Processing High Quality Toughened Zirconia Ceramics by Controlling the Chemistry at the Particle-Particle Interface
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S.T.THURAISINGHAM* AND J.L. HENSHALL** *Colloids-In-Industry,P.O.Box 263, Concord, MA 01742-0263, USA **School of Engineering, University of Exeter, Exeter, EX4 4QF, England
ABSTRACT A novel lower cost wet-processing technique, for producing toughened zirconia ceramics with complex geometries, is presented. It is based mainly on modifications to the particle surface chemistry. The sintered materials produced so far are significantly harder (ca 50%) as well as being tougher (ca 10%) than a commercial CeTZP.
INTRODUCTION The principles of colloid science and surface chemistry have been exploited by materials scientists for over a decade with the aim of producing higher quality green compacts during the "colloidal or wet-processing" of ceramics [1-4]. Previous work on altering the surface chemistry of fine particles in liquid suspension together with changes in the physical variables of a wet-processing system has produced no clear processing paradigm [5,6]. Thus no effective procedure has emerged that enables ceramic materials exhibiting superior mechanical properties to be produced. An outline of a novel high quality lower cost wet-processing technique for ceramics, based on presureless sintering of green compacts with complex shapes exhibiting low porosity recently developed by Colloids-In-Industry, is presented. The frictional effects associated with the dry-pressing of ceramic components and the high cost of hot isostatic pressing are thus avoided. Zirconia was chosen because of the recent commercial interest shown for such high toughness ceramic materials [7].
EXPERIMENTAL PROCEDURE Details of the experimental procedure cannot be presented here for proprietary reasons but experimental parameters of importance will be discussed. Central to this technique is the control of the particle surface chemistry which influences the packing process during compaction as well as promoting fusion during sintering. 343 Mat. Res. Soc. Symp. Proc. Vol. 318. ©1994 Materials Research Society
The powder was a 12 mol% ceria stabilised tetragonal zirconia (CeTZP) of median particle size 0.58 g~m (UNITEC plc, Stoke, England). The preparation procedures were conducted under clean conditions as is often required in studies of the physics and chemistry of surfaces [8]. Aqueous suspensions of the CeTZP particles were freshly prepared and left to equilibriate for at least one day after which different combinations of chemicals were added to each of the suspensions. This ensured that for each suspension a particular chemical type or group existed at the surface of the fine particles. The suspensions were then shaken intermittently during storage. This was followed by compaction at various pressures using a filtration technique common to chemical engineering practice [6]. The parameters relevant to this process are thus the chemistry at the particle-particle interface (SC), the volume fraction and rheology of the suspension (SR) and the compaction pressure (ratio PR). After the initial compaction the specimens were placed in a thin-wall
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