Abstract
Shrinkage, grain growth, dimensional changes and homogenization during liquid phase sintering of ceramics or metallic systems, are influenced by the presence of additives. Additives are intentionally added to control the microstructural and dimensional development during sintering. Additives may be present in solid state only or may form liquid phases. In this paper the effect of additives on sintering will be described using examples which cover some of the essential basic principles of interactions of additives with the host material.
The addition of Ni to the refractory metals W and Mo is used to describe all three stages of solid state sintering as well as postdensification by hot isostatic pressing. It is shown that the densification phenomena during solid state sintering are changed by the presence of additives via simultaneous increase of the grain boundary diffusivity and of the surface diffusivity in this system. From experiments with W-Ni, Mo-Ni and Fe-Cu it is concluded, that in the presence of liquid additives the densification phenomena, rearrangement and center to center approach occur by elementary mechanisms as particle disintegration, coarsening and directional grain growth.
The influence of driving forces provided by chemical reaction on the kinetics of mass transfer is discussed for diffusion induced grain boundary migration and for liquid film migration. These types of interface migration were found to be caused by the interdiffusion of the additives with the host metal. Liquid film migration is thought to be of great importance for the homogenization of Fe-Cu and Fe-Cu-C alloys during liquid phase sintering.
