POST SINTERING HEAT TREATMENTS AND CREEP OF Si-Y-Al-O-N CERAMICS

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POST SINTERING HEAT TREATMENTS AND CREEP OF Si-Y-Al-O-N CERAMICS

Thierry Chartier, J. Besson, P. Goursat, W. Mustel

To cite this version:

Thierry Chartier, J. Besson, P. Goursat, W. Mustel. POST SINTERING HEAT TREATMENTS AND CREEP OF Si-Y-Al-O-N CERAMICS. Journal de Physique Colloques, 1986, Thirteenth International Conference on Science of Ceramics 47 (C1), pp.C1-673-C1-678. �10.1051/jphyscol:19861103�. �jpa- 00225498�

JOURNAL DE PHYSIQUE

Colloque Cl, supplkment au n 0 2 , Tome 47, fbvrier 1986 page cl-673

POST SINTERING HEAT TREATMENTS AND CREEP O F Si-Y-A1-0-N CERAMICS

T. CHARTIER, J.L. BESSON, P. GOURSAT* and W. MUSTEL"

LA C. N. R. S. 320, E. N. S. C. I. , 47, Avenue Albert Thomas, F-87065 Limoges Cedex, France

* L A C.N.R.S. 320, U.E.R. des Sciences, 123, Avenue Albert Thomas, F-87060 Limoges Cedex, France

* * C E R A V E R , Usine de Buzet, B.P. 113, F-65001 Tarbes, France

~ 6 s u m 6 - S u r deux n u a n c e s de c k r a m i q u e s de t y p e SiYAlON, o b t e n u e s p a r f r i t t a g e n a t u r e l , i l e s t montr6 1 1 i n t 6 r E t , p o u r a m h l i o r e r l a r k s i s t a n c e a u f l u a g e , d e t r a i t e m e n t s t h e r m i q u e s en l i t de p o u d r e s o u s a t m o s p h 6 r e d ' a z o t e .

A b s t r a c t

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I - I N T R O D U C T I O N

I t i s now w e l l e s t a b l i s h e d t h a t t h e h i g h t e m p e r a t u r e b e h a v i o u r of n i t r o g e n c e r a m i c s is l i n k e d t o t h e p r o p e r t i e s o f t h e r e s i d u a l

i n t e r g r a n u l a r phas.e, i . e . , t o t h e n a t u r e and q u a n t i t y o f d e n s i f i c a t i o n a i d s and t o t h e m e t a l l i c i m p u r i t i e s i n t h e r a w m a t e r i a l s . T h i s

i n t e r g r a n u l a r p h a s e may b e m o d i f i e d by p o s t - s i n t e r i n g h e a t t r e a t m e n t i n o x i d i z i n g o r n e u t r a l a t m o s p h e r e s /l/. These p o s s i b i l i t i e s have b e e n i l l u s t r a t e d by t h e s t u d y o f t h e c r e e p b e h a v i o u r o f two SiYAlON

m a t e r i a l s w i t h t h e same nominal c c m p o s i t i o n , b u t w i t h a d i f f e r e n t i m p u r i t y c o n t e n t ( C a , F e ) .

I1

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Two b a t c h e s o f s a m p l e s ( ~ and N5) were p r e p a r e d from s i l i c o n , 3 aluminium and y t t r i a powders. The nominal c o m p o s i t i o n i s t h e same, e x c e p t f o r t h e p u r i t y of t h e s i l i c o n powder ( T a b l e 1 ) . The powders a r e mixed i n a l c o o h o l , d r i e d , t h e n n i t r i d e d d u r i n g a 70 h o u r t h e r m a l c y c l e . The r e a c t i o n k i n e t i c s a r e c o n t r o l l e d by n i t r o g e n consumption / 2 / . The b r i c k s a r e c r u s h e d and f i n e l y ground ( g < 2 p m ) . A f t e r

s h a p i n g , t h e s a m p l e s a r e d e n s i f i e d by p r e s s u r e l e s s s i n t e r i n g a t 1 7 0 0 ° C i n n i t r o g e n t o 95% t h e o r e t i c a l d e n s i t y .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19861103

