Sintering and microstructure of glass-coated nanocrystalline yttria-stabilized tetragonal zirconia powder: Effect of the
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Nanocrystalline yttria-stabilized zirconia (nc-Y-TZP) powders coated with 10 vol% sodium strontium silicate glass were prepared by the sol-gel method. The glass is found to uniformly coat the zirconia particles or particle aggregates. The presence of the glass was found to enhance the densification of the powder compacts leading to 97% of the theoretical density compared to 90% in the pure nc-Y-TZP. It also resulted in a fine microstructure, homogeneous particle shape, and narrow size distribution. The polyhedral-shaped faceted grains in the sintered pure Y-TZP were converted to round-shaped grains in the glass containing specimens.
I. INTRODUCTION 1
Since the report of Wakai et al. on the superplastic behavior of fine-grained yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) and further studies by Nieh et al.2 and others, the study of ceramic superplasticity has steadily been approached with an increasing interest. Though the fundamental issues in superplasticity have been well addressed in the literature (for a recent review see Ref. 3), further research on modifying the microstructural parameters of the ceramics, underlying the phenomena, is necessary and useful. Large plastic deformations may be achieved by viscous flow of grain boundary (GB) glassy phases.4 This was the original motivation for introducing a glassy phase into the ceramics in a controlled manner5–13 in order to benefit from their enhanced superplastic behavior. As the diffusional creep rate in polycrystalline ceramics is inversely proportional to the cube power of the grain size,14 the fine or ultrafine grain size seems to be a necessary parameter for the superplasticity. Even though the grain size refinement was found to lower the superplastic forming temperature and stress or increase the strain rate,15–17 nanocrystalline (nc) ceramics do not exhibit the expected improvement of several orders of magnitude in the strain rate. Nevertheless, the nanocrystalline character can be used to incorporate high volume fraction of the glassy phase into the ceramic microstructure as a grain boundary film and thus improve the superplastic forming ability of the ceramic. 18 Some studies13,19,20 reveal that the glassy phase exists only at the grain boundary corners and the multiple junctions rather than at the grain boundaries. The presence of the glassy phase at the grain boundaries or at the multiple junctions, generally depends on the processing conditions, the composition, and the volume 296
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J. Mater. Res., Vol. 16, No. 1, Jan 2001 Downloaded: 18 Mar 2015
fraction of the glass. The microstructure of the zirconia alloys can be modified by an addition of the glassy phase.21,22 Coating the powder particles with the glass through the sol-gel method23–25 is efficient in preserving the nanostructure character of the powder, as opposed to powder mixing. The volume fraction of the grain boundaries changes with the grain size, especially in the nanometer regime.26–28 Thus, assuming a grain boundary thickness of 1 nm for a
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