Sintering: physico-chemical aspects - Part 2: Liquid phase sintering

When a liquid is introduced into any granular stack, various phenomena can be observed: the cohesion between grains increases (slightly damp sand heap), part of the porosity is eliminated as the liquid migrates into the pores, sometimes accompanied by a contraction of the stack. What's more, some very slow transformations can be accelerated by faster diffusion in the liquid phase.

These properties of liquids have long been used to facilitate sintering. In silicate ceramics, for example, sandstone formation is the rule, where reactions between the starting constituents form compounds that melt at fairly low temperatures, with the development of an abundant quantity of viscous liquid.

Various technical ceramics are also sintered in the presence of a liquid phase. This is particularly true of highly covalent products such as nitrides, carbides and borides, for which the temperatures required to achieve significant densification by solid-phase sintering are either too high, or incompatible with the chemical stability of the compound. The solution generally adopted to densify these products is to promote the appearance of a liquid in the microstructure by means of a suitably chosen addition. Sintering can be carried out with or without the application of external pressure.

Liquid phase sintering rarely does not involve chemical reactions; when the influence of these reactions is not predominant, interface effects predominate.