C l - 6 7 4 JOURNAL DE PHYSIQUE

Powder ___-_-_______---_-_--- PURITY w t %

C a 1 Fe 1 A l

0 , 0 2 - 0 , 1 4 1 0 , 4 8 - 0 , 5 8 1 0 , 0 2 - 0 , 1 5

_-_-_________)_____---I---

n . d . - 0 , 0 2 1 0 , 3 5 - 0 , 4 8 1 0 , 0 7 - 0 , 0 8

_-_-_-_______(_____---I---

n . d [ 50 ppm [ n.d.

_-_---_-_-___I_-_-_---l---

43 ppm l ^{2000 ppm}

!

I l I

Table 1 : I m p u r i t y c o n t e n t of t h e powders.

i - A s s i n t e r e d m a t e r i a l s c o n s i s t i n R

'Si5.6A10.400.4N7.6 as the

major p h a s e , w i t h a s m a l l amount of y t t r i u m s i l i c a t e ( B Y S i 0 ) and a r a t h e r l a r g e q u a n t i t y o f g l a s s y p h a s e ( F i g . 1 ) . N o y t t r i u $ a?uxinium g a r n e t h a s been d e t e c t e d e i t h e r i n N3 o r N5 m a t e r i a l s .

ii - A f t e r h e a t - t r e a t m e n t i n a i r (48 h - 1 3 5 0 ° C ) , m i c r o a n a l y s i s (E.D.A.X.) r e v e a l s a m i g r a t i o n of c a l c i u m and y t t r i u m which

c o n c e n t r a t e i n t h e o x i d e s c a l e a t t h e e x t e r n a l i n t e r f a c e . T.E.M.

m i c r o g r a p h s show a p a r t i a l m i c r o c r y s t a l l i z a t i o n o f t h e g l a s s y phase p o c k e t s ( F i g . 2 ) .

P i g . 1 : T.E.M. m i c r o g r a p h s o f a s - s i n t e r e d m a t e r i a l .

a - b r i g h t f i e l d , b - d a r k f i e l d and a s s o c i a t e d d i f f r a c t i o n p a t t e r n .

iii - H e a t - t r e a t m e g t s have a l s o been performed i n a powder bed.

The b a r s ( 4 X 4 X 25 mm ) a r e embedded i n s i l i c o n n i t r i d e powder ( p u r i t y 9 9 . 9 % ) and t r e a t e d i n n i t r o g e h f o r 100 h o u r s a t t e m p e r a t u r e s between 1150 and 1300°C. A complete r e c r y s t a l l i z a t i o n o f t h e g l a s s y phase i s evidenced by l a c k o f any a l t e r a t i o n u n d e r t h e e l e c t r o n beam d u r i n g TEM i n v e s t i g a t i o n ( f i g . 3 ) . X-Ray d i f f r a c t i o n p a t t e r n s show an i n c r e a s e i n Y S i 2 0 7 and t h e a p p e a r a n c e o f A1Y03 and of new p h a s e s which have no$ y e t been i d e n t i f i e d . No outward diffusion of y t t r i u m was d e t e c t e d by EDAX m i c r o a n a l y s e s of t h e sample s u r f a c e .

a b

F i g . 2 : T . E . M . m i c r o g r a p h s of a sample o x i d i z e d 48 h a t 1350°C.

a - d a r k f i e l d , b - s p o t t e d r i n g s i n t h e d i f f r a c t i o n p a t t e r n .

F i g . 3 : T.E.M. micrograph of a sample h e a t - t r e a t e d i n a powder b e d .

