|Ph.D Student||Rachel Marder|
|Subject||Densification Mechanisms and Kinetics of Ceramic|
Nano-Powders during Spark Plasma Sintering
|Department||Department of Materials Science and Engineering||Supervisor||Professor Chaim Rachman|
|Full Thesis text|
Spark plasma sintering (SPS) is a modified hot-pressing technique in which sintering is performed by the use of a pulsed DC current that is forced to flow through the powder-die assembly. This direct way of heating allows fast heating rates (up to 1000ºC/min) and together with the applied load results in rapid and full densification of ceramic powders within a few minutes. This is often related to the applied electric field, which may provide an additional driving force for diffusion. However, the effects of the electric field such as spark and plasma formation were yet under debate especially in the non-conducting ceramic powder aggregates.
Lithium fluoride (LiF) microcrystals and Yttrium Aluminum Garnet (YAG) nano powders were used, as model systems for soft and rigid ceramics, respectively, and densified by SPS at different temperature and pressure conditions. The density of the sintered specimens together with their microstructures was characterized in order to investigate the densification mechanisms and kinetics with respect to the material properties and the SPS parameters.
SEM examination of the fracture surfaces of the partially dense LiF and YAG specimens revealed traces of liquid, which were related to sparking and plasma formation. Partial melting of the particle surfaces was observed in LiF sintered at low pressure and 500°C, while material jets connecting between the spherical nano-particles were observed in YAG sintered to 1200°C. The observed microstructure is characteristics for a dusty plasma. Densification in LiF occurred by plastic deformation and strain hardening at pressures above the yield stress, and by particle rearrangement assisted by viscous flow at the particle surfaces below the yield stress. A modified hot-isostatic pressing model was used to describe the densification, where considering the changes in the material properties and microstructure versus the SPS pressure and temperature. Very good agreement was found between the experimental and the theoretical densification curves. For YAG, rapid densification occurred above 1200°C, which was related to surface softening due to plasma formation. Aligned nanograin clusters in sintered YAG specimens pointed to the strong effects of the electric field on the sintering mechanism. A Matlab model of aggregates was developed to describe discharges in ceramic powders, considering the SPS parameters.