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TETRAGONAL ZrO2 - Y2O3 PART I : PREPARATION METHODS AND
CHARACTERIZATION
A. Winnubst, M.A.C.G. van de Graaf, K. Keizer, A. Burggraaf
To cite this version:
A. Winnubst, M.A.C.G. van de Graaf, K. Keizer, A. Burggraaf. TETRAGONAL ZrO2 - Y2O3 PART I : PREPARATION METHODS AND CHARACTERIZATION. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-569-C1-573. �10.1051/jphyscol:1986186�. �jpa-00225617�
JOURNAL DE PHYSIQUE
Colloque C1, suppl6ment au n 0 2 , Tome 47, f6vrier 1986 page c1-569
TETRAGONAL ZrO, - Y 2 0, PART I : PREPARATION METHODS AND CHARACTERIZATION
A . J . A . WINNUBST, M.A.C.G. VAN DE GRAAF, K. KEIZER and A. J . BURGGRAAF
m e n t e University of Technology, Department of Chemical
Engineering, Laboratory of Inorganic Chemistry, Materials Science and Catalysis, P.0. Box 217, NL-7500 AE Enschede, The Netherlands
Resume - Des echantil lons p o l y c r i s t a l l ins de zircone tetragonale (Y-TZP) sont
-- prepares par des methodes chimiques en voie humide. On obtient ainsi par f r i t t a g e r e a c t i f , des ceramiques avec des t a i l l e s de grainsaussi f a i blesque 0 , l pm. Le vieillissement dans l ' a i r e t dans l ' e a u a haute temperature a e t @
Ctudi& par d i f f r a c t i o n X. Les principaux facteurs du vieillissement sont l a teneur en yttrium, l e taux d'impurete e t l a t a i l l e des qrains. Les Y-TZP avec 6 , l moles % Y01,5 e t une t a i l l e de grains de 0 , l urn ne pr6sentent pas de de- gradation en phase monoclinique m6me aprss 5 heures a 450 K dans l ' e a u . Abstract - Y20 - c o n t a i n i n g t e t r a g o n a l Zr02 p o l y c r y s t a l s (Y-TZP) a r e prepared by wet chemica? methods. T h i s r e s u l t s i n s i n t e r r e a c t i v e ceramics with g r a i n s i z e s down t o 0.lpm. The aging behaviour i n a i r and water a t high temperatures has been s t u d i e d by X-ray d i f f r a c t i o n . Important f a c t o r s on aging a r e
Y-concentration, impurity l e v e l and g r a i n s i z e . Y-TZP with 6.1 mol% Y O l e 5 and a g r a i n s i z e of O.lpm do not show degradation t o monoclinic phase even a f t e r 5 hours a t 450 K i n water.
I - INTRODUCTION
I n t h e system (100-x)Zr02-xY01~5 s e v e r a l phases e x i s t a t low y t t r i a concentrations (0<x<15) / I / . A t room temperature t h e s e phase mixtures c o n s i s t of a monoclinic and a cubic f l u o r i t e (m+f) phase, while a t higher temperatures t e t r a g o n a l ( t ) and f phases a r e p r e s e n t ( 1 ) . According t o S c o t t / 1 / a monophasic t-phase r e g i o n e x i s t s a t Y O l a 5 c o n c e n t r a t i o n s lower than 5 a t % (O<x<5) and temperatures between 800 K and 2500 K.
T h i s high temperature t-phase can be llfrozenw t o room temperature when small ceramic g r a i n s (<0.3pm) a r e p r e s e n t / 2 / . Lange /3/ r e p o r t e d t h a t t h i s c r i t i c a l g r a i n s i z e (D ), which a r r e s t s t h e t + m transformation during cooling i n c r e a s e s from 0.2 t o 1 .Sum when x i n c r e a s e s from 4 t o 7 a t $ YO1 .5. This t e t r a g d n a l m a t e r i a l (afterwards c a l l e d Y-TZP) is r e l a t i v e l y tough due t o t h e m a r t e n s i t i c s t r e s s induced
transformation f rom t + m /2,3/.
