Determination of interfacial properties between AlN and aluminum beneath salt at high temperature

Aluminum nitride is of interest as a material for electrolysis cells in the aluminum industry due to its chemical stability when in contact with molten aluminum and/or cryolite-based salt melts. It has also been considered in combination with electrically conductive materials (i.e. AlN/Al-composite) as a material for drained cathode systems in Hall-Heroult processes. Knowledge of the interfacial properties of AlN in contact with molten aluminum and/or cryolite-based melts is therefore important. This paper reports observations of the wettability of AlN by aluminum under salt cover at high temperature using an X-ray technique. Results obtained in this work combined with previously published data are used for the assessment of the work of adhesion of molten aluminum on AlN under vacuum as well as under a cryolitic salt. Scanning electron microscopy examination of metallographic sections was used to confirm the nature of the interfaces. The measured contact angle between AlN and molten aluminum beneath a salt cover at 850 °C is 136° demonstrating the non-wettability of AlN by liquid aluminum under these conditions. The work of adhesion of molten aluminum on AlN is higher under vacuum than under salt. Previously published data allowed the determination of the interfacial properties between liquid aluminum and AlN under a salt cover. The interfacial energy between molten aluminum and salt is 773 mN/m at 850 °C. The work of adhesion of aluminum on AlN is 217 and 1322 mN/m under salt and under vacuum, respectively.