RHEOLOGICAL PROPERTIES OF AQUEOUS SUSPENSIONS OF Si3N4-Si-Al2O3-ZrO2 POWDERS

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RHEOLOGICAL PROPERTIES OF AQUEOUS SUSPENSIONS OF Si3N4-Si-Al2O3-ZrO2 POWDERS

G. Babini, C. Galassi, R. Lapasin, E. Lucchini

To cite this version:

G. Babini, C. Galassi, R. Lapasin, E. Lucchini. RHEOLOGICAL PROPERTIES OF AQUEOUS SUSPENSIONS OF Si3N4-Si-Al2O3-ZrO2 POWDERS. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-73-C1-78. �10.1051/jphyscol:1986111�. �jpa-00225505�

JOURNAL DE PHYSIQUE

Colloque C1, supplkment au n 0 2 , Tome 47, f6vrier 1986 page cl-73

RHEOLOGICAL PROPERTIES OF AQUEOUS SUSPENSIONS OF Si,N,-Si-Al,O,-Zr02 POWDERS

G.N. BABINI, C. GALASSI, R. LAPASIN* and E. L U C C H I N I "

IRTEC- C . N . R . , Via Granarolo 64, I-48018 Faenza, Italy

* ~ s t i t u t o d i Chimica Applicata e Industriale, Via A. Valerio, 2 , I-34127 Trieste, Italy

Resume - Le faqonnage est tres important dans la production des c6ramiyues puisque les defauts qui s e produisent pendant cette phase de la fabrication demeurent aussi apr6s le frittage.Dans ce travail on Gtudie les proprietes rheologiques des suspensions des poudres de Si N -Si-A1 03-ZrO dans Te but d'obtenir des barbotines de bonne qualite pour ?a proguction 3.4 de ceramiques pour les hautes temperatures.

Abstract - The forrning process is very important in the production of ceramics because the defects introduced during this manufacturing step will remain in the product even after the sintering process.In this work the rheological properties of aqueous suspensions of Si3N4-Si-A1203-Zr0 powders are studied in order to define the appropriate conditions for 2 preparing good slips for the production o f high temperature ceramics.

I - _- INTRODUCTION _- -

Reaction sintering is proposed as an useful intermediate stage to obtain sintered silicon nitride based compacts in order to decrease the total shrinkage,to allow the finishing of the presintered bodies,to ootain higher density in the initial sintering stage.Reducing the nitridation time and the complexity of the nitri- ding atmosphere,the addition of silicon nitride to the powder mixtlnre leads to an easier reaction bonding process.The presence of sintering and touqhening agent in the powder mixture must be taken into account for the sintering stage and to improve the mechanical properties of the bodies,which are in any case strictly related to texture of green body's microstructure,i.e. to the forging technology.

Well known and currently used in the traditional ceramic field,slip casting is a suited way for shaping complex geometry,but a lack of knowledge exists on the contrclling microstructure parameters during casting on plaster mould.The present work is a first approach to the characterization of the rheological pro- perties and the castability features o f the slips with the final aim to find

- 2 3 . 2 3

(

'

)si3N Starck,$ET=lS.O m /g,d=3.18 g/cm ;SI Starck,BET=O.S m /g,d=2.32 g/cml;Al 0

&lcoa,~$~=17.7 m /g,d=3.98 g/cm3;~r0 Harsaw (U.S.A. ₂ ) ,BET=28 m /g,d=5.89 g/cm 2 . ^{2 3}

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986111

C 1 - 7 4 J O U R N A L DE PHYSIQUE

suitable and accessible controlling parameters and their correlation to the microstructure.

I1 - EXPERIMENTAL METHODS a! Materials.

All experiments were performed with a powder mixture of 31.1 wt% Si N ,46.6 wt% Si,6.6 wt% A1 0 and 15.7 wt% ZrO ( I ! dispersed in distilled water and 3 4

2 3 2

different amounts of industrial deflocculants.

b ) Rheological tests.

The suspensions were prepared through dispersion with an impeller of the powders r,iixture.The solid concentrations were 60,65 and 7 0 wt%.The deflocculants concentrati- ons were calculated on a dry material basis-The rheological tests were carried at 25"C,with a rotational viscometer R towisko RV 100:measuring device a bob/cup

-

9

system ZB 15;shear rate range:3-700 s

.

c) Casting measurements.

Tne suspensions were prepared by dispersion of the components in a horizontal roller using A1 0 balls and a polythene container.The pH was checked ranging hetween 7.0 to

3.3

for all the tested systems.The kinetics of the slip casting was followed using an apparatus proposed by Melton et Al./l/.It was designed in or- der to determine the rate of removal of water from the slip by the mould.The

,pparatus consists in a plaster mould (plaster/water ratio: 100/64) which is put in contact with the suspension hold in a cup.The volume of water abstracted from the slip is replaced by a reservoir burette,and so it is possible to take readings of volume against the time.The measurements were performed during not over few minuts immediatly after the slip preparation in order t o avoid the :,ydrogen evolution.Bulk densities of cast bodies were measured according to inercury picnometry technique.