A phase transformation t + m a l s o t a k e s place a f t e r aging a t e l e v a t e d temperatures /4,5,6,7/. This e f f e c t i s s t r i k i n g a t T=470-570 K i n a i r /4,5/ o r i n H20 a t T=400- 500 K /6,7/. The degradation depends on g r a i n s i z e and y t t r i a content of t h e ceramic m a t e r i a l . Here a l s o a c r i t i c a l g r a i n s i z e i s found which i n c r e a s e s from 0.2 t o 0.5 Llm
when x i n c r e a s e s from 4 t o 9.5 a t % /'I/. A m a t e r i a l aged f o r 2000 hours a t 480 K shows a decrease i n f l e x u r a l s t r e n g t h from 1000 t o 100 MPa i f compared with
s i n t e r e d m a t e r i a l s /5/. Tsukuma /8/ has found t h e h i g h e s t f r a c t u r e toughness value (KIC) f o r x=2 a t $ (KIC= 10 MPa dm). T h i s m a t e r i a l with a g r a i n s i z e of 0.4 pm,
d e s i n t e g r a t e s , however, t o powder a f t e r s e v e r a l thermal c y c l e t e s t s between room temperature and 1070 K.
I t i s obvious t h a t degradation of Y20 - s t a b i l i z e d t e t r a g o n a l z i r c o n i a (Y-TZP) e s p e c i a l l y t a k e s p l a c e a t r e l a t i v e l y ?arge ceramic g r a i n s i z e s (0.2-0.5pm). The
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986186
~ 1 - 5 7 0 JOURNAL DE PHYSIQUE
commercially a v a i l a b l e Y-TZP powders r e s u l t i n dense ceramics a f t e r ( h o t ) p r e s s i n g and s i n t e r i n g a t 1670 K o r higher temperatures. The g r a i n s i z e of t h e ceramic obtained i n t h i s way i s 0.411~1 o r more, which i n most c a s e s l e a d s t o thermal degradation
/4,516,718/.
For t h e p r e p a r a t i o n of Y-TZP, with g r a i n s i z e s smaller t h a n 0.4vm, lower s i n t e r i n g temperatures have t o be used. I t i s necessary t o s t a r t with u l t r a f i n e , homogeneous and weakly agglomorated powders t o o b t a i n a dense ceramic i n t h i s case. I n t h i s paper two wet-chemical powder p r e p a r a t i o n methods a r e used a s described elsewhere /9/. The s i n - t e r i n g c h a r a c t e r i s t i c s and thermal s t a b i l i t y of t h i s m a t e r i a l a r e s t u d i e d . I n a second paper t h e mechanical and e l e c t r i c a l p r o p e r t i e s a r e described / l o / .
I1 - EXPERIMENTAL
The z i r c o n i a powders were prepared by two hydrous g e l p r e c i p i t a t i o n techniques. These imply h y d r o l y s i s of a metal alkoxide-benzene s o l u t i o n i n water o r of a metal chloride- H C 1 s o l u t i o n i n NH4 OH. After thoroughly washing with water and a l c o h o l t h e r e s u l t i n g g e l s were d r i e d a t 390 K and c a l c i n e d a t 820 K f o r two hours. More d e t a i l s of t h e s e wet chemical p r e p a r a t i o n methods a r e described elsewhere /9/. The powder
(100-x)Zr02.xY01 w i t h x = 5.5 (ZY5.5) w a s prepared by t h e "alkoxidett method while f o r t h e powders wizh composition x = 6.1 o r 11 ( r e s p . ZY6.1 and ZYll) t h e "chloridett method w a s used. X-ray fluorescence spectroscopy w a s a p p l i e d f o r t h e a n a l y s i s of t h e o v e r a l l composition. The powders were pressed i s o s t a t i c a l l y a t 400 MPa, p r e s i n t e r e d a t 1170 K f o r 3 hours and f i n a l l y s i n t e r e d a t temperatures between 1380 and 1660 K.
The ceramic samples were sawn t o t h e d e s i r e d dimensions f o r mechanical and e l e c t r i c a l measurements / I T / . X-ray d i f f r a c t i o n experiments were performed on a s sawn and aged specimens using a P h i l i p s X-ray d i f f r a c t o m e t e r PW 1710 with CuKa-radiation. The sam- p l e s were aged i n a i r f o r 1000 hours a t 525 K o r 1025 K. Also some aging experiments w e r e performed a t 450 K i n H20 f o r 5 h o u r s i n a n a u t o c l a v e . The g r a i n s i z e s o f polished and thermally etched samples were i n v e s t i g a t e d using a scanning e l e c t r o n microscope (SEMI type JEOL-JSM 35 CF. The g r a i n s i z e was determined according t o Mendelsohn /11/. Ceramic d e n s i t i e s were measured using t h e Archimedes technique ( i n Hg).
