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NOTE ON THE RELATION BETWEEN THE COMPRESSIVE STRENGTH OF DEBASED ALUMINA AND ITS USE AS HOT-PRESSING DIE
MATERIAL
G. de With
To cite this version:
G. de With. NOTE ON THE RELATION BETWEEN THE COMPRESSIVE STRENGTH OF
DEBASED ALUMINA AND ITS USE AS HOT-PRESSING DIE MATERIAL. Journal de Physique
Colloques, 1986, 47 (C1), pp.C1-667-C1-671. �10.1051/jphyscol:19861102�. �jpa-00225497�
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Colloque Cl, suppl6ment au n 0 2 , Tome 47, fdvrier 1986 page cl-667
NOTE ON THE RELATION BETWEEN THE COMPRESSIVE STRENGTH OF DEBASED ALUMINA AND ITS USE AS HOT-PRESSING DIE MATERIAL
G. DE WITH
Philips Research Laboratories, P.O. Box 80.000.
NL-5600 J A Eindhoven, The Netherlands
Resume
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La d u r a b i l i t 6 d e s c e r a m i q u e s d ' a l u m i n e employ6es d a n s les m a t r i c e s d e p r e s s a g e h chaud est tr&s v a r i a b l e . C ' e s t p o u r q u o i l a r e s i s t a n c e & l a c o m p r e s s i o n d e d i v e r s e s c 6 r a m i q u e s d t a l u m i n e a 6 t 6 d 6 t e r m i n 6 e e n f o n c t i o n d e l a t e m p e r a t u r e . Les r 6 s u l t a t s s o n t a n a l y s e s d a n s l e u r s r a p p o r t s a v e c l a m i - c r o s t r u c t u r e e t a v e c l a m o r p h o l o g i e d e s f r a c t u r e s . I1 n t y a aucune c o r r e l a - t i o n e n t r e les donn6es o b t e n u e s et l a d u r a b i l i t 6 d e s m a t e r i a u x a u p r e s s a g e B chaud. C e t t e p r o p r i 6 t e s e m b l e e t r e d e t e r m i n 6 e e x c l u s i v e m e n t p a r l t h o m o g 6 n 6 i - t 6 d e l a c e r a m i q u e e t p a r s o n a b s e n c e d e d e f a u t s .A b s t r a c t
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D u r a b i l i t y o f a l u m i n a c e r a m i c s when u s e d a s d i e m a t e r i a l i n com- p r e s s i v e l o a d i n g v a r i e s w i d e l y . The c o m p r e s s i v e s t r e n g t h o f v a r i o u s a l u m i n a c e r a m i c s was t h e r e f o r e d e t e r m i n e d as a f u n c t i o n o f t e m p e r a t u r e . The r e s u l t s a r e d i s c u s s e d i n terms o f t h e m i c r o s t r u c t u r e and f r a c t u r e morphology. The ( h o t - p r e s s i n g ) d u r a b i l i t y o f t h e m a t e r i a l s d o e s n o t c o r r e l a t e a t a l l w i t h t h e c o m p r e s s i v e s t r e n g t h d a t a o b t a i n e d . I n s t e a d , t h i s p r o p e r t y seems t o b e d e t e r m i n e d e n t i r e l y by t h e homogeneity and l a c k o f f l a w s i n t h e c e r a m i c .I
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INTRODUCTIONI n o u r l a b o r a t o r y d u r i n g h o t - p r e s s i n g e x p e r i m e n t s i n t h e t e m p e r a t u r e r a n g e o f 8 0 0 t o 1200°C and a t p r e s s u r e s o f 0.05 t o 0.15 GPa, major d i f f e r e n c e s i n d u r a b i l i t y were found between v a r i o u s a l u m i n a c e r a m i c s . The p r i m e p r o p e r t y t o s t u d y t h e n seems t o be t h e c o m p r e s s i v e s t r e n g t h o f t h e s e a l u m i n a ceramics. C o n t r a r y t o t h e t e n s i l e ( o r b e n d i n g ) b e h a v i o u r o f c e r a m i c s t h e c o m p r e s s i v e s t r e n g t h h a s r e c e i v e d o n l y s c a n t y a t t e n t i o n . A g e n e r a l d i s c u s s i o n on t h e i n t e r p r e t a t i o n o f t h e c o m p r e s s i v e s t r e n g h o f c e r a m i c s h a s been g i v e n by R i c e ( 1 ) . He r e l a t e s c o m p r e s s i v e s t r e n g t h t o h a r d n e s s . D a t a f o r o u r p a r t i c u l a r ceramics were l a c k i n g , however, and t h e r e f o r e t h e c o m p r e s s i v e s t r e n g t h was measured a s f u n c t i o n o f t e m p e r a t u r e .