I11 - RESULTS AND DISCUSSION Rheological properties

The possibility o f formulating concentrated suspensions having satisfactory stability and fluidity and,consequently,of obtaining cast bodies with high bulk density strongly depends upon the use of powerful dispersing agents which control the aggregation conditions of the solid phase because of their adsorption onto the particle surfaces.The rheological measurements are the best quantitative method to achieve informations on both fluidity and stability of conce trated suspen-

i?

sions.Therefore,the ability of various commercial deflocculants ( ) was tested by examining the rheological behaviour of a number of suspensions,having the same solids content (60 wt%) and the same deflocculant concentration (0.3 wt%), according to the Camina-Roffey procedure /2/.It consists in the alternate and

sequential application of the same reference shear rate and of different shear rates.

Each shear rate is applied until a steady value of the shear stress is attained.

So,the shear- and the time- dependent properties of the slips can be investigated and,at the same time,their stability can be measured by comparing the shear stress values attained at the reference shear rate ,repeatedly applied during the experimental test.A progressive decrease of the shear stress clearly indicates

( ) 2 Fratelli Lamberti:Reotan LA,Reotan 747,;?eotan LA/7,Reotan LP4.

scarce stability of the slip and the occurrence of settling.The yost gromising results were obtained with ammonium alginates (A50 and A500) ! since they allow stable slips to be prepared without collateral and detrimental aerating effects-Alginate solutions are pseudoplastic fluids at the concentration examined in the present work.The use of A50 alginate allows castable suspensions with high solids content up to 7 0 wt% to be easily prepared .Figure 1 reports the experimental data for the systems with a solid phase content of 60,65 and 70 wt% and 0.3 wt% of A50.

In the shear rate range explored, the rheological behaviour of the first two systems is plastic, whereas at the highest solids content it is plastic-dilatant and resem- bles that of other very concentrated suspensions /3-5/.

0 50 Z (Pa) 100

Fig. 1 - Shear rate vs. shear stress for the s'ips examined (tile first two number are referred to the solids content, the last to the A50 concentration).

In order to evaluate the deflocculant amount to be added for optin~um fluidification, the shear-dependent behaviour of a number of slips with 7 0 wt% of sol id phase and different deflocculant concentrations was examined. As reported in Fig. 1, all the systems exhibit plastic-dilatant behaviour. It can be described satisfactorily by the Herschel-Bulkey equation.

c = c + k Y .n

0

The alginate concentration does not affect appreciably the yield value T and the exponent n. 0

All the systems exam'ned show thixotropic properties which are significant at lo,d -

7

shear rates (96 10 s 1. When a sudden decrease of the shear rate is applied, a pro- gressive shear stress increase at the lower constant shear rate is generally observ- ed, as a consequence of structural buildup processes which take place inside the system. Interesting results can be obtained by applying the Tapeznikov-Fedotova procedure /6/ with the aim of investigating the buildup properties in rest conditions.

Accordingly, the syster~~ is subjected to a constant shear rate until a steady value of the shear stress is attained, and then the shear is stopped. The same shear rate ('1 CECA Algum A 50,Algum A 500

~ 1 - 7 6 JOURNAL DE PHYSIQUE

i s a p p l i e d a f t e r a g i v e n r e s t t i a l e t and t h e i n i t i a l s t r e s s v a l u e z i s recorded.

The p r o c e d u r e i s r e p e a t e d f o r d i f f e r e n t r e s t t i n e s . The i n c r e a s e oYa% r w i t h tr, t h a t i s t h e v i s c o s i t y b u i l d u p w i t h r e s t t i m e , g i v e s an i n s i g h t o f t h e m % % i x o t r o p i c processes w h i c h i n v o l v e t h e d i s p e r s e phase o f t h e system when i t suddenly passes f r o m shear t o no-shear c o n d i t i o n s . F i g u r e 2 r e p o r t s an example o f t h e pronounced v i s c o s i t y b u i l d u p f o r t h e system 704.

The shear-dependent b e h a v i o u r of t h e s l i p s r e s u l t s f r o m t h e non-Newtonian c o n t r i b u - t i o n of t h e d i s p e r s i n g medium and t h e a g g r e g a t i o n s t a t e o f t h e d i s p e r s e phase. A f i r s t e s t i m a t e of t h e a g g r e g a t i o n s t a t e c a n be d e r i v e d f r o m t h e r e l a t i v e v i s c o s i t y Q, ( t h e v i s c o s i t y of t h e s l i p d i v i d e d b y t h e v i s c o s i t y o f t h e d i s p e r s i n g illedium).

