FROM ZrO2
MICROPOWDERS
A. Smith, B. Cales, J. Baumard
To cite this version:
A. Smith, B. Cales, J. Baumard. PREPARATION OF DENSE TETRAGONAL ZIRCONIA CE- RAMICS FROM ZrO2 MICROPOWDERS. Journal de Physique Colloques, 1986, 47 (C1), pp.C1- 237-C1-241. �10.1051/jphyscol:1986135�. �jpa-00225564�
JOURNAL DE PHYSIQUE
Colloque Cl, suppl6ment au n02, Tome 47, fhvrier 1986 page c l - 2 3 7
PREPARATION OF DENSE TETRAGONAL ZIRCONIA CERAMICS FROM ZrOz MICROPOWDERS
A. SMITH, B. CALES and J.F. BAUMARD
Centre de Recherche sur la Physique des Hautes Temperatures, C.N.R.S., F-45045 Orleans Cedex, France
F.dsumd - Cette dtude se rapporte 2 lf61aboration de c6ramiques denses de zir- -- cone quadratique. Le comportement de trois poudres de zircone dopde par 3% en moles d'oxyde d'yttrium a dt6 analys6. Leur aptitude au frittage a QtS reli6e B la nature des prdcurseurs utilisds et B la morphologie du mat6riau avant et au cours de la densification. Cette dernibre est maxirnale pour des tempgratures d'environ 1500°C et sfaccompagne de retraits importants.
Abstract - The elaboration of dense TZP ceramics has been studied. The beha- viour of three zirconia powders doped with 3 mo1.Z of yttrium oxide has been investigated. The sintering mechanisms have been related to the nature of the precursors and to the morphology of the material before and during heating.
The maximum densification is achieved in the vicinity of 1500°C.
I - INTRODUCTION
In the last few years, many investigations have been devoted to the processing of ceramics for structural applications. Tetragonal zirconia is one of the most promis- sing candidate in this domain as reported during the two last conferenceson the IrScience and Technology of Zirconia" /1,2/. Achievement of the best mechanical proper- tie's requires the production of dense materials, while keeping the grain size to a small value 13-5/. Thus, fine zirconia powders with narrow size distribution must be used and the yttrium oxide content lies generally between 2 and 3 mol.% Y2O3 appro- ximately.
In the present work, the natural sintering of three zirconia powders prepared accor- ding to several routes has been investigated. The influence of the precursors on the microstructure of ceramic specimens during and after firing has been described.
I1 - POWDER SYNTHESIS
The first powder (A) has been obtained using a coprecipitation method from a solution of zirconium sulfate and yttrium nitrate. After a calcination at 1150°C, the solid was ball-milled in alcohol / 6 / . The f i ~ a l powder is characterized by a very small crys-
tallite size, of the order of 300 A, and by a large amount of monoclinic phase, about 70% instead of 18% before milling.
Another powder (B) also issues from a coprecipitation using chlorides or ox:~chlorides solutions 171. The process does not necessitate a ball-milling of the soft agglomera- tes. The X-ray diffraction pattern indicates a lower content of monoclinic phase, of the order of 38%.
The powder C has been produced after hydrolysis of a zirconyl nitrate solution / 6 / . The amorphous zirconia is washed, dispersed into an ~ttrium nitrate solution with an appropriate concentration and spray-dried. After calcination at 1000"C,Qthe powder is ball-milled in alcohol. The mean crystallite size is then about 200 A and the percentage of monoclinic phase increases to 75%.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986135
JOURNAL DE PHYSIQUE
111 - RESULTS AND DISCUSSION SINTERING
The shrinkage curves of compacted samples obtained from powders A,B and C are depic- ted in Fig. 1. They have been measured with the aid of a laboratory built high temperature dilatometer, described previously 161. Three different domains may be identified. In the range 20-100O0C, the three zirconia compacts exhibit a normal volume expansion. The thermal expansion coefficients of samples A and B are close to
10.10-~ "c-1 and are in good agreement with those reported for tetragonal and mono- clinic zirconia 181.
f Shrinkage (%l
Fig. 1 - Dimensional change of sample A,B and C during firing (3.4 "Clmn).
Fig. 2 - The variation of density versus temperature At temperatures beyond 1000°C about, fast and important shrinkages are observed for all the three samples. The variations of density for samples A,B and C, fired for 4 hours under usual isothermal conditions, are reported versus temperature in Fig. 2.
A maximum density, larger than 95% it^
of the theoretical density estima- ted to 6.1, is reached by 1500°C for samples A and C. In the case
of sample B, a value of 6.07 (99.5% of theoretical density) --- -0--- Theoretical density 6.1 is obtained in the vicinity of
1400°C. Densified ceramics A,B and C are fully stabilized under tetra- gonal symmetry. Mercury porosimetric measurements reveal that the pore size distribution is narrow.
