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REACTIONS BETWEEN THE PHASES IN GLASS-ALUMINA CERAMIC COMPOSITES
W. Schiller, U. Mücke, D. Linke
To cite this version:
W. Schiller, U. Mücke, D. Linke. REACTIONS BETWEEN THE PHASES IN GLASS-ALUMINA CERAMIC COMPOSITES. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-509-C1-514.
�10.1051/jphyscol:1986176�. �jpa-00225607�
REACTIONS BETWEEN THE PHASES IN GLASS-ALUMINA CERAMIC COMPOSITES
W. SCHILLER, U. ~ C K E and D. LINKE
Akademie der Wissenschaften der DDR, Zentralinstitut fiir
Anorganische Chemie, D D R - 1 1 9 9 Berlin, R u d o w e r Chaussee 5, D . R . G .
R6sum6 - Des r e a c t i o n s qui se produisent e n t r e 700 e t 1200 OC ont m u d i 6 e s p a r d i v e r s mkthodes (ATD d i f f r a c t i o n X e t RMN
& haute r 8 s o l u t i o n s u r Iy~l par exemplej. On a indlque l a f o r t e influence de c e s r e a c t i o n s s u r l e f r i t t a g e e t s u r l e a proprig- t 8 s d e s materiaux form6s.
Abstract - Reactions occuring a t temperatures between 700 and -were i n v e s t i g a t e d by d i f f e r e n t ethods, e. g. by DTA, X-ray d i f f r a c t i o n and high r e s o l u t i o n ~ % 1 IYMR. The strong in-
fluence 09 these r e a c t i o n s on the s i n t e r i n g behaviour of the compacts and on t h e p r o p e r t i e s of the r e s u l t i n g m a t e r i a l s i s i n d i c a t e d , too.
Mixtures c o n s i s t i n g of 40 vol% corundum p a r t i c l e s and 60 vol% o f a powdered notaasium boroalumosilicate g l a s s s i n t e r a t t e m p e r a t u r e s o f
800 - 1000 OC t o dense ceramic bodies. Although one assumes f o r such
"Kesamovitron" m a t e r i a l s /1,2/ a simple s t r u c t u r e of the type matrix/
disperse phase/pore, some complications a r i s e from the f a c t t h a t mu- t u a l i n t e r a c t i o n of the two components occurs /3/. This corresponds with e a r l i e r r e s u l t s reported by Pask /4/ and a l s o by S t e t t and Ful- r a t h /5/; these authors found t h a t nepheline i s formed a t hot-pres- s i n g of corundum with a sodium s i l i c a t e g l a s s . Nevertheless, what we know about such processes was s t i l l very i n s u f f i c i e n t .
Our aim was t h e r e f o r e
- t o study the mentioned r e a c t i o n s i n g r e a t e r d e t a i l ,
- t o i n d i c a t e the c o r r e l a t i o n s between the r e a c t i o n behaviour of glass/corundum compacts on the one s i d e , and the s i n t e r i n g and c r y s t a l l i z a t i o n processes on the o t h e r ,
- t o show the consequences of t h e phase boundary r e a c t i o n s f o r the formation of t h e m i c r o s t m c t u r e and f o r the p r o p e r t i e s of the sin- t e r e d products.
The present paper i s not intended t o be a complete t r e a t i s e of the subject. It i s predominantly concerned w i t h the first problem. F'ur- t h e r information about the s i n t e r i n g behaviour can be obtained from
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986176
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o u r paper /6/; f o r a more d e t a i l e d d e s c r i p t i o n of some consequences o f t h e r e a c t i o n s i n r e g a r d t o m i c r o s t r u c t u r e and p r o p e r t i e s s e e /7/.
I1 - EXPERIMENTAL WTHODS
The i n i t i a l compositions of t h e potassium b o r o s i l i c a t e g l a s s e s used f o r t h i s study a r e s i t u a t e d i n t h e composition t r i a n g l e K 0-B 0 -Si02 i n s i d e t h e f i e l d s o f primary c r y s t a l l i z a t i o n o f boroleuc&e, %?e.
