DENSIFICATION OF LARGE PORES .2. DRIVING POTENTIALS AND KINETICS

A kinetic analysis of densification is presented for the case of isolated, identical pores separated by dense, polycrystalline material where the grain size is a variable (smaller to larger, relative to the pore size). The analysis includes the influence of both grain growth and mass transport rates on the driving potential for mass transport to the pore. For the expected condition where the rate of grain growth is much greater than the rate of pore shrinkage, it is shown that the driving potential is relatively independent of pore surface curvature, and approaches (2gamma(s)/R(po)) sin (PSI(e)/2) during grain growth, where gamma(s) is the surface energy per unit area of the material, PSI(e) is the dihedral angle, and R(po)) is the initial pore radius. Using this driving potential, an expression is derived for the current densification rate. The proposed mechanism for mass transport is radial diffusion through a spherical polycrystalline unit cell containing a spherical pore, where diffusion is restricted to grain boundaries that intersect the pore. The expression includes the average separation distance between pores and the grain boundary area intersecting each pore. This expression is in qualitative agreement for data reported in Part I for a Zr(3Y)O2 material where the grain size is always smaller than the pore size, and a Zr(8Y)O2 material where the grain size is larger than the pore size.