INFLUENCE OF PROCESSING METHOD ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF MULLITE/ZrO2 COMPOSITES

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INFLUENCE OF PROCESSING METHOD ON THE MICROSTRUCTURE AND MECHANICAL

PROPERTIES OF MULLITE/ZrO2 COMPOSITES

P. Miranzo, M. Osendi, J. Moya

To cite this version:

P. Miranzo, M. Osendi, J. Moya. INFLUENCE OF PROCESSING METHOD ON THE MI- CROSTRUCTURE AND MECHANICAL PROPERTIES OF MULLITE/ZrO2 COMPOSITES.

Journal de Physique Colloques, 1986, 47 (C1), pp.C1-417-C1-421. �10.1051/jphyscol:1986162�. �jpa-

00225593�

INFLUENCE OF PROCESSING METHOD ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF MULLITE/ZrO, COMPOSITES

P. MIRANZO. M . I . OSENDI and J.S. MOYA

Institute d e ergm mica y V i d r i o , C.S.I.C., Km 24.3 C t r a . , Madrid-Valencia, Arganda d e l Rey, Madrid, S p a i n

Resum6 - Des m a t e r i a u x c o m p o s i t e s M u l l i t e / Z r O n o n t 6 t & p r e p a r g s p a r d e w mgthodes d i f f g r e n t e s : f r i t t a g e e t f r i t t a g e r g a c t i f . Les p r o p r i g t g s mgcaniques e t l e s mecanismes de t e n a c i t g d e s deux com- p o s i t e s o n t 6 t 6 a n a l y s g s s u r l a b a s e d e c o n s i d 6 r a t i o n s m i c r o s t r u c - t u r a l e s .

A b s t r a c t - M u l l i t e / Z r 0 2 c o m p o s i t e s have been p r e p a r e d b y two d i f -

-- f e r e n t methods: s i n t e r i n g and r e a c t i o n s i n t e r i n g . The mechanical p r o p e r t i e s and toughening mechanisms of b o t h c o m p o s i t e s a r e ana- l i z e d on t h e b a s i s o f m i c r o s t r u c t u r a l c o n s i d e r a t i o n s .

I - INTRODUCTION

High t e c h n o l o g y m u l l i t e / Z r 0 2 c o m p o s i t e s have c r e a t e d a g r e a t d e a l of i n t e r e s t due t o t h e i r p o t e n t i a l t e c h n i c a l a p p l i c a t i o n s . T o p r o d u c e t h e - s e m a t e r i a l s two d i f f e r e n t p r o c e s s e s have been f o l l o w e d i n t h e p r e s e n t work: i ) S i n t e r i n g o f m u l l i t e p r e c u r s o r ( p r e m u l l i t e ) and z i r c o n i a pow d e r s ( t h i s sample w i l l be named MZI), ii) r e a c t i o n s i n t e r i n g o f ZrSiOb and A l 2 O 3 powders (MZ2 sample) i n s t o i c h i o m e t r i c p r o p o r t i o n s a c c o r d i n g t o t h e e q u a t i o n :

2ZrSiO,, + 3A1,0,<- 2Zr0, + 2Si02.3A1203 111

A c o m p a r a t i v e s t u d y of t h e s i n t e r i n g b e h a v i o u r , m i c r o 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 and mechanical p r o p e r t i e s o f b o t h c o m p o s i t e s h a s been made.

I1 - EXPERIMENTAL

The p r o c e s s i n g of t h e powders was t h e same f o r b o t h samples: a t t r i t i o n m i l l i n g i n i s o p r o p y l a l c o h o l and i s o s t a t i c p r e s s i n g a t 200 MPa.

The h e a t t r e a t m e n t was: a ) 2 . 5 h o u r s a t 1 5 7 0 ' ~ f o r t h e sample MZ1 and b ) a s t e p a t 1400°c d u r i n g 3 h o u r s and 4 h o u r s a t 1 5 7 0 ' ~ f o r MZ2.

A f t e r t h e s e t h e r m a l t r e a t m e n t s b o t h samples r e a c h e d a d e n s i f i c a t i o n of - 9 8 % t h and have 20 v o l % o f Z r O 2 .

