Sintering and Grain Growth

Abstract

Sintering is the process of transforming a powder into a solid body using heat. Why is there a whole chapter on sintering? Partly for historical reasons—it has traditionally been the principal method of processing ceramic bodies. Sintering is still the most important process in making bulk ceramics, but the process is not unique to ceramics. There is a large field known as “powder metallurgy” that considers many of the concepts and problems that we address for ceramics. One of the reasons for processing metals by sintering is to control the grain size, which is precisely what we usually need to do in ceramics.

We can discuss the topic at different levels of complexity. The phenomenon is quite straightforward and involves deciding how best to pack particles (that are usually modeled as spheres), understanding the movement of grain boundaries (GBs), and knowing how the packing geometry and GB migration is affected by the need to balance surface tensions (interface energies). The quantitative analysis of the process is much more difficult since it involves transport of several different species with different chemical driving forces. In multiphase systems, quantitative analysis is not yet possible.

The other point to emphasize is that for many years the aim of sintering has been to make dense ceramics. The terms sintering and densification were almost used interchangeably. Today, there are many uses for porous ceramics and these materials must also be sintered. The aim is then different and the process must change accordingly. Most of the time we assume that the material we are sintering is single phase, so we make some assumptions that will not necessarily be valid for multiphase materials that also need to be sintered.