Kinetics of Initial Coarsening During Sintering of Nanosized Powders

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INTRODUCTION

GRAIN growth during sintering is an issue that plays a critical role in the manufacture of bulk nanocrystalline materials by sintering nanosized powders. Most reported studies of the sintering of nanosized metallic or ceramic powders[1–10] found that grain growth is so rapid and so signiﬁcant that the nanosized powders would grow to greater than one or several hundred nanometers as soon as a powder compact is heated, losing nanocrystalline features. Controlling and minimizing grain growth is indeed one of the most important challenges for manufacturing bulk nanocrystalline materials by the route of sintering of nanosized powders. The general characteristics of grain growth during sintering can be illustrated by studying the trajectory of grain growth vs relative density. A characteristic phenomenon of sintering that was observed for many diﬀerent materials[11–14] is that porous compacts densify without signiﬁcant grain growth until the density of the compact reaches ~90 pct of the bulk density of the material. Then grain growth becomes much more rapid after reaching 90 pct relative density. In other words, grain growth is slow and limited in very porous solids when relative density is less than 90 pct, while grain growth is rapid and dominant in less porous solids when HONGTAO WANG, Research Associate, Z. ZAK FANG, Professor, and KYU SUP HWANG, Graduate Student, are with the Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112. Contact e-mail: [email protected] Manuscript submitted October 29, 2010. Article published online June 9, 2011 3534—VOLUME 42A, NOVEMBER 2011

relative density is greater than 90 pct. In this study, we use the term ‘‘initial coarsening’’ to describe the grain growth in very porous compact 90 pct relative density. The latter grain growth has attracted considerable interest from researchers in the past (and still is today). The issue of initial coarsening, however, is often ignored by researchers. For example, Chen and Wang[15] proposed a two-step sintering technique that was able to successfully control the latter grain growth of nanosized zirconia powders, but the initial coarsening, during which the grain size increased by 4 to 6 times the initial grain size of the nanopowder, was not elaborated on by the authors. It should be recognized, however, that the initial coarsening during sintering of nanosized powders is also a critical issue that cannot be ignored, because it may determine if the grain size after heating would still be at nanoscale. To date, however, there have been very few studies dealing with the initial coarsening in the literature. There has been no detailed analysis of experimental observations of the initial coarsening, partially due to the complexity of the microstructures in very porous compacts (> RT and T >> T0, the temperature integral can be expressed using the following expression without sacriﬁcing precision:[28] ZT

1 QG R QG exp T exp dT ½7 T QG RT RT

T0

Therefore, the grain growth behavior as a f

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