Grain-growth kinetics in a nanocrystalline 2 yttria-stabilized tetragonal zirconia polycrystals ceramic with a silica-ba
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Grain-growth kinetics of a nanocrystalline 2 yttria-stabilized tetragonal zirconia polycrystals ceramic containing a silica-based glassy phase was determined at 1200 to 1600 °C. At short durations below 1300 °C, the slow grain growth was associated with zirconia dissolution for composition equilibration. The significant increase in the grain size started only after 10 h at 1400 °C or at shorter durations at higher temperatures. Clusters of the cubic grains formed at the two-phase field confirm the inhibited tetragonal grain growth to be independent of the cubic grains. The microstructure evolution during the tetragonal grain growth was interpreted in terms of grain coalescence. Grain growth was initiated by contact flattening and followed by grain-boundary diffusion through the grain-boundary glassy phase. Some aspects of cation diffusion within the viscous glass were also discussed. I. INTRODUCTION
Grain growth in nanocrystalline ceramics is the main process that accompanies the final stages of the densification during sintering. Generally, in pure glass-free ceramics, the ionic diffusivity at the sintering temperatures is high enough to enable grain-boundary migration where the porosity becomes discrete. Therefore, grain growth is an inevitable process in the final stage sintering of glassfree nanocrystalline ceramics.1,2 However, addition of an appropriate grain-boundary glassy phase may be helpful in both enhancing the densification process at lower temperatures and limiting the diffusion through the glass; hence, the grain growth may be inhibited.3–6 Such effect was observed in the nanocrystalline yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) with silica-based glass additions.7 This paper describes the kinetics and the microstructural evolution during the grain growth of nanocrystalline 2Y-TZP ceramic containing 10 vol% silica-based glassy phase. II. EXPERIMENTAL
Commercial nanocrystalline 2Y-TZP powder (Tioxide; 2.2 mol% Y2O3) with average particle diameter of 73 nm was coated with a glassy phase layer through the sol-gel technique. The detailed description of this procedure is published elsewhere.7 Ethanol–water-based solution was prepared using silicon tetrachloride, strontium, and sodium nitrates. The final composition (mol%) of the glass was determined by chemical wet analysis as 93% SiO2–6% SrO–1% Na2O. Both SrO and Na2O were added to lower the viscosity of the silica glass. The present glass composition with glass transition temperature 950
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J. Mater. Res., Vol. 18, No. 4, Apr 2003 Downloaded: 17 Mar 2015
of 760 °C was planned to enhance the wetting ability of the zirconia grains by the glass and to increase the solubility of Zr and Y cations within the glass. An appropriate amount of the glass solution was added to the water-based dispersion of the nano-2Y-TZP powder to reach the glass concentration of 10 vol% in the calcined (600 °C for 1 h) powder. A disc-shape green pellet was isostatically cold pressed at 250 MPa, followed by sintering at 1400 °C for 1
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