11- EXPERIMENTAL
Although t h e measurement of t h e c o m p r e s s i v e s t r e n g t h , Sc, is c o n c e p t u a l l y s i m - p l e , e x p e r i e n c e w i t h t h i s t y p e o f measurement is n o t wide s p r e a d . A b r o a d d i s c u s - s i o n on t h e e x p e r i m e n t a l p r o b l e m s i s g i v e n by S i n e s and Adam ( 2 ) .
F o r t h r e e d i f f e r e n t t y p e s o f d e b a s e d a l u m i n a s p e c i m e n s were c u t o f l e n g t h 1 8 mm and d i a m e t e r 6 mm. These were t e s t e d between a l u m i n a d i e ' s ( d i a m e t e r 40 mm) a t a s t r a i n r a t e o f a b o u t 0 . 3 + 1 0 - ~ S-' a t v a r i o u s t e m p e r a t u r e s i n a i r . No i n t e r f a c e mate-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19861102
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r i a l was used. The measurements were done on a T i n i u s Olsen E l e c t o m a t i c u n i v e r s a l t e s t i n g machine i n a Kanthal wound alumina t u b e furnace temperature c o n t r o l l e d by a microcomputer. The ceramics were a l s o t e s t e d a t room temperature i n water, i n o r d e r t o check on e f f e c t s o f t h e environment on t h e compressive s t r e n g t h , Sc. The f r a c - t u r e s u r f a c e s o f t h e compression specimens were examined by scanning e l e c t r o n m i - croscopy (SEMI.
The m i c r o s t r u c t u r e was r e v e a l e d by o p t i c a l microscopy (OM) and SEM a f t e r p o l - i s h i n g (4-7 pm diamond) and t h e r m a l e t c h i n g i n a i r a t 1 2 5 0 " ~ f o r 4 hours. The g r a i n s i z e d i s t r i b u t i o n was determined by l i n e a r i n t e r c e p t measurements from approximate- l y 700 g r a i n s . The o v e r a l l chemical composition was determined by chemical a n a l y s i s w h i l e X - d i f f r a c t i o n was used t o determine t h e n a t u r e o f t h e second phase. The den- s i t y , q, was determined from w e i g t h and geometric data. The l o n g i t u d i n a l wave ve- l o c i t y , v l , a t 10 MHz and t h e t r a n s v e r s e wave v e l o c i t y , vs, a t 20 MHz were d e t e r - mined a t room temperature by t h e pulse-echo technique u s i n g Panametrics 5223 equip- ment. Young's modulus, E, and P o i s s o n ' s r a t i o , v, were c a l c u l a t e d from q, v 1 and vs a c c o r d i n g t o t h e u s u a l formulae f o r i s o t r o p i c m a t e r i a l s . The f r a c t u r e toughness, K p , a t room temperature was measured i n a t h r e e - p o i n t bending set-up i n d r y a i r
(dew p o i n t
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42°C) u s i n g specimens o f s i z e 3*9*45 mm3 a t a cross-head speed o f 1.0 mm/min. A n o t c h w i t h a r e l a t i v e depth o f 0.15 and a w i d t h o f about 150 pm was sawn i n each specimen. P r e c r a c k i n g was done b y a Knoop i n d e n t a t i o n ( 5 N l o a d ) a t t h e n o t c h r o o t on b o t h s i d e s o f t h e specimen.B l o c k s o f t h e d i f f e r e n t m a t e r i a l s n o r m a l l y used a t h o t - p r e s s i n g d i e ' s t y p i - c a l l y o f 100 mm diameter and 50 mm h e i g h t , were t e s t e d u l t r a s o n i c a l l y u s i n g Sonic Mark I V equipment. Test f r e q u e n c i e s used were 5, 10 o r 15 MHz w i t h t r a n s d u c e r s h a v i n g a diameter o f 1/4 o r 1/2 inch.
