ON THE VARIOUS TYPES OF ALUMINA DUE TO THE HIGH TEMPERATURE INTERACTION OF AlCu-SiO₂ AND Al-ZnO COUPLES

Comprehensive studies on the high-temperature interaction between liquid Al or AlCu32 alloy and SiO₂(quartz) and ZnO substrates were performed. The AlCu32/SiO₂and Al/ZnO couples were produced in a classical sessile drop tests at a temperature of 800°C, 900°C and 1000 °C for different time of contact under a vacuum. In order to clarify which phases are exactly formed at processing temperature, i.e. when the structure of the reaction product region formed due to redox reaction is less affected by cooling history, the drop/substrate interface was in situ “opened” at a test temperature under vacuum by using drop pushing procedure [1].

After the tests, the samples were examined using scanning and transmission electron microscopy techniques.The detailed structural characterization of drop/substrate interface evidenced that in both couples the main solid reaction product was α-Al₂O₃phase forming the mutually interpenetrating channels either with the Al(Zn) or formed most probably during the cooling Al₂Cu phase. Moreover, a careful TEM examination showed the presence of four different types of alumina, depending on their localization with respect to the substrate. In the AlCu32/SiO₂sample, the substrate-side interface is composed of the mixture of θ-Al₂O₃and γ-Al₂O₃phases interspersed with small and deformed precipitates of the Al₂Cu and Si. On the contrary, the substrate-side interface in the Al/ZnO couple formed after 2 h contact at 800 °C presents a thin layer of δ-Al₂O₃ phase of a different morphology depending on the type of ZnO used. In the case of <0001> ZnO single crystal, the formation of continuous δ-Al₂O₃ layer took place while for the polycrystalline ZnO, this layer had a complex morphology, i.e. it has a fine-grained structure in the area closer to the substrate and columnar one at the reaction product region side.However, the short time of interaction (5 and 20 minutes) at 1000 °C between the <1-100> ZnO single crystal and liquid Al led to the formation of two layers of about 150-200 nm in thickness. The first one, located at the substrate-side, was identified as ZnAl₂O₄spinel while the second one, formed next to the ZnAl₂O₄ layer, was suggested to be alumina of unknown type. The well-distinguished epitaxial growth of these both layers together with the appearance of the selected area electron diffraction patterns typical for the hexagonal lattice allowed to make an assumption that the lattice of this Al₂O₃can be hexagonal. If so, the calculated cell parameters would be c= 16 Å and a=5.5 Å.