A THEORETICAL-STUDY OF GRAIN-GROWTH IN POROUS SOLIDS DURING SINTERING

The processes of grain boundary migration, pore drag and pore/boundary separation are described on the basis of the phenomenological equations for boundary migration and surface diffusion. Cylindrical pores on triple grain junctions are assumed to represent the open porosity during intermediate-stage sintering. It is found that cylindrical pores can hardly detach from migrating boundaries. Three-dimensional closed pores, however, which predominate during final stage sintering, can separate from migrating grain junctions. The separation process is modelled numerically and the conditions for separation are formulated. Analytical approximations for the pore mobility are shown to describe the numerical results well. They serve to establish effective mobilities of grain boundaries bearing pores in various configurations. Classical theories of grain coarsening are modified by using these effective mobilities. Mechanical constitutive models of sintering contain the grain size as an internal variable. The present analysis leads to an evolution equation for the average grain size, which depends on the volume fraction of the pores and on their configuration.