I11
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RESULTSI n t a b l e 1 t h e main c h a r a c t e r i s t i c s o f t h e v a r i o u s ceramics a r e presented.
Major d i f f e r e n c e s between t h e m a t e r i a l s a r e found i n t h e amount and n a t u r e o f t h e a d d i t i v e s and i n t h e g r a i n s i z e . I n a l l cases t h e g r a i n s i z e d i s t r i b u t i o n was mul- t i - m o d a l log-normal and t h e o v e r a l l modal g r a i n s i z e , D, i s i n d i c a t e d . The d i f f e r - ences between t h e m a t e r i a l s are a l s o r e f l e c t e d by t h e values o f K.,I A s i g n i f i - cant decrease o f t h e compressive s t r e n g t h i n water was observed.
Table 1: C h a r a c t e r i s t i c s o f t h e v a r i o u s alumina ceramics
A t low temperatures t h e specimens o f a l l m a t e r i a l s e s s e n t i a l l y exploded form- i n g powder, many t i n y c h i p s and some l a r g e r pieces. I n t h e temperature range of1000 t o 1200°C t h e specimens were m o s t l y sheared i n t h e middle.
M a t e r i a l
q( g/cm3
X-ray (2nd phase) D(Y~[))
w t ,O S i w t X Ca wt X Mg E(GPa)
;Ic(MPa.m1/2) Sc(GPa), a i r Sc(GPa), water
A B C
3.85 3.86 3.80
s p i n e 1
-
s p i n e 115 3 1 6.4
0.050 0.10 1.9
0.046 0.020 0.31
0.58 0.037 0.27
369 369 347
0.236 0.235 0.240
4.10(0.22) 383(0.08) 5.66(0.20)
2.11(0.11) 1.90(0.13) 1.83(0.21)
1.33(0.14) 0.96(0.20) 1.67(0.33)
Sample s t a n d a r d d e v i a t i o n s a r e g i v e n i n parentheses.
2 . 5
o : ceramic C n : cerarnlc A X : ceramic B
1 . 0
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I n f i g . 1 t h e compressive
s t r e n g t h o f t h e v a r i o u s c e r a m i c s i s p r e s e n t e d . A t room t e m p e r a t u r e t h e compressive s t r e n g t h c o r r e - l a t e s w e l l w i t h t h e f r a c t u r e toughness. Apart from t h e maximum i n t h e compressive s t r e n g t h val- ues f o r one o f t h e m a t e r i a l s , no l a r g e d i f f e r e n c e s i n t e m p e r a t u r e dependence between t h e v a r i o u s c e r a m i c s a r e observed. The r e l a - t i v e change with t e m p e r a t u r e R ( = l/Sc.dSc/dT) was about 1.1 * 1 0 - ~ K-I i n t h e t e m p e r a t u r e range o f 400 t o 1200°C.
The d a t a f o r ceramic C a r e a c t u - a l l y t h e a v e r a g e o f d a t a from two nominally t h e same m a t e r i a l s b u t produced a s rod and p l a t e . A s s h o u l d be t h e c a s e , t h e micro- s t r u c t u r e , chemical composition, mechanical p r o p e r t i e s and f r a c -
, , , , , t u r e morphology were t h e same
400 600 1000 1200 1400 w i t h i n e x p e r i m e n t a l accuracy.
