In situ observation of the thermally induced reduction of fused silica coatings in contact with molten aluminumWednesday (07.10.2020) 17:20 - 17:40
Metastable alumina phases covering the surface of ceramic foam filters are regarded as efficient functional materials for segregation of non-metallic inclusions from aluminum melts directly in the casting process. Desired metastable alumina phases are often produced, when silica is reduced by molten aluminum. Thus, the high-temperature reaction between a silica coating deposited on the surface of a ceramic foam filter and the aluminum melt can be utilized for the functionalization of the metal melt filters. The aims of this study were to describe the reaction kinetics and the sequence of phases that form at the interface between fused silica and molten aluminum, and to specify the conditions, which lead to the formation of γ-Al2O3.
The reaction kinetics and the phase formation were monitored by in situ high-temperature X-ray diffraction (HTXRD). The HTXRD experiments were carried out at slowly increasing temperature up to 750°C. The samples under study were polished fused silica substrates that were coated with thin aluminum layers prior to the in situ HTXRD experiment. The thickness of the aluminum layers was adjusted in the way that also the XRD signal from the original Al/SiO2 interface could be registered. Reduction of SiO2 to Si started at approx. 660°C and proceeded until the original Al layer was consumed. The aluminum oxide crystallized as γ-Al2O3. The reaction kinetics in the interface zone will be discussed in detail. Metastable γ-Al2O3 remained stable for several hours at 750°C, which is the typical casting temperature of aluminum alloys. Complementary microstructure analysis of the reaction layer, which was done post mortem on quenched samples using scanning electron microscopy, revealed nano-sized dendritic intergrowth zone of γ-Al2O3 and Si at the former metal ceramic interface.