Densification and structural development in liquid phase sintering

Abstract

This paper discusses current ideas on densification and structural development in liquid phase sintering (LPS). As far as possible, it is couched in comparative terms in that currently accepted views on structural development and densification are contrasted with processes envisioned to take place in the earlier (approximately 1950) descriptions of LPS. For example, it is shown that the initially perceived “particle rearrangement” process itself consists of two stages and that secondary rearrangement, in which liquid penetrates solid grain boundaries in an initially polycrystalline solid particle array, is requisite for full densification in LPS. Likewise the “solution-precipitation” phenomena of the classical description are divided into two distinct processes. One of them, “Ostwald ripening”, leads to microstructural coarsening and is observed ubiquitously during LPS but is unrelated to ndensification occurring therein. Another solution-precipitation process can, as shown in the earlier descriptions, lead to compact densification, but it is argued here that its relevance to actual densification in LPS systems is suspect. Solid skeletons are also always formed during LPS. Conditions for the formations of a skeleton are summarized and it is argued that, in distinction to the earlier description, the formation of a solid skeleton does not,per se, lead to a cessation of densification. This is a reflection of the mobility of pores in an LPS structure and, regardless of the solid phase morphology, densification can take place by buoyancy driven pore migration. However, densification by this mechanism necessitates the fulfillment of certain thermodynamic and morphological criteria, and these are summarized herein. The paper concludes with a brief description of morphological changes taking place during extended sintering of LPS materials. Although the time scales described are much longer than those used commercially, the structural evolution taking place during long time sintering can produce microstructures which may have commercial potential.