INFLUENCE OF SINTERING CONDITIONS ON MECHANICAL PROPERTIES AT ROOM AND HIGH TEMPERATURES FOR SELECTED Y-Al-Si-O-N MATERIALS

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INFLUENCE OF SINTERING CONDITIONS ON MECHANICAL PROPERTIES AT ROOM AND HIGH

TEMPERATURES FOR SELECTED Y-Al-Si-O-N MATERIALS

W. Braue, G. Wötting, G. Ziegler

To cite this version:

W. Braue, G. Wötting, G. Ziegler. INFLUENCE OF SINTERING CONDITIONS ON ME- CHANICAL PROPERTIES AT ROOM AND HIGH TEMPERATURES FOR SELECTED Y- Al-Si-O-N MATERIALS. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-341-C1-345.

�10.1051/jphyscol:1986150�. �jpa-00225580�

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JOURNAL DE PHYSIQUE

Colloque C1, supplbment au n02, Tome 47, fbvrier 1986 page cl-341

INFLUENCE OF SINTERING CONDITIONS ON MECHANICAL PROPERTIES AT ROOM AND HIGH TEMPERATURES FOR SELECTED Y-A1-Si-0-N MATERIALS

W. BRAUE, G. WOTTING and G. ZIEGLER

Deutsche Forschungs- und versuchsanstalt fiir Luft- und Raumfahrt (DFVLR), Institut fiir Werkstoff-Forschung, D-5000 Koln 90, F.R.G.

Resume - La m i c r o s t r u c t u r e e t l e s p r o p r i 6 t e s massiques de SigNq I'ritte sont

@turdii.es en f o n c t i o n des c o n d i t i o n s de f r i t t a g e e t de c e l l e s du r e c u i t . A temperature ambiante l a r e s i s t a n c e a l a r u p t u r e depend de l a morphologie des g r a i n s de l a phase P-Si3N4; pour des temperatures sup@rieures ii 1000°C e l l e depend de l ' e q u i l i b r e e n t r e l e s d i f f e r e n t e s phases du l i q u i d e residue1 e t l e s phases o x i n i t r u r e s .

A b s t r a c t - Development o f m i c r o s t r u c t u r e and m a t e r i a l p r o p e r t i e s o f s i n t e r e d Si3N4 i s s t u d i e d as a f u n c t i o n o f s i n t e r i n g and post-heat treatment c o n d i - t i o n s . Room-temperature s t r e n g t h depends on t h e g r a i n morphology o f t h e P-Si3N4 phase, s t r e n g t h a t temperatures exceeding 1000 ^OCon phase r e l a t i o n - ships between t h e r e s i d u a l l i q u i d and o x i n i t r i d e phases, r e s p e c t i v e l y .

I - INTRODUCTION

I n t h e case o f polyphase ceramics such as dense Si3N development o f m i c r o s t r u c t u r e and o f mechanical p r o p e r t i e s i s c l o s e l y c o r r e l a t e d . % i s complex i n t e r a c t i o n depends on t h e c h a r a c t e r i s t i c s o f t h e s t a r t i n g powder, t h e a d d i t i v e amount and composition as w e l l as t h e processing and s i n t e r i n g c o n d i t i o n s . From room temperature up t o about 1000 OC i t may be q u a n t i t a t i v e l y described i n terms o f t h e B-Si3N4 g r a i n morphology /I/. The mechanism o f 6-phase g r a i n growth i s s t r o n g l y a f f e c t e d by b o t h t h e amount and v i s c o s i t y o f the l i q u i d phase and t h e s i n t e r i n g c o n d i t i o n s d u r i n g l i q u i d phase s i n t e r i n g 121. Focusing on environmental c o n d i t i o n s exceeding 1000 ^OC, m a t e r i a l p r o p e r t i e s a r e i n p a r t i c u l a r c o n t r o l l e d by t h e composition - v i s c o s i t y r e l a t i o n s h i p o f t h e amorphous o x i n i t r i d e phase a t t r i p l e j u n c t i o n s and i n t e r g r a n u l a r c o n t a c t s as w e l l as i t s s t a b i l i t y w i t h r e s p e c t t o c o e x i s t i n g c r y s t a l 1 i n e o x i n i t r i d e s i n the micro- s t r u c t u r e . I n t h e scope o f r e a l i z i n g concepts f o r improving t h e high-temperature p r o p e r t i e s o f dense S i N4,the i n f l u e n c e o f e s s e n t i a l parameters on mechanical proper- t i e s a t room and h i g h zemperatures i s demonstrated f o r a constant (Y203 + ^Al203)-

f l u x e d s t a r t i n g composition as a f u n c t i o n o f s i n t e r i n g c o n d i t i o n s . Furthermore, the c a p a b i l i t i e s o f post-heat treatment procedures a r e evaluated.