I11 - RESULTS AND DISCUSSION S i n t e r i n g behaviour
The ceramic powders had a composition a s given i n t a b l e I . The BET-surfaces of t h e powders a r e a b o u t 100 m2/g and t h e r e i s n o t much d i f f e r e n c e between t h e alkoxide and c h l o r i d e syntheses. A l l specimen had a d e n s i t y of 47% of t h e t h e o r e t i c a l one a f t e r i s o s t a b i c a l l y pressing a t 400 MPa.
I n t a b l e I t h e f i n a l s i n t e r i n g temperature and time is given a f t e r t h e twostep s i n t e r - i n g procedure r e s u l t i n g i n a r e l a t i v e d e n s i t y of 97% o r more. No d i f f e r e n c e was ob- served i n t h e s i n t e r i n g behaviour between t h e samples prepared by t h e "chloridett o r by t h e more time consuming "alkoxideV method / 9 / . The t a b l e shows t h a t - a r e l a t i v e l y low s i n t e r i n g temperature can be obtained i f compared with l i t e r a t u r e . The r e s u l t i n g cera- mic g r a i n s i z e s can be kept s m a l l e r , e s p e c i a l l y when s i n t e r e d a t 1380 K f o r 70 hours
(O.lum). These c r y s t a l l i t e s i z e s a r e i n t h e o r d e r of t e t r a g o n a l microdomain s i z e s (60 nm) i n s i n g l e c r y s t a l l i n e ZY5.8 /12/.
I n Fig.1 t y p i c a l m i c r o s t r u c t u r e s a r e shown of polished and thermally etched specimens s i n t e r e d a t 1660 K. I t i s c l e a r from Fig.lb t h a t ZY11 c o n s i s t s of two d i f f e r e n t types of g r a i n s a f t e r s l n t e r i n g a t 1660 K: small g r a i n s with a s i z e of 0.5pm and l a r g e g r a i n s of 2.3pm ( t a b l e I ) .
The ceramic g r a i n s i z e s obtained i n Y-TZP m a t e r i a l s a r e g e n e r a l l y s m a l l e r i f compared with c u b i c y t t r i a s t a b i l i z e d z i r c o n i a prepared by t h e t'alkoxidett method /13/. The g r a i n s i z e s of cubic ZY17 amount 0.75pm and 2.5pm when s i n t e r e d a t 1525 K o r 1680 K r e s p e c t i v e l y /13/. The slow g r a i n growth i n t h e s i n t e r e d t e t r a g o n a l m a t e r i a l s may be due t o t h e t-phase i t s e l f o r t o t h e lower y t t r i a c o n t e n t , i f compared with cubic ma- t e r i a l . Preliminary Auger experiments on ZY5.5 showed a n enrichment of Y a t t h e s u r - f a c e with a f a c t o r 5 a f t e r a temperature treatment of 48 hours a t 1030 K. T h e s a m e temperature treatment shows f o r cubic ZY17 an Y s e g r e g a t i o n t o t h e s u r f a c e and g r a i n
boundaries with an Y-enrichment f a c t o r of 1.8 /14/. I t i s suggested t h a t t h e g r a i n growth c a n be impeded by t h e f a c t t h a t Y s e g r e g a t i o n t o t h e grain-boundary i n t e r f a c e keeps up w i t h t h e g r a i n boundary (llimpurityll d r a g mechanism) /15/. A h i g h e r
Y-enrichment f a c t o r a t t h e g r a i n boundary of Y-TZP may t h e r e f o r e a c t a s a s t r o n g e r grain-growth i n h i b i t o r .
Table It S i n t e r i n g and X-ray d i f f r a c t i o n r e s u l t s of t h e i n v e s t i g a t e d m a t e r i a l s .
* t : t e t r a g o n a l p h a s e ; t h e s i z e o f t h e a a x i s i s g i v e n i n 1 0 - ~ ~ m ; t h e s t a n d a r d d e v i a t i o n i n t h e l a s t decimal f i g u r e is g i v e n i n p a r e n t h e s e s ; t h e r a t i o of t h e t e t r a g o n a l c and a a x e s i3 1 .015 i n a l l c a s e s .
m: monoclinic phase; t h e i n t e n s i t y r a t i o { I ~)+IT1 l ) ) / ( ~ m ( i l ( ~ ~ ~ 1 )+ITl 11 ) + I t 1 1 1 ) ) is g i v e n i n p a r e n t h e s e s .
f : c u b i c f l u o r i t e phase w i t h a a s t h e l a t t i c e c o n s t a n t (1 0-'Om).