Both the mean pore size and the pore volume decrease during densi- 5
fication as shown in Fig. 3 for a ceramic of zirconia A.
For firing temperatures higher than 1500°C, the density of zirconia
ceramics B and C keeps a constant Sample A
value while an expansion of sample
A is observed (Fig. 1). The latter 5 I I I
-
is associated with a fast decrease 1400 1m 1600 T ('c)
of density down to only 5.24 for
samples sintered at 1600°C (Fig. 2). In the same temperature range, thermogravimetry and mass spectrometry reveal a noticeable exhaust of gaseous species from the powder arising from the decomposition of residual sulfates which did not decompose during the calcination process.
-
m 0.02-m' - Sample A -
-
Pore size (pm)
Fig. 3 - The variation of pore volume versus pore size for various temperatures From the weigh loss of powder A between 1400 and 1600°C ( 0.38 wt.%), the residual pressure in the ceramic at 1600°C has been estimated on assuming that the gases are constrained, as S02 species, in the closed pore volume deduced from the density mea- surements. Such a pressure reaches the large value of 30 MPa. It should explain the formation of a new porosity at high temperature, owing to the increasing plasticity of the ceramic, (Fig. 3) and the swellings observed by microscopy (Fig. 4).
Fig. 4 - Microstructure of sample A fired at 1600 'C.
MICROSTRUCTURE
Micrographic investigations have been performed on polished and thermally etched samoles. Fie. 5 shows the microstructure of zirconia C fired at 1400°C. The morpho- logy of samples A and B is quite similar. The variation of the mean grain size for ceramics A,B and C versus temperature of sintering is reported in Fig. 6. The mean grain size does not exceed the average critical value reported by Lange 131 for the retention of the metastable tetragonal Zr02 phase at room temperature, i.e.wlp for a content of 3 mol.% Y2O3. Larger grains transform during cooling from the tetra- gonal to the monoclinic phase and can induce microcracking in the ceramic specimens.
This is probably one of the origins of the density decrease (Fig. 2) for the ceramics
J O U R N A L DE PHYSIQUE
heated at temperatures higher than 1 5 0 0 ° C and cooled to room temperature. A new porosity is observed and the equivalent pore size distribution is larger than before and during densification (Fig. 3).
Fig. 5 - Microstructure of sample C sintered at 1 4 0 0 ° C . In the case of zirconia A, some very
large grains (>> lpm), a few percent
in volume, are also observed up to 1 8 (pm)
1 5 0 0 ° C about (Fig. 7 ) . They are as-
sociated with higher local contents of yttrium oxide. Analysis by STEM
0.8- indicates that the large grains are formed of cubic zirconia. Beyond
1 5 0 O 0 C , the cubic large grains dis- 07-
appear. This would arise from the chemical diffusion of yttrium at
high temperature that induces the Q6-
nucleation of small tetragonal A
grains from cubic domains. X B
05 - 0 C
Fig. 6 - Thz variation of the mean I
grain size D for samples A,B and C 1300 14M) 1 ! x l T ("C) versus temperature.
Fig. 7 - Microstructure of sample A fired at 1 4 7 0 ° C
IV - CONCLUSION
The three zirconia powders used in this study are characterized by relatively low sintering temperatures (-1500°C). Dense TZP ceramics with small grain sizes (<lpm) have been obtained. For temperatures higher than 1500°c, the behaviour of the specimens is dependent on the nature of the precursors used for the zirconia powder synthesis and on the process itself. When the distribution of yttrium
oxide is heterogeneous, irregular grain growth can occur which induces microcracking during cooling the ceramic. Moreover, in the case of sulfate precursors, gas
desorption phenomena are observed up to 1600°C. They give rise to a rapid decrease of density at high temperature.
Acknowledgements
The authors are grateful to J.P. Torre and Y. Bigay from Desmarquet Cie. for their co-operation throughout the course of the present work.
REFERENCES
/l/ "Science and Technology of Zirconia", Ed. Heuer, A. H. and Hobbs, L. W., The American Ceramic Society, Columbus, USA (1981).
/2/ "Science and Technology of Zirconia II", Ed. Claussen, N., Riihle, M. and Heuer, A. H., The American Ceramic Society. Columbus, USA (1984).
/ 3 / Lange, F.F.,J. Mat. Sci. 17 (1982) 225.
141 Evans, A. G. and Heuer, A ~ H . , J. Am. Ceram. Soc.,= (1980) 241.
151 Evans, A. G., Burlingame, N., Drory, M. and Kriven, W. N., Act. Met. 2 (1981)
447.
161 Smith, A., Ph. D. Thesis, University of OrlSans, France (1984).
/7/ Tagaki, H., Sano, S. and Ishii, E., Ber. Dt. Keram. Ges. 51 (1974) 234.
/8/ Patil, R. N. and Subbarao, E. C., J. Appl. Cryst. 2 (196% 281.
