153. OPTIMIZATION OF MATERIAL STRUCTURE DURING SPARK PLASMA SINTERING
Name: Diletta Giuntini
Grad Year: 2016
Spark Plasma Sintering (SPS) is an innovative powder technology for the production of materials with outstanding properties for a variety of applications, from aerospace to biomedical components, and from defense to energy sectors. Powder media are densified by means of the flow of electric current through the surrounding tooling or the specimen itself, leading to strong Joule heating effects, and thus mass transfer through diffusion and viscous flow mechanisms, activated by the high temperatures reached. SPS is characterized by very high heating rates, which allow short processing times and therefore the retaining of small grain sizes, an important condition for the final dense materials to have high mechanical strength. This process rapidity renders SPS particularly suitable for the production of nanograined materials, starting from powders with submicron particle sizes. Nanosized powders, nevertheless, are prone to undesired agglomeration phenomena, which consist of the formation of particles clusters due to weak interactions such as Van der Waals forces. These agglomerates create hierarchical porous structures, constituted by smallsize pores inside the clusters (intraagglomerate porosity) and largesize pores among them (interagglomerate porosity). During sintering, the small pores undergo preferential densification, and the large pores become extremely hard to eliminate, causing microstructural inhomogeneities and leaving residual voids that greatly hamper the material characteristics. The conventional strategies to address agglomeration issues consist of preSPS treatments aimed at breaking the clusters, but the subsequent handling of the powder prior to the densification process itself often render these procedures vain. We therefore propose to operate in situ deagglomeration. An analytical model for the densification of hierarchical porous structures is developed, in which the nonlinear viscous rheology characterizing the consolidation of crystalline materials is embedded. The nonlinearity parameter, strain rate sensitivity, is dependent on the applied temperature, and it is on this fact that our porous material structure optimization is based. The continuum theory of sintering is employed to derive the shrinkage kinetics of the agglomerated powder sample, as functions of porosities and strain rate sensitivity. Thus, an appropriate choice of the SPS thermal regime leads to a more homogeneous densification.
Industry Application Area(s)