Table 1 - Chemical composition, s o f t e n i n g resp. t r a n s f o r m a t i o n temperatures and c o e f f i c i e n t s of thermal expansion OL of t h e used g l a s s e s
Composition (mol%) ( a)
P r o p e r t i e s T s o f t (K)
Glass no.
1 2 3 4
( a ) composition of b o r o l e u c i t e f o r comparison (molb):
Si02 66,67 / B203 16,67 / K20 16,67 ( b ) f o r corundum: 1 0 6 a 7 , 5 K"
K20*B203*4 Si02 ( g l a s s e s No. 1 and 2 ) , and c r i s t o b a l i t e (No. 3 ) , respectxvely. I n o r d e r t o o b t a i n g l a s s e s s t a b l e a g a i n s t r e c r y s t a l l i - z a t i o n , v a r y i n g amounts o f alumina were added t o t h e i n i t i a l mixtures ( e . g. g l a s s Wo. 4). Table 1 g i v e s t h e d a t a f o r t h e s e f o u r s e l e c t e d g l a s s e s . The chemical composition of each p a r t i c u l a r g l a s s w a s d e t e r - mined by wet chemical a n a l y s i s .
The p r e p a r a t i o n method f o r t h e compaction of t h e i n i t i a l powders w i t h mean g r a i n s i z e s of some micrometers t o t h e f i n a l specimen ready f o r
f u r t h e r i n v e s t i g a t i o n s c o n s i s t e d of following s t e p s : S l i p p r e p a r a t i o n and spray-drying, dry-pressing, burn o u t o f temporary a d d i t i v e s , s i n - t e r i n g and f i n i s h i n g . A t s i n t e r i n g temperatures o f about 1200 K we g o t ceramic m a t e r i a l s w i t h about 5 % r e s i d u a l p o r o s i t y .
( 1 ) Right p a r a l l e l e p i p e d s of p r e - s i n t e r e d dense alumina were prepa- r e d c o n t a i n i n g a s l i t f i l l e d w i t h g l a s s ; t h e s l i t width was about 200 micrometers, i. e. small enough t o avoid melt convec-
t i o n d u r i n g t h e f u r t h e r thermal treatment ( s e e a l s o Fig. 1 o r /12/).
e l e c t r o n micmprobe a n a l y s i s ? I ) , DTA and d i l a t o m e t r y /a/, q u a n t i t a - t i v e ceramography, i. e. automatic l i n e a r a n a l y s i s u s i n g a n a n a l y s a t o r EPIQUANT (VEB C a r l Z e i s s Jena) /9/, s e l e c t i v e d i s s o l u t i o n of t h e g l a s s y phase, followed by chemical a n a l y s i s of t h e s o l u t e / l o / , and h i g h r e s o l u t i o n 2 7 ~ 1 NMR measurements / I I / .
111 - RESULTS
Depending on t h e a c t u a l f i r i n g temperature one h a s t o d i f f e r e n t i a t e between t h e t h r e e r e a c t i o n s t e p s A, B, C:
A: The r e a c t i o n s t a r t s a t about 1000 K , i n t h e temperature r e g i o n
- between t h e lower l i m i t f o r s 'nkage and t h e s o f t e n i m temperature of t h e used g l a s s e s , T s o f t , i Kf. Rom e l e c t r o n microprobe a n a l y s e s i t may be deduced t h a t potasslum aluminates a r e formed a t t h e phaseboun- dary between corundum and g l a s s m e 1 t . T h i s r e a c t i o n provides t h e d r i - v i n g f o r c e f o r t h e s t e e p r i s e o f t h e potassium resp. oxygen c o n t e n t i n c l o s e proximity t o t h e zone o f c o n t a c t (Fig. 1 ) . The r e a c t i o n - induced boundary l a y e r s w i t h a t h i c k n e s s of some micrometers can be v i s u a l i z e d by e t c h i n g ( F i g . 2 ) . Microscopically one can a l s o see micro-cracks around t h e corundum p a r t i c l e s , d e s p i t e o f t h e c l o s e pro- ximity of thermal expansion f o r corundum and f o r t h e g l a s s under con- s i d e r a t i o n , r e s p e c t i v e l y (Table 1 ) . I n a preceding paper /7/ we concluded i n a q u a n t i t a t i v e manner t h a t t h e c r a c k s a r e r e a l l y due t o changes i n composition and p r o p e r t i e s of t h e boundary l a y e r .
