Sintering is the process when powders bond together when heated to above
approximately half of their melting temperature. The main driving force for sintering
is the reduction of total surface energy by the formation of interparticle bonds, as
particle surface energies are larger in magnitude than grain boundary energies.
At liquid phase sintering, a liquid phase coexists with a solid at the sintering
temperature. It is a common production process for the fabrication of near net-shape
products like hard metals ( cemented carbides) and for dense tungsten based heavy
metals. A mixture of different metal or ceramic powders is heated to the melting
temperature of one constituent. At melting, the liquid phase, the matrix , wet and
infiltrate the solid grain structure by a combination of reaction and capillary forces,
followed by a dissolving and growth of the solid particles by coalescence and
Ostwald -ripening. The driving force for liquid phase sintering is not only the reduction
of surface energy by capillary forces but also the reduction of chemical potential by
dissolving of original and growth of equilibrium solid phase.
A major advantage of liquid phase sintering is the enhanced liquid atomic diffusion
and mass transport, which results in rapid sintering of the components. The wetting by
capillary attraction also provides a smooth rearrangement of the solid particles and a
densification without the need of external pressure. The disadvantages is related to the
parameters that control the sintering process, the solubility, diffusivity and the surface
energies of the phases present, which coupled with the rapid rates of sintering give less
predictability of structure and properties for the components produced.