Densification of refractory aggregates at temperatures below their melting points (sintering) connotes shrinkage, as the void spaces between the particles of refractory aggregate decrease in size and are thus, eliminated. Hence, more than any other property, shrinkage is characteristic and one of the immediate effects of sintering process. The adjacent figure shows schematic representation of densification of a compact of a single particle size and spherical shape. With gradual heating of the compact (consider green refractory brick) during firing a temperature is reached when the random movements of ions (diffusion) begin and necks are formed between the adjacent particles which causes shrinkage. With shrinkage, pores come closer and the process continues until all pores e closed. The pore shrinkage decreases the overall size as much as surface tension and gas pressure inside the pore will allow. The temperature at this point is considered to be the ideal sintering temperature for that particular refractory composition. Above this temperature, any coalescence of pores and increase in pressure inside the pores will cause the sintered compact to bloat or swell called, overfiring. In order to develop good properties in the brick, proper sintering becomes necessary. A well sintered refractory brick would have high density, high strength (CCS), low porosity, high MOR, high thermal conductivity among many other properties.
Mostly there are two types of sintering in refractories -
1) Solid - solid sintering
2) Solid - liquid sintering
There are three stages of sintering:
1) The early stages
2) Stage of densification and grain growth
3) Final stages of closed pore space.
The three stages of sintering can be characterized by three changes:
1) Changes in grain size and shape
2) Changes in pore shape
3) Changes in pore size.
The literature in the field of Sintering is quite extensive. Although the term ‘Sintering’ has been defined differently by so many authors, yet it is difficult to find a single definition which completely covers all the complex features of sintering and distinguish between the mechanisms of so many types of sintering like, cold sintering, pre-sintering, pressure sintering, high sintering, solid-state sintering, liquid phase sintering, reaction sintering, activated sintering, and many more.
The first attempt for a scientific approach of sintering was presented by J. Frenkel followed by the model experiments of Kuczynski which was widely discussed during “Symposium on Physics of Powder Metallurgy” in Bayside, New York (1949). Perhaps Rhines and his fellow workers were the first to study and describe sintering in terms of porosity and its changes as affected by material movement during sintering.A more comprehensive definition of sintering is, “the consolidation, densification, recrystallization and bonding obtained by heating agglomerated powders during or following compaction (pressing) at temperature below the melting point of the principal component”.