To proof t h e formation of potassium aluminates we analyzed t h e s o l u t e s obtained by s e l e c t i v e e t c h i n g t h e s i n t e r e d and p o l i a h e d samples a t room temperature w i t h a very d i l u t e a c i d i c s o l u t i o n ( 1 % HC1/ 0 , l % HF) . Under t h e s e mild c o n d i t i o n s d i s s o l u t i o n i s most- l y r e s t r i c t e d on t h e boundary l a y e r s around t h e corundum g r a i n s . The microanalyses /lo/ showed w i t h i n the l i m i t s o f s t a t i s t i c a l e r r o r t h a t
f o r samples s i n t e r e d a t temperatures up t o 1220 K v a l u e s f o r t h e mo- l a r r a t i o K20/A120 n e a r 1,O a r e obeyed. Therefore, t h e boundary l a y e r i s b e l i e v e d $0 c o n s i s t o f KA102 and the r e a c t i o n proceeds according t o t h e s i m p l i f i e d equation:
A1203 + K20-B203*4 Si02 d 2 KA102 + B203 + 4 Si02.
b o r o l e u c i t e
Due t o t h i s r e a c t i o n t h e l a s s melt n e a r t o t h e product l a y e r becomes poor i n potass2um (Fig. 17 which f o r c e s t h e boron atoms o f t h e boro- l e u c i t e - l i k e u n i t s t o undergo a rearrangement from t h e c o o r d i n a t i o n number 4 t o 3; simultaneously sixfold-coordinated aluminium becomes a t e t r a h e d r a l environment. e change i n t h e c o o r d i n a t i o n number o f boron could be v e r i f i e d by RB I U R s p e c t r a / I I / . I n a d d i t i o n , t h e v a l u e s !Pg of t h e l a s s y phase i n t h e s i n t e r e d m a t e r i a l , determined by d i l a t o m e t r y /87, a r e i n d i c a t i v e of such a change.
( 2 ) t h e l e t t e r ' i ' denotes t h e i n i t i a l composition of t h e g l a s s under c o n s i d e r a t i o n ; Tsoft w i l l be a l t e r e d d u r i n g r e a c t i o n according t o t h e a c t u a l composition of t h e g l a s s y phase.
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B: A t temperatures above T s o f t i, i. e. a t 1100 K and beyond f o r the z a s s e s under consideration, t&e p r i m a r i l formed aluminate l a y e r s
begin t o d i s s o l v e i n the g l a s s melt. The 3 7 ~ 1 NMR s p e c t r a / ? I / speak i n favour of the formation of symmetric alumosilicate groups; X-ray and DTA d a t a /8/ make evident t h a t sometimes well-defined alumosili- c a f e s ( e . g. l e u c i t e ) a r e p r e c i p i t a t e d from the melt. Such a crystal- l i z a t i o n i s f a c i l i t a t e d by presence of seed c r y s t a l s with ~ i m i l a r s t r u c t u r e (e. g. boroleucite) ; simultaneously, the g l a s s melt w i l l be enriched i n boron.
