Interface reactions of differently coated carbon-bonded alumina filters with an AZ91 magnesium alloy melt

To investigate possible reactions between differently coated carbon-bonded alumina filters and an AZ91 magnesium alloy melt, immersion tests were carried out. Uncoated as well as MgAl₂O_4-, Al₂O_3-, nano- (carbon nano tubes/alumina nano sheets) and MgO-C-coated filters were tested. Thermodynamic calculations showed that only magnesia (MgO) and carbon are stable against molten magnesium; alumina (Al₂O₃) and spinel (MgAl₂O₄) will be reduced under the formation of magnesia. Optical and scanning electron microscopy as well as EDX analysis were performed near and at the filter-magnesium alloy-interface of the cooled and sectioned filter samples after their immersion into the AZ91 melt. The results of the thermodynamic calculations were confirmed by the experiments. The MgO-C-coated filter was the only one that did not show an in situ-formed layer on its surface after being in contact with the magnesium alloy melt. The alumina- or spinel-containing filter surfaces displayed platelet-like in situ layers after their contact with the molten AZ91. The results of the EDX analysis of these layers suggest their composition of MgO, since notable respective Mg and O contents were detected, as predicted by the calculations.