First stage of reaction of molten Al with MgO substrate

• New unique evidence of first stages of interaction of liquid Al with MgO substrates
• Interaction of liquid Al with MgO starts with the formation of a layer MgAl2O4.
• Growth of MgAl2O4 is slow as controlled by solid state out-diffusion of Mg and O.
• MgAl2O4 serves as a nucleation site for Al2O3 and consumed by it soon after.
• Growth of Al2O3 is fast as controlled by diffusion in liquid state.

The Al/MgO couple was produced in vacuum (~ 5 × 10− 4 Pa) by contact heating from RT up to 1000 °C and holding at that temperature for 1 h of a small 4 × 4 × 4 mm aluminium (5 N) sample placed on the [100] MgO single crystal substrate. TEM observations backed with electron diffraction analysis indicated that the interaction between liquid aluminium and MgO starts from a redox reaction producing a continuous layer of MgAl2O4 spinel on the substrate surface. Its growth is controlled by solid state out-diffusion of magnesium and oxygen towards the surface being in contact with liquid metal. The thickening of spinel layer is accompanied by its cracking and infiltration with aluminium. The above process enables local dissolution of the MgO substrate and formation in it of a thin region of interpenetrating metallic channels walled with spinel. The removal of dissolved magnesium through open aluminium channels towards the drop and to vacuum locally produces areas of aluminium enriched with dissolved oxygen, which results in the nucleation of α-Al2O3 at spinel clad walls. The growth of α-Al2O3 is controlled only by the dissolution rate of MgO by aluminium, liquid state diffusion of Mg to drop/vacuum and oxygen to the front of the of α-Al2O3 crystallites growing into MgO substrate.