TRANSFORMATION TOUGHENING IN CERAMICS : MECHANICAL PROPERTIES AND TEMPERATURE DEPENDENCE OF TETRAGONAL POLYCRYSTALLINE ZIRCONIA (TZP)

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TRANSFORMATION TOUGHENING IN

CERAMICS : MECHANICAL PROPERTIES AND TEMPERATURE DEPENDENCE OF TETRAGONAL

POLYCRYSTALLINE ZIRCONIA (TZP)

G. Orange, Gilbert Fantozzi, Y. Bigay, J. Torre

To cite this version:

G. Orange, Gilbert Fantozzi, Y. Bigay, J. Torre. TRANSFORMATION TOUGHENING IN CERAM- ICS : MECHANICAL PROPERTIES AND TEMPERATURE DEPENDENCE OF TETRAGONAL POLYCRYSTALLINE ZIRCONIA (TZP). Journal de Physique Colloques, 1986, 47 (C1), pp.C1-655- C1-659. �10.1051/jphyscol:19861100�. �jpa-00225495�

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TRANSFORMATION TOUGHENING IN CERAMICS : MECHANICAL PROPERTIES AND TEMPERATURE DEPENDENCE OF TETRAGONAL POLYCRYSTALLINE ZIRCONIA (TZP)

G. ORANGE, G. FANTOZZI, Y. BIGAY* a n d J.P. TORRE*

Groupe d'Etudes de Metallurgie Physique et de Physique des MatBriaux, C.R.R.A.C.S., U.A. 341, I.N.S.A., Bdt. 502, F-69621 Villeurbanne Cedex, France

" ~ 6 r a m i q u e s Techniques DESMARQUEST, Laboratoire de Recherche et Developpement, 2, Avenue Albert Einstein, F-78190 Trappes, France

& s d

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Des materiaux de type zircone tetraganale p o l y c r i s t a l l i n e (TZP) o n t e r o b t e n u s par f r i t t a g e nature1 a p a r t i r de poudres submicroniques avec a d d i t i o n s d e f a i b l e s q u a n t i t e s d1Y203. Us p r o p r i e t e s k a n i q u e s o n t ete determinees en fonction de la tenp6rature ( j u s q u l a 900°C). Ainsi la tenacite (environ 10 MPa r m a t e e r a t u r e anbiante) d k r o i t lineairement de 200" a 600°C. Ies r e s u l t a t s m n t d i s c u t e s , e n termes de mkanismes de renforcement ( t r a n s f o r m a t i o n d e p h a s e ) , e n f o n c t i o n d e l a m i c r o s t r u c t u r e e t d e l a s t a b i l i t 6 de l a phase tetragonale.

Abstract

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P o l y c r y s t a l l i n e t e t r a g o n a l z i r c o n i a mater i a l s (TZP) have been obtained by normal s i n t e r i n g from submicronic powders with lcw a d d i t i o n s of Y203. Mechanical p r o p e r t i e s have been studied as a function of tenperature, up t o 900°C. F r a c t u r e toughness (about 10 M P a m a t room t e n p e r a t u r e ) decreases l i n e a r l y from 200" to 600°C. Results are discussed according in t o u g h e n i n g mechanisms ( p h a s e t r a n s f o r m a t i o n ) o n t h e b a s i s o f t h e microstructure and s t a b i l i t y o f the tetragonal phase.

The a p p l i c a t i o n of phase transformations to enhance toughness of ceramic materials h a s been studied f i r s t i n the case of p a r t i a l l y s t a b i l i z e d zirconia (P.S.Z.). I n these ceramics, t h e metastable phase is present a s f i n e w h e r e n t p r e c i p i t a t e s i n a large-grain cubic matrix / L I Z / . ~ e s e m a t e r i a l s are usually made by s i n t e r i n g i n t h e c u b i c / t e t r agonal f i e l d followed by c o n t r o l l e d p r e c i p i t a t i o n treatments, o r i n a s i n g l e s t e p process by s i n t e r i n g i n the cubic/tetragonal f i e l d . I n both cases, the microstructure is made up of l a r g e g r a i n s (cubic phase), and t h i s is a limiting f a c t o r f o r mechanical p r o p e r t i e s . J?urthermore, t h e s e p r o p e r t i e s a r e s t r o n g l y dependent on thermal h i s t o r y of t h e m t e r i a l .