Consequently t h e d a t a o b t a i n e d t e m p e r a t u r e ["Cl were averaged.
F i g . 1. Compressive s t r e n g t h o f t h e v a r i o u s c e r a m i c s a s a f u n c t i o n o f t e m p e r a t u r e .
For a l l c e r a m i c s c l e a r twinning phenomena were observed upto 800°C. These phe- nomena were o c c a s i o n a l l y observed a t much h i g h e r t e m p e r a t u r e s . I n g e n e r a l t h e f r a c - t u r e mode was mainly t r a n s g r a n u l a r below about 80O0C, changing t o mainly i n t e r g r a n - u l a r above t h a t t e m p e r a t u r e .
On t h e f r a c t o g r a p h s v a r i o u s m i c r o f r a c t u r e phenomena were observed. Microcracks due t o twinning o r o r i g i n a t i n g from g r a i n boundary o r t r i p l e j u n c t i o n v o i d s but a l s o from p o r e s w i t h i n a s i n g l e g r a i n . I n a number o f c a s e s t h e l i n k i n g o f microcracks
Fig. 2. M i c r o f r a c t u r e phenomena a s d e t e c t e d on t h e f r a c t u r e s u r f a c e s . L e f t : m i c r o f r a c t u r e o r i g i n a t i n g from a p o r e i n s i d e a g r a i n .
Right: twinning l i n k e d t o m i c r o f r a c t u r e o r i g i n a t i n g from a g r a i n boundary pore.
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IV
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DISCUSSION OF PREVIOUS DATA AND MEASUREMENTSFrom t h e s c a r c e l i t e r a t u r e d a t a one c a n d i s t i n g u i s h two c l a s s e s o f a l u m i n a c e r a m i c s a s f a r a s S, is c o n c e r n e d . R a t h e r p u r e m a t e r i a l s w i t h a n S, v a l u e t y p i c a l - l y above 3 GPa and a n R v a l u e o f a b o u t 5
*
10-4 K-I (3-8) and t h e l e s s p u r e m a t e r i - a l s w i t h Sc t y p i c a l l y a r o u n d 2 GPa and an R v a l u e o f a b o u t 1*
10-3 K-I (4-9). (The m a t e r i a l o f Nash ( 9 ) , a l t h o u g h n o t e x p l i c i t l y s t a t e d , p r o b a b l y c o n t a i n s a l s o SiOp s i n c e h e u s e d c o m m e r c i a l l y a v a i l a b l e m a t e r i a l s ) .The v a l u e s o b t a i n e d f o r o u r r a t h e r i m p u r e c e r a m i c s ( s e e t a b l e ) a t room temper- a t u r e a r e t y p i c a l l y 2 GPa and t h u s c o n s i s t e n t w i t h t h o s e o f t h e s e c o n d c l a s s . A l s o t h e R v a l u e o f o u r c e r a m i c s is s i m i l a r t o t h e rate o b s e r v e d by Dawihl and D o r r e ( 4 ) f o r t h e i r s i l i c a t e c o n t a i n i n g m a t e r i a l . C o n s e q u e n t l y t h e c o n c l u s i o n must be t h a t f o r t h e s e c e r a m i c s t h e f l a w d e n s i t y is h i g h e r ( l o w e r s t r e n g t h v a l u e ) and t h a t c a v i t a t i o n is e a s i e r ( l a r g e r r a t e o f d e c r e a s e ) .
