The kind of heat treatment that allows to obtain the final properties of the material and product is called sintering. It consists in heating and holding the molded product (billet) at a temperature below the melting point of the main component. For multicomponent systems, solid-phase and liquid-phase sintering are distinguished.
Solid-phase sintering is accompanied by the appearance and development of bonds between particles, the formation and growth of contacts (necks), the closure of through porosity, enlargement and spheroidization of the pores, compacting the workpiece due to shrinkage. In the process of sintering, mass transfer of matter through the gas phase occurs due to surface and bulk diffusion, viscous flow, flow caused by external loads (sintering under pressure). During sintering, recrystallization is also observed (the growth of some grains at the expense of the others of the same phase). The heat seal is mainly due to the volume deformation of the particles, which is realized by the bulk self-diffusion of the atoms.
Liquid-phase sintering takes place in the presence of the liquid phase of the low-melting component, which wets the solid phase well, improves cohesion between the particles, increases the diffusion rate of the components, facilitates the movement of the particles relative to each other. Poor wettability prevents compaction. The solid phase in the contact zone can dissolve in the liquid, intensifying the processes of mass transfer (Fig. 4, b). Distinguish systems with insoluble components, with limited solubility and with significant mutual solubility of components. Liquid-phase sintering of such systems has its own peculiarities associated with the predominance of one of the stages:
– viscous flow of liquid – rearrangement of particles;
– dissolution – precipitation; Formation of a rigid skeleton.
Combination of the pressing and sintering process is observed during hot pressing, which is carried out at a temperature (0.5-0.9 melting point) of the main component. The high temperature of pressing makes it possible to reduce the pressing pressure by several tens of times. The holding time is from 15-30 minutes. Up to several hours. Hot pressing is used for difficult-to-apply powders in order to obtain high physical-mechanical properties. Hot-pressed parts have a fine-grained structure. The mold in which hot pressing is carried out is made of heat-resistant materials, and when pressing the refractory compounds it is made of graphite, the strength of which increases with increasing temperature.