Sample Code
zy5.5 zy5.5 ZY6.1 zY6.1 ZYll ZY11
Fig.1 : SEM p i c t u r e s of p o l i s h e d and t h e r m a l l y e t c h e d specimens; s i n t e r e d a t 1660 K (a)ZY 6.1 ( b ) Z Y 11 .
s i n t e r i n g c o n d i t i o n s temp. ( t i m e ) 1525K ( 3 h ) 1660K (3h) 1380K(70h) 1660K (3h) 1525K ( 3 h ) 1660K (3h)
g r a i n s i z e (urn) 0.2 0.4 0.1 0.4 0.3 0.5+
2.3
X-ray d i f f r a c t i o n * a s sawn
t;a=5.101(1) t;a=5.097(1) t ; a = 5 . 0 9 8 ( 2 ) t;a=5.096(2) f ; a = 5 . 1 3 6 ( 1 ) f;a=5.136(1)
525 K 1 000 hours
t ; a = 5 . 0 9 8 ( 1 ) t;a=5.096(1) t;a=5.102(2) t ; a = 5 . 0 9 2 ( 2 ) f ; a = 5 . 1 3 7 ( 2 ) f ; a = 5 . 1 3 3 ( 2 )
1025 K 1000 hours
t ; a = 5 . 1 0 0 ( 1 ) t;a=5.097(1) t ; a = 5 . 0 9 6 ( 2 ) t ; a = 5 . 0 8 8 ( 4 ) f;a=5.138(1) f;a=5.137(1)
450 K 5h. (H20)
t;a=5.102(1) +m (0.1) m (0.9) t;a=5.099(1) t ; a = 5 . 1 0 0 ( 1 )
+m (0.7) f;a=5.139(2) f;a=5.135(1)
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X-ray d i f f r a c t i o n
X-Ray d i f f r a c t i o n d a t a a r e given i n t a b l e I. I t i s i n t e r e s t i n g t h a t a f t e r sawing of t h e ceramic no d e t e c t a b l e q u a n t i t i e s of a monoclinic phase a r e found. T h i s means t h a t i n a l l c a s e s t h e ceramic g r a i n s a r e small enough t o s t a b i l i z e t h e high temperature t e t r a g o n a l phase t o room temperature. This i s i n agreement with l i t e r a t u r e /3/. The sample ZY 11 showed some broadening o f the 31 1 and 400 f l u o r i t e X-ray d i f f r a c t i o n peaks. This may be due t o some t e t r a g o n a l d i s t o r t i o n which i s very d i f f i c u l t t o determine with t h i s composition . I n t h i s sample g r a i n g o w t h k i n e t i c s proceed i n a complicated way. The small g r a i n s i n F i g . l b I1behaven l i k e t e t r a g o n a l g r a i n s while t h e l a r g e r ones resemble c u b i c g r a i n s /13/ ( s e e a l s o previous s e c t i o n ) . These r e s u l t s may i n d i c a t e t h a t t h e t e t r a g o n a l phase i t s e l f e x h i b i t a slower g r a i n growth k i n e t i c s . However Nakajima e t . a l /6/ found two d i f f e r e n t phases i n a ZY 10 m a t e r i a l s i n t e r e d a t 1820 K. This specimen c o n s i s t e d of two types of grains. The small g r a i n s ( < 1 um) had a composition of ZY6.5 a s determined by EPMA. These ZY6.5 g r a i n s were assigned t o t h e metastable t e t r a g o n a l phase. The l a r g e r g r a i n s with s i z e s of s e v e r a l micrometers had a composition of ZY14 and were cubic. From t h i s l i t e r a t u r e r e s u l t s one may conclude t h a t t h e l a r g e g r a i n s i n F i g . l b a r e c u b i c g r a i n s , which c o n t a i n more Y203 than t h e s m a l l e r t e t r a g o n a l grains.