I 1

-

EXPERIMENTAL PROCEDURE

S i n t e r e d Si3N4 m a t e r i a l s i n v e s t i g a t e d were prepared from a m i x t u r e o f Si3N4 + 15 wt.%

Y203 + 3.4 wt.% A1 O3 (Si3N4 : LC12, H.C. Starck, FRG). S i n t e r i n g was c a r r i e d o u t i n a 1 bar n i t r o g e n af.mosphere. The soaking time was v a r i e d from 10 min t o two-step procedures f o r several hours a t 1800 OC and 1820 OC r e s p e c t i v e l y . Thg (1800°C/2h +

1820°c/2h) c o n d i t i o n was submitted t o a post-heat treatment a t 1400 C, 48 h a t p ~ ~ = 1 bar. Mechanical p r o p e r t i e s were c h a r a c t e r i z e d by t h e KI, value applying t h e

Vickers hardness i n d e n t a t i o n method 131 and f l e x u r a l s t r e n g t h data a t room and 1 2 5 0 ~ ~ ( 4 - p o i n t bending, span 40120 mm, specimen dimensions 3.5 x 4.5 x 45 mm). M i c r o a n a l y t - i c a l methods employed a r e described elsewhere 141.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986150

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JOURNAL DE PHYSIQUE

I 1 1

-

RESULTS AND DISCUSSION

The e f f e c t o f d i f f e r e n t s i n t e r i n g c o n d i t i o n s ranging from 1 8 0 0 ~ ~ / 1 0 min t o (1800°c/

2h

+

1820°c/5h) on m i c r o s t r u c t u r a l v a r i a b l e s and f l e x u r a l s t r e n g t h a t room tempera- t u r e i s demonstrated i n Fig.1. The h i g h e s t values f o r b o t h f l e x u r a l s t r e n g t h and KCI a r e obtained f o r 10 min soaking time, although f i n a l d e n s i t y has n o t y e t been reached. The reason behind t h i s s i t u a t i o n may be understood i n terms o f t h e model o f t h e @-phase g r a i n morphology /2/: S t a r t i n g w i t h a h i g h l y a c t i v e a-Si3N4 powder, spontaneous c r y s t a l l i z a t i o n o f idiomorphic p a r t i c l e s occurs d u r i n g t h e e a r l y stages o f t h e l i q u i d phase s i n t e r i n g process. With i n c r e a s i n g soaking time, t h e p r i m a r i l y p r e c i p i t a t e d @-grains w i t h an h i g h aspect r a t i o change towards a more equi-axed morphology i n o r d e r t o minimize t h e i n t e r f a c i a l energy associated w i t h f u r t h e r den- s i f i c a t i o n . The r e d u c t i o n o f f l e x u r a l s t r e n g t h and o f K I ~ r e f l e c t s t h e marked i n - f l u e n c e o f t h e aspect r a t i o o f t h e @-phase compared w i t h t h e e f f e c t o f r e s i d u a l p o r o s i t y .

Powder: LC 12

Additives: 15wt.%Y203+3.1wt.%A12~

9 0 0 4 9 t \,

TlOO

. .

l l l O O / l h 111011b 18~0.12h ~d./lh

Sintering Conditions

F i g . 1

-

M i c r o s t r u c t u r a l v a r i a b l e s ( t o p ) and mechanical p r o p e r t i e s OF and KJ o f s i n t e r e d Si3N4 a t room temperature (bottom) as a f u n c t i o n o f s i n t e r i n g c o n d ~ F i o n s .

I n c o n t r a s t t o these r e l a t i o n s a t room t mperature, an increase o f t h e f l e x u r a l s t r e n g t h a t 1250 OC from 340 t o 160 M N ~ - $ was observed (Fig.2). This v a r i a t i o n i s t o be discussed i n t h e l i g h t o f i n t e g r a l secondary phase compositions and, i n p a r - t i c u l a r , o f t h e amount and d i s t r i b u t i o n o f amorphous and c r y s t a l l i n e secondary phases. The occurrence o f t e r n a r y and quaternary o x i n i t r i d e s i n the Y-A1-Si-0-N system i s dependent on t h e b u l k composition, l o c a l chemical g r a d i e n t s w i t h i n t h e m i c r o s t r u c t u r e and b o t h t h e processing and s i n t e r i n g c o n d i t i o n s /5/.