Most s t u d i e s o f t h e c o m p r e s s i v e s t r e n g t h o f a l u m i n a were done on t r a n s l u c e n t m a t e r i a l ( L u c a l o x ) , p a r t i c u l a r l y by L a n k f o r d (5-8). A s i g n i f i c a n t s t r a i n r a t e e f - f e c t was o b s e r v e d ( 5 ) b u t a n i n f l u e n c e o f t h e e n v i r o n m e n t , s o m a n i f e s t l y p r e s e n t i n t e n s i l e t e s t i n g , was n o t found. I n t e s t i n g t h e s e r a t h e r p u r e m a t e r i a l s a t room tem- p e r a t u r e , t w i n n i n g was o b s e r v e d a t l e a s t t o some e x t e n t o v e r a w i d e l y v a r y i n g s t r a i n - r a t e r a n g e ( 5 ) . The t w i n n i n g l e d t o m i c r o c r a c k i n g s t a r t i n g m o s t l y a t g r a i n b o u n d a r i e s . M i c r o c r a c k s a l s o s t a r t e d a t v o i d s a t t r i p l e j u n c t i o n s . After n u c l e a t i n g some growth of t h e m i c r o c r a c k s t o o k p l a c e . The c o a l e s c e n c e o f t h e m i c r o c r a c k s t h e n l e d t o c a t a s t r o p h i c a l f r a c t u r e . A t h i g h e r t e m p e r a t u r e s c a v i t a t i o n i n t h e (supposed- l y g l a s s y ) g r a i n boundary p h a s e was s a i d t o t a k e p l a c e . Growth and c o a l e s c e n c e a g a i n l e d t o c a t a s t r o p h i c a l f r a c t u r e . The o b s e r v e d maximum i n c o m p r e s s i v e s t r e n g t h is e x p l a i n e d b y a d e c r e a s e d e f f e c t i v e n e s s o f t h e t w i n n i n g mechanism w h i l e t h e g r a i n boundary p h a s e i s n o t y e t e a s y f l o w i n g ( 6 ) .
A l l t h e phenomena d i s c u s s e d e x t e n s i v e l y by L a n k f o r d f o r t r a n s l u c e n t a l u m i n a were o b s e r v e d on t h e s e d e b a s e d a l u m i n a ' s a s w e l l . I n p a r t i c u l a r t h e s u g g e s t i o n t h a t p o r e s a r e p r o b a b l y a l s o a s o u r c e o f m i c r o f r a c t u r e ( 7 ) h a s b e e n f o u n d t r u e .
A maximum i n S, was o b s e r v e d f o r c e r a m i c C o n l y . T h i s is p r o b a b l y r e l a t e d t o t h e l a r g e amount o f S i 0 2 m a i n l y p r e s e n t i n t h e g r a i n boundary p h a s e . The f r a c t u r e t o u g h n e s s o f d e b a s e d a l u m i n a shows a maximum a t a b o u t 800°C a s shown by Davidge a n d Tappin ( I O ) , which is e x p l a i n e d by a r e d u c t i o n o f t h e c r a c k t i p stress by v i s c o u s f l o w o f t h e g l a s s y s e c o n d p h a s e , e f f e c t i v e l y i n c r e a s i n g e n e r g y d i s s i p a t i o n . A t s l i g h t l y h i g h e r t e m p e r a t u r e v i s c o u s g r a i n boundary s l i d i n g , i n t r o d u c i n g p o r e s , re- s u l t s i n a s t r e n g t h d e c r e a s e . The v i s c o u s f l o w d i s s i p a t i o n mechanism i s l i k e l y t o o p e r a t e i n c o m p r e s s i v e f r a c t u r e as w e l l , b e s i d e s t h e a l r e a d y mentioned t w i n n i n g and c a v i t a t i o n mechanisms, b u t o n l y i f enough g l a s s y g r a i n boundary p h a s e i s p r e s e n t . The a b s e n c e of a maximum i n S, f o r t h e o t h e r c e r a m i c s i s p r o b a b l y d u e t o t h e much e a s i e r c a v i t a t i o n a s compared w i t h t r a n s l u c e n t alurnina.
