6.2. Liquid Phase and Activated Sintering

Liquid phase sintering method is getting more and more common, in which the presence of liquid phase during all or part of the sintering cycle of material is used for enhanced densification. There are two variations of the process:

1. normal liquid phase sintering for which the formation of the liquid phase is associated with one or more components contained in the original green compact.

2. infiltration of the original green compact with a liquid formed outside the compact during the very early period of sintering. Although simultaneous infiltration and sintering appears to be dominant, infiltration of a previously sintered part is also practised.

During liquid phase sintering three stages 'rearrangement' or 'liquid flow', 'accommodation' or 'dissolution and reprecipitation and 'coalescence' or 'solid phase sintering' take place. These stages follow in the approximate order of their occurrence, but there may be significant overlapping for any specific system. Figure 6.1 shows the densification stages during liquid phase sintering. With progress in liquid phase sintering the densification kinetics is lowered. Increasing the liquid content up to approximately 35 volume percent aids initial densification. A coarse particle size and a high green density act to offset the favourable effects of the melt. In case the second stage is not effective, the melt will penetrate along the interparticle interfaces and cause particle separation. This will contribute in swelling. A small dihedral angle inhibits coalescence of neighbouring particles. A detailed account on the theory of liquid phase sintering in given in...