**6. Sintering**

Sintering is a non-equilibrium thermodynamic process in which a system of particles (powder or compacted aggregate) acquires a coherent solid structure by reducing the specific surface area. The result is the formation of grain boundaries and growth of necks and inter-particle bond, typically leading the system to densification and volumetric shrinkage (Gomes, 1995).

Solid-phase sintering occurs at temperature where none of the system elements reaches their melting point. This is accomplished with material transport (atomic diffusion, vapor transport, viscous flow, etc) (Costa, 2004).

In the second case, liquid-phase sintering leads to the formation of a liquid phase capable of dissolving a percentage of particles. This yields a diffusion pathway analogous to the grain boundary in the solid-phase sintering, causing a rapid initial density increase and then, dissolution of solid particles in the liquid and precipitation on the neck region occur (Gomes, 1995).

The sintering of metal matrix composites can be divided into two categories: a) solid-phase sintering (including powder metallurgy) and b) liquid-phase sintering (Kocjak, *et al*, 1993). This is a complex step in the powder technology route and is influenced by several parameters such as: solubility, self-diffusivity and inter-diffusivity, mutual solubility and wettability, etc., which act simultaneously on the process and systems are so diverse that it has not yet been possible to develop a single sintering model capable of meeting the most varied and possible systems. Therefore, the solution is to develop a model for each system to be studied.