With an a c c u r a t e c o n t r o l o f g r a i n s i z e and a d d i t i v e s c o n t e n t , s i n g l e phase t e t r a g o n a l z i r c o n i a m a t e r i a l s (TZP) can be obtained /3/. I n t h i s type o f material t h e t o u g h e n i n g p h a s e , i.e. t h e p h a s e which l e a d s to e n e r g y a b s o r p t i o n by stress-induced transformatian, is n o t r e s t r i c t e d because a l l t h e material is i n the tetragonal form : a l s o , t h e toughening e f f e c t can be p o t e n t i a l l y m i m i z e d and the f r a c t u r e toughness is c o n s i d e r a b l y i n c r e a s e d compared t o a l l o t h e r P.S.Z..

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19861100

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JOURNAL DE PHYSIQUE

Furthermre, t h e g r a i n s i z e is very f i n e (between 0.1 and 1

km),

and the f r a c t u r e s t r e n g t h can also be considerably enhanced. W i n r e s u l t s have been obtained with snnll Q O j a d d i t i o n s (Y-TZP), and a dependence of mechanical behaviour on Y ~ o j c o n t e n t h a s been observed /4/; t h e e f f e c t o f raw powder c h a r a c t e r i s t i c s and s i n t e r i n g conditions has also been reported /5,6/. Thus, it has been observed that Y-TZP f r a c t u r e s t r e n g t h increases i f the tetragonal g r a i n s i z e is reduced, whereas f r a c t u r e toughness is maximized a t the l a r g e s t possible tetragonal g r a i n s i z e

-

^for

a given Y203 content

-.

A l l high

Cf

^and ^{K b}values of TZP reported are concerned with rocen temperature conditions. I n t h i s work, we have observed the m ~ c h a n i c a l p r o p e r t i e s tenperature dependence of Y-TZP materials. The r e s u l t s a r e discussed i n view of the t h e o r e t i c a l p r e d i c t i o n s based a? the v a r i a t i o n o f chemical f r e e energy change / 7 / .

11-1

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^Material

Materials studied a r e f i n e grained tetragonal zirconia with 5.2 w t % Y2O3 ( W 3 mle

%). 'Ikbatches have been obtained (Y-TZP 1 and Y-TZP 2) by pressureless s i n t e r i n g (1500°C

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4 h) from m i x e d submicron pawders. Only milling conditions have been m d i f i e d from YqZP 1 to Y " P P 2, t o obtain f i n e r microstructure. Densities are about 94 to 96.5% of t h e o r e t i c a l density, and the mean g r a i n s i z e is between 0.65 and 0.8pm.

Relative contents of tetragonal and m o c l i n i c phases have been calculated by X-Ray d i f f r a c t i o n from ( l l l ) m , (1ll)m and ( I l l ) t r e f l e x i o n i n t e n s i t i e s /8/.

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M h a n i c a l experiments

The f r a c t u r e s t r e n g t h (Gf) and the f r a c t u r e toughness ( K I C ) v a l u e s have been obtained with 3 points bending tests (15 mn span), a t a cross-head speed of 0.1 mn/min. Specimens (4 x 3 x 22 mn3) were machined and polished ( 6 y m and

lpm

^d ⁱ ^d

paste) from s i n t e r e d bodies. K ~ c measurements have been performed on notched beams (S.E.N.B.) with a s t r a i g h t through notch of 1.6 mn depth (a/w n 0.4) and a t i p radius of 80 m. Tb eliminate induced r e s i d u a l stresses, annealing treatments a t 950°C (15 m i n r have been made with a l l the specimens. E x p r i n s n t s were performed i n a i r , u p to 900°C, with a s p e c i a l high tenperature t e s t i n g apparatus /9/.

