In this study, spray drying of alumina-zirconia slurries on a pilot scale, and their pressing behaviour were investigated. Slurries were prepared at 50 wt-% solid loading. An acrylic polymer emulsion, polyethylene glycol, and stearic acid were used as a binder, plasticizer, and lubricant, respectively. Granules produced had a mean size of 85 μm with spherical morphology and smooth surfaces with considerably good flow behaviour. When polyethylene glycol was not used, granules were rather hard such that it was very difficult to break down the granule structure even at a pressure of 195 MPa. Introduction Dry pressing is one of the most commonly used forming techniques for making engineering ceramics. It is particularly preferred for compacting simple shapes at high speeds. Dry pressing is used to produce a uniform size and green density compact, consistent part-to-part green density and a defect-free compact [1]. Pressing at high speeds requires granules with excellent flow properties in order to ensure rapid die fill, to achieve uniform filling of the die cavity, and consequently to produce homogeneous compacts. Additionally, the granules should have high fill density, should easily be deformable under normal pressing conditions, and should not stick to the die walls. Flow behaviour is mainly a function of shape, surface smoothness, and size distribution of the granules. Fill density is determined by the density and shape of the granules. External factors such as humidity, conveying environment, vibration, and aeration should also be considered. Spray drying is widely used to produce spherical granules of controlled size, and granule variables can be optimised by adjusting slurry parameters and spray drying conditions [2]. The amount of organic additives and their composition and properties are also important parameters for obtaining good pressing granules. The binder should have optimum plasticity, which is a function of its glass transition temperature and the plasticizer used. In the present work, spray drying of aluminazirconia slurries containing 2 to 10 vol % ZrO2 with two different organic additive systems were studied. The flow and compaction behaviour of the granules obtained are reported. Experimental procedures Commercially available powders, namely AES-11C (99.8 % Al2O3, Sumitomo, Japan) and 3Y-ZrO2 (Tosoh, Japan), were used as alumina and zirconia sources. Alumina and zirconia powders were mixed with distilled water at 50 wt. % and ammonium polyacrylate (Darvan C, Rt Vandebilt, USA) was used as a deflocculant. Milling was conducted in a 12-liters attrition mill, half-filled with 3 mm diameter 3Y-ZrO2 grinding media, for two hours in order to break any possible aggregates and agglomerates. Then, organic pressing aids were introduced with the aid of a mixer. In order to observe the effect of organics on pressing behaviour, two different organic systems were prepared, as shown in Table 1. One contained an acrylic emulsion binder (Duramax B-1007, Rohm and Haas Company, France) and a stearic acid (Selasol 920, Chukyo, Japan) lubricant. The other contained, in addition to the binder and the lubricant, polyethylene glycol (PEG-1500S, Clarient, Germany) as Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 233-236 doi:10.4028/www.scientific.net/KEM.264-268.233 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,18:13:05) a plasticizer. After mixing with the organics, the slurries were passed through a 32 μm sieve in order to remove any possible large agglomerates or inclusions. Table 1 Organic additive compositions for granules. Binder Plasticizer Lubricant Granule I 3 wt-% B-1007 1 wt-% stearic acid Granule II 1.5 wt-% B-1007 1.5 wt% PEG 1 wt-% stearic acid Spray drying was carried out in a pilot scale spray-drying unit with a water evaporation capacity 1 to 5 kg/h (Nubilosa, Germany) using a 1.8 mm diameter two-fluid nozzle. In order to prevent burning of the organics during drying, the outlet temperature was kept at 125°C while the inlet temperature was 240°C. The size distribution of the granules produced was measured using laser diffraction (Malvern Mastersizer, (Hydro 2000G, UK). Granule morphologies were observed using SEM (CamScan S4, UK) after sputtering with gold-palladium to prevent charging. SEM was also employed to examine the microstructural features of the pressed and sintered compacts. In order to estimate granule flowability, methods that are easy to use in the industrial field, namely flow through an orifice (FTO), Hausner ratio (HR) and compressibility index (CI) were preferred [3]. Fill density and tapped density values were also measured [4]. Note that each test was carried out three times and an average value was calculated. Consolidation behaviour of the granules during high speed pressing was studied in a stainless steel square die (16 x 16 mm), mounted in a fully automatic mechanical press (Dorst DACS15, Germany). Filling height was 20 mm and the speed of compaction was adjusted to 30 pieces/min. Sintering was achieved in air in an electric furnace at a heating rate of 3°C/min up to 600°C for debinding and of 10°C/min to the maximum temperatures of 1550° and 1600°C. Samples were sintered for two hours at either temperature. Results and discussion Fig. 1 shows a typical SEM image of the granules (II) obtained. They are fairly spherical, but mainly have a donut shape. It is well known that the granule morphology depends highly on the formulation of the slurry, or more precisely the degree of flocculation [5]. Fig. 2 is the size distribution of the spray-dried granules (II) measured by laser diffraction. Mean granule size was measured to be around 85 μm while 90 % of the granules were below 190 μm. The relative humidity of the granules was measured to be around 0.35 %. Fig.1. A typical SEM image of the granules. Fig. 2. Size distribution of the granules. The flow behaviour of the granules (II) from the shoe of the press during compaction was assessed by measuring the compaction pressure, which depends on the amount of filling powder in the die Particle Size Distribution 0.01 0.1 1 10 10