|M.Sc Student||Marder Rachel|
|Subject||Densification Mechanisms during Spark Plasma Sintering of|
Nanocrystalline Yttrium Oxide (Y2O3)
|Department||Department of Materials Science and Engineering||Supervisor||Professor Rachman Chaim|
|Full Thesis text|
In the present work the densification mechanisms during spark plasma sintering (SPS) of nanocrystalline yttria (nc-Y2O3) powder were investigated.
Pure, cubic nc-Y2O3 powder with 18 nm crystallite size was sintered using SPS at 1100°C and 100 MPa for different durations. Specimens with 98% density and 106 ? 33 nm mean grain size were formed after 20 min. The grain size first increased and then tended to stagnation with the SPS duration. The nanostructure consisted of convex tetrahedron shaped nano-pores at part of the grain boundary junctions. Theoretical calculations were made for grain growth stagnation imposed by either drag from nano-pores at grain junctions or from dense triple junctions. The experimental results were in agreement with grain growth stagnation due to nano-pores drag in pure nc-Y2O3. Extended SPS duration up to 40 min led to sudden grain coarsening and loss of the nanocrystalline character.
1wt% LiF was added to cubic nc-Y2O3 powder to investigate the effect of the sintering additive on the densification mechanism. Specimens were fabricated by SPS within the temperature range 700°C to 1500°C. Sintering at 700°C for 5 min resulted in 95% density, compared to 65% in nc-Y2O3 without sintering additive. Sintering at higher temperatures resulted in higher densities, however, with much larger grain size. The average grain size increased from 84 nm at 700°C to several microns (~17 ?m) at 1500°C after SPS at 100 MPa for 5 min. The presence of LiF assisted rapid densification by particle sliding at low temperatures, while at higher temperatures high capillary forces were responsible for the very fast sintering process. However, at higher sintering temperatures, liquid phase sintering resulted in LiF entrapment as globular second-phase within the fast growing grains.
SPS of multiphase (monoclinic and cubic) nc-Y2O3 powders was performed between 1000°C and 1500°C. The densification was accompanied by interface controlled monoclinic to cubic phase transformation around 1000°C. Moreover, SPS at 1000°C showed a clear limitation in the densification of the multiphase powder in comparison to the pure cubic counterpart; the final density was 76%, in comparison to 94% for the pure cubic Y2O3. The microstructure showed a vermicular nanometric matrix, with large spherical cubic Y2O3 grains. It was deduced that metastable monoclinic particles suppress densification by early neck formation.
 R. Marder, R. Chaim, C. Estournès, Mat Sci Eng A 527 (2010) 1577-1585
 R. Marder, R. Chaim, G. Chevallier, C. Estournès, submitted to J Eur Ceram Soc