Various model experiments were done to explain an overall densification process during liquid phase sintering, because the previous results had been obtained independently and then were not proper to explain a whole densification process.
The relations between pore radii and average grain radii at the critical pore filling conditions were measured experimentally and compared with results of theoretical analysis. Exponential relations were found between pore radii an average grain radii, which were different from theoretically derived linear relations. These exponential relations were attributed to shape accommodation of solid grains. During liquid phase sintering, a local equilibrium between capillary force and sphering force was not maintained. Solid grains could not grow with similar shape and were deformed continuously. It was though that a critical condition for liquid flow into the pore was determined by grain shape, which also was determined by grain growth process.
Sequential pore filling was observed in a bimodal pore system. Smaller pores were filled first and filling of lager pores was delayed because of liquid segregation in the filled pore sites and slow microstructure homogenization. Homogenization assisted by grain growth was a very slow process and could control a continuous pore filling. The specimens were thought to be slowly shrunken by shape accommodation of solid grains with increased capillary pressure after pore filling. Shape accommodation of solid grains and homogenization of liquid pools were thought to act synergetically for continuous pore filling.
Microstructure changes of practical W-Ni-Fe compacts in solid state sintering during heating to the liquid phase sintering temperature as well as during isothermal liquid phase sintering were observed. The fine powder compacts were almost completely densified in solid state sintering and reached directly to final stage sintering from beginning of liquid phase sintering. Pore elimination by liquid flow also operated during sintering of practical powder compacts.
Besides spherical grain systems, microstructure change of a WC-Co system with angular grain shape were observed during liquid phase sintering. Ball-milled WC-Co compacts showed an almost same densification pattern with those of metallic liquid phase sintering system. Unmilled WC-Co compacts showed a remarkable local densification at the moment of liquid formation with simultaneous recovery of grain shape. The severe local densification was attributed to particle motion (rotation or translation) due to unbalanced capillary force between faceted solid grains at the moment of liquid formation. Additionally artificial large pores were also found to be filled with liquid flow, but a relation between pore radii and average grain radii could not be found.
