Sintering: physico-chemical aspects - Part 1: Solid phase sintering

Sintering can be described as the transformation of a powdery compact into a coherent material under the action of heat. During this transformation, the shape of the part is preserved, but its volume generally decreases (shrinkage phenomenon). Consolidation of the compact takes place through the formation of bonds between the grains; if fusion occurs during sintering, it can only be very localized, in order to maintain the coherence of the part as a whole. For refractory materials, or those that decompose before melting, sintering is an essential means of producing parts at temperatures below melting or decomposition temperatures; on the other hand, as soon as a powder is heated to high temperatures, the sintering that occurs tends to weld the grains together, reducing the surface area reactive with the outside, and often opposing the reactivity of the powder (reduced activity of a catalyst, for example).

Prior to the sintering stage, the part is shaped using techniques that depend on its size and complexity: uniaxial pressing for simple cylindrical shapes, casting for hollow parts, and injection molding for precision parts. The part thus prepared, known as the green part, is then consolidated and densified by sintering heat treatment. From a technological point of view, a distinction is made between natural sintering, carried out without mechanical stress (heating in air or in a controlled atmosphere), and loaded sintering, carried out by applying external pressure simultaneously with heating. The latter is particularly used for materials that are difficult to sinter, or to obtain densities close to the theoretical maximum.

From a physico-chemical point of view, there are three types of sintering, depending on the reactions taking place inside the tablet:

• solid phase sintering, during which all components remain solid. Densification is achieved by welding and changing the shape of the grains. Sintering can be single-phase with one component, or multi-phase. Additives can be added to accelerate densification without creating a liquid phase. This sintering mode is particularly used for technical ceramics where the purity of the grain boundaries is important for the properties of use: thermomechanical (cutting tools, thermal barriers, nuclear fuel), electrical or magnetic (dielectrics, varistors, superconductors), optical or even biological;

• vitrification, which corresponds to the appearance of a large vitreous phase during heating, sufficient to fill the pores. This process is widely used to densify traditional ceramics such as porcelain... Obtaining a liquid phase in addition to the solid phase requires the reaction of several constituents, or the fusion of one of them to the exclusion...