Solid-state contributions to densification during liquid-phase sintering

Abstract

Densification via liquid-phase sintering generally requires transport of substantial amounts of dissolved solid through the liquid. However, in composite systems, such as W-Cu, solid solubility in the liquid is almost negligible, and densification is hindered by the low amount of total mass transport. In this case, solid-state sintering of the skeletal solid structure in the presence of the liquid is a significant densification mechanism. In this article, the relative contributions to densification of both liquid and solid mass transport mechanisms are considered. A computer simulation is constructed to predict the densification behavior and concurrent microstructural development of liquidphase sintered composites for realistic heating cycles. Governing differential equations for densification are derived from idealized models of the microstructure, considering grain size, diffusion distance from vacancy source to sink, pore size, and pore morphology. Temperature-dependent terms, including the diffusivity, solubility, and surface energy, govern densification and microstructural parameters, such as the grain size, dihedral angle, and contiguity. Predictions for the sintered density, grain size, and contiguity are compared to experimental results for the W-Cu and W-Cu-Ni systems with approximately 20 vol pct liquid. For W-Cu, which has almost no intersolubility, solid-state sintering of W in the presence of liquid Cu is the dominant densification mechanism. Nickel additions increase solid solubility in the liquid and improve typical liquid-phase sintering contributions to densification. Alternatively, high sintered densities can be achieved in the absence of solubility with a sufficiently small particle size due to the solid-state contribution.