111 - CREEP

The 3 - p o i n t bending c r e e p t e s t s i n a i r a r e performed between 1100 and 1250°C a t s t r e s s e s r a n g i n g from 70 t o 165 MPa. The s t r e s s and s t r a i n a t t h e o u t e r m o s t f i b r e a r e c a l c u l a t e d under e l a s t i c t h e o r y

a s s u m p t i o n s . T h i s a p p r o x i m a t i o n i s j u s t i f i e d by t h e low v a l u e of t h e measured d e f l e c t i o n and t h e newtonian b e h a v i o u r o f t h e f l o w i n t h e s t a t i o n a r y s t a g e / 3 / . T y p i c a l c r e e p c u r v e s a r e shown i n f i g u r e 4.

A f t e r a r a p i d d e c r e a s e a t t h e b e g i n n i n g , t h e c r e e p r a t e c o n t i n u e s t o d i m i n i s h s l o w l y . A t r u e s t e a d y s t a t e i s r e a c h e d o n l y a f t e r s e v e r a l hundred h o u r s . A t e r t i a r y c r e e p h a s n e v e r been o b s e r v e d under o u r e x p e r i m e n t a l c o n d i t i o n s .

i - Creep of t h e a s - s i n t e r e d N m a t e r i a l h a s been s t u d i e d u s i n g t h e phenomenological r e l a t i o n ; ⁼ A ~ S ) o n exp - E/RT where E i s t h e c r e e p r a t e a t t h e o u t e r t e n s i l e f i b r e , t h e maximum t e n s i l e s t r e s s , n t h e s t r e s s e x p o n e n t , E t h e a p p a r e n t a c t i v a t i o n e n e r g y , R t h e Boltzmann c o n s t a n t , T t h e a b s o l u t e t e m p e r a t u r e and A ( S ) a f a c t o r depending on t h e m i c r o s t r u c t u r e .

D e t e r m i n a t i o n of n , through s t r e s s - s t e p s e x p e r i m e n t s , shows a l i n e a r and r e v e r s i b l e v a r i a t i o n of I n C v e r s u s I n ^U f o r t i m e s g r e a t e r t h a n 340 h o u r s ( f i g . 5 ) . During t h e l o n g p r e c e e d i n g p e r i o d when t h e c r e e p r a t e i s s l o w l y d e c r e a s i n g , t h e m i c r o s t r u c t u r e changes due t o t h e o x i d a t i o n and t h e d e c r e a s e i n q u a n t i t y of t h e i n t e r g r a n u l a r g l a s s y

J O U R N A L DE PHYSIQUE

Fig. 4 : Creep curves at 1150°C and 120 MPa for N3-samples and 120O0C- 240 MPa for N -samples.

1 - as sintered N 5 2 - as sintered N

2 - preoxidized N ~ ' N -samples heat-treated in a powder bed at 3 (48 h - 1350°C) ⁵ 4 - 1?50°c ; 5 - 12OO0C ; 6

-

phase (fig. 6). This oxidation leads to an outward migration of yttrium and calcium /4/. Studies of the oxidation behaviour have

allowed us to link the onset of the stationary stage with an

intergranular oxidation completed in the bulk of the samples. At this stage, the evolution of the microstructure is achieved ; the

contribution of the viscoelastic effects to the creep rate becomes negligeable ; the only remaining mechanism is a diffusional one

(n = 1 , E = 780 kJ/mol). This mechanism corresponds to a solution- migration-precipitation process induced by chemical potential gradients which arise from the buttressing of adjacent grains on ledges along grain-boundaries.

ii - N5 material shows a similar behaviour with a better resistance (fig. 4). The steady stage is reached more rapidly. The lower calcium and iron contents increase the refractoriness of the glassy phase.

iii - For N3 pre-oxidized samples also, the stationary stage is observed for shorter times than in as-sintered N3 samples, but the steady creep rate is higher (fig. 4). After creep test, no noticeable microstructural changes have been observed, which means that the microstructure has been stabilized by the oxidation

treatment. A consequence of the partial microcrystallization of the glassy phase pockets may be that the solution-migration-precipitation process can now occur at buttressing points between crytallites. This possibility of matter flow between silicate crystallites through the oxidized residual glassy hhase might explain a higher value for the creep rate in the steady-state creep.

