Nanoscopic metal fluorides as promising sintering aids for high-performance alumina ceramics

Nanoscopic aluminium fluoride and magnesium fluoride were investigated as sintering additives for high-purity alumina ceramic production.Nanoscopic magnesium fluoride has been found to be an excellent sintering additive. It enables an effective limitation of grain growth compared to standard ceramic production, thus resulting in ceramics with high density, sub-µm grain size and low concentration of defects, which are perfect preconditions for an improvement of mechanical strength, transparency as well as optical properties. As a result, transparent corundum ceramics with a Vickers hardness of about 3200 were obtained. A mechanistic interpretation of the possible chemical role of nano-MgF2 in the course of texture formation, crystal growth, and densification of the corundum particles is presented based on DTA-TG and 19F MAS NMR-investigations. However, in contrast to literature reports, AlF3 is not suitable for the preparation of slurries and spray granulates with application of the standard technology mainly due to its strong tendency toward pyrohydrolysis, resulting altogether in outgassing effects.