Table I shows that. a f t e r aging i n a i r f u l l y t e t r a g o n a l ( o r c u b i c ) m a t e r i a l s a r e found. The i n f l u e n c e of water on t h e phase d i s t r i b u t i o n i s c l e a r l y observed and depends on t h e ceramic g r a i n s i z e . For t h e samples aged i n a i r no t + m phase
t r a n s f o r m a t i o n was observed. According t o Watanabe e t a1./4/ t h e c r i t i c a l g r a i n s i z e f o r ZY5.5 is l e s s than 0 , 3 pm t o avoid t h e t + m transformation a f t e r aging f o r 1000 hours a t 575K. The c r i t e c a l g r a i n s i z e (Dc) f o r our ZY5.5 sample is a t l e a s t 0.4 urn i n t h i s case. T h i s d i f f e r e n c e may be due t o t h e impurity l e v e l of t h e samples. Watanabe e t a1./4/ used commercial Zr02 and Y203 powders which subsequently were mixed and ground by wet b a l l - m i l l i n g . These type of powders can c o n t a i n A1203 (up t o 2 wt%) and Si02 /7/. TEM/EDS a n a l y s i s on t h e s e s i n t e r e d samples i n d i c a t e an amorphous
grain-boundary phase around t h e t and c g r a i n s which is r i c h i n Y203, Si02and A1?03 /7/. When t h e t h i c k n e s s of t h e amorphous phase l a y e r is s m a l l e r t h e degradation 1s r e t a r d e d during aging /7/.The ZY5.5 sample used i n t h i s s t u d y , which i s prepared by t h e llalkoxidee method, o n l y c o n t a i n s 0.04 wtg S i and 0.03 wt$ A 1 /13/. These small amounts of i m p u r i t i e s may e n l a r g e Dc.
A f t e r aging i n water a t 450 K (5h) t h e degradation t o a monoclinic Z r i s dependent on g r a i n s i z e and Y amount ( s e e Table I ) . ZY5.5 with a g r a i n s i z e of 0.4 um almost shows complete degradation a f t e r water aging. ZY6.1 and ZY11 with about t h e same g r a i n s i z e s showed some o r no degradation r e s p e c i v e l y . The dense ceramic s i n t e r e d a t 1380K and a g r a i n s i z e of 0.1 um (ZY6.1) remained f u l l y t e t r a g o n a l . The t + m transformation of a l l t e t r a g o n a l g r a i n s was r e p o r t e d by Nakajima / 6 / f o r ceramic samples with composition between ZY5 and ZY9.5 a f t e r aging under t h e same c o n d i t i o n s (5h a t 450K i n water).
This d i f f e r e n c e with our r e s u l t s may be due t o t h e f a c t t h a t i n /6/ a r e l a t i v e l y high s i n t e r i n g temperature of 1820K is used which l e a d s t o l a r g e r g r a i n s and t h e r e f o r e a f a s t e r degradation.
The p r e p a r a t i o n of Y-TZP by h y d r o l y s i s of metal alkoxide o r metal c h l o r i d e compounds r e s u l t i n s i n t e r r e a c t i v e powders. Dense ceramics with a low impurity l e v e l can be obtained a f t e r s i n t e r i n g f o r 70h a t 1380K with a r e s u l t i n g g r a i n s i z e of O.lum. The g r a i n growth r a t e and g r a i n s i z e of t h e s e Y-TZP m a t e r i a l s i s smaller a f t e r s i n t e r i n g a t higher temperatures i f compared with c u b i c m a t e r i a l s . T h i s may be due t o t h e higher grainboundary enrichment f a c t o r of Y i n Y-TZP which i n h i b i t s g r a i n growth.
The i n v e s t i g a t e d m a t e r i a l s do n o t show any degradation a f t e r aging i n a i r f o r 1000 hours a t 525K o r 1025K.
The amount of t e t r a g o n a l g r a i n s which transforms t o monoclinic ones during aging i n water a t 450K f o r 5 hours d e c r e a s e s with i n c r e a s i n g amount of Y and decreasing g r a i n s i z e . No aging i s found f o r a Zr02 ceramic with 6.1 mol% Y O l s 5 and a g r a i n s i z e of 0. lum. So i t can be concluded t h a t chemical degradation i s suppressed by t h e use of very f ine-grained ceramic.
Acknowledgement.
We would l i k e t o thank M r . H . van Benthem and Mrs . Y . Roman f o r t h e i r a s s i s t a n c e w i t h s y n t h e s i s and X-ray d i f f r a c t i o n experiments.
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