F i g . 2 t i o n o f c o n s t a n t shear r a t e : a ) j = 9 0 s - V i s c o s i t y b u i l d u p i n r e s t c o n d ' t i o n s f o r t h ~ -

1

, b ) j = 3 0 s - , c ) j = 9 s- ). system 704 ( a f t e r t h e a p p l i c a -

F i g u r e 3 r e p o r t s a comparison between t h e r e l a t i v e v i s c o s i t i e s o f t h e systems w i t h 0.2 w t % and 0.5 w t % d e f l o c c u l a n t c o n c e n t r a t i o n s and w i t h t h e same s o l i d c o n t e n t ( 7 0 w t % ) .One c a n see t h a t , w h i l e t h e 0.5 wt% system i s more v i s c o u s t h a n t h e 0.2 wt% one i n t h e whole shear r a t e r a n g e , i t s r e l a t i v e v i s c o s i t y i s s e n s i b l y lower a t a n y shear r a t e . I t means t h a t a t h i g h e r a l g i r ~ a t e c o n c e n t r a t i o n s t h e d i s p e r s e phase i s l e s s a g g r e g a t e d , t h a t i s t h e p a r t i c l e s a r e b e t t e r d i s p e r s e d ,

and t h e more e f f e c t i v e d i s p e r s i n g action,due t o t h e 0.5 wt% a l g i n a t e a d d i t i o n , i s s i m p l y h i d d e n b y t h e c o r r e s p o n d i n g v i s c o s i t y enhancement.

C a s t i n g p r o p e r t i e s

C a s t i n g t e s t s were performed w i t h t h e above mentioned a p p a r a t u s , f o r suspensions c o n - t a i n i n g 60,65 and 70 wt% o f s o l i d phase and d i f f e r e n t amount o f A 50 and A 500 de- f l o c c u l a n t . C a s t i n g k i n e t i c s a r e u s u a l l y d e s c r i b e d b y t h e r e l a t i o n s h i p :

L =K,t 2

where L i s t h e t h i c k n e s s o f t h e c a s t a t t h e t i m e t . I f t h e i n i t i a l d a t a , r e l a t i v e t o t h e t i m e d u r i n g w h i c h t h e gap between t h e suspension and t h e p l a s t e r mould i s f i l l e d , a r e e x c l u d e d and i f t h e c a s t produced has c o n s t a n t d e n s i t y t h r o u g h o u t i t s t h i c k n e s s

t h e above equation can be adapted t o t h e experimental data by t h e f o l l o w i n g one ,as described b y / 7 / :

V=Kt 1 / 2

where K i s t h e c a s t i n g constant and V i s t h e volume o f water a b s t r a c t e d a t t i m e t from t h e s l i p b y t h e mould,normalized t o t h e u n i t area o f t h e mould / s l i p i n t e r f a c e . The c a s t i n g constants r e s u l t t o be s t r i c t l y r e l a t e d t o t h e d e f l o c c u l a n t concentra- t i o n and t o t h e p o r o s i t y o f t h e c a s t (Fig.4),higher c a s t i n g r a t e b e i n g o b t a i n e d w i t h lower d e f l o c c u l a n t c o n c e n t r a t i o n and h i g h e r d e n s i t y being obtained w i t h lower c a s t i n g rate.Anomalous behaviour o f t h i s general t r e n d has been v e r i f i e d and are p o i n t e d o u t by t h e anomalous values o f c a s t body's p o r o s i t y (see system 655 and 705 i n F i g . 4 ) .

F i g . 3 - R e l a t i v e v i s c o s i t y vs. shear r a t e f o r t h e systems 702 and 705.

0.4 0.5 0.6 0.4 0.5 0.6

R e l a t i v e D e n s i t y

Fig.4 - Dependence o f t h e c a s t i n g constant on t h e r e l a t i v e d e n s i t y f o r t h 3 systems w i t h A 50 and A 500 def l o c c u l a n t s ( t h e o r e t i c a l d e n s i t y =2.92 g/cm 1.

c I - 7 8 J O U R N A L DE PHYSIQUE

They are probably related to the variation of the effectiveness of the def locculant on dispersing the solid phase.The different dispersing degree of the solid phase, also confirmed by the viscosity measurements,directly influences the microstructure of the cast bodies as shown in Fig.5 where only the concentration of 0.6 wt%

gives satisfactory microstructure.The general trend and the anomalous behaviours :an be explained according to the Carman-Kozeny model /8/.In fact the permeability of the cast (that is casting constant) is strictly related both to the amount and to the distribution of the porosity,being the stress to remove water from the pores inversely proportional to the pore radius.

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REFERENCES

/1/ Celik,N.,Melton,I.E. and Rand,B.,Trans.J.Brit.Cerarn.Soc.,g (1983) 136 /2/ Can~ina,M. and Roffey,C.G.,Rheol .Acta,lO (1971) 606.

/3/ Metzner,A.B. and Whitlock,M. , ~ r a n s . ~ o c T ~ h e o l .

,2

(1958) 239.

/4/ Lapasin,R. and Lucchini,E.,E.Science of Ceramics,ll - (1981) 91.

/5/ Sacks,M.D.,Am.Ceram.Soc.Bu11.,~ (1984) 1510.

/6/ Trapeznikov,A.A. and Fedotova,V.A.,Dokl.Akad.Nauk.SSSR,95 (1954) 595.

/7/ Adcock,D.S. and McDowall ,I .C. ,J.Am.Ceram.Soc. ,% ( 1957)355.

/8/ Funk,J.E.,Forming of Cerarnics,Am.Ceram.Soc. 76-84.