6.1. Introduction

Sintering may be considered the process by which an assembly of particles, compacted under pressure or simply confined in a container, chemically bond themselves into a coherent body under the influence of an elevated temperature. The temperature is usually below the melting point of the major constituent. Much of the difficulty in defining and analyzing sintering is based on the many changes within the material that may take place simultaneously or consecutively. Densification or shrinkage of the sintered part is very often associated with all types of sintering. However, sintering can take place without any shrinkage; expansion or no net dimensional change is quite possible. From the tooling point of view it is preferred to avoid very large amount of dimensional changes. The driving force for solid state sintering is the excess surface free energy. Sintering is a complex process and for any given metal and set of sintering conditions there are likely to be different stages, driving forces and material transport mechanisms associated with the process.

Various stages of sintering can be grouped in the following sequence:

1. Initial bonding among particles

2. Neck growth

3. Pore channel closure

4. Pore rounding

5. Densification or pore shrinkage

6. Pore coarsening

A detailed description of these stages is given by Hirschhorn1 and German.2 Sometimes some stage overlaps over the later stages.

Among various mechanisms evaporation and condensation, surface, grain boundary and volume diffusion and plastic deformation mechanisms are prevalent. The description of such topic is beyond the scope of this book. However, a detailed...