Sintering is a process of bonding small particles without melting them. It is a simple and cheap way of fabricating parts of metals, ceramics, and some polymers. The driving force for sintering is the reduction energy resulting from decreased surface area. Most ceramics are consolidated by sintering. These include clay products as well as refractory oxides. These ceramics cannot be fabricated by melting and freezing. Sintering is also used to produce parts of metals that are difficult to melt. Examples include carbide tools and tungsten for lamp filaments. Mixed powders are sintered to make composites that are not otherwise possible, such as friction materials for brakes and clutches. Porous parts for filters or oilless bearings are made by incomplete sintering. Even some polymeric materials are sintered. Teflon cannot be melted without decomposing so Teflon parts are made by sintering powder.

Mechanisms

During sintering adjacent particles adhere and a neck is formed at the area of contact. Figure 14.1 is a micrograph of such a neck formed between two nickel spheres. There are two groups of sintering mechanisms, as shown in Figure 14.2. Mechanisms like vapor and surface diffusion transport material from the surface to form the neck. These do not change the distance between the centers of particles so they contribute little to densification. Mechanisms that transport material from the interface between the particles to form the neck (grain boundary and lattice diffusion) do cause densification.