Fig. 1 - Weight f r a c t i o n s ( i n $1 f o r s i l i c o n , oxygen, otassium and aluminium versus the distance from the boundary l a y e r PBL) corundum/
g l a s s No. 3 , obtained by e l e c t r o n microprobe a n a l y s i s onto s p e c i a l prepared model samples; t r e a t e d 2 hours a t 1170 K (open symbols, bro- ken l i n e s ) resp. a t 1720 K ( f i l l e d symbols, s o l i d l i n e s ) . The abao- l u t e v a l u e s f o r oxygen a r e only approximative.
C: A t s i n t e r i n g t e m p e r a k r e s above 1300 K the degree of the r e a c t i o n between melt and corundum hardly increases. There i s a strong tendency
t o d i s s o l u t i o n of a l l c r y s t a l l i n e phases fomed meanwhile. A s a con- sequence, the molten g l a s s becomes more and more homogeneous; the p l o t s of the weight f r a c t i o n s of potassium resp. oxygen v e r s u s the d i s t a n c e fromthephase boundary y i e l d s t r a i g h t l i n e s p a r a l l e l t o the a b s c i s s a (Fig. 1 ) . The r e a c t i o n degree approaches a l i m i t when the melt a t the given temperature i s s a t u r a t e d with aluminium.
BL BL .-
1 60-
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, - 0 - - - - - 0 - - - - - 0
a s -
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- - - - - - - -
A
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1225 K, p o l i s h i n g and e t c h i n g ( 1 0 % s o l u t i o n of HC1, 60 min a t 80 "c) :
on t h e l e f t : l i g h t m i c r o s c o p e , reflectedlight,magnific3ation 1400 : 1;
on t h e r i g h t : scanning e l e c t r o n microscope, m a g n i f i c a t i o n 2250 : 1 ; ( 1 ) corundum p a r t i c l e s ( 2 ) micro-cracks around them
( 3 ) g l a s s y m a t r i x ( 4 ) reaction-induced boundary l a y e r s .
IIJ - CONCLUSIONS
1 ) The u n d e r s t a n d i n g o f t h e reaction-induced boundary l a y e r s i n s i n t e r e d glass/corundum compacts g i v e s an i n s i g h t i n t o t h e mechani- c a l and o t h e r p r o p e r t i e s of glass/corundum m a t e r i a l s . For i n s t a n c e , t h e thermal s t r e s s e s caused by m i m a t o h i n thermal expansion of t h e b o t h p h a s e s and o b s e r v a b l e by X-ray d i f f r a c t i o n /13/ a r e much i n f l u - enced by composition changes of t h e boundary l a y e r s /7/.
2) I t i s supposed t h a t i n many o t h e r c a s e s one h a s t o t a k e i n t o con- s i d e r a t i o n t h e formation o f boundary l a y e r s a s r e s u l t o f r e a c t i o n s o c c u r i n g a t s i n t e r i n g between m a t r i x and d i s p e r s e phase o f ceramic compacts. A t l e a s t , o u r r e s u l t s can be g e n e r a l i z e d f o r o t h e r g l a s s / corundum m a t e r i a l s , e. g. f o r t'Keramovitron" m a t e r i a l s w i t h s i l i c a t e g l a s s e s c o n t a i n i n g Na 0 , CaO o r ZnO i n s t e a d of K20, f o r m a t e r i a l s w i t h much h i g h e r alumfna c o n t e n t s and/or h i g h e r p o r o s i t y /12/.
ACKNOWLEDGEMENT
The a u t h o r s g r a t e f u l l y acknowledge t h e experimental a s s i s t a n c e and c o o p e r a t i o n of t h e i r c o l l e a g u e s J. Graul, I. Kosche, Th. Moelders, I. Schiemann, Ch. SchriSter, E. Sonntag and D r . J. Wiegmann. They l i k e t o thank a l s o f o r t h e e l e c t r o n microprobe a n a l y s e s undertaken by D r . J. Klosowski, C e n t r a l I n s t i t u t e of S o l i d - S t a t e P h y s i c s and M a t e r i a l s Research o f t h e Academy of S c i e n c e s of t h e GDR, Dresden.
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