I n the studied Y-TZP m a t e r i a l s case, we d i d not observe any s i g n i f i c a n t m t of cubic (c) phase. l b e t e t r a g o n a l phase proportion is between 78% and 90% in a s machined specimens and increases, i n a l l specimens, t o values higher than 95% a f t e r 950°C annealing.

Worn temperature mechanical c h a r a c t e r i s t i c s a r e s i m i l a r to those reported f o r s i m i l a r materials /4, 10, 11/. The high toughness values ( w 10 MPa Y-m) can be c o r r e l a t e d to the c o n t r i b u t i o n of a stress-induced phase transformation to t h e f r a c t u r e energy. T h i s mechanism is confirmed by a systematic a n a l y s i s of t h e d i f f e r e n t cr i s t a l l o g r a p h i c v a r i e t i e s : one observes a reduction o f the tetragonal phase proportion from 95% (before f r a c t u r e ) to 65% on f r a c t u r e faces /12/. According to t h e thermdynamic approach proposed by F.F. IANGE, t h e phase transformation toughening e f f e c t can be expressed a s , /13/ :

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I

*\

^{T Z P 1}

K-

T Z P 2

+

T ("C)

F i g . 2

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Influence of temperature on the fracture toughness of Y-TZP materials

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JOURNAL DE PHYSIQUE

With b = 3 MPa Y-m (toughness of material without any transformation toughening phenomemn)

,

Vi=95% (volume fraction of retained tetragollal phase), %=I80 B a r Uc=0.25 and F ~ 0 . 8 F m ( s i z e of the transformation zone associated with the crack), one obtains (for w 9 . 2 MPa Vm) : d e f AU,=258 M . J . ~ - ~ . This value expresses the work done, per u n i t volume, by the s t r e s s f i e l d to induce the transformation.

Microstructure observations of TZP materials reveal the presence of twinning i n transformed (monoclinic) g r a i n s and o f microcracks along g r a i n boundaries /13/. This p i n t contributes to relieving s t r a i n energy during f r a c t u r e and so increases the term @ %( -4 u,. f ) i n equation (1) : t h i s can explain the high toughness values we observed. Hcwever, the fracture strength values (Y-TZP 1 and Y-TZP 2) are q u i t e low i f c o m p a r e d t o o t h e r T . Z . P . : t h i s i s d u e t o t h e p r e s e n c e of important residual porosity (d=95% dth) and a l s o the presence of large inclusions

(from raw powder and milling media). According to these observations, processing has been s l i g h t l y modified and one observes, on new T%P batches (%), f r a c t u r e strength values of about 1000 MPa.

The influence of tenperature on mechanical properties is i l l u s t r a t e d i n fig. 1 and fig. 2. The f r a c t u r e toughness decreases q u i t e l i n e a r l y between 200°C and 600°C, and is similar t o t h a t one o f s t a b i l i z e d z i r c o n i a a t 800°C. The f r a c t u r e strength v a r i a t i o n is s i m i l a r , except an important decrease a t about 200°C. This low tenperature degradation can be correlated to an activated formation of mxroclinic phase on the specimens surface. Such a behaviour has been reported by d i f f e r e n t authors, and seem to reach a maximum a t 200°C /14/. The decrease of K l C with temperature is similar to the chemical f r e e energy change( 1 b Gc 1 ) temperature dependence : for the tetragonal nwmoclinic transformation, Id(;c) decreases with increasing tenperature.

