Sintering joins a compact of particles by heating it below the temperature at which the entire body becomes a liquid. The main thermodynamic driving force is reduction of interfacial energy: a collection of small particles has much more surface area than one dense body of the same mass.

Early in sintering, neighboring particles form necks. Atomic transport enlarges those necks. Depending on material and conditions, transport can occur by surface diffusion, grain-boundary diffusion, lattice diffusion, evaporation-condensation or viscous flow.

Not every transport path causes densification. Moving matter along a free surface can grow necks while leaving pore volume nearly unchanged. Densification requires transport that shrinks pore space and brings particle centers closer together.

As sintering proceeds, interconnected pores can become isolated, grain boundaries migrate and grains may grow. Excessive grain growth can trap pores and make full densification harder.

A useful design tradeoff is therefore temperature and time versus microstructure: stronger heating accelerates diffusion and densification but can also coarsen grains or trigger unwanted reactions.

Sintering is central to ceramics and powder metallurgy because it converts a shaped powder compact into a coherent solid while allowing composition and near-net geometry to be established before final densification.