The s i n t e r i n g b e h a v i o u r h a s been s t u d i e d u s i n g a d i l a t o m e t e r f u r n a c e . The m i c r o s t r u c t u r e s have been o b s e r v e d by a s c a n n i n g e l e c t r o n microsco- p e (SEM) on p o l i s h e d s u r f a c e s . The bend s t r e n g t h (OF) h a s b e e n m e a s u r e d on non machined c y l i n d r i c a l b a r s by t h e 3 - p o i n t bending method a n d t h e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986162

JOURNAL DE PHYSIQUE

critical stress intensity factor (KIc) has been determined by indenta- tion on polished surfaces.

The grain size distribution was determined using a MORFHOMAT30 Imaging Analyser on SF24 micrographs corresponding to different areas of thepo- lished surfaces of the samples.

I11 - ^RESULTS AND DISCUSSION

The relative shrinkage v.s. temperature of both the samples is plotted

F i g . 1 . - R e l a t i v e s h r i n k a g e 0 . s . t e m p e r a t u r e f o r t h e s a m p l e s M Z Z and M Z 2 ( . H e a t i n g r a t e - 1 0 ~ / m i n l .

TEMPERATURE

('CI

in Fig. 1. The sample MZ1 shows a very high shrinkage up to 1 2 0 0 ~ ~ due to the large specific surface of premullite (-300 mz/g); the anomalous behaviour of the curve between 1150 and 13500C has been explained by the crystallization of mullite/l/. In MZ2 the shrinkage starts at12000C and shows a higher shrinkage rate than MZ1 beyond 14500C.

The grain size distribution of ZrO, particles in both samples is shown in Fig. 2. The distribution for MZ2 is wider than that for MZI. To de-

g ^{- 2 .}

2

: ^-4.

-

3

Y

-

z

3-10

W

ix -12

F i g . 2.- G r a i n s i z e d i s t r i b u t i o n o f Z r O , p a r t i c l e s f o r a ) MZ1 a n d b ) MZ2.

3m ~m, iim im ooo u m ism

I I 1

- - -

-

termine the average grain size of ZrO, particles and, also, the criti-

cal size for the t-ZrO, + m-ZrO, transformation, the cumulative fre-

1.6 urn for MZ2 sample; the average grain size is 0.6 and 1 ~ m , and the critical grain size is 0.3 and 0.7 ym for MZ1 and MZ2 respectively.

The critical grain size of ZrO, particles is determined by considering the relative tetragonal contents /2/ obtained by XRD of the sintered samples: 10% for MZ1 and 25% for MZ2.

The microstructure of both the samples is shown in Fig. 4. From these microphotographs the following points can be stated:

100

,- _E-"

80

-

><,<

_a6 ^{, ,}

^{, , ,} ₁₄ ^, ₁₈ ^{. ,} ₂₂ ^,

_2.6 ^,

DIAMETER Ounl

Fig. 3 . - A c c u m u l a t i v e f r e q u e n c y v s d i a m e t e r of Z r 0 2 p a r t i c l e s .

a) In the MZ1 sample the mullite grains are equiaxed showing flat mu- llite-mullite grain boundaries. The Zr02 grains are polygonal and main- ly located in intergranular positions.

b) In the MZ2 sample mullite-niullite as well as mullite-Zr0,grainboun- daries are curved, and the 210, grains are mainly intragranular, roun- ded, and exhibit convex and concave curvatures, filling the mullite grain interstices.

XRD analysis of MZ2 samples showed a fraction (-10%) of unreacted alu- mina, which is very hard to resolve in the SEM micrographs. A similar microstructure (i.e. rounded ZrOn grains filling the mullite intersti- ces) has been observed by the present authors /3/, in the case of ter- nary mullite/A1,03/Zr02 composites obtained from premullite. This sin- gular microstructure was attributed to a decrease in the mullite-Zr0, grain boundary energy by the presence of free alumina.

The mechanical properties (OF and KIC) of the samples are summarized in Table I. Considering the Lange's expression for transformation toughening /4/, values of ~ ~ ~ ~ = 2 . 4 M N ~ - ~ / ' and 3.3 M N ~ - ~ / ' were obtain- ed for MZ1 and MZ2 respectively. These results suggest that transforma- tion toughnening is not the only operative mechanism in these composi- tes.

