COMPUTER SIMULATION OF SINTERING KINETICS IN TWO-PARTICLE AND POWDER SYSTEMS.

摘要

New models have been developed to describe the dynamic sintering characteristics of both amorphous and crystalline materials, based on the finite element analysis procedure. The problem of relating the sintering behaviour of a powder compact to that of a two-particle sintering model is also addressed.; Amorphous sintering is treated as a process of slow, nearly incompressible viscous flow, with finite element analysis being used to determine the instantaneous deformation rates of a sinter body in response to the capillarity-induced surface stresses. The crystalline sintering model uses finite element analysis to determine the gradient in chemical potential throughout the sinter body by solving Laplace's equation subject to appropriate curvature-related boundary values of chemical potential. The potential gradients are then used to determine surface migration rates by surface and volume diffusion, acting both separately and in unison. Both models are completely general and neither require simplifying assumptions concerning neck geometry.; The models were employed to describe the sintering of infinite lines of cylinders and spheres. A reasonable progression of surface profiles was obtained in all cases and it was observed that neck curvature in amorphous sintering is sharper than that predicted by a simple tangent-circle neck geometry. In all cases, logarithmic plots of neck radius versus time showed a distinctly non-linear character. The slopes of such plots for intermediate neck sizes indicate that the models are in reasonable agreement with simpler fixed-geometry models of sintering.; It is proposed that powder compacts are well modelled by random close packings of hard spheres. A computer program was written to simulate the shrinkage due to sintering of such a compact based on a grain boundary diffusion mechanism by balancing or accommodating shrinkage rates throughout the packing. The model was used to simulate the sintering of loose and densely packed arrays of spheres in both two and three dimensions. It was found that the shrinkage behaviour of compacts is critically dependent on the coordination number of particles in the packing: highly coordinated packings shrink nearly homogeneously, while those with low coordination numbers densify slowly with little overall shrinkage of the compact.; Finally, it is shown that the Voronoi polyhedral net of a random packing may be used to model the grain structure of a sinter body in the late-intermediate stage of sintering. This approach should allow a realistic grain structure to evolve naturally in response to capillarity, thus avoiding the necessity to assume a constant grain shape.