S e l e c t i n g t h e 1 8 0 0 ~ ~ / 1 0 min, t h e (1800°c/2h

+

1820°c/2h) and t h e post-heat t r e a t e d c o n d i t i o n s as r e p r e s e n t a t i v e examples o f t h e m i c r o s t r u c t u r a l development w i t h i n t h e m a t e r i a l s sequence i n v e s t i g a t e d , X-ray r e s u l t s (Table 1) i n d i c a t e t h a t besides

@-Si3N4 t h e J-phase Y4Si207N2 i s t h e major secondary c r y s t a l l i n e phase i n t h e s i n t e r e d specimens which increases from j u s t t r a c e s i n t h e 1800~C/10 min c o n d i t i o n t o s i g n i f i c a n t amounts i n t h e (1800°C/2h + 1820°c/2h) c o n d i t i o n . The post-heat t r e a t - ment procedure o f t h i s c o n d i t i o n , however, completely changes t h e c r y s t a l l i n e second- a r y phase assemblage t o H-phase Y5Si3OI2N and small amounts o f K-phase YSi02N.

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700

T

, 600.- f g ^500.-

* "7

d

5

^400.-

E U

. ^-f

300.-

.I--

Powder: LC 12

Additives: 15wt.%Y24+3.4wt.%A12h

zoo+ ^{V H T}Post-Heat Treated

Sintering Conditions

Fig. 2

-

Variation of flexural strength a t 1250 OC with s i n t e r i n g conditions and one post-heat treatment (flexural strength a t room temperature f o r comparison).

*In f a c t a sialon type Si6-xAlxOxN8-x solid solution with small f a c t o r x /4/

Sintering condition 1 8 0 0 ~ ~ / 1 0 m i n

1 8 0 0 ~ ~ / 2 h + 1820°c/2h post-heat treatment

Tab1 e 1 - X-ray r e s u l t s of sintered and post-heat treated Si3N4 materials.

Phases observed

8-Si3N4* J-phase ( t r a c e s ) 8-Si3N4* J-phase (major amount) (3-Si3N4* H-phase (major amount),

K-phase (small amount)

The liquid phase coexisting with 8-Si3N and J-phase a t s i n t e r i n g temperature in the system Y203

-

^Si02

-

SijN4 (neglecting ?he influence of the A1-component) e x h i b i t s a large compositional v a r i a b i l i t y , e s e c i a l l y in the

g

Y Og/SiO r a t i o a s v e r i f i e d by microanalytical data from the (1800 C/2h

+

1820°c/2hf condition (Fig. 3a). Thus, comparing the l a t t e r w i t h the 1 8 0 0 ~ ~ / 2 0 min condition, crystal 1 ization of J-phase will induce a homogenization e f f e c t with respect t o Y and S i . Moreover, precipita- tion of J-phase will cause the residual liquid to s h i f t t o Si02-richer compositions.

Liquid immiscibility on a submicron s c a l e as described t o e x i s t i n several oxi- n i t r i d e glass systems /7/ was not confirmed in t h i s study. However, there i s evi- denke f o r a Si-rich phase (grain s i z e 9 0 pm) r e s t r i c t e d t o extended soaking times only (including the post-heat treated specimens) which may r e s u l t from thermal decomposition of Si02-rich Y-A1-Si-0-N glassy pockets, according to /8/. The replace- ment of glassy phase by more refractory crystal 1 in e phases i n the t r i p l e junctions with increasing s i n t e r i n g conditions i s evident when comparing representative TEM micrographs in Figs. 4 and 5. As a consequence of the post-heat treatment procedure, the secondary phase assemblage moves t o mainly H-phase a t the expense of the residual liquid a f t e r nearly complete resorption of the J-phase. This i s demonstrated in the microanalytical data from Fig. 3b and the TEM micrograph in Fig. 6, respectively.

The r e s u l t s confirm the general concept t h a t additional improvement of high-tempera- t u r e properties of sintered Si3N4 may be achieved by means of partial c r y s t a l l i z a - tion of the amorphous phase when adopting optimized methods from glass ceramic tech- no1 ogy / 9 / .

