Packing-dependent viscous sintering of glass powder from wet deposition

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Spraying, brushing and dipping of glass powder slurries on/of metal and ceramic substrates are state-of-the-art coating technologies in the enamel and dental-ceramic industries. Process parameters influencing the structure of the green body and the fired glassy coat are mostly empirically developed. A knowledge-driven understanding of the underlying mechanisms that control viscous sintering is up to now still missing. In particular the effect of non-regular and inhomogeneous packing may largely control sintering due to the relative low process temperatures, adhesion of submicron-particle fractions and jagged particles of non-spherical shape. The project aims therefore to gain a deeper understanding of the parameters that control the sinter kinetics. On one hand, this is tackled by exploring the 3D microstructure of the green body and the changes after short periods of firing using X-ray tomography (XCT), X-ray microscopy (XRM) and TEM tomography to visualize local particle contacts in the micron and sub-micron range, respectively (flanked by Hg-porosimetry and BET analysis to cover the development of the overall open porosity). For visualizing the packing structure of the glass powder systems, XCT and XRM seems the method of choice, as these non-destructive techniques give both local and global information on the desired length scales. On the other hand, the densification during sintering of the glass particles is modelled numerically using discrete descriptions of the shrinkage, to account for the local densifying structures of the particulate. A three term model (for particle rearrangement, neck formation and pore spheroidization) will be used with the benefit of direct experimental validation by utilizing geometric parameters of the 3D data. The microstructure of dried glass powder deposition will be varied systematically with respect to the submicron-particle fraction, broadness of the particle size distribution, and thermal history of the glass particles (high vs. low fictive temperature). Sintered glass beads will work as reference in this project with respect to the effect of particle shape on packing and rearrangement.