Sintering processes can be categorized into two basic types: solid-state sintering (SSS) and liquid-phase sintering (LPS). For both types, the basic phenomena occurring during sintering are densification and grain growth.
Since the late 1940s, several models and theories have been developed to understand the densification kinetics during sintering. In SSS, densification occurs via diffusional atom transport from the grain boundary to pore surface under the driving force stemming from the capillary pressure of pores. As a result of the atom transport, the pore size decreases continuously if pore coalescence, which is due to grain growth, does not occur during sintering and no insoluble gases are entrapped within the isolated pores. Many small pores can be present in the solid-state sintering body. Unlike SSS, there is a critical contribution of liquid to densification in LPS as liquid can flow whenever the pressure within it is uneven. When the surface of a pore is completely wetted by a liquid with an increase in the liquid meniscus radius during grain growth, the liquid flows instantaneously into the pore (pore filling). This is a fast bulk flow of material for densification. Provision of a path for atom transport from the contact area between grains to the pore region (contact flattening) is another role of liquid during LPS, if a liquid film is present at the grain boundary. This process is similar to that of atom transport from the grain boundary to the pore surface in SSS. Experimental observations and theoretical calculations indicate that pore filling is the major densification mechanism of LPS and the contribution of contact flattening, if any, is limited to the very early stage of LPS. According to the pore filling theory, the pore filling occurs in a temporal sequence, with smaller pores earlier and larger ones later. Small pores are rarely observed during LPS, unlike the case of SSS.
A fundamental difference in grain growth between SSS and LPS likely resides in its controlling step. In the case of SSS, grain growth takes place with the migration of the grain boundary and its kinetics is governed by the atom transport across the boundary under the capillary pressure difference between two adjacent grains. In LPS, unlike in SSS, as the boundary is anchored at the neck because of the dihedral angle condition between the grains, the atom transport across the boundary is suppressed. Grain growth can take place with the atom transport through the liquid matrix between grains (Ostwald ripening). Therefore, the grain growth in LPS is governed by the material transport via the dissolution of atoms from the shrinking grains in the liquid and the precipitation of atoms at the growing grains. The role of liquid in grain growth is obviously the provision of a diffusion medium for atom transport.
Suggested readings:
1. R. K. Bordia, et al., J. Am. Ceram. Soc. 100 (2017) 2314.
2. S.-J. L. Kang, "Liquid phase sintering: Fundamentals" in Encyclopedia of Materials: Technical Ceramics and Glasses," A. Leriche and F. Cambier (eds), Elsevier (2020).