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~ 1 - 3 4 4 JOURNAL DE PHYSIQUE

St3N, ( SNI Y2S$03NI I N-MI Y2S~,03N, ( N - M I '2 '3

Fig. 3 - Isothermal s e c t i o n s o f t h e Y203-SiO2-S!3N4 system (mol .%) a t 1 7 3 0 ' ~ (a) and 1550'~ ( b ) / 6 / , m o d e l l i n g coexistence o f o x i n i t r i d e s and l i q u i d phase i n s i n t e r e d Si3N4. Data p l o t t e d represent secondary phase compositions obtained by EDX from the (1800°C/2h + 1 8 2 0 ~ ~ / 2 h ) c o n d i t i o n ( a ) and t h e post-heat t r e a t e d m a t e r i a l ( b ) .

F i g . 4

-

TEM b r i g h t f i e l d images from t h e 1 8 0 0 ~ ~ / 1 0 min c o n d i t i o n ,

( a ) f3-Si3N4 m a t r i x phase w i t h h i g h aspect r a t i o , ( b ) glassy pocket a t t r i p l e j u n c - t i o n , confirmed by i t s t y p i c a l halo i n t h e s e l e c t e d area d i f f r a c t i o n ( S A D ) p a t t e r n .

F i g . 5 - TEM b r i g h t f i e l d images from t h e (1800'~/2h + 1820°c/2h) c o n d i t i o n , ( a ) g l o b u l a r i z e d 8-Si3N4 m a t r i x phase, ( b ) J-phase confirmed by SAD p a t t e r n .

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F i g . 6 - TEM b r i g h t f i e l d images from t h e post-heat t r e a t e d c o n d i t i o n ,

( a ) H-phase confirmed by SAD pattern, (b) area of homogeneous a d d i t i v e d i s t r i b u t i o n i n the m i c r o s t r u c t u r e .

I V

-

^SUMMARY

Mechanical p r o p e r t i e s o f s i n t e r e d (Yp03+A1 0 3 ) - f l u x e d Si3N4 a t room and h i g h temperatures e x h i b i t c o n t r a r y dependenc~es on $he s i n t e r i n g c o n d i t i o n s and/or t h e micro- s t r u c t u r a l c h a r a c t e r i s t i c s . Besides density, room-temperature s t r e n g t h i s m a i n l y a f u n c t i o n o f t h e p r i s m a t i c p-Si N4 g r a i n c h a r a c t e r i s t i c s , w h i l e i n excess o f 1000 OC

s t r e n g t h i s s t r o n g l y a f f e c t e d

gy

t h e c o n s t i t u t i o n o f t h e g r a i n boundary phases.

Prolonged soaking time r e s u l t s i n t h e p r e c i p i t a t i o n o f t h e J-phase which gets r e - sorbed d u r i n g a post-heat treatment due t o c r y s t a l l i z a t i o n o f H- and K-phases.

C r y s t a l l i z a t i o n o f these o x i n i t r i d e s , which i s associated w i t h t h e simultaneous f o r m a t i o n o f a S i 0 2 - r i c h glassy phase residue, improves t h e mechanical p r o p e r t i e s above 1000 ^OC.

REFERENCES

/1/ Wotting, G. and Z i e g l e r , G., Science o f Ceramics, 12, 361 (1983).

/2/ Wotting, G. and Z i e g l e r , G., CFI F o r t s c h r i t t s b e r . Gr DKG,

1,

33 (1985).

/3/ Majdic, M. and Z i e g l e r , G., Ber. Dt. Keram. Ges. 57, 140 (1980).

141 Braue, W., Dudek, H.J. and Z i e g l e r , G., subm. t o K k r o c h i m i c a Acta Suppl. 11, (1985).

/5/ Krivanek, O.L., Shaw, T.M. and Thomas, G., J . Am. Ceram. Soc.

62,

585 (1979).

/6/ Kaufman, L., Hayes, F. and B i r n i e , D., Calphad, 5, 163 (1981).

/7/ Drew, R.A.L., Hamsphire, S. and Jack, K.H., ProcT o f I n t e r n a t i o n a l Symposium on Ceramic Components f o r Engine, Hakone, Japan, 394 (1983).

/8/ Messier, D.R. and Deguire, E.J., J . Am. Ceram. Soc. 67, 602 (1984).

/9/ Braue, W., Wotting, G. and Z i e g l e r , G., subm. t o Second I n t e r n a t i o n a l Symposium an Ceramic M a t e r i a l s and Components f o r Engines, Lubeck-Travemunde, A p r i l 14-17, 1986.