Cold isostatic compaction of nano-size powders: Surface densification and dimensional asymmetry
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Cold isostatic pressing (CIP) is often used in the compaction of nano-sized powders. For technological reasons, however, uniaxial pressing prior to CIP takes place. This paper reveals the first quantitative measurements of density gradients within and the asymmetric sintering response of nanoscale zirconia compacts formed by (i) simple uniaxial compaction and (ii) specific ratios of uniaxial and CIP pressure. We find that CIP forms an exterior “skin” of higher but variable surface density and decreases the width of the density distribution. It does not eliminate density gradients; nonuniform shrinkage still occurs during sintering. The high- and low-density zones (the moving and fixed ram ends, respectively) that form during uniaxial compaction are reversed during CIP. Considering both density distribution width and spring-back cracking, the “best” uniaxial-CIP pressure combination is 1–20 ksi for this particular powder and an L/D of 1.0. The greater final compaction of the low-density zone during CIP causes relatively large variations in final dimensions (nearly 400 microns) in spite of the smaller density distribution width. The usually neglected uniaxial pressing step has definite technological impacts on the production of nanostructured components via compaction.
I. INTRODUCTION
Nanoscale powders can, if properly utilized, be used to produce components possessing highly desirable properties.1,2 However, many of the same economic factors that govern the manufacture of compacts from submicronsized powders also control the foreseeable future of nanopowders. For high volume manufacturing of threedimensional products, dry pressing remains the route of choice. In this context, cold isostatic pressing (CIP) has often been used on the basis of the unquantified assumption that it produces more uniform compacts having higher densities. However, nearly all of these components require uniaxial pressing before CIP3–5 to achieve even a relatively simple final shape. In the compaction area, previous efforts have examined either (i) the effect of axial pressure alone or (ii) the effect of the subsequent CIP pressure on density, sintering shrinkage, and microstructural evolution.6–9 No published work investigating both uniaxial pressure and CIP exists. Given the everpresent need to form any powder with greater precision, a better understanding of the effects of these pressure combinations has considerable relevance. Nanopowders present special challenges revealed by examining studies that relate to submicron-sized powders: (i) Particles of decreased size disperse compaction 2794
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J. Mater. Res., Vol. 17, No. 11, Nov 2002 Downloaded: 29 Jun 2014
pressure more efficiently.10 (ii) The inherently larger driving force for microstructural change 3,11,12 in nanopowder compacts must, in combination with density gradients, result in larger temporal and final differences in external shrinkage. Quantitative proof of the influence of density gradients on dimensional tolerance have only recently appeared in
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