F i g . 5

f

a-

F i g . 6

F i g . 5 : Creep r a t e a t 1150°C v e r s u s s t r e s s f o r a s - s i n t e r e d N -samples.

-

F i g . 6 : T.E.M. micrograph o f N -sample a f t e r c r e e p . 3

i . v - A f t e r t r e a t m e n t i n a powder bed a t 1150°C, t h e b e h a v i o u r i s s i m i l a r t o t h a t o f t h e a s - s i n t e r e d m a t e r i a l , b u t improves w i t h

i n c r e a s i n g h e a t t r e a t m e n t t e m p e r a t u r e . I n t h e c a s e of a sample h e a t t r e a t e d a t 1250°C, t h e s t e a d y s t a t e c r e e p r a t e i s l o w e r and t h e s t r a i n i s t h r e e t i m e s l e s s t h a n i n t h e c a s e o f t h e a s - s i n t e r e d m a t e r i a l , a l l e l s e b e i n g e q u a l . T h i s improvement i s a s c r i b e d t o t h e c r y s t a l l i z a t i o n l e a d i n g t o t h e a l m o s t complete d i s a p p e a r a n c e o f t h e v i t r o u s p h a s e . The powder bed p r o t e c t s t h e samples from any d e c o m p o s i t i o n and f o r

r e a c t i o n w i t h oxygen t r a c e s i n t h e atmosphere.

I V - CONCLUSION

T h i s s t u d y d e m o n s t r a t e s t h a t t h e d i f f e r e n c e i n b e h a v i o u r between t h e two SiYAlON m a t e r i a l s , N3 and N 5 , of t h e same nominal c o m p o s i t i o n , i s r e l a t e d t o t h e i r d i f f e r e n t c a l c i u m i m p u r i t y c o n t e n t s ; i r o n d o e s n o t seem t o p l a y any r o l e . Thermal t r e a t m e n t s e i t h e r i n o x i d i z i n g o r n e u t r a l a t m o s p h e r e s , l e a d t o t h e c r y s t a l l i z a t i o n , more o r l e s s c o m p l e t e , of t h e s e c o n d a r y g l a s s y p h a s e . I n o x i d i z i n g e n v i r o n m e n t s , s u r f a c e p i t t i n g l i m i t s t h e d u r a t i o n o f t h e t r e a t m e n t , whereas i n t h e c a s e o f t h e powder bed t e c h n i q u e , t h e samples a r e p r o t e c t e d from s u r f a c e d e g r a d a t i o n and r e a c h t h e s t a b l e thermodynamical s t a t e c h a r a c t e r i z e d by t h e e l i m i n a t i o n of t h e t r a n s i e n t g l a s s y p h a s e .

For t h i s SiYAlON c e r a m i c , h e a t t r e a t m e n t s i n n i t r o g e n u s i n g a powder bed i n d u c e a marked improvement i n t h e c r e e p r e s i s t a n c e .

JOURNAL DE PHYSIQUE

REFERENCES

/ l / BESSON J . L . , BOUARROUDJ A . a n d GOURSAT P . - ^{R e v .} I n t . H a u t e s T e r n p e r . R e f r a c t . , 19 ( 1 9 8 2 ) , 381.

/ 2 / MUSTEL W. a n d BROUSSAUD D. - I 1 S c i e n c e o f C e r a m i c s 12", e d i t e d by P . V i n c e n z i n i C e r a r n u r g i a p u b l i s h e r ( 1 9 8 3 ) , p . 131.

/ 3 / HOLLENBERG G . W . , T E R W I L L I G E R G . R . and GORDON R . S .

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/ 4 / CHARTIER T . - T h & s e D o c t e u r I n g g n i e u r , L i r n o g e s (1985).