"Is densification during crystalline sintering diffusion limited?"
Sintering is an industrially important powder consolidation process that has gained renewed interest as a process to densify 3D printed powder compacts. Despite ≈70 years of scientific investigation, practical aspects of sintering continue to present challenges to our fundamental understanding of the process. This is highlighted by ongoing contentious debates around the roles of electric fields, high heating rates, and chemical additives in affecting sintering kinetics. Such experiments are primarily interpreted in the context of diffusion rate limited sintering models, where geometry, surface and grain boundary diffusivities, and surface and grain boundary energies are the primary variables. Determining all of these variables well in any particular system has always been experimentally challenging and laborious, which makes quantitative assessment of the models difficult.
We developed ultrahigh temperature small-scale mechanical testing methods based on laser heating and in situ transmission electron microscopy imaging. The methodology is applied to performing a series of bicrystal Coble creep, zero-creep, and unconstrained sintering experiments that provide a basis for measuring interfacial energies and diffusivities, and grain boundary point defect formation and migration volumes that identify the diffusion mediating defect. These data are applied to interpreting the sintering behavior of model 2-particle sintering configurations in cubic ZrO2 and Al2O3-GdAlO3 composites observed in situ. The associated kinetic analyses indicate that, for the particles characterized, densification kinetics are interface reaction rate limited. The densification observed occurs during discrete steps, whose kinetics follow measured diffusivities, with longer intermittent incubation periods. It is hypothesized that these events are preceded by nucleation of climb mediating grain boundary dislocations. A large energy barrier is observed whose magnitude agrees with the work associated resulting from the vacancy flux between the surface and the dislocation sink in the grain boundary. The new understanding derived from these experiments is primarily discussed in the context of heating rate effects on sintering kinetics. The results, however, have broad impacts on how other variables such as chemistry, applied stress, or electric field affect sintering kinetics.