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

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 2 powders are studied in order to define the appropriate conditions for preparing good slips for the production of 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 nitriding 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 properties and the castability features of the slips with the final aim to find 2 3 . 2 3 ( ' )si3N Starck,$ET=lS.O m /g,d=3.18 g/cm ;SI Starck,BET=O.S m /g,d=2.32 g/cml;Al 0 2 &lcoa,~$~=17.7 m /g,d=3.98 g/cm3;~r0 Harsaw (U.S.A. ) ,BET=28 m /g,d=5.89 g/cm . 2 3 2 Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986111 C1-74 JOURNAL 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 3 4 wt% Si,6.6 wt% A1 0 and 15.7 wt% ZrO ( I ! dispersed in distilled water and 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 70 wt%.The deflocculants concentrations 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 order 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 to avoid the :,ydrogen evolution.Bulk densities of cast bodies were measured according to inercury picnometry technique. I11 RESULTS AND DISCUSSION Rheological properties The possibility of 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 suspeni? 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 shearand the timedependent 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 70 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 resembles that of other very concentrated suspensions /3-5/.