Sintering

Sintering is a heat treatment process that compacts and solidifies powder materials by heating them to a temperature below their melting point. The process bonds particles through diffusion and capillary forces, reducing porosity and producing a cohesive solid with a stronger, continuous microstructure.

The typical sequence begins with powder preparation and shaping, such as uniaxial pressing or cold isostatic pressing, followed by binder removal if organic binders are used. The compact is then heated in a controlled atmosphere to the sintering temperature, chosen to promote diffusion without melting the material. In pressureless sintering, densification occurs mainly through diffusion; in pressure-assisted variants such as hot pressing or spark plasma sintering, applied pressure accelerates bonding and can yield higher densities at shorter times.

Mechanisms of densification include lattice diffusion, grain boundary diffusion, surface diffusion, and, in some ceramics, viscous flow. Neck formation between particles reduces surface area and porosity, while grain growth accompanies densification and influences final properties. The presence of a transient liquid phase in liquid-phase sintering can enhance diffusion and shrink porosity more rapidly.

Types of sintering encompass solid-state sintering, liquid-phase sintering, and reactive or transient-liquid processes. Variants based on applied pressure, such as hot pressing or spark plasma sintering, produce denser components with finer control over microstructure.

Applications are widespread in ceramics and metal powders, including alumina and silicon carbide parts, steel and copper components, dental implants, turbine and bearing parts, and cermets. The resulting materials typically exhibit increased strength and stiffness, reduced porosity, and altered toughness, with final properties governed by density, grain size, and porosity distribution.