A R I 0.8 gm

Y-TZP

T ( " C ) ^'from

Pig. 3

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Wrk (per u n i t volume) to induce t h e transformatian versus temperature The c r i t i c a l tenperature To, i.e. the tenperature corresponding to the value (lAGc\

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^Ause.f) ⁼ o, is defined i n our materials a t a b u t 625OC : there is m r e toughening e f f e c t a t higher terrperatures and &=K, (equ. (1)). We have p l o t t e d

(2) ZFYT grade : DES-T CJ3RAMIQUE TECHNIQUE

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²⁷⁰²⁵ENREUX (France)

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g r e a t e s t temperature dependence f a c t o r i n mu. ( l ) , t h e slope nust be nearly the

same a s d I A GcfdT f o r pure ZrO;! /15/. Recalculated values o f ( d ^Gc-A Us. f ) with R-1.38pm g i v e a r e s u l t i n g slope i n good agreement with reported thermdynamic d a t a (0.248 M7. m-3.

cl),

^{arid the}new value oflAC+l a t room tenperature is about 150 MS.

m-3. This nay value is s l i g h t l y lower than the calculated value for hot-pressed Y-TZP with 2 mle% Y2O3 (188 ~ . m - ~ ) /7/. This is c o n s i s t e n t with the e f f e c t of a l l o y content :IA+jdecreases with increasing Y2O3.

.

High toughness values ( 10 MPa r m ) have been obtained i n pressureless s i n t e r e d t e t r a g o n a l z i r c o n i a m a t e r i a l s (Y-TZP)

.

The toughening e f f e c t can be explained by the contribution o f a stress-induced phase transformation.

.

^{I n}^them a t e r i a l s studied, t h e c r i t i c a l tenperature is 625OC. Xhe ener y

3

change at room tenperature associated with the transformation is about 150 M3.m-

,

w i t h a t r a n s f o r m a t i o n zone s i z e o f 1.38 p m . R e s u l t s a r e c o n s i s t e n t w i t h thermodynamic data.

/1/ Garvie, R.C., Hannink, R.H. J. and Pascoe, R.T., Nature 258 (1975) 703.

/2/ P o r t e r , D.L. and Heuer, A.H., J. Am. Gram. Soc. 60 (1977) 183.

/3/ Gupta, T.K., Bechtold, J.H., Kuznicki, R.C., Cadoff, L.H. and Rossing, B.R., J. Mater. Sci.

21

(1977) 2421.

/4/ %ukuma, K-, Kubota, Y. and Tsukidate, T., Science and Technology of Zirconia 2, Advances in Ceramics Vol. 12, ed. Claussen N., Eiihle M. arid Heuer A.H., (1984) 382.

/5/ rciihle, M., Claussen, N. and Heuer, A.H., ibid., 352.

/6/ Matsui, M., Soma, T., and Oda, I., ibid., 371.

/7/ Lange, F.F., J. Mater. Sci.

17

(1982) 255 (Part. 5).

/8/ Garvie, R C . and Nicholson, P.S., J. Am. Ceram. Soc. 55 (1972) 303.

/9/ Orange, G., -is, J., Fantozzi, G. and Gobin, P.F., Mem. Sci. Rev. Met.

2

(1980) 131.

/lo/

GUpta, T.K., Lange, F.F. and Beckitold, J.H., J. h t e r . Sci.

2

(1978) 1464.

/11/ Lange, F.F., J. Mater. Sci.

2

(1982) 240 (Part. 3 ) .

/12/ _Orange, G. and Fantozzi, G., Fourth Inter. Conf. on Fracture Mechanics o f Ceramics (Blacksburg), June 1985, U.S.A. be published.

/U/ Lange, F.F., J. Mater. Sci.

17

(1982) 225 (Part. 1) and 235 (Part. 2).

/14/ Sato, T., Ohtaki, S. and Shimada, M., J. Mater. Sci.

g

(1985) 1466.

/15/ Whitney E.D., J. Am. Ceram. Soc. 45 (1962) 612.