The present thesis consists of two parts: one for particle rearrangement which may occur at very early stage of liquid-phase sintering and the other for densification during liquid-phase sintering.
In the first part, a model of particle rearrangement has been developed to describe the motion of particles which are connected by liquid bridges. The model takes into account capillary force, inertia of particles, viscous drag of liquid, elastic collision and torque which induces particle rotation with respect to each other. Below a critical thickness of liquid film between particles, the liquid is assumed to follow the Maxwell model. For a single pair of particles, the model predicts that, with increasing viscosity, the first contact of particles occurs later and subsequent particle bounce is weaker. When the model is applied to planar arrays of particles, the particles are rearranged and agglomerated, generating pores between particle clusters during the rearrangement. The particle packing density inside the particle clusters increases with the presence of torque component during the rearrangement, resulting in higher shrinkage. The time to reach a stable microstructure is in an order of $10 \mu sec$, which is much shorter than the heat transfer time into the center of a bulk specimen for the melting of liquid-forming particles. This result implies that the overall kinetics of rearrangement may be controlled by heat transfer rate in real processing.
In the second part, a new theory of liquid-phase sintering, namely the pore filling theory, has been developed. So far, the densification during liquid-phase sintering has been explained by contact flattening between two particles since the development of a related theory by Kingery. The contact flattening theory, however, has critical defect because of ignorance of grain growth, which is another basic phenomenon occurring during liquid-phase sintering. Some attempts have been made to overcome the basic problem in the contact flattening theory. Nevertheless, the developed theories also have problems because the grain growth was assumed to occur with a self-similar shape which is far from real microstructure development. The developed pore filling theory can overcome the intrinsic problem of the contact-flattening-based theories, by considering both densification and grain growth into account for densification calculation. The pore filling theory allows to predict the effects of such various processing parameters as pore size distribution, pore and liquid volume fraction, dihedral and wetting angle, particle size (scale), entrapped gas, etc.
It has been found that pore size distribution has a considerable effect on sintering kinetics of powder compacts with same porosity. The densification is faster as the distribution of pores is narrower and average pore size smaller. The effect of porosity also is found to be considerable but less than the effect of liquid volume fraction. Dihedral angle increase enhances densification but not by much. In contrast, wetting angle increase considerably retards densification. The effect of scale is determined by the kinetics of grain growth, resulting in the same scale exponent as that of grain growth. The effect of the entrapped gas also can be predicted by the present theory.
Based on the present theory, the microstructure evolution during liquid-phase sintering also has been analyzed in terms of interrelationship between average grain size and relative density. For constant liquid volume fraction, the microstructure trajectories reduce to a single curve in a grain size(x)-density(y) map, regardless of grain growth constant. The slope of curves in the map is inversely proportional to average pore size, while it increases rapidly with liquid volume fraction increase. Increase in pore volume fraction retards the densification considerably, but shows marginal effect on the slope. The activation energy of densification is predicted to be the same as that of grain growth as long as the liquid volume fraction is constant for any temperature range studied. The analyses on microstructure evolution may demonstrate the usefulness of pore filling theory and provide a guideline for process optimization of liquid-phase sintering.