C o n t r a r y t o t h e r e s u l t s o f L a n k f o r d ( 5 ) , a s i g n i f i c a n t e n v i r o n m e n t a l e f f e c t was o b s e r v e d by Nash ( 9 ) . F o r a 97.52 p u r e a l u m i n a t h e v a l u e o f S, d r o p p e d from 1.7 GPa when t e s t e d i n a i r t o 1.2 GPa when t e s t e d i n a p h y s i o l o g i c a l s a l t s o l u t i o n . S i m i l a r l y f o r 99.5% p u r e m a t e r i a l , S, dropped from 2.2 GPA t o 1.7 GPA. A s i g n i f i - c a n t d e c r e a s e i n S, was a l s o o b s e r v e d i n o u r c a s e . T h i s p r o b a b l y due t o t h e f a c t t h a t t h e s p e c i m e n s u s e d by Nash ( a n d o u r s ) were ground on t h e o u t e r s u r f a c e s w h i l e f o r t h e s p e c i m e n s o f L a n k f o r d t h e s e s u r f a c e s were a s - f i r e d . The d e n s i t y o f t h e f l a w s on t h e o u t e r s u r f a c e s , s e n s i t i v e t o t h e e n v i r o n m e n t , is much s m a l l e r i n t h e l a t t e r c a s e t h u s r e d u c i n g t h e e n v i r o n m e n t a l e f f e c t .
V
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RELATION WITH HOT-PRESSING PRACTICEComparing t h e v a l u e s of Sc w i t h t h e p r e s s u r e s n o r m a l l y a p p l i e d d u r i n g h o t - p r e s s i n g it is c l e a r t h a t t h e l a t t e r are g e n e r a l l y much l o w e r . N e v e r t h e l e s s f r a c - t u r e o c c u r s . The o r d e r o f l o n g e r d u r a b i l i t y i n ( h o t - p r e s s i n g ) p r a c t i c e is: A>>B) C. T h i s o r d e r d o e s c o r r e l a t e weakly w i t h t h e room t e m p e r a t u r e c o m p r e s s i v e s t r e n g t h
(order: A ) B = C) b u t t h i s c o r r e l a t i o n i s i r r e l e v a n t s i n c e h o t - p r e s s i n g i s done a t h i g h e r temperatures. I t does n o t c o r r e l a t e w i t h t h e h i g h temperature compressive s t r e n g t h ( A = B = C). Slow c r a c k growth may be i m p o r t a n t s i n c e a t room temper- a t u r e a c l e a r environmental e f f e c t was observed. More i m p o r t a n t seems t o be, howev- e r , t h e homogeneity o f t h e m a t e r i a l when d e l i v e r e d i n l a r g e blocks. For m a t e r i a l A t h e a s - d e l i v e r e d b l o c k s used f o r h o t - p r e s s i n g a r e e s s e n t i a l l y f r e e o f macroflaws (as t e s t e d b y pulse-echo technique) b u t t h e o t h e r ceramics o f t e n show some l a r g e inhomogeneities probably due t o e x t r u d i n g o r p r e s s i n g ( f i g . 3). F a i l u r e o f t h e b l o c k s d u r i n g h o t - p r e s s i n g i s o f t e n o r i g i n a t i n g from these d e f e c t s . T h i s make t h e S, measurements i r r e l e v a n t . The p r o c e s s i n g o f t h e ceramics i s thus, again, shown t o be o f v i t a l importance even i n t h e case o f compressive l o a d i n g which i s u s u a l l y considered as a s a f e t y p e o f loading.
F i g . 3. U l t r a s o n i c p u l s e echo p a t t e r n s (A-scans) o f a p a r t i c u l a r b l o c k o f ceramic B t a k e n a t 10 MHz and g a i n 63 dB.
L e f t : proper p a t t e r n from t h e edge showing o n l y t h e t r a n s m i t t i n g p u l s e ( l e f t ) and backside echo ( r i g h t ) .
R i g h t : p a t t e r n from t h e c e n t r e showing a l s o two defects.
V 1
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ACKNOWLEDGEMENTMany thanks a r e due t o Mr. H.C. Smulders f o r t h e compressive s t r e n g t h measure- ments and M r . C.J. Geenen f o r t h e SEM photography.
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