Sintering

Sintering
Sintering is a manufacturing process in which a fine powder that has been formed into a shape is subsequently fired at temperature below melting point. The compact, when fired, densifies and becomes non-porous . The powder particles form bonds at this temperatures and the contact points between particles, which were originally formed under the mechanical pressure during compaction, increase in size and strength, improving mechanical properties.
sintering is a thermal treatment that densification accompanies it. Densification almost always requires shrinkage. The shrinkage takes place as a result of materials being transported by one or more of several diffusion processes. This may involve a liquid or reactive liquid or diffusion at grain boundaries or through the volume of particle.
Therefore, it is a densification process where porosity is usually reduced and the grain size increases significantly through mass transport. This is important because it allows an engineer to form a dense ceramic body at temperatures below the often very high melting temperature of the ceramic. For many properties such as strength, thermal conductivity, and translucency, in order to reach a maximum value for a product, it is necessary to remove as much porosity as possible. A formed powder compact can be anywhere from 40% to 75% dense, with little strength. A sintered body can have much higher strengths and densities exceeding 99% .
The ability to achieve dimensional tolerances in powder metallurgy parts arises directly from the control of the sintering process and in particular shrinkage. Thus a detailed understanding of the sintering process and the associated shrinkage is of considerable importance.

Sintering Mechanisms
Sintering proceeds from various mass-transport mechanisms. These can be divided into surface transport and bulk transport mechanisms. In surface transport mechanisms, atoms move from the surface of one particle to the surface of another particle. In bulk transport mechanisms, atoms move from the particle interior to the surface. Surface transport mechanisms lead to neck growth without shrinkage or densification, while bulk transport mechanisms result in net particle movement, leading to shrinkage and densification. Densification means an increase in packing density. The surface transport mechanisms are surface diffusion and vapor transport, and the bulk transport mechanisms are lattice diffusion, grain boundary diffusion, and viscous flow. In powders composed of different materials, chemical reactions (also called reactive Process) may also provide additional mass-transport mechanisms .
Different mechanisms dominate at different points in the sintering process, and different materials exhibit different mechanisms. For instance, viscous flow is the diffusion in the liquid phase and evaporation condensation (vapor diffusion) is in the gaseous phase. Evaporation condensation and surface diffusion are called non-densifying mechanisms because they do not contribute to the pore shrinkage. They only increase the cohesion of the grains by the increase of the grain contacts. While other mechanisms contribute to the decrease of the volume fraction of pores; i.e.; the shrinkage .
The compaction powder (called a green compact) has a large surface area relative to its volume. This surface area provides the driving force in sintering, which is the reduction of free surface energy resulting from the high surface area of the particles.