Considering the e x p r e s s i o n ~ k / ~ = ( ~ ~ ~ ~ ~ p - ^K=c~)/K and substituting the experimental KIC values (Table I) Ak/K=0.33 is obtained for both the samples. This fact suggests that the mechanism which explains the ad- ditional toughness observed in both the samples can be the same.

At this respect, it has been theoretically /5/ and experimentally /6/

proved that in the case of MZ1 sample the AK observed is due to the presence of a metastable solid solution at the grain boundary mullite/

ZrO,.

Consequently, it appears reasonable to assume that a similar mechanism

is operative in MZ2.

JOURNAL DE PHYSIQUE

TABLE I

Mechanical P r o p e r t i e s of samples MZ1 and MZ2

F i g . 4 . - SEM m i c r o p h o t o g r a p h s c o r r e s p o n d i n g t o M Z Z f a ) a n d MZ2 ( b ) . SAMPLE

M Z 1 MZ 2

I n t h i s c o n t e x t , t h e h i g h e r mechanical p r o p e r t i e s o b s e r v e d i n MZ2 compo- s i t e s ( T a b l e I ) must be r e l a t e d t o t h e h i g h e r Zr02 c r i t i c a l d i a m e t e r and h i g h e r t e t r a g o n a l c o n t e n t . However t h e l a s t p o i n t i s d i f f i c u l t t o prove due t o t h e c o n s t r a i t s imposed by s t o i c h i o m e t r y of r e a c t i o n { I } .

K ~ - ~ ( M N ~ - ~ / ~ ) 3.2 + 0.1

4.4 ? 0.1

c r ~ (MN m- )

280 + 20

354 + ²³

1 . The s a m p l e o b t a i n e d by s i n t e r i n g h a s a n a v e r a g e - Z r 0 2 g r a i n s i z e a n d a t - Z r 0 2 + m - Z r O z t r a n s f o r m a t i o n c r i t i c a l g r a i n s i z e o f a b o u t o n e h a l f of t h a t c o r r e s p o n d i n g t o t h e r e a c t i o n s i n t e r e d one.

2 . The m i c r o s t r u c t u r e o f t h e sample o b t a i n e d by r e a c t i o n s i n t e r i n g p r o - cess, shows rounded i n t r a g r a n u l a r Z r O z g r a i n s a n d rounded i n t e r g r a n u l a r Z r O z g r a i n s f i l l i n g t h e m u l l i t e m a t r i x i n t e r s t i c e s , which i s c o m p l e t e l y d i f f e r e n t from t h a t o b t a i n e d by s i n t e r i n g .

3 . The r e a c t i o n s i n t e r e d specimen e x h i b i t s -30% h i g h e r v a l u e s of KIC a n d c r ~

REFERENCES

/ I / . Moya J . S . a n d O s e n d i M . I . , J . Mat. S c i . , 1 9 ( 1 9 8 4 ) 2 9 0 9 . / 2 / . Green D . J . , J. A m e r . Ceram. Soc. 65(1982)-0.

/ 3 / . Rincdn J . M . , D i n g e r T.R., Thomas c, Moya J.S., O s e n d i M . I . M i - c r o s t r u c t u r e o f M u l l i t e / Z r 0 2 and M u l l i t e / A 1 2 0 3 / Z r 0 ~ to u g h c e r a m i c c o m p o s i t e s , a c c e p t e d f o r p u b l i c a t i o n i n A c t a . M e t a l l .

/ 4 / . Lange F . I . , J. Mater. S c i . 1 7 ( 1 9 8 2 ) 2 3 5 .

/ 5 / . O s e n d i M . I . , Miranzo P . a n d x o y a J . S . , S o l i d s o l u t i o n s e f f e c t o n t h e f r a c t u r e t o u g h n e s s , a c c e p t e d f o r p u b l i c a t i o n i n J. M a t e r . S c i , L e t t e r s .

/6/. D i n g e r T.R., K r i s h n a n K.M., Thomas G. O s e n d i M . I . a n d Moya J . S . ,

A c t a N e t a l l z